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Slow Sand Filtration of
domestic water supplies
By: Dave Murray

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.

 

Treatment:

Rapid Sand Filters

 

Rapid sand filters are large filter vessels (tanks), most frequently manufactured from carbon or mild steel. Many vessels are manufactured from fiber glass or composite materials.

Here is a small illustration
of a rapid sand filtration vessel:
 

The image above is of the internal structure of a filter vessel and does not show the sand installed. The vessel would normally be filled with sand to a level just below the upper pipes called the influent header and laterals.

The influent (incoming) and effluent (outgoing) headers and laterals are shown here:

These Influent headers and laterals are designed to distribute water over the sand bed evenly and to prevent disturbing the top layers of sand. If the filter is operating properly the top of the sand bed is smooth with no mounds or divots.

The effluent headers are designed to collect the water flowing down through the sand bed and to insure the entire sand bed is being utilized. A sand filter of any design, (especially slow sand filters) is prone to developing areas where flows of water are slowed and are not adequately cleaned. This is why the laterals are spread out all the way to the sides of the filter vessel.

Rapid Sand filters can filter 8-10 gallons per minute (gpm)/square foot of surface area of sand bed. If these filters are upstream of additional filters, such as cartridge filters they can be operated at a high a rate of 20 gpm/square foot of surface area of sand bed.

Here is an illustration of a design that I would reject:

Notice that the header and laterals do not extend out to the far sides of the tank. Because the tank is round and the laterals must be placed at the top and bottom, the laterals are placed at the thinnest regions of the tank. Water takes the course of least resistance and will go straight down missing the regions farthest from the direct route.

The filter bed on this design is only 18 inches thick, as well. This type of sand filter is often referred to as a "horizontal sand filter". The sales pitch for a horizontal design is: Horizontals have more surface area of sand bed and can, thus, filter more water in less area.

This is true. Slow sand filters are horizontal, as well.

BUT: Note the areas in red where water does not flow . The water will take the course of least resistance and not flow through the areas marked in red. In my opinion, horizontal filters must be designed to eliminate this lack of flow area. I have not seen one, yet, that qualifies.

I have used this design and manufactured horizontal tanks but not in a drinking water environment.

I reject this design in a potable water application because this horizontal design, as well as slow sand filters have a serious problem with cleaning themselves.

On the left, you can click on the article on Slow Sand Filters. Get your stomach ready for how the water is filtered. Or, you can click here for the same article.