Effect of Ozone and Ozone/Persulfate on Biodegradability
The effect of ozone and ozone/persulfate in the advanced oxidation process (O3/S2O8) on the biodegradability of stabilized leachate was investigated and determined for future research. COD, BOD5, and BOD5/COD were tested before and after ozonation of stabilized leachate. Figure 2 presents the effect of O3 and O3/S2O8 on COD, BOD5, and BOD5/COD. To investigate the performance of the O3/S2O8 system and compare it with other treatment techniques, biodegradability was also examined after persulfate oxidation only and persulfate oxidation followed by ozonation. Accordingly, the system O3/S2O82– in AOPs was found to be more efficient in enhancing the biodegradability of stabilized leachate than other treatment techniques. As shown in Figure 2, COD decreased from 2480 to 695 mg/L with a total removal efficiency of 72%, and BOD5 was increased from 107 to 202 mg/L. Consequently, the biodegradability described in terms of BOD5/COD increased from 0.043 to 0.29, whereas the ratio only improved to 0.05, 0.12, and 0.16 after ozonation only, persulfate oxidation only, and persulfate oxidation + ozonation, respectively. Although ozonation reduces and converts large refractory organic molecules found in mature leachates into smaller more biodegradable molecules , the combination of ozone and persulfate is more efficient in increasing BOD and enhancing biodegradability. Persulfate, as a new oxidant, can severely destroy organic molecules, and the degradation products are easily biodegraded. Xu et al.  improved the biodegradability index of leachate from 0.13 to 0.95 using potassium persulfate combined with activated carbon adsorption. Cortez et al.  obtained an improvement in biodegradability from 0.01 to 0.17 using O3/H2O2 in the advanced oxidation process.
In the O3/S2O82– system, S2O82– releases sulfate radicals (Eqs. (2) and (3)), which powerfully oxidizes organic molecules [19, 21].
The generation of sulfate radicals during S2O82– oxidation can be significantly enhanced by some catalysts, such as heat and UV radiation (Eq. (4)), which can initiate sulfate radical generation [28, 29].
The performance of O3/S2O82– was higher at high pH (8–11) because O3 can initiate the formation of hydroxyl radical, whose oxidation potential (E0 = 2.80) was higher than that of O3 (E0 = 2.07) in a direct reaction under an acidic condition [30, 31]. Furthermore, S2O82– was more active at high pH. Deng and Ezyske  reported the same result, and low removal rates of COD and NH3–N are obtained when S2O82– oxidation is applied at pH 4. The removal efficiency improves with increased pH to 8.3.
Biodegradability can also be measured by removing COD through aeration, which can be defined as CODi – CODf during lab-scale aeration (Eq. (2)). Control over the process depends on the regulation of aeration to satisfy oxygen requirements, which are closely linked to organic matter biodegradability and biodegradation kinetics. Therefore, better knowledge of biodegradation kinetics enables the prediction of composting time.
The effect of ozone and ozone/S2O82– in the advanced oxidation process on the biodegradable COD fraction in stabilized leachate was investigated. Figure 3 presents the degradable COD in the batch aeration method before and after applying the ozone/persulfate process of stabilized leachate. The initial kinetic removal of COD in the first 5 days of aeration gradually increased for both samples. Furthermore, during this period, the degradation of COD after O3/persulfate was slightly higher than that in raw leachate. The increased removal of COD stabilized in raw leachate on the fifth day of aeration, whereas removal of COD continued to increase after ozonation and stabilized on the eighth day of aeration.
Figure 3. Biodegradation of COD in batch method for stabilized leachate before and after treatment by ozone and ozone/persulfate (AOP). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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The biodegradable and nonbiodegradable COD fractions were also calculated after the aeration process (Eq. (1)). The biodegradable COD improved from 24 to 39% after the ozone/persulfate process, whereas the nonbiodegradable fraction decreased from 76 to 61% (Figure 4). These results revealed that the O3/S2O8 system improved the removal of COD through extended aeration. Therefore, the biological processes were observed to be generally affected by new leachate, containing mainly volatile fatty acids, but were less effective for stabilized leachate . Bilgili et al.  obtained 40 and 30% removal of COD using aerobic and anaerobic reactors of the leachate treatment system, respectively. These results revealed that the O3/S2O8 system improved the removal of COD using extended aeration.
