Effect of the association of coagulation/flocculation, hydrodynamic cavitation, ozonation and activated carbon in landfill leachate treatment system

Mature landfill wastewater is a complex effluent due to its low biodegradability and high organic matter content. Currently, mature leachate is treated on-site or transported to wastewater treatment plants (WWTPs). Many WWTPs do not have the capacity to receive mature leachate due to its high organic load leading to an increase in the cost of transportation to treatment plants more adapted to this type of wastewater and the possibility of environmental impacts. Many techniques are used in the treatment of mature leachates, such as coagulation/flocculation, biological reactors, membranes, and advanced oxidative processes. However, the isolated application of these techniques does not achieve efficiency to meet environmental standards. In this regard, this work developed a compact system that combines coagulation and flocculation (1st Stage), hydrodynamic cavitation and ozonation (2nd Stage), and activated carbon polishing (3rd Stage) for the treatment of mature landfill leachate. The synergetic combination of physicochemical and advanced oxidative processes showed a chemical oxygen demand (COD) removal efficiency of over 90% in less than three hours of treatment using the bioflocculant PGα21Ca. Also, the almost absolute removal of apparent color and turbidity was achieved. The remaining CODs of the treated mature leachate were lower when compared to typical domestic sewage of large capitals (COD ~ 600 mg L−1), which allows the interconnection of the sanitary landfill to the urban sewage collection network after treatment in this proposed system. The results obtained with the compact system can help in the design of landfill leachate treatment plants, as well as in the treatment of urban and industrial effluents which contains different compounds of emerging concern and persistence in the environment.


Material and methods
The leachate used in this experiment was collected at the sanitary landfill Delta A, at the geographical coordinates 22° 54′ 47.53″ S and 47° 8′ 35.55″ W, in the municipality of Sorocaba, São Paulo, Brazil, where this landfill no longer receives waste solids, so the leachate generated can be considered stabilized. The experiment was carried out in the laboratories of the State University of Campinas (UNICAMP), São Paulo, Brazil.
To meet the objectives of this work, the characterization of landfill leachate was carried out between the years 2018 and 2021 (Table 1), analyzing the pH, color, turbidity, electrical conductivity (EC), and chemical oxygen demand (COD), carried out according to what was proposed by APHA 29 . The stages proposed for this leachate treatment system were: 1st-Coagulation/Flocculation; 2nd-Hydrodynamic Cavitation plus Ozonation; and 3rd-Activated Carbon, where the efficiencies of the steps were tested according to the color, turbidity and COD analyses, carried out in accordance with APHA 29 .
To meet the 1st Stage of this experiment, four types of coagulants were tested, PGα21Ca, FeCl 3 , AlCl 3 e Al 2 (SO 4 ) 3 , these being considered as treatments of this experiment, the applied dose was 1 g L −1 , varying the pH between 4 and 8, maintaining fast and slow mixing conditions in a Jar Test system. The best condition results were used for application in the next step.
In the 2nd Stage, a system was assembled containing a 20 L reactor coupled to hydrodynamic cavitation, containing a recirculation pump coupled to a Venturi plate with a concentric orifice (10 mm) for extrusion, the system also had an ozonizer, whose application geometry was adapted and patented according to the process "BR 10 2021 025779 2", this methodology used was based on and adapted from Wu et al. to Bis et al. 7,30 . Ozone (O 3 ) generation used the off-gas measurement system in potassium iodide (KI) solution analyzed according to the Iodometric Method 2350 E 31 . Figure 1 shows the illustration of the 2nd Stage system.
The 2nd Stage was operated in a closed circuit (366 mg O 3 L −1 ), therefore, before testing the complete system (all stages), tests were carried out to define the operating time without application of the coagulant, being tested 1, 2, 3, 4, 5 and 6 h, where after each test the effluent was passed through a system that simulated a conventional decanter. The best results were applied to the complete system.   www.nature.com/scientificreports/ In the 3rd Stage, 1 g L −1 of powdered activated carbon was applied in a rapid mixing system to maintain the contact time, after which it was passed through a system that simulated a conventional decanter. This step was the last of this experiment, where the results were used to define the total efficiency of the system.
Statistical analyses consisted of comparative tests of central tendencies, using the analysis of variance (ANOVA) followed by the Tukey test, in which the level of significance was set at 5%, comparing the total efficiencies of the system (all steps).

