Transformation of roxarsone in the anoxic–oxic process when treating the livestock wastewater

Highlights: • A-O process was used to treat synthetic livestock wastewater containing ROX. • ROX and its metabolites could affect the bacterial communities in the process. • Different forms of As could be detected in the effluent, sludge and biogas. In order to evaluate the influence of roxarsone (ROX) on the livestock wastewater treatment, a lab-scale pilot employing an anoxic–oxic (A-O) process was investigated by adding different concentrations of ROX at different periods. The mass balance of arsenic (As) in the A-O system was established through the analysis of As speciation and As migration in the gas, liquid and solid phases. The results showed that around 80% of total ROX (initial concentration was 50 mg ROX L^{− 1}) was eliminated in the anoxic reactor (R1) in which at least about 11% of total ROX was transformed to inorganic As^v (iAs^v) due to the direct breaking of the C-As bond of ROX. Inorganic As^III (iAs^III) and arsine (AsH₃) were produced in R1, while the generated iAs^III in the effluent of R1 was almost completely oxidized to iAs^V in the aerobic reactor (R2). However, the concentration of ROX in the effluent of R2 was almost the same as that in the effluent of R1. After 85 days operation, iAs^V and residual ROX as the main forms of As were observed after the A-O process. Furthermore, the mass balance of As at steady state revealed that around 0.08%, 3.91% and 96.01% of total As was transformed into gas (biogas), solid (excess sludge) and liquid (effluent). Additionally, the 16S rRNA analysis demonstrated that the existence of ROX in livestock wastewater may play a crucial role in the diversity of bacterial community in the A-O system.