Upgrade of waste stabilization ponds to improve effluents for reuse purposes

Waste stabilization pond (WSP) systems exist in many countries affected by climate change causing increasing demand for irrigation water. There is little long-term experience with WSP in Africa, and thus, this study provides a comprehensive investigation of a WSP in Namibia. One of its treatment trains was upgraded with mechanical and anaerobic pre-treatment (PreT) and post-treatment (PostT) and compared to a second train without upgrades. The PreT showed positive results on chemical oxygen demand (COD), total suspended solids and partially pathogen removal. Escherichia coli were reduced to the new European Union reuse standard of 1,000 MPN/100 mL for fodder irrigation, Pseudomonas aeruginosa stagnated and Enterococci levels increased. The main pathogen reduction happened during PreT and in the baffled facultative pond. On the contrary, the PostT only reduced 5% of algae-based chlorophyll-a. Among the top 20 genera, Pseudomonas was most dominant. With different upgrades, hygiene requirements especially for restricted fodder irrigation can be reached. At the same time, high total COD (tCOD) and total nitrogen (TN) effluent values did not meet different reuse standards. But as the tCOD mainly consists of algae, adding biomass and TN fertilizer to the barren soil, it raised the question if all parameters set in the standards are applicable for WSP or should be adapted to their specific purpose.