How to obtain quality compost?
Obtaining quality compost is not easy! Even less so on a collective or industrial scale! So how can you compost large volumes of bio-waste without any nuisance or odour, and with ease?
Composting, you know: a bin at the bottom of the garden, filled with peelings and grass clippings, and found 12 to 18 months later filled with compost ready to be spread in the vegetable garden.
And yet, obtaining quality compost is not at all easy, especially on a collective or industrial scale! Piling up organic matter or so-called "surface" composting (which incorporates waste into the soil in a superficial manner) does not allow the matter to decompose properly.
With these methods, you even run the risk that the bio-waste, packed in an environment without oxygen, will produce methane. Methane is a gas that has 25 times the global warming power of CO2! This is a shame, as composting is fully in line with the ecological approach of the circular economy.
What is quality compost?
Compost is basically humus! Like the one you find in the undergrowth, under the carpet of dead leaves, and which has that smell so characteristic of autumn walks.
Humus is the result of the decomposition of organic matter in an aerated and humid environment. Nitrogen and carbon, which make up the majority of biowaste, are assimilated and transformed into energy by an incredible fauna: insects, myriapods, woodlice, worms, mites, bacteria, moulds, yeasts, micro-algae, etc. This is how the leaves that fall from the trees are transformed into humus every autumn!
When compost is created by composting, the process is more controlled for a controlled result. The mature compost, ready for use, is brown or black in colour. It is homogeneous in appearance, without lumps, and can be crumbled and spread easily. It is stable, with a stabilised carbon/nitrogen ratio of 10-15, which means that it no longer contains nitrogen that can be assimilated by micro-organisms. It is also hygienised, i.e. free of any pathogens and weed seeds (the famous weeds).
What elements are found in good compost?
When compost is spread on the soil or on crops, there are two main benefits: fertilisation and soil improvement.
Fertilisation: this is the capacity of compost to provide nutrients that can be assimilated by plants. In other words, to feed them directly. These fertilising elements are provided in organic form: nitrogen (N), phosphorus (P), potassium (K) and magnesium (Mg), as well as various trace elements in very small quantities.
Beware! Even if they are present in mass in the compost, these elements have a low fertilising power. Less than 10% of the nitrogen supplied is available in the first year, and about 30% to 40% of the phosphorus.
This does not mean that using compost is useless, on the contrary. But its amending value is much more interesting than its fertilising value! It stores organic matter in the soil and therefore, in the long term, stimulates its microbial activity, improves its structure and its capacity to retain water. With compost, the soil is first nourished to feed the plant.
Compost has a strong organic amendment effect: 1 tonne of compost provides about 300kg of organic matter, i.e. 216kg of humus, for an organic matter stability index (OSI) of 72%)[2].
It is precisely this indicator, the OMSI, that allows us to measure the quality of a compost. It indicates the percentage of compost that will turn into stable organic matter, i.e. humus. The higher this index is, the more stable the organic matter is and the more difficult it is to degrade, the richer the soil is.
A quality compost is therefore one that has a high OMSI and, to a lesser extent, contains nutrients that can be assimilated by plants.
What does it take to make good compost?
Composting is a living process! To make good compost, you need to create the right conditions for microbial life. These conditions are quite simple: you need organic carbon, sufficient aeration and constant humidity.
Carbon-rich waste
To produce quality compost, the first thing to do is to use a nutrient-balanced starting material for the micro-organisms, which can promote good fermentation.
Organic materials are composed mainly of nitrogen and carbon. The ratio of carbon to nitrogen, noted C/N, determines the humic potential of the compost, i.e. its capacity to generate humus. The higher the C/N ratio, the slower the compost degrades and provides stable humus. For optimal bacterial activity, the C/N ratio should ideally be between 25 and 35, i.e. 25 to 35 times more carbon than nitrogen in the organic material entering the compost.
Carbon-rich bio-waste is woody material such as branches, wood, cobs, dead leaves, sawdust or cardboard. On the other hand, soft, moist materials such as fresh lawn clippings, sludge, effluent, fruit and vegetable peelings are rich in nitrogen.
Meat and animal protein materials already have a satisfactory C/N ratio. They can be added without worrying about carbon and nitrogen rations. Kitchen scraps, including meat and fish, are also welcome in compost. Like all organic matter, they will eventually decompose.
However, in wooden composters, it is preferable not to put meat in order to avoid bad smells and pests. On the other hand, in electromechanical composters, it is perfectly possible to throw away all the leftovers from plates and food preparation because the hygienisation process avoids this kind of unpleasantness!
Water
Water is essential to life and to compost! It is water that allows bacteria, moulds, worms and other insects to live in the compost. Good quality compost is slightly damp. If it is too dry, the mixture heats up and carbonises the organic matter.
But be careful not to overdo it! If the mixture is too wet, the fermentation process does not have enough oxygen: it synthesises methane and leads to rotting. In the end, the compost is of poorer quality.
It is estimated that the ideal moisture content ranges from 40% to 70% by mass, depending on the composting phase. It can be controlled, especially in an electro-mechanical composter that will take measurements.
Air
Like water, air is essential for the bacteria that decompose organic matter. Their action must take place in aerobic conditions, i.e. in the presence of oxygen.
In the absence of oxygen, we are in anaerobic conditions: we no longer have the desired degradation, but a simple fermentation, source of bad smells and methane.
To have a well aerated compost, there are two possibilities, which can be combined:
either, you incorporate green waste that will let the air through, such as straw or shredded wood.
or, the compost is regularly mechanically aerated, i.e. turned or mixed. This can be done with a simple fork in a garden, or with more powerful and automated tools when the composter is a good size.
Controlled acidity
To create the right conditions for the degradation of carbonaceous materials, it is preferable to have a rather neutral environment. The pH varies according to the type of waste in the compost and its maturity. If the materials used are very acidic, calcium can be added during the composting process to reinforce the alkalinity of the medium.
If you need to guarantee the quality of the compost at the end of the process, then it may be necessary to know what waste you are going to throw in. This is why UPCYCLE assists most of its clients in identifying the biowaste produced, in order to establish the best composting recipe, and to guarantee the quality of the compost output.
A controlled temperature
The compost naturally rises in temperature during the first few days. It can reach 60 to 70°C due to the activity of the microbial populations which release heat.
Maintaining high temperatures makes it possible to hygienise the material to be composted, i.e. to eliminate pathogens, diseases (salmonella) and pests (helminth eggs, for example). It can also destroy or inhibit the germination of certain seeds, such as weeds.
At temperatures below 45°C, the survival of most germs is 180 to 240 days. Between 45 and 55°C, their survival is only a few days. A temperature of 55°C for several days reduces the risk to almost zero.
If you produce more than 1 tonne of compost per day, you will need to obtain a health permit. The latter is conditional on composting that respects the time/temperature pairing (more than 55°C for 2 weeks). You will then have to provide proof of this sanitisation.
Below this production threshold, you are not required to obtain this approval to distribute it locally. However, some of your partners (nurserymen, green space managers) may ask you to provide proof of its hygienisation.