Identification of the mechanisms involved in the removal of pesticides in waste stabilization pond: Laboratory scale study

It is suggested that for the attenuation of Emeging Contaminants (ECs) in waste stabilization ponds (WSPs), possible mechanisms are: biodegradation, photo-degradation, sorption, volatilization and uptake by microalgae. No study has been carried out to find out the removal mechanisms of pesticides in WSPs. Bench scale batch studies using microalgae and bacteria grown on municipal wastewater or synthetic wastewater were carried out to assess the mechanisms of removal of pesticides (aldrin, endosulfan, dichlorodiphenyltrichloroethane, hexachlorocyclohexane, carbaryl and malathion). Factors which were considered to evaluate the removal processes were solar radiation (light or dark), microalgae (present or absent), wastewater (real or synthetic) and aeration (aerated or non aerated). A total of 6 sets of experiments with different combinations were carried out with each reactor having 3 replicates. Microalgae consortium was collected from WSP located at Rishiskesh. Dominant species of microalgae were chlorella sp., scenedesmus, chlorophyta sp. and tetradesmus. Complete removal of ammonical-nitrogen was observed during the experimental run (12 days) with microalgae showing a growth rate of 0.19 d^-1. COD, BOD₅ and total phosphate show removals of 77, 85 and 89%, respectively. ECs which had Henry law constant >3.0 x 10^-1 Pa.m³mol^-1, i.e. aldrin, endosulfan, dichlorodiphenyltrichloroethane, hexachlorocyclohexane were removed upto 99% due to volatilization (air stripping). Biodegradation was found to be responsible for the removal of carbaryl, malathion. The use of microalgae helped in increasing the removal efficiencies of carbaryl and malathion. The results from the reactor with only light does not show any evidence of photodegradation. Among carbaryl and malathion, uptake of carbaryl by microalgae was found to be maximum, i.e. 54%. Laboratory experiments combining microalage, real wastewater, synthetic wastewater and aeration prove that volatilization is a relevant removal process for ECs having Henry's law constant >3.0 x 10^-1 Pa.m³mol^-1. Biodegradation is the main mechanism for ECs which have lower Henry's law constant. Microalage enhances the biodegradation process of ECs.