Evaluation and microbial risk assessment of a sand filter for greywater filtration
Slow sand filters have been used for drinking and wastewater treatment due to their simplicity of operation and high filtration capacity. They consist of a vessel filled with sand where the water passes through at low flow rate e.g. 0.3 m3/m2/hour (Graham, 1988). For the purposes of treating greywater (GW), the GW passes through the filter and is cleaned by physical, chemical and biological processes (Wotton, 2002). In this research, three slow sand filters for treatment of GW were constructed. These filters with the same characteristics were made using acrylic tubes filled with fine sand were dosed with GW daily. Four different designs were developed, to improve the filtering system. A pre-filter filled with coarse sand was found to be the most effective pre-treatment method retaining part of the suspended solids and lowering the turbidity. To prevent clogs from forming, pre-filters were replaced after 30 days of continuous use. A quantitative microbial risk assessment was performed for three different scenarios which corresponded to (i) the accidental water intake of filtered GW; (ii) the water ingestion of a child of 24 months of age, playing on wet grass irrigated with GW and (iii) the accidental intake of water when changing the pre-filter. Risk values were found to be acceptable for two scenarios but not for the one that involved the pre-filter. Recommendations were made for the pre-filter use according to the results found in the risk analysis.