flow return line to the storage tank, as shown by Figure 10-4. Centrifugal pumps should be direct-connected but not close-coupled because of possible leakage into the motor, and should be constructed of type 316 stainless steel, FRP, and plastics. Metering pumps, currently available, allow a wide range of capacity to compare with the rotodip and rotameter systems. Hydraulic diaphragm type pumps are preferable to other type pumps and should be protected with an internal or external relief valve. A back pressure valve is usually required in the pump discharge to provide efficient check valve action. Materials of construction for feeding equipment should be as recommended by the manufacturer for the service, but depending on the type of system, will generally include type 316 stainless steel, FRP, plastics, and rubber.

Piping and Accessories. Piping systems for alum should be FRP, plastics (subject to temperature limits), type 316 stainless steel, or lead. Piping and valves used for alum solutions are also discussed in the preceding section on dry alum.

Pacing and Control. The feeding systems described above are volumetric, and the feeders generally available can be adapted to receive standard instrument pacing signals. The signals can be used to vary motor speed, variable speed transmission setting, stroke speed and stroke length where applicable. A totalizer is usually furnished with a rotodip type feeder, and remote instruments are available. Instrumentation is rarely used with rotameters and metering pumps.

10.1.3 Dry Sodium Aluminate

Properties and Availability. Dry sodium aluminate, Na2Al204, is available from two major manufacturers in the United States. They are:

Nalco Chemical Company

Chicago, Illinois (Plant located at Clearing, Ill.)

Reynolds Chemicals

Richmond, Virginia (Plant located at Bauxite, Ark.)

Sodium aluminate is shipped in 50 lb bags and has a bulk density ranging from 40 to

50 lb/ft3. The Al2O3 content ranges from 41 to 46%. Sodium aluminate is

noncorrosive and the pH of a 1% solution is about 11.9. Manufacturers should be consulted for more precise specifications of their product.

The current price of dry sodium aluminate in 40,000 lb shipments ranges from $0.11 to $0.14/lb, F.O.B. the point of manufacture. Prices increase substantially for smaller shipments.

General Design Considerations. Requirements for dry sodium aluminate feed systems are generally similar to those for dry aluminum sulfate. Dry sodium aluminate is not available in bulk quantities. Therefore, the small, day type hoppers with manual filling arrangements as shown by Figure 10-1 are used. Precautionary measures for handling sodium aluminate are similar to those for strong alkalies, such as caustic soda.

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Contact with skin, eyes, and clothing should be avoided. Aluminate dust or solution spray should not be breathed.

Storage. Dry sodium aluminate is stored as received, in bags, and at optimum conditions of 60 to 90° F, the recommended storage limit is 6 months. Hopper material of mild steel is completely adequate. This chemical may or may not be free flowing, depending on the manufacturer and grade used. Therefore, hopper agitation may be required. Sodium aluminate deteriorates on exposure to the atmosphere and care should be taken to avoid tearing of bags.

Feeding Equipment. Materials of construction for dissolving chambers may be mild steel or stainless steel and selection may be influenced by conformity with adjacent equipment. Equipment similar to that shown by Figure 10-1 is applicable. Standard practice for the free-flowing grade of sodium aluminate calls for dissolvers sized for 0.5 lb/gal. or 6% solution strength with a dissolver detention time of 5 minutes at the maximum feed rate.

After dissolving sodium aluminate in the preparation of batch solutions, agitation should be minimized or eliminated to prevent deterioration of the solution. Air agitation is not recommended, and solution tanks should be covered to prevent carbonation of the solution.

Piping and Accessories. Materials for piping and transporting sodium aluminate solution may be mild steel, iron, type 304 stainless steel, concrete, or plastics. The use of copper, copper alloys, and rubber should be avoided.

Pacing and Control. Pacing and control fundamentals are similar to those described for dry alum.

The amount of dilution does not appear to be a consideration in the use of sodium aluminate. Therefore, the use of float valves to satisfy centrifugal pump suction, and the use of eductors are permissible.

10.1.4 Liquid Sodium Aluminate

Properties and Availability. Liquid sodium aluminate is available from the following major manufacturers in the United States:

Nalco Chemical Company
Chicago, Illinois

Vinings Chemical Company

Vinings, Georgia

Conservation Chemical Company

Kansas City, Missouri

There is considerable variety in the composition of sodium aluminate from the manufacturers listed. The Al2O3 content varies from 4.9 to 26.7%. The lower solution

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strengths are usually more expensive because of the cost of freighting the solution water. Because of the variety of solution strengths available, the manufacturers should be contacted for more specific information on density, viscosity, and cost.

Liquid sodium aluminate is available in 30 gal. drums (380 lb), tank truck, and tank car quantities. The current price of sodium aluminate in 40,000 lb quantities is about $0.08/lb (Al2O3 content, 26.4%) F.O.B. the point of manufacture.

General Design Considerations. Material selection and dilution restrictions are not as limited as for liquid alum, because of the noncorrosive nature of sodium aluminate. Sodium aluminate is a strong alkali and the same precautions should be exercised in handling it as in handling caustic soda.

Storage. Liquid sodium aluminate is usually stored at the shipping concentration, either in the shipping drums or in mild steel tanks. Storage tanks may be located indoors or outdoors, however, outdoor tanks should be provided with facilities for indirect heating. The maximum recommended length of storage is two months. Bulk shipments can be unloaded by gravity, pumping, or air pressure. However, if air is used, it should first be passed through lime-caustic soda breathers to remove the CO2. Steam injection facilities are required at the unloading site.

Feeding Equipment. Feeding equipment and systems as described for liquid alum generally apply to sodium aluminate except with changes of requirements regarding dilution and materials of construction as described above.

