STP retrofitting and upgradation: bringing an existing plant back inside its consent conditions.

Most commonly because actual flow or organic load has outgrown the design basis; because aeration capacity has degraded through worn blowers or choked diffusers; because sludge is not being wasted and dewatered, so solids build up and carry over; because tertiary treatment was never completed or is bypassed; or simply because the plant is running without a trained operator. Membrane and media fouling add to the list on those technologies.

Often yes, and it is one of the more effective retrofits. Adding media to an existing aeration tank converts it towards an attached-growth process, raising treatable load within the same volume, provided aeration, retention and the downstream clarifier are re-checked and upgraded to suit. Conversion to SBR involves more change, since it alters the plant's hydraulic and control regime, and needs the tankage to suit batch operation.

When the existing structures cannot hold the volume the required load needs, when the tanks are structurally unsound or leaking, or when the actual flow has so far exceeded the original design that any upgrade would be a new plant inside an old shell. We state that plainly in the audit, because a retrofit stretched past what the civil works can carry only defers the failure.

By measurement, not inspection alone. Flow is metered against design, influent and inter-stage samples are analysed to see which unit is failing, dissolved oxygen and mixed-liquor solids are profiled, sludge settleability and age are checked, blower output and diffuser condition are verified, and the operating logs are read. Retrofits designed without this data tend to replace whatever is most visible rather than whatever is actually limiting.

A change in capacity or in the treatment process generally needs the Consent to Operate to be amended, and where capacity is being increased, Consent to Establish for the modification as well. Where the plant is tied to conditions in an Environment Clearance, those conditions should be checked at the same time. We identify the approval route as part of the retrofit design rather than after the work is done.

It depends on whether the plant can be worked in stages. Where the train has parallel streams or where temporary storage or tankering can absorb flow, much of the work proceeds with the plant partly live; where a single stream must be taken out, an interim arrangement has to be planned before work starts. Biological stabilisation after restart takes its own time and is not compressible.

Within limits, and that is exactly what an attached-growth retrofit is for, carrying more biomass in the same volume. The ceiling is set by aeration capacity, hydraulic retention time, clarifier or membrane surface area and the sludge line. Beyond that ceiling, additional tankage is unavoidable, and the audit should say where the ceiling is instead of leaving it to be found in operation.