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Biosolid 101: All You Need to Know

Wastewater treatment plants have several by products and biosolids is the main type of by-products. However, this by products can be reused if carefully managed and treated.

In its most basic form biosolids is sewage sludge that is treated in a plant. There is a clear distinction between biosolids and sludge. Biosolids are only considered once they pass a few guidelines that manage biosolids.

Compared to the 3 percent solid content of sewage sludge, biosolids will usually have around 15 percent to 90 percent solids.

What is found in biosolids?

Sewage sludge is the solids that accumulate from the wastewater treatment plant. They’ve only been collected but no further processing has been done on them. Sewage sludge usually only contains around 3 percent solids. The sludge will have biosolids once they are processed and treated accordingly. This further biosolid reduction is necessary to lessen the pathogens that carry diseases and organic matter that is volatile. 

As mentioned above biosolids comes from sewage sludge, which consists of both water and organic materials. This is a by-product of sewage wastewater treatment. Wastewater usually comes from bathrooms, kitchens and laundries.

Biosolids can be made up of macronutrients like Sulphur, potassium, nitrogen or phosphorus and micronutrients like manganese, boron, iron, magnesium, calcium, zinc, copper or molybdenum.

Apart from organic materials, biosolids can also contain a little bit of synthetic metals and organic compounds like chromium, lead, nickel, arsenic, cadmium and selenium. These synthetic compounds are what limits the usefulness of biosolids. Uses of biosolids are regulated by the government in each state and also at a federal level. Australia has a very strict regulatory requirement for biosolids.

Last year the annual biosolid production in Australia was around 371,000 tonnes(dry). Around 70 percent was used in the agriculture industry and around 24 percent was used for either land rehabilitation or landscaping. The rest was just discharged or stockpiled.

What are biosolids used for?

Biosolids are mainly used for fertilizer to maintain and improve productive soil. It is also used to promote plant growth.

The fertilizer can be used in parks and gardens as well. Rehabilitation of mining sites can also be done using biosolids. Below are some more examples of biosolids.

  •  Road Base
  • In the agriculture industry for cereal, olive, pasture, vine etc
  • Power production/co-generation
  • Forestry operations
  • Landfills and land rehabilitation
  • Topsoil
  • Landscaping
  • Composting
  • Bio-fuel
  • Construction material
  • Glass manufacturing
  • Cement works

Biosolids have grades and these are given according to their chemical and pathogen composition after treatment. Due to this reason specific grades are used for different purposes. Low quality biosolids will be less useful these will be used for mining site rehabilitation or as bases for roads. The agriculture industry uses top quality biosolids because most plants are grown for human consumptions.

It is the responsibility of regulators to see the proper segregation of biosolids into relevant industries.

This is a quick guide as to what biosolids are and what they are used for. The process used to produce biosolids is a lengthy one and will require an understanding of sewage treatment processes.

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