Stable Brown Foam

1Symptom

A thick, stable, chocolate brown foam (sometimes like scum) covers parts of the aeration basin and can carry into the clarifier. It does not collapse with water sprays and may grow in the digester.

2Check first

1. Look at the foam under the microscope: branched Gram positive filaments suggest Nocardioforms; Gram positive coiled filaments suggest M. parvicella.
2. Check grease and oil loading.
3. Check SRT: long SRT favors Nocardioforms.
4. Check whether foam is being trapped and recycled (scum returned to the basin).

3Possible causes

• Nocardioform organisms, favored by long SRT, grease and warm water.
• M. parvicella, favored by low F/M, cold water and grease.
• Foam trapping: baffles or scum removal that send foam back to the process.

4Next investigation

• Measure foam depth and coverage daily.
• Quantify filaments in the foam versus the mixed liquor.

5Corrective actions

1. Remove foam from the system; do not return it to the aeration basin or digester feed.
2. Reduce SRT if nitrification allows.
3. Control grease at the source.
4. Spray foam with chlorine solution in a targeted way (with care for nitrification).
5. Use a selector to discourage foam formers.