Introduction:

Releasing nanoparticles from products and materials causes great concern connected with a potential risk for man and environment. Increase of nanoparticles concentration, including nanosilver, in domestic sewage is caused by  more and more frequent use of products containing nanoparticles in everyday life ( fabrics, cosmetics, cleaning supplies, paints...). An influence of released nanoparticles on efficiency of wastewater  treatment  by activated sludge method is particularly important but little known issue. Biocidal action of nanosilver on activated sludge biocenosis could reduce  wastewater  treatment efficiency and make negative consequence for health.

Materials and methods:

The purpose of the studies was to prepare a new methodology to analyze the influence of silver nanoparticles on the beta hemolytic Gram-positive bacteria that exist in activated sludge and to investigate biocidal activity of nanosilver. The studies were conducted for various concentrations of nanosilver and different times of its contact with a suspension of beta hemolytic bacteria. The effect of influence of nanoparticles on live organisms depends on the properties of nanoparticles, and these change with the surrounding medium and within time. Among many properties of nanoparticles, zeta potential and the size of particles are considered to be of decisive importance in respect of biological effects.

Bacteria were isolated from activated sludge collected from residential wastewater treatment plant by spread-plate method on medium COLUMBIA (bioMerieux). A colloidal nanosilver solution received by the physical method was used in the experiment. The size of particles, their concentration and zeta potential (Zetasizer 3000, Nanosight NS500) were determined in the nanosilver solutions.

Results:

The results obtained indicate that nanosilver in solution forms a solution with heterogeneous structure containing nanoparticles and their agglomerates. Together with a change of colloid concentration its properties also modify – the size of particles and zeta potential are changing. Because of aging of the colloidal nanosilver solutions their characteristics and biocidal properties change. The experiments showed that nanosilver shows biocidal activity against beta hemolytic bacteria isolated from activated sludge.  It was found that there exists a relation between the nanosilver dose and the time of contact with bacteria. The higher the volume of nanosilver dose is, the more effective it is and the shorter is the time required to obtain the biocidal effect.

The conducted studies have shown the usefulness of the test based on the viability of beta hemolytic bacteria in the screening assessment of potential biocidal properties of nanosilver. The developed method is characterized by low costs, simplicity in conducting and rapidity of determination of nanosilver biocidal effectiveness. The method, after some modifications, may be also applied in effectiveness tests of other substances toxically acting on microorganisms.

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