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SLOW SAND AND DIATOMACEOUS EARTH FILTRATION 9.3

TABLE 9.1 Source Water Quality Limitations of Slow Sand Filters

Parameter Recommended limit
Turbidity 5 to 10 ntu*
Algae 200,000/Lt
True color 15 to 25 platinum color units
Trihalomethanes 50/zg/L
Nonpurgeable dissolved organic carbon 2.5 mg/L
UV absorbance~: 0.080 cm-l
Dissolved oxygen >6 mg/L
Phosphorus (PO4) 30/zg/L
Ammonia 3 mg/L
Iron < 1 mg/L
Manganese < 1 mg/L

*Presence of clay particles must be determined.
tDependent on identification of algal species and assumes covered filter.
:~Absorbance at wavelength of 254 nm.

found that some biological activity also extends deeper into the bed, where particulate re-
moval is accomplished by bioadsorption and attachment to the sand grains.

Source Water Quality Considerations

In considering whether slow sand filtration is an appropriate treatment method, source
water quality must be carefully evaluated. If source water quality data are not available,
pilot testing of the source water is essential to determine the applicability of the slow sand
treatment option. Table 9.1 lists source water quality parameters with recommended lim-
its (Spencer and Collins, 1991).
Turbidity. Both the level and type of turbidity in source water must be considered. In
general, most existing slow sand plants successfully treat source water turbidity of less
than 10 ntu (Slezak and Sims, 1984), which is recommended for an upper limit in de-
signing new facilities. Also of some importance is the stability of the water. Slow sand
facilities operate more efficiently if source turbidity is relatively constant and generally
--< 5.0 ntu.
Of equal importance is the nature of particulates. Source waters that normally contain
clay particulates or that pick up clay after storm events will cause problems for slow sand
filters. This difficulty for slow sand filters occurs because clay penetrates deep into the
bed or may even carry through the filter, causing an immediate problem of elevated fil-
tered water turbidity and a long-term problem of filter clogging and reduced length of fil-
ter runs.
Algae. In a few instances, it has been found that the presence of certain types of algae
actually enhances the filtration process by providing greater surface area for biological
activity. In general, however, the presence of algae in the source water reduces filter run