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Department of Civil Engineering-I.I.T. Delhi
CEL796 Biological HW (Sem 2, 2013-14)
(no submission)
Q1. An organic waste with 250 mg/L BOD5 (S0) needs to be treated using a completely-mixed activated sludge
process. The effluent BOD is to <=10 mg/L (Seff). Assume that the temperature is 20°C, the flow rate is 5 million
gallons/day and following information are available:
(i) Influent volatile suspended solids to reactor are negligible.
(ii) Return sludge concentration = 10000 mg/L of suspended solids = 8000 volatile suspended solids
(iii) Mixed-liquor volatile suspended solids (MLVSS)=X=3500 mg/L
(iv) Mean cell-residence time (θc)= 10 days
(v) Microbial parameters: kd =0.06/day; Y=0.65 mg cells/mg BOD5 utilized.
(vi) Effluent has 20 mg/L biological solids (here 80% is volatile and 65% is biodegradable). Assume that the
biodegradable biological solids can be converted from ultimate BOD demand to a BOD5 demand using the
factor 0.68 [i.e., BOD K value =0.1/day (base 10)].
Calculate (i) Biomass production rate; (ii) Observed yield (i.e., Y/ [1+kdθc]); (iii) Food to microorganisms ratio
(i.e., S0/ [Xθ]).
[2×3=6 points]
Q2. Assuming that the endogenous coefficient (kd) can be neglected, develop expressions for determining substrate
and cell concentration as a function of time for a batch reactor. If the initial concentration of substrate and cell is
100 and 200 mg/L, respectively, calculate the amount of substrate remaining after 1 h. If the endogenous
coefficient is equal to 0.04/day, estimate the error made be neglecting this factor. Assume: k=2/h; Ks = 80 mg/L;
Y=0.4 mg/mg.
[4+4=8 points]
Q3. An anaerobic digester is designed to remove 85% of BOD5 of an industrial organic waste with an ultimate
BOD =2000 mg/L. If (θc) = 10 days, estimate the amount of sludge to be wasted daily and the quantity of gas
produced each day. Assume that the flow =0.1 million gallons/day; Y=0.1; kd=0.01/day.
[3+3=6 points]
Q4. In the activated sludge reactor, bacterial oxidation and synthesis can be described using Eq. (1) and its
endogenous respiration can be described using Eq. (2). Calculate amounts of oxygen required for oxidation and
synthesis of 3500 mg/L MLVSS (i.e., bacterial concentration)?
Oxidation and synthesis (in presence of bacteria):
COHNS (organic matter) +O2 + nutrients  CO2 +NH3 + C5H7NO2 (new cells) + other end products (1)
Endogenous respiration (in presence of bacteria):
C5H7NO2 (cells) + 5O2  5CO2 +2H2O+ NH3 + energy
……
……………(2)
Q5. From Q3, how much gas and sludge will be produced if the θc is increased to 20 days?
Q6. Calculate amount of methane produced per mg of ultimate BOD5 stabilized during anaerobic digestion
process? Assume the starting compound is tricarboxylic acid (C3H8O6).
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Q7. Compare organic matter degradation of distillery industry wastewater (i.e., spent wash) using eight different
anaerobic reactors. Also comment on methane gas production from each of these eight reactors.
Q8. Compare COD removal effectiveness of different treatment types.
Q9. Comment on removal of Salmonella over time at different locations of wastewater treatment plant (anaerobic
sludge blanket).
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Q10. Compare concentrations of fecal indicators and pathogens at different locations of wastewater treatment plant
(anaerobic sludge blanket).
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