Primary, Secondary and Tertiary are three stages or options of waster water filtration and septic treatment depending on environment and individual requirements.
Primary systems relays on a basic screen filter system before waste water is dispersed into the environment via LPED dripper lines and is generally suitable for a non sensitive 
receiving environment with good soil structure
Secondary systems have an increased level of treatment and the biogradation is enhanced by aerobic conditions and would most likely involve aeration. A secondary system is suited to a more sensitive receiving environment and sub-standard soil structure
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. You would require a tertiary system in very sensative 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 Consulting Engineering designs 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 Consulting Engineering.