This guide offers a comprehensive introduction to biosolids, covering their origin, treatment processes and main types or classifications. It also outlines common endpoints, including land application, thermal treatment and landfilling. Finally, it highlights how biosolids can support nutrient recycling, soil improvement and energy recovery, while offering insight into the importance of long-term sludge planning.
Biosolids are treated wastewater solids, or sewage sludge, that have undergone processes to reduce pathogens, odours and other concerns so they can be managed, recycled or disposed of in accordance with regulations.
Biosolids originate from wastewater treatment plants. In simple terms, the process follows five main steps:
The words “sludge” and “biosolids” are sometimes used interchangeably. In technical and regulatory settings, however, the distinction can matter.
Sewage sludge is the broader term for the solid, semi-solid or liquid residue generated during wastewater treatment. Depending on the context, it may refer to untreated, partially treated or treated wastewater solids.
Biosolids refers to sewage sludge or wastewater solids that have been treated and managed to meet applicable requirements for handling, use or disposal. In the United States, the term is closely associated with treated sewage sludge that meets the requirements of the Environmental Protection Agency (EPA) biosolids rule, particularly when used on land as a soil conditioner or fertiliser.
Terminology also varies by location. Some countries and regulatory systems use the term “biosolids” widely; others continue to use “sewage sludge”, “treated sludge”, “bioresources” or other local terms.
Sewage sludge, or wastewater solids, must be treated or “stabilised” before it can be safely managed, recycled or disposed of.
Stabilisation means reducing the biological activity of sludge so that it is less likely to produce strong odours, attract pests or continue to decompose rapidly. Depending on the process used, stabilisation can also reduce pathogens, lower sludge volume, improve handling characteristics and support beneficial use.
Wastewater treatment plants rarely rely on one process alone. In many cases, stabilisation is combined with dewatering, drying, composting or another treatment step to produce a material that meets local regulatory requirements and is suitable for its intended outlet.
The most common approaches include conventional anaerobic digestion, aerobic digestion, lime stabilisation, composting and thermal drying. More advanced plants may also use pretreatment technologies, such as thermal hydrolysis, or other advanced digestion processes to improve performance, reduce treatment time and produce higher-quality biosolids.
|
Technology |
Basic principle |
Typical benefits |
Main drawbacks |
Pathogen reduction |
|
Conventional anaerobic digestion |
Microorganisms break down organic matter without oxygen, producing biogas. |
Produces renewable energy, reduces volatile solids and odour potential, widely proven. |
Requires large tanks, long retention times and careful process control. |
Good |
|
Aerobic digestion |
Microorganisms stabilise sludge in the presence of oxygen. |
Relatively simple to operate and suitable for smaller plants. |
High energy demand for aeration and typically less energy recovery than anaerobic digestion. |
Moderate |
|
Lime stabilisation |
Lime raises pH and, in some cases, temperature to inhibit biological activity and reduce pathogens. |
Fast, relatively low capital cost and effective for odour and pathogen control. |
Ongoing chemical cost, increased solids mass, dust, and handling considerations. |
High |
|
Composting |
Biosolids are mixed with bulking agents and treated under warm, aerobic conditions. |
Produces a soil-like material with good public acceptance and useful organic matter. |
Requires space, bulking material, odour control and vector management. |
High |
|
Thermal drying |
Heat evaporates water from dewatered biosolids. |
Major volume reduction, improved storage and transport, and potential for a more consistent product. |
Energy-intensive and requires careful management of dust, odours, and fire risk. |
Very high |
|
Advanced anaerobic digestion
|
Sludge is pre-treated or digested under enhanced conditions to improve breakdown before or during anaerobic digestion. Examples include thermal hydrolysis, thermophilic digestion, two-stage digestion, mechanical disintegration and ultrasonic pre-treatment. |
Benefits vary by technology, but may include higher biogas production, greater solids reduction, improved dewaterability, reduced odour potential and the potential to produce higher-quality biosolids. |
Drawbacks vary by technology, but may include higher capital cost, more complex operation, additional energy or chemical use, and greater need for process control. |
High to very high, depending on the technology and operating conditions. |
After stabilisation and further treatment, biosolids can be removed from a wastewater treatment plant in several physical forms. The final form depends on the treatment process, site infrastructure, regulatory requirements and intended end use.
