## 3 Results and Discussion
### BOD and OUR Measurements  
Biodegradability is often characterized by the biochemical oxygen demand (BOD) values. In BOD measurements, a mixed microorganism population (activated sludge, collected from a biological degradation unit of a wastewater treatment plant) is utilized for oxidation, and usually O\({}_{2}\) consumption is measured over a 5-day incubation period (Figure 2). As Figure 2D shows, in a pure water matrix BOD increased with the measuring time, and around 5 days it showed a tendency to level off. The microorganisms (bacteria, fungi) produce extracellular enzymes for breaking down large molecules into smaller, readily biodegradable ones, thus producing nutrients for the microorganism. In the presence of nutrients, the microorganisms multiply. This is a self-stimulating process; the more enzymes the bacteria release, the more ofloxacin will be decomposed by these enzymes, resulting in more nutrients. Because there are more nutrients, the bacteria multiply faster. The enzymes can decompose only a small part of the original molecules; therefore, the BOD value at zero dose is small. With an increasing dose, the biodegradability improves and the BOD values increase. On the fifth day, the values were 15.3 in pure water, 20 in tap water, and 41.4 mg dm\({}^{-3}\) O\({}_{2}\) in purified wastewater at a 4 kGy absorbed dose. The high value measured in the purified wastewater is due to the biodegradable impurities in this matrix. The organic content of a solution is considered to be easily biodegradable when the BOD\({}_{5}\)/COD ratio is above 0.4 [37]. This ratio in the untreated sample was \(\sim\)0.09, showing that ofloxacin is poorly biodegradable at the concentration used in the experiments, 0.1 mmol dm\({}^{-3}\). As a result of irradiation, the BOD\({}_{5}\)/COD ratio increased above 0.4 at higher doses, reflecting good biodegradability in the treated solution. The ratio also increased with the absorbed dose in both the tap water and purified wastewater matrices. The nonbiodegradable organic content transformed into biodegradable upon irradiation of the solutions in these cases, too. The effect of degradation products on the metabolic activity of a mixed microbial population was investigated in oxygen uptake rate measurements (OUR, Figure 3). Similarly to BOD, this test also uses activated sludge with a complex mixture of bacterial species. However, OUR measurements take only three hours, whereas BOD measurement is at least five days long. The number of microorganisms used in the OUR test is around two orders of magnitude higher than the initial number of microorganisms in BOD measurement. However, this number does not change during OUR measurement; microorganisms have no time for producing extracellular enzymes, and they cannot multiply either, while at the end of the five-day measurement, the number of microorganisms in BOD measurement is two orders of magnitude higher (four orders of magnitude compared to the starting solution) than in the OUR test. Therefore, in OUR measurements, only the readily biodegradable compounds are digested. In pure water, the blank solution containing activated sludge had an OUR value of 0.70 mg dm\({}^{-3}\) h\({}^{-1}\). When ofloxacin was added to the blank solution, the OUR value increased to 1.13 mg dm\({}^{-3}\) h\({}^{-1}\). The value corrected with the blank is 0.43 (in Figure 3 the corrected values are shown). The corrected values increased at higher doses compared to the unirradiated one, showing more intensive metabolic activity in the treated samples in agreement with the results of the BOD\({}_{5}\) measurements. The values indicate that the degradation products not only are nontoxic, but they used as a source of nutrients by the microbes. zone diameters decreased with the dose, showing that irradiation decreased the antibiotic potency of the solutions (Figure 4). In pure water and tap water matrices at 1 kGy dose, the antibacterial activity was strongly reduced, whereas at 2 kGy no inhibition was observed.