Piece of biocake: Investigating the proteins that foul membrane bioreactors
- Published: Jan 19, 2015
- Author: Jon Evans
- Channels: Electrophoresis
What's in the biocake?
Membrane bioreactors remain the state-of-the-art for treating wastewater. Rather than wait for microbes and suspended solids to settle out via gravity, membrane bioreactors actively remove them by pumping wastewater through a semi-permeable membrane. This saves a great deal of time, both because membrane filtration is faster than the settling process, and because it removes the need for several other treatment steps, such as sand filtration and disinfection.
The big downside to membrane bioreactors is fouling, whereby microbes and suspended solids that can’t pass through the membrane form a thick layer on its surface, known as biocake. As this layer is simultaneously built up and compacted by the pressure of the water, it gradually blocks the surface of the membrane, reducing the flow rate and thus requiring the pumping strength to be increased, adding expense. Eventually, the biocake needs to be scraped off the surface of the membrane, adding even more expense.
Thus, scientists are keen to determine the composition of biocake, and investigate how this composition varies with different flows and membrane pressures, in order to find ways to slow its formation. The main components of biocake are microbes, polysaccharides and proteins, but scientists have so far focused on analysing the microbes and polysaccharides in biocake rather than the proteins. To remedy this oversight, scientists from China and the US, led by Fangang Meng at Sun Yat-sen University in Guangzhou, have now used two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) to analyse the proteins present in various samples of biocake. In addition, they also tried to identify a selection of these proteins using matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS).
The samples of biocake were produced by a model membrane bioreactor consisting of two semi-permeable polymer membranes placed in a tank in series, through which the scientists pumped a synthetic wastewater. Using two membranes allowed Yat-sen and his colleagues to investigate the biocake produced at two different flow rates, as the flow rate experienced by the first membrane in the series will obviously be higher than that experienced by the second membrane. By keeping the flow rates constant during the course of the experiment, they were also able to investigate how the composition of the biocake changed in response to rising pressures, as the fouling caused the resistance at the membranes to increase.
What they discovered was that the type of proteins found in the biocake differed as the membrane pressure increased, especially for the higher flow rates experienced by the first membrane. When they analysed the proteins in the biocake samples with 2D-PAGE, they found higher concentrations of proteins at neutral pHs (5.5–7.0) early in the process, when the pressure was low. Later in the process, at higher pressures, they found that the biocake samples contained higher concentrations of proteins at lower pHs (4.0–5.5).
From neutral to acidic
Neutral proteins in biocake tend to be soluble microbial products (SMPs), which are released when cells die, while acidic proteins tend to be extracellular polymeric substances (EPSs), which living cells secrete into their environment. So this suggests that at early stages and low pressures, the biocake consists mainly of dead cells and the SMPs they have released. At later stages and high pressures, live cells start to be trapped into the growing biocake, where they feed on the SMPs while releasing EPSs that become trapped in the biocake.
Further evidence that the composition of the biocake changes over time comes from the discovery that 183 proteins differed in abundance between the different samples. Yat-sen and his colleagues were able to determine the identity of 23 of these proteins, with over half being outer membrane proteins. Next, Yat-sen and his colleagues want to investigate the microbes that are present in these biocakes, in order to determine exactly which microbes are producing which proteins.
Environmental Science & Technology (Article in Press): "Metaproteomic analysis of biocake proteins to understand membrane fouling in a submerged membrane bioreactor"
Article by Jon Evans
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.
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