Anaerobic Digestion Supply Chain Options

Barony College already operates an anaerobic digester and reed bed as a septic system to dispose of human waste and filter water on the main campus. It seemed appropriate to investigate the potential for anaerobic digestion (AD) technology in CH₄ and therefore power production. For this to be an economically viable proposition only a large scale automatic CHP system would be appropriate; large waste streams (at least one of which with a significant nitrogen and/or bacterial content) must therefore be considered.

As part of the initial Scottish Enterprise proposals, our team identified two sources of waste of sufficient size to support CHP. One of these – slurry from dairy cattle – would be available on-site; the other – waste vegetables – would require to be transported.

The vegetable waste in this case consists mainly of onion skins. It is wet, and this together with intrinsic properties produces a total water content of about 95%, with a corresponding dry solids content of 5%. Presently used for composting, the waste commands a gate fee of £16.00/tonne plus transport costs of around £7.00/tonne. Approximately 3500 tonnes of waste are available per annum. It should be noted that aerobic digestion by means of composting produces little in the way of CH₄.

Slurry is produced from dairy operations on-site, with water content being seasonally variable. Some 2x10⁶ British gallons are available (a 5x10⁵ gallon slurry tower emptied four times a year), equating to around 9000 tonnes of per annum. Slurry has a nitrogen and bacteria content suitable for AD. The slurry is presently stored and spread on the land for nutrient value.

Combined, these two waste streams provide the mineral and biological content that would be necessary for anaerobic digestion.

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