NC0085359_Regional Office Historical File Pre 2018

HAZENAND SAWYER
Environmental Engineers & Scientists
August 14, 2006
North Carolina Division of Water Quality
Water Quality Section
Point Service Branch
1617 Mail Service Center
Raleigh, North Carolina 27699-1617
Re:
Al
Union County, NC
Twelve Mile Creek WWTP
Permit No. NC0085359
SOC Application
Hazen and Sawyer, P,C.
4944 Parkway Plaza Blvd.
Suite 375
Charlotte, NC 28217
AUG 1 6 2Q06
To Point Source Branch Manager:
On behalf of the Union County Public Works Department, Hazen and
Sawyer is submitting three (3) final reports which detail the Phosphorus Removal
Optimization Study at the County's Twelve Mile Creek Wastewater Treatment
Plant. The submission of this report is in accordance with the schedule proposed
in the Special Order by Consent (SOC) application package dated July 7, 2006.
Should you have any questions or require additional information, please
do not hesitate to contact me at [phone] or at
istruve c(?hazenandsawyer.com.
JNS/gb
Cc:
Very truly yours,
HAZEN AND SAWYER, RC.
James N. Struve, P.E.
Senior Associate
Christie Putnam — Union County
Mark Tye — Union County
Mike Parker — Mooresville DWQ Regional Office
New York, NY • Armonk, NY • Upper Saddle River, NJ • Raleigh, NC • Charlotte, NC • Vienna, VA • Hollywood, FL • Boca Raton, FL • Fort pierce, FL • Jupiter, FL • Miami, FL • Bogota, D_E. Colombia
TOO,

ION COUNTY PUBLIC
Christie L. .Putnr rn, P. E , Interim Director
July 7, 2006
North Carolina Division of Water Quality
Water Quality Section
Point Service Branch
1617 Mail Service Center
Raleigh, North Carolina 27699-1617
Re:
To Point Source Branch Manager:
ors
Union County, NC
Twelve Mile Creek VWVVTP
Permit No. NC0085359
SOC Application
. 1—
AND I`Z!ts $. .
Enclosed for your review and consideration is an application for a Special Order by
Consent (SOC) for the County's Twelve Mile Creek Wastewater Treatment. Plant
(WVVTP), NPDES Permit No. NC0085359. Primarily within the Last year, the plant has
had violations of BOD5, NH3-N, TSS, phosphorus, and fecal coliforms. Upon completion
of the on -going 6 mgd expansion project (scheduled for October 2008), it is anticipated
that the plant will be able to reliably comply with their NPDES effluent limits. In the
interim, however, the County is respectfully requesting that their current NPDES effluent
limits be relaxed.
We look forward to your favorable response and are available to meet at your
convenience to discuss our request in greater detail. Please let me know if you require
additional information or clarification to facilitate your review process.
Sincerely,
UN.QN COUNTY PUBLIC WORKS
hristie Putnam, P.E.
Interim Director of Public Works
Enclosure
Cc: Mark Tye— Union County
Mike Parker Mooresville DWQ Regional Office
Jim Struve — Hazen and Sawyer
400 North Church St.
e, North Carolina 28112-4804 • Phone: [phone] • Fax: (74}4)296-4232

AUG 1 2 0o
ATER
Phosphorus Removal Opt Optimization Study
for the
Twelve Mile Creek Wastewater Treatment Plant
Union County, North Carolina
Final Report
August 2006
HAZEN AND SAWYER
Environmental Engineers & Scientists

Twelve Mile Creek WWTP Phosphorus. Removal Optimization Study
Table of Contents
Executive Summary . 1
1.0 introduction 3
1.1. Existing Facilities 3
1.2. Effluent Quality Requirements 5
2.0 Wastewater Characterization 10
3.0 Operations and Performance 11
4.0 Supplemental Sampling 14
4.1. Supplemental Wastewater Characterization 14
4.2. Process Profiles 14
4.3. Clarifier Settling 15
5.0 Process Modeling and Evaluation 16
5.1. Model Calibration 16
5.2. Process Evaluation 17
5.2.1. Plant Recycle Streams 18
5.2.2. Nitrified Recycle Rate 19
5.2.3. Dissolved Oxygen Return 20
5.2.4. Readily Biodegradable Carbon 20
5.2.5. Secondary Clarifier Operation 21
5.2.6. SRT Analysis and Optimization 22
5.2.7. Chemical Phosphorus Removal 24
6.0 Operational Recommendations 25
6.1. Reduce Digester Decant Recycle 25
6.2. SRT Optimization 25
6.3. Reliable Solids Removal 26
6.4. Chemical Phosphorus Precipitation 27
7.0 Capital Improvement Recommendations 28
7.1. Additional Aeration 28
7.2. Phosphorus Monitoring 29
7.3. Particulate Phosphorus Removal 29
7.4. Chemical Addition to Digester Decant 30
7.5. Solids Handling Improvements 30
8.0 Opinion of Probable Cost ..32
Hazen and Sawyer, P.C. i Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
8.1. Operational Costs 32
8.2. Capital Costs .32
9.0 References 35
Appendix A — Flow and Load Analysis .36
Appendix B — Plant Operational Data 43
Appendix C — Supplemental Sampling Results and Profiles 46
Appendix D — Clarifier Settling Tests and Solids Flux Analysis 56
List of Figures
Figure 1-1 Total Phosphorus Effluent Limits — Current TMDL 6
Figure 3-1 Effluent BOD5 and TSS Concentrations 11
Figure 3-2 Effluent Nitrogen Concentrations 12
Figure 3-3 Effluent Total Phosphorus Concentrations 12
Figure 5-1 BioWin Calibration Model — Twelve Mile Creek WWTP ..16
Figure 5-2 Dynamic Simulation — Sampling Week ..17
Figure 5-3 Influence of Nitrified Recycle Rate 19
Figure 5-4 Effluent TP vs. Influent BOD:TP 21
Figure 5-5 Effluent Phosphate vs. SRT 23
Figure 5-6 Wasting Requirements vs. SRT .23
Figure A-1 Historical Influent Flows 38
Figure A-2 Influent BOD5 Concentration 39
Figure A-3 Influent TSS Concentration 40
Figure A-4 influent Ammonia Concentration 40
Figure A-5 Influent Total Nitrogen Concentration 41
Figure A-6 Influent Total Phosphorus Concentration 41
L. f Figure A-7 Wastewater Effluent Temperatures 42
r,o Figure B-1 Historical MLSS Concentrations 44
t Figure B-2 Historical Solids Retention Times 44
1--k Figure B-3 Historical SVI 45
L ti Figure B-4 Historical Oxidation Ditch Dissolved Oxygen 45
Figure C-1 Nutrient Profile — Oxidation Ditch No. 1—1/24/06 50
ry Figure C-2 Nutrient Profile — Oxidation Ditch No. 2 — 1/24/06 50
Figure C-3 Nutrient Profile — Oxidation Ditch No. 1—1/25/06 51
Hazen and Sawyer, P.C. ii Revised:8/11/2006

