NC0023965_Authorization to Construct_20141103

NPDES DOCIMENT SCANNING COVER SHEET
NPDES Permit:
NC0023965
Wilmington Northside WWTP
Document Type:
Permit Issuance
Wasteload Allocation
-°'
Authorization to Construct (AtC)_
Permit Modification
Complete File - Historical
Engineering Alternatives (EAA)
Correspondence
Owner Name Change
Instream Assessment (67b)
Speculative Limits
Environmental Assessment (EA)
Document Date:
November 3, 2014
This document iec printed on reams paper - ignore any
content on the reirerisce made

Mr. Caldwell
Page 4
N CO v 2 q & S
Engineer's Certification
Co2r745-e- - K o laze 02 316,5401 - ! ss„t d 10 1 %
1, Anthony W. Boahn , as a duly registered Professional Engineer in the State of
North Carolina, having been authorized to observe (periodically, weekly. full time) the construction
of
the project,
James A. Loughlin (Northsidc) WWII' Expansion
(NPDES Permit NC00239651
Project Name
New Hanover County
Location
for the
Permittee hereby states that, to the best of my abilities, due care and diligence was used in the
observation of the following installations:
• Expansion & modification of the existing 8 MGD treatment plant to accommodate a design
flow of 16 MGD,
• Addition of new screening and grit removal facilities,
• .Addition of two new primary clarifiers (105 ft. diameter) and primary sludge pumps,
• Addition of four aeration tanks (1.55 MG each) with fine bubble diffusers (four blowers at
5000 icfm),
• Addition of two new secondary clarifiers (130 ft. diameter); two existing clarifiers to remain
in place (90 ft. diameter),
• Addition of new secondary sludge pumps/pump station for both the two existing and
two new secondary clarifiers,
• Addition of four deep bed filters (10 ft. by 70 fL each),
• Addition of two backwash tanks and two backwash reclaim tanks (and associated
pumps), two air scour blowers, with diversion of backwash to head of plant,
• Addition of UV disinfection (four channels, 360 lamps) and associated equipment designed
for 40 MGD peak flow,
• Addition of four pumps and associated piping for in -plant reuse of treated effluent,
• Addition of effluent parshall flume,
• Addition of post aeration (aeration diffusers in effluent sampling area, as well as backwash
tank area),
• Addition of effluent sampling area,
• Addition of equal split piping to the existing effluent pump station and the new effluent
pump station,
Refurbishment of existing effluent pump station to add 5 new effluent pumps rated at 5
MGD each (with flow to the existing effluent force main and diffuser to NE Cape Fear
River),
• Addition of new effluent pump station with 3 effluent pumps rated at 10 MGD each,
• Addition of new parallel effluent force main and diffuser to NE Cape Fear River,
• Refurbishment of existingthickening facilities (2 refurbished belt filter presses), 2 new belt
filter presses and associated thickening equipment,
• Addition of 2 new, 3m gravity belt thickeners,
• Addition of 2 new anaerobic digesters (appx. 65 ft. diameter) with heat exchangers, sludge
transfer pumps; refurbishment of 3 existing anaerobic digesters,
• Addition of sludge truck loading station,
• Addition of 1 rotary, drum thermal dryer and associated equipment,
• Addition of a new generator with automatic transfer switch to accommodate the new
treatment units (and retention of existing generator),
• and all additional chemical feed systems, piping, valves, control equipment and
appurtenances.
• This expansion phase also includes odor control facilities. septage receiving station, supply
building, and waste gas facilities.
• The existing aeration basin and blower will be retained (not part of the expansion, but
retained for possible future use in some capacity).

Mr. Caldwell
Page 5
Engineer's Certification (continued)
11-aditionally, nitrification may require a greater detention time than what will be used at
the City. As stated below in this Authorization to Construct permit, in the event that the facilities
fail to perform satisfactorily at the increased design flow, the City of Wilmington shall take
corrective action, such as the construction of additional wastewater treatment units.
Once construction under this ATC has been completed and an Engineer's Certificate has been
subrated, the permittee: is authorized to operate the units listed above. with permitted limits as
spec Pied in the NPDES permit issued September 22, 2004. This Authorization to Construct
permit only approves beat rent units to the design f low of 16 MGD for the Wilmington Nor thside
(James A. Loughlin) WWTF. The Division of Water Quality approves this project in terms of
compliance with NPDES limits; we defer to the Capc Fear P.U.A. and their consultants for
approval of structural integrity, etc. of the various treatment units. All treatment units shall be
protected from the flood elevation (as specified by I5A NCAC 2H .02190)(4)).
Signature
Date i v — t 4
Mail this Certification to the
Registration No. Q255 +Wil l i/ t�.►l,,
,,k% k �iiit�c , ;.
ti i. -
NPDES Unit
DENR/DWQ
1617 Mail Service Center
Raleigh, NC 27699-1617

Michael F. Easley, Governor
State of North Carolina
William G. Ross, Jr., Secretary
Department of Environment and Natural Resources
Alan W. Klimek, P.E., Director
Division of Water Quality
October 4, 2004
Mr. Hugh Caldwell, P.E.
Public Utilities Director
City of Wilmington
P.O. Box 1810
Wilmington, North Carolina 28402
Subject: ERROR CORRECTION
Authorization to Construct Permit Issuance
ATC Number 0023965A01
Wilmington Northside WWTF Expansion
(James A. Loughlin WWTF)
NPDES Permit NC0023965
New Hanover County
Dear Mr. Caldwell:
The Authorization to Construct permit for Wilmington Northside (James A. Loughlin
WWF f) issued September 29, 2004 contained an error in the description of the discharge point for
the effluent force main and diffuser. Thank you for pointing out this error, the Division of Water
Quality apologizes for any inconvenience.
The following correction has been made to p. 1 of the permit and p. 4 (Engineer's
Certification):
• Refurbishment of existing effluent pump station to add 5 new effluent pumps rated at 5
MGD each (with flow to the existing effluent force main and diffuser to the Cape Fear
River).
• Addition of new parallel effluent force main and diffuser to the Cape Fear River.
Please find enclosed the revised pages for the Authorization to Construct permit. The
revised pages should be inserted into your permit. The old pages may then be discarded. All
other terms and conditions contained in the original permit remain unchanged and in full effect.
This correction does not change the effective date of the Authorization to Construct permit
(September 29, 2004).
If you have any questions regarding the Authorization to Construct permit, please contact
Susan Wilson at (919) 733 - 5083, extension 510.
cc: Central Files
NPDES Unit, Permit File
Wilmington Regional Office,
Mr. Ronald L. Taylor, P.E.
Mr. Tony Boahn, P.E.
Si� v)
an W. Klimek, P.E.
Surface Water Protection
Hazen and Sawyer, P.C.
4011 WestChase Blvd.
Raleigh, North Carolina 27607
McKim & Creed
243 North Front Street
Wilmington, North Carolina 28401
North Carolina Division of Water Quality
1617 Mall Service Center
Raleigh, North Carolina 27699-1617
[phone]
FAX [phone]
On the Internet at http://h2o.enr.state.nc.us/

