marsh plants to plant in grey water runoff for filtration.

Hey permies.

We've been working on a grey water catchment and I need to find some plants for filtration. Water from the kitchen sink and outdoor sink were just running from a tube into the ground, making an unused wet spot. So we dug a channel to spread out the water usage and its been working beautifully. I'm making terra cotta banks for it and a thin layer of rock in the midddle. Both are to keep and move the water. I've planted two different mints near the spout in the most wet area but I need something that can handle the food, dish soap and grease to plant amongst the rocks to help filter the grey water so its not so gross as it moves down our little creek in the making. Suggestions? I'm in spain, sea coast, mountain valley, resaonable amount of shade in the spot it will be planted.

Eleisia

I use "water Irises" of different species. I like the purple and blue flowered ones. I would give you seeds or starts, but mostly that is not possible trans nationally. I also use Juncus species. There are native Asters in this area that seem to grow in moist roadside ditches. These all have seeds this time of year where I live in Washington State USA. I guess what you would need are Facultative Wetland species.

I have noticed that with such areas of my own I need to add more small stones or gravel to low spots to prevent water puddles which would create mosquito breeding spots.

One might be able to collect seeds or notice a ditch clearing road project where "weeds" are being dug oiut and thown away so take home the "weeds" and plant them.

I did a Google search like this:
"small wetlands of Castellon, Spain"
that had many interesting results for me, including some constructed wetlands for various purposes.. YRMV

and another:
"constructed wetlands Castellon, Spain"
many good results possibly close to you and you might quietly ask for seeds or starts at one of these projects.

and another google search:
"small wetland plants of Castellon,Spain"
and many useful results in addition to this turned up..."Ethnobotanical review of wild edible plants in Spain" https://www.researchgate.net/publication/227510655_Ethnobotanical_review_of_wild_edible_plants_in_Spain

The paper here: https://www.researchgate.net/publication/40424182_Soil_Requirements_of_Three_Salt_Tolerant_Endemic_Species_from_South-East_Spain
has references at the end of the paper that could be interesting for you also. Soap by definition is a salt, although the dish soap might be a detergent.

Finally. While the writing is somewhat pie in the sky for my taste, Anna Eddy
https://www.solvivagreenlight.com/green-clean-wastewater-management-with-flush-toilets/
has produced some of the most interesting publicly accessible work on water filtration as she documents in her book "Solviva" which I have read. I borrowed it from our local library. If you had money you could purchase a copy. I have not read her newer book “Green Light at the End of the Tunnel”, but I would like to. The main reasons I point to her work is that it is written clearly, approaches problems with down to earth common sense, and is not afraid to detail failings of her own and of "the system" we are all so familiar with while at the same time she perseveres and finds compelling solutions. Like growing onions in urine water!!! Other practitioners of waste water management seem reluctant to divulge their methods, the species they utilize, or even patent their "system".

Cattails are traditionally used in grey water holding ponds.  Switchgrass or canebrake is a native (to US) and does well in moist wet soil and is hardy enough to handle the hard to digest material.  Willow trees will also soak up an amazing amount of water and then be coppiced.   Many bamboo species will do well in wet soil as long as they are not inundated so the rhizome can breath.  Bald Cypress would be an option.  Cedar trees are heavy drinkers as well.

Also, in my mind, it is a two pronged approach to soil filtration.  Keep the excess water absorbed by thirsty plant species to keep the soil damp but not saturated, so the earth worms can eat all that wonderful carbon matter that the plants can't process.  Vermiculture is the best way to deal with that waste; and they will, so long as they can breathe.  

Jack: I will have to look into if those american species can thrive in this climate but as I love willow, that's a great idea. I thought of them even it's just that I want to make this space as productive as possible so was thinking of herbs and/or plants that attract bees. Worms, of course! We found a gigantic earthworm while digging the trough so they are certainly eating and being drawn to the area. It helps we have a compost pile very close by. I appreciate your suggestions.

Dan: I had an iris planted there but it was too wet for it. I'm going to replant it now that the water is more dispersed and add other species. There is a reed species that grows rampant around here but any juncus is going to overwhelm the other plants. I found that adding small stones was a great idea and since there are rocks everywhere here, it was easy to source. That's a great suggestion, they are actually doing some work like that next to a river here so I will go and see what I can collect. I didn't even think to google those! Great references, thank you. I will look into the author and links you have sent me. There are wetlands very close to here, I drove through them the other day even. Thank you for your help.

Yep ordinary irises will not thrive with wet feet. I should have included the idea of googling "water irises" as most are not familiar with that section of the iris family. They make great filtration plants, some are facultative in that they can live in very wet soil or in water OR in ordinarily moist soil. They also can be used to stabilize small islands and peninsulas, creating higher ground that can be homes for other plants. To take that idea farther, those higher ground patches can be keyholed or woven making the water path less straight which gives more filtration.

I admired the iris growing along canals when I recently lived in the netherlands and was aware of them long before that. I think they will be a good idea along with several herbs I already had in mind. That's interesting about them being island builders, I like that fact.