NC0024945_Authorization to Construct_19981005

NPDES DOCUHENT SCANNING COVER SHEET
NC0024945
Irwin Creek 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
Draft Permit
Instream Assessment (67b)
Speculative Limits
Environmental Assessment (EA)
Document Date:
October 5, 1998
Thins document ins printed on re -tame paper - ignore any
content on the re'reuse Bide

State of North Carolina
Department of Environment
and Natural Resources
Division of Water Quality
James B. Hunt, Jr., Governor
Wayne McDevitt, Secretary
A. Preston Howard, Jr., P.E., Director
October 5, 1998
Mr. Thomas W. Vandeventer, P.E.
Chief Engineer
CMUD, Engineering Division
5100 Brookshire Blvd.
Charlotte, North Carolina 28216
AtTf�
NCDENR
NORTH CAROLINA DEPARTMENT OF
ENVIRONMENT AND NATURAL RESOURCES
Subject: Authorization to Construct
Permit No. NC0024945
Project No. AC0024945
Irwin Creek WWTP
Mecklenburg County
Dear Mr.Vandeventer:
A letter of request for an Authorization to Construct and a check for $400.00 (#228062) was received June
24, 1998 by the Division, and final plans and specifications for the subject project have been reviewed and
found to be satisfactory. Authorization is hereby granted for the construction and installation of the
following:
• Three (3) secondary clarifiers
• One (1) secondary clarifier splitter box
• Replace existing rock media in two (2) trickling filters (Nos. 1 and 4) with plastic media
• RAS pumping system, including four (4) horizontal non -clog centrifugal pumps (one for each clarifier
and standby) with a 5.0 MGD capacity per pump.
• WAS pumping system, including three (3) horizontal non -clog centrifugal pumps with a 0.432 MGD
capacity per pump.
• Associated piping and appurtenances.
at Irwin Creek WWTP, with discharge of treated water into Irwin Creek in the Catawba River Basin.
This Authorization to Construct is issued in accordance with Part III, Paragraph A. of NPDES Permit No.
NC0024945 issued November 1, 1996 and shall be subject to revocation unless the water treatment facilities
are constructed in accordance with the conditions and limitations specified in Permit No. NC0024945.
Any sludge generated from this facility must be disposed of in accordance with North Carolina General
Statute 143-215.1 and in a manner approved by the North Carolina Division of Water Quality.
In the event that the facility fails to perform satisfactorily, including the creation of nuisance conditions, the
Permittee shall take immediate corrective actions, including those as may be required by this Division, such
as the construction of additional or replacement water treatment or disposal facilities.
P.O. Box 29535, Raleigh, North Carolina 27626-0535 Telephone [phone] FAX [phone]
An Equal Opportunity Affirmative Action Employer 50% recycled/ 10% post -consumer paper

CMUD—Irwin Creek WWTP
Permit No. NC0024945
Project No. AC0024945
Authorization to Construct (October 5, 1998)
The Mooresville Regional Office, telephone number [phone], 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 performed.
Such notification to the regional supervisor shall be made during normal office hours from 8:00 a.m. until
5:00 p.m., Monday through Friday, excluding State Holidays.
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. Please
mail the certification to the NPDES Unit, P.O. Box 29535, Raleigh, NC 27626-0535.
A copy of the approved plans and specifications shall be maintained on file by the Permittee for the life of
the facility.
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 G.S. 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) that have jurisdiction.
One (1) set of approved plans and specifications is being forwarded to you. If you have any questions or
need additional information, please contact Bethany Bolt at telephone number [phone], extension
551.
Sincerely,
WIWI Signed By
d A. Goodrich
A. Preston Howard, Jr., P.E.
Enclosure
cc: Central Files
Mooresville Regional Office/Water Quality Section
Cecil Madden/Construction Grants and Loans
L. Kevin Mosteller, P.E./HDR Engineering, Inc./128 S. Tryon St., Suite 1400/Charlotte, NC 28202-5001
Technical Assistance and Certification Unit
NPDES Unit

