Lime is among a family of chemicals which are alkaline in nature and contain principally calcium, oxygen and, in some cases, magnesium. In this grouping are included quicklime, dolomitic lime, hydrated lime, dolomitic hydrated lime, limestone, and dolomite. The most commonly used additives are quicklime and hydrated lime, but the dolomitic counterparts of these chemicals (i.e., the high-magnesium forms) are also widely used in wastewater treatment and are generally similar in physical requirements.
Quicklime, CaO, has a density range of approximately 55 to 75 lb/cu ft, and a molecular weight of 56.08. A slurry for feeding, called milk of lime, can be prepared with up to 45 percent solids. Lime is only slightly soluble, and both lime dust and slurries are caustic in nature. A saturated solution of lime has a pH of about 12.4.
The CaO content of commercially available quicklime can vary quite widely over an approximate range of 70 percent to 96 percent. Content below 88 percent is generally considered below standard in the municipal use field. Purchase contracts are often based on 90 percent CaO content with provisions for payment of a bonus for each 1 percent over and a penalty for each 1 percent under the standard. A CaO content less than 75 percent probably should be rejected because of excessive grit and difficulties in slaking.
Pebble quicklime, all passing a 34-in. screen and not more than 5 percent passing a No. 100 screen, is normally specified because of easier handling and less dust. Hopper agitation is generally not required with the pebble form. Published slaker capacity ratings require "soft or normally burned" limes which provide fast slaking and temperature rise, but poorer grades of limes may also be satisfactorily slaked by selection of the appropriate slaker retention time and capacity. Storage of bagged lime should be in a dry place, and preferably elevated on pallets to avoid absorption of moisture. System capacities often make the use of bagged quicklime impractical. Maximum storage period is about 60 days. Bulk lime is stored in airtight concrete or steel bins having a 55-degree to 60-degree slope on the bin outlet. Bulk lime can be conveyed by conventional bucket elevators and screw, belt, apron, drag-chain, and bulk conveyors of mild steel construction. Pneumatic conveyors subject the lime to air slaking and particle sizes may be reduced by attrition. Dust collectors should be provided on manually and pneumatically-filled bins.
Quicklime feeders are usually limited to the belt or loss-in-weight gravimetric types because of the wide variation of the bulk density. Feed equipment should have an adjustable feed range of at least 20:1 to match the operating range of the associated slaker. The feeders should have an over-under feed rate alarm to immediately warn of operation beyond set limits of control. The feeder drive should be instrumented to be interrupted in the event of excessive temperature in the slaker compartment. Lime slakers for wastewater treatment should be of the continuous type, and the major components should include one or more slaking compartments, a dilution compartment, a grit separation compartment, and a continuous grit remover. Commercial designs vary in regard to the combination of water to lime, slaking temperature, and slaking time in obtaining the "milk of lime" suspensions. The paste-type slaker admits water as required to maintain a desired mixing viscosity. This viscosity therefore sets the operating retention time of the slaker. The paste slaker usually operates with a low water-to-lime ratio (approximately 2:1 by weight), elevated temperature, and 5-minute slaking time at maximum capacity. The detention-type slaker admits water to maintain a desired ratio with the lime, and therefore the lime feed rate sets the retention time of the slaker. The detention slaker operates with a wide range of water-to-lime ratios (2.5:1 and 6:1), moderate temperature, and a 1 0-minute slaking time at maximum capacity. A water-to-lime ratio of from 3.5:1 to 4:1 is most often used. The operating temperature in lime slakers is a function of the water-to-lime ratio, lime quality, heat transfer, and water temperature. Lime slaking evolves heat in hydrating the CaO to Ca(OH)2 and therefore vapor removers are required for feeder protection. Lime slurry should be transported by gravity in open channels wherever possible. Piping channels and accessories may be rubber, iron, steel, concrete, and plastics. Glass tubing, such as that in rotameters, will cloud rapidly and therefore should not be used. Any abrupt directional changes in piping should include plugged tees or crosses to allow rodding out of deposits. Long sweep elbows should be provided to allow the piping to be cleaned by the use of a cleaning "pig. " Daily cleaning is desirable.
