A Filter can be described as a device or material, which separates one entity from another. This process of separation in Industrial processes falls into four main categories.

  • LIQUID from LIQUID separation
  • SOLID from LIQUID separation
  • SOLID from SOLID separation
  • SOLID from GAS separation

A Filter can be described as a device or material, which separates one entity from another. This process of separation in Industrial processes falls into four main categories.

  • LIQUID from LIQUID separation
  • SOLID from LIQUID separation
  • SOLID from SOLID separation
  • SOLID from GAS separation

Water filters use a variety of different media (the components of the filter) to remove contaminants from water. Charcoal is a common media for many water filters. To answer your question simply, a charcoal filter does not make water even dirtier because the charcoal used in water filters is generally solid or granulated (like grains of sand) and not ashy like the charcoal used in art pencils. In a water filter, as water passes over the media, the granules of that media trap large water contaminants and stop them from passing through with the water. The process is very similar to river water passing through rocks and emerging cleaner on the other side. While this method of filtration is certainly useful, it is not perfect. Water filters can only remove certain contaminants and smaller contaminants will easily pass through the granules of the media with the water. Most modern filters now use chemical processes to filter water, in addition to simple physical filtration. Modern filters use a media that will attract contaminants (using a process of positive and negative charges similar to magnets) and encourage these contaminants to break their bond with water. For example, chlorine is a chemical that often contaminates water. Chlorine is very difficult to remove by simply passing water over a granular media. Chemically, chlorine can be removed by using a carbon-based media to attract atoms of chlorine and to encourage these atoms to break their bond with the water. To sum it all up, water filters remove contaminants by either physically blocking their passage through the filter media or chemically attracting them to the filter media. In general, they use a granulated or solid filter media that will not travel with water and further dirty it.

A backwashing filter is a water filter that cleans itself periodically by rinsing away impurities it has filtered from water. Although a backwashing filter may look like a water softener and be the same size, it’s a different animal. Softeners are “ion exchangers,” not filters. A backwashing filter is a simple device that consists of a large tank called a “mineral tank” that is filled with a filtering substance called a filter medium. (The plural is media.) Water enters the top of the tank through a special control valve and passes downward through the medium, which removes impurities and holds them. Some media do not hold impurities, but cause a change to occur. Calcite, for example, dissolves and in the process increases the pH of acidic water. The treated water then enters a tube at the bottom of the mineral tank, passes upward through the tube (called a riser), and exits the filter via the control valve. When the filter medium is saturated with contaminants, the control valve initiates a backwash. The backwash is an operation in which water passes backward through the filter at a rapid rate. It enters the tank at the bottom via the riser tube, and then passes upward through the filter medium, exiting at the top, via the control valve. The rapid upward flow, in addition to washing away stored impurities, fluffs and resettles the medium bed, preparing it for another filtering cycle.

A number of things could be causing the problem. It could mean that a lateral is cracked or broken, or a valve may need servicing. Or, air may be passing through the filter, causing a channeling effect that permits dirt to get by the filter. In the latter case, there may be nothing wrong with the filter

The general recommendation is to backwash your filter when the pressure reaches 10 psi over the initial start up pressure. For example, if the start up pressure is 15psi, you should backwash when the pressure reaches 25psi. If the pressure does not return to the start up pressure after backwashing, it is time to change the sand.

To replace the sand in your filter you must first disconnect the pipes going into the valve on top of the filter. Next, unscrew the complete valve counter-clockwise. Once the valve has been removed, you will be able to see the sand bed. The best way to remove the sand is to either use a portable vac or just dip it out with a small can. Once the old sand has been removed, fill the tank 2/3 full of water and just pour the new sand in. DO NOT get sand in the vertical pipe sticking up from the bottom of the filter tank.

Filter Operation

Basic operation of Pressure Filter, Dual Media Filter and Activated Carbon is as follows:

Mode of Operation: All Units operate in down flow mode, where the water enters from the top, percolates through the media and treated water is collected from the bottom.

Sequence of Operation: (see figure)

Service: The water to be filtered enters from the top of the shell, percolates downward through the media and is drawn off from the bottom.

