THE IMPACT OF SLUDGE RETURN LIQUORS IN SOUTH AFRICAN WASTEWATER TREATMENT PLANTS.

Water pollution, particularly, the eutrophication of South Africa's scarce water resources is a major concern due to the threat it poses to our nation's water security. One of the major sources of pollution of surface waters are effluent streams from municipal wastewater treatment plants. Pollution loads, namely phosphorus and nitrogen compounds are the two primary contributors to the eutrophication of surface waters. To combat this problem, the Department of Water & Sanitation is setting more stringent effluent requirements via treatment plant licensing for both inland and coastal regions. At the same time, many of South Africa's treatment plants are approaching or have reached their design load capacity, or are in fact, overloaded. Furthermore, many of these overloaded treatment systems have nutrient removal constraints. Most treatment plants in South Africa do not consider the return of sludge liquors back to the main plant for further treatment, or when it is applied, the whole infra-structure is not prepared to deal with these additional loads and flows. Unsustainable measures, such as the dosing of costly chemicals (typically metal salts) for phosphate removal, are typically applied at overloaded plants. The causes the capital and operational costs for municipal wastewater treatment plants to run into billions of Rand per annum. There is, therefore, an acute need to look at optimisation measures within treatment systems to reduce the nutrient loads on overloaded treatment plants in South Africa. The main goal of the research study is to improve the knowledge on the impact and mitigating measures of the sludge return liquors from anaerobic sludge digestion on the wastewater treatment process. During the anaerobic digestion, the organic matter in the sludge is converted into biogas with the stored nutrients in the biomass being released into the water phase. This sludge return liquors should be recycled to the main process train, usually to the head of works or biological reactors, instead of being discharged into ponds or water bodies without any treatment. The additional load from the sludge return liquors may result in a deterioration of the plant effluent quality, due to an overloading of the plant, increase the energy requirements (for aeration) and increase the chemical dosage (metal salts to precipitate phosphorous).