1
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i,
Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
Figure C-4
Figure C-5
Figure C-6
Figure C-7
Figure C-8
Figure C-9
Figure D-1
Figure D-2
Figure D-3
Figure D-4
Exhibit 1-1
Exhibit 1-2
Exhibit 1-3
Exhibit C-1
Table ES-1
Table 1-1
Table 1-2
Table 1-3
Table 2-1
Table 3-1
Table 4-1
Table 5-1
Table 5-2
Table 6-1
Table 7-1
Table 7-2
Table 8-1
Table 8-2
Nutrient Profile — Oxidation Ditch No. 2 — 1/25/06 51
pH Profile— 1/24/06 52
pH Profile— 1/25/06 52
Dissolved Oxygen Profiles — 1/23/06 53
Dissolved Oxygen Profiles -- 1/24/06 53
Dissolved Oxygen Profiles — 1/25/06 54
Column Settling Test Results —1/24/06 58
Column Settling Test Results --1/25/06 58
Initial Settling Velocity vs. Solids Concentration 59
State Point Analysis — Current Conditions 60
List of Exhibits
Site Plan 7
Process Flow Schematic — Liquid Stream 8
Process Flow Schematic — Solid Stream 9
Oxidation Ditch Profile Sampling Locations 55
List of Tables
Estimated Capital Costs 2
Process Design Criteria — Existing Facility 4
Current Effluent Quality Requirements 5
Effluent Quality Requirements -- 6.0 mgd Design Flow 6
Historical Wastewater Characteristics 10
SVI Percentile Values 13
Solids Flux Analysis Results 15
Twelve Mile Creek WWTP Influent Fractions 16
Maximum Month Conditions 18
Recommended SRT Optimization 26
Total Phosphorus Effluent Limits 28
Recommended Capital Improvements 31
Estimated Annual Alum Costs 32
Estimated Capital Costs for Phosphorus Removal 34
Hazen and Sawyer, P.C. iii Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
f •
Table A-1 Historical Flows and Peaking Factors 37
Table A-2 Annual Average Influent Concentrations 38
Table A-3 Average Influent Ratios 38
Table A-4 Historical Load Peaking Factors 39
Table A-5 Historical Effluent Temperatures 42
Table C-1 Raw Influent Characterization 47
Table C-2 Raw Influent + Recycle Characterization 47
Table C-3 Secondary Effluent Characterization 48
Table C-4 Filtered Effluent Characterization ...48
Table C-5 Digester Decant Characteristics 49
Table C-6 Reported Plant Operational Data — Supplemental Sampling Week 49
Table C-7 MLSS Properties — Samples Collected During Secondary Profiles 49
Table D-1 Column Settling Test Solids Concentrations 57
Hazen and Sawyer, P.C. iv Revised:S/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
Executive Summary
The Union County Public Works Department owns and operates the Twelve Mile Creek WWTP
located in Waxhaw, North Carolina. The facility is currently permitted to discharge a maximum
month flow of 2.5 mgd to Twelve Mile Creek. Effluent total phosphorus (TP) concentrations are
limited by Total Maximum Daily Loads (TMDLs) corresponding to current permitted maximum
effluent TP concentrations of 2.0 mg/L and 1.0 mg/L on a maximum month and 12-month
average basis, respectively. The monthly and 12-month average TMDLs have been exceeded
in the past year. Furthermore, the facility is currently being expanded to a design flow of 6.0
mgd, resulting in maximum effluent TP concentrations of 0.83 mg/L and 0.42 mg/L on a
maximum month and 12-month average basis, respectively, at the 6.0 mgd design flow. Future
expansions to the Twelve Mile Creek WWTP will require even further phosphorus removal since
it is not anticipated that the current TMDL will ever be increased. The purpose of this
optimization study is to develop strategies to meet the current and future phosphorus removal
goals, giving consideration to optimizing the existing facilities for phosphorus removal.
Historical operational data over the past two years were evaluated to determine influent
characteristics, operational performance and effluent quality. Additional site specific sampling
was performed to supplement the historical wastewater characterization. The supplemental
characterizations included detailed analyses of plant influent, effluent and decant constituents,
as well as secondary process nutrient profiles and solids settling tests.
A process model of the Twelve Mile Creek WWTP was developed using BioWin process
simulation software. The historical and supplemental wastewater evaluations were used to
calibrate the model. The calibrated model was used to develop various treatment scenarios to
optimize biological phosphorus removal. The process evaluation included investigations of the
following:
• Plant recycle streams
• Nitrified recycle rate
• Dissolved oxygen return
• Readily biodegradable carbon
• Secondary clarifier operation
• SRT analysis and optimization
• Chemical phosphorus precipitation
The existing 2.5 mgd Twelve Mile Creek WWTP has facilities in place to provide efficient
biological phosphorus removal and meet existing permit limitations. However,
increasing/variable loads, particularly in regards to phosphorus returned in the digester decant,
operational inconsistencies and aeration deficiencies have prevented optimal biological
phosphorus removal from occurring. The following operational modifications are recommended
to optimize phosphorus removal under the current conditions at the Twelve Mile Creek WWTP:
• Place the existing rotary drum thickener into operation to reduce thickening
through digester decanting.
• Increase solids hauling frequency to minimize aerobic digester SRT.
• Minimize the aerobic solids retention time to maximize biological phosphorus
removal while maintaining nitrification.
• Analyze effluent ortho-phosphate daily to monitor increases in effluent
phosphorus and manually control alum feed.
Hazen and Sawyer, P.C. 1 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
JIB
• Improve solids settling through polymer addition and/or RAS chlorination.
• Feed alum upstream of the secondary clarifiers to precipitate soluble
phosphorus.
Allowable effluent total phosphorus concentrations will continue to decrease as effluent flow to
Twelve Mile Creek increases. In order to meet the strict future effluent phosphorus
requirements, capital improvements to address phosphorus removal will be necessary.
Recommended capital improvements to optimize phosphorus removal to ensure permitted
effluent limits are met at the Twelve Mile Creek WWTP include:
• Provide adequate aeration to the oxidation ditch to ensure full nitrification and
luxury phosphorus uptake.
• Online analysis of ortho-phosphate to monitor effluent phosphorus
concentrations and provide automatic control of alum feed.
• Feed alum directly to the digester decant to precipitate soluble phosphorus in the
decant.
• Construction of gravity belt thickening facilities to reduce digester decant flow.
• Construction of deep bed filters once design flows exceed 6.0 mgd to provide
effective removal of particulate phosphorus.
• Construction of a secondary effluent pump station to convey flow to the deep bed
filters.
• Construction of tertiary clarification and/or membrane filtration if future TMDLs
require limits of technology (LOT).
Estimated operational and capital costs were developed for the recommended phosphorus
removal strategies. Increases in operational costs include chemical costs for alum, additional
pumping costs associated with greater wasting at lower SRTs, polymer addition to the rotary
drum thickener and possible additional staffing requirements. Capital costs include construction
of deep bed filter and pumping facilities, phosphorus monitoring equipment and additional
aeration. Capital costs associated with the various design flows are summarized in Table ES-1
below:
Table ES-1 — Estimated Capital Costs
Design Condition Estimated Cost
Current Design Flow (2.5 mgd) $100,000
Expansion to 6.0 mgd $300,000
Expansion to 9.0 mgd $9,500,000
Expansion to 12.0 mgd $1,600,000
Expansion to 15.0 mgd $1,100,000
Limits of Technology @ 15.0 mgd $17,200,000
Hazen and Sawyer, P.C. 2 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
1.0 Introduction
The Union County Public Works Department owns and operates the Twelve Mile Creek WWTP
located in Waxhaw, North Carolina. The facility is currently permitted to discharge a maximum
month flow of 2.5 mgd to Twelve Mile Creek. Effluent total phosphorus (TP) concentrations are
limited by Total Maximum Daily Loads (TMDLs) of 41.70 and 20.85 pounds per day (lb/day) on
a maximum month and 12-month average basis, respectively. The TMDL corresponds to a
maximum 12-month effluent TP concentration of 1.0 mg/L. The Twelve Mile Creek had
historically met the required TMDLs through 2004. However, the monthly and 12-month
average TMDLs have been exceeded in the past year.
The facility is currently being expanded to a design flow of 6.0 mgd. The current TMDLs will
need to be met at the expanded flow, resulting in maximum effluent TP concentrations of 0.83
mg/L and 0.42 mg/L on a maximum month and 12-month average basis, respectively. In
addition, future expansions to the facility will be required to meet the current TMDL or a more
stringent limit, requiring additional phosphorus removal strategies.
The purpose of this optimization study is to develop strategies to meet the current and future
phosphorus removal goals, giving consideration to optimizing the existing facilities for
phosphorus removal. This study will review the existing facilities and historical operational data
and recommended operational and capital improvements to optimize phosphorus removal to
comply with the TMDLs at the current and future design flows. Biological, chemical and
physical phosphorus removal processes will be considered and discussed in this report.
1.1. Existing Facilities
Raw influent enters the Twelve Mile Creek WWTP through a 48-inch main and is pumped from
the influent pump station to the headworks, where the flow is screened and grit is removed.
Plant recycle flows are also introduced at the headworks. Return activated sludge (RAS) is
introduced downstream of grit removal, and the combined influent/RAS flows by gravity to the
secondary process.
The Twelve Mile Creek WWTP has two Eimco Carrousel® A2CTM Systems for biochemical
oxygen demand (BOD) removal and biological nutrient removal (BNR). The systems consist of
an oxidation ditch preceded by an anaerobic and anoxic zone. Aeration is provided via low
speed mechanical surface aerators. One 100-horsepower (hp) aerator is installed at each
oxidation ditch. It is reported that the existing aerators operate well below the rated motor
horsepower. Based on amperage measurements, the aerator motors draw a maximum of 85
horsepower, which substantially decreases the oxygen delivered to the system. The cause of
this discrepancy is currently under investigation, and supplemental aeration will be provided at
the oxidation ditches until the expansion to 6.0 mgd is complete.
Nitrified recycle is returned from the oxidation ditch to the anoxic zone via an adjustable gate.
Two secondary clarifiers settle the mixed liquor suspended solids (MLSS) from the oxidation
ditches. RAS is pumped from a common sludge well back to the secondary process influent
pipe. Waste activated sludge (WAS) is pumped from the sludge well to aerobic digesters.
Secondary effluent is filtered through two traveling bridge filters, followed by ultraviolet (UV)
disinfection. Effluent is conveyed by gravity to a cascade aerator for re -aeration and ultimate
disposal to Twelve Mile Creek.
Hazen and Sawyer, P.C. 3 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
A
The solids handling facilities consist of three aerobic digesters. Aeration is provided through
coarse bubble diffusers. A rotary drum thickener is also installed at the facility. Sludge is
thickened in the digesters either by mechanical means (i.e., operating the rotary drum thickener)
or by gravity solids/liquid separation (Le., returning decant to the head of the plant). Sludge is
stored in the digesters until it is removed and hauled for disposal via land application. Solids
are typically removed from the facility four times per year.
The layout of the existing facility is shown in Exhibit 1-1. Exhibits 1-2 and 1-3 present the liquid
stream and solids stream process flow schematics, respectively. Table 1-1 summarizes the key
process design criteria and existing facilities at the Twelve Mile Creek WWTP.
Table 1-1 — Process Design Criteria — Existing Facility
Design Parameter Design Value
Influent
Flow, mgd 2.5
BOD5, mg/L 250
TSS, mg/L 250
TKN, mg/L 40
TP, mg/L 8
Effluent
BOD5, mg/L < 5.0
TSS, mg/L < 15
NH3-N, mg/L <. 1.0
TN, mg/L < 10
TP,mg/L <1.0
Anaerobic Basin Design
Number of Anaerobic Basins per Train Two
Anaerobic Volume per Train, MG 0.104
A Number of Anaerobic Mixers per Train Two
Horsepower per Anaerobic Mixer, hp 5
Carrousel Basin Design
Number of Trains Two
Aerobic Volume per Train, MG 0.91
Anoxic Volume per Train, MG 0.23
Number of Aerators per Train One
Horsepower per Aerator, hp 100
Number of Anoxic Mixers per Train One
Horsepower per Anoxic Mixer, hp 10
Process Design
Aerobic SRT, days 15
Internal recycle, mgd 10
MLSS, mg/L 4,000
Sludge Production, Ibs TSS/day 4,036
Hazen and Sawyer, P.C. 4 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
Table 1-1 — Process Design Criteria — Existing Facility (Con't)
Secondary Clarifiers
Number of Clarifiers 2
Diameter, ft 70
Depth, ft 14
Total Area, ft2 7,697
Average Overflow Rate, gpd/ft2 325
Peak Overflow Rate, gpd/ft2 812
Effluent Filters
Number of Filters 2
j 5 Type Traveling Bridge
Total Area, ft2 864
Average Hydraulic Loading Rate, gpm/ft2 2.0
Peak Hydraulic Loading Rate, gpm/ft2 5.0
Solids Handling
Number of Aerobic Digesters 3
Digester Nos. 1 and 2 Volume each, MG 0.244
Digester No. 3 Volume, MG 1.069
Total Digester Volume, MG 1.557
Rotary Drum Thickener Capacity, gpm 200
1.2. Effluent Quality Requirements
The Twelve Mile Creek WWTP is currently permitted by the North Carolina Department of
Environment and Natural Resources (NCDENR) Division of Water Quality (DWQ) to discharge a
maximum monthly average daily flow (MMADF) of 2.5 mgd of treated effluent to Twelve Mile
Creek. The discharge is permitted through National Pollutant Discharge Elimination System
(NPDES) Permit Number NC0085359. The permit was effective on November 1, 2005 and
expires on June 30, 2010. Current permitted effluent requirements pertaining to the secondary
treatment process are summarized in Table 1-2.
Table 1-2 — Current Effluent Quality Requirements
Flow, mgd
BOD5 (Apr. 1 — Oct. 31), mg/L
BOD5 (Nov. 1 — Mar. 31), mg/L
TSS, mg/L
NH3-N (Apr. 1 — Oct. 31), mg/L
NH3-N (Nov. 1 — Mar. 31), mg/L
TP Load, lb/day
TP Concentration, mg/L
12-Month Avg. Monthly Avg. Weekly Avg.
20.85
1.00
2.5
5.0 7.5
10 15
30 45
2.0 6.0
4.0 12.0
41.70
2.00
Revised permit requirements will come into effect once the facility is expanded to 6.0 mgd. The
effluent ammonia concentration will be decreased to half the current limit. The total phosphorus
Hazen and Sawyer, P.C. 5 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
load will remain the same, effectively decreasing the maximum TP concentration allowable.
Table 1-3 presents the revised effluent limits.
Table 1-3 —Effluent Quality Requirements 6.0 mgd Design Flow
Flow, mgd
BOD5 (Apr. 1 — Oct. 31), mg/L
BOD5 (Nov. 1 — Mar. 31), mg/L
TSS, mg/L
NH3-N (Apr. 1 — Oct. 31), mg/L
NH3-N (Nov. 1 — Mar, 31). mg/L
TP Load, lb/day
TP Concentration, mg/L
12-Month Avg. Monthly Avg. Weekly Avg.
20.85
0.42
6.0
5.0 7.5
10 15
30 45
1.0 3.0
2.0 6.0
41,70
0.83
It is not expected that the total maximum daily load (TMDL) for total phosphorus will ever be
increased above the current load. In fact, it is possible that the TMDL will be reduced at some
point in the future. Based on the current permitted TMDL, maximum month and twelve-month
total phosphorus effluent concentrations of 0.33 mg/L and 0.17 mg/L, respectively, will be
required at the ultimate build -out design flow of 15.0 mgd. Figure 1-1 shows the decrease in
permitted effluent concentrations from the current design flow through build -out.
22
2.0
1.8
1 6
71'
1 4
9
5
E 0 8
0 6
0.4
0.2
0 0
Figure 1-1 — Total Phosphorus Effluent Limits — Current TMDL
2.00
1 2 3 4 5 6 7 8 9 10 11 12 13 14 16 16
Flow (mgd)
-41—Mon1bly Limit -4/4-12-Mon1h Avg. Lo-nil
Hazen and Sawyer. P.C. 6 Revised:8/1 I /2006

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PHOSPHORUS REMOVAL OPTIMIZATION STUDY
SITE PLAN

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UNION COUNTY
NORTH CAROLINA
TWELVE MILE CREEK
PHOSPHORUS REMOVAL OPTIMIZATION STUDY
PROCESS FLOW SCHEMATIC - LIQUID STREAM

EXHIBIT 1-3
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UNION COUNTY
NORTH CAROLINA
TWELVE MILE CREEK
PHOSPHORUS REMOVAL OPTIMIZATION STUDY
PROCESS FLOW SCHEMATIC - SOLID STREAM

