Sewage sludge as a climate protector

The Barsinghausen group sewage treatment plant is located near Hanover and treats the wastewater of around 35,000 people, as well as that of the biscuit manufacturer Bahlsen. In recent years, it has been thoroughly modernised to reduce operating costs and CO₂ emissions. Against this backdrop, it launched the project “Construction of an anaerobic sludge stabilisation with digester gas utilisation to save CO₂” in 2018. The objective was to extract methane from the sewage sludge and use it to produce electricity and heat in their own cogeneration unit.

The cogeneration unit began operation in June 2022 and has since been generating an average of 2,300 kilowatt hours of electricity and 4,300 kilowatt hours of heat per day – including planned downtime for maintenance work. It was supplied by MAN partner Sokratherm, which integrated a 120 kilowatt MAN gas engine into the small power plant. “This means we can cover around half of our energy needs ourselves,” calculates Heiko Bartling, who is responsible for the technical operations at the group sewage treatment plant. “What’s more, we are completely self-sufficient when it comes to heating the operations buildings.” In view of the high energy prices, this provides noticeable relief for the municipal enterprise.

Smooth continuous operation

The cogeneration unit also contributes to climate protection. “We eliminate 438 grams of CO₂ for every kilowatt hour of electricity we produce ourselves, as we would otherwise have had to buy electricity from the public grid,” reports Bartling. He is extremely satisfied with the reliability of the cogeneration unit because, aside from the maintenance intervals, the plant runs continuously around the clock and completely silently in a container in the depot. “It doesn’t require any work from us other than occasionally pulling up the weeds around it,” says Bartling.

The group sewage treatment plant has a multi-stage process to thank for the energy for the cogeneration unit operation. In this process, the sludge in the wastewater drops into basins and migrates to the digestion tower as “primary sludge”. In parallel, bacteria break down the nitrogen in the remaining wastewater and become “secondary sludge”, which also ends up in the digestion tower. There, different bacteria produce methane, the main component in natural gas, at around 37 degrees Celsius. The methane is pumped into the gas storage tank and later converted into electricity and heat in the cogeneration unit.

Additional income from electricity fed into the grid

The Hagenow wastewater association – which mainly purifies industrial waste water – produces sewage gas for its two cogeneration units in the same way. These were also supplied by Sokratherm and, thanks to the MAN engines used, deliver 400 or 250 kilowatts of electrical power. “We produce more electricity than we can use ourselves,” reports Thomas Zahn, who is responsible for the technical plants in Hagenow. “We feed the surplus of around 150 kilowatts into the public grid.”

The two cogeneration units have been in operation since 2015 and 2019 and have also proven to be extremely reliable. “The plants are extremely compact because Sokratherm has packed all the technology into a very small space,” says Zahn. “This means that everything fits into our existing machine building.” He has noticed that all larger sewage treatment plants in the region are already operating cogeneration units with gas from the digestion tower. “The investment is worthwhile for a population of around 30,000 or more or the corresponding amount of wastewater from industry.” What's more, the climate also benefits from intelligent usage through anaerobic wastewater treatment or the use of digestion towers.