Mechanical biological treatment
In addition to clean and dirty MRFs, a third type of recovery system has entered the picture. Mechanical biological treatment (MBT) – also known as a wet MRF – treats solid waste both mechanically and biologically. A mixed waste stream enters these facilities and magnets, shredders and other types of separators mechanically remove metals, plastics, glass and paper. Some MBT facilities will also separate combustible elements from the waste stream, such as plastics and organics, and convert them to refuse-derived fuel (RDF). RDF is typically used as a fuel in power plants.
After mechanically recovering recyclables from the waste, the remaining organic material is processed using biological methods. These include anaerobic digestion, in which microorganisms break down the waste to produce biogas, soil amendments and materials suitable for composting.
Because MRF technologies and systems can vary depending on each community’s waste stream profile and management goals, no two MRFs look exactly alike.
However, the main objectives generally overlap:
- Reduce the volume of waste to be landfilled
- Improve resource recovery through recycling and production of a degradable or combustible residue
- Stabilize all waste residuals that end up in landfills
Smart energy recovery
Because municipal solid waste contains plastics, organics and other carbon-rich material, waste managers more and more are viewing their garbage as a potential source of renewable energy. Methods of converting waste to energy – or WTE in waste industry parlance – is the focus of this section .
WTE conversion occurs in two basic ways. One is incineration. This typically implies burning solid waste to heat steam-powered generators that produce electricity. The other way is to process waste in a manner that produces gases and liquid fuels that are used for commercial heat and power.
In some respects, WTE solutions fall outside of our smart cities focus on information and communications technologies. Moreover, some argue that because incineration generates emissions and is a relatively low-value use for previously manufactured materials, it falls outside the smart cities sustainability emphasis.
With those caveats in mind, let’s briefly take a look at where WTE is headed as a waste management tactic.