Michael F. Easley, Governor
State of North Carolina
William G. Ross, Jr., Secretary
Department of Environment and Natural Resources
Alan W. Klimek, P.E., Director
Division of Water Quality
September 29, 2004
Mr. Hugh Caldwell, P.E.
Public Utilities Director
City of Wilmington
P.O. Box 1810
Wilmington, North Carolina 28402
Subject: Authorization to Construct Permit Issuance
ATC Number 0023965A01
Wilmington Northside WWTF Expansion
(James A. Loughlin WWTF)
NPDES Permit NC0023965
New Hanover County
Dear Mr. Caldwell:
A request for an Authorization to Construct (ATC) was received by the Division May 3,
2004, and final plans and specifications for this project have been reviewed and found to be
satisfactory. Authorization is hereby granted for improvements to and expansion of Wilmington
Northside's James A. Loughlin Wastewater Treatment Facility (WWTF) to a design flow 16 MGD.
In general (due to the complexity of the improvements and expansion please refer to the
plans/specifications for greater detail), this ATC allows:
• Expansion & modification of the existing 8 MGD treatment plant to accommodate a design
flow of 16 MGD,
• Addition of new screening and grit removal facilities,
• Addition of two new primary clarifiers (105 ft. diameter) and primary sludge pumps,
• Addition of four aeration tanks (1.55 MG each) with fine bubble diffusers (four blowers at
5000 icfm),
• Addition of two new secondary clarifiers (130 ft. diameter); two existing clarifiers to remain
in place (90 ft. diameter),
• Addition of new secondary sludge pumps/pump station for both the two existing and
two new secondary clarifiers,
• Addition of four deep bed filters (10 ft. by 70 ft. each),
• Addition of two backwash tanks and two backwash reclaim tanks (and associated
pumps), two air scour blowers, with diversion of backwash to head of plant,
• Addition of UV disinfection (four channels, 360 lamps) and associated equipment designed
for 40 MGD peak flow,
• Addition of four pumps and associated piping for in -plant reuse of treated effluent,
• Addition of effluent parshall flume,
• Addition of post aeration (aeration diffusers in effluent sampling area, as well as backwash
tank area),
• Addition of effluent sampling area,
• Addition of equal split piping to the existing effluent pump station and the new effluent
pump station,
• Refurbishment of existing effluent pump station to add 5 new effluent pumps rated at 5
MGD each (with flow to the existing effluent force main and diffuser to the Cape Fear
River),
• Addition of new effluent pump station with 3 effluent pumps rated at 10 MGD each,
• Addition of new parallel effluent force main and diffuser to the Cape Fear River,
• Refurbishment of existing thickening facilities (2 refurbished belt filter presses), 2 new belt
filter presses and associated thickening equipment,
• Addition of 2 new, 3m gravity belt thickeners,
• Addition of 2 new anaerobic digesters (appx. 65 ft. diameter) with heat exchangers, sludge
transfer pumps; refurbishment of 3 existing anaerobic digesters,
• Addition of sludge truck loading station,
North Carolina Division of Water Quality
1617 Mail Service Center
Raleigh, North Carolina 27699-1617
[phone]
FAX [phone]
On the Internet at http://h2o.enr.state.nc.us/

OriA-.�F'Q
0 QG
y
O
Michael F. Easley, Governor
State of North Carolina
William G. Ross, Jr., Secretary
Department of Environment and Natural Resources
Alan W. Klimek, P.E., Director
Division of Water Quality
September 29, 2004
Mr. Hugh Caldwell, P.E.
Public Utilities Director
City of Wilmington
P.O. Box 1810
Wilmington, North Carolina 28402
Subject: Authorization to Construct Permit Issuance
ATC Number 0023965A01
Wilmington Northside WWTF Expansion
(James A. Loughlin WWTF)
NPDES Permit NC0023965
New Hanover County
Dear Mr. Caldwell:
A request for an Authorization to Construct (ATC) was received by the Division May 3,
2004, and final plans and specifications for this project have been reviewed and found to be
satisfactory. Authorization is hereby granted for improvements to and expansion of Wilmington
Northside's James A. Loughlin Wastewater Treatment Facility (WWTF) to a design flow 16 MGD.
In general (due to the complexity of the improvements and expansion please refer to the
plans/specifications for greater detail), this ATC allows:
• Expansion & modification of the existing 8 MGD treatment plant to accommodate a design
flow of 16 MGD,
• Addition of new screening and grit removal facilities,
• Addition of two new primary clarifiers (105 ft. diameter) and primary sludge pumps,
• Addition of four aeration tanks (1.55 MG each) with fine bubble diffusers (four blowers at
5000 icfm),
• Addition of two new secondary clarifiers (130 ft. diameter); two existing clarifiers to remain
in place (90 ft. diameter),
• Addition of new secondary sludge pumps/pump station for both the two existing and
two new secondary clarifiers,
• Addition of four deep bed filters (10 ft. by 70 ft. each),
• Addition of two backwash tanks and two backwash reclaim tanks (and associated
pumps), two air scour blowers, with diversion of backwash to head of plant,
• Addition of UV disinfection (four channels, 360 lamps) and associated equipment designed
for 40 MGD peak flow,
• Addition of four pumps and associated piping for in -plant reuse of treated effluent,
• Addition of effluent parshall flume,
• Addition of post aeration (aeration diffusers in effluent sampling area, as well as backwash
tank area),
• Addition of effluent sampling area,
• Addition of equal split piping to the existing effluent pump station and the new effluent
pump station,
• Refurbishment of existing effluent pump station to add 5 new effluent pumps rated at 5
MGD each (with flow to the existing effluent force main and diffuser to NE Cape Fear
River),
• Addition of new effluent pump station with 3 effluent pumps rated at 10 MGD each,
• Addition of new parallel effluent force main and diffuser to NE Cape Fear River,
• Refurbishment of existing thickening facilities (2 refurbished belt filter presses), 2 new belt
filter presses and associated thickening equipment,
• Addition of 2 new, 3m gravity belt thickeners,
• Addition of 2 new anaerobic digesters (appx. 65 ft. diameter) with heat exchangers, sludge
transfer pumps; refurbishment of 3 existing anaerobic digesters,
• Addition of sludge truck loading station,
North Carolina Division of Water Quality
1617 Mail Service Center
Raleigh, North Carolina 27699-1617
[phone]
FAX [phone]
On the Internet at http://h2o.enr.state.nc.us/

