Technical Research

Technical research is needed for various parameters, especially in intermittent sand filtration.

Re-design of concrete filter & field testing

• Results from recent research on pause time, hydraulic loading and sand size indicate that perhaps the current design of the concrete filter could change in order to give more consistent results in water quality. To do this, the filter might have to be widened with less freeboard in the reservoir part of the filter – discussion on this available here. However, a wider filter would mean a heavier and more expensive filter, and one that also needs more sand to fill it. Given that the current design can be produced cheaply and has proved affordable to the rural poor, it could be that a total re-design based on only the research might be unrealistic in terms of production, sales and transport. This is an area of field-based research that would be well worth looking into.

Microbiological & biological testing.

• Actual pathogen breakthroughs need to be studied under varying conditions, rather than only indicator organisms. This is especially needed for household intermittent filters. Most previous studies have concentrated solely on the detection of E. coli indicator bacteria as a measure of efficacy, rather than the detection of the actual particular disease-causing organisms (for example Hepatitis A). There are reliability issues related to using these indicator bacteria to correlate to pathogen removal, especially viruses. Admittedly, it is expensive and complicated work to look for individual pathogens, and therefore the nature of the testing rules out fieldwork as an effective mechanism. Rather, laboratory testing is needed.
• More research is needed on the mechanics of virus removal in the intermittent biosand filter. Research carried out by Elliott et al (2008 [ref.01]Ref.01: Elliott, M.A.; Stauber, C.E.; Koksal, F.; DiGiano, F.A.; Sobsey, M.D. (2008). Reductions of E. coli, echovirus type 12 and bacteriophages in an intermittently operated household-scale slow sand filter. Water Research Vol 42 (10-11) pp.2662 – 2670. (DOI: 10.1016/j.watres.2008.01.016). Available here.) on the intermittently-operated filter investigated reduction in viruses in filtered water. They found that echovirus 12 reductions (99% removal) were greater than those of coliphages MS2 and PRD-1 (90% removal), indicating that as far as viruses are concerned, the ability of the filter to reduce them may depend on the viral agent. They suggested further investigation is needed.
• Pathogen removal efficiency with differing daily filtered water volume and differing raw water quality. This is especially needed for household intermittent filters. It seems that pathogen removal may be more efficient initially, decreasing the more litres go through the filter. The reason has to do with the quality of water produced in pause time compared to that which flows through afterwards which has not been standing in the filter, and which consequently has had a lower contact time in the filter. This research is ongoing at the University of North Carolina at present using intermittent sand filters and actual pathogens. They intend to seed the raw water with pathogens and test directly for them in the filtrate. The pathogens they intend to study are the human enteric viruses Hepatitis A virus and Echo 12 virus, the indicator viruses MS-2 and PRD1, Clostridium perfringens bacteria spores, Cryptosporidium parvum oocysts, and Escherichia coli bacteria. The testing is set to run for most of 2005. In this first year they are focusing on looking at pathogen removal using two different daily water volumes (20 or 40 litres per day) and two different water qualities (good and poor).
• Pathogen removal efficiency with quarry sand versus washed and unwashed river sand. This is especially needed for household intermittent filters. It has been recommended that the best sand to use for slow sand filters is quarry sand that has been graded to the correct effective size and uniformity coefficient. Reasons to use quarry sand have to do with its availability in certain regions, its standard size and cleanliness. While these points are valid, they are not a reality in the field. Most filter projects use what sand is available – in Africa this is invariably river sand. While this is usually washed, often water is scarce and the sand is not washed thoroughly enough. Research that looks into the pathogen removal efficiency of different conditions would be very useful – for example quarry sand, washed river sand, partially washed river sand and unwashed river sand all of the same effective size and uniformity coefficient. The testing should be carried out in the field on various types of river sand. Indicator organism testing should be fine, since what needs to be researched is possible regrowth of pathogens within the filter using remnant organic matter as food and shelter to multiply from. The result would give some concrete results to determine how important clean sand is for reliable test results.
• Pathogen removal efficiency of filters with differing standing water levels above the sand. This is especially needed for household intermittent filters. Projects using the standard concrete filter design usually train technicians to install the filters with 5cm of water standing above the sand when the water has stopped flowing. While Buzunis (1995) used experimental filters with 12.5cm of water above the sand with still good results, it would be interesting to know how much pathogen removal depends on water depth when the filter is in pause time, and how much it improves with having a shallower water depth above the sand. Indicator testing should provide sufficient trends for analysis. Presumably a shallower depth allows more oxygen transfer to occur to the biological layer, and this is probably the reason why Palmateer et al (1999, [ref.02]Ref.02: Palmateer, G.; Manz, D.; Jurkovic, A.; McInnis, R.; Unger, S.; Kwan, K.K. and Dutka, B.J. (1999). Toxicant and Parasite Challenge of Manz Intermittent Slow Sand Filter.Environmental Toxicology, vol. 14, pp. 217- 225. Available here.) suggested an ideal level of 2 – 3 cm of water above the sand but this was not based on test data:
• It is believed that the height of the standing water may be important in the development of the biofilm; 2 – 3 cm appears to be an efficient level.
• Pathogen removal efficiency of filters with varying types of diffuser plates. This is especially needed for household intermittent filters. Diffuser plates differ in material from project to project – some fit better than others, and they can have differing sizes of holes that let the water through. Using a single sand type and raw water quality, it would be interesting to know the effect of ill-fitting diffuser plates that allow scouring around the edge of the sand layer, or the effect of having holes that are too small and possibly restrict oxygen transfer or too large and disturb the biofilm. Testing for indicator organisms should produce good results for analysis.
• The application of using metal-coated sand for emergencies. A study carried out by Lukasik et al (1999, [ref.03]Ref.03: Lukasik, J.; Cheng, Y-F.; Lu, F.; Tamplin, M.; Farrah, S.R. (1999) Removal of microorganisms from water by columns containing sand coated with ferric and aluminum hydroxides. Res. Vol. 33, No. 3, p.775.) found that during the initial 48 days of comparing microbiological removal efficiencies for columns of either modified (coated with metallic hydroxides) or unmodified sand, that the modified sand columns had higher removal rates during this period, after which both columns had similar removal efficiencies due to the development of a biological layer in the unmodified column. Also a study done by Truesdail et al (1998, [ref.04]Ref.04: Truesdail, S.E.; Lukasik, J.; Farrah, S.R.; Shah, D.O.; Dickinson, R.B. (1998). Analysis of Bacterial Deposition on Metal (Hydr)oxide-Coated Sand Filter Media.Journal of Colloid and Interface Science, 203, 369-378.) indicated that metal (hydr)oxide surface coatings have the ability to increase bacterial deposition by up to a factor of 5 over that observed for untreated sand. It would therefore be good to know whether this could have an application in emergency settings where a water treatment system is set up, and where good quality water is required before the usual ripening period. It would be good to repeat similar tests using full-scale sand filters.

