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Plant problem

Sludge, foam or odour is abnormal

Sludge behaviour and odour are the biology reporting its condition. They are readable — and worth reading before they are masked.

Typically arises in

  • STP
  • ETP
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

  • Sludge settles slowly, or will not settle at all (sludge bulking)
  • Sludge rises in the clarifier and carries over the weir (rising sludge)
  • Persistent thick foam on the aeration tank, white or brown (foaming)
  • A bad smell that has changed character, not merely increased
  • Sludge volume has grown without a matching change in load

What helps, if you have it

  • Recent settleability or SVI records
  • Photographs of the foam, the clarifier surface and the settled sludge
  • Sludge wasting and return rates as actually operated

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

Sludge condition

  • Settleability and sludge volume index over time, not a single reading
  • Floc structure and filamentous content under a microscope
  • Sludge age and wasting practice against what the process needs

Line of enquiry02

Aeration and mixing

  • Dissolved oxygen profile across the tank, including the low corners
  • Diffuser condition, distribution and blockage
  • Zones that have gone anoxic or septic through poor mixing

Line of enquiry03

What the biology is being fed

  • Organic loading and its variability
  • Nutrient balance where the effluent is nitrogen- or phosphorus-deficient
  • Oil, grease, surfactants or inhibitory constituents entering the stage

Line of enquiry04

Upstream and holding

  • Retention in sumps, wet wells and collection systems generating sulphide
  • Return streams from sludge handling recycling load back to the front
How an investigation runs, step by step

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