Kinetics of Biological Treatment of Low Level Pesticide Wastewater

Abstract

Pesticides are chemical substances intended to protect food crops and livestock from pests in order to pro-mote agricultural productivity and protect public health. Contamination of soil, air and water and threat to human and animal health are the major constraints in the use of pesticides. Treatment of pesticide contaminated water is, therefore, paramount. Biological treatment provides the most economical option when compared to other treatment methods. The aim of the study was to develop a safe and effective in the farm biological treatment for low level agricultural pesticide wastewater. The degradation of the fungicide captan was evaluated under batch and continuous modes of operation with a retention time of 15 days. The initial cell number (30.1×10⁶ cells/mL) in the soil water mixture first declined with time during the 24 h reaching 15.6×10⁶ and 11.1×10⁶ cells/mL in the batch and continuous bioreactors, respectively. This was due to the inhibitory effect of pesticide on some of the soil microbial species that had less tolerance to captan at the initial concentration of 144 mg L^-1. Then, the microbial population started growing, reaching its maximum after 5 and 12 days from the start in the batch and continuous bioreactors, respectively. The lag period and the specific growth rate for the batch bioreactor were 22 h and 0.096 h^-1, respectively. A captan degradation efficiency of 89.6% was achieved after 10 days in the continuous bioreactor compared to a degradation efficiency of 100% after 5 d in the batch bioreactor. This study showed that the effluent from the continuous bioreactor has a captan concentration of 12 mg L^-1 which is not acceptable for livestock water according to Health Canada Guidelines. A half life of 52 h is observed in the batch bioreactor.