Animal or human waste may be fed to domestic animals, causing their contamination.
For the authorities of some countries, the products of the physical separation stages (primary and secondary stages) of waste water treatment operations do not represent sewage sludge (an illegal additive in many countries), but rather animal waste that can be used in animal feed.
The EU has been investigating the use of septic sludge in the Netherlands, at two slaughter house facilities where sludge is known to have been used (6/99). In France it has been discovered that industrial sewage has been sold to animal feed additive producers at the beginning of 1999. Tests identified three slaughter houses and a gelatine manufacturer who was using sludge from the local water purification centre. A Belgian animal waste-processing plant has been routinely mixing 5,000 tonnes of animal slurry a year into feed. This additive is not considered sludge.