Results and discussion
Mature leachate physicochemical characterization. The leachate produced in mature landfills is characterized by the considerable biodegradability difference from young and medium-age landfill leachates due to the stabilization process of the urban solid waste. The mature leachate generally has a stable physicochemical composition, for instance high concentration of refractory organics compounds (e.g., humic and fulvic substances). Due to these characteristics, the first stage of this work was to characterize the landfill to better understand it and define the appropriate treatment methods to be applied. The sampling campaign comprised the wet and dry season throughout 3 years of study. Table 1 presents the physicochemical characteristics of the raw leachate from the landfill studied.
The pH is the parameter that influences the coagulation/flocculation conditions and varied between 7.9 and 8.5 in untreated mature landfill leachate, similar conditions were found in other studies 4,7,[32][33][34][35][36][37] . This low variation (CV-2.2%) and the tendency towards basicity of this effluent is common due to the natural stabilization through which the medium undergoes over time, because of the decrease in the concentration of partially ionized free volatile fatty acids, which are consumed by methane-producing bacteria 31 . Thus, as with the pH values, the results obtained in the other parameters were very close to studies with similar stabilization conditions, such as those carried out in Florida (USA) and also in Croatia 35,38 .
The color parameter (Table 1) expressed as apparent color (mg Pt-Co L −1 ) presented high concentrations values when compared with different mature landfills in the literature. As mentioned, with age the landfill changes its composition and becomes dominated by refractory compounds. The dark brownish color ( Fig. 2) is due to the presence of high concentration of DOM compounds which are correlated to the relative abundance of aromatic substances and chromogenic functional groups. The presence of high organic concentrations can become a serious environmental problem manly because color hinders the growth of aquatic life by decreasing the sunlight penetration, subsequently disturbing photosynthetic activity. The values of apparent color and turbidity are important to the coagulation and flocculation process due to the colloidal concentration fraction in the mature leachate [39][40][41][42] .
The turbidity parameter (Table 1) showed the highest coefficient of variation and low values when compared with other types of wastewaters (e.g., domestic sewage). Low colloidal or suspend particle concentrations can affect the coagulant dosage and charge neutralization in the landfill treatment process. Also, the low values and variation of turbidity can be associated to the high concentrations of apparent color which can affects the readings of the nephelometric method.
Studies show that the COD value of landfill leachate is usually below 10,000 mg L −1 , where the lower this value, the older is the leachate 4 , therefore, the leachate studied in this work is in an advanced stage of stabilization. After 10 years, a landfill will be composed with less biodegradable organic matter, influencing the COD of the leachate for values below 4000 mg L −1 . When it reaches this stage, the organic matter is composed of complex macromolecular organic compounds that present significant resistance to biodegradation and is characterized as a refractory organic wastewater. Therefore, the leachate will present high pH values, ammoniacal nitrogen concentration, low CH 4 production and BOD 5 /COD ratio less than 0.1 37,[43][44][45] . In the present study, a BOD 5 /COD ratio of less than 0.06 was observed from the database provided by the company responsible for monitoring the landfill under study.
Accordingly, to Chen et al. 2020 and Liu 2022 the DOM presented in the mature leachate has unbalanced carbon-to-nitrogen ratio, and high molecular weight (highly aromatic benzene ring compounds) and high degree of unsaturation compounds which can cause great difficulty to be treated. The data observed by Chen    www.nature.com/scientificreports/ Krevelen diagrams. In addition, Liu et al. 2022 investigated and compared the molecular weight distribution of DOM in mature and young leachate. The authors conclude that the molecular weight distribution of DOM in mature leachate was wider and contained more DOM with molecular weights greater than 400 m/z. Whereas, for young leachate the distribution range of DOM was smaller and the density center of the molecular weight distribution of DOM was significantly lower than that of the mature leachate 39,40,46 . Kulikowska & Klimiuk (2008) report that there is significant variation in the quality of the leachate produced in different sanitary landfills around the world, even among those considered mature for chemical composition, in particular, for organic compounds and nitrogen 47 . Therefore, the characteristics of the leachate used in this work (Table 1) show that the traditional treatment methods reported in the literature are not efficient to treat this type of effluent, justifying the need for a new treatment process based on the combination of advanced techniques to obtain a satisfactory result.
