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9.4 CHAPTER NINE

TABLE 9.2 Algal Species Classification for Slow Sand
Filtration
Filter clogging Filamentous Floating
Tabellaria Hydrodictyon Protococcus
Asterionella Oscillaria Scenedesmus
Stephanodiscus Cladophora Synura
Synedra Aphanizomenon Anabaena
Melosira Euglena

lengths. Table 9.2 presents a list of commonly found algal species, divided into categories
related to their effect on filter performance (Spencer and Collins, 1991). Filter-clogging
species are detrimental to filter performance, while filamentous species may actually en-
hance filter performance by providing greater surface area. Floating species would not re-
sult in direct clogging of the filter, but may shorten run lengths based on poorer-quality
raw water.
Algae may be present in source water delivered to the filter and may also occur in an
uncovered filter bed open to sunlight. In general, it is prudent to reduce algal content in
source water to as low a level as possible to limit its effect on filter performance. Obser-
vation of algal growths, as well as identification, will aid with assessing the need for pre-
treatment, such as copper sulfate, and in determining when filter run lengths may be short-
ened. Some researchers have suggested the measurement of chlorophyll at concentrations
of 5 mg/m 3 as a limit in source water (Cleasby et al., 1984).
Color. Color in treated water is currently categorized by USEPA as a secondary conta-
minant in drinking water supplies, with the focus being aesthetic concerns. As identified
by Christman and Oglesby (1971), the yellow to brown color of many source waters can
be the result of microbial breakdown of lignins from woody plants. True color removals
of 25% or less were reported by Cleasby et al. (1984). Other research has indicated a re-
moval range between 15% and 20% for total organic carbon (Fox et al., 1994; Collins et
al., 1989).
When one is evaluating the applicability of slow sand filtration for a specific source
water, a review of historical trihalomethane (THM) data can reveal whether the expected
low removal efficiency of aquatic organic substances by the process is a concern. Where
historical color and THM data are unavailable, a sampling program can be initiated to aid
in evaluating whether slow sand filtration is an appropriate treatment method.

Iron and Manganese. Slow sand filters remove iron and manganese through precipita-
tion on the sand surface in a scaling-like action, but an upper limit of 1 mg/L of iron is
suggested to avoid forming an iron precipitate that could clog filters. A similar limit for
manganese would also appear to be acceptable. Collins et al. (1989) showed that iron pre-
cipitate on a slow sand filter enhanced the removal of organic precursors.
Dissolved Oxygen. The presence of dissolved oxygen in source water is critical for stim-
ulating a healthy schmutzdecke for proper slow sand filter operation. Some slow sand
plants use aeration of the water as a pretreatment. Reduction of dissolved oxygen levels
commonly occurs following algal blooms, so that the importance of dissolved oxygen in
the source water is another reason to control algal growth in the source. Potential prob-