Filamentous Foaming Causes and Remedies

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

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

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

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

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

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.

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

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

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

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

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

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.

Identifying Microthrix parvicella Shape: Irregular and coiled Size: 50-200 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.

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

Identifying Type 1863 Shape: Irregularly shaped cells with no attached growth Size: 10-50 µm long, and cells 0.8-1.0 µm in width Staining: Gram negative Neisser negative. N positive granules common Flexible chain of irregular cell shapes.

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

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.

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.

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.

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.

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.

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

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.

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.

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