Monitoring of nutrient removal within integrated constructed wetlands (ICW)

The aim of the project was to assess the nutrient removal within integrated constructed wetlands (ICW). The study was based on twelve free-water surface ICW treating farmyard runoff in Ireland. The principal design criterion leading to adequate effluent water quality (i.e. molybdate reactive phosphate (MRP) less than 1 mg/L) from ICW is that the total wetland area needs to be sized by a factor of at least 1.3 times the farmyard area. It is common practice to construct at least four wetland cells in series with individual aspect ratios of less than 1:2 (width to length). Most of the MRP concentrations after ICW treatment were in agreement with Irish urban wastewater standards for discharge to sensitive waters, which can be used as a benchmark to measure ICW performance and which are usually applied to ICW even if it may appear to be too stringent. Furthermore, the number of denitrifying bacteria detected in the benthos of ICW systems is higher than the number of ammonia-oxidising bacteria. Anoxic conditions at the wetland bottom create conditions which are favourable for denitrifying bacteria. It follows that sediment and litter components of the ICW system support denitrification. The outflow water quality was within recommended guidelines. The study suggests that phosphorus exported from an ICW system was similar to the typical background concentrations of phosphorus export rates from land to water.