Determination of kinetic constants of activated sludge microorganisms responsible for degradation of xenobiotics

Abstract

Experiments with the degradation of morpholine (MO), sulphanilic acid (SA), nitrilotriacetic acid (NTA) and methanol (MA) were carried out in completely-mixed systems at constant volumetric loading ( B v) and variable solids retention time (SRT). In the first series, a mixed substrate composed of the above compounds was used (each compound formed 25% of the total COD), in the second, each of the above compounds was used as a sole source of organic carbon and energy. Respirometric studies were carried out with excess biomass in order to determine the following kinetic parameters: yield coefficient ( Y), half-velocity coefficient ( K s), maximum volumetric removal rate ( q v,max) and maximum specific removal rate ( q max). It has been found that both q v,max and q max depend on the SRT of a mixed culture. A method of determining the critical solids retention time, π x,cr, at which q v,max = B v, was developed. The values of π x,cr found were 2.4, 1.5 and 0.62 d in the first series for SA, NTA and MA, respectively, and 3.8, 3.6, 1.3 and 0.52 d in the second series for MO, SA, NTA and MA, respectively. Below π x,cr, there is q v,max < B v, and a tested compound behaves as only partially degradable or nondegradable. Above π x,cr, there is q v,max B v and a system has the potential capability for shock loading treatment. At the minimum solids retention time, π x,min, all microorganisms responsible for the degradation of a given compound are washed out from the cultivation system. The developed method makes it also possible to determine q max, μ max and the decay coefficient, b, of the microorganisms in a mixed culture responsible for the degradation of xenobiotics. The results indicate that the microorganisms responsible for the degradation of the xenobiotics are slow-growing. A kinetic equation based on q v,max and K s was derived and shown to predict well effluent concentrations of the tested compounds at all SRT. None of the tested compounds supported filamentous bulking in completely-mixed reactors under the technological parameters used.