CIVL 456: Wastewater Treatment
Homework #6- Fall 1996
Due Date: 14 October 1996
Problem 1
Suppose that you are in charge of the West Lafayette WWTP
and that you adjust the secondary systems SRT in order
to evaluate the corresponding impact on MLVSS in the aeration
tank. You should make the following assumptions:
HRT = 3.5 hours
Influent CBOD = 180 mg/L
Theo. Yield=0.5;
Max. Specific Substrate Utilization Rate: 15 mg CBOD/mg MLVSS-day
Death and Decay Constant = 0.05/day
Influent Flow = 8 MGD
Waste Sludge Concentration = 3 x Reactor MLVSS (assume to be constant!)
Determine the steady-state values for observed
yield, effluent substrate (CBOD) concentration,
reactor cell mass (MLVSS) concentration,
and expected secondary sludge mass (lbs per day)
for varied SRT's of 2, 4, 8, and 20 days.
In each case, you'll need to run through the following
stepwise solutions: 1) get the spec. growth rate based
on HRT, 2) get the effluent CBOD concentration, 3) get the
observed yield, 4) get the reactor MLVSS concentration,
5) get the effluent waste sludge flow value, and 6) derive
the waste sludge poundage.
HOW-TO-HINTS First, for each SRT, determine the spec. growth
rate. Second, determine the maximal spec. growth rate, using Yt
and the maximal specific substrate utilization rate. Third, for
each spec. growth rate, determine the corresponding value for
effluent (and reactor) 'S'. Fourth, determine the reactor 'X'
concentrations based on the SRT, HRT, S, Sin, and Yobs.
Fifth, determine the waste mass based on flow, substrate
removed, and Yobs.
Problem 2
Suppose that you run the aforementioned system at an SRT of
8 days. Determine the corresponding requisite aeration horsepower
based on: a) the horsepower required to mix this reactor volume (assuming
0.1 HP per 1000 gallons of reactor volume), and b)
the oxygen requirement for the given reduction in CBOD. You
should assume that their new fine-bubble diffuser system provides
an oxygen transfer capacity of 3.5 lbs of oxygen per HP per hour
(as measured at a zero D.O.), that they want to maintain a minimum
D.O. in the aeration system of 2.0 mg/L, and that the maximal D.O.
for their wastewater (at a worst-case [i.e., least oxygen
solubility] temperature of 29 degrees C) would be 7.1 mg/L.
HOW-TO-HINTS First, determine HP based on the volume.
Second, determine HP based on oxygen demand. Figure out how much
CBOD is being used at the 8 day SRT, and then compute an hourly
oxygen demand (perhaps using a 10 to 20% safety margin for oxygen
demand versus CBOD demand). Then compute the actual oxygen transfer
rate based on the existing D.O. (which will slightly reduce the
maximal transfer efficiency which manufacturers quote for a D.O. of
0 mg/L. Finally, divide the demand by the efficiency and determine
the necessary HP.
Last Modified: 5 October 1996; alleman@ce.ecn.purdue.edu