I'm not an expert on any of this but I saw a couple systems in action while in europe last summer. In germany, I saw a wetlands wastewater treatment system that had been going for about a decade and working well. Also, in Scotland I saw a greenhouse artificial wetland system called the Living Water Machine. Both were impressive for rural/village models. I wonder about the possible applications for urban areas though. Does anyone know what's being worked on?

Also, anybody here familiar with biogas/methane digester, or composting toilets for solid waste?

On a permaculture video I saw a system in India where the outhouses fed a below ground biogas tank, the methane collected fed into lines for cooking in the adjacent kitchen area. pretty cool.