Influences And Mechanisms Of D-amino Acids On The Microbial Aggregates In Wastewater Treatment Reactors

With the increasing growth of population and the rapid development of economy, the environment has been polluted heavily. The biological way of waste water treatment has been extensively used with its unique advantages.Microbial aggregates are the main block in treatment process and their stability is essential for the solid/liquid separation in biological wastewater treatment. As the technology and analytical methods improved, scientists have focused more on signal molecular influencing the stability of microbial aggregates compared with before on traditional operation factors. D-amino acids were discovered as one of new signal molecular, for they can regulate gene expression and their group behaviors. The synthesis of D-amino acids may be a common strategy for bacteria to adapt to the changing environmental conditions. So it is of great significance to investigate D-amino acids on the stability of microbial aggregates and the application of D-amino acids in wastewater treatment field. In this study, the effect of an exogenous D-tyrosine on the shear stability and surface characteristics of activated sludge flocs were investigated and the interaction between D-amino acids and membrane was elucidated. The main contents and conclusions are following:Sludge flocs were found to be less stable in the addition of D-tyrosine. The constant of shear stability increased with the concentration of D-tyrosine. The value of md,t is linear with the concentration of D-tyrosine. In addition, D-tyrosine changed the surface characteristics of sludge flocs.The hydrophobicity of sludge flocs was reduced and the zeta potential was more negative with the content of D-tyrosine increasing, which made the sludge flocs unstable. D-tyrosine inhibited the production of extracellular polymeric substances (EPS) especially for the proteins but did not change the function groups of sludge EPS. A high correlation coefficient was observed between the composition of EPS fraction and D-tyrosine content. A linear relationship between the extracellular polymeric substances and surface characteristics for sludge flocs indicated that the inhibited EPS production may be responsible for the instability of sludge upon the addition of D-tyrosine.D-amino acids effectively relieved membrane pollution caused by biofouling and the effect is universal in both hydrophilic PES membrane and hydrophobic PVDF membrane. The effect of relieving pollution is more apparent in Gram-positive bacteria than in Gram-negative bacteria. D-amino acids inhibited the production of EPS and attachment of bacteria to the membrane surface. Soluble microbial products (SMP) are important substances in biofouling process and D-amino acids did not have any effect on SMP. They had the same behavior of biofouling. After physical cleaning and chemical cleaning, the polluted membrane recovered some flux. The membrane polluted by Gram-positive bacteria can be cleaned more easily by physical methods while membrane polluted by Gram-negative bacteria can be cleaned more easily by chemical methods. It is the same as sludge floes in membrane; D-amino acids inhibited the production of EPS but did not change the function groups of EPS.The reduced production of EPS made bacterium attaching to the membrane surface more loosely, alleviating biofouling. EPS are also contaminants in filtration process, decreasing the content of EPS would also be beneficial to alleviating biofouling.This study investigated the influences of D-amino acids on microbial aggregates and elucidated the mechanism between them. It provides a potential new way to control membrane pollution and is of great significance in theoretical and practical field.