Designing, Construction and Characterization of Field Scale Constructed Wetland for the Treatment of Domestic Wastewater

Abstract

Pakistan, as many developing countries, is facing serious water quantity and water quality issues due to industrial, agricultural and municipal activities. Water availability among others is predicted to be less than 700 m3 per capita by 2025 which is far below the international standard of 1500 m3 per capita. In addition, water pollution is causing unfavorable health conditions by raising the level of water borne diseases. Reuse of the wastewater after treatment is one of the possible option to comply with water shortage. With the designing, construction and long term treatment process of wastewater other factors such as less energy consumption, operation and management and cost effectiveness should also be considered in developing countries like Pakistan. Constructed wetlands considered effective in developing countries for their low construction, management and operational cost. Constructed wetlands are considered as attached growth bioreactors in which microbial colonization occur either in substrate in the form of biofilm or they form association in plants roots. The present study aimed to construct a treatment system (constructed wetland) for the treatment of domestic wastewater of Quaid-i-Azam University residential colony. For this purpose initially laboratory scale wetland study was designed and evaluated. Laboratory scale horizontal subsurface flow (HSSF) and hybrid Horizontal and Vertical subsurface flow (HSSF+VSSF) constructed wetland with a surface area of 0.75 m2 were built for the treatment of domestic wastewater. The wetland units were planted with Typha latifolia. Both units were fed with domestic wastewater at flow rate of 0.0384 m3/d. Removal efficiencies of the both systems were increased with time and showed removal of COD and BOD upto 90 and 96 %, TSS 70 and 79 %, Nitrate 97 and 79 %, Nitrite 95 and 97 %, Phosphate 75 and 80 % and sulphates 59 and 53 % for HSSF and HSSF-VSSF setups respectively. Furthermore, for a single house hybrid constructed wetland system was constructed at pilot scale and was operated at continuous average flow rate of 1.6 liter per day with measured HRT of 8.6 Days. Year round performance of the system was monitored for physio chemical and microbial analysis. Significant results were found during the study and effluent of the system meets the national standards of water discharge by giving average removal efficiencies of 89.61 % for COD, 89.0 % for BOD, 94.0 % for NO-2, 81.13 % for NO-3, 36.94 % for SO4-2, 66.29 % for PO4-4, 94.5 % for TSS and 96.36 % for MPN. Based on the results of Lab-scale and Household units two systems at field scale were constructed receiving continuous flow of wastewater from residential colony of QAU. System-I consisted of an anaerobic baffled reactor (ABR) followed by a saturated vertical subsurface-flow (VSSF) CW and a free-water-surface (FWS) CW as a tertiary treatment; system-II consisted of an ABR followed by a horizontal subsurface-flow (HSSF) CW and FWS constructed wetland. Maximum reduction of 80 and 78 %, 81 and 82 %, 63 and 69 %, 79 and 89 % for COD, BOD, TKN and TSS was achieved in Systems I and II respectively. There was also effective removal (93-94 %) of the bacterial population in both the systems while more than 94 % of pathogenic microorganisms were removed. Data from both the systems were further used to compute the first–order rate constants for the k–C* model commonly used in CW design. The treatment performance was confirmed to follow a first-order reaction rate, in which the k20 values of Chemical oxygen demand, Biological oxygen demand, Total Kjeldahl nitrogen, Total phosphorus and Total suspended solids were calculated as 165, 117, 133, 7.5 and 78 m.yr-1 respectively for VSSF and 226, 134, 199, 22 and 73 m.yr-1 respectively for HSSF. A positive correlation with temperature was observed for all the parameters studied in the systems. Treated water from Field scale wetland was used for irrigation of different vegetables in summer and winter season. For this purpose vegetables were irrigated with raw sewage, treated sewage and tap water. A significant difference in aerobic plate count (APC) was found between vegetables irrigated with sewage water and wetland treated water. Significant difference in microbial load was observed in summer and winter season for both groups of vegetables irrigated with treated and untreated water. Non-significant difference was found between the growth of vegetables irrigated with sewage water and treated water, after 60 days of sowing. Therefore, it could be inferred from this study that wetland treatment significantly reduced the risk of pathogen spread in vegetables irrigated with sewage water. Application of constructed wetland 2D (CW2D) model HYDRUS Wetland module was used for the Vertical subsurface flow constructed wetland that was running in continuous mode without having any sequential dry and wet periods. Collected data was processed to examine whether the model could work for simulation of VSSF treating sewer wastewater with varying hydraulic loading rate (HLR). Results of simulation from CW2D was better fit in with the measured values for COD, Ammonia and Nitrates. CW2D can be used for the constructed wetlands receiving varying concentration of wastewater. In conclusion this study is the first study which contributed to the environment protection at the local level and a wastewater treatment system was constructed at Quaid-i-Azam University. This would lead to the duplication of such systems throughout the country for wastewater treatment at site on those areas which are not connected to main treatment systems of the city.