PRESSURE SAND FILTERS
PRESSURE SAND FILTERS
These filters are the most popular method for removal of turbidity from water. The Pressure Sand Filter consists of a multiple layer of sand with a variety in size and specific gravity. These Filters are designed to remove turbidity and suspended particles present in the feed water with minimum pressure drop. These Filters are custom designed to suit the process requirement. These filters are offered in Mild Steel, FRP or SS construction with face piping and associated Valves.
The Pressure sand filter consists of a pressure vessel either vertical or horizontal, with a set of frontal pipe work and valves, graded silica quartz sand supported by layers of graded under bed consisting of pebbles and gravels, a top distributor to distribute the incoming water uniformly throughout the cross section of the filter, and an under drain system to uniformly collect the filtered water.
Raw water flows down wards through the filter bed and as the suspended matter, which is treated by addition of a coagulant like alum or poly electrolyte, is retained on the sand surface and between the sand grains immediately below the surface. There is steady rise in the loss of head over a period of time and the flow reduces once the pressure drop across the filter is excessive. The filter is then taken out of service and cleaning of the filter media is effected by flow reversal also called as backwash. To assist in cleaning the bed, the backwash operation is sometimes preceded by air scouring by way of agitation through the under drain system. The air scouring agitates the sand with a scrubbing action, which loosens the intercepted particles. The filter is now ready to be put back into service.
We Offer the following Range of Pressure Sand Filters:
• FRP Pressure Sand Filters up to 2000 mm Diameter
• Vertical MS Pressure Sand Filters up to 5000 mm Diameter
• Horizontal MS Pressure Sand Filters up to 4000 mm Diameter
Pressure Sand Filter – Salient Features:
• Efficient Turbidity Removal
• MOC – FRP, MSRL, MSEP & SS
• Standard and effective multi grade sand media.
• Low Pressure drop across the vessel
• Optional Air scouring available.
• Manual, Semi Automatic & Automatic Options
Fluid Systems offers Chemical handling and Dosing Systems for dosing Coagulants like Lime. Ferrous Sulfate, PAC, Alum and Feed Conditioning chemicals like Anti-scalant, Antioxidant, Membrane cleaners etc. Dosing systems are Skid mounted, compact and rugged, engineered to handle and work efficiently under highly corrosive and varying conditions.
The areas of application include
• Raw Water Treatment
• Waste Water Treatment
• Reverse Osmosis
• Ultra Filtration
• Cooling Water Treatment
• Boiler Feed Treatment
Typical Dosing systems consist of Metering type pumps with varied material of construction to suit the chemical handling. All Dosing Systems can be upgraded to a high level of automation for link up to a localized PLC or client’s DCS as required.
Pre-engineered designs conform to International safety norms with respective to spillage and containment.
PRESSURE GRANULAR BED FILTERS
Pressure filters are sometimes used for rapid filtration. The filter medium is contained in a steel pressure vessel.The water to be filtered enters the filter under pressure and leaves at slightly reduced pressure because of the head loss encountered in the filter medium, underdrain, and piping connections.
Description of Pressure Filters
The pressure vessel may be a cylindrical tank with a vertical axis such as shown in Figure, or it may be a horizontal-axis cylindrical tank.The horizontal cylindrical configuration has the disadvantage that the width of the filter medium is not constant from top to bottom, usually being wider at the top.This leaves dead areas along the walls that do not receive adequate fluidization during backwashing and therefore, may not be washed effectively.
In some horizontal-axis cylindrical tanks, the filter is divided into multiple (four or five) cells by vertical bulkheads. Cost advantages are gained by this configuration because only a single pressure vessel is needed. One filter cell can be backwashed by the production of the other cells that remain in service.This requires a filtration rate sufficiently high so that the total production of the operating cells is sufficient to fluidize the medium of the single cell being backwashed.
