The Freeze Dry Unit process consists of these main steps: freezing, thawing and drying. The freezing step enables drying to high a high dryness of the sludge. Thawing and drying uses low temperatures to dewater and dry the sludge. After this, a fourth step could be added, hygienization. The high dryness opens up a lot of different ways to dispose or use the sludge, specifically, for the extraction of phosphorus.
Water resides in sludge in four different ways in and around the sludge particles. It can be categorised in four different ways: free water is water that surrounds the sludge but is not bound to anything; interstitial water is water that is located in tiny holes and is held in place with capillary force; surface water covers the surface of the sludge particle and is held in place through surface tension and lastly intercellular or bound water that is chemically bound to the sludge particles and can only be released if the sludge particle is destroyed.
Freezing transforms bound, interstitial and surface water to free water. This makes it possible to remove more water from the material.