Technologies using materials other than lime for alkaline stabilization have been used by a number of municipalities. These technologies are modifications of traditional dry lime stabilization, and most of them use materials such as cement kiln dust, lime kiln dust, portland cement, or fly ash. The most common modifications are the supplemental drying and addition of other chemicals or bulking agents.
The principal advantages of advanced alkaline stabilization processes include: (1) the product meets class A stabilization requirements and is easy to handle; (2) the product has decreased odor potential and has value as a liming agent; (3) the capital cost is low compared with other class A stabilization processes; and (4) the processes are easy to operate, start up, and shut down. The disadvantages include high operating costs, the extensive odor control system that is required to treat ammonia and other off-gases, the increase in total solids/chemical mass to transport, and the product not being appropriate for alkaline soils.
Many of these advanced alkaline stabilization technologies are proprietary and are available through private companies. In one of the systems, class A pasteurization criteria are met by utilizing the exothermic reaction of quicklime with water in the sludge. Each kilogram of 100% quicklime produces approximately 15,300 cal/g-mol (27,500 Btu/lb-mol) of heat. The reaction between quicklime and carbon dioxide, also exothermic, releases approximately 43,300 cal/g-mol (78,000 Btu/lb-mol) of heat. Depending on the quantity of lime used, this reaction achieves a process temperature in excess of 70°C for more than 30 minutes as required to meet class A criteria. Odor control reagents and nutrient enhancements may also be incorporated as required.
In another pasteurization process, dewatered sludge is preheated in an insulated electrically heated screw conveyor prior to being transferred to a pug mill mixer, where the heated sludge and quicklime are mixed. Because supplemental heat is used to elevate the temperature of the sludge, quicklime must only be added in sufficient quantities to elevate the pH. This results in a lower lime dose than if the exothermic reaction of lime and water is the only source available to elevate sludge temperature. From the pug mill, the sludge-lime mixture is conveyed to a heated and insulated vessel reactor, where it is retained at a minimum temperature of 70°C for 30 minutes to meet the class A requirements.
In another proprietary process that satisfies the PFRP requirements of 40 CFR Part 503, an alkaline material such as cement kiln dust or lime kiln dust is mixed with sludge in sufficient quantity to achieve a pH of 12 or greater for at least 7 days. The alkaline-stabilized sludge is then dried for at least 30 days, until a minimum solids concentration of 65% is achieved. The biosolids need to be kept above 60% solids concentration before the pH drops below 12, and the mean ambient temperature must be above 5°C for the first 7 days. In a variation of this, the pH is increased to 12 or greater for only 72 hours, and the mixture is then heated to a temperature of at least 52°C and maintained for a minimum of 12 hours. The stabilized sludge is then dried to achieve a solids concentration of minimum 50%.
In a chemical stabilization/fixation technology process, pozzolanic materials are added to dewatered sludge. The addition of these materials causes cementitious reactions and after drying, produces a soil-like material of 35 to 50% solids content that is not subject to liquefaction under mechanical stress. The principal use of this material is as landfill cover.
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