Waste water is not just a contaminant but also a risk to health. A range of diseases are spread via water among other things. Not only bacteria , but also worm eggs and viruses can be contained in the waste water. Only some of these pathogens are killed on passing through a sewage plant – they accumulate in the sludge. Decontamination is therefore necessary in the event of agricultural use on grassland and sometimes also in the event of spreading on arable land.