SOC PRIORITY PROJECT: No
To: Permits and Engineering Unit
Water Quality Section
Attention: Steve Pellei
Date: July 13, 1998
AUTHORIZATION TO CONSTRUCT
NPDES PERMIT
REPORT AND RECOMMENDATIONS
County: Mecklenburg
MRO No.: 98-42
Permit No. ACD024945
PART I - GENERAL INFORMATION
1. Facility and Address: Irwin Creek WWTP
c/o CMUD
5100 Brookshire Blvd.
Charlotte, N.C. 28216
2. Date of On -Site Investigation (if conducted): N/A
3. Report Prepared By: Michael L. Parker, Environ. Engr. II
4. Persons Contacted and Telephone Number: N/A
5. Verified Discharge Point(s), List for All Discharge Points:
Latitude: 35° 11' 44" Longitude: 80° 54' 27"
USGS Quad No.: G 15 NW
6. Site size and expansion area consistent with application? Yes. Ample area exists for the
construction of the proposed facilities.
7. Topography (relationship to flood plain included): Generally flat to moderate slopes.
Portions of the WWTP site appear to be located in the 100 year flood plain. Flooding of
the WWTP occured during July, 1997, even though a berm existed between the plant and
Irwin Creek. CMUD, however, initiated berm improvements in the fall of 97 that will
hopefully minimize the potential for future flooding occurrences.
8. Location of Nearest Dwelling: None within 500 feet of the site.

Page Two
PART II - DESCRIPTION OF DISCHARGE AND TREATMENT WORKS
1. Existing Treatment Facility
a. Current permitted capacity: 15.0 MGD.
b. Date(s) and construction activities allowed by previous ATCs issued in the
previous two (2) years: ACCO24945 - sludge storage area modifications.
c. Actual treatment capacity of the current facility (design volume): 15.0 MGD.
d. Description of existing or substantially constructed WWT facilities: screening, a
Pista grit removal system, influent flow measurement, three (3) primary clarifiers,
four (4) high rate trickling filters, pH adjustment, aeration basins (diffused), three
(3) final clarifiers, tertiary filters, polishing lagoon (currently not in use), dual
chlorine contact chambers, bisulfite addition (dechlorination), six (6) anaerobic
sludge digesters, sludge storage tanks, a belt filter press, and cascade aeration.
2. Type of Proposed Treatment Facility: The applicant proposes to construct three (3) 110
foot diameter secondary clarifiers and a RAS/WAS pumping stationS1YlUD also plans
on replacing he rock media in two (2) of the existing trickling filters with plastic media.
3. Residuals Handling and Utilization/Disposal Scheme: Land Applied.
a. DWQ Permit No. WQ0000057
Residuals Contractor: AMSCO, Inc.
Telephone No. [phone]
b. Residuals Stabilization: Class B
4. Treatment Plant Classification (attached completed rating sheet): Class IV (The proposed
modifications/additions will not change the present WWTP classification)
5. SIC Code(s): 4952
6. Wastewater Code(s): 01
Main Treatment Unit Code: 04603
6. Important SOC/JOC or Compliance Schedule Dates: N/A

Page Three
PART III - EVALUATIONS AND RECOMMENDATIONS
CMUD proposes to construct three (3) new secondary clarifiers and a new RAS/WAS
pumping station. The four (4) existing clarifiers would eventually have to be abandoned due to
their age, equipment reliability, shallow side water depth, and difficulty in achieving adequate
flow splits. The three (3) new clarifiers will be designed to handle the WWTP's permitted and
maximum hydraulic loading.
Also proposed in this ATC request is the replacement of the existing stone media in 2 of
the 4 existing trickling filters (Trickling Filters No. 1 & 4) with random dump type plastic filter
media. The concrete wall in Trickling Filter No. 1 will also have to be replaced.
This Office has reviewed CMUD's request and, pending a technical review by P&E, it is
recommended that an ATC be issued.
tore of Re o re
Date
/,,r
Water Quality Regi 1 Supervisor Date
h:\atc\atc98\irwinckl.atc
parer
p parer

AC0024945
09/16/98
BAB
CMUD-Irwin Creek WWTP: ATC Summary
ATC request for: three (3) secondary clarifiers, media replacement for two (2) trickling filters (with
plastic), one (1) splitter box, and an RAS/WAS pumping station, including four (4) horizontal non -clog
centrifugal RAS pumps and three (3) WAS pumps.