Mr. Caldwell
Page 2
• Addition of 1 rotary drum thermal dryer and associated equipment,
• Addition of a new generator with automatic transfer switch to accommodate the new
treatment units (and retention of existing generator),
• And all additional chemical feed systems, piping, valves, control equipment and
appurtenances.
• This expansion phase also includes odor control facilities, septage receiving station, supply
building, and waste gas facilities.
• The existing aeration basin and blower will be retained (not part of the expansion, but
retained for possible future use in some capacity).
Traditionally, nitrification may require a greater detention time than what will be used at
the City. As stated below in this Authorization to Construct permit, in the event that the facilities
fail to perform satisfactorily at the increased design flow, the City of Wilmington shall take
corrective action, such as the construction of additional wastewater treatment units.
Once construction under this ATC has been completed and an Engineer's Certificate has been
submitted, the permittee is authorized to operate the units listed above, with permitted limits as
speed in the IVPDES permit issued September 22, 2004. This Authorization to Construct
permit only approves treatment units to the designflow Iow of 16 MGD for the Wilmington Northside
(James A. Loughlin) WWTF. The Division of Water Quality approves this project in terms of
compliance with NPDES limits; we defer to the City of Wilmington and their consultants for
approval of structural integrity, etc. of the various treatment units. All treatment units shall be
protected from the flood elevation (as specified by 15A NCAC 2H .0219(j)(4)).
This Authorization to Construct is issued in accordance with NPDES Permit No.
NC0023965 issued September 22, 2004, and shall be subject to revocation unless the
wastewater treatment facilities are constructed in accordance with the conditions and limitations
specified in the permit.
The sludge generated from these treatment facilities must be disposed of in accordance
with G.S. 143-215.1 and in a manner approved by the North Carolina Division of Water Quality.
In the event that the facilities fail to perform satisfactorily, including the creation of
nuisance conditions, the Permittee shall take immediate corrective action, including those as may
be required by this Division, such as the construction of additional or replacement wastewater
treatment or disposal facilities.
The Wilmington Regional Office, telephone number (910) 395 - 3900, shall be notified
at least forty-eight (48) hours in advance of operation of the installed facilities so that an in -place
inspection can be made. Such notification to the regional supervisor shall be made during the
normal office hours from 8:00 a.m. until 5:00 p.m. on Monday through Friday, excluding State
Holidays.
Upon classification of the facility by the Certification Commission, the Permittee shall
employ a certified water pollution control treatment system operator to be in responsible charge
(ORC) of the water pollution control treatment system. The operator must hold a certificate of the
type and grade at least equivalent to or greater than the classification assigned to the water
pollution control treatment system by the Certification Commission. The Permittee must also
employ a certified back-up operator of the appropriate type and grade to comply with the
conditions of Title 15A, Chapter 8G, .0202. The ORC of the facility must visit each Class I facility
at least weekly and each Class II, III, and IV facility at least daily, excluding weekends and
holidays, and must properly manage and document daily operation and maintenance of the facility
and must comply with all other conditions outlined in Title 15A, Chapter 8G, .0204. Once the
facility is classified, the Permittee must submit a letter to the Certification Commission which
designates the operator in responsible charge within: (A) Sixty calendar days prior to wastewater
being introduced into a new system or (B) within 120 calendar days of the following, (i) after
receiving notification of a change in the classification of the system requiring the designation of a
new ORC and back-up ORC or (ii) a vacancy in the position of ORC or back-up ORC.

Mr. Caldwell
Page 3
A copy of the approved plans and specifications shall be maintained on file by the
Permittee for the life of the facility.
The Operational Agreement between the Permittee and the Environmental Management
Commission is incorporated herein by reference and is a condition of this Permit. Noncompliance
with the terms of the Operational Agreement shall subject the Permittee to all sanctions provided
by G. S. 143-215.6 for violation of or failure to act in accordance with the terms and conditions of
this Permit.
Failure to abide by the requirements contained in this Authorization to Construct may
subject the Permittee to an enforcement action by the Division of Water Quality in accordance
with North Carolina General Statute 143-215.6A to 143-215.6C.
The issuance of this Authorization to Construct does not preclude the Permittee from
complying with any and all statutes, rules, regulations, or ordinances which may be imposed by
other government agencies (local, state, and federal) which have jurisdiction.
Upon completion of construction and prior to operation of this permitted facility, a
certification must be received from a professional engineer certifying that the permitted facility has
been installed in accordance with the NPDES Permit, this Authorization to Construct and the
approved plans and specifications. Mail the Certification to the NPDES Unit, Division of Water
Quality, 1617 Mail Service Center, Raleigh, NC 27699-1617.
One (1) set of approved plans and specifications is being forwarded to you. If you have any
questions or need additional information, please contact Ms. Susan A. Wilson, P.E., telephone
number [phone], extension 510.
cc: Central Files
NPDES Unit, Permit File
Wilmington Regional Office,
Mr. Ronald L. Taylor, P.E.
Mr. Tony Boahn, P.E.
Sincerely,
Alan W. Klimek, P.E.
Surface Water Protection
Hazen and Sawyer, P.C.
4011 WestChase Blvd.
Raleigh, North Carolina 27607
McKim & Creed
243 North Front Street
Wilmington, North Carolina 28401

PROPOSED
FENCE
AERATION
PROCESS
BUILDING
(875)
PORTABLE
EQUIPMENT
STORAGE 1
BUILDING
(580)
STORMWATER TREATMENT
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243 N. Irani 51, Wilmingion, North Carolina 284D1
HAZEN AND SAWYER
Environmental Engineers & Scientists
4011 WeatChase Blvd, Ralaish. North Carolina 27607
05
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NEW FACILITIES
CITY OF WILMINGTON
JAMES A. LOUGHLIN WWTP
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PLOT DATE 06/25/03 2:09pm FILE= H:\30374\NORTHSIDE\FIGURES\001 by PLAWANSON XREF FILE = NONE
CLARIFIER
DISTRIBUTION
BOX
COUNTY
INFLUENT
WASTEWATER
BAR SCREENS/
GRIT REMOVAL
FACILITY
CITY
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WASTEWATER
PS
PRIMARY
CLARIFIER
PRIMARY
CLARIFIER
TRICKLING
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TRICKLING
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TRICKLING FILTER
Q
PS RECIRCULATION
CLARIFIER
SPLITTER
BOX
RECIRCULATION
PUMP STATION
— INTERMEDIATE
PUMP STATION
AERATION
TANK
RAS
SECONDARY
CLARIFIER
SECONDARY
CLARIFIER
RAS
RAS
PS
PRIMARY SLUDGE
PUMP STATION
PS
ANAEROBIC ANAEROBIC
DIGESTER J DIGESTER
PRIMARY
SLUDGE
PUMP
STATION
DIGESTER
BUILDING
ANAEROBIC
DIGESTER
TWAS
DIGESTER
GAS STORAGE
TWAS
STORAGE
TANK
t
DIGESTED
SLUDGE
STORAGE TANK
ALUM SLUDGE
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ALUM SLUDGE
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CITY OF WILMINGTON
NORTHSIDE WASTEWATER TREATMENT PLANT

Wilmington ATC
Subject: Wilmington ATC
From: Ed Beck <[e-mail]>
Date: Tue, 31 Aug 2004 15:51:01 -0400
To: Susan Wilson <[e-mail]>
CC: Rick Shiver <[e-mail]>
Susan
ok
ALLY `�''�
I have reviewed the response to your request for supporting information from Hazen
Sawyer. It confirms that the designer and the applicant are aware of our concerns and are
confident in the proposed design. I, therefor, have no further objections to the treatment
plant design.
Concerning the question about the suitability of the influent pump stations operated by the
city and county, I don't desire additional information. As flow rates increase the city and
county have made modifications in the past and can be expected to do so in the future.
Thanks
Ed Beck
1 of 1 9/1/2004 9:07 AM

