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9.2 CHAPTER NINE

With establishment of the Stage 1 Disinfectants/Disinfection By-product Rule, filtra-
tion of surface water must achieve specified reductions in total organic content (TOC)
and/or reductions in disinfection by-product levels [total trihalamethanes (TTHM) and
sum of five haloacetic acids (HAA5)]. The Interim Enhanced Surface Water Treatment
Rule calls for lower drinking water turbidity. This means that use of slow sand and
DE filters may be limited further without the use of supplemental measures or additional
treatment.

SLOW SAND FILTRATION
Slow sand filtration was the first type of porous media filtration used in water treatment.
The first recorded installations occurred in Scotland and England in the early 1800s. By
the mid-1800s, legislation was passed in London, England, requiring filtration of water
to be consumed. The first recorded installation of slow sand filtration facilities in the
United States was in Poughkeepsie, New York, in 1872. Subsequent development of rapid
rate filtration then slowed the pace of construction of slow sand plants in the United States
in the early 1900s.
Around 1980, interest in using slow sand filtration was rekindled as the U.S. Envi-
ronmental Protection Agency (USEPA) conducted research to develop treatment options
that are simple to operate for use by small communities and that produce high-quality ef-
fluent. Research reconfirmed that, at recommended filter rates and with appropriate me-
dia and source water quality, slow sand filtration can produce a low-turbidity effluent and
can effectively remove microbiological contaminants.
When the USEPA passed the Surface Water Treatment Rule (SWTR) in 1989, further
pressure was placed on communities that were not filtering surface water supplies to add
filtration. Slow sand filters were rated along with rapid sand and DE filters as baseline
treatment in the regulations. As a result, slow sand filtration has once again become a
treatment method routinely considered in evaluating filtration options in many U.S. com-
munities. Abroad, it is used to provide safe drinking water to many poor or rural com-
munities. A major portion of the city of London water supply is treated by slow sand
filtration.
Renewed interest in the slow sand process has generated new research into improving
treatment performance. The focus has been on expanding use of slow sand filtration in
treating poorer-quality source water, especially with higher turbidity and organic content.

Slow Sand Treatment Mechanism

As its name implies, slow sand filtration is accomplished by passing water at a relatively
low rate through a sand medium. The filtration rate is on the order of one-hundredth of
the rate used in a typical rapid sand filter.
Because of the relatively low filter rate, head loss across the bed occurs gradually over
a much longer time. Average filter run length is normally between 45 and 60 days. In
some newer installations, filter run lengths in excess of 6 months and even greater than
1 year have been reported.
Slow sand filtration accomplishes its treatment primarily through biological activity,
with the bulk of this activity taking place on the surface of the sand bed. A layer devel-
ops on the sand surface that is called schmutzdecke, an accumulation of organic and in-
organic debris and particulate matter in which biological activity is stimulated. It has been