What Happens to the Sludge in Wastewater Treatment

Modern sewage treatment means that raw sewage is rarely released directly into the earth’s water supplies anymore, and it is processed to be less environmentally hazardous that it once was. However, that does not mean that disposing of large amounts of solid or semi-solid sludge is easy.

Ocean dumping, once a common practice, is now banned. The state of Pennsylvania and other states now contend that most of the sewage sludge is of high enough quality to be classified as biosolids, and it can be recycled to the soil for use in agriculture, mine reclamation, landscaping or horticulture. There are, though, still some risks involved, according to critics.

Landfill disposal is said to be a relatively simple disposal solution from the management and materials handling standpoint, and it compares favorably with other options based on cost. Environmentally, risks are minimal if the landfill is properly constructed and maintained. Yet, dumping charges are rising, and landfill space is also limited in many areas. The potential breakdown of organic waste in landfills is also problematic. The methane produced by such breakdown could be released into the atmosphere. And methane can have other implications. Finally, if a landfill liner fails or the leachate collection system is inadequate, the released organic nutrients could contaminate surrounding surfaces and groundwater.

Sewage sludge incineration destroys pathogens and decomposes most organic chemicals in addition to recovering the heat value of the sludge. The stable residual ask is only 10 to 20% of the original volume, but incineration releases another greenhouse gas, carbon dioxide, and other potential pollutants and requires some sophisticated equipment. In addition, the potential benefits of organic sludge are lost.

Capturing the Benefits of Biosolids

Land application, say many experts, is the most promising and beneficial reuse of the organic matter and plant nutrients in biosolids. As the Pennsylvania study notes, “The source of most of the organic matter and nutrients in biosolids ultimately is from crops grown on agricultural lands. Land application of biosolids returns those materials to the soil so they can be used to produce another crop.”

In Pennsylvania and in other states, such application occurs most often on agricultural fields or for mine reclamation where there is little soil left. The biosolids are of great commercial value to such end users, and that value is often increased through processing that includes composting, pasteurizing, heat drying and pelletizing. Large-scale farmers, nursery owners, landscapers and homeowners all appreciate the nutritional benefits.

Many of the plant nutrients are present in a “slow-release” organic form, and they are more efficient than similar amounts of inorganic fertilizer. One concern, though, is that there may also be additional pollutants and pathogens contained in the biosolids that could conceivably affect human and animal health or soil quality. Overapplication or improper use is seen as a potential risk.

Safeguards Surrounding Use of Land-applied Biosolids

In 1993, the U.S. Environmental Protection Agency (EPA) established the regulations that are currently in effect for land application of biosolids. Those regulations contain risk-management requirements that are designed to limit the potential for pollutant or pathogens. Additionally, several states have regulations that are even more stringent than federal guidelines in order to protect sensitive environments, soils, water supplies, crops and people. In addition, there are guidelines that try to limit the contact between biosolids and known disease vectors that include mosquitoes, flies and rodents.

Kentucky is another state that has addressed the use of biosolids in a unique category. Along with sewage sludge, the Kentucky “special waste” designation applies to water treatment plant sludge, utility coal ash, gas and oil drilling muds and other wastes of “high volume, low hazard.” The state issues four types of permits for managing sewage sludge by “means other than disposal at a landfill.” They include sludge giveaway, storage and treatment, composting and land farming. Interestingly, though, incineration is not discussed in the special waste regulations.

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