A Terrace Compost Bin

I have a large terrace where I grow many flowering plants and different types of vegetables.

To make my plants grow better I add fertilizer to the soil (I prefer the one made from horse manure). In addition to it I use kitchen waste, such as fruit peels, rinds and cores, potato peels, vegetable parts and so on, that I cut in small pieces adding them directly to the soil. A small problem lies in those pieces that remain in the ground for a long time before rotting.

Besides I really like the fava-beans that I grow every year. I like to eat them with slices of salami or fresh cheese (and a glass of wine, of course). Do you know what I'm talking about? In a small village close to my town there is an annual fair just about this kind of food.

So every year, after harvesting these fava-beans, a good amount of waste remains and I never know how to dispose of it correctly.

This year I thought that, instead of throwing them in the trash bin, I could build a home composter.

This is the material/tool list:

• A big plastic bin 100 litres
• A PVC grey soil pipe one meter long and 90 mm in diameter
• A 110 mm PVC end cap
• A drill with bits
• A hacksaw
• A hole saw
• Few bolts

After reading the Instructable "Easy Home Compost Bin" (by jsrubianoch) and the good ideas expressed in that tutorial, I tried to create my own home composter, trying to improve something.

I bought a bin similar to that one proposed (the store was selling only a black type) that is 100 litres, and a piece of PVC grey soil pipe one meter long and 90 mm in diameter. The first thing I noted is that in the original project the pipe was free to move. Since the material put in the composter must be mixed frequently, I thought of a way to fix the pipe to the bottom of the bin.

The pipe is flanged and the flange has an external diameter of 110 mm, which fits perfectly in an end cap of that size. I bought the 110 mm PVC end cap, and I fixed it on the bottom of the bin with three small bolts. (In any case an end cap with 90 mm in diameter could work).

After I cut the pipe to the correct length, divided the circumference into four, drawing a longitudinal line for each division and drilled a series of holes 80 mm away from each other, with a 6 mm tip.

Before placing the pipe inside, I drilled with a hole saw the bottom of the bin and the PVC cap for airflow. Without the presence of oxygen the system will be anaerobic and it will start to ferment and smell.

Finally, I also drilled the bin following the same procedure as the pipe, but with a 10 mm tip, only on four rows. If it will be necessary (and I will understand it from how the compost process will evolve), I can increase the number of these aeration holes. Just to remove any liquid that could form on the bottom of the composter bin during the fermentation process (leachate) I also drilled, threaded and screwed a 6 mm bolt, to be able to unscrew and drain the liquid in an easy way.

This liquid, properly diluted, can also be used as fertilizer. Make sure to clean all the holes properly to avoid plastic shavings in the compost. Remember that the bottom of the bin must be raised above the ground using, for example, just two bricks, leaving the central hole free for air inflow. For my wife delight, since the decaying material is not easily accessible from the outside, it will be difficult for some mouse to enter and hide inside.

Now let me talk about what is a compost, how to get it and what to insert in the composter bin. Compost is decomposed organic material that consist of four equally important components. Let's see them in detail:

• Carbon sources or browns
• Nitrogen sources or greens
• Oxygen
• Water

Browns

Browns are the source of carbon, a very important component for your mix, and will provide the energy microbes need to do the decomposition work. High-carbon materials tend to be brown and dry.

For a domestic composter perhaps in a small garden or on a large terrace in the city centre, finding brown components is not that easy. Checking the internet the best advice is to add:

• Tree twigs
• Hay
• Straw
• Wood chops
• Sawdust

Each of these components is difficult, if not impossible, to be found at home, in the neighbourhood or even in the town. Believe me, it is not easy to come across a bale of hay in the town hall gardens downtown.

Luckily the list includes other components:

• Dry healthy leaves
• Paper towel
• Pressed paper egg cartons
• Brown paper shopping bags
• Not printed shred paper (inks can be toxic)
• Cardboard (not with glossy coatings)

And I have quite a lot of them, or I can get them easily.

Greens

Greens are what some consider to be the main components of compost, that will introduce nitrogen into your mixture. They will then provide the protein to grow and reproduce more microorganisms, the main contributors to the decomposition of waste. Most kitchen waste is classified as green waste and it contains moisture.

These are a few examples of green waste that you can add to your compost bin:

• Fruit and vegetable scraps
• Clippings of fresh green grass and leaves (without pesticides)
• Food leftovers except for meat and bones
• Coffee grounds and tea leaves
• Fertilizer derived from animal manure (cow and horse)
• Stale bread

Oxygen

Oxygen is fundamental to oxidize carbon. Bacteria need an oxygen level of more than 5% to perform the processes necessary for composting.

Water

Water is needed in the right amount to maintain composting activity alive. Keep your compost mix moist but not soaked.

What not to put in the compost bin

Any other component not listed above and never:

• Toxic ingredients
• Plastic
• Glass
• Any kind of metal
• Meats and bones
• Weed seeds
• Pet droppings
• Glossy paper
• Cooking oils and fatty foods
• Diseased or pest infested plants
• Chemical fertilizer

As a rule of thumb, if you're not sure, don't throw it in the compost bin.

Making compost is not just throwing ingredients into a bin and waiting for them to turn into humus. You have to take care of the ratio between greens and browns, aerate the compost and maintain the air flow, check that the microorganisms and enzymes are working properly and finally verify the degree of humidity.

Process accelerators are additives that will speed up the process of introducing microbes to your compost, so it can start maturing right away. You can also just buy accelerators from your local gardening store if you want a fool-proof way to activate your compost, or you can just use some beer, that doesn't have to be a good one. Another important component is stale bread, which can act as a booster for the development of compost, but if in excess it becomes counterproductive because it releases acetic acid and ethyl alcohol (in the picture you can see pieces of "piadina romagnola")

Things like too many greens (nitrogenous material), too little aeration, too much moisture, and not being mixed well can cause a bad smell of a compost pile. Check your compost every now and then. If it smells badly, try turning it over to ventilate the mix.

Bacteria, or microorganisms, and enzymes help in breaking down organic material. Adding earthworms to the decomposition process, a practice known as vermicomposting, will help.

All of these ingredients mix to form an optimal blend that breaks down into a final product similar to humus, which is the dark, organic matter found in soil as elements decay.

Browns/greens (C/N) ratio

Browns are rich in Carbon while greens are rich in Nitrogen. If you search in internet the ideal ratio of carbon/nitrogen ingredients, you'll find that is about 25:1 (30:1 will speed up the process. With low values like 15:1 ammonia could be released). Be aware that the ratio is intended in weight, anyway if you put a kilogram of greens in your bin you don’t have to put 30 kg of browns. You have to consider that every component has its own C/N ratio as the following examples:

Browns

• Dry leaves 50:1
• Paper 100:1
• Cardboard 400:1
• Sawdust 500:1
• Cardboard (corrugated) 600:1
• Wood chips 700:1

Greens

• Coffee grounds 20:1
• Fresh green grass 15:1
• Vegetable scraps 15:1
• Fruit wastes 35:1
• Cow manure 15:1
• Horse manure 30:1

You have to mix the components proportionally to reach the target value (i.e.: 1 kg of coffee grounds [20:1] with 1 kg of dry leaves [50:1] and you'll have 2 kg of a mix with a 35:1 ratio). To help your ingredients break down faster, make sure they are as small as possible before adding them to the bin.

I hope you enjoy this Instructable. Feel free to leave your comments and feedback in the comment section.