The process used by the Alchemy is actually a hybrid aerobic and anaerobic digestion process. It deals with the majority of kitchen waste (fruit, vegetable peelings, dairy and bakery products, eggs, eggshells, pasta, plate scrapings and small volumes of brown cardboard). However due to the ABPR regulations, and nature of the macerator used and heating thresholds achieved, meat, fish, poultry and bone must be excluded.
Primary Processing:
Raw kitchen waste from kerbside collections is placed in a macerator which chops the material into 50mm particles, and is fed via a tapering chute into one of two "pregesters" - skip shaped vessels containing hot water heating pipes. Here water is added to the macerated waste as required and is heated to 57 degree centigrade for a minimum of 5 hours.
The liquid is then drained off and enters the anaerobic part of the process and the solid material is strained and hydraulically tipped out and enters the aerobic part of the process.
The Aerobic process:
The solid material is blended with miscanthus (elephant grass) or shredded cardboard and allowed to settle and mature for 18 days. Any leachate is contained.
The material is then placed in a Keenan mixer where it is constantly mixed, temperatures often naturally reaching 60 degree centigrade and accelerating the composting process.
Finally, the material is placed in pallet based worm beds to refine the compost for as long as is required.
The Anaerobic process:
The liquid from the primary process drains into a 20m 3 digester tank where anaerobic microbes digest the fatty acids, proteins and sugars present.
If the temperature in this tank exceeds 35 degree centigrade (either naturally, or by adding heat from the hot water exchange system) methane gas is also produced which can be used to fuel a dual fuel generator which can then supply electricity for the plant or for charging batteries for subsequent use.
Any excess liquid in the digester tank is collected in a sump tank which can then be reintroduced into the pregesters with the next batch of raw kitchen waste. Should a surplus of liquid build up, it can be tested and used as a liquid fertiliser.
Additional information:
The Alchemy Project's hot water exchange system was heated by a wood fuelled boiler, fuel typically being waste wood from local industry. In addition solar and engine heat recovery could be utilised.
The Alchemy Project operated "off grid" in terms of electricity. If the plant were operating at capacity enough methane "biogas" would be generated to supply all the plants electrical needs, and generate a surplus to aid fuelling an electric collection vehicle. |
