We are hoping to compost organic matter in a plastic bottle and collect biogas in a balloon over 10 days. Is this a safe experiment to do in a school environment?

John Richardson of SSERC (Scottish Schools Equipment Research Centre) comments:-

The biogas report from CLEAPSS (Consortium of Local Education Authorities for the Provision of Science Service) is in fact out of print. It might just be possible to get an old copy from CLEAPSS.

What I would say, from our recent experiences with composting on a relatively small scale, is that I have doubts as to whether this methane production will work very reliably in a plastic bottle. For example we found initially that when composting grass cuttings we had insufficient mass of material to get any significant fermentation going. It's quite tricky getting the right moisture levels and compost mix etc.

I have seen this biogas production work on a somewhat bigger scale though and no doubt with trial and error it can be made to work.

As far as health and safety issues are concerned there are specific risks when anaerobic fermentation is set up and particular care is needed to avoid contamination if soil is to form any part of the inoculum since several of the serious soil borne pathogens are anaerobes. Animal material (manure etc) is also best avoided - stick to vegetable matter such as peelings and grass cuttings.

The risk of explosion is slight, so long as the room is well ventilated and the set up is not in a confined space. The explosive limits for methane are quite high and the amount of methane produced is very unlikely to provide an explosive mixture. I also have some doubts as to whether there will be sufficient pressure to inflate a balloon (even a pre-stretched one). A taped plastic bag might be a better bet.

Anyway, all of this is mere speculation since we've never tried this actual method ourselves.