Biofilm Thickness Influences Biodiversity in Nitrifying MBBRs—Implications on Micropollutant Removal

Abstract

In biofilm systems for wastewater treatment (e.g., moving bed biofilms reactors-MBBRs) biofilm thickness is typically not under direct control. Nevertheless, biofilm thickness is likely to have a profound effect on the microbial diversity and activity, as a result of diffusion limitation and thus substrate penetration in the biofilm. In this study, we investigated the impact of biofilm thickness on nitrification and on the removal of more than 20 organic micropollutants in laboratory-scale nitrifying MBBRs. We used novel carriers (Z-carriers, AnoxKaldnes) that allowed controlling biofilm thickness at 50, 200, 300, 400, and 500 μm. The impact of biofilm thickness on microbial community was assessed via 16S rRNA gene amplicon sequencing and ammonia monooxygenase (amoA) abundance quantification through quantitative PCR (qPCR). Results from batch experiments and microbial analysis showed that (i) the thickest biofilm (500 μm) presented the highest specific biotransformation rate constants (kbio, L g(-1) d(-1)) for 14 out of 22 micropollutants; (ii) biofilm thickness positively associated with biodiversity, which was suggested as the main factor for the observed enhancement of kbio; (iii) the thinnest biofilm (50 μm) exhibited the highest nitrification rate (gN d(-1) g(-1)), amoA gene abundance and kbio values for some of the most recalcitrant micropollutants (i.e., diclofenac and targeted sulfonamides). Although thin biofilms favored nitrification activity and the removal of some micropollutants, treatment systems based on thicker biofilms should be considered to enhance the elimination of a broad spectrum of micropollutants.

Regulating exopolysaccharide gene wcaF allows control of Escherichia coli biofilm formation

“…To the best of our knowledge, this is the first study to demonstrate the maintenance of biofilm thickness through the inhibition of wcaF gene using CRISPRi. This could be particularly useful for applications related to wastewater treatment and bioproduction in which the thickness of biofilm has shown to influence pollutant removal efficiency in wastewater treatment and bioproduction 15,16,18,21,22 .…”

“…Consequently, the cells in the inner layers of biofilm would become less active. For the use of biofilms in wastewater treatment, the thickness of biofilm has shown to influence pollutant removal efficiency 16,21,22 . Pollutant removal efficiency of biofilm increases within certain thickness due to increased biomass, but starts to decrease as the biofilm becomes thicker because of the diffusion limitation 21,22 .…”

“…For the use of biofilms in wastewater treatment, the thickness of biofilm has shown to influence pollutant removal efficiency 16,21,22 . Pollutant removal efficiency of biofilm increases within certain thickness due to increased biomass, but starts to decrease as the biofilm becomes thicker because of the diffusion limitation 21,22 . In bioproduction, excess growth in biofilm limits the feed availability, thus reducing the bioreactor efficiency and productivity 15,18 .…”

Influence of light and temperature on the development and denitrification potential of periphytic biofilms

“…The CLSM images clearly demonstrated the role of algae morphology in determining the structure of periphytic biofilms. Biofilm thickness has been found to be related to the biodiversity in nitrifying moving bed biofilms reactors (MBBRs), and thin biofilm favored nitrification activity and the removal of some micropollutants but thicker biofilms should be considered for the removal of a broad spectrum of micropollutants (Torresi et al, 2016).…”

A review on the advances in nitrifying biofilm reactors and their removal rates in wastewater treatment

“…studied the effect of thickness of biofilm on the microbial community through the qPCR technique. Their results showed that the biofilm with the highest thickness (500 μm) achieved the highest constants for specific biotransformation rate and the biofilm with least thickness (50 μm) showed the highest nitrification rate …”