How much liquid is produced when composting?

Question

Composting is a great way of turning common wastes into a usable product. The compost should be wet enough to allow the composting process to happen. But when composting wet wastes there is some excess of liquid which drains out.
I wouldn't usually care much about it unless there was too much draining liquid (making the vicinity of compost muddy and slippery).

• How much excess liquid is produced when composting common household and garden wastes?
• Is it advisable to try to lower the excess liquid somehow? If yes, what would be a good strategy to do so?
• Is there any connection between excess liquid and C/N ratio in the compost?

Answer 1

Let's assume that you are composting mostly plant material. Some water comes in with that. As the structure of the plant material breaks down, that water is free to move to the bottom of the container.

However plant structure is mostly cellulose (C₆H₁₀O₅)_n.

When munched this reacts with air to create CO₂ and H₂0. Ten pounds of dry cellulose will produce roughly 4 pounds of water.

Plant sugars and starches have similar breakdown ratios.

Most fresh plant material -- grass clippings, say -- will be 60 to 80% water to start with. For the sake of easy math suppose it's 2/3 water. That means that 10 lbs of dry plant material started out as 30 lbs of fresh material. So now you've got 24 lbs of water coming out of the bottom of your pile.

Answer 2

If we don't consider water from rainfall, then all water coming out was present in the materials you placed on your compost heap or bin, in one form or the other. Generally speaking greens (e.g. grass clippings, fruit and vegetable wastes) contain the most water, and browns (e.g. leaves, cardboard, coffee filters) not as much. Since greens contain the most nitrogen and browns the most carbon the C:N ratio is related to the amount of water, unless you add extra water or dry materials before adding.

A moisture content of 40-60% is ideal for a compost pile. A handful of compost should feel like a wrung out sponge; moist is good, soggy is not (unless you want anaerobic composting). This means that only 1 or 2 waterdrops should come out if you sqeeze the compost in your hand. Theoretically moisture contents can be over 90% and still allow aerobic composting, but in practice once you are over 70% moisture, the heap/bin contents becomes soggy and not enough oxygen is present for the organisms that are doing the aerobic composting. How much water is ok does depend a bit on the type of materials you put in there. Grass clippings, cardboard and saw dust can quickly become a soggy lump, but if you mix it with stronger materials such as straw and twigs then this will happen less fast and the heap/bin will be better aerated.

If your heap or bin is too wet, you can reduce the amount of water by:

• temporarily stop throwing in greens, or dry them first before adding.
• turning and aerating the contents regularly (if you put the most wet stuff on top, water can evaporate more quickly)
• adding dry pieces of cardboard (in the most wet places)
• adding a draining system, e.g. put your heap/bin on a small platform with holes in it
• sheltering your heap from rain

From your question I take it that you've already got some draining system. You can't say if the drained liquids are a problem just by looking at the amount. It could be that your draining system is working too well and your heap isn't wet enough or that you've added a lot of very wet materials. Or if you have an open heap or bin, perhaps there has just been more rain than usual. It's the moisture level of the compost in your bin that counts, so you'll have to check that before deciding on taking measures.