Slow Sand Filters

How it Works:

Contaminated water is poured into the top of the filter, and virtually all of the bad organisms or pathogens are removed in the top two inches of the filter. The layers of sand remove suspended solids and most micro-organisms.

The Result: Clean, fresh water flows out of the filter using only gravity.

The Benefits: Childhood diseases due to contaminated drinking water are ended.

Filter Attributes

• Water runs clear with no chlorine smell
• Low cost
• Maintenance free
• No power required
• Located in the home, school and clinic
• No moving parts
• Simple
• Locally available materials
• 98% proven effectiveness

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 and gravel. 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.

The 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