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As can be seen from the diagram the puraflo system goes in after the septic tank (not instead of the septic tank). Its purpose is to treat the liquid effluent after it comes out of the septic tank. If you have good percolation there there is no need for a puraflo system.
Stage One: Primary Settlement - Physical Treatment
1. The wastewater from the house flows into the septic tank.
2. Initial settlement occurs.
3. The gross solids sink to form a sludge layer at bottom of the tank.
4. The liquid effluent flows by gravity into the pump chamber.
Stage Two: Puraflo Biofiltration - Secondary Treatment
5. The liquid effluent is pumped to the Puraflo modules.
6. A pipework system at top level in the modules evenly distributes the effluent onto a naturally occurring filter media.
7. A combination of biological, chemical and physical processes treat the effluent as it filters through the media in the modules.
8. The treated effluent emerges from the modules through the outlet pipework for an approved disposal method.
Puraflo is one of a number of competing systems that essentially do the same thing but with differing technologies.
Bord na Mona seem to be in the process of Changing their name to Anua.