Performance Evaluation of Constructed Wetland at NUST

Abstract

The rising trend in human population, water utilization and release of wastewater in fresh water has posed serious stress on available water resources. Recently constructed wetland have emerged as novel, cost effective and eco friendly technology for wastewater management. Reuse of wastewater after treatment reduces stress on fresh water in terms of quantity and quality. In Pakistan, wastewater treatment through constructed wetland is a relatively new trend. Although a lot of work is being carried out internationally for assessing the performance of constructed wetland. There exists very little or no data on how the CWs will perform in the climatic condition of Pakistan. This research work was aimed at performance evaluation of constructed wetland in terms of nutrient removal, hydraulic efficiency and cost effectiveness. The site is established at NUST, H-12 as a three stage treatment system comprising of sedimentation tank, constructed wetland and free floating detention ponds. On daily basis, 75000 US gallons of low to medium strength domestic wastewater is treated with six days of hydraulic retention time. The pH values of treatment system for the studied period remained in the narrow width of 7-8, within the allowable range of 6-9. The average treatment efficiency in reduction of EC, Turbidity and TDS was 30.69, 74.93 and 30.78% for whole treatment system. The overall treatment performance of horizontal subsurface flow constructed wetland was higher than free floating detention ponds. The treatment efficiency for reduction in nitrates, ammonium and total phosphorus was 42.40, 33.59 and 48.50% for constructed wetland, 21.63, 26.47 and 15.13% for free floating detention ponds and 12.65 , 12.14 and 14.84 % sedimentation tank. To evaluate the hydraulic efficiency of treatment facility in terms hydraulic conductivity, short circuiting and dead zone, finite elemental analysis was applied. Underutilization of available treatment volume was identified at three points, which was improved through hypothetical modifications. It was observed that use of longitudinal baffles, perforated inlet walls and multiple inlets enhanced hydraulic efficiency. It was further assessed that uniform velocity profile of wastewater in constructed wetland is more dependent on width and diameter of interconnecting pipes as compared to the length of basin. The width of 8- 10 feet and pipe diameter of 1 feet provided most suitable conditions for uniform velocity profile in a baffled constructed wetland. The major cost components were construction cost and human resource for operation and maintenance. Monetary benefits of the studied system ranged between 0.252 to 0.504 million PKR.