If you are discharging to a ditch or stream you must use a treatment plant. However, if you are discharging to a drainage field (through the ground), you are legally entitled to install a septic tank. However, many people upgrade this to a treatment plant for the following reasons:
This being said, a septic tank can be a perfectly good solution for home owners, and some (like the millennium septic tank) have filters to help prevent blockages in a drainage field. It has been proven that waste from a septic tank is cleaned in the ground. Therefore, the environmental impact (specifically our underground water sources) are safe from harm. However, for the drainage field to act as a good filter for water from a septic tank, the water percolating through it must do so slowly. Too fast and there is risk of contamination. If you are installing a drainage field then you may want to read our guide to give you tips and advice.