Chemical testing

• Pesticide removal efficiency. This is especially needed for household intermittent filters. An investigation  carried out in Nicaragua on the intermittent filter appeared to show that the filter was ineffective at removing organochloride pesticides in the field. However, one of the factors that came into play in producing the results was that there were not enough values to undertake proper statistical testing, making the results not statistically significant. This was partly an outcome of the small sample size (10) and a limited budget of the research team. Further investigation is therefore needed to investigate the removal efficiency of the intermittent filter on pesticides, especially since the study carried out by Palmateer et al (1999, [ref.02]Ref.02: Palmateer, G.; Manz, D.; Jurkovic, A.; McInnis, R.; Unger, S.; Kwan, K.K. and Dutka, B.J. (1999). Toxicant and Parasite Challenge of Manz Intermittent Slow Sand Filter.Environmental Toxicology, vol. 14, pp. 217- 225. Available here.) showed that the filter was capable of removing between 50 – 99% of organic and inorganic toxicants including a herbicide.
• Organic and inorganic toxicant (a Polyaromatic Hydrocarbon (PAH) called phenanthrene, the herbicide metolachlor, a chemical toxicant HgCl2, Mercury and nonylphenol) removal at normal environmental concentrations. The study by Palmateer et al (1999, [ref.02]Ref.02: Palmateer, G.; Manz, D.; Jurkovic, A.; McInnis, R.; Unger, S.; Kwan, K.K. and Dutka, B.J. (1999). Toxicant and Parasite Challenge of Manz Intermittent Slow Sand Filter.Environmental Toxicology, vol. 14, pp. 217- 225. Available here.) on the intermittent filter tested raw water with toxicant concentrations of between 10 and 100 times normal environmental levels. They felt that the filter could easily remove greater than 50% of organic and inorganic toxicants at normal environmental concentrations, from waters used as potable water source water. Although easily proven, it will involve costly detailed chemical analyses of input waters and filtered waters at ppb and ppt levels.

Other

• Shortened development of the biological layer. It would be interesting to look into whether the biological layer could be kick-started using some kind of starter sachets to enhance development of schmutzdecke. This would be very useful in areas where the biological productivity of the raw water is low, or where satisfactory filtration is required sooner.

References:

Ref.01: Elliott, M.A.; Stauber, C.E.; Koksal, F.; DiGiano, F.A.; Sobsey, M.D. (2008). Reductions of E. coli, echovirus type 12 and bacteriophages in an intermittently operated household-scale slow sand filter. Water Research Vol 42 (10-11) pp.2662 – 2670. (DOI: 10.1016/j.watres.2008.01.016). Available here.

Ref.02: Palmateer, G.; Manz, D.; Jurkovic, A.; McInnis, R.; Unger, S.; Kwan, K.K. and Dutka, B.J. (1999). Toxicant and Parasite Challenge of Manz Intermittent Slow Sand Filter.Environmental Toxicology, vol. 14, pp. 217- 225. Available here.

Ref.03: Lukasik, J.; Cheng, Y-F.; Lu, F.; Tamplin, M.; Farrah, S.R. (1999) Removal of microorganisms from water by columns containing sand coated with ferric and aluminum hydroxides. Res. Vol. 33, No. 3, p.775.

Ref.04: Truesdail, S.E.; Lukasik, J.; Farrah, S.R.; Shah, D.O.; Dickinson, R.B. (1998). Analysis of Bacterial Deposition on Metal (Hydr)oxide-Coated Sand Filter Media.Journal of Colloid and Interface Science, 203, 369-378.