Dry alum is not corrosive unless it absorbs moisture from the air, such as during prolonged exposure to humid atmospheres. Therefore, precautions should be taken to ensure that the storage space is free of moisture.
Alum is shipped in 100 lb bags, drums, or in bulk (minimum of 40,000 lb) by truck or rail. Bag shipments may be ordered on wood pallets if desired. Ground and rice alum are the grades most commonly used by utilities because of their superior flow characteristics. These grades have less tendency to lump or arch in storage and therefore provide more consistent feeding qualities. Hopper agitation is seldom required with these grades, and in fact may be detrimental to feeding because of the possibility of packing the bin.
Alum dust is present in the ground grade and will cause minor irritation of the eyes and nose on breathing. A respirator may be worn for protection against alum dust. Gloves may be work to protect the hands. Because of minor irritation in handling and the possibility of alum dust causing rusting of adjacent machinery, dust removal equipment is desirable. Alum dust should be thoroughly flushed from the eyes immediately and washed from the skin with water.
Bulk alum can be stored in mild steel or concrete bins with dust collector vents located in, above, or adjacent to the equipment room. Recommended storage capacity is about 30 days. Dry alum in bulk form can be transferred or metered by means of screw conveyors, pneumatic conveyors, or bucket elevators made of mild steel. Pneumatic conveyor elbows should have a reinforced backing as the alum can contain abrasive impurities.
Bags and drums of alum should be stored in a dry location to avoid caking. Bag or drum-loaded hoppers should have a nominal Storage capacity for eight hours at the nominal maximum feed rate so that personnel are not required to charge the hopper more than once per shift. Converging hopper sections should have a minimum slope of 60 degrees to prevent arching.
Bulk storage hoppers should terminate at a bin gate so that the feeding equipment may be isolated for servicing. The bin gate should be followed by a flexible connection, and a transition hopper chute or hopper which acts as a conditioning chamber over the feeder.
A typical feed system includes all of the components required for the proper preparation of the chemical solution. Capacities and assemblies should be selected to fulfill individual system requirements. Three basic types of chemical feed equipment are used: volumetric, belt gravimetric, and loss-in-weight gravimetric. Volumetric feeders are usually used where initial low cost and usually lower capacities are the basis of selection. Volumetric feeder mechanisms are usually exposed to the corrosive dissolving chamber vapors which can cause corrosion of discharge areas. Manufacturers usually control this problem by use of an electric heater to keep the feeder housing dry or by using plastic components in the exposed areas.
Volumetric dry feeders in general use are of the screw type. Screw-feed mechanisms allow even withdrawal across the bottom of the feeder hopper to prevent hopper dead zones. Some screw designs are based on a variable-pitch configuration with the pitch expanding unevenly to the discharge point. Other screw designs are based on constant-pitch type expanding evenly to the discharge point. This type of screw design is known as the constant-pitch-reciprocating type. This type has each half of the screw turned in opposite directions so that the turning and reciprocating motion alternately fills one half of the screw while the other half of the screw is discharging. The variable-pitch screw has one point of discharge, while the constant-pitch-reciprocating screw has two points of discharge, one at each end of the screw. The accuracy of volumetric feeders is influenced by the character of the material being fed and ranges between ± 1 percent for free-flowing materials and + 7 percent for cohesive materials. This accuracy is volumetric and should not be related to accuracy by weight (gravimetric).
Where the greatest accuracy and the most economical use of chemicals is desired, the loss-in-weight-type feeder should be selected. This feeder is limited to the low and intermediate feed rates up to a maximum rate of approximately 4,000 lb/hr. The loss-in-weight-type feeder consists of a material hopper and feeding mechanism mounted on enclosed scales. The feed-rate controller retracts the scale poise weight to deliver the dry chemical at the desired rate. The feeding mechanism must feed at this rate to maintain the balance of the scale. Any unbalance of the scale beam causes a corrective change in the output of the feeding mechanism. Continuous comparison of actual hopper weight with set hopper weight prevents cumulative errors.
Belt-type gravimetric feeders span the capacity ranges of volumetric and loss-in-weight feeders and can usually be sized for all applications encountered in wastewater treatment applications. Initial expense is greater than for the volumetric feeder and slightly less than for the loss-in-weight feeder. Belt-type gravimetric feeders consist of a basic belt feeder incorporating a weighing and control system. Feed rates can be varied by changing either the weight per foot of belt, or the belt speed, or both. Controllers in general use are mechanical, pneumatic, electric, and mechanical-vibrating. Accuracy specified for belt-type gravimetric feeders should be within + 1 percent of set rate. Materials of construction of feed equipment normally include mild steel hoppers, stainless steel mechanism components, and rubber-surfaced feed belts.
Because alum solution is corrosive, dissolving or solution chambers should be constructed of type 316 stainless steel, fiberglass reinforced plastic (FRP), or plastics. Dissolvers should be sized for preparation of the desired solution strength. The solution strength usually recommended is 0.5 lb of alum to 1 gal. of water, or a 6 percent solution. The dissolving chamber is designed for a minimum detention time of 5 minutes at the maximum feed rate. Because excessive dilution may be detrimental to coagulation, eductors, or float valves that would ordinarily be used ahead of centrifugal pumps are not recommended. Dissolvers should be equipped with water meters and mechanical mixers so that the water-to-alum ratio may be properly established and controlled.
