Filtration Systems

General Description

 

For thousands of years, filtration has been used to reduce the level of dirt, rust, suspended matter and other impurities from water. This is achieved by passing the dirty input water (influent) through a filter media. As the water passes through the media, the impurities are held in the filter media material. Depending on the nature of impurities and the media, several different physical and chemical mechanisms are active in removing impurities from the water.

Some of the equipment used to employ these mechanisms has changed dramatically over time. Other systems, such as depth filters, have undergone very little change.The fundamental physical and chemical mechanisms that occur during filtration have become better understood over the years. These advances have allowed optimization of the removal of impurities from the water. Filtration systems remove particulate matter and, because of the large surface area of filter media, they also can be used to drive chemical reactions that result in the removal of several contaminants.

Sand And Dual Media Filters:

Occlusion:

Removal due to the impurity’s particle size. The filtration of suspended solids by occlusion removes particles based on size. Particles are occluded, or held back, due to their inability to pass through the pores of a barrier of some sort. The barrier might be a packed bed of sand, a fiber mat, or a membrane surface. Filtration by occlusion is often called “surface filtration”, since it occurs on the surface of the filtering media. Sand and Multi-Media filters are some of the filters working on this principle.

Activated Carbon Filters: Reduction:

Removal of free residual chlorine through conversion to chloride ions in the presence of activated carbon media. Chlorine is often added to water as a treatment chemical (e.g., for disinfection), and some residual chlorine may remain in the water after the treatment is complete. Residual Chlorine is the total amount of free and combined chlorine that remains in water after a designated contact time. Free available residual chlorine is the chlorine that exists in the water as hypochlorous acid and hypochlorite ions. De-chlorination partially or completely reduces the residual chlorine by chemical means. Free residual chlorine is converted to chloride ions in the presence of activated carbon by the following reaction:

Activated carbon has a nearly unlimited capacity for chlorine removal due to its large surface area and the above reaction. Activated carbon is a special form of carbon that is produced by heating organic material (such as coconut shells, walnut shells or coal) in the absence of oxygen. The heat removes trapped moisture and gases and dry activate most of the remaining organic material; it also leaves the remaining material with a slightly positive surface charge. Sodium bisulfite (SBS) injection is also frequently employed for chlorine removal in some systems.

These systems tend to have a much lower capital cost than the pressure vessel system that uses activated carbon. However, activated carbon filtration has the advantage of being a passive technology with no normal risk of “non-treatment”. Removal of chlorine with SBS may create side effects on subsequent treatment units such as Reverse Osmosis and was found to encourage the growth of bacteria in some cases. On the opposite side, the removal of the chlorine using activated carbon did not show such a phenomenon.

Adsorption:

removal due to the impurity’s adherence to the media. Adsorption refers to the removal of an impurity from a liquid to the surface of a solid. A water-born, suspended particle adheres to a solid surface when adsorption occurs. Adsorption differs from occlusion in that occluded particles are removed from a process flow because they are too large to pass through a physical restriction in the media. In most cases, adsorbed particles are affected by weak chemical interactions that allow them to adhere to the surface of a solid. Adsorbed particles become attached to the surface of a given media, becoming a weakly held part of the solid.

One Example is activated carbon bed, which can remove minute suspended particles, colloidal particles and dissolved organics due to its ability to adsorb or electro-statically hold particles. These particles would pass between the grains of carbon if not for the weak electrostatic attraction between the positive surface charge of the carbon and the negative surface charge of the particles. Particles can also be trapped in the porous structure of the activated carbon where they are then weakly held. Note that an activated carbon filter is not very efficient at removing most organic compounds from water and is rarely used in this manner.

Birm Filters :

Oxidation: removal of iron and manganese using oxidation, precipitation and filtration in the presence of Dissolved Oxygen and Birm media.Iron and manganese are commonly present in water in their soluble ferrous (Fe2+) and manganous (Mn2+) forms. They must be removed from the water to prevent fouling of downstream equipment and processes. Reverse osmosis (RO) can remove both ions if oxygen is kept out of the system. This can be a risky proposition. Removal before RO is a safer design. Before they can be removed, the iron and manganese are oxidized to create insoluble products that precipitate out of the water with the following reactions:For Iron:

As water passes through a bed of Birm media, the ferrous and manganous ions react with the oxygen in the water catalyzed by the surface of the media grains and get oxidized. With oxidation, the iron and manganese ions are converted to their insoluble ferric (Fe3+) and manganese (Mn3+) forms. Birm is not consumed in the iron removal operation. The birm operation is enhanced by the presence of dissolved oxygen of at least 15% of the iron content and high pH (more than 6.5).

