GRANULAR ACTIVATED CARBON FILTER
GRANULAR ACTIVATED CARBON FILTER
Activated Carbon Filter
The Activated carbon Filters are designed to remove free chlorine, organic matter, odour and Colour present in the raw water and waste water.
Due to its high degree of micro porosity, activated carbons provide a huge surface area. Activated carbon filter operates through adsorption. Adsorption is directly related to the surface area of the media. This great surface area furnishes a huge adsorption area for organic as well as chlorine molecules to attach themselves.
The Activated Carbon Filters consist of Activated carbon granules supported by very fine quartz filter media. Various grades of carbon are available for specialized treatments.
Filtration is the most frequently used, robust method to remove suspended solids from water. The Filter consists of a multiple layer of sand/ Filter Media with a variety in size and specific gravity. The Filters are designed to remove turbidity and suspended particles present in the feed water with minimum pressure drop.
Fluid Systems offers a wide range of filters for varying applications with single/multiple media options to meet specific process requirements.
Water Treatment Using Carbon Filters: GAC Filter Information
A filter with granular activated carbon (GAC) is a proven option to remove certain chemicals, particularly organic chemicals, from water. GAC filters also can be used to remove chemicals that give objectionable odors or tastes to water such as hydrogen sulfide (rotten eggs odor) or chlorine.
However, other chemicals, like iron and nitrate, are not attracted to the carbon and are not removed and another type of filter, such as reverse osmosis (RO) or green sand may be needed. RO filters will also remove certain organic chemicals. This information only addresses GAC filters.
Granular activated carbon
Granular activated carbon is made from raw organic materials (such as coconut shells or coal) that are high in carbon. Heat, in the absence of oxygen, is used to increase (activate) the surface area of the carbon; this is why these filters are sometimes referred to as “charcoal” filters. The activated carbon removes certain chemicals that are dissolved in water passing through a filter containing GAC by trapping (adsorbing) the chemical in the GAC.
It is very important that the type and concentration of contaminants, and average water use, be known in order to determine the correct size and components of the system. All treatment systems require proper installation and periodic maintenance. Eventually, the ability of the GAC to bind and remove chemicals is used up and the GAC needs to be changed. How often the GAC should be changed needs to be based on contaminant levels and water use. While some filters may last for several years if contaminant levels and/or water use are low, higher levels or use may require more frequent change-outs.
Use of GAC in Rapid Filtration
Granular activated carbon is being used in filter-adsorbers that serve both for filtration of particles and for adsorption of organic compounds, as well as in biological filters and post-filter-adsorbers. The principal application of filter-adsorbers to date is for removal of taste and odor, where fullscale experience shows successful removal for periods of from 1 to 5 years before the GAC must be regenerated . High concentrations of competing organic compounds, however, can reduce this duration. Most existing GAC filter-adsorbers are retrofitted rapid filters where GAC has replaced part or all of the sand in rapid sand filters, or replaced anthracite in dual- or triple-media filters.
Several filter adsorbers, however, have been initially constructed with GAC. Granular activated carbon used in retrofitted filters is typically 15 to 30 in (0.38 to 0.76 m) of 12 × 40 mesh GAC (ES 0.55 to 0.65 mm) or 8 × 30 mesh GAC (ES 0.80 to 1.0 mm) placed over several inches of sand (ES 0.35 to 0.60 mm) in a dual-media configuration. These GAC materials have a higher UC (≤2.4) than traditionally used filter materials (≤1.6).The high UC can contribute to more fine grains in the upper layers and shorter filter cycles, but it also results in less size intermixing during backwashing, which is beneficial to adsorption.
The use of retrofitted filters as filter-adsorbers is primarily for removal of trihalomethane (THM) precursors.The removal of less strongly adsorbed compounds such as trihalomethanes and volatile organic compounds is limited. The short emptybed contact times typical of such filter-adsorbers (about 9 min) would result in frequent regeneration or replacement of the GAC if used for THM adsorption.