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
2.0 Wastewater Characterization
Historical data from the past two years (January 2004 through December 2005) was used to
characterize the influent wastewater and develop annual average flows, concentrations and
Toads as well as maximum month peaking factors. Daily influent flow, temperature, total
suspended solids (TSS) and 5-day biochemical oxygen demand (BOD5) data were available.
TSS and BOD5 samples are collected and analyzed on weekdays only. In addition, influent
samples are analyzed weekly for ammonia (NH3-N), total nitrogen (TN) and total phosphorus
(TP). It is assumed, for the purposes of the wastewater characterization, that the influent
concentrations of nitrate (NO3) and nitrite (NO2) are insignificant and that the influent TN is
equivalent to the total Kjeldahl nitrogen (TKN) concentration.
A composite influent sampler collects influent samples over a twenty-four hour period based on
flow proportion. The influent sampler is located at the headworks just downstream of the
screens. Plant recycle flows including plant drains, filter backwash and digester decant are
returned to treatment upstream of the headworks. Therefore, the influence of these return
streams is accounted for in the influent samples.
Historical wastewater characteristics are summarized in Table 2-1. A more detailed analysis of
the influent wastewater characteristics is provided in Appendix A. Maximum month wastewater
characteristics, including the peak factor, are based upon a Toad based analysis, discussed
further in Appendix A.
Table 2-1 — Historical Wastewater Characteristics
Annual Average Maximum Month Load Peak Factor
Flow, mgd 2.2 2.6 1.21
BOD, mg/L 237 274 1.40
TSS, mg/L 279 401 1.74
TN, mg/L 40.4 41.1 1.23
NH3-N, mg/L 25.5 24.9 1.18
TP, mg/L 7.6 9.8 1.56
The current annual average daily flow is approximately 2.15 mgd (from June 2005 through May
2006). The peak maximum month flow has already exceeded the permitted capacity of the
Twelve Mile Creek WWTP. Maximum month load peaking factors for TSS and TP are skewed
by the influence of the plant recycle streams. The average temperature during the two years
evaluated was 19.3°C, and the minimum and maximum month temperatures were 12.7°C and
25.3°C, respectively.
Hazen and Sawyer, P.C. 10 Revised:8/I I/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
3.0 Operations and Performance
Union County Twelve Mile Creek WWTP operational and performance data from the past two
years were evaluated to understand current operations and determine the treatment efficiency
of the existing process. Daily laboratory reports provided weekday MLSS concentrations,
sludge volume indices (SVIs) and dissolved oxygen (DO) concentrations in each of the two
oxidation ditches. Monthly operating logs provided daily RAS and WAS flow rates as well as a
record of decant operations, solids hauling, operational upsets and equipment malfunctions.
Historical plant effluent concentrations are presented in Figures 3-1 through 3-3.
1000
100
10
9
•
•
t. A
8
rr
AA At
0
1/1
Figure 3-1 — Effluent BOD5 and TSS Concentrations
4
A A
A4
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A •
* • • At.4, '
.di: et
A,
A A4.4 .6 i
4 A A
a) An An LAn LAO A-0
0 9 9 9 o
> 6, 9
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z -, 2 :',2. 0,1' .,..„
,,0: i=
•
43$
• 000 TSS —1111—BOD lor,0h4 Avg, —0— TSS - Monthly Avg,.
0
The most notable observation from the operational data was a major process upset that
occurred during the first part of 2005. An increase in effluent solids occurred from January until
May 2005, and nitrification was lost from the middle of January until April 2005. The process
upset can be attributed to a variety of factors including loss of electrical power to the WWTP,
filter blinding and maintenance issues. In particular, a power outage damaged the high speed
starters on the aerator motors, and the aerators were not able to deliver sufficient oxygen for
nitrification. Low dissolved oxygen concentrations were reported at times during January and
February 2005 (see Figure B-4 in Appendix B). The process has since recovered and effluent
BOD5, TSS, NH3 and TN concentrations returned to typical concentrations before the upset. An
increase in these concentrations was seen again in December 2005. This may also be
attributed to a lack of oxygen for effective nitrification, as flows and influent loads exceeded
design conditions. Excluding these occurrences, effluent TN concentrations are typically below
5 mg/L, indicating efficient denitrification as well.
Hazen anti Sawyer, 1 Revised:8/1 II2OO6

Twelve ile Creek WTP Phosphorus oval Optimization Study
40
35
30
'E 25
2
.2 20
c
15
1
0 la
3
2
0
'a 6
2
tE 4
3
Figure 3-2 - Effluent Nitrogen Concentrations
4,5
9
4
a
•
•
a A
4
0
0 0
-▪ 5
NH3-N a TN A.— TN Meng -Ma Avg
* •
4
,a •
.04 4
• la* 4
*Aa a • **"
Figure 3-3 - Effluent Total Phosphorus Concentrations
8
t * *
44 * * 88
E.22
0
4
4
**
•
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9 0 0
n
25 a •..--,
—, as o
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4
4
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5
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9
• TP manamTP Monthly Avg. 12-Month Avg. Urod ----tyllammum Month Avg. Um*
Hazen and Sawyer, RC. 12
Revised:8/11/2006

r)
Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
j
Effluent total phosphorus concentrations increased in the last half of 2005. The average effluent
TP concentration during this period was approximately 3 mg/L, and Union County has exceeded
their permitted phosphorus Load to Twelve Mile Creek. In contrast, the average effluent TP
concentration before the process upset was approximately 1.2 mg/L. The effluent TP
concentration reached a maximum in January 2005, which coincided with the beginning of the
process upset. Effluent `TP was reduced to previous concentrations during the upset (from
February through May 2005). This may be attributed to, alum being added at the onset of the
process upset and an increase in the effective anaerobic volume, which promotes biological
phosphorus reduction, due to the loss of nitrification and subsequent absence of nitrate in the
internal recycle and RAS return flows. In May 2005 nitrification was restored and denitrification
resumed in the anoxic zones, reducing the available anaerobic volume and decreasing
biological phosphorus removal. Alum addition has been discontinued since the process has
recovered.
The Twelve Mile Creek WWTP typically operates at a MLSS concentration between 4,000 and
5,500 mg/L. The average MLSS over the two year period evaluated was approximately 4,700
mg/L. Operations staff reports a typical volatile content (f,) of 75%.
Historical solids retention times (SRT) were also evaluated to determine the impact on the loss
of nitrification. The solids retention time was computed using only the volume in the oxidation
ditch (aerobic SRT). The volumes of the anaerobic and anoxic tanks were omitted. It should be
noted that solids are only wasted on weekdays, and even then solids are not wasted every day.
This results in variable MLSS concentrations and resulting SRTs. The average SRT during the
period evaluated was over 26 days.
j The historical sludge volume index (SVI) was also evaluated. The SVI dropped at the onset of
i_, r the process upset due to a reduction in bulking organisms through RAS chlorination. The RAS
was also chlorinated in December 2005, as evidenced by another decrease in SVI. Table 3-1
presents the historical 50, 90 and 99-percentile SVI values.
Table 3-1 - SVI Percentile Values
SVI (mL/g)
50th-percentile 166
90th-percentile 204
99th-percentile 243
Dissolved oxygen concentrations are monitored between the aerator and effluent weir. The
average DO concentrations in Oxidation Ditch No. 1 and Oxidation Ditch No. 2 were 2.0 mg/L
and 1.8 mg/L, respectively. With the exception of Spring 2004 and January 2006, the DO
concentrations in each basin followed a similar trend, with slightly greater DO concentrations in
Oxidation Ditch No. 1. Plots of the plant operational data are attached as Appendix B.
Hazen and Sawyer, P.C.
13 Revised:8/ 11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
�i
4.0 Supplemental Sampling
4.1. Supplemental Wastewater Characterization
Supplemental sampling was performed at the Twelve Mile WWTP from January 23rd through
January 27th, 2006. Twenty-four hour composite samples of raw influent, raw influent with
recycle, secondary effluent and final effluent were taken daily during this period, and a nutrient
profile through the process was performed on January 24th and January 25th. In addition, daily
composite samples of the digester decant were taken from January 24th through January 26th.
The results of the composite samples taken at each location are summarized in Appendix C.
The wastewater characterization indicated a significant variance in influent wastewater
characteristics, particularly in regard to the raw influent (without recycle) sample. The raw
influent sample was taken at the plant influent pump station. Several of the COD, BOD and
TSS concentrations reported are not typical of municipal wastewater and therefore are likely
attributed to sampling or analytical error. Furthermore, the plant operations log reports that one
of the plant influent pumps had an operational issue during the supplemental sampling week.
Therefore, the raw influent sample is not considered to be indicative of typical wastewater
quality at the facility.
The raw plus recycle influent characteristics were more consistent over the supplemental
sampling period and indicate more typical municipal wastewater characteristics. Since the
samples were taken at the same location and in the same manner as the daily samples
collected at the Twelve Mile Creek WWTP, it is expected that these samples are more indicative
of typical influent wastewater quality at the facility. These samples include the influence of the
digester decant and filter backwash return. The filter backwash return is not expected to
contribute a significant solids or nutrient load. However, the digester decant provides significant
solids, nitrogen and phosphorus loads to the process. Since the digesters were decanted from
Monday (1/23/06) afternoon through Wednesday (1/25/06), the raw plus recycle composite
samples collected Tuesday, Wednesday and Thursday include digester decant.
4.2. Process Profiles
Nutrient, pH and dissolved oxygen (DO) profiles were performed through the secondary
treatment process. Nutrient and pH profiles were taken through each oxidation ditch on January
24th and January 25th, 2006. DO profiles were performed on January 23`d through January 25th.
Results of the profiles are presented in Appendix C.
Biological phosphorus removal was effective in each train. Greater nitrification was observed in
Oxidation Ditch No. 2 on both days. This corresponds to Oxidation Ditch No. 2 having a greater
dissolved oxygen concentration. This is likely attributed to uneven flow and load distribution to
each train. Re-release of phosphate in the secondary clarifiers was observed on one day of
profiling (January 25th).
Plant operations staff reports that uneven flow distribution is quite significant at the facility, and
Oxidation Ditch No. 1 was receiving significantly more flow during the sampling period. The
nutrient profiles appear to confirm this observation. Greater phosphorus release was seen in
the Oxidation Ditch No. 2 anaerobic zones, which may indicate a longer hydraulic retention time.
Mass balances were performed to determine the approximate difference in flows between the
two basins. These balances indicate that Oxidation Ditch No. 2 received only 40% of the
influent flow of Oxidation Ditch No. 1 during the profile periods.
Hazen and Sawyer, P.C. 14 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
a
4.3. Clarifier Settling
Solids settling tests were performed at the Twelve Mile Creek WWTP on the afternoons of
January 24th and January 25th, 2006. The testing comprised of calculating the settling velocity
of the solids -liquid interface at various MLSS concentrations. Settling parameters for the
Twelve Mile Creek WWTP were developed from the test and can be used to perform site
specific solids flux analyses and/or model clarifiers through the BioWin process simulation
software. The clarifier testing is discussed in further detail in Appendix D.
The clarifier settling tests indicated that the Twelve Mile Creek WWTP had a well settling sludge
during the supplemental sampling period. A solids flux analysis was performed to determine the
maximum MLSS concentration that can be effectively settled by the existing secondary
clarifiers, as well as the 6.0 mgd design condition with an additional two 85-foot diameter
clarifiers. The RAS rate was assumed to be 100% of the maximum month flow for both
conditions. The analysis was performed for both the field observed settling characteristics (SVI
- 180) and the 99th percentile SVI condition. Table 4-1 summarizes the results of the solids flux
analysis.
Table 4-1 - Solids Flux Analysis Results
Condition
Max. MLSS, mg/L
Flow, mgd
SVI99th%o Field
2.5 MGD - Max Month - all in service 2.5 5,300 6,500
2.5 MGD - Max Month -1 out of service 2.5 3,800 5,500
2.5 MGD - Peak Day - all in service 4.6 3,700 4,700
2.5 MGD - Peak Day - 1 out of service 4.6 2,400 3,900
6.0 MGD - Max Month - all in service 6.0 5,400 6,800
6.0 MGD - Max Month - 1 out of service 6.0 4,700 6,200
6.0 MGD - Peak Day - all in service 11.0 3,800 4,800
6.0 MGD - Peak Day -1 out of service 11.0 3,200 4,400
Hazen and Sawyer, P.C. 15 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
5.0 Process Modeling and Evaluation
Computer modeling of the biological system using BioWin software was used in order to assess
the capacity of the existing oxidation ditches and identify potential operational and/or capital
improvements that will enable the Twelve Mile Creek WWTP to reliably meet total phosphorus
effluent requirements. BioWin is a dynamic wastewater treatment process modeling and
simulation software package widely utilized for process design and optimization.
5.1. Model Calibration
A model of the Twelve Mile Creek WWTP was developed in BioWin and calibrated based upon
historical data and the results of the supplemental sampling program. Figure 5-1 presents the
BioWin model used in the calibration.
Plant Influent
Figure 5-1 — BioWin Calibration Model — Twelve Mile Creek WWTP
Anaerobic Anoxic
OX 1.2 OX 1.3
OX;1.1
AnaerobicAnoxic OX 2-2 OX 24
HJ
Secondary Clarifier eft -blew
WAS
Influent wastewater was characterized based on the historical data and supplemental sampling.
Due to the inconsistencies in the raw influent (without recycle streams) characterization
performed during the supplemental sampling period, the influent wastewater characteristics and
calibration model are based on the plant influent including recycle flows. Influent fractions used
in the BioWin model are summarized in Table 5-1.
Table 5-1 — Twelve Mile Creek WWTP Influent Fractions
Constituent Fraction BioWin Default
Fbs - Readily Biodegradable COD 0,22 0.16
Fac - Acetate Fraction RBCOD 0.05 0.15
Fxsp - Slowly Biodegradable COD 0.78 0.75
Fus - Soluble Unbiodegradable COD 0.10 0.05
Fup - Particulate Unbiodegradable COD 0.22 0.13
Fna - Ammonia:TKN Ratio 0.66 0.66
Fnus - Soluble Unbiodegradable TKN 0.02 0.02
Fpo4 Phosphate:TP Ratio 0.60 0.50
The model was calibrated based on running steady state and dynamic simulations. Steady
state simulations were run for the sampling week and month of January 2006. Dynamic
Hazen and Sawyer, P.C.
16 Revised:/1 6

