Just an idea.  We know that waste water can be treated in a wetland using reeds and wetland plants to clean out the water.  Could the same be done using banana trees in a horizontal subsurface flow CW?  Instead of planting wetland plants on top could you plant banana trees instead.

I know that there has been a lot of great success with permies and natural farmers applying the banana circle and mulch basin concept.  Directing the flow of grey water in to the much basin and having the banana plants suck up all the water.  I would assume the excess water would percolate through the mulch and soil and into the ground.

Would it be bad for the soil if the grey water were to percolate and be absorbed into the ground? Or would it be better for the grey water to pass through a gravel filter system similar to a constructed wetland before being released in to the ground or water way.

I havent came across any information on whether bananas actually help to purify the water just like wet land plants do.  Does anyone know?

Banana plant roots themselves won't do much in the way of filtration (by the way, most roots don't actually do the water treatment in any system it is the bacteria and fungi that that do most of that) roots need to be the type that form mats to trap larger particles, hence the reeds and grasses are usually used for that purpose.

You may be able to get a system similar to the zero discharge willow system: http://www.wetlandsystems.ie/zerodischarge.html

The effluent is pump fed across the system and the trees mop up the liquid and nutrients as they grow.

If you knew the evapotranspiration uptake of the banana trees, the local evapotranspiration rates and the local rainfall rates you should be able to design a system to the correct size. Give me a shout if you need any pointers with it at reeds@wetlandsystems.ie