The energy consumption for mechanical processes can be several orders of magnitude lower compared to thermal drying processes, see Figure 3.44.
As long as the material to be dried lets it, it is recommendable to use predominantly mechanical primary separation processes to reduce the amount of energy used for the entire process. Generally speaking, the majority of products can be mechanically pretreated to average moisture content levels (the ratio between the liquid mass of the liquid to be removed and the mass of dry substance) of between 40 and 70 per cent. In practice, the use of the mechanical process is limited by the permissible material loads and/or economic draining times.
Sometimes mechanical processes are also recommendable prior to thermal treatment. When drying solutions or suspensions (spray drying, for instance), the pretreatment can be membrane filtration (reverse osmosis, nanofiltration, ultrafiltration or microfiltration). For example, in the dairy industry, milk can be concentrated to 76 % moisture content before spray drying.