In-home performance and variability of biosand filters treating turbid surface and rain water in rural Kenya

Abstract

Thirty low-income Kenyan households using turbid river and relatively cleaner rain water participated in a 6 month in-home Biosand filter (BSF) performance study comprised of surveys and monthly monitoring of BSF influent and effluent water for turbidity and fecal coliforms (FC). River–river (influent–effluent) sample pairs (n = 155; 90% of observations) resulted in average BSF instantaneous FC and turbidity removals of 1.41 log10 (96.1%) and 32.5%, respectively. Cumulative distributions of influent and effluent quality demonstrated unambiguous improvement of river water but rain water improvement was limited and less reliable. Filter performance varied considerably within and across units. A hierarchical set of hypothesized factors affecting filter bacterial performance variability was assessed. BSF effluent FCs were positively correlated with influent (flush water) FCs and influent and effluent turbidity, and negatively correlated with turbidity applied to-date and days since maintenance. Interrupted use and moving the BSF negatively impacted effluent quality. Households with children age 6–10 collecting BSF filtered drinking water, or with more members, had higher effluent FCs. BSFs fed only river water performed better, on average, than mixed-source filters. Implications for BSF implementation in developing countries are discussed, including aqueous chemistry aspects of performance.

Slow Sand Filters for the 21st Century: A Review

“…In comparison, another study reported that while microbial loading in the feed water only marginally affected the removal of E. coli and coliform bacteria, a loading of 10 6 MPN/L resulted in an 85% removal rate for both [17]. A Kenyan study of 30 households using inhome BSFs reported lower level of effluent fecal coliform in homes (a) that used rainwater only, (b) where children aged 6 to 10 did not collect the filtered water, or (c) that housed fewer family members [65]. The last factor was likely due to the over-use of the filters in homes with larger families, essentially decreasing the HRT, whereas the first factor was directly related to the influent water quality.…”

“…A randomized controlled trial in rural communities in Tamale, Ghana, reported a 67% geometric mean reduction in turbidity using biosand filters [73]. A six-month study involving 30 Kenyan households each using in-home biosand filters reported a 32% reduction in turbidity [65]. The wide variation in the removal of turbidity is associated with filter characteristics (including media characteristics); operating conditions (including HRT); and, most notably, source water quality, including the specific type of particles that are responsible for the turbidity.…”

A critical overview of household slow sand filters for water treatment

“…The cleaning methods most often used were superficial agitation (< 1 cm) and 5 cm stirring (Table S1 -supplementary material). Some studies also carried out maintenance by removing 5 cm of the filtration media (Singer et al, 2017) and by the wet harrowing method (Jenkins et al, 2011;McKenzie et al, 2013;Tiwari et al, 2009). Singer et al (2017) compared three cleaning methods for I-HSSF media: surface agitation (< 1 cm), 5 cm stirring, and 5 cm replacement.…”

Investigation of the Effect of Grain Size, Flow Rate and Diffuser Design on the CAWST Biosand Filter Performance

“…Hydraulic loading rate, defined as the ratio of volumetric flow rate and the filtration cross-sectional area has been reported to have varying degrees of improvement on bacteria efficiencies 6,7,10,21 . It is hypothesized that a slower hydraulic loading rate leads to increased contact time between the bacteria and the surface of the sand grain and the biolayer.…”