Effect on (micro)biological water quality

Slow sand filters can greatly improve the biological and microbiological quality of water. In contrast to rapid sand filters, slow sand filters are biological in nature, and much of the effectiveness of the process depends on biological activity within the sand.  For more information about these processes click here.

Tests on the efficiency of sand filtration to be able to remove pathogens (disease-causing organisms) tend to be in one of two main types – direct pathogen testing and indicator organism testing.  The first type looks for actual pathogenic organisms that are found in the water. In these cases the raw water is usually seeded with the pathogens of interest and the tests tend to be complicated and expensive, requiring significant laboratory equipment. The second type of test looks for indicator organisms that are usually associated with pathogenic organisms, but which may not be inherently harmful themselves. In contrast, these tests are carried out more easily and with less expensive equipment which can be used in the field. Results from various testing programmes are detailed here.  The results are presented according to the type of test carried out – direct pathogen or indicator organism, and whether they were carried out on continually operated or intermittently operated systems.

Evidence from continually operated slow sand filters

Slow sand filters have been in use for the past 200 years, even before they were known to purify water. The first known sand filter was made in 1804 by John Gibb in Scotland. Filtration for domestic supply began in 1829, when James Simpson constructed filters for London. However, it was not until 1855 when John Snow proved disease could be transmitted by contaminated water, that filtration was discovered to remove the agents of disease. Following on from this, in 1876 Robert Koch developed the Germ Theory of Disease, proving that microorganisms cause sickness, and by 1886, Koch’s bacteriological techniques were used to demonstrate that slow sand filters removed bacteria.

Direct pathogen tests
A well-operated slow sand filter will remove protozoa such as Endamoeba histolytica and helminths such as Schistosoma haematobium and Ascaris lumbricoides (IRC, 1981 [ref.01]Ref.01: IRC (1981). Small Community Water Supplies: Technology of Small Water Supply Systems in Developing Countries. Hofkes, E.H. (Ed.) Technical Paper Series 18. IRC, Rijswijk, The Netherlands. Available here). In percentage reduction, this translates to a 95% removal of coliforms and a 99% removal of Cryptosporidium and Giardia cysts. This is partly because turbidities, which are related to pathogen presence in raw water, can also be reduced to less than 1 NTU (WEDC, 1999 [ref.02]Ref.02: Unpublished information supplied by WEDC, 1999.).

Wheeler et al (1988 [ref.03]Ref.03: Wheeler, D. ; Bartman, J. ; Loyd, B.J. (1988) The removal of viruses by filtration through slow sand filtration : recent developments in water treatment technology. Graham, N.J.D. (Ed.) John Wiley & Sons, New York, USA.) studied sand filtration systems and found that in 50cm of sand, removal rates of between 95 – 100% can be achieved for polio viruses, and 99.75 – 99.996% of NS2 coliphage. Bellamy et al (1985 [ref.04]Ref.04: Bellamy, W.D.; Silverman, G.P.; Hendricks, D.W.; Logsdon, G.S. (1985) Removing Giardia cysts with slow sand filtration. American Water Works Association Journal (77), 2, p.52.) found that slow sand filters can remove Giardia cysts by over 99.99%.

Indirect indicator tests
Studies show that continually operated filters are able to remove pathogenic organisms from untreated water with a high efficiency rate. In general, filters can reduce the total bacteria count (faecal and non-faecal organisms) by a factor of 103 to 104, and faecal bacteria (mainly E. coli) by a factor of 102 to 103 (IRC, 1981 [ref.01]Ref.01: IRC (1981). Small Community Water Supplies: Technology of Small Water Supply Systems in Developing Countries. Hofkes, E.H. (Ed.) Technical Paper Series 18. IRC, Rijswijk, The Netherlands. Available here).

Evidence from intermittently operated slow sand filters

With the development of household-level intermittent sand filtration developed by Dr. Manz and the University of Calgary in the early 1990’s, there have been increasingly more studies to show the effectiveness of the intermittent treatment process. These are listed here.

Direct pathogen tests
Testing for the actual pathogenic organisms in the filtrate produces absolute results, rather than indications of the likelihood of pathogen presence, as is the case with indicator tests.  Two such studies that have been carried out on intermittent filters are outlined below.