City .of Wilmington Northside WWTP
Subject: City of Wilmington Northside WWTP
From: "Taylor, Ronald L." <[e-mail]>
Date: Tue, 24 Aug 2004 13:48:37 -0400
To: <[e-mail]>
CC: 'Tony Boahn" <[e-mail]>, "Dave Heiser"
<[e-mail]>, "Nailor, David A." <[e-mail]>
Hi Susan,
The City of Wilmington is planning to defer installation of some of the equipment shown on
the plans you are reviewing for the Northside WWTP. Specifically, at the new aeration
basins, they plan to defer installation of the mixers in Cells 2, 3, 4, 5, 9 & 10 and NRCY
pumps and piping for each of the four trains. This equipment is all planned for future
nutrient removal and would not be used under the current NPDES permit. Openings for
the equipment will be constructed and covered and electrical conduits will be installed to
these locations to ease conversion to a biological nutrient removal process in the future. /
What do you need from us to approve that change (does this e-mail suffice or do you need
a letter)?
Also, we sent you a package of information on the process design for nitrification last
week. Does that information satisfactorily address your concern?
Thanks
Ron
Ronald L. Taylor, P.E.
Hazen and Sawyer, P.C.
4011 Westchase Blvd., Suite 500
Raleigh, North Carolina 27607
(T) 919-833-7152
(F) 919-833-1828
(M) 919-606-6408
[e-mail]
1 of 1 8/25/2004 2:14 PM

DIVISION OF WATER QUALITY
August 18, 2004
MEMORANDUM
To: Susan Wilson
NPDES Permits Unit
From: Ed Beck
Wilmington Regional Office
Through: Rick Shiver
al AUGU G 2 3 2003
Subject: City of Wilmington WWTP Expansion
ATC Number 023965A01
New Hanover County
PONTDENR - WATER QUALITY
ISOURCE BRANCH
A draft NPDES permit has been developed to increase the flow rate for the
City of Wilmington Northside wastewater treatment plant from 8.0 MGD to 16.0
MGD. Plans have now been submitted for expansion of the wastewater treatment
plant. The old trickling filters are to be demolished, as well as two existing primary
clarifiers, two secondary clarifiers, and the sludge drying beds. The expanded
system is to include a new screening and grit removal system, two new primary
clarifiers, a single stage nitrification process aeration basin with four basins and
eleven cells each, two new secondary clarifiers, a four cell tertiary filter system, an
ultraviolet disinfection system, upgraded sludge thickening, dewatering, and drying
facilities and upgraded anaerobic digesters. The aeration basins are to have
mechanical mixers and fine bubble air diffusers.
The primary clarifiers appear to be capable of removing 30 % to 55 % of TSS
as designed. Design criteria for other essential units appear to be marginal for a
system that must meet very stringent effluent limits. The hydraulic retention time of
9.3 hours in the aeration basin appears to be the minimum that conventional
guidance literature recommends. Also, the BOD loading rate of 28.2 lb/d/1000 cu ft
is quite high and the proposed SRT of 9 days is quite low. Metcalf and Eddy states

that SRT is the most critical parameter in plant performance and recommends 3 to
18 days for complete nitrification. The proposed tertiary filter loading rate of 3.97
GPM/sq ft is quite high.
It is understood that concerns about the design have been expressed to the
consultant and a response is anticipated.
eb: WilmingtonATC . 084
cc: Wilmington Regional Files
Central Files
Ed Beck

HAZEN NOD SAWYER
Environmental Engineers & Scientists
August 13, 2004
Ms. Susan Wilson
North Carolina Department of Environment
and Natural Resources
Water Quality Section
1617 Mail Service Center
Raleigh, North Carolina 27699-1617
Dear Ms. Wilson:
Hazen and Sawyer, P.C.
4011 WestChase Blvd.
Raleigh, NC 27607
919 833-7152
Fax: [phone]
[AUG 1 7 2003
DENR - WATER DUALITY
POINT SOURCE BRANCH
Re: Process and Hydraulic Evaluation
Expansion from 8 mgd to 16 mgd
City of Wilmington Northside WWTP
This letter is to provide additional information to address your concerns regarding
aeration tank retention times to achieve sufficient biological treatment and post
aeration facilities to ensure compliance with the DO standard in the NPDES permit.
Information is provided on nitrification process evaluations at the Wilmington Northside
WWTP and aeration tank retention times at similar facilities. Information is also provided
on the approach proposed to address your concerns about the adequacy of post -
aeration facilities.
Process Design Model for Nitrification
Hazen and Sawyer has used the nitrification process design model from the EPA Process
Design Manual for Nitrogen Control since 1978 for the evaluation and design of
nitrification at wastewater plants (See attached). The model was confirmed by
nitrification performance at the North Buffalo Creek Plant in Greensboro through the
1980's and at the Osborne Plant in Greensboro in 1994. A yield coefficient of 0.65 and a
decay coefficient of 0.05 are used in the model. A safety factor >_ 1.0 is required to
maintain nitrification performance.
The process design model for nitrification was used to evaluate aeration tank
requirements at the Northside WWTP. Operating data from the past several years was
analyzed to determine worst case operating conditions for the proposed process
design. Input data to the process model included operating data as follows:
DwQ-wilmington lir
New York, NY • Philadelphia. PA • Detroit. MI • Raleigh, NC • Charlotte, NC • Greensboro. NC • Atlanta. GA • Fairfax. VA • Baltimore. MD • Hollywood. FL • Boca Raton, FL • Fort Pierce. FL • Sarasota, FL • Miami, FL

HAZEN AND SAWYER
Ms. Susan Wilson
August 13, 2004
Page 2
Average Annual Flow, mgd 16
Max. Month Flow, mgd 22.4
Minimum wastewater temperature, °C 15°C
Effluent pH 6.8
Annual Average Influent BOD, mg/I 250
Max. Month Influent BOD, mg/I 205
MLVSS, mg/I 1875
The minimum hydraulic retention time (HRT) for nitrification depends upon many factors
including flows, loads, and operating temperatures. The proposed design provides a
design hydraulic retention time at 16 mgd of 9.3 hours. This corresponds to a process
safety factor of 1.77 in the summer and 1.52 in winter conditions. At maximum month
flows and loads and under winter time minimum temperature conditions the minimum
process safety factor is 1.10, still in a range to produce stable and effective nitrification.
Nitrification Models and HRT's at other WWTP's in North Carolina
Hazen and Sawyer has used the nitrification process design model from the Nitrogen
Control Manual to design activated sludge facilities at many wastewater treatment
plants in North Carolina. A partial listing of plants and HRT's provided for nitrification
based on the model are as follows:
HRT for Nitrification
Plant/Location (hrs)
Neuse River Plant — Raleigh, NC 9.7
North Buffalo Creek Plant — Greensboro, NC 6.5
T.Z. Osborne Plant — Greensboro, NC 16.1
Eastside Plant — High Point, NC 4.6
Monroe Plant — Monroe, NC 10.9
North Durham Plant — Durham, NC 9.8
South Durham Plant — Durham, NC 12.8
All HRT's are calculated based on permitted capacity. The hydraulic retention times
vary over a wide range depending on the BOD loading at each plant, minimum
wastewater temperature and process safety factor provided. All of these facilities have
DWQ-Wilmington Itr

