Biological control of microbial attachment and membrane biofouling.
Date of Issue2012
School of Civil and Environmental Engineering
Microbial attachment to solid surfaces has been widely observed in natural and engineered systems. A number of factors have been considered to contribute to microbial attachment, such as surface charge, hydrophobic interactions, hydrodynamic shear force, bacteria surface properties, extracellular polymeric substances and cell species. Unwanted microbial attachment can cause serious problems, such as membrane biofouling. Increasing global need for water and wastewater treatment has driven the widespread development of membrane processes. It should be realized that the main drawback hindering the wide application and further improvement of membrane systems is membrane biofouling due to microbial attachment onto membrane surface, which leads to increased operation cost due to frequent cleaning and replacement of the clogged membrane. So far, extensive research efforts have been devoted to prevention and cleaning of membrane biofouling. The traditional methods developed for reducing membrane biofouling are mainly based on physicochemical principles, such as modification of membrane surface, optimization of operation conditions, regular physical and chemical cleaning. However, research on biological control of membrane biofouling is limited. Therefore, this study was aimed to investigate novel strategies for biological control of microbial attachment and membrane biofouling through energy dissipation and D-amino acid. In the first phase of study, biological control of microbial attachment and membrane biofouling through energy dissipation was investigated. For this objective, 2, 4-dinitrophenol (DNP), a typical metabolic uncoupler, which can dissipate the proton motive force and further disrupt adenosine-5’-triphosphate (ATP) synthesis, was employed. It was shown that energy dissipation through energy uncoupling resulted in reduced microbial attachment on various solid surfaces, and subsequently mitigated membrane biofouling on both hydrophobic polytetrafluoroethylene (PTFE) and hydrophilic nylon membranes. Results revealed that higher cell ATP level of suspended microorganisms favored microbial attachment to different solid surfaces, suggesting that energy metabolism was essentially involved in initial attachment of microorganisms onto a solid surface. In addition, dissipation of ATP synthesis by DNP also led to lowered autoinducer-2 (AI-2) production in suspended microorganisms, and a positive link between the ATP and AI-2 levels in suspended microorganisms was established, i.e., AI-2 synthesis is energy dependent. Moreover, it was found that inhibition of ATP synthesis of suspended microorganisms facilitated prevention and mitigation of membrane biofouling. It appears from this study that energy metabolism is involved in microbial attachment, and inhibition of ATP and ATP-mediated AI-2 of suspended microorganisms would be a promising alternative for control of microbial attachment and membrane biofouling. Physiology and structure of biofilms is biofilm age-dependent. To explore biological cleaning of membrane biofouling by ATP dissipation, the responses of different-age biofilms developed on membrane surfaces to a metabolic uncoupler 3, 3’, 4’, 5-tetrachlorosalicylanilide (TCS) were investigated. Results showed that ATP dissipation caused by TCS would promote detachment of different-age biofilms from membrane surfaces. It was observed that chemically inhibited cellular ATP synthesis also suppressed production of AI-2 and extracellular polymeric substances (EPS) of fixed biomass. The extent of biofilm detachment was found to be closely related to AI-2-regulated EPS content of fixed biomass. These results suggest that energy dissipation would lead to a new cleaning strategy of biologically fouled membrane. Since most amino acids in microorganisms are present in the form of L-isomers, the existence and function of D-amino acids is not well understood yet. In this phase of study, how D-amino acid would affect attachment of mixed-species microorganisms to hydrophilic glass and hydrophobic polypropylene surfaces was investigated, and D-tyrosine was employed as a model D-amino acid. Results showed that D-tyrosine did not influence ATP synthesis, microbial growth and substrate utilization, but significantly inhibited the synthesis of AI-2, extracellular DNA (eDNA) and extracellular polysaccharides and proteins of suspended microorganisms, and subsequently reduced microbial attachment onto glass and polypropylene surfaces. These in turn provide a plausible explanation about how D-tyrosine suppressed microbial attachment. It appears that D-amino acid would be a non-toxic agent for control of microbial attachment. In addition, results further revealed that D-tyrosine effectively promoted mixed-species biofilm detachment from membrane surfaces. It was found that D-tyrosine at studied concentration negatively affected AI-2 secretion and extracellular polymeric substances production of fixed biomass on membrane surface. More importantly, the positive correlation between fixed biomass on membrane surface and corresponding AI-2 content of fixed biomass observed in detachment strongly suggest that AI-2 may mediate biofilm detachment from membrane surfaces. These indicate that D-tyrosine not only suppresses microbial attachment, but also promotes biofilm detachment through inhibition of cellular communication and EPS production. In conclusion, this study clearly showed that energy dissipation and D-amino acid would be possible new alternatives for biological control of microbial attachment and membrane biofouling.
DRNTU::Engineering::Environmental engineering::Water treatment