Performance of subsurface vertical flow constructed wetlands receiving municipal wastewater

The efficiency of pollution removal from municipal sewage in two vertical flow constructed wetlands consisting of gravel filters with a surface area of 4 × 5 m, depth 60 cm, planted with reed (Phragmites) was assessed over a period of about two years. The flow of wastewater was 50 mm per day. Wastewater underwent only primary treatment before application to reed bed B, but reed bed A was supplied with wastewater after mechanical and biological treatment. Measurements were taken of sewage supply and discharge, precipitation and wastewater temperatures. The main indicator of efficiency was the elimination of suspended solids, BOD5, nitrogen and phosphorus from the wastewater during treatment. The elimination of the pollution load was 2-25g O2 per square meter per day for the BOD5 and 0-3.5 g per square meter per day for so-called "total nitrogen". Rates of pollution removal were between 2 and 4 times as high in bed B (after primary treatment) as in bed A (after biological treatment), but the loading rate of bed B was also substantially higher. The rate of BOD5 removal and the coefficient k for BOD5 were greatly dependent on temperature for reed bed B (primary treatment); less so for bed A (biological treatment). The difference between summer and winter temperatures indicates that the surface area of constructed wetland B with wastewater after mechanical treatment should be about 3 times greater during winter, to obtain the summer rate of BOD5 pollution removal in the climatic conditions of Northern Poland (54oN).