Biodegradation behavior of agricultural pesticides in anaerobic batch reactors

This study explored the biodegradation potential of two agricultural pesticides (2,4-D and isoproturon) as well as their effect on the performance of the anaerobic digestion process. Three 3.5 L batch reactors were used, having the same initial isoproturon concentration (25 mg/L) and different 2,4-D concentrations (i.e. 0, 100, or 300 mg/L, respectively). All systems were fed with equal amounts of primary sludge and digested sludge and operated at the low mesophilic range (32 +/- 2 degrees C). Following an acclimation period of approximately 30 days, complete 2,4-D removal was achieved, whereas isoproturon biodegradation was practically negligible. The presence of 2,4-D did not have a direct effect on acidogenesis since soluble organic carbon expressed either as volatile fatty acids (VFAs) or as total organic carbon (TOC)] peaked within the first 10 days of operation in all bioreactors. Utilization of VFAs however appeared to follow two distinct patterns: one pattern was represented by acetate and butyrate (i.e. no acid accumulation) while the other was followed by propionate, isobuturate, valerate and isovalerate (i.e. acid accumulation, duration of which was related to the initial 2,4-D concentration). On the whole, all reactors exhibited a successful digestion performance demonstrated by complete VFAs utilization, considerable gas production (containing 45 to 65% methane by volume), substantial volatile suspended solids (VSS) reduction (42 to 50%), as well as pH and alkalinity recovery.