Environmental Impact Assessment (EIA) of Effluents from Constructed Wetlands on Water Quality of Receiving Watercourses

The amount of animal manure produced in Flanders—Belgium by intensive animal farming generates a surplus that needs to be treated in order to achieve quality objectives set by the Nitrates Directive (91/676/EEC) and the European Water Framework Directive (2000/60/EU). After the physical separation and biological nitrification/denitrification processes, the liquid fraction of manure can be cost-efficiently and effectively treated by constructed wetlands (CWs). However, current discharge criteria limits do not evaluate whether nutrient loads from specific point sources (such as CWs) affect the water quality of their receiving waterway. Thus, we investigated whether a site-specific analysis, based on local environmental conditions, would yield more relevant discharge thresholds. In the present study, a standardized framework was developed for environmental impact assessment (EIA) of effluents from CWs on the water quality of receiving watercourses. This framework was tested as a case study on a manure treatment installation located in Langemark—Belgium. The effect of different impact scenarios on water quality and flow of the effluent and the receiving waterway was studied. Standardized EIA guidelines and sensitivity analyses were applied to determine the expected impacts of total nitrogen (TN), total phosphorous (P), chlorides (Cl⁻) and sulphates (SO₄²⁻) on the receiving watercourse. From this study, we concluded that the methodology currently applied requires adaptation when assessing the discharge from wetlands as current estimations of impact are overly conservative when compared with actual impact. In addition, results showed that expected impact might be mitigated by differentiating discharge limits between dry and wet periods.