NC0023965_Report_20071018

NPDES DOCUHENT SCANNING; COVER SHEET
NC0023965
Wilmington Northside WWTP
NPDES Permit:
Document Type:
Permit Issuance
Wasteload Allocation
Authorization to Construct (AtC)
Permit Modification
Complete File - Historical
Engineering Alternatives (EAA)
Correspondence
Owner Name Change
Report
Instream Assessment (67b)
Speculative Limits
Environmental Assessment (EA)
Document Date:
October 18, 2007
This document is prizited on reuse paper - igrzore any
content on the rezrex-se side

Ink
.01116,
HAZEN AND SAWYER
Environmental Engineers & Scientists
October 18, 2007
Mr. Tony Boahn, P.E.
McKim and Creed, Inc.
243 North Front Street
Wilmington, NC 28401
Hazen and Sawyer, P.C.
4011 WestChase Blvd.
Suite 500
Raleigh, NC 27607
[phone]
[phone] (Fax)
5 2007
R • WATER QUAi IT
SOURCE c"
Re: City of Wilmington Northside WWTP
Interim Capacity Evaluation
Dear Tony:
Hazen and Sawyer is pleased to submit for review and distribution are twelve (12) copies of the
Northside Wastewater Treatment Plant Interim Capacity Evaluation report. The report presents our
findings and recommendations related to rerating the plant to a higher interim flow rate prior to
completion of the overall construction project expanding the plant capacity to 16 million gallons per
day (mgd). Several of the new treatment facilities are anticipated to be operational in March 2008,
and at that time the Northside WWTP can be rerated from its current 8 mgd capacity to a capacity of
10 mgd utilizing the following processes:
• New headworks
• Existing primary clarifiers
• New aeration tanks
• Existing secondary clarifiers
• New filters
• New UV disinfection
• New effluent pump station
• New anaerobic digesters
Our analysis indicates that the interim facility can provide robust treatment to meet new permit limits
derived from allowable mass loadings in the current 8 mgd NPDES permit. If we can answer any
questions related to this report, please do not hesitate to contact our office.
Very truly yours,
HAZEN AND SAWYER, P.C.
Ronald L. Taylor, P.E.
Vice President
cc: Katya Bilyk
New York, NY • Philadelphia, PA • Raleigh. NC • Charlotte, NC • Greensboro, NC • Charleston, SC • Atlanta, GA • Fairfax, VA • Baltimore, MD • Cincinnati, CH • Hollywood, FL • Boca Raton, FL • Sarasota, FL • Miami. FL

Table of Contents
Section Page
ES Executive Summary ES-1
1.0 Introduction 1-1
1.1 Project Purpose 1-1
1.2 Regulatory Objectives 1-2
1.3 Organization of Report 1-2
2.0 Model Development and Calibration 2-1
2.1 Overview 2-1
2.2 Historical Data Analysis 2-1
2.2.1 Flow 2-1
2.2.2 Influent Characteristics 2-3
2.2.3 Process Data 2-8
2.2.3.1 Primary Clarifier and Trickling Filter Performance 2-9
2.2.3.2 Mixed Liquor Suspended Solids Concentrations 2-11
2.2.3.3 RAS Concentration and Rate 2-12
2.2.3.4 Sludge Volume Index 2-12
2.2.3.5 Solids Retention Time 2-14
2.2.3.6 Temperature 2-14
2.2.4 Effluent Performance 2-15
2.2.5 Mass Balance on Solids 2-16
2.3 Model Calibration 2-16
2.3.1 Comparison of Observed and Modeled Parameters 2-17
3.0 Alternatives Analysis 3-1
3.1 Overview 3-1
NWWTP Interim Capacity Evaluation
HAZEN AND SAWYER
Environments' Engineers & Scientists

Section Page
3.2 Construction Sequencing 3-1
3.2.1 Critical Facilities 3-2
3.2.2 Hydraulic Limitations 3-3
3.2.3 Process Limitations 3-4
3.3 Design Flow and Loads 3-7
3.4 Alternatives Analysis 3-9
3.4.1 Steady State Evaluation 3-10
3.4.2 Dynamic Wet Weather Evaluation 3-11
3.5 Summary 3-12
4.0 Unit Process Evaluation at Rerated Capacity of 10 mgd
4.1 Overview 4-1
4.1.1 Anticipated NPDES Permit Limits 4-1
4.1.2 Hydraulic Peak Flow Capacity 4-1
4.2 Headworks 4-2
4.3 Primary Clarifiers 4-2
4.4 Primary Sludge Pumping 4-2
4.5 Intermediate Pump Station 4-3
4.6 Aeration Basins and Aeration Blowers 4-3
4.7 Secondary Clarifiers and Return Activated Sludge 4-3
4.8 Tertiary Filters 4-4
4.9 UV Disinfection 4-5
4.10 Effluent Pump Station 4-5
4.11 Waste Activated Sludge Thickening 4-5
4.12 Anaerobic Digestion 4-6
4.13 Dewatering 4-6
4.14 Other Construction Related Considerations 4-7
4.14.1 Manual Operation 4-7
4.14.2 Operator Accessibility 4-8
4.14.3 Standby Power 4-8
4.14.4 Other Facilities 4-8
NWWTP Interim Capacity Evaluation I-ii
HAZENAND SAWYER
Environmental Engineers 8 Scientists

Section Page
4.15 Summary of Recommended Improvements 4-9
4.15.1 Critical Facilities for Rerate 4-9
4.15.2 Critical Yard Piping for Rerate 4-9
4.15.3 Additional Expenses 4-9
5.0 Recommendations 5-1
5.1 Overview 5-1
5.2 Model Calibration 5-1
5.3 Alternatives Analysis 5-2
5.4 Unit Process Evaluation 5-3
NWWTP Interim Capacity Evaluation
-iii HAZEN AND SAWYER
Environmental Engineers & Scientist!

List of Tables
Table Page
1-1 Flow Projections for NWWTP Service Area 1-1
2-1 NWWTP Flow Peaking Factors 2-2
2-2 NWWTP Flow Peaking Factors Excluding Storms
Larger than 25 Year Storms 2-3
2-3 NWWTP Annual Average Influent Concentrations 2-4
2-4 NWWTP Historical Load Peaking Factors 2-4
2-5 NWWTP Key Influent Ratios 2-5
2-6 NWWTP Primary Clarifier and Trickling Filter Removal Efficiencies 2-9
2-7 NWWTP Temperature Data 2-15
2-8 Comparison of Observed and Modeled Parameters
from Calibrated Model 2-17
3-1 NWWTP Construction Schedule 3-2
3-2 NWWTP Influent Pumping Capacity 3-4
3-3 Calculated Secondary Clarifier Surface Overflow
and Solids Loading Rates 3-5
3-4 NWWTP Design Flow and Loads 3-8
3-5 NWWTP Design Flow and Loads for Wet Weather Evaluations 3-9
3-6 Comparison of Steady State BioWin Model Simulations
of Alternatives 3-11
3-7 Comparison of Dynamic Wet Weather Model Simulations
of Alternatives 3-12
4-1 Anticipated NPDES Permit Limits at 10 mgd 4-1
4-2 NWWTP Flow Peaking Factors Excluding Storms
with over 9 inches of Rainfall (25 Year Storm) 4-2
5-1 Anticipated NPDES Permit Limits at 10 mgd 5-1
5-2 Comparison of Observed and Modeled Parameters
from Calibrated Model 5-2
NWWTP Interim Capacity Evaluation I -iv i7AZENAND SAWYER
Environmental Engineers & Scientists

List of Figures
Figure Page
2-1 NWWTP Flows 2-2
2-2 NWWTP Influent COD Concentration and Mass Load 2-6
2-3 NWWTP Influent BOD Concentration and Mass Load 2-6
2-4 NWWTP Influent TSS Concentration and Mass Load 2-7
2-5 NWWTP Influent Ammonia Concentration and Mass Load 2-7
2-6 NWWTP Influent pH 2-8
2-7 NWWTP Primary Effluent TSS 2-10
2-8 NWWTP Primary Effluent BOD 2-11
2-9 NWWTP MLSS Concentration 2-12
2-10 NWWTP RAS to Influent Flow Ratio 2-13
2-11 NWWTP RAS Concentration 2-13
2-12 NWWTP SVI 2-14
2-13 NWWTP SRT 2-15
2-14 NWWTP Effluent BOD and TSS 2-16
3-1 State Point Analysis at 10 mgd without Polymer 3-6
3-2 State Point Analysis at 10 mgd with Polymer 3-7
NWWTP Interim Capacity Evaluation I -v
HAZENAND SAWYER
Environmental Engineers & Scientists

a
Executive Summary
ES.1 Project Background
The James A. Loughlin Wastewater Treatment Plant, also called the Northside
Wastewater Treatment Plant (NWWTP), is one of two treatment plants owned and
operated by the City of Wilmington. The NWWTP is currently undergoing an expansion
to increase its permitted capacity from 8 million gallons per day (mgd) to 16 mgd.
Construction is anticipated to be completed in February 2009.
Flow projections for the NWWTP are shown in Table ES-1. These projections include a
diversion of flow from the City's overloaded Northeast Interceptor, which currently
conveys wastewater to the City's Southside Wastewater Treatment Plant. In 2008, the
projected flow will exceed the permitted capacity of the NWWTP.
Table ES-1
Flow Projections for NWWTP Service Area
Year
2006
2007
2008
2009
NVVV TP Projected Flow (mgd)
6.8
7.61
10
10.5
ES.2 Project Purpose
The purpose of this evaluation is (1) to determine if the NWWTP can be rerated to an
increased interim flow capacity higher than 8 mgd with a portion but not all of the new
facilities online and if so, (2) to quantify that interim flow. An interim rerating of the
NWWTP would allow the City to utilize a portion of the newly constructed capacity prior
to the completion of the entire construction project.
It is intended that this evaluation serve as the basis for the North Carolina Division of
Water Quality to issue a new, or revise the existing, Special Order of Consent (SOC) for
the NWWTP. The SOC is expected to set a compliance schedule for the City to
complete the interim improvements and complete the interceptor flow diversions to the
NWWTP to relieve the current overload conditions on the Northeast Interceptor. Once
the NWWTP construction project is complete, the plant will be permitted to treat 16 mgd.
NWWTP Interim Capacity Evaluation
ES-1
HAZENAND SAWYER
Environmental Engineers & Scientists

