Towards Net Zero Energy: Enhancing Wastewater Treatment Sustainability with Energy Efficiency and Biogas Integration. A case study of the Western Cape

Abstract

Urban wastewater treatment is vital for removing pollutants and preserving the water cycle by collecting and treating community-generated wastewater. Historically, wastewater treatment plants (WWTPs) prioritized meeting community demand and adhering to stringent effluent quality standards, especially concerning organic releases into water bodies. In the face of global concerns about climate change, carbon footprints, and energy efficiency, developing countries, such as South Africa, confront additional challenges like energy supply shortages and escalating electricity costs. This underscores the pressing need for implementing energy-efficient practices and exploring alternative sources in wastewater treatment. This research aims to evaluate the feasibility of transforming a Wastewater Treatment Plant in the Western Cape, South Africa, into a Net Zero Energy (NZE) facility. The study begins by analysing the current energy consumption of the Wastewater Treatment Works (WWTW) and then applies various energy efficiency measures to estimate potential savings and project energy consumption over twelve months. Additionally, the research assesses the plant's potential biogas production and calculates the theoretical electrical energy yield, with the overarching goal of determining whether energy efficiency measures and biogas utilization can render the plant a NZE user. The research had two main components. First, a qualitative literature review identified energy efficiency best practices and assessed potential savings for retrofitting wastewater treatment plants. The second phase involved a case study, utilizing quantitative data provided by the City of Cape Town (CoCT) over twelve months, including Eskom electricity bills, a list of mechanical equipment with power ratings, and daily sludge production and key sludge characteristics. The study was able to conclude that an energy consumption reduction in the range of 28% could be achieved by implementing energy-efficient best practices. In addition, the theoretical average biogas production was calculated to be in excess of 505 MWh per month. Despite these promising results, the research concluded that these two interventions alone would not result in a NZE site. This does however open avenues for further study including a full energy audit of the site, based on energy monitoring equipment and real-time data. The study also invites further opportunities for improved site-based energy production through pre-treatment and co-digestion, as well as additional renewable energy sources including, but not limited to, solar energy, wind energy, heat energy recovery, hydrogen production and microbial fuel cell technology. General limitations were faced during the study, such as time constraints, data collection challenges, limitations in analysis depth, and expertise. The study's scope had to be restricted to fit within the available time frame. Consequently, reliance on pre-existing data from a third party was necessary, as there wasn't sufficient time for a twelve-month observational data collection, crucial for the analysis. This reliance introduces a level of uncertainty. The analysis of the results was also constrained by the allocated timeframe and expertise. For the research question to be answered, the study required an in- depth understanding of both the wastewater process and plant configuration, as well as the intricacies of the mechanical equipment and subsequent electrical load. Where experience and time frame did not allow for an in-depth analysis, assumptions had to be made based on available literature. Limitations specific to this research are acknowledged, including the exclusion of the impact of sludge variability on biogas production due to time and scope constraints. The study does not cover variables affecting methane quantity, such as sludge temperature and pH. It also does not explore the consequences of retrofitting a WWTP with additional equipment and processes, and what effect this would have on the effluent quality. Additionally, while recognising the complexity of defining a NZE facility and potential scenarios, the study does not delve into these cases. Instead, this report considers only the case to which the monthly energy consumption is offset by the theoretical energy production on site for the same month.