10 Questions on Composting – Lisboa Green Capital 2020
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10 Questions on Composting

Environmental sustainability is undoubtedly one of the main challenges of the 21st century. In addition to being present on global policy agendas, it is also a focus of interest for citizens who are concerned about reducing their ecological footprint.

As far as waste separation is concerned, home composting is gaining more and more followers, not only because it is an ecologically sustainable process but also because it is economic. By composting, the amount of waste that needs to be transported and treated is reduced and materials that are considered “waste” are transformed into organic fertilizers. This reduces the volume of waste while also avoiding the need to purchase fertilisers for a garden, a yard or a flower shop.

The Lisbon City Hall, together with Valorsul, took up this challenge and created Lisboa a Compostar, a project to encourage home composting, as part of the Municipal Waste Management Plan for the City of Lisbon and the European Project FORCE – Cities Cooperating For Circular Economy (HORIZON 2020).

This project aims at composting training, with the offer of a composer to those who have space to install it and the continued support by the City Hall to residents who, through registration, show interest in reducing their household waste. If there is no space to install the composer but there’s an interest in this process, there is always the possibility of using community composters.

For more information about this project or about the composting process just visit the website of Lisboa a Compostar.

1. What is composting?
Composting is a biological process through which micro organisms transform organic matter into compost. This compost, being rich in nutrients, works as fertilizer or soil fertilizer, thus improving the growth of plants, lawns and gardens, while also being an excellent alternative to chemical fertilizers.

2. Who can do home composting?
Anyone who has a small outdoor space. Just collect the leftovers from the food preparation and dump them in a compost. Then cover it with some dry branches and leaves and let nature take its course.

3. Where is composting done?
In domestic composters which, in some regions of the country, may be offered by entities which manage solid urban waste, or acquired in the market, with different forms and volumes. It can also be done in piles of compost in the garden.

4. Where you can put the composer?
The composer should be easily accessible, have water nearby and be protected from the wind, preferably under a deciduous tree, in order to avoid high temperatures in summer and low temperatures in winter (good mixture of shade and sun). The compost should be placed in contact with the earth, which should have a good drainage so that the water can drain and infiltrate when it rains.

5. What can be composed?

As general rule, all natural materials from the kitchen, garden or yard can be placed in the composer. However, some care must be taken to ensure that the process runs smoothly. For example, if you deposit bones or cooked food in the composer, you may attract rats or other unwanted animals. Waste that can and should be composted is usually classified as ‘green’ and ‘brown’ according to moisture content and proportion of nutrients. For composting to take place in the best possible way, it is important to have the widest possible range of residues in an equal proportion of ‘green’ and ‘brown’.

Green waste (usually wet and rich in nitrogen):

  • Green leaves;
  • Fresh grass cuttings;
  • Flowers;
  • Seedless weeds;
  • Vegetable and fruit remains;
  • Crushed egg shells;
  • Coffee grounds, including filters.

Brown Residues (usually dry and rich in carbon):

  • Dry leaves;
  • Mowed dry grass;
  • Dry cut and pruned residues;
  • Wood shavings and sawdust;
  • Potato peelings;
  • Straw or hay.

6. What should not be composed?
In order to avoid bad odors, pests or the delay of the process, there are some residues to avoid, such as:

  • Non-biodegradable waste – glass, plastics, paints, metals, batteries, among others;
  • Seasoned cooked food or with fat – may release fatty acids that lead to the delay of the composting process, decreasing the quality of the final compound produced;
  • Remains of meat, fish and seafood;
  • Dairy products;
  • Cigarette ashes/butts – are alkalinising and can be harmful as they increase the pH and cause ammonia volatilisation, contributing to the emission of unpleasant odours and reducing the nitrogen available for plants;
  • Medicinal products;
  • Plant residues treated with chemicals;
  • Domestic animal excretions;
  • Paper – should not exceed 10% of the stack. The wax (newspapers and magazines, for example) should be avoided because of its difficult decomposition, as well as the color, as it can contain heavy metals.

7. How long does composting take?
Composting may require a period of three months followed by another equal period for the maturation of the compost. However, these periods may vary depending on the type of material (greens decompose faster than browns), the volume of compost mass and the size of the particles. Small particles break down faster but need more aeration; larger particles break down slower and may require more watering. In the end, the compost should look homogenous, brownish in colour and smell like wet soil.

8. What factors influence the process?
It is necessary to check the composer regularly because there are several factors that influence the composting process:

  • Oxygen – The presence of oxygen inside the materials to be composted is essential for the survival and activity of the microorganisms that promote composting. The lack of oxygen leads to the production of bad odors – airing the pile allows a quick decomposition of the materials free of odors. One way to aerate the pile is to revolve the materials periodically (once a week).
  • Humidity – Water is fundamental to decomposing micro-organisms, also resulting from their activity when transforming biodegradable waste. Excess or lack of moisture in the environment negatively conditions the activity of these living beings. The way to control humidity is to do the “sponge test”: squeeze with your hand the waste that you are composting. If it drips, it is too moist, which means you need to add more brown residue. If, on the other hand, you notice that it is too dry, then you need to add green and water.
  • Temperature – The activity of microorganisms causes temperature variations. High values are essential to maximize the efficiency of decomposition and hygienization of materials. In the absence of a thermometer, stick a bar or iron tube in the pile and wait a few minutes. When removing, place your hand: if the bar is hot, the compound is good. The temperature considered ideal is around 60° and 65°C.
  • Size of materials – The material to be decomposed should be in small pieces in order to maximize the contact surface with the microorganisms. On the other hand, too small particles favour compaction and consequently limit the circulation of oxygen and water. Structuring materials (such as branches) help to ensure proper spacing.

9. How/where can the compound be used?
The compost can be mixed, using a 1% dose in relation to the soil, i.e. 100g in a 10 kg pot of soil. If the compost is very fresh (not very mature) or very rich in nitrogen, the amounts should be reduced by half.

  • Gardens and gardens – can serve as a cover or incorporated into the ground;
  • Vessels and seeds – twice a year, beginning of autumn/winter and spring/summer production seasons.

10. How to compost?
1. Choose the location – preferably in the shade, with a water point and no wind;
2. Prepare the bottom – place a layer of small branches to allow aeration and prevent compaction;
3 . Mix the materials – arrange the green and brown residues in alternate layers – the last one being the brown residues – in order to reduce odour problems and the proliferation of insects and other undesirable animals;
4. Let it air – place the pile of organic waste in contact with the earth to allow the entry of microorganisms and the drainage of water;
5. Keep the compost moist – water whenever necessary.