# Rationale for the Combined Use of Biological Processes and AOPs in Wastewater Treatment Tasks
## 3 Results and Discussion  
### Investigation of Purification of Wastewater from Nitrogen Compounds Using AOPs  
The water treatment quality results using AOPs are shown in Figure 5; these are quite consistent with the results obtained in previous studies using a combination of UV irradiation and the addition of hydrogen peroxide [23]. The comparison with these results  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Microbial Sensitivity Category** &
\begin{tabular}{c} **Growth** \\ **Retardation, mm** \\ \end{tabular} & **Microbiological Characteristics** \\ \hline Sensitive (S—susceptible) & \(\leq\)4 & Does not have resistance mechanisms \\ Intermediately resistant or moderately & 14–18 & Subpopulation between sensitive and \\ sensitive (I—intermediate) & & resistant \\ Resistant (R—resistant) & \(\geq\)19 & Has resistance mechanisms \\ \hline \hline \end{tabular}
\end{table}
Table 1: Interpretation criteria for the sensitivity of microorganisms of the storage culture of activated sludge.  
is fair because, before reaching the photolysis unit, the aqueous solution passes through an electrolysis unit where, among other things, it is also saturated with hydrogen peroxide.  
At the same time, the increase in values of pollutants at the stage of anodic oxidation (on average by 59%) should be noted: ammoniacal nitrogen by 50%, nitrites by 67%, and nitrates by 60%. This can be explained by the process of oxidation of organic nitrogen (total nitrogen) along the chain \"total nitrogen \(\rightarrow\) ammonium nitrogen \(\rightarrow\) nitrites \(\rightarrow\) nitrates\",  
Figure 5: The results of experimental studies on the purification of wastewater from nitrogen compounds from CLWSI using AOPs.  
and by the insufficient oxidative potential for the destruction of pollutants by anode oxidation alone.  
Such conclusions are confirmed by the results of purification of the solution sampled 2 h after fish feeding, and its 1x turnover through the filter sieve and laminar settling tank. An improvement in the reduction of pollutants only by the indicator \"nitrate concentration\" was recorded: by 20% after anodic oxidation, and by 15% after photolysis. On the other hand, a negative result was obtained for ammoniacal nitrogen and nitrites (the situation worsened on average by 150% for all of the processes)--an increase in the concentration of pollutants. The concentration of ammoniacal nitrogen increased by 150% after anodic oxidation and photolysis; nitrite concentration increased after anodic oxidation by 100%, and after photolysis by 200%. The presented results are consistent with the findings of [34]. It can be reasonably assumed that the deterioration of the cleaning effect was caused by the excessive presence of organic pollutants--primarily organic (total) nitrogen--compared to the complex oxidative potential of the system \"anodic oxidation + photolysis\". In this case, the term \"complex oxidative potential\" implies not only the process of pollutant destruction itself, but also all accompanying and critically necessary physical and chemical processes.  
Accordingly, the experimental studies (Figure 6) fully confirmed the preliminary conclusions about the blocking of oxidant synthesis by organic pollutants, their transportation to the necessary reaction zones, and their effective use.  
Figure 6: Deflocculation of activated sludge flakes when adding a sludge mixture with non-ionic surfactant at different concentrations, per gram, magnification \(\times\)40: (**a**)—control; (**b**)—surfactant = 10 mg/dm\({}^{3}\); (**c**)—surfactant = 30 mg/dm\({}^{3}\); (**d**)—surfactant = 50 mg/dm\({}^{3}\).  
At the same time, based on the experimental evaluation of the action of AOPs on nitrogenous pollutants, the need to combine oxidative processes with other methods of water treatment--for example, based on biological approaches--is fully confirmed.