Prospects of hybrid rainwater-greywater decentralised system for water recycling and reuse: A review

This review examines the prospects of a decentralised hybrid rainwater-greywater system to simultaneously alleviate water scarcity and address technical, environmental, and cost constraints. This includes (1) a review on the physicochemical and microbiological characteristics of rainwater and greywater to determine the necessary treatment options; (2) a review of individual components and potential treatment trains for hybrid systems; and (3) an evaluation of configurations for hybrid systems. The literature review reveals that both untreated rainwater and greywater are highly variable in quality and quantity, and so an equalisation basin is proposed to normalise influent into a hybrid system. Both rainwater and greywater should not be reused without treatment due to the presence of pathogens such as Aeromonas, Salmonella, Pseudomonas, and Staphylococcus. Based on the literature, hybrid systems are categorised under three configurations: (a) separate treatments of rainwater and greywater; (b) rainwater reused in washing machines prior to reuse as greywater; and (c) combined treatment of rainwater and greywater. In all three designs, rainwater requires only first-flush diversion and disinfection. Combined rainwater-greywater mixtures should be treated as greywater. Greywater requires chemical, biological, and physical treatment to meet non-potable reuse standards. Chemical processes are effective at removing solids, organics, and surfactants in light greywater, whereas aerobic biological processes are effective at organics removal in mixed and dark greywaters with high organic strength. Physical processes, particularly membrane filtration, are recommended for polishing effluents from chemical or biological treatment as membranes foul frequently and are costly. Subsequently, a combination of ozone or UV with chlorine is recommended to eradicate chlorine-resistant Cryptosporidium oocysts from hybrid rainwater-greywater systems and prevent microbial regrowth. (C) 2016 Elsevier Ltd. All rights reserved.