Effects of reduction of wastewater volumes on sewerage systems and wastewater treatment plants

This study aimed to assess and quantify the extent of negative impacts on the proper functionality of sewerage systems (collection and conveyance systems) and wastewater treatment works (WWTW) resulting from drought and water conservation and demand management measures such as reduced wastewater influent flows and quality, and to identify mitigation measures to minimise these impacts. A review of the impacts and mitigation measures employed during reduced wastewater flow conditions in international case studies was conducted, as well as a local case study on the City of Cape Town (CoCT). The CoCT has experienced the most severe drought conditions of any Metro in the country with respect to the recent drought, with water restrictions reaching Level 5 in September 2017. The findings of the international literature review revealed that the conditions such as climate, integration of storm water with sewerage and the disposal of garbage, such as kitchen waste to the sewer, differed from the South African conditions. It was also noted that in most cases, the per capita consumption during water conserving conditions, was significantly higher than the local average normal water consumption of 235 l/cap.d. While wastewater flow reduction as a result of drought conditions or demand management in South Africa resulted in many of the impacts described in the international case studies, the different conditions highlighted the need for the development of a guideline document that also draws from local observations and experiences. This guideline has thus been compiled from the findings of both the international studies and the local contextualisation of this information, to provide practical and applicable adaptive interventions for negative impact mitigation within the framework of the South African best practices and operational requirements. Municipal officials can make use of this guideline to not only educate on the expected impacts of drought conditions on sewage conveyance and plant infrastructure and performance, but also to assist with planning and implementation of mitigation measures. The target audience of this guide is municipalities with established wastewater conveyance and treatment infrastructure, but the design guidelines can also be applied to un-developed and rural municipalities where such systems are planned.