• In 1999, a study was carried out by Palmateer, et al [ref.05]Ref.05: 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. Article available here. They found that intermittent household sand filters could remove 83+% of total heterotrophic bacterial populations, 100% of Giardia cysts, and 99.98% of Cryptosporidium oocysts when administered in concentrations varying from 10 – 100 times environmental pollution levels. As such, they concluded that intermittent sand filters should be considered to be much more than just a bacteriological filter, since it also has an impact on parasitic cyst removal and decreasing toxicant – genotoxicant concentrations from drinking waters in developing and developed countries.The test was run for 29 days. Cryptosporidium oocysts showed some breakthroughs up to day 22, and from day 22 to day 29 no further oocysts were encountered. Although the filter removed almost all of the oocysts, the remaining oocysts that broke through in the effluent were compared to the potential infective dose. The infective dose of a pathogen is how much of that particular organism a human needs to ingest before contracting the disease. For some pathogens, the infective dose is small while for others it is larger. For Cryptosporidium, some studies have already been done to determine this dose. In a review by Kindzierski and Gagos (1996), it was noted that in an experimental feeding trial where 16 people were given doses of approximately 300 Cryptosporidium parvum oocysts, 14 of these became infected – an infection rate of 88%. In contrast, Haas and Rose (1995) have suggested that in highly vulnerable populations, the possibility of an outbreak may exist at concentrations varying from 10 – 30 oocysts per 100 ml of finished water. If this is the case, even taking the lower figure of 10 oocysts per 100 ml, this would mean that drinking even 3 litres on a day when the oocysts broke through, would mean a high chance of infection. During the study this potentially infective dose for susceptible individuals was reached on only two days; day 7 when 28.12 oocysts per 100 ml, and on day 11 when 25 oocysts per 100 ml were detected.The conclusions from this study indicate that intermittent slow sand filtration is a very efficient system for removing parasitic cysts from water intended for human consumption.
• In 2001, a study was published by Logan et al [ref.06]Ref.06: Logan, A.J.; Stevik, T.K.; Siegrist, R.L.; Rønn, R.N. (2001). Transport and fate of Cryptosporidium parvum oocysts in intermittent sand filters. Wat. Res. Vol. 35, No. 18, pp.4359-4369. concerning the transport and removal of Cryptosporidium parvum oocysts. The transport potential of C. parvum through intermittent, unsaturated, sand filters used for water and wastewater treatment was investigated. Sixteen columns were dosed eight times daily for up to 61 days with 65,000 Cryptosporidium parvum oocysts per litre, and samples were taken from the effluent as well as from the sand beds themselves. The fine-grained sand columns (D10 0.16 mm) effectively removed oocysts under the variety of conditions examined, with low concentrations of oocysts infrequently detected in the effluent. Coarse-grained media columns (D10 0.90 mm) yielded larger numbers of oocysts, which were commonly observed in the effluent regardless of operating conditions. Factorial design analysis indicated that grain size was the variable that most affected the oocyst effluent concentrations in these intermittent filters. Loading rate had a significant effect when coarse-grained media was used and lesser effect with fine-grained media while the effect of feed composition was inconclusive. No correlations between turbidity, pH, and effluent oocyst concentrations were found. Pore-size calculations indicated that adequate space for oocyst transport existed in the filters. It was therefore concluded that processes other than physical straining mechanisms are mainly responsible for the removal of Cryptosporidium parvum oocysts from aqueous fluids in intermittent sand filters used under the conditions studied in this research.On most of the sand columns the test was run for 27 days, while two of the test continued for a further 26 days with a with higher hydraulic loading rate. Cryptosporidium oocysts showed some breakthroughs depending on the sand type and loading rate. The fine sand columns had a maximum breakthrough of 2.5 oocysts per 100 ml regardless of hydraulic loading rate (4, 10 or 20 cm). The coarse sand that had a 4cm hydraulic loading also had breakthroughs of only 1 oocyst per 100 ml. A potential infective dose (greater than 10 oocysts per 100 ml) was therefore only reached in coarser sands with hydraulic loads of 10cm and 20cm. The two columns with coarse sands and hydraulic load of 10cm had a total of 3 breakthroughs in 27 days of 40, 40 and 20 oocysts per 100 ml, similar to the study of Palmateer, et al (1999 [ref.05]Ref.05: 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. Article available here). The one column that was loaded with 20cm water had breakthroughs at each test during the following 26 days – results were 17.6, 82.4 and 25 oocysts per 100 ml.Althougth the tests were carried out on a different filter set up to that of the household intermittent sand filter, it is interesting to draw some possible conclusions. It could be that if coarser sand media has to be used for intermittent slow sand filtration, effective filtration of Cryptosporidium parvum could still be achieved through lowering the hydraulic loading on the filter, which in turn would slow down the filtration rate per unit area. This is especially applicable for household sand filtration in developing countries, where often quality control of the sand media used is difficult to achieve. The filter used in the study by Palmateer, et al (1999 [ref.05]Ref.05: 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. Article available here) probably had a significant hydraulic load, which could have been responsible for oocyst breakthroughs. However, this is difficult to know for sure since the sand size used was not specified. More research is needed on whether hydraulic loads of greater than 20cm will affect even fine sand.Reducing the hydraulic load and consequent flow rate as an idea is backed up by Buzunis (1995 [ref.07]Ref.07: Buzunis, B.J. (1995) Intermittently Operated Slow Sand Filtration: A New Water Treatment Process. MSc Thesis, University of Calgary, Canada, p.163.). He noted that there was a dip in removal efficiency for water that had not been held in the filter during a pause period.  It was found that incompletely metabolized contaminants from the new water were being swept through the filter because of the higher initial flow rate.  The higher flow rate was a result of increased hydraulic conductivity within the biologically active zone, since during pause time the deposited matter was matabolized so efficiently as to open up the pore spaces further. Presumably with a lower hydraulic load and thus flow rate, fewer breakthroughs would be observed. This is certainly an area for further research – for more information about further research needed on the intermittent bio-sand filter, click here.
• Research carried out by Elliott et al (2008 [ref.17]Ref.17: 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. These removal rates were also for water that had been sitting in the filter during pause time, and for new flush water the removal rates dropped to 93% and 82% respectively.

