Assessment of water quality and associated impacts of the sewage discharges into the Mooi River Catchment

Abstract

Population growth in developing cities is putting pressure on the wastewater treatment plants. The improper treated sewage entering the aquatic environment deteriorates the water quality of the receiving water resource. The Mooi River Catchment has been a centre of attention and a number of research studies on significant pollution sources have been undertaken. However, very little has been done about the impacts of sewage discharges into the water bodies in the Mooi River Catchment. The aim of the study was to determine the possible negative impact of pollution injection by sewage treatment plants located within the Mooi River Catchment area. The study looked at how the discharges affect the water quality of the Mooi River system in relation to the regularised discharge of treated water into the receiving water environment. This study assesses the impact of wastewater discharges from the Kokosi, Flip Human and Potchefstroom Wastewater Treatment Works, on the Loop Spruit; Wonderfontein Spruit and Mooi River, respectively. Data received from Department of Water and Sanitation (DWS) was analysed for the sites upstream and downstream of the wastewater discharges. The results were then analysed using a student’s t-test to determine if there is any significant change in water quality between the points upstream and downstream. The data analysed for physico-chemical and microbiological parameters were checked against compliance with the national and international water and wastewater guidelines and standards. Water quality results upstream and downstream of the various wastewater treatment works were evaluated and tested for significant differences between upstream and downstream. The data on the physico-chemical and the microbiological parameters such as pH, electrical conductivity, suspended solids, ammonia, nitrate, phosphate, chemical oxygen demand, faecal coliforms and E. coli of the water of the Loop Spruit, Wonderfontein Spruit and the Mooi River for the period 2015 to 2018 was received from DWS and collated for analysis. Furthermore, included for comparison was data from the period 2001 to 2002 as the reference data. The river water quality data were for samples taken downstream of the discharge points of wastewater treatment works (WWTWs) located in the three rivers. Upstream samples were also included for comparison purposes. To evaluate the quality of the receiving water, the combined data were compared against set national and international water and wastewater guidelines and standards as well as the Water Use Licences for the three wastewater works (Kokosi, Flip Human and Potchefstroom). Data revealed instances during the period July 2015 to January 2018 at which the concentrations and values of most of the physical, biochemical and microbiological quality indicators were higher than those expected for natural surface water. Comparative analysis of the data at the sampling points located downstream against their respective upstream of the discharge points into the rivers suggested there were occasions where the WWTWs discharged treated wastewater which was of poor quality into the rivers, leading to an increase in the water quality parameters such as conductivity, nitrates, ammonia, orthophosphate, suspended solids, COD, E. coli and faecal coliforms. It is possible that on these occasions, the quality of the treated wastewater from the WWTWs were non-compliant with the guidelines on Water Use Licence for authorised discharge. This could arise if there was incidences of lapse and failure to adhere to the process quality control protocols on the water treatment process at the WWTWs. Thus, the results indicate that discharging treated water from the WWTWs deteriorated the quality of water of the Mooi River and its tributaries. More concerning were the elevated levels of the pathogenic bacteria as was observed by high values in the microbiological quality parameters (total coliform (faecal) count and E. coli count) of the receiving water. Ideally, surface water should be free from any form of pathogens (E. coli = 0; Faecal less than 0), as these pose a serious health risk, ranging from diarrhoea to sudden death. The results from the t-test statistical analyses indicated that there was significant difference between the upstream and downstream water quality for the following parameters and sites: electrical conductivity at Flip Human (71.8 and 84.6 mS/m) and Potchefstroom (72 and 101 mS/m), nitrates at Kokosi (5 and 6.08 mg/l), ammonia at Kokosi (0.002 and 1.88 mg/l), Flip Human (0.006 and 0.56 mg/l) and Potchefstroom (0.01 and 0.32 mg/l), orthophosphate at Kokosi (1.079 and 2.26 mg/l) and Flip Human (0.07 and and 2.7 mg/l) and E. coli at Kokosi (23 and 928.9 cfu/100mL). Even when the discharge is regularised and planned there might be a long term effect on the self-sustainability of the aquatic ecosystems along the three rivers as well as the attaching a health risk to users, livestock and wild life. Most of the monitored parameters relevant to wastewater discharge in the receiving river system exceeded the National and international quality standards and the water use licence limits set for discharging WWTWs.