Oxygen transfer studies in the completely mixed activated sludge process

Utilization of the activated sludge process is widespread although many of the mechanisms that make it work are still relatively misunderstood. Recent studies have indicated that dual substrate limitations may occur in the process. Several misconceptions in the basic fundamentals regarding the rates and mechanisms involved in oxygen transfer to wastewater systems also exist.

This research investigation examined the effects of the mean cell residence time and wastewater stoichiometry on the operation of the completely mixed activated sludge process under a dual substrate limitation. At low mean cell residence times (θ_c) the system was growth C limited with respect to carbon and at high mean cell residence times the system was oxygen limited. Oxygen transfer studies were conducted to ascertain the relationship between the steady state oxygen transfer coefficient (K_La) and the oxygen uptake rate of the mixed liquor (R).

The objectives of this research were accomplished by operating two continuous flow bench scale activated sludge units at COD:TKN ratios of 6.07:1 and 0.65:1. Reactor-1 was operated at a COD:TKN = 6.07:1 and was always growth limited with respect to organic carbon while Reactor-2 was operated at a COD:TKN = 0.65:1 and was carbon limited at low mean cell residence times and oxygen limited at high θ_c values. Mean cell residence time served as the primary control parameter during the laboratory studies and was varied form approximately 2.5 to 21.0 days.

Theoretical studies were also conducted in which biokinetic and stoichiometric equations were used to develop a model to simulate the process operating under carbon and oxygen limitations. The model was found to yield results that were similar to the actual experimental data collected. Further refinement of the model by including inhibition functions would result in a model with better predictability.

Examination of the experimental data collected during the laboratory study revealed several interesting conclusions. Operation of the activated sludge process at a low COD:TKN ratio (0.65:1) and under an oxygen limitation at high mean cell residence times can result in high levels of free ammonia and nitrite that will lead to a deterioration in effluent quality. Increased removal efficiencies for COD, TKN and NH₃-N can be achieved by operating the process at a high COD:TKN ratio (6.07:1). Steady state oxygen transfer coefficients determined in the mixed liquor of the reactors indicated there was a direct relationship to the oxygen uptake rate of the activated sludge (R). This observation is quite significant since standard aeration theory states that K_La is constant for a given aeration device. Nonsteady state K_La values determined on the effluent from each reactor indicated that K_La was a constant. Alpha and beta coefficients determined from nonsteady state tests on wastewater effluent from each reactor showed no trend with the mean cell residence time.