Biosolids are solid, semisolid, or liquid wastes produced during the primary, secondary, or advanced treatment of residential sanitary sewage using one or more regulated methods to minimize pathogens and vector attraction (flies, mosquitos, rodents).

Anaerobic digestion, aerobic digestion, and lime stabilization are examples of these processes.

The word “biosolids” is similar to the definition of sewage sludge water Resources Protection of the Natural Resources and Environmental Protection Act, 1994 PA 451, as amended; however, biosolids are only that fraction of wastewater that has been adequately treated to allow the application to land.

Industrial sludge or seepage are not regulated as biosolids under Michigan Administrative Code Rule 323.2405.

What is the Problem with biosolids?

Aside from the potential for pathogenic and other contaminants present in biosolids, odor is another issue.

Biosolids may have a different odor depending on the treatment procedure and technologies utilized.

Ammonia, amines and reduced sulfur-containing compounds are the most often created and observed odorous chemicals.

Wind speed and direction, relative humidity, and temperature are all factors that might influence unpleasant smells.

The existence of smells from biosolids does not imply that they are harmful to health and the environment.

Technologies for biosolids processing

Water removal from biosolids is critical for producing a solid product that may be used to make fertilizer or biofuel.

De-watering is the initial step in the process that eliminates the liquid sludge from the wastewater and changes it to a solid product.

Biosolids drying

Direct dryers employ hot air or gas in direct contact with the product to raise the temperature of the wet solid, allowing evaporation to dry the product.

Direct contact dryers include rotary drums and belt dryers.

A heat source, such as a furnace, generates the exhaust gas, which is fed directly into the drum and heats the product.

The dewatered solid flows along a slow-moving belt, exposed to hot air created by the furnace, drying the product as it goes along the belt in the case of a belt dryer.

Solar drying is an alternative biosolid drying technology that employs solar energy and a greenhouse enclosure to heat and dry the wet solid.

The enclosed construction prevents rain from adding moisture back into the solid, although cloud cover, humidity, temperature and wind speed can all alter and delay the drying process.

Biosolids cooling

The temperature must be controlled to prevent decomposition, burning, and auto igniting when storing and packaging biosolids.

Indirect heat exchangers are a new device that is useful in cooling it to a thermally stable end temperature.

The cooling agent is cooled by conduction rather than direct contact with the biosolids in this method.

Why it should be treated?

The wastewater treatment residuals (sewage sludge) must be treated to minimize pathogens and attraction to vectors to fulfill the regulatory standards (40 CFR Part 503, outlined below) for land application.

Aerobic digestion, anaerobic digestion, composting, alkaline stabilization and thermal drying are some of the treatments available.

They can also be used to reduce odor and neutralize heavy metals.

The Name Change Task Force at the Water Environment Federation (WEF), the water and wastewater industry’s main trade and lobbying organization in the United States, coined the term “biosolids” in 1991 to identify treated sewage sludge from untreated wastewater sludge and make land application of treated sludge more public-friendly.

Where can biosolids be used?

Biosolids as a soil conditioner

Biosolids are nutrient-dense organic materials with more than 50% organic matter content.

It can thus be used as a soil conditioner to enhance the physical, chemical and biological aspects of soils, particularly those that have been degraded or disturbed.

Organic elements are the principal binding agents for aggregate formation and stability and function as a food source for bacteria.

Optimal soil structure, in turn, enhances many other essential physical and chemical aspects of the soil, including bulk density, porosity, water and cation exchange capacity, aeration and drainage, microbial communities, and soil fauna, resulting in disease control and lessening soil erosion.

In the near term, biosolids’ direct diluting impact, extremely high organic matter content and influence on aggregate formation and stability may all contribute to lower bulk density and increased water holding capacity.

Long-term application of it in the soil can modify particle size distribution by increasing the volume of macropores or micropores, depending on the soil texture.

Because of the increased pore capacity, biosolids-treated soil was much less susceptible to compaction than untreated soil.

Biosolids in agriculture

Biosolids must be applied to land at the proper agronomic rate, which is the sludge application rate that provides the quantity of nitrogen required by the crop or vegetation cultivated on the land.

Crop type, geographic location, and soil variables all influence agronomic rates.

A qualified expert should be consulted for developing the agronomic rate.

Sites of reclamation

Biosolids have also been effectively used to grow continuous vegetation, lower the bioavailability of hazardous chemicals often present in soils, prevent soil erosion and regenerate soil layers in disturbed locations.

For restoring places with little or no topsoil, soil regeneration is critical.

Generation of biogas

Anaerobic digestion (AD) is frequently used for biomass stabilization and energy production since roughly 50% of the carbon entering the anaerobic digester is recovered as methane (CH4).

The biogas produced during the digestion process can be used as a source of biogenic energy for both process heating and electricity generation.

Recognizing Pollutants in biosolids

The EPA’s Biosolids Program’s major goal is to determine the possible danger of contaminants contained in biosolids.

EPA uses swage sludge surveys to identify contaminants in biosolids and assess their potential damage to public health and the environment.

Since the promulgation of 40 CFR Part 503 in 1993, when EPA began documenting their prevalence, more than 500 contaminants have been discovered in biosolids.

Not every wastewater treatment facility will have all of the approximately 500 contaminants discovered in biosolids.

Pollutants identified in biosolids will change over time, based on the inputs to specific wastewater Treatment plants.

The existence of a contaminant does not always imply that they are harmful to human health and the environment.

Pollutants such as Microplastics (MPs), Nano plastics (NPS), synthetics, heavy metals, medicines and designed nanoparticles have all been found in processed waste-water sludge (biosolids) that have been recycled in agricultural areas.

Classification of biosolids

Depending on treatment procedures, biosolids are classified as “Class A” or “Class B.”

Treatment standards for pollutants, infections, and vector attraction reduction, as well as general requirements and management techniques, vary by class.

Class A

Is certification for dewatered and heated sewage sludge that complies with all U.S. EPA criteria for land application.

As a result, class A can be lawfully utilized as fertilizer on farms and vegetable gardens, as well as marketed as compost or fertilizer to home gardeners.

Class A Biosolids are extremely similar to Class B, but with tougher pathogen and “vector attraction” limitations (i.e., class A biosolids must not attract disease-carrying insects or rodents, etc.).

The Environmental Protection Agency asserts that Class A biosolids must “contain no detectable amounts of pathogens, however, the only pathogens that are assessed and controlled by the legislation are faucal coliform and salmonella.

Class B

Is a classification for treated sewage sludge that fulfills US EPA requirements for use as fertilizer on land.

Unlike Class A, Class B biosolids are permitted to include detectable pathogens.

All other requirements for the two kinds of pollutants are the same.

Although the United States Environmental Protection Agency (EPA) guarantees, the safety regulations for Class B are insufficient to ensure safety.

References

[1] Basic Information about Biosolids (online) is available at: https://www.epa.gov/biosolids/basic-information-about-biosolids

[2] Biosolids definition (online) available at:https://www.michigan.gov/egle/about/organization/water-resources/biosolids/definition

[3] Biosolids Soil Application: Agronomic and Environmental Implications 2012 (1 Aug) (online) available at: https://www.hindawi.com/journals/aess/2012/201462/

[4] Drying and processing biosolids (online) available at: https://www.solexthermal.com/resources/articles/drying-and-processing-biosolids/