Indirect indicator tests
Most testing on intermittent sand filters has been done using indirect methods. A summary of such methods and their drawbacks can be found here.

• During an 8-month study in Ghana in 2008, Stauber et al (2012, [ref.20]Ref.20: Stauber, C.E.; Kominek, B.; Liang, K.R.; Osman, M.K.; Sobsey, M.D. (2012) Evaluation of the Impact of the Plastic BioSand Filter on Health and Drinking Water Quality in Rural Tamale, Ghana. Int. J. Environ. Res. Public Health 2012, 9, 3806-3823; doi:10.3390/ijerph9113806. Full article is available here.) found that the plastic biosand filter achieved a geometric mean reduction of 97% for E. coli. The results were part of a wider health impact stidy that suggest the plastic biosand filter significantly improved drinking water quality and reduced diarrheal disease during the short trial in rural Tamale, Ghana. The results are similar to other trials of household drinking water treatment technologies.
• Water quality test results from field samples from two and three-year old filters collected during a demand-led BSF project by Tearfund in Afghanistan showed an average water treatment efficiency of 91.7% for E. coli bacteria. 13.3% of the filtered water samples achieved the WHO guideline value of 0 cfu/100ml, while 64.4% achieved the “low risk” level of <10 cfu/100ml, and 22.2% measured greater than 10 cfu/100ml and are considered “intermediate risk” (Burt, 2012; [ref.19]Ref.19: Burt, M. (2012) Evaluation of a demand led biosand filter programme in the complex emergency context of Afghanistan. Tearfund, Teddington, UK.).
• A 2008 evaluation in Nicaragua of filters that were installed between 1999 and 2004 revealed that out of the 234 installed filters, only 24 were still in use, the low rate of use being largely attributed to construction and maintenance issues. However, the average filtration efficiency was found to be 98% for total coliforms, 96% for E. coli and 88% for turbidity. The thesis can be downloaded by clicking here.
• During two laboratory studies documented by Stauber et al (2006,  [ref.18]Ref.18: Stauber, C.E.; Elliott, M.A.; Koksal, F.; Ortiz, G.M.; DiGiano, F.A.; Sobsey, M.D. (2006). Characterisation of the biosand filter for E. coli reductions from household drinking water under controlled laboratory and field use conditions. Water Sci Technol. 2006; 54(3): 1-7. Abstract available here.), mean E. coli reductions were 94% and they improved over the period of filter use, reaching a maximum of 99%. Field analysis conducted on 55 household filters in the Dominican Republic averaged E. coli reductions of 93%. The E. coli reductions by the BSF in laboratory and field studies were less than those typically observed for traditional slow sand filters, although as for traditional filters microbial reductions were found to improve over the period of filter use.
• A Medair evaluation carried out in 2003 by Fewster, Mol and Wiesent-Brandsma [ref.08]Ref.08: Fewster, E.; Mol, A.; Wiessent-Brandsma, C. (2004) The Bio-Sand Filter. Long term sustainability: user habits and technical performance evaluated. Presentation given at the 2003 International Symposium on Household Technologies for Safe Water, 16-17 June 2004, Nairobi, Kenya. Available online. tested water from 51 sand filters in Kenya that had been sold to householders in 1999. Testing was done using membrane filtration to detect faecal coliforms. While just over half of the filters showed a removal rate of more than 95%, the percentage decrease or increase was found to only be a rough indicator for filter performance. This was because the figures did not accurately reflect whether the filtrate itself had an acceptable level of coliforms (which for the study was taken as 10 coliforms or less per 100 ml filtrate). For example, all the households that showed no change in coliform levels actually had no initial levels. Alternatively, some households showing high percentage reductions could still end up having unacceptable levels of coliforms in the filtrate. Because of this, the evaluation concentrated on actual coliforms remaining in the filtrate, finding that 71% of filters still produced filtrate with acceptable levels of coliforms, with several just above the acceptable level. Note that these filters had been sold between 2½ – 4 years previously, and had been used since being sold with no outside assistance. Considering this, the filters have performed very well taking into account the ‘bush’ factor. Given what is known about intermittent sand filtration on the mechanical removal of pathogens such as Giardia, it can be concluded that even the owners of underperforming filters still have access to water of greatly increased quality compared to the untreated water they were using previously. All filters, whether performing optimally or not, therefore are likely to contribute significantly to improved health.
