CVEN 5534: Wastewater Treatment
Spring 2008
Assignment 3: Due Thursday, February 21
Using the growth-decay stoichiometry and rate matrix on the next page:
1. Wastewater containing 300 g/m3 COD is to be treated for 90% COD removal. The reaction rate
is assumed to be first order with respect to COD concentration because the concentration of cells
is constant.
rS = -kSS
where: rS = rate of COD consumption (g/m3/d), k = first-order reaction coefficient = 5 d-1, and SS
= COD concentration (g/m3)
a. Compare the required system volumes for
i. An ideal single tank CSTR
ii. An ideal plug flow reactor
iii. A CSTR cascade with 2 cells
iv. A CSTR cascade with 4 cells
b. How will the ratio of required volumes change for 99% removal?
c. What would the relative volume ratios in part a be for a zero-order reaction: rS = -k’ where the
zero-order reaction coefficient, k’ = 75 g/m3/d?
2. Chlorine contact basins for disinfection of water are typically designed as a plug flow reactor?
Why is that? (Explain by means of formulas, graphs or text.)
3. Develop rate expressions for:
a. Net growth rates for heterotrophic and autotrophic (nitrifying bacteria)
b. Net consumption/production rates for soluble COD, ammonia nitrogen, nitrate, and
oxygen
4. Develop a formula for the steady-state concentration of viable cells, “X,” as a function of, So,ˆ
, Ks, b, and for the chemostat example problem worked in class. Using the conditions in the
problem what is the cell concentration for a hydraulic retention time of 50 hours?
5. A completely mixed lagoon treatment system is to treat wastewater with 450 mg/L influent
COD. The discharge permit requires that maximum effluent COD ≤ 45 mg/L. Assume that the
heterotrophic cell growth rate is described by Monod kinetics, withˆ = 1 d-1, Ks = 75 mg/L
COD, and b = 0.03 d-1. The wastewater design average flow rate is 400 m3/d. Assuming that
COD is the rate determining substrate and that the cells can be modeled as CSTR’s, compare the
required total volume for a one-cell lagoon to that required for a two-cell lagoon in order to meet
the discharge COD standard.
a. calculate the effluent concentration of cells for the one-cell and two-cell lagoon systems.