HAZEN AND SAWYER
Ms. Susan Wilson
August 13, 2004
Page 3
been successfully nitrifying for a number of years. All of the above process designs for
nitrification have been accepted by DWQ. The proposed HRT for nitrification in
Wilmington of 9.3 hours is within the range of HRT's normally provided for nitrification at
similar plants in North Carolina. It is also noted that the Northside WWTP minimum
temperature data indicates a design condition of 15°C, which is higher than most North
Carolina locations and has a significant impact on hydraulic retention time
requirements for nitrification. Hazen and Sawyer is confident that the proposed design
will consistently comply with the limits proposed in the permit for 16 MGD (BOD5 = 5 mg/I
and NH3-N = 1 mg/I).
Post -Aeration Facilities
The Northside WWTP will have an effluent parameter requiring a minimum dissolved
oxygen of 6.0 mg/I. We have discussed the design's provision of fine bubble diffusers in
the backwash storage tank which will provide re -aeration of the plant effluent prior to
effluent sampling and discharge to the receiving stream. This approach was used at
the North Durham Water Reclamation Facility, which has been in operation
approximately 10 years with no problems in complying with the same limits. At the City
of Wilmington Northside WWTP, to further address this issue, fine bubble diffusers are also
being added to the effluent box downstream of the effluent flow measurement parshall
flume. This is the effluent sampling point and will ensure that the dissolved oxygen
standard is complied with under all circumstances.
We believe the enclosed information should address your concerns. Please call if you
need additional information or if you would like to meet to discuss these issues.
Very truly yours,
HAZEN AND SAWYER, P.C.
o d L. T
Senior Associate
cc: Ken Vogt
Tony Boahn
DWQ-Wdmngton di

•
e �,k:4 PC4;3y1 : 4 !t J0 ' 1'4 ►tV.6 10.11 ,i0„t f� l,1 » i° 1 o� 1
Virtually all applications of the high purity oxygen activated sludge process to nitrification
have been for separate st!ge nitrification applications t,44.6), rather than for
combined carbon oxidation -nitrification applications.
4.3.2 Utility of Nitrification Kinetic Theory in Design
The nitrification kinetic theory presented in Chapter 3 may be directly applied to the design
of those activated sludge modifications compatible with nitrification. The equations must be
adapted to the hydraulic configuration under consideration, but in all cases this adaptation
is relatively straightforward.
Nitrification kinetic theory can be very usefully applied to define the following parameters:
1. The safety factor required to handle diurnal transients in loading to prevent
significant ammonia bleed through under peak load conditions.
2. The design solids retention time under the most adverse conditions of pH, DO and
temperature.
3. The allowable organic loading on the combined ci;rbon oxidation -nitrification
stage.
G. The required hydraulic detention time in the aeration tank at ADWF.
5. The excess sludge wasting schedule.
The following sections present the design procedures in terms of a number of specific
examples. The procedure developed for each ease has often been termed the "solids
retention time" design approach.
4.3.3 Complete Mix Activated Sludge Kinetics
As a design example, consider a 1 mgd treatment plant that must achieve complete
nitrification at 15 C. The plant incorporates primary treatment. Primary effluent BOD5 is
150 mg/1, including solids handling return streams to the primary. Total Kjeldahl Nitrogen
(TKN) is 25 mg/1 as N. As a simplifying assumption, neglect that portion of the TKN that is
assimilated into biomass or associated with refractory organics. The wastewater has an
alkalinity of 280 mg/1 as CaCO3. The procedure is as follows:
1. Establish the safety factor, SF. The SF is affected by the desired effluent quality.
Assume a minimum SF of 2.5 is required due to transient loading conditions at
this particular plant (see Section 4.3.3.2).
Establish the minimum mixed liquor dissolved oxygen (DO) concentration.
47

Consideration of aera tion efficiency a the peak hourly load i:: required (;ace
Section 4.8). Assume .i minimum DO of 2.0 nag/1 is selected as a compromise
between power requirements and a consideration of the depressing effects of low
D) levels on the rate of nitrification as discussed in Section 3.2.5.5.
3. E5:timatc the process operating pH (see Section 4.9.2). Approximately 7.14 mg/1
of alkalinity as CaCO3 is destroyed per n-rg/i of NH, -N oxidized. Neglecting the
incorporation or nitrogen into biomass, the alkalinity remaining after nitrification
will be at least:
280 — 17.14(25) j= 102 mg/i
If a coarse bubble aeration system is chosen, the pH should remain above pH 7.2
and chemical addition is not required for pH control (see Section 4.9.2).
4. Calculate the maximum growth rate of nitrifiers at 15 C, DO = 2 mg/ 1, and
pH 7.2. The appropriate equation to be used was presented in Section 3.2.6 and
is as follows:
A
U.
_ �N D+� 1 - 8.33(7.2 - p}i)(3-23)
U2 DO
\L ,' \there: f;N - maximum possible nitrifier growth rate, day-1,
environmental conditions of pH, temperature, and DO,
A
NN
maximum nitrifier growth rate, day-1, and
KO 2 = half -saturation constant for oxygen, mg/l.
The last bracketed term is taken as unity at a pH above 7.2. Using the specific
values adopted in Section 3.2.5 for and KO-) leads to the following expres-
sion:
0.098(T - 15) DO
,ul�, = 0.47 e - 0.833(7.2 - p1-I)
_11
DO+ 1.3
Using the numbers given above:
-1
pN = (0.47)(0.61) = 0.285 day
(4-1)
5. Calculate the minimum solids retention time for nitrification:. From Equation
3-1 5, the correct expression is:
4-8

where: 0
c
For this example:
0rn _ 1
(4-2)
solids retention time, d2ys, for nitrification at pH,
temperature and DO.
0m = 1— - 3.51 days
c 0.285
6. Calculate the design solids retention time. From Equation 3-29, the correct
expression is:
where: 0 d
For this example:
ad= SF• Om
solids retention time of design, days.
0c = 2.5(3.51) = 8.78 days.
(4-3)
7. Calculate the design nitrifier growth rate. From Equation 3-12, the correct
expression is:
where: µN = nitrifier growth rate ,VilrosOmonas , day-1.
For this example:
PN 8.78
= 0.114 day-1
(4-4)
8. Calculate the half -saturation constant for ammonia oxidation at 15 C. The proper
expression is:
K .100.051T-1.158
N
where: KN = half -saturation constant for NH4 - N, mg/1, and
T = Temperature, C
For this example:
KN = 10-0.393 = 0.405 mg/1
(3-13)
4-9

f.:alcul ate.. the :�t::aciy statc .i 1 1 ;'# ont'.° t o 1 �J r i .q ! i �.
� 1.{tit r .J. fir= ���,�iel t. �: • �a ::%17 �• �!� i,
directly applicable to { ornplete mix activated sli2d . systi;rri;' whe!' ' 1'' , is tll+'
effluent ammoma-m trogen content:
1\j r 111v1
where: lv 1 - effluent nt ?`�l1•I4 - ;tip, nig/1
For this case:
uN = 0.114 = 0.285 l,\, 1 -+- 0.405
1'J1 _ 0.27 mg/1
Transient loading effects on effluent quality are presented in Section 4.3.3.2.
10. Calculate -the organic, removal rite. The design solids retention time, applies to
both the nitrifier ,population and the heterotrophic population. Equation 3-27 can
be applied to determine substrate removal rates:
N
where: Yb
1
pb = Dd = Yhyb - Kd
(3-27)
heterotrophic yield coefficient, lb VSS grown per lb 1301)5
removed,
clb = rate of substrate removal. lb Bt}I)5 removed/lb VSS/day, and
I d "decay' coefficient, day-1.
Assume representative values for Y1. and Kd .29
Y, = 0.65 lb VSS/lb 13C}I} rem.
Kd = 0.05 day.-1
Therefore:
0.114 = 0.65% - 0.05
4i+ 0.252 lb BOD re,n./lb ,MLVSSiday
4-10

