The performance of six multi-stage hybrid wetland systems, which were designed and constructed for treating high-content wastewater, was evaluated in the cold climate of Hokkaido, northern Japan. The systems were designed to treat four kinds of wastewater: dairy wastewater (three systems, average inflow 4.9-46.6 m 3 .d -1 , average inflow content 2,400-5,000mg.COD.l -1 , 2-5 years of operation); wastewater from a pig farm, including liquid food washing wastewater (one system, 4.1 m Keywords hybrid constructed wetland; cold climate; dairy milking parlor;potato starch processing wastewater; swine urine; oxygen transfer rate 4
IntroductionJapan's food self-sufficiency ratio is only about 40%, which is remarkably low compared to other industrialized countries. A large surplus of nitrogen and phosphorus is given to animals as feed, which is mostly imported from abroad (Yano et al. 1999).Moreover, the nitrogen and phosphorus from animal manure exceed fertilizer needs because of the small area of farmland in Japan. Treatment of dairy milking parlor wastewater, potato starch processing wastewater, and swine urine wastewater has been a big problem in Hokkaido, northern Japan, where such wastewaters are polluting rivers and groundwater. Conventional mechanical wastewater treatments are expensive. Thus, there is an urgent need for a low-cost technology for treating such wastewater.Constructed wetlands for pollution control have progressed greatly over the past 20 years (Cooper 2009;Vymazal 2009; Kadlec et al. 2000). Moreover, hybrid systems have been in use since the 1980s (Vymazal 2011). Several important studies about the selection of filter media, the treatment performance at low temperature, and the recirculation effect have been conducted in cold climates (Poldvere et al. 2009(Poldvere et al. , 2010Jenssen et al. 2005Jenssen et al. , 2010Speer et al. 2012). However, issues of clogging and freezing for treating high-content wastewater in cold climates still remain. To overcome these issues, we designed and constructed multi-stage hybrid reed bed systems in 2005 with a safety bypass and a floating cover to treat high-content wastewater in the cold climate of Hokkaido, Japan (Kato et al. 2006(Kato et al. , 2009 Mean temperature, rainfall, assessment period, and assessment number are shown in ** All systems operated throughout the year except the system for the potato starch factory P, which was designed to work from May to November. *** P1 was preserved wastewater from May to August and P2 was fresh wastewater from September to November.
Schematic diagramA schematic diagram of the hybrid wetland system is shown in Fig. 1. Our systems are composed of three to four V flow beds with a self-priming siphon and no or one horizontal (H) flow bed with a total of three to five beds. A French-type self-priming siphon was applied for the V flow bed with minor modifications; each dosing pipe was single for simplicity and easy maintenance (Kato et al. 2006(Kato et al. , 2009. Some effluents were recirculated (Vr) to the inlet...