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
simulations were performed for the sampling week and for the months of November 2004 and
January 2006. Figure 5-2 compares the BioWin predicted secondary effluent nutrient
concentrations to the composite effluent samples for the week, based on the results of the
dynamic simulation.
Secondary Effluent Concentration
9
©L
1/22/2006
Figure 5-2 — Dynamic Simulation — Sampling Week
112312006 1/2412006
—Total P Soluble P
—Ammonia-N —Nitrate=N
• Field Total N 4, Field TKN
1;25,2006
1126/2006
—Total N *TKN
Field Tot,[ P 4). Field Soluble PO4-P
• Field Amrnonia-N • Field Nitrate-N
1/2712006
The calibrated BioWin model predicted effluent nitrogen concentrations fairly accurately. Actual
effluent phosphorus concentrations were lower than those predicted by the model. The
dynamic model predicted MLSS and MLVSS concentrations of approximately 3,400 mg/L and
2,500 mg/L respectively. This correlated well to the solids concentrations found during the two
days of secondary effluent sampling.
5.2. Process Evaluation
The calibrated model was used to develop various treatment scenarios to optimize biological
phosphorus removal. The evaluation assumes that the flow distribution issue apparent during
the supplemental sampling period will be corrected and that sufficient aeration will be provided
to the oxidation ditch. The process evaluation included investigations of the following:
• Plant Recycle Streams
• Nitrified Recycle Rate
• Dissolved Oxygen Return
• Readily Biodegradable Carbon
• Secondary Clarifier Operation
• SRT Analysis and Optimization
• Chemical Phosphorus Precipitation
,Hazen and Sawyer, P.C. 1 7 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
The process evaluation was based on maximum month Toad conditions. Table 5-2 summarizes
the maximum month influent concentrations used in the modeling and analysis. The maximum
month TSS concentration was reduced from the historical concentration presented previously to
a more realistic value since reported peak TSS concentrations are likely overstated due to the
proximity of the influent sampler to the recycle stream return. Process options were evaluated
at an SRT of twelve days and the minimum month temperature of 12°C, except where otherwise
noted.
Table 5-2 — Maximum Month Conditions
Constituent Maximum Month
Flow, mgd 2.5
BOD5, mg/L 274
TSS, mg/L 288
TKN, mg/L 41
NH3-N, mg/L 27
TP, mg/L 9.8
PO4-P, mg/L 5.9
5.2.1. Plant Recycle Streams
Filter backwash, plant drain flows, aerobic digester decant and rotary drum thickener centrate
flow by gravity to the in -plant pump station, which pumps the flow back to the headworks. The
plant recycle streams combine with the plant influent flow upstream of the secondary process.
The digester decant is a significant source of phosphorus load to the secondary process.
Decant total phosphorus concentrations up to 85 mg/L were measured during the supplemental
sampling period, and operations staff report historical decant total phosphorus concentrations
between 130 and 180 mg/L. The remaining recycle flows likely do not contribute much to the
phosphorus loading of the process.
The digesters are decanted several times a month for a period of two to three days at a time.
There is no direct flow measurement of the decant flow, but operations estimates the flow to be
approximately 80,000 gpd and a typical total flow of 180,000 gallons over the decant period.
The digesters are decanted to thicken the sludge, which is hauled off -site quarterly for land
application.
The process model was used to determine the effect of removing the digester decant return
flow. Predicted total phosphorus effluent concentrations decreased from 0.81 mg/L to 0.53
mg/L and orthophosphate effluent concentrations were reduced from 0.65 mg/L to 0.41 mg/L.
In addition, removing the digester decant return flow significantly reduces the variability of the
phosphorus load to the secondary process, which may substantially increase the reliability of
the process. Reducing or eliminating the digester decant flow will also reduce the inert solids
loading to the process, reducing MLSS concentrations.
The process model was also used to simulate introducing the recycle flow directly to the aerobic
portion of the oxidation ditch. The model predicted no significant change in effluent phosphorus
concentrations in comparison to current operations.
Hazen and Sawyer, P.C. 18 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
5.2.2. Nitrified Recycle Rate
Nitrate is returned from the oxidation ditch to the anoxic zone through an adjustable gate prior to
the oxidation ditch aerator zone. The nitrified recycle rate can range from 3 to 15 times the
influent flow rate, depending on the position of the gate. The gate may also be fully shut to
avoid recycle of nitrate to the anoxic zone. If the gate is fully shut, the anoxic zone acts as
additional anaerobic volume.
The internal recycle gates were fully open at both ditches during the supplemental sampling
period. Based on a mass balance of ammonia around the anoxic zone, it was estimated that
each oxidation ditch was recycling approximately 715% of the influent flow for a total nitrified
recycle rate of 1430%. This is consistent with the manufacturers stated maximum internal
recycle rate of 1500%.
The effects of reducing the nitrified recycle rate and shutting the gates completely to avoid
nitrified return were evaluated using the process model. Effluent nitrate and total nitrogen
concentrations were evaluated in addition to phosphorus removal. Figure 5-3 presents the
results of the evaluation.
Figure 5-3 - Influence of Nitrified Recycle Rate
0.9
0 .6
ECc
E
0
0
7
15
12
#*
.frt
8
9
0
0
6 ?
z
"E0
0
3
071 0
0% 200% 400% 600% 800% 1000% 1200% 1400%. 1600%
Nitrified Recycle Rate
Effluent PO4 990-- Effluent TP —44—Efflue5 NO3 Effluent TN
Phosphorus removal increased with a reduction in the nitrified recycle rate, and effluent nitrate
and total nitrogen concentrations increased with a decreased nitrified recycle rate. However,
the effect of adjusting the nitrified recycle rate on effluent nutrient concentrations diminished as
recycle rates exceeded 400%. The model estimated that an additional 0.15 mg/L. of phosphorus
could be removed by shutting the gates, eliminating nitrified recycle.
14a-/en and Sawyer., P.C. 19 Revised:8/1 1/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
Although additional biological phosphorus removal may occur if the nitrified recycle gate is
adjusted to reduce or prevent denitrification, it is not recommended since it may cause
operational issues, including pH depression due to reduced alkalinity recovery via denitrification.
A decrease in pH would inhibit nitrification. The process modeling indicated a pH depression of
0.4 units when the gate was fully closed compared to allowing 400% nitrified recycle. Oxygen
requirements would also increase if denitrification was prevented. The nitrified recycle gate
should be set at 27% open to allow a minimum of 400% of the influent flow to be recycled back
to the anoxic zone.
5.2.3. Dissolved Oxygen Return
In order to optimize biological phosphorus removal, introduction of dissolved oxygen to the
anaerobic zone must be avoided. In some facilities, RAS can contribute significant dissolved
oxygen to the anaerobic zone, inhibiting biological phosphorus removal. RAS deoxygenation
zones can be implemented to strip oxygen from the RAS to improve biological phosphorus
removal.
The dissolved oxygen profiles performed during the supplemental sampling week indicated that
there was no significant dissolved oxygen in the anaerobic zone. In addition, the high
secondary sludge blankets reported indicate that the RAS is likely anoxic. It is unlikely the RAS
is returning significant dissolved oxygen to the anaerobic zone and a significant contributor to
the recent increases in effluent phosphorus concentrations. Incorporation of a RAS dissolved
oxygen stripping zone will have little effect on biological phosphorus treatment under current
conditions at the Twelve Mile Creek WWTP.
5.2.4. Readily Biodegradable Carbon
Readily biodegradable carbon is required for biological phosphorus removal. The influent BOD
to TP ratio (BOD:TP) is often used as an indicator of biological phosphorus potential. In
general, an influent BOD:TP ratio of at least 25:1 is considered favorable for biological
phosphorus removal. However, a recent Water Environment Research Foundation (WERF)
study indicates that variability in the BOD:TP ratio has a greater effect on effluent phosphorus
concentrations than absolute BOD:TP ratios (Neethling, et. al, ES-9). Effluent phosphorus was
observed to increase on occasions where the BOD:TP ratio decreased. It was hypothesized
that changes in the steady state operation of the phosphorus accumulating organisms (PAOs)
associated with the variable BOD:TP ratio resulted in excessive release of accumulated
phosphorus in the biomass.
The Twelve Mile Creek WWTP has a historical average influent BOD:TP ratio of approximately
33, which is typically indicative of a system that can achieve reliable biological phosphorus
removal. Ninety percent of the influent samples that were analyzed for both BOD and TP had
ratios above 25. Plant recycle stream influences are included in these ratios, since the plant
influent is sampled downstream of the recycle stream return. The historical data was further
evaluated (Figure 5-4) to determine if there was a correlation between influent BOD:TP ratio
and effluent total phosphorus concentration.
Hazen and Sawyer, P.C. 20 Revised:8/11/2006

Twelvc Mile Creek WWTP Phosphorus Removal Optimization Study
70
65
60
55
50
45
40
o
5 30
a
20
•
8
Figure 5-4 - Effluent TP vs. Influent BOD:TP
•
8
84
** a
8
• *
4
13
12
15. 1 3
•
•
• 1 2
•• •
• * * *
* * •
* * * 1 1
* , * * *
*
0 i*•• * * • • 8 • ** •
0
in
9
0 1oftuent BOD:TP * Effluer3 TP
The variability of the BOD:TP ratio appeared to influence the effluent phosphorus concentration
on several occasions. An increase in the BOD:TP ratio in Spring 2005 correlated to a decrease
in effluent TP concentration, and effluent TP concentrations increased when the influent ratio
decreased in the second half of the year. Similarly, effluent TP concentrations decreased in
January 2006, corresponding to an increase in BOD:TP. A decrease in the BOD:TP ratio in
February correlates to an increase in effluent TP concentration as well.
Typical influent BOD:TP ratios at the Twelve Mile Creek WWTP would tend to support reliable
biological phosphorus removal. It is uncertain whether variability in the influent BOD:TP ratio
may be partly responsible for inconsistent effluent TP concentrations. However, recent
research indicates that variable ratios may lessen reliability of biological phosphorus removal.
Variability in this influent ratio at the Twelve Mile Creek WWTP may be reduced by removing the
reintroduction of solids recycle streams (decant) to the process. This would also have the
added benefit of increasing the BOD:TP ratio, since the recycle streams contain high amounts
of phosphorus but limited readily biodegradable carbon (the recycle stream BOD:TP ratio was
found to be -11:1 during the supplemental sampling period).
5.2.5. Secondary Clarifier Operation
Phosphate accumulated in the biomass may be released during anaerobic/anoxic conditions,
These conditions may occur during secondary clarification, particularly if high sludge blankets
and low dissolved oxygen concentrations in the clarifiers are present. High secondary clarifier
blanket levels are reported to be common at the Twelve Mile Creek WWTP. Operational data
indicates that blankets can exceed 10 feet at times. Furthermore, blanket levels in the two
clarifiers can be significantly different at the same time. Operations staff reports that it is very
'Hazen and Sawyer, P.C. 21 Revised:8/11/2006

