The functioning of a wetland system used for polishing effluent from a sewage treatment plant

Abstract

A surface-flow wetland system designed for polishing effluent from a sewage treatment plant (STP) on the island of Texel, The Netherlands, was studied between April 1996 and March 1997. The wetland system was composed of a sequence of several units with different water depth, hydraulic retention time and vegetation. The system had a relatively short hydraulic retention time of 2.4 days (hydraulic loading rate 25 cm day −1). The wetland system showed 92% removal of faecal coliforms (3.7 × 10 10 cfu m −2 yr −1), a 26% reduction of nitrogen (126 g N m −2 yr −1) and less than 5% reduction of phosphorus (5 g P m −2 yr −1). The oxygen concentration, which was less than 3 mg l −1 in the STP effluent, showed a strong increase during passage through the wetland all year with a clear diurnal shift between 1 and 12 mg l −1 in summer. Turbidity of the surface water doubled, but the suspended solids changed from sewage sludge particles at the beginning of the system to microscopic biota characteristic for a wetland at the end. A presettling basin produced substantial reductions of faecal coliforms (11 × 10 10 cfu m −2 yr −1) and also intercepted incidental peaks in organic N and P load. N removal was highest in the shallow front sections of the subsequent parallel ditches (240 g N m −2 yr −1), largely owing to denitrification. These ditch sections contained Phragmites australis or Typha latifolia. The increase of the oxygen dynamics predominantly occurred in the rear, deeper sections of the parallel ditches, due to the presence of submerged macrophytes, macro-algae and periphyton. The treatment of the wetland resulted in water with acceptable faecal coliform concentrations that closely resembled the quality of the receiving surface water. However, the removal of nutrients was insufficient to meet the criteria for good ecological quality, probably due to the short hydraulic retention time.