Slow Sand Filtration
by Mary Rust and Katie McArthur
The history of slow sand filtration in the United States has been one of
reluctant acceptance. Many European cities choose slow sand filtration as a
water treatment method because of its simplicity, reliability, and economy
(Collins). Slow sand filters in the United States are found primarily in smaller
communities with fewer than 10,000 people, 45% of which serve fewer than 1,000
people (Sims). This is primarily due to the associated low cost of slow sand
treatment facilities compared with alternative water treatment technologies
(Sims). The number of operating slow sand filters by state in the United States
as of 1991 can be seen in Figure 1.
Slow sand filtration is a water purification process in which water is passed
through a porous bed of filter medium. Slow sand filters are typically
characterized by certain design components: the supernatant (water above the
filter sand that provides hydraulic head for the process), filter sand varying
in depth, the underdrain medium (usually consisting of graded gravel), and a set
of control devices (Sims). In a mature sand bed, a thin upper sand layer called
a Schmutzedecke forms. The Schmutzedecke consists of biologically active
microorganisms that break down organic matter while suspended inorganic matter
is removed by straining (Van Duk). Slow sand filters are distinguished from
rapid sand filters by the biologically active sand medium (including the
Schmutzedecke), and slow detention times. Rapid sand filters utilize primarily a
physical removal process, are periodically backwashed for cleaning, and operate
with long detention times. Slow sand filters are cleaned by periodically
scraping the existing Schmutzedecke (Van Duk). Figure 2 is a schematic of a
common cross section of a slow sand filter.
The supernatant serves two distinct purposes. First, it provides a head of water sufficient to pass the raw water through the filter bed. Second, the supernatant creates a detention time of several hours for the treatment of the raw water. The supernatant should not be considered as a reservoir for sedimentation. If the raw water has a high content of suspended mater, then pretreatment should be considered to prevent rapid clogging of the filter bed. The supernatant depth is typically a meter (Van Duk).
The physical characteristics of a sand bed are important in maintaining the
slow sand filterís efficiency. The effective size is the size opening that will
pass ten percent by weight of the filter material (Haarhoff). Effective sizes in
the range of 0.15 mm to 0.35 mm are used (Van Duk). The uniformity coefficient
is the ratio of the size openings that pass sixty percent of filter material to
the size openings that pass ten percent of filter material, e.g. the effective
size (Haarhoff). Uniformity coefficients range between two and five; most
facilities maintain uniformity coefficients less than three (Haarhoff). The
filter medium itself should consist of inert and durable grains; sand should be
washed so that it is free of clays, loams, and organic matter. Depth of a filter
bed ranges between 1.0 and 1.4 meters (Van Duk). The clean filter medium from a
slow sand filter in a treatment plant near Paris, France can be seen in Figure
The underdrain system serves two purposes. It provides unobstructed passage
for the collection of treated water and it supports the bed of filter medium. It
is important that the underdrain system provide a uniform velocity over the
entire filter area (Van Duk). The underdrain gravel is placed so that the finest
gravel is directly underneath the sand and the coarsest gravel is surrounding
the underdrain pipes or covering the underdrain block (Pyper). This prevents the
filter sand grains from being carried into the treated water system. An example
of support media and underdrain system is shown in Figure 4.
Biological and Physical Mechanisms
Biological activity in the sand bed is not well understood. Scientists have a vague idea of the processes involved, but specific interactions are still unknown. Suggested biological removal mechanisms are predation, scavenging, natural death and inactivation, and metabolic breakdown (Haarhoff). In the Schmutzedecke, algae, plankton, diatoms, and bacteria break down organic matter through biological activity. It has been hypothesized that as the raw water passes through the bed, it constantly changes direction. Thus, the sand grains develop a uniform sticky layer of organic material that absorbs to the particles by various attachment mechanisms. The sticky layer around the sand grains is biologically active (bacteria, protozoa, bacteriophages) and the organic impurities are biologically converted to water, carbon dioxide and harmless salts. According to a study by Collins, the bacterial concentrations in the Schmutzedecke were a function of the elapsed time and potential for cell growth rather than the filtration of free-living bacteria from the source water (Collins). The biologically active section of the entire filter bed extends 0.4-0.5 m downward from the surface of the Schmutzedecke (Van Duk).
Physical processes are also inherent to slow sand filter mechanisms. As the biological activity of the filter bed decreases, the physical processes of adsorption and chemical oxidation are the primary mechanisms (Van Duk). Adsorption accounts for removals that were traditionally thought to be purely biological. For example, the removal of chlorinated organics and the distribution of viruses are thought to follow adsorption isotherms (Haarhoff). Furthermore, suspended inorganic matter may be removed by the physical process of straining (Van Duk).
Organic Carbon Removal
Adsorption and biodegradation are considered to be the primary natural organic matter removal mechanisms (Collins). Literature cited by Collins suggests that large hydrophobic-humic organic molecules are removed by adsorption, and smaller organic molecules are removed by both adsorption and biodegradation. The smaller hydrophilic material (carbohydrates, aldehydes, and simple organic acids) are considered to be primarily removed by biodegradation. A common oxidant for the treatment of water in the United States is chlorine; the hydrophobic-humic organics (considered to be the more trihalomethane reactive) were removed in greater than 80% of all comparisons of organic parameters cited in the Collins study.
Another interesting aspect of the Collins paper was the fact that natural organic matter and organic precursor material were a function of filter media biomass: the greater the biomass, the greater the organic carbon removals. Three US sand filters were compared; the West Hartford filters use a unique Schmutzedecke cleaning procedure called the filter-harrowing cleaning technique (discussed in more detail under Filter Scraping). This procedure allows for the minimization of biomass removal from mature sand filters resulting in increased removals of biodegradation and bioadsorption (Collins).
Removal of Giardia and Cryptosporidia
In the past decade the protozoan parasite Cryptosporidium parvum has been recognized as a significant threat to public water supplies. The resistant stage of Cryptosporidia is called an oocyst; this stage is relatively untouched by a chlorination disinfection process. Slow sand filtration has been looked at in numerous studies to determine the viability of this treatment process for the removal of Cyrptosporidia. A study in England by Timms found reductions of oocysts greater than 99.97%; the oocysts were found in the filter media above 2.5 cm. Another study in British Columbia by Fogel contradicts the aforementioned study. Fogel found removal efficiencies of 48%; this figure is significantly different than the 100% removals Fogel cites from previous literature. However, a point to note concerning the British Columbia filters is that they were operating well out of the range of the recommended design limits for the uniformity coefficient at 3.5 (Fogel). Furthermore, temperature can adversely affect the performance of a slow sand filter; the British Columbia filters were operating at extremely low temperatures of less than 1įC (Fogel). Overall, the literature supports data that strongly suggests slow sand filtration is a viable alternative for Cryptosporidia removals.
Slow sand filters have also been proven highly efficient in removing Giardia lamblia, a frequently identified pathogenic intestinal protozoa. The same study by Fogel found that despite the uniformity coefficient parameter and the low temperatures, Giardia removals were complete. This data was further supported by literature cited by Fogel. Furthermore, fecal and total coliform counts were below the detection limit, and the removal rates were similar to Giardia removals (Fogel).
Schmutzedecke Scraping Operations
Scraping typically involves the removal of the Schmutzedecke and the
operation is site specific. Frequency of scraping depends on the available head,
the media grain-size distribution, the influent water quality, and the water
temperature (Letterman). Higher frequencies of scraping are associated with
increased water temperature, high solids concentrations in the influent, low
head, and small media pore size. A typical operation involves draining the
supernatant (usually by continuing filtration with no influent) to 20 cm below
the sand surface, skimming off one inch of the Schmutzedecke and associated
sand, and then filling the filter from the bottom of the bed using filtered
water to prevent air entrapment. The bed should be refilled until depth is
sufficient to continue normal operations (Letterman). A filter in the process of
having the Schmutzdecke scraped is shown in Figure 5.
If you are interested in the latest concerns about the operation of slow sand
filters in the United States you may want to take a look at the AWWA Slow Sand Filtration
Send comments or suggestions to:
Student Authors: Katie McArthur, mailto:firstname.lastname@example.org Mary Rust, email@example.com
Faculty Advisor: Daniel Gallagher, firstname.lastname@example.org
Copyright © 1996 Daniel Gallagher
Last Modified: 02/24/1998