• An evaluation carried out by Kaiser et al (2002 [ref.09]Ref.09: Kaiser, N.; Liang, K.; Maertens, M.; Snider, R. (2002) BSF Evaluation Report. Submitted to Samaritan’s Purse, Canada.) of 577 intermittent household sand filters located in 6 countries found that on average, 93% of faecal coliforms in the raw water were removed by the filters. Faecal removal rates varied for individual countries as follows:
  Honduras 100%
  Nicaragua 99%
  Mozambique 98%
  Kenya 94%
  Cambodia 83%
  Vietnam 81%
• In 2002, the Family Bible Fellowship carried out 31 field tests on filters installed in Guatemala and El Salvador. The average removal rate for E. coli was 83%, representing between 5 and 100 coliforms in the effluent. Possible reasons for the higher results were attributed to flow rates that were too fast (2 to 3.3 litres/min). In addition, most of the filters had caps on the exit spouts, which resulted in water filling to the top of the reservoir and reducing oxygen transfer to the biological layer.
• In 2001, a total of 39 sets of samples were tested from intermittent bio-sand filters in Nepal by Hurd et al [ref.10]Ref.10: Hurd, J.; Tse, L.; Paynter, N.; Smith, M. (2001) Nepal Water Project. Massachusetts Institute of Technology, USA. Summary available online.. Of these, 14 did not show favourable results in terms of the removal of microbial contamination using tests to detect H2S bacteria, total coliforms and E. coli). Of the subset that did not work, 63% were found to have problems either with the diffuser plate, the resting water level or the maturity of the biofilm. Since these filters were not considered representative of the microbial removal efficiency of the filters, they were excluded in the results. The results of filters that were working properly showed using presence/absence tests that 75% removed total coliforms, 83% removed E. coli and 89% removed H2S-producing bacteria.
• In 2001, a set of presence/absence tests carried out by Lee [ref.11]Ref.11: Lee, T-L. (2001). Biosand household water filter project in Nepal. MSc Thesis, Massachusetts Institute of Technology, USA. on filters in Nepal found that while filtered water from the filters had low turbidities, 9 out of 12 properly functioning filters completely removed total coliforms and 10 out of 12 properly functioning filters completely removed E. coli. The presence/absence tests could not give bacteria numbers, but could only confirm the presence or absence of the organisms. Membrane filtration tests (in which numbers of organisms present are counted) were carried out at the Massachusetts Institute of Technology. The results indicated that the filter technology was effective at removal of total coliforms with an average removal of 99.5%. Out of 5 tests, the filter reduced total coliforms from an average of 630 to 3 per 100 ml. The study recommended that the filters should be replicated but only with proper construction and operation and maintenance supervision.
• The Global Outreach Student’s Association (GOSA [ref.12]Ref.12: Global Outreach Student’s Association (GOSA) Guatemala 2001.) in Guatemala carried out 3 tests in 2001. About 25 filters were built and installed during the project. The tests revealed a raw water coliform average of 1781 per 100 ml that reduced to an average of 7 coliforms per 100 ml after 14 days. The average removal rate was 99.6%.
• Medair carried out two sets of bacteriological tests in 1999 and 2000 on filters installed in Machakos District, Kenya (Medair, 2000 [ref.13]Ref.13: Medair (2000). Evaluation / Final Report – Family Bio-Sand Filtration Project in Machakos District, June 1999 – September 2000.). Random testing of 160 installed filters showed an average coliform removal rate between 91% -93%. It has to be mentioned that this average was brought down by 6 samples with a count of less than 80%, caused by owners misusing the filter. Excluding these samples an average removal rate of 96% was established, while in all but 11 cases turbidity was reduced to less than 5 NTU. Except for 17 cases drinking water was produced with less than 10 coliforming units per 100 ml – an acceptable standard for most of rural Africa.
