Selection of an effective substrate to treat gray water in horizontal flow constructed wetlands

Abstract

A study was carried out to compare the performances of different substrates used in a laboratory scale subsurface flow constructed wetland systems to treat gray water released from the university staff canteen. Three wooden boxes with the dimensions of 60cm x 25cm x 25cm were fabricated and 15mm rock aggregates, pebbles (15mm) and broken tiles (25mm) were used as substrates (which were selected considering the availability) in each of the boxes. Each horizontal subsurface flow bed was fed in a batch process with gray water with a flow rate of 20 litres per day. The retention time for each batch was maintained as 6 days. BOD, COO, pH, conductivity, N03-, P043-, turbidity and temperature in the influent and the effluent were monitored for every batch of waste water passed through the systems and their removal efficiencies were determined. The highest removal efficiencies of COD and, BOD,and P043- were 80%-90%and 60%- 80%, respectively were recorded from broken tiles, while highest removal efficiency of N03-, of 60%- 90%, was recorded from aggregates. Removal of turbidity in all three tanks was similar being between 60%- 90%. Hence, crushed aggregates and broken tiles are found to be the best substrates, considering the removal efficiencies to treat gray water.