Slow Sand Filters for the 21st Century: A Review

Abstract

Safe drinking water remains a major global challenge, especially in rural areas where, according to UNICEF, 80% of those without access to improved water systems reside. While water, sanitation, and hygiene (WASH)-related diseases and deaths are common outcomes of unsafe water, there is also an economic burden associated with unsafe water. These burdens are most prominent in rural areas in less-developed nations. Slow sand filters (SSFs), or biological sand filters (BSFs), are ideal water treatment solutions for these low-resource regions. SSFs are the oldest municipal drinking water treatment systems and improve water quality by removing suspended particles, dissolved organic chemicals, and other contaminants, effectively reducing turbidity and associated taste and odor problems. The removal of turbidity and dissolved organic compounds from the water enables the use of low-cost disinfection methods, such as chlorination. While the working principles of slow sand filtration have remained the same for over two centuries, the design, sizes, and application of slow sand filters have been customized over the years. This paper reviews these adaptations and recent reports on performance regarding contaminant removal. We specifically address the removal of turbidity and microbial contaminants, which are of great concern to rural populations in developing countries.

Review of Slow Sand Filtration for Raw Water Treatment with Potential Application in Less-Developed Countries

“…Nonpathogenic aerobic microorganisms deposited on the surface of the sand filter can metabolize organic matter that enters the filter with the incoming water. These microorganisms can prey on bacteria and viruses present in the water [65].…”

“…Traditional slow filters are usually tanks up to 6 m wide, up to 60 m long and consisting of four layers (Fig. 1) [65,75,80]. Drainage is placed on the bottom of the tank.…”

“…Changing the thickness of the sand layer affects the removal rates of bacteria and viruses. For example, a decrease in sand layer thickness from 0.6 m to 0.3 m resulted in a 0.04% decrease in poliovirus (poliovirus) removal (from 99.98% to 99.94%) [65,86], and a 2% decrease in coliform removal (from 97% to 95%) observed when filter layer thickness was reduced from 0.97 m to 0.48 m [61,87].…”

“…According to [65], the key design parameter of SSF controlling water quality is the filter's hydraulic residence time (HRT). HRT is determined by:…”

“…Therefore, the sand must have a sufficiently high homogeneity. According to [65], the use of a sand layer consisting of particles with a size of 0.35-1.5 mm provides a high degree of water purification at HRT from 8 to 12 hours.…”