ES.3 Conclusions
Based on a thorough evaluation of the construction schedule and wastewater process
modeling, the NWWTP can be rerated for a permitted capacity of 10 mgd utilizing the
following processes:
• New headworks
• Existing primary clarifiers
• New aeration tanks
• Existing secondary clarifiers
• New filters
• New UV disinfection
• New effluent pump station
• New anaerobic digesters
The new facilities listed above are anticipated to be online by March 2008. Therefore,
based on the current construction schedule, this would allow the plant to accept a higher
flow approximately ten months earlier than the anticipated completion of the overall
construction project.
This evaluation assumed that the interim solution shall comply with the NWWTP's
current permit on a mass basis. The anticipated interim mass -based limits at NWWTP
are summarized in Table ES-2.
HAZENAND SAWYER
NWWTP Interim Capacity Evaluation ES-2
Environmental Enpineere & Scientists

Table ES-2
Anticipated NPDES Permit Limits at 10 mgd
Parameter
Permitted Flow
Effluent TSS
Monthly average
Weekly maximum
Effluent BOD
Monthly average
Weekly maximum
Fecal Coliforms
Monthly average
Weekly maximum
Ammonia
Total Phosphorus
Dissolved Oxygen
Units
mgd
mg/L
mg/L
mg/L
mg/L
CFU/100 mL
CFU/100 mL
mg/L
mg/L
mg/L
Quantity
10
30
45
24
36
800
1600
report
report
report
The process limiting step for the interim treatment facilities is the existing secondary
clarifiers, as the new clarifiers are not planned to be online until close to the end of the
overall construction project. To ensure adequate treatment under varying conditions at
the higher rated capacity, it is recommended that polymer feed facilities be added to the
NVVWTP to allow the addition of polymer to the existing secondary clarifiers on an as -
needed basis. The estimated cost of a Polyblend® or comparable polymer feed system,
as described in Section 4, is $25,000. There will also be some increased operating costs
associated with increased flows to the NVWVTP in the interim period. However, these
operating costs are not likely to be significant in comparison to the overall plant
operating costs.
NWWTP Interim Capacity Evaluation
ES-3
HAZENAND SAWYER
Environmental Englneere & Solentlate

Section 1.0
Introduction
1.1 Project Purpose
The James A. Loughlin Wastewater Treatment Plant, also called the Northside
Wastewater Treatment Plant (NWWTP), owned and operated by the City of Wilmington,
provides wastewater treatment for a design average flow of 8 million gallons per day
(mgd). Construction is currently underway at the NWWTP to expand the permitted
capacity from 8 mgd to 16 mgd. The purpose of this evaluation is (1) to determine if the
ANN
NWWTP can be rerated to an increased interim flow capacity higher than 8 mgd with a
AMIN
portion but not all of the new facilities online and if so, (2) to quantify that interim flow. An
interim rerating of the NWWTP would allow the City to utilize a portion of the newly
constructed capacity prior to the completion of the entire construction project.
The City owns and operates two wastewater treatment facilities, which together receive
and treat wastewater flows from Wilmington, New Hanover County, Wrightsville Beach,
and Pender County. The City desires additional capacity as soon as possible at the
NWWTP to provide flexibility to manage flows on the City's Northeast Interceptor.
Wastewater flow projections for the NWWTP service area are shown in Table 1-1 and
include flow from the interceptor diversion.
Table 1-1
Flow Projections for NWWTP Service Area
Year
2006
2007
2008
2009
NWWTP Projected Flow (mgd)
6.8
7.61
10
10.5
It is intended that this evaluation serve as the basis for the North Carolina Division of
Water Quality (NCDWQ) to issue a new, or revise the existing, Special Order of Consent
(SOC) for the NWWTP. The SOC is expected to set a compliance schedule for the City
to complete interim improvements and complete the interceptor flow diversions to the
NWWTP to relieve the current overload conditions on the Northeast Interceptor. Once
the NWWTP construction project is complete, the plant will be permitted to treat 16 mgd.
NWWTP Interim Capacity Evaluation 1-1
HAZENAND SAWYER
Environmental Engineers & Scientists

1.2 Regulatory Objectives
This evaluation assumes that the interim solution shall comply with the NWWTP's
current permit on a mass basis. Therefore, the regulatory objective is to have no net
increase in the permitted mass discharged. For instance, the NWWTP currently has an
effluent monthly biochemical oxygen demand (BOD5) limit of 30 mg/L at 8 mgd. If the
plant is rerated for 10 mgd, the new effluent limit would be 24 mg/L (30 mg/L x 8 mgd/10
mgd = 24 mg/L).
1.3 Organization of Report
This report is organized into the following sections.
• Section 1 — Introduction
• Section 2 — Model Development and Calibration
• Section 3 — Alternatives Analysis
• Section 4 — Unit Process Evaluation
• Section 5 — Recommendations
NWWTP Interim Capacity Evaluation 1-2
HAZENAND SAWYER
Environmental Engineers & Scientists

Section 2.0
Model Development and Calibration
2.1 Overview
This section of the report describes the technical approach used to develop and calibrate
a wastewater process model of the Northside Wastewater Treatment Plant (NWWTP).
Historical process and operations data were reviewed to understand the plant's
performance and how it typically operates. Then, the existing plant configuration was
constructed using BioWinTM software and calibrated to the historical data. The calibrated
model serves as the basis for the alternatives analysis discussed in Section 3.
2.2 Historical Data Analysis
The first step in the model calibration was to obtain and analyze historical data. Influent,
primary effluent, trickling filter effluent, final effluent, solids processing, and aeration
basin data from January 2005 through May 2007 were obtained. This information was
used to evaluate historical flows and peaking factors, annual average concentrations,
load peaking factors, influent ratios, and typical operating conditions at the NWWTP.
••i The plant keeps extensive records of influent biochemical oxygen demand (BOD),
chemical oxygen demand (COD), total suspended solids (TSS), temperature, and
ammonia. To a lesser extent, the plant also has some historical influent total Kjeldahl
nitrogen (TKN) and total phosphorus (TP) measurements. The historical data is
summarized in this section.
Aft 2.2.1 Flow
Aft
Table 2-1 summarizes the historical flow in millions of gallons per day (mgd) and flow
peaking factors at the NWWTP. Figure 2-1 shows a graphical representation of the total
influent flow for the same period.
Anik
sink
oak
- NWWTP Interim Capacity Evaluation 2- I HAZEN AND SAWYER
Environmental Engineers & Scientists

Anes
AMA
ink
Aq
22 -
dak 20
ink
IIlk
Ale
ONA
vellak
ink
AlIlk
Ask
18
16
14
E 12
IJ
ii 10
LL
8
6
4
2
Table 2-1
NWWTP Flow Peaking Factors
For the period of January 2005 — May 2007
Flow Criteria
Historical
Flow
(mgd)
Peaking
Factor
Minimum Day
5.6
0.76
Average Annual
7.4
1.00
Maximum 30-Day
10.5
1.40
Maximum 7-Day
14.8
2.00
Maximum Day
20.9
2.81
Figure 2-1
NWWTP Flows
For the period of January 2005 — May 2007
• Total Influent Flow
—7 per. Mov. Avg. (Total Influent Flow)
—30 per. Mov. Avg. (Total Influent Flow)
— 0
Nov-04 Feb-05 May-05 Sep-05 Dec-05 Mar-06 Jul-06 Oct-06 Jan-07 Apr-07 Aug-07
The maximum day, maximum week, and maximum month flow peaking factors are fairly
high. These values are influenced by two significant rainfall events —namely Tropical
Storm Tammy (early October 2005) and Tropical Storm Ernesto (late August 2006),
which inundated the Wilmington area with over 16 and 10 inches of rain, respectively.
Hurricane Ophelia also passed through the area in early September 2005 leaving
approximately 8 inches of rain, but the effects of the storm on the values shown in Table
NWWTP Interim Capacity Evaluation
2-2
HAZENAND SAWYER
Environmental Engineers & Scientists

Aft
Aft
•► 2-1 are less significant. To quantify the impacts of Tropical Storm Tammy and Tropical
Aft. Storm Ernesto on the data set, the same statistics were determined excluding those
events. Excluding these two storms with over 9 inches of rain (25 year storm event)
" provides the flow and peaking factors shown in Table 2-2.
"
"
Arms
ANON
Table 2-2
NWWTP Flow Peaking Factors Excluding Storms
Larger Than 25 Year Storm Events
Flow Criteria
Historical
Flow
(mgd)
Peaking
Factor
Minimum Day
5.6
0.78
Average Annual
7.2
1.0
Maximum 30-Day
9.8
1.3
Maximum 7-Day
1YO
_._. _
1.5
Maximum Day
14.6
2.0
The maximum month peaking factor is lower, as are the maximum week and maximum
day flows. The statistics shown in Table 2-2 are more representative of plant flow
variations and will be used in the alternatives analysis.
2.2.2 Influent Characteristics
Influent data was analyzed to develop average influent concentrations, influent load
peaking factors, and quantify important ratios.
A" Table 2-3 summarizes the historical annual average influent, trickling filter effluent, and
primary effluent concentrations of COD, BOD, carbonaceous BOD (CBOD), TSS, and
Adak ammonia. The influent TKN and TP data is limited (approximately 10 to 20 data points),
Aft and therefore this information should not be used to draw conclusions about varying
influent conditions.
S•u
Ask
Ana
HAZENAND SAWYER
NWWTP Interim Capacity Evaluation
Ask
2-3
Environmental Engineers 8 Scientiate

AIM
41119..
Adilk
0
Table 2-3
NWWTP Average Influent Concentrations
For the period of January 2005 - May 2007
Criteria
Historical
Average
Concentration
BOD, mg/L
198.9
CBOD, mg/L
152.7
COD, mg/L
480.8
TSS, mg/L
168.1
TKN, mg/L
34.7
Ammonia, mg/L
23.8
TP, mg/L
5.2
Historical COD, BOD, CBOD, TSS, ammonia, TKN, and TP load peaking factors for both
plants are summarized in Table 2-4. The TKN and TP data sets are small and should be
interpreted cautiously. Several key influent ratios are summarized in Table 2-5. Although
these ratios are more significant for biological nutrient removal (BNR) facilities, the
values were quantified as part of the influent analysis. The COD to BOD ratio is higher
than average for municipal wastewater, which tends to vary from 2.0 to 2.2. The influent
characteristics also appear favorable for future biological phosphorus removal and
nitrogen removal, although those processes are not required at this time.
Table 2-4
NWWTP Historical Load Peaking Factors
For the period of January 2005 - Ma v 2007
Criteria
BOD5
CBOD
COD
TSS
TKN
NH3
TP
Minimum Day
0.62
0.77
0.32
0.33
0.88
0.23
0.78
Average Annual
1.00
1.00
1.00
1.00
1.00
1.00
1.00
Maximum 30-Day
1.12
-
1.14
1.39
-
1.20
-
Maximum 7-Day
1.33
1.16
1.35
1.87
-
1.61
-
Maximum Day
1.89
1.35
2.47
3.94
1.16
2.46
1.15
NWWTP Interim Capacity Evaluation 2-4
HAZENAND SAWYER
Environmental Engineers & Scientists

