Cogeneration processes in WWTPs: technology contributing to environmental and energy sustainability

5 June, 2023

Article by Fernando Díaz, Director of Infrastructures at Incatema, on the occasion of the celebration of World Environment Day.

In a world where natural resources are increasingly scarce and environmental concerns are growing, wastewater treatment plants (WWTPs) play a key role in the preservation of ecosystems and public health. These facilities are essential to treat wastewater before it is reintroduced into the environment, thus ensuring that harmful pollution does not occur.

One of the factors to be taken into account in the circular economy is that WWTPs hold a significant potential for the production and use of energy of the biogas generated from treated sewage sludge. From an economic and environmental point of view, this is very attractive since, on the one hand, waste management costs are minimised and, on the other hand, the use of the waste generated has a positive impact on the environment.

Sewage sludge will be generated through the different stages of the WWTP wastewater treatment. During anaerobic digestion (in the absence of oxygen), the sludge produced undergoes a digestion process by anaerobic organisms, which results in the production of methane (CH4) and carbon dioxide (CO2).

Methane gas, which is highly flammable, can be exploited for energy by combustion in engines, turbines or boilers, either on its own or mixed with other fuels. In addition, it can be used as biofuel in the plant itself, or it can be used to produce thermal and electrical energy.

For this reason, WWTPs are ideal facilities to host cogeneration processes, where cogeneration is understood as the joint production and use of two or more different types of energy, normally thermal energy and electrical energy, from the same fuel. The most commonly used cogeneration technology in WWTPs is the one generated via a reciprocating engine. In this type of system, the gas (which functions as a fuel) is introduced to perform a combustion reaction in which a large amount of thermal energy is released and converted into mechanical and electrical energy.

Thanks to the cogeneration process, the wastewater treatment plants use the thermal energy produced to maintain the temperature of the anaerobic digester and to heat the sludge before the dewatering process. In addition, the electricity generated can be used to operate the wastewater treatment plant whilst increasing its efficiency and environmental commitment. It is used for the WWTP's own self-consumption, and the excess, if produced, is sold to the electricity network.

Therefore, instead of simply releasing the biogas into the atmosphere, WWTPs can use it to generate electricity and heat through a cogeneration system that offers only benefits: firstly, it reduces dependence on conventional energy sources, such as coal or natural gas, by harnessing a renewable and locally available resource such as biogas.

Secondly, as the process allows WWTPs to be energy self-sufficient, significant savings in energy costs and more economically efficient facilities are achieved. Last but not least, additional socio-economic benefits are obtained for the local communities where these WWTPs are located. It creates employment for the maintenance and operation of these cogeneration systems. This is why cogeneration systems in WWTPs can be an outstanding example of how technology and innovation contribute to environmental and energy sustainability.