A simple low-cost technology ...

A simple low-cost technology for onsite treatment is Constructed Wetlands. Usually the costs for operation and maintenance are very low, since these facilities are easy to run (no big mechanical parts, no need for personnel, almost automatic-natural operation, very low power consumption only for pumps). However, surface area needs are higher. There are numerous such facilities around the world now, we have also designed many and it is well proven that treated effluent can be reused for garden irrigation or toilet flushing. For applications like yours, I think you should seriously think about CWs. Let me know if I can provide any assistance.