The following figure is from an article on a bench-scale process to treat wastew

Question

The following figure is from an article on a bench-scale process to treat wastewater. A-stage HRAS is aerobic tank but with a short SRT (~3 days). B-stage AVN is also aerobic but with a much longer SRT (~10 days). An engineer now decides to modify the A-stage of the treatment so that it can achieve nitrogen removal via simultaneous nitrification and denitrification. The idea is to operate A-stage treatment at a low DO concentration of 0.2 mg/L to promote simultaneous nitrification and denitrification. The target of the effluent NH4-N concentration from Astage HRAS is 0.5 mg/L-N, and the whole system operates at 15 °C. Given the following nitrification kinetic information:
a) What SRT is required?
b) Comment on whether you think the A-stage HRAS would take more or less volume compared to the alternative MLE process?
c) How would this simultaneous nitrification and denitrification impact the Anammox process in B-stage?
Kinetic coefficients at 20 oC
nm=0.75 g VSS/g VSSd q for nm = 1.07
Kn=0.74 mg NH4-N/L q for Kn = 1.053
Yn=0.12 g VSS/g NH4-N q for kdn =1.04
kdn=0.08 g VSS/g VSSd q for Ko = 1.00
Ko=0.5 mg O2/L

Explanation / Answer

Answer:

a) high level of denitrification may be achived for DO conc. used 0.1 to 0.4 mg/ l however long SRT are to be used

Initial SRT of stage A = 3 days

As a function of DO conc. the rate of nitrification in stage A is given by

n = [(nm*N/Kn + N)* (DO/DO+Ko)] - kdn

SRT = 1/ n

n = 0.66

srt = 1.51 days

b) MLE process will require more volume for the same given process as the amount of recylce required to obtain same amount of nitrogen removal is high increasing the volume of the reactor

c) the load on the anammox process will increase due to higher ammonia load leading to higher cost of removal in the stage B

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