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Water treatment

organic compounds in the

organic compounds in the range of 3 to 86 % (filtration rate of 0.21mh -1 ) were presented. The removal mechanism for most of these compounds was considered to be adsorption/desorption to the sand surfaces. The previous observations also are in harmony with the data reviewed by Lambert and Graham (1995) and grouped and reported under different parameters. For different pesticides they report efficiencies in the range of -20 to 100%. Low rates of biological biodegradation were considered to be an important factor in the poorer removals of some of the pesticides. For non-specific organic compounds that comprise organic carbon and organic colour components, it was found that removal efficiencies appear to be site-specific, and vary with both the nature of the raw water source, ambient conditions, and the filter management practice applied. The literature review shows that SSF units generally remove between 5 and 40% of DOC, although the mean value was only 16%, and the difference between upland and lowland water sources was not significant. It was found that there was no particular relationship between DOC and UV-absorbance or colour removal between one plant and another, and in some instances, the relationship that exists in a particular plant was found to vary seasonally or between successive filter runs. The data reviewed showed that UV-absorbance (254 nm) is reduced in the range 5 to 35%. For all water sources, reported colour removals (absorbance at 400 nm or 0 Hazen) in the literature ranged from 15 to 80%, with a mean colour removal of 34%. However, mean colour removals were 42% for upland water sources, compared to 26% for lowland waters. True colour removals, as colour units of Pt-Co in filtered or centrifuged samples, includes only colloidal and soluble substances, especially natural organic matter. The removal of true colour is normally reported to be in the range 25 to 30% (Cleasby et al., 1984; Ellis, 1985; Collins et al., 1991). As a result of the potential formation of disinfection by-products in the presence of organic material, low colour levels are desirable (Spencer and Collins 1991). The colour level, however, should not determine the application of final disinfection, as the risk of acute microbiological contamination is far more significant (Craun et al, 1994b). Heavy microbiological contamination. In some communities, the only source available for water supply may be so heavily contaminated with harmful microorganisms that SSF alone will not be able to produce a good quality effluent. Whilst long term efforts are directed to protecting catchment, pretreatment of the raw water may be necessary before SSF can be properly applied. 2.6.2 Conditions that inhibit or reduce the efficiency of the treatment process Various circumstances can interfere with the treatment process in the SSF units and prevent the expected efficiencies being obtained. Some of these are related to the short filter runs considered in Section 2.6.1. Other important inhibiting conditions are low temperatures, low nutrient content and low dissolved oxygen content. A low temperature increases the viscosity of water and reduces the bio-chemical activity in the sand bed, affecting the treatment efficiency. E. coli removal may be reduced from 99 to 36

50% when the temperature falls from 20ºC to 2ºC (Huisman and Wood 1974). In London, conventional SSF running at 0.3 mh -1 could not produce a filtrate below 50 CFU/100 ml, at temperatures below 4ºC (Toms and Bayley 1988), whereas in summer results were much better, usually below 10 CFU/100 ml. The strategy in countries that face cold periods during the year has been to cover the filters or to build underground to prevent the freezing of the units and reduce the impact of low temperatures which, of course, has considerable economic implications. Reducing the filtration rate is another way to reduce the impact of low temperature on the treatment process. Toms and Bayley (1988) present evidence that the same filters in London operating at filtration rates below 0.20 mh -1 instead of 0.30 mh -1 produced effluents with less than 10 CFU/ml, even though temperatures were low. Nutrients. The microorganisms active in the sand bed require nutrients such as carbon, nitrogen, phosphorus and sulphur for their metabolism and growth. The humic and fulvic acids are rich in carbon but low in the other elements (Spencer and Collins, 1991). This may be part of the explanation for the low removal of natural colour in SSF treating water sources that are well protected. Bellamy et al. (1985) report that adding nutrients permits increasing the biological activity in experimental SSF units, and improves the removal efficiency for turbidity and for microbiological contamination. Dissolved oxygen. When the flow velocities and the dissolved oxygen level in the water source are low, particularly if this is combined with a high amount of biodegradable material, the oxygen in the water can be depleted resulting in anaerobic conditions in the filter skin (Joshi et al., 1982). This anaerobic condition in the filter must be avoided because it may create serious water quality problems such as bad smell and taste, as well as re-suspension of heavy metals with implications of aesthetic nature or interference with the final disinfection stage. In summary, in spite of the potential of the SSF process illustrated in table 2.5, surface waters presenting relatively moderate to high levels of contamination could not be treated directly by conventional SSF units, as recommended by some authors in table 2.6. Far too great a strain would be placed on the terminal disinfection, limiting its role as a final safety barrier. This would be critical in the Latin American and Caribbean countries, where the reliability of disinfection is low (Reiff and Win, 1995). Table 2.6 Some water quality guidelines that permit direct slow sand filtration treatment Water quality Quality limitations based on references of 1991 parameters Spencer, et al. Cleasby Di Bernardo Turbidity (NTU) (1) 5 - 10 5 10 Algae (Units/ml) 200 (2) 5 µgl -1 (3) 250 True colour (PCU) 15 – 25 5 Absorbance at 254 nm (cm -1 ) 0.08 Dissolved oxygen (mgl -1 ) > 6 Phosphate (PO 4 ) (mgl -1 ) 30 Ammonia (mgl -1 ) 3 Total Iron (mgl -1 ) 1 0.3 2.0 Manganese (mgl -1 ) 0.05 0.2 Faecal Coliforms (CFU/100ml) 200 (1) The type of turbidity and the particle distribution may produce changes in the water quality of the effluent of the SSF. (2) Both the number and the type of species present in the water source are important. This reference suggests covered filters. (3) This limit corresponds with chlorophyll-a in the supernatant water as an indirect measure for the algae content. 37