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An In-Depth Look at the Green Cone Food Waste Digester
Function
The function of the Green Cone food waste digester is to:
- Accelerate the natural decomposition process by
- raising temperatures;
- maintaining aerobic conditions; and
- encouraging the growth of micro-organisms.
- Contain and enclose the food waste to prevent dispersion and eliminate odours.
- Create barriers to human, farm animal, wild animal, pet and bird activity.
- Prevent surface and top soil contamination.
- Meet all relevant health, safety and environmental legislation.
The system takes all household food waste, including vegetable scraps, raw and cooked meat or fish, bones, dairy products and other organic food waste such as bread and pasta.
Design
The Green Cone is a four-part plastic injection moulded system comprising a digestion basket that is installed below ground and which forms the base for an above ground double-walled solar chamber with an access lid. The design of the Green Cone utilises solar heating in the double-walled chamber to facilitate and accelerate the aerobic decomposition process within the digestion basket. The system should therefore be installed to obtain the maximum sunlight in a household’s garden. A four-litre receptacle, which can be sealed, is provided for collecting and carrying the food waste to the Green Cone.
The diameter of the 42 cm deep digestion basket is 59 cm at the top and 40 cm at the bottom. A series of slots designed into the basket commence 19 cm below ground level and effectively remove about half of the side wall and base material. The inner and outer walls of the conical solar chamber are 2 cm part, providing a gap that is open at the top and bottom of the unit. The diameter of the outer chamber reduces from 59 cm at its base to 28 cm at the lid end. The chamber is 70 cm high but when assembled and installed, the Green Cone stands 70 cm above ground level and extends 42 cm below ground level. Access is through a 28 cm diameter hole in the top of the solar chamber, which is sealed by a hinged lid with a turnbuckle security catch.
The Green Cone will dispose of more than 5 kg of food waste a week, which is over 25% greater than that produced by the average household. The food waste is converted into water, carbon dioxide and a small amount residue, without the need for user intervention such as the mixing or turning of the waste. In a well operating system the residue will occupy the bottom 250 mm of the digestion basket after the decomposition of about a tonne of food waste. Thus, after about five years this small quantity of residue must be removed and dug into the garden sub-soil.
Process
The decomposition of organic material is a natural process. Much of the dry weight of plant and animals is attributable to proteins and their remains are eventually converted into soil through a continuous cycle of activity by a wide range of interdependent organisms and micro-organisms. The distribution of these organisms follows that of organic matter and is therefore not uniform, with over ninety percent concentrated in the top 10 cm of the soil. By locating the digestion basket below ground, the Green Cone takes advantage of this distribution of soil organisms. The system works best in fertile, well-drained soil, which means that areas of solid rock or with a high water table are unsuitable. In heavy clay soils, drainage should be improved by using a mixture of gravel and compost around the digestion basket. Soil fertility can be enhanced by the addition of suitable natural bacteria, as described later.
The smallest and most numerous micro-organisms are bacteria, with one gram of fertile soil containing around a billion bacteria. Bacteria are unicellular micro-organisms and amongst the smallest living creatures known. There are three bacterial cell shapes, spherical (coccus), rodlike (bacillus) and spiral (spirillum). Under favourable conditions bacteria numbers grow rapidly. Some survive in a dormant or spore state when conditions are not suitable, reviving when they become favourable again.
On the basis of temperature requirements for growth, bacteria are grouped into psychrophiles (0 - 30oC), mesophiles (15 - 45oC) and thermophiles (45 - 60oC). Bacteria within each group exhibit specific minimum, maximum and optimum temperatures for growth, where, for example, mesophiles grow best in the temperature range 25 – 40oC. To ensure a healthy population of bacteria, the Green Cone is provided with a mixture of these natural bacteria on a cereal base for use when the system is first installed and if the decomposition process slows because of an imbalance of organic material and bacteria.
Most bacteria grow in a near neutral environment (neither acidic or alkaline) and without light. Atmospheric oxygen is required by some but not all bacteria, others are inhibited by its presence. Bacteria are classified as aerobes when they require oxygen to grow and anaerobes when they cannot grow in the presence of oxygen. Facultative anaerobes do not require oxygen but can grow in its presence; obligate anaerobes are poisoned by free oxygen. Under poor oxygen conditions some micro-organisms can produce toxins that inhibit the growth of higher plants and other micro-organisms. These toxins include methane, hydrogen sulphide, phosphine, skatole, indol and various organic acids. It is for this reason that the Green Cone is designed to maintain aerobic conditions through generating air movement, which results from the temperature gradients created by the double walled solar chamber.
In addition to bacteria, other soil micro-organisms are intimately involved in the natural decomposition process. Actinomycetes resemble both bacteria and fungi. Their spores, although similar to those of bacteria, germinate into very fine colourless threads (mycelia) that resemble those of fungi. Fungi such as moulds, mildews or mushrooms are usually more variable in form than either bacteria or actinomycetes. There are also algae, which are found as motile single cells or non-motile filaments.
Soil fauna ranges from microfauna, usually defined as animals less than 100 microns long, through macrofauna to megafauna, which are the largest soil organisms. Microfauna includes single-celled protozoans, some smaller nematodes, small flatworms, rotifers and tardigrades. Many microfauna only exist in the water films on the organic matter. The most common macrofauna are the small white segmented enchytraeidae that feed on fungi, bacteria and decaying matter. Soil macrofauna play a valuable role in fragmenting organic waste and increasing its surface area. In addition, with the help of symbiotic organisms in their guts, some also break down complex substances such as cellulose, keratin and chitin. Megafauna includes the larger earthworms, which also pass both soil and organic matter through their guts. The fragmented organic waste and soil fauna excretions create an environment suitable for the growth of micro-organisms. The continuous cycle of consumption, digestion and excretion by soil fauna alternates with increases in the population of micro-organisms.
