Page 431 - Water and wastewater engineering

11-4 WATER AND WASTEWATER ENGINEERING

into service after cleaning. The peak occurs because of residual backwash water being flushed
from the media, and from particles in the influent water that are too small to be captured. As the
clean media captures particles, it becomes more efficient because the particles that are captured
become part of the collector surface in the filter. Amirtharajah (1988) suggests that up to 90 per-
cent of the particles that pass through a well-operating filter do so during the ripening stage.
After ripening, the effluent turbidity is essentially constant and, under steady-state condi-
tions, can be maintained below 0.1 NTU. On the other hand, headloss continues to rise as par-
ticles collect in the filter. At some point the number of particles that can be effectively captured
begins to decline and breakthrough occurs.

Nomenclature
There are several methods of classifying filters. One way is to classify them according to the type
of medium used, such as sand, coal (called anthracite ), dual media (coal plus sand), or mixed
media (coal, sand, and garnet). Another common way to classify the filters is by nominal filtra-
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tion rate or hydraulic loading rate (m of water applied/d · m of surface area, or m/d). A third
alternative is to classify the filters by the pretreatment level.
Based on the hydraulic rate, the filters are described as being slow sand filters, rapid sand
filters, or high-rate filters.
Slow sand filters were first introduced in the 1800s. The water is applied to the sand at a
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loading rate of 3 to 8 m /d · m . As the suspended or colloidal material is applied to the sand,
the particles begin to collect in the top 75 mm and to clog the pore spaces. As the pores become
clogged, water will no longer pass through the sand. At this point the top layer of sand is scraped
off, cleaned, and replaced. Although slow sand filters require large areas of land and are labor
intensive, the structures are inexpensive in comparison to the other types, and they have a long
history of success.
In the late 1800s, health authorities began to understand that clean water was a major fac-
tor in preventing disease. The limitations of slow sand filters in meeting the need for filtration
systems to serve large populations became readily apparent. Rapid sand filters were developed to
meet this need. These filters have graded (layered) sand in a bed. The sand grain size distribution
is selected to optimize the passage of water while minimizing the passage of particulate matter.
Rapid sand filters are cleaned in place by backwashing. The wash water flow rate is such that
the sand bed is expanded and the filtered particles are removed from the bed. After backwashing,
the sand settles back into place. The largest particles settle first, resulting in a fine sand layer on
top and a coarse sand layer on the bottom. Rapid sand filters are the most common type of filter
in service in water treatment plants today.
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Traditionally, rapid sand filters have been designed to operate at a loading rate of 120 m /
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d · m (5 m/h). Filters now operate successfully at even higher loading rates through the use of
proper media selection and improved pretreatment.
In the wartime era of the early 1940s, dual media filters were developed. They are designed
to utilize more of the filter depth for particle removal. In a rapid sand filter, the finest sand is on
the top; hence, the smallest pore spaces are also on the top. Therefore, most of the particles will
clog in the top layer of the filter. In order to use more of the filter depth for particle removal, it is
necessary to have the large particles on top of the small particles. This is accomplished by placing
a layer of coarse coal on top of a layer of fine sand to form a dual-media filter. Coal has a lower
specific gravity than sand, so, after backwash, it settles more slowly than the sand and ends up on
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top. Some dual-media filters are operated up to loading rates of 480 m /d · m (20 m/h).