• In Nicaragua, an investigation was carried out in 1999 into pesticide and bacterial contamination of rural drinking water wells and the efficiency of the bio-sand filter in their removal. With respect to bacterial removal, the removal rates ranged from 64.4 to 95.0%, with an average of 79.9% for faecal coliforms. They did not provide any information on how many water tests were performed and during what conditions. It appears to have been a fairly short test over a short time period.
• In Kenya and Uganda, a Samaritan’s Purse project distributed 25 bio-sand filters, which benefited at least 200 people. A report produced by Snider (1999 [ref.14]Ref.14: Snider, R. (1998) Clean Water for Londiani – A Final Report on the Biosand Water Filter Project In East Africa, June 20 – Oct 20, 1998.) showed removal rates on the 25 filters. For the first filter, the total coliform removal rate was 88.9% during the first week, improving to 98.3% by week 8. Similarly, the E. coli removal rate started at 86.1% in the first week and went up to 97.9% by week 8. The average % reduction was 95 % for total coliforms and 94 % for E. coli. At the time, the raw water had total coliforms of 400 to 600 while the levels of E. coli were about 375. The other 24 filters were also tested with the average E. coli reduction being 93.32 % and the average total coliform reduction being 96.07 %.
• The Lai Yen Water Filter Project run by Samaritan’s Purse in Vietnam in 1998 set up 100 bio-sand filters in the community. Testing was done via ColiStrips. Raw water bacteria levels were were off the scale. Nevertheless, the average filter efficiency was 95.8%. The results were as follows:
  7 out of 38 filters (22.5%) indicated 99% or greater efficiency
  17 out of 38 filters (55%) indicated an efficiency of 90 – 99% efficiency
  7 out of 38 filters (22.5%) indicated less than 90% efficiency
  Considering the fact that some of the source water was more than 10 000 coliforms/100 ml, and that the average was over 2500 coliforms/100 ml, the efficiency of the filter was remarkable.
• A report was carried out on a bio-sand filter project in Brazil in 1998 (Liang, 1998 [ref.15]Ref.15: Liang, K. (1998) A report on the BioSand Water Filter Project in the Amazon Basin. Agua de Saude, Santarem, Brazil.). 55 filters were constructed and installed in the Amazon area east of the city of Santarem from July to November 1998. After the filters had been in operation for at least two weeks, testing was done using colistrips and colibags, which tested for faecal coliforms and E. Coli. The average removal rate for the project was 99.7% for faecal coliform removal, and 98.6% for removal of E. Coli.
• In 1996 and with funding from UNCHS, SERVE began research on an appropriate slow sand filter for use in households (Gresham, 1998 [ref.16]Ref.16: Gresham, B. (1998) The household slow sand filter. Footsteps, No.35, June 1998, p.11.). A number of filters were designed and tested before a particular model was settled on. After three months of testing with heavily polluted water, the filter was removing 98% to 99% of all contaminating organisms.
• An in-depth thesis on the functioning of the household bio-sand filter was carried out in 1995 (Buzunis, 1995 [ref.07]Ref.07: Buzunis, B.J. (1995) Intermittently Operated Slow Sand Filtration: A New Water Treatment Process. MSc Thesis, University of Calgary, Canada, p.163.). Considerable data was taken over a 55 day test period using influent water averaging 1300 coliforms/100 ml taken from a river lagoon. The experimental filter, which had a water layer of 12.5 cm, was found to be effective in removing 96% of faecal coliform indicators while reducing turbidity levels to < 1 NTU. A mathematical model to describe the diffusion of oxygen transfer into the filter bio-layer was developed and supported by experimental data.
• 55 filters were installed by Dr Manz and Buzunis in Nicaragua in 1993. The filters were installed mainly in households in Valle Menier. Each filter had 3 faecal coliform tests in June 1994 and November 1994. After 21 days of operations, faecal coliform removal ranged from a low of 86.67% to a high of 100 %, with an average of 97% removal. After 2 months of operation, the faecal coliform removal averaged 96.4% for the 55 filters.