1
Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
difficult to evenly withdraw sludge from the two clarifiers. Sludge currently flows by gravity to a
common wetwell, where it is pumped back to the head of the plant. Provisions for isolating the
wetwell such that separate RAS pumps are used to control blanket depths in each clarifier are
being incorporated into the current construction of the expansion.
The nutrient profile performed on January 25, 2006 indicated a release of phosphate in the
secondary clarifier. The secondary effluent ortho-phosphate concentration was 0.45 mg/L,
whereas aeration basin effluent concentrations were less than 0.10 mg/L. However, secondary
release was not apparent on the first day of nutrient profiling (January 24th). Sludge blankets
recorded for these two days were not significantly different. However, diurnal variations in
sludge blanket may attribute to the observation of phosphate release on only one day.
Release of phosphate in the secondary clarifiers appears to occur at times. High sludge
blankets and low oxidation ditch effluent dissolved oxygen create favorable conditions for
phosphorus release at the Twelve Mile Creek WWTP. Phosphorus release can be reduced by
maintaining low sludge blanket levels, if possible, and ensuring the mixed liquor exiting the
oxidation ditches is sufficiently aerated prior to introduction in the clarifier. Providing aeration in
lieu of mechanical mixing at the mixed liquor splitter box, currently under construction, may
reduce the potential of phosphate release at the clarifiers. Maintaining lower sludge blankets
will also optimize total phosphorus removal by reducing the amount of solids loss over the
clarifier weirs.
5.2.6. SRT Analysis and Optimization
The solids retention time can significantly affect biological phosphorus removal. Increased
solids retention times decrease the efficiency of biological phosphorus removal due to cell
decay and resulting phosphorus release experienced at long sludge ages. However, it is
important to maintain the minimum aerobic SRT required for nitrification to meet effluent
ammonia limitations. Therefore, the aerobic SRT should be optimized for biological phosphorus
removal and ammonia removal.
The process model was used to evaluate biological phosphorus removal over a range of SRTs
for various temperatures. The SRT was calculated using only the volume of the oxidation ditch
(anaerobic and anoxic zones were omitted) to provide an aerobic SRT. This evaluation was
based upon maintaining aerobic conditions throughout the oxidation ditch. Figure 5-5 presents
the results of the SRT analysis.
As expected, the model indicated better phosphorus removal at lower SRTs. However,
particularly under cold weather conditions, the aerobic SRT has a significant effect on ammonia
removal. In order to assure that cold weather ammonia effluent limits are met (2.0 mg/L
maximum month average at the 6.0 mgd design condition), a minimum aerobic SRT of 12 days
should be maintained during the winter. The aerobic SRT can be reduced to 6 days during the
summer to optimize biological phosphorus removal while still meeting the 1.0 mg/L effluent
ammonia limit. An 8 day aerobic SRT is recommended during the remainder of the year to
optimize biological phosphorus removal and maintain nitrification.
Minimizing the SRT will increase the wasting requirements at the Twelve Mile Creek WWTP.
Figure 5-6 presents the WAS production and required waste flow rates at the various SRTs
evaluated.
f TE
III(
Hazen and Sawyer, P.C. 22 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
0 4
Figure 5-5 — Effluent Phosphate vs. SRT
* a nun 00*
5 6 7 8 9 10 11 12
Oxidation Ditch SRT (days)
13
4'
4
3 5
3 2-
2 5
0
0
2 4z
c
14 15 16
PO4 „in 120 * PO4 D 190, PO4 id 26C owx ix NH3 12C —A- NXI3 190 -we iii-NH3 43 26C
5,500
hio
5,400 Ct.18
5,300 , 3 13,
Figure 5-6 — Wasting Requirements vs. SAT
0 5
5,200
5,103
g 5 000 l
= 4 900
4,800
4,700
4,600
500
6
8 9 13 11 12 13 14 15 16
Oxidation Ditch SDI (days)
3-44-12C 3-30-ixi 19C 4 266 --A WAS Flow
Hazen and Samyer, Pr,
RCN Ised:8/1 I /2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
5.2.7. Chemical Phosphorus Removal
The process modeling indicated that SRT control could produce effluent soluble phosphate
concentrations between 0.5 mg/L to 0.6 mg/L. This is adequate to reduce effluent total
phosphorus concentrations to less than 1.0 mg/L provided that solids are removed below 10
mg/L TSS. Upset conditions at the Twelve Mile Creek WWTP have led to excessive solids in
the plant effluent, resulting in increased effluent phosphorus concentrations. Chemical
phosphorus removal can be used to further reduce effluent soluble phosphate concentrations,
assuming reliable solids removal is provided. The Twelve Mile Creek WWTP has existing alum
facilities and has used them in the past for phosphorus removal.
Process modeling indicated that an alum feed rate of 150 gallons per day at 2.5 mgd would
reduce soluble phosphate concentrations from 0.6 mg/L to below 0.05 mg/L and total
phosphorus concentrations below 0.25 mglL, based on an effluent TSS of 5 mg/L. The
predicted alum feed rate is based upon providing 300% of the stoichiometric alum requirement
to precipitate phosphorus. Jar testing is recommended to better quantify the required feed rate
under the specific site conditions.
Hazen and Sawyer, P.C. 24 Revised:8/11/2006

•
1

Twelve N1i1e Creek WWTP Phosphorus Removal Optimization Study
6.0 Operational Recommendations
The existing 2.5 mgd Twelve Mile Creek WWTP has facilities in place to provide biological
nitrogen and phosphorus removal. However, increasing/variable loads, operatored issues and
aeration deficiencies have prevented the facility from being able to reliably meet permitted
effluent phosphorus requirements. The existing processes at the plant were evaluated with
respect to optimization for phosphorous removal. Several eperational changes can be math in
order to optimize biological phosphorus removal at the Twelve Mile Creek WWTP and allow the
facility to reliably meet current effluent total phosphorus permit requirements. The
recommended operational modifications include:
a Reduce Digester Decant Recycle
• SRT Optimization
• Reliable Solids Removal
• Chemical Phosphorus Trim
6.1. Reduce Digester Decant Recycle
The results of the supplemental sampling program indicate that the recycle of phosphorus from
the aerobic digesters can increase the phosphorus loading to the process by 50% to 100°,4,,
Dynamic simulations of historical operating data indicate the potential for elevated effluent
phosphorus concentrations due to the increased loads during and after decant operations. it is
recommended that the influence of decant from the digesters be reduced in order to consistently
reduce total phosphorus concentrations below permitted effluent limits.
Cell lysis and decay, resulting in release of stored phosphorus, occurs due to the prolonged
solids retention times (- 90 days) in the digesters. The impact of the recycle stream could be
diminished by reducing the solids retention time in the digesters, Increased solids hauling
frequency is required in order to decrease the retention time in the digester. Reducing the need
for decanting from the digesters would also be beneficial to provide reliable biological
phosphorus removal.
it is recommended that the existing rotary drum thickener be utilized to reduce the reliance on
decanting operations to thicken waste activated sludge. WAS would be routed through the
rotary drum thickener prior to entering the digester. Plant staff has indicated that they have had
problems aerating thickened sludge. Thickened and un-thickened sludge could be blended to
achieve the desired solids percentage (3.5% to 4.0% solids). Operations staff will still need to
cycle air on and off to allow for denitrification to maintain pH and reduce dissolved oxygen
returned in the decant. Reducing digester decant operations will reduce the quantity and
variability of phosphorus loads and improve biological phosphorus process reliabiiity.
6.2. SRT Optimization
Optimizing the solids retention time is recommended to promote biological phosphorus removal
while maintaining nitrification, The SRT should be limited to the minimum required for full
nitrification. A minimum aerobic SRT of 12 days should be maintained when wastewater
temperatures are below 14 C, provided full nitrification is maintained. The aerobic SRT can be
lowered to 6 days when temperatures exceed 19 "C to maximize biological phosphorus
removal. Minimizing the SRT has the added benefit of reducing the operational MLSS, which
will reduce the solids loading to the secondary clarifiers and increase clarifier reliability.
.Hatcn and Sawyer„ Re vised:8/1 I /2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
Maximum month operational MLSS concentrations at the reduced SRTs are predicted to be
2,000 mg/L in the summer to 3,500 mg/L in winter. These concentrations do not include any
addition& solids load attributed to the use of alum for chemical phosphorus removal. The
process modeling predicted an additional 200 mg/L of MLSS when alum was added upstream of
the secondary clarifiers for chemical phosphorus trim.
Additional solids wasting will be required in order to minimize the SAT. It is estimated that
between 5,000 and 5,200 Ibs/day of solids will need to be wasted from the secondary process at
the design flow of 2.5 mgd. This is approximately 25% greater than the design solids production
rate of approximately 4,000 lbs/day. It is recommended that solids be continually wasted to
maintain the target SAT and improve process reliability. Recommended solids retention times
and resulting MLSS concentrations and wasting rates for a range of wastewater temperatures
are summarized in Table 6-1.
Table 6-1 - Recommended SRT Optimization
Parameter T < 14°C 14°C <T < 19°C T >19°C
Aerobic SAT, days 12 8 6
MLSS, mglL 3.500 2,700 2,000
Solids Prod., Ibs/day 5,000 5,100 5,200
WAS Flow, mgd 0.09 0.13 0.18
WAS Flow, gpm 62 90 125
6.3. Reliable Solids Removal
The historical operating data indicates frequent loss of solids through the secondary clarifiers
and blinding of the effluent filters. In addition to exceeding permitted TSS and OBOD
concentrations, total phosphorus limits are being exceeded due to particulate phosphorus.
Consistent solids settling and removal are imperative in providing reliable phosphorus removal.
The solids settling tests conducted during supplemental sampling indicate a well settling sludge.
In addition, current solids and hydraulic loading rates are within acceptable guidelines. Plant
operations staff has had problems with uneven sludge withdrawal from the existing clarifiers.
The clarifiers operate at high mixed liquor concentrations, averaging 4,700 mg/L with maximum
reported concentrations above 6,000 mg/L. Limiting the operational solids retention time as
discussed above wffl reduce the mixed liquor concentrations to the clarifiers and improve solids
separation reliability and clarifier performance.
An MLSS distribution structure and modifications to the RAS pumping system are included in
the construction of the expansion to 6.0 mgd and should greatly increase secondary clarifier
reliability. Individual submersible RAS pumps are being provided for each clarifier, which will
help provide more even sludge withdrawal. it is also recommended that the Twelve Mile Creek
WWTP feed polymer to the secondary clarifiers when settling problems are encountered. In
addition, chlorination of RAS should also be considered when settling is impacted by an
abundance of filamentous bacteria.
The effluent filters provide an additional solids barrier prior to effluent disinfection and disposal.
However, these filters become blinded when secondary effluent solids concentrations increase.
Therefore, it is necessary that the secondary clarifiers be operated to achieve maximum solids
removal prior to the filters.
Hazen and Sawyer, P,C,
26 Revise

Twelve Mile Creek WWTP Phosphorus emoyal Optimization Study
6.4. Chemical Phosphorus Precipitation
Alum feed facilities are currently in place at the Twelve Mile Creek VVWTP. Alum has been fed
at times to reduce effluent phosphorus, but is not currently being applied, it is recommended
that alum be fed upstream of the secondary clarifiers to ensure total phosphorus knits are met,
Alum feed should be 9 lirdinated with effluent phosphorus monitoring to avoid overfeed of alum,
which increar s operational costs and may inhibit biologiest phosphorus as well as BOG and
nitrogen) uptake by decreasing the availability of phosphate for biomass gris h.
Hazen and Sawye!., P,C, 27 Revised:8/1 112(X)fi