Review summary:
• Surface overflow rates in gpd/ft2 are close to range of typical design values (Metcalf &Eddy, p. 588:
400-800); peak = 815 and average = 326.
• Tank radius does not exceed five times the sidewater depth (appropriate, M&E, p. 581).
• The design surface area for these clarifiers (12,271 ft2) is adequate based on the required surface area
(based on average flow/overflow rate), which is 12,270 ft2.
• Three clarifiers will be installed, and a fourth is planned for the future.
• Solids loading (based on engneer's estimate of 8-28 lbs/day-ft2) is approx. 0.33-1.17 lbs/h- ft2, which is
suitable considering typical range for this type of clarifier (M&E, p. 588: 0.8-1.2)
• Scum pumping system operated off float switches in individual scum well. Spray system included to
reduce foam. Scum pumps rated at 50 gpm included for each clarifier, transferring scum back to head
of aeration basins. Interconnecting wetwells and suction piping allow for any one pump to be out of
service and still maintain scum flow.
• Splitter box looks okay; design allows for future fourth clarifier.
• Tank bottom slope is 1"/12" (typical for clarifiers).
• Sludge collected in a center hopper at bottom of each clarifier.
• Inboard launders and solids density current baffle around perimeter of launder for even flow and solids
distribution.
• Sludge line sizes reasonable.
• Hydraulic modifications:
aeration basin effluent pipes from basins 1-6 being modified for side discharge to eliminate
vortexing and provide hydraulic head; plugging up pipes to route effluent to secondary clarifiers'
splitter box in 42" line instead of the previous path.
Also, weir on chlorine contact chamber being lowered by 8" to provide additional hydraulic head
in secondary effluent piping. New residence time sufficient/calcs?*
• Trickling Filters: Rock media being replaced with random dump type plastic in filters No. 1 and 4.
Vent pipes to be placed at 10° increments, and vitrified clay underdrain tile will be replaced with
polypropylene panels.
• Floor and underdrains should typically have a 1-5% grade (M&E, p. 622); this design is approx. 1%.
• Typical hydraulic loading for plastic media is 0.2-1.2 g/ft2-min. (M&E, p. 615); this design = 0.35-
0.57.
• Typical design depth is listed as 10-40' in M&E (p. 615); however, these filters are not Irwin Cr
WWTP's primary treatment component for removing BOD, so that design criterion does not apply
(height not critical). This design simply replaces rock with plastic at a depth ranging from 4-4.5'.
Since this is a modification of existing tanks, height of media is limited.
• RAS/WAS pumping station: four (4) horizontal non -clog centrifugal RAS pumps (5.0 MGD/ea.) and
three (3) WAS pumps (432,000 gpd/ea.). This allows for a standby RAS pump (max. monthly flow is
15.0 MGD). No standby WAS pump is required since a total capacity of only 330,000 gpd is required,
and additional WAS flow can be accomplished by wasting greater amounts of flow from the remaining
units (966,000 gpd extra capacity). Extra capacity allotted for future gravity belt thickeners.
(Additional pumps would need to be installed in the event a fourth clarifier is added.)
*See calculations faxed by Scott Alpert 9/15/98. Detention time adequate with lowering of weirs (8"
lower) in chlorine contact chamber, even if filters bypassed (15 minutes minimum at 30 MGD peak
day flow, 30 minutes minimum at 12 MGD annual average flow.)

SECONDARY CLARIFICATION
Surface overflow rates (GPD/ft2):
a. Peak 5/ 5
b. Average 3Z6 �.-
(Typical values range frw 400-$,air activated sludge and oxygen
activated sludge, 200-400 for extended aeration, and 400-600 for trickling
filters)
Detention time at average flow: 7 8 iv
Tank Dimensions:
Rectangular:
/LenJh(ft)
Depth(ft)
Sidewater Depth(ft)
(Sidewater depth typically measured at effluent end; the length
should not exceed 10-15 times the sidewater depth)
Circular:
Diameter(ft) vi it -1-Dvki /) 9.47 / (r = . R'
Sidewater Depth(ft) 1 it-. 3 3 ' ,S ;339 = 7 /. 6 / 1 /. 66
(Sidewater depth measured at tank sidewall• _radius -should not
exceed 5 times the sidewater depth, typical -range is 12-20 ft)
f 1 lr 7.cv,Jcrl 7)i.?''
