Thermohydrolysis: a new process at the Tobl wastewater treatment plant

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.

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.

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.