Determination of Dehydrogenase Activity in Anammox Sludge: Principles, Methods, and Applications

Dehydrogenase activity (DHA) determination is a sensitive biochemical assay used to quantify the metabolic vitality of anaerobic ammonium oxidation (anammox) sludge, providing critical insight into reactor performance, microbial inhibition, and enrichment progress. This article presents the technical principles, sampling requirements, TTC- and INT-based testing methods, and typical applications of DHA measurement for anammox systems in wastewater treatment research and engineering. Accredited third-party laboratories routinely perform this assay to support process commissioning, troubleshooting, and scientific studies.

Technical Overview

Anaerobic ammonium oxidation (anammox) is a chemolithoautotrophic process in which specialized bacteria of the phylum Planctomycetes, including genera such as Candidatus Brocadia, Kuenenia, Jettenia, and Scalindua, convert ammonium and nitrite directly into nitrogen gas. The overall stoichiometry is approximately NH4+ + NO2− → N2 + 2H2O, proceeding through hydrazine and hydroxylamine intermediates catalyzed by hydrazine synthase and hydrazine dehydrogenase. Because these microorganisms are strictly anaerobic, extremely slow-growing, and difficult to cultivate by conventional microbiological methods, enzyme activity assays have become the preferred tool for evaluating their physiological state.

Dehydrogenases are intracellular oxidoreductase enzymes that transfer electrons from organic and inorganic substrates to electron carriers such as NAD+ and FAD. In sludge systems, the total dehydrogenase activity is widely recognized as an integrated indicator of active biomass metabolism, because it reflects the rate of electron transport within the cell. For anammox sludge, which derives energy from a purely inorganic redox reaction, DHA measurement captures the electron transfer intensity of the catabolic pathway and correlates closely with the nitrogen removal rate.

The most common analytical principle is the tetrazolium salt reduction method. A colorless, water-soluble tetrazolium compound, typically 2,3,5-triphenyltetrazolium chloride (TTC) or 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyl tetrazolium chloride (INT), is introduced as an artificial electron acceptor that competes with the natural electron transport chain. Upon accepting electrons, the salt is reduced intracellularly to a red-colored, water-insoluble formazan pigment (triphenylformazan, TPF, from TTC). The formazan is extracted with an organic solvent and quantified spectrophotometrically at approximately 485 nm for TPF or 490 nm for INT-formazan. The measured amount of formazan produced per unit of volatile suspended solids per unit time is directly proportional to the dehydrogenase activity of the sludge.

Testing Samples

Granular Anammox Sludge

Granular biomass harvested from upflow anaerobic sludge blanket (UASB), expanded granular sludge bed (EGSB), or sequencing batch reactors operating in sidestream or mainstream deammonification mode. Granules typically range from 0.5 mm to 3 mm in diameter, and the assay may be performed on whole granules or gently homogenized granule suspensions depending on the study objective.

Flocculent Anammox Sludge

Suspended flocs collected from continuously stirred tank reactors, membrane bioreactors, or integrated fixed-film activated sludge systems. Flocculent samples generally require less pretreatment than granules and are suitable for rapid activity screening.

Biofilm and Carrier-Based Samples

Biofilm detached from plastic carriers, nonwoven fabrics, or magnetic particles used in moving bed biofilm and hybrid anammox processes. The detached biofilm is washed, resuspended, and assayed in the same manner as flocculent sludge.

Stored and Preserved Sludge

Sludge preserved for storage and revival studies, including samples maintained at 4 °C for short periods or cryopreserved at −80 °C with cryoprotective agents. DHA testing is frequently used to quantify the loss and recovery of activity during long-term storage.

Testing Items

Testing programs for anammox sludge dehydrogenase activity typically include the following items:

• TTC-dehydrogenase activity (TTC-DHA), expressed as micrograms of TPF produced per gram of volatile suspended solids per hour (μg TPF/(g VSS·h)).
• INT-dehydrogenase activity, expressed as micrograms of INT-formazan per gram of VSS per hour, used as an alternative or confirmatory electron acceptor.
• Specific dehydrogenase activity normalized to mixed liquor suspended solids (MLSS) for comparison across sludges with different ash contents.
• Volumetric dehydrogenase activity normalized to reactor volume, used to relate enzyme activity to process loading.
• Conditional DHA profiles measured under varied substrate concentrations, temperatures, pH values, or inhibitor dosages to determine kinetic or inhibition parameters.
• Supporting physicochemical indices, including MLSS, MLVSS, ammonium nitrogen, nitrite nitrogen, and nitrate nitrogen, determined in accordance with recognized water analysis standards such as ISO 7150-1 for ammonium and ISO 6777 for nitrite.

Testing Methods

Sampling and Pretreatment

Samples are collected following good practice consistent with the ISO 5667 series on water quality sampling. Because anammox bacteria are oxygen sensitive, biomass is transferred rapidly under minimized air exposure, preferably with nitrogen flushing. The sludge is washed two to three times with oxygen-free phosphate buffer (pH 7.2 to 7.5) to remove residual soluble substrate and inhibitors, then homogenized to a uniform suspension. Mixed liquor suspended solids and volatile suspended solids are measured gravimetrically so that enzyme activity can be expressed on a biomass-specific basis. If nitrite is present at high concentrations, it is reduced or diluted to avoid chemical interference.

