Greywater drainage issues

Our greywater from our shower and sink runs straight out the back of the house, down a tube, and down a long dirt slope (20ft ish) into a garden. We have never quite got the hang of how to do this properly and have issues with smell, flies, and just plain yuckiness. Anyone have advice, or links to plans that I could use to redesign our drainage? I am becoming pretty skilled at swatting flies, but it is a skill I would be happy to relinquish

It sounds like what you need is a reed bed filter for that gray water. You can also incorporate a sand bed pre filter so the water has time to loose any grunge and bacteria will eat up that grunge.

Check out the Watson Wick. I found this through info Brad Lancaster spoke or wrote about. I don't have a system set up at this time. Long range dream.

Watson Wick     http://oasisdesign.net/compostingtoilets/watsonwick.htm

Brad Lancaster's water harvesting site, has some info on grey water harvesting.     https://www.harvestingrainwater.com/

Note that there is a consensus that storing grey water in tanks, results in black water. He harvests grey water to store it in the landscape.

Personally I've always thought sand or gravel filters for greywater sound like they'll be a hassle years from now when you have to wash the sand or gravel. I like wood chips better as a filter. They gradually decompose along with the gunk and dissolve and go out, and from time to  time you can add some more wood chips. You could just have the greywater go through a wood chip box before it goes out to the ground. It will reduce the smell a bit, and if you screen or cover it to keep flies out, it will largely reduce flies. I saw Ana Edey of Solviva's wood chip and compost worm box for a flush toilet, and it worked great, but the effluent went into perforated pipes in a gravel filled trench along some pine trees. Since you're dealing with only greywater, and you've tolerated it being on the surface this long, I'd lean towards trying a simple solution like adding a wood chip (and maybe compost worm) filter and continue to send it out to the surface.

Our school has had greywater to surface canals for 20 or 25 years now and it works great, but in the hot weather, and as we have more and more people living here every year, it does smell a bit. One thing that helps with smell and bugs is to change the canal that it directs into every day or two, and let the soil of the other canal dry out a bit.

I've finally convinced the guys at the school to try out a wood-chip and compost-worm chamber to filter the kitchen greywater before it daylights into the irrigation canal along the trees, where it's been smelling and seething a bit.