1/1
Oak
,a^
*nk
/114
r►
Table 2-5
NVWVTP Key Influent Ratios
Ratio
Value
Influent COD/BOD
2.48
Influent COD/CBOD
3.20
Influent NH3-N/TKN
0.71
Influent COD/TKN
14.15
Influent BOD/TKN
5.89
Influent COD/TP
94.20
Influent BOD/TP
37.60
Influent COD/NH3
20.65
Influent BOD/NH3
8.41
Influent COD, BOD, TSS, and ammonia concentrations and loads for the period
analyzed are shown in Figures 2-2, 2-3, 2-4, and 2-5, respectively. Seven-day and thirty -
day running averages, representing the average weekly and monthly trends, are shown
on the figures as well. Influent pH is shown in Figure 2-6.
oak
Ak
11►
0111
oak
1114
1/1
4114
Ank
411
4111,
411114.
.e,
NWWTP Interim Capacity Evaluation
2-5
HAZENAND SAWYER
Environmental Engineers & Scientists

800
700 -
600 -
500 -
J
E 400
0
0
300
200
100
0
Figure 2-2
NWWTP Influent COD Concentration and Mass Load
•
•
•
•
•
• •
• • •$
• ••
• •• •
•
• •• •
r ▪ •
•
•
•• •
•
• •
•
• • COD Concentration • COD Loading
—7 per. Mov. Avg. (COD Loading) —30 per. Mov. Avg. (COD Loading)
—7 per. Mov. Avg. (COD Concentration) —30 per. Mov. Avg. (COD Concentration)
•
•
T 80000
— 70000
— 60000
— 50000
n
0
0
— 40000 c
— 30000
20000
10000
0
Nov-04 Feb-05 May-05 Sep-05 Dec-05 Mar-06 Jul-06 Oct-06 Jan-07 Apr-07 Aug-07
400.00
350.00 -
300.00 -
250.00 -
J
E 200.00
0
150.00
100.00
50.00
0.00
Figure 2-3
NWWTP Influent BOD Concentration and Mass Load
• BOD Concentration • BOD Loading
— 7 per. Mov. Avg. (BOD Loading) —30 per. Mov. Avg. (BOD Loading)
— 7 per. Mov. Avg. (BOD Concentration) —30 per. Mov. Avg. (80D Concentration)
• ••w. .
•• •
• it �••
S.• •
1
•
• = w •
•
1 • 1.••
.•
• ••
rn1r•
•
•
•
ti
• k - • •
• - •
•
•)11
:•.1' • .
• '
'•-•
• r}
••
.
•
m
— 50000
— 45000
— 40000
35000
30000
0
0
25000
20000
15000
10000
— 5000
0
Nov-04 Feb-05 May-05 Sep-05 Dec-05 Mar-06 Jul-06 Oct-06 Jan-07 Apr-07 Aug-07
HAZENAND SAWYER
NWWTP Interim Capacity Evaluation 2-6
Environmental Engineers & Scientists

400 -
350 -
300 -
250 -
J
-61
E. 200
rn
150 -
100 -
50 -
45 -
40 -
35 -
30 -
J
Ol
E 25 -
M
z
20 -
15 -
10-
5-
0
50 -
Figure 2-4
NWWTP Influent TSS Concentration and Mass Load
• TSS Loading
— 7 per. Mov. Avg. (TSS Loading) —30 per. Mov. Avg. (TSS Loading)
— 7 per. Mov. Avg. (TSS Concentration) —30 per. Mov. Avg. (TSS Concentration)
.. •
• TSS Concentration
— 50000
— 45000
— 40000
— 35000
— 30000
co
co
— 25000 0
— 20000
— 15000
— 10000
— 5000
0 0
Nov-04 Feb-05 May-05 Sep-05 Dec-05 Mar-06 Jul-06 Oct-06 Jan-07 Apr-07 Aug-07
Figure 2-5
NWWTP Influent Ammonia Concentration and Mass Load
• • NH3 Concentration • NH3 Loading
—7 per. Mov. Avg. (NH3 Loading) —30 per. Mov. Avg. (NH3 Loading)
—7 per. Mov. Avg. (NH3 Concentration) —30 per. Mov. Avg. (NH3 Concentration)
— 4000
— 3500
— 3000
— 2500
z
2
w
- 2000 r
a
a)
— 1500
1000
500
0
Nov-04 Feb-05 May-05 Sep-05 Dec-05 Mar-06 Jul-06 Oct-06 Jan-07 Apr-07 Aug-07
HAZENAND SAWYER
NWWTP Interim Capacity Evaluation 2-7
Environmental Engineers 8 Scientists

8
7.8
7.6
7.4
7.2
= 7
a
6.8
6.6
6.4
6.2
6
Figure 2-6
NWWTP Influent pH
•
• pH
— 7 per. Mov. Avg. (pH)
- 30 per. Mov. Avg. (pH)
Nov-04 Feb-05 May-05 Sep-05 Dec-05 Mar-06 Jul-06 Oct-06 Jan-07 Apr-07 Aug-07
The influent pH typically varies from 6.8 to 7.3, and averages approximately 7.1. This is
typical of influent wastewater. The primary effluent alkalinity has averaged approximately
439 mg/L, which is considered high. Higher alkalinities help buffer the aerobic tanks from
changes in pH that could affect biological kinetics. When the plant nitrifies, this alkalinity
will help buffer the activated sludge process from a pH drop due to nitrification. The
anaerobic digestion process introduces alkalinity into the liquid stream when belt filter
press filtrate rejoins the liquid treatment train. Insufficient influent alkalinity
measurements were available to determine how much of this alkalinity is contributed by
the influent and how much is contributed by the recycle stream.
2.2.3 Process Data
Information on the following parameters was used to evaluate the current performance
of the plant and its various processes:
• Primary clarifier effluent BOD and TSS
• Trickling filter effluent BOD and TSS
• Mixed liquor suspended solids (MLSS) concentration
• Dissolved oxygen (DO) in the aeration basin
NWWTP Interim Capacity Evaluation 2-8 HAZENAND SAWYER
Environmental Engineers & Scientists

0
• Return activated sludge (RAS) concentration and rate
• Sludge volume index (SVI)
• Solids wasting rates
• Temperatures
These parameters are routinely monitored. This section presents a summary of process
performance based on this data.
2.2.3.1 Primary Clarifier and Trickling Filter Performance
Available data on primary clarifier and trickling filter effluent BOD and TSS was analyzed
to assess the efficiency of these processes, as summarized in Table 2-6.
Table 2-6
NWWTP Primary Clarifier and Trickling Filter Removal Efficiencies
Criteria
Primary
Effluent
Primary
Effluent
Trickling Filter
Effluent
Trickling
Filter
Effluent
Average
Concentration
Percent
Removal
Average
Concentration
Percent
Removal
BOD, mg/L
TSS, mg/L
134
56.5
31.7%
64.1 %
65.01
68.01
51.6%1
-20.6%1
The trickling filter effluent sampling location failed to capture all of the suspended sords material, and therefore the values
shown are presumed to be underestimates of he true BOD and TSS. In addition, the trickling filter effluent TSS is expected
to be higher than the primary clarifier effluent TSS due to sloughing of biomass from the filter media.
The primary clarifiers remove approximately 64 percent of the influent TSS and 32
percent of the influent BOD. This is in the typical range for well performing clarifiers. The
surface overflow rate of the primaries at 8 mgd is approximately 700 gallons per day per
square foot (gpd/sf), which is well below the typical design values of 1000 gpd/sf. Thus,
it is expected that this system would perform well.
The trickling filter effluent quality is estimated on a mass balance basis with the
understanding that the trickling filter biofilm on the media will periodically slough off of
the media and move into the liquid phase. Using a mass balance approach, TSS in the
trickling filter effluent is estimated to be approximately 92 mg/L, and BOD is estimated to
be approximately 116 mg/L. This represents approximately 14 percent removal of BOD
„MI by the trickling filters. Since the trickling filters are not optimized, i.e., they do not
recirculate the influent, this estimate of BOD removal appears reasonable.
NWWTP Interim Capacity Evaluation 2-9 HAZEN AND SAWYER
Environmental Engineers & Scientists

Primary effluent TSS and BOD are shown graphically in Figures 2-7 and 2-8,
respectively.
Figure 2-7
NWWTP Primary Effluent TSS
160 - - 12000
140 -
120
100
E • 80 -
cn
60 -
40 -
20
• • •i
• . ; ♦♦• t
'r
t .t. •
▪ •:: .:I •: . t
▪ +.�•l i .-.,�L . It 1 t
,erg. i r� 'r,'•,7.ft•• , ••," . +'
•
•
• • • ♦• •
�.`• ••
•
.b ■ aU _.
•
• TSS Concentration
— 7 per. Mov. Avg. (TSS Loading)
— 7 per. Mov. Avg. (TSS Concentration)
0
•
•
• .♦
•
• .
• TSS Loading
— 30 per. Mov. Avg. (TSS Loading)
— 30 per. Mov. Avg. (TSS Concentration)
10000
8000
1
to
m
— 6000 o-
'n
-- 4000
— 2000
0
Nov-04 Feb-05 May-05 Sep-05 Dec-05 Mar-06 Jul-06 Oct-06 Jan-07 Apr-07 Aug-07
NWWTP Interim Capacity Evaluation 2-ID
HAZEN AND SAWYER
Environmental Engineers & Scientists

