title: Waste stabilization ponds for water reuse in water scarce regions
Creators: name: Sinn, Jochen
Publisher: name: Verein zur Förderung des Insituts IWAR der Technischen Universität Darmstadt e.V. Darmstadt: Eigenverlag 2023
Year: 2023

description: Waste stabilization pond (WSP) systems are widely applied for communal wastewater treatment especially in countries of the global south with warm climates and sufficient available land area. At the same time many of these regions are affected by climate change and recently experience erratic rainfalls which distress the local subsistence agriculture. In these water scarce regions farmers often rely on surface or groundwater sources to irrigate staple foods and fodder or to water their animals. This in turn puts a burden on the water supply of the population. Therefore, reuse of treated WSP effluent, which is often only evaporated, presents a valuable source for irrigation water and at the same time of plant nutrients. But due to rapid population growth and lack of regular operation and maintenance many of these systems are overloaded and the effluents are overflowing into the surrounding environment causing environmental degradation and health risks to humans and animals. Especially in sub-Saharan Africa there is little documented long-term experience with WSP operation and performance and their potential for water reuse. Therefore, this research presents an overview of the existing situation of nine WSP systems in north-central Namibia which in their current state do not fulfil the national Namibian and the newly published European standard for water reuse. As part of the research project EPoNa one WSP system was enhanced at full scale with different pre- and post-treatment technologies. These included a 250 μm micro sieve as mechanical pre-treatment for the removal of solids and organic carbon, an upstream anaerobic sludge blanket (UASB) as biological pre-treatment also for the removal of solids and organic carbon, sludge removal to restore the original volume of the ponds, floating baffles to improve flow conditions in the facultative pond and a rock filter as post-treatment in the final maturation pond for algae and pathogen reduction. The effects of these enhancements were compared with a second, parallel treatment train operated with its original setup. Compliance with the national and European reuse requirements was evaluated regarding the physical and biological wastewater parameters and further the microbial community was analysed. The main results of this dissertation and relevant aspects for further applications are the following: • In their current state, none of the researched WSP systems in north-central Namibia adhere with the Namibian and European reuse standards, which is mainly due to total organic carbon concentrations above 100 mg/L caused by high algal fractions in the particulate organic carbon. • The algae related chlorophyll-a concentrations correlate linearly with the particulate organic carbon and this correlation can be used to fractionate the total organic carbon for further judgement. • The microbial community is divers with different dominating genera in the influent than in the effluent. • The mechanical pre-treatment with micro sieve (MS) (250 μm) and the anaerobic biological pre-treatment with an upstream anaerobic sludge blanket (UASB) reactor are both operational under the local conditions and can be implemented on large scale to reduce organic carbon, suspended solids and partially pathogens. • The UASB achieves better average removal of chemical oxygen demand (50 %) and total suspended solids (57 %) but the MS is more flexible in handling changing inflow patterns and has a much smaller footprint. A maximum particulate chemical oxygen demand reduction of 89 % is reached with the UASB and 72 % with the MS . • With the pre-treatment there is only limited nitrogen and phosphorus reduction which therefore remain as nutrients in the water and are valuable for further irrigation purposes. • After one year of operation the rock filter as post-treatment reduces only 5 % of chlorophyll-a and shows no additional removal of algae compared to the original treatment train. • Algae concentrations are best reduced with pre-treatment, sludge removal and baffles in the facultative pond. • With enhancements E. coli concentrations are reduced down to the new EU water reuse standard of 1,000 MPN/100 mL for fodder irrigation whilst concentrations of P. aeruginosa stagnate and Enterococci levels increase. With this divergence the function of E. coli as indicator for broader pathogen reduction is questioned. Main pathogen reduction happens during pre-treatment and in the facultative pond with baffles and not as expected in the maturation ponds. • Due to high carbon and nitrogen concentration the effluent does not meet the Namibian and European reuse standards but the high algal content would add valuable biomass and fertilizer to the barren soil. Therefore, a review of the standards considering particularly WSP effluents is suggested. For the first time WSP in north-central Namibia are comprehensively analysed and the reuse potential of their effluents compared. At one pilot plant different enhancement technologies are tested at large scale and the results evaluated with regards to their applicability under similar conditions. Therefore, this dissertation contributes valuable information for the upgrade of existing WSP to improve the environmental situation and produce irrigation water in water scarce regions.