Treatment wetlands, yes we can implement these systems. I'm back now from Algeria where many towns and villages haven't any treatment solutions, where city with 200,000 habitants discharge wastewaters and solids directly in oasis or shallow desert lands without any basic pre-treatment like sedimentation. Don't believe...see the photo below!
A report of my visit is here: http://www.elwatan.com/Universite-Mohamed-Kheider-de Treatment wetlands are the right way also in Africa: costless, easy maintenance, reuse of treated waters in desert land for different purposes. In Algeria there is only one wetlands made by WWG international team (http://www.wastewatergardens.net/).
(this is not petroleum...is liquid shit!) [sorry for the bad word]