1. Filamentous Foaming
Causes and Remedies

2. Contents What is foaming? How to recognise types of foaming
The filamentous bacteria responsible
Identifying which filaments you have
Getting rid of foams

3. Settling problems
Once settling problems become apparent in the clarifier…
…it is usually too late to fix quickly.

4. Sludge quality problems
Bulking
filamentous bacteria or zooglea prevent sludge settling
Foaming
scum on the top of the reactor/clarifier
Pin-point floc
turbid effluent, small flocs remain suspended
Rising sludge blanket
nitrogen gas from denitrification

5. Filamentous bulking Cause: Impact:
Filamentous bacteria so prevalent that bridging between flocs causes poor settling, compaction, and thickening
Impact:
Reduced sludge settling (poor SVI)
Suspended solids overflow
Reduced solids concentration in RAS and WAS
Can hydraulically overload solids handling processes
Worldwide problem

6. Foaming Cause: Impact: Young sludge Toxic/organic shock
Specific filaments
Fats/oils/greases
Low DO
Nutrient deficiencies and denitrification (scums)
Impact:
Can accumulate and putrefy
Foams can overflow tank freeboards

7. Billowy stiff white foam
Young sludge (3-4d)
Process start-up
High organic loading
Toxic conditions
Excessive wasting
High effluent turbidity
Zoogloea
Surfactants
Filament Type 1863
Young Sludge can be often <2d SRT, but also often within 3-4d of startup.

8. Brown foam Thick and scummy Viscous greasy, chocolate brown
Old sludge
Viscous greasy, chocolate brown
Specific filaments e.g. Nocardia & Microthrix
Rich chocolate brown
Plants operating in nitrifying mode

9. Very dark, black foams Insufficient aeration Coloured influent
e.g. dyes and inks
Fats, oils or greases

10. Filamentous foams Brown foams – usually associated with: Nocardioforms
Microthrix parvicella
Type 1863

11. Nocardiaforms Persistent and difficult to break mechanically
Hydrophobic cell surfaces
High numbers mean flocs attract air bubbles and floats
Can identify

12. Identifying Nocardiaforms
Shape – True branched filaments
Size – 5-30 µm long, and cells 1.0 µm in width
Staining:
Gram positive – strongly
Neisser negative. N positive granules common
Many variants (genera), e.g. Nocardia, Gordona, Skermania.
Can identify

13. Microthrix parvicella
Persistent and difficult to break mechanically
Hydrophobic cell surfaces
More common in high SRT biological nutrient removal plants
Thrives in anoxic / anaerobic conditions, competitive advantage with lipids.

14. Identifying Microthrix parvicella
Shape: Irregular and coiled
Size: µm long, and cells 0.8 µm in width
Staining:
Gram positive – strongly
Neisser negative. N positive granules common
Coiled growth and beaded granules feature.

15. Type 1863 White-grey foam which collapses easily
When plant operated at <2d sludge age
Much higher filament concentrations in foam than in mixed liquor

16. Identifying Type 1863
Shape: Irregularly shaped cells with no attached growth
Size: µm long, and cells µm in width
Staining:
Gram negative
Neisser negative. N positive granules common
Flexible chain of irregular cell shapes.

17. How to remove foams Depends on the cause
Must identify your filament first!

18. Dealing with Nocardioforms
Most important control – removing foam from process
Selective foam wasting and dewatering.
Side weirs, channels, or gates – continuously remove top 1-3 cm of water from surface.
Chlorination – not effective, as the filaments are protected by the floc which surrounds them.
Can achieve some success misting chlorine over the foam surface.
Cationic polymer addition to foam (0.5-1 mg/L wastewater). Typically dosed into RAS or mixed liquor stream.

19. Dealing with Nocardioforms
Sludge age – encouraged by high SRT (>9d).
Consider reducing SRT by increased wasting.
Automated wasting to maintain SRT can significantly reduce occurrence of Nocardioforms.
Also encouraged by high temperatures – difficult to control, but allows preparation.
Selectors – aerobic selectors effective at >5d SRT, anoxic somewhat effective. Must ensure foam not recycled.

20. Dealing with Microthrix
Causes both foaming and bulking
Plug flow systems preferred (avoid intermittently aerated zones).
Ensure anaerobic and anoxic recycles enter at aerobic zones and DO is maintained above 2 mg/L.
Must not trap foam – separate and dewater.

21. Dealing with Microthrix
Sludge age – encouraged by high SRT, increase wasting if process allows.
Addition of polyaluminium chloride based flocculants can be effective (has little effect on Nocardioforms).
Also encouraged by low temperatures – difficult to control, but you can prepare when temperatures drop.

22. Dealing with Type 1863 The foam is typically billowy and weak.
Should reduce as the plant biomass matures (i.e. after start up or recovery from toxic shock).
Increasing biomass SRT should assist – i.e. consider reduced wasting until a suitable SRT is established.
Not typically a significant, ongoing problem.

23. Digesters
Nocardioforms and M. Parvicella will persist and foam in anaerobic digesters too

24. Preventative monitoring
Regular monitoring of your plant biology helps.
Identify the problem filaments before they get established, and adjust your plant to select against them.
Far more cost effective than reacting to a major foaming incident.

25. References Images and data sourced from the following:
Mott MacDonald microscopy image database
FNDAE No.2 and 33
Jenkins D, Richard M, Daigger G; “Manual on the causes and control of activated sludge bulking, foaming, and other solids separation problems”. 3rd edition, 2003.