The effect of the combined system of hydrodynamic cavitation, ozone, and hydrogen peroxide on chlorophyll a and organic substances removal in the raw water

Increased levels of nutrients and algae can cause drinking water problems in communities. Harmful algal blooms affect humans, fish, marine mammals, birds, and other animals. In the present study, we investigated the use of a combined system [Hydrodynamic Cavitation, Ozone (O3), and Hydrogen Peroxide (H2O2)] on the removal of Chlorophyll a and Organic substances in the raw water was investigated. The Effect of different operating conditions such as pH, cavitation time, pressure, distance, flow rate, ozone dose, and hydrogen peroxide concentration was studied. Utilizing the Taguchi design method, experiments were planned and optimized. The combined system treatment yielded a maximum reduction in Chlorophyll a and Total Organic Carbon (TOC) at an optimum condition of pH 5, cavitation pressure 5 bar, flow rate of 1 m3/h, a distance of 25 cm from the orifice plate, O3 3 g/h and 2 g/l of H2O2 concentrations. The most efficient factor in the degradation of TOC and Chlorophyll a, was cavitation pressure based on the percentage contributions of each parameter (38.64 percent and 35.05 percent, respectively). H2O2 was found to have the most negligible impact on degradation efficiency (4.24 percent and 4.11 percent, respectively).


Materials and methods
pilot and materials. This water was used for the pilot because it had the characteristics of the incoming water to the Sanandaj water treatment plant, which are variable ( Table 1). The experimental setup for using ozone and hydrogen peroxide in conjunction with hydrodynamic cavitation is shown in Fig. 1. There is a jet flow loop and a 20-L polyethylene tank in it. The cooling water circulation system controls the temperature of the water Table 1. Characteristics of the raw water used in the cavitation reactor.
Designation and optimization of experiment. The Taguchi method is a potent approach to problemsolving that raises productivity, yield, and process performance. Through the systematic use of the statistical design of experiments, also known as robust design, Taguchi's main goal is to reduce variability around the target value of product properties. Taguchi explained that the framework could be seen in three main components: (1) Systems Design (it may include the best fusion of materials and procedures), (2) Design of the parameters (which includes the best set of guidelines for the recognized design components), (3) Tolerance Design: Look at the factors that play a significant role in product quality 41,42 . The required variation in the design is then provided by tolerance limits that are then identified. By evaluating the impact of variables on removal efficiency (response), experimental results can be analyzed using the signal-to-noise ratio (S/N). A dimensionless parameter (metric) known as the signal-to-noise ratio measures the discrepancy between a response and the desired value. Three types of signal-to-noise analysis are commonly used. Lower is better (LB), nominally better (NB), and higher is better (HB), respectively. A larger S/N was chosen since the primary objective of optimization in this study was to achieve the highest removal efficiency. Thus, for the larger one, the S/N ratio is a better criterion in the Eq. (1) is:  www.nature.com/scientificreports/ The n represents the number of repetitions of the experiment and the EF represents the outcomes of the measurements. The removal efficiency of chlorophyll a and TOC is obtained from Eq. (2), where C1 and C2 are the initial and equilibrium concentrations of pollutants (chlorophyll a and TOC, respectively). After designing the experiment using the Taguchi method for the seven parameters, 27 proposed run steps. Tables 2 and 3 provide specifics about each experiment. Every experiment was run twice, put into the model, and then analyzed. Here, the analysis of the statistical mean value approach (ANOM) is utilized to produce ideal conditions 43,44 . First, the average of the S/N ratio of each factor at a given level should be calculated (Eq. 3).
where [(S/N) Factor = I] is the S/N ratio of Factor I in Level i, I is the mean S/N ratio of Factor I at Level i, nIi denotes the number of instances of Factor I in Level I, and the order of appearance in Tables 4 and 5 is the jth. Each controllable factor's.
Impact on the separation of Chlorophyll a and TOC is also examined using the statistical technique of analysis of variance (ANOVA). The percentage contribution of each factor, RF, is given by Eq. (4): Each parameter's degree of freedom (DF) is one number less than the number of levels in the factor, which in this study are two.
The total sum of squares, SST, is given by Eq. (5) Equation (6) is used to determine the value of EF T . Where m (27 experiments) and n (two repetitions) denote the number of experiments and number of experiments, respectively.    www.nature.com/scientificreports/ The sum of factor squares (SSF) is calculated using Eq. (7): EF k F the average of the measurement results of a certain factor in the kth level. In addition, the error variance, VEr, is given by Eq. (8):

Results and discussions
Optimization. For each test condition, the S/N ratio is calculated in Tables 2 and 3. The maximum signalto-noise ratio among the 27 tests is indicated in bold type in these tables. According to Tables 4 and 5, the ideal conditions for removing TOC and chlorophyll a are as follows: pH = 5, retention time = 90 min, cavitation pressure = 5 bar, water flow = 1 m 3 /h, orifice plate distance = 25 cm, ozone value = 3 g/h, and H 2 O 2 concentration = 2 g/l. The confirmation experiment was carried out under the aforementioned ideal circumstances, the EF of chlorophyll a and TOC were measured, and the S/N ratio was computed. Table 6 shows the efficiency difference between the optimal and test 5 conditions were about 1.7 percent for chlorophyll a, and it was about 19.75 www.nature.com/scientificreports/ percent for TOC. Due to the difference in the 30-min retention time and the requirement to adjust the pH, Run 5 is more cost-effective than the ideal state when it comes to the consumption of ozone and hydrogen peroxide.

