Improving filter media: additions and enhancements

There are several ways for improving the performance of filter media. Such enhancements can improve water quality, flow rate and corrosion control.

Improving water quality

There are two ways to improve water quality. The first is using metal oxide coated sand, the second through the addition of nails to the sand.

Metal oxide coated sand
Some research has been done on enhancing the sand media itself to improve the sand’s filtration effect. Sands that have been coated with metal oxides and hydroxides improve removal efficiencies of pathogens through enhancing microbial adhesion to sand grains, as well as removal efficiencies of heavy metals. Although the interactions are still poorly understood, they appear to be a result of adsorption mechanisms, such as van der Waals forces and electrostatic interactions (Truesdail et al, 1998 [ref.01]Ref.01: 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. and Lukasik et al, 1999 [ref.02]Ref.02: 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. Wat. Res. Vol. 33, No. 3, p.775.).

A study done by Lukasik et al (1999) [ref.02]Ref.02: 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. Wat. Res. Vol. 33, No. 3, p.775. tested tap water seeded with different microorganisms or untreated wastewater which was passed through columns containing sand modified by the in situ precipitation of metallic hydroxides or unmodified sand. Small columns (35.5 x 5.0 cm) packed with 1 kg of sand modified with a combination of ferric and aluminum hydroxide were compared to unmodified columns. For the first 30 days, the modified columns removed greater than 99% of Escherichia coli, Vibrio cholerae, poliovirus 1 and coliphage MS-2 from dechlorinated tap water. This represented more than a 4 log10 reduction in the numbers of these microorganisms as opposed to less than a 1 log10 reduction for untreated sand columns. However, it is interesting to note that by day 48, while the modified columns were still able to remove 80% of E. coli and 90% of poliovirus 1, no significant differences were seen in the ability of columns containing modified or untreated sand in their removal. This was attributed to the possible development of a microbial biofilm on the sand. Interestingly, the columns containing modified sand were still able to remove 99.9% of coliphage MS-2, which represented more than a 3 log10 reduction at the end of the test – a much greater result than for unmodified sand. The optimum pH for viral removals differed between modified and unmodified sand columns. The findings of this study would indicate that metal-coated sand might be useful when good quality water is needed rapidly. One application might be where a water treatment system is set up in an emergency setting, and where good quality water is required before the usual ripening period. Further research might be useful to repeat the test using full-scale sand filters. Click here for further details about possible future research.

Another study by Truesdail et al, (1998) [ref.01]Ref.01: 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. quantitatively compared different metal (hydr)oxide coatings in enhancing bacterial deposition. Specifically, the deposition rates for bacterial strains Streptococcus faecalis, Staphylococcus aureus, Salmonella typhimurium, and Escherichia coli were compared for Ottawa sand and surface coatings consisting of aluminum (hydr)oxide, iron (hydr)oxide, and mixed iron and aluminum (hydr)oxide. The deposition results indicated that the metal (hydr)oxide surface coatings have the ability to increase bacterial deposition by up to a factor of 5 over that observed for untreated Ottawa sand.

In addition to increased pathogen removals, iron oxides are known to adsorb arsenic. A study carried out by Poole (2001) [ref.03]Ref.03: 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. The 2001-2002 MIT Nepal Project investigated iron oxide coated sand as a point-of-use arsenic removal technology. Various technologies were investigated that would remove total arsenic (As (III) + As (V)) effectively below the Interim Nepali Standard of 50 µg/litre. The results showed that the total percent of arsenic removed varied from 11 – 99%. This variability led to only 27% of the effluent sample concentrations to fall below the Nepali Interim Standard of 50µg/L. This was attributed to differences of the sand types used on adsorption properties, since previous studies had shown the success of arsenic removal by iron oxide coated sand media from raw waters with arsenic concentrations up to at least 300 µg/L (Ali et al, 2001 [ref.04]Ref.04: Ali, M.A.; Badruzzaman, A.B.M.; Jalil, M.A.; Hossain, M.D.; Hussainuzzaman, M.M.; Badruzzaman M.; Mohammad, O.I.; Akter, N. (2001) Development of Low-cost Technologies for Removal of Arsenic from Groundwater. University of Engineering and Technology, Bangladesh.). The study noted that further detailed investigation was needed concerning iron oxide properties, as well as sufficient resources allocated to production of the media, which was crucial before iron oxide coated sand technology could be implemented for point-of-use arsenic removal in Nepal or other developing countries.

One concern about using metal-coated media is the negative effect of metal traces in the effluent that might have an effect on health. In the study by Lukasik et al (1999) [ref.02]Ref.02: 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. Wat. Res. Vol. 33, No. 3, p.775. the metal used for coating the sand could not be detected in the column effluents, indicating that the coatings were stable.

Addition of nails in sand
Since numerous studies have shown that iron hydroxide (iron rust) is an excellent adsorbent for arsenic, 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. studied how the simple addition of nails to a sand filter would affect arsenic removal. The Arsenic Biosand Filter was found to be effective in removing arsenic (range = 87 to 96%, mean = 93%). The nails, which were exposed to air and water, rusted very quickly, producing iron hydroxide particles. As such, the process compared to other studies using iron oxide coated sand media since both species of arsenic found in water (arsenide and arsenate) are effectively and tightly bound to the iron hydroxide. The arsenic was rapidly adsorbed onto the surface of the ferric hydroxide particles, which, because of the very small pore spaces of the fine sand layer, were mostly settled out on top of the layer, effectively removing arsenic from the effluent.

Improving flow rate

Anthracite is a material that is commonly used to increase flow rate in sand filtration. However, it is used only in rapid sand filtration. For details, see Schulz and Okun (1984) [ref.06]Ref.06: Schulz, C.R.; Okun, D.A. (1984). Surface water Treatment for Communities in Developing Countries. IT, London. Available from www.developmentbookshop.com.

Improving corrosion control

The associated decrease of pH in slow sand filters, due to CO2 conversion and biological activity, may produce effluent that is slightly corrosive to downstream distribution pipe material in larger scale filters. Rooklidge and Ketchum (2002) [ref.07]Ref.07: Rooklidge, S.J.; Ketchum, L.H. (2002) Corrosion control enhancement from a dolomite-amended slow sand filter. Water Research 36, pp.2689-2694. carried out a pilot study to examine the use of a 3 cm deep crushed dolomite limestone media layer placed within the filter column of a slow sand filter. This was to try to enhance effluent corrosion control. The result was that alkalinity was increased by 19%, and effluent pH averaged 7.7. Note however, that lower pH is preferable for viral removal – for further information, click here.

References:

Ref 01: 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.

Ref 02: 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. Wat. Res. Vol. 33, No. 3, p.775.

Ref 03: 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 04: Ali, M.A.; Badruzzaman, A.B.M.; Jalil, M.A.; Hossain, M.D.; Hussainuzzaman, M.M.; Badruzzaman M.; Mohammad, O.I.; Akter, N. (2001) Development of Low-cost Technologies for Removal of Arsenic from Groundwater. University of Engineering and Technology, Bangladesh.

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: Schulz, C.R.; Okun, D.A. (1984). Surface water Treatment for Communities in Developing Countries. IT, London. Available from www.developmentbookshop.com

Ref 07: Rooklidge, S.J.; Ketchum, L.H. (2002) Corrosion control enhancement from a dolomite-amended slow sand filter. Water Research 36, pp.2689-2694.