If the sludge is to be spead on farmlands it can not contain any pathogens or infectious matter that could lead to harm on human health.
Infectious matter have to be destroyed through hygienization. The hygienization can be performed in different ways. Usually the sludge is placed outside for at least six months where pathogens and microorganisms die.
Another method is pasteurization where the sludge is helt in above 70 degrees Celsius for a minimum of one hour (https://www.svensktvatten.se/globalassets/avlopp-och-miljo/uppstromsarbete-och-kretslopp/m137-slamanvandning-och-strategier-for-slamanvandning-mars-2013.pdf).
Hygienization can also be achieved by adding ammonia where the toxic haracteristics of the ammonia hygienize the sludge (http://vav.griffel.net/filer/SVU-rapport_2017-10.pdf).
Freeze Dry Unit uses pasteurization as a hygienization method. To achieve this excess heat from the freezing process can be used.