A Feasibility Study on Sustainable Wastewater Treatment Using Constructed Wetlands - An Example from Cochabamba, Bolivia

Abstract: Cochabamba in Bolivia is an example of a city that faces increasing environmental problems and health risks due to insufficient sewage system coverage and municipal wastewater treatment. The situation is especially severe in peri-urban fast growing settlements. A decentralized alternative to conventional wastewater treatment plants is constructed wetlands with horizontal subsurface flow (HSF). This treatment method exists in many parts of the world and a few wetlands have been constructed in Cochabamba. The present study includes an evaluation of the treatment efficiency of one of these systems, a pilot plant built by the foundation AGUATUYA treating wastewater coming from a kindergarten, and an investigation of the applicability of the method in Cochabamba. Field work was performed in Cochabamba during November and December 2008. The treatment plant shows 80-97 % reduction in biochemical oxygen demand (BOD5), 80-90 % reduction in chemical oxygen demand (COD) and 50-80 % in turbidity. The pH and temperature conditions are favourable for degradation by microorganisms. Nutrients were not observed to be removed from the wastewater. The wetland is largely anaerobic which is likely to be the reason why no nitrogen removal was observed. The microbiological analyses were few but the results indicate that faecal coliform bacteria are reduced by 90 %. The effluent concentration of coliforms does not meet treatment requirements for unrestricted re-use of the water. The wetland has two sections containing different media: gravel and plastic pieces. The plastic medium has significantly higher surface area per bed volume but the analyses showed little or no difference in treatment efficiency between the sections. To better understand the difference in treatment capacity between the sections and to be sure that they work properly, the outlet of the wetland has to be redesigned. Tracer experiments were performed and showed a large variation in transport time for the gravel section. The transport time of the plastic section could not be determined from the experiments. The nominal retention time was calculated to 6.2 and 8 days for the gravel and the plastic section respectively. Cochabamba has a long dry season, creating a great need for water re-use and especially for irrigation of agricultural and recreational areas. Treated water from constructed wetlands can be used for this purpose in the vicinity of the wetlands, reducing the consumption of potable water, shortening water transports, reducing the risk of eutrophication in adjacent waterbodies and lowering energy requirements. Climate and terrain conditions in Cochabamba are suitable for constructed wetlands. HSF constructed wetlands require little energy, construction material and maintenance. They are a good alternative in sparsely populated, poor areas like peri-urban Cochabamba. Use of constructed wetlands could lead to more green areas with lawns and trees, and this in turn decreases soil erosion.