Evaluation of crushed slates as a suitable capping material for rapid gravity sand filters

Abstract

In the potable water treatment process, Rapid Gravity Sand Filters (RGSF) are commonly adopted as the last solid-liquid separation stage. Cleaning of the RGSF is done through backwashing. RGSF is widely adopted all over the World due to its ease of operation and high filtration rates. However, these filters suffer from stratification of the sand media, which causes floc removal to occur only at the topmost layer of the filter bed, leaving the remaining depth unutilized. Capping is a technique whereby a thin layer of sand filter media is replaced with a suitable coarse material to overcome the problem of stratification and transform a singlemedia RGSF into a dual-media filter. The objective of this study is to determine the suitability of crushed slates as a capping material. The study evaluated the performance of a crushed expanded slate-capped filter against a conventional single-media RGSF, the effects of its physical and chemical characteristics, and varying the depth of the capping material. Laboratory tests were conducted to assess the physical and chemical characteristics of slates from Maji ya Chumvi (Coast, Kenya). This included specific gravity, acid solubility, water extractable substances, silica content, and friability. A performance comparison was carried out by means of a fabricated model filtration unit set up within an existing community water treatment plant. The model filtration unit was fed with pretreated raw water of varying influent turbidities. Crushed expanded slates met the minimum physical and chemical requirements for use as a capping material for RGSF. The crushed expanded slate-capped RGSF model demonstrated high robustness under high shock turbidity loads (above 150 NTU), which is illustrated by an increased length of filter run of 27% (50–150 NTU) and 45% (150–300 NTU). Increasing the depth of capping material from 25mm to 50mm did not yield any significant improvement or deterioration in the filter run length. At influent turbidities below 150 NTU, the effluent water quality for all three scenarios (uncapped, 25 mm, and 50mm crushed expanded slates capped) is below 5 NTU and therefore meets the Kenyan drinking water standards. Above 150 NTU influent turbidity, the effluent water quality for the uncapped RGSF deteriorates, whereas for the 25mm and 50mm capped RGSF, it remains consistently below 5 NTU. This demonstrates the usefulness of the crushed expanded slates in improving the turbidity removal of RGSF for high (above 150 NTU) turbidity loads. This study recommends a full-scale trial of crushed expanded slates to facilitate a more precise estimation of the overall benefit of full-scale community water filtration systems.