Gabriela Schaule

Biofouling—the Achilles heel of membrane processes☆

Microorganisms in membrane systems tend to adhere to surfaces and to form a gel layer called biofilm, which participates in the separation process as a secondary membrane. On the raw water side, it causes an increase of fluid friction resistance which increases Δpfeed/brine. Also, overall hydraulic resistance of the membrane Δpmembrane can increase due to the biofilm. If these effects exceed a certain threshold of interference, they are addressed as biofouling. Countermeasures require a three step protocol: (1) detection, (2) sanitation, and (3) prevention. Detection has to be performed on the surface as planctonic cell numbers released randomly from the biofilm do reflect neither site nor extent of biofilm growth. The analysis includes microbiological and biochemical parameters; the differentiation between other kinds of fouling such as scaling or organic fouling can be performed by FTIR-ATR spectroscopical analysis. Sanitation should be focused on removal of the biomass rather than on killing the microorganisms attached to the surface. First, the slime matrix, consisting mainly of polysaccharides and proteins, must be weakened. This requires interference with the binding forces, which are weak physico-chemical interactions such as hydrogen bonds, van der Waals and electrostatical interactions. Then, increased shear forces can remove the biomass. A preventive concept should acknowledge the fact that biodegradable substances in the water represent the biofouling potential. Biofouling can be regarded as a “biofilm reactor in the wrong place”. Reduction of the nutrient content of the raw water can be achieved by a “biofilm reactor in the right place”, i.e., a biofilter on which microorganisms form biofilms and sequester the nutrients from the water phase. Mandatory for any optimized antifouling strategy is monitoring of biofilm development; a fiber optical device which provides real-time, on-line, in situ information non-destructively is proposed which can be adjusted to membrane modules.

Contamination of drinking water by coliforms from biofilms grown on rubber-coated valves

In water samples from drinking water distribution systems, coliform bacteria (predominantly Citrobacter species) were repeatedly detected. Disinfection and flushing of the systems did not erase the problem. The pattern of the coliform occurrences indicated contamination originating from biofilms. After inspection of internal surfaces of the systems, no significant biofilm growth was observed on pipe surfaces, but in a number of cases, visible biofilms were detected on rubber-coated valves which harboured the same coliform species as those found in the drinking water samples. In these cases, the rubber-coated valves seemed to act as point sources for the contamination of water.

The permeability of biofouling layers on membranes

Abstract During operation of membrane systems, biofilms develop from the onset and participate in the separation process as a secondary membrane. Biofilms mainly consist of bacteria, the extracellular polymer substances - which are excreted by the cells and which immobilize these cells and entrap particles on the membrane surfaces. The biofilm has an hydraulic resistance. If this leads to an intolerable loss of performance, the effect is called “biofouling” and cleaning is needed. The assumed action of cleaners is to remove the biofouling layer. However, in many cases removal of the biomass is not actually observed, although performance improves after cleaning. An explanation is that the cleaner improves the permeation properties of the biofilm instead of removing it. It is shown here that the application of commercial cleaning agents substantially affects the permeability of the matrices used to model biofouling. Improved performance due to redispersion of the matrix is small compared to the improved performance due to increased matrix permeability. Formaldehyde tends to decrease biofilm permeability. Thus, the hydraulic characteristics of biofilms on membranes are very important for the separation process and can be improved or made worse by adding chemicals for cleaning and disinfection.

Grafted glycopolymer-based receptor mimics on polymer support for selective adhesion of bacteria.

A sugar-containing monomer (2-lactobionamidoethyl methacrylate, LAMA) was grafted on a polypropylene (PP) microfiltration membrane surface by UV-induced graft copolymerization. The degree of grafting can be controlled by variation of monomer concentration, UV irradiation time, and photoinitiator concentration. Fourier transform infrared spectroscopy and scanning electron microscopy were employed to confirm the surface modification on the membranes. The water contact angle was used to evaluate the hydrophilicity change of the membrane surface before and after modification. Bacteria capture experiments showed that the membrane could selectively bind E. faecalis while adhesion of S. maltophilia was not influenced by the functionalization of PP with grafted poly(LAMA). The adhesion of E. faecalis onto poly(LAMA) grafted membrane could be inhibited by 200 mM galactose solution; however, glucose solution showed no inhibition effect. Moreover, occupying sugar residues on the membrane surface primarily by a galactose targeting lectin, peanut agglutinin, could significantly suppress the following adhesion of E. faecalis. All these results clearly demonstrate that this poly(LAMA) grafted PP membrane can selectively capture E. faecalis and that this selection is based on the interaction between galactose side groups on grafted flexible functional polymer chains on the membrane surface and galactose binding protein on the E. faecalis cell membrane.

Biofouling on Membranes — A Short Review

“Fouling” is referred to as the unwanted deposition of material from the bulk water phase on surfaces, such as membranes. This term has been adopted from heat exchanger technology (Epstein, 1981; Characklis, 1990). For membrane technology, the most important types of fouling include: crystalline fouling (“scaling”, deposition of minerals due to excess of the solution product) organic fouling (deposition of oil, grease, lipids etc.) particle fouling (deposition of clay, silt, humic substances, debris etc.) colloidal fouling (silica, humic acids) biofouling (adhesion and accumulation of microorganisms)

Effects of electric polarization of indium tin oxide (ITO) and polypyrrole on biofilm formation

The influence of electric polarization on primary adhesion and on biofilm formation was investigated. As substrata, indium tin oxide (ITO) and polypyrrole coatings were used because of their electric conductivity. The materials were polarized from -600 mV to +600 mV, switching every 60 seconds. Control was non-polarized substrata. Primary adhesion under this regime was not strongly influenced, however, the morphology of the primary biofilm was obviously different from that of the control. Biofilm formation of the natural population of non-chlorinated drinking water, supplemented with nutrient in low concentration, was determined over 164 hours. While the biofilm on the control surface developed to a thickness of about 100 microm, on the pulsed polarized surface it reproducibly developed only to a very thin biofilm. Faster switching of the polarization (10 second) had no further influence. If the polarization routine was reduced to only twice a day (one hour), no influence on biofilm development was observed. These results indicate that fluctuating polarization at a rate of once per minute inhibits the physiological processes during biofilm formation during one week. Investigations are in process to determine further details of this effect in order to employ it for inhibition of biofouling.

Bestimmung der stoffwechselaktiven Bakterien im Belebtschlamm

Die Bestimmung der Stoffwechselaktivitat ist ein zentrales Anliegen der Okologie der Abwasser-Mikroorganismen, da der Abbau von organischen Wasserinhaltsstoffen und die Stoffumsatzrate nur durch physiologisch aktive Organismen bestimmt werden. In der Regel wird die Aktivitat von Mikroorganismen im Abwasser uber die summarische Bestimmung von Stoffumsatzraten und Biomasseparametern charakterisiert. Diese summarischen Kenngrosen ermoglichen keine Aussagen uber die Verteilung der Stoffwechselaktivitat in den einzelnen Populationen. Fur das Verstandnis der Populationsdynamik und der Stoffumsatzraten im Abwasser ist zunachst die Erfassung der physiologisch aktiven Mikroorganismen auf zellularer Ebene eine wesentliche Voraussetzung. Die gleichzeitige Identifikation der stoffwechselaktiven Mikroorganismen uber Gensonden, um die wirklichen Akteure und deren Bedeutung fur die Stoffumsetzung im Abwasser zu charakterisieren, stellt in einem weiteren Schritt eine wichtige Aufgabe der Okologie dar.