Ben Brading 5 min read

How sewage is used to power Britain

Britain has over 7,000 sewage treatment works, all producing sewage sludge, a solid organic by-product of wastewater treatment.

In the past, sewage sludge was routinely disposed of in British landfills, where it decomposed and produced harmful greenhouse gases.

Today, sewage sludge is recognised as a valuable biomass source that can generate green biogas, renewable electricity, and organic fertiliser.

This guide explains how sewage sludge helps offset some of the harmful environmental impacts of the water industry.

Can sewage be used to generate energy?

Yes. Sewage is a valuable source of renewable energy.

Organic solids removed during sewage treatment are rich in biochemical energy, which can be captured as biogas and used to generate electricity or to produce green gas for the grid.

In Britain, this already happens at scale. It is common for regional water companies to process sewage into energy at their larger treatment works, and to transport the sewage sludge they collect at smaller sites to a dedicated centre for conversion to energy.

What is sewage sludge?

Sewage sludge is the solid organic material left over after wastewater is treated.

When sewage from homes and businesses arrives at a treatment works, the process separates it into two streams:

Treated water: Water that can be safely returned to the water cycle.
Sewage sludge: The remaining solids that settle out during the treatment process.

For more detail about the sewage treatment process, visit our complete guide to how sewage systems and treatment works operate.

Sewage sludge is made up of the organic matter, nutrients and microorganisms removed during the screening, settlement and aeration stages of treatment.

It is rich in biochemical energy, which can be captured as biogas through anaerobic digestion. It also contains nitrogen, phosphorus and potassium, which make the treated end product, called digestate, a useful agricultural fertiliser.

What is a sewage digester?

A sewage digester is an anaerobic digester system specifically designed to convert sewage sludge into biogas.

Some sewage treatment works have a digester on site, while some water companies transport sludge to dedicated industrial-scale anaerobic digestion facilities.

Here is an overview of how a sewage digester converts sewage sludge into biogas:

Feeding the digester: Sewage sludge is pumped into a sealed, airtight anaerobic digester tank.
Heating and mixing: The digester is heated and stirred to provide optimal conditions for microorganism growth.
Organic breakdown: The microorganisms convert organic matter into simpler molecules, producing biogas as a by-product.
Biogas collection: Methane-rich biogas is captured and extracted from the top of the tank.
Digestate collection: The remaining sludge is nutrient-rich and safe to use as an agricultural fertiliser, as the microbes will also have killed all pathogens.

The next two sections explain how the biogas produced by the sewage digester can be used to produce energy.

How biogas from sewage becomes electricity

Biogas can be burned directly in an on-site Combined Heat and Power (CHP) unit to generate electricity and to provide a source of heat for the sewage digester.

A CHP unit is a large internal combustion engine that works in a similar way to a car engine, except that instead of consuming petrol it consumes biogas. The mechanical energy produced by the engine is used to generate electricity by means of a generator.

Here is a step-by-step explanation of how this works:

Gas conditioning: The biogas produced by the sewage digester is first conditioned to remove corrosive components, including hydrogen sulphide and siloxanes, which can damage a CHP unit.
Gas is combusted within the CHP: The cleaned biogas is fed into the cylinders of the CHP unit. The gas is combusted, which releases chemical energy to drive the pistons, which in turn rotate the crankshaft.
The spinning crankshaft generates electricity: The crankshaft is connected to a generator. As it rotates, it spins a magnet within a coil of wires, converting the mechanical energy into electricity.
Recycling of heat: CHP units produce large quantities of waste heat. A heat exchanger captures this heat, which is then used by the sewage digester to maintain the optimal temperature for microorganism growth.

The CHP unit is connected to the local Distribution Network Operator grid via a business electricity connection. This allows the unit to export the electricity it generates.

The water company sells the electricity on the wholesale market using a Corporate PPA or the Smart Export Guarantee Scheme.

💡 Combined Heat and Power units at sewage treatment works generate REGO certificates, the system of certification Ofgem uses to guarantee renewable energy generation.

How sewage becomes biomethane for the gas grid

An alternative way to use the biogas produced by the sewage digester is to purify it to a standard at which it can be injected into the gas grid.

The biogas produced by the sewage digester is typically 60% methane and 40% carbon dioxide. It can be converted into a form acceptable to the gas grid by:

Stripping carbon dioxide: Carbon dioxide must be removed so that the gas is at least 98% methane. Modern systems typically use membrane separation, in which a specialist material allows CO2 molecules to pass through while holding back the methane.
Propane dosing: Natural gas on the grid contains small amounts of heavier hydrocarbons such as propane. A small amount of propane is dosed into the biogas to match the composition of natural gas.
Adding an odorant: Methane is naturally odourless. A sulphur compound called mercaptan is added to give the gas its distinctive smell. This improves the safety of the gas grid.

The treated biomethane is compressed to match the pressure of the local gas distribution network, metered, and then injected into the grid using a business gas connection.

Biomethane production is supported by the Green Gas Support Scheme, which incentivises the injection of renewable gas into the national grid.

How much energy can sewage generate?

The latest renewable energy data published by the UK government shows that in 2024, 988 GWh of grid electricity was generated from sewage gas. This equates to 0.4% of overall grid electricity generation.

The contribution of sewage gas is significantly smaller than that of other sources of low carbon energy, such as wind farms and nuclear power plants.

What are the benefits of turning sewage into energy

This section explains both the environmental and economic benefits of converting sewage into energy.

Renewable and low-carbon energy source

In Britain, the gas and electricity grids are heavily reliant on the fossil fuel natural gas.

In contrast, the electricity or biomethane produced from sewage is renewable, as it relies on a naturally replenishing organic waste source.

The process also has a positive impact on greenhouse gas emissions. If sewage sludge is not processed in a sewage digester, it will rot, releasing methane directly into the atmosphere.

Methane is a far more harmful greenhouse gas than the carbon dioxide produced when biogas is combusted.

Reduced customer water rates

In England and Wales, the profits of regional water companies are tightly controlled by the regulator Ofwat.

The electricity and gas produced using sewage sludge provide these local water companies with a source of revenue that offsets the cost of operating sewage works.

Through the regulated charging process, this means that the rates paid by water customers are reduced.

Businesses in Britain pay the local water company indirectly via a business water supplier. However, the pass-through of wholesale costs from the local water company means that they too benefit from cheaper business water rates, thanks to the generation of renewable energy from sewage.

Using sewage as biomass fuel

Sewage sludge can be converted into valuable biomass fuel at central facilities, such as Daldowie in Scotland, which processes 35% of all sewage sludge generated by Scottish Water.

A central facility uses centrifuges (machines that separate liquids and solids by spinning them rapidly) to remove any remaining water from the sewage sludge.

The remaining solid sludge is heated and dried into biomass pellets, a concentrated source of energy.

Biomass pellets are used at power plants, such as the Drax Power Station, where they are incinerated to generate renewable electricity.

Using sewage as a fertiliser

Digestate is a valuable by-product of the sludge-to-energy process. It contains key nutrients needed by plants and crops, specifically:

Nitrogen (N): Promotes leafy growth.
Phosphorus (P): Essential for root development and flowering.
Potassium (K): Improves overall plant health and resistance to disease.
Micronutrients: Includes trace elements like zinc, copper, and magnesium.
Organic matter: Improves soil structure, water retention, and microbial activity.

Before it can be safely used in agriculture, the pathogens present in sewage sludge must be neutralised, either through:

Lime stabilisation – Adding chemicals to the sludge to make it less acidic, killing off pathogens and reducing odours.
Thermal drying – Drying the sludge at high temperatures to kill pathogens and reduce moisture content, converting the sludge into slow-release fertiliser granules.

The treated sludge is then applied to farmland as a cheaper alternative to synthetic fertilisers, helping to encourage crop growth.

💡 The runoff of fertilisers into British rivers poses a significant threat to the health of British waterways.