Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | JUNE 2017 | 13 suspended in fluid and passed through an electronic detection instrument. The method allows the bacteria present to be visualised, enabling all microbiological cells to be counted. It is also much faster than the standard plate counting method, as the presence of bacteria can be detected in a matter of minutes compared to a matter of days. When water is treated with disinfectant, the bacterial cell membranes are damaged. The cell membrane protects the contents of the cell and so a damaged cell membrane is a very good indicator of a cell being "dead". In the FCM, fluorescent dyes are added that enter different cells depending on the state of their cell membrane; a green dye enters all cells regardless of the state of their cell membrane, while a red dye only enters the bacteria that have cell membrane damage. The intact and damaged bacteria can then be visualised on a scatter plot. Those that appear within a defined area, called a gate, can be identified as those with intact cell membranes and are "living" bacteria. Those outside of the gate are identified as damaged and therefore "dead" bacteria. The scatter plot shows a transition from intact to damaged bacteria cells, or from "living" bacteria to "dead" bacteria, as the bacteria appear to move across the plot from the gated to the non-gated area. A Cranfield Water Science Institute team recently published a paper in the journal Environmental Technology, which reports the findings of a study looking at the use of this method for water quality testing compared to traditional plate-counting methods. The study found that, under laboratory conditions, the two methods showed similar trends when used to test whether E. coli bacteria are killed by treatment with different concentrations of chlorine as a disinfectant. However, the FCM analysis showed that cells could still be deemed as "living" at higher concentrations of disinfectant compared to the plate-counting method. When no bacteria were identified by plate- counting (i.e. all bacteria were deemed "dead"), the bacteria still appeared in transition from the gated area to the non-gated area on the FCM scatter plot and so some bacteria were still "living". This suggests that FCM analysis is more accurate in identifying the presence of "living" bacteria. When using real water samples, bacteria were identified as present using plate-counting when they were in transition on the FCM scatter plot, so both methods show that there were still some "living" bacteria. This suggests even greater congruence between the two methods for identifying when bacteria are effectively killed in real water samples. Water service providers invest a large amount of expertise and resources into ensuring that our water supplies are safe to drink. This new study demonstrates the potential that FCM has to improve both the accuracy and the speed of water quality testing. Further research is needed to identify which bacteria remain present a‰er disinfection and whether these are harmful to human health. The permanence of disinfecting bacteria in real water samples also needs to be investigated. The paper reports a welcome starting point in this line of research and shows great promise in the use of FCM analysis in water quality testing. As the plate-counting approach is currently mandated by the water quality regulator, revisions to regulation and policy would need to be devised if widespread water quality testing with FCM is to become a reality. If FCM is proven to be an accurate method as well as a faster method for monitoring water quality, this will help to ensure the safety of our water, for drinking, farming and industrial use. SPONSORED BY Dr. JohN LEar, TEchNicaL DirEcTor, BioLogicaL PrEParaTioNs LTD Microbes can deal with surfactants A wide variety of surfactants are used in industry and need to be removed from wastewater, but a similar variety of bacteria can be applied to the task G lobal production of surfactants ("surface active agents") now exceeds 15 million tonnes per year and these compounds find extensive use in household cleaners, personal care products, paints, pesticides, textiles, petroleum recovery, polymers and the pulp and paper industry. Hence it is not surprising that significant quantities of surfactant may end up in the wastewater treatment plant, where they must be removed as part of the treatment process. Most surfactants in significant use today are readily biodegradable by a range of microbial species but high levels can cause effluent toxicity, disruption of the microbial floc, increased growth of filament formers and excessive foaming. This means that breakdown may not happen to the required degree in the treatment plant and partially degraded surfactants can pass through to discharge, elevating BOD and COD and risking breach of consent limits, as well as the clearly undesirable result of releasing them into the environment. The addition of suitable bacteria can greatly enhance surfactant breakdown in the wastewater treatment plant and helps to avoid these problems. Surfactants are amphiphilic molecules meaning that they consist of both hydrophilic ("water-loving") and hydrophobic ("water-hating") portions. This enables them to adsorb at liquid/liquid, solid/liquid and gas/ liquid interfaces, lowering liquid surface tension and endowing them with their cleaning, emulsifying, dispersing and wetting properties. According to the charge carried by the hydrophilic head portion of the molecule, surfactants may be non-ionic (e.g. alcohol ethoxylate), anionic (e.g. sodium lauryl sulphate), cationic, (e.g. quaternary ammonium compounds) or amphoteric (e.g. cocamidopropyl betaine). This gives rise to a wide variety of surfactants with different properties, and microbial breakdown of these complex molecules tends to occur in stages. For example, a typical breakdown pathway for the anionic surfactant linear alkylbenzene sulphonate would be: ω- and β-oxidation of the straight alkyl chain; removal of the sulphonate group by desulphonation; and then breakdown of the remaining benzene ring, e.g. oxidation of phenylacetic acid. As for other complex molecules, it is therefore essential to select bacteria with suitable metabolic abilities and a consortium of organisms is o‰en required for complete biodegradation as a particular species or strain can only carry out one step of the breakdown pathway. This synergism between members of the microbial population is very important for surfactant breakdown. Biological Preparations supplies consortia of bacteria that are capable of breaking down non-ionic, anionic, cationic and amphoteric surfactants. The Talk: opinion "Most surfactants in significant use today are readily biodegradable by a range of microbial spe- cies but high levels can cause effluent toxicity, disruption of the micro- bial floc and excessive foaming."

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