Tank bottom slope: 1 /1 7,-
Have inlet baffles been installed for even flow distribution (circle): YES// NO
•
Number of tanks: 3 (-1�� 'r ru'l-et re
t,. 7. Total clarifier surface area; required surface area is Average flow divided by
overflow rate. The actual surface area (total number of tanks times tank surface ir
area) should be checked against the required area calculated. ^' 2 o S . -e
I/ /y, 27Z sf.,C1 ' Vs. 4, 000, 000/3 26 = /2, 270
8. Solids loading rate (lb/ft2-hr): • 3 — /. /7
$-z0air/�.s p- . s/z4 - z/z� - — o.g3 I./7 J1,s//.s1 1. o.8 2.
✓9 . Flow Control to tank: 4p 1,' er bo x
1
v10. Coagulants added are:-
• 11. Type of sludge collector: C..f i4 i-'PX f/i 0 ti
12. Size of sludge collector: --�^
W A 13. Number of sludge pumps: WIkS U urn\2S (0_11?1,2 MN)/r/o s 4 AS �Y3
'/4.7vvyr
Capacity of slude pumps: 5.0 MIA r) / Q 141 S ►FYI ps �� M�f�! }
gP P rtA
.+r ` II
71T); Size of sludge lines: l r� £(CTh. Ceti 4e,r kODDQr 0 paw? 51d1-; » 1
--� RA S - �9 cm'pair'
sl a+t c'r —~ Man hole I (rc1i t.. t%d s1_h
WAS 6 OM►'► --� d t13 erO7, C Ci e5'1-erS
g

•
•
1:$
;J•
588 DESIGN OF FACILITIES FOR THE BIOLOGICAL TREATMENT OF WASTEWATER
in Table 10-12 are typical values used for the design of biological systems kh
values are based on wastewater flowrates instead of on the mixed -liquor flo
because the overflow rate is equivalent to an upward flow velocity. The return I
flow is drawn off the bottom of the tank and does not contribute to the upward
velocity. •
The solids -Loading rate on an activated -sludge settling tank may be compu
dividing the total solids applied by the surface area of the tank. The preferred it
which are the same as those used to compute the solids flux discussed previotisl
pounds per square foot per hour, although units of pounds per square foot pe
are common in the literature. The former is favored because the solids -loading f
should be evaluated at both peak and average flow conditions. If peaks al-6:i;
duration, average 24 h values may govern: if peaks are of long duration, peak3v
should be assumed to govern to prevent the solids from overflowing the tank
In effect, the solids -loading rate represents a characteristic value for the's'
sion under consideration. In a settling tank of fixed surface area, the effluent
will deteriorate if the solids loading is increased beyond the characteristic value
suspension. Typical solids -loading values used for the design of biological, sy
are given in Table 10-12. Without extensive experimental work covering all s
and operating variables, higher rates should not be used for design.
- -p
Sidewater Depth. Liquid depth in a secondary clarifier is normally nieas
the sidewall in circular tanks and at the effluent end wall for rectangular
The liquid depth is a factor in the effectiveness of suspended -solids .remo
in the concentration of the return sludge. In recent years, the trend has bee
-t'.+F.;t7
TABLE 10-12 '+
Typical design information for secondary clarifiersa
•
Overflow rate, Solids loadin
gal/ft2 • d lb/ft2;
Type of treatment Average Peak Average
Settling following air activated -sludge
(excluding extended aeration) 400-800 1,000-1,200 0.8-1.2 ':•
Settling following oxygen activated -sludge 400-800 1,000-1,200 1.01.1.4
Settling following extended aeration 200-400 600-800 0.2=1•:0
Settling following tricklin. filtration 400-600 1,000-1,200 0.6 ,110
Settling following rotating biological contractors:
Secondary effluent 400-800 1,000-1,200 0.842-
Nitrified effluent 400-600 800-1,000 0.6=;L•0
" Adapted in part from Ref. 60. t�.
Note: gal/ft2 • d x 0.0407 = m3/m2 • d • -a+-
Iblft2 • h x 4.8824 = kg/m2 • h
ft x 0.3048 = m
Yti,
•
y.

