I'm a permie and a licensed civil engineer in New York, so this is a topic of definite interest to me. One of the big challenges I've found with the state regs here is that there is no leeway from a traditional septic tank if you want to have a flush toilet. Having read Joe Jenkins' Humanure Handbook, I personally would have no issue with abandoning them and composting -- but for the vast majority of folks (my lovely wife included), this is a non-starter. The need for a septic tank for primary treatment (settlement) means that you have to have someone come in every so often and pump it out.
I've been doing some minor consulting work on the side with a group that wants to found an ecovillage in my town, and the whole septic arrangement has been frustrating. One idea I had was to tie several housing units into one system that used a low-HP grinder pump to push the sewage uphill to a small greenhouse with digesters, use the sewage to produce cooking gas through anaerobic digestion, and then allow the effluent to flow back downhill for fertigation purposes when the process is complete. All well and good -- except that by the regs you have to have a licensed wastewater treatment plant operator on site 24/7 for any system that involved any moving parts/processes.
One idea I've had is to look at bringing in a university department and approach the site as an experimental station in order to get the DEC to relax the regs. Wondering if you have any experience dealing with these kinds of thorny regulatory issues surrounding blackwater treatment for a small community. Thanks!
Something that I've found here in Ireland - and it may well be in operation in the US as well - is that while the Irish EPA Code of Practice appears to be limited in it's scope around dry toilets, willow facilities and the like, in actual fact it's not as limited as it seems. Not all local authority personnel would agree with me; but some do. Many people in the local council here are as frustrated with the legislation as are those trying to do the eco-friendly thing. They need to tick their boxes, but they also really want to do their best or the local environment. Sadly there are also many examples where the eco-friendly thing just isn't allowed. In such cases - conserve your energy and do the next right thing rather than worrying unduly about what you can't do.
Your site meanwhile… be sure you have enough houses to power the anaerobic digester in a cold climate. You could just put in the septic tanks and then filter the water through a reed bed or constructed wetland system. You can get water clean enough to irrigate at surface level, but actually it's easier if you have a subterranean irrigation system feeding to fruittrees or a biomass woodland.
The university link is a good idea. We've used similar approaches here to good effect.
Constructed wetlands for secondary treatment were a direction that I looked into as well. But this addresses mainly the secondary treatment. What I was looking for was a way to get around the regular pump-outs for a septic tank -- especially since the community is looking to largely prevent auto transport of all types from traveling through the property.
I had a couple of conversations with Bob Hamburg (aka Biogas Bob of Dragon Husbandry) about methods to do anaerobic digestion in a cold temperate climate. He's also worked on biodigester designs that more closely mimic an intestinal tract -- flexible rubber with long horizontal runs instead of batch runs -- that would work inside of a greenhouse.
One possibility is to use septic tanks (if you need to), and then pump the effluent on-site to a sludge reed bed or sludge constructed wetland. I'm not sure how enthusiastic the local authorities would be about that. It would need to be done really carefully following either best practice from Danish sludge reed beds or a soil based constructed wetland. This would be for annual use only, dose fed from the septic tank - either by gravity syphon if you have the right gradient, or pump fed.