Some filter-adsorbers designed for GAC initially use up to 48 in (1.2 m) of coarser GAC (ES-1.3 mm) with lower uniformity coefficients (1.4). This provision should result in longer filter cycles, and longer periods between GAC regenerations. Granular activated carbon is being used successfully on both monomedium and dual media filter-adsorbers. It is as effective in its filtration function as conventional (sand or dual) filter media, provided an appropriately sized medium has been selected. Particulates removed in the filteradsorber, however, do impede the adsorption function somewhat, compared to postfilter adsorbers with the same contact time.
The layer of fine sand in the dual-media filter-adsorber may be essential where very low turbidity filtered water is the goal.The use of sand and GAC in dual-media beds, however, causes problems during regeneration because of the difficulty of removing sand-free GAC from the filter-adsorber, and of separating the sand afterremoval. Sand causes difficulties in regeneration furnaces.
Bacteria proliferate in GAC beds, making periodic backwashing essential. Backwashing should be minimized, however, because of its possible effect on adsorption efficiency. Application of chlorine to the influent of the adsorbers does not prevent bacterial growth and can cause other detrimental effects. Post adsorption disinfection is essential.
Granular activated carbon is softer than anthracite, but the attrition of GAC media in full-scale plants has not been excessive. Losses of from 1 to 6 percent per year have been reported, which are not higher than typical losses of anthracite. A modest reduction in grain size of GAC has been measured at some plants. Granular activated carbon has a lower density than anthracite, posing some concerns about losses during backwashing (a topic that will be discussed later).
Granular Activated Carbon Types
Activated carbon is commonly used to adsorb natural organic compounds, taste and odor compounds, and synthetic organic chemicals in drinking water treatment. Adsorption is both the physical and chemical process of accumulating a substance at the interface between liquid and solids phases. Activated carbon is an effective adsorbent because it is a highly porous material and provides a large surface area to which contaminants may adsorb. The two main types of activated carbon used in water treatment applications are granular activated carbon (GAC) and powdered activated carbon (PAC).
GAC is made from organic materials with high carbon contents such as wood, lignite and coal. The primary characteristic that differentiates GAC to PAC is its particle size. GAC typically has a diameter ranging between 1.2 to 1.6 mm and an apparent density ranging between 25 and 31 lb/ft3), depending on the material used and manufacturing process. The bed density is about 10 percent less than the apparent density and is used to determine the amount of GAC required to fill a given size filter. The uniformity coefficient of GAC is quite large, typically about 1.9, to promote stratification after backwashing and minimize desorption and premature breakthrough that can result from mixing activated carbon particles with adsorbed compounds with activated carbon particles with smaller amounts of adsorbed compounds. Iodine and molasses numbers are typically used to characterize GAC. These numbers describe the quantity of small and large pore volumes in a sample of GAC. A minimum iodine number of 500 is specified for activated carbon by AWWA standards.
The two most common options for locating a GAC treatment unit in water treatment plants are:
(1) post-filtration adsorption, where the GAC unit is located after the conventional filtration process (post-filter contactors or adsorbers); and
Figure 1: Post-filtration adsorption.
(2) filtration-adsorption, in which some or all of the filter media in a granular media filter is replaced with GAC. Examples of these configurations are shown in Figures 1 and 2, respectively.
Figure 2: Filtration/adsorption.
In post-filtration applications, the GAC contactor receives the highest quality water and, thus, has as its only objective the removal of dissolved organic compounds. Backwashing of these adsorbers is usually unnecessary, unless excessive biological growth occurs. This option provides the most flexibility for handling GAC and for designing specific adsorption conditions by providing longer contact times than filter-adsorbers.