WEIN
..a
AMIN
200
180
160
140
120
J
100
O
m
80
60
40
20
0
Figure 2-8
NWWTP Primary Effluent BOD
•
• BOD Concentration
• BOD Loading
— 7 per. Mov. Avg. (BOD Loading)
—30 per. Mov. Avg. (BOD Loading)
— 7 per. Mov. Avg. (BOD Concentration)
— 30 per. Mov. Avg. (BOD Concentration)
18000
16000
14000
12000
10000 co
o
0
8000
6000
4000
2000
0
Nov-04 Feb-05 May-05 Sep-05 Dec-05 Mar-06 Jul-06 Oct-06 Jan-07 Apr-07 Aug-07
2.2.3.2 Mixed Liquor Suspended Solids Concentrations
MLSS concentrations are shown in Figure 2-9. The MLSS concentration averages
approximately 1,055 mg/L. The volatile fraction averages 83 percent of the total MLSS.
There has been significant fluctuation in the MLSS concentration and it appears that the
fluctuation is seasonal. The MLSS average is higher in the winter than in the summer.
This is anticipated as kinetics slow with declining temperature, and for a given SRT a
higher population of microorganisms is needed to achieve the same amount of BOD
removal.
NWWTP Interim Capacity Evaluation 2-1 I 17AZENAND SAWYER
Environmental Engineers d Scientists

3000
2500 -
2000 -
W
En
1500
co
CO
J
2
1000
500
0
Figure 2-9
NWWTP MLSS Concentration
—30 day average MLSS
Nov-04 Feb-05 May-05 Sep-05 Dec-05 Mar-06 Jul-06 Oct-06 Jan-07 Apr-07 Aug-07
2.2.3.3 RAS Concentration and Rate
The historical RAS rate and concentration have averaged 1 mgd and 5,400 mg/L,
respectively. Historical RAS rates as a fraction of the influent flow rate are shown in
Figure 2-10. Figure 2-11 illustrates the RAS concentration as a function of time. The
average secondary clarifier sludge blanket height has been less than one foot, which is
ideal.
2.2.3.4 Sludge Volume Index
SVI is a measure of the settlability of secondary sludge. SVI is calculated by dividing the
liquid -sludge interface from a settleometer test after thirty minutes (mL/L) by the MLSS
concentration (mg/L) and is measured in units of mL/g. Figure 2-12 shows SVI over
time, which averages approximately 140. There are varying opinions on what SVI
qualifies as having excellent, moderate, and poor settling and compaction
characteristics, but most references classify an SVI of 140 as having moderate sludge
settling and compaction properties.
NWWTP Interim Capacity Evaluation
2-12 HAZEN AND SAWYER
Environmental Engineers & Scientists

RAS/Q;01 [dimensionless]
Concentration [mg/L]
1.00 -
0.90 -
0.80 -
0.70 -
0.60 -
0.50 -
0.40 -
0.30 -
0.20 -
0.10 -
Figure 2-10
NWWTP RAS to Influent Flow Ratio
• RAS/Qinf.
—7 per. Mov. Avg. (RAS/Qinf.)
—30 per. Mov. Avg. (RAS/Qinf.)
•
0.00
Nov-04 Feb-05 May-05 Sep-05 Dec-05 Mar-06 Jul-06 Oct-06 Jan-07 Apr-07 Aug-07
14.000
12,000
10,000
8,000
6,000
4,000
2,000
Figure 2-11
NWWTP RAS Concentration
• Waste SS
— 7 per. Mov Avg. (Waste SS)
— 30 per. Mov. Avg. (Waste SS)
• • •
•
•
•
0
Nov-04 Feb-05 May-05 Sep-05 Dec-05 Mar-06 JuI-06 Oct-06 Jan-07 Apr-07 Aug-07
NWWTP Interim Capacity Evaluation
2-13 HAZEN AND SAWYER
Environmental Engineers & Scientists

AMA
600
500
400
rn
E 300
rn
200
100
0
Figure 2-12
NWWTP SVI
•
• svl
— 7 per. Mov. Avg. (SVI)
— 30 per Mov. Avg. (SVI)
•
•
•
Nov-04 Feb-05 May-05 Sep-05 Dec-05 Mar-06 Jul-06 Oct-06 Jan-07 Apr-07 Aug-07
2.2.3.5 Solids Retention Time
SRT is a measure of the age of the sludge. SRT is calculated by dividing the mass of
solids in the secondary system by the mass of solids wasted daily. SRT as a function of
time is shown in Figure 2-13. The SRT has averaged approximately 1.6 day and has
varied from 1 to 3 days.
2.2.3.6 Temperature
Temperature is recorded daily at the NWWTP. Table 2-7 summarizes the temperature
trends at the NWWTP. The minimum week winter temperature is 14.6°C. A minimum
temperature of 13°C is used for the analysis.
NWWTP Interim Capacity Evaluation
2-14 HAZEN AND SAWYER
Environmental Engineers & Scientists

Oak
024
Ik
oak
Ik
5.0
4.5 -
4.0 -
3.5 -
3.0 -
2.0
1.5
1.0
014
0.0
AWN
a
0.5
Figure 2-13
NWWTP SRT
• SRT
—7 per. Mov. Avg. (SRT)
•—•-•30 per. Mov. Avg. (SRT)
Nov-04 Feb-05 May-05 Sep-05 Dec-05 Mar-06 Jul-06 Oct-06 Jan-07 Apr-07 Aug-07
Table 2-7
NWWTP Temperature Data
Criteria
Historical
Temperature,
°C
Minimum Day
12.0
Minimum 7-Day
14.6
Minimum Month
15.2
Average Annual
20.9
Maximum 30-Day
27.4
Maximum 7-Day
28.0
2.2.4 Effluent Performance
The NWWTP records effluent BOD, TSS, and ammonia on a daily basis. The plant does
not currently have an ammonia limit. The plant also records effluent total phosphorus,
total nitrogen, and COD on a less frequent basis.
Monthly average effluent BOD and TSS concentrations are summarized in Figure 2-14.
As shown by this figure, the plant has had no trouble complying with the 30 mg/L BOD
2- 15 H z +S AND SAWYER
NWWTP Interim Capacity Evaluation
Environmental Engineers & Scientists

and TSS monthly average limits in its current configuration. It is anticipated that
treatment quality will improve when the interim capacity facilities are placed into service.
• 0 N 0 0 0 0 0
c6 - U
tL 2
'4)
0 0
> c
0
Figure 2-14
NVWVTP Effluent BOD and TSS
0 ▪ 0
o) a
Q • (/)
.L) N
0 0
o
0 0 0 0
c C 1) t9
❑ ( u_
Month
9
(0 0 co co
0 0 09
@ ' ' - cn
co
0
(/)
O o0000
• > U C
• cis
o Z ❑ COU 2
2.2.5 Mass Balance on Solids
Solids mass balances using historical information are a component of all calibration
efforts. These mass balances are based on the principle that the mass of solids entering
the plant must equal the mass of solids exiting the plant as either effluent or biosolids.
A solids mass balance was conducted on the primary clarifiers and secondary clarifiers.
In both cases the mass entering the clarifier was approximately equal to the mass exiting
the clarifier with the difference being less than fifteen percent. This agreement confirms
the reliability of the plant wasting records.
2.3 Model Calibration
BioWin model calibrations can include varying levels of complexity depending on the
amount of historical and site -specific data available. For this study, historical data was
NWWTP Interim Capacity Evaluation
2-16 HAZEN AND SAWYER
Environmental Engineers & Scientists

used to calibrate the model. The calibration process consisted of the following
components.
• Model construction
• Comparison of observed and modeled parameters
2.3.1 Comparison of Observed and Modeled Parameters
The various processes at the existing NWWTP were simulated using the available
process elements in BioWin. A steady state simulation using historical averages for each
parameter was modeled. As part of the calibration, observed and modeled parameters
naas
are compared. The calibrated model accurately reflected the influent characteristics and
treatment plant performance based on an annual average steady state simulation. A
comparison of the key modeled and observed parameters from the calibrated model is
shown in Table 2-8. This table demonstrates good agreement between the observed
and modeled parameters and therefore a good calibration of the model.
Table 2-8
Comparison of Observed and Modeled Parameters from Calibrated Model
A
0114
+a
ink
Parameter
Units
Observed
Calibrated
Model
Influent Flow
mgd
7.05
7.05
Influent COD
mg/L
483
483
Influent BOD
mg/L
199
198
Influent TKN
mg/L
33.4
33.4
Influent TP
mg/L
5
5
Influent TSS
mg/L
168
174
PE TSS
mg/L
57
54
PE BOD
mg/L
134
114 -
MLSS
mg/L
1050
1036
MLVSS/MLSS
-
0.83
0.86
RAS
mg/L
5400
5300
Effluent TSS
mg/L
5
6.4 -
Temp
C
21
20
Primary Solids
%
4.3
4.1
TWAS Solids
%
4.8
4.7
Anaerobically Digested Solids
%
1.9
2.6
Cake
%
18.5
18.3
The calibrated model was used as the basis for the alternatives analysis that is
discussed in Section 3.
NWWTP Interim Capacity Evaluation
014
2-17
HAZEN AND SAWYER
Environmental Engineers & Scientists

Section 3.0
Alternatives Analysis
3.1 Overview
This section of the report describes the factors and alternatives that were considered
when evaluating interim operating solutions that would allow the Northside Wastewater
Treatment Plant (NWWTP) to be rerated for an increased interim flow with a portion but
not all of the new facilities online. Specifically, this section details construction
sequencing, critical facilities, hydraulic and process limitations; outlines the design flow
and loads used in the alternatives analysis; and summarizes the alternatives analysis.
3.2 Construction Sequencing
Construction is currently underway to expand the NWWTP from 8 million gallons per day
(mgd) to 16 mgd. The following process approach was utilized to develop process
configurations that could be utilized to rerate the plant on an interim basis.
• Identify what critical facilities need to be online to rerate the facility on an interim
basis taking into account completion dates relative to completion of the overall
construction project.
• Identify and evaluate hydraulic limitations.
• Identify and evaluate process limitations.
The construction contractor is proposing the schedule shown in Table 3-1 for completing
the major liquid treatment unit processes remaining for the NWWTP expansion. The
NWWTP is undergoing significant changes to the liquid treatment facilities including
construction of new headworks facilities, new primary clarifiers, new aeration tanks,
additional secondary clarifiers, new tertiary filtration facilities, new UV disinfection
facilities and an additional effluent pump station. Modification of the hydraulic profile as
part of the plant improvements will eliminate use of the existing trickling filters,
intermediate pump station and the existing aeration tank. Completion of the new
headworks, currently scheduled for March 2008; will allow operation through the final
hydraulic profile such that these facilities would not be required.
NWWTP Interim Capacity Evaluation 3-1
HAZEN AND SAWYER
Environmental Enolneers & Scientists