Effect of O3 and O3/Persulfate on Soluble and PCOD
The fractionation of COD is the most important parameter for evaluating the quality of leachate. Soluble COD is one of the main fractions, and PCOD is the other important fraction of COD. Determining soluble COD is important in identifying other COD fractions. In this study, soluble COD was obtained through ZnSO4 coagulation and filtration . The fractions of soluble and PCOD are given by the following equations:
where COD is the total COD (mg/l), PCOD is the PCOD (mg/l), and COD(s) is the total soluble COD (mg/L; passed through a 0.45 μm filter after coagulation).
Figure 5 presents the effect of O3 and O3/persulfate on soluble and PCOD of semiaerobic stabilized leachate. Soluble COD was improved from 59% in raw leachate to 72% after O3/S2O8 process under optimal operational conditions, whereas PCOD was reduced from 41 to 35% and 28% after O3 and O3/S2O82–, respectively. These results indicated that the new treatment process was efficient for improving the removal efficiency of PCOD and converting nonsoluble COD to soluble COD. In addition to the ability of ozone to reduce soluble inert COD in wastewater and increase residual soluble COD , persulfate can react with aromatic and aliphatic components, and then abstract hydrogen by breaking the C[sbond]H bond , resulting in the reduction of high-strength nonsoluble organics. The results revealed that the combination of ozonation and persulfate efficiently reduced nonsoluble COD and increased soluble COD in stabilized leachate. Previous studies showed a 60% increase in soluble COD using the Fenton process in the pretreatment of stabilized activated sludge . The removal efficiency of total COD in stabilized leachate through ozone alone was 15%, which then improved to 72% through the O3/persulfate-based advanced oxidation process. Previous studies reported a COD removal of 47 and 65% [16, 17] using the Fenton process, 96%  using the photo-assisted Fenton process (UV/Fenton), and 27 and 50%  using ozone and ozone/H2O2 in advanced oxidation, respectively.
Effect of Ozone Applications on Biodegradable Soluble COD(Sbi) and Nonbiodegradable Soluble COD(Subi)
COD(s) contains biodegradable soluble COD(Sbi) and nonbiodegradable soluble COD(Subi) in stabilized leachate. The fractions of COD(Sbi) and COD(Subi) were determined using the following formulas:
where COD(Sbi) is the biodegradable soluble COD (mg/L), COD(Subi) is the nonbiodegradable soluble COD (mg/L), COD(bi) is the biodegradable COD (mg/L) in a batch aeration system, and COD(s) (mg/L) is the total soluble COD (mg/L).
The obtained fractions of COD(Sbi) and COD(Subi) of the stabilized leachate were 38 and 62%, respectively. The COD(Sbi) fraction in stabilized leachate was generally very low (Figure 6). Bilgili et al.  showed that fresh aerobic landfill leachate contains approximately 40% of COD(Sbi) and approximately 60% of COD(Subi). The effects of ozone applications on COD(Sbi) and COD(Subi) in semiaerobic stabilized leachate are presented in Figure 6. The biodegradability of leachate during ozonation is enhanced because of the fragmented organic compounds with long chains to decrease chains degraded to carbon dioxide . The most biodegradable organic material was produced after oxidation using ozone alone . The results obtained in the current study showed improved COD(Sbi) fraction in stabilized leachate from 38 to 43% after 60 min of O3 alone and to 55% after ozonation using O3/S2O8. Meanwhile, the COD(Subi) fraction decreased to 57 and 45% using O3 and O3/S2O8, respectively. Previous studies showed that the Fenton process enables 19.5% removal of nonbiodegradable COD . In the current work, the results revealed that ozonation converts nonbiodegradable organics to biodegradable components, suggesting the enhanced availability of applying the biological treatment of stabilized leachate after ozonation.
Treatment Efficiency and Statistical Analysis
A total of 30 runs were executed using the CCD experimental design. Interactions between the four independent variables were considered in each run to investigate the validity of treating stabilized leachate using ozone and persulfate during advanced oxidation. The removal efficiencies of total COD ranged from 29 to 75.8% for COD, and the optimum removal of total COD was 72% under optimal operational conditions (O3 = 30 g/m3, pH 10, persulfate dosage = 1 g/1 g COD/S2O82–, and reaction time = 210 min). Table 3 presents the ANOVA regression parameters for the predicted response surface quadratic models and other statistical parameters for COD removal. The effect of initial pH variation was examined to determine the optimal pH for the O3/S2O8 system. The removal of COD increased with increased pH. These phenomena can be attributed to the ability of O3 to initiate the formation of hydroxyl radical at high pH, which has an oxidation potential (E0 = 2.80) higher than O3 (E0 = 2.07) in a direct reaction under an acidic condition . Although S2O82– is more active at high pH, Deng and Ezyske  reported higher removal for COD and NH3–N at lower pH (4), higher temperature (40 °C), and higher persulfate dosage. In this study, the optimal removal efficiency was obtained at optimal ozone temperature (15 °C), high pH (10), and minimum persulfate dosage. Furthermore, the reaction time was varied between 30 and 210 min to determine optimal experimental conditions. The results demonstrated that the degradation of organics in the leachate improved with increased reaction time.
