Primary, Secondary and Tertiary are three stages or options of waster water filtration and septic treatment depending on environment and individual requirements.
Primary systems relys on a primary anerobic treatment septic tank with a basic screen filter system allowing the liquids to flow into a pump chamber before being dispersed to a disposal field via simple LPED squirt lines. Primary systems are generally suitable for a non-sensitive 
receiving environment with good soil structures.
Secondary systems have an increased level of treatment within the system itself prior to distribution to the disposal field typically in metred dripperlines. There are two common types of secondary systems- aerated systems and packed bed reactors. A secondary system is suited to a more sensitive receiving environment and/or sub-standard soil structures.
Tertiary systems carry the same treatment process as a secondary system with an additional filtration or "polishing" process often in the form of UV treatment. This process will further remove the remaining organic matter and bacteria. Tertiary systems are required in very sensitive receiving environments.
It is important to consider what is right for your environment as there is always the “newest thing” out there but newest isn’t always best.
Ashby Consultants design solutions for your site, we don’t sell specific systems. We will work with you to choose the best value, most reliable system for your specific site based on our years of experience and our knowledge of independent comparative testing of systems available in New Zealand.
For your requirements contact your local team, Ashby Consultants.