Investigation on the effects of design and operational variables on the efficacy of biosand filters

The following paper reports on the efforts made to assist in the overall implementation of one specific household water treatment (HWT) for improving water quality for people in developing countries, biosand filters (BSFs). It is recognized that BSFs are not applicable for every situation or community. When BSFs were first developed for household applications, the minimum sand bed depth was determined to be 50 cm, based on existing Canadian regulations for water treatment through large-scale, high-capacity sand filters. We questioned this basic assumption, and investigated whether smaller, lighter, and cheaper BSFs (with a shorter sand bed depth) are as effective as the traditional large, concrete filter. The overall project objective was to assess the efficacy, effectiveness, and acceptability of a smaller biosand filter, both in the laboratory and in the field, with the overall goal of demonstrating successful performance and acceptability of the smaller BSFs to reduce implementation costs, allowing more households to be reached. Hopefully, the results presented herein will provide additional insight and quantified data on the operational considerations and removal capabilities of various types of full-scale BSFs to aid in the justification and support for future implementation efforts. In section one, the background and scope of the problem of water access and quality in developing countries is reviewed, including a brief overview of several household water treatment technologies that are currently used. The introduction, section two, provides a detailed description of the biosand filter and the experimental setup that was the focus of the laboratory research. Sections three through six contain the manuscript style descriptions of the four studies conducted, including the results and conclusions. The last and final section, section seven, is a summary of conclusions including findings and lessons learned gained in from the execution and evaluation of this research. The research conducted and reported herein tested the general hypothesis that biosand filtration can be effective on a smaller, cheaper scale than currently practiced with the concrete BSF. In particular, we investigated how the efficacy of the CAWST BSF compared to smaller bucket-sized BSFs with respect to removal of turbidity, total coliforms, E. coli, MS2 coliphage, and Cryptosporidium parvum oocysts from raw drinking water supplies. Specifically, the research attempted to answer the following questions regarding BSF performance: (1) Are the removal efficiencies of smaller BSFs significantly different from the concrete BSF? (2) Is removal efficiency impacted by the turbidity of the source water? (3) To what extent do slight disturbances affect the performance of the bucket BSFs? (4) Can the BSF be modified (i.e., by the addition of rusty nails in the diffuser basin) to significantly improve the removal of viruses in the BSF? (5) How is the removal efficiency impacted by the length of the pause period? (6) If smaller sized BSFs can offer an acceptable level of removal (based on the laboratory results), how will a smaller BSF perform in the field and will it be acceptable to end-users?Four separate studies (Sections 3.0 - 7.0 and summarized below), were conducted to answer the questions outlined above. Effect of sand bed depth and media age on bacteria and turbidity removal The main objective of the first study was to build several full-scale BSFs, simulate real-world usage conditions, and assess the long-term efficacy (9-month study period) for particulate and bacteria removal. Four replicates of three different filter designs were built: the traditional concrete BSF, and two scaled-down versions that use a 5-gal and 2-gal bucket, respectively, as the casing material. The major difference among the three BSF designs was the depth of the sand layer: approximately 54, 15, and 10 cm for the concrete, 5-gal bucket, and 2-gal bucket BSFs, respectively. This study investigated (1) how the efficacy of the CAWST (Centre for Affordable Water and Sanitation Technology version 10) BSF performed with respect to removal of turbidity and E. coli from raw drinking water supplies, (2) whether biosand filtration could be effective with scaled-down 5-gal and 2-gal bucket BSFs, (3) the effects of low and high turbidity feed water on filter performance and maintenance, and (4) the effects of filter maintenance (i.e., cleaning) on filter performance. All bucket-sized filters, and two of the concrete filters, had hydraulic loading rates (HLRs) in the range of 0.2-0.3 m3/(m2*hr) for the majority of the testing period. The smaller sand bed depths in the bucket-sized filters did not impact filter performance with respect to turbidity and E. coli removal or the effluent levels of turbidity and E. coli. All filters produced effluents with a mean turbidity of \u0026lt;0.6 NTU. In addition, 78%, 74%, and 72% percent of effluent samples for the concrete, 5-gal, and 2-gal filters, respectively, had E. coli concentrations \u0026lt;1 CFU/100 mL. Based on the data collected in this study, the CAWST v10 concrete filter was able to achieve 98.1 - 98.4% turbidity removal and 3.8 - 4.0 log E. coli removal. The scaled-down BSFs, constructed in 5-gal (15cm bed depth) and 2-gal (10cm bed depth) buckets, were shown to be as effective (p-values \u0026gt;0.05) as the CAWST v10 concrete (54cm bed depth) configuration for both turbidity and E. coli removal. Alternating the influent turbidity between periods of high and low turbidity (~50 and ~5 NTU, respectively) did not influence either turbidity removal or E. coli removal. Periodic filter maintenance (i.e., cleaning the top of the sand bed) exhibited no correlation to either removal values or effluent levels of either E. coli or turbidity (p\u0026lt;0.05 and

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