dealing with waste water
In this case we mean both grey water and black water (from toilets), in other words, drainage. A septic tank is the traditional answer but these need emptying at least annually. The alternative is a sewage treatment plant or biodigester, like the Klargester BioDisc treatment system, which breaks down the material which then outfalls to a drainage field. Sludge build up will need clearing out over time by a licensed waste removal contractor.
Capital cost of this option is higher than a septic tank but the running cost is less than £20 per year (typically to power the biodigester) compared to £200 to empty a septic tank. There are also models which work without electricity.