Sequential Anaerobic–Aerobic Co-Treatment of Three Herbicides Mixture in Water: A Comprehensive Study on Biotransformation

Abstract

Anaerobic reducing conditions tend the herbicides to undergo dehalogenation; dechlorination and demethylation reactions form simpler end products. Thus formed intermediate compounds can be mineralized in the aerobic reactor followed after anaerobic treatment. Therefore, this study was conducted to evaluate the sequential anaerobic–aerobic treatment of three herbicides mixture namely (2-ethylamino)-4-(isopropylamino)-6-(methylthio)-s-trazine) (ametryn), 3,6-dichloro-2-methoxybenzoic acid (dicamba) and 2,4-dichlorophenoxyacetic acid (2,4-d) with different formulations. The reactors were operated at hydraulic retention time (HRT) of 48 h, pH between 6.5–7.5 and at ambient reactor liquid temperature (27–32.2 °C). The long term study was conducted using two anaerobic reactors namely R1 (anaerobic control with no herbicide) and R2 (anaerobic reactor fed with 2, 4-d + ametryn + dicamba mixtures). The effect of increased herbicides concentration on the anaerobic–aerobic reactor during 400 days of the treatment period was evaluated. Two aerobic reactors were operated simultaneously to give post-treatment to the anaerobic effluent. The reactor performance was evaluated by monitoring herbicide removal efficiency of chemical oxygen demand (COD) and biogas production. The reactor's stability parameters pH, alkalinity, volatile fatty acids (VFA) and oxidation–reduction potential (ORP) were monitored on a daily basis. Both the anaerobic reactors were stabilized using 2 g/L of starch with total organic loading rate (OLR) of 0.21–0.215 kg-COD/m3/d during 48 days, and aerobic reactors were stabilized in 14 days using anaerobic effluent as feed having OLR of 0.02 to 0.038 kg-COD/m3/d. After achieving the quasi-state condition the influent was fed with known herbicide concentrations to the R2 reactor. The maximum COD removal efficiency obtained for different influent herbicide concentrations under anaerobic treatment from R2 reactor was 77–88%. Sequential anaerobic–aerobic removal efficiency for A2 reactor was found to be >85%. Addition of 5–10 mg/L of anthraquinone-2,6-disulphonate (AQS) as a redox mediator enhanced the herbicides biotransformation efficiency in the anaerobic reactor (R2) by 5–10%. The samples were analyzed using GC-HRMS to identify the intermediate compounds.KeywordsAmetrynDicamba2,4-dBiotransformationSBRAQSSequential anaerobic–aerobic treatment