Depth filters are the variety of filters that use a porous filtration medium to retain particles throughout the medium, rather than just on the surface of it. These filters are commonly used when the water to be filtered contains a load of particles because, in comparison to the other types of filters, they can retain a large mass of particles before becoming clogged.
Depth filtration works by collecting particles within the filter media and passing a clean outlet flow of the fluid. Depth filtration typified by multiple porous layers with depth are used to capture the solid contaminants from the liquid phase.
Due to the tortuous and channel-like nature of the filtration medium, the particles are retained throughout the medium within its structure, as opposed to on the surface. Depth filters pose the added advantage that they are able to attain a high quantity of particles without compromising the separation efficiency. Depth filters are commonly characterized by the sand filter and have the ability to be used with substantially higher filter rates than in other designs. It is these characteristics that have cemented the use and popularity of depth filters as an effective medium for separation. With ongoing advances in process technologies, depth filter designs are continuously adapting and improving to meet the needs of industry.
A sand bed filter is a kind of depth filter used to separate small amounts (<10 parts per million or <10 g per cubic metre) of fine solids (<100 micrometres) from aqueous solutions.
Pre treated water passes through Sand filters to remove suspended impurites. Sand bed filters are used for both fresh and waste water filteration.
Carbon filtering is a method of filtering that uses a bed of activated carbon to remove contaminants and impurities, using chemical adsorption. Activated carbon works via a process called adsorption, whereby pollutant molecules in the fluid to be treated are trapped inside the pore structure of the carbon substrate. Carbon filtering is commonly used for water purification.
Iron can be removed from water, either in its soluble form or insoluble form. As soluble ferrous ions, iron removal can be carried out by the following process :
(1) Ion exchange using a metal selective (chelating) strong acid cation resin.
(2) Reverse osmosis provided the total iron in the feed water is < 0.05 mg/l.
There are several main types of processes that can achieve removal of total iron from water, via its insoluble forms :
(1) Chemical oxidation - Strong oxidants such as ozone, hydrogen peroxide and chlorine can be dosed to fully oxidise soluble iron to insoluble ferric hydroxide for its removal by clarification or filtration.
(2) Media adsorption - Various active medias can be used in presence of chlorine which acts as catalyse for oxidation reactions. The media also serves to filter out the suspended particles of ferric hydroxide produced.
(3) pH adjustment - The pH can be adjusted to above 8 by aeration and/or dosing caustic. Flocculation is then carried out to maximise particle size, before removal by either clarification or filtration. The above processes are all capable of achieving total iron reductions down to the 0.3 mg/l limit set for potable water.
To achieve better results in terms of specific flow and outlet quality multigrade filters are being used. Multigrade filter have more than one media for filteration. Other than filtering sand more poros media are utilized for filteration, entrasite and garnet are the examples of those kind of medias.
Because of their better porosity they are used as the primary filtering media along with the filtering sand as second level. The outlet of multigrade fiter can give less than 2 ppm turbidity.