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.
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.
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.
Advantages:
- It has a much higher flow rate than a slow sand filter;
- Requires relatively small area of land
- Is less sensitive to changes in raw water quality
- It delivers about 150 to 200 million gallons of water per acre per day.
Disadvantages:
- The rapid sand filter is not an adequate treatment on its own.
- It requires greater maintenance than a slow sand filter and generally requires mechanical pumping of water and air, at least for backwashing. For this reason, it is not usually classed as an appropriate technology.
- Generally ineffective against taste and odor.
- Produces large volumes of sludge for disposal.
- Requires on-going investment in costly flocculation reagents.
Besides of these types of filters, we can find such ones as media filters, screen filters, disk filters and cloth filters.
A Media Filter is a type of filter utilizing a bed of sand, crushed granite or other material to filter water for drinking, swimming pools, aquaculture, irrigation, and other applications.
One design brings the water in the top of a container through a “header” which distributes the water evenly. The filter “media” start with fine sand on the top and then graduatingly coarser sand in a number of layers followed by gravel on the bottom, in gradually larger sizes. The top sand physically removes particles from the water. The job of the subsequent layers is to support the finer layer above and provide efficient drainage.
A Disk Filter is a type of water filter used primarily in irrigation, similar to a screen filter, except that the filter cartridge is made of a number of disks stacked on top of each other like a pile of poker chips. The water passes through the small grooves in between and the impurities are trapped behind. Some types of disk filters can be backflushed in such a way that the disks are able to separate and spin during the cleaning cycle.
A Screen Filteris a type of filter utilizing a rigid or flexible screen to separate sand and other fine particles out of water for irrigation or other applications. These are generally not recommended for filtering out organic matter such as algae, since these types of contaminants can be extruded into spaghetti-like strings through the filter if enough pressure drop occurs across the filter surface. Also, screen filters do not have the capacity to filter large amounts of contaminants as do media filters. Typical screen materials include stainless steel (Mesh), polypropylene, and nylon. Similar devices with larger openings designed only to keep out large objects are called strainers.
Developed for use in Bangladesh, the Cloth Filter is a simple and cost-effective appropriate technology method for reducing the contamination of drinking water. Water collected in this way has a greatly reduced pathogen count - though it will not necessarily be perfectly safe, it is an improvement for poor people with limited options. The cloth filter has been studied and reported on by Rita Colwell and Anwar Huq from the University of Maryland Biotechnology Institute, together with other researchers from the USA and Bangladesh. They report that: It is common practice in villages in Bangladesh to use cloth, frequently a flat, unfolded piece of an old sari, to filter home-prepared drinks. The researchers studied the application of this technique to drinking water, with folded cloth. They studied the pore size of the cloth, the effect of folding the cloth on the effective pore size, the ability of the cloth to remove particles and plankton, as well as the effect on rates of cholera when used in a Bangladesh village.