Two types of organophosphate pesticides and their combined effects on heterotrophic growth rates in activated sludge process

BACKGROUND: Pesticides are sometimes non-biodegradable and, moreover, toxic to microorganisms. If pesticides exceed the tolerance of microorganisms, failure of the activated sludge process (ASP) occurs. Therefore the effects of two types of organophosphate pesticides on heterotrophic growth rate constant in sludge from ASP were investigated. Oxygen uptake rate was employed to measure the rate constants. RESULTS: The results indicated that the value of heterotrophic growth rate constant decreased from 3.88 d(-1) to 1.46 d(-1) or by 62% when 0.5 mg L-1 of glyphosate was added. When adding 0.5 mg L-1 of malathion, the value of heterotrophic growth rate constant decreased to 1.33 d(-1) or by 66%. The value of heterotrophic growth rate constant decreased to 1.98 d(-1) or by 49% when 0.5 mg L-1 of pesticide combination (50% for each) was added. CONCLUSIONS: The inhibitory effects of glyphosate and malathion were in good agreement with non-competitive inhibition kinetics, but pesticide combination did not follow non-competitive kinetics. The inhibition coefficient values for glyphosate, malathion and their combination were 0.29, 0.29 and 0.58 mg L-1, respectively. For comparison, linear and exponential types of models were derived by regression. According to non-competitive kinetics, and linear and exponential models, the inhibitory effects of glyphosate and malathion were almost consistent. Finally, the degree of inhibition was simulated using different types of model. It was found that the toxicity of the two pesticides agreed with the antagonism well. (C) 2009 Society of Chemical Industry