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.
.png)
1
