Influence of Feed Water, Substrate Concentration and Permeate Flux On Biofilm Resistance

Introduction

Biofouling of membranes is caused by biofilms. The biofilm acts as a secondary membrane on top of the filtration membrane, influencing the overall membrane resistance. In this study, the contribution of the biofilm should be determined. In order to do so, a microfiltration membrane was chosen because its intrinsic resistance is expected to be much lower than that of the biofilm, and concentration polarization of salts is avoided. This approach allows separating the biofilm influence from that of the membrane. Aim of the study was to investigate the influence of nutrient supply and permeate flux on biofilm resistance.

Experimental

Biofilms were generated in a lab scale crossflow microfiltration system at two constant fluxes (20 and 100 L m^-2 h^-1). Sodium acetate (CH₃COONa), sodium nitrate (NaNO₃) and sodium dihydrogen orthophosphate (NaH₂PO₄) in the ratio of 100:20:10, respectively, was employed as a substrate source and added to the feed water in the following amount: a final concentration of 1.0, 0.1 mg L^-1 of organic carbon in comparison to a control without added nutrients.

The following parameters were determined: biofilm resistance, biofilm thickness, total cell number, total organic carbon, and concentration of extracellular polymers. All parameters are expressed per square centimeter. Furthermore, microscopic analyses such as scanning electron microscopy (SEM) and confocal laser scanning microscopy were applied.

Results and Discussion

It could be verified that the fouling layer consisted of a growing biofilm and not of a cake of cells accumulated by the filtration process. SEM micrographs demonstrated that no internal fouling of the membrane occurred. Reproducibility tests showed that biofilms that grow at similar operation conditions show reproducible resistances as well as similar thickness, TOC, total cell numbers and EPS content. This was proven for biofilms grown not only simultaneously but also at different starting times. Biofilm resistance is very responsive to the substrate concentration in the feed water: the higher the substrate concentration the higher the biofilm resistance and the other measured biofilm parameters. This was the case for both operational fluxes, where all parameters showed higher values for higher flux operation (100 Lm^-2 h^-1). Contrary to the biomass parameters TCN, TOC and EPS, resistance was not proportional to biofilm thickness, suggesting that a higher flux possibly caused biofilm compaction and, thus, a higher biofilm density. Flux increase (from 20 to 100 L m^-2 h^-1) resulted in resistance increase; at return to the original flux, the original resistance was restored. For flux decrease from 100 to 20 L m^-2 h^-1), restoration of the original resistance was also observed. The resistance is attributed to the hydrodynamic effect of the EPS molecules. They represent a highly hydrated gel which is de-swelling under pressure, resulting in a higher resistance. The reversible nature of the effect suggests a dynamic behavior of the EPS matrix.