Anammox Sludge Hydrophobicity Testing: Methods, Standards, and Applications

Anaerobic ammonium oxidation (anammox) sludge hydrophobicity testing quantifies the surface water-repelling properties of anammox granules and biofilms, which directly govern granule formation, settleability, and biomass retention in deammonification systems. This article outlines the technical principles, sample requirements, analytical methods, applicable standards, and practical applications of hydrophobicity measurement for anammox sludge in both research and full-scale wastewater treatment engineering.

Technical Overview

Anaerobic ammonium oxidation (anammox) is a chemolithoautotrophic process in which planctomycete-type bacteria such as Candidatus Brocadia, Kuenenia, Jettenia, and Scalindua convert ammonium and nitrite directly into nitrogen gas. Because these microorganisms exhibit extremely slow growth rates, with doubling times often exceeding 7 to 20 days, the retention of active biomass is the decisive factor in reactor performance. Surface hydrophobicity is one of the most important physicochemical indicators of this retention capacity.

Hydrophobicity of anammox sludge is determined primarily by the composition of extracellular polymeric substances (EPS), particularly the ratio of proteins, which contain hydrophobic amino acid domains, to polysaccharides, which are predominantly hydrophilic. Under DLVO and extended DLVO theory, higher surface hydrophobicity strengthens cell-to-cell adhesion, promotes dense granulation, improves settleability, and enhances resistance to hydraulic shear. Conversely, a decline in hydrophobicity frequently precedes granule disintegration, filamentous overgrowth, biomass washout, and granule flotation caused by trapped nitrogen gas. Routine hydrophobicity monitoring therefore serves as an early-warning diagnostic parameter for anammox and partial nitritation-anammox (PN/A) systems.

Testing Samples

Anammox Granular Sludge

Granular biomass harvested from upflow anaerobic sludge blanket (UASB), expanded granular sludge bed (EGSB), or internal circulation (IC) reactors is the most common sample type. Representative sampling across reactor height is recommended, and granules are typically sieved into defined size fractions (for example, 0.5–1.0 mm, 1.0–2.0 mm, and above 2.0 mm) before analysis.

Attached-Growth Biofilm Samples

Biofilm carriers from moving bed biofilm reactors (MBBR), rotating biological contactors, and packed-bed systems are accepted. The biofilm is gently detached from carrier media by mild sonication or mechanical agitation, then washed and standardized to a consistent biomass concentration.

Flocculent and Hybrid Sludge

Flocculent anammox sludge from sequencing batch reactors (SBR), continuously stirred tank reactors (CSTR), and hybrid suspended-attached systems can be tested using the same procedures, with results interpreted against floc-specific baseline values.

Sample Handling and Preservation

• Transport samples in airtight containers at 2–8 °C under anoxic or anaerobic conditions.
• Avoid freezing, which disrupts EPS structure and alters measured hydrophobicity.
• Complete testing within 24–48 hours of sampling whenever possible.
• Rinse biomass with phosphate or PUM buffer before analysis to remove soluble organics and interstitial nitrogen gas.
• Provide a minimum of 50–100 mL of mixed liquor or 5–10 g wet weight of granular sludge, along with process metadata (reactor type, nitrogen loading rate, temperature, salinity).

Testing Items

Core hydrophobicity parameters and associated surface characterization items include:

• Relative hydrophobicity by microbial adhesion to hydrocarbons (MATH), expressed as a percentage of biomass partitioned into the hydrocarbon phase.
• Water contact angle of compacted sludge cakes, expressed in degrees, with values above 90° conventionally classified as hydrophobic.
• Total surface free energy and its Lifshitz–van der Waals and acid–base components, expressed in mN/m.
• Zeta potential at defined pH and ionic strength, reported in mV, to characterize surface charge alongside hydrophobicity.
• EPS characterization, including protein concentration (Lowry method), polysaccharide concentration (anthrone–sulfuric acid method), and the PN/PS ratio.
• Ancillary supporting parameters: granule size distribution, granule integrity strength, settling velocity, and mixed liquor suspended solids (MLSS/MLVSS).

