||The intermittent slow sand filter
shown here was developed by Dr. David Manz of the University
of Calgary, Alberta.
It has been extensively tested
in laboratories at the University of Calgary, MIT and Dartmouth
College, and has proven economical and effective in thousands
of field installations. In its commercial format, it is about
the size of an office water cooler, constructed of concrete
and plastic pipe, and filled with multiple grades of sand
The cement container filled with sand and
the plastic pipe exiting at the bottom of the filter and returning
up the outside combine to form a "trap", similar
to the trap under a sink.
As water in both sides of a
trap seeks a uniform level, the height of the outlet of the
pipe is adjusted to maintain approximately 2 inches of water
on top of the sand in the container.
As polluted water is poured into the container
through a diffuser plate that controls the rate of flow, clean
water that has traveled through the sand filtration process
exits the pipe into a clean water receptacle.
removal of harmful contaminants and pathogens takes place on the
top surface of the sand 2 inches below the water surface. A natural
biomass layer of microorganisms present in the contaminated water
forms at this sand interface, which in turn actively degrades further
organic material and removes it from the water that then filters
through the layers of sand. The resulting water flowing from the
outlet pipe is free of contaminants, clear in color, clean in taste
and smell and safe for human consumption.
Many technical reports have been published attesting to the effectiveness
of intermittent slow sand filtration. These reports confirm that
under optimal operating conditions, the bio-sand filter is capable
of removing 97% of fecal coliform, 100% of giardia cysts, 99.98%
cyptosporidium oocysts, 100% of worms, 100% of parasites, and up
to 90% of organic and inorganic toxicants from contaminated water.
No moving parts. No external power or sunlight required. Effective
in intermittent usage. Centuries of proven results.
Low cost. Simple construction in-country with locally available
materials. All-in-all, the slow sand household filter can and will
make a measurable difference in family health and peoples' lives.
Pure water has now developed a plastic filter and a portable molding
machine so that these filters can also be produced in country. These
filters are light and much easier to distribute.
What is the effective cost of each unit?
The cost includes:
- manufacturing the filter,
- the disfusser and cover,
- providing 2 buckets,
- monitoring and education,
- and purchasing and administration of the parasite
treatment for the household
The total cost for a lifetime of clean drinking
water for a household in on average $75 per filter. This cost covers
only in-country expense and does not cover administration costs.
$450 buys a filter manufacturing mold