Comparison of Pressure and Gravity Filtration
While the outward appearance of pressure and gravity filters are quite different, the filtration process is the same.The same mechanisms for capturing particles are functioning in both.The same filter medium, the same filtration rates, and the same terminal head loss should be utilized if comparable filtrate quality is desired. The use of higher filtration rates and high terminal head loss in a pressure filter is tempting because the influent is under pressure and more potential head loss is available. This temptation should be resisted, however, unless no detriment to the quality of the filtrate can be demonstrated on a case-by-case basis. One advantage gained by pressure filtration is that water leaves the filter under a positive gauge pressure, and therefore no negative pressure can ever exist in the
filter medium. The potential problems associated with negative pressure discussed earlier are therefore avoided.
Operation of Pressure Filters
Because of the similarities between pressure and gravity filters, the operating principles are identical. For example, appropriate pretreatment is equally important to pressure and gravity filters; patterns of filtrate quality will be the same; the impact of sudden rate increases will be just as detrimental; and the importance of proper backwashing is equally important.
The operation of a pressure filter is similar in most respects to the operation of a gravity filter. Proper backwashing of a pressure filter is more difficult, however, because the filter medium is not conveniently visible to the operator during the backwash operation. The various analyses that can be conveniently made by visual
observation for gravity filters are difficult or impossible to perform with pressure filters.
1. Presence of filter cracks before the backwash, or mudballs after the backwash
2. Uniformity of backwash water distribution
3. Uniformity of rate of cleanup of the wash water over the full filter areas
4. Proper functioning of the auxiliary scour device such as surface wash or air scour
5. Elevation and appearance of the top surface of the filter medium after the backwash
6. Whether the medium is fully fluidized during the backwash (if that is the
intended washing method), and the extent of expansion of the filter medium
7. Extent of loss of the filter medium
Because of these difficulties, pressure filters have been implicated in waterborne disease
outbreaks that can be partially attributed to poor condition of the filter medium. . Prior concerns about the reliability of pressure filters and other concerns have caused some state regulatory agencies to exclude the use of pressure filters in the treatment of surface waters or other polluted source waters and lime softened waters .
Rate Control for Pressure Filters
Rate control for pressure filters is equally as important as it is for gravity filters
because the same filtration mechanisms are functioning in both cases. Fewer options are available for pressure filters, however, because the pressure filter operates full of water under pressure. As a result, options involving changing water levels are not available, and influent gravity flow splitting is not available. Therefore, the benefits of slow rate changes by allowing water levels to change are not available to pressure filters.
The usual arrangement for a bank of pressure filters is based on the assumption that flow through the system will self-equalize. The total filter area of the filter bank is sized appropriately to meet local regulatory requirements based on feed pump capacity. Individual filter flow controllers and flow meters are not provided. The sys tem is designed symmetrically so that flow is distributed equally to each filter in the bank. Thus, after backwashing all of the filters in short succession, if one filter is passing more flow than it should, it is assumed that that filter will clog more quickly than the other filters, and that the flow will be reduced because of the clogging resistance.
Thus, the system is considered self-equalizing during operation. However, if the multiple filters are backwashed at random intervals during a filter cycle, the cleanest filter will provide the highest flow and the dirtiest filter the lowest flow. The extent of these differences will not be known if individual flow meters are not provided.
In spite of the common practice just described, it is desirable to have a flow meter on each pressure filter in a bank of several filters. Flow metering is useful to the plant operators for diagnostic purposes to observe if something is wrong with one filter, causing the flows to not be equally split.This could happen if one filter is not cleaning up properly during backwashing, perhaps due to a clogged underdrain. Or, one filter may flow at an excessive rate due to the loss of some or all of its filtering medium.
Applications of Pressure Filters
Pressure filters tend to be used in small water systems. Many pressure filters are used in industrial water and wastewater filtration applications.They also are used widely in swimming pool filtration.
Advantages of pressure filters over gravity filters include the following:
_ The filtrate, which is under pressure, can be delivered to the point of use without
repumping. In some treatment plants, source water can be pumped from the source through the treatment plant and directly to the point of use by the source water pumps.
_ A treatment plant equipped with pressure filters is somewhat easier to automate.
_ Some groundwaters containing iron can be treated by pressure aeration and/or chemical oxidation and then filtered directly on pressure filters.This approach has received considerable application for small communities. (Not all iron-bearing waters respond successfully to this method of treatment, however, and prior pilot testing is usually required.)
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