On-site Effluent Testing (OSET)

****  The On-site Effluent Treatment National Testing Site has been closed down indefinitely ****

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.

Update August 2021 

Due to a number of issues the Partners Advisory Group (PAG) have decided to stop Trial 16 and pause the OSET -NTP facility.


The treatment plant site 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 Local Authority 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 under a Partners Agreement with financial and technical management under the auspices of Water New Zealand. Testing was initially undertaken in accordance with AS/NZS 1547 (which cross referenced to AS/NZS 1546.3:2008 for treatment performance requirements). In 2018 the plant was upgraded so that testing is aligned with the new standard as far as practicable. This meant that subsequent to Trial 13 all plants are now tested at their nominated plant capacity, rather than just at 1000 L/day as before. In addition a review of plant signage, emergency storage capacity, noise, structural integrity (compliance with AS/NZS 1546.1), and the Installation and Operations & Maintenance Manuals is undertaken.

The facility is funded by partner councils grants and testing fees from manufacturers using the facility.

A list of contacts for the facility is available here.

Accreditation / Audit Process

Initially each trial ran 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. From 2019 (Trial 14) onwards each trial is undertaken within a calendar year, commencing in January with a 2 month settling in period followed by a 7 month performance test period during which there are 6 stress tests and 41 sampling days. ALL test results including those after a stress test are required to comply with the standard and ALL are used in the OSET-NTP Benchmark analysis. 

Data sets from the laboratory testing are analysed and compiled into an Evaluation Report and a Performance Certificate, both including the OSET-NTP Benchmark analysis.  This report assesses treatment performance in meeting the secondary effluent treatment requirements as set out in AS 1546.3:2017. A list of parameters tested at the facility is outlined in the  laboratory services section of this website.  In addition a review of plant signage, emergency storage capacity, noise, structural integrity (compliance with AS/NZS 1546.1), and the Installation and Operations & Maintenance Manuals as outlined in AS 1546.3 is included in the above documents. 

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 Noel Roberts for more details 

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 CCTV surveillance cameras installed.  Screened raw municipal sewage at the Rotorua Lakes Council WWTP is pumped to a manifold which is pressure controlled to 3m via a header pipe. The manifold has 7 Tee discharges one to each plant under test. Each has an automatic ball valve connected to a mag-flow meter via the plant PLC which controls both the discharge flow rate and the total flow discharged at pre-set time intervals dependent upon each plants nominated plant capacity. These discharge via gravitate to each test plant.

Each site is 8m x 3m and is provided with a 50mm slip fitting at an invert of approximately 2.7m for the raw sewage influent supply, a 25mm discharge point at ground level to an effluent mag-flow meter discharging to a 2.7m high riser pipe, plus a 240V hard wired electrical point and a kW hour meter.

The OSET-NTP facility is fitted with alarms controlling all influent and electrical equipment, but each plant is expected to have its own control and alarm panel with both visual and audible alarms.  

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.