Treatment of domestic wastewater by subsurface flow constructed wetland

Abstract

Constructed wetlands have been used for decades mostly for the treatment of domestic or municipal sewage. For a small community with limited funds for expanding or updating wastewater treatment plants, constructed wetlands are an attractive option. In addition, rural municipalities have access to adequate inexpensive land, and wetlands blend into a natural landscape setting and once the wetlands are designed and constructed, annual maintenance costs are low. Therefore, this study focuses on the design of constructed wetlands that might serve as a treatment plant for domestic wastewater, will be economic and help re-use of wastewater to reduce demand on current available sources. In urban areas like Dhaka in Bangladesh, water is a growing crisis with the rapidly growing rate of urbanization. It is necessary now to explore ways of using resources in a sustainable way to reduce pressure on centralized systems. Through recycling and reuse of generated wastewater, this crisis could definitely be reduced which also will make the currently available fresh water sufficient for the city. This study works on sub-surface flow constructed wetland systems to test whether it can be a reliable treatment method for recycling grey water which have low organic and nutrient loading. The performance of sub-surface constructed wetlands to treat grey water at household level is tested in this experiment where the removal efficiency of BOD5, COD, ammonia (total), orthophosphate, TSS, and Fecal Coliform (FC) were tested. The findings from the study indicate that a basin with surface area less than 5 m2 is good enough to treat almost 18,000 L of grey water per day. Normally, 60-70% of total wastewater from households is grey water. Assuming that per capita use per day is 225 L, then a constructed wetland with surface area of 5 m2 would treat grey water of almost 20 families (6 person/family). But it should be noted that the flow rate should not be above 12 L/min. In that case a sedimentation tank should be designed to control the flow rate into the basin. The results show that removal efficiency of TSS varies in the range from 70-95%. Removal efficiency of BOD is more than 90%. Removal efficiency of BOD is more than 90%. Removal efficiency of total ammonia varies in the range from 70-95%. Removal efficiency of orthophosphate varies in the range from 45-70%. Removal of FC was found high in constructed wetlands. Vertical flow constructed wetlands are more efficient in removing pollutants from water. It was also experienced in the study that removal rate is higher with vegetation, except for suspended solids. Basin 1 also shows higher efficiency than basin 2 that indicates that higher travel time and length increase removal efficiency. The study shows that subsurface constructed wetlands can reduce the concentration of pollutants that are present in domestic grey water, to improve downstream water quality. As the systems produce an effluent low in BOD, which has a much lower total-NH3 and P, and show good color and less turbidity, the system can be used at community/household level that will help to reduce the water crisis of Dhaka city.