Ultimate Disposal

Even after treatment, we are left with a large volume of sludge that needs a final resting place. The choices for ultimate disposal of sludge are limited to air, water, and land. Until quite recently, incineration ("air disposal") was viewed as an effective sludge reduction method, if not exactly an ultimate sludge disposal method (the residual ash still required disposal). However, strict controls on air pollution and increasing concern over global warming are making incineration an increasingly unlikely option. Disposal of sludges in deep water (such as oceans) is decreasing owing to adverse or unknown detrimental effects on aquatic ecology. Land disposal, particularly the use of sludge as fertilizer or soil conditioner, has historically been a favored disposal method, and is currently growing in popularity as other options become more problematic.

Incineration is actually not a method of disposal at all, but rather a sludge treatment step in which the organics are converted to H2O and CO2 and the inorganics drop out as a nonputrescible residue. Two types of incinerators have found use in sludge treatment: multiple hearth and fluid bed. The multiple-hearth incinerator, as the name implies, has several hearths stacked vertically, with rabble arms pushing the sludge progressively downward through the hottest layers and finally into the ash pit (Fig. 10-16).

Figure 10-16. Multiple-hearth incinerator. (Courtesy of Nichols Engineering and Research Corp.)

The fluidized bed incinerator is full of hot sand suspended by air injection; the sludge is incinerated within the moving sand. Owing to the violent motion within the fluid bed, scraper arms are unnecessary. The sand acts as a "thermal flywheel," allowing intermittent operation. Despite a flurry of interest during the previous decade, sludge incineration is no longer considered the best available technology by many state regulatory agencies because of environmental concerns about atmospheric emissions and ash disposal.

The second method of disposal, land disposal, is becoming more popular, particularly in areas where there are restrictions on industrial contaminants entering the wastewater treatment. (Sludges contaminated with industrial chemicals may not be suitable for land application.) The ability of land to absorb sludge and to assimilate it depends on such variables as soil type, vegetation, rainfall, and slope. In addition, the important variable of the sludge itself will influence the capacity of a soil to assimilate sludge. Generally, sandy soils with lush vegetation, low rainfall, and gentle slopes have proven most successful. Mixed digested sludges have been spread from tank trucks, and activated sludges have been sprayed from both fixed and moving nozzles. The application rate has been variable, but 100 dry tons/acre-yr is not an unreasonable estimate. Most unsuccessful land application systems may be traced to overloading the soil. Given enough time, and the absence of toxic materials, soils will assimilate sprayed liquid sludge.

There has been some successful use of land application for sludge for fertilization, particularly in silviculture operations. Forests and tree nurseries are far enough from population centers to minimize aesthetic objections, and the variable nature of sludge is not so problematical in silviculture as in other agricultural applications. Sludge may also be treated as packaged fertilizer and plant food. The city of Milwaukee has pioneered the drying, disinfection, and deodorizing of sludge, which is packaged and marketed as the fertilizer Milorganite.

Transporting liquid sludge is often expensive, and volume reduction by dewatering is necessary. The solid sludge may then be deposited on land and disked into the soil. A higher application rate (tons/acre-yr) may be achieved by trenching, where 1- m2 (3-ft2) trenches are dug with a backhoe, and the sludge is deposited in the trench, then covered with soil.

In the past few years chemicalfixation, which involves chemically bonding sludge solids so that the mixture "sets" in a few days, has found use in industries that have especially critical sludge problems. Although chemical fixation is expensive, it is often the only alternative for besieged industrial plants. The leaching from the solid seems to be minimal.

Sludge often contains compounds that are potentially harmful to vegetation and animals (including people), or that can cause degradation of surface water and groundwater supplies. Although most domestic sludges do not contain sufficient concentrations of toxins such as heavy metals to cause immediate harm to vegetation, the overall concentration of toxins or metals may bioaccumulate in plants and animals if the sludge is applied to the same land for an extended period of time. Because of this, sludges to be applied as fertilizer or soil conditioners must be tested to certify that they meet state and federal guidelines. It is possible to remove some toxins during sludge treatment, but the most effective means of controlling toxicity is to prevent the toxins from entering the sewerage system. Strongly enforced sewerage ordinances are necessary, particularly given the increasingly difficult problem of providing for the ultimate disposal of sludge.

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