Heterotrophic soil micro-organisms, which derive their carbon and energy from organic materials, are concerned mainly with the breakdown of organic matter, the carbon cycle and nitrogen fixation. Autotrophic micro-organisms, which obtain carbon from carbon dioxide and energy from the oxidation of simple organic compounds, form nitrites and nitrates and oxidise sulphur and iron compounds. Most micro-organisms produce carbon dioxide, which dissolves in water to form carbonic acid. Mineral elements such as sodium, potassium and magnesium are released to the soil during the decomposition process. The weak carbonic acid dissolves relatively insoluble soil minerals.
The different bacteria outlined above produce different enzymes, which are the protein catalysts responsible for the metabolism of organic waste. The principal enzyme types important in the decomposition of food waste are:
- Lipases to digest the fats in foods such as dairy produce, oil and meat.
- Amylases to digest the carbohydrates in foods such potato peelings, bread, biscuits, rice and pasta.
- Proteases to digest the proteins in foods such meat, milk and eggs.
- Cellulases, or cytases, to digest the cellulose in fruit and vegetable matter.
Where conditions are such that the decomposition process is very efficient, such as within the Green Cone, only a small residue of humic substances comprising lignin and protein remains.
Relevant Legislation
The UK has amongst the most comprehensive legislation and controls related to food waste and animal by-products in the world. The recent EU Animal By-Products Regulations and the UK’s National Regulations introduced controls for the processing and end use of composted material derived from food waste. Under the legislation, all food waste that contains, or has been in contact with, meat or other products of animal origin must be disposed of so that animals and birds cannot gain access. This does not just apply to the collection, transportation and centralised treatment of food waste but also to household treatment. Detailed guidance on the treatment of animal by-products and catering (food) waste has been provided by the UK Department for Environment, Food and Rural Affairs (Defra).
Meat or other products of animal origin fit for human consumption are classified under legislation as low risk category 3 animal by-products. Regulation 16 of the Animal By-Products Regulations states that the composting requirements for centralised treatments “do not apply to the composting of category 3 catering waste on the premises on which it originates provided that (a) the decomposed material is only applied to land at those premises: (b) no ruminant animals or pigs are kept on the premises; and (c) if poultry is kept at the premises the material is composted in a secure container which prevents the poultry having access to it during decomposition.” As a consequence, the household treatment of food can only take place in an enclosed container that prevents access by poultry, wild and domestic animals and birds. In addition, a household food waste digester must be physically separated from pigs or ruminants (e.g. sheep, cows, goats, deer) by a suitable barrier, such as a fence. This applies to both farmed and pet animals.
The design and operation of the Green Cone meets all the relevant legislation.
Health, Safety and Environmental Impact
The Green Cone is a benign technology and as such causes minimal health, safety and environmental impact. The below ground digestion basket and the sealed solar chamber provide sound physical barriers to odours, insects, birds and animals. With 40% of the unit below ground, the green conical solar chamber of the system creates negligible visual impact. The Green Cone is constructed from as much recycled material as possible, which in practice means that 100% of the digestion basket and inner solar chamber are made from recycled plastic.
Common to all handling of food and food waste, good housekeeping practises are the cornerstone of health and safety. Such practices include not spilling or leaving food uncovered in the home or elsewhere and the washing of hands before and after handling food. One advantage of the Green Cone is that no third party is involved in the collection and treatment of the waste, with householders handling their own food waste of which they know the provenance. In addition, the individual choices made in the home production or purchasing of food regarding organic growing and farming methods, GM derivatives, animal welfare and pesticide residues can be carried though to the household food waste disposed of in the Green Cone.
In use, food waste is transferred as soon as practical from the kitchen to the Green Cone, which allows food waste to be removed daily as opposed to weekly or fortnightly with a centralised collection approach. Once deposited in the below ground digestion basket, there is no possibility for unintentional access to the food waste by human activity. As with any gardening activity, particularly those involving soil or compost, gloves should be worn when removing the small amount of residue that accumulates in the digestion basket after several years of operation.
As noted in the previous section, much of the legislation relevant to the treatment of food waste is concerned with the risk to pigs, ruminants and poultry of infection from pathogens potentially present in meat and the subsequent risk to humans. A detailed risk assessment of the centralised treatment of food waste and land disposal of the compost produced has been carried out for Defra, which concluded that the approach is acceptable provided that a number of key conditions are met.
The derivation of reliable absolute risk values covering human intervention in environmental systems is extremely complex. The Green Cone has been assessed relative to the benchmark established by Defra using both qualitative and quantitative information. Environmental risk assessment is not concerned with the complete elimination of a pathogen by any one barrier but relies on a multiple barrier approach. Many of the barriers involved in the centralised treatment of food waste are common to the household treatment using the Green Cone system. These include the reduction and elimination of by-pass to the treatment process using physical barriers, the effectiveness of the process itself, decay in the soil, dilution in the soil and the fact that only household food fit for human consumption is being treated, of which a few per cent is uncooked meat.
Temperatures in the solar chamber of the Green Cone reach up to 50oC for long periods during summer months in the UK. Although temperatures are lower with natural decomposition in the Green Cone than in commercial centralised treatments, the indigenous micro-organisms can be preserved to grow at the expense of any pathogens present through competition for nutrients and predation. In addition, the process is performed over extremely long periods of up to five years, which allows for effective pathogen destruction.
Due to the effectiveness of the Green Cone in controlling process by-pass, the efficiency of the decomposition process itself and the inaccessibility to animals, birds and humans, the overall health, safety and environmental risk is extremely small and comparable to that for centralised treatments.
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