Testing Methods

Microbial Adhesion to Hydrocarbons (MATH/BATH) Assay

Washed biomass is suspended in PUM buffer to a standardized optical density, mixed with a hydrocarbon phase such as n-hexadecane, xylene, or n-octane, vortexed under controlled conditions, and allowed to phase-separate. The reduction in optical density (typically OD600) of the aqueous phase is used to calculate the percentage adhesion, which is directly proportional to relative hydrophobicity.

Static Contact Angle Measurement

Sludge is filtered onto a 0.45 µm membrane to form a uniform cake, conditioned to constant humidity, and analyzed by the sessile drop method using ultrapure water and reference liquids such as formamide and diiodomethane. Surface free energy components are computed using the van Oss–Chaudhury–Good approach. Measurements follow the general principles of contact angle practice described in ASTM D7334 and ISO 15989, adapted for soft biological biomass.

Salt Aggregation Test and Chromatographic Methods

The salt aggregation test evaluates cell surface hydrophobicity through reversible aggregation in graded ammonium sulfate solutions, while hydrophobic interaction chromatography assesses adsorption onto hydrophobic gels. These methods are useful as corroborating techniques.

EPS Extraction and Quantification

Loosely bound and tightly bound EPS fractions are extracted by heat treatment, cation exchange resin (CER), or formaldehyde–NaOH protocols, followed by protein and polysaccharide quantification. Extraction and analytical procedures reference national sludge examination standards (GB series methods) and Standard Methods for the Examination of Water and Wastewater.

Supporting Electrophoretic Analysis

Zeta potential is determined by electrophoretic light scattering in accordance with ISO 13099-1 and ISO 13099-2, providing complementary surface charge data for comprehensive DLVO-based interpretation.

All tests are performed in replicate with reagent blanks, calibrated instruments, and standardized biomass concentrations to ensure reproducibility. Results are reported with measurement uncertainty, temperature, ionic strength, and pH conditions clearly documented.

Applications

• Startup supervision and bioaugmentation of new anammox reactors, where rising hydrophobicity confirms successful granulation and acclimation.
• Quality control of commercial seeding sludge and inoculum transfer between full-scale plants.
• Diagnosis of granule flotation, sludge washout, and granulation deterioration events in sidestream deammonification systems treating digester reject water.
• Research into EPS regulation, quorum sensing effects, and microbial community selection in mainstream anammox applications.
• Evaluation of tolerance to inhibitory shocks, including salinity, free ammonia, free nitrous acid, and organic carbon loading, all of which alter surface hydrophobicity.
• Engineering optimization for landfill leachate treatment, anaerobic digester supernatant polishing, and low-temperature mainstream partial nitritation-anammox processes.

FAQ

Why is hydrophobicity so critical for anammox sludge?

Because anammox bacteria grow extremely slowly, the process depends on long-term biomass retention. Higher hydrophobicity strengthens microbial aggregation, produces denser and faster-settling granules, and reduces washout risk, directly translating into higher nitrogen removal capacity per reactor volume.

What hydrophobicity values are considered healthy?

Reported values vary with reactor configuration and method, but well-performing anammox granular sludge typically shows MATH adhesion above approximately 40–50% and water contact angles in the range of 50–90°. Trend monitoring over time is more diagnostic than any single absolute value.

Which standards apply to this testing?

No single international standard is dedicated exclusively to sludge hydrophobicity. Testing is performed using established scientific methods validated against general contact angle practices (ASTM D7334, ISO 15989), zeta potential measurement standards (ISO 13099 series), and national sludge examination procedures (GB series) for supporting EPS and solids parameters. Accredited third-party laboratories integrate these references into validated in-house methods.

Can hydrophobicity testing diagnose flotation problems?

Yes. Sudden decreases in surface hydrophobicity, often accompanied by a falling PN/PS ratio in EPS, are an established precursor of granule instability and flotation. Combined hydrophobicity, EPS, and zeta potential profiling enables root-cause analysis and timely corrective dosing or operational adjustment.