DESCRIPTIONGranular activated carbon (GAC) absorption has been used successfully for the advanced (tertiary) treatment of municipal and industrial wastewater. GAC is used to adsorb the relatively small quantities of soluble organics (See Table 1) and inorganic compounds such as nitrogen, sulfides, and heavy metals remaining in the wastewater following biological or physical-chemical treatment. Adsorption occurs when molecules adhere to the internal walls of pores in carbon particles produced by thermal activation.TABLE 1 ORGANIC COMPOUNDS AMENABLE TO ABSORPTION BY GACClassExampleAromatic solventsBenzene, toluene, xylenePolynuclear aromaticsNaphthalene, biphenylChlorinated aromaticsChlorobenzene, PCBs, endrin, toxaphene, DDTPhenolicsPhenol, cresol, resorcinol, nitrophenols, chlorophenols, alkyl phenolsAromatic amines & high molecular weight aliphatic aminesAniline, toluene diamineSurfactantsAlkyl benzene sulfonatesSoluble organic dyesMethylene blue, textiles, dyesFuelsGasoline, kerosene, oilChlorinated solventsCarbon tetrachloride, percholoroethyleneAliphatic & aromatic acidsTar acids, benzoic acidsPesticides/herbicides2,4-D, atrazine, simazine, aldicarb, alachlor,carbofuranGAC systems are generally composed of carbon contactors, virgin and spent carbon storage, carbon transport systems, and carbon regeneration systems (See Figure 1). The carbon contactor consists of a lined steel column or a steel or concrete rectangular tank in which the carbon is placed to form a “filter” bed. A fixed bed downflow column contactor (See Figure 2) is often used to contact wastewater with GAC. Wastewater is applied at the top of the column, flows downward through the carbon bed, and is withdrawn at the bottom of the column. The carbon is held in place with an underdrain system at the bottom of the contactor. Provisions for backwash and surface wash of the carbon bed are required to prevent buildup of excessive headloss due to accumulation of solids and to prevent the bed surface from clogging.I审貿irCjirbco 厂 鲁:Sumrr PumpPips Dari:iMnwgTrjnipoirl Waur 、蹲lEhsiMr !U FwiwbJI1Spvni 亡时or fiufTySZjltHin -r-* XimnSfitmi Cirti? ft frtan Ri *etd Ivii.3 说 n P钓*门*11论|1 Futkpftjr的呻fW凸linKjSource: WEF MOP 8, 1998.FIGURE 1 GAC ADSORPTIONSCHEMATICExpanded bed and moving bed carbon contactors have been developed to overcome problems associated with headloss buildup experienced with fixed bed downflow contactors. In an expanded bed system, wastewater is introduced at the bottom of the contactor and flows upward, expanding the carbon bed, much as the bed expandsSource: Tchobanoglous and Burton, 1991.FIGURE 2 TYPICAL DOWNFLOW CARBON CONTACTORduring backwash of a fixed bed downflow waiting trucRke.generated or virgin carbon is then contactorI.n the moving bed system, spent carhbyodnraulicaltlryansportedfrom a second truckor is replaced continuously so that the headlosfsrodmoeas separate compartment in the first truck to the not build up. Carbon contactors may be opercaotnetdactor, then to a commercial reactivation facility under either pressure or gravity flow. The Gcehnoeircaelly,systems which containat leastone between pressure and gravity flow generally million pounds of carbon find on-site regeneration depends on the available pressure (head) witthoinbethceost effective.wastewater treatment plant and cost.Carbon regeneratioinsaccomplishedprimarilyby thermal means.Organic matter within the pores of the carbon is oxidized and thus removed from the carbon surface. The two most widely used regeneratiomnethods arerotarykilnand multiple hearth furnaces. Approximately 5 to 10 percent of the carbon is destroyed in the regeneration process or lost during transport and must be replaced with virgincarbon. The capacityof the regenerated carbon isslightllyessthanthatof virgincarbon. Repeated regeneration degrades the carbon particles until an equilibrium is eventually reached providing predictablleongterm systemperformance. See Figure 3 for a schematic of the carbon regeneration process.All carbon contactors must be equipped with removal and loadingmechanisms to allowspent carbonto be removed and virginor regenerated carbon to be addeSdp.ent, regenerated, and vir carbon is typicallytransportedhydraulicallbyy pumping as a slurry. Carbon slurriemsay be transportedwith water or compressed air,Source: WEF MOP 8, 1998 centrifugal or diaphragm pumps, or eductors.When the carbon contactor effluent quality reFaIcGhUeRsE 3 REGENERATION SCHEMATICminimum water quality standards, the spent carbonisremoved from thecontactorforregeneration.APPLICABILITYSmall systemsusuallyfindregeneratioonf theirspent carbon at an off-site commercial reactTiyvpaitciaolnly, GAC adsorption is utilized in wastewater facility to be the most convenient and econotmriecatlment as a tertiaryprocess following method. In thiscase,the spent carbon is conventi