The technology to treat sludge with lime and to pasteurize is known, but knowing this is not enough. Technology data must be consolidated and applied to the facility's design, and design parameters and requirements must be considered in these process systems.

Lime stabilization offers many advantages when compared to digestion or composting, including lower operating costs, improved pathogen reduction, and ease of use. In addition, upgrading to a PFRP level may be done easily by transferring the lime-treated sludge from the mixer directly to a heated storage hopper.

LIME-SLUDGE MIXING STATUS

Three lime-sludge mixing method shave been developed. In one method lime is mixed with liquid sludge and deposited from tank trucks onto land. In the second method, lime is mixed with liquid sludge before the mixture is mechanically dewatered. The third end most feasible method involves mixing lime with sludge that has been dewatered to a concentration of about 20% solids.

Several installations have been constructed in which lime is mixed with dewatered sludge. The expected product is a sludge with a pH of 12.

BASIC CHEMISTRY

The sludge pH is raised by adding calcium hydroxide (Ca(OH)₂). It is important to remember that quicklime(CaO) must first be converted to calcium hydroxide by hydration to raise a sludge's pH. The lime manufacturing process demonstrates the reaction and the variables involved in lime sludge treatment systems. Figure 1 illustrates the major operations used for high calcium (arogomite) or low-calcium (dolomitic) lime. The arogomite is 98% calcium carbonate (CaCO₃) and the dolomitic material is a mix of 58% CaCO₃ and 40% magnesium carbonate MgCO₃. The chemical and physical characteristics of lime are important to the system designer. For example,particle size affects the rate of hydration and, therefore, the rate of pH change.These rates are used in the design of the lime sludge mixing equipment. A very fine (pulverized) product will improve the hydration rate, but may adversely affect the function of the storage and feeding equipment. The optimum selection, therefore, is a ground product that has a satisfactory hydration rate and yet minimizes storage and feeding problems.

The chemical reaction that occurs in the hydrator is similar to that which takes place in lime-sludge treatment mixing. The hydration reaction is expressed as:

CaO + H₂O = Ca(OH)₂ + heat (1)

Dolomitic lime, hydrated under atmospheric conditions:

CaO · MgO + H₂O = Ca (OH)₂ . MgO + heat (2).

It is important to note that each pound of 100% quicklime produces 491 Btu of heat, 1.32 lb of Ca(OH)₂, and extracts 0.32 lb of free water from the sludge. Note that none of the magnesium the dolomitic lime is converted to the hydroxide Mg(OH)₂ form under atmospheric conditions. The pH does not change unless the sludge is mixed with a hydroxide compound. A pressure hydrator must be used to convert magnesium to a hydroxide, which is required for pH control.

Precise reaction equations cannot be developed with a heterogeneous product such as sludge, but a generalization may serve as a foundation for design criteria. Thus, a 1-mgd treatment plant will produce approximately 1 ton/day of dry solids (DS). These solids will be mechanically dewatered to a cake having about 20% solids. The lime required To raise the pH of these solids to a value of 12.0 or higher may only be accurately determined by testing. Experience shows, however, that the lime demand may vary from 0.25 lb to 2.0 lb for every 1.0 lb of DS.

Under low lime-demand conditions of 0.25 lb to 1.0 lb (DS) the mixer contains heat, 2000 lb DS, 500 lb lime, and 8000 lb water. Equation 1 shows that the theoretical water required to hydrate quicklime is 160 lb:

0.32 lb H₂O · 500 lb lime = 160 lb H₂O

Thus, the mixer holds much more water than the amount theoretically required for hydration.

An excess amount of water is, however, required for slaking the lime (see Box). The water-to-lime ratio is normally about 3:1. The example shows a ratio of 16:1, water to lime, simplyby virtue of the free water in the dewatered sludge cake which is to be treated. This excess water absorbs some of the generated heat and can adversely affect both the rate and magnitude of pH change. This must be considered in the system design.

DESIGN FACTORS

Several variables must be considered by the designer of the system. Some of the more critical variables are quality and lime type, heat produced from their action and its use in the process reaction, water-to-lime ratio, and lime-to ratio. Testing can be useful in determining the lime quality and

Hydration

Versus Slaking

The terms hydration and slaking are frequently used interchangeably. Technically, hydration entails mixing lime with water in a ratio that will yield a dry powder. Slaking of lime requires the use of water in a ratio of 3:1 or more to produce a wet hydrate. Slaking and hydration produce anexothermic reaction. Lime sludge treatment is a slaking process with the required water provided by the dewatered sludge. The amount of water in the sludge results in much higher ratios of water to lime,usually in the range of 10:1 or higher. The reaction is still exothermic, but the slurry temperature is depressed by the large excess of water.

Reaction rate. The temperature of the products (lime, sludge, and water) in the mixer correlates to the reaction rate. Reaction rates determine mixer size. Some lime-sludge mixtures exhibit a rate of reaction doubling for every 10°C temperature rise. Figure 3 depicts the results of a test using high-calcium quicklime on a biologically treated sludge. The mixer was insulated to take advantage of the heat generated by the reaction, and it was also equipped to supply auxiliary heat. In the absence of supplementary heat, the temperature peaked at 38°C with a pH of 11.8 after a retention of 3 minutes. Raising the temperature with auxiliary heat to 650º C yielded the desired 12.0 pH after 45 seconds of mixing. The solids concentration in the 65° C sludge was 29.1% as compared to 27.4% for the 38°C sludge.

The value of the insulated mixer was investigated using a sludge composed of 80% primary and 20% secondary sludges. Mixing time was extended to 3 minutes. Temperature and pH were then recorded. The final temperature was 33°C with a final pH of ll.0.

Dewatering. The dewatering device used in a lime-sludge stabilization should remove most of the water from the sludge. Ideally, the water to lime ratio should be in the range of 3:1 to 5:1. Equipment is available that can generally produce a sludge with a water to lime ratio of 10:1. Figure 4 illustrates the impact of the high ratio on the final temperature in the mixer. This mutation of the generated heat affects the rate at which the reaction goes forth. An increase in the lime dosage rate would compensate for the excess water in the sludge.

Other factors. The MgO content inhibits slaking and with excessive agitation and retention of material in the mixer it becomes thixotropic. Increasing the mass of material in the mixer will accelerate the reaction rate. Sulfates will adversely affect reaction rates and chlorides will accelerate the reaction rate. These factors affect the lime process, however, to a lesser degree than those which have been previously discussed.

Design. The many variables involved in the mixing of lime and sludge necessitate testing of both before selecting the process and equipment design. The type of lime commercially available for the particular location must be known and the required mixer retention must be determined. The required data may be determined in bench-scale tests. Onsite pilot tests can ascertain the effects of other factors.

Two basic lime-sludge systems are available to meet PSRP requirements. Figure 5 shows System A, an arrangement that uses a conveyor to transport the material and to mix the sludge. Mixing can be accomplished only if the conveyor length is properly sized and the conveyor flights are properly constructed to provide the desired retention.

System B is used when the conveyor cannot be adapted to provide mixing or if other variables indicate that more system flexibility is required than that which is available without the mixer. Note that auxiliary heat is available for preheating the sludge in the conveyor. This will reduce the time required to retain the sludge and lime in the mixer. Once mixed, the sludge is stored and retained in the heated hopper if it is required to further reduce pathogens. Systems A or B must precede the retention hopper.

Middletown, Pa.,

Operating Data

Sewage:

Flow rate - 1 mgd

Treatment Process -activated sludge using primary and secondary settling. Phosphorus is removed using ferrous sulfate at a rate of 9 mg/L, introduced at the aeration tank.

Sludge treatment:

Aerobically digested

Dewatered with a 1 .5-m belt filter press producing a cake with 15% solids at a rate of 510 Ib/hour of dry solids. Lime added to sludge discharged from belt filter press at a rate of 220 Ib/hour of 96% pure calcium oxide.

Sludge nutrients, expressed as percent of dry weight, is:

Total N - 2.93

P₂O₅ - 3.54

K₂O - 0.19

Non-nutrients in Ib/dry ton are:

Lead - 0.12

Chromium - 0.03

Mercury - 0.00

Nickel - 0.04

Cadmium - 0.00

Sludge Disposal:

The final pH of sludge after land application is 11.6. Final product is transported by Borough trucks to croplands and distributed by manure spreader.

Six different types of screws are available for the mixing of lime and

Belt filter press sludge screw conveyor with a variable speed drive is directly discharged into single screw conveyor with a variable speed drive.

Getting to Know Your Sludge Regulations

Current federal regulations governing land applications of municipal sludges are contained in 40 CFR 257 -Criteria for Classification of Solid Waste Disposal Facilities and Practices. Pathogen reduction is broken into two categories: Process to Significantly Reduce Pathogens (PSRP) and Process to Further Reduce Pathogens (PFRP). PSRP reduces but does not eliminate pathogens. Therefore, to protect public health, the regulations minimize the potential for direct and indirect exposure to sludge. PFRPs reduce pathogens below detectable levels, therefore there are no pathogen related restrictions to managing sites where PFRP sludges have been applied.

Passage of the Water Quality Act of 1987 required EPA to comprehensively regulate the use and disposal of sewage sludge. The new sludge regulations will be included in 40 CFR Part 503— Standards for Disposal of Sewage Sludge.

Under the proposed 503 regulations, pathogen reduction is broken into three categories, Class A, B and C. Class A is similar to PFRP. Class B and C are similar to PSRP. In addition, proposed 503 regulations require adequate reduction in vector attraction.

Congress directed EPA to begin immediate regulation of wastewater sludge use and disposal. Before promulgation of the standards, EPA is required to begin to address sewage sludge disposal in NPDES permits issued to POTWs. Therefore, designers should be knowledgeable in the proposed regulations and incorporate the requirements in the design of any sludge-treatment facilities.

Operating cost. Operating cost with a lime-stabilization system for PSRP requirements is primarily that associated with the lime. Average cost for high calcium quicklime is $100.00/ton. At an average rate of 0.5 Ib/lb of DS lime addition, the major operating cost per mgd is $50.00/day. Average capital cost of $150,000 can be expected for 5-mgd and smaller facilities.

The wastewater treatment industry may choose to substitute a chemical system of waste solids stabilization for a biological system and meet present health requirements proposed by EPA. A live, bottom-heated storage hopper can be added to the system when conditions require a pasteurized sludge for land application or disposal.

The type and quantity of lime to be used along with the mixer retention time are critical to lime stabilization. Testing is imperative for a system with a process guarantee and anything less than a bench test would not be prudent. If possible, an on-site pilot test should be conducted.

REFERENCES

1. Westphal, P.A., and Christensen, G.L., "Lime Stabilization: Effectiveness of Two Process Modifications." J. Water Pollut. Control Fed., 55, ( 1983).

2. "Sludge Treatment and Disposal: Disinfection, Technology Transfer." U.S. EPA 625/1-79-011 (1979).