Drying of dehydrated sludge: an increasingly strategic solution

The disposal of sludge from wastewater treatment plants is becoming an escalating challenge: finding landfill sites is increasingly difficult and costly, while its use in agriculture has often become unfeasible, if not outright forbidden.

To address the issue, there are three available treatment methods:

• Thickening, which enables a dry solids content of 2–10%;

• Dehydration, using technologies such as plate or belt filter presses, decanters, and screw compactors, achieving a dry content of 25–35%;

• Thermal drying, which removes nearly all remaining moisture, reaching up to 90% dry content.

Finally, there is also the lesser-used option of incinerating sludge by mixing it with municipal solid waste (MSW).

Technological evolution: from high-temperature processes to low-energy systems

In the 1980s and 1990s, only fluidized bed or drum dryers with direct or indirect heating up to 200–300°C, or pyrolytic systems, were used. In recent years, low-temperature systems (50–80°C) have gained traction for their much higher energy efficiency. Their operation is simple: a flow of dry air passes through the sludge, dehydrating it until saturation is reached.

Operation of closed-cycle dryers

Modern modular dryers consist of stacked stainless steel belts, through which hot air (50–80°C) is circulated. The dehydrated sludge, pre-pelletized using extruders, is distributed in thin layers. The hot air can be generated by:

• heat exchangers that recover energy from hot fluids already present in the plant;

• or transcritical CO₂ heat pumps.

In closed-loop systems, the air is continuously recirculated. This avoids emissions into the atmosphere and exempts the system from additional environmental permits. The cycle works as follows:

1. Hot air extracts moisture from the sludge;

2. It passes through a condenser that separates out water as condensate;

3. The now-dry air is reheated and fed back into the system.

The result? Zero emissions, no odors, no dust.

Economic and environmental benefits

By measuring the amount of water condensed, one can precisely estimate the sludge’s weight reduction. In many cases, there’s a decrease of up to 70%, bringing tangible benefits in:

• transportation costs;

• disposal expenses;

• management overhead.

Features of the dried product and the system

The dried sludge is extracted as low-density granules (about 85% dry solids) using a screw conveyor into special containers. The material is:

• sanitized and pasteurized;

• free of pathogenic germs;

• reusable in agriculture or as filler material.

The entire plant is built from stainless steel and aluminum, resistant to wear, and is automatically managed via remote PLC systems, ensuring reliability and ease of operation.

In summary

Compared to traditional dryers, the new systems are:

• economical: lower consumption and reduced operating costs;

• clean: no emissions or bad odors;

• reliable: automatic and continuous operation;

• easy to manage: low maintenance and remote control.

Will you consider a system like this?