Biological action occurs in slow sand filter beds. Because of the low hydraulic loading and smaller sand size found in slow sand filters, most of the solid particles are removed within the top 0.5 - 2 cm of sand, as opposed to rapid filters where the penetration is much deeper. With time, this area develops a biological film called the schmutzdecke as well as a zone of biological activity within the sand bed. The various purification processes in both the schmutzdecke and deeper biological zones, are interdependent and are best described together. One example of such interdependence is where trapped impurities on and within the filter bed are broken down and rendered innocuous, with by-products of one process fuelling another deeper down.
The two principal actions that contribute to the overall purification effect are chemical and microbiological oxidation, but other biological processes involving various forms of animal and vegetable life may play a significant part.
The schmutzdecke defined
The word schmutzdecke is derived from the German for 'dirty layer'. This sticky film, which is reddish-brown in colour, consists of decomposing organic matter, iron, manganese and silica and therefore acts as a fine filter that contributes to the removal of fine colloidal particles in the raw water. The schmutzdecke also doubles up as an initial zone of biological activity, providing some degradation of soluble organics in the raw water, which is useful for reducing tastes, odours and colour (WEDC, 1999 [ref 01]).
It is important to note that the schmutzdecke film is not the only area of biological activity in a slow sand filter. Often the term schmutzdecke appears to be used sometimes to denote the general zone of biological activity that occurs within the sand bed. However, this zone is distinct. Due to its double function, which includes mechanical filtration, the depth of the schmutzdecke can be said to correlate to the penetration zone of solid particles, that is, the top 0.5 - 2 cm of a slow sand filter bed. At this depth range, the schmutzdecke merges with the deeper biological layer, and particle-free raw water flows into this zone after passing through the schmutzdecke. This deeper zone is therefore not largely a mechanical filtration zone but rather a continuation of the area of biological action.
The schmutzdecke should remain undisturbed. This is so that the biological population in the top centimetres of sand are not disturbed or stressed, while not allowing any film that has built up to be broken, which would reduce the film's straining effect while pushing solids further into the sand. A device is usually used to disperse the energy of water as it enters a filter. This can take the form of a baffle plate, or in the case of intermittent sand filters, a diffuser plate which sits close to the water surface.
Purification processes within the schmutzdecke and the biological zone
The purification mechanisms described here originate mainly from Huisman and Wood (1974 [ref 02]).
While the exact nature and dominance of the various processes is still not completely clear, a review of the literature suggests that there are four dominant processes which contribute to the purification of the raw water:
- Hostile Environment - Conditions found within a slow sand filter are generally unsuitable for the multiplication of intestinal bacteria. Normally used to a body temperature of 37°C, they do not thrive at temperatures below 3O°C.
- Competition for food - Food is required by the bacteriological population for metabolism. Oxidisation processes during metabolism consume organic matter in the raw water, including dead pathogens. The filter bed does not usually contain enough organic matter of animal origin to meet their nutritional needs. Within the upper layers there is competition for food from other microbes while at lower depths suitable food becomes even scarcer so that they starve, particularly at higher temperatures when their metabolic rate increases.
- Predation - Many types of predatory organisms (such as protozoa and lower metazoa) abound in the upper part of the bed and feed on other cells.
- Excretion of poisons - while there is little quantitative data available, it is known that the microorganisms in a slow sand filter produce various substances that act as chemical or biological poisons to intestinal bacteria.
The combined effects of this selectively hostile environment results in the death and inactivation of many pathogens (disease causing organisms). The overall result is a substantial reduction in the number of indicator bacteria such as E. coli, and an even greater proportional decrease in pathogens themselves. This effect becomes greater as the flora and fauna of the filter develop in the presence of adequate food, oxygen, and suitable temperatures.
Bacteria produce a slimy substance consisting of exocellular polymers as well as living and dead cells - this substance is known as zoogeal (Brock and Madigan, 1991 [ref 03]). Within the schmutzdecke and this zoogeal film, bacteria derived initially from the raw water multiply selectively, the deposited organic matter being used as food. The bacteria oxidize part of the food to provide the energy they need for their metabolism (dissimilation), and they convert part of it into cell material for their growth (assimilation). Thus dead organic substances are converted into living matter. The dissimilation products are carried away by the water, to be used again at greater depth by other organisms.
The bacterial population is limited by the amount of organic material supplied by the inflowing raw water; the growth (assimilation) is therefore accompanied by an equivalent dying off. This in its turn liberates organic matter, which becomes available to bacteria at lower depths. In this way the whole of the degradable organic matter present in the raw water is gradually broken down and converted into water, carbon dioxide, and relatively innocuous inorganic salts such as sulphates, nitrates, and phosphates (mineralization) to be discharged in the filter effluent. The bacterial activity described is most pronounced in the upper part of the filter bed and gradually decreases with depth as food becomes scarcer.
