The ecology of protozoan populations of slow sand filters with particular reference to the ciliates

Abstract

The protozoan populations of slow sand filter beds, used by the Metropolitan Water Division of the Thames Water Authority for the purification of stored water, were studied over the period of a year. Most of the work was conducted on beds where the water approached the sand at a flow rate of 0.4 mh-1, while a bed run at 0.2 mhl was used for comparative studies. Physical and chemical parameters of the water were monitored by the Thames Water Authority. A method for the qualitative and quantitative examination of live protozoa was developed using a sedimentation technique. The time required for sedimentation limited the number of samples which could be identified, measured and counted. These were concentrated on defining the depth, rather than horizontal, distribution of protozoa as the bed functions vertically, and samples were taken as frequently as possible in order to adequately estimate the population growth rates of the protozoa. Depth distributions of particulate organic carbon content of the sand were determined, as an indication of food availability for the microorganisms, in parallel with the protozoan samples. The depth distribution of bacteria associated with the sand grains was also studied, but only over the initial period of a filter bed run. Scanning electron microscopy was employed to examine the localised distributions of bacteria on the sand grain surfaces. The availability of a pilot-scale sand column enabled interstitial water to be sampled and analysed for bacteria .Maximum population densities of ciliates and flagellates, and variations in mean body size of the ciliates were found to be related to temperature. Maximum ciliate densities were also associated with surface accumulation of carbon, rates of head loss change and, on two occasions, with the onset of a dissolved oxygen deficit in the filtrate water. The pattern of density-depth distributions of ciliates and flagellates varied with time during the filtration runs, and deeper penetration into the sand occurred in the filter bed with the faster rate of approach water flow. Rates of growth of the ciliate and flagellate populations increased with temperature; the ciliate population Qio equalled 1.6 between 4° and 20°C. Rates of growth also increased with flow rate and accummulation of particulate organic carbon in the filter beds. After maximum densities had been achieved and the ciliate population was in numerical decline, the total ciliate biovolume continued to increase,' but at a more gradual rate; this was due to a succession to larger species during this part of the run. 2A high initial growth rate of bacteria, doubling time equal to 10 hours, was recorded over the first few days of a filter bed run. Bacterial enumeration in the interstitial water of the pilot-scale sand column was less informative, as very little change in density distribution occurred during the initial part of a run. Oxidation-reduction profiles of the interstitial water were also found to change little with time, maintaining a characteristic increase in potential with depth.