Compaction and relaxation of biofilms

Abstract

A B S T R A C TOperation of membrane systems for water treatment can be seriously hampered by biofouling. A better characterization of biofilms in membrane systems and their impact on membrane performance may help to develop effective biofouling control strategies. The objective of this study was to determine the occurrence, extent and timescale of biofilm compaction and relaxation (decompaction), caused by permeate flux variations. The impact of permeate flux changes on biofilm thickness, structure and stiffness was investigated in situ and non-destructively with optical coherence tomography using membrane fouling monitors operated at a constant crossflow velocity of 0.1 m s −1 with permeate production. The permeate flux was varied sequentially from 20 to 60 and back to 20 L m −2 h −1 . The study showed that the average biofilm thickness on the membrane decreased after elevating the permeate flux from 20 to 60 L m −2 h −1 while the biofilm thickness increased again after restoring the original flux of 20 L m −2 h −1 , indicating the occurrence of biofilm compaction and relaxation. Within a few seconds after the flux change, the biofilm thickness was changed and stabilized, biofilm compaction occurred faster than the relaxation after restoring the original permeate flux. The initial biofilm parameters were not fully reinstated: the biofilm thickness was reduced by 21%, biofilm stiffness had increased and the hydraulic biofilm resistance was elevated by 16%. Biofilm thickness was related to the hydraulic biofilm resistance. Membrane performance losses are related to the biofilm thickness, density and morphology, which are influenced by (variations in) hydraulic conditions. A (temporarily) permeate flux increase caused biofilm compaction, together with membrane performance losses. The impact of biofilms on membrane performance can be influenced (increased and reduced) by operational parameters. The article shows that a (temporary) pressure increase leads to more compact biofilms with a higher hydraulic resistance.

The composition and compression of biofilms developed on ultrafiltration membranes determine hydraulic biofilm resistance

“…Dreszer et al (2014) proposed that two mechanisms govern the biofilm compressibility: a physical compression of the EPS and a chemical EPS consolidation. However in our study, similar hydraulic resistances were measured for the 1st and 2nd compressibility tests, indicating that in our test the compression of the biofilms was entirely reversible These results are in accordance with the study of Vallares Linares et al (2016), who reported that the effect of permeate flux on the hydraulic biofilm resistance is reversible (Vallares Linares et al, 2016).Our results also suggests that chemical EPS consolidation did not occur during the 1st compression test and did not modify the mechanical properties of the biofilms submitted to the 2nd test. The elasticity of biofilms also explains why similar mesoscale physical structures were observed without TMP for the two types of biofilm (Fig.…”

Optical coherence tomography in biofilm research: A comprehensive review

“…Since OCT detects reflection and scattering signals, it is well suited to visualize the overall distribution of particular matter in biofilms. More importantly the main limitation of especially fluorescence imaging is overcome: regions of background signals surrounded by biofilm are identified as voids and cavities (Valladares Linares et al, ; Wagner et al, ). To date none of the biofilm research groups applying OCT have reported if these cavities contain other matters than water (e.g., EPS).…”

“…This might be an acceptable assumption for an unaltered, sponge‐like biofilm structure containing a high fraction of water. But when the biofilm is compressed (Valladares Linares et al, ) this simplification becomes to some extend obsolete the more water is released. However, Ratheesh Kumar et al () estimated the refractive index of biofilm during a cultivation by means of a swept‐source OCT. Their measurements revealed a refractive index of confirming the general simplification made in most studies.…”

“…Thickness measurements were also conducted to evaluate the compression behavior of biofilms (Blauert et al, ). Valladares Linares et al () monitored the compression‐relaxation characteristics of biofilm cultivated in a MFS in a series of B‐scans (200–300 μm). A flux increase by 200% led to a reduction of the mean biofilm thickness by 40%.…”

“…Blauert et al () have been the first to show the power of OCT imaging for the estimation of the shear modulus and Young's modulus. The extremely fast imaging capability allows for the acquisition of 2D videos in “real‐time” (Blauert et al, ; Valladares Linares et al, ) and time‐lapsed 3D imaging (Blauert et al, ) showing overall biofilm deformation. The latter is exactly what is needed for the establishment of a fast and reliable method to determine mechanical biofilm properties.…”