But it's only supposed to be biodegradeable and create a bigger surface area for the moisture to evaporate from, combined with probably keeping your deposit off the bottom of your highly expensive bucket. So, grass for instance. Paper. Heather, moss, wood shavings, apple cores etc etc etc. I'm not sure why people think it has to be a purchased bag of zip locked pre-processed sawdust/coco shell from a shop? The loos are used in remote Swedish houses with just bracken for instance.
Neil maybe you could post up some pics of your system from the outside because it is both quite cool and not quite a normal black tank perhaps? If it works well maybe more would try it. How can you tell when the cassette is almost but not quite full?