Current and future microbiological strategies to remove As and Cd from drinking water

Access to clean drinking water is one of the most fundamental of all human rights; however, many millions of people around the world, particularly in low-income, developing countries, are regularly exposed to water sources that are contaminated by pathogenic bacteria, or toxic levels of pollutants such as the metalloid arsenic (As) and heavy metal cadmium (Cd) (Smedley et al., 2002; UN-EP, 2010). These pollutants can lead to the development of many health issues such as rashes, lesions, cancers or even death (Smith et al., 2000; UNEP, 2010). Perhaps the most famous case of As release into drinking water supplies has affected South-East Asia and the Bengal basin over recent decades. Widespread issues of pathogen-contaminated surface water supplies and high mortality rate amongst under 5-year olds led to the installation of millions of boreholes in the early 70s to access clean groundwater. Installing these boreholes led to changes in the local geochemistry and subsequent mobilization of As, which then became entrenched within the drinking water. Consequently, an approach conceived to ensure clean drinking water for millions of people, instead led the devastating effect of slowly poisoning them. Cadmium contamination is often associated with agricultural soils, due to the use of Cd-containing phosphate fertilizers, mining or other industrial activities (Muehe et al., 2013a, b). These uses have led to contamination in groundwater via run-off, in addition to the potentially harmful effects of bioaccumulation in plants, which can reach the food chain. A sustainable approach to addressing these contamination issues is to either change the water source or treat the water source before it is consumed. Often, however, simply changing the water supply is not an option as entire regions may be affected and it becomes economically unviable to transport clean water over many hundreds of kilometres. Instead, it may be easiest and cheapest to treat the water before it is drunk or used for other purposes (e.g. agriculture, washing, etc.). Various remediation approaches have been used to date including membrane-based technology, photochemical oxidation, ion exchangers, adsorbents, coagulants or flocculants (Berg et al., 2006; Hering et al., 2017). Here we discuss bioremediation methods which make use of microbiological processes in order to remove As and Cd from drinking water. Bioremediation strategies where bacteria are actively involved with the transformation of a species (e.g. through oxidation or reduction) are referred to it herewith as ‘direct microbial bioremediation’, whereas treatments by secondary processes (e.g. mineralization) stimulated by bacteria are referred to as ‘indirect microbial bioremediation’.