Potential nitrification and denitrification and the corresponding composition of the bacterial communities in a compact constructed wetland treating landfill leachates.

Constructed wetlands can be used to decrease the high ammonium concentrations in landfill leachates. We investigated nitrification/denitrification activity and the corresponding bacterial communities in landfill leachate that was treated in a compact constructed wetland, Tveta Recycling Facility, Sweden. Samples were collected at three depths in a filter bed and the sediment from a connected open pond in July, September and November 2004. Potential ammonia oxidation was measured by short-term incubation method and potential denitrification by the acetylene inhibition technique. The ammonia-oxidising and the denitrifying bacterial communities were investigated using group-specific PCR primers targeting 16S rRNA genes and the functional gene nosZ, respectively. PCR products were analysed by denaturing gradient gel electrophoresis and nucleotide sequencing. The same degree of nitrification activity was observed in the pond sediment and at all levels in the filter bed, whereas the denitrification activity decreased with filter bed depth. Denitrification rates were higher in the open pond, even though the denitrifying bacterial community was more diverse in the filter bed. The ammonia-oxidising community was also more varied in the filter bed. In the filter bed and the open pond, there was no obvious relationship between the nitrification/denitrification activities and the composition of the corresponding bacterial communities.

Can constructed wetlands reduce the diffuse phosphorus loads to eutrophic water in cold temperate regions?

Construction of wetlands is a possible supplement to best management practices (BMP) at the field level to mitigate phosphorus (P) pollution from agricultural areas. In this paper, annual results from 17 intensively studied wetlands in the cold temperate or boreal climatic zone are reported and analyzed. Surface areas varied from 0.007 to 8.7% of the catchment area. The average total phosphorus (TP) retention varied from 1 to 88%, and the dissolved reactive phosphorus (DRP) retention from -19 to 89%. Retention varied substantially from site to site, indicating the existence of site-specific factors in the catchment and wetlands that influenced the P removal. Factors important for P retention in wetlands were evaluated through multiple statistical analyses by dividing P into two fractions: particulate phosphorus (PP) and DRP. Both relative (%) PP and DRP retention increased with wetland surface area. However, PP retention was not as sensitive as DRP in terms of wetland size and retention: specific PP retention (gram P retention per m(2) and year) decreased as wetland area (A(w)) increased, suggesting the existence of a site-specific optimal wetland to catchment area (A(c)) ratio. Particulate P retention decreased with increasing DRP to TP ratio, while the opposite was found for DRP. Dissolved reactive P retention was higher in new than in old wetlands, while increasing age did not influence PP retention negatively. Effective BMP in the catchment is important to keep the P loss low, because the outlet concentration of P from wetlands is often positively correlated to the input concentration. However, wetlands act as the last buffer in a catchment, since the retention often increases as the P concentration in streams increases.

Free water surface wetlands for wastewater treatment in Sweden: nitrogen and phosphorus removal.

In South Sweden, free water surface wetlands have been built to treat wastewater from municipal wastewater treatment plants. Commonly, nitrogen removal has been the prime aim, though a significant removal of tot-P and BOD7 has been observed. In this study, performance data for 3-8 years from four large (20-28 ha) FWS wetlands have been evaluated. Two of them receive effluent from WWTP with only mechanical and chemical treatment. At the other two, the wastewater has also been treated biologically resulting in lower concentrations of BOD7 and NH4+-N. The wetlands performed satisfactorily and removed 0.7-1.5 ton N ha(-1) yr(-1) as an average for the time period investigated, with loads between 1.7 and 6.3 ton N ha(-1)yr(-1). Treatment capacity depended on the pre-treatment of the water, as reflected in the k20-values for N removal (first order area based model). In the wetlands with no biological pre-treatment, the k20-values were 0.61 and 1.1 m month(-1), whereas for the other two they were 1.7 and 2.5 m month(-1). P removal varied between 10 and 41 kg ha(-1) yr(-1), and was related to differences in loads, P speciation and to the internal cycling of P in the wetlands.