Symptoms

• High TSS content in the secondary clarifier effluent

• Filamentous microorganisms in mixed liquor

Main causes

• Low nutrient concentration in the incoming wastewater

• Toxic compounds in the incoming wastewater

• Wide pH and temperature oscillations

• High organic loading rate

• Insufficient aeration

Investigations and analyses

• Measure the Sludge Volume Index (SVI)

• Microscopic observations

• Check the C:N:P ratio or BOD₅:N:P ratio

• Measure temperature, pH and DO (in different sections of the aeration tank)

• Check F/M, volumetric organic loading rate, SRT

• Check toxic compounds in the incoming wastewater

Remedial actions

• Chlorine or oxygen peroxide dosage in the return sludge line (5 – 15 g Cl kg^-1SS d^-1)

• Inorganic coagulants (cake, ferric chloride, etc.) dosage

• Increase SRT

• PH and DO

• BOD₅:N:P ratio correction in the incoming wastewater