Hi Gavin, good to have you on the forum. Chris Buckley is from your area. Email me on
reeds@wetlandsystems.ie if you'd like his email address. He's a researcher and has worked extensively with dry toilet systems. He was an opening speaker and an Irish wastewater conference some years back and I was sitting in the audience in raptures as he expounded upon the many benefits of dry toilets to our flush toilet expert devotees. In fact it's not so much that standard sanitary engineering is devoted to the flush loo - it's more that no other option is even considered as a viable method.
If you need a septic tank for technical reasons, then I'd guess that the embodied energy in a new tank is probably a pretty low percentage of your overall house build. Thus, if you follow the law and then put in a reed bed for all the septic tank effluent to get the water really clean prior to discharge to ground, you'll at least be able to keep the local groundwater cleaner.
A 3/4 acre plot is about right for any reed bed option. Where rainfall is limited, you may wish to filter your grey water separately and to use it for watering food crops. The septic tank effluent (or reed bed effluent serving the septic tank) may also be reused by routing it to an area of trees. Here we use the Salix viminalis willow, but in Durban you'll have local varieties of evergreen willows that may be possible to use. Other trees would also be fine. In fact, food crop trees would be ideal if you can ensure that the inputs to the septic tank are all food grade. Eco-friendly detergents etc. Remember that any piping with trees will need to be modified to prevent damage or blocking by roots. I usually recommend a splitter system from www.ribbit.ie and then instead of the standard 4" perforated piping, cut a 1' pipe down the middle to create two upside down gutters on the base of your percolation trench. These form a lower cost trench infiltrator unit which allows the roots to come down around the effluent without clogging the pipe.
Anyway, just some thoughts. Let me know if this prompts any questions.
:-)