EFFECT OF GRAIN SIZE, BED DEPTH AND RETENTION TIME IN INTERMITTENT BIOSAND FILTERS FOR IMPROVEMENT OF SECONDARY SEWAGE EFFLUENT QUALITY

Abstract

Biosand filtration (BsF) is a simple technique used for pathogen and particle removal in drinking water purification. It may be adapted for wastewater stabilization but only a few studies have been done on improvement of wastewater quality using BsF. Application of Biosand filters (BsFs) for wastewater treatment has spread rapidly across the globe in recent years, creating a need for sound scientific understanding of mechanisms and factors controlling BsF microbial removal. The primary aim of this study was to evaluate the effects of sand grain size, bed depth and retention time in BsFs on secondary sewage effluent quality before disposal into waterways. This was achieved by comparing effluent quality data from filtered and unfiltered effluent. The BSFs were assembled with varying sand bed depths and sand grain sizes. The study was a three factorial design consisting of 27 treatments with three levels each of sand bed depth, sand grain size and retention time. The samples of filtered and unfiltered effluents were collected on a monthly basis for six consecutive months and analyzed for efficiency of the various designs of BsFs. Removal efficiency was determined in terms of selected parameters such as total coliforms (TC), faecal coliforms (FC), faecal Streptococcus (FS), total bacterial counts (TBC) and physicochemical parameters such as biochemical oxygen demand (BOD), conductivity, pH, Total Suspended Solids (TSS) and nutrients as well as percentage increase in dissolved oxygen (DO). ANOVA was used to evaluate performance of the various BSFs and Duncan’s Multiple Range Test to separate means if the differences were significant. Results of this study indicated that there was a significant number of bacteria present in the raw or unfiltered effluent compared to water treated by filtration through various filters. In addition, there were significant differences in the removal efficiency of the different filters. Sand bed depth, retention time and sand grain size influenced filter performance for all the dependent variables under investigation. The physicochemical parameters decreased significantly in effluents from fine sand grain size (0.1- 0.5) mm filters while the bacteriological parameters decreased significantly in effluents from large sand (1.1-2.0) mm filters. Sand bed depth affected removal of most physicochemical parameters (except PO43-) and removal was better for the longest depth (0.7 m). However, removal of bacteria in effluents was not significantly affected by sand bed depth. The highest removal of both physicochemical and bacteriological parameters under study was achieved for the longest period of retention (72 hours) in the filters. The study recommends that for efficiency of BsFs in removal of bacteria, retention time should be extended to 72 hours, sand grain size should be between 1.1-2.0 mm and sand bed depth need not be longer than 0.5 m. However, if the removal of physicochemical parameters is targeted, sand bed depth should be increased to 1.0 m, sand grain size should be fine (between 0.1-0.5 mm) and the retention time should also be extended to 72 hours