Here we have sewerage system via a cesspit and soak-away. .....strangely the gardens in the hamlet downhill from us are extremely green ad fertile!!!
If mains water is cut, loo flushing will only continue using excess harvested rain water, or using hand lifted water from the well. The second option quite be very labour intensive.
Backup is a dry toilet. Next year we will start using this in a limited way ( me only I suspect) to evaluate humanure methodology. Toilet now refurbished and composting bins constructed in readiness...... I'm now looking forward to getting down to business