Milk-of-lime transfer pumps should be of the open impeller centrifugal type. Pumps having an iron body and impeller with bronze trim are suitable for this purpose. Rubber-lined pumps with rubber-covered impellers are also frequently used. Makeup tanks are usually provided ahead of centrifugal pumps to ensure a flooded suction at all times. Plating out of lime is minimized by the use of soft water in the makeup tank and slurry recirculation. Turbine pumps and eductors should be avoided in transferring milk of lime because of scaling problems. Lime slaker water proportioning is integrally controlled or paced from the feeder. Therefore, the feeder-slaker system will follow pacing controls applied to the feeder only. As discussed previously, gravimetric feeders are adaptable to receive most standard instrumentation pacing signals. Systems can be instrumented to allow remote pacing with telemetering of temperature and feed rate to a central panel for control purposes.
The lime feeding system may be controlled by an instrumentation system integrating both plant flow and pH of the wastewater after lime addition. However, it should be recognized that pH probes require daily maintenance in this application to monitor the pH accurately. Deposits tend to build up on the probe and necessitate frequent maintenance. The low pH lime treatment systems (pH 9.5 to 10.0) can be more readily adapted to this method of control than high-lime treatment systems (pH 11.0 or greater) because less maintenance of the pH equipment is required. In a close-loop pH-flow control system, milk of lime is prepared on a batch basis and transferred to a holding tank with variable output feeders set by the flow and pH meters to proportion the feed rate.
Hydrated lime, Ca(OH)2> is usually a white powder (200 to 400 mesh); has a bulk density of 20 to 50 lb/cu ft, contains 82 percent to 98 percent Ca(OH)2, is slightly hydroscopic, tends to flood the feeder, and will arch in storage bins if packed. The modular weight is 74.08. The dust and slurry of hydrated lime are caustic in nature. The pH of hydrated lime solution is the same as that given for quicklime. Hydrated lime is slaked lime and needs only enough water added to form milk of lime. Wetting or dissolving chambers are usually designed to provide 5-minutes detention with a ratio of 0.5 lb/gal of water or 6 percent slurry at the maximum feed rate. Hydrated lime is usually used where maximum feed rates do not exceed 250 lb/hr., i.e., in smaller plants. Hydrated lime and milk of lime will irritate the eyes, nose, and respiratory system and will dry the skin. Affected areas should be washed with water.
Information given for quicklime also applies to hydrated lime except that bin agitation must be provided. Bulk bin outlets should be provided with nonflooding rotary feeders. Hopper slopes vary from 60 degrees to 66 degrees. Volumetric or gravimetric feeders may be used, but volumetric feeders are usually selected only for installations where comparatively low feed rates are required. Dilution does not appear to be important, therefore, control of the amount of water used in the feeding operation is not considered necessary. Inexpensive hydraulic jet agitation may be furnished in the wetting chamber of the feeder as an alternative to mechanical agitation. The jets should be sized for the available water supply pressure to obtain proper mixing.
Controls as listed for dry alum apply to hydrated lime. Hydraulic jets should operate continuously and only shut off when the feeder is taken out of service. Control of the feed rate with pH as well as pacing with the plant flow may be used with hydrated lime as well as quicklime.
Lime is somewhat different from the hydrolyzing coagulants. When added to wastewater it increases pH and reacts with the carbonate alkalinity to precipitate calcium carbonate. If sufficient lime is added to reach a high pH, approximately 10.5, magnesium hydroxide is also precipitated. This latter precipitation enhances clarification due to the flocculant nature of the Mg(OH)2. Excess calcium ions at high pH levels may be precipitated by the addition of soda ash. The preceding reactions are shown as follows: Ca(OH)2 +Ca(HC03)2 - 2 CaC03l + 2H20
2 Ca(OH)2 + Mg(HC03)2 - 2 CaC03 1 + Mg(OH)2 1 + 2H,0
Reduction of the resulting high pH levels may be accomplished in one or two stages. The first stage of the two-stage method results in the precipitation of calcium carbonate through the addition of carbon dioxide according to the following reaction:
Single-stage pH reduction is generally accomplished by the addition of carbon dioxide, although acids have been employed, This reaction, which also represents the second stage of the two-stage method, is as follows:
As noted for the other chemicals, the preceding reactions are merely approximations to the more complex interactions which actually occur in waste waters. The lime demand of a given wastewater is a function of the buffer capacity or alkalinity of the wastewater.
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