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

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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.

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