unit includes fat traps, influent pit, grit chamber, and
two balancing tanks with mechanical aerator. In the
balancing tank, a part of recycle sludge is mixed and
aeration is carried out, which results in some degradation
of organic material present in the wastewater.
Therefore, wastewater from the outlet of the balancing
tank was not used for this study. So the sample was
collected from the outlet of the fat trap.
The laboratory-scale reactor was operated for
about 90 days; by varying the hydraulic retention
time of 2–12 days, the corresponding flow rates are
in Table 1.
Before the start of the work, the reactor was
seeded with the sludge taken from the secondary
treatment unit of Nestle treatment plant for three days.
The influent was subjected to settling in the effluent
bottle. Due to lack of mechanical return sludge
facility, the settled sludge was daily removed from
the final clarifier in a beaker. The nitrogen (N) and
phosphorus (P) requirements are based on the BOD
of the wastewater, where a BOD: N: P of 100: 5: 1
is considered adequate11. In this study, the average
value of BOD was 1520 mg/l. Thus the BOD: N: P
for this wastewater came out to be 1520: 310: 3.3 or
100: 20: 0.22. These calculations show that sufficient
amount of N was present in the wastewater. But
the P was 0.22 as against the desirable value of 1.
Hence it was deficient. The deficiency was met by
adding a calculated amount of potassium di-hydrogen
phosphate (KH2PO4) salt.
Flow, temperature, and pH values of the reactor
were measured daily to ensure favourable environmental
conditions in the reactor for biological
treatment. Mixed liquor volatile suspended solids
(MLVSS) in the reactor, COD, and BOD5 of influent
and effluent were measured thrice a week to determine
kinetic coefficients. All the tests were performed according
to the procedures laid down in the “Standard
Methods”12.