The depths of processes
Research done on continually operated slow sand filters shows that the majority of biological processes occur in the top 0.4m of the sand bed (ASCE, 1991 [ref 04]). This is confirmed by results that show that bacteriological purification occurs mostly in the top 40 cm of the sand bed below the schmutzdecke (more). Biological activity does, however, occur deeper within the filter bed. Below a depth of 30-40 cm (depending on the filtration rate) bacterial activity is small, but biochemical reactions take place converting organic materials such as amino acids into ammonia, nitrites, and nitrates (nitrification). These amino acids are liberated by the bacterial life cycle in the upper sand layer (Huisman and Wood, 1974 [ref 05]). This has been confirmed by test data where oxidation of nitrogenous organic compounds at depths shallower than 40cm was found to be incomplete (Muhammad et al, 1996 [ref 05]).
For intermittently operated sand filters, the depth of biological processes is also dependent on how much water is standing on top of the sand during pause times. A shallower water depth means that more oxygen is able to diffuse to the biological layer, and as a result the biologically active zone can grow deeper within the sand. A study done by Buzunis (1995 [ref 06]) found that with 12.5cm of standing water, the biological zone was only 10cm deep.
Keeping the sand bed wet
For the survival of the microorganisms within the biological zone, the sand must be kept wet. The sand filter bed is kept wet by sand filter design, where the outlet level is made above the level of the sand. This always ensures that the filter bed does not dry out.
For the survival of the microorganisms within the biological zone, there needs to be a supply of food in the raw water. Seeding the filter with biologically productive raw water ensures more efficient biological filtration (Palmateer et al, 1999 [ref 07]).
For the survival of the microorganisms within the biological zone, there needs to be a supply of oxygen. Oxygen is used in the metabolism of biodegradable components and the inactivation and consumption of pathogens. If it falls to zero during filtration anaerobic decomposition occurs, with consequent production of hydrogen sulphide, ammonia, and other taste- and odour-producing substances together with dissolved iron and manganese, which make the treated water unsuitable for washing clothes and other purposes. Thus the average oxygen content of the filtered water should not be allowed to fall below 3 mg/l if anaerobic conditions are to be avoided throughout the whole area of the filter bed (Huisman and Wood, 1974 [ref 02]). This requirement may call for aeration of the raw water to increase its oxygen content or pre-treatment to lower its oxygen demand.
Finding a way to allow enough oxygen transfer to sustain the biological layer has been essential in the design of the intermittent household slow sand filter. An in-depth study was carried out in 1995 (Buzunis, 1995 [ref 06]) and a mathematical model to describe the diffusion of oxygen transfer into the filter bio-layer was developed and supported by experimental data.
For satisfactory biochemical oxidation of organic matter by the organisms in the biological layer, sufficient time must also be allowed to maintain a long enough contact time with the sand bed.
Research published by Elliott et al (2008 [ref 08]) and carried out by the University of North Carolina confirmed the importance of residence time of water in a filter. During six-to-eight week long studies conducted on the biosand filter, filters were fed with surface water spiked with E. coli, echovirus type 12 and bacteriophages (MS2 and PRD-1). They found that the performance of the filter in reducing microbial concentrations in water fed into the filter depended largely on
- The amount of time that the filter bed had to ripen and
- The daily volume of water put through the filter each day.
Regarding the daily amount of water put through the filter, they found that microbial reductions were greater with a greater residence time within the filter, especially for water retained in the filter bed overnight. The researchers showed this through taking samples of filtered water at various stages when the filter was re-started after pause time – a significant drop in filtrate quality was noted after the pore volume threshold had been filtered which in their case was 18.3 litres. This showed that water that had been sitting in the filter during pause time had a much better quality, and this seems to be largely due to the increased contact time for biological and chemical processes in the sand.
Research done by Jenkins et al (2009, [ref 09]; 2011, [ref 10]) of the University of California, Davis, has built on this understanding about the importance of residence time. They investigated the effect of pause time on efficiency at removing viruses, bacteria and turbidity but also looked at other parameters that influence flow rate and therefore residence time, namely hydraulic loading and sand size. The results were:
- They confirmed previous findings about the impact of pause time on water quality. In general, they found that water quality improved with longer residence times (mean of 16 hours) compared to shorter residence times (mean of 5 hours).