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
7.0 Capital Improvement Recommendations
Allowable effluent total phosphorus concentrations will continue to decrease as effluent flow to
Twelve Mile Creek increases. It is not anticipated that the current 12-month and maximum
month average TMDLs of 20.85 lb/day and 41.70 lb/day will ever be increased, although there is
a strong possibility that limits of technology for phosphorus removal will need to be met in the
future. Table 7-1 summarizes the maximum effluent concentrations (in mg/L) based on the
current TMDLs at various design flows through the 15 mgd build out condition.
Table 7-1 - Total Phosphorus Effluent Limits
Design Flow
Current (2.5 mgd)
6 mgd
9 mgd
12 mgd
15 mgd
12 Month Average
1.00
0.42
0.28
0.21
0.17
Maximum Month
2.00
0.83
0.56
0.42
0.33
In order to meet the strict future effluent phosphorus requirements, capital improvements to
address phosphorus removal will be necessary. Recommended capital improvements to
optimize phosphorus removal at the Twelve Mile Creek WWTP include:
• Additional Aeration
• Phosphorus Monitoring
• Particulate Phosphorus Removal
• Chemical Addition to Digester Decant
• Solids Handling Improvements
7.1. Additional Aeration
The existing mechanical surface aerators do not have sufficient aeration capacity. Nitrification
is commonly lost, even at extended solids retention times. Biological phosphorus removal
depends on luxury phosphorus uptake under aerobic conditions. The historical data and
supplemental sampling indicate that portions of the oxidation ditch are likely operating
anoxically,decreasing the aerobic volume which may lead to a reduction in luxury phosphorus
uptake. The additional anoxic volume may also promote re-release of phosphate, further
reducing biological phosphorus removal.
Additional aeration is required in order to provide adequate aerobic volume for complete
nitrification, and may also benefit biological phosphorus removal by promoting luxury
phosphorus uptake. It is estimated that a total aeration input of 220 hp is required to meet
maximum month oxygen demands to provide full nitrification at the current 2.5 mgd design flow
(88 hp/mgd). Assuming each existing aerator currently draws 85 hp, an additional 50 hp of
installed aeration capacity is required to meet maximum month load conditions. Union County
is currently pursuing the installation of additional aeration equipment in the oxidation ditches as
a temporary measure to provide adequate aeration while the expansion to 6.0 mgd facilities are
constructed. In addition, a change order is being issued to the contractor performing the
expansion to 6.0 mgd to increase the horsepower of the existing aerators and aerators to be
installed in the two new oxidation ditches from 100 hp to 150 hp each, for a total installed
capacity of 600 hp (100 hp/mgd).
Hazen and Sawyer, P.C. 28 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
The results of the process modeling indicate that the existing oxidation ditch configuration can
likely provide reliable phosphorus removal if sufficient aeration capacity is provided and the
process is carefully controlled. However, conversion of the oxidation ditches to three stage plug
flow reactors with diffused aeration will increase reliability by providing operational flexibility and
better control of dissolved oxygen through the secondary process. Additional capacity may be
available in the existing oxidation ditches due to the increase in aeration efficiency of this
configuration, and the oxidation ditches can likely be re -rated once the expansion to 6.0 mgd is
constructed. It is recommended that this option be carefully considered during the design of the
expansion beyond 6.0 mgd.
7.2. Phosphorus Monitoring
Effluent total phosphorus samples are collected and analyzed weekly and reported to the
NCDENR DWQ. The weekly samples are averaged to develop the monthly effluent total
phosphorus concentration and load that are used to determine compliance with the TMDLs.
Due to the infrequency of phosphorus sampling and analysis, operations staff may not know
about elevated effluent phosphorus concentrations until the weekly sample is analyzed. One
elevated total phosphorus concentration during the month may cause noncompliance.
Therefore, it is recommended that effluent ortho-phosphate be analyzed daily to detect any
elevations which may cause a violation of the TMDLs. Alum feed could be implemented upon
elevation of effluent phosphorus concentrations. Effluent phosphate concentrations could either
be monitored on-line by a nutrient analyzer or tested in the lab with the daily composite
samples.
7.3. Particulate Phosphorus Removal
As the allowable effluent total phosphorus concentration decreases, more reliance on chemical
trim and solids removal will be required. Chemical feed facilities to provide alum or another
precipitant should be maintained, and chemical feed points should be established prior to the
secondary clarifiers and effluent filters. Due to the shallow bed depth of the existing traveling
bridge filters, it is recommended that provisions for more reliable solids removal be included in
facility designs beyond the 6.0 mgd design condition.
Tertiary clarification upstream of the traveling bridge filters or replacement of the traveling bridge
filters with deep bed filters will provide more reliable solids removal. Due to hydraulic limitations
at the facility, these options may require intermediate pumping. It is recommended that deep
bed filters for solids, and particularly particulate phosphorus, removal be incorporated into the
design of any expansion beyond 6.0 mgd. Biological phosphorus removal, followed by chemical
trim and deep bed filtration should be sufficient to meet the effluent phosphorus concentrations
(0.17 mg/L and 0.33 mg/L 12-month and maximum month average, respectively) required at the
ultimate build out to 15.0 mgd design flow.
i
r In order to reduce total phosphorus below 0.10 mg/L, additional solids removal processes will
need to be considered. Tertiary clarification with chemical precipitation followed by deep bed
r ; filtration, ballasted flocculation and membrane filtration are technologies that have been used in
I f conjunction with biological phosphorus removal to reduce total phosphorus levels to the limits of
technology.
Hazen and Sawyer, P.C. 29 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
7.4. Chemical Addition to Digester Decant
Provisions to feed alum directly to the digester decant should be provided to precipitate the
soluble phosphorus before it is returned to the process. Precipitating phosphorus in the decant
can reduce operational costs in comparison to providing alum prior to secondary clarification
since the required alum to phosphorus ratio will be lower when applied to a concentrated
stream as opposed to a more dilute flow. It is recommended that alum feed to the digester
decant be incorporated into design of the expansion beyond 6.0 mgd.
7.5 Solids Handling Improvements
Additional solids thickening capacity is recommended in order to reduce and/or eliminate the
return of digester decant to the secondary treatment process beyond the 6.0 mgd design flow.
It is recommended that Union County initiate a capital improvements project to provide gravity
belt thickeners (GBTs) at the Twelve Mile Creek WWTP. The installation of two 2-meter
thickeners should provide sufficient thickening capacity to limit daily GBT runtime to eight hours
at minimum oxidation ditch aerobic SRTs based on daily operation.
In addition to the thickening units, feed piping, polymer feed systems, washwater booster pumps
and thickened waste activated sludge (TWAS) pumps and piping will need to be provided.
Modifications to the existing WAS pumping system may also be necessary. It is recommended
that the GBT facilities be located in a separate solids thickening building located adjacent to the
existing digesters. The solids thickening building should be designed to facilitate the expansion
of the structure to allow additional thickening equipment and/or dewatering facilities to be
installed at future design flows.
Recommended capital improvements to the Twelve Mile Creek WWTP to ensure compliance
with the current total phosphorus TMDLs at the various future design flows are summarized in
Table 7-2. These improvements are recommended in addition to the operational
recommendations previously discussed.
Hazen and Sawyer, P.C. 30 Revised:8/11/2006

LJ
Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
Table 7-2 — Recommended Capital Improvements
Design Flow Capital Improvements
Current (2.5 a Additional Aeration
mgd) • Laboratory Effluent Ortho-P Monitoring
6.0 mgd
9.0 mgd
12.0 mgd
15.0 mgd
• Additional Aeration
• On -Line Effluent Ortho-P Monitoring
• Conversion of Oxidation Ditches to 3 Stage Plug Flow
BNR Process
• On -Line Effluent Ortho-P Monitoring
• Chemical P Removal Prior to Filters
• Chemical P Removal at Digester Decant
• Deep Bed Filters
• Gravity Belt Thickening
• 3 Stage Plug Flow BNR
• On -Line Effluent Ortho-P Monitoring
• Chemical P Removal Prior to Filters
• Chemical P Removal at Digester Decant
• Deep Bed Filters
• 3 Stage Plug Flow BNR
• On -Line Effluent Ortho-P Monitoring
• Chemical P Removal Prior to Filters
• Chemical P Removal at Digester Decant
• Deep Bed Filters
Limits of • 3 Stage Plug Flow BNR
Technology • On -Line Effluent Ortho-P Monitoring
• Chemical P Removal Prior to Tertiary Clarifiers
• Chemical P Removal at Digester Decant
• Deep Bed or Membrane Filters
Hazen and Sawyer, P.C. 31 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
8.0 Opinion of Probable Cost
Preliminary cost estimates were developed for the recommended phosphorus removal
alternatives. Both capital and operational costs were evaluated and are discussed below. All
costs are in 2006 dollars.
8.1. Operational Costs
Additional operational costs will be necessary in order to reliably meet effluent TP limitations.
These costs include alum, polymer for use with the rotary drum thickener and increased energy
costs. Additional staffing requirements for solids handling and process control may be
necessary as well.
Estimated alum costs at each design flow are presented in Table 8-1. The alum costs were
prepared based on biological phosphorus reduction to 1 mg/L phosphate and a 3 to 1 AI:P
molar ratio. The current Twelve Mile Creek WWTP alum cost of $0.52Jgallon was used. The
optimization of biological phosphorus removal facilities will likely reduce these costs.
Table 8-1 — Estimated Annual Alum Costs
Design Flow
Current (2.5 mgd)
6 mgd
9 mgd
12 mgd
15 mgd
Annual Alum Cost
$7,200
$42,200
$56,900
$81,800
$106,600
Alum CostIMGD
$3,500
$8,500
$7,600
$8,200
_ $8,600.
Additional pumping costs associated with decreasing the SRT is expected to be Tess significant.
The estimated additional annual energy cost for lowering the SRT is $350/mgd, based on $0.06/
kilowatt-hour (kWh), about 10% of the expected alum cost. However, additional solids handling
costs will be required with an increased wasting rate. Polymer will be required when the rotary
drum thickeners are put into operation, which will incur additional power costs. The increase in
solids wasted may increase solids hauling requirements, and additional staff may be necessary
to operate the rotary drum thickeners.
It is recommended that alum trim be practiced in conjunction with effluent phosphorus
monitoring as an immediate solution to enhancing phosphorus removal without significant
capital expenditures. The impact of digester decant should be reduced by activating the rotary
drum thickeners and/or increasing solids hauling frequency. If digester decant cannot be
significantly reduced or eliminated, it is recommended that additional alum be fed to digester
decant to reduce the phosphorus load to the process. Process modifications to enhance
biological phosphorus removal will decrease the dependence on chemical trim to remove
soluble phosphorus and reduce alum costs.
8.2. Capital Costs
Current effluent TP limits (1.0 mglL) can be met with limited capital cost. The only significant
capital costs associated with the recommendations for the current design flow of 2.5 mgd are for
phosphorus monitoring and analysis and additional aeration. Supplemental aeration is required
to ensure ammonia limits are met and is not considered a necessary capital expenditure for
Hazen and Sawyer, P.C. 32 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
19
meeting the current phosphorus limit. The County recently purchased and installed floating high
speed mechanical aerators in the existing oxidation ditches. The capital cost of the
supplemental aeration equipment was approximately $100,000.
Phosphate can be analyzed through an online monitor or in the lab. Portable colorimeters
capable of measuring phosphate can be purchased for under $400. On-line phosphorus
analyzers can be purchased and installed for approximately $50,000. It is recommended that
Union County purchase a portable phosphate analyzer now for use in manual alum feed and
adjustment and incorporate on-line phosphate analyzers into the construction of the expansion
to 6.0 mgd.
The capital cost of the recommended gravity belt thickener facility is estimated to be $1.3
million, and includes installation of two 2-meter GBTs, TWAS pumps, polymer systems,
appurtenant piping, electrical costs and construction of a solids thickening building. Engineering
design and construction services are not included in the estimated cost.
i ( Significant capital improvements will be required beyond the expansion to 6.0 mgd. These
capital improvements address the need to effectively remove solids containing particulate
phosphorus from the Twelve Mile Creek WWTP effluent. Deep bed filters are recommended for
particulate phosphorus removal when the design flow exceeds 6.0 mgd. In addition to
construction of the filters, a secondary effluent pump station will be necessary to pump flow to
the filters since insufficient hydraulic capacity exists to provide gravity flow to the filters. For the
purposes of this cost estimate, it is assumed that the gravity belt thickeners, deep bed filters and
secondary pump station would be built at the 9.0 mgd design condition. Additional filter cells
would be added at the 12 mgd and 15 mgd design flows. It is also assumed that the effluent
�a pump station would initially be constructed to provide for the 10 mgd of design flow, and an
_ , additional 5 mgd of capacity would be installed during the expansion to 12.0 mgd.
The limit of technology costs are based on providing tertiary membrane filtration for the full 15.0
mgd design flow. Tertiary membrane filtration can be used in lieu of deep bed filters if
reductions in the current total phosphorus TMDL require effluent TP limits below 0.10 mg/L.
Io
Table 8-2 summarizes the preliminary capital costs specifically pertaining to the removal of
phosphorus at the various design conditions.
Hazen and Sawyer, P.C. 33 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
Table 8-2 — Estimated Capital Costs for Phosphorus Removal
Design Condition Estimated Cost
Current Design Flow (2.5 mgd)
Portable Phosphate Analyzer $400
Supplemental Aerators $100,000
Expansion to 6.0 mgd
Effluent Phosphate On-line Analyzer $50,000
Additional Aeration $250,000
Expansion to 9.0 mgd
Deep Bed Filters $5,872,000
Secondary Effluent Pump Station $2,300,000
Alum Feed to Digester Decant TBD
Gravity Belt Thickening $1,300,000
Expansion to 12.0 mgd
Deep Bed Filters $1,142,000
Secondary Effluent Pump $410,000
Expansion to 15.0 mgd
Deep Bed Filters $1,142,000
Limits of Technology @ 15.0 mgd
Tertiary Membrane Filtration $17,250,000
TBD — To Be Determined
Hazen and Sawyer, P.C. 34 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
9.0 References
Neethling, J.B. et al. Factors Influencing the Reliability of Enhanced Biological Phosphorus
Removal. Alexandria, Virginia: Water Environment Research Foundation, 2005.
Hazen and Sawyer, P.C. 35 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
r-
Appendix A
Flow and Load Analysis
Hazen and Sawyer, P.C. 36 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
Historical data from the past two years (January 2004 through December 2005) was used to
characterize the influent wastewater and develop annual average flows, concentrations and
loads as well as peaking factors. Daily influent flow, temperature, total suspended solids (TSS)
and 5-day biochemical oxygen demand (BOD5) data were available. TSS and BOD5 samples
are collected and analyzed on weekdays only. In addition, influent samples are analyzed
weekly for ammonia (NH3-N), total nitrogen (TN) and total phosphorus (TP). It is assumed, for
the purposes of the wastewater characterization, that the influent concentrations of nitrate (NO3)
and nitrite (NO2) are insignificant and that the influent TN is equivalent to the total Kjeldahl
nitrogen (TKN) concentration.
A composite influent sampler collects influent samples over a twenty-four hour period based on
flow proportion. The influent sampler is located in the headworks just downstream of the
screens. Plant recycle flows including plant drains, filter backwash and digester decant are
returned to treatment upstream of the headworks. Therefore, the influence of these return
streams is accounted for in the influent samples.
The recycle streams have had a significant effect on the influent concentrations reported. TSS
and BOD5 concentrations above 1,000 mg/L have been reported throughout the historical period
analyzed. A closer evaluation of the data shows that these periods of high influent
concentrations often coincide with reported process upsets and equipment repair and
maintenance periods. Examples of these events, which are included on the daily operations
log, include filter blinding, draining of a clarifier for maintenance and returning activated sludge
through the plant drain system during return activated sludge (RAS) pump repair periods. Plant
influent data during these occurrences, including a significant portion of the data from April and
May 2005, were removed from the data set during initial screening of the data.
Historical daily influent flows are presented in Figure A-1. The annual average flow for the 2
year period was 2.0 mgd. The current annual average daily flow is approximately 2.15 mgd.
Historical flows and peaking factors for the two year period analyzed are summarized in Table
A-1.
Table A-1 - Historical Flows and Peaking Factors
Criteria Flow (MGD) Peaking Factor
Minimum Day
Average Annual
Maximum 30-Day
Maximum 7-Day
Maximum Day
1.1
2.0
2.6
3.0
4.9
0.56
1.00
1.21
1.40
2.22
Hazen and Sawyer, P.C. 37 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
4.5
3 5
3
2
2.5
0
1,5
:4
0 5
Figure A-1 — Historical Influent Flows
RI„mning Avq, m4.30-Day Running Av.
4,
'8
0 yi
z
Annual average influent BOD5, TSS, NH3, TN and TP concentrations were calculated.
Concentrations outside of two standard deviations were not included in the annual average
calculation to eliminate the influence of sampling, analytical or other errors that may skew the
average. The annual average concentrations are summarized in Table A-2. Resulting average
influent ratios are provided in Table A-3. Ratios are typical for domestic wastewater and are
favorable for biological nutrient removal,
Table A-2 - Annual Average Influent Concentrations
Criteria Concentration
BOD, mg/L 237
TSS, mg/L 279
TN, mg/L 40,4
NH3-N, mg/L 25.5
TP, mg/L 7.6
Table A-3 — Average Influent Ratios
Ratio Value
Influent BODTTN
Influent BOD/NH3
Influent BOD/TP
6,22
9,55
333
An influent load analysis of the historical data was used to determine influent load peaking
factors. Historical influent concentrations were converted to loads by multiplying the
Hazen and Sawyer. P.C. 38 Revised:8/I I/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
concentration by the daily flow by a unit conversion factor. The following equation is used to
determine the loads in pounds per day (lb/day) from flow and concentration data.
Load (119 I day) = Oingd)* C(mg I 1)* 8.34
Minimum month, maximum month, maximum week and maximum day load peaking factors
were developed and are presented in Table A-4. Maximum week NH3, TN and TP were not
computed since only one sample is taken weekly. Concentrations outside of three standard
deviations were omitted to avoid the influence of sampling and analytical errors.
Table A-4 - Historical Load Peaking Factors
Criteria BOD5 TSS TN NH3 TP
Minimum Day 0.29 0.20 0.50 0.60 0.51
Average Annual 1.00 1.00 1.00 1.00 1.00
Maximum 30-Day 1.40 1.74 1.23 1.18 1.56
Maximum 7-Day 1.79 2.29 NA NA NA
Maximum Day 3.55 3.03 1.61 1.31 2.22
The load peaking factors are typical of a municipal wastewater with the exception of maximum
day BOD5 and TSS loads, which are greater than typically encountered. This is likely due to the
influence of plant recycle streams (filter backwash and digester decant) being introduced
upstream of the sampling point. Plots of historical influent concentrations are given in Figures
A-2 through A-6.
700
600
500
E 4 0
8 300
20 0
•
4.
Figure A-2 - Influent BOD5 Concentration
• •
444 +
• •
100.
9
2
4.
,T ,o
9 o
8 m
z ,
•
4.
• 4.
# •
•
•
rn 41
9 0t
`L
9 a
> , 0
0 --,
Z 0 z
70ay Running Avg. '"""-30-Da.y Running Average
*
•
•
• •
Hazen and Sawyer, P.C. 39
ReviscW8

