Large In-vessel Composting Systems: What are They and How Do They Work?

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Progressive cities and many urban centers around the world are now installing large-scale in-vessel composting systems as part of their total MSW management.

Large scale in-vessel composting, which makes high quality compost through the use of enclosed “reactors” that closely monitor temperature and oxygen levels throughout the biodegradation of organic materials, is popular.

In-vessel composting systems generally contain all their process air and liquids so that they can be captured and treated prior to discharge.

Using in-vessel composting, the mixture is mechanically “churned” and moved along bays, or moved from one tunnel to another half way through the in-vessel stage.

 in-vessel composting systems
A new In-vessel Composting Tunnel. (Image courtesy: Hytech-Water Ltd)
Air is blown through the mixture to keep it from overheating and to provide oxygen to the microbes.
During this period, helpful microbes feed on organic materials in the mix, raising the temperature to about 130 degrees F.

The heat stabilizes the material and further neutralizes potentially harmful bacteria, while maintaining the right temperature for beneficial, “composting” microbes to do their work

The following items must be considered when estimate costs for a specific in-vessel composting facility: C C Land acquisition.

Equipment procurement, of these plants entails the production of a detailed specification which should include at least the following:

  1. land acquisition
  2. composting vessel, loading equipment, conveyors,
  3. air supply equipment,
  4. temperature monitoring equipment, and
  5. odor control equipment. Operation and maintenance labor.
  6. energy (electricity and fuel for equipment)
  7. water and wastewater treatment
  8. equipment maintenance and upkeep
  9. product distribution expenses and marketing revenues
  10. regulatory compliance expenses such as permitting, product analysis, process monitoring, record keeping and reporting.
(This is not necessarily a complete list.)
In rotating vessel in-vessel composting plant designs agitation and aeration are performed by the vessel. Since, they are normally fully enclosed, in-vessel composting systems do not usually emit the foul odors often associated with some other composting operations. Odor in exhaust air is vented through a biofiltration system which removes the odor before it is discharged.Environmental benefits of producing and using compost include the recycling of a valuable resource, reduction of dependence on chemical fertilizers, and offsetting the use of natural resources such as trees or peat moss as mulch material.

 

 Deerdykes In-vessel Composting Plant
In-vessel Composting Plant showing ventialtion fans. (Image courtesy: Hytech-Water Ltd
Some versions are known as vertical compost units (VCUs), and they rely on gravity and the upward movement of ventilation air to provide movement.The compost then needs to be further processed once it has matured and is ready for selling, to make a good saleable compost.

This is often done by passing it through a screener, such as a Powerscreen 615 trommel with a half-inch screen to separate the remaining larger woody material and any other contaminants that escaped earlier detection. Oversized material (often called “overs”) are composted again and inert material not suitable for compost use can be are used for daily landfill cover.

In vessel composting involves larger capital costs than Windrow or Aerated static pile, but needs less land and labour.

After the waste is processed in-vessel it must cure for an additional period which might be 90 days or a similar period, before it is ready to be used as a soil amendment.

Air is forced into and circulated through the box from a series of holes within the floor. It is recycled continually until the carbon dioxide concentration in the reactor reaches, (in some designs) four percent by mass, which triggers release of the reactor air and introduction of fresh air.


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