Optimisation of Onsite Wastewater Treatment Systems

I am currently working on decentralised options for water and wastewater treatment in my hometown.

What are the possible energy-efficient and cost-effective optimisation options for the onsite wastewater treatment system?

My intention is to treat (and reuse domestic wastewater) to at best drinking water quality.

The dominant onsite treatment system in the targeted area are the septic tanks. l am, thus, looking for alternative technologies or setups that can be adapted to reclaim wastewater from this system.