Twelve il Creek WWTP Phosphorus Removal Optimization Study
Figure A-3 — Influent TSS Concentration
9
J
Z
'.....•7-1.1.1y ',Cunning Avg. ° °"""3D-Daay Auro urrq .Av
Figure A-4 - Influent Ammonia Concentration
*3O-D y Runnmq Average
r1zen and Sawyer
).C,
40

Twe1ve leCasek WWTP PhosphorusRemoval optim za ion Study
10
Figure _ - Influent Total Nitrogen Concentration
Figure — Influent Tote l r on ntration
Ha en and P.
41
Revi ed: /11/2006

Twelve Mile Creek WYVTP Phosphorus Removal Optimization Study
Daily wastewater temperatures (based on effluent data obtained from daily monitoring reports)
were also evaluated for the two year period. Table A-5 summarizes the temperature data, and
Figure A-7 presents a plot of the temperature trend over the past two years.
E
30
?5
.20
5
Table A-5 - Historical Effluent Temperatures
Criteria Temperature, "C
Minimum Day
Minimum 7-Day
Minimum Month
Average Annual
Maximum 30-Day
Maximum 7-Day
11.9
12.5
12,7
19.3
25.3
25,8
Figure A-7 - Wastewater Effluent Temperatures
'97 ottn ci,trt tet`tt,Lfl
-37
(3'
"65 ("3
Running Avg..."?..30-Day Running Avg.
Hazen and Sawyer, P.C. 42 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
Appendix B
Plant Operational Data
Hazen and Sawyer, P.C. 43 Revised:8/11/2006

'Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
8000
7000 'E
Figure B-1 ® Historical MLSS Concentrations
♦
6000 •/*
� M_ ♦ ♦ ar
♦
2000
1000
80
♦
•
•
kiOR-14h y Avg.
Figure B-2 — Historical Solids Retention Times
♦
♦ 2-day Running Avg, —30-day Running Avg
•
Hazen and , a 'yet-, P.E.'.
44
6

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
300
50
0
o o
os .....,
VI
8
Figure B-3 — Historical SVI
!r} nLC, t,to
8 9 0 0 9 0 0 9
6.
NV) 0
—Mr- Monthly Avg,
Figure B-4 — Historical Oxidation Ditch Dissolved Oxygen
• #
•
• •
• •
• v. $
••
•
•
\ if
•
•
• *•
•
ID,
^7 `,1'
o 9 o o
.t="41 ;... i
2 2
•
•
•
• •
*.
•
•
•
•
•
't •
• •
• *
•
••• •
• C
• • - • n
•
441
9 9 9
4
•
-
*
•
* *
•
•
•
•
•
Litt kat 0 0
9 o 0 9
, .) o at
01 Z --)
• Ox. Ditch g I —4*-0x. Ditch it 1 = Monthly Avg, Ox, Ditch g2 • Ox. Ditch g2 Monthly Avg,
Hazen and Sawyer, PC., 45
Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
Appendix C
Supplemental Sampling Results
and Profiles
Hazen and Sawyer, P.C. 46 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
a
Table C-1 - Raw Influent Characterization
Parameter Monday Tuesday Wednesday Thursday Friday
1/23/06 1/24/06 1/25/06 1 /26/06 1/27/06
CODxx 690 440 440 4100 600
CODxG 260 220 170 3500 250
CODxF 250 170 160 2400 200
BODxx 231 141 247 567 192
BODxG 92.8 4.8 94.4 284 71.8
TSS 824 104 140 75 145
VSS 513 52 93 9.105 105
TKNxx 29 28 32 20 38
TKNxG 22 18 25 18 25
TPxx 6.32 5.11 5.32 3.06 6.24
TPxG 3.86 3.56 5.88 2.34 3.75
NH3-N 24.4 21.7 48.4 15.6 25
NOx-N 0 0 0 0.96 0
PO4-P 4.1 3.1 5.1 1.6 3.6
ALK 180 140 140 120 120
Ca 20 19 23 13 18
Mg 5.4 5.5 6.8 3.5 5.5
XX - Not Filtered - Standard Methods preservation and preparation
XG - Filtered with glass fiber filter (1.2 µm)
XF - Flocculated and filtered with 0.45 µm membrane filter
Table C-2 - Raw Influent + Recycle Characterization
Parameter Monday Tuesday Wednesday Thursday Friday
1/23/06 1/24/06 1/25/06 1 /26/06 1/27/06
CODxx 550 310 400 460 420
BODxx 229 242 202.6 228 202.4
TSS 400 190 170 156 156
VSS 222 127 91 96 97.5
TKNxx 30 23 33 29 28
TPxx 5.88 8.7 8.39 11.1 5.06
NH3-N 23.7 19.0 20.5 20.9 20.6
NOx-N 0 0 0 0 0
PO4-P 2.9 6.5 6.3 8.1 3.4
ALK 180 140 140 140 180
Hazen and Sawyer, P.C. 47 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
Table C-3 - Secondary Effluent Characterization
Parameter Monday Tuesday Wednesday Thursday Friday
1 /23/06 1/24/06 1/25/06 1/26/06 1/27/06
CODxx 61 56 85 83 110
CODxG 61 77 77 86 120
CODxF 56 77 120 150 130
BODxx 8.0 8.4 5.9 8.5 6.3
BO DxG 1.5 1.6 1.5 1.55 1.39
TSS 4.6 6.4 9 3.5 2.5
VSS 0.986 1.6 3.0 0.583 0.192
TKNxx 2.7 3.5 5.1 4.0 4.2
TKNxG 2.5 4.1 4.0 3.5 3.8
TPxx 0.436 0.303 0.344 0.375 0.24
TPxG 0.322 0.185 0.283 0.256 0.143
NH3-N 1.9 2.69 1.9 2.5 2.7
NOx-N 4.03 1.88 1.98 1.73 1.96
PO4-P 0.23 0.097 0.16 0.17 0
ALK 100 100 100 80 100
Table C-4 - Filtered Effluent Characterization
Parameter Monday Tuesday Wednesday Thursday Friday
1/23/06 1/24/06 1/25/06 1 /26/06 1/27/06
CODxx 56 B5 64 86 81
BODxx 4.6 9.8 7.5 4.2 3.3
TSS 1.6 0.8 0.4 0.4 0.8
VSS 0 0
TKNxx 1.3 2.4 3.8 2.5 3.5
TKNxG 1.4 2.2 3.9 2.3 3.7
TPxx 0.313 0.202 0.216 0.257 0.18
TPXG 0.286 0.182 0.249 0.232 0.157
NH3-N 0.45 1.23 1.1 1.3 1.6
NOx-N 3.5 6.06 1.97 4.32 2.58
PO4-P 0.2 0.097 0.12 0.15 0.071
Hazen and Sawyer, P.C. 48 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
Table C-5 - Digester Decant Characteristics
Parameter Tuesday
1/24/06
Wednesday Thursday
1/25/06 1/26/06
COD,
BODxx
TSS
VSS
TKNxx
TPxx
NH3-N
NOx-N
PO4-P
ALK
pH
1300
924
1265
759
2.2
84.8
13.4
0.42
62
120
6.92
1200
537.5
575
389
65
46.3
9.5
0.62
29
180
7.23
280
121
115
52
20
45.2
10.4
5.78
41
180
7.01
Table C-6 - Reported Plant Operational Data - Supplemental Sampling Week
Parameter
Monday Tuesday
1 /23/06 1/24/06
Wednesday Thursday Friday
1/25/06 1/26/06 1/27/06
Flow, mgd 2.64 2.66
MLSS, mg/L 4,040 4,105
RAS, mgd 2.43 2.42
WAS, mgd 0.058 0
SVI, mlJg 144 117
OX. Ditch No. 1 DO, mg/L 2.4 2.4
OX. Ditch No. 2 DO, mg/L 3.6 2.8
2.58
4,500
2.24
0
133
2.4
3.3
2.33
4,675
2.44
0.081
169
2.3
2.6
2.14
6,085
2.75
0.086
146
2.2
1.8
Table C-7 - MLSS Properties - Samples Collected During Secondary Profiles
Parameter
Tuesday Wednesday
1/24/06 1/25/06
Oxidation Ditch No. 1
MLSS, mg/L
MLVSS, mg/L
Volatile Fraction, %
Oxidation Ditch No. 2
MLSS, mg/L
MLVSS, mg/L
Volatile Fraction,
SVI, m[Jg
3,410
2,525
74
3,630
2,680
74
174
3,310
2,525
76
3,495
2,664
76
185
Hazen and Sawyer, P.C. 49 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
5
18
18
14
12
1
4 1
2
0
18
Figure C-1 - Nutrient Profile - Oxidation Ditch No. 1 - 1/24/06
Haw influent
C/x. DIGO rt -
Anaerobic 1
Ox. Ditch 1 - Oxi Ditch 1 - Ox Dirtch 1 - Ox. Ditch 1 -
Anaerobic Elf Anoxic Aerobic 1 Aerobic, 2
NH3-N • NOil-N 0 PO4-P
Oxi Ditch 1 - Secondary
Aerobic 3 EIrt0en1
Figure C-2 - Nutrient Profile - Oxidation Ditch No. 2 - 1/24/06
Haw influent O. Ditch 2 • Q. Ditch 2 - O. Ditch 2 �, DirtCh 2
Anaerobic. 1 Anaerobic Eft Anoxic
Aerobic, 1
iiiN1-13-N • NO3 14 0 PO4-P
Ox, Ditch 2
Aerobic 2
3x, Ditch 2 - Secondary
Aerotk 3 Effluent
Hazen and Sawyer,. P.C.
50
Revised:S/U2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
Figure C-3 - Nutrient Profile - Oxidation Ditch No. 1 - 1/25/06
Flaw IoIur2 O. Ditch. 1 - Ox. Ditch - O. Ditch 1 - Ox„ Ditch 1 - O. Ditch - O. Ditch 1-
Anaerobic 4 Anaerobic tiiirt. Ancnitic Aerobic Aerobic.? Aerobic 3
NH3-N • NO3-N D0C114P
Figure C-4 - Nutrient Profile - Oxidation Ditch No. 2 - 1/25/06
O. Ditch 2 Ox, Ditch 2
Anaerobir; 1 Anaerobic, Etl.
NL
Ox, Ditch 2 - Ox. NOT 2 -
Anoxic Aerobic 1
NHIN • NO3-N D Pt.14-P
Ox Ditch 2 -
Aerobic2
Secondary
Effluent
D. Ditch 2 - Sectandaq
Aerobic a Effluent
Hazen and Sawyer, P.C.
51
Revised:8/11/2006

