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Advanced Oxidation Photochemical Processes (AOPs) for Landfill Leachate Treatments Using Ozonation Process and Hydrogen Peroxide Systems

Ozonation process and hydrogen peroxide systems can be used as advanced oxidation photochemical processes (AOPs) to treat leachate from the municipal landfill (El Waffa Wa El Amal) in the greater Cairo, Egypt. This study aims to determine the effectiveness of these systems for the treatment of leachate, and therefore a 1.5 L semi-batch reactor was used in this work containing a sample of 1 L at pH values 7.5 to 8.5 with an average rate of 4 L/min of oxygen supply taken from ambient air, ozone dose 20 mg/dm3 with various concentrations of H2O2 (0, 5, 10, 20, and 40 mg/L). It was found that both O3 and O3/H2O2 were very effective in reducing the concentration of organic compounds in optimal experimental conditions; biodegradability was achieved by ozone alone or combined with H2O2, but the O3/H2O2 was the most effective process and best results were obtained at 40 mg/L of H2O2 after increasing treatment time to 60 minutes, and the efficiency of COD removal was 84% to 92%, The efficiency of TOC removal was 29.21 mg/L to 58.42 mg/L; the biodegradation indicated by BOD5/COD ratio increased from 0.17 to 0.74 after the processes, and the efficiency of turbidity removal was 75.60% and 82.80%, respectively.

key words: ozonation; hydrogen peroxide systems; Advanced Oxidation Photochemical Processes (AOPs); landfills; leachate treatment

Introduction

High population growth especially in developing countries increases the amount of solid waste which is commonly disposed to landfills. This waste undergoes several simultaneous physical, chemical, and biological changes that produce a harmful liquid called leachate which consists of a highly concentrated mixture of organic and inorganic contaminants, high chemical oxygen demand (COD) (Seredynska-Sobenka et al., 2005; Deng, 2009; Boonyaroj et al., 2012), low biodegradation biodegradability, ammonia-nitrogen, inorganic salts, heavy metal (Eggen et al., 2010), leachate quantity. Leachate characteristics depend on the age of the landfill and the type of substances disposed in it (Behr et al., 2011).
The quantity of leachate is small as compared to other wastewaters, but the contents are extremely hazardous with significant adverse effects on human health and the aquatic environment matrices and may contaminate ground waters (Stuart 2012). Leachate is considered as a significant source of new and emerging pollutants. The occurrence of micro-pollutants in landfill leachate is affected by the compound specific physical chemical properties such as its molecular size, shape, and configuration, its acid base properties, and hydrophilicity (Petrisor and Lazar, 2006).
AOPs were first proposed in USA in the 1980s for drinking water treatment by Glaze et al. (1987) and later were widely studied for treatment of different wastewaters. (Yang et al. 2016). It has been known that the common principle is the generation of highly reactive oxidizing species called free radicals such as hydroxyl radical (OH•), superoxide ion (O2•–), ozonide anion (O3•–), hydrogen peroxide (H2O2•), HO2• and HO3•– radicals, and their oxidation potentials (vs. SHE) are given in the following order: OH– (E0 = 2.80 V), O3 (E0 = 2.07 V), H2O2 (E0 = 1.77 V), HO2 (E0 = 1.70 V), O2 (E0 = 1.23 V). (Amaral-Silva N., et al. 2016). Once the hydroxyl radicals are formed, the organic pollutants are oxidized producing biodegradable intermediates, and are then mineralized to water, carbon dioxide, and inorganic salts (Tizaoui Ch., et al. 2007).
AOPs can be classified according to the nature of the phases (homogeneous or heterogeneous) or depending on the mode of production of hydroxyl radicals: chemical, photochemical, sonochemical, and electrochemical. Moreover, they can also be coupled with other techniques, such as physicochemical or biological processes.
Other investigators employed physicochemical methods, such as adsorption, to promote the biodegradability of organic compounds by using several adsorbents: activated carbon (Azzam et al. 2010), zeolite (Aly et al. 2018), sawdust (Jeguirim et al. 2012), bentonite (Al-Essa 2018), natural clays (Azzam et al. 2015) and different adsorbent matrices such as Azolla, granular active carbon, resin, and zeolite (Padovani et al. 2013, Ververi andand Goula 2019). It was proved that AOPs are very successful in treating wastewaters including tannery wastewaters (Vilardi et al. 2018), textile wastewaters (Pourgholi et al. 2018), municipal wastewaters (Serna-Galvis et al. 2019), oil refinery wastewaters (Bustillo-Lecompte et al. 2018), pulp and paper wastewaters (Klidi et al. 2018), dairy wastewaters (Markou et al. 2017), pharmaceutical wastewaters (Yang et al. 2018), surfactant wastewater (Arslan et al. 2018) and mine wastewaters (Meng et al. 2019). AOPs are also used to remove heavy metals (Li et al. 2019), landfill leachate (Joshi and Gogate 2019), phenol (Niu et al. 2018) and many other components (Huang et al. 2018, Sable et al. 2018, Tawabini and Makkawi 2018, Rubio-Clemente et al. 2019).
Advantages of AOPs. (i) Significant reduction of inorganic and organic compounds (Miklos et al. 2018); (ii) complete mineralization of organic contaminants to carbon dioxide, water, and mineral salts (Leal et al. 2018); (iii) generating nontoxic by-products without release of secondary pollutants (Bustillo-Lecompte et al. 2018); (iv) high reactivity and nonselectivity of hydroxyl radicals towards pollutants (Yang et al. 2019); (v) used as treatment or pretreatment of wastewater (Ramteke and Gogate 2015).
AOPs processes are involved in the production of free hydroxyl radical (•OH) which is produced from single oxidants such as ozone (O3) (Amr et al. 2016), or from a combination of strong oxidants such as O3 and hydrogen peroxide (H2O2), under optimum pH (Chen et al. 2019). The initiation of ozone decomposition can be accelerated by increasing the pH or by the addition of hydrogen peroxide (Kudlek E. et al. 2017).
Ozonation and the addition of hydrogen peroxide (O3/H2O2) processes are attractive means for the treatment of landfill leachates due to the high oxidative power that possesses. These systems favor the production of hydroxyl radicals (•OH), which are highly reactive species. OH− and H2O2 initiate a series of radical reactions that enhance ozone decomposition to yield •OH.In the system O3/OH−, the hydroxide ion reacts with ozone to yield superoxide anion radicals (O2•−), which in their turn are involved in a series of reactions that yield •OH. Overall 1 mol of O3 yields 1 mol of •OH. Hydroxyl radicals are very reactive, nonselective oxidants, and are the most important species in an advanced oxidation process (J.J. Wu et al. 2004). In the system O3/H2O2, when H2O2 is dissolved in water, it partially dissociates into hydro-peroxide ion (HO2−, the conjugate base of hydrogen peroxide), which reacts rapidly with ozone to initiate a radical chain mechanism that leads to hydroxyl radicals (R. Tosik 2005). Thus ozone-based advanced oxidation processes are attractive processes to oxidize the complex leachate mixtures (F. Wang et al. 2003).
In this study, advanced oxidation processes (AOPs) such as ozone and ozone/hydrogen peroxide systems (O3/H2O2) were investigated in laboratory scale experiments as an effective alternative for municipal landfill leachate treatment. The study showed that O3/H2O2 systems were the most effective treatment process.

