Pasteurization of sludge

• The device consists of a recuperative heat exchanger, a pasteurization heat exchanger, and a pasteurization tank.
• It operates with non-dewatered sludge containing 5 to 8% dry solids.
• Raw sludge is preheated in the recuperative heat exchanger by recovering heat from the pasteurized sludge.
• The preheated sludge is heated to the required pasteurization temperature in the pasteurization heat exchanger (water/sludge) using hot water.
• The required retention time for sludge hygienization is achieved in the pasteurization tank.
• The pasteurization temperature guarantees the devitalization of pathogenic microorganisms at the specified retention time.
• Before entering the digestion tank, the pasteurized sludge is cooled in the recuperative heat exchanger by transferring heat to the raw sludge.
• In the mesophilic digestion tank, energy-rich biogas is produced.
• Biogas combustion is used to generate electricity and heat.

Pasteurization is a process during which sludge is heated to a specific temperature only for a short time, e.g., 70 °C for at least 30 minutes. The optimal temperature range is 64 to 70 °C, which ensures a high degree of devitalization of pathogenic microorganisms. Pasteurization must be combined with subsequent sludge stabilization, most commonly anaerobic in the digestion tank. The reason is that weakened pathogens in reduced numbers cannot compete for substrate with the anaerobic biomass in the digestion tanks and thus perish. Another advantage of pre-pasteurized sludge is its better stabilization and improved dewatering properties.

The pasteurization process takes place in three devices: a recuperative sludge/sludge heat exchanger, a water/sludge pasteurization heat exchanger, and a pasteurization tank. Fresh sludge is pumped into the inner chamber of the recuperative heat exchanger. The outer chamber of the exchanger is filled with heated hygienized (pasteurized) sludge pumped from the pasteurization tank. Proper mixing in the inner chamber is essential for heat transfer between hot and cold sludge, enabling heat recovery.

The preheated fresh sludge is pumped into the water/sludge pasteurization heat exchanger, where it is heated to 70 °C. This temperature must be maintained for at least 30 minutes, which is achieved in the pasteurization tank. The pasteurization tank temperature is monitored by temperature sensors. Hot water at 80 °C is supplied to the pasteurization heat exchanger for sludge heating, coming from a boiler or a cogeneration unit (if installed).

The pasteurized (hygienized) sludge is reused to preheat the incoming raw sludge in the recuperative heat exchanger. This cools the pasteurized sludge before it flows to the digestion tank, where sludge stabilization occurs.

Advantages of Pasteurization

• The sludge is not only hygienized during pasteurization but also favorably prepared for further processing (biogas production, thickening).
• The external energy demand is relatively low due to heat recovery and is even lower if biogas is produced.
• The sludge fully complies with regulations on sewage sludge management from a microbiological standpoint.
• The equipment requires minimal space.
• Simple design and easy operation.
• The system is operationally proven, including the sludge/sludge heat exchanger, and can flexibly respond to changes in sludge input by adjusting the number of batches.

Energy Consumption

• Energy for sludge heating is supplied by hot water delivered to the water/sludge pasteurization heat exchanger.
• When biogas is produced, the external energy demand is relatively low.

Selected References

The pasteurization method is successfully operated at many wastewater treatment plants in Western Europe.

List of selected applications

• WWTP Maur, Switzerland
• WWTP Nesslau, Switzerland
• WWTP Wüeri, Switzerland

Performance range according to the amount of sludge processed

Technological diagram