Mile Creek 4ti'TP Phosphorus Removal Optimization Study
6.9
4".
6.7
6
65
6.4
67
F.�
Raw Influent
Flaw Bnlfumnl
Figure C-5 - pH Profile - 1/24/06
Anaerobic t Anaerobic Eft
Mom
Aerctbrc
x. Ditch f •w)x. Writ 2
Aerobic; 2
Figure C-6 - pH Profile - 1/25/06
Anaerobic t Anaerobic Eff.
Anoxic
Aerobic 1
Aerobic 2
Aerobic 3
Aerobic 3
Secondary
Effluent
Secondary
Effluent
Hazen and Sawyer, R.C.
sed:8/I l /2(l(J(

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
0
1.5
0.5
2.5
r 1 5
0 '
Figure C-7 - Dissolved Oxygen Profiles - 1/23/06
11
Anaerobic, Basin Anaerobic Basin Anoxic Basin
Effluent Effluent
Aerobinl
Aerobic ttr Probe Aerobic 2 Est Weir) Aerobic 3
Ditch Ox. Ditch 2
Figure C-8 — Dissolved Oxygen Profiles — 1/24/06
MOO
EAski
rietfret,
x3E1%0
Proiseiss Anaerobic Anaerobic Anoxic. Basin Anoxic Basin
Influent Eno. Basin Basin Effluent Effluent
RAS)
Ox Ditch 1
Aerobic f Aerobic al ,Aerobic 2E3 Aerobic 3
Probe (near battles)
Hazen and Sawyer, P.C.
Re v sed
/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
5
4.5
4
3.5
E 3
2 5
0
2 2
5
) 5
0.5
Figure C-9 - Dissolved Oxygen Profiles - 1/25/06
nat
Bat*
0 lc'
Process Anaerobic Anaerobic Anoxic Basin AFIC1,6(7, f3asin Aerobic) Aerobic at Aerobic 2 (at (Aerobic 2B Aerobic 3
infiiient Ono_ Basin Basin EtIluent
RAS) Effluent
Cx. Dilc0 • Ox, 04ch 2
Probe weir) Vaear battles
Hazen and Sawyer, P.C. 54 Revised:8

J
EXHIBIT C-1
1
E
0,
0
0
2
tn
co
0
0
a
—n
4
m
m
0
AEROBIC B
U
\—ANAEROBIC EFF (TYP)
DO PROBE -
& TRHNSW
5 HP SUBNEROED
MIXER (TIP. 4 PLACES)
10 HP SUBMERGED
TURBINE MIXER (TIP. 2 PLACES)
1 ST ANOXIC BASIN
ANAEROBIC 1 (TYP)
FLOW _
EY
FRP COVER
100 HP
2 PLACES)
ti
ANOXIC (TYP)
AEROBIC 3 (TYP)
a
FRP COVER
FLOW
OXIDATION CARROUSEL- No.1
IJ
C
FLOW _
1 ST ANOXIC BASIN FRP COVER
Lw
HAZEN AND SAWYER
Environmental Engineers & Scientists
DO PROBE
& TRANSMITTER
AEROBIC 1 (TYP)
FLOW
DO PROBE AND
TRANSMITTER
AEROBIC 2 (TYP)
1
FRP COVER
PLAN
NOT TO SCALE
r
now
OXIDATION CARROUSEL No.2
FLOW
UNION COUNTY
NORTH CAROLINA
TWELVE MILE CREEK
PHOSPHORUS REMOVAL OPTIMIZATION STUDY
OXIDATION DITCH PROFILE SAMPLING LOCATIONS

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
1,1
Appendix D
Clarifier Settling Tests and Solids Flux Analysis
Hazen and Sawyer, P.C. 56 Revised:8/11/2006

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
'I
Solids settling tests were performed at the Twelve Mile Creek WWTP on the afternoon of
January 24th and January 25th, 2006. The testing comprised of calculating the settling velocity
of the solids -liquid interface at various MLSS concentrations. Settling parameters for the
Twelve Mile Creek WWTP were developed from the test and can be used to perform site
specific solids flux analyses and/or model clarifiers through the BioWin process simulation
software.
Five column settling tests were conducted on each day. Various solids concentrations were
produced by mixing MLSS with RAS and secondary effluent. SV1 tests were run during the
settleability testing, and the MLSS SVI was found to be 174 and 185 mUg, which is greater than
the historical median (166 mUg). The resulting solids concentrations for each test are given in
Table D-1.
Table D-1 - Column Settling Test Solids Concentrations
Tuesday Wednesday
1/24/06 1/25/06
Run #1, mg/L 3,450 3,750
Run #2, mg/L 10,680 5,590
Run #3, mg/L 5,190 1,790
Run #4, mg/L 6,030 3,690
Run #5, mg/L 1,450 6,440
The solids concentration for Run #3 on January 24th was produced by mixing MLSS and
secondary effluent. Since the reported solids concentration for Run #3 is greater than the
reported MLSS concentration, it is assumed that the solids concentration is incorrect and Run
#3 was omitted from further evaluation.
The column settling tests were used to calculate the initial settling velocity for each solids
concentration. The initial settling velocity is the slope of the linear portion of the settling curve;
therefore, only the linear portion of each curve is used to develop the initial settling velocity for
each concentration. Figures D-1 and D-2 show the settling curve for each column test, and
denote the linear region of the curve. There is an excellent fit for the data in the linear portion of
the curve.
The initial settling velocities were then plotted as functions of the solids concentrations. The
Vesilind equation is typically used to describe activated sludge settling behavior, and is
described as follows:
Vs - Voe'kx
VS is the settling velocity in ft/hour and X is the solids concentration in mg/L. V0 is the sludge
specific initial settling velocity (in ft/hour) and k is a sludge specific parameter in Umg. Vo and k
are determined through a non -linear regression based on the plot shown in Figure D-3. The
regression indicated an excellent fit of the available data. The site specific Vesilind parameters
for the Twelve Mile Creek WWTP are as follows:
• Vo = 38.2 ft/hour
• k = 4.05x10-4 Umg or 0.405 Ugram
Hazen and Sawyer, P.C. 57 Revised:8/11/2006

Twelve k WTP Phosphorus Re oval Optin zatim Study
Figure -1 Colu n ling Test Results 1/2006
Figure - lu 'rig Test Results 1/25/06

Twelve Mile Creek WWTP Phosphorus Removal Optimization Study
25 r-
20.
15
10
5
0
0
Figure D-3 — Initial Settling Velocity vs. Solids Concentration
2000 4000
6000
MLSS (mg/L)
8000
t, 0 954
10000 12000
The settling solids fiux is calculated using the Vesilind Equation,
GsF = XVs (0.001497)*X*Voe'kX
GsF is the settling solids flux in units of lb/ft2-day. This equation helps model the solids
concentrations at which the settler can operate without washing -out or accumulating suspended
solids, when plotted in conjunction with the underflow and overflow rate lines. The overflow rate
line is calculated using the following equation:
Gom = 8.34*X*Q/A
G0FR is the overflow solids flux in units of lb/ft2-day. Q is the clarifier effluent flow (mgd) and A is
the total clarifier surface area (ft2).
The underflow rate line is determined from this equation:
Gum = (834/A)[QXmL + QriAs(XmL-X)]
Gur-F-4 is the underflow solids flux (Ib/ft2-day). Xml, is the MLSS concentration (mg/L) and OHAs is
the RAS flow (mgd).
The settling solids flux (GsF), overflow solids flux (Gof p) and underflow solids flux (Gum) curves
are plotted as a function of solids concentration to develop a solids flux graph. The point of
intersection of the GOER and IG0FP curves is referred to as the "state point". The settling solids
flux curves were created for four conditions:
Hazen and Sawyer. PC, ,59 Revised:8/ I I /2006

Twelve Creek WWTP )hosphorus Re ptintization Study
• Site specific settling flux, based on the Vesilind parameters developed above,
• Settling flux based on correlations at the observed SVI
• Settling flux based on correlations at the historical 9011percentile SVI
• Settling flux based on correlations at the historical 99th percentile 5V1
Figure D-4 presents the solids flux analysis for the 2,5 mgd design flow at the historical average
MLSS of 4,700 mg/L.
Figure 0-4 - State Point Analysis - Current Conditions
Solais Concentration (rngl )
—•Setthry Flux Si.te SpecJIc Overflow Underliew
• — Seqfing Flux -SV 9O Settling Flux - .= — ,Serting Fox - SV Observed
The clarifier is considered underloaded with respect to thickening if the GuFH- curve passes
underneath the descending portion of the settling flux curve. The clarifier is critical loaded with
respect to thickening if the GUFR curve is tangential to the descending portion of the settling flux
curve. The clarifier is overloaded with respect to thickening if the Gum curve passes above the
descending portion of the settling flux curve, Under the current conditions, the clarifier is
underloaded at the 99% percentile SVI value at a MLSS concentration of 4,700 mg/L. The
solids flux analysis was repeated for maximum month and peak day scenarios for the current
design flow and expansion to 6.0 mgd design flow. Results of the subsequent analyses are
summarized in Table 4-1 in the report.
The solids flux analysis assumes ideal hydraulic conditions are present at the clarifiers,
Hydraulic inefficiencies, such as those reported at the Twelve Mile Creek WWTP (uneven flow
distribution, uneven RAS withdrawal) are not factored into the solids flux analysis, The analysis
strictly predicts the ability of solids to settle when applied at a specific loading rate and removed
at a specific withdrawal rate,
Hazen and Sawyer, P.c. Revise1:8/ I/2(iU(