it
can ,c.:ection i f(d- di:.;(.ip;si:In of i.his
. fic,termi:I:7.: the hyth-,:tilic detention time at t.hiF
ancl c flluenis4)1i.lb1e B01.) must be 1:nov.m. Thf.: effh;ent (icluble BO1)5
can be asn;mied to be very low (say 2 mg/1). The lql..VSS conie.riti dependent en
the mixed liciurs- tt.:.)tal suspended solids, which is in turn dependent on the
operation of aw.: nitrifivition sedimentation tank. (Se.....tirn 4.10). AssurrieTOF the
purposes of this example that the design mixed liquor content at 15 C is
2500 mg/1. At a volatile content of 75 percent, the MLVSS is 0.75 (2500)
1875 mg./I. F:-om Equation 3-23, the expression for hydraulic detention Om:: is:
X n
1
where: HT hydraulic deternion time, day,
.1 mixed liquor volatile :r.uspendeci solids; tviLVSS, mg,'1,
So = influent total BOD'_, mg/1, and /
.) /
I
S I M le mg/I.Tluent soluble BOD,-.fi •
../ i
i
, /•
For this example, the hydraulic detention time at ADM:: is: \....
148
HT 0.313 days
(1875)(0.252)
= 7.5 hours
12. Determine the organic loading per unit volume. The volume required in the
aeration basin for 1 mid flow is:
volume, Q .Li1 1(0.313) 0.313 mil gal
= 41,844 cu ft
‘vhere: Q
The BOD.. loading is:
influent flow rate, nigd
(1)(8.33)( ! 50)
41 1

The BOD5 load per 1000 cu ft is:
1249 = 29.9 lb BOG / 1000 cu ft/day
41.84 5
13. Determine the sludge wasting schedule. Sludge is wasted from the system from
two sources: (1) solids contained in the effluent from the secondary sedimenta-
tion tank, and (2) intentional sludge wasting from the return sludge or mixed
liquor. The sludge to tie wasted under steady state conditions can be calculated
from the solids retention time. The total sludge wasted per day is:
S = 8.33(Q • X2 + W • Xw) (4-6)
where: S = total sludge wasted in Ib/day,
W = waste sludge flow rate, mgd
X, = effluent volatile suspended solids, mg/I, and
X = waste sludge volatile suspended solids, mg/1
The inventory of sludge in the system is:
where: 1
I = 8.33(X 1 • V)
inventory of VSS under aeration, Ib, and
= volume of aeration tank, mil gal
The solids retention time is defined as:
d_ 1
0c S
In this case, application of Equation 4-7 yields:
1 = 8.33(1875)(0.313) = 4889 lb VSS
(4-7)
(4-8)
Using Equation 4-8 and a design 0d of 8.78 days, the sludge wasted from the
system is:
S = 4889/8.78 = 557 lb/VSS day
4-12

The sludge contained in the effluent at. 1 mfcd can be calcu:ated assuming iha the efflu-
ent volatile suspended solids is equal to 12 mg/1:
8.33 ( 1 ) (12) = 100 lb VSS/clay
By difference, the Ib of h 1LVSS to be wasted from the mixed liquor or return :sludge is:
557 -- 100 = 457 lb VSS/day
4.3.3.1 Effect of Temperature and Safety Factor on Design
The design example presented in the previous section provided one solution to a set of
stated conditions. Alteration of the lowest temperature at which nitrification will be
supported, or the design safety factor, or the wastewater strength, or the assumption of
different kinetic constants can materially alter the design.
To give one illustration, Table 4-2 has been prepared using differing safety factors (2.0 to
3.0) and differing minimum wastewater temperatures with design calculations to derive the
computed quantities shown. Assumptions have been made for illustrative purposes as to the
allowable MLSS. Allowable mixed liquor levels are a function of sedimentation tank
operation. The mixed liquor level that can be maintained will be affected by reduced
sedimentation efficiency at lower temperatures. Consideration of aeration tank -secondary
sedimentation tank interactions is presented in Section 4.10.
As can be seen from Table 4-2, low temperature applications (10 C) of combined carbon
oxidation -nitrification in complete mix activated sludge systems require very long hydraulic
residence times to achieve favorable conditions for nitrification. This factor was one of the
reasons for the development of separate stage nitrification systems. As temperatures rise,
required residence times are materially reduced. At 20 C, less than five hours is required for
virtually complete nitrification in the specific case examined. While it is possible to design
for nitrification using the relatively low detention times given in Table 4-2 for 20 C, special
attention must be given to oxygen transfer as a very high oxygen demand is expressed per
unit volume. Considerations for oxygen transfer are given in Section 4.8.
4.3.3.2 Consideration in the Selection of SF
In introducing the safety factor concept to the design of biological treatment systems,
Lawrence and McCarty29 noted that the SF was necessary to achieve high efficiency of
treatment, to insure process stability and to provide resistance to toxic upsets. Excessively
high safety factors resulted in higher operating and capital costs. It was noted that the safety
factor concept had been implicitly incorporated into treatment plant design practice by the
selection of solids retention times in excess of Om
c
4-13

•{� 10
15
20
a At ADWF
TABLE 4-2
CALCULATED DESIGN PARAMETERS FOR A I MCD
COMPLETE MIX ACTIVATED SLUDGE PLANT
I .`tIaximum
Minimum I possible
temp. for I nit rifler
nitrification, 1 growth rate,
t�f1 , ciay-t
fit
0. 175
0. 265
0. 465
b 62.4 lb/1000 cl/cia
Assumed
a'towable
MSS/ MLV SS
mg/1
2, 000
1. 500
2,500
i� 1aa
, f$
I
Safety
Factor,
SF +Ii
2.0
2.5 {
3.0
3,000 2.0
+I
2.5
2.250 3.0
1
iJ{-•S j [t• I I Steady
soli:1s state
retention I 'ff.uent
ti me,`� days i 111.1*-N.
V •
Z•!1/1
7.0 j L. 40 1 rf. 29
I , _
,• �; 1 0. 7 I f1. 25
i .
10.5 0. 20 j 0. 1:2
Organic.
run' evaI
rate_
ih :i0D rpm/
lb 11I.. t'wS-Lin
C. 23
0. 15
0._7
-I
4..• i O. 73 i
5.4 O.4f;
6.
0. 3t�
O. 44
0. 3'.r
:3.32
1
r
•
•
a ,
7.
I . .
I .,-;.
5.2
. rig . ..

