An in-situ communal ...

An in-situ communal recirculating sand filter with disinfection could do the job at a minimal cost. The USEPA has guidelines for the design and construction of on-site wastewater treatment system, which are excellent for isolated communities and certain activities (military fields, golf courses, large farms, isolated communities). A two-chamber storage system with a volume proportional to the flow is needed, either underground or above the surface. Solids decant in the first chamber like in most septic tanks, liquids flowing by gravity to the second chamber. Either by gravity or with a small pump, and equivalent amount of the inflow is discharged into a sand filter, near the surface, allowing the water to trickle through the sand filter (depth of sand is proportial to area of treatment and/or inflow). The sand filter is encapsulated with a rubber membrane welded in situ to a seal. Collection pipes at the bottom of the sand filter drain by gravity into a sump, where a low-head pumping system with a recirculating solenoid valve returns a certain % of the flow to the second chamber and the balance to the irrigation fields. Disinfection is provided to the line going to the irrigation field through an automatic chlorinator or UV system. In a small system we built for a golf course in Puerto Rico, BOD's were reduced by 95%, equivalent to secondary treatment, and the transparency of the water was almost 100%. I have a paper on the subject that you can request at my email fquinon@msn.com, but it is in Spanish. The USEPA page on In Situ Treatment Systems provides a lot of the design criteria.