XJY Transforming Sewage Sludge into Biosolids: A Journey Through Wastewater Treatment and Beyond

Introduction of the XJY  sludge treatment

Before 1950, most communities in the United States discharged their wastewater, or sewage, into streams and rivers with little if any treatment. As urban populations increased, the natural ability of streams and rivers to handle the wastewater was overwhelmed and caused water quality to deteriorate in many regions. In response to concerns about water quality degradation, thousands of communities throughout the United States constructed wastewater treatment systems during the 1950s and 1960s. This resulted in greatly improved stream and river water quality, but created another material to deal with: sewage sludge. Approximately 99% of the wastewater stream that enters a treatment plant is discharged as rejuvenated water. The remainder is a dilute suspension of solids that has been captured by the treatment process. These wastewater treatment solids are commonly referred to as sewage sludge.

"Sewage sludge" or "biosolids"--what's in a name?

The term "biosolids" recently has been introduced by the wastewater treatment industry. The industry defines biosolids as sewage sludge that has undergone sufficient treatment for stabilization and pathogen reduction, and that is of sufficiently high quality to be land applied. The term is intended to distinguish high-quality, treated sewage sludge from raw sewage sludge and from sewage sludge that contains large quantities of environmental pollutants. The term "biosolids" also helps to distinguish sewage sludge from industrial sludge by emphasizing that the former is produced by a biological process. The term has been criticized by some as an attempt to disguise the real nature of sewage sludge, thereby making land application of this material less objectionable to the general public. Although "biosolids" undoubtedly does not conjure up the same negative images as does "sewage sludge" or simply "sludge," it is a legitimate and functional term when correctly used to make the distinction described above. In this document, "sewage sludge" will be used to refer to wastewater treatment solids generally, and "biosolids" will be used to refer specifically to material that is suitable for land application.

picture  6 Sewage sludge

Production of Municipal XJY Sewage Sludge

Municipal wastewater, or sewage, refers to water that has been used in urban and suburban area homes or businesses for washing, bathing, and flushing toilets. Municipal wastewater also may include water from industrial sources. To remove chemicals or pollutants resulting from industrial processes, industrial contributors to municipal wastewater systems must pretreat their wastewater before it is discharged into the sewerage system. The wastewater is conveyed via the sanitary sewerage system to a centralized wastewater treatment plant (sometimes called a Publicly Owned Treatment Works, or POTW). At the POTW, the sewage passes through a series of treatment steps that use physical, biological, and chemical processes to remove nutrients and solids, break down organic materials, and destroy pathogens (disease-causing organisms) in the water. The rejuvenated water is released to streams and rivers, or may be sprayed over large areas of land.

picture  7 Municipal sewage sludge

Preliminary treatment of raw sewage involves screening to remove large objects such as sticks, bottles, paper, and rags, and a grit removal stage during which inorganic solids (sand, grit, cinders) rapidly settle out of the water. The screenings and grit removed in this stage of treatment typically are landfilled and do not become part of the sewage sludge.

Primary treatment involves gravity sedimentation and flotation processes that remove approximately half of the solid material that enters this stage. Solid material (both organic and inorganic) that settles out during this stage of treatment is drawn from the bottom and constitutes the primary sludge. In most POTWs, the floating material (oil, grease, wood, and vegetable matter) that is skimmed from the water surface during primary treatment is disposed of separately and does not become part of the primary sludge.

Secondary treatment is a carefully controlled and accelerated biological process in which naturally occurring microorganisms are used to degrade (break down or digest) suspended and dissolved organic material in the wastewater. This material is converted into carbon dioxide that is released to the atmosphere and into microbial cell mass.

In secondary sedimentation basins, the microbial cell mass settles to the bottom and is removed. This mainly organic material is called secondary sludge.

Some treatment plants also include tertiary treatment steps designed to further reduce plant nutrients (nitrogen and phosphorus), suspended solids, or biological oxygen demand in the wastewater. Chemically precipitated phosphorus and filtration produce a tertiary sludge.