El Waffa Wa El Amal Landfill
In Egypt, solid waste quantities are increasing due to demographic and rapid urban growth that the country is experiencing. El Waffa Wa El Amal landfill is the largest landfill site in Cairo serving the Greater Cairo area, and is used mainly for disposal of domestic solid wastes generated from an estimated Cairo Governorate population of 9,840,591 according to the estimates of the Central Agency for Public Mobilization and Statistics 2019, site area nearly 350 fed, and contains accumulations of solid waste of which 79% are organic materials over a period of more than twenty years, with about 13 million tons and a height of up to 25 meters. Regulations would not allow direct discharge of the leachates into the sewer system. Therefore different ozone-doses-based AOPs were used. The efficiency of the selected methods depends on many factors including the presence of inorganic and organic substances in the leachate, pH, reaction time, and oxidants concentration. Studies were carried out to determine the optimum conditions of each process and its effects on biodegradability. These data might also lead to a better understanding of the chemical oxidation by different ozone-doses-based AOPs, and different hydrogen peroxide concentrations and contact time during treatment processes in order to ensure compliance and safe discharge.



Figure 1a. Location of landfill leachate, El Waffa Wa El Amal landfill, Cairo

Figure 1b. Pollution due to landfill leachate, El Waffa Wa El Amal landfill, Cairo.

