On-site Effluent Testing (OSET)

The On-site Effluent Treatment National Testing Programme undertakes performance testing of ex-factory on-site wastewater treatment units at its testing facility located within the Rotorua City Wastewater Treatment Plant Site.


It provides a testing and benchmarking facility for up to seven on-site domestic wastewater treatment units annually. Trials are managed by staff at the Rotorua wastewater treatment facility and overseen by technical experts from around New Zealand.

Current test certificates for units which have been trialled at the facility are available here. Detailed performance reports which can be used to assist with design and environmental limit setting are available to funding partners.

The OSET Test Facility was established in 2008 by the Bay of Plenty Regional Council, Rotorua Lakes District Council Water New Zealand, and the Small Wastewater and Natural Systems group. The facility is funded by partner councils and manufacturers using the facility.

A list of contacts for the facility is available here.

Accreditation / Audit Process

Each trial runs from November through to July, with a 2 month settling in period followed by a 6 month test period (of which the last three months involves benchmarking of six treated effluent quality parameters) and a 1 month high load test.

Data sets from the laboratory testing are analysed and compiled into a benchmarking and performance certification report. This report assesses treatment performance in meeting the secondary effluent treatment requirements of AS/NZS 1547 as set out in AS/NZS 1546.3. A list of parameters tested at the facility is outlined in the laboratory services section of this website.

Benchmarking and performance certification reports are overseen by an independent advisory panel of experts from around New Zealand, the Small Wastewater and Natural Systems Management Advisory Group (SWANS-MAG).

Testing and benchmarking procedures are outlined in this document.

Research and Development

Depending on availability of test beds, the facility may also be used by manufacturers for research and development. Contact the technical manager to discuss opportunities.

The OSET Testing facility

The OSET testing facility is focused on security and repeatability for every trial by ensuring that there is a fair and accurate comparison for every participant.

The trial sites are fully fenced (as shown in the photo below) and have multiple surveillance cameras installed. Each of the 7 sites has its own header tank with a certified volume. Once these tanks have been filled by the raw sewage pump the process is then gravity fed to the individual sites with the release of wastewater controlled by a PLC controlled solenoid valve to follow a typical household flow as closely as possible.

Each header tank is currently filled with 500 litres twice per day with raw sewage to ensure the wastewater is fresh and has not settled. The 1,000 litres per day test flow volume represents the daily domestic wastewater flow from a 3-bedroom dwelling with occupancy of 5 to 6 persons.

Each site is 8m x 3m and is fitted with a 50mm slip fitting at an invert of 2.7m for the raw sewage supply, 25mm discharge point with a flow meter and 5lb (3.5m) back pressure valve for the treated effluent, 240V hard wired electrical point and a kW hour meter.

The system is fitted with many fail safe measures that are fed into a SCADA system with any failures sent to the operator on call via cellphone. There is a redox probe that monitors the wastewater, a flow switch which alarms if the pump is called to start and there is no flow and each individual header tank is fitted with a level monitor that will sound an alarm if the tanks fail to fill in a set time.

Laboratory Services

Samples are tested at the Rotorua Lakes Council Environmental Laboratory, which is IANZ accredited and Ministry of Health approved to provide water testing services, and offers a comprehensive range of tests.

Tests for the OSET NTP include:

  • Alkalinity
  • Biochemical oxygen demand
  • Faecal coliforms
  • Temperature
  • Total suspended solids
  • pH
  • Total phosphorus
  • Ammonia
  • Nitrite
  • Nitrate
  • Total kjeldahl nitrogen
  • Total nitrogen
  • Total oxidised nitrogen

Operation and Funding of the Testing Programme

Establishment of the OSET test facility was funded by grants from the Ministry for the Environment, the Water Services Managers' Group of Water New Zealand, contributions in kind from Bay of Plenty Regional Council and Rotorua District Council, and grants from Establishment Funding Partners Waikato Regional Council, Gisborne District Council and Horizons Regional Council.

Manufacturers cover their own costs in setting up their test units at the facility, and are then charged a fee which includes site rental/operational costs during the 9 month test period together with all sampling and analytical costs and provision of a testing results review report.

Regional and District Council Funding Partner provide financial grants which support the supervision and audit costs required for benchmarking.

Councils are one of the main beneficiaries of the national testing programme in that they will have certainty regarding performance certification of on-site domestic wastewater treatment units in replacing septic tank systems in areas with increasing demand for un-sewered wastewater servicing. These include new subdivisions in rural residential and holiday area locations, and existing development where conventional on-site systems do not adequately protect environmentally sensitive land and natural water.

Only those councils supporting the OSET NTP as funding partners have exclusive access to detailed individual reports and reporting on the comparative test results for each trial. Performance information is accessible via this weblink.

Funding Partners for the 2017-18 trial (trial 12) were: Ashburton District Council, Auckland Council, Bay of Plenty Regional Council. Christchurch City Council, Environment Canterbury. Gisborne District Council. Hawke’s Bay Regional Council Horizons Regional Council Horowhenua District Council Hutt City Council, New Plymouth District Council, Northland Regional Council, South Wairarapa District Council, Tasman District Council, Taupo District Council, Tauranga City Council, Waikato District Council, Waikato Regional Council, Western Bay of Plenty District Council and Whangarei District Council