Table 3-1
NWWTP Construction Schedule
Process
Scheduled
Completion
Date
New UV system
Feb-08
New effluent pump station
Feb-08
New tertiary filters
Feb-08
New headworks
Mar-08
New aeration tanks
Mar-08
Demolish tricking filters
May-08
New secondary clarifiers
Aug-08
New primary clarifiers
Oct-08
Final construction
Feb-09 -
The solids treatment facilities will also be capable of treating 16 mgd at the end of the
construction phase. The critical facility in the solids treatment process is anaerobic
digester capacity, as the frequency of thickening and dewatering operations can
increase to accommodate a moderate increase in flow. The new anaerobic digesters
sized for 16 mgd are scheduled to be complete in March 2008.
3.2.1 Critical Facilities
The new aeration tanks are the key process facility under construction that is required to
rerate the NWWTP. The aeration tanks are sized for treating a 16 mgd flow capacity with
year round nitrification, and thus would easily be capable of treating in excess of 8 mgd
once the tanks are available. The contractor's schedule anticipated that the new
aeration tanks will be available for service by March 2008. At that time several other
important improvements will also be available including:
• New headworks
• New UV system
• New tertiary filters
• New effluent pump station
• New anaerobic digesters
NWWTP Interim Capacity Evaluation 3-2
HAZENAND SAWYER
Environmental Engineers & Scientists

In combination with existing processes that will remain in service as part of the upgrade,
these new facilities allow the plant to be rerated for a higher flow. Based on the
construction schedule, the following two treatment strategies were evaluated.
• Alternative 1 (without Primary Treatment) — New headworks, new aeration tanks,
existing secondary clarifiers, new filters, UV disinfection, new effluent pump
station, and new anaerobic digesters
• Alternative 2 (with Primary Treatment) — New headworks, existing primary
clarifiers, diversion of excess wet weather flows directly to new aeration tanks,
new aeration tanks, existing secondary clarifiers, new filters, UV disinfection,
new effluent pump station, and new anaerobic digesters
The existing trickling filters are scheduled to be demolished in May 2008. In regards to
Alternative 2, this evaluation assumes that temporary piping will be installed from the
existing primary clarifiers to the intermediate pump station and that this piping will be
utilized during demolition, and therefore the existing primary clarifiers would be available
to remain online throughout construction.
The schedule for installing interconnecting piping associated with the critical facilities
was reviewed as well. The necessary piping is anticipated to be in place.
3.2.2 Hydraulic Limitations
Rerating the NVVVVTP during construction requires a reevaluation of the plant hydraulic
grade line. Hydraulic constraints common to both alternatives are as follows:
• The maximum allowable flow through the treatment plant is limited by the
elevation of the existing secondary clarifier weirs. Conservative calculations show
that the weirs will be submerged at flows above 20 mgd.
• The firm effluent pumping capacity is 20 mgd during the interim rerate period.
This analysis is based on using the single existing effluent force main to the
receiving stream. The planned second parallel force main will be installed later in
the project.
• The firm return activated sludge (RAS) pumping capacity is 5.2 mgd.
NWWTP Interim Capacity Evaluation 3-3
HAZENAND SAWYER
Environmental Engineers & Scientists

AMR
In addition, Alternative 2 is hydraulically restricted by the capacity of the intermediate
pump station. The firm capacity of this station is 18 mgd. The alternatives evaluation will
be conducted based on routing only 13 mgd of primary effluent to the intermediate pump
station and diverting any excess flows around the primary clarifiers and directly to the
aeration tanks. This is a conservative evaluation approach since all flows can be routed
through the primary clarifiers before pumping to the aeration tanks when all of the pumps
are operational. The evaluation assumes that excess flows above 13 mgd will be routed
directly to the new aeration tanks without primary treatment.
The firm effluent pumping capacity in the interim rerate period is 20 mgd, which matches
the hydraulic capacity of the secondary clarifier. The firm influent pumping capacity
(combination of City and County pump stations) is 21.6 mgd, as shown in Table 3-2. The
NWWTP has the ability to control influent flows to the treatment plant by avoiding the
condition where all four pump stations are running at the same time. Since the maximum
day flow to the NWWTP excluding rain events over 9 inches (25 year storm) has been
14.6 mgd historically (see Table 2-2), this appears to be a realistic interim strategy.
Table 3-2
NWWTP Influent Pumping Capacity
PS No.
Firm Pumping Capacity
(mgd)
City 10
8.4
City 44
0.9
County 62
0.5
County 89
11.9
Total Firm Capacity
21.6
Given these hydraulic restrictions on the secondary clarifiers and effluent pumping to a
maximum of 20 mgd, influent flows will be restricted during construction and the interim
rerate period. On this basis, the City of Wilmington would request a variance on the
design peak hour flow factor to allow an interim peak flow factor of 2.0 times the
permitted capacity. A peak flow of 20 mgd is used in this evaluation, and this appears
conservative given the historical peak flow has only been higher than 14.6 mgd when
greater than 25 year storm events have occurred (hurricanes/tropical storms).
NWWTP Interim Capacity Evaluation
3-4
HAZENAND SAWYER
Environmental Engineers & Scientists

3.2.3 Process Limitations
Both alternatives under consideration assume that the new headworks, new aeration
basins, new tertiary filters, and new UV disinfection facilities rated for 16 mgd are in
service. Therefore, the process limitations are likely to be the existing facilities—
, —
secondary clarifiers and in the case of Alternative 2, primary clarifiers.
Albs
^a
elMie
Primary clarifiers are typically designed to perform well at average overflow rates Tess
than 1,000 gallons per day per square foot (gpd/sf) and at peak overflow rates less than
2,500 gpd/sf. At the anticipated peak flow of 20 mgd, the loading rate is approximately
1,800 gpd/sf, and this process as currently sized does not restrict the capacity to a lower
flow.
To properly assess the process capacity of the secondary clarifiers, predicted overflow
rates and solids loading rates to the clarifiers were compared to typical design values.
Table 3-3 compares calculated surface overflow rates and solids loading rates of the
existing secondary clarifiers for a range of permitted flows with typically recommended
values. Based on this cursory comparison of surface overflow rates and solids loading
rates (based on MLSS concentrations of 1,500 mg/L), it appears that the secondary
clarifiers could treat an average flow of between 10 and 12 mgd. Based on this analysis,
the secondary clarifiers may represent the process limiting step for rerating the NWWTP.
Table 3-3
Calculated Secondary Clarifier Surface Overflow and Solids Loading Rates
Permitted
Flow (mgd)
Average
Surface
Overflow
Rate
(gpd/sf)
Average
Solids
Loading
Rate
(Ib/d/sf)
Peak
Overflow
Rate
(gpd/sf)
Peak
Solids
Loading
Rate
(Ib/d/sf)
Recommended
400
15
1,200
30
8
630
8
947
12
10
787
10
1,184
15
12
945
12
1,421
18
14
1,102
14
1,658
21
NWWTP Interim Capacity Evaluation
Ate,
3-5
HAZENAND SAWYER
Environmental Engineers 5 Scientists

Additional evaluations of the existing secondary clarifiers were also conducted using
state point analysis with conservative settling parameters. State point analysis is a more
detailed assessment of clarifier capacity and considers influent flow, RAS flow, MLSS
concentrations, surface area of the clarifiers, and settling parameters; and assesses
whether the clarifiers would fail at a given condition. Provided the intersection of the RAS
and solids overflow rate lines is below the flux curve, the clarifiers will not fail for a given
set of conditions.
The benefit of using polymer in the secondary clarifiers was also evaluated using
empirical data on the impact of polymer on settling coefficients. The results of the state
point analysis without and with polymer addition are shown in Figure 3-1 and 3-2,
respectively. These evaluations assume a flow rate of 10 mgd, a RAS rate of 1.44 mgd,
and a MLSS concentration of 1,200 mg/L. These figures show that adding polymer
increases the factor -of -safety within which the clarifiers would operate. Therefore,
polymer usage is recommended on an as -needed basis, and generally speaking, should
be used when the flow exceeds 15 mgd during the interim period.
40.0
35.0
E 30.0
—0 25.0
20.0
+�► cn 15.0
in 10.0
41114
IIlk
.11
5.0
0.0
Figure 3-1
State Point Analysis at 10 mgd without Polymer
Gravity Settling Flux
Surface Overflow Rate
-RAS Underflow Rate
r/A, -
0 2,000 4,000 6,000 8,000 10,000 12,000 14,000
Solids Concentration (kg/m3)
NWWTP Interim Capacity Evaluation 3-6
HAZENAND SAWYER
Environmental Engineer8 Sclentlete

40.0
Figure 3-2
State Point Analysis at 10 mgd with Polymer
35.0 -
30.0
-0 25.0
x 20.0 -
• 15.0
-0
in - - 10.0 -
5.0
0.0
Gravity Settling Flux
- Surface Overflow Rate
- RAS Underflow Rate
r
0 2,000 4,000 6,000 8,000 10,000 12,000 14,000
Solids Concentration (kg/m3)
The state point analysis evaluation was also conducted at a peak flow of 20 mgd. The
results of the secondary clarifier evaluation show that the plant is expected to comply
with its permit with regards to TSS and BOD provided the following conditions are met:
• Peak hour flow remains at or below 20 mgd
• The RAS rate is at least 3.5 mgd during a storm event
• Polymer is used as -needed and when the flow exceeds 15 mgd
• Polymer dose is adjusted in response to settling
• All equipment is properly operated and maintained
3.3 Design Flow and Loads
Design flow and loads for this interim capacity evaluation were developed based on
influent data from the past two and a half years. Although these values differ slightly
from those used in design of the 16 mgd upgrade, they are representative of the current
condition and therefore appropriate for use in an interim capacity evaluation.
For illustration purposes, design flow and loads will be presented based upon the
assumption that the plant can be rerated to 10 mgd permitted capacity. However, it
should be noted that either alternative could reliably treat a flow higher than 10 mgd from
a process standpoint were it not for the limitations of the secondary clarifier capacity.
NWWTP Interim Capacity Evaluation
3-7
HAZENAND SAWYER
E nvironme ntel Enpineere & Scientists

