Is vermicomposting our food rubbish sustainable in term of climate change?

Question

I'm a fan of indoor, cold, worm composting (vermicomposting) to reduce my rubbish. This uses a box with many worms that eat your vegetable remains in a very efficient way, without any odor, and outputs earth and vermicompost "juice". And it is also very entertaining to have such pets!!

The benefit of this practice are:

• reducing the amount of rubbish to be sent to landfill,
• having rubbish less humid to avoid the use of additives to burn the rubbish in the garbage processing centers,
• reducing the weight of rubbish and reduce the frequency and number of bin lorry,
• transform this rubbish into a re-usable product (fertilizer), can be used indoor in city where people usually doesnt have a garden to make traditional composting.

However, it seems that vermicomposting creates massive greenhouse gas due to the intrinsic chemical process in the worm gut which produce nitrous oxyde (N2O), a greenhouse gas hundreds time more powerful than CO2. However, vermicomposting produces less mathane (CH4) greenhouse gas than hot composting; also N2O emission in vermicomposting can be reduced with several factors (temperature, humididy rate, aeration, carbon etc) (2, 3).

My question is: globally, and in the context of global warming, is it more appropriate to throw their vegetable waste to the bin or to use vermicomposting at home?

Answer 1

If I understand, the overall CO₂ (and weighted N2O, CH3 equivalents) net balance of of in home vermicomposting (or other composting) is at the crux of the question here.

Some rigorous and easily repeatable testing of the compost vs vermicultivation has been done, the results put vermicomposting as in improvement in general; and that CO₂ and CH₃ reduction performance could be improved with the sufficient inclusion of carbonaceous material, aka 'Brown' and also improves with increasing density of worms.

It seems than any net CO₂ (N₂H, NO_x , H₂S, plastic nanoparticles) discussion could include the net effects of the total emissions caused by transportation systems for large scale collection of wastes. Simply not adding to that source of emissions makes some sense in reducing net CO₂ and other equivalents. Whatever that amount is, zero is less than any.

Then, in the case of the resulting fertilizer from on-site vermicomposting, there is the potential for further net reductions by not purchasing or transporting fertilizer to your garden; however small that might be. Add to that the further reduction in transportation of the vegetables costing only the breath walking from garden to kitchen. Truly, this isn't a facetious argument; the net total of all CO2 from consumer produce shipping is made up of exactly all our little bits we didn't grow or gather ourselves, and by conservative estimates most produce contributes an average of 0.1kg of CO2 equivalent per 1.0 kg of food due to transportation of that food.

source https://science.sciencemag.org/content/360/6392/987

a comprehensive meta-analysis of 1530 studies, supplemented with additional data received from 139 authors, assessed against 11 criteria designed to standardize methodology, resulting in 570 suitable studies with a median reference year of 2010. The data set covers ~38,700 commercially viable farms in 119 countries and 40 products representing ~90% of global protein and calorie consumption'.

One reference for actual numbers comparing composting to worms: http://dx.doi.org/10.1016/j.jclepro.2016.08.058

Vermicomposting as a technology for reducing nitrogen losses and greenhouse gas emissions from small-scale composting

As all food waste byproduct not incinterated will eventually undergo decomposition, whether intentional or simply buried in a landfill; any situation which worms participate in the process appears to reduce harmful emissions overall.

In both composing cases; at home or collected/municipal; the total food waste emissions might change in rate of release vs duration of decay, but not in aggregate total emissions. So, it would appear that little net difference in direct emissions is made if none, any, or all homes compost. As the ratio of vermicomposting is used to replace composting; the specific emissions should decrease as shown.

Any reduction of transportation related emissions in the pursuit of a transition from composting to vermicultivation appears to be a tangental but beneficial outcome. In some specific situation where the transition would increase the distance, total net mass, or utilize dirtier energy sources this might work against those gains.

TLDR; Vermicomposting reduces nitrogen loss 10–20% over composting, decreasing N2O 25–36% and methane emissions by 22–26% and may be less overall when including transportation related CO₂ and other greenhouse emissions.

So, if forced to choose worms or composting, the worms are marginally better overall. If the total volume of food waste was handled at home; compost or vermiculture, the reduction in transportation is the same, and reduced emissions would occur to the extent that worms are preferred over compost.

Answer 2

Any kind of home composting will result in less greenhouse gas emissions than sending the waste to landfill. The waste composting at home will indeed release CO2, as a result of the worms and microbes respiring, but this is exactly balanced out by CO2 removed from the atmosphere by photosynthesis when the food was growing. However, in a landfill, there is less oxygen available than in a home composting situation, so some anaerobic digestion will take place, and methane, a more potent greenhouse gas than CO2, will be released. Even in the best case, the waste would decompose in landfill and emit the same amount of CO2 as in the home composter anyway. I suppose that in some cases the methane may be captured for use as fuel, but for general landfill I would suspect that the proportion captured this way is not high enough to counter the negatives that the original poster mentions. I have seen a few projects based on collecting food waste for anaerobic digestion to methane, but these tend to be small scale operations focused on very local sources of waste, often using bikes to transport the waste, rather than long distance trasnport in trucks.