Table 3. ANOVA for analysis of variance and adequacy of the quadratic model for total COD removal
| ||Source||Sum of squares||Degree of freedom||Mean square||F-Value||Prob > F|| |
|COD Removal (%)||Model||4478.49||9||497.61||48.44||< 0.0001||Mean = 51.31|
|A||61.60||1||61.60||6.00||0.0237||R2 = 0.9376|
|B||56.89||1||56.89||5.54||0.0289||SD = 5.719|
|C||3695.13||1||3695.13||359.73||< 0.0001||PRESS = 2027|
|D||33.89||1||33.89||3.30||0.0843||Adj R2 = 0.9272|
|C2||316.48||1||316.48||30.81||< 0.0001||Adeq. precision = 32.87|
| ||Residual||205.44||20||10.27|| || || |
| ||Lack of Fit||203.62||15||13.57||37.43||0.0004|| |
| ||Pure Error||1.81||5||0.36|| || || |
Table 3 demonstrates that all models were significant at a 5% confidence level and the P values were <0.05. The correlation coefficients obtained in this study for COD removal, that is, R2 = 0.9561, were greater than 0.80, the cutoff for a model with good fit. A high R2 value indicates good agreement between the calculated and observed results, as well as desirable and reasonable agreement with the adjusted R2 [39, 40]. The “adequate precision” (AP) ratio of the models varied between 16.29 and 32.87, which was adequate. AP values above 4 were desirable and confirmed that the predicted models can be used to navigate the space defined by the CCD. Figure 7 demonstrates the normal probability plots of the Studentized residuals for COD removal. The normal probability plots predicted that if the residuals followed a normal distribution, as shown in Figure 7, then the points would fall along a straight line for each case. However, some scattering is expected even with normal data; thus, the data can be considered to be normally distributed.
Compared with ozone/Fenton in AOPs, our previous work  obtained 65% for COD removal. Furthermore, the effect of ozone/Fenton oxidation on the biodegradable characteristics of stabilized leachate has been recently investigated . The biodegradability (BOD5/COD ratio) only improved from 0.034 to 0.1 following O3/H2O2/Fe2+. The fractions of COD(bi), COD(S), and COD(bsi) increased to 36, 72, and 51% after O3/Fenton application in AOPs, respectively. COD(ubi), PCOD, and COD(ubsi) decreased to 68, 28, and 49%, respectively, following O3/Fenton application in AOPs. These results revealed that the new oxidation process (ozone/persulfate) was more efficient in improving the biodegradable characteristics of stabilized leachate than the ozone/Fenton process.
Statistical analysis of COD fractions before and after O3 and O3/persulfate was performed using the Statistical Package for Social Sciences, and the descriptive data are summarized in Table 4. The standard errors for all COD fractions (3.75–4.48) in the three leachate types (raw, after O3, and after O3/persulfate) were statistically applicable. The variances ranged between 42.333 and 63, which were relatively high, and revealed the significant effects of ozonation processes on the COD fractions of raw leachate.
Table 4. Summary of descriptive statistical analysis of COD fractions before and after ozonation processes
| || || || || ||95% confidence interval for mean|| || || || |
| ||N||Mean||Std. deviation||Std. error||Lower bound||Upper bound||Range||Variance||Min.||Max.|
|Biodegradable COD (%)||3||30.3333||7.76745||4.48454||11.0379||49.6288||15.00||60.333||24.00||39.00|
|Nonbiodegradable COD (%)||3||69.6667||7.76745||4.48454||50.3712||88.9621||13.00||42.333||61.00||76.00|
|Soluble COD (%)||3||65.3333||6.50641||3.75648||49.1705||81.4961||15.00||60.333||59.00||72.00|
|Particulate COD (%)||3||34.6667||6.50641||3.75648||18.5039||50.8295||13.00||42.333||28.00||41.00|
|Biodegradable soluble COD (%)||3||46.0000||7.93725||4.58258||26.2828||65.7172||15.00||63.000||40.00||55.00|
|Nonbiodegradable soluble COD (%)||3||54.0000||7.93725||4.58258||34.2828||73.7172||15.00||63.000||45.00||60.00|