Impact of hydraulic loading rate on performance of constructed wetlands for wastewater treatment

Abstract

Wastewater generation is increasing at an exponential rate as a result of rapid population growth and urbanization. Sri Lanka in its way forward to become a hub in Asia will have to give an immense attention for treatment of wastewater in the field of pollution control, which is a challenge in developing countries like Sri Lanka, due to the limitations of resources and expertise. Constructed wetlands where water, plants and microorganisms interact to improve the quality of water are growing popular as an effective, low-cost wastewater treatment technology, which does not necessarily require skilled personnel to run the system. However, these systems are not yet widely spread in Sri Lanka due to lack of information. This study investigated the performance of laboratory scale vertical subsurface flow (VSSF) and horizontal subsurface flow (HSSF) constructed wetland systems subjected to varying hydraulic loading rates (HLRs) at tropical condition. Four wetland beds of size 1.4 m x 0.5 m x 0.6 m (Length x Width x Height) were constructed and arranged - two beds as VSSF systems and the remaining two beds as HSSF systems. All four beds were filled with 10 - 20 mm gravel as the wetland media and planted with locally available emergent macrophyte, narrow-leaf cattail (Typha angustifolia). Each wetland system was supplied continuously with synthetic wastewater at different HLRs of 2.5, 3.5, 5, 7.5, 10, 12.5, 15, 20 and 30 cm/day at 12 days cycles over a period of four months. Samples were collected at the end of each HLR application and analysed for five day biochemical oxygen demand (BOD5), total suspended solids (TSS), fecal coliform (FC) and Total coliform (TC). Results showed a decrease of removal efficiencies of BOD5, TSS, FC and TC when increasing the HLR. However it was observed over 80% of BOD5, 40% of TSS, 86% of FC, 90% of TC removal efficiencies in HSSF wetland systems and over 85% of BOD5, 29% of TSS, 89% of FC, 88% of TC removal efficiencies in VSSF systems respectively from 2.5 - 30 cm/day HLRs. Results of this study can be used in field applications of CWs where frequent water flow fluctuations occur.