Backwash: The water enters from the bottom of the vessel, passes through the media and is drained from the top. This is called BACKWASH and it is done to carry the dirt accumulated on the top. Generally back washing is done once in every 24 hrs or when the pressure drop exceeds 8 psi. (0.5 kg/cm2)

Rinse: The water enters from the top passed through the media and is drained off from the bottom.

Thumb rules for designing a filter: Calculate area of vessel by required volumetric flow rate and the velocity as mentioned in the following table.
Area (m2) =  Volumetric Flow Rate (m3/hr)
Velocity  (m/hr)
Based on above calculated area calculate dia. of the vessel by the following formulae:
Dia (m) = [Area (m2)/ 0.7856]½

Packaging Chart

Parameters Sand Filters Dual Media Filters Activated Carbon
Velocity (m3/m2/hr) 7.5-12 12-20 15-20
Effective size of Media (mm) 0.45 – 0.6 (fine sand) 0.65 – 0.76 (Anthracite) 0.35 – 0.5
Uniform coefficient 1.6 max 1.85 < 2 (115 typical)
Density (kg/m3) 2650 1600

Important Points on Filter

Normally, pressure sand filter is used to filter suspended solids up to 30 ppm and dual filter for 50-55 ppm and higher suspended solids would require coagulation. Output quality of water from a Pressure Sand Filter is 25 to 50 microns. Normally, velocity for Sand velocity is taken for water treatment / residential filters are taken as 7.5 to 18 m3/m2/hr; for institutional filters 20 to 30 m3/m2/hr. For recirculation of water like swimming pool velocities can be taken greater than 35 m3/m2/hr for low turbidity application higher velocity will induce higher head loss through the bed and frequency of backwash will increase. Back washing of filter should always be carried out using clean water. Whenever air scouring is provided, it should be done before back washing step. Where strainers are provided at bottom, pebbles and gravels need not be put. 1NTU is approximately between 2.5 to 3 ppm

Impurities in Water

Impurities Effect Method of removal
Suspended silica
Can clog pipelines and equipments  can choke ion exchange resin and RO membranes Coagulation, setting and filtration
Color Indication of organic, iron etc. and can be harmful to the unit operation ahead. Coagulation, settling filtration, followed by activated carbon filter.
Organic matter Can foul ion exchange resins membranes and may be detrimental to process. Coagulation, setting, filtration, followed by activated carbon filtration.
Bacteria Will depend upon the type of bacteria, can induce corrosion and also harmful to RO membrane. Coagulation, filtration, setting and super chlorination, UV, ozonation
Iron Red water, corrosion, deposit, interferes with dyeing, bleaching etc. Aeration, coagulation, filtration, filtration through Manganese Zeolite
pH High pH or low pH can both induce corrosion. Ion exchange, addition of acid or alkali.
Calcium, Magnesium       (Hardness) Scaling, cruds with soap interfere with dyeing and also harmful to other process. Ion exchange Lime Soda
Sodium Unharmful when low in concentration, increases TDS, high concentration can induce corrosion. Ion Exchange through cation H+ resin. Reverse Osmosis
Bicarbonates, Carbonates,                Alkalinity, Hydroxide
Corrosion, foaming and carry over Acid addition Ion Exchange by weak acid cation Split stream by hydrogen cation resin Degasification
Sulphate Scaling if associated with Calcium, harmful in construction water. Ion Exchange Reverse Osmosis
Evaporation Electrolysis
Chloride Corrosion Ion Exchange Reverse Osmosis
Evaporation Electro dialysis
Impurities Effect Method of removal
Nitrate Normally not found in raw water. Harmful in food processes (especially baby food). Ion Exchange Reverse Osmosis
Silica Scaling and deposition on equipment. Ion Exchange
Carbon Dioxide Corrosion Open aeration, Degasification, and Vacuum de-aeration.
Hydrogen Sulphide Corrosion Aeration, filtration through Manganese Zeolite, aeration plus chlorination.
Oxygen Corrosion De-aeration
Addition of chemicals likes sodium sulphite or hydrazine.
Anion exchanger
Ammonia Corroding especially of Copper and Zinc Aeration Hydrogenations exchange if ammonia is present in Ionic form.
Free chlorine Corrosion By adding chemicals
Activated carbon


To select vessel model for a selected media quantity, approx, flow rates based on linear velocity min = 8m3/m2/hr and max = 25m3/m2/hr