o �
41.
hD: 1
FAA' TRANSMITTAL SHEET
Date
09f15/98
Number of Pages (Includes cover page)
.8
MESSAGE TO:
Authorization Code:
1300
Name: Ms. ietlrnn halt
Firm: NC DENR DWQ
City: —_ Raleigh
HIMProject No.'
Telecopy No.:
Telephone No.:
State:
00121-300-018
919-733-0719
9J9-733. 5083
NC
if you do not receive all the pages. please call [phone] as soon as possible.
MESSAGE FROM:
Natnc:
Scott Alpert, UDR -Charlotte
MESSAGE:
RE: Charlotte-Mecldetuburg Utilities, Irwin Creek WWTP-Secondary Clarifiers
Bethany,
Please find attached the design calculations for the chlorine contact time based on the lowered
weir elevation. Sorry for the delay.
1 hope this information answers your questions. If not. please do not hesitate to call me.
Thank you,
Scott
1.11)1t Engineering.1ric.
raxPnaMi hnr.
68Z-d 80/10'd 0911-1
Suite 1400
128 South Tryon Street
r" harl&+r Mr' 7Q;IY?.CfA1
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',/(2)
r 2}
May 28. 1998
Ms. Julie McLelland, P.E.
Project Manager
Charlotte -Mecklenburg Utilities (CMUD)
5100 Brookshire Blvd.
Charlotte, NC 28216
Subject: Irwin Creek WWTP — Facilities Upgrade Project
Secondary Clarifiers and Trickling Filters — Design Documents
HDR Project No. 00121-300-018
Dear Ms. McLelland:
Five copies of the design documents for the Irwin Creek WWTP Secondary Clarifiers and
Trickling Filters Project were delivered to you on May 21, 1998. These documents were
provided for your distribution to the appropriate CMUD design team review members. To
maintain our proposed schedule, we request CMUD review and comment by June 2, 1998. A
progress meeting has already been established for 1:00 p.m. on June 2, 1998 to afford CMUD the
opportunity to transmit written comment for discussion and subsequent incorporation.
In addition to CMUD's review of the design drawings and specifications. the following sections
outline the project history, design criteria, and layout of facilities.
Project History
HDR Engineering, Inc. of North Carolina's (HDR) 1991 Irwin Creek WWTP Prelimin 'v
Design Report recommended the construction of two 110-foot diameter clarifiers with a 14foot
side water depth (SWD). These clarifiers were proposed as an addition to the four existing 100-
foot diameter clarifiers which have a 9-foot SWD. Additional return activated sludge (RAS),
waste activated sludge (WAS), and scum pumping facilities were also planned along with the
new clarifiers. At that time, the proposed clarifiers and pumping facilities were deferred from
the proposed capital improvements program.
Recent process developments, including effluent filter problems and high secondary effluent
suspended solids, have led CMUD staff to pursue the construction of additional secondary
clarifiers. Project Team discussions have determined that the existing clarifiers should be
abandoned based on the age and reliability of the equipment. the shallow SWD, and difficulty in
achieving adequate flow splits. As a result, three new clarifiers and an RAS/WAS pumping
station with firm capacity to handle the plant's permitted and maximum day loadings are now
proposed for construction. Consideration has been given to adding a fourth clarifier with respect
to underground piping and concrete construction.
CMUD operations staff has elected to operate only two of the plant's four existing trickling
filters. The trickling filters are currently equipped with rock media with an approximate 4-foot
HDR Engineering, Inc.
Employee -owned
Suite 1400 Telephone
128 S. Tryon Street 704 338-6700
Charlotte, North Carolina Fax
28202-5001 704 338-6760
t echlett secclart f. doc

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Ms. Julie McLelland, P.E.
May 28, 1998
Page 2
depth. The proposed plan for the trickling filters is to replace two of the units with random -
dump type, plastic media. This media will have a higher (BOD) removal rate than the existing
rock media. During review of this modification. it was determined that the structural integrity of
the trickling filter walls was in question. Concrete coring was initiated for the walls and floor in
the trickling filter area. It was determined that the concrete structural walls must be replaced in
Trickling Filter No. 1. while the existing walls for Trickling Filter No. 4 were adequate.