Operational parameters of coagulation and hydrodynamic cavitation plus ozonation. Based on the characterization of the mature leachate, physicochemical techniques and advanced oxidative processes were chosen to evaluate the removal efficiency of the high concentrations of COD and apparent color. Initially, coagulation/flocculation processes were tested using traditional chemical coagulants (AlCl 3 , Al 2 SO 4 , FeCl 3 ) and compared with a bioflocculant (Poly-Glu-PGα21Ca) proposed for the treatment of leachate. Similarly, hydrodynamic cavitation associated with ozonation were tested and compared with the coagulation/flocculation in terms of removal efficiency of COD and apparent color.
There are several studies regarding pH in optimizing coagulation in the production of drinking water and in the treatment of effluents, combining the ideal pH and system conditions with the efficiency of pollutant removal 48 . More recently, the application of coagulation/flocculation processes has proven to be a successful method for the removal of contaminants that can be adsorbed by colloids such as toxic organic matter, viruses and metals. Coagulation/flocculation is necessary to reduce the colloidal fraction, the total suspended solids and color in order to improve the efficiency of subsequent treatment process. The results found in this study show that for FeCl 3 , AlCl 3 e Al 2 (SO 4 ) 3 the ideal pH value is close to 5.0 and for PGα21Ca the ideal pH would be the range of 4.0. Trivalent salts of aluminum and iron, produce many species by hydrolysis producing positively charged metallic hydroxyoxide complexes (e.g., Al 6 (OH) 15 3+ and Fe 3 (OH) 4 5+ ). These hydrolysis species act on charge neutralization of negatively charged colloids which permits the van der Waals force of attraction to induce colloidal particle aggregation to form flocs.
When FeCl 3 was used, the COD reduction reached 56.8% and the color 39.5% efficiency, with AlCl 3 the COD removal efficiency was 62.4% and the color 74.6%, with the coagulant PGα21Ca the removal was 52.5% for COD and the color 76.3%, and when Al 2 (SO 4 ) 3 was used the COD removal was 52.0% and the color 64.4% under the indicated pH conditions. Different factors have impact on the coagulation/floculation efficency, such as the chemical compositon of coagulant, the coagulation pH, and dosage. To obttain high effiency of treatment is mandatory to evaluate the combination of these factors. In a study carried out in Malaysia using ferric sulfate in a coagulation/flocculation test of leachate from a landfill, at a dosage of 5500 mg L −1 at pH 6, it was possible to achieve a removal of 47% of color in the study 38 . This variation indicates that with the test of new coagulants in different dosages it is possible to improve the percentages of color removal and consequently the COD at this stage 38 .
To date, very few studies were reported on the application of PGα21Ca in the coagulation/flocculation of landfill leachate, in a study in the treatment of wastewater from the production of potato starch, it was found that the pH equal to 4.0 presented the best results, like this study 26 . In a study carried out with vinasse, even working with temperatures above natural, they show that the best coagulation conditions with PGα21Ca are with a pH below 4.0 27 . In terms of COD removal, the PGα21Ca has shown higher efficiency than FeCl 3 , AlCl 3 e Al 2 (SO 4 ) 3 salts. This can be explained because PGα21Ca has in addition to γ-PGA in its composition, calcium and aluminum sulfate/carbonates acting as auxiliary coagulants.
The Table 2 presents the results of the hydrodynamic cavitation tests together with ozonation, without the application of coagulants to define the best operating time for the system. The table shows that the increase in removal efficiency in percentage occurred more significantly from the 1st to 2nd hour of the test, with an increase of 15.2% for COD, 27.9% for turbidity and 11.5% for color. The reduction of COD can be explained because hydrodynamic cavitation is based in the formation, growth, and collapse of vapor cavities induced by a restrictive flow device such as a pump, jet nozzle, propeller, or orifice plate [49][50][51] . As cavities collapse, bubble formation can www.nature.com/scientificreports/ reach elevated temperatures and pressures. This process, called hot spots, it is capable to produce turbulence, highly reactive free radicals, and high-speed jets within the fluid. When associated to ozonation the process contributes for rapid absorption of ozone molecules in the system. The combined process of cavitation with ozone produces ·OH radicals therefore can significantly improve the degradation efficiency of the process 18,22,52 . The increments from the 2nd to 3rd hour of system operation in efficiency were smaller compared to the increment from the 1st to 2nd hour, being 5.4% for COD, 10.8% for turbidity and 3.1%, being similar in the other times tested (  22 . After the second hour of treatment the efficiencies removal of the parameters reached a state of greater system stability where trends in organic matter removal were not observed. With these results it was defined that the operating time of the 2nd Stage, hydrodynamic cavitation plus ozonation was two hours in the complete tests (all stages).
