Wastewater Treatment and Reuse through Constructed Wetlands Restoration of EU-Facultative Lagoons in Vientiane, Lao PDR

Abstract: The massive infrastructure investments of the last 10 years in Vientiane (capital of Lao PDR) saw the city’s drainage network and wetlands being replaced by hydraulically more efficient cement-lined channels designed to ensure flood protection. As a result, water quality entering the receiving That Luang wetland has significantly degraded. It was initially intended that sewage water from the city centre would be prevented from entering the drainage network and pumped to large facultative wastewater stabilization ponds when stormwater contribution was low. Nowadays, this treatment system is inoperative and housing and commercial buildings have started to appear around the upper part of the site (i.e., inflow area). Considering that agriculture is still a significant activity toward the lower part of the site (i.e., outflow area) located in the That Luang wetland, this project demonstrates that simple designs improvement could in theory produce 8 000 to 50 000m3 of treated wastewater per day suitable for reuse. The highest water quality level, necessary for irrigation of crops likely to be eaten uncooked, could be achieved if a vertical subsurface-flow component was added. This however means that the hydraulic retention time would have to be kept to a minimum of 2,7 days (< 30,000 m3/d) to accommodate a security margin for bot BOD5 and TSS. Only a less restrictive reuse could be achieved if only a free water surface flow design is maintained. Finally, a reversal of the original flow direction is suggested to ensure that air quality and amenity is maintained in the section surrounded by habitations.