Surface water is sometimes the only available safe water source for rural homeowners. But treating surface water can present real headaches for the water treatment professional. Typical problems encountered can be caused by high suspended solids, turbidity, coliform bacteria, viruses, Giardia and agricultural runoff.

Conventional remedies usually involve media filtration, cartridge filters and chlorination, which can require high maintenance and cost. The careful metering of chemicals makes some health officials wary of their use by homeowners. However, slow sand filtration can help solve some of these problems.

Slow sand filters have been used for more than 150 years. Today, they are popular in developing countries and in some municipalities and state parks in the United States. The proven ability of slow sand filters to remove turbidity, bacteria and viruses resulted in renewed interest by the U.S. Environmental Protection Agency (EPA) and the American Water Works Association (AWWA) in the 1980s. Recently, innovative packaging of slow sand filter plants has made this technology available to rural homeowners.

Common features of slow sand filters include the following:

* 2 or more vessels 6 to 8 feet deep

* An underdrain assembly for collecting treated water

* A gravel support layer around and over the underdrain

* A sand layer on top of the gravel 3 to 4 feet deep

* Controls to regulate flow rate

* Head loss measurement devices

* Filtration rate of .04-.16 g/ft²/minute.

Raw water enters the vessel at the top (diagram 1). When operating, the vessels are kept full, leaving a three-foot layer of water -- the supernatent -- above the filter sand. This layer provides some treatment through sedimentation, but more importantly provides head to drive the water through the filter sand for extended periods of time. The slow filtration rate allows the development of a naturally occurring layer of organisms to thrive in the top few inches of sand.

This layer, the Schmutzedecke, is responsible for removing up to 99.99 percent of bacteria, viruses, Giardia cysts and turbidity in the water passing across it. The Schmutzedecke is composed of a wide variety of life forms including algae, bacteria copopods, rotifers and many other invertebrates. Deeper in the bed, other processes occur that remove even more contaminants. Sedimentation, mechanical filtration and static electrical charge continue to polish the water.

Go With the Flow

The flow rate -- the speed and consistency of the flow -- is very important. Violent pressure changes that occur by opening and closing valves and rapidly changing the rate of flow can affect the filter's ability to work properly. Careful design of flow controls and the use of storage tanks to accomodate varying usage demands are important features in slow sand filter systems.

As the filter works, suspended solids build up in the Schmutzedecke, which will eventually need cleaning. Head-loss meters are necessary to monitor solids accumulation. Site glasses open to the atmosphere and attached to the bottom of the filter vessel show the condition of the filter. As solids accumulate, the water level in the tube drops. The filter should be cleaned when the level drops between 12 inches and 24 inches. The frequency of cleaning depends on raw water quality, but typically filters are cleaned every 2 to 10 weeks.

Slow sand filters are cleaned by draining the filter and scraping the top inch or so of sand and discarding it. However, this destroys the Schmutzedecke and requires a re-ripening period that can take weeks.

Recently, a method called wet harrowing has resulted in rapid turnaround times, usually in hours. In this method, water is drained above the sand layer while the Schmutzedecke is vigorously harrowed, or raked. This releases the suspended solids into the water above the filter, which is then drained to waste. The sand layer itself is never removed or exposed to air and the Schmutzedecke is not destroyed.

Sand Filter Drawbacks

Two problems often encountered by end-users of slow sand filters are high turbidity and organics in the water. High turbidity -- greater than 20 nephelometric turbidity units (ntu) -- can quickly clog a slow sand filter, resulting in short periods between cleaning. This may be one reason why slow sand filters became unpopular in the early 20th century.

There are simple remedies, however. Sedimentation tanks and gravel roughing filters are very effective at reducing turbidity. Roughing filters can reduce turbidity by 50 to 80 percent and can provide excellent pretreatment for slow sand filters.

Conventional slow sand filters are not effective at removing organics, which tie up chlorine, or pesticides. However, by using a layer of granular activated carbon (GAC), organic carbon and pesticides can be significantly reduced. This minimizes exposure to pesticides and reduces the ainount of chlorine required to establish any necessary residual. Reducing chlorine and organics lowers costs and the risk of producing and exposing customers to trihalomethanes (THMs).

Another problem with slow sand filters is construction costs. Traditionally, slow sand filters have been made of concrete and pipe. The innovative use of roto-moulded polyethylene structures has distinct advantages. First, the engineering cost is reduced because it is amortized over the number of filters manufactured. Second, the cost of producing polyethylene filter vessels is much less than the construction and pouring of concrete forms. Finally, the materials used in polyethylene components can be certified as non-toxic, while materials for cast-in-place filters require testing of each installation.

Additional advantages of polyethylene slow sand filters include transportability, durability and rapid set-up time. A polyethylene filter plant can be installed, loaded and started in hours compared with a built-in unit, which might require months to construct.

The best and simplest slow sand filter setup for a rural user is gravity-fed from a spring (See diagram 2). Water is piped from the spring directly into the slow sand filter. Treated water flows into a storage tank that distributes the water downhill to the end-user; gravity ensures continual water pressure.

But this configuration is not always possible. From a stream or pond, water may be pumped directly into the filter using an appropriately sized pump or a pump, pressure tank and pressure switch. After the storage tank, another pump, pressure tank and pressure switch can be used to provide water at suitable pressure to the user. Additions to the system may include chlorinators after the filter, ultraviolet (UV) after the filter and roughing filters before the filter if high turbidity or solids loads are anticipated.

When designed and operated properly, the filter should be harrowed every two months. The sand should last 5 to 10 years. Even a small slow sand filter is capable of filtering 1,400 gallons per day.

For the last 150 years, slow sand filters have been making drinking water safe throughout the world. With the recent discoveries of better maintenance methods, amendments to media and innovative packaging, this technology provides another tool for treating challenging water problems.

Humphrey Blackburn is a designer, developer and promoter of low-technology water treatment systems. He holds a master's degree from Humboldt State University, where he researched slow sand filters for the Mayan Highlands of Chiapas, Mexico.