TTC Reduction Assay Procedure

The reaction mixture typically contains a defined volume of washed sludge suspension, phosphate or Tris-HCl buffer at pH 7.2 to 7.8, a TTC solution at a final concentration of approximately 0.3 to 0.5 percent, and substrate solution providing ammonium chloride and sodium nitrite at concentrations matching the target test condition, often around 50 to 100 mg N/L for each nitrogen species. Incubation is carried out at 35 ± 1 °C, near the optimum temperature for anammox metabolism, in the dark under anoxic or anaerobic conditions for 2 to 6 hours with gentle shaking. The reaction is terminated by addition of a stopping reagent or by direct addition of the extraction solvent.

Formazan Extraction and Quantification

The produced TPF is extracted from the biomass using an organic solvent such as acetone, methanol, ethanol, or toluene, sometimes with heating at moderate temperature to improve extraction efficiency. After centrifugation or filtration to remove solids, the absorbance of the extract is measured spectrophotometrically at 485 nm against a reagent blank. The formazan concentration is calculated from a TPF external calibration curve prepared under identical conditions, and the enzyme activity is computed as micrograms of TPF per gram of VSS per hour after subtracting the blank value.

INT Alternative Method

The INT method follows the same overall workflow, substituting INT for TTC and quantifying the resulting INT-formazan at approximately 490 nm. INT penetrates cells readily and is sometimes preferred for fast assays, although both tetrazolium salts are widely accepted for sludge DHA determination.

Quality Assurance and Quality Control

Reliable results depend on rigorous controls:

• Autoclaved or boiled sludge blanks to subtract abiotic and adsorption effects.
• Reagent blanks without biomass to correct for non-enzymatic tetrazolium reduction.
• Multi-point TPF or INT-formazan calibration curves verified with each analytical batch.
• Replicate incubations with relative deviation acceptance criteria, commonly within 10 percent.
• Parallel MLSS and MLVSS determinations to ensure valid biomass normalization.
• Substrate-free and nitrite-free controls to distinguish anammox-linked electron transport from endogenous respiration.

Applications

Dehydrogenase activity determination is applied throughout the life cycle of anammox process development and operation:

• Startup and enrichment monitoring, where rising DHA tracks the successful proliferation of anammox microorganisms during long acclimation phases.
• Inhibition and toxicity assessment, quantifying the adverse effects of heavy metals, antibiotics, sulfide, salinity, free ammonia, and free nitrous acid on anammox metabolism.
• Shock load and recovery evaluation, comparing DHA before and after hydraulic, temperature, or substrate shocks to gauge resilience.
• Granulation and biofilm studies, mapping activity distribution across granule size fractions or biofilm depths.
• Sludge storage, transport, and revival programs, in which DHA serves as the key acceptance criterion for seeded biomass in engineering projects.
• Process optimization of temperature, pH, substrate ratio, and loading rate in sidestream and mainstream deammonification reactors.
• Academic research and regulatory reporting, providing reproducible enzyme activity data for publications, patents, and technology verification.

FAQ

Why is dehydrogenase activity preferred over oxygen uptake rate for anammox sludge?

Anammox bacteria are obligate anaerobes, so oxygen-based respirometry is not a suitable vitality indicator. Tetrazolium-based DHA assays intercept electrons directly from the intracellular transport chain using an artificial electron acceptor, which functions independently of oxygen and matches the anaerobic physiology of the biomass.

How should samples be transported and stored before testing?

Wet sludge should be collected in airtight containers with minimal headspace, protected from temperature extremes, transported on ice packs, and analyzed within 24 to 48 hours. For longer holding times, samples should be refrigerated at 4 °C and oxygen exposure minimized, or cryopreserved with an appropriate cryoprotectant if the study design permits.

What are the typical reporting units and result ranges?

Results are most commonly reported as μg TPF/(g VSS·h). Absolute values vary widely depending on enrichment level, granule morphology, and operating conditions, so trend analysis against a baseline sample from the same system is generally more meaningful than comparison of isolated numbers.

What factors can interfere with the assay?

High residual nitrite, exposure to oxygen during handling, strong sample color or turbidity, toxicant carryover, and incomplete formazan extraction are the principal sources of bias. Proper washing, blank correction, anaerobic technique, and validated extraction procedures effectively control these interferences.

How long does routine DHA testing take?

After sample receipt and pretreatment, a standard TTC-DHA determination including incubation, extraction, and spectrophotometric quantification is typically completed within one to two working days, with full reporting from accredited third-party laboratories generally issued within three to seven working days depending on the scope of conditional testing.

Can DHA results be correlated with nitrogen removal performance?

Yes. Under stable operating conditions, biomass-specific DHA correlates positively with volumetric nitrogen removal rates. Deviations between the two signals often indicate changes in the active fraction of the microbial community, substrate diffusion limitations within granules, or partial inhibition, making combined monitoring a powerful diagnostic approach.