Quantitative Microbial Risk Assessment and Microbial Barrier Analysis for a standard water treatment plant in Ghana

Abstract

Safe drinking water is essential for human health and has even been declared as a human right. However, this is still a critical challenge in Ghana, Africa. In Ghana, many drinking water sources are microbiologically contaminated, and unsafe drinking water is commonly consumed. There is a need for better water treatment to reduce the health risk for consumers. The overall aim of this master thesis was therefore to perform a Quantitative Microbial Risk Assessment (QMRA) and a Microbial Barrier Analysis (MBA) for a standard water treatment plant in Ghana, with the Tano River as the raw water source. The MBA analysis evaluate if there are enough microbial barriers in relation to the raw water quality, to guarantee a safe drinking water for the consumer. Three pathogen groups were considered, bacteria, viruses and parasites. In the QMRA, different scenarios were modelled and the annual risk of infection of E.coli O157, Norovirus and Cryptosporidium were compared to the guideline limit of one infected per 10 000 consumers per year (set by the United States Environmental Protection Agency). Due to the lack of studies of microbiological water quality in Ghana, the input data was taken from existing research from Ghana and Sweden, used in combination with some carefully considered estimations and assumptions. The microbial water quality in the Tano River is affected by wastewater discharge, which increases the risk of microbial contamination. The results from the MBA and QMRA showed that conventional treatment including chemical precipitation and chlorination is not enough to guarantee a safe drinking water. The separation and inactivation of viruses and parasites was not efficient enough, and parasites was especially critical. Additional treatment with UV-disinfection and ultrafiltration, is recommended for removal of the parasite Cryptosporidium, which the results showed to be of high risk. However, the results differed between the MBA and the QMRA. The MBA addition of UV-disinfection would be sufficient whereas, the QMRA suggested both UVdisinfection and ultrafiltration was needed. The way water quality was characterised for the two methods could explain the different result. Possible improvements of raw water quality could be better upstream control and improved wastewater collection system. In addition, sampling on pathogen concentrations would be beneficial for further studies.