Sand filtration of domestic water supplies
By: Dave Murray

Sand filtration of a water supply is as old as Egyptian technology.

The concept is very simple. If you create a layer of fine sands, the space between each grain can still allow water to pass through, but is still small enough to trap and retain fine particles within the water flowing through it. The sand bed can also assist in filtering out many types of small biological organisms.

Slow sand filters utilize sand beds to support a biological layer that exists atop a bed of sand. This layer is called the Schmutzdecke.

Schmutzdecke - (German, "grime or filth cover") is a complex biological layer formed on the surface of a slow sand filter.

The Schmutzdecke is the layer that provides the primary filtering of potable water treatment. The underlying sand providing the support medium for this biological treatment layer.

The composition of any particular Schmutzdecke varies, but will typically consist of a gelatinous matrix of bacteria , fungi, protozoa, rotifera and a range of aquatic insect larvae. As a Schmutzdecke ages, more algae tend to develop, and larger aquatic organisms may be present including some bryozoa, snails and worms

1. Unlike other rapid sand filtration methods, slow sand filters use biological processes to clean the water, and are non-pressurized systems. Slow sand filters do not require chemicals or electricity to operate.

2. Cleaning is traditionally by use of a mechanical scraper, which is usually driven into the filter bed once it has been dried out. However, some slow sand filter operators use a method called "wet harrowing", where the sand is scraped while still under water, and the water used for cleaning is drained to waste;

3. For municipal systems, it is desirable for the maximum required throughput of water to be achievable with one or more beds out of service;

4. Slow sand filters require relatively low turbidity levels to operate efficiently. In summer conditions and in conditions when the raw water is turbid, binding of the filters occurs more quickly, and pre-treatment is recommended.

5. Unlike other water filtration technologies that produce water on demand, slow sand filters produce water at a slow, constant flow rate and are usually used in conjunction with a storage tank for peak usage. This slow rate is necessary for healthy development of the biological processes in the filter.

While many municipal water treatment works will have 12 or more beds in use at any one time, smaller communities or households may only have one or two filter beds.

If drinking water after it travels through the Schmutzdecke makes your stomach squirm, a bit, read about what we think is a better alternate, to slow sand filtration, in the articles on the left.

In the base of each bed is a series of herringbone drains that are covered with a layer of pebbles which in turn is covered with coarse gravel. Further layers of sand are placed on top followed by a thick layer of fine sand. The whole depth of filter material may be more than 1 meter in depth, the majority of which will be fine sand material.

Slow sand filters slowly lose their performance as the Schmutzdecke grows and thereby reduces the rate of flow through the filter.

Eventually it is necessary to refurbish the filter.

Two methods are commonly used to do this.

In the first, the top few millimeters of fine sand is very carefully scraped off using mechanical plant and this exposes a new layer of clean sand. Water is then decanted back into the filter and re-circulated for a few hours to allow a new Schmutzdecke to develop.

The filter is then filled to full depth and brought back into service.

The second method, sometimes called wet harrowing, involves lowering the water level to just above the Schmutzdecke, stirring the sand and thereby suspending any solids held in that layer, and then running the water to waste.

The filter is then filled to full depth and brought back into service. Wet harrowing can allow the filter to be brought back into service more quickly.

If the thought of your drinking water traveling through the Schmutzdecke makes your stomach squirm a bit, read about what we think is a better alternate, to slow sand filtration, in the articles on the left.