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 single-media 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 impact of introducing crushed expanded slate on length of filter run and turbidity removal efficiency, physical and chemical characterization and the cost benefit of introducing crushed expanded slate to RGSF. 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. Length of the filter run and turbidity removal efficiency 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 slate met the chemical and physical characterization for use as a capping material and increased the length of filter run by an average of 25% and 52% for medium (50 - 150NTU) and high (150 – 300NTU) influent turbidities respectively. Improvement in turbidity removal was insignificant. The total present value (CAPEX and OPEX) for capping with 25mm crushed expanded slates gave an overall cost reduction of 11%. Considering the highlighted benefits and the non-hazardous nature of the material, the study recommends the use of crushed expanded slates from Maji ya Chumvi for use as a capping material for Rapid Sand Filters.