presently accepted conventional treatments, and it offers the flexibility of a built-in capacity to meet more stringent water quality standards in the future," says W. W. Hopwood, president of Calgon. The use of polymeric coagulators in primary treatment is a comparatively recent development which Dow Chemical has been spearheading (C&EN, Oct. 10, 1966, page 4 0 ) . Dow began marketing such polymers in 1964. Both Dow and Calgon point out that these polymers can greatly improve sewage treatment in plants using only primary treatment. Primary treatment typically removes about 35% of the biological oxygen demand (BOD) of raw sewage. Addition of polymeric flocculants can increase BOD removal to 50 to 70%. In the Calgon process, alum is added to the primary tanks; the presence of coagulators with the alum improves phosphate removal, too. Removal of suspended solids can be boosted from 45% to between 70 and 90%, Calgon says. The efficient removal of solids during primary treatment, the company notes, makes possible the elimination of conventional secondary treatment. These involve large aerators and clarifiers. Solids in the effluent from the modified primary treatment are at a level low enough to be filtered by the carbon and backwashed from it without difficulty. Activated carbon has been used as an adsorber for many years, but its use as a simultaneous filter and adsorber is a recent development of Calgon's Pittsburgh Activated Carbon Co. subsidiary. In 1962, Pittsburgh Activated Carbon installed a dual-purpose car-
Calgon's Hopwood Flexibility for the future
bon system in a potable water plant at Hopewell, Va. Two years ago, tertiary waste-water treatments using carbon were installed in a pilot plant at Corona, Calif., and a full-scale plant at South Lake Tahoe, Calif. The first full-scale use of the dual-purpose system for a potable water treatment occurred last year at Nitro, W.Va. The 15 million gallon-per-day Nitro plant includes an on-site, high-temperature furnace for regenerating spent carbon. Regeneration, also a part of the Calgon process, can be done 10 times or more. Its Filtrasorb carbon granules are strong enough to withstand repeated regeneration in the furnace and repeated backwashings in the filters, the company says. Activated carbon treatments as a tertiary process can improve BOD removal from 85 to 90% (conventional secondary quality) to 95 to 99%. In the process, Calgon says, primary treatment of effluent from which 50 to 70% of the BOD has been removed can be improved to whatever level is desired—(between 85 and 99%. Variation of final effluent quality is done by changing the amount of activated carbon on the beds, by regenerating the carbon fairly frequently, or by changing the number of filter beds. This flexibility, a key quality of the activated carbon system, enables the system to automatically handle sudden increases in the amount of pollutants caused by increased sewage volume or concentration of contaminants. The plant can also be easily expanded to meet future volume or quality demands.
GE launches roofing silicone Continuing a four-year campaign to sell construction silicones to individual consumers through distributors, General Electric is now selling a third line of construction silicones—a silicone rubber roofing compound. (The other two silicone products are construction sealants and traffic coatings.) Although conservative about immediate penetration of an estimated $1 billion-plus annual market for roofing materials, GE believes a modernizing construction field will eventually accommodate large volumes of the nowspecialty elastomers. Other silicone producers share GE's interest in the construction industry as a potential mass outlet for the historically small-volume, premium-quality silicone materials. Dow Corning, like GE, has had a construction sealant on the market for several years, and its interest in the construction field is growing. Similar interest
Silicone application Breathes but watertight is expressed by Stauffer, although the company has no product for construction now. The only other U.S. silicones maker, Union Carbide, also has no offerings directly to the construction industry. GE's new roofing material is a paintlike silicone polymer which goes on plywood, concrete, fiberboard, and other surfaces in the form of a continuous, breathing (but watertight) membrane 0.022 inch thick. The product, GE says, stays flexible down to - 6 5 ° F. and will not soften at 300° F. It adapts to any configuration and should last for 30 years (GE makes a limited guarantee of five years, however). GE is counting on such properties to counter the cost advantage of traditional asphalt materials. The latter can easily halve the silicone's estimated total cost of 55 cents per square foot. Even so, GE states, the silicones are competitive for some architectural designs and industrial repair needs. And the silicones are competitive with other roofing materials, such as neoprene. The silicone roofing compounds are applied as primer and two coats. The primer is a silicone polymer used to penetrate the base material surface and provide a bonding surface for the other layers. The next application (base coat) is a polymer-rich elastomer containing about 7 5 % solids and a volatile solvent. Top coat is a tougher surface with about 8 5 % solids. In application, roofing materials are mixed with a catalyst which crosslinks (hardens) the silicone polymer chains in about 30 minutes to two hours. Structure of the compounds is the normal silicon-oxygen backbone of silicones with organic substituents, mainly methyl groups. The chains are linked by oxygen bridges between silicon atoms. The bridges form tetrahedrally around the silicon centers, GE explains. FEB.
13, 1967 C&EN 13
