Thanks for the thoughts so far. To respond to some of the questions/thoughts:
Right now, the summer kitchen is just a roof over sloping land just downhill from our "grazing garden." That's grazing for us, not the deer, hopefully! The fill to level the floor hasn't even been brought in yet, let alone any flooring, counters, fixtures, etc. So there are no restrictions at this point on what I can do regarding installing diverter valves and the like.
Currently, there are no fruit trees or anything downhill from the summer kitchen location. It's location was chosen primarily due to traffic flows around the property. I don't want to deal with a pump that might break. Unless I use the greywater to irrigate mature white pines in the wind break, which rarely need any supplemental water, anything watered with it will be chosen and planted to accommodate the greywater, not the other way around. I can go two directions with it, down about 120 horizontal feet of about 10% grade, or down about 40 feet of 40% grade, to reach what to me are logical places for the water to end up, neither of which have been developed at all yet (and I have to mow them. I hate mowing). I'd welcome suggestions for the final use of the water. Would need to be frost tolerant, because the bottom of the hill is a severe frost pocket.
I'm concerned with diverting the veggie wash water directly to plants/ground, because of the proximity of the swamp, and the water table. Our infiltration rate is so high that water flows vertically down through the sand faster than it flows horizontally along the ground anywhere that isn't heavily compacted by livestock or vehicle traffic. Our garden soil is heavy in nutrients because we add large amounts of
compost, poultry-, alpaca- and horse manure, and worm castings, a fair bit of greensand and aragonite, and a bit of boron to correct its deficiencies in various micronutrients and organic matter. So, when we're washing veggies, and particularly root veggies, there's going to be a lot of nutrient runoff in that wash water that I want to be used by whatever we plant, but not make its way past that. Or settled out and returned to the garden from whence it came.
I don't want to divert to septic for two reasons: 1) septic tank is uphill. Well, the tank itself is probably close to level with the summer kitchen, but, still... and 2) getting the permit to add a building to our existing septic is difficult, time consuming, and expensive in our county (in part because we are the absolute northeast corner of the county, and the county seat is in the southeast quadrant of the county. Inspectors hate coming out that far in general). However, I could see diverting "regular" kitchen wastes to a wood chip biofilter, which would drain to the rest of the greywater system, while the veggie wash water bypasses the biofilter. Does that seem logical?
How would you go about constructing the settling tank so that it drains slow, but doesn't clog with the sand and such, and removing the settled solids is fairly simple and pleasant?