Sand filters use a single grade of sand media to remove particulates, typically suspended solids larger than 50 microns. Sand filters come in two varieties:
- Gravity filters - used quite often in municipal applications on surface water supplies
- Pressure filters – used in industrial water reclaim applications, swimming pool filters, cooling tower side stream filtration, and many other applications
Flow rates are sized in gallons per minute, per square foot of filtration area. Filtration rates are usually between 2 gpm and 5 gpm per square foot. Because of their slow filtration rate, sand filters require a large foot print. To clean the filter bed, sand filters can be backwashed with either finished water or source water. Backwash rates are typically twice the filtration rate.
Multimedia filters are similar to sand filters in terms of mechanical function. The difference is, as the name would suggest, they use more than one media for improved filtration.
Multimedia filtration, also known as depth filtration, is a process in which impurities are removed by multiple media layers. The differing characteristics of each layer facilitate diverse particulate removal. In a typical sand filter with one media, 90+% of the filtration takes place in the top 3” of media. In a multimedia filter, filtration takes place in each layer, which provides the following advantages over a single-layer sand filter:
- More dirt holding capacity per cubic foot of media, providing longer service runs
- Reduced water usage during backwash, due to the media layers’ diversity of weight
- Higher flow rates; some multimedia filters are rated up to 10, 15, or even 20 gallons per square foot, resulting in a smaller foot print than traditional sand filters and often a lower capital cost
- Improved particulate filtration, since the bottom layer of most multimedia filters is finer and more dense than the sand used in a sand filter
Cartridge and bag filters are "dead-end" filters. In this process, water passes directly through an expendable filter, which catches particles and eventually becomes fouled and must be replaced. This type of filtration has three distinct advantages:
- Lowest capital cost
- Reduced use of space
- Flexibility in size of particulate removal (1 to 100 micron)
Cartridge and bag filters have two disadvantages:
- Higher operating cost
- Greater need for operator maintenance and attention
Cartridge and bag filters can also be used for final high purity applications requiring sub-micron filtration.
Greensand media filtration is used to remove iron, manganese or hydrogen sulfide from water. The density of the greensand media, combined with its ion exchange capabilities, make it an ideal method to remove these impurities. The greensand bed is regenerated using potassium permanganate, which is introduced to the media in one of two ways: continuous regeneration and batch regeneration.
Continuous regeneration is accomplished by constantly feeding a small, threshold amount of potassium permanganate in proportion to the feed water flow. The potassium permanganate oxidizes the iron, manganese or hydrogen sulfide. This oxidation causes the formation of a particulate that can be trapped by the media. The filter bed is backwashed either periodically or by the use of a pressure differential switch. Continuous feed/regeneration is the most efficient way to operate a greensand media filter, and it is used in most large applications.
Batch regeneration is the process in which the greensand media bed is intermittently saturated with potassium permanganate, a procedure similar to the function of a common softener.
Greensand filters are sized at around 5 gpm per square foot of filter area. Backwash rates are typically twice the service flow rates.
Activated carbon filtration is used to remove organics, tastes, and odors, as well as chlorine and other oxidizers. Additionally, activated carbon filters can remove turbidity, though its removal should not be the filter’s primary function, as the presence of turbidity will hinder the media’s ability to remove organics.
When water is passed through an activated carbon filter, organics are adsorbed and absorbed into the media’s characteristically large, porous surface area.
Filtration rate is determined by the specific organic to be removed; some organics require longer contact time than others. Filtration rates are usually between 5gpm and 15gpm per square foot of filtration area.