Combined treatment of mature leachate. After analyzing the processes individually, a study was carried out to evaluate the synergistic effect of physicochemical and advanced oxidative processes. From the results shown in Table 3, it can be observed a high total removal efficiency in the COD (greater than 80%) for all types of applied coagulants, it also shows the efficiency part of the 2nd stage between the 1st and 2nd hour of application of hydrodynamic cavitation plus ozonation and polishing with activated carbon.
The stablished condition for the coagulation/flocculation process contributed to the removal of organic substances and chromaticity which can be observed in the reduction of COD and apparent color. The differences in efficiency removal of color and COD between the coagulants used can be explained by the interaction of DOM with different types of flocculants. As stated by Wang et al. 2022 aluminum salt flocculants preferentially remove unsaturated larger compounds and iron salt can remove smaller saturated compounds. This could explain the lower efficiency of FeCl 3 in the studied experimental conditions.
The combination of coagulation/flocculation with hydrodynamic cavitation/ozonation was effective to reduce refractory organic matter concentration and color in mature landfill leachate. The color reduction can be associated with the destruction of chromophores and auxochrome groups, and the organic-containing benzene ring  39 . Also, since ozone is very effective in the oxidation of aromatic compounds susceptible to electrophilic attack, the cavitation process can enhance the interaction of DOM with ·OH radicals formed in the systems, thus significantly improving the degradation efficiency. Among the coagulants studied, PGα21Ca was the one that presented the highest total efficiency of COD removal from the crude leachate, which initially had an average concentration of 2425 mg L −1 and at the end of 183 mg L −1 of COD, that is, treatment efficiency of 92.5%, being significantly different from other coagulants. The second coagulant that showed the best performance in reducing COD was AlCl 3 with 85.7%, followed by Al 2 (SO 4 ) 3 with 83.3% and finally FeCl 3 showing a reduction of 81.7%, however this did not show significant difference between each other (Table 3).
There are records in the literature of the use of bioflocculants, such as Ocimum basilicum, extracted from the basil seed, together with ozone for the treatment of landfill leachate at doses of 0.2 g O 3 L −1 h −1 during 30 min. The technique carried out in the province of Guilan (Iran), integrated coagulation/flocculation with ozonation and obtained an efficiency of 87% and 92% of COD and color, respectively 53 .
Through the data in Table 3, it can be observed that the best results obtained for reducing color and turbidity were achieved from the use of PGα21Ca, a compound recently used in water treatment plants for supply, with a significant difference when compared to with FeCl 3 . The PGα21Ca is a product developed by Nippon Poly-Glu Co and obtained from Bacillus licheniformis and Bacillus subtilis through fermentative processes. It is a natural bioflocculant mixture containing γ-PGA with cross-link product of γ-PGA with the addition of calcium sulfate, calcium carbonate, sodium bicarbonate and aluminum sulfate. The γ-PGA is anionic polymer which is ideal for colloid removals and DOM compounds. With the addition of alkalinity species and coagulants, such as aluminum, the PGα21Ca is a versatile flocculant and can be applied in wide range of pH. The mechanism of action of PGα21Ca can be explained firstly by the effect of the agglomerating of particles and suspended solids carried out by the presence of aluminum salts and then by the increase in interaction and colloidal nucleation stimulated by γ-PGA. This process allows the formation of denser aggregates and heavy flakes 28,54,55 .
When FeCl 3 was used, the results were not satisfactory, and in the end the effluent had a cloudy, yellowish appearance, which influenced its turbidity to remain present even after the adsorption of activated carbon. The turbidity efficiency result of the system that used FeCl 3 was negative, going from 11.2 to 15.1 NTU, an increase of 34.5% (Table 3).
As shown, the color of the effluent after using FeCl 3 was not satisfactory, but when COD removal was observed, the scenario was different. According to Oloibiri et al. (2015) the time required for the treatment of leachate by coagulation-flocculation, with the coagulant in question, associated with the adsorption of activated carbon, is 258 min to achieve a reduction of 53% of COD 33 . As for the combination of ozonation and adsorption by activated carbon, it took 240 min to reach a removal of 77%. In the case of the present study, it took 200 min to reach a COD removal of 81.7%, that is, both the time and the efficiency of organic matter removal were superior to the study described above.
