STP commissioning and performance testing: proving the plant meets its design before handover.

Commissioning brings the plant into stable operation: checks, trials, calibration, seeding and tuning. The performance test is the demonstration afterwards, in which treated water is sampled over a defined run at or near design flow and analysed against the contracted parameters. One makes the plant work; the other proves it, on record.

Seeding introduces active biomass, usually sludge from an operating plant. So the biological stage does not have to grow a culture from nothing. Stabilisation then takes weeks rather than days, and depends on temperature, the consistency of incoming load, and how promptly the process is tuned. It is the stage of a project least amenable to compression, and testing before it completes produces misleading results.

Those written into the contract, which normally mirror the consent: BOD, COD, total suspended solids, pH, oil and grease, residual disinfection, and where relevant ammoniacal nitrogen and turbidity for reuse quality. The test run duration, sampling points and frequency, and the analysing laboratory should be specified alongside the parameters.

It can be run and tuned at partial flow, and often has to be on a development that is still filling up, but a guarantee test at a fraction of design flow does not demonstrate design capacity. Where full flow is genuinely unavailable at handover, the contract should say how the test will be conducted and what obligation survives until design load arrives.

The client or the client's consultant, with sampling and analysis by an appropriately recognised laboratory rather than by the contractor alone. Witnessed sampling and third-party analysis are what make the result usable later: with the board, in a compliance report, or in any dispute over the guarantee.

As-built drawings, the commissioning report with all test data and analysis certificates, equipment manuals and warranty certificates, the O&M manual with the control philosophy and set points, the recommended spares list, the calibration records, and evidence of operator training. Plants handed over without this documentation are markedly harder and costlier to operate afterwards.

The contract should already say. Normally the contractor identifies and rectifies the cause and the test is repeated, with the guarantee period running from successful demonstration. Failure is often traced to influent characteristics differing from the design basis rather than to the plant, which is why the design basis and the test conditions both belong in the contract from the start.