The application of different methods for indirect microbial development assessment in pilot scale drinking water biofilters

The biofiltration process in the biologically activated carbon filters (BAC) is one of advanced methods of water treatment. It enables efficient elimination of dissolved organic matter and some inorganic pollutants. The production of high-quality drinking water requires an appropriate method of filter work control based on biofilm growth assessment. The first aim of the study was to assess the microbial development in beds of two BAC filters with the use of various methods. The second aim was to compare the obtained results and indicate the method which could support filter operators during routine control of biofiltration process. The study was carried out in a pilot scale on models of BAC filters during two filter runs. The analysis of microorganisms was performed in water samples collected from different depths of the filter beds with the use of culture method (HPC), metabolic activity assay (with the FDA), epifluorescence microscopy – total cell count method (TCC) and biochemical method (system Vitek 2 Compact). No statistical correlation between HPC and metabolic activity assay was noted. Total bacteria number determined with the use of TCC was approx. 100–900 times higher than in the HPC method. The biochemical tests revealed the presence of several Gram-negative species. The comparison of the applied methods shows that microbial activity assay is the most useful, fast and low-cost method which may be applied additionally to the HPC method at standard water treatment plant laboratory. 38 D. Holc, B. Mądrecka-Witkowska, M. Komorowska-Kaufman, E. Szeląg-Wasilewska, A. Pruss, Z. Cybulski process occurs. It leads to gradual filling the pores of carbon grains with DOM, what induces the microbial colonization. The phase of full acclimation of bacteria lasts for 2–3 months. The biofilm is formed by the bacteria attached to the carbon grains surface as a results of the cell divisions. Bacteria living in biofilm secrete extracellular polymeric substances (EPSs) which act as a matrix stabilizing cells and protecting them against external factors. Biodegradation of DOC occurs as degradation of substances present in the treated water as well as the result of desorption of partially degraded compounds from the GAC pores. The microbial activity in the BAC extends the filtration time and reduces the necessity of activated carbon bed regeneration (Seredyńska-Sobecka et al. 2006, Simpson 2008, Kołwzan 2011). The filtration trough the BAC filter beds enables great removal efficiency of contaminants of emerging concern, e.g., pharmaceuticals, pesticides (Zang et al. 2017), endocrine disrupting chemicals, personal care products (Snyder et al. 2007), disinfection by-product precursors (Chaukura 2020), cyanotoxins, geosmin and 2-methylisoborneol, amines, aliphatic aldehydes, phenols/chlorinated phenols (Simpson 2008). It can be useful in perand polyfluoroalkyl ether acids elimination (Hopkins et al. 2018). The achievement of the benefits of BAC application in water treatment technology depends on the appropriate methods of filter work’s control. The effects of biofiltration process are affected by many factors such as the quality of the treated water, water temperature, DO and pH level, flow velocity, the stage of biofilm growth, backwashing frequency (Korotta-Gamage and Sathasivan 2017). The number, activity or biodiversity of microorganisms living in the bed can be studied by many methods, e.g., culture method – heterotrophic plate count (HPC), Eberhardt, Madsen and Sontheimer (EMS) test, the determination of microbial activity with the use of specific chemical compounds, e.g., fluorescein diacetate (FDA) or 2,3,5-triphenyltetrazolium chloride (TTC), the quantification of adenosine triphosphate (ATP), the use of glucose-labeled 14C, the observation with the use of microscopic techniques, the determination of chemical compounds of the biofilm (phospholipids, polysaccharides and proteins) and biochemical tests (Mądrecka et al. 2018). Nowadays, also the techniques of molecular biology are more frequently used to discover and describe the taxonomical diversity and functioning of microorganisms living in a biofilm of BAC, e.g., polymerase chain reaction (PCR) based methods, 16S rRNA analysis, denaturing gradient gel electrophoresis (DGGE), temperature gradient gel electrophoresis (TGGE) and next generation sequencing (NGS) techniques used in metagenomics (Douterelo et al. 2014). Most of these methods are still too expensive, demand special equipment or qualified laboratory personnel to use them in the routine analyses at water treatment plants. Studies on biofilm formation and functioning with the use of commonly known and old methods as well as comparing them with modern ones are still very useful. The first aim of the study was to determine and compare the amount, activity and diversity of bacteria growing in beds of two BAC filter columns which differ with the method of bacterial inoculation. Research was conducted in a pilot scale and included two filter runs during the full microbiological activity of BAC. The evaluation of the changes of microbiological activity in the filters was made in water samples taken from different depths of filters bed with the use of traditional culture method (HPC), the metabolic activity assay (with the FDA) and the epifluorescence microscopy techniques. Additionally, taxonomical identification based on biochemical method (system Vitek 2 Compact) was used. The microbiological studies were carried out in water samples collected in the filter bed cross-section and were accompanied by chemical analysis of inflowing and outflowing water. The second aim of the research was to compare the results obtained from chosen methods and indicate the most useful method, which could support filter operators during routine control of the biofiltration process.