Types of Filters

Filters can be categorized in a variety of ways.   The table below shows the characteristics of four types of filters which can be used in water treatment.

Slow Sand Filter
Rapid Sand Filter
Pressure Filter
Diatomaceous earth filter
(Diatomite filter)
Filtration rate
0.015-0.15 2-3 2-3 1-2
Pros Reliable.  Minimum operation and maintenance requirements.  Usually does not require chemical pretreatment. Relatively small and compact. Lower installation and operation costs in small filtration plants. Small size.   Efficiency.  Ease of operation.  Relatively low cost.  Produces high clarity water.  Usually does not require chemical pretreatment.
Cons Large land area required.  Need to manually clean filters. Requires chemical pretreatment.  Doesn't remove pathogens as well as slow sand filters. Less reliable than gravity filters.  Filter bed cannot be observed during operation. Sludge disposal problems.  High head loss.  Potential decreased reliability.  High maintenance and repair costs.
Filter Media Sand. Sand.  Or sand and anthracite coal.  Or sand and anthracite coal and garnet. Sand.  Or sand and anthracite coal.  Or sand and anthracite coal and garnet. Diatomaceous earth.
Gravity or Pressure? Gravity. Gravity. Pressure. Pressure, gravity, or vacuum.
Filtration Mechanism Biological action, straining, and adsorption. Primarily adsorption.  Also some straining. Primarily adsorption.  Also some straining. Primarily straining.
Cleaning Method Manually removing the top 2 inches of sand. Backwashing. Backwashing. Backwashing.
Common Applications Small groundwater systems. Most commonly used type of filter for surface water treatment. Iron and manganese removal in small groundwater systems. Beverage and food industries and swimming pools.  Smaller systems.

We will discuss two types of filters below - the slow sand filter and the rapid sand filter.  The pressure sand filter is essentially a rapid sand filter placed inside a pressurized chamber while the diatomaceous earth filter is not commonly used in treatment of drinking water. 


The history of water treatment dates back to approximately the thirteenth century B.C. in Egypt.  However, modern filtration began much later.  John Gibb's slow sand filter, built in 1804 in Scotland, was the first filter used for treating potable water in large quantities.  Slow sand filters spread rapidly, with the first one in the United States built in Richmond, VA, in 1832. A set of slow sand filters adapted from English designs was built in 1870 in Poughkeepsie, NY, and is still in operation. 

A few decades after the first slow sand filters were built in the U.S., the first rapid sand filters were installed.  The advent of rapid sand filtration is linked to the discovery of coagulation.  By adding certain chemicals (coagulants) to turbid water, the material in the water could be made to clump together and quickly settle out.  Using coagulation, clear water for filtration could be produced from turbid, polluted streams. 

By the end of the nineteenth century, there were ten times as many rapid sand filters in service as the slow sand type.  Currently, slow sand filtration is only considered economical in unusual cases. 

The diatomaceous earth filter was developed by the U.S. Army during WWII.  They needed a filter that was easily transportable, lightweight, and able to produce pure drinking water.  The diatomaceous earth filter is used in smaller systems, but is not commonly part of water treatment plants. 

Slow Sand Filter

The slow sand filter is the oldest type of large-scale filter.  In the slow sand filter, water passes first through about 36 inches of sand, then through a layer of gravel, before entering the underdrain.  The sand removes particles from the water through adsorption and straining. 
Slow sand filter.
Unlike other filters, slow sand filters also remove a great deal of turbidity from water using biological action.  A layer of dirt, debris, and microorganisms builds up on the top of the sand.  This layer is known as schmutzdecke, which is German for "dirty skin."  The schmutzdecke breaks down organic particles in the water biologically, and is also very effective in straining out even very small inorganic particles from water.

Maintenance of a slow sand filter consists of raking the sand periodically and cleaning the filter by removing the top two inches of sand from the filter surface.  After a few cleanings, new sand must be added to replace the removed sand. 

Cleaning the filter removes the schmutzdecke layer, without which the filter does not produce potable water.  After a cleaning the filter must be operated for two weeks, with the filtered water sent to waste, to allow the schmutzdecke layer to rebuild.  As a result, a treatment plant must have two slow sand filters for continuous operation. 

Slow sand filters are very reliable filters which do not usually require coagulation/flocculation before filtration.  However, water passes through the slow sand filter very slowly, and the rate is slowed yet further by the schmutzdecke layer.  As a result, large land areas must be devoted to filters when slow sand filters are part of a treatment plant.  Only a few slow sand filters are operating in the United States although this type of filter is more widely used in Europe.

Number of slow sand filters by state.
Number of slow sand filters operating in each state as of 1991. (Sims)

Rapid Sand Filter

The rapid sand filter differs from the slow sand filter in a variety of ways, the most important of which are the much greater filtration rate and the ability to clean automatically using backwashing.  The mechanism of particle removal also differs in the two types of filters - rapid sand filters do not use biological filtration and depend primarily on adsorption and some straining. 

Since rapid sand filters are the primary filtration type used in water treatment in the United States, we will discuss this filter in more detail. 

Cut-away view of a rapid sand filter.

A diagram of a typical rapid sand filter is shown above.  The filter is contained within a filter box, usually made of concrete.  Inside the filter box are layers of filter media (sand, anthracite, etc.) and gravel.  Below the gravel, a network of pipes makes up the underdrain which collects the filtered water and evenly distributes the backwash water.  Backwash troughs help distribute the influent water and are also used in backwashing (which will be discussed in a later section.) 

In addition to the parts mentioned above, most rapid sand filters contain a controller, or filter control system, which regulates flow rates of water through the filter.  Other parts, such as valves, a loss of head gauge, surface washers, and a backwash pump, are used while cleaning the filter.

Rapid sand filter.

Operation of a rapid sand filter during filtration is similar to operation of a slow sand filter.  The influent flows down through the sand and support gravel and is captured by the underdrain.  However, the influent water in a rapid sand filter is already relatively clear due to coagulation/flocculation and sedimentation, so rapid sand filters operate much more quickly than slow sand filters.