Effect on chemical water quality

Sand filters can improve the chemical quality of raw water. Traditionally, much of the treatment capacity of sand filters has been attributed to the action of the biological layer, which greatly improves microbiological quality.  However, the results below show that slow sand filters have particular significance to developing countries, which often suffer from chemical contamination of water supplies, largely due to uncontrolled industrial activity.  A water treatment technique that is able to deal with such pollutants in an effective way is very valuable.

Chemicals in water include arsenic, fluoride, chloride, conductivity, dissolved oxygen, iron, manganese, nitrate, nitrite, sulphates, pesticides and heavy metals.

In general, chemical contaminants at low concentrations are difficult to remove from water. Chemical precipitation, reverse osmosis and other methods become inefficient when contaminants are present in trace concentrations. The process of adsorption is one of the few alternatives available for such situations. A number of adsorbent materials have been studied for their capacity to remove heavy metals, including activated carbon, activated alumina, ion exchange resins and crushed coals. Some of these materials, such as ion exchange resins are totally effective but expensive and others, such as coal and straw, are inexpensive but ineffective. Activated carbon is very effective in removing heavy metals, but is readily soluble under extreme pH conditions. Peat moss has been found to be very effective in adsorbing heavy metals.  However, as one of the mechanical processes in slow sand filtration is adsorption, it provides a means of significantly lowering chemical concentrations.  For more information about mechanical processes involved in sand filtration, click here.  Evidence from several studies, looking at both intermittently and continually operated slow sand filters, is detailed below.

Evidence from continually operated slow sand filters

• Slow sand filters have been shown to remove organic chemicals such as cyanobacterial hepatotoxins (or microcystins). Cyanobacterial toxins, or cyanotoxins, encompass a variety of toxic substances produced by cyanobacteria (blue-green algae), which can be observed in surface waters worldwide. Under certain circumstances (particularly elevated nutrient concentrations and low turbulence) they can proliferate massively, causing so-called cyanobacterial blooms, sometimes forming thick scums on water surfaces. Cyanotoxins pose a hazard for animals and people, particularly through oral ingestion. A study done by Grützmacher, et al (2002 [ref.01]Ref.01: Grützmacher, G.; Böttcher, G.; Chorus, I.; Bartel, H. (2002). Removal of microcystins by slow sand filtration. Wiley periodicals. German Federal Environmental Agency (Umweltbundesamt, UBA), Berlin, Germany. Abstract available online) showed that under moderate temperatures, with an intact schmutzdecke (biofilm) with previous contact with microcystins, slow sand filtration is an effective treatment for eliminating microcystins from drinking water.
• In general, slow sand filters are known to be able to achieve a 10% removal of Total Organic Content (WEDC, 1999 [ref.02]Ref.02: Unpublished information supplied by WEDC, 1999.).
• Experimental results on the influences of process variables in removing heavy metals by slow sand filters demonstrated that adsorption was one of the mechanisms of the removal of heavy metals (Muhammad et al, 1997 [ref.03]Ref.03: Muhammad, N.; Parr, J.; Smith, M.D.; Wheatley, A.D. (1997) Removal of Heavy Metals by Slow Sand Filtration. Proceedings of the 23rd WEDC International Conference on Water Supply and Sanitation, Durban, South Africa, pp 167-170.). To confirm this prediction/hypothesis, batch adsorption tests were carried out by Muhammad et al (1998 [ref.04]Ref.04: Muhammad, N.; Parr, J.; Smith, M.D.; Wheatley, A.D. (1998) Adsorption of heavy metals in slow sand filters. 24th WEDC Conference: Sanitation and Water for All, Islamabad, Pakistan, pp.346-9.) to establish adsorption isotherms and adsorption capacity of the sand for the selected heavy metals. The study found that adsorption of heavy metals in slow sand filters is a significant mechanism of removal. Their results are summarized as follows:
  • Adsorption of Cu, Cr, Pb and Cd onto sand satisfied the Langmuir and Freundlich isotherms.
  • The adsorption capacity of sand was the highest for Pb followed by Cu, Cr and Cd.
  • The values of Freundlich exponent n were greater than one for all four heavy metals confirming the adsorption of these metals onto sand.

Evidence from intermittently operated slow sand filters

• In 2010 in Peru, a 2-month study ([ref.09]Ref.09: Laboratorios Analíticos del Sur E.I.R.L. (2010) EVALUACION DE LA EFICIENCIA DE PRECIPITACION ELECTROQUIMICA PARA LA REMOCION DE METALES PESADOS (Cr, Cd, Pb y Fe) BAJO DIFERENTES NIVELES DE CONTAMINACION. Unpublished report.) showed the ability of the biosand filter to remove 3 metals – chromium, cadmium, and iron. Challenge water of three different concentrations of these metals was prepared. The first contained these metals at the maximum allowable limits set by the Peru government (Cr = 50 µg/L; Cd = 3 µg/L; Fe = 300 µg/L). The second challenge water contained 10 times the regulatory limits. The third challenge water contained 100 times the regulatory limits. The average removal percentages for these metals are shown below. All filtered water samples met the Peruvian regulatory limits for drinking water. While the results appear promising, this study lasted only 2 months, which is a relatively short period. Longer-term study is recommended to determine if the removal effectiveness can be maintained and for how long.
  • Chromium
    • 87% reduction
    • 97% reduction
    • 99% reduction
  • Cadmium
    • 91% reduction
    • 98% reduction
    • 99% reduction
  • Iron
    • 85% reduction
    • 89% reduction
    • 99% reduction

• In Bangladesh, pond sand filters (a type of intermittent filter) were shown to reduce iron and manganese by 74% and 51% respectively (Islam et al, 2011 [ref.10]Ref.10: Islam, M.A.; Karim, M.R.; Higuchi, T.; Sakakibara, H.; Sekine, M. (2011) Survey of trace metals in drinking water supply options in coastal areas of Bangladesh. The future of water, sanitation and hygiene: innovation, adaptation and engagement in a changing world. 35th WEDC International Conference, Loughborough, UK. Available online.).
• The intermittent sand filter can also be modified for arsenic and iron removal. The Arsenic Biosand Filter (ABF) was found to be effective in removing arsenic (range = 87 to 96%, mean = 93%) and iron (range = >90 to >97%, mean = >93%) (Ngai and Walewijk, 2003 [ref.05]Ref.05: Ngai, T.; Walewijk, S. (2003) The Arsenic Biosand Filter (ABF) project: design of an appropriate household drinking water filter for rural Nepal. Final Report. Massachusetts Institute of Technology and Stanford University, USA.). On-going research at MIT and Stanford will seek to further enhance filter performance, and user-friendliness.
• A study carried out by Poole (2001 [ref.06]Ref.06: Poole, B.R. (2001) Point-of-use water treatment for arsenic removal through iron oxide coated sand: application for the Terai region of Nepal. MSc Thesis, Massachusetts Institute of Technology, USA.) found that metal-coated sand has been shown to remove arsenic.
• Intermittent slow sand filters, when challenged with organic and inorganic toxicants at levels 10 – 100 times normal environmental levels, were found to be able to retain between 50 – 99% of toxicants. The toxicants that were added during the study were: a Polyaromatic Hydrocarbon (PAH) called phenanthrene, the herbicide metolachlor, a chemical toxicant HgCl2, mercury and nonylphenol (Palmateer, et al, 1999 [ref.07]Ref.07: 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. Abstract available online).  It was concluded that under normal working conditions and at normal environmental concentrations, intermittent filters could easily remove greater than 50% of organic and inorganic toxicants from waters used as potable source water.  This would be costly to cary out through detailed chemical analysis of input waters and filtered waters at ppb and ppt levels, but is an area of further research. It would appear that many of the toxicants are entrapped, biodegraded, or biotransformed in the schmutzdecke as shown by increased toxicant levels over time that were recorded in the schmutzdecke.
• An investigation was carried out in Nicaragua to determine how intermittent sand filters cope with pesticides. The results from this study indicated that the bio-sand filter appeared to be ineffective at removing organochloride pesticides in the field. However, results must be interpreted with care since many factors may have come into play in producing the results. One of these factors was that there were not enough values to undertake proper statistical testing, making the results not statistically significant. This was partly a result of the small sample size (10) and a limited budget of the research team. Further investigation is needed.
• Oxidation of nitrogenous organic compounds occurs in intermittent slow sand filters (Muhammad et al, 1996 [ref.08]Ref.08: Muhammad, N.; Ellis, K.; Parr, J.; Smith, M.D. (1996) Optimization of slow sand filtration. Reaching the unreached: challenges for the 21st century. 22nd WEDC Conference New Delhi, India, 1996. pp.283-5. Available online). Although oxidation was not found to be sensitive to sand size or filtration rates, it was found to increase with sand bed depth. More details can be found within the pages explaining sand as a filter media.

References:

Ref.01: Grützmacher, G.; Böttcher, G.; Chorus, I.; Bartel, H. (2002). Removal of microcystins by slow sand filtration. Wiley periodicals. German Federal Environmental Agency (Umweltbundesamt, UBA), Berlin, Germany. Abstract available online

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

Ref.03: Muhammad, N.; Parr, J.; Smith, M.D.; Wheatley, A.D. (1997) Removal of Heavy Metals by Slow Sand Filtration. Proceedings of the 23rd WEDC International Conference on Water Supply and Sanitation, Durban, South Africa, pp 167-170.

Ref.04: Muhammad, N.; Parr, J.; Smith, M.D.; Wheatley, A.D. (1998) Adsorption of heavy metals in slow sand filters. 24th WEDC Conference: Sanitation and Water for All, Islamabad, Pakistan, pp.346-9.

Ref.05: Ngai, T.; Walewijk, S. (2003) The Arsenic Biosand Filter (ABF) project: design of an appropriate household drinking water filter for rural Nepal. Final Report. Massachusetts Institute of Technology and Stanford University, USA.

Ref.06: Poole, B.R. (2001) Point-of-use water treatment for arsenic removal through iron oxide coated sand: application for the Terai region of Nepal. MSc Thesis, Massachusetts Institute of Technology, USA.

Ref.07: 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. Abstract available online

Ref.08: Muhammad, N.; Ellis, K.; Parr, J.; Smith, M.D. (1996) Optimization of slow sand filtration. Reaching the unreached: challenges for the 21st century. 22nd WEDC Conference New Delhi, India, 1996. pp.283-5. Available online

Ref.09: Laboratorios Analíticos del Sur E.I.R.L. (2010) EVALUACION DE LA EFICIENCIA DE PRECIPITACION ELECTROQUIMICA PARA LA REMOCION DE METALES PESADOS (Cr, Cd, Pb y Fe) BAJO DIFERENTES NIVELES DE CONTAMINACION. Unpublished report.

Ref.10: Islam, M.A.; Karim, M.R.; Higuchi, T.; Sakakibara, H.; Sekine, M. (2011) Survey of trace metals in drinking water supply options in coastal areas of Bangladesh. The future of water, sanitation and hygiene: innovation, adaptation and engagement in a changing world. 35th WEDC International Conference, Loughborough, UK. Available online.