Optimizing the Performance of Sand Filters and Packed Bed

Plantas Purificadoras de agua .

Filters

Through Media Selection and Dosing Methods

More than 100 years ago sand filters were first used to treat
wastewater in North America. Since the

1960s, they have enjoyed a resurgence of interest and today sand
filters are among the most successful

methods for onsite wastewater treatment wherever high
groundwater, poor soils, or other site constraints

rule out conventional septic systems. Their capability for
nutrient and pathogen removal, their low

maintenance and power requirements, and their tolerance for
periodic surges in loading rates make them

practical and economical. (plantas
purificadoras de agua)

Nevertheless, obtaining sand of the proper size, uniformity, and
cleanliness has in some locations been a

stumbling block in the spread of this technology. When sand
filters are introduced into an area, sand

meeting the required specifications may require long-distance
transport if a local supplier is not willing

to gear up to produce the small amount required for the first
few installations.

How Media Size and Gradation Affect Filter Performance

An ideal filter medium has both large surface area permitting
wastewater maximum contact with the

zoogleal film and sufficient pore space to allow aeration and unsaturated
flow

even as the diameter of the spheres changes. While the surface
area of a 0.3 mm diameter medium, for

example, is much 0.3 mm diameter medium medium, the
size of the pores between the 0.3 mm

diameter particles is much smaller and maintenance of
unsaturated flow, therefore, would be more

difficult to achieve. (filtros
para agua)

Sand from most sources contains a variety of grain sizes with
smaller grains of sand filling in the pores

between the larger grains. Sand filters benefit from this
condition as surface area per unit of volume

increases, but can suffer when pore size becomes too small for unsaturated
flow to occur.

In fact, sand filters have suffered where ASTM’s C-33 standard
has been adopted unconditionally as the

specification for sand filter medium. At first glance, the C-33
specification appears similar in size and

uniformity to what is needed for a sand filter. However, C-33
sand has too large a percentage of fine

particles to make it permissible for this use. Developed for the
manufacture of concrete, the C-33

specification is designed to minimize voids. That runs counter
to the objective for sand filters:

sufficiently large pore space to allow ample oxygenation and unsaturated flow
around the sand particles.

production cost-effective. One important caveat: relying on the
ASTM C-33 concrete sand specification

as a filter medium specification may be dangerous to a sand
filter’s health.

When it’s available, affordable, clean, and properly sized,
sand is an excellent filter medium. It is not,

however, the only alternative. In fact, sand filters are just
one example of what is known, generically, as

packed bed filters. Packed bed filters may use any of a variety
of filter media, including sand, glass,

slag, ceramic, plastic and even textile. Their success in
treating wastewater depends largely on selection

of the proper form and correct placement of the medium as well
as on the dosing methods used in

applying wastewater to the filter medium.

To understand how filter media and dosing methods affect
performance, it’s instructive to review some

of the conditions essential for a properly operating sand
filter.

The Hidden Life of a Sand Filter

As wastewater percolates slowly through the filter medium,
physical, biological, and chemical processes

remove contaminants. On the surfaces of the grains of sand or
other medium grows a naturally

occurring, microscopically thin zoogleal film composed of large
populations of bacteria and other

microorganisms. As septic tank effluent flows over the surface
of the zoogleal film, organic material

contained in the wastewater is absorbed onto the film where it
becomes food for the bacteria. For

maximum treatment, then, it is essential that all the wastewater
have sustained contact with the film

attached to the medium. And because the aerobic organisms in the
zoogleal film need oxygen to live, it

is also essential to maintain unsaturated flow conditions

through the filter medium.

Unsaturated flow and sustained contact are achieved by
distributing the wastewater evenly over the

surface of the filter medium and by keeping doses small and frequent over the
course of the day. (negocios)

Negocios !