Water Quality Assessment of Rooftop-Harvested Rainwater and its Potential Uses in Vhembe District Municipality

Abstract

Rainwater is a major water source which can be harvested and used for various domestic purposes. It is believed that rainwater is free from impurities and soft in nature, making it an alternative source of water for various potential uses, if harvested. This study explores the potential uses of harvested rainwater in the Vhembe District of South Africa. To achieve this, the perception of people towards rainwater’s uses, was assessed as well as the quality of rainwater harvested from various roof types. The physicochemical (pH, turbidity, electrical conductivity, salinity, temperature, and total dissolved solids), microbiological (faecal coliform and total coliform) characteristics, and heavy metals levels in the water were monitored using standard protocols, such as multimeter, membrane filtration technique, and ICP-OES. The relationship between water quality and roof types was also evaluated. Using 110 questionnaires, a survey was conducted with community members who harvest rainwater. The results showed that washing of clothes accounted for the most use of harvested rainwater at 82.73%, in Vhembe District Municipality (VDM). The people of VDM perceived the roof-harvested rainwater as their alternative source of water after municipal-supplied water; 3.6% of the respondents indicated that they have not experienced any adverse effects from the use of harvested rainwater. The reason why residents consider rainwater harvesting has been reported to be mainly due to poor service delivery with respect to water supply by the municipalities. Most of the harvested rainwater’s physical parameters analysed were within the permissible limit of the South African National Standards and World Health Organisation Standards. In comparing all the roof types (slate, steel, aluminium, concrete and thatched) used as catchments for harvesting rainwater, thatched roofs showed very poor standards in terms of physicochemical and microbiological quality of the collected rainwater. The Water Quality Index was evaluated based on the physicochemical parameters focusing on heavy metals and E. coli. All the roof-harvested samples collected in the VDM from the three research areas have an excellent water quality, except water collected from thatched roofs. The results showed that the inclusion of E. coli as a parameter to compute WQI will result in high water quality index value, thereby, proving that pathogenic microorganism play a major role in reducing the quality of water. No carcinogenic risk was computed, based on the levels of trace metals recorded in this study, as the computed Hazard Index and Quotient were all less than 1. Quantitative microbial risk assessment (QMRA), a modelling technique used to estimate the probability of infection and subsequent illness when exposed to pathogenic microorganisms was also calculated. The QMRA calculations all showed the possibility of risk associated with the consumption of rooftop harvested rainwater. This means that continuous consumption of rooftop harvested rainwater without any treatment could pose a great risk to human health, therefore, simple point-of-use water treatment method is recommended prior to the consumption of rooftop-harvested rainwater such as boiling and addition of bleach in order to disinfect harvested rainwater before consumption and food preparation.