What is a Combined heat and power (CHP)

Energy efficiency and sustainability are central to modern engineering practice, particularly in water and wastewater management. One of the most effective technologies that supports these goals is Combined Heat and Power, often abbreviated as CHP. Also known as cogeneration, CHP is a process that produces both electricity and useful heat from a single fuel source. In the context of wastewater treatment, the fuel is frequently biogas generated from anaerobic digestion of sewage sludge.

By using a CHP unit, treatment plants can transform what was once considered a waste by-product into a valuable energy resource. This not only reduces operating costs but also contributes to national and global goals for carbon reduction and renewable energy. CHP has become an integral part of sustainable wastewater treatment in the United Kingdom and around the world.

What is Combined Heat and Power

Combined Heat and Power is a system designed to generate electricity while simultaneously capturing and utilising the heat produced in the process. Conventional power stations waste large amounts of heat energy during electricity generation, often losing it through cooling towers or exhaust gases. CHP systems capture that heat and put it to use for space heating, hot water, or industrial processes.

In wastewater treatment plants, CHP units are typically powered by biogas. Biogas is produced during anaerobic digestion, a process in which microorganisms break down organic material in the absence of oxygen, generating a gas rich in methane. Rather than flaring this gas or allowing it to escape into the atmosphere, CHP units combust it in engines or turbines to produce renewable electricity and heat.

Historical background

The principle of cogeneration has been known for more than a century. In the late nineteenth and early twentieth centuries, many industrial sites generated their own electricity and reused the waste heat for factory processes. With the rise of centralised power generation in the mid-twentieth century, this practice declined.

The modern resurgence of CHP is linked to environmental concerns and the need for energy efficiency. Wastewater treatment plants, in particular, recognised the opportunity to harness biogas, which had historically been treated as a disposal problem. By the late twentieth century, CHP units were being widely installed at treatment works across Europe, North America, and Asia. Today, CHP is regarded as a best practice for sustainable wastewater management.

How CHP works

The operation of a CHP unit follows several key stages:

  1. Biogas production: Organic matter in sewage sludge is digested anaerobically to produce biogas, typically containing 55–65% methane.

  2. Gas treatment: Biogas is cleaned to remove moisture, hydrogen sulphide, and siloxanes, which could damage engines.

  3. Combustion: The treated biogas is burned in a gas engine or turbine.

  4. Electricity generation: The combustion drives a generator to produce electricity, which can be used on-site or exported to the grid.

  5. Heat recovery: Waste heat from the engine exhaust and cooling system is captured and reused, often to maintain digester temperature or to provide space heating.

This integrated process maximises the energy value of biogas and significantly improves overall efficiency compared with separate heat and power generation.

Benefits of CHP in wastewater treatment

The use of CHP in wastewater treatment plants offers numerous advantages:

  • Energy self-sufficiency: Plants can generate much of their own electricity, reducing reliance on the grid.

  • Cost savings: Lower energy bills and potential income from exporting surplus electricity.

  • Waste reduction: Biogas is used productively rather than flared or wasted.

  • Heat utilisation: Heat recovered from the process can be used to maintain optimal digester conditions or provide heating for facilities.

  • Carbon reduction: Replacing fossil fuel energy with renewable biogas reduces greenhouse gas emissions.

  • Resilience: CHP units provide a reliable local power source, improving energy security for critical infrastructure.

These benefits make CHP an attractive option not only for large municipal works but also for smaller facilities seeking to improve sustainability.

Design considerations

When installing CHP systems in wastewater treatment plants, several design factors must be considered:

  • Biogas yield: The volume and consistency of biogas production determine the scale of the CHP unit.

  • Gas quality: Treatment systems must ensure the gas is clean enough for engines to operate efficiently.

  • Heat demand: The plant must have sufficient uses for recovered heat to maximise efficiency.

  • Grid connection: Infrastructure is needed to integrate electricity generation with the local distribution network.

  • Maintenance: CHP engines require regular servicing, especially when operating on biogas with variable quality.

  • Regulatory compliance: Systems must meet air quality, safety, and environmental standards.

Designing CHP installations requires collaboration between civil, mechanical, and electrical engineers, as well as environmental specialists.

Environmental significance

The environmental advantages of CHP are considerable. Biogas is a renewable resource that would otherwise emit methane, a potent greenhouse gas, if left uncontrolled. By capturing and combusting it in CHP units, methane emissions are reduced while renewable energy is generated.

In addition, CHP reduces reliance on fossil fuels, cutting carbon dioxide emissions and contributing to national renewable energy targets. In the UK, water companies using CHP from sewage sludge digestion play a major role in reducing the sector’s carbon footprint.

Regulations and standards

In the United Kingdom, CHP systems in wastewater treatment must comply with a range of regulations. These include environmental permitting requirements, air quality standards, and health and safety regulations for pressure vessels, engines, and electrical systems.

Government incentives such as the Renewable Heat Incentive (RHI) and Feed-in Tariffs (FITs), though now phased out for new entrants, historically encouraged widespread adoption of CHP. Today, broader commitments to net-zero carbon emissions continue to support investment in CHP and renewable biogas technologies.

Limitations and challenges

Despite its many benefits, CHP is not without challenges:

  • High capital cost: Installing CHP units requires significant upfront investment.

  • Maintenance intensity: Engines running on biogas are prone to wear from contaminants.

  • Variable gas production: Seasonal and operational changes in sludge digestion can affect output.

  • Heat demand mismatch: Some plants may not use all the heat generated, reducing efficiency.

  • Regulatory complexity: Navigating energy markets, grid connections, and permits can be difficult.

These limitations highlight the need for careful feasibility studies and ongoing operational management.

Applications beyond wastewater

While particularly associated with wastewater treatment, CHP is widely applied in other sectors:

  • Food processing plants generating organic residues.

  • Agricultural facilities with anaerobic digestion of manure and crop waste.

  • Hospitals, universities, and large buildings where both heat and power demand are high.

  • Industrial sites with continuous process heat requirements.

These broader applications demonstrate the versatility of CHP as a sustainable energy solution.

Future perspectives

The role of CHP in wastewater treatment is expected to grow as technology advances. Innovations in gas cleaning, high-efficiency engines, and microturbines are improving performance and reducing costs. Integration with digital monitoring and smart grids will further optimise energy management.

There is also increasing interest in upgrading biogas to biomethane, which can be injected into the gas grid or used as vehicle fuel. In such cases, CHP may be combined with biogas upgrading systems to provide flexible, site-specific energy solutions.

As the UK and other countries pursue net-zero targets, wastewater CHP will play a vital role in decarbonising essential public infrastructure.

Conclusion

Combined Heat and Power is a proven and effective technology that transforms wastewater treatment plants into energy producers. By using biogas from anaerobic digestion to generate both heat and electricity, CHP delivers efficiency, sustainability, and economic benefits.

Although it requires careful planning, investment, and maintenance, CHP represents a cornerstone of modern resource recovery in the water industry. Its environmental significance, cost savings, and contribution to carbon reduction ensure that it will remain an essential technology for the future of wastewater management.