The presence of chlorine and organic matter greatly reduces the Birm efficiency. Many other media are available in the market that have different application and specialized in the removal and treatment of one or more of the elements in the treated water. To mention a few: – Greensand Media – Lime Stone Media – Oil Removal Media – KDF Media AES Arabia should be consulted for any desirable application in this regard.

Filtration Process:

Vessels with sand or other loose filtration media are used in AES industrial filtration applications. These filters are cleaned by backwashing the media. During a backwash cycle, the filter bed is lifted and fluidized to remove accumulated particles. After the backwash cycle, the filter bed is allowed to settle. While it settles, the filter bed media will classify with the heaviest media particles settling first, and the lightest particles settling on the top. A single media (sand) filter bed will classify differently than a multi-media filter. Since all sand particles in a single media bed have approximately the same density, the largest particles are heaviest, and the smallest are lightest.

Larger/heavier particles settle at the bottom, while the smallest/lightest particles settle on top. While this does not provide very efficient filtration capacity, since filtration occurs mostly at the upper surface of the filter bed where the spaces between media particles are smallest.

However, due to the formed filtration cake on the top of the filter, smaller and smaller particles can be prevented from passing through, which results in a better outlet quality. Dual media beds use two or more filtration media. The media have selected densities to ensure that the bed settles in a more efficient manner. Anthracite, the largest particle, is the lightest (least dense) and settles on top. It provides large pore spaces that trap larger particles while allowing smaller particles to travel through to the layers below.

Sand is intermediate in size and density. It settles as the middle layer. The sand layer filters out particles of intermediate size while allowing smaller particles to flow through to the gravel layer. Gravel, the heaviest (most dense), settles out as the bottom layer in a filter vessel. A dual-media filter needs fewer backwashing for a given volume of water filtered. The filtering capacity is significantly increased for the same volume of media as compared to a single media bed. The whole bed filters, not just the surface.

Operating Principle:

During operation, water enters the vessel under pressure and is distributed over the top layer of the media bed by an inlet distributor. The media layers sit on top of a layer of subfill that supports the under-drain assembly. The sand fills the bottom of the vessel up to the weld seam of the straight shell with the bottom head, and covers the under-drain assembly. The subfill is not involved in water filtration. The lower distributor assembly collects the water and directs it out of the vessel through the service outlet.

When a sufficient quantity of particulate matter collects in the media bed, the filter is cleaned with a backwash cycle. Valves redirect the flow of water into the vessel through the lower distributor assembly. The water flows up through the media bed and carries the impurities out of the vessel through the inlet distributor and the backwash outlet. The backwash flow rate is much higher than the service flow rate. A media filter requires the addition of sufficient freeboard to allow expansion of the media bed during the backwash cycle. An optional air scour system can also be used to clean the media bed. The air scour system is used if the impurities on the media bed are particularly difficult to break up with a normal backwash. This is often the case when polymers are added to enhance the performance of the media bed.

To clean the media bed, the vessel is drained and air is blown into the under-drain and up through the media bed, causing the filter particles to scrub impurities off of each other. After the air scour cycle, the vessel is refilled and a backwash cycle is performed to remove loose impurities and to re-classify the media. The backwash step that follows an air scour is shorter and uses less water than a normal backwash cycle. The following modes of operation for a multi-media filter are described below: – Normal service – Drain-down (part of air scour option) – Air scour (optional) – Backwash – Rinse

Normal Service:

The valve configuration and water flow for normal service:Service Inlet valve open (to provide a supply of water to be filtered)Service Outlet valve open (to provide filtered water to downstream equipment) – Backwash Inlet valve closed (to prevent water from flowing downstream without being filtered, in common feed/ backwash header) – Backwash Outlet valve closed (to prevent incoming water from going to drain) – Rinse Outlet valve closed (to prevent filtered water from exiting the vessel and going to drain) – Air Inlet valve for Air Scour closed (to prevent the introduction of air into the vessel)

Drain-down for Air Scour (optional):

Before the air scour system can be used on the media, the water level must be lowered to a few inches above the top of the media. The valve configuration listed below is used during the drain-down cycle of the media filter system.- Service Inlet valve closed (to prevent water from entering the vessel) – Service Outlet valve open (to provide filtered water to downstream equipment) Service Outlet valve closed (to prevent unfiltered water from going to downstream equipment) – Backwash Inlet valve closed (to prevent water from entering the vessel) – Backwash Outlet valve opened (to allow air entering as water drain from the vessel) – Rinse Outlet valve open (to allow water to go to drain and to lower the water level) – Air Inlet valve for Air Scour closed (to prevent air from entering the vessel at the wrong location) When the water reaches the proper level, the air scour cycle can be initiated. The level is set by routing the rinse outlet pipe up to the level of the top of the media bed before turning down.

Air Scour (optional):

The optional air scour cleans the media more thoroughly than a backwash cycle alone. Service Inlet valve closed (to prevent incoming water from entering the vessel during the air scour cycle) Service Outlet valve closed (to prevent dirty water from contaminating down stream equipment) Backwash Inlet valve closed (to prevent incoming water from entering the vessel during the air scour cycle) – Backwash Outlet valve opened (to allow air to exit the vessel) – Rinse Outlet valve closed (to prevent water from going to drain) – Air Scour Inlet valve open (to provide an air supply for the air scour system to operate for a specified time).
Before the air scour system is used, the vessel must be drained until the water level is just above the top of the media bed. After the air scour, the filter must be refilled with water and backwashed to remove the loosened impurities. After the air scour is complete, the vessel is refilled with water via the backwash inlet valve.

Backwash:

The backwash cycle is used to remove impurities that have collected in the media bed. During the backwash cycle, the valves are oriented to reverse the flow of water from normal operation. With sufficient flow, impurities are loosened from the media bed and carried out of the vessel through the inlet distributor and service inlet. The media bed must be expanded by 30% for the backwash to be effective. To prevent filter media particles escaping from the vessel, the inlet distributor must be sufficiently higher than the top of the expanded bed. – In case of manual filter the operator does backwash process initiation manually. In case of automatic filter the backwash process is generally initiated through the timer or differential pressure switch. In AES standard automatic filter, timer based backwash is provided.

The following valve configuration is used during the backwash cycle of the media filter system: Service Inlet valve closed (to prevent incoming water from flowing against the backwash water flow) Service Outlet valve closed (to prevent dirty backwash water from contaminating downstream equipment) – Backwash Inlet valve open (to supply water to backwash the media bed). – Backwash Outlet valve open (to set the flow rate and carry away the dirty back wash water to drain) – Rinse Outlet valve closed (to prevent water from going to drain) – Air Scour Inlet valve closed (prevent air from entering system)

After the backwash cycle is complete, the vessel is rinsed and can then return to normal service.

Rinse:

The rinse cycle is used to remove any residual backwash water in the media bed. The rinse mode is the same as the service mode, except the water is sent to drain instead of to service. The valve configuration listed below is used during the rinse step of the multi-media filter system:- Service Inlet valve open (to provide a supply of water for the rinse step) – Service Outlet valve closed (to prevent dirty water from contaminating down stream equipment) – Backwash Inlet valve closed (to prevent incoming water from entering the vessel at the wrong location) – Backwash Outlet valve closed (to prevent water from leaving the vessel during the rinse step) – Rinse Outlet valve open (to carry away the dirty rinse water to drain) – Air Scour Inlet valve closed (to prevent air from entering system) After the rinse cycle, the vessel can be returned to normal service.

Type Of Filters :

AES Industrial Series Filters are mainly classified according to media used-such as : – Sand Filters – Dual Media or Sand / Anthracite Filters – Activated Carbon Filters – Birm Filters. Depending upon mode of operation, these filters can further be classified as: – Manual IF Filters – Automatic IF Filters.
Applications :-

Sand Filters:

Removal of turbidity and suspended solids from lightly contaminated water sources such as deep wells and municipality water supplies. Used frequently upstream of reverse osmosis and demineralization systems.

Dual Media or Sand / Anthracite Filters: emoval of turbidity and suspended solids from heavily contaminated water sources such as gray water and domestic sewage tertiary treatment. Also used as a roughening-filters a head of two stage filtration systems.

Activated Carbon Filters:

Removal of free chlorine upstream of RO or Softening plants and removing from smell and odor from lightly contaminated water supplies.

Brim Filters:

Removal of Iron and Manganese from contaminated water supplies.
For applications other than above & if you require help in proper selection, please consult AES & we will be happy to assist you.