Sequential anaerobic-aerobic treatment of herbicides in water

Abstract

Herbicides are toxic compounds which cause deterioration of the surface and ground water resources, cause harm to all living organisms. Various treatment methods like physicochemical and biological processes and in combination of aforementioned treatment techniques have been suggested for removal of pesticides from water. Under anaerobic reducing conditions, herbicides undergo dehalogenation, dechlorination and demethylation reactions and form substituent which can be further mineralized under aerobic conditions. Therefore, this study was conducted to evaluate the sequential anaerobic-aerobic treatment of three herbicides 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), and their mixtures in different formulations. The performance was evaluated at hydraulic retention time (HRT) of 48 h, neutral pH between 6.5 – 7.5 and at ambient reactor liquid temperature (27 – 32.2oC). A preliminary study was conducted in four set of sequential anaerobic-aerobic system influent herbicides concentrations of 25 mg/L of 2,4-d, ametryn and dicamba separately and keeping one set as control. The preliminary study was conducted to evaluate the treatment potential of the reactors; significant removal efficiency was achieved for both the herbicides. The long term study was conducted using 4 anaerobic and aerobic reactors namely R1 (anaerobic control with no herbicide), R2 (anaerobic reactor fed with ametryn), R3 (anaerobic reactor fed with dicamba), R4 (anaerobic reactor fed with 2,4-d and ametryn mixture), and R5 (anaerobic reactor fed with 2,4-d ametryn and dicamba mixtures). Effects of increased herbicides concentration when they are treated separately (ametryn and dicamba), and in mixtures (2,4-d with ametryn and 2,4-d, ametryn with dicamba) during 400 – 430 days of treatment period. Five aerobic reactors were operated simultaneously to give post treatment to the anaerobic effluent. The reactors performance was evaluated by monitoring herbicide removal efficiency of ametryn, dicamba, chemical oxygen demand (COD) and biogas production. The reactors stability parameters pH, alkalinity, volatile fatty acids (VFA) and oxidation reduction potential (ORP) were monitored on daily basis. All the anaerobic reactors wereii 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 respective anaerobic reactors. The maximum removal efficiency obtained for different influent herbicide concentrations under anaerobic treatment from R2 reactor was 88 – 100% for ametryn and 85 – 92% for COD, similarly from R3 about 68 – 80% for dicamba and 77 – 85% for COD respectively. Sequential anaerobic-aerobic removal efficiency was found to be greater than the efficiency of anaerobic reactor, complete removal of ametryn with COD >95% in A2, and >88% for dicamba and COD in A3 was achieved. The mixed herbicides removal efficiency was evaluated based on COD removal efficiency only, the overall COD removal efficiency achieved for different influent concentrations of herbicides mixture was >85%, and >88% respectively from A4 and A5 respectively. Addition of anthraquinone-2,6-disulphonate (AQS) as a redox mediator enhanced the herbicides removal efficiency in the anaerobic reactors R2 and R3 by 12 – 20%, and a slight improvement in the COD removal in the R3 and R4 reactors by 5 – 10%. The GCHRMS and LC-MS analysis was conducted to identify the transformation products (TPs) formed during the treatment process. Commonly identified TPs of anaerobic treatment include long chain fatty acids, esters, and alcohols from all the reactors, which were oxidised in the aerobic reactors and TPs of herbicides were different for the specific herbicides, ametryn TPs were biodegradable under anaerobic condition itself (in R2), while some TPs of dicamba were mineralised in aerobic post treatment step. The effluent from R4 – A4 and R5 – A5 contained different TPs which were not mineralised completely, but removed to a maximum level. Therefore, sequential anaerobic-aerobic treatment is found to be effective and efficient for the removal of selected herbicides from wastewater.