On-site Effluent Testing (OSET)

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


An OSET NTP Testing and Auditing Structure (Figure 1) was established by SWANS-SIG (Water NZ Small Wastewater And Natural Systems Special Interest Group) in 2008. This comprised three Strands with Strand 1 comprising Benchmark Appraisal of ex factory units, Strand 2 comprising a Field Performance and Maintenance Appraisal, and Strand 3 comprising a Product and Component Appraisal of the ex factory units.

Fig 1 Structure of the OSET Testing and Auditing Structure

The OSET NTP Strand 1 Test Facility was established in 2008 under a Memorandum of Understanding (MOU) between Bay of Plenty Regional Council (BOPRC), Rotorua District Council (RDC), Water NZ, and SWANS-SIG. [RDC changed its name to Rotorua Lakes Council (RLC) in 2014].

It provides a testing and benchmarking facility for up to seven ex-factory on-site domestic wastewater treatment units at a time. Benchmark testing is a pre-cursor to a parallel field testing programme (Strand 2), however only Strand 1 has been actioned to date.

The first NTP trial commenced November 2008 as Trial 4 following the Bay of Plenty Regional Council/Rotorua District Council earlier nitrogen reduction Trials 1 to 3 during 2005 to 2008. Trials 4 to 11 have been overseen by an Operations Team comprising the Technical Manager (SWANS-SIG), Operations and Laboratory Manager (RLC), and the Data and Report Reviewer (BOPRC).

Companies with systems which have been tested or are under test at the OSET TestFac since is inception are shown in Figure 2.

Figure 2: Companies and Systems Tested

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 reported on by the Technical Manager in association with the Data and Report Reviewer. A SWANS-SIG Management and Auditing Group (SWANS-MAG) has been established to review the test results and compile 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 for biochemical oxygen demand and total suspended solids, and in addition benchmarks the median values of treated effluent quality for biochemical oxygen demand, total suspended solids, total nitrogen, ammonia nitrogen, total phosphorus and faecal coliforms. Average daily energy consumption is also assessed.

SWANS-MAG is made up of the Operations Team plus three independent wastewater specialists, and is administered by the NTP Technical Manager. The benchmarking and audit reports are provided to the individual manufacturers, the MOU Partners, and those Regional and District Councils providing financial grants as Funding Partners to the NTP.

Funding Partners for Trial 11 in 2016 were:

  • Bay of Plenty Regional Council
  • Rotorua Lakes Council
  • Auckland Council
  • Environment Canterbury
  • Hawke's Bay Regional Council
  • Horizons Regional Council
  • Northland Regional Council
  • Waikato Regional Council
  • Christchurch City Council
  • Hutt City Council
  • Ashburton District Council
  • Gisborne District Council
  • Horowhenua District Council
  • New Plymouth District Council
  • Tasman District Council
  • Taupo District Council
  • Waikato District Council
  • Whangarei District Council

The OSET TestFac

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 (Figure 3 - 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 closely as possible follow a typical household flow.

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.

Figure 3

Laboratory Services

The RLC Environmental Laboratory is IANZ accredited (ISO17025:2005) 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 NTP 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. The NTP involves benchmark testing at the Rotorua National OSET Test Facility, field testing of specific treatment unit models installed around NZ following benchmark testing, and product testing and certification to ensure structural and component operational integrity (Figure 3 - above).

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.

Technical management and SWANS-MAG benchmarking and auditing activities are covered by Regional and District Council Funding Partner financial grants. A funding pool has been established to cover the supervision and audit costs for the benchmarking, field testing and product certification procedures.

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 unsewered 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 will have exclusive access to detailed individual reports and reporting on the comparative test results for each trial.

View the trial results here.

View the OSET NTP Contacts here.