This operation mainly involves UWW. Its purpose is to cut up solid matter drawn along in the water, thus enabling this matter to continue along the circuit towards subsequent treatment stages. The objective consists in removing the constraints generated by the removal and discharge of bar-screening waste and in taking advantage of a certain increase in the volume of digestion gas output. However, in practice, comminution does have drawbacks, especially the danger that comminuted fibres will combine with grease (blocking pumps and pipes) and the need for relatively frequent maintenance work on fairly delicate equipment.
On these grounds, comminution is now only rarely advocated. On the other hand, it sometimes replaces fine barscreening in sludge treatment. A pressurised comminutor is used in these cases, ensuring that the requisite comminuted fineness is achieved.