As mentioned above, the use of the AlCl 3 coagulant provided a COD removal of 85.7%, a high result (greater than 80%) in a leachate treatment with high levels of contaminants. A study carried out with treatment by coagulation and flocculation associated with ozonation, with stabilized leachate and already coming from secondary treatments (anaerobic and facultative pond), the final removal of COD was close to 72%, a value lower than that achieved by the present study 56 . In comparison, another study in the treatment of landfill leachate, used coagulation-flocculation, with polymeric ferric sulfate (250 mg L −1 ), combined with ozonation, at the end of the process an efficiency of 88.3% of COD was obtained 57 . Rivas et al. (2003) carried out a study similar to the present work, the leachate used had already been submitted to the ozonation process, then it went through the adsorption process with activated carbon (30 g L −1 ) during the period of 120 min, under these conditions a COD removal efficiency of 90% was reached. It should be emphasized that in the present study it was possible to reduce COD values in an efficiency range between 81.7 to 87.2% using only 1 g L −1 of activated charcoal and 200 min of treatment 58 . Figure 2 shows an image of the raw leachate and its evolution as the treatment progresses, in which Al 2 (SO 4 ) 3 was used as a coagulant and showed high removal efficiency in terms of color. The initial dark brown coloration can be attributed to the presence of humic substances, which are gradually lost due to the dosages and time of exposure to ozone, changing to a light-yellow color until it becomes almost colorless after the application of activated carbon. This same evolution was observed and reported in the study carried out by Ntampou et al. 56 .
The work by Gottshalk et al. (2020), Rivas et al. (2003) and Silva et al. (2004) described that the removal of color may be related to the direct attack of ozone on the double bonds of carbon atoms located in the chromophore groups of the compounds comprising the leachate, in addition to the attack itself on the aliphatic bonds, ketones and aldehydes [58][59][60] . In summary, all coagulants showed little differentiation regarding the removal efficiency of the parameters monitored in the treatability tests, with the exception of ferric chloride, which showed lower removal levels. Currently, there has been a concern about the management of coagulants used in wastewater treatment in general (sludge volume), however, studies are more comprehensive for sludge generated in supply water treatment, which is known to have few toxic pollutants. In any case, the ways of managing the coagulation sludge can be the use as a substrate in constructing wetlands, building materials (ceramic, brick, and cement chemicals), abacation of Geopolymers (GPs) and removal of pollutants 61 . Table 4 shows the efficiencies of mature landfill leachate treatments using processes that involve the synergy of physicochemical treatments with advanced oxidative processes.
It can be seen that the combination of coagulation/flocculation with advanced oxaditive processes is efficient for COD removal of mature landfill leachate with different BOD 5 /COD levels. Removal efficiencies greater than 86% can be observed, however treatment time, energy costs and infrastructure are factors that need to be considered when applying these processes on full scale.

Conclusion
The combination and association of techniques of coagulation/flocculation, hydrodynamic cavitation, ozonation and activated carbon were efficient for the treatment of mature landfill leachate with the use of different coagulants, with emphasis on PGα21Ca as greater efficiencies.
The operational parameters defined in this work show that in coagulation/flocculation for FeCl 3 , AlCl 3 and Al 2 (SO 4 ) 3 the ideal pH value is close to 5.0 and for PGα21Ca the ideal pH would be the range of 4.0. In hydrodynamic cavitation/ozonation, the ideal operating time was close to 2 h, with the first hour showing the greatest increase in efficiency.
The coagulants evaluated and used in the coagulation/flocculation stage of the landfill leachate treatment of the proposed system, showed that the efficiency of DOC removal were all greater than 80%, with emphasis on the coagulant PGα21Ca, with removal of 92.5% of COD, followed by AlCl 3 with 85.7%, Al 2 (SO 4 ) 3 with 83.3% and FeCl 3 with the lowest performance, with removal of 81.7% of COD, with a significant difference between PGα21Ca and the other coagulants.
The color removal in the proposed system was higher with the application of the PGα21Ca coagulant, when compared with the FeCl 3 , being 97.7% and 90%, respectively. About turbidity, the system with the application of FeCl 3 coagulant was not satisfactory, being significantly lower than the other treatments.
The results obtained show that the proposed system can significantly reduce the concentration of organic matter in the treated leachate, making it possible to release and dilute it in the urban sewage collection network, considering that in large cities the COD values of 600 mg L −1 are normally found.
It is noteworthy that many mature sanitary landfills need to send their leachate for treatment in effluent treatment plants, where several requirements are required to receive this type of waste. In this scenario, the implementation of the proposed system can contribute to solve this problem.

Data availability
All data generated or analyzed during this study are included in this published article in the form of figures, tables, and graphs.