OUTLOOK
Activated Carbon Used for Large Scale Water Treatment Efective for both water purification and pollution abatement, granular activated carbon treatment systems ofler economies that derive from their high throughput and regeneration qualities Donald G. Hager Calgon Corp., Pittsburgh, Pa. 15230 The first full scale granular activated carbon water purification plant of its kind in the world has recently gone onstream a t Nitro, W. Va. Culminating three years of related research and design studies, plant managers point out that the plant supplies more than 8 million gallons of pure water daily to approximately 13,000 rzsidents of Nitro, Dunbar, and South Charleston, W. Va. The principal features of the new water purification system are: replacement of sand with granular carbon in the plant’s filter beds to achieve simultaneous filtration of turbidity as well as adsorption of organic materials; and on-site reactivation of the carbon. The ability of charred substances such as wood char, bone char, or charcoal to remove undesirable tastes and odors from water has been known for many years. A literature search reveals that real efforts, some partially successful and others not, were made a t water purification using these materials in filter beds. The successful use of animal charcoal for filtering the East London Water Company’s water in lodging houses with 700 occupants in Columbia Square, London, in 1866 led to public demand for its use on a larger scale. Some advocated filtering the entire London supply through beds of animal charcoal. Byrne, in 1866, investigated the use of charcoal and bone char for water filtration and concluded that it was not suitable for a large supply. Efforts in
the US.during the early 1930’s toward large scale filtration-purification resulted in similar findings. These early evaluations were based upon charcoal which had relatively little adsorptive capacity compared to present day activated carbon products. Furthermore, the modern practice of reactivating spent granular carbon for many cycles of reuse has completely changed earlier conclusions regarding economics of granular carbon for water treatment. Coal-based granular carbon combines density with hardness, making it the preferred material for reactivation procedures. In addition, these same properties make coal-based carbon useful in conventional water filtration equipment. The use of granular carbon in place of filter media such as sand or coal in the purification of municipal water supplies is a recent development of Pittsburgh Activated Carbon Co., a wholly-owned subsidiary of Calgon Corp. The technique of simultaneous adsorption-filtration was pioneered in 1962 in Hopewell, Va., a t the Old Dominion Water Co., a subsidiary of American Water Works Co. Application experience a t Hopewell has since been applied in other water plants with added refinements such as on-site reactivation. The Nitro plant of the West Virginia Water Co. is an example of this type of complete system. The Kanawha River, which serves as the source of water for the Nitro plant,
is heavily polluted with various organic industrial wastes. For many years, water plant officials tried to improve the finished water while upstream industries attempted to minimize pollution loads. Nevertheless, in spite of double aeration and relatively high powdered carbon and chlorine addition at the water plant, the odor and carbon-chloroform extract (CCE) values frequently exceeded the recommended levels. Because of this odor problem, the company officials agreed to conduct a large-scale experiment with granular activated carbon which led to development and construction of the full-scale system. During 1963, a series of small-scale experiments and a plant-scale test were conducted to obtain granular carbon design data for removal of odorous and nonodorous organic material from Kanawha River water. An accompanying illustration gives a flow chart of the original purification system and the revisions made for the plant-scale experiment. The principal change for the Nitro test was discontinuance of powdered carbon addition. Also, two full-scale carbon beds were added following the sand filters. (The carbon beds were housed in rebuilt gravity sand filter structures.) Data derived from both the largeand small-scale tests led to the following conclusions : * Granular carbon removed both the odorous and nonodorous organic material to meet the recommended Volume 1, Number 4, April 1967 287
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Dewatering screw conveyor
Granular car hon D LI ri fi c a tio t i s Y s t ern
Carbon slurry tanE
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Water from main supply
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Main shutoff valve Reactivated carbon
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Spent carbon Collector pipes in bottom of cell
Equipment Summary Adsorption-Filtration
Equipment
