Post-treatment of UASB reactor effluent in an integrated duckweed and stabilization pond system

Post-treatment of effluent from an Upflow Anaerobic Sludge Blanket (UASB) reactor, that was fed with domestic sewage, was conducted in an integrated pond system. The system consisted of a series of shallow duckweed and stabilization ponds. The main objective of post-treatment is removal of bacterial pathogens and further polishing of effluent quality. Rapid and efficient pathogen removal can be achieved in shallow stabilization ponds but their effluent BOD and TSS is relatively high, due to presence of algae Passing stabilization pond effluent through duckweed ponds was expected to remove algae due to reduced light penetration. Duckweed ponds have revenue generating potential since the produced biomass can be used as animal fodder. However, when applied separately, their pathogen removal is poor. A pilot plant system with an overall retention time of 4.2 days, was tested for this purpose. This system consisted of 10 ponds in series, arranged in 3 stages. The first stage consisted of 2 duckweed ponds the second stage of 3 stabilization ponds and the third stage of 5 duckweed ponds. The system's effluent median fecal coliform count in two experimental periods of 6 months was 3.3* 102 5.0* 103 per 100 ml. Increasing the retention time of the stabilization ponds to 3-4 days is suggested for consistently satisfying the WHO criterion for unlimited irrigation. Rapid removal took place in the stabilization ponds. A first order fecal coliform decay constant Kd was calculated for each of the three stages. The values obtained were 0.7 -3.2, 4.0∼5.9 and about 1.4 d-1, respectively. The shading by the duckweed cover in the last stage proved to be able to remove practically all algae. Therefore, an excellent effluent quality with respect to TSS was achieved (11 mg/l). It was demonstrated that duck-weed biomass-production and wastewater treatment for reuse in irrigation can be achieved in one simple system.