Decentralized wetland-based treatment of oil-rich farm wastewater for reuse in an arid environment

► Decentralized wetland-based treatment was effective for oil-rich dairy wastewater. ► Effluent met quality criteria for agricultural reuse. ► Salinization was minimal in vertical flow wetlands. ► Seasonal effects were minimal in the recirculating vertical flow wetland. A novel system of anaerobic treatment and constructed wetlands (CWs) was tested for decentralized treatment of high-strength wastewater discharged at a dairy farm located in an arid environment. The wastewater volume averaged 2.6 m 3 d −1 from the household, milking parlor, and cheese-making facility. Raw wastewater was highly variable with mean chemical oxygen demand (COD), 5-d biochemical oxygen demand (BOD 5), total suspended solids (TSS), and oil and grease (O&G) of 2700 ± 1700, 840 ± 140, 920 ± 590, and 520 ± 670 mg L −1, respectively. The treatment system consisted of: (1) anaerobic tanks (5-d hydraulic retention time); (2) vertical-flow CW (VFCW, 60-m 2); and (3) recirculating VFCW (RVFCW, 4-m 2). The anaerobic treatment provided solids reduction and waste-strength equalization. The combined VFCW and RVFCW reduced concentrations of COD, BOD 5, TSS, and O&G by 94, 96, 97, and 99%, respectively. Total nitrogen (TN) and total phosphorus (TP) were both reduced 73%, and fecal coliforms were reduced 2 orders-of-magnitude to 10 5 CFU/100 mL. Sodium (Na), sodium adsorption ratio (SAR), electrical conductivity (EC), and boron (B) were not statistically altered by the system's treatment, indicating that evapotranspiration did not significantly increase wastewater salinity. Treatment efficiency of the VFCW was reduced during low winter temperatures; however, final effluent quality from the RVFCW remained stable throughout the year, due to multiple passes of wastewater through its treatment bed. Effluent met WHO health risk guidelines for reuse in mechanized agriculture, with slight to moderate restriction due to potential salinization.