I am looking for sustainable alternatives for carrying out human waste. WAG bags, though convenient, sadly have to end up in land-fills. I am looking for a system where the non-biodegrading components can be reused hygienically, and the biodegrading substances can be disposed of in pit toilets or composting toilets. In addition to sustainability and hygiene, weight, bulk, convenience when used on multi-day trips, and longterm cost are also of concern. Poop tubes (either the PVC or dry-bag version) strike me as a good starting point. Sadly (possibly due to the sensitive nature of the topic) I found it difficult to find useful first-hand information on how to make that system work with the criteria stated above.
Use a biodegradable (e.g. cornstarch-based) bag to pick up waste and then pack it in a airtight container. For obvious reasons the container should only be used for that purpose and disinfected when you return home ready to be reused next time.
Also, look out for schemes such as Keep Cairngorm Snow White which provides biodegradable bags, a pot to carry the waste in, and a site to dispose of the waste into sewage treatment on your return (also described here).