FRP, plastics (polyvinyl chloride, polyethylene, polypropylene, and other similar materials), and rubber are general use and are recommended for alum solutions. Care must be taken to provide adequate support for these piping systems, with close attention given to spans between supports so that objectionable deflection will not be experienced. The alum solution should be injected into a zone of rapid mixing or turbulent flow.
Solution flow by gravity to the point of discharge is desirable. When gravity flow is not possible, transfer components should be selected that require little or no dilution. When metering pumps or proportioning weir tanks are used, return of excess flow to a holding tank should be considered. Metering pumps are discussed
Standard instrument control and pacing signals are generally acceptable for common feeder system operation. Volumetric and gravimetric feeders are usually adaptable to operation from any standard instrument signals.
When solution must be pumped, consideration should be given to use of holding tanks between the dry feed system and feed pumps, and the solution water supply should be controlled to prevent excessive dilution. The dry feeders may be started and stopped by tank level probes. Variable-control metering pumps can then transfer the alum stock solution to the point of application without further dilution. Means should be provided for calibration of the chemical feeders. Volumetric feeders may be mounted on platform scales. Belt feeders should include a sample chute and box to catch samples for checking actual delivery with set delivery. Gravimetric feeders are usually furnished with totalizers only. Remote instrumentation is frequently used with gravimetric equipment, but seldom used
Liquid alum is shipped in rubber-lined or stainless steel insulated tank cars or trucks. Alum shipped during the winter is heated prior to shipment so that crystallization will not occur during transit. Liquid alum is shipped at a solution strength of about 8.3 percent as A1203 or about 49 percent as A12(S04)3'14H20. The latter solution weighs about 11 lb/gal at 60°F and contains about 5.4 lb dry alum (17 percent A1203) per gal of liquid. This solution will begin to crystallize at
Bulk unloading facilities usually must be provided at the treatment plant. Rail cars are constructed for top unloading and therefore require an air supply system and flexible connectors to pneumatically displace the alum from the car. U.S. Department of Transportation regulations concerning chemical tank car unloading should be observed. Tank truck unloading is usually accomplished by gravity or by
Established practice in the treatment field has been to dilute liquid alum prior to application. However, recent studies have shown that feeding undiluted liquid alum results in better coagulation and settling. This is reportedly due to prevention of
No particular industrial hazards are encountered in handling liquid alum. However, a face shield and gloves should be worn around leaking equipment. The eyes or skin should be flushed and washed upon contact with liquid alum. Liquid alum becomes very sick upon evaporation and therefore spillage should be avoided. Storage tanks may be open if indoors but must be closed and vented if outdoors. Outdoor tanks should also be heated, if necessary, to keep the temperature above 450F to prevent crystallization. Storage tanks should be constructed of type 316 stainless steel, FRP, steel lined with rubber, polyvinyl chloride, or lead. Liquid alum can be stored indefinitely without deterioration.
Storage tanks should be sized according to maximum feed rate, shipping time required, and quantity of shipment. Tanks should generally be sized for 1.5 times the quantity of shipments. A ten-day to two-week supply should be provided to allow for unforeseen shipping delays.
A12(S04)3 in aqueous solution is most commonly known as aluminum sulfate solution. The CAS (Chemical Abstract Service) Index is "Sulfuric Acid, Aluminum Salt (3:2)". Its CAS number is 10043-01-3. Aluminum sulfate, solution, technical grade is a clear, white to slightly yellow brown liquid.
Reactions between alum and the normal constituents of wastewaters are influenced by many factors; hence, it is impossible to predict accurately the amount of alum that will react with a given amount of alkalinity, lime, or soda ash which may have been added to the wastewater. Theoretical reactions can be written which will serve as a general guide, but in general the optimum dosage in each case must be determined by laboratory jar tests.
The simplest case is the reaction of Al3+ with OH" ions made available by the ionization of water or by the alkalinity of the water.
Solution of alum in water produces:
Hydroxyl ions become available from ionization of water: H20 » H+ + OH"
The aluminum ions (Al3+) then react: 2 Al3+ + 60H » 2 Al(OH)3
Consumption of hydroxy 1 ions will result in a decrease in the alkalinity. Where the alkalinity of the wastewater is inadequate for the alum dosage, the pH must be increased by the addition of hydrated lime, soda ash, or caustic soda. When amounts of alkali when added to wastewater they will maintain the alkalinity of the water unchanged when 1 mg/1 of alum is added. For example, if no alkalinity is added, 1 mg/1 of alum will reduce the alkalinity of 0.50 mg/1 as CaC03 but alkalinity can be
The reactions of alum with the common alkaline reagents are...
A12(S04)3 + 3 Ca(HC03)3 - 2 Al(OH)3 I + 3 CaS04 + 6 C02 !
A12(S04)3 + 3 Na2C03 + 3 H.0 - 2 Al(OH)3 1 + 3 C02 t
maintained unchanged if 0.39 mg/1 of hydrated lime is added. This lowering of natural alkalinity is desirable in many cases to attain the pH range for optimum coagulation. For each mg/1 of alum dosage, the sulfate (S04) content of the water will be increased approximately 0.49 mg/1 and theC02 content of the water will be
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