Liquid biosolids have high water content and leave the treatment facility without mechanical dewatering or drying. They may be suitable where approved land application sites are nearby, but they require careful planning for storage, weather conditions, hauling, odour control, nutrient management and public communication.
Dewatered biosolids cake is a damp, semi-solid material and is often one of the most practical forms for off-site transport. It is usually produced by first conditioning the sludge with polymers or other aids, then using equipment such as centrifuges, belt filter presses or screw presses to separate water from solids.
Sludge dewatering reduces the amount of water being hauled, which can lower transport volumes, costs and handling challenges. However, cake still contains significant moisture, so dewatering performance can have a significant impact on operating costs, the carbon footprint and overall flexibility.
Dried biosolids have much lower moisture content, making them easier to store, transport, blend and apply. They are typically produced by passing dewatered cake through thermal dryers, which use heat to evaporate additional water.
Some drying systems also form the material into granules or pellets, although not all dried biosolids are pelletised. Sludge drying can improve product consistency and marketability, but it also adds energy demand, process control requirements and quality assurance needs.
Composted biosolids are produced by blending biosolids with bulking materials such as wood chips, green waste or similar organic materials, then managing the mixture under aerobic conditions.
Composting creates a more stable, soil-like product that may support landscaping, land restoration or agricultural use where regulations allow. However, it requires sufficient space, odour control, feedstock management and ongoing monitoring.
Did you know?
Biosolids are often described in terms of dry solids (DS). This refers to the percentage of material that remains after the water has been removed.
For example, if a biosolids cake is 25% DS, that means it contains approximately 25% dry matter and 75% water. This matters because water content affects almost every aspect of biosolids management, including transport costs, storage needs, treatment capacity, energy use and the choice of final outlet.
Biosolids quality needs to be assessed because treated wastewater solids have various final endpoints, from agricultural land application and landscaping to land restoration, incineration or landfill disposal. Each route carries different requirements for public health, environmental protection, nutrient management, odour control and public confidence.
Quality assessment helps determine whether biosolids have been treated sufficiently, whether they meet regulatory limits, and where they can be used safely and responsibly.
Because regulations differ by country, biosolids classification systems are not identical worldwide. However, most frameworks ask the same practical question: has the sludge been treated, tested and managed to a standard that makes it suitable for its intended route?
In the United States, the most widely recognised biosolids categories are Class A and Class B, defined under the US Environmental Protection Agency’s biosolids rule. These terms are sometimes used more loosely in other countries to distinguish between higher-quality and lower-quality biosolids, although the regulatory meaning may differ.
Class A biosolids are treated to reduce disease-causing microorganisms, known as pathogens, to very low levels. In the United States, the EPA’s Class A requirements focus on reducing organisms such as faecal coliform bacteria, Salmonella, viruses and parasite eggs, depending on the treatment method used.
To reach Class A quality, biosolids usually undergo more advanced treatment. This may include composting, thermal drying, high-temperature treatment, alkaline treatment with lime, or other approved processes that achieve a similar level of pathogen reduction.
Because they are treated to a higher standard, Class A biosolids can often be used in more ways than less-treated biosolids, including some landscaping, soil improvement and agricultural applications.
Class B biosolids have also been treated, but to a lower pathogen reduction standard than Class A. In the US, Class B material may be used in land application, but additional controls apply. These can include crop harvesting restrictions, grazing restrictions and limits on public access after application.
Other countries use different terminology. In the UK, for example, biosolids used in agriculture are commonly described under the Biosolids Assurance Scheme and Safe Sludge Matrix as either conventionally treated or enhanced treated.
Did you know?
Biosolids do not all smell the same. Fresh or poorly stabilised sludge can have strong odours because it still contains readily degradable organic matter and sulphur- or nitrogen-containing compounds produced during decomposition. Well-treated biosolids are usually much less offensive, although they may still have an earthy, musty or ammonia-like smell depending on the treatment process.
Odour is more than a nuisance issue. It can affect public acceptance, transport options, storage requirements and the suitability of biosolids for land application, landscaping or other beneficial uses. For this reason, lower-odour biosolids are often associated with better stabilisation, improved process control and higher product quality.
Biosolids quality is assessed using a range of parameters, which may vary according to applicable national, regional or local requirements. A typical assessment includes the following elements.