Maximum month design loads were developed based upon the historical load peaking
^�^ factors presented in Section 2. Table 3-4 summarizes the annual average design load,
maximum month design load, and load peaking factors utilized. As shown in Table 3-4,
slightly higher BOD and COD Toad peaking factors were used to be more conservative.
Further data analysis was conducted to develop a "worst case" wet weather treatment
scenario. This analysis assumes a combination of higher than normal loads and higher
than normal flows. The resulting combination increases the total wet weather loads by
,MP,
approximately 10% to 15% above the maximum month loading condition as shown in
Table 3-5. Therefore, wet weather evaluations are based upon the loads shown in Table
3-5. The wet weather load analysis determined that the limiting process will be the
secondary clarifiers and not the activated sludge process. The hydraulic capacity of the
secondary clarifiers will limit the treatable capacity long before the design loads limit the
is treatment capacity.
eels
ease
eine
elate
AMA
Ales
Alloe
oak
Ark
Table 3-4
NWWTP Design Flow and Loads
Parameter
Annual
Average
(mg/L)
Annual
Average
Design
Load
(Ib/d)
Maximum
Month
Peaking
Factor
from
Data
Maximum
Month
Peaking
Factor for
Design
Maximum
Month
(mg/L)
Maximum
Month
Design
Load
(lb/d)
COD
483
40,300
1.1
1.2
580
48,300
BOD
199
16,600
1.1
1.2
239
19,900
TKN
33.4
2,800
1.2
1.2
38.7
3,200
TSS
168
14,000
1.4
1.4
234
19,500
VSS
143
11,900
1.4
1.4
199
16,600
ISS
25.2
2,100
1.0
1.0
35.0
2,900
TP
5
420
1.2
1.2
5.8
480
— NWWTP Interim Capacity Evaluation 3-8 HAZENAND SAWYER
Environmental Engineers & Scientists
464

ANN
AMIN
Table 3-5
NWWTP Design Loads for Wet Weather Evaluations
Parameter
Wet Weather
Design Load
(Ib/d)
COD
53,600
BOD
22,100
TKN
3,700
TSS
18,600
VSS
15,800
ISS
2,800
TP
550
3.4 Alternatives Analysis
The following two alternatives were evaluated using a combination of steady state and
dynamic modeling.
• Alternative 1 — New headworks, new aeration tanks, existing secondary
clarifiers, new filters, new UV disinfection, new effluent pump station, and new
anaerobic digesters
• Alternative 2 (with Primary Treatment) — New headworks, existing primary
clarifiers, diversion of excess wet weather flows directly to new aeration tanks,
new aeration tanks, existing secondary clarifiers, new filters, UV disinfection,
new effluent pump station, and new anaerobic digesters
60,
Each alternative was evaluated at the following conditions:
oak
Ink
vall
• Steady state simulation, annual average load, average temperature
• Steady state simulation, maximum month load, minimum week temperature
• Dynamic wet weather simulation, maximum month load, minimum week
temperature
In general, a minimum solids retention time (SRT) of three days was assumed for BOD
removal. The target mixed liquor concentration was 1,200 mg/L. If a higher SRT was
possible without exceeding a 1,200 mg/L MLSS concentration, then a higher SRT was
Aft used in the simulation. In the dynamic wet weather simulations, the BioWin
ighb, computational -fluid -dynamics -based active clarifier element was utilized to simulate the
behavior of the clarifier during wet weather. This clarifier analysis tool enhances the
�+ state point analysis discussed earlier.
HAZEN AND SAWYER
NWWTP Interim Capacity Evaluation 3-9
Environmental Engineers & Scientists

3.4.1 Steady State Evaluation
The results of steady state simulations for both alternatives are compared in Table 3-6.
The results of the steady state comparison show that Alternative 2, the alternative that
leaves the existing primary clarifiers in service, allows for a longer SRT, which gives
Alternative 2 the advantage of providing more robust treatment and being, therefore, the
more conservative choice. Under both scenarios the effluent TSS and BOD are
anticipated to be less than 10 mg/L during dry weather with all of the clarifiers and filters
in service. As is apparent from Table 3-6, Alternative 2 is expected to provide year-
round nitrification resulting in low ammonia in the plant effluent.
NWWTP Interim Capacity Evaluation
3-1 o HAZENAND SAWYER
Enrl ronmentel Enpineera & SClenhIet7

ink
Table 3-6
Oki
0114
AlMk
AlMk
oink
014
ink
Ilfea
INK
41114
Comparison of Steady State BioWin Model Simulations of Alternatives
Parameter
Units
Alternative 1
Alternative 2
Max
Month
Load
Average
Load
Max
Month
Load
Average
Load
Influent Flow
Mgd
10
10
10
10
Influent COD
mg/L
580
483
580
483
Influent TKN
mg/L
38.7
33.4
38.7
33.4
Influent TP
mg/L
5.8
5
5.8
5
Influent Nitrate
mg/L
0
0
0
0
Influent pH
mg/L
7.1
7.1
7.1
7.1
Influent Alkalinity
mmol/L
8.8
8.8
8.8
8.8
Influent ISS
mg/L
35
25.2
35
25.2
Influent DO
mg/L
0
0
0
0
Aeration volume
MG
6.2
6.2
6.2
6.2
SRT
Days
3
4.1
6.6
9.2
MLSS
mg/L
1173
1194
1195
1173
VSS
mg/L
943
967
972
956
PE TSS
mg/L
-
-
91
74
PE BOD
mg/L
-
-
146
122
Effluent TSS
mg/L
<10
<10
<10
<10
Effluent BOD
mg/L
<10
<10
<10
<10
Effluent Ammonia
mg/L
20.9
0.22
0.54
0.08
Temp
C
13
_21_
13
21
Flow bypassing PC
Mgd
-
-
0
0
PC Removal
%
-
-
56
56
Polymer usage
y/n
y
y
y
y
3.4.2 Dynamic Wet Weather Evaluation
ilk
The results of dynamic wet weather simulations for both alternatives are compared in
Table 3-7. Secondary clarifier settling characteristics were adjusted to reflect the benefits
,s, of polymer addition. During the simulated storm, effluent TSS and BOD values remained
in compliance with the plant's maximum weekly and average monthly criteria with either
alternative. The new tertiary filters are expected to further limit solids and particulate
oak BOD in the plant effluent under these "worst case' conditions.
— NWWTP Interim Capacity Evaluation 3- I I HAZEN AND SAWYER
Environmental Engineers & Scientists

A-,
Table 3-7
Comparison of Dynamic Wet Weather Model Simulations of Alternatives
Parameter
Units
Alternative 1
Alternative 2
Wet Weather
Load
Wet Weather
Load
Influent Flow
mgd
6.6 - 14.7
6.6 - 14.7
Influent COD
Ib/d
53,600
53,600
Influent TKN
Ib/d
3,700
3,700
Influent TP
Ib/d
550
550
Influent Nitrate
mg/L
0
0
Influent pH
mg/L
7.1
7.1
Influent Alkalinity
mmol/L
8.8
8.8
Influent ISS
Ib/d
2,800
2,800
Influent DO
mg/L
0
0
Flow to PC
mgd
-
<=13
Aeration Volume
MG
6.2
6.2
SRT
days
4
9
MLSS
mg/L
950 - 1100
1000 - 1100
SE TSS
mg/L
<=48
<=45
SE BOD
mg/L
<=23 --
<=20 -
Effluent TSS
mg/L
below effluent limits
Effluent BOD
mg/L
below effluent limits
Temp
C
13
13
Polymer Usage
y/n
y
y
3.5 Summary
The following two alternatives were evaluated using a combination of steady state and
dynamic modeling.
• Alternative 1 — New headworks, new aeration tanks, existing secondary
clarifiers, new filters, new UV disinfection, new effluent pump station, and new
anaerobic digesters
• Alternative 2 (with Primary Treatment) — New headworks, existing primary
clarifiers, diversion of excess wet weather flows directly to new aeration tanks,
new aeration tanks, existing secondary clarifiers, new filters, UV disinfection,
new effluent pump station, and new anaerobic digesters
NWWTP Interim Capacity Evaluation
3-12 HAZEN AND SAWYER
Environmental Engineers E. Scientists

Based on a thorough evaluation of the construction schedule and wastewater process
modeling, Alternative 2 is the preferred alternative because it is more conservative. For a
given MLSS, it provides a higher factor of safety by operating at a higher SRT.
The NVVWTP can be rerated for a permitted capacity of 10 mgd utilizing the following
processes:
• New headworks
• Existing primary clarifiers
• New aeration tanks
• Existing secondary clarifiers
• New filters
• New UV disinfection
• New effluent pump station
• New anaerobic digesters
These new facilities are anticipated to be online by March 2008. Therefore, based on the
current construction schedule, this would allow the plant to accept more flow ten months
before the scheduled construction completion date.
The process limiting step is the existing secondary clarifiers, as the new clarifiers will not
be online until close to the end of the construction project. Therefore, the addition of
polymer is recommended on an as -needed basis, and generally speaking, should be
used when the flow exceeds 15 mgd during the interim period. The results of the
secondary clarifier evaluation show that the plant is expected to comply with its permit
with regards to TSS and BOD provided the following conditions are met:
• Peak hour flow remains at or below 20 mgd
• The RAS rate is at least 3.5 mgd during a storm event
• Polymer is used as -needed and when the flow exceeds 15 mgd
• Polymer dose is adjusted in response to settling
• All equipment is properly operated and maintained
NWWTP Interim Capacity Evaluation
3-13 HAZENAND SAWYER
Environmental Enoineers & Scientists

Based on these conclusions, the NWWTP may be permitted at a capacity of 10 mgd and
will reliably comply with an interim mass -based permit with a BOD limit of 24 mg/L on an
average monthly basis and 36 mg/L on a weekly average basis.
3-14 HAZEN AND SAWYER
Environmental Engineers & Scienllsls
NWWTP Interim Capacity Evaluation

