What is Secondary Treatment?
A secondary treatment system is an onsite wastewater system that treats to a secondary level. Secondary treatment in New Zealand is most commonly 20/30 (20 mg/L of BOD and 30 mg/L of TSS; in some areas it is 30 mg/L BOD and 45 mg/L of TSS).
What is BOD?
BOD stands for Biological Oxygen Demand. When aerobic bacteria decompose organic matter they use dissolved oxygen. Therefore high oxygen usage indicates high levels of organic matter in the water. This test is undertaken by a certified laboratory over a period of 5 days.
Standard household effluent has a BOD of about 350-400 mg/L, so your treatment plant needs to reduce this to 20 mg/L to meet secondary treatment levels.
What is TSS?
TSS stands for Total Suspended Solids. This is solid particles, both organic and inorganic, which indicate levels of contamination in the water.
Why 20/30?
These values are recognised by the local authorities as a safe level of contamination to be released onto the land. It is also the minimum requirement for the wastewater to be discharged from Pressure Compensating Drip Irrigation (PCDI). PCDI is designed for irrigating plants and has been widely adopted in the wastewater industry in New Zealand. At levels exceeding the secondary standard, PCDI irrigation will block more quickly potentially resulting in system failure.
When do I need secondary treatment?
Mostly this is governed by council regulations, the only expectation being if you particularly want to use PCDI irrigation.
Property Size: This is approached in different ways by different councils. Under the Horizons regional council any property under 4 Hectares must have secondary treatment. In Auckland a lot to discharge ratio is used, where any property with a ratio under 3 must have secondary treatment. The ratio is calculated by dividing the property area by the volume being discharged; so if you have a 2900sqm property and are discharging 1000 L/day, then your ratio will be 2.9 and you will require secondary treatment.
Waterways: The councils have regulations around how close you can discharge to a waterway or water body. The most common distances are 20m of separation for primary treated effluent and 15m for secondary treated effluent. So if you have limited space and need to discharge closer than 20m to a waterway, you will be required to treat your wastewater to a secondary level (There are also further rules around tertiary treatment that further reduce separation requirements, though these often require a resource consent).
Irrigation: Some of the regulation doesn't actually specify secondary treatment, but rather specifies that you need PCDI irrigation; this in turn will require you to have secondary treatment. Also if you particularly wanted PCDI irrigation for your planted areas, you would require a secondary treatment system.
Topsoil Depth: if the depth of your topsoil is less than 250mm, the council regulations require PCDI irrigation; to use PCDI irrigation you must have a secondary treatment system. With shallow topsoil there is still the possibility that soakage beds may be suitable, which only require primary treatment.
Slope Angle: Slope angle requirements vary from council to council. Some specify that slopes exceeding 25 deg should use PCDI irrigation, others have reduced this parameter to 10 deg. This applies specifically to the area in which the irrigation system is being laid.
