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Plant problem

The plant cannot keep up with flow

The plant was sized for a duty it now cannot hold. Whether the limit is hydraulic, biological or operational is the question — and they look identical from the top of the tank.

Typically arises in

  • STP
  • ETP
  • WTP
Observation record
Reported
in the words of the person at the plant
5 signs
that often come with it
4 lines
of enquiry to check on site
Cause
not assumed from the symptom

Symptoms, not causes

You may also be seeing

  • Tanks overflow or run to bypass during peak hours: the plant is overloaded
  • Equalisation empties within a shift, or never empties at all
  • Pumps run continuously and still do not clear the flow
  • The plant only performs when production is deliberately held back
  • Capacity was never demonstrated at the rated flow after commissioning

What helps, if you have it

  • Flow records, or production data the flow can be inferred from
  • Hydraulic profile or general arrangement drawing
  • Pump and blower nameplate data

None of it is required to start.

What an investigation looks at.

Lines of enquiry, not conclusions. Which of them matters is decided at the plant, by evidence.

Line of enquiry01

The real duty

  • Measured flow and peak factor against the design flow
  • Whether flow is metered at all, and whether the meter is trustworthy
  • Streams added to the plant since it was built

Line of enquiry02

The hydraulic path

  • Where the profile actually restricts — the bottleneck is rarely where the overflow appears
  • Pump operating point against the real system head
  • Blockages, silting and accumulated solids reducing effective volume

Line of enquiry03

Process loading

  • Clarifier surface loading and solids loading rate
  • Aeration capacity against actual oxygen demand at peak
  • Whether retention time collapses before or after the failing stage

Line of enquiry04

How it is operated

  • Whether equalisation is being used as designed, or as storage
  • Manual interventions that shift the peak rather than absorb it
How an investigation runs, step by step

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