Effect of studied factors. Influence of factors under investigation
The production of hydroxyl radicals (Eq. 1) is the primary mechanism of pollutant degradation by the hydrodynamic cavitation process [9][10][11] . Hydrodynamic cavitation produces hydroxyl radicals, and the amount and rate of formation are influenced by variables and reactor conditions. The main influences on the removal of TOC and chlorophyll a are depicted in Figs. 2 and 3, respectively. According to these figures, hydrogen peroxide has the most negligible impact on cavitation pressure and a greater impact on cavitation pressure.
pH effects. The pH of the water is an important parameter in determining the extent of the degradation of the organic pollutants by the HC process. Figure 4 shows the percentage reduction in chlorophyll a and TOC of the water sample as the pH changed. It was discovered that as the pH increased from 5 to 9, the percentage reduction also started to decrease (Fig. 5). Thus, based on the findings of this study, cavitation produces the largest degradation when operating in an acidic environment, and less severe degradation when used in an alkaline environment. Algae in water sources usually have a negative charge (zeta potential), and the ideal values also   www.nature.com/scientificreports/ depend on the specific compounds' pKa during processing. The zeta potential (ZP) of the algal cells needs to be destabilized to improve the removal of algal cells during water purification. In acidic media, the generation of % • OH radicals is preferred and also has a higher oxidizing capacity. "Additionally, the recombination probability of the • OH radicals is low, resulting in a higher number of • OH radicals in the solution to degrade the target contaminant. With an increase in pH, the recombination of •OH radicals takes place, reducing their ability to degrade the target contaminant. Numerous studies support this finding, highlighting that as pH increases, the removal efficiency of parameters decreases." Several studies have supported this as the pH increases, the removal efficiency of the parameters decreases [47][48][49][50][51][52] .
Time effect. The cavitation flow, ozone, and hydrogen peroxide injection all increase with increasing cavitation time, which also accelerates the rate at which free radicals are produced and pollutants are broken down. The production of free radicals and the rate at which organic matter and chlorophyll decompose also rise as the cavitation time increases along with the cavitation flow, ozone injection, and hydrogen peroxide injection. The Chlorophyll a and TOC values of the cavitated liquid are likely to continue to decline as the processing time increases, but this is also likely to result in a higher energy need for the procedure. This figure illustrates the relationship between the contact time and the removal efficiency of chlorophyll a and TOC. Other studies have supported these results 50-53 .
Pressure effects. As the liquid passes through strictures like the orifice, the pressure at the vena contract falls below the vapor pressure of the liquid, causing the liquid to flash and produce a series of bubbles that later burst when pressure is restored downstream of the stricture. The photosynthetic system and membrane structure of algal cells can be damaged by high pressure and the hydroxyl radicals that are produced during the HC process. Thus, cavitation effectiveness and the generation of free radicals are influenced by pressure. Results for how inlet pressure affected this study's findings. Figure 6 illustrates how changes in inlet pressure result in an increase in the percentage of chlorophyll a and TOC removal. This is because more cavities are created as the inlet pressure rises, which leads to an increase in the percentage of OH radical formation and organic degradation 20,54-56 . Jadhav et al. demonstrated Imidacloprid removal using a cavitation device combined with oxidants and reported that increasing the inlet pressure from 5 to 15 bar increased the degradation efficiency of Imidacloprid 57 . The research's findings corroborated earlier studies that found cavitation efficiency rises with pressure up to 5 bar 58,60 .
Increasing the cavitation pressure leads to increase in the velocity of the fluid at the orifice hole and then, more cavitation bubbles are generated and the intensity of cavitation also increases, thereby leading to the formation of more % • OH radicals and more degradation of the organic content 49 .
Flow effect. The ability of hydrodynamic cavitation reactors to process contaminants more than once during a single operation is the desired benefit. Under other studies, decomposition is made simple and the number of free radicals produced per pollutant unit rises when the flow rate is reduced 60,61 . Figure 7 also illustrates how this study's findings adjust with those of other studies. www.nature.com/scientificreports/ Distance effect. The removal efficiency of chlorophyll a and TOC increased with decreasing orifice plate distance from the cavitation tube, as shown by the study's findings (Fig. 8). This might be connected to a hole in the low-pressure region having a longer retention time. Additionally, holes and heat are produced on the orifice plate when the water that has been infused with energy and pressure from the pump strikes it. The geometry of the cavitation devices affects the hydrodynamic cavitation reactor's efficiency. So, the geometry of cavitation devices is dependent on how long the cavity remains in the low-pressure region. Thus, it is likely that the cause of the increase in algae removal efficiency at distances near the orifice plate with the primary cavitation tube is the increase in the cavitation time of a cavity in a low-pressure region. Since, over a short distance, the incident energy and pressure change are increased and the distance between the orifice plate and the cavitation tube's starting point is decreased, the cavitation intensity is probably increased. Thus, the effectiveness of hydrodynamic cavitation depends on the orifice plate's position 20,59,[63][64][65][66] .
Ozone effect. Ozone-assisted hydrodynamic cavitation can be used to increase the oxidation intensity of pollutants while reducing ozone consumption. In hydrodynamic cavitation by ozone, it has been confirmed that the oxidation of contaminants occurs immediately after the injection of ozone. The combined operation of ozone and cavitation ensures that, in addition to being directly attacked by ozone, pollutants are also decomposed by hydroxyl radicals. Also, the local turbulence generated by cavitation contributes to the mass transfer of ozone from the gas phase to the bulk, so the rate of reaction of ozone with pollutant molecules is not very significant due to high mass transfer resistances in water. In addition to this, ozone dissociates in the presence of cavitation and generates atomic oxygen ( • O), which further reacts with water molecules to generate highly reactive • OH radicals. The combined effect of adding ozone and HC improved the degradation efficiency in both pH ranges  www.nature.com/scientificreports/ (acidity and alkalinity) compared to HC treatment alone or ozone treatment alone. This is because the combined operation of ozonation and HC renders a synergistic effect 52,[67][68][69][70] . It can be seen in Fig. 9 that the effect of ozone on removing chlorophyll is greater than that of TOC. The reason for this is probably the destruction of algal gas vacuoles by ozone. Also shown in this graph, the pollutant removal efficiency increases with the increase in ozone concentration. The reason is the increase in free radical production per pollutant, which is consistent with previous studies 70,71 .
Hydrogen peroxide effect. With increasing H 2 O 2 dosages (0-2 g/l), the results are shown in Fig. 10 for the degradation of chlorophyll a and TOC. The creation and then attack of hydroxyl radicals is a critical mechanism that aids in the degradation of pollutants by HC-H 2 O 2 (Eqs. [16][17][18] Algae + · OH → CO 2 + H 2 O + Intermediate products of degradation (20).   (7) the factorial sum of squares,SSF, for each factor was calculated for each factor and determined in Table 9. Using Eq. (5), the total sum of squares, SS T , was calculated. By changing SSF and SS T in Eq. (8), the error variance, V ER , was obtained. Finally, by substituting SS F , SS T , V ER , and DOF F in Eq. (4) to determine the percentage contribution of each factor, RF, the results are shown in Table 9.