Section 4.0
Unit Process Evaluation at Rerated Capacity of 10 mgd
4.1 Overview
This section provides a process by process evaluation of the Northside Wastewater
Treatment Plant (NWWTP) at the recommended rerated capacity of 10 mgd. Other
construction related considerations such as manual operation, accessibility, and standby
power are also discussed.
4.1.1 Anticipated NPDES Permit Limits
This evaluation assumes that the interim solution shall comply with the NWWTP's
current permit on a mass basis. The regulatory objectives are based on the current
permit and there is no increase in the permitted mass discharge. Table 4-1 summarizes
the effluent treatment goals of the NWWTP at 10 mgd.
Table 4-1
Anticipated NPDES Permit Limits at 10 mgd
Parameter
Permitted Flow
Effluent TSS
Monthly average
Weekly maximum
Effluent BOD
Monthly average
Weekly maximum
Fecal Coliforms
Monthly average
Weekly maximum
Ammonia
Total Phosphorus
Dissolved Oxygen
Units
mgd
mg/L
mg/L
mg/L
mg/L
CFU/100 mL
CFU/100 mL
mg/L
mg/L
mg/L
Quantity
10
30
45
24
36
800
1600
report
report
report
4.1.2 Hydraulic Peak Flow Capacity
Based on the evaluation of influent and effluent pumping capacity, historical wastewater
flows, and existing hydraulic restrictions, a peak flow of 2.0 times the permitted capacity
is recommended for the interim plant. The limiting hydraulic capacities are the firm
effluent pumping capacity and secondary clarifier hydraulic capacity, both of which are
20 mgd. Influent pumping will be modulated to maintain flows at or below this level. The
NWWTP Interim Capacity Evaluation 4-1
HAZENAND SAWYER
Environmental Engineers & Scientists

historical maximum day flow peaking factor to the plant has been 2.0, as shown in Table
4-2. Based on these factors, rerating the NVWVfP for an annual average flow of 10 mgd
and a peak flow of 20 mgd is appropriate in the interim period. This basis of design is
confirmed by the fact that historical peaks have only exceeded 14.6 mgd during wet
weather events in excess of a 25 year storm.
Table 4-2
NWWTP Flow Peaking Factors Excluding Storms
with over 9 inches of Rainfall (25 Year Storm)
Flow Criteria
Historical
Flow
(mgd)
Peaking
Factor
Minimum Day
5.6
0.78
Average Annual
7.2
1.0
Maximum 30-Day
9.8
1.3
Maximum 7-Day
11.0
1.5
Maximum Day
14.6
2.0
4.2 Headworks
New step screens (1/4" openings) are being provided for the 16 mgd expansion. These
facilities are presently scheduled to be completed in March 2008. Therefore, these
facilities, designed for 16 mgd, will provide superior screening to the existing facilities
and be more than sufficient for treating the rerated interim flow of 10 mgd.
4.3 Primary Clarifiers
As was stated in Section 3, the two existing 85-foot primary clarifiers will operate at an
average overflow rates less than the recommended 1,000 gallons per day per square
foot (gpd/sf) at 10 mgd, and at peak overflow rates of approximately 1,800 gpd/sf at 20
mgd. This process as currently sized is sufficient for treating the rerated interim capacity
of 10 mgd.
4.4 Primary Sludge Pumping
There are two existing primary sludge pump stations that are configured such that one is
dedicated to each of the clarifiers. Each station has two piston pumps with capacities of
90 gpm and 100 gpm. It is assumed that primary sludge will be pumped from the
clarifiers at approximately 4% solids at the 10 mgd design flow. The existing pumps
transfer primary solids continuously to the anaerobic digesters. The current required
NWWTP Interim Capacity Evaluation 4-2 HAZEN AND SAWYER
Environmental Engineers d Scientists

pumping rates are well below the pump capacity, and therefore are sufficient for a flow of
10 mgd.
4.5 Intermediate Pump Station
The intermediate pump station contains three submersible pumps for pumping trickling
filter effluent to the aeration basin. Each of the three pumps has a capacity of
approximately 9 mgd for a firm pumping capacity of 18 mgd. One of the pumps did not
pump at its rated capacity during the most recent drawdown testing, and therefore the
firm capacity is derated for this evaluation to 13 mgd. During the rerate period, the
analysis assumes that flows in excess of the intermediate pump station firm capacity will
be diverted around the existing primary clarifiers using the new primary effluent
distribution box and flow directly to the new aeration tanks. This is a conservative
assumption since all intermediate pumps will normally be in service, allowing all flow to
receive primary treatment.
4.6 Aeration Tanks and Aeration Blowers
The new aeration tanks are configured to provide full nitrification at the design capacity.
of 16 mgd. The aeration tanks consist of four parallel trains of cells with eleven baffled
zones per train for future operation in a biological nutrient removal (BNR) mode. Fine
bubble diffusers are being provided in all cells. The existing blower building is not of
adequate size for the 16 mgd air requirements, and therefore a new blower building is
being constructed to provide aeration for the new tanks.
The aeration tanks and aeration blowers are scheduled to be commissioned in March
2008. Since these facilities are sized for flows of 16 mgd, they will be more than
sufficient for treating the rerated interim flow of 10 mgd.
4.7 Secondary Clarifiers and Return Activated Sludge
There are two existing 90-foot diameter secondary clarifiers that have a sidewater depth
of 16 feet. A state point analysis evaluation was conducted at an average flow of 10
mgd and at a peak flow of 20 mgd. A wet weather dynamic simulation was also
performed. The results of the secondary clarifier evaluation show that the plant is
expected to comply with its permit provided the following conditions are met:
NWWTP Interim Capacity Evaluation 4-3
HAZEN AND SAWYER
Envlronmentel Engineers & Scientists

ANN
• Peak hour flow remains at or below 20 mgd
• The RAS rate is at least 3.5 mgd during wet weather
• Polymer is used as -needed and when flow exceeds 15 mgd
• Polymer dose is adjusted in response to settling
• All equipment is properly operated and maintained
A cost-effective polymer system for the NWWTP would be to use polymer totes, typically
available in 550 gallon quantities, along with an emulsion polymer preparation system
such as the Polyblend® system by Stranco or other comparable system to activate the
polymer. This equipment could be located outdoors near the secondary clarifier influent
distribution box. Plastic tubing would be inserted into the polymer tote, and this tubing
would transfer polymer from the tote to the polymer makeup unit, and then from the
makeup unit to the distribution box. A 120 volt power source and a water connection are
required.
Activated Pump Station No. 1 will be completed in January 2008. This pump station is
designed to return activated sludge from the existing Secondary Clarifiers to the Aeration
Tanks as needed. For the 16 mgd plant design, a second Activated Pump Station No.2
will be constructed to serve new Secondary Clarifiers No.3 and No.4. Activated Sludge
Pump Station No.1 will include two duty pumps with a capacity of 2.5 mgd each plus one
spare pump to pump RAS from the existing 90-foot diameter secondary clarifiers.
Therefore, the firm capacity of these RAS pumping facilities is more than sufficient to
meet the conditions outlined by the state point analysis and is sufficient for treating the
rerated interim capacity of 10 mgd.
4.8 Tertiary Filters
Deep bed filters are being provided for the 16 mgd expansion. These facilities are
presently scheduled to be completed in February 2008. It is anticipated that these deep -
bed tertiary filter facilities will routinely produce an effluent TSS of less than 5 mg/L.
These facilities, designed for 16 mgd, will be more than sufficient for treating the rerated
interim capacity of 10 mgd.
4.9 UV Disinfection
NWWTP Interim Capacity Evaluation 4-4 HAZEN AND SAWYER
Environmental Engineers & Scientists

An Ultraviolet (UV) Disinfection Facility is being provided to replace the existing
disinfection facilities as part of the 16 mgd expansion. These facilities are presently
scheduled to be completed in February 2008. At the 16 mgd plant design flow, four
channels are sized to provide a hydraulic peak capacity of approximately 40 mgd with all
channels in service, or 10 mgd per channel. The new UV disinfection facilities, designed
for 16 mgd, are more than sufficient for treating the rerated interim capacity of 10 mgd.
4.10 Effluent Pump Station
A new effluent pump station is being provided as part of the 16 mgd expansion. These
facilities are presently scheduled to be completed in February 2008. The pump station
will be comprised of three 10 mgd vertical turbine pumps to provide a firm capacity of 20
mgd. Along with upgrades to the existing pump station, the combination of the two pump
stations will have a firm capacity of 40 mgd. All pumps will operate with variable
frequency drives to meet varying flow requirements.
The pumps in both pump stations are sized assuming installation of a second parallel 30
inch force main to the Cape Fear River. The existing discharge force main line will be
utilized in the interim period since the second force main is anticipated to be completed
later in the project. Based on the interim rerated peak flow rate of 20 mgd, the existing
force main along with the new effluent pump station will be sufficient for the rerated
interim capacity of 10 mgd.
4.11 Waste Activated Sludge Thickening
The existing waste activated sludge (WAS) thickening facilities include three WAS
pumps, one 2-meter gravity belt thickener (GBT), a polymer system, two thickened WAS
(TWAS) pumps, and a TWAS storage tank. The three WAS pumps, each with a
capacity of 300 gpm, are located adjacent to the Sludge Thickening Building. New
thickening facilities are under construction and are scheduled to be complete close to
the end of final construction. WAS flow will be maintained to the existing GBT until the
new WAS/GBT feed pumps are operational. WAS production and GBT operation at 10
mgd will increase slightly over that anticipated for 8 mgd. Assuming a proportional
increase in WAS production and that the GBT is loaded at its maximum rate of 350
gallons per minute, the GBT will operate 11.6 hours per day at maximum conditions and
8.8 hours under average conditions.
Aftts
NWWTP Interim Capacity Evaluation 4-5
HAZEN AND SAWYER
Environmental Engineers & Scientists

The TWAS storage tank allows for providing a steady flow of TWAS to the digesters
while thickening equipment is operated on an intermittent basis. The tank has a capacity
of 32,000 gallons, providing capacity for approximately 1.2 days of TVVAS production at
the 10 mgd maximum condition.
This thickening schedule demonstrates that the existing thickening system is sufficient
for thickening the solids associated with the rerated interim capacity of 10 mgd.
4.12 Anaerobic Digestion
Two Anaerobic Digesters will be installed to provide, in conjunction with three existing
anaerobic digesters, digestion of the primary sludge and the waste activated sludge
associated with the 16 mgd plant expansion to meet Class B biosolids stabilization
criteria. These digesters are scheduled to be operational in March 2008. The two new
digesters each have a volume of 695,000 gal, with a diameter of 65 feet and a sidewater
depth of 28 feet. The two new digesters are sized to provide the following approximate
mean cell residence time (MCRT) under various conditions at a design flow of 10 mgd:
Average Loading
Maximum Loading
Maintenance Provision
MCRT of 30 days (all units in service)
MCRT of 20 days (all units in service)
MCRT of 15 days (max loading with one out of service)
The two new anaerobic digesters are therefore sufficient for the rerated interim capacity
of 10 mgd.
4.13 Dewatering
The existing dewatering facilities consist of two 2-meter belt filter presses (BFPs) located
in the Dewatering Building, three BFP feed pumps, two polymer systems, and alum
sludge and digested sludge storage tanks. The three BFP feed pumps include two
progressive cavity pumps and one rotary lobe pump. These pumps pump directly to the
BFPs. Presently, one BFP is used primarily for digested sludge and one for alum sludge
from the water plant.
NWWTP Interim Capacity Evaluation 4-6 IIAZENAND SAWYER
Environmental Engineers 8 Scientlele

