If you have customers who rely on private surface water sources for household use, slow sand filtration - or more accurately biologically active filtration - may be an effective choice for water treatment.

Slow sand filters can remove up to 99.99 percent of turbidity, bacteria, viruses, and Giardia lamblia cysts without the need for chemical flocculents or the use of electrical power.

Slow sand filtration is a preferred technology for customers who:

- Wish to use surface water (ponds, streams, springs)

- Use daily volumes that make cartridge use impractical

- Have no access to electrical power

- Cannot or do not wish to use chemical treatment

Two types of sand filters are used for water treatment - rapid sand and slow sand. Rapid sand filters filter at a rate of 1 to 2 gallons per minute per square feet (gpm/ft2) and use physical straining to trap solids in the pores between sand particles throughout the bed.

To increase the capacity to filter, rapid sand filters often use flocculent (polymers) to stick particles together into larger particles that can then be more easily strained out. To clean the filter, flow is reversed through the filter bed at a high rate that fluidizes the bed, actually expanding the spaces between sand particles and flushing trapped material to waste.

This process is performed frequently, often daily. The injection of flocculent needs to be constant and precisely metered. Jar testing is necessary to determine the effectiveness of dosing of the flocculent. A skilled worker is necessary to maintain a rapid sand filter properly.

Slow sand filters work very differently. The much slower flow rate of .04 to .16 gpm/ft2 results in a much different character to the sand bed. Because of the slow rate, particles tend to settle in the very top layers of the sand.

More importantly, a rich biological matrix develops in the top layers of the sand. The matrix, commonly called the Schmutzedecke, is composed of a wide variety of tiny organisms including bacteria, algae and various other single and multiple cell organisms.

This matrix lives off whatever is passing through it in the water stream. Pore size of the bed is less important because a bacteria passing through can't touch anything without being stuck and consumed.

This way slow sand filters can remove particles smaller than the space between sand particles. At some point, the biological layer begins to plug up with the filtered material and debris. Flow becomes substantially reduced.

Most of the maintenance issues involving slow sand filtration can be minimized with careful preparation and planning.

Backwashing a slow sand filter using the same method as in a rapid sand filter would create havoc with the biological layer because fluidizing of the bed would damage the matrix and disrupt the intricate interrelationships of sand and microscopic life.

How do you clean a slow sand filter? Slow sand filters usually are returned to operational status by scraping and removing the top layer of sand because that is where the clogging takes place.

To accomplish this, the filter vessel is drained. Workers using either shovels or tractors scrape the top 1/2-inch of sand and discard it. Then the filter is refilled with water. Since the biological layer has been removed, filtered water is run to waste until the biological layer re-establishes itself - ranging anywhere from days to weeks. After many scrapings, the sand bed gets shallower and at some point new sand needs to be placed on the bed. After some years, small amounts of material accumulate deeper in the sand bed to an extent that require removal of all the sand and replacement. Scraping the filter bed is probably the most significant labor expense in slow sand filter maintenance. One report estimated that scraping requires 25 to 50 hours per 1,000 square feet per year, approximately two to five hours per month.

Compare this with the maintenance required to monitor flocculent levels and backwash a rapid sand filter several times a week though, and the maintenance of slow sand filters seems quite minimal.

In recent years, a new method of cleaning slow sand filters has emerged. Operators of a West Hartford, CT, slow sand filter developed a method called wet harrowing. Water above the sand is drained slowly to a depth of 15 inches or so.

The upper layer of sand is raked or harrowed while water is being drained off above the filter bed. The harrowing releases fine suspended particles that have accumulated. These particles are washed off by the water as it drains through valves above the filter bed.

Advantages of wet harrowing are significant. The most obvious is the labor savings. The filter does not have to be drained completely, and the sand does not have to be physically removed.

More importantly, the biological layer is stirred up, but not destroyed by exposure to air. This means that the filter does not need to go through a lengthy re-ripening period. Often the filter is back to normal operation within hours instead of days to weeks.

Since sand is not being removed, the sand bed retains its original depth and does not require periodic additions of new sand. Typically, if a slow sand filter is designed and operated properly, the sand should not need replacement for five to ten years.

How do you know when the filter needs to be cleaned? As material is pulled from the water in the Schmutzedecke layer, head loss increases. This head loss can be easily measured with the use of piezometers, clear tubes open to the atmosphere extending on the outside from the bottom of the filter to above the normal waterline. As head loss increases, the level of water in the tube will drop as atmospheric pressure overcomes the push of water through the filter. When the water level in the tube drops to 15 to 18 inches below the level in the filter, the filter needs to be cleaned.

Sediment tanks to remove settled solids, and course media roughing filters to reduce excess turbidity, reduce the solids load on slow sand filters.

If turbidity exceeds 20 nephelometric turbidity unit (ntu) for any time exceeding a few days, a roughing filter should be installed. Roughing filters can reduce turbidity by 50 to 80 percent and reduce the amount of maintenance on slow sand filters considerably.

Roughing filters are built as either upflow, downflow or horizontal flow depending on solids loading and method of cleaning desired. Media is usually layered course to fine gravel.

Selecting the right filter for the application is a very important step. For high demand situations, slow sand filters may require too much land area to be practical. Where high turbidity is a problem certain times of the year, pretreatment must be addressed.

These things considered, slow sand filters can provide high quality water from surface water sources. They are particularly valuable for the small rural homestead or community. New methods including wet harrowing, reduce the steps involved in maintaining slow sand filters and allow the filters to be returned to service rapidly by personnel with minimal training.

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