you could make a poor man's drain field:
cut a few plastic 33- or 55-gallon barrels in half lengthwise. plumb your drain pipes into them however you like and bury them cut side down where your trees need water. a little colander full of
straw to filter bits of food out of your sink water is probably about all you would need. the only way this system is likely to clog up is if you wash a lot of grease down your drains. showers and washing machines shouldn't give you any trouble.
more complicated setups involve reeds and grasses planted in gravel for a living filter and maybe a surge tank to prevent overflow. something like that would be more suited to a situation where the water is headed to a
pond or some other body of surface water where stripping out some of the nutrient load is important. do a system like that well and you could drink the water coming out the other end. doesn't sound like that's what you're after, though.
soap: make your own. it's fun. if that doesn't appeal to you, more and more soap is available that lists ingredients. get some with ingredients you can pronounce. I recommend the bulk bins at your
local hippie supply outlet.