Biogas Digesters are tools that process organic waste such as food scraps through Anaerobic Digestion into Biogas and fertiliser. For the purposes of this article, this process will be referred to as Biogas Digestion. The tool is simply a vessel or container that organic matter can be put into, where it will be submerged in water, and the gas that is produced by the digestion can be trapped and typically fed into a storage vessel. This is in most regards similar to a typical compost bin, with the distinction that the gases are trapped instead of released. Trapping this gas allows it to be used for heating, cooking and electricity consumption, with the added benefit that burning trapped methane is a method of greenhouse gas removal.
This tool essentially allows operators to turn household food scraps, and other organic wastes into free cooking and heating gas. Some biogas digesters are also used instead of sewerage or septic systems to recycle Human Waste. When combined with scrubbers, which remove contaminants from the gas, it can also be pressurised in commonly available gas bottles, which will provide more consistent output of gas for electricity generation and water heaters. Biogas Digesters produce methane when their interior temperature is above 20 degrees celsius and process material faster at higher temperatures. In cold climates a heater can be used to keep the digestion process running. Most gas appliances can run on Biogas with little or no modification.
How Digesters function
Digesters are best explained as a compost bin that captures and controls the gasses produced by the breakdown of the waste inside. The composting matter is generally contained within water, though some Biogas systems operate dry, and the gas is captured in an airlock created by the water, and an airtight ceiling. This ceiling will generally have a valve, often with a tap for turning gas output on or off, which leads to a hose and a storage vessel or tool. Most Biogas digesters also have an inlet for organic matter to be fed into the machine, which should go well below the water level, as well another outlet for water/fertiliser. Biogas production at temperatures lower than 50 degrees centigrade requires agitation of the organic matter as well, and many digesters as a result also have a stirring mechanism built into the design, or are designed to be agitated by hand. Excluding the gas outlet, all of these design functions can utilise the water level to prevent gas escaping.
An inlet that takes organic material right to the bottom of the digester ensures that fresh material begins to decompose at the bottom of the vessel, so that gasses rise into the containment area at the top.
Biogas systems operate in two temperature ranges. From 20 degrees centigrade to 50 degrees, known as the Mesophilic range is the standard for most digesters, and is easily achieved in most climates without additional heating. A temperature of 35-40c is ideal for this mode of production where it is achievable, as it will result in faster conversion of material into byproducts. Falling below 20 degrees will stop the production of methane, but unless the system freezes, it will begin methane production again when it rises above 20 degrees. However you should not continue to feed a system that is not productive. In situations where it is impossible to avoid the Digester freezing, draining the bulk of the liquid from the vessel is suggested as a way to prevent damage. In temperate and cold climates greenhouses, insulation and active heating are often used to maintain Mesophilic activity. HomeBiogas advocates the use of commonly available aquarium heaters for their systems.
The second temperature range of 41 to 122 degrees centigrade (typically optimised to 70 degrees) is Thermophilic digestion, and this is usually utilised in industrial plants.
Types of Digesters
Digesters vary widely in size and complexity, depending on the application and the materials or resources available for construction. The below list is effectively in order of complexity and scale, with the exception of the DIY systems, which are arranged in the order in which they were designed, with the later designs described after the ones they are derived from.
Do it yourself systems
A small note on manufacturing Biogas Digesters out of plastic vessels; It is possible to weld plastic, so if you're removing some material from the vessel, set it aside and heat it up later, to approximately 200 degrees c, and apply it to the gaps around the piping you install to reduce or eliminate the need for sealants.
Solar Cities IBC Biodigester
Solar Cities, an open-source group of Biogas 'Innoventors and Practitioners', designed a BioGas Digester utilising commonly available IBC storage tanks, with detailed instructions available on their website; www.solarcities.eu/education/388. Their design has been replicated with many variations and derivatives including different vessels, such as generic, blue 200lt plastic drums.
MH4 TECH Drum Digester
This design is demonstrated in a build video on the Youtube Channel MH4 TECH and could be described as an intermediary concept between the Solar Cities Biodigester and the Lucky Hill Farm design, though it was published after the latter design, and it lacks an access port for a stirring mechanism. The build video also doesn't mention the potential applications for fertiliser, though it does demonstrate the burning of methane and installation of gas pipes, taps and tire for gas storage.
Lucky Hill Farm design
The proprietor of Lucky Hill Farm, designed a derivative of the Solar Cities Digester that reduces the requirement for maintenance. A video demonstrating the construction of that design can be found on their channel here: www.youtube.com/watch?v=i7onXeTKiRM.
The core design difference between the Solar Cities and Lucky Hill Digester is that the water/fertiliser outlet is placed on the side of the vessel, a little way down, so as to stop the internal water level raising to the fixtures at the top for the gas outlet. Provision is also made for a stirrer, with a recessed pipe installed into the roof of the vessel, down to below the water level. This access pipe allows for manual stirring, via the rotation of a T shaped pipe as well as somewhere to install an aquarium heater, if local temperatures are too low for methane production.
HomeBioGas produce and sell a combined BioGas Digester & storage system, along with appliances such as gas hotplates and toilets for feeding human waste into their Digesters. The system was first deployed in Bedouin and Palestinian villages, and then provided for sale to a wider audience after a successful Indiegogo campaign. The digester design was updated in 2017, and ships with instructions for installation and operation. A large community of users discuss the operation of these systems on Facebook.
MyGug is a BioGas Digester developed by Kieran Coffey, with funding from the Sustainable Energy Authority of Ireland. This digester has integrated heating to allow it to work in colder climates.
Industrial Biogas systems
Large scale biogas systems are often utilised in or adjacent to food processing and agricultural industrial centers, to process waste and byproducts from their processes. These systems are often produced to site specific considerations, such as the Biogas system produced by Barwon Water in Colac. This system receives inputs from Bulla, a dairy company and ALC, a meatworks, which allows them to reduce their wastewater costs and gas bills. The Colac system also allowed Barwon Water to power it's water reclamation plant, and at the same time feed additional renewable electricity into the national grid.