Biosolids recycling

A cost-effective and environmentally sustainable fertiliser for your farm. 

Our products

You should CONTACT YOUR FARM LIAISON OFFICER to discuss your farm’s needs, the fertilsier produced in your area and the best option for you.

Harnessing biomethane for our sites and the national grid

More than 20 of our sites use anerobic digestion to conventionally treat sewage sludge.

This involves storing the sludge in a container, without oxygen, for a set period and temperature – usually 15 days at 37.5 0C. 

This produces methane gas. We convert this methane into a renewable source of energy and use it to power our sites. 

We put any surplus energy back into the national grid where it’s used by people all over the region. 

Enhanced treated biosolids using a thermal hydrolysis process

At some of our larger sites we optimise sludge treatment thorugh the use of a thermal hydrolysis plant, which cooks our sludge at high temperature and pressure. 

This makes the sludge more biodegradable, results in a higher production of biogas and the final biosolids product is also dryer than those created using conventional digestion.

We’re working to constantly improve this process and bring to more of our sites in the future. 

Lime treated biosolids 

On occasion we blend the sludge with quicklime. This raises the pH value of the sludge to a neutral level.

Clean water sludges

As well as selling just under 600,000 tonnes of biosolid products each year, we also produce around 60,000 tonnes of clean water sludge from our water treatment processes.

Clean water sludges are made up of the soil dragged in from reservoirs and rivers and filtered out during the production of clean drinking water.

The clean water sludge is free and can be used as a soil-conditioner.

Biosolid product prices and cost benefits

There is a fantastic cost benefit of using biosolids in comparison to manufactured fertilisers. 

The actual saving depends on a wide variety of factors, but typical applications will save your business well over £200 per hectare on nitrogen, phosphate, potash and sulphur. 

The organic matter, trace elements and other nutrients supplied with the biosolids adds further value and savings. 

How we calculate prices

Our biosolids are charged on a price per hectare basis, dependent on the distance of your farm from our sludge treatment centres.

And you’re paying for more than just the fertiliser. When you buy from us, you get:

The nutrients in our biosolids fertiliser

Our biosolids are typically spread at a rate of 20 to 25 tons per hectare, limited to the nitrogen content remaining within Nitrate Vulnerable Zone regulations at under 250 kg per hectare. 

Biosolids are a sustainable fertiliser acting as a useful source of organic matter and contain a wide range of key nutrients such as nitrogen, phosphate, magnesium, sulphur and a range of trace elements. 

This combination of nutrients can improve your soil health and benefit both your soils, your crops and your farm business. 

The specific content of nutrients in biosolids varies depending on the sludge treatment centre and its catchment area. 

You should speak to your local Farm Liaison Officer for details of the biosolids produced closet to your farm. However, we’ve listed typical nutrient information in these tables as a guide. 

Typical nitrogen contents of biosolids

Biosolids product Dry matter (%) Total Nitrogen Readily Available Nitrogen
    (kg/t fresh weight)
Digested cake 25 11 1.6 (15%)
Lime stabilised 25 8.5 0.9 (11%)


Biosolids contains up to 250kg of nitrogen per hectare, with about 15% of that present as Readily Available Nitrogen (RAN) for crop offtake. 

This is because the nitrogen is in an organic form and will be locked up in the soil, to be released to the crop over a longer period. 


Typical phosphate contents of biosolids

Biosolids product Dry matter (%) Total Phosphate
    (kg/t fresh weight)
Digested cake 25 11
Lime stabilised 25 7.0


Biosolids act as a fantastic source of phosphate to crops, with 50% of the phosphate available in the first year and the remainder available over the following 3 to 5 years. 

As biosolids are so rich in phosphate, they act as a beneficial replacement to manufactured phosphate fertiliser. 

Phosphate is a finite natural resource, and so biosolids recycling supports the drive to create a circular economy and reduce our carbon footprint.

In the UK biosolids supply an average of 35,000 tonnes of phosphate annually, which benefits the UK agricultural industry by £25 million per year.  


Typical potash, sulphur and magnesium contents of biosolids

Biosolids product Dry matter (%) Total Potash Total Sulphur Total Magnesium
    (kg/t fresh weight)
Digested cake 25 0.6 8.2 1.6
Lime stabilised 25 0.8 7.4 2.4


The small amount of potash that Biosolids contain is typically 90% available for uptake to the following crops. Similarly, magnesium is present in a small volume and acts as a contribution to soil reserves. 

The sulphur in biosolids however plays a much larger benefit to soils health. Sulphur is known to improve the nitrogen uptake in crops and a recent
issue emerging across arable farming is sulphur deficiency, thought to be linked to the reduction of atmospheric sulphur disposition caused by improvements in air quality. 

