off the net:
Primary treatment involves screening the solids from the water and allowing a proportion of the suspended solids and organic matter to settle from the wastewater.
Secondary treatment takes primary treated effluent and with the aid of biological processes breaks down a further proportion of the dissolved or suspended organic matter to a form that reduces its environmental impact if discharged. Disinfection by means of chlorination, ozonisation or UV radiation, is also often considered to be part of the secondary treatment step.
In tertiary treatment, the secondary treated effluent is further processed using various techniques including flocculation, coagulation, clarification and filtration. The main aim is to remove nutrients such as nitrogen and phosphorus and further reduce the small amount of organic material and any remaining harmful micro-organisms in the secondary treated effluent.
the septic tanks would receive pumpout i would imagine, rather than absorption into the ground.