Not all digesters are the same

Anaerobic digesters or in-vessel composters are in their 5th generation of evolution. Not all digesters are the same nor do they perform the same function. Some just chew the waste up, others just overcook the organic material to produce an inert waste matter. 

Here we take a look at some of the organic resource recovery methods currently on the market.

Aerobic. These systems work on a centralised large-scale basis, occupying a large land tract to spread the organic material out, using the sun and piling to heat the mass up, and periodic turnover to ensure even processing. Some sites add aeration, microbial additives and/or heating to the process. Trucks collect the matter and bring it to a central point. This is presently the most common resource recovery method for organics in Australia. Relative to other methods below, this is the most energy, land and time intensive methodology.

Domestically this process is replicated at a household level with your own “compost heap” however meat, dairy, fish, eggs, citrus peel, onion and garlic skins and other food sources that can harbour salmonella and ecoli are not recommended in home compost systems. It’s an incomplete solution and the longest processing time of any of the methods discussed here

Macerators. As the name suggests, these units take the organic resource you’re trying to recover and chop it up into small pieces. Often the processing unit is separated from the storage unit with a pump and pipe in between the two: the mashed-up organic matter is pumped into a storage receptacle and collected on a periodic basis. These systems attain a 50% volume reduction and advance the decomposition process as far as the length of time between periodic collection. Although the macerator itself can be quite small often the storage unit is large. Recovering the organic matter is at the discretion of the collection agent.

Dehydrators: These units heat the organic resource up to evaporate moisture, basically “overcooking” food and other organic matter to reduce it’s volume. Mass reduction is equivalent to the moisture content of the matter processed ie if you put 10kg of lettuce in a dehydrator you will get a larger volume reduction than if you put in 10kg of dry biscuits. These units can be energy intensive and malodourous. Turning the collected matter into compost is at the discretion of the collection agent.

Liquidators.  These units both macerate the organic resource and then add water in order to flush the resultant mass down the sewage system or to a septic tank for periodic collection. 1 to 1 ratios are common – 1kg of organics is mixed with 1 litre of water to form a slurry. Domestically these units were popular in the 70’s and 80’s in household kitchen sinks.  These units use a lot of water relative to other methodologies.

Digesters. These units add microbial agents to enclosed vessels to speed decomposition. Sometimes they are combined with some of the other methods above for instance, a large steel tube with solar reflectors and a turning handle combines heating, microbial action and agitation as the basis for it’s faster processing speed. Microbes are a living organism and thus require oxygen – the mass must be aerated in order for it to decompose effectively. Viable compost is usually achieved in 4 to 8 weeks dependent on the system. 

Gen 5 digesters. Generation 5 units combine maceration, dehydration, microbial digestion and emissions control in one system, achieving a better than 90% volume reduction and producing live, viable compost as an output whilst eliminating malodourous and climate-changing emissions.  Viable compost is achieved in 24 hours for volumes up to 20 tonnes and 48 hours for larger quantities. Jorgji systems are a Gen 5 digester.