Design and performance of hybrid constructed wetland systems for high-content wastewater treatment in the cold climate of Hokkaido, northern Japan

The performance of six multistage hybrid constructed wetland systems was evaluated. The systems were designed to treat four kinds of high-content wastewater: dairy wastewater (three systems, average inflow content 2,400–5,000 mg·COD l⁻¹, 3–6 years of operation); pig farm wastewater, including liquid food washing wastewater (one system, 9,500 mg·COD l⁻¹, 3 years); potato starch processing wastewater (one system, 20,000–60,000 mg·COD l⁻¹, 3 years); and wastewater containing pig farm swine urine (one system, 6,600 mg·COD l⁻¹, 2.8 years) (COD = chemical oxygen demand). The systems contained three or four vertical (V) flow beds with self-priming siphons and surface partitions and no or one horizontal (H) flow bed (three to five beds). In some V flow beds, treated effluents were recirculated (Vr) through the inlet to improve performance. Mean annual temperature was 5–8 °C at all locations. To overcome clogging due to the high load in a cold climate, we applied a safety bypass structure and floating cover material to the V flow beds. Calculated average oxygen transfer rates (OTRs) increased proportionally with the influent load, and the OTR value was Vr > V> H. The relations of load–OTR, COD–ammonium, and a Arrhenius temperature-dependent equation enable the basic design of a reed bed system.