Carbon filters backwashing

Adsorption efficiency can be optimized by using finer particle size products which will improve the diffusion rate to the surface of the activated carbon. However, there is a tradeoff in using finer particles with pressure drop and, hence energy use. Note that during start-up of an activated carbon filter bed, a bed expansion of 25 to 35 % is recommended in order to remove soluble matter and to stratify particles in order to ensure that the MTZ is maintained when future backwashing is performed. 

Activated carbon filters are employed primarily as RW contaminant removal systems for chlorine (by chemisorption) and various organics such as trihalomethanes (THMs), petroleum products, and pesticides (by adsorption). In addition, they act as physical filters and therefore incorporate sufficient freeboard in their designs to permit periodic backwashing. 

A definite schedule should be followed for backwashing and disinfecting the carbon filter. Instructions for this are furnished by the manufacturer of the equipment. 

Porous carbon filters for mercury usually operate at pressures up to 8 bar. The housings are nickel or nickel-coated carbon steel. The pores in the tubes have diameters less than 50 xm. Filtration face velocities range from about 0.5 to 2.0mhr . The content of suspended mercury is reduced from 10-30 ppm to less than 0.5 ppm. Results are better with lower velocity, lower caustic temperature, and the recycle of some of the filtrate. Many plants use multiple filtrations, perhaps with higher unit rates in the second step, and achieve lower mercury content in the effluent. Leaf filters packed with specially treated activated carbon are an alternative. These usually operate at lower temperature (60-65°C) and sometimes are used as two stages in series. The mercury concentration is reduced to 1-5 ppm in the first stage and to less than lOppb in the second. The spent carbon is backwashed from the filters and allowed to settle in a hold tank. The supernatant weak caustic can be used in brine treatment or pH control, and the carbon itself is stripped in a mercury furnace before disposal. 

ABSTRACT Ultrafiltration membrane was used to treat Spent Filter Backwash Water (SFBW) of carbon filter and sand filter in a certain Water Treatment Plant (WTP). The results showed that the removal rate of turbidity by ultrafiltration was above 98%, the total bacteria count was less than 3 CFU/mL and the CODMn was about 1 mg/L. Ultrafiltration membrane ran stably when inflow flux was 60 L/(m h) and the cumulative water yield was about 1135 m after continuously running of 26 d, the reclamation rate was higher than 95% and the water producing cost was about 0.15 CNY/m. ... 

Sand filters vary in sophistication. A simple filter will remove most particles down to 5 pm. Multi-media filters which use sand and anthracite, and possibly a third medium, in discrete layers, can yield very efficient filtration down to 2 pm. Granular activated carbon can be used instead of sand to add some measure of organic removal to the filtration process. The quality produced by any filter depends largely on the efficiency of the backwash. Sand filters in some form provide a satisfactory solution for the majority of water-filtration problems. 

In a typical fixed-bed carbon column, the column is similar to a pressure filter and has an inlet distributor, an underdrain system, and a surface wash. During the adsorption cycle, the influent flow enters through the inlet distributor at the top of the column, and the groundwater flows downward through the bed and exits through the underdrain system. The unit hydraulic flow rate is usually 2 to 5 gpm/ft2. When the head loss becomes excessive due to the accumulated suspended solids, the column is taken off-line and backwashed. 

The filters are backwashed occasionally to remove any suspended solids that may have accumulated on the surface of the bed. Backwashing does not remove material adsorbed in the pores of the carbon. Although a few installations regenerate their carbon using thermal, steam, solvent extraction, or other techniques, most applications see replacement of carbon when exhausted. Typical "life" of carbon used for TOC removal is 6 -12 months. For chlorine removal, the typical "life" is 12 - 18 months. 

GAC may be used in fixed or moving beds and in downflow or upflow mode. Fixed beds are operated in downflow mode and as such, provide some amount of solids filtration however, influent solids concentration must be kept low (less than 5 mg/L suspended soHds) to prevent rapid plugging of the bed. Filtered solids are periodically removed by backwashing. Upflow beds are more tolerant of soHds because they are fluidized and expanded by the wastewater entering at the bottom. In moving beds, the flow is countercurrent and makeup, fresh carbon is added continuously at the top of the unit while an equal amount of spent carbon is removed from the bottom. 

In the mid-1990s, two spiral-wound nanofiltration plants were installed for the treatment of effluent from a paper mill. In both cases, the nanofiltration systems were installed to remove color, organic carbon, and dissolved solids from effluents for reuse or for further processing. Both plants had a very efficient pretreatment before the NF spiral-wound modules to prevent plugging of the filtration elements. For instance, the pretreatment included settling at several stages with chemicals, a sand filter, a backwashable screen filter, and a bag filter (5 pm) [106]. Neither of the plant is in operation today (P. Eriksson, personal communication, 2005). 

Zeist, Netherlands [83 1 Groundwater Aeration Filtration over CaCOjMgO GAC fixation Removal of trichloroethane Particle size 1 mm Depth of bed 1.5 m Filter diameter 3.3 m Vol. carbon per filter 12 m 0)ntact time 12 min. Empty bed vol until regeneration 25,000 No. filters 3 by 2 Backwash 1st filter only... 

At the present time, Cummins has decided to install a pretreatment system using dissolved air flotation as the primary treatment followed by an anthracite/sand backwashable filter and an activated carbon polishing filter. The effluent from the filters then goes to the RO unit. The RO will remove the last traces of the most soluble oils and organics, it is Cummins intention to have the effluent from the filters at a turbidity of less than kS JTU s. This should keep the RO units from fouling and will allow the RO to be used with minimal cleaning. 

Multimedia filters, which consist of a top layer of coarse and low density anthracite, layers of silicas, and then dense finest media vitreous silicate, remove about 98% of particulates >20 pm. These filters are regularly backwashed to avoid build-up of particulates. Finer filters (5-10 pm) are used to remove suspended matter and colloidal materials. To prevent scaling due to water hardness, sodium ions generated from brine are exchanged with calcium and magnesium ions in the water. Activated carbon or metabisulfite is used to remove chlorine. 

The desulfurized slurry from both processes is pumped to a filter press. Soluble sodium sulfate and carbonate/hydroxide (depending on the reagent used) are removed through a series of backwashing and rinsing cycles. The solid product, once dried, is ready to be charged to the furnace whilst the soluble sodium sulfate is either disposed of via the plant s effluent system or is reclaimed as a solid product, similar to that in the caustic soda/acid neutralization process outlined above. 

Carbon can be placed on the filters by adding a carbon slurry to the filters after backwashing. Particles of carbon are thus distributed throughout the water on top of the filter and they then deposit on the sand surface of the filter. As the filtration continues, water passes through a layer of carbon, thus removing odorous materials. This is the point usually selected in small plants where feeding facilities do not exist. 

This method was developed by Stefan Huber (Karlsruhe, Germany) and consists of three size exclusion chromatography columns which divide the organic carbon into several fractions as a function of size, but also hydrophobic and ionogenic characteristics. A sample of up to 3 mL is injected into the instrument and filtered in-line with a 0.45 )um filter. The deposit on the filter is backwashed after 5 minutes and directly analysed with the TOC analyser to determine the particulate organic carbon content (POC). 

Frequent backwash of carbon media filters is more effective than hot water or steam sanitisation since the microbial load reduction by sanitisation is temporary due to the rapid recolonisation of the sanitised biofihn in the carbon bed. 

The experiment has been carried out in a WTP which is located in Shenzhen City of the subtropical zone, the current water yield is about 250,000 t/d. The backwash cycle of sand filter and carbon... 

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