Types of Contactors  
            For activated carbon to adequately complete the job of removing impurities from wastewater or municipal water sources, the activated carbon needs to be intimately in contact with the water to be treated.  Contactors or adsorbers are used to create the intimate environment needed.  The design and the configuration of the activated carbon contactor system should be chosen based on the characteristics of the influent stream and the treatment objective for the influent.  However, the tradeoffs between the size and the cost of the system should not be overlooked.   
            The two fundamental varieties of activated carbon contactors can be categorized into fixed bed and moving bed contactors.  As the name implies, the carbon bed in the fixed bed contactor remains at a standstill or fixed during operation.  Fixed bed contactors can be operated in a downflow or upflow nature with respect to the flow of the wastewater through the contactor.  Downflow operation has the ability of performing dual functions: adsorption and filtration.  The secondary function of filtration is necessary when the influent has high concentrations of suspended solids.  Periodically, the contactor should be backwashed to remove the suspended solids from the carbon bed.  Given that, build up of suspended solids will hinder the adsorption capacity of the carbon and increase the bed pressure drop or head loss.  When a carbon bed is used in conjunction with filtration media such as sand and gravel, the frequency of backwashing can be reduced.  Below is a diagram of a typical downflow fixed bed carbon contactor with secondary filtration media commonly used in the treatment of municipal water sources.  
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  Figure 6.  Typical Downflow Fixed Bed Contactor Used in Treatment of Municipal Water Sources  
            Fixed bed contactors and moving bed contactors can be operated by means of gravity flow or pressure flow.  Figure 6 is an example of a pressure flow contactor.  The advantages of such operation are a higher hydraulic loading.  A higher hydraulic loading can significantly reduce the contactor cross-sectional area required to obtain the same performance.  Pressure flow contactors can also handle influent streams with a higher concentration of suspended solids with less frequent backwashing.  This is direct result of increased head to surmount the increasing head loss due to the higher concentration of suspended solids.  The advantage to gravity flow is the lower energy cost associated with the process.  Gravity flow is used only in downflow contactors, and below is a diagram of a typical setup.   
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  Figure 7.  Typical Downflow Gravity Contactor  
            In the other set of activated carbon contactors is the category of moving bed contactors.  This class of contactors is always operated in a pressurized upflow nature.  The class can be broken up into: expanded beds, and pulsed bed contactors.  In expanded bed contactors, the flow rate of the influent is maintained at a velocity in which the flow rate is high enough to cause the carbon bed to expand upwards slightly.  This type of configuration can be used for influents that have both high and low concentrations of suspended solids.  With an exceedingly high concentration, a downflow contactor would result in excessive head loss due to the accumulation of suspended solids on the carbon bed when used as a secondary filtration media.  Upflow moving beds are limiting to those applications where downstream contamination by suspended solids or carbon residue would pose a problem.   
            The second type of moving bed contactor is called a pulsed bed.  In this type of contactor, the carbon bed is occasionally pulsed to remove exhausted carbon out the bottom of the contactor and the same amount of exhausted carbon that was discharge is replaced with fresh or reactivated carbon through the top of the contactor.  The flow between the carbon and the wastewater is countercurrent.  The advantage to this contactor configuration is optimal carbon utilization when properly executed, as only completely exhausted carbon is withdrawn from the contactor.  As result of the continuous replacement of exhausted carbon the system when operated correctly will never be completely exhausted.  This inhibits contamination of the effluent stream.  Another result of the continuous replacement of the exhausted carbon is that the set concentration of impurities in the effluent is constant over an extended time period.  In fixed bed contactors, the set concentration of the impurities allowed in the effluent increases with respect to time in operation.  Below is a diagram of a typical pulsed bed contactor.   
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  Figure 8.  Typical Pressurized Pulsed Bed Carbon Contactor  
            Single stage contactors are generally used at small-scale plants where the contaminants removed result in a low carbon exhaustion rate.  At larger scale water treatment facilities, multiple contactors are often used where rapid carbon exhaustion rate occur.  When using multiple contactors lower carbon usage rate can be achieved by arranging the contactors in either a series or parallel setup.  As shown in Figure 8, the influent stream enters one contactor at a time and flows downward through the carbon bed.  In this type of setup, the lead unit will be the first contactor to have its carbon bed go to exhaustion.  When this happens, the lead unit is taken offline and the next unit in line becomes the lead unit.  If the effluent requirements are very stringent and all units are needed to achieve this requirement, a standby contactor can be used while unit are taken offline for regeneration.  When there is a build up of suspended solids, backwashing will only need to be done on the lead unit of the system.  Lower carbon usage rates can be achieved in a series setup, because units are taken offline for regeneration only after the entire carbon bed is exhausted.  In single stage systems, the carbon will have to be replaced as so as the mass transfer zone reaches the bottom of the contactor.   
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  Figure 9.  Series Contactor System  
            Opposed to the series setup, in a parallel arrangement (Figure 10), the influent is flown through each individual contactor at the same time.  This means that each of the units receives the same quantity and quality of the influent.  The startup, however, is staggered so that only one unit will approach exhaustion at a time.  Like the series setup, a standby unit can be used when another unit is taken offline for regeneration.  Unlike the series setup, the parallel arrangement cannot obtain as high of carbon utilization because in the parallel arrangement no unit is run to complete exhaustion.  It does have a lower carbon usage rate than a single stage contactor system.  The applicability of this setup is for system where there is a large total flow rate is required, because the distribution of the flow amongst the individual units will minimize the pressure drop.   
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  Figure 10.  Parallel Contactor System  
            Other configurations can be used to produce lower carbon usage rates.  These configurations are combinations of series and parallel setups.  These types of setups are arranged for specific applications using the desired attributes from each of the different contactor setups.