Filtration is an important part of most water treatment systems. Filters range from simple cartridge systems to large commercial/industrial multi-tank systems, not to mention the large municipal systems that filter drinking water.
Filtration is the removal of suspended solids from a liquid such as water. Most water treatment systems begin with or include some type of filtration. The most common problem that is encountered with filters is that they can become fouled with contaminants in the water that is being filtered. This can result in a high pressure drop across the filter and/or a decrease in flow through the filter.
Why do filters become fouled? The answer is that they have become saturated with contaminants found in the incoming water. When using a cartridge-type filter, it’s possible to simply change out the cartridge and insert a clean one. However, a tank-type filter with filter media inside must be backwashed to remove the contaminate and then the bed must be returned to a clean working condition. The backwash rate of a filter is one of the most misunderstood aspects of water treatment. Most filters have either too much or not enough backwashing. What factors must be considered when designing a proper filter and reducing the tendency to foul?
We must first examine what it is we are trying to accomplish by asking the following questions.
- What is the total flow of water available?
- What are the service flow requirements?
- What is the contaminate that is necessary to remove, and how much of it is there?
- What else is in the water, and how will it affect the system?
- What are the quality requirements of the finished water?
- Is there a turbidity spec that must be met?
- What is the end use of the water? (Are there membranes down stream that must be protected?)
Proper Backwash Rate
To answer these questions, begin with determining the backwash rate required by the filter. What is the service rate desired? The proper design is calculated by using the proper flows as gallons per minute (gpm) per square foot of bed area. A typical sediment filter often requires a backwash rate that is twice the service flow rate. If a 20 gpm service flow is needed, a 40 gpm backwash rate may be required or the utilization of two tanks and backwash each sequentially at 20 gpm.
Carbon filters, iron removal media filters and other specialty media have specific backwashing requirements. Refer to manufacturers specifications when designing this type of filter.
In designing sediment filters, it often is an advantage to use multimedia filters in place of the dated sand filters. The sand in a sand filter is of a uniform particle size while a mutimedia filter uses up to five different media for different particle sizes and weights. A one-media filter becomes clogged when the top portion becomes loaded with contaminants. Often only six inches of bed are utilized. A multimedia allows the use of the entire bed depth due to the various densities of the filter media used. The big advantage to multimedia filters is that they often can use the same backwash rate as the service rate. Typically, 12 gpm per square foot of bed area is allowed.
For example, a 14-inch diameter tank has a square foot bed area of 1.07 square feet. A service and backwash rate of 13 gpm should not cause a problem. A single media filter of the same size would require a backwash rate of 26 gpm. (See Table 1.) Also note that the proper bed depth is required to accomplish proper filtration. Too shallow a bed depth will allow particles to pass through the bed.
The distribution system used inside the tank also must be properly designed. The lower distributor must be designed to allow the backwash water to reach all the bed area and purge the contaminants from the filter. The larger tanks will require either a hub/lateral or a header/lateral system. The correct pipe and slot size is of extreme importance.
Backwash With Filtered Water
One common design flaw is to use the incoming dirty water to backwash the bed. It is impossible to clean a filter bed when backwashing with the same contaminated water. Proper design indicates that filtered water be used as the backwash water. Use a storage tank to hold filtered water and pump this water to the filter during backwash. In multitank systems, use the filtered water from the system to backwash each tank sequentially to help keep the filter bed clean.
Filter fouling also can come from other sources. Oil, grease, iron and other contaminants can cause problems. Eliminate most fouling problems by giving some thought to the design of the filter system before installation.
SIDEBAR: Multimedia Depth Filters
Multimedia depth filters contain two or more types of media and gravel underbedding. There are several types of depth filters, each with the following features.
- Multimedia filtration is a design concept; the coarse media layers in the top of the tank trap large particles and successively smaller particles are trapped in the finer layers of media deeper in the bed. The result is a highly efficient filtering since removal takes place throughout the entire bed. Mulltimedia depth filters typically remove particles 5 to 15 microns in size or larger.
- Correct layering is the key to proper performance in the depth filter. All media are selected according to particle size and density, so the media retains its stratification during backwashing and rinse.
- In addition to efficient production of high clarity water, depth filters generally allow higher service flow rates of 10 gpm to 20 gpm per square foot of surface area, and longer service runs between backwashes.
- When compared to gravity filters, multimedia pressure filters installed in municipal applications generally require less space and can out perform single media filters.