3e cruse the Sr' concept is relatively new, her" is no plant f experience th its
. � ;., ,..an scale e.�{JerleilC., with �.
application ;.iccurriulated as yet On which to base broad recommendations. Rather, i:ir1 tic
itselftheory ;s•i.. eci this section to f_stalali„h :771)-?/n?.Lril factors of :7.afety considering the
i_ ef:ircdd degree of nitrification under steady :: t.ite and transient liaad conditions. It must be
emphasized that these are minimum values and individual designs may exceed these values
for a variety Of reasons. For instance, the presence. of industrial wastes may adversely affect
nitrification rates, requiring conservatism in the selection of the SF.
Figure 4-2 provides a wider array of safety factors for the design example presented in Table
4-2. As may be seen, the selection of the SF has a marked effect on the calculated steady
state values of ammonia in the effluent. If relatively complete nitrification is to be obtained
(at steady-state) resulting in 0.5-2 mg/l of ammonia nitrogen in the effluent, a minimum SF
of 1.5 is appropriate for application to complete mix activated sludge systems. Further,
effluent values for a comparable plug flow system are also shown in Figure 4-2 (see Section
4.3.5 for plug flow data). A.s may be seen, complete mix systems have higher effluent
ammonia levels than plug flow systems .at the san'ie SF.
In all practical applications, waste treatment plants do not operate at "steady state."
Significant diurnal variation in the nitrogen loading on such systems occurs. Figure 4-3
shows the diurnal variations in influent flow and TKN loading experienced at the Chapel
Hill, N.C. treatment plant. The ratio of the maximum TKN loading to the average was 2.17,
while the ratio of the maximum to minimum was 6.72. Tlie Chapel Hill system is a relatively
small system (1.8 mgd) with high peak to average ratios for all constituents.30 The variation
in load for each community will be a function of the unique characteristics of that
community (see Section 4.8), and data must be inciividtially developed for each situation.
TKN load variations have a significant impact on nitrification kinetics, and ammonia
bleedthrough can occur under peak load situations.31,32 Kinetic theory can be applied to
these situations, however, and the safety factor established at levels which will prevent
ammonia bleedthrough from causing significant deterioration of effluent quality.
A mass balance on nitrogen in the organic and ammonia form can be made at any time
during a diurnal cycle which states that the influent TKN load is equal to the effluent
ammonia load plus that nitrified in the complete -mix reactor during any time, Lt:
N0Qat=gNfX1VAt+N1QAt (4-9)
where: No = influent TKN concentration, mg/1,
N1 = effluent ammonia nitrogen concentration, mg/l,
Q = influent or effluent flow rate,.mgd,
At = time increment,
4-15

V = volume of aeration basin, mil gal,
f = riitrifier fraction of the mixed liquor :solids
FIGURE 4-2
EFFECT OF THE SAFETY FACTOR ON STEADY STA TE EFFLUEN F
AMMONIA LEVELS IN SUSPENDED GROWTH SYSTEMS
7
COMPLETE MIX
PLUG FLOW
0 1
1.0 1.2
2.5
A. at 20C
1.4 1.6 1.8 2.0 2.2 2.4
SAFETY FACTOR, SF
2.6 2.8 3.0
COMPLETE MIX
0.5 PLUG
FLOW
0 I 1 1_,—L
1.0 1.2 1.4 1.6 1,8
B. at 10 C
2.0 2.2 2.4 2.6 2.8 3.0
SAFETY FACTOR, SF
4-16

PERCENT OF AVERAGE FLOW
200
180
160
140
120
100
80
60
40
20
0
220
200
180
160
140
120
100
80
60
40
20
FIGURE 4-3
DIURNAL VARIATIONS AT THE CHAPLL HILL, N.C.
TREATMENT PLANT (AFTER HANSON, ET AL. (30))
1 I 1 I 1 1 1
2400 0400 0800 1200 1600 2000 2400
T IME
DIURNAL VARIATION IN WASTEWATER FLOW
INFLUENT
LOAD
INFLUENT
CONCENTRATION
p 1 1 1 I 1 1 I
24 00 0400 0800 1200 1600 2000 2400
T 1ME
DIURNAL VARIATION IN NITROGEN LOAD AND CONCENTRATION
4-17

:i•t _ :t1•..:1:: `r rms. t_S;il_!ine•s %fl! }:l;l:ii:ijt !� 1`\ r
jj tii_ili: E �
:i' _ ._f•'\,,y I.�.:�i i1� !':i:I }.I�I •• E _1 ei t:� !'� ... " (��'-' E:^ �•� : r' of rc:!}�. aifln1c;?:;ci i:Ei?irf'i:!: il'ii ;n 017.
1,'�Llfi�. i�l•if':li SC)!!..: 001; Il'''r1111•,0.!e2 �1:..1C 1e tll n i, a•r , c
'i.� 1, ;:. .;1`ti;lar.!''. ti: 11 '�?'jr:b; t 1f.)afi c f :c:. ;1•.(•m
uselui fc: .:i`{'`rC,xirnati!'•t� the effect!,; l' •{•� ... •1� il� t.'� 1 . •t. r' 1.i� !lii}:::i �''3�L loads
• 'E•c:til_rjj �-= '.� �!!: -i�ti..y'+•!1 ;-i:Y T.'t
-'i f_Y :l: ,r I'ri by `,UtJ:titi.jli+.•!1 fc;I i1i: ti'ii}�5 tr+i E.- iiltrifiration rate,
.and:l1r:. "Ll.: i^: iiJ �� "' f•• :. -i. �! ' r^f�. `n ��t
E
ii :1f• Iri ei1tOry" r.) :Iltriiyirg organisms. ' h . iinvc:'tory of
ni rif%iiilg c.r a;-li;aiis can be }'t•lilE!:r: to tilt. solids ret::ntion time +.t.l:c' :ht'nclg.'•a the fc:ilciwll•r`.?
`�d_ IX lv
)
;`•1 ,•I 1
24 hr-average 'nf liierlt TKN, mg'1
.1
24 hr-averace effluent. NH4 -- N, mg/1
mean flow rate (ADW•F), mad, and
nitrifier yield coefficient, lb V /lb NI•14 - N removed.
(4-10)
The tern} t_>`()\.i(t7f:) Crl) represents the quantity of nitrifie'r: grown per day, \which. must be
weteC1 each day toestablish a steady -stage Itf
((. "rage terms, No
. solids retention time, 0 The :1�t,,
and Ni are now weighted averages of nitrogen concentration of an entire. dray (the
equivalent of composite samples). Q represents the average dry weather flow (AMP).
The nitrification rate from Equations 3-20 and 3-24 is.
N / Nl
)
Yl,� K� + Nl
Substitution of Equations 4-10, 4-1 1 and Equation 3-29 into Equation 4-9 yields:
�) NSF Nl ��
I
Q K 1,� + N 1
J
(4-1 1 )
(4-12)
Equation 4- I 2 can be used to solve for N 1 over a 24-hr cycle since all other quan tities in
Equation 4-12 are known or can be estimated. Initially, N1 can be estimated to be the
'calculated steady-state value. Once Equation 4-12 has been applied to generate a 24-hr cycle
of N1 values, a new value of Fl 1 may be calculated. If N 1 differs significantly from the
initial assumption, the calculation process can he repeated.
4-18

