MFEM recently announced that a reticulated wastewater system with tertiary-level treatment and land-based disposal is the preferred approach to the Mei Te Vai Ki Te Vai (MTVKTV) project. The project’s lead water engineer Chris Wium explains what we mean by ‘tertiary’ wastewater treatment:
Tertiary Treatment is the final process in wastewater treatment. It follows pre-treatment, primary treatment, and secondary treatment. Tertiary treatment improves the quality of the wastewater before it’s discharged.
Here is a short description of the stages of wastewater treatment:
Pre-treatment is the minimum level of treatment. It only removes debris and large particles from the raw wastewater.
Primary treatment is an additional step after pre-treatment. It involves separating suspended solids (fine particles) and grease from the wastewater.
Secondary treatment follows pre-treatment and primary treatment. It is a biological process which digests and removes dissolved organic matter from wastewater. This reduces the nutrient and pathogen levels in the treated wastewater.
Tertiary treatment is an optional final process, following pre-treatment, primary treatment, and secondary treatment. Tertiary treatment makes the treated wastewater suitable for most disposal options, including land-based disposal with commercial crops.
Tertiary treatment can further reduce any combination of:
remaining inorganic compounds and suspended solids
nutrients (nitrogen, phosphorus)
pathogens (bacteria, viruses and other microorganisms that can cause disease) and parasites.
For the MTVKTV tertiary treatment, we propose using disinfection using UV (ultra violet light) treatment, possibly combined with further nutrient removal.
Disinfection destroys pathogens (bacteria, viruses and other microorganisms that can cause disease) and parasites. UV treatment works by damaging the microorganisms’ DNA so it is unable to replicate or infect.Nutrient removal will remove excess nitrogen, phosphorus.
The UV disinfection would be the last step in the treatment process before the treated wastewater gets piped to land for disposal by sub-soil irrigation into the soil under plants.
The plants then use the remaining nutrients in the treated wastewater, and the soil has a natural ability to filter out any remaining pathogens. A wide range of plants can be grown on land being fed by the treated wastewater.
Chris Wium, Civil Engineer, MTVKTV Project Management Unit.