The case of Anammox

Anammox granules are a cornerstone of energy-efficient nitrogen removal in wastewater treatment. They are typically conceptualized as diffusion-limited microbial aggregates in which solute transport occurs exclusively through molecular diffusion. In our recent work, we demonstrate that this assumption is incomplete. We provide the first experimental evidence for the existence of strong electric fields within anammox granules and show that these fields play a critical role in governing ion transport and reaction limitations.

Experimental Evidence for Electric Fields in Anammox Granules

In a recent study, we directly measured electric potentials inside anammox granules collected from full-scale wastewater treatment systems (Tucci et al. 2025, Water Research). The granules ranged in diameter from approximately 0.3 to 3 mm. Across these systems, we consistently observed electric potential gradients within the biomass.

The measured electric fields reached intensities of up to 360 V/m, which is exceptionally high for a biological system. Initial measurements indicated a sharp decrease in electric potential immediately below the granule surface, corresponding to relatively modest electric fields in the outer layers. However, subsequent experiments focusing on smaller granules (approximately 500 µm in diameter) revealed substantially stronger fields, ranging from 189 to 360 V/m, with an average intensity of 270 ± 56 V/m.

These results demonstrate that strong and spatially structured electric fields are an intrinsic property of anammox granules.

Electric potential profiles measured within one anammox granule immersed in wastewater. Each potential profile represents the average of three repeated measurements. Electric field intensities obtained from electric potential microprofiles measured in different spots of six different granules, plotted against the granule volume dindicate that the potential gradient is independent from the granule size. The granules were sampled at the Marselisborg plant and incubated at 32 ◦C in synthetic wastewater (granule age: 1–3 days).

Mechanistic Interpretation and Modeling Approach

To elucidate the origin of these electric fields, we developed a mathematical model that explicitly accounts for ion transport in charged biofilms. The model is based on the Nernst–Planck equation, which describes ion flux as the combined result of molecular diffusion and ionic migration in response to electric potential gradients.

Our analysis indicates that anammox biomass behaves as a weak ion exchanger, particularly toward ammonium (NH₄⁺). The negatively charged surfaces of anammox cells and their associated extracellular polymeric substances provide ion exchange sites that selectively interact with cations. This interaction reduces the effective mobility of NH₄⁺ relative to anions such as nitrite (NO₂⁻), resulting in the formation of a diffusion potential.

Model simulations show that the presence of ion exchange is essential to reproduce the experimentally observed electric field strengths. Moreover, the model predicts that ion exchange can alter the identity of the rate-limiting substrate within the granule. Under these conditions, nitrite may become fully depleted toward the granule core, shifting the limitation from ammonium to nitrite.

Consequences for Mass Transport and Reaction Rates

The electric fields generated within anammox granules have a pronounced impact on mass transport. Ionic migration driven by the electric potential contributes to solute fluxes at a magnitude comparable to that of molecular diffusion.

At the granule–bulk liquid interface, ionic migration increases the inward flux of ammonium by approximately 40%, while simultaneously counteracting the diffusional influx of nitrite by a similar magnitude. Nitrate (NO₃⁻), produced within the granule, is likewise affected by the electric field. The net effect is a preferential transport of cations into the granule and anions out of the granule.

Neglecting ionic migration in transport models therefore leads to substantial errors in estimating substrate fluxes, reaction rates, and the location of reaction-limiting zones within anammox granules.

Broader Implications and Future Perspectives

These findings indicate that the conventional diffusion-only framework used to describe solute transport in biofilms is insufficient for anammox systems and potentially for other highly active, densely packed microbial aggregates with charged biomass matrices.

Recognizing the presence of internal electric fields has important implications for both fundamental understanding and engineering practice. Incorporating ionic migration into biofilm models will improve predictive accuracy and enable more reliable process design and optimization. From an applied perspective, strategies such as electrolyte composition control, substrate concentration and stoichiometry optimization, or the application of external electric fields may offer new opportunities to influence mass transport and enhance process performance.

In summary, this preliminary evidences establish that strong electric fields are an inherent feature of anammox granules and that these fields play a central role in controlling ion transport and bioprocess efficiency. Accounting for electric-field-driven migration is therefore essential for a comprehensive description of mass transport in biofilm-based technologies.