Growth in production and use of nanoparticles (NPs) will result increased concentrations of these in industrial and urban wastewaters and, consequently, in wastewater-treatment facilities. The effect of this increase on the performance of the wastewater-treatment process has not been studied systematically and including all the microbial communities involved in wastewater treatment. The present work investigates, by using respiration tests and biogas-production analysis, the inhibitory effect of four different commonly used metal oxide (CeO(2) and TiO(2)) and zero-valent metal (Ag and Au) nanoparticles on the activity of the most important microbial communities present in a modern wastewater-treatment plant. Specifically, the actions of ordinary heterotrophic organisms, ammonia oxidizing bacteria, and thermophilic and mesophilic anaerobic bacteria were tested in the presence and absence of the nanoparticles. In general, CeO(2) nanoparticles caused the greatest inhibition in biogas production (nearly 100%) and a strong inhibitory action of other biomasses; Ag nanoparticles caused an intermediate inhibition in biogas production (within 33-50%) and a slight inhibition in the action of other biomasses, and Au and TiO(2) nanoparticles caused only slight or no inhibition for all tested biomasses.

In this study, several standard toxicity tests have been performed on selected inorganic nanoparticles. Acute toxicity tests were selected according to their extensive use in toxicological studies and included phytotoxicity using several seeds, Daphnia magna and a bioluminescent test (Microtox®). All of them have been used in several international regulations as toxicity assays. In the case of nanoparticles (NPs), we have studied those of cerium oxide, titanium dioxide and iron oxide. Iron oxide NPs are well known and broadly used and were selected because of their low toxicity. Titanium dioxide and cerium oxide NPs are currently being used in several fields such as photocatalysis and medical applications, but their toxicity effects have been scarcely studied. Our results revealed that cerium NPs are extremely toxic in the entire set of tests conducted (inhibition higher than 80% at very low concentrations for the bioluminescence test and LC50 = 0.012 mg/ml of mortality in the assays of Daphnia magna), whereas titanium NPs were practically inert in terms of toxicity (values similar to those of controls). The possible toxicological effect of the solvents necessary to stabilize NPs in liquid medium for the three cases (stabilizers) has been also studied.Only in the germination test (phytotoxicity) of some seeds they showed some detrimental effect to germination. In general, the standardized tests proposed in this study have proved to be very useful in the determination of NPs toxicity when no or few data are available, although further work is necessary in the case of the germination test.

A novel concept of dosing iron ions using Fe3O4 engineered nanoparticles is used to improve biogas production in anaerobic digestion processes. Since small nanoparticles are unstable, they can be designed to provide ions in a controlled manner, and the highest ever reported improvement of biogas production is obtained. The nanoparticles evolution during operation is followed by an array of spectroscopic techniques.

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