Since 1991 the institute for water provision has run two vertical flow constructed wetlands for domestic wastewater treatment at two farm houses (8 p.e.) in Upper Austria. The systems are designed for elimination of organic compounds and for nitrification which was no problem even under winter conditions. In 1995 two methods were tested to achieve denitrification also in both systems. System A is a one-stage system, system B has two stages operated in series. The approach to the one-stage system consisted in pumping a part of the nitrified effluent from the soil filter back to the settling tank of the influent, where the raw wastewater mixes with the nitrified effluent-water. The approach to the two-stage system B consisted in adding an external carbon source (methanol) into the second, water saturated stage. The research was undertaken during several experimental stages to test the influence of the recirculation ratio (system A) respectively of the feeding (system B; intermittent, continuous, batch). The highest elimination rates could be achieved with system B (dosage of methanol) during experimental stage 1 (intermittent loading four times a day). The mean elimination rates were 82% for Ninorg and 78% for TN. The TN elimination performance of system A was only a little lower (72%). The elimination of COD, BOD5 and TOC was no problem throughout the whole experimental period (effluent concentrations of both systems were well below the Austrian standards).

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