One of the questions that often comes up in relation to Earthships is why do they use flush toilets instead of composting toilets. The design for the Waiheke earthship intends to follow the Earthship approach to sewage treatment and recycle used water (grey water) through interior plant filters/ botanical cells which is then used for flushing toilets and then treated using a Simple Wastewater Solutions vermi-composting wastewater system. This wastewater system is an advancement over a standard septic tank as all the toilet wastewater goes into an exterior solid waste digestor where the worms turn it to compost. Instead of having to pump out a septic tank every few years (and send the sludge to landfill), the end result is nutrient rich compost which can be used in the orchard. The Simple Wastewater Solutions system also involves a series of exterior plant filters which further treats the wastewater by filtering it and transforming it to an aerobic state by using natural processes instead of electric air pumps which most conventional onsite waste water treatment systems require.
The Simple Wastewater Solution system has been approved within the resource consent for the Waiheke earthship as its an approved wastewater system in Auckland as long as you have sufficient land area to absorb the wastewater. The internal workings of recycling the greywater to flush the toilets falls under the building consent process rather than resource consent so this still has yet to be approved.
So why not just have a composting toilet? Composting toilets require a well ventilated chamber under the toilet that you can access to extract the compost and for maintenance purposes. Given that earthships are built on the ground/ concrete pad, into a slope and/or surrounded by earth this is not usually practical, unless your composting toilet is in a separate out-house or you have your toilet raised high off the ground above the chamber and get the compost out from the top in which case you have to carry it though your house to remove it.