Earth is calling... Hengelo generates power with sewage soup

They are impossible to miss when travelling between Hengelo and Borne by train: the three large fermentation bulbs are right next to the railroad. They are the pride and joy of Energiefabriek Hengelo (Energy Plant Hengelo): an advanced water purification plant that not only purifies sewage water but also generates energy in the process. How do you go about purifying dirty water and generating power at the same time? It goes as follows, from flushing the toilet to generating electricity: All the wastewater from Hengelo’s toilets, kitchens, showers, and rain pipes is collected on the Westermaat industrial site. A coarse rake is used to dredge out anything that does not belong there. This includes things like wipes and toilet paper, but also dentures and cell phones that slip out of someone’s pocket while they are flushing. This so-called ‘grate waste’, around 20 cubic metres per month, is transported to the landfill in containers.

In short:

  • De energiefabriek Hengelo zuivert water en wekt tegelijkertijd stroom op. De manier waarop dat gebeurt, is niet alleen uniek in Twente, maar in het hele land.
  • Het secundaire slib dat bij de zuivering ontstaat, wordt omgezet in biogas. Via een verbrandingsmotor en een dynamo levert het elektriciteit op. Genoeg zelfs voor 3500 huishoudens.

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Fresh sludge

The remaining water goes to a large basin where gravity can do its work. Faeces and other debris sink to the bottom. Everything that collects on the bottom is called ‘primary’ or ‘fresh’ sludge by the experts. This is extracted at the bottom of the basin and pumped directly into the characteristic fermentation bulbs. Bacteria will continue to nibble at the remains of the excrements in another basin afterwards, which causes the bacterial colonies to grow considerably. Their surplus is skimmed off in the form of ‘secondary’ sludge.

Pressure cooker

The crux of the Energiefabriek is in the treatment of this secondary sludge. Before it goes to the fermentation bulbs, it is thickened and boiled to pieces in a pressure cooker: the Thermal Pressure Hydrolysis Reactor. The sludge boils there for half an hour, at a temperature of 160 degrees Celsius, under the pressure of 6 bars. This creates a thickened sludge that can be efficiently converted into biogas. The fermentation bulbs have a temperature of 40 degrees Celsius, the ideal climate for the methane gas bacteria that ultimately produce the biogas. This biogas is then converted into electricity on-site, via an internal combustion engine and dynamo. The purified water is discharged into streams and ditches; the plant in Hengelo does not produce drinking water.


The ‘cooking’ of secondary sludge is unique in the Netherlands. Hengelo is home to the first TDH reactor, a technique developed in Norway. The process is complex and requires a great deal of precision. It is monitored 24 hours a day, 7 days a week. The entire system can grind to a halt if a sensor somewhere falters. But it is certainly profitable for the purification plant. 28 per cent less ‘sludge cake’, a residual project, is transported to the sludge incinerator in Moerdijk because more sludge can be fermented on site. This saves around 500 trucks per year and reduces costs by 1.5 million euros. The extra biogas that is generated by the pressure cooker is also a major benefit because the entire plant is now completely energy neutral. What’s more, it supplies energy to 3,500 households. The Energiefabriek is a wonderful example of circular thinking: the plant proves that you can perfectly convert a product that everyone turns their nose up at into energy. 16 million kWh of electricity is produced in this way every year.

Date: 23 March 2022 |

Author: Maaike Thüss

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