- But they also tested filters with varying levels of hydraulic loading above the sand surface (10, 20 and 30 cm) and two sand sizes (0.17 mm and 0.52 mm). They found that bacteria and virus removal was significantly better for filters with finer sand and those with lower head, independently from each other and for both short and long term residence times, but that results were enhanced with longer residence times. The best combination was 0.17 mm sand with 10 cm head over longer residence times.
There are design considerations that should be considered as a result of this research. Click here for discussion on application of the research to the current concrete filter design.
The effect of temperature
The temperature of the water must not be allowed to fall too low for satisfactory biochemical oxidation of organic matter to take place by the organisms in the biological layer. The efficiency of slow sand filtration may also be seriously reduced by low temperatures, owing to the influence of temperature both on the speed at which chemical reactions take place and on the rate of metabolism of bacteria and other microorganisms.
This is illustrated by the effect of temperature on the reduction in permanganate consumption brought about by slow filtration. If T is the temperature in degrees Celsius, the reduction in permanganate consumption in milligrams per litre is equal to (T+ 11)/9. Thus at 25°C permanganate consumption is reduced by 4 mg/l, while at 7°C it is reduced by only 2 mg/l.
Below 6°C the oxidation of ammonia practically comes to a standstill. When air temperatures drop below 2°C for any considerable period it is necessary either to cover the filters to reduce heat losses or to provide for subsequent chlorination as a precaution against incomplete purification in the filtration plant.
At low temperatures, the activity of bacteria-consuming protozoa and nematodes drops sharply, and at the same time the metabolism of the intestinal bacteria themselves slows down, increasing the chance of survival of those that are carried through the bed. The factor by which the numbers of E. coli are reduced, which is normally in the range 100-1000, may fall as low as 2 at temperatures of 2°C or less, and chlorination is then essential if the quality of the delivered water is to be maintained.
The biological layer is made up of a variety of microorganisms. These include algae, bacteria, protozoa and small invertebrates. The types of microorganisms and the relative number of each species are specifically adapted to the characteristics of the influent water source and the environment of the filter (Buzunis, 1995 [ref 06]).
Different types of bacteria are normally found at various depths below the filter surface, the true water bacteria predominating at deeper levels. This indicates a subdivision of the filter bed into zones, in each of which specific bacteria abound, each producing well-defined effects.
A biofilm takes some time to develop naturally. This is usually at least around 2 - 3 weeks. Palmateer, et al (1998 [ref 07]) measured the development of a biofilm and found that at 21 degrees C it took 16 days for the biological film to develop to 85-90% cover. They noted that having a raw water that is more biologically productive will mean that the biofilm will develop more quickly and that the filter will operate more efficiently.
Research done by Elliott et al (2008 [ref 08]) showed that microbial reductions improved over time with ripening, but an enhanced reduction was noted after 30 days. Microbial reductions continued to improve even up to 53 days during the research, indicating that the longer the better and that ripening can take some time.
References: (jump back)
Ref 01: Unpublished information supplied by WEDC, 1999.
Ref 02: Huisman, L; Wood, W.E. (1974). Slow Sand Filtration. WHO, Geneva, Switzerland. pp. 31-4. Available from IRC.
Ref 03: Brock, T.D.; Madigan, M.T. (1991). Biology of Microorganisms. 6th Edition. Prentice Hall, New Jersey.
Ref 04: ASCE (1991). Slow sand filtration. Logsdon, G.S. (Ed). American Society of Civil Engineers, New York, USA.
Ref 05: 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 here.
Ref 06: Buzunis, B.J. (1995) Intermittently Operated Slow Sand Filtration: A New Water Treatment Process. MSc Thesis, University of Calgary, Canada. p.163.
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. Available here.
Ref 08: 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 09: Jenkins, M.W.; Tiwari, S.K.; Darby, J.; Nyakash, D.; Saenyi, W.; Langenbach, K. (2009). The BioSand Filter for Improved Drinking Water Quality in High Risk Communities in the Njoro Watershed, Kenya. Research Brief 09-06-SUMAWA, Global Livestock Collaborative Research Support Program. University of California, Davis, USA. Available here.
Ref 10: Jenkins, M.W.; Tiwari, S.K.; Darby, J. (2011) Bacterial, viral and turbidity removal by intermittent slow sand filtration for household use in developing countries: Experimental investigation and modeling. Final draft of submitted paper. Dept of Civil & Environmental Engineering, University of California, Davis, USA. The draft is available here, and the published article is available at http://www.sciencedirect.com/science/article/pii/S0043135411005410. A poster submitted at a household water treatment conference in 2008 also illustrates the findings well, and is available here.