References:

Ref.01: IRC (1981). Small Community Water Supplies: Technology of Small Water Supply Systems in Developing Countries. Hofkes, E.H. (Ed.) Technical Paper Series 18. IRC, Rijswijk, The Netherlands. Available here

Ref.02: Unpublished information supplied by WEDC, 1999.

Ref.03: Wheeler, D. ; Bartman, J. ; Loyd, B.J. (1988) The removal of viruses by filtration through slow sand filtration : recent developments in water treatment technology. Graham, N.J.D. (Ed.) John Wiley & Sons, New York, USA.

Ref.04: Bellamy, W.D.; Silverman, G.P.; Hendricks, D.W.; Logsdon, G.S. (1985) Removing Giardia cysts with slow sand filtration. American Water Works Association Journal (77), 2, p.52.

Ref.05: 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. Article available here

Ref.06: Logan, A.J.; Stevik, T.K.; Siegrist, R.L.; Rønn, R.N. (2001). Transport and fate of Cryptosporidium parvum oocysts in intermittent sand filters. Wat. Res. Vol. 35, No. 18, pp.4359-4369.

Ref.07: Buzunis, B.J. (1995) Intermittently Operated Slow Sand Filtration: A New Water Treatment Process. MSc Thesis, University of Calgary, Canada, p.163.

Ref.08: Fewster, E.; Mol, A.; Wiessent-Brandsma, C. (2004) The Bio-Sand Filter. Long term sustainability: user habits and technical performance evaluated. Presentation given at the 2003 International Symposium on Household Technologies for Safe Water, 16-17 June 2004, Nairobi, Kenya. Available online.

Ref.09: Kaiser, N.; Liang, K.; Maertens, M.; Snider, R. (2002) BSF Evaluation Report. Submitted to Samaritan’s Purse, Canada.

Ref.10: Hurd, J.; Tse, L.; Paynter, N.; Smith, M. (2001) Nepal Water Project. Massachusetts Institute of Technology, USA. Summary available online

Ref.11: Lee, T-L. (2001). Biosand household water filter project in Nepal. MSc Thesis, Massachusetts Institute of Technology, USA.

Ref.12: Global Outreach Student’s Association (GOSA) Guatemala 2001.

Ref.13: Medair (2000). Evaluation / Final Report – Family Bio-Sand Filtration Project in Machakos District, June 1999 – September 2000.

Ref.14: Snider, R. (1998) Clean Water for Londiani – A Final Report on the Biosand Water Filter Project In East Africa, June 20 – Oct 20, 1998.

Ref.15: Liang, K. (1998) A report on the BioSand Water Filter Project in the Amazon Basin. Agua de Saude, Santarem, Brazil.

Ref.16: Gresham, B. (1998) The household slow sand filter. Footsteps, No.35, June 1998, p.11.

Ref.17: 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.18: Stauber, C.E.; Elliott, M.A.; Koksal, F.; Ortiz, G.M.; DiGiano, F.A.; Sobsey, M.D. (2006). Characterisation of the biosand filter for E. coli reductions from household drinking water under controlled laboratory and field use conditions. Water Sci Technol. 2006; 54(3): 1-7. Abstract available here.

Ref.19: Burt, M. (2012) Evaluation of a demand led biosand filter programme in the complex emergency context of Afghanistan. Tearfund, Teddington, UK.

Ref.20: Stauber, C.E.; Kominek, B.; Liang, K.R.; Osman, M.K.; Sobsey, M.D. (2012) Evaluation of the Impact of the Plastic BioSand Filter on Health and Drinking Water Quality in Rural Tamale, Ghana. Int. J. Environ. Res. Public Health 2012, 9, 3806-3823; doi:10.3390/ijerph9113806. Full article is available here.