The solid organic material left over after sewage treatment is known as biosolid. In 2019 Australia produced about 371,000 t of dry biosolids and just over 11,000 t was added to existing stockpiles, see this link. Biosolids are apparently rich in nutrients similar to soil once dried, and this link discusses the ways dry biosolids can be reused.
Measurements show that the process used to dry biosolids produces significant quantities of methane, carbon dioxide and nitrous oxide – approximately 380 kg CO2-e per tonne of dry biosolid, see this link.
Stockpiles of dried bisolids are also significant sources of green house gas emissions – around 90 kg CO2-e per tonne for the first year, then dropping to around 60 kg CO2-e per tonne for the next two years then to around 28 kg CO2-e per tonne from then on, see this link.
In 2016 Polish researchers published findings on the gasification of dry biosolids to produce syngas at a relatively low temperature, see this link. Syngas, or synthesis gas, is a fuel gas mixture consisting primarily of hydrogen, carbon monoxide, and very often some carbon dioxide. A sketch of their test rig is shown below.
Syngas is a well understood chemical feed stock that could feed a Syngas platform biorefinery to make liquid fuels and chemicals, see this link. The high temperature processes used within the biorefinery are introduced in this listing.
This link provides information on another Australian project involving the gasification of biosolids. This project, however, operates at a higher temperature, uses wet biosolids as feed material and does not appear to produce excess syngas as it all seems to be required to operate the plant.
There is an opportunity to continue development of the low temperature gasification process identified earlier, utilising feed material extracted directly from existing dry stacked stockpiles, some of which are decades old. This would result in the production of “green syngas” which could be either burnt in a stationary engine for power generation or used as a feed material for a biorefinery, which usually uses well understood technology.
A biorefinery producing liquid chemicals from a low value “waste” would be an excellent example of a cyclic economy, where a perceived waste is elevated to a useful resource for another end user.
Contact us if you want to discuss this idea further.
