Rainwater to potable water: Mini review

Abstract

Rainwater can serve as a safe and sustainable source of drinking water. From its formation and collection, rainwater can be contaminated by the surrounding atmosphere, dissolving various gases like carbon dioxide, sulphur oxides and nitrogen oxides to form various inorganic acids. These acids further promote contamination by dissolving metals and metal oxides that the rainwater encounters. On the other hand, depending on the cleanliness of the rainwater harvesting system, rainwater may be exposed to biological contaminants like pathogenic organisms from plant and animal origins and dust from dry deposition of particulate matter from the atmosphere. While all these contaminations can be dealt with relatively easily with modern water treatment technologies like media filtration, membrane technology and chlorination, rainwater is not as widely used as a source of drinking water as it should be, primarily due to a lack of incentives from the policy makers and availability of more accessible and abundant water sources like municipal water and groundwater. Above all, rainwater is not a reliable source of water as it is intermittent and can be unpredictable even with state-of-the-art weather forecasting. Thus, using rainwater as potable water source might only be suitable during rainy season and floods. The objective of this paper is to briefly review the technologies in purifying rainwater to potable water, and subsequently recommend an alternative technology called progressive freeze concentration (PFC) for the water purification purpose. Preliminary results with the new setup show that PFC can reduce the total dissolved solids (TDS) in rainwater by 83 - 96 % to produce ice that has less than 10 ppm TDS, with 75 – 90 % recovery. By comparing with reverse osmosis (RO), PFC has considerably lower capital and operating cost. Therefore, more research will be conducted to establish the feasibility of using PFC to purify rainwater and encourage the use of rainwater as a source of potable drinking water.