Therefore, the sulphur in biosolids is a valuable nutrient to the soils. Recent research shows that 10% to 20% of total sulphur in biosolids is available to the next crop following an autumn application, and 35% to the next crop following a spring application. 


Organic matter

As well as providing key nutrients, biosolids provide a superb source of organic matter to improve soil health and benefit both the environment and agriculture. 


Impact Changes Benefits
Improved soil structure

Reduces compaction and increases workability

Improves root development

Increased drainage capability

Less soil work and energy required 

Increased crop yields

Reduced risk of yield loss

Increased water retention capability

Increased drought resistance

Reduced risk of erosion and nutrient run-off

Increased crop yields

 Maintain soil structure and nutrient composition

 Reduced risk of diffuse pollution

Increased life in soil (from microbes to earthworms)

Improves drainage

Improves organic material breakdown and release of nutrients

Natural provision of nutrients

Increased crop yields

Increased carbon sequestration Organic matter locks up carbon in soil Greenhouse gas reduction

Biosolids compliance, quality assurance standards and regulations

On each of our wastewater treatment sites, we follow approved site procedures to be confident all hazards are analysed and critical control points are implemented to consistently deliver a safe, high-quality product. 

We regularly test the biosolids to confirm compliance and have an annual external audit to assess our processes.

Biosolids Assurance Scheme

We’re a fully certified participant in the Biosolids Assurance Scheme – our BAS certification number is NSFBAS/002/1/15.

The Biosolids Assurance Scheme is a not-for-profit company set up by the 11 UK mainland water and sewerage companies. 

The scheme’s purpose is to provide confidence and reassurance to food chain stakeholders – including farmers, retailers and the public – that biosolids can be safely and sustainably recycled for use in agriculture. 

The scheme combines legislative and non-legislative action with best practice standards to make sure the right controls and measures are in place for recycling biosolids. 

The scheme examines and sets standards for:

  • compliance
  • testing
  • treatment
  • delivery
  • storage
  • application of biosolids

To ensure compliance, we’re audited every year by NSF Certification UK Limited, who act as the external certification body.  

The Biosolids Assurance Scheme also works across the industry to investigate potential risks, hazards and concerns related to biosolids. 

The Safe Sludge Matrix

The Safe Sludge Matrix has been created and agreed by Water UK and the British Retail Consortium, representing all wastewater companies, key regulators and the associated major retailers.

The Safe Sludge Matrix outlines different types of sludge treatment and gives guidance on when and on which crop type particular biosolids can be used.


Crop group Untreated sludges Conventionally treated sludges Enhanced treated sludges
Fruit (10 month harvest interval applies)
Salads (30 month harvest interval applies) (10 month harvest interval applies)
Vegetables ✕ (12 month harvest interbal applies) (10 month harvest interval applies)
Horticulture (10 month harvest interval applies)
Combinable and animal feed crops

Grass and forage: Grazed*
(Deep injected or ploughed down only)
Grass and forage: Harvested*
(No grazing in season of application)



✓ - All applications must comply with the Sludge (Use in Agriculture) Regulations and DEFRA Code of Practice for Agricultural Use of Sewage Sludge

✕ - Applications not allowed

* - 3 week no grazing and harvest interval applies


Biosolids Nutrient Management matrix

The Biosolids Nutrient Management Matrix acts as a nutrient management tool in outlining the rules of recycling biosolids to different indices of phosphate in the soil. 


ADAS soil Phosphorous Index Maximum potential application of lime stabilised biosolids (a) Maximum potential application of all other biosolids types
0/1/2 250kg/ha total N in any twelve month period  
3 250 kg/ha total N in any twelve month period – application 1 year in 4 on sandy soils and 1 year in 2 on all other soils 250 kg/ha total N in any twelve month period – application 1 year in 2 on sandy soils (b)
4 250 kg/ha total N in any twelve month period – application 1 year in 5 on sandy soils and 1 year in 3 on all other soils 250 kg/ha total N in any twelve month period – application 1 year in 4 on sandy soils (c) and 1 year in 2 on all other soils
5 and above No application No application


a. Lime addition rate >5% w/w on a dry solids basis.

b. Composted biosolids can be applied annually and.

c. Can be applied 1 year in 2

  • Soil extractable P analysis must be less than 5 years old
  • 0-15cm soil sampling depth on arable land; 0-7.5cm on grassland.
  • Soil types based on Cross Compliance soil categories
  • No biosolids applications directly in front of legumes (e.g. peas, beans), except for composted biosolids which is very low in readily available N.
  • Septic tank sludge is not included within the scope of the Matrix. 

Contact our team

Our teams are always on hand to help. Get in touch with our biosolids team and our Farm Liasion Officers.