Alum sludge is pumped from the water plant to the sludge storage tank adjacent to the
dewatering facility. The plant receives approximately 60,000-70,000 gallons of alum
sludge per day at a concentration of approximately 2%. These parameters are not
expected to change during the rerate and thus this process requires no modification as a
result of the rerate.
The second BFP is dedicated primarily to dewatering of digested sludge. Digested
sludge is transferred to and stored in the digested sludge storage tank adjacent to the
Dewatering Building prior to dewatering. BFP operation at 10 mgd will increase slightly
over that anticipated for 8 mgd. Assuming a proportional increase in digested sludge and
that the BFP is loaded at its limiting rate, the BFP will operate 9.9 hours per day at
maximum conditions and 6.8 hours under average conditions. The solids dewatering
facilities are therefore sufficient for the 10 mgd interim capacity rating.
4.14 Other Construction Related Considerations
Additional construction related considerations such as manual operation, operator
accessibility, and standby power are addressed in this section.
4.14.1 Manual Operation
The NWWTP does not currently have a supervisory control and data acquisition
(SCADA) system for its treatment processes. Therefore, operation and modification of all
processes is achieved through manual adjustments at the location of each process. This
mode of operation will continue for existing equipment through the interim rerate period.
A new plant -wide SCADA system is planned for the expanded plant; however, this
system will only be partially functional during the interim rerate period. Continued use of
manual control for the existing facilities is not anticipated to create a hardship for the
operations staff since much of the existing plant will be decommissioned as new facilities
are placed into service in the interim rerate. New equipment that is part of the rerate
(i.e., new screening and headworks, new aeration tanks and blowers, new tertiary filters,
,.,, new UV, new effluent pump station, new anaerobic digesters) will be tied into the new
SCADA system by its commissioning date. Therefore, monitoring and automatic control
of the new equipment will be possible. Local automatic controls will be fully functional for
each process that has been identified as part of the rerate. For example, the local
programmable logic controller panel will automatically initiate filter backwashing at the
NWWTP Interim Capacity Evaluation 4-7 HAZEN AND SAWYER
Environmental Engineers 6 Scientists

filters as needed and the new aeration tanks will be capable of automated dissolved
oxygen control. These features will increase the reliability of treatment during the interim
rerate operation.
4.14.2 Operator Accessibility
It is anticipated that all points of access and safety handrails will be in place by the
commissioning dates cited for all of the new processes. Therefore, the full functionality,
accessibility and safety features of each new facility that is placed into service will be
available for the interim rerated plant.
4.14.3 Standby Power
The existing plant provides standby power through an on -site generator set. The 16
mgd expansion will require a second generator set to allow powering of all critical
facilities during a utility power outage. The new generator set is anticipated to be fully
functional when the other critical facilities for the interim rerated capacity are placed into
service. Therefore, standby power for all of the plant's critical treatment processes will
be provided at all times for the interim rerated plant capacity of 10 mgd.
4.14.4 Other Facilities
The filtrate pump station receives side stream flows from the solids building as well as
drainage from most of the unit processes. The filtrate pump station pumps these flows to
the headworks for treatment. Therefore, the pump station must be completed
concurrently with the commissioning of the new headworks facility for the headworks to
be functional. The current completion date is listed planned for February 2008, which
allows this facility to be operational for the interim rerated plant.
NWWTP Interim Capacity Evaluation 4-8 HAZEN AND SAWYER
Environmental Engineers & Scientists

4.15 Summary of Recommended Improvements
Critical facilities and additional expenses associated with the rerate are summarized in
this section.
4.15.1 Critical Facilities for Rerate
The NVWVTP can be rerated for a permitted capacity of 10 mgd utilizing the following
processes:
• New headworks
• Existing primary clarifiers
• New aeration tanks
• Existing secondary clarifiers
• New filters
• New UV disinfection
• New effluent pump station
• New anaerobic digesters
4.15.2 Critical Yard Piping for Rerate
The schedule for installing interconnecting piping associated with the critical facilities
was reviewed and the necessary piping is anticipated to be in place.
4.15.3 Additional Expenses
A polymer feed system should be purchased for the existing secondary clarifiers to make
the NWVVTP reliable at a rerated capacity of 10 mgd. The estimated cost of a
Polyblend® or comparable system, as described earlier, is $25,000. There will also be
some increased operating costs associated with treating additional wastewater. For
instance, at 10 mgd the solids processing equipment will need to run about 25 percent
longer to process the additional solids. This will result in increased energy consumption
and may impact hours of operation. The plant will also have to purchase polymer for use
in the existing secondary clarifiers under high flow conditions. In addition, operating the
UV disinfection system with increase energy costs; however, the operating cost of the
UV system will typically be less than the current cost of disinfection chemicals. These
operating costs will not be significant in comparison to the overall plant operating costs.
NWWTP Interim Capacity Evaluation 4-9
HAZENAND SAWYER
Environmental Engineers & Scientists

Section 5.0
Recommendations
5.1 Overview
This section summarizes the interim capacity evaluation and conclusions. The purpose
of this evaluation was (1) to determine if the NWWTP can be rerated to an increased
interim flow capacity higher than 8 mgd with a portion but not all of the new facilities
online and if so, (2) to quantify that interim flow. An interim rerating of the NWWTP would
allow the City to utilize a portion of the newly constructed capacity prior to the completion
of the entire construction project.
This evaluation assumed that the interim solution shall comply with the NWWTP's
current permit on a mass basis. The anticipated interim mass -based limits at NWWTP
are summarized in Table 5-1.
Table 5-1
Anticipated NPDES Permit Limits at 10 mgd
Parameter
Permitted Flow
Effluent TSS
Monthly average
Weekly maximum
Effluent BOD
Monthly average
Weekly maximum
Fecal Coliforms
Monthly average
Weekly maximum
Ammonia
Total Phosphorus
Dissolved Oxygen
Units
mgd
mg/L
mg/L
mg/L
mg/L
CFU/100 mL
CFU/100 mL
mg/L
mg/L
mg/L
Quantity
10
30
45
24
36
800
1600
report
report
report
5.2 Model Calibration
The results of the historical data collection and analysis were used to construct and
calibrate a model of the NWWTP. Table 5-2 demonstrates that this model predicted the
observed parameters well.
NWWTP Interim Capacity Evaluation 5-1
HAZENAND SAWYER
Environmenlel Engineers & Scientists

Table 5-2
Comparison of Observed and Modeled Parameters from Calibrated Model
Parameter
Units
Observed
Calibrated
Model
Influent Flow
mgd
7.05
7.05
Influent COD
mg/L
483
483
Influent BOD
mg/L
199
198
Influent TKN
mg/L
33.4
33.4
Influent TP
mg/L
5
5
Influent TSS
mg/L
168
174
PE TSS
mg/L
57
54
PE BOD
mg/L
134
114
MLSS
mg/L
1050
1036
MLVSS/MLSS
-
0.83
0.86
RAS
mg/L
5400
5300
Effluent TSS
mg/L
5
6.4
Temp
C
21
20
Primary Solids
%
4.3
4.1
TWAS Solids
%
4.8
4.7
Anaerobically Digested Solids
%
1.9
2.6
Cake
%
18.5
18.3
5.3 Alternatives Analysis
The following two alternatives were evaluated using a combination of steady state and
dynamic modeling.
• Alternative 1 — New headworks, new aeration tanks, existing secondary
clarifiers, new filters, and new UV disinfection
• Alternative 2 — New headworks, existing primary clarifiers, diversion of excess
wet weather flows directly to new aeration tanks, new aeration tanks, existing
secondary clarifiers, new filters, and new UV disinfection
Based on a thorough evaluation of the construction schedule and wastewater process
modeling, Alternative 2 is the preferred alternative because it is more conservative. For a
given MLSS, it provides a higher factor of safety by operating at a higher SRT.
The NWWTP can be rerated for a permitted capacity of 10 mgd utilizing the following
processes:
NWWTP Interim Capacity Evaluation 5-2
HAZEN AND SAWYER
Environmental Engineers & Scientists

• New headworks
• Existing primary clarifiers
• New aeration tanks
• Existing secondary clarifiers
• New filters
• New UV disinfection
• New effluent pump station
,. • New anaerobic digesters
The new facilities are anticipated to be online by March 2008. Therefore, based on the
current construction schedule, this would allow the plant to accept a higher flow
approximately ten months earlier than anticipated.
The process limiting step is the existing secondary clarifiers, as the new clarifiers will not
be online until close to the end of the construction project. Therefore, the addition of
,ate
polymer is recommended on an as -needed basis, and generally speaking, should be
used when the flow exceeds 15 mgd during the interim period. In summary, the results
of the secondary clarifier evaluation show that the plant is expected to comply with its
permit with regards to TSS and BOD provided the following conditions are met:
• Peak hour flow remains at or below 20 mgd
• The RAS rate is at least 3.5 mgd during a storm event
• Polymer is used as -needed and when the flow exceeds 15 mgd
• Polymer dose is adjusted in response to settling
• All equipment is properly operated and maintained
5.4 Unit Process Evaluation
A process by process evaluation of the NWWTP at 10 mgd concluded that the
recommended plan is viable; provided a polymer feed system is installed for the existing
secondary clarifiers. The temporary polymer system is estimated to cost $25,000
installed. There will also be some increased operating costs associated with treating
additional flow. However, these operating costs are not likely to be significant in
comparison to the overall plant operating costs.
NWWTP Interim Capacity Evaluation 5-3 11AZENAND SAWYER
.g. Environmental Engineers 3 Scientists