Conclusion
• Hydrodynamic cavitation systems have been recognized as a new form of multiphase reactors capable of producing favorable oxidation, including localized hotspots, turbulence, and reactive free radicals within the system. In this study, chlorophyll a and TOC were removed from water using a combination of hydrodynamic cavitation, ozone, and hydrogen peroxide. The ideal conditions for removing chlorophyll a and TOC are as follows: cavitation pressure of 5 bar, retention time of 90 min, pH: 5, flow of 1 m 3 /h, distance from the orifice of 25 cm, ozone of 3 g/h, and hydrogen peroxide of 2 g/l. According to the percentage contributions of each factor, cavitation pressure was identified as the factor that was most effective in the degradation of TOC and chlorophyll a (38.64 percent and 35.5 percent, respectively). H 2 O 2 was found to have the least impact on degradation efficiency (4.24 percent and 4.11 percent, respectively). The issues and future research directions that merit careful consideration are listed below in light of the study's findings.
(18) Algae + · OH → CO 2 + H 2 O + Intermediate products of degradation.   www.nature.com/scientificreports/ • The orifice plate's distance from the cavitation tube's beginning was considered in this study, but it was not in earlier ones. In this study, distances of 25, 50, and 75 cm were assessed; however, future studies may assess extra distances. • The addition of ozone increased the effectiveness of removing pollutants while decreasing the amount of time needed to achieve this result. The main drawback of using ozone alone in water treatment plants is mass transfer, but hydrodynamic cavitation can increase the mass transfer of ozone from a gaseous phase to water. • In this study, the synergistic effect of combining hydrodynamic cavitation with hydrogen peroxide and ozone oxidants was minimal. Changes in the organic load and algal input to the sewage treatment plant may be to blame for this (Table 1). This is so that the research could be done using actual raw water that entered the Sanandaj treatment facility. • Throughout this study, several parameters, including a load of organic matter, rainfall, the temperature of the inlet water, and the load of algae, changed, which had an impact on how effectively the algae were removed. • Due to its high efficiency in destroying microalgae, hydrodynamic cavitation has a lot of potential for treating nutrient-rich waters. Because it doesn't produce secondary pollution, hydrodynamic cavitation is also a sustainable abatement technique.

Data availability
Contains data required for analysis in manuscript. The corresponding author is willing to clarify the data and will provide all necessary datasets according to the request.