What are biosolids?
Sewage sludge is produced during wastewater treatment. After the wastewater is treated, the remaining solids (sewage sludge) are further treated to produce material called biosolids.
Biosolids can be recycled to agricultural land under regulatory controls and the Biosolids Assurance Scheme (BAS).
From wastewater to biosolids
Around 12 million tonnes of wastewater is treated in the UK every day, generating sewage sludge that must be managed responsibly.
The sewage sludge undergoes controlled treatment processes to reduce pathogens and stabilise the material before recycling to agricultural land as biosolids.
Treatment methods commonly include anaerobic digestion and the addition of lime, with smaller quantities treated through drying. Process controls ensure treatment operates within defined parameters.
Anaerobic digestion also produces biogas, which is used to generate renewable electricity, enough to power around 370,000 homes.
How biosolids are used
Most sewage sludge in Great Britain is recycled to agricultural land. A smaller proportion is used for incineration with energy recovery, industrial processes or land restoration.
Biosolids are usually applied as treated cake. Application rates are based on soil analysis, crop nutrient requirements and regulatory controls.
When applied to agricultural land in line with regulatory requirements and the Biosolids Assurance Scheme (BAS) Standard, biosolids return nutrients and organic matter to the soil.
Nutrients and soil function
Biosolids contain nutrients such as nitrogen and phosphorus, along with organic matter.
Organic matter contributes to soil structure and function, including:
- Water retention
- Increased soil biological activity
- Long-term soil function
Each year in Great Britain, recycling biosolids contributes an estimated £50 million in nutrient value to the farming industry.
Regulation and controls
Recycling biosolids to agricultural land is regulated under environmental and agricultural legislation designed to protect soil quality, water quality and the food chain.
This includes controls on treatment standards, application rates, crop restrictions and record keeping under the Sludge (Use in Agriculture) Regulations, the Environmental Authorisation (Scotland) Regulations and related agricultural regulations.
In addition to meeting statutory requirements, participating organisations can gain certification for activities conforming to the BAS Standard.
The BAS provides a standards-based scheme covering source controls, treatment validation, storage and recycling, with an independent audit.
Organisations must demonstrate that defined standards and control measures are applied throughout the biosolids lifecycle.
Certification demonstrates that specified activities of organisations meet the requirements of the BAS Standard, as verified at the time of audit by a UKAS-accredited Certification Body.
Together, regulation and independent assurance provide structured oversight of biosolids recycling to agricultural land.
Ongoing review and transparency
The wastewater system receives inputs from domestic, commercial and industrial sources. Managing this complexity is supported by regulation, operational controls, independent audit and ongoing research to assess and manage potential risks.
The water sector works with regulators and research organisations to improve understanding of substances in wastewater and biosolids.
Regulatory frameworks and the BAS Standard are subject to periodic review, taking into account emerging evidence and wider regulatory considerations.
The BAS provides a structured mechanism to incorporate new knowledge into operational practice while maintaining defined requirements and independent verification.