Project #41706 - Wastewater Treatment

1. The following are related to design and operate the activated sludge process for wastewater treatment (2 marks for each problem).


Q: influent flow rate (m3/d)

Qr: flow rate for recycling (m3/d)

Qw: flow rate for waste ((m3/d)

X: active cell conc. (g VSS/L)

S:  substrate conc. (g phenol/L)


1) The activated sludge process is operated at steady state.

2) Let’s assume the wastewater contains only the phenol without any other contaminants, cells and solids. 3) The Hawaii State Environmental Protection Agency (EPA) regulates release of phenol below 20 mg/L.

4) The following are the biokinetic constants and operating conditions.

So (inlet substrate conc.) : 10 g/L phenol

Ks=10 mg phenol/L,   k (max. specific substrate utilization rate) = 1 g phenol/g VSS-d

Y=0.40 g VSS/g phenol,   fd (fraction of cell debris) = 0.15 g VSS/g VSS

Kd (cell decay constant) =0.1 g VSS/g VSS-d

Q (influent flow rate) = 1000 m3/d,   inlet nb VSS (Xo,i)=30 g/m3

Hydraulic retention time = 0.179 d-1     Xin(cell conc. in influent)=Xe (cell conc. in effluent)=negligible








1-1. Determine the sludge retention time (SRT) to meet the Hawaii EPA’s regulation.


1-2. Calculate the active cell concentration (X, g VSS/L) in the reactor.


1-3. Calculate total solid production rate (PXT,VSS, g VSS/d) using the above conditions. The outlet concentration of phenol is set to 20 mg phenol/L.


1-4. If the sludge recycle ratio is changed from 0.67 to 0.5 while maintaining the same effluent phenol concentration, the same reactor volume and the same Xr (cell conc. to be recycled), how will the active cell concentration (X, g VSS/L) in the reactor


1-5. What is the sludge recycle ratio?



2. You are ready to design an activated sludge process (2 marks for each problem)..


The influent values are: Q=4000 m3/d, S0=300 mg BOD/L, Xin=0


The biokinetic parameters were obtained from the lab tests and stoichiometry:

Y=0.4 g VSS/g BOD

k=22 g BOD/g VSS-d

Ks=200 mg BOD/L




The design factors are:   safety factor (SF)=60, MLVSS = 2500 mg/L


2-1. Calculate SRT,min.


2-2. Calculate SRT, design.


2-3. Calculate S (effluent BOD).


2-4. Calculate the kg of substrate (BOD) removed per a day.


2-5. Calculate X (in the reactor).


2-6. What is the sludge recycling ratio in this design?






Subject Science
Due By (Pacific Time) 09/30/2014 09:00 am
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