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Number of Units 10 Type Gravity flow, concrete, filter boxes U n d e r d r a i n System - P i p e L a t e r a l s Media Support-Graded Gravel Media - $0” Bed MWT (14x40 Mesh) Granular Carbon effective size 0 60 m m uniforrnitycoefficient 1 7 Flow Rates 1.2 g p m / f t 2 Backwash Rate 10 gpm/ft2
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288 Environmental Science and Technology
Granular Carbon Reactivation Equipment
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Total Space Requirement 400 ft2 Furnace 8 hearth, 6‘0’OD x 4’6’ ID Nominal Capacity - 500 Ibs/hr Furnace Feed System 8’0’ diameter x 8 0 ’ cone bottom slurry tank rotary vane feeder/water eductor dewatering screw conveyor Carbon Return System 2’0” x 2’6” cone bottom stainless steel carbon quench tank water eductor Carbon Transfer Lines - mild steel pipe and ffexible rubber hose
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standards of the US. Public Health Service. Standard size, 8 X 30-mesh, granular-activated carbon beds were able to produce an odor-free water as the raw water threshold odor number varied from 300 to 1300. A finer carbon, 20 X 50-mesh, produced an odor-free water for twice as long as the standard size carbon. Within the tested range of physical and hydraulic conditions, a linear relationship existed between contact time and carbon performance. When the threshold odor number was reduced to less than 3, the carbon-chloroform extract value was reduced to less than 200 p.p.b., the recommended maximum value. This successful test work was reviewed and approved by the U. s. Public Health Service and was reported in a paper titled, "Carbon Bed Design Criteria Study at Nitro, W. Va." Hopewell experience
Experimental work at the Old Dominion Water Co. in Hopewell, Va., during 1963-64 revealed that granular carbon could be used in place of sand in conventional filters. The granular carbon beds, still in use, are 2 feet deep and operate at a nominal water flow rate of 2 gallons per minute per square
foot. In an earlier paper with M. E. Flentje (1965), we reported that ken, manganese, and turbidity were reduced to levels equal to, or better than, those of sand media of the same or smaller effective size. Odor removal in the carbon beds was essentially complete while comparative sand filters were, as expected, not effecting odor removal. Nitro water purification plant
The complete granular carbon purification system at Nitro resulted from previous test work at both Nitro, W. Va., and Hopewell, Va. As designed, it is the first full-scale plant of its kind in the world. The plant has a nominal capacity of 15 million gallons per day and produces close to 8 million gallons per day on a yearly average. Raw Kanawha River water is first double-aerated to remove volatile contaminants. Following aeration, the water enters a 24-hour sedimentation basin where alum is added to effect coagulation and chlorine is used for disinfection purposes. After coagulated impurities have settled out, the water enters filter beds of granularactivated carbon. The carbon beds remove any remaining suspended matter, as well as tastes and odors, from the water. Post chlorination completes the treatment prior to distribution. The Nitro filiers are conventional
rapid sand grav'ty units equipped with pipe lateral underdrain systems. HOWever, the filter sand has been replaced by 14 X 40-mesh granular carbon. Carbon is supported above the underdrain system by coarse gravel. The granular carbon removes both turbidity and dissolved organic contaminants which persist through the prior treatmeni sequence. Eight filters are onstream in parallel operation with the effluents of each blended to produce a finished water of the highest quality. The totai carbon system includes dualpurpose granular carbon filters, carbon handling equipment, and a multihearth, high temperature furnace for on-site reactivation of the granular carbon. Reactivation of the spent granular carbon begins with its removal as a water slurry from the filters. Excess water is drained from the carbon in special dewatering equipment and the carbon fed to the furnace where it is subjected to temperatures in the 1600" to 1800" F. range. The adsorbed organic impurities are burned from the carbon thus reactivating the granular carbon for another adsorption cycle. The carbon is then quenched in water and returned as a water slurry to the filters for reuse. The following table presents test results obtained at Nitro during the period September-December 1966:
Flow diagram of Nitro, W. Va., water purification plant
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Raw water
Coagulation and sedimentation
Double aeration Alum
Filtration
Clearwell
Distribution system
Chlorine
Powdered carbon slurry I
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Kanawha River
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Double aeration I
Coagulation and sedimentation
Granular carbon b&
Filtration
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Alum I
