Alternative filter media instead of sand

It is possible to use alternative filter media instead of sand. This can be useful in some areas where sand is not available. Such alternatives include burnt rice husks, crushed glass or crushed stone.

Burnt rice husks

Combustion of rice hulls produces an ash that is 90% silica. As such, rice hull ash has been investigated in Australia as an alternative filter media by Barnes and Mampitiyarachichi (1983 [ref.01]Ref.01: Barnes and Mampitiyarachichi, (1983) Water filtration using rice hull ash. Waterlines, vol. 2, pp.21-3.). A pilot filter containing rice hull ash with a D₁₀ of 0.135 and a uniformity coefficient of 2.96 was tested against a sand filter, with filtration rates intermediate between slow and rapid sand filtration (0.25 – 2 m/hour). They found that the performance of the rice hull ash was superior to that of the sand with respect to the following parameters when treating water with an influent turbidity of between 40 and 60 NTU – see table below.

The burnt rice husk medium was found to become unstable at higher flow rates of 4 m/hour due to its porosity.

Other tests were carried out between 1974 and 1977 in Thailand using burnt rice husks as a filter medium as part of a 2-stage filter, with a pre-filter consisting of shredded coconut husks (Frankel, 1979 [ref.02]Ref.02: Frankel, R.J. (1979) Operation of the coconut fibre/burnt rice husks filter for supplying drinking water to rural communities in Southeast Asia. AJPH vol.69, no.1, pp.75-6.). Filtration rates were between 10 and 15 times faster than standard sand filters. The results showed that raw water turbidities were reduced from 15 – 150 JTU to below 5 JTU, while coliform removals averaged 60 – 85%. Iron removals averaged 80 – 95%.

Crushed glass

CWC (1995 [ref.03]Ref 03: CWC (1995) Evaluation of Crushed Recycled Glass as a Filtration Medium in Slow Rate Sand Filtration. CWC, Seattle, USA. Full copy of report available to order here.) investigated the possible use of crushed recycled glass as an alternative medium for slow sand filters. The pilot study tested crushed glass against 3 other types of sand. The conclusions were that in general, all of the filter media appeared to have similar coliform removal efficiencies. The removal of bacteriological contaminants demonstrated that the glass filter media obtained the activity level typically expected with slow rate filtration. However, the study cautioned against applying the conclusions of the project to other water systems, since the effectiveness of a filter medium and slow sand filtration treatment of drinking water are site specific, and each water system must be evaluated on a case-by-case basis.

In comparison to these results, another study on using crushed glass in place of sand for dual-media rapid sand filters concluded that particle removal capabilities of a crushed glass filter were slightly poorer than those of a sand filter (Rutledge and Gagnon, 2002 [ref.04]Ref.04: Rutledge, S.O.; Gagnon, G.A. (2002) Comparing crushed recycled glass to silica sand for dual media filtration. J. Environ. Eng. Sci. Vol. 1.). It was found that the crushed glass used in the project had a higher angularity and slightly higher uniformity coefficient then the sand that was tested. Removal rates of particles were 1.4-log for the crushed glass filter, as opposed to 1.6-log for the sand filter.

Crushed stone

In a study of alternative media for rapid sand filtration, Rao (1981 [ref.05]Ref.05: Rao, D.R.J. (1981) Evolving high rate filter and use of crushed stone as filter media. J. Inst. Eng., vol. 61, pp.92-6 (India).) found that crushed stone with a D₁₀ of 0.47mm was more effective than sand of the same grading, with respect to (a) turbidity removal, (b) bacterial removal, and (c) length of filter run. The stone dust used was a waste product from quarries. Note that for sand filtration media, limestone-rich media should not be used as this may dissolve over time.

References:

Ref 01: Barnes and Mampitiyarachichi, (1983) Water filtration using rice hull ash. Waterlines, vol. 2, pp.21-3.

Ref 02: Frankel, R.J. (1979) Operation of the coconut fibre/burnt rice husks filter for supplying drinking water to rural communities in Southeast Asia. AJPH vol.69, no.1, pp.75-6.

Ref 03: CWC (1995) Evaluation of Crushed Recycled Glass as a Filtration Medium in Slow Rate Sand Filtration. CWC, Seattle, USA. Full copy of report available to order here.

Ref 04: Rutledge, S.O.; Gagnon, G.A. (2002) Comparing crushed recycled glass to silica sand for dual media filtration. J. Environ. Eng. Sci. Vol. 1.

Ref 05: Rao, D.R.J. (1981) Evolving high rate filter and use of crushed stone as filter media. J. Inst. Eng., vol. 61, pp.92-6 (India).