What are the different forms of biosolids?

Biosolids is a general term for treated sewage sludges. More specific terms that describe the four most common forms of biosolids are:

  • Biosolids cake – raw sludge is stabilised biologically (most commonly by anaerobic digestion) and the liquid biosolids are dewatered to produce biosolids ‘cake’. The cake varies between 15 to 30 % solids for consistency, with the balance being water. The cake can be dried further by air drying. A similar result occurs with lagoon stabilisation and air drying.
  • Biosolids pellets – raw sludge or liquid biosolids are dewatered to approximately 20% total solids then further heated in a dryer and dried to 95% total solids and then pelletised.
  • Lime amended biosolids – raw sludge or biosolids are first dewatered followed by stabilisation with the addition of lime.
  • Composted biosolids – dewatered raw sludge or biosolids are composted, typically with other organic materials, to produce a high quality product suitable for use in domestic gardens as well as public parks and gardens.