Intermittent versus continuous filtration

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]Ref.01: Buzunis, B.J. (1995) Intermittently Operated Slow Sand Filtration: A New Water Treatment Process. MSc Thesis, University of Calgary, Canada.). 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]Ref.01: Buzunis, B.J. (1995) Intermittently Operated Slow Sand Filtration: A New Water Treatment Process. MSc Thesis, University of Calgary, Canada.). 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]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) suggest that 2 – 3 cm is an efficient level.

Sudden changes in filtration rate have 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 microbes making up the biological population 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. These were some of the reasons why continuous operation of filters was thought to be the only valid way in which to operate a filter. Indeed, recent research (Young-Rojanschi and Madramootoo, 2014 [ref.03]Ref.03: Young-Rojanschi, C.; Madramootoo, C. (2014) Intermittent versus continuous operation of biosand filters. Water Research, Vol. 49, 1 Feb 2014.) has looked at continually-operated filters comapred to intermittently-operated ones. As suspected, continuous operation of the filters resulted in significantly better reduction of Escherichia coli (3.71 log10 versus 1.67 log10), bacteriophage MS2 (2.25 log10 versus 0.85 log10), and turbidity (96% versus 87%), and dissolved oxygen levels at 5 and 10 cm of media depth in intermittent filters reached an average of 0 mg/L by 24 h of residence time on day 60 of the experiment. This confirms that filters perform significantly better when operated continuously, and yet shows that both operational modes result in a significant reduction of microbial indicators.

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]Ref.01: Buzunis, B.J. (1995) Intermittently Operated Slow Sand Filtration: A New Water Treatment Process. MSc Thesis, University of Calgary, Canada.) 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:

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

Ref 03: Young-Rojanschi, C.; Madramootoo, C. (2014) Intermittent versus continuous operation of biosand filters. Water Research, Vol. 49, 1 Feb 2014.