Liquid sodium aluminate may be fed at shipping strength or diluted to a stable 5 to 10% solution. Stable solutions are prepared by direct addition of low hardness water and mild agitation. Air agitation is not recommended.

Piping and Accessories. Material requirements are the same as previously indicated for solutions of dry aluminate.

Pacing and Control. System pacing and control requirements are the same as described for liquid alum.

10.1.5 Estimated Initial Costs of Adding Liquid Alum Feed Facilities

to Existing Wastewater Treatment Plants

Cost estimates of chemical storage and feeding equipment for 1, 10, and 100 mgd plants have been prepared based on the use of liquid alum (8.3% Al2O3) for phosphorus precipitation. A mole ratio of Al:P of 2.0:1 was assumed to treat an influent phosphorus concentration of 10 mg/1 as P. This corresponds to an alum dose of 191 mg/1 (17.4 mg/1 as A13+).

Chemical feed equipment was sized for a peak feed rate of twice that calculated from the mole ratio. Storage was provided for at least 15 days at the average feed rate. This storage time is arbitrary and will vary at each installation depending on the distance to and the reliability of the source of chemical supply. Piping and buildings to house the

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feeding equipment are not included in the estimates. At many locations the chemical feeding equipment can be installed in existing buildings. In all estimates, the costs include an allowance for the contractor's installation, overhead, and profit, and an allowance of 20% of the construction cost for engineering and contingencies. The estimated costs for each size plant are:

For the 1.0 mgd plant, feeding equipment for liquid alum includes two 25 gph hydraulic diaphragm pumps (one operating and one standby) with the necessary accessories and equipment to pace the feed rate with plant flow. Two 3,000 gal. FRP tanks with accessories are provided for storage of liquid alum. The total capacity of 6,000 gal. allows some flexibility in ordering and receiving 4,000 gal. tank truck shipments.

For the 10 mgd plant, liquid alum is fed by three 125 gph rotodip-type feeders (two operating and one standby) with the necessary accessories and control panel for proportioning chemical feed to flow. Alum is stored in four 11,500 gal. FRP tanks. For the 100 mgd plant, liquid alum is fed with seven 415 gph rotodip-type feeders (six operating and one standby) with the necessary control equipment. Storage costs are based on ten 50,000 gal. underground concrete tanks with a rubber lining. The underground storage necessitates transfer pumps and day tanks for the rotodip feeders but eliminates the need for heating the tanks. Three 500 gal. FRP day tanks are included in the estimate; one for each pair of feeders. Four alum transfer pumps (three operating and one standby), with a capacity of 50 gpm at 50 ft of head are provided.

The cost of feeding and storage facilities for the 100 mgd plant is greater than 10 times the cost of facilities for the 10 mgd plant. This is because underground storage facilities were the basis of design for the 100 mgd plant in contrast to FRP tanks located in an existing building for the 10 mgd plant. The cost of a building for the 1.0 and 10.0 mgd plants would raise the cost substantially. FRP tanks could be used for the larger plant, however, structural design may necessitate special construction requirements for underground FRP tanks. FRP tanks located above ground and on concrete foundations would require special insulation and heating.

10.2 Iron Compounds

10.2.1 Liquid Ferric Chloride

Properties and Availability. Liquid ferric chloride is a corrosive, dark brown

oily-appearing solution having a density as shipped and stored of 11.2 to 12.4 lb/gal. (35 to 45% FeCl3) (2). The ferric chloride content of these solutions, as FeCl3, is 3.95 to 5.58 lb/gal. Shipping concentrations vary from summer to winter due to the relatively high crystallization temperature of the more concentrated solutions as shown by Figure 10-5. The pH of a 1% solution is 2.0.

The molecular weight of ferric chloride is 162.22. Viscosities of ferric chloride solutions at various temperatures are presented in Figure 10-6.

Liquid ferric chloride is shipped in 3,000 to 4,000 gal. bulk truckload lots, in 4,000 to 10,000 gal. bulk carload lots, and in 5 and 13 gal. carboys. Liquid ferric chloride is produced at the following locations:

Dow Chemical Co.

Midland, Michigan

Pennwalt Corp.

Philadelphia, Pa. (Plant at Wyandotte, Mich.)

The current price of liquid ferric chloride in bulk quantities is about $0.04 to $0.045/lb (as FeCl3), F.O.B. the point of manufacture.

Tank trucks and cars are normally unloaded pneumatically, and operating procedures must be closely followed to avoid spills and accidents. The safety vent cap and assembly (painted red) should be removed prior to opening the unloading connection to depressurize the tank car or truck, prior to unloading.

General Design Considerations. Ferric chloride solutions are corrosive to many common materials and cause stains which are difficult to remove. Areas which are subject to staining should be protected with resistant paint or rubber mats.

Normal precautions should be employed when cleaning ferric chloride handling equipment. Workmen should wear rubber gloves, rubber apron, and goggles or a face shield. If ferric chloride comes in contact with the eyes or skin, flush with copious quantities of running water and call a physician. If ferric chloride is ingested, induce vomiting and call a physician.

Storage. Ferric chloride solution can be stored as shipped. Storage tanks should have a free vent or vacuum relief valve. Tanks may be constructed of FRP, rubber lined steel, or plastic lined steel. Resin impregnated carbon or graphite are also suitable materials for storage containers.

It may be necessary in most instances to house liquid ferric chloride tanks in heated areas or provide tank heaters or insulation to prevent crystallization. Ferric chloride can be stored for long periods of time without deterioration. The total storage capacity should be 11⁄2 times the largest anticipated shipment, and should provide at least a 10 day to 2 week supply of the chemical at the design average dosage.

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