Sustainability of municipal wastewater treatment plants operations in Ethekwini municipality

Abstract

Municipal wastewater pollution in developing countries is a great concern due to partially treated or untreated wastewater. Inflow and Infiltration (I/I) is a challenge in municipalities that receive stormwater as it impacts the capacity and efficiency of the sewage plants. As such this study sought to evaluate the impacts of the I/I on the 3 wastewater treatment plants (WWTPs) namely: Kwa-Mashu, Phoenix and Verulam in eThekwini that were assessed. The operating data for twelve-months in 2019 calendar year of each plant were evaluated which factored in the storm water contribution. The total monthly and average daily flows and influent loads were more variable especially during the wet weather seasons. Kwa-Mashu with operating capacity of 65 000 m3/d after recording up to 51,1 mm average daily rainfall resulting in inflows up to 96 305 m3/d. Phoenix had an operating capacity of 29 000 m3/d however, received inflows up to 35 284 m3/d whilst it recorded the maximum of 8 mm rainfall in December 2019. Verulam mainly treats 8 000 m3 wastewater but could receive up to 10 675 m3/d whilst it recorded rainfall up to 16 mm. Using the IBM SPSS statistical package, moderate to strong correlations between rainfall intensity (I) and flowrate (Q) in the 3 WWTPs assessed were observed. Kwa-Mashu showed a strong/ large correlation (0.756). Verulam showed a weak/small correlation (-0.42) between rainfall and inflow, while Phoenix also had a week/small correlation (0.164). The wastewater influent (Qw) characteristics as well as effluent quality indicators are also impacted by the additional pollution in the storm water contribution. This can result in a short-term risk of non-compliance in the discharged effluent. Thus the treatment efficiency based on Chemical Oxygen Demand (COD), Ammonia (NH3) Total Suspended Solid (TSS) and PV4 removal were evaluated in the 3 (WWTPs) that were assessed. These involved analysing both the influent and the effluent data. Kwa-Mashu showed high concentrations of COD whilst Phoenix had the least and Verulam was in between. Kwa-Mashu influent COD ranged from 720 – 940 mg/l, Verulam received up to 848 mg/l COD whilst Phoenix influent COD concentrations ranged from 456 – 615 mg/l . The influent NH3 in Kwa-Mashu ranged from 17 – 33 mg/l in, about 26 – 44 mg/l and 30,8 – 41 mg/l in Verulam and Phoenix, respectively. The treatment efficiency in Kwa-Mashu was equally high for all the key parameters evaluated recording as high as 96% for COD removal followed by Verulam then Phoenix. Phoenix was receiving wastewater with relatively low concentrations however,the effluent was equally high, showing generally poor treatment by the WWTP, as low as 34% for COD. Municipal wastewater are designed to treat wastewater, however, sludge treatment and disposal are a challenge. Sludge produced may be re-used to produce bio-energy and other bio-commodities however, it contains pollutant loads such as pathogens and heavy metals. Sludge can be stabilised with anaerobic digestion which is becoming an integral part of a modern WWTPs. Hence there is a surge in anaerobic digestion with a potential of recovery and reuse of biogas globally. Thus anaerobic digestion optimises the financial and environmental footprint of the WWTP. This study sought to review the anaerobic digestion fundamentals, the applicable process parameters, the types of digesters, the biogas utilisation, challenges and opportunities, and the biogas developments with focus in South Africa. Biochemical methane potential (BMP) experiments using the AMPTS II were performed in order to assess the potential to produce biogas. Triplicates samples of thickened sewage sludge and digested sewage sludge from the 3 WWTPs assessed were used as subtrates and inoculum, respectively. Each 50 mL bottle reactor sample operating at mesophilic temperature of 37 oC was incubated for 31 days where the gas produced was measured in the thermostatic water bath through the water displacement. Connected to the computer, a digital pulse for every 2 mL of gas that flows was recorded and the values were read off through a data-logger. The BMP results were as follows: Phoenix 264,18 NmL/g VS, Kwa-Mashu 147,96 - 170,50 NmL/g VS and Verulam 181,79 NmL/g VS. The electrical energy potential was estimated where Phoenix showed an electrical energy potential of 7 kWh, Kwa-Mashu’s Mash-S1a - c and Mash-S2a – c had 4,51 and 3,91 kWh, respectively and Verulam had a potential electrical energy of 4,81 kWh.