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Water treatment

Bellamy (1985) reports a

Bellamy (1985) reports a total coliform removal reduction from 99.4% at d 10 of 0.1mm to 96% at d 10 of 0.6mm. Di Bernardo and Escobar (1996) studied four pilot units in parallel having sand with d 10 values in the range of 0.21 to 0.23 mm, and u c values of 2.24, 2.85, 4.20, and 4.29. The tested filtration rates were 0.1, 0.2 and 0.25 mh -1 . For the lower velocities (0.1 and 0.2 mh -1 ), the pilot units with higher u c values had longer filter runs, deeper dirty penetration, and better effluent quality in terms of turbidity, apparent color, total iron and number of particles. The higher quality seems to be associated with a greater amount of data after the maturation period in the filters having sand with greater u c values. Van der Hoek et al (1996) studied the impact of grain size and filtration rate on operation and performance in SSFs in Amsterdam. They fed four pilot units with highly pretreated surface waters. Two different filter rates were applied to two different filter sand types (d 10-90 =0.19- 0.35 mm and d 10-90 =0.25-0.84 mm). All the units were producing water within the Amsterdam Water Supply standards. The filters with the smaller grain size showed slightly better performance with respect to the filtrate, and presented a shorter filtration rate, as predicted by filtration theory. However, these results also show that pretreatment in this case practically overrides the effect of grain size and filtration rate on the SSF performance. Deeper sand beds should result in improved removal of particles. However, due to the development of the filter skin and the biological activity concentrated mainly in the upper sand layers, particle removal is more effectively accomplished in this part of the SSF units. After a field survey Lloyd (1974) found evidence showing the significance of the uppermost 5-10 cm of SSF as the functional zone in purification at a filtration rate of 0.15 mh -1 , where around 1 log reduction of mesophilic and thermophilic bacteria usually takes place. For the same grain size grading, this functional zone increases with higher filtration rates, although population quantities and densities tend to decrease for the most active protozoa predators. Bellamy et al (1985) found in their pilot units that 93 percent of the total coliforms were removed in the top 0.5-m bed depth and 95 percent above 1.0-m bed depth. Experimental evidence like this supports the practice of having a minimum sand depth above 0.3 to 0.5 m in the SSF units to achieve more than one log reduction of indicator bacteria. This is relevant for small systems working with low flow rates (0.1 to 0.2 mh -1 ), but having to filter at higher rates during short periods due to their lower buffering capacity when one of the units is out of operation. The sand to be put into the SSF units should be clean, free of clay, earth and organic material (Visscher et al 1987; Ives, 1990). The presence of dust or fine material produces high initial head losses and seems to limit the essential development of an active and effective microbial population in the filters bed. Placing dirty sand in the filter may interfere with the treatment process and makes it necessary to remove the sand earlier for correct washing. 2.5.3 Operation and maintenance procedures SSF units must operate continuously since this contributes to better quality effluents, and a smaller filtration area is required for a given daily water production. Declining rate filtration 30

can be applied, but intermittent operation should be avoided since oxygen depletion in the bed compromises biological activities. Sundaresan and Paramasivan (1982) report deterioration of effluent bacteriological quality when filters recommence operation after 5 hs. Haarhoff and Cleasby (1991) identified initial-ripening periods in the range of 35 to 100 days before the effluents of the SSF units became stabilized for parameters such as viruses, indicator bacteria, and turbidity. Hendricks and Bellamy (1991) report data of Giardia cyst removal above 2 log units even with clean sand. Bellamy et al (1985), experimenting with low polluted raw water, found that the initial ripening period could be reduced from seven weeks for the control filter, to three to four weeks with nutrients added to the other filters. This initial period is not necessarily related to head loss in the SSF units, since the filter skin may be made up of fine detritus and deprived of an active biological population. After several weeks or months of running, the SSF unit will gradually become clogged as a result of the accumulation of inorganic and organic material, including the biomass that is formed on top of the filter bed. In this top layer the major increase in head-loss occurs thus allowing, by scraping off this layer, the restoration of the hydraulic conductivity to the level at the beginning of the filter run. Classically, this is achieved by scraping the top 1 to 3 cm of the filtering bed. After several scrapings, when the filter bed measures its minimum (0.3 to 0.5 m), resanding is required. Manual cleaning has been the option for most small SSF units. In general, high frequency of scraping is associated with one or more of the following factors, high solids concentration in the raw water, the grow of algae in the water supernatant, small media grains, low available head, and high water temperature (Letterman, 1991). The filter runs (periods between scrapings) of SSF units in the USA, with a water production of 112 and 650 m 3 per m 2 of filter surface area, ranges from one week to one year, with the average about 1.5 months (Letterman, 1991). After reviewing five references about small systems in the USA, Letterman (1991) reports manual scraping labor requirements in the range of 1.3 to 8 (average 4.2) person-hour per 100 m 2 of area scraped. The labor requirement increases significantly when the depth scraped is greater than about 2.5 cm. Filter runs for small systems are reported in the range of 20 to 60 days by Shulz and Okun (1992), quoting Fair et al (1968). Cleasby (1991) considers that with cycles shorter than 1.5 months labor costs will escalate and operator satisfaction with the plant will diminish. After scraping the sand surface, a secondary-ripening period may be necessary for the SSF to recover their previous treatment capacity. Letterman and Cullen (1985) report values in the range of 0 to 10 days for this secondary period. They found no correlation between filter design and operation conditions with the presence or absence of a ripening period. Hendricks and Bellamy (1991) consider that the most important factor affecting the duration of a secondary-ripening period appears not to be the removal of the filter skin, but the dewatering of the sand bed. This consideration is in harmony with Lloyd's (1974; 1996) recommendations of doing cleaning procedures whenever possible in warm periods and keeping the water table within 10 cm below the sand surface. According to Lloyd, this should contribute to retain spirotrichs and peritrichs protozoa which are susceptible to desiccation and are unable to reestablish themselves at less than 3 0 C. 31

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