In addition to dissolved organics removal, the filter-adsorber configuration uses the GAC for turbidity and solids removal, and biological stabilization. Existing rapid sand filters can frequently be retrofitted for filtration-adsorption by replacing all or a portion of the granular media with GAC. Retrofitting existing high rate granular media filters can significantly reduce capital costs since no additional filter boxes, underdrains and backwashing systems may be required. However, filter-adsorbers have shorter filter run times and must be backwashed more frequently than post-filter adsorbers (filter-adsorber units are backwashed about as frequently as conventional high rate granular filters). In addition, filter-adsorbers may incur greater carbon losses because of increased backwashing and may cost more to operate because carbon usage is less effective.
Primary factors in determining the required GAC contactor volume are the (1) breakthrough, (2) empty bed contact time (EBCT), and (3) design flow rate. The breakthrough time is the time when the concentration of a contaminant in the effluent of the GAC unit exceeds the treatment requirement. As a rule of thumb, if the GAC effluent concentration is greater than the performance standard for over three consecutive days, the GAC is exhausted and must be replaced/regenerated. The EBCT is calculated as the empty bed volume divided by the flowrate through the carbon. Longer EBCTs can be achieved by increasing the bed volume or reducing the flow rate through the filter. The EBCT and the design flow rate define the amount of carbon to be contained in the adsorption units. A longer EBCT can delay breakthrough and reduce the GAC replacement/regeneration frequency. The carbon depth and adsorber volume can be determined once the optimum EBCT is established. Typical EBCTs for water treatment applications range between 5 to 25 minutes.
The surface loading rate for GAC filters is the flow rate through a given area of GAC filter bed and is expressed in units of gpm/ft2. Surface loading rates for GAC filters typically range between 2 to 10 gpm/ft2. High surface loading rates can be used when highly adsorbable compounds (such as SOCs) are targeted for removal. The surface loading rate is not important when mass transfer is controlled by the rate of adsorption as is the case for less-adsorbable compounds.
The carbon usage rate (CUR) determines the rate at which carbon will be exhausted and how often carbon must be replaced/regenerated. Carbon treatment effectiveness improves with increasing contact times. Deeper beds will increase the percentage of carbon that is exhausted at breakthrough. The optimum bed depth and volume are typically selected after carefully evaluating capital and operating costs associated with reactivation frequency and contactor construction costs.
GAC contactors can be configured as either (1) downflow fixed beds, (2) upflow fixed or expanded beds, or (3) pulsed beds; with single or multiple adsorbers operated in series or in parallel. In downflow fixed beds in series, each unit is connected in series with the first adsorber receiving the highest contaminant loading and the last unit receiving the lightest contaminant load. Carbon is removed for reactivation from the first unit, with the next adsorber becoming the lead unit. For downflow fixed beds in parallel, each unit receives the same flow and contaminant load. To maximize carbon usage, multiple contactors are frequently operated in parallel-staggered mode in which each contactor is at a different stage of carbon exhaustion. Since effluent from each contactor is blended, individual contactors can be operated beyond breakthrough such that the blended flow still meets the treatment goal. Upflow expanded beds permit removal of suspended solids by periodic bed expansion and allow using smaller carbon particles without significantly increasing head loss. In pulsed bed adsorbers, removal of spent carbon occurs from the bottom of the bed while fresh carbon is added at the top without system shutdown. A pulsed bed cannot be completely exhausted, which prevents contaminant breakthrough in the effluent.
Depending on the economics, facilities may have on-site or off-site regeneration systems or may waste spent carbon and replace it with new. Spent GAC must be disposed of recognizing that contaminants can be desorbed, which can potentially result in leaching of contaminants from the spent GAC when exposed to percolating water, contaminating soils or groundwater. Due to contamination concerns, spent GAC regeneration is typically favored over disposal. The three most common GAC regeneration methods are steam, thermal and chemical; of which thermal regeneration is the most common method used. Available thermal regeneration technologies used to remove adsorbed organics from activated carbon include: (1) electric infrared ovens, (2) fluidized bed furnaces, (3) multiple hearth furnaces, and (4) rotary kilns.