Recognising that sludge imports will place considerable pressure on the waste water treatment plant at Tollebeek in the future, the client – Waterschap Zuiderzeeland – needed a sustainable solution that would be able to treat larger volumes of sludge with energy- and resource-recovery facilities, to generate (green) energy, reduce the plants’ carbon footprint and reduce sludge volumes.

The Challenge

The existing system, a conventional mesophilic anaerobic digestion with a single tank, was only capable of treating the sludge from WWTP Tollebeek. Danny Traksel, Director Business Development at Royal HaskoningDHV, says that Waterschap Zuiderzeeland acted as a professional and enthusiastic launching customer for the Ephyra® technology. 

Royal HaskoningDHV, together with partners Stichting Toegepast Onderzoek Waterbeheer (STOWA), TUDelft, Logisticon, HHSchieland & Krimpenerwaard, applied for the LIVE EU funding to develop the Ephyra® innovation from lab to pilot and from pilot to full scale at Tollebeek. Royal HaskoningDHV’s major strengths include our expertise and innovation in this sector, proactive approach to business development, ability to create partnerships, and our in-depth knowledge of anaerobic digestion.

Separating Key Processes 

The Ephyra® concept is based upon a plug flow digestion in which the hydraulic retention time (HRT) and solids retention time (SRT) are separated. The Ephyra® plug flow digester consists of three or four compartments, either separated as individual tanks or integrated as compartments in a single tank. Separating the different processes in the anaerobic degradation of organic matter allows Ephyra® to operate at much shorter retention times, as short as six to eight days. 

Optimisation of reactor design, process control and operations have led to significantly improved performances. In comparison to a conventional sludge digestion installation, the Ephyra® installation has demonstrated a breakdown of 20% to 30% more organic substance for different sludge types. This results in more sustainable energy in terms of biogas and a lower amount of residual sludge. 
Compared to alternative solutions, Ephyra® is compact and easy to install or retrofit, thus reducing the need for building materials. The major consideration is sufficient space on site to install an additional Ephyra® digester, or to install interconnecting piping between the digesters in such a way that they can be operated in a serial configuration. The system has a small footprint and is considered to be the most effective mesophilic anaerobic digestion technology on the market with relatively low investment costs (CAPEX and OPEX) for its application. With the Ephyra® controller, the process can be fully controlled and operated, including self-learning abilities.


Ben Roelfzema, senior engineer at Waterschap Zuiderzeeland, comments, “We can now treat more sludge quicker using smaller digestion tanks. And we now have an excess of biogas, which we use to provide 120 households with green energy.”

Traksel adds, “This is a major step forward for the sector in battling climate change by not consuming fossil-fuel based electricity. Beyond 2020, even more sludge will be treated as the volume transported from Lelystad to Tollebeek will increase. Our Ephyra® solution can handle these additional sludge imports without any adaptation or modifications.”

The system has provided additional cost benefits for the water authority as higher sludge degradations and better dewatering are produced with less chemical usage, resulting in lower sludge disposal costs. And the ability to process additional sludge from Lelystad has reduced the cost per volume treated (economies of scale).

In addition, high biogas yields, resulting in the generation of additional green energy, have significantly reduced the energy bill. In under five years, savings in electricity are €50.000 - €100.000 per year. The solution has also contributed towards positive public reputation building for Waterschap Zuiderzeeland in terms of piloting a more sustainable, climate neutral solution within the waste water treatment sector.

Contact us

Waterschap Zuiderzeeland

To pilot and implement an innovative, compact and cost-efficient solution for processing additional sludge imports.

Development of Ephyra®, which processes larger volumes of sludge with energy- and resource-recovery facilities, while reducing volumes of digested sludge. 


  • Quicker treatment of sludge
  • Smaller built footprint resulting from smaller digestion tanks
  • Enhanced biogas production due to a higher breakdown of sludge
  • Energy generation from excess of biogas – smaller carbon footprint
  • Can be scaled up without any adaptation or modifications
  • Higher sludge degradations and better dewatering with less chemical usage

Related markets