In the UK, approximately 12 million tonnes of wastewater is processed each day = 140 tonnes per second (on average).
Further to this a considerable amount of rainfall enters the wastewater network.
Around 53 million tonnes per annum of untreated sludge is output from 8,500 wastewater treatment works servicing small villages to large cities.
The untreated sludge is transported (from the smaller wastewater treatment works) and processed at around 200 Sludge Treatment Centres (usually located at the larger wastewater treatment works).
Sewage Sludge Treatment
The main objective of sludge treatment is to create a product that is safe and acceptable to recycle to agriculture. This includes reducing or eliminating potentially harmful micro-organisms e.g. E. Coli and Salmonella spp. in biosolids and reducing the fermentability of the final product to acceptable levels. Various treatment technologies are employed to produce biosolids; about 73% by anaerobic digestion and 22% by lime stabilisation.
Biosolids can also be treated by thermal drying and composting sometimes with green waste. Process control points and monitoring are used to ensure that continuous treatment processes operate within defined limits and therefore a consistent quality of biosolids output can be expected.
Digestion of sludge in the UK produces significant amounts methane gas which generates enough renewable electricity to supply about 200,000 homes and there is potential for much more.
Currently around 78% of biosolids are recycled to agricultural land, 12% are incinerated, 5% goes for industrial use (e.g. as a fuel for cement production or for soil manufacturing) and 5% are used for land reclamation or restoration.
“Gas from biosolids production in the UK supplies about 200,000 homes with renewable electricity”
Recycling to Agricultural Land
Biosolids are recycled to agricultural land in various forms:
- Digested cake is around 23% dry solids and can be safely and securely stored in field heaps before application and soil incorporation.
- Lime treated cake is about 25% dry solids and contains quick lime or slaked lime and can also be stacked in field heaps before application.
- Digested liquid at only 4% dry solids is normally injected directly into the soil.
- Granules and pellets at 98% dry solids are broadcast on the land.
Around 3.6 million tonnes per annum (or 170,000 truckloads) of biosolids are recycled to agricultural land in the UK. This is applied to about 150,000 hectares per annum, or 1.3% of the UK’s agricultural land.
The financial value to UK agriculture of nutrients in biosolids is around £25 million per annum – mainly as phosphate and nitrogen as well as sulphur, potash and magnesium. There is a strong demand from farmers as it is worth about £170 per hectare in nutrients alone.
Benefits to Soil and the Environment
In addition to valuable nutrients, biosolids also contain organic matter, which further benefits agriculture and the environment as indicated in this table:
Improves root development
Increased drainage capability
Reduced risk of erosion and nutrient run-off
Improves organic material breakdown and release of nutrients
Increased crop yields
Reduced risk of yield loss
Maintain soil structure and nutrient composition
Reduced risk of diffuse pollution
Natural provision of nutrients
Increased crop yields
The UK Government considers biosolids recycling to agricultural land in most circumstances to be the Best Practicable Environmental Option and it is the most sustainable practice aligning with Government Waste Hierarchy as illustrated below.
“Biosolids recycling to land is consistent with the EC Circular Economy”
Here are some documents that provide further information on biosolids: