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Designs for slow sand filters have to consider all of the interlinked mechanical and biological processes that occur in slow sand filtration, in order to create effective filters. Both continuous and intermittently operated systems have been successfully designed that factor in the essential processes.
Continually operated slow sand filters are designed to function with a continual flow of raw water through the filter. This water brings food and oxygen to the biological layer within the sand bed. In comparison, intermittently operated slow sand filters are designed to function without a continual flow of water into the filter. The organisms within the biological zone in the sand bed tend to be more mobile, moving towards the sand surface during pause time when the water is not flowing (Buzunis, 1995 [ref 01]). During flow times, oxygen and food pass into the filter with the raw water, while during pause times, oxygen transfer takes place by diffusion from the air through a shallow level of supernatant water to the biological layer within the sand (Buzunis, 1995 [ref 01]). The level of standing water during pause times is important in controlling diffusion of oxygen and therefore the development of the biofilm, Palmateer, et al (1998 [ref 02]) suggest that 2 - 3 cm is an efficient level.
Sudden changes in filtration rate had been thought to upset the equilibrium of microorganisms, resulting in deterioration of effluent quality. For this reason, it was considered most desirable to design a slow sand filter to operate continuously without interruption and at as constant a filtration rate as possible. In a similar way the various bacterial populations were thought to be adapted to the type and amount of food supplied by the passing water, and it was recommended that sudden fluctuations in raw water quality should be avoided. While these points may be valid, tests on intermittently operated slow sand filters are encouraging. With the correct design parameters, intermittent filters can operate in a very efficient way.
One interesting difference has been how an intermittent filter responds during the pause time (when water has stopped flowing). During the filter run, contaminants are captured, yet during the pause time, the filter skin pores are opened due to biological degradation of the contaminants that were captured. Buzunis (1995 [ref 01]) found that there was a dip in the removal efficiency in the effluent associated with the water in the top layers of the sand during pause time. However, this was not caused by a simultaneous dip in oxygen levels. Rather, the decline in removal rate resulted from the incomplete metabolism of contaminants trapped in the upper sand layers. These were swept through the filter because of the high flow rate resulting from increased hydraulic conductivity of the biologically active zone.
References: (jump back)
Ref 01: Buzunis, B.J. (1995) Intermittently Operated Slow Sand Filtration: A New Water Treatment Process. MSc Thesis, University of Calgary, Canada.
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. Article available online |