Once sewage sludge has been stabilised, dewatered, dried or otherwise treated, it must be sent to an approved final outlet. The right outlet depends on biosolids quality, local regulation, contaminant levels, available infrastructure, transport distance, public acceptance and whether the material can be safely reused.
Land application is one of the most common beneficial-use routes for treated biosolids. When quality requirements are met, biosolids can be applied to land as a source of organic matter and nutrients, particularly nitrogen, phosphorus and micronutrients. This can reduce reliance on mineral fertilisers and help improve soil structure.
Typical land application outlets include:
Where land application is restricted, unsuitable or not publicly accepted, biosolids may be sent to thermal treatment. These routes reduce the volume of material requiring final management and can, in some cases, support energy or nutrient recovery.
Incineration is the most established and widely used thermal route for sewage sludge. In mono-incineration, sewage sludge is combusted alone, usually after dewatering and often after drying. This greatly reduces volume, destroys pathogens and many organic contaminants, and produces a mineral ash stream. Because ash is derived solely from sludge, it may be more suitable for phosphorus recovery than mixed ash, provided contaminant levels, technology and market conditions allow.
In co-incineration, sludge is combusted with other fuels or waste streams, such as municipal solid waste, coal, biomass or industrial residues. Some utilities also use dried biosolids as a supplementary fuel or raw material in cement kilns, where the organic fraction contributes energy and the mineral fraction may be incorporated into the clinker or cement product. Co-incineration can leverage existing thermal infrastructure, but the resulting ash or mineral residue is usually mixed with other materials, which can limit resource recovery options.
Pyrolysis and gasification are emerging thermal options, but unlike incineration, they are not yet commercially widespread. Pyrolysis heats biosolids with little or no oxygen to produce biochar, gas and oil fractions, while gasification uses limited oxygen or steam to generate a combustible gas. Both may support energy or resource recovery, but their use depends on regulation, product quality, contaminant management and market acceptance.
Wet air oxidation is another thermal treatment route, but it differs from incineration because it treats wet sludge under elevated temperature and pressure, using oxygen to oxidise organic material in the liquid phase. This can reduce organic solids and improve sludge manageability without first requiring full drying. However, it requires specialised equipment and careful management of energy use, process conditions and residual streams.
Thermal routes can be attractive where sludge quality limits land use, transport distances are high, or policy is moving towards contaminant destruction and resource recovery. However, they usually require significant capital investment, energy management, emissions control and a reliable outlet for ash or residual products.
Landfill disposal remains an outlet in some regions, especially where reuse or thermal treatment capacity is limited. Biosolids may be disposed of in a conventional municipal solid waste landfill or in a dedicated sewage-sludge-only landfill, often called a monofill.
Landfilling is often seen as a lower-value endpoint because nutrients and organic matter are not returned to productive use. It can also create long-term liabilities, including leachate, methane generation, odour and space constraints. However, controlled landfill or monofill disposal may still be used when biosolids do not meet land application standards, when other outlets are unavailable, or as a contingency route during market disruptions, weather constraints or treatment plant outages.
In some cases, storage is not a final outlet but a temporary step before land application, thermal treatment or disposal. Long-term storage lagoons, stockpiles or dedicated containment sites must still be managed to control odour, runoff, vector attraction and environmental risk.
Biosolids are more than a residual material to be managed at the end of wastewater treatment. When properly treated and controlled, they can become part of a wider resource recovery strategy, helping communities return nutrients to soils, improve soil condition, recover energy and reduce reliance on linear disposal routes.
Realising this value starts with long-term sludge planning: understanding current solids production, future capacity needs, regulatory requirements, outlet options, energy opportunities, costs and community expectations. A clear sludge strategy helps utilities make informed decisions about treatment, storage, transport and final use, rather than reacting to constraints as they arise.
Depending on local goals and infrastructure, this strategy may include digestion, dewatering, drying, composting, advanced treatment or combinations of these approaches. In some facilities, technologies such as thermal hydrolysis and anaerobic digestion can support broader resource recovery by improving treatment performance, energy recovery and the consistency of the final bioresource.
For municipalities and utilities, the future of biosolids management lies in planning systems that are safe, compliant, cost-effective and adaptable. With the right strategy, biosolids can move from being a disposal challenge to a managed resource with practical environmental and operational value.