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Pressure Filtration

Overview
 

Included Processes: Bag Filtration and Cartridge Filtration

Pressure filters are in many respects similar to conventional rapid filters. The main differences are that the media is contained in a pressure vessel (such as a steel tank) and that they are operated under pressure provided by means of a pump or high-pressure water source on the influent side rather than gravity.

Typically, pressure filters are used for the filtration from groundwater or high quality surface water that does not require clarification. In groundwater systems they can be used for the removal of iron and manganese in addition to turbidity. Greensand filters can also be configured as pressure filters for the removal or arsenic. Pretreatment, such as oxidation, may be added before the filter to enhance removal of some contaminants.

Like conventional rapid filtration, pressure filtration is a physical chemical process. The main parameters controlling the efficiency of filtration are linked to influent water quality (pH, turbidity, TOC, alkalinity, and temperature), filter media characteristics (media type, grain size, distribution coefficient, age and general conditions) and operation (filter loading). Loading rates, criteria for backwashing and other operational factors remain similar to conventional filtration.

The differences between pressure filters and rapid sand filters are that it is not possible to visually inspect the filter media during operation and backwash, flow splitting between filters is generally unregulated, and the addition of pretreatment chemicals may be done under pressure depending on the system configuration. Pressure filters are constructed in vertical or horizontal configurations. Vertical pressure filters are generally used for smaller plants because the design offers the flexibility of incremental plant expansion and the ability to easily isolate a single filter cell. Horizontal pressure filters typically are used for larger plants since their layout maximizes filter area and minimizes footprint.

Like rapid sand filters the residuals from pressure filters consist primarily of spent filter backwash. Spent filter backwash can be recycled to the treatment process to improve performance and reduce water losses, or it may be discharged to a sanitary sewer or receiving stream. Filter backwash will contain elevated concentrations of the contaminants removed and should be properly evaluated prior to disposal.

Pressure filtration processes include bag filtration and cartridge filtration.  In bag filtration systems, water to be treated passes through a bag-shaped filtration unit where the particles are collected on the bag's filter media while allowing filtered water to pass to the outside of the bag.  Bag filters are are availble in several micron ratings (typically from 1 to 40 microns).  The sizing of the bag filtration component is conditional on the raw water quality, including the amount of particulate matter and turbidity. 

Cartridge filtration typicallty includes pressure filters with pleated fabrics, membranes or strings wrapped around a filter element and housed in a pressure vessel.  The pleating allows for higher surface area for filtration.  These filters are available in several micron ratings (from 0.3 to 80 microns) and materials.  Similar to bag filtration, these units are very compact and do not require much space.  





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