Abstract :
Reduction of biomass production coefficient is an ideal solution for the reduction of excess sludge especially in
industrial wastewater treatment plants. Studies were carried out in two sequencing batch reactors, which were
controlled online. After presenting a stable situation in reactors, during 24 months of the study, sampling and
examining of chemical oxygen demand (COD), biochemical oxygen demand, pH, sludge volume index (SVI),
specific oxygen uptake rate (SOUR), remaining pNP, and biomass yield (Y) were implemented.
Results have shown that among different retention times (5, 10, 15, 25 days), maximum COD removal efficiency
(95%) was achieved in 10 days, without bulking and foaming problems. In 10 days of sludge retention time,
average Y and kinetic coefficient (Kd) were calculated: 0.58 mg biomass/mg COD and 0.058 1/day, respectively,
and correlation coefficient (R2) was 0.98. Different concentration of pNP were used due to energy spilling effect in
the reactor and the results show that injection of 100 mg/L pNP to the reactor can reduce synthetic coefficient Y
from 0.58 to 0.27 mg biomass/mg COD without pNP injection, so that the excess sludge was reduced by 0.56%.
Although, an increase of 193 mg/L soluble COD in the effluent was observed. On the other hand, in this
concentration of pNP, SOUR rate reached 31 mg O2/h/g volatile suspended solids, and SVI rate reached less than
48 mL/g. In the concentration of 150 mg/L pNP, no sludge was produced, but COD rate of the effluent increased
to 480 mg/L. Otherwise, pNP rate is an environmental limitation in effluent and sludge disposal.