Trickling Filters Nos. 2 and 3 will require structural wall modifications in the future. should they
be placed back into service.
Basis of Design - Secondary Clarifiers and RAS/WAS Pumping Station
The Secondary Clarifiers are sized for firm capacity of the plant's rated flow and maximum day
conditions. The physical parameters associated with the clarifiers are as follows:
-4 Inside Diameter -
- Effective Diameter -
-+ Surface Area -
- > Weir Length -
- Side Water Depth -
-* Weir Elevation -
-+ Volume -
125.00
(ctrevc to w f-AA-)19.67
12.271
376
14.33
600.90
1.31
The Irwin Creek WWTP design flow characteristics
feet 71 r /2 272
feet (5 ' i4 ; i0' ? 1,0'..
square feet (each)
feet (each)
feet
MSL
million gallons (each)
175, E>S8
are as follows: 'a
Annual Average Daily Flow - 12.0 MGD = 3 - 4. o M p
Maximum Month Daily Flow - 15.0 MGD = 3 i (.)
Maximum Daily Flow - 30.0 MGD / 0_0 V,
(maximum daily flow is conservative considering future flow equalization facilities).
1
=-;;. g
These flows result in the following process parameters at full capacity and firm capacity
(i.e. with one clarifier out of service):
Full Capacity
Average Day
Max Month:
Max Day /
Overflow Rate, gpd/sf*
V 326
V97
`'815
Detention Time, hrs
t/ 7.8
✓ 6.3
✓ 3.1
Weir Loading Rate, gpd/lf
✓10,638
-713,297
‘z 26.596
Firm Capacity
Overflow Rate, gpd/sf
✓488
'7611
✓1,222
Detention Time, hrs
/5.2
�4.1
-2_1-
Weir Loading Rate, gpd/if
A5,957
`19,946
V'39,894
*Does not include RAS flow.
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Ms. Julie McLelland, P.E. ( role -
May ' - ' { Q Vic-
28, 1998 '2- '(aQ.,. _-- ''`' 6hi A 1.
Page 3 ��= —: ; " `� , �;'-
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7
The maximum RAS rate will be 10Q% of the desi n maxum month flow. 'or-- -1-5.7Q The
WAS capacity will be designed at 330.000 gallons pe day (GPD) based on the maximum
average daily wasting rate from the 1991 Predesign Re ort. olids flux will vary with
process operation and flow .resulting in an estimated range f 8 to 281bs%dar sf . full capacity.
Facility Site Layout — Secondary Clarifiers and RAS/WAS Pumping Station
The new secondary clarifiers and RAS/WAS pumping station will be located in the space
occupied by the existing, yet abandoned, polishing lagoon. CMUD is currently in the process of
dredging and removing residual sludge from the lagoon, and will then dewater the lagoon for
construction of the proposed structures. Excavation of material will be required in this area to
allow the secondary clarifiers to be placed in the appropriate elevation along the plant's
hydraulic grade line to allow gravity flow both into and out of the new clarifiers.
Two existing plant access drives will be extended to serve the new secondary clarifiers and
RAS/WAS pumping station. One drive will be extended by the effluent filters. while the other
drive will be extended beyond the plant's maintenance building. These access drives will be
constructed of asphalt paving along the north, east, and west sides of the proposed structures,
while a gravel access drive will be installed around clarifier No. 3 and future clarifier No. 4.
This gravel access road will provide plant access around the entire perimeter, and allow access to
new electrical equipment pads on the south side of the new construction.
station.
Site Piping — Secondary Clarifiers and RAS/WAS Pumping Station
Influent to the new secondary clarifiers will be conveyed by a 42-inch line from aeration basins
Nos. 1-6. This flow will join with flow from aeration basins Nos.7-11 at a new,b
.junctionox, and
then the combined flow will be conveyed to the secondary ifiers via a 60-inch line. The 60-
inch line will discharge into a new arafier er box hich will be constructed
along with the clarifiers and pumping station using common wa1I^construction. Flow from the
splitter box will discharge into one of three 36-inch influent lines to each clarifier. The splitter
box will be equipped with three 8-foot downward acting weir gates to facilitate equal flow
control to each clarifier. The splitter box will have a fourth overflow chamber for construction
of a futurefourth clarifier.
Site drainage will be conveyed by drainage ditches, and storm sewer piping to a 30-inch pipe
leading to Irwin Creek. Since the disch e--ele _ 'on o he pipe___will_be-.lower than the creek
surface eleva ion during. flood events alv�J
1 the placed on the . -' ch line jo inhibit_
backflow of water from the creek to the area of the new clarifiers and RAS/WAS_ umpi _
Effluent from each clarifier will be routed into a common 48-inch header pipe. The 48-inch
header pipe will have two discharge locations — a new 48-inch line that ties into an existing 42-
inch line, and a new 36-inch line that ties into an existing 24-inch line. Both of the existing tie-
techkttsecclanfdoc

Ms. Julie McLelland, P.E.
May 28, 1998
Page 4
in locations feed back to the plant's existing liquid stream process just upstream of the chlorine
contact chamber.
Return -activated sludge will be piped via a 24-inch line from the new RAS/WAS pumping
station back to existing Manhole No. 16. which will permit RAS to be mixed with aeration basin
influent flow prior to splitting into the aeration basins. Waste -activated sludge will be piped via
an 8-inch line to the anaerobic digesters. Adequate pipe extensions and valuing will be provided
to allow for future connection of the 8-inch WAS line to the proposed WAS thickening facility.
Several hydraulic modifications a bee deemed necessary for construction of the new
secondary clarifiers. First, the two wei e_existin chlorine contact chamber must be
lowered by a t - _ es to pxoxide--additional hydraulic head in the secondary
e . Second, individual aeration basin discharge pipes from basins nos. 1-6 should be
modified for side discharge to eliminate vortexing problems and provide additional hydraulic
head on the influent side of the secondary clarifiers.
Secondary Clarifier and RAS/WAS Pumping Station Layout
The secondary clarifiers and RAS/WAS pumping station have been designed for common wall
concrete construction with the RAS/WAS pumping station nested between the three clarifiers.
The RAS/WAS pumping station has double -tee roof construction, that provides access to each
clarifier bridge and to the electrical room, which is located in a masonry enclosure on top of the
pumping station. The electrical room will house the motor control center.. PLCs. VFDs, and
electrical panels.
The upper level of the complex has three stair access points on the north, south, and west sides.
The lower level has two means of entry and egress. The primary means of egress is located on
the south wall and is enclosed along with the electrical room. The secondary/emergency means
of egress is an alternating tread type system on the east side of the complex and is located
outdoors.
The secondary clarifiers will be equipped with high -rate spiral scraper sludge removal
equipment. The sludge will be transferred to a center hopper, which will feed into a concrete
encased 18-inch RAS/WASlinelbelow each cl iIr anTl elow the pumping station slab. The
clarifiers will also be equipped with ducking skimmer type scum removal mechanisms which
discharge into individual scum wells. The second clarifiers are desi ned with inboard
f-.. launders and a solids density current baffle around the perimeter of the launder._ Access
center p a o ill be via an alum` inum grated walkway from the top of the pumping station.
G`�-6ett
The S and WAS pumping systems are manifolded off of a common suction header. The total
RASJim.pumping capacity is 15.0 MGD (5.0 MGD per ip ). The RAS pumping system is
designed to have one horizontal non -clog _centrifugal pump designated for each clarifier, :with a
standb um rovided-to serve as backup for anyone of units. The RAS pumping system
r�—X-P p- P p P P g Y
includes VFDs on each unit and is metered using a magnetic -type flowmeter on the common
so, LOOS 1 As uri
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techlettseccitnf.doc .tV fi a r l LA e RA 5 ?it ,t ' 'O (' 4O {- ` !u 1�

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Ms. Julie McLelland, P.E.
May 28. 1998
Page 5
discharge line. Operator setpoints for RAS rates will control total flow from the pumping station
and can be established locally or through the plant's superviso y control and data acquisition
system (SCADA). v, 3 - 330 : ytY?
The WAS pumping system is designed to have one horizontal non -clog centrifugal pump
designated for each clarifier. TheAol. WAS pumping capacity required is 330.000 GPD. Each
WAS pump, however, will have a capacity of 432,000 GPD to match the design criteria of the
future gravity belt thickeners. No standby pumping capability is provided since additional WAS
flow can be accomplished by wasting greater amounts of flow from the remaining units. WAS
flow, metered with a magnetic type flowmeter, will be transferred to the anaerobic digesters
(interim) or the proposed WAS thickening facility (future). Operator setpoints for WAS rates
will control total daily WAS flow and can be input locally or through the plant's SCADA
system.
The proposed design includes chlorination of each individual RAS pump suction line.
Chlorination of RAS will be manually controlled by the opening and closing of valves. An
emergency eyewash and shower station are proposed in the lower level for safety considerations
associated with chlorine solution.
The scum pumping system will operate off of float switches in each individual scum well (one
per clarifier). The wetwells will include a spray system to reduce foam. The scum will be
pumped from the wetwells with progressing cavity pumps rated at 50 GPM each (one per
clarifier) into the RAS bypass line. With the influent going into the bypass line, the flow will not
be metered as RAS, but will be transferred back to the head of the aeration basins. If necessary.
the scum flow can be routed to the WAS discharge line to remove it from the plant's liquid
process stream. In the event of a scum pumpsg off line, backup capability is provided by
interconnecting the wetwells and suction piping to allow for any one of the pumps to be out of
service and still maintain scum flow.
Maintenance access from the lower level will be achieved with a motorized, constant -speed
monorail located above the RAS and WAS pumps. The monorail can move the equipment to a
location adjacent to a roof hatch in the upper level. From this point, a movable cart can be used
to center the equipment under the hatch where a jib crane will be used to hoist equipment
through the roof hatch and deposit the equipment on a maintenance vehicle. Scum pumps will
be conveyed to the hatch area with movable carts.
Trickling Filter Modifications - Basis of Design
The CMUD operations team has selected random dump type plastic media for replacement of the
existing rock media in trickling filters No. 1 and No. 4. As compared to rock media, the plastic
media is lighter, provides more void space, and provides a greater surface area for bacterial
growth. It is expected that the random media will result in higher BOD removal efficiencies than
currently achieved by the traditional rock media trickling filters. Once the rock media is
removed, the plastic media will be placed in the filters to a depth of approximately 4.5 feet near'
techlettsecclart£doc

Ms. Julie McLelland, P.E.
May 28, 1998
Page 6
the effluent channel of the filters,_and decreasing to approximately 4 feet at the high point of the
10 40 �aGk - , � T
GYP 3
floor.
`VP
In addition to the replacement of the rock media, the existing vitrified clay underdrain tile will be
replaced with polypropylene underdrain panels. Similar to the plastic media. the underdrain will
provide greater void space for enhanced air circulation through the filters. Vent pipes will be
placed at 10-degree increments around the filter to further enhance air circulation.
Based on manufacturer -supplied data, the original hydraulic capacity of the rotary distributor
arms was approximately 10 MGD to each filter3with an available working head of 3.6 feet in the
center column. Recently, the original 28-inch tubs of the rotary distributors were replaced with
30-inch tubs. Larger tubs, combined with modifications to the distributor arms will likely result
in increased hydraulic capacity. Based on the dimensions of the plastic media. the original
capacity of the rotary distributors, and plant data over the past three years. the following process
operating parameters have been dete ed: 6t r �l
6�5)C
Description
' Average Day
Maximum Month
Peak Flow*
Plant Flow (MGD)
✓12 -
'715
`Y?r0
Hydraulic Loading (gpl/ft2)
0.35
1i0.42
rA:57
CBODS Loading (lbs/1000 ft'-d)
115 S 7. S
149 -74 , s-
198
99
T-SS Loading (mg/1)
115
120
120
Air Temperature
48 to 85 °F
Wastewater Temperature
0 to 110 °F
/ Maximum total flow to two filters based on original hydraulic capacity of rotary distributors.
Tyr. {?-*
0.2- -1. 2o_
Under average conditions, the random dump plastic media has a projected BOD removal rate in
the range of 40% to 70%, depending on wastewater and air temperatures.
We appreciate your review regarding the design information presented herein and the contract
documents previously submitted. We look forward to our continued cooperation and success
with CMUD on this Project.
Best regards,
HDR Engineering, Inc. of North Carolina
L. Kevin Mosteller, P.E.
Project Manager
; 'Ark, d
A
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