Table 1. Characteristics of raw and settled (4 h) leachate.
Parameter Raw Settled
pH 7.6–7.85 8.15–8.48
Colour (PtCo) 2,173–2,572 1,998–2,194
Turbidity (NTU) 312–319 218–276
BOD5 (mg/L) 175–198 170–194
COD (mg/L) 1,197–2,370 1,210–2,440
Total solids (mg/L) 6,397–6,616 6,337–6,583
Total suspended solids (mg/L) 172–196 123–172
Total dissolved solids (mg/L) 6,225–6,420 6,214–6,411
Total phosphorus (mg/L) 147–168 143–164
Ammonia-nitrogen (mg/L) 724–972 652–893
Material and Methods
Leachate samples were collected from the site in drums and transported to the laboratory, which is situated at about 1 hour drive from the site. A 20 L quantity of the leachate was refrigerated immediately and used in this study. The leachate used in this study is classified as of intermediate age tending to stabilization (raw leachate in order to minimize biological and chemical reactions). The samples were stored in a cold room in the laboratory at 4°C. Before the experiment, the samples consisted of preliminary coagulation which is, in the separation of the sediment and other substances, precipitating after pH decreases, and producing a necessary step in this process for 4 h. This work was carried out in a semi-batch reactor having a cylindrical shape with 1.5 L volume with continuous ozone gas supply to the contactor. The equipment used in this study consisted of an aerator, a flow meter, an ozone generator, and an ozone contactor. The flow rate of oxygen supply was at 4 L/min taken from ambient air as connected to an ozone generator. The ozone generator which changes the O2 into O3 has a specification of OZF-1G, 200 W, and 220 V. The ozone generated was applied to an ozone contactor (1500 ml) which had previously been filled with 1000 mL of leachate sample. To make a homogeneous ozone condition throughout the leachate sample, the contactor was equipped with magnetic stirrers. The process is carried out for 60 minutes, with samples taken ± 30 mL every 10 minutes. Parameters such as pH, the total organic carbon (TOC), turbidity, BOD5, and COD were measured every 10 minutes. Standard methods were used to characterize the leachate. COD measurements were performed using a Behr COD work station according to DIN 38409. Relative error for COD measurement was estimated to about 10%. BOD5 was measured using the respirometric method by measuring oxygen pressure decrease using Oxitop® bottles (10% relative error). TOC measurements were performed using Euroglass TOC 1200 instrument according to NF EN 1484 (10% relative error). A pH meter, Orion 420A, was used for pH measurements. In order to stop ozone reaction at the end of an experiment, a stream of nitrogen was used to purge any residual ozone.

Results and Discussion



Figure 2. pH change as function of reaction time.

Figure 2 shows evolution of pH change as function of reaction time from 0 to 60 minutes with ozone alone and ozone combined with various concentrations of hydrogen peroxide H2O2 (5, 10, 20, and 40 mg/L), small changes in pH following ozonation of a leachate. It was found that pH presented only a slight change of less than 0.05–0.2 (initially, lower pH values by about 0.54–0.64 pH units in all experiments and then constant values at about 8.69–8.79 on average). pH units were obtained with the O3/H2O2 system as compared to O3 alone. This small lowering in pH may be attributed to hydrogen peroxide, which is a weak acid (H2O2⇔HO2 − +H+). pH is used as an indicator for hydrogen ion activity, A high pH value indicates more hydroxide ions (OH-) acting as an initiator in the decomposition of ozone, In the ozonation system, pH usually gives a positive effect on the removal rate of COD. A high pH value is important because the hydroxide ions will start the ozone decomposition involving the following reactions.
Though, it was expected that ozonation would decrease the pH to acidic values due to the formation of carboxylic acids. pH lowering was not observed in this work because it may due to oxidation of the organic matter. They may result from certain construction and demolition wastes, which can always end up in the landfill.
O3 + OH-→ O2● + HO2● (1)
HO2● ↔ O2● - + H+ (2)
O3 + O2● - → O3● - + O2 (3)
O3● - + H+↔ HO3● (4)
HO3● → ●OH + O2 (5)

3O3 + OH- + H+→ 2OH° + 4O2 (6)
●OH + H2O2 → HO2● (7)
HO- + O3 → OH2- + O2 (8)
In O3/H2O2 system, the addition of H2O2 can accelerate the decomposition of ozone and subsequently enhance the production of •OH radicals, as in reactions (9), (10) ,(11):
HO• + HO• → H2O2 (9)
H2O2+ HO• → H2O + HO2• (10)
HO2• + HO• → H2O + O2 (11)
O2-+ O3→ O2 + O3- (12)
O3-+ H+ → HO3 (13)
HO3→ OH + O2 (14)
OH + O2 → HO2 + O2 (15)
CO2 + H2O ⇌ H2CO3 ⇌H+ + HCO3− ⇌ 2H+ + CO32− (16)

These reactions will increase pH by time as can be seen in Figure 2. The pH of all variations increases with increasing contact time as indicated by the increase of hydroxide ions. It resulted in a higher pH than the control condition, and the other variations during the ozonation process will result in higher pH values. Based on reactions (1) and (2), the initiation of ozone decomposition can be accelerated by increasing the pH or by adding hydrogen peroxide.



Figure 3. COD removal efficiency by time.

