Nitrogen removal from pretreated wastewater in surface flow wetlands

The wastewater treatment plant in the town of Oxelösund (12,500 inhabitants), Sweden, has mechanical and chemical treatment for removal of BOD and phosphorus. With the aim to achieve 50% nitrogen removal, a surface flow wetland system of 21 ha was created as a final step during 1993. It consists of 5 cells, where 2+2 are operated in parallel with a final common cell. This allows intermittent filling and emptying, the goal of which is to promote both nitrification and denitrification for a design flow of 6000 m³ day⁻¹. During the first year of operation, August 1993 to July 1994, the wetland removed 720 kg ha⁻¹ of total nitrogen from the load of 1810 kg ha⁻¹. Ammonium-N was the dominant fraction at the inlet as well as at the outlet, 79% and 90% of total nitrogen, respectively. The large fraction of NH₄⁺ at the outlet showed that nitrification was the limiting step. An intensive monitoring effort in May 1994 indicated that neither wastewater toxicity nor oxygen deficiency were likely to limit nitrification. Instead, sub-optimal hydraulic loading conditions; a lack of suitable surfaces for ion exchange of NH₄⁺, as well as for attachment of nitrifiers; and phosphorus deficiency, were considered potentially important factors in limiting nitrification. In addition to nitrogen removal, the wetland system reduced total phosphorus, BOD₇ and E. coli (44°C) to very low levels at the outlet.