Surface Skew Effects on Biofilms in Wastewater Treatment

This research is a collaboration between the University of New Mexico and Flow Science. Flow Science contributed free CFD software and CFD expertise as part of their Research Assistance Program.

The sensitivity of bacteria to physical, chemical and environmental disturbances presents a challenge to maintaining microbial nitrification in wastewater treatment plants. Researchers at the University of New Mexico conducted a series of experiments to understand the effects of surface roughness on annular nitrifying biofilm reactors (Roveto et al., 2019). Their team used FLOW-3D to find possible explanations for their experimental observations.

Nitrifying Biofilms

A biofilm is a community of bacterial cells in which cells stick to each other and to a surface. Biofilms can cause chronic infection and disease, and are difficult to eradicate, and are a problem in agriculture and medicine. But in the wastewater treatment industry, biofilms can be harnessed to extract and digest organic compounds from sewage.

Recently, specific types of biofilms containing ammonia-oxidizing bacteria have been identified in wastewater systems. These nitrifying biofilms are used to improve the efficiency of the nitrification process in wastewater treatment systems. They are also used in trickling filters (van den Akker et al., 2011) and moving bed biofilm reactors (MBBRs, Song et al., 2019).

Annular Biofilm Reactors with Skewness

Annular biofilm reactors are made up of a cylindrical reactor with a submerged rotating inner cylinder – a standard Taylor-Couette flow setup (Fig. 1, bottom left). These reactors have been used to study microbial community dynamics in drinking water systems and to determine the growth rate in response to increasing rotation speed (Gomez-Alvarez et al., 2014). Because of this research, annular biofilm reactors are used with added skew to the inner cylinder of the reactor.

‘Skewness’ is a roughness parameter that describes asymmetry in surface feature distribution and is thought to affect biofilm formation. In order to quantify the effects of skewness, well-defined positive and negative skew surfaces (Fig. 1, bottom right) were compared against the biofilm growth on a flat (non-skewed) surface.

Surface Skew Effects Blog
Figure 1. Annular reactors (top), plan view of annular bioreactor scheme (bottom left) and detail of rotating attachment surface, and outer stationary cylinder illustrating cylindrical Couette flow (bottom right).

CFD Modeling

FLOW-3D was used to calculate the distribution of shear forces on the positive and negative skew surfaces. To compare the cumulative shear stresses on the skewed surfaces to the flat control surface and to each other, a representative 300 micrometer section of each skewed surface was evaluated for wall shear stress along its length (Figure 2). Values over the experimentally observed constant flat surface value of 0.35 Pa were categorized as “high shear” and those below were designated as “low shear.”

CFD-generated 2D shear stress profiles
Figure 2. CFD-generated 2D shear stress profiles along (a) positive and (b) negative skew surfaces at steady-state flow and predicted logarithmic shear stress values at the surface for the (c) positive and (d) negative skew surfaces along representative feature length (300 m). Horizontal red lines indicate shear stress of flat surface (0.35 Pa). Yellow and red shaded sections indicate shear values less than or greater than the flat surface shear, respectively.

Low shear stress values were predicted along most of the positive skew surface, with 86% of the representative section demonstrating values below 0.35 Pa and 14% in the high shear range (Fig. 2c). For the negative skew surface, 62% had shear values below that of the flat surface, while 38% was categorized as high shear (Fig. 2d). The negative skew surface had 2.5 times more area with predicted shear wall stress values in the high shear range over the positive skew surface.


Experimentally, it was found that negative skew surfaces show the highest rates of complete nitrification during biofilm growth. CFD simulations from FLOW-3D provide a possible explanation for this phenomenon. They illustrate that negative skew surfaces had larger regions with high shear than positive skew surfaces. These high shear regions may cause increased rates of attachment and rates of mass transfer which in turn increase the nitrification rate in biofilms.

This research suggests a method of increasing the efficiency of nitrifying biofilms in wastewater treatment plants.

About Flow Science’s Research Assistance Program

If you are a part of an academic program, like the researchers at the University of New Mexico featured in this blog, you may be eligible for access to FLOW-3D through Flow Science’s Academic Assistance Program. Through this program, researchers and educators at universities can apply for a free FLOW-3D license. For more information, check out our Academic Program. We look forward to working with you!


Roveto, P.M., Gupta, A., Schuler, A.J., 2019. Effects of Attachment Surface Skew on Growth and Community Dynamics of Nitrifying Biofilms. Water Research (in review)

Van den Akker, B., Holmes, M., Pearce, P., Cromar, N.J., Fallowfield, H.J., 2011. Structure of nitrifying biofilms in a high-rate trickling filter designed for potable water pre-treatment. Water Res. 45, 3489–3498.

Song, Z., Zhang, X., Ngo, H.H., Guo, W., Song, P., Zhang, Y., Wen, H., Guo, J., 2019. Zeolite powder based polyurethane sponges as biocarriers in moving bed biofilm reactor for improving nitrogen removal of municipal wastewater. Sci. Total Environ. 651, 1078–1086.

Gomez-Alvarez, V., Schrantz, K.A., Pressman, J.G., Wahman, D.G., 2014. Biofilm Community Dynamics in Bench-Scale Annular Reactors Simulating Arrestment of Chloraminated Drinking Water Nitrification. Environ. Sci. Technol. 48, 5448–5457.

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