If you comply with the requirements in this regulatory position statement you can treat solid waste from composting toilets at the depot where they were hired, without an environmental permit.
Conventional portable toilets at events are sucked out by tanker and the waste is generally taken directly from the event to a waste water treatment works for disposal. The empty toilets are then returned to a depot for cleaning ready for re-hire.
Composting toilets are different in that the waste is returned to the hire depot where it is treated. This requires either a waste exemption or an environmental permit.
If you follow the requirements in this guidance you will not need to apply for an environmental permit for very small-scale composting activities taking place at the depot where composting toilets are hired.