Figure 6.4 Theoretical quicklime dosages for class A and class B dry lime stabilization.
lower line shows the dose required for class B pH requirements, and the upper line shows the dose for class A temperature requirements. The figure shows that the quicklime requirement for class B stabilization theoretically increases with increased solids, whereas the quicklime requirement for class A stabilization decreases with increased solids. This is because of the predominance of the heating requirements of the water for class A versus the pH effects of class B. In practice, the amount of quicklime required to achieve class A can be 50% greater the theoretical class A temperature line in Figure 6.4. A drier, more easily crumbled product can be obtained by increasing the quicklime addition up to as much as twice the theoretical value shown.
Chemical dosages for advanced alkaline stabilization technologies vary by process, type of chemical and final product requirements. The exact dosage for any particular technology can be estimated through bench-scale testing.
Several types of alkaline chemicals are available: quicklime (CaO), hydrated lime [Ca(OH)2], lime kiln dust, cement kiln dust, and portland cement. In dry lime and advanced alkaline stabilization technologies, different types of additives result in different product texture and granulation. Quicklime is available in pebble, granular, and pulverized (virtually all passes a No. 20 sieve) forms. Hydrated lime is available in powder form (at least 75% passes a No. 200 sieve). Lime kiln dust and cement kiln dust are by-products from the respective industries, and it is important to ensure that these materials do not introduce contaminants or additional pollutants, jeopardizing the quality of the product.
Lime can be delivered in bags or in bulk. Bagged lime is preferred only where daily requirements are less than 300 kg (660 lb) of lime per day. Bagged lime must be stored under cover to prevent it from getting wet. Proper handling is especially important when quicklime is used, because it is highly reactive with water, producing heat and swelling that can cause the bags to burst.
Hydrated lime may be stored under dry conditions for periods up to a year without serious deterioration with atmospheric carbon dioxide, known as recarbonation. Quicklime deteriorates more rapidly by reacting with moisture from the air, which causes caking. Under good storage conditions, quicklime may be held as long as six months, but in general should not be stored for more than three months.
Bulk alkaline material is stored in conventional steel silos with hopper bottoms that have a side slope of 60° from the horizontal. The storage silos must be airtight to prevent slaking and recarbonation. Pebble quicklime is free flowing and will discharge readily from storage bins. Pulverized quicklime and especially hydrated lime have a tendency to arch and therefore require some type of mechanical or pneumatic agitation to ensure uniform discharge from silos.
Storage silos should be sized on the basis of the daily lime demand, the type and reliability of delivery, and an allowance for flexibility and expansion. As a minimum, storage should be provided to supply a 7-day lime demand, with a two- to four-week supply preferred. In any case, the total storage volume should be at least 50% greater than the capacity of the delivery truck to endure adequate lime supply between shipments.
In a liquid sludge stabilization process, lime is always delivered to the liquid sludge mixing vessel as a calcium hydroxide slurry (milk of lime). Dry lime should not be added to liquid sludge because caking will occur. After being mixed into slurry, hydrated lime and quicklime are chemically the same.
Feeding of Hydrated Lime In small treatment plants where bagged hydrated lime is used, the dry chemical is simply mixed with water in a batch tank. Lime solution is not corrosive, so that an unlined steel tank is sufficient for mixing and storing the slurry. An impeller type of mixer is generally used for mixing. Lime slurry is fed as a 10 to 30% solution by weight, the percent age depending on application and operator preference. The slurry can be discharged to the sludge mixing tank in one batch or metered continuously through a solution feeder.
In larger operations, where hydrated lime is stored in a silo, a more automated mixing and feeding scheme is appropriate. A dry chemical feeder that is positioned at the base of the bulk storage silo is used for continuous delivery of a measured amount of dry lime to a dilution tank. Two general types of automated dry feeders are available: volumetric feeders and gravimetric feeders. Volumetric feeders deliver a constant, preset volume of chemical in a unit time, regardless of changes in material density. Gravimetric feeders discharge a constant mass of chemical. Gravimetric feeders cost approximately twice as much as volumetric feeders, but they are more accurate. Gravimetric feeders are preferred because of their accuracy and dependability, but less expensive volumetric feeders may be sufficient when greater chemical feeding accuracy is not required. The feeder should be isolated from the storage silo with a slide gate to allow easy removal of the metering equipment if it becomes clogged.
Dust is a major problem associated with most dry chemical feed systems. Poorly fitting slide gates and leaking feeders are obvious sources of dust. The vertical drop between the feeder and slurry tank should be reduced or enclosed to reduce dust problems.
Dry hydrated lime is delivered by the feeder to a dilution tank that is normally located directly under the feeder. Level sensors in the tank and an automated waterline valve and feeder allow a preset amount of lime and water to be mixed in batches. The tank content is agitated by compressed air, water jets, or impeller-type mixers. The lime slurry is then transferred to the sludge mixing tanks. This transfer operation is the most troublesome single operation in the lime-handling process. Milk of lime reacts with atmospheric carbon dioxide or carbonates in the dilution water to form hard calcium carbonate scales, which, with time, can plug the slurry transfer line. Because the magnitude of this problem is directly proportional to the distance over which the slurry must be transferred, lime feed facilities should be located as close as possible to the lime-sludge mixing tanks. The slurry tank and all appurtenances should be cleaned and the line flushed thoroughly at the end of each day's operation.
Slaking and Feeding of Quicklime Feeding of quicklime is similar to that for hydrated lime, except that there is the additional step of slaking, in which the quicklime reacts spontaneously with water to form hydrated lime. Bagged quicklime can be slaked in batches simply by mixing 1 part quicklime with 2 to 3 parts water in a steel trough while blending with a hoe. Proportions should be adjusted so that the heat of hydration maintains the temperature of the reacting mass near 93°C (200°F). The resulting paste should be held for 15 to 30 minutes after mixing to complete hydration. Manually operated batch slaking is a potentially hazardous operation and should be avoided if possible. Uneven distribution of water can produce explosive boiling and splattering of lime slurry.
Continuous slaking is accomplished in automated machines that also degrit the lime slurry. Several types of continuous slakers are available. They vary mainly in the proportion of lime to water mixing initially. A volumetric or gravimetric dry chemical feeder is used to measure and deliver quicklime to the slaker. Since quicklime is available in a wide range of particle sizes, it is important to match the dry feeder with the type of quicklime to be used in the particular application. A paste is created in the slaker by a water-to-lime ratio of 2 : 1. The paste is then diluted further, as required, in a slurry tank. The paste should be held for approximately 5 minutes to allow complete hydration in the slaking chamber before discharg into the slurry tank.
A tank must be provided for mixing raw sludge with lime slurry and then holding the mixture for a minimum contact time. The contact time recommended is a minimum of 30 minutes after the pH reaches 12.5. The tank can be constructed of steel or concrete, although circular steel tanks with a coating that can withstand the high pH and the abrasion from the particles in sludge and lime, and with a concrete floor, are the most common. The size of the tank depends on whether the mixing is done on a batch or a continuous basis.
In the batch mode, the tank is filled with sludge and sufficient lime is added and mixed to maintain the pH of the sludge-lime mixture above 12.5 for the next 30 minutes. After this minimum contact time, the mixture is held for an additional 2 hours before the stabilized sludge is transferred either to the dewatering facilities or tank trucks for land application. Once the tank is emptied, the cycle begins again. In land application of liquid sludge, a storage tank may be required to store the biosolids during periods of the year when biosolids cannot be land applied.
In the continuous-flow mode, the pH and volume of the sludge in the mixing tank are held constant. Feed sludge displaces an equal volume of treated sludge. Lime is added continuously in proportion to the flow of the incoming raw sludge. The lime dose must be sufficient to keep the contents of the tank at a pH of 12.5 for at least 2 hours.
It is most common to operate lime stabilization systems in batch mode. Batch operations are very simple and are well suited for small-scale and manually operated systems. In very small treatment plants, the tank should be sized to treat the daily sludge production in one batch, because small plants have operating staff present during only one shift. In such instances, the mixing tank can also be used to gravity-thicken the lime-treated sludge before disposal. Continuous-flow systems require automated control of lime feeding and therefore are not usually cost-effective for small treatment plants. The primary advantage of continuous-flow systems over batch systems is that a smaller tank size may be possible.
Thickening of raw sludge before lime stabilization will reduce the mixing tank capacity requirement in direct proportion to the reduction in sludge volume. However, the lime requirement will be reduced only slightly because most of the lime demand is associated with the quantity of sludge solids.
Sludge-lime mixing can be accomplished with either diffused air or mechanical mixers. The agitation should be great enough to keep the sludge solids suspended and to distribute the lime slurry evenly and rapidly. Diffused air systems have the added advantage of keeping feed sludge fresh preceding lime addition in batch mode operations. However, ammonia will be stripped from sludge, producing odors and reducing the fertilizer value of the bio-solids. Although mechanical mixers are subject to fouling with rags and other debris in the sludge, newer nonclog mechanical systems are available.
With air mixing, coarse bubble diffusers should be used. An air supply of 1.2 to 2.4 m3/m3 • h (20 to 40 cfm per 1000 ft3) is required for adequate mixing. Mechanical mixing design criteria is based on (1) maintaining the bulk fluid velocity (defined as the turbine agitator pumping capacity divided by the cross-sectional area of the mixing tank) above 8.5 m/min (26 ft/min), and (2) using an impeller Reynolds number greater than 1000. For mechanical mixing in circular tanks, baffles that are one-twelfth the diameter of the tank should be placed 90° apart at the tank periphery. Consultation with agitation equipment manufacturers is recommended to select the most efficient and nonclog mechanical mixers.
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