Earth:Hot container composting

Hot container composting (also referred to as in-vessel composting for larger industrial batches) is different to cold composting, in that compost is created without losing valuable heat. Heat loss is the reason why a compost pile takes so long to decompose. Observers have noted that the time taken to create compost can be dramatically reduced by retaining the heat in a suitably insulated container. Another observation noted is the way the compostable items are added and mixed which allows for aeration. This is an important step in the process. Compost becomes anaerobic due to the absence of air, and this can result in it becoming smelly. By adding large, medium and fine items to the top of the container, space is provided in between the items for the air to flow from bottom to top. This creates a trickle ventilation. Compost made with this method is called aerobic composting.

Requirements

• Insulated container: Polystyrene with polypropylene thick walls or cavity/ceiling insulation vents on top and bottom
• A starter composting medium: A handful of soil from garden that is a few months old containing natural microbes and bacteria
• Regular feeding of household and compostable waste: Food scraps, Fruit and vegetable peelings, Egg shells, Bones, Grass cuttings
• Carbon: Shredded or torn paper, Cardboard, Egg cartons, Cereal boxes
• Thermometer (optional)

Method

Place the waste you want to compost in a bucket or bag, add large pieces (bones, large bark pieces or anything the size of large egg), medium pieces (carrot tops, potato skins, chopped up branches etc.) and fine pieces (grass cuttings, ground egg shells) and carbon (bag of shredded paper, cut up cardboard, torn up cereal boxes, egg cartons, old magazines, newspaper).

Mix well or shake the bag. Empty the bag into top of container and close the lid. It must be able to be sealed and not let air enter or escape. Only the top vent should allow a controlled flow of steam to escape. Too much and most of the beneficial heat will escape and too little and the steam turns to streams of water in the inside and drops the internal temperature which in turn lowers the rate of decomposition.

How it works

The hyperthermic bacteria present in the container, derived from the handful of garden soil you added, reproduces and multiplies very quickly, which gives off a lot of heat.

Retaining this heat is the secret to hot container composting. By keeping the container sealed, insulated and controlling the escaping steam, the temperature can be kept with the range needed for hot container composting. Temperatures higher than 70 °C cause the important hyperthermic bacteria to die off and defeats the plan.

The smaller the pieces, the quicker the decomposition. Typing paper takes days to decompose, grass clippings take weeks, branches a couple of months, and bark takes several months.

Hot container composters aim for a temperature ranging between 40-60 °C (110-140 °F) to aid optimal bacterial decomposition. A suitable thermometer, analogue or digital, is needed for monitoring this.

If you don't have a thermometer, you have to wait for the final product to inspect if it is suitable.

The bottom door is only opened to extract the compost when the top is full. The first time you open the door, there will be partially decomposed material.

This is due to most of the decomposition work taking place on the top of the pile (where the heat is concentrated) and this can simply be added to the top during your next top up. Your next collection will probably have been through the process and be ready for use.

Let the collected material stand for aeration for 30 minutes to let the bad odor dissipate. Either remove the larger and medium-sized particles by hand or use a sifter. The larger pieces can be used again.

Cold composting versus hot composting

There are advantages and disadvantages of cold and hot composting.

Cold compost pile

Advantages

• Low cost
• Ease of maintenance - does not require a bulking agent or paper to be added, or the temperature to be monitored

Disadvantages

• Attracts vermin e.g. mice and rats due to odour of decomposing materials and some warmth it can offer.
• Attracts flying insects, e.g. flies
• Attracts other wildlife such as otters, foxes, raccoons, hedgehogs, and birds
• Foul odors from the anaerobic processes
• Little decomposition takes place in winter
• Requires space for the pile, as well as a space for the new pile while the old pile is turned

Hot container composting

Advantages

• Retains the heat all winter, allowing the compost to decompose all winter
• Any odours can be eliminated by adding an odour trap to the escaping steam (optional)
• No place for rodents, birds or wildlife to enter the container
• Takes up less space
• Sterilises compost, the high heat in the container kills any seeds, harmful bacteria or fungi
• Creates compost in 20% of the time (the higher the temperature inside the quicker the material decomposes)

Disadvantages

• More expensive, as it requires an insulated container and thermometer
• Requirement to monitor the temperature and add bulking agent and paper

Animation

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Commercial systems

Commercial products are available for home hot composting, consisting of an insulated container incorporating a thermometer and vents, plus bulking agents to help improve airflow. Manufacturers include HotBin.

Environmental benefits

Large quantities of waste streams are made up of "organic" waste products. These organic compounds can be made into a useful soil when composted. For example, "Mandatory Recycling and Composting Ordinance" passed in San Francisco in 2009 requiring residents of the city to separate their trash into recyclables. The city currently diverts about 1.5 million tons of waste every year into soil.

Composting helps to reduce the need for water by holding more moisture. The landfills are less filled because the organic material is being used for soil, not taking up space. Current waste management methods prevent all waste from being exposed to oxygen. The anaerobic conditions significantly slow down the decay process.

Composting helps grow the next generation of crops. In 2017 over 27 million tons of municipal solid waste or MSW was recovered through composting. This is equivalent to 0.45 pounds per person per day for composting, according to the Environmental Protection Agency (EPA). Reducing organic waste in landfills reduces methane. By composting we turn what would've been greenhouse gases in our atmosphere into higher yields of crops.

See also

• Aerated static pile composting
• Mulch