Figure 3 shows the COD removal efficiency for different systems during different reaction times (0 to 60 minutes). It was measured to find out how much organic compound can be eliminated, especially the nonbiodegradables. Figure 3 also shows that each variation increases COD removal by time; COD removal rate is highest at the beginning of the experiment due to availability of easily oxidizable compounds. Further increase in reaction time led to a slow change in removal rate and a tendency to level off at a final COD removal value. This indicates that an increase in hydrogen peroxide level will not always increase COD removal rates. In effect, increasing hydrogen peroxide concentration will change its role from being initiator for the production of hydroxyl radicals to inhibitor of ozone decomposition through free radical reactions. The scavenging effect of hydrogen peroxide presented a barrier for treatment improvement. Ozone combined with hydrogen peroxide removed COD by 60.23%–92% which was higher than ozone alone that has a removal efficiency for COD of 46.12%–84%. The maximum eemoval efficiency of COD (92%) was obtained by ozone combined with hydrogen peroxide at 40 mg/L but by ozone alone it was 84%, at an ozone dosage of 2 g O3/L. This phenomenon can also be attributed to the high pH value of this variation. In addition, this indicates that ozone decomposes to OH●. The decrease in COD concentration is caused by a nonbiodegradable organic substance which is oxidized either directly with ozone which is selective or indirectly by OH● which has a nonselective property and converts it into an inorganic substance such as CO2.

Figure 4. COD variation with time.

Figure 4 shows that COD decreased gradually during the initial O3 and O3/H2O2 processes due to the availability of OH radicals. Further, an increase in COD concentration was observed at 60 minutes, indicating the formation of organic acids.
There was an improvement in COD removal from 1,825.67 mg/L to 1,533.56 mg/L; COD value was 292.11 mg/L, using O3 treatment only during 60 minutes; maximun COD removal was 1,825.67 mg/L to 1,679.62 mg/L; COD value was 146.05 mg/L using O3/ H2O2 40 mg/L. The presence of H2O2 (5, 10, 20, and 40 mg/L) resulted in an increased COD removal. Removal of COD indicates that the addition of H2O2 increased the formation of OH radicals, which in turn mineralized the organic compounds.
As the O3 /L H2O2 treatment continued, COD decreased further due to additional oxidation of the intermediates by OH radicals to the final COD. The COD removal for an optimum dosage of O3 and 40 mg/L H2O2 shows that an increased H2O2 concentration does not always result in increased COD removal rates. The increase in H2O2 concentration leads to a change in its role from OH radical producer to inhibitor of O3 decomposition by free radicals.
O3+HOO−→O3⋅−+HO2⋅O3+HOO−→O3·−+HO2·
Increasing the concentration of H2O2 can cause further decomposition of ozone while yielding the ozonide radical anion O3, which could be detected as dissolved ozone. This may result in a pseudo increase in the dissolved ozone concentration.


Figure 5. The effect of O3 and O3/H2O2 systems on biodegradability of BOD5/COD.

Figure 5 shows results for the effect of O3 alone and O3/H2O2 systems on COD and biodegradability of the leachate that presented low biodegradability (BOD5/COD = 0.1), which was not enhanced when both systems were used except for O3/H2O2 system at H2O2 = 40 mg/L that gave a biodegradability value of 0.73, which was based on ozone that gave a biodegradability value of 0.57, used ozone in conjunction with H2O2 systems. The process enhanced biodegradability from 0.1 to 0.73 and reduced COD by 92%, and it was found that ozonation alone was capable of enhancing biodegradability from 0.1 to 0.57 at an ozone dose of 2 g/L and reducing COD by about 84%. The scavenging effect of hydrogen peroxide presented a barrier for treatment improvement. It is therefore always inevitable to search for an optimal hydrogen peroxide concentration above which a decline or no effect of H2O2 on treatment would be observed. Hydrogen peroxide concentration of 40 mg/L remained optimal for the parameters studied so far (i.e., COD removals, and biodegradability enhancement). Our work showed good enhancement of biodegradability by the O3/H2O2 systems, which is a very important result. Indeed a process of O3/H2O2 followed by a biological treatment could be an economical means for the treatment of this leachate.



Figure 6. Changes in TOC with ozone and ozone combined with various concentrations of hydrogen peroxide.
Figure 6 shows changes in TOC with ozone alone and ozone combined with hydrogen peroxide with various concentrations of H2O2 (5, 10, 20, and 40 mg/L) during different reaction times (0 to 60 minuntes). The degree of mineralization is typically determined by measuring the relative change in TOC. Typically, much longer degradation times are required after the removal of the parent compound to achieve complete mineralization. The TOC removal for each AOP was analyzed within the 60-minute treatment period. After 10 minutes of the process, the TOC removal rate was increased and its extension to 60 minutes caused an increase in the efficiency of oxidation of con¬taminants, as shown in Figure 4. Both ozone and O3/H2O2 reduced the TOC. degradation in 60 min and had a TOC removal of 70%, complete TOC removal after 