A really really basic constructed wetland for your waste water is not difficult to build and you can even use it for 'greywater' from your shower and laundry. You'd probably want to check if you need a permit in your location, but just saying 'it's for my washing machine' won't arouse suspicion.

If I had to build one, I'd start with a gently sloping area of soft soil, not rock, and build up a berm. Then I'd line it with a pond liner that has some holes in it, or clay so it is not TOO permeable, then coarse gravel, then upside-down chunks of sod from the lawn beneath. Add some tolerant wetland plants like cattails and rushes, or whatever grows best in your area (in mine it seems lately to be invasive purple loosestrife- pretty, but irresponsible to propagate), and start using it for your leftover ferts.
It would be worth looking into; no doubt plans for settling basins are all over the place on the internet. Look into the ones used to remediate road salt runoff areas, as the issues are similar.