Abstract:
The Activated Sludge Process (ASP) is the largely used biological treatment of wastewater,
fundamentally because it is a cheap technology, which can be adapted to vast range of
wastewater. In the ASP, a bacterial biomass suspension (the activated sludge) is responsible for
the removal of pollutants. Biodegradation of organic matter plays a key role in ASP to treat the
wastewater. A mathematical model was developed in order to describe the degradation
coefficient and order of decay in organic matter and nutrients. Experiments were conducted in
pilot-scale activated sludge reactor under steady-state conditions and varying Hydraulic
Retention Time (HRT) (6, 12 and 18h). Peristaltic pump was used to automatically supply
wastewater to the reactor from the feed tank and sludge was approximately 100% recycled.
Biochemical Oxygen Demand (BOD) was measured to carry out the mathematical modeling for
ASP. Average temperature during the experiment was maintained between 27.2-28.2o C. stating
from mass balance equation and derive differential equation for calculate the BOD degradation
coefficient and order of the decay. Derived equation cannot solve analytically and 4th Order
Runge-Kutta equation used to performed the mathematical model. The model was run under
different t values in order to determine the suitable t value for each HRT. t=1 take long time
to run the process and t=8 take short time to run the process. Then t=3 chose as best t value
for this ASP. Order of decay and degradation coefficient of BOD at t=3 ware (0.9≈1.0) mgL-1d-
1 and 0.1 d-1 respectively under HRT 6h whereas (0.7≈1.0) mgL-1d-1 and 0.1d-1 respectively under
HRT 12h. In HRT 18h, order of decay was (0.1≈0.0) mgL-1d-1 and degradation coefficient was
16.0 mgL-1d-1. Results showed that first order process take place at HRT 06 and zero order
process take place at HRT 18. Process does not depend on the number of microorganism cell at
first order reaction and process depends on the number of cells at zero order reaction. It can
concluded that BOD degradation at HRT 18 depend on microbial cell number and BOD
degradation at HRT 6 independent from microbial cell number. Experimental data and model
data plot at t=3 clearly showed that 6 and 18h HRT experimental data remarkably fit with
mathematical model and 12h HRT experimental data fit with the model data at start and deviate
in end. It was expected that BOD degradation at 12h HRT was mix order reaction. It clearly
indicates that BOD concentration was rapidly decreased at 18h HRT. Because microorganism
had sufficient time to degrade the organic matter contain in the wastewater at 18h HRT than 06
and 12h HRTs. Rapid BOD removal caused to higher BOD rate constant at 18h HRT.
