Renovation of Hengelo sludge plant

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Energy-neutral in 2025

Any train passenger will immediately notice Hengelo’s ‘egg cups’. This is the distinctive look of the sewage treatment plant (RWZI) in Hengelo. Sewage sludge, which remains as a residual product from wastewater treatment, has been fermented here since the 1970s. The Vechtstromen water board aims to be fully energy-neutral by 2025. To this end, the Vechtstromen water board decided in 2013 on a full renovation of the sludge plant in Hengelo, and to convert it into a central sludge treatment plant for all the water board’s sewage sludge.

This aspiration necessitates improving and optimising the technical condition of Hengelo’s sludge installation, along with the water board’s entire sludge processing. Some of the sludge components are broken down during the digestion process, producing biogas that can be used as an energy source for heat and electricity production in a combined heat and power installation (CHP).


In the new set-up, the sludge is ‘pre-cooked’ at 160 ºC in a thermal pressure hydrolysis (TDH) process. This pre-treatment makes the sludge more biodegradable, increasing the biogas production in the digestion process. A CHP uses the biogas as fuel and produces electricity and heat. The heat generated is used to produce steam for the TDH, which in turn keeps the fermentation tanks at temperature and heats the company building. The CHP produces 16 million kWh of electricity annually. This is more than enough to cover the electricity needs of the water treatment plant and 3,500 households. This lets Hengelo WWTP serve as an ‘energy factory’ for the surrounding area. After digestion, the sludge is dewatered in centrifuges to reduce its volume. Then the compact residual product is transported for final processing.

Phased construction process

But even during renovations and alterations, ‘the show must go on’. The complexity lies not just in the technology, but also in keeping the installations running with the same quality and efficiency. This meant the construction process being organised in phases, so that RWZI Hengelo’s water treatment and sludge processing could continue throughout the project. The original digesters and sludge installations were gradually converted and renovated on a large scale. Among other things the digesters were adapted to the new process, with a higher sludge concentration and biogas production. The sludge dewatering, storage and loading were also replaced in their entirety. Finally, the TDH installation was fitted in-between existing and new process components.

The experiences and learning points from the first project phases have been incorporated into the project’s subsequent phases. The TDH installation was realised in the first phase as a ‘full-scale demo’. This provided the opportunity to test the operation of the process and its effect on the fermentation, and to acquire experience in this new technology. Points for improvement to increase the installation’s robustness were included in the project’s follow-up phases. This has led to a robust and cost-efficient set-up for the central sludge treatment in Hengelo.

Tendering strategy

Alongside smart construction phasing, a good tendering strategy has also ensured a successful project. The contract experts of the water board and Witteveen+Bos selected the best possible tender form for each phase. The digesters and CHPs were renovated in a construction team, the TDH plant was put out to tender with a UAV-GC contract, and the final dewatering centrifuges were put out to tender in a unique mini-competition. In this mini-competition, full-scale machines were tested under controlled conditions to see which machine achieved the highest possible dry-matter content. This enabled the best solution to be chosen. The renovation of the other sludge-processing components was put to the market as a UAV (Uniform Administrative Conditions) contract.