Finally, the water undergoes disinfection treatment to destroy pathogenic microorganisms. The rejuvenated water is then released to a stream or river or may be sprayed over large areas of land.

XJY Treatment methods for municipal sewage sludge

Primary, secondary, and tertiary sludges normally are combined, and the resulting mixture, which contains from 1 to 4% solids, is called "raw" sewage sludge. Because of its pathogen content and its unstable, decomposable nature, raw sewage sludge is a potential health and environmental hazard; however, several treatment processes now are used to stabilize sewage sludge, decrease its pathogen content, and increase its solids content. Some of the more commonly used processes for stabilizing and reducing pathogen levels in sewage sludge are listed and briefly described in Table 1.

What is in sewage sludge?

Sewage sludge is composed of both inorganic and organic materials, large concentrations of some plant nutrients, much smaller concentrations of numerous trace elements¹ and organic chemicals, and some pathogens. The compositions of sewage sludges vary considerably depending on the wastewater composition and the treatment processes used. Table 2 gives median and 95th percentile concentrations of plant nutrients and some of the trace elements found in sewage sludge. These data are from an extensive survey of sewage sludges produced in Pennsylvania during 1996 and 1997.

Options for Dealing with Sewage Sludge

Sewage sludge can be viewed either as an organic and nutrient resource to be used beneficially or as a waste material to be disposed of. Before 1991, large amounts of sewage sludge, including some from Pennsylvania, were disposed of by ocean dumping. Concerns about excess nutrient loading of ocean waters led to the banning of this practice. At present, almost all sewage sludge produced in Pennsylvania has been treated and is of sufficiently high quality to be classified as biosolids. Somewhat less than half of this material is disposed of by landfilling or incineration, while the remaining biosolids are recycled to the soil by use in agriculture, mine reclamation, landscaping, or horticulture. Each of these options has economic and environmental benefits, problems, and risks associated with it.

Landfill disposal

From a management and materials handling perspective, landfilling is perhaps the simplest solution. From an economic standpoint, landfilling presently compares favorably with other options. This undoubtedly will change, however, as landfill space becomes more limited and tipping fees (waste-dumping costs) increase. From an environmental standpoint, landfilling prevents the release of any sludge-borne pollutants or pathogens by concentrating the sludge into a single location. If the landfill is properly constructed and maintained, environmental risks are minimal.

There are, however, risks associated with landfill disposal of sewage sludge. Organic wastes undergo anaerobic decomposition in landfills, producing methane gas that could be released to the atmosphere. Methane is a greenhouse gas that has been implicated in global warming. Other gasses released from landfills can cause unpleasant odors. The large quantities of nutrients that sewage sludge adds to a landfill pose a risk to the local environment. Should a failure of the landfill liner or leachate collection system occur, these nutrients could contaminate local groundwater and surface water. Landfilling sewage sludge also takes up valuable landfill space and forfeits the potential benefits of the organic matter and plant nutrients in the sludge.

picture  8 Landfill disposal

Incineration disposal

Sewage sludge incineration reduces the volume of the material to be disposed of, completely destroys pathogens, decomposes most organic chemicals, and recovers the small amount of heat value contained in sewage sludge. The residual ash is a stable, relatively inert, inorganic material that has just 10 to 20% of the original sludge's volume. Most trace metals in the sewage sludge become concentrated in the ash (a five- to tenfold increase in concentration). This material most commonly is landfilled, although it potentially could be used in construction materials.

Incineration also releases carbon dioxide (another greenhouse gas) and possibly other volatile pollutants (cadmium, mercury, lead, dioxins) into the atmosphere. Incinerator operation requires sophisticated systems to remove fine particulate matter (fly ash) and volatile pollutants from stack gasses. This makes incineration one of the more expensive options for sewage sludge disposal. As with landfilling, the potential benefits from organic matter and plant nutrients in sewage sludge are lost.

picture  9 Incineration disposal