Eqliation 4-•i 2 has been applied to the. vari:-Itions in load observed at Cilupel Hill, and using
the design information used to generate Table 4-2 at a temperature c.n: 15 C. The result::: of
this analysis are plotted in Figure 4-4 for three different nssumed safety factois, 1.5, 2.0,
2.5. As may be seen, th.t. assumption of the sarety facto] has a matkif.d cifext on the
average effluent amminia content, N . For this particular case, the ratio of peak to average
TKN loadir.g was the SF had to -exceed this ratio (2.5) to produce an effluent that had,
on the average, less than 1 mg,./1 of ammonia-N.
The application of Equation 2 to several other such cases.. showed the same. effect;
namely, the minimum safety factor should equal or exceed. the ratio of peak ammonia load
to average load to prevent high ammonia bleedthrough at peak loads. This F tatment may be
used as "a rule. of thumb" for designing 'suspended growth nitrification systems operated in
the complete mix mode.
A flow equalization procedure applicable to reducing diurnal peaking on nitrification
systems is presented in • Chapter 3 of the Process Design Manual for Upgrading Existing
Wastewater Treatment Plants. 25 By incorporating flow equalization into treatment plants,
COIVC•ENTR AT ION ,
v.
15
10
0
2400
0400
FIGURE 4-4
EFFECT OF SF ON DIURNAL VARIATION IN EFFLUENT AMMONIA
SF = 1.5
NI = 5.5
A .
/
S F 2 .5 /
NI = O. 6
J.
--f
1600 2000 2400
= 24 hr overage
composite N1-114-- N
concentrat ion •
SF = 2.0
/ NI = 1,5
0800 /200
TIME, HR
.4-19

the safety factor used in kinetic design of the nitrification tanks may be reduced. Case
examples for treatment plants incorporating flow equalization are presented in Sections
9.S.I.1,9.5.l.2and 9.5.2.1.
4.3.4 Extended Aeration Activated Sludge Kinetics
The procedure presented in Section 4.3.3 for complete mix activated sludge kinetics is
directly applicable to ex tended aeration activated sludge. Extended aeration systems arc
usually operated at such long solids retention times that except during cold temperatures
(5-10 C) nitrification is usually obtained in properly operated systems.
4.3.5 Conventional Activated Sludge (Plug Flow) Kinetics
The approach for conventional activated sludge plants is similar to that for complete. mix
plants with the exception of the equations used to predict effluent quality. The plug flow
model may be applied to approximate the hydraulic regime in these plants. The Monod
expression for substrate removal rate (Equation 3-24) must be integrated over the period of
time an element of liquid remains in the nitrification tank. The following is a solution for
plug flow kinetics that can be adapted to this problem as shown:29
1 / N(No - NI)
0d N
(N°-Nl)+K,vin °
NI
for r < I (4-13)
where: r = recycle ratio (or return sludge ratio).
or
SF
(No - NI)
N
(No-Nl)+KNln No
1
for r < 1 (4-14)
Equation 4-14 is evaluated in Figure 4-2 for the design example presented in Section 4.3.3.
Comparing the safety factor to the safety factor producing the same effluent ammonia in
the complete mix case, it can be seen that lower values of the SF are required for plug flow
nitrification processes than for complete mix nitrification processes. This means that plug
flow processes theoretically can be more efficient at the same SF, or alternatively, require
less aeration tank volume for the same level of nitrification efficiency. However, plug flow
type reactors have the disadvantage that the carbonaceous oxygen demand is concentrated
at the head end of the tank, making it difficult to supply enough air in that area for both
carbonaceous oxidation and nitrification. Air diffusion systems must be specifically
designed to handle this concentrated load. Otherwise, the first portion of the tank will not
be available for nitrification and thus effective volume for nitrification will be reduced.
4-20

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RE: City of Wilmington Northside WWTP
Subject: RE: City of Wilmington Northside WWTP
From: 'Tony Boahn" I'[e-mail]>
Date: Fri, 6 Aug 2004 10:07:17 -0400
To: "Susan Wilson" <[e-mail]>
CC: <[e-mail]>, <[e-mail]>,
<[e-mail]>, "Dave Heiser" <[e-mail]>
Susan,
Thanks for your response - I will have a map prepared that shows what is to be demolished and what is to be
added as requested. I'II also ask Ron to follow up with your questions on the detention time, etc.
From a schedule standpoint, could you give us a time frame of when we would might recieve your comments?
Thanks,
Tony
From: Susan Wilson [mailto:[e-mail]]
Sent: Friday, August 06, 2004 9:52 AM
To: Tony Boahn
Cc: [e-mail]; [e-mail]; [e-mail]; Dave Heiser
Subject: Re: City of Wilmington Northside WWTP
Tony,
Sorry I am just now getting up with you - I'm just back from vacation. I'm keeping tabs
with Joe and Matt (the hearing officer) so that I can keep on schedule with the ATC. I know
that Ed Beck (WiRO) was reviewing the plans/specs before I left on vacation and we
discussed them a bit (don't know if he contacted you guys - he had the same concerns, as I
did, re. the 9 hour detention time; he also had concerns that this design was different
than what he had seen in the past). One thing I need from you (really Ron) - I would like a
signed/sealed letter from you and/or Ron T. stating that the proposed design will
consistently comply with the limits proposed in the permit for 16 MGD (BODS = 5 mg/1 and
NH3-N = 1 mg/1). I would also like to see something from Ron to the effect that adding
aeration to the backwash tank will still allow the facility to comply with the D.O. effluent
limit in the permit (based on where the sampling is relative to where the aeration occurs). I
believe I voiced my concerns about this during our meeting and Ron said they had used a
similar design at another facility (which I have not had a chance to look at their data).
Also - as we discussed during our meeting - I'd like even just a cryptic drawing of what's
existing vs. what's to be removed and added. (nothing fancy - just something to help me
with the review). Sorry I did not make it for the site visits prior to the hearing (that would
have been helpful).
Susan
Tony Boahn wrote:
I Dave -Susan,
Could you please provide a status of the ATC review for this facility. It was our understanding that ATC review
1 of 2 8/6/2004 3:41 PM

RE: City of Wilmington Northside WWTP
would continue even though the public hearing issue has delayed the NPDES permit. If there is anything we can
do to help expedite this please let me know.
Please feel free to give me a call if you have any questions.
Thank you
Tony Boahn, P.E.
McKim & Creed, P.A.
243 N. Front Street
Wilmington, NC 28401
[phone]
[phone] (Fax)
[phone] (Cell)
www.mckimcreed.com
2 of 2 8/6/2004 3:41 PM

O\-\oc. WA 1-.QG Governor Michael . e
r William G. Ross, Jr., Secretary
- North Carolina Department of Environment and Natural Resources
Y Alan W. Klimek, P.E., Director
Division of Water Quality
Water Quality Section/ NPDES Unit
FAX: [phone]
9/29/04
FAX TO: Hugh Caldwell, P.E.
Re.: Wilmington Northside ATC
FROM: Susan A. Wilson, P.E.kiiik(1--Z
PHONE: [phone], ext. 510
FAX NUMBER: 910 341 5881
NO. OF PAGES INCLUDING THIS SHEET: 67
Hugh,
Here's the ATC. Hardcopy will follow. (Sorry — I didn't find out until late Monday that the NPDES permit was
issued).
Susan