In the case of BOD5, authors have reported reductions between 15% (Nebel et al.
1973) and 67% (Paraskeva et al. 1998). In our tests (Fig. 8), the effect of ozone on
this parameter proved negligible, although it did remove up to 68% in one test.
In the cases of both COD and BOD5, the initial wastewater level was very small
(Seoánez 1999) and this could explain why the rate of reduction for both parameters
was not very high. However, as shown in Fig. 9, we should highlight that ozone treatment
increased the biodegradability of the effluent, because the BOD5/COD ratio
increased from poorly biodegradable to biodegradable. This increase in biodegradability
allowed certain chemical compounds and organic matter to be degraded by
microbial attacks. This tends to increase water quality, as these compounds tend to be transformed into compounds that are less toxic than the original ones. These results
support the findings reported by Alfafara et al. (2000) and Chun-du et al. (2007).