NC0057151_NC0057151_ATC Design Calculations_20171205_20171205

DESIGN CALCULATIONS
SEWER SYSTEM IMPROVEMENTS
WWTP AND I-26 INTERCHANGE
TOWN OF MARS HILL
MADISON COUNTY, NORTH CAROLINA
CA
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MIKE
MIKE DOWD, PE _ ¢ SEAL
024487
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McGill
ASSOCIATES
Engineering • Planning • Finance
Asheville, North Carolina
NOVEMBER 2017

WWTP Design Flow:
NPDES Permit No. NC0057151
Discharge Limit = Design Daily Flow = ddf = 425,000 gallons/day (295
gallons/minute)
Average Annual Daily Discharge = 160,000 gallons/day (111 gallons/minute)
Design Return Activated Sludge Flow:
Return Activated Sludge Flow = 50% to 150% of design average flow per Ten State
Standards 92.41
RAS Q = 148 to 443 gallons/minute (37 to 111 gpm per sludge eductor)
Return Sludge Pipe Capacity
The controlling (smallest, and longest) return sludge line connects Clarifier #2 to the
digester. This return sludge line must be capable of carrying half of the RAS, or up to 222
gpm.
Pipe Diameter = 6 inches
Inlet elevation = 2201.99'
Outlet elevation = 2202.15'
Length = 20.68 feet
Slope — 2202.15-2201.99 = 0.0077
20.68
RH _ diameter (in) = 6 = 0.125
48 48
1.486 2 1 1.486 2 1 ft
V = x (RHP x S2 = x (0.125)3 x (0.0077)2 = 2.51-
0.013 0.013 s
ftn(12)2 ft3 7.48 gall (Eos l
Q = VA = 2.51 s X 4 = 0.49 s ( ft3 I `mint = 222 gpm
Air Lift Pump Capacity
The previous Authorization to Construct application for the WWTP expansion sized the
sludge holding basin's blower at 225 cfin, with 200 cfin required for basin aeration and 25
cfin to operate two sludge air lift pumps. Consequently, 50 cfin should operate four air lift
PUMPS.
The McGraw Hill Pump Handbook, 1St ed. by Karassik, Messina, Krutzsch, & Fraser states
that —0.15 cfm is required to pump 1 gpm for an air lift pump with a Head/Submergence
W

ratio of 0.4. The amount of liquid that can be pumped per cfm increases as H/S ratio
decreases, implying that for this application, the air required should be less than 0.15
cfm/gpm.
Head= 3 ft
Submergence = 17.3 ft (according to original design drawings)
H/S Ratio = 0.17
443 gpm - 4 air lift pumps = 111 gpm/pump
50 cfm - 4 air lift pumps = 12.5 cfm/pump
12.5 cfm_ cfm
111 gpm 0.11 gpm
Transactions of the American Institute of Mining and Metallurgical Engineers Vol. 63, p.
444 discusses calculating air requirements in the article "Unwatering a Mine by Air -Lift,"
and gives an air lift formula attributed to the Ingersoll-Rand Co. as follows:
Li = length of the lift section (ft) = 3
LS = length of the submerged section (ft) = 17.3
C = constant that depends on Li (given as 245 for 10-60 ft of lift) 245
_ 0.8L1 _
Va L34
Clog10 (s+ 34
0.8(3)
245log10 173+34
34
Air= 0.055 CFM X 443 Lal = 24.3 CFM
gal min
=
CFM
0.055
gal
While it may not be appropriate to extrapolate given that this formula is meant for a
minimum lift of 10 ft, this result as well as the previous citation suggests that a total of 50
CFM should be at least adequate for the return sludge flow required.
Air Lift Pump Discharge Pressure
While the intake of the sludge eductors extends almost to the full depth of the sludge
hoppers, the proposed PD blower will discharge into sludge eductors at a depth of
approximately 12 feet according to design drawings.
12 ft depth
2.31 rt
psi
= 5.2 psi
Head loss through blower silencer, inlet filter, air piping and fittings = 0.5 psi
Pressure = 5.2 psi + 0.5 psi = 5.7 psi = 6.0 psi
3

Main Aeration Blower VFD Controls
As the aeration basins are currently operated, dissolved oxygen in aeration basin #3 is
frequently observed to be 3.0 mg/l or higher. Operators would prefer to maintain dissolved
oxygen concentrations closer to the 2.0 mg/l minimum stated in Ten State Standards 92.33.
The main blowers for the plant are two 75 hp Roots 616J blowers driven by 1750 rpm
motors with belts and pulleys
Motor pulley dia. = 18"
Blower pulley dia. = 13"
Blower speed = 1750 rpm (13„) = 2420 rpm
Pressure at the blowers will be the sum of piping losses, water pressure at the diffusers,
and head loss through the diffusers.
Diffuser depth = 11.25 ft
11.25 ft depth
2.31 ft
= 4.87 psi
psi
Head loss through coarse bubble diffusers = 0.5 psi
Head loss through blower silencers and inlet filter = 1.0 psi
Head loss through air piping and fittings = 0.5 psi
Pressure = (4.87 + 0.5 + 1.0 + 0.5) psi = 6.87 psi
According to the pressure -performance curves provided by the manufacturer for the
existing blowers, the existing blowers should discharge approximately 1600 cfm at 7.0 psi.
The replacement of the existing motors with newer 75 hp motors on variable frequency
drives capable of turning the motor rpm down from 1750 to 700 rpm or slower would
permit the blowers to discharge at any point on their curve.
2

REP
800-536-9933
www.pdblowers.com
2500
2000
LL
LL
U
a 1500
3
0
J
u 1000
F
W
J
Z
500
04
250
LL 200
W
w 150
a
a 100
m
IL
-u 50
0
160.00
140.00
PRESSURE PERFORMANCE
FRAME 616 RAM WHISPAIR &
WHISPAIR GAS PUMP
MAX. PRESSURE RISE = 16 PSI
MAX. SPEED = 3000 RPM
PERFORMANCE BASED ON AIR,
INLET AT 14.7 PSIA & 68°F
AUGUST, 1998
4 PSI
8 PSI
12 PSI
15 PSI
120.00
a 100.00
W
LO
It 80.00
0
X
W 60.00
Y
C 40.00
20.00
0.00
500
1000 1500 2000 2500 3000 3500
SPEED -RPM
RC -19-616R
Rotating Engineered Products Inc.. 2405 Murphy Blvd • Gainesville • GA • 30504
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