Sorting Through the Muck

An overview of the technologies used to convert municipal wastewater residuals into useful products

Biosolids -- in the United States, more than 19,000 municipal wastewater treatment facilities generate 7 million dry tons of this material every year. Biosolids are the nutrient-rich organic materials resulting from the treatment of sewage sludge, which is the name for the solid, semi-solid, or liquid untreated residue generated during the treatment of domestic sewage in a treatment facility. Wastewater treatment plants spend $1.1 billion to treat and dispose of the stuff. Over 80 percent of it is recycled and reused.

As the numbers suggest, biosolids management is a significant industry in and of itself. It is closely regulated by federal guidelines and supports a widening variety of technical solutions.

What follows is a primer on how to sort through the muck and find the right technical solution to treat biosolids so they can be beneficially recycled to the environment.

Class A and B
To ensure that biosolids applied to the land do not threaten public health, the U.S. Environmental Protection Agency (EPA) created the 40 Code of Federal Regulations (CFR) Part 503 Rule. It categorizes biosolids as Class A or B, depending on the level of pathogenic organisms in the material, and describes specific processes to reduce pathogens to these levels. The rule also requires vector attraction reduction (VAR) -- reducing the potential for spreading of infectious disease agents by vectors (i.e., flies, rodents, and birds) -- and spells out specific management practices, monitoring frequencies, recordkeeping, and reporting requirements. Incineration of biosolids is also covered in the regulation.

Understanding 40 CFR Part 503

Class A biosolids contain minute levels of pathogens. To achieve Class A certification, biosolids must undergo heating, composting, digestion, or increased pH that reduces pathogens to below detectable levels. Some treatment processes change the composition of the biosolids to a pellet or granular substance, which can be used as a commercial fertilizer. Once these goals are achieved, Class A biosolids can be land applied without any pathogen-related restrictions at the site. Class A biosolids can be bagged and marketed to the public for application to lawns and gardens.

Class B biosolids have less stringent standards for treatment and contain small but compliant amounts of bacteria. Class B requirements ensure that pathogens in the biosolids have been reduced to levels that protect public health and the environment and include certain restrictions for crop harvesting, grazing animals, and public contact for all forms of Class B biosolids. As is true of their Class A counterpart, Class B biosolids are treated in a wastewater treatment facility and undergo heating, composting, digestion, or increased pH processes before leaving the plant. This semi-solid material can receive further treatment when exposed to the natural environment as a fertilizer, where heat, sunlight, wind, and soil microbes naturally stabilize the biosolids.

The biosolids rule spells out specific treatment processes and treatment conditions that must be met for both A or B classifications.

Class A Treatment Technologies
Technologies that can meet Class A standards include thermal treatment methods like composting, heat drying, heat treatment, thermophilic (heat generating) aerobic digestion, and pasteurization. Class A technologies are known as processes that can further reduce pathogens (PFRP). The technologies must process the biosolids for a specific length of time at a specific temperature.

Composting. This is an environmentally friendly way to recycle the nutrients and organic matter found in wastewater solids. Composting systems turn wastewater biosolids, sawdust, yard waste, and wood chips into high-quality compost. As the material decomposes, oxygen filters through the compost site, releasing water, heat, and carbon dioxide. This process helps dry the organic material, while the generated heat increases the rate of decomposition and kills pathogens.

Heat Drying. This process applies direct or indirect heat to reduce the moisture in biosolids. It eliminates pathogens, reduces volume, and results in a product that can be used as a fertilizer or soil amendment. Because dryers produce a 90 percent dry material, additional VAR is not required.

Class A Heat Drying Technologies: Case Studies

Digestion. In autothermal thermophilic aerobic digestion (ATAD) systems, biosolids are heated to 131 degrees to 140 degrees Fahrenheit (55 degrees to 60 degrees Celsius) and aerated for about 10 days. This autothermal process generates its own heat and reduces volume. The result is a high-quality Class A product acceptable for reuse as a liquid fertilizer.

Pasteurization. Pasteurization produces a Class A material when the biosolids are heated to at least 158 degrees Fahrenheit (70 degrees Celsius) for 30 minutes. This extreme heat kills pathogens in the organic matter. When followed by anaerobic digestion, the VAR is attained and the biosolids can be land applied with minimal restrictions. The majority of the energy used in the pasteurization process is recovered with an innovative heat exchanger system and used to maintain the proper temperature in downstream anaerobic digesters.

Class B Treatment Technologies
EPA regulations list a number of technologies that, under certain operating conditions, can treat and reduce pathogens so that the material qualifies as Class B biosolids. These are known as processes that can significantly reduce pathogens (PSRP). Class B technologies include anaerobic digestion, aerobic digestion, composting, air drying, and lime stabilization.

As is true of Class A, a number of technologies are available to help achieve Class B biosolids.

Digestion. Several EPA-approved stabilization technologies are available for anaerobic and aerobic digestion, including:

  • Heaters, heat exchangers, digester covers, gas, and hydraulic mixing systems are all important components in conventional anaerobic digestion systems.
  • Temperature-phased anaerobic digestion (TPAD) systems optimize anaerobic digestion through a heat recovery system that pre-heats raw material and simultaneously cools the digested biosolids.
  • Membrane gas storage systems include an expandable membrane cover that provides variable digester gas storage, optimizes digester gas utilization for heating and electrical generation, and increases storage capacity.
  • Hydraulic mixers use a multi-port discharge valve to greatly improve biosolids mixing in the digestion process.
  • Air diffusers and aerators can be incorporated in any aerobic digester configuration.

Lime Stabilization. Adding lime can stabilize biosolids by raising the pH and temperature. While adding sufficient amounts of lime to wastewater solids produces Class B biosolids, adding higher amounts will yield Class A biosolids. Combining low amounts of lime with anoxic oxygen deprived storage can also yield Class A biosolids.

More Processes, Services
For maximum biosolids volume reduction, fluidized bed furnace systems operate cost-effectively and in compliance with EPA biosolids management and clean air requirements. These furnaces also use an innovative heat recovery system to maintain low energy costs.

Odor Control. A broad range of both vapor-phase and liquid-phase odor control products and technologies are available. These are incorporated directly into the equipment design and into integrated biosolids treatment systems, providing cost-effective odor control for thickening, dewatering, digestion, composting, thermal drying, alkaline stabilization, conveyance, and storage equipment.

Thicker is Better

Biosolids are the nutrient-rich organic materials that can be separated and treated from the wastewater treatment process. Depending on how they are processed, biosolids may contain varying amounts of pathogens. This bacterial level determines the material's EPA classification, as well as the treated biosolids' end use.

Communities across the country and around the world are developing biosolids management programs. More and more of them are viewing biosolids as a resource to be used (as fertilizer or soil amendment) and not wasted (sent to landfills).

Depending on the methods used, Class A or Class B biosolids result from the treatment process for stabilizing the solids. Liquid or solids stabilization treatment processes are used to treat biosolids, reducing odors and controlling pathogens and other organisms that spread illness or disease. Thermophilic aerobic digestion, thermal treatment, and pasteurization are used to stabilize biosolids in a semi-solid or liquid state; heat drying, composting, and lime addition produce wastewater residuals in a solid state.

Forward-looking communities wishing to produce Class A or Class B biosolids now have a vast field of solutions, technologies, and services from which to choose, as well as a number of disposal methods.

Understanding 40 CFR Part 503
In 1993, EPA issued the "Standards for the Use and Disposal of Sewage Sludge" at 40 Code of Federal Regulations (CFR) Part 503. This rule defined the management practices and numerical criteria for the three major use and disposal options -- land application, incineration, and surface disposal -- that protect public health and the environment. In addition to limiting where and when biosolids can be applied, the rule requires processes to kill pathogens and strictly limits amounts of metals that can be applied to any piece of land.

Federal, state, and local governments play crucial roles in enforcing the Part 503 rule. Local government also is responsible for addressing related local concerns.

The National Academy of Sciences' National Research Council reviewed the regulations and concluded in a July 2002 report that "the use of these materials in the production of crops for human consumption, when practiced in accordance with existing federal guidelines and regulations, presents negligible risk to the consumer, to crop production, and to the environment."

According to EPA, beneficial use is one of several appropriate methods for use or disposal of biosolids. "I believe we've reached a point where the quality of biosolids is good enough that many environmentalists should be able to agree that beneficial use is something that should happen," said Ned Beecher, executive director of the New England Biosolids & Residuals Association (NEBRA). "In my view, it's hard to make the argument that a well-managed biosolids recycling program has anything but a net environmental benefit."

Thicker is Better
There is more to biosolids than achieving Class A or Class B status. Effective biosolids management systems feature efficient thickening, dewatering, and transportation processes to reduce moisture and convey and store the dewatered "cake." Without reliable thickening, dewatering, and handling technologies, biosolids management would be a more difficult and expensive proposition.

Thickening. To thicken or concentrate biosolids (in excess of 5 percent to 6 percent solids), USFilter offers a variety of systems, depending on biosolids characteristics and results desired. Among these are gravity belt and rotary drum thickeners, centrifuges, dissolved air flotation thickeners, and gravity thickeners.

Dewatering. For dewatering, USFilter provides belt presses and centrifuges that are capable of producing biosolids cake of 25 percent to 35 percent solids. The USFilter line of filter presses can achieve solids levels as high as 45 percent. The J-Vap® system as well as direct and indirect drying systems can dry biosolids in excess of 90 percent solids.

Biosolids Handling. USFilter's biosolids handling capabilities include belt conveyors, shafted and shaftless screw conveyors, and bucket elevators, as well as a wide range of live bottom silos and hoppers for sludge collection and storage. USFilter also provides biosolids mixing and pumping equipment.

Class A Heat Drying Technologies: Case Studies
USFilter heat drying technologies have been installed in several U.S. locations.

Corona, Calif., is the first U.S. city to employ the Convective Thermal Dryer™ (CTD) system, licensed to USFilter by the Italian company Sernagiotto. The system dries biosolids and organic wastes to 95 percent, producing 1 millimeter (mm) to 4 mm, dust-free Class A pellets. The dryer's heat recovery system ensures energy efficiency and low operating costs.

Forest City, N.C., installed a Dragon Dryer® process at its wastewater treatment plant in 1997. Provided by USFilter, the unit employs indirect heat in a unique rotating chamber to convert biosolids cake to a granular byproduct with over 90 percent dry solids. Forest City can safely store 30-days' worth of processed Class A biosolids in one silo for up to four weeks.

Mountain City, Tenn., is using a J-Vap dewatering/drying system from USFilter to produce pathogen-free Class A biosolids. The city replaced the drying beds at its wastewater treatment plant, resulting in reduced labor costs. J-Vap systems remove water from liquid biosolids and dry the material up to 99 percent solids in a single, fully automated process.

This article originally appeared in the 09/01/2004 issue of Environmental Protection.

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