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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\u0026lt;0.4). The smaller bucket-sized filters were found to be a viable alternative to the concrete BSFs for the removal of bacteria and turbidity from drinking water.Transport effects on hydraulic loading rate and removal performance BSFs designed using smaller and/or lighter casing material can result in reduced logistical requirements and implementation costs. However, the increased portability of a smaller, lighter design presents a potential negative consequence: the ability to move the installed/operational filter by the homeowner and potentially disturb the system. This study investigated the effects of moving and agitation on filter performance, using mature BSFs which had been in use for over nine months prior to the move. Data were analyzed for four replicate filters of three different filter types: the traditional concrete BSF and two plastic bucket (5-gal and 2-gal, respectively) BSFs. Filters were moved approximately 1 km and monitored for hydraulic loading rates (HLRs) and E. coli removal for eight weeks following the move. Moving the filters resulted in reduced HLRs, likely due to sand compaction, but E. coli removal remained high (log10 removal ≥ 2.8 for all sizes) and increased significantly as compared to data collected prior to the move. The resulting operational implications of moving BSFs are discussed.Influence of sand depth and pause period on microbial removal in traditional and modified BSFsThe results of the first study showed that small biosand filters (sand bed depths of 10-15 cm) were effective at removing bacteria and turbidity. However, the impact of shorter bed depths on removal rates for smaller, sub-micron particles (such as viruses), as well as the impact of shorter pause periods on filter performance, remained unknown. For the third study, biosand filters with three different sand bed depths were modified with the addition of iron nails in the diffuser basin and evaluated for bacterial, protozoal, and virus removal over six different pause periods (1, 3, 6, 12, 24, and 72 hours). The BSF configurations tested proved effective at removing the microbial contaminants over a range of pause periods. Removal of bacteria and protozoan cysts for all filter types and sizes ranged from 3 log10 to 4 log10. The addition of nails resulted in significantly better bacteria removal for all filter sizes, while only the smallest filters exhibited significantly better protozoan removal with the addition of nails. Virus removal for all filter types and sizes ranged from \u0026lt;1 log10 to 6 log10. Both the pause period and filter type (size/configuration) influenced virus removal, and the addition of nails to the filter significantly improved virus removal at the shorter pause periods. Field evaluation of plastic-cased filters in NicaraguaThe fourth study was a field investigation to assess 1) the effectiveness of plastic-cased BSFs for improving water quality, 2) user acceptability and use, and 3) operational performance of the units. Two types of household BSFs were built, installed, and monitored over a three month period in four rural communities near San Juan del Sur, specifically a large filter made from (10in diameter) PVC pipe and a small filter made from a 5-gallon plastic bucket. The filters were designed based on the proportions of the CAWST v10 concrete BSF, that is there were proportionally designed with respect to filter media layers (i.e., sand, rock, and gravel) with the major differences between the types being the sand bed depths and reservoir volumes, which were 54cm and 15cm, and 12L and 3.6L for the large (PVC pipe) and small (5-gal) filters, respectively. From the results of this study, the 5-gal bucket and PVC BSFs performed similarly with respect to E. coli removal. After approximately 6 months of use, the median log reduction values (LRVs) for the bucket and PVC BSFs were 1.73 and 0.95, respectively."}