Optimization of low energy sonication treatment for granular activated carbon colonizing biomass assessment

Abstract

This study is aimed at optimizing a low energy sonication (LES) treatment for granular activated carbon (GAC)-colonizing biomass detachment and determination, evaluating detachment efficiency and the effects of ultrasound exposure on bacterial cell viability. GAC samples were collected from two filters fed with groundwater. Conventional heterotrophic plate count (HPC) and fluorescence microscopy with a double staining method were used to evaluate cell viability, comparing two LES procedures, without and with periodical bulk substitution. A 20 min LES treatment, with bulk substitution after cycles of 5 min as maximum treatment time, allowed to recover 87% ÷ % of attached biomass, protecting detached bacteria from ultrasound damaging effects. Observed viable cell inactivation rate was 6.5÷ 7.9% cell/min, with membrane-compromised cell damage appearing to be even higher (11.5% ÷ 13.1% cell/min). Assessing bacterial detachment and damaging ultrasound effects, fluorescence microscopy turned out to be more sensitive compared to conventional HPC. The optimized method revealed a GAC-colonizing biomass of 9.9×10⁷ cell/g_GAC for plant 1 and 8.8×10⁷ cell/g_GAC for plant 2, 2 log lower than reported in literature. The difference between the two GAC-colonizing biomasses is higher in terms of viable cells (46.3% of total cells in plant 1 GAC-colonizing biomass compared to the 33.3% in plant 2). Studying influent water contamination through multivariate statistical analyses, a possible combined toxic and genotoxic effect of chromium VI and trichloroethylene was suggested as a reason for the lower viable cell fraction observed in plant 2 GAC-colonizing population.

• granular activated carbon
• drinking water
• biofilm detachment
• ultrasound
• fluorescence microscopy

The work presented here was carried out in collaboration between all authors. M. Antonelli conceived and planned the research; she designed the experiments and provided suggestions for data analysis and results interpretation; she supervised the drafting and revised the work. M. Bernasconi collaborated in designing and developing the experiments, teaching and training G. Saccani on microbiological analysis techniques. G. Saccani carried out the laboratory experiments, analyzed the data, interpreted the results and wrote the paper. All authors have seen and approved the manuscript.