Thermohydrolysis: a new process at the Tobl wastewater treatment plant


The problem with the overflow sludge

Due to structural conditions, the excess sludge had to be pumped over a long transport distance (approx. 350m) to the storage tank feeding the digestion tower. This caused the following problems:

  • high pressure losses caused by the long pumping lines (>10bar) which in turn resulted in:

-high wear and tear on the eccentric screw pumps
-high spare parts costs
-high maintenance effort
-high power consumption

  • Fat deposits in the sludge pipe, which had to be cleaned again and again with the help of a flushing vehicle. In winter, this problem, caused by the low temperatures, occurred even more frequently.

To counteract these problems, we tried to keep the dry substance below 5% dry matter. This resulted in a high water content, which caused the amount of sludge to rise sharply, which in turn shortened the residence time in the digestion tower. As a result, the gas yield and the gas quality decreased. At the same time, the risk of acidification ("tipping") of the digestion tower increased. Likewise, shorter residence times in the digestion tower increased the proportion of organic substances and the sludge is more difficult to dewater. This in turn led to an increase in costs.


A solution was needed. The small project T19-18 (thermal pre-treatment) was developed.

Laboratory tests showed that thermal pre-treatment of the overburden at 60-70°C significantly improved the pumpability of the sludge and increased the degradability and methane yield. This results in the following advantages for the operation of the sewage treatment plant:

  • The viscosity of the USS decreases by approx. 90%.
  • The residence time in the digestion tower is extended.

The engineering office BioTreaT, which accompanied us in the project, also expects a possible increase in methane gas production of 30,000m³ or an increase in electrical energy of around 100,000kWh per year.

The new plant is in operation

The new plant is now in operation and the first results are very promising:

  • The pumping pressure could be halved. From 12-14 bar to about 4-6 bar.
  • Smaller pumps with lower drive power were used
  • The wear of the pumps is reduced
  • The excess sludge can now be thickened from 4-5% DM to 6-8% DM, which means a considerable reduction of the sludge quantity
  • Due to the longer residence time in the digestion tower, the organic content in the digested sludge decreases and the gas production increases by 20-30%
  • Due to the lower organic content in the digested sludge (according to Koop approx. 5%), the dry substance after sludge dewatering increases with lower polymer consumption (> approx. 3kg WS/kg dry substance)

According to the literature, the thermal treatment of excess sludge should still have a positive effect on foam formation in the digestion tower. We will observe this yet.

What our process engineer says

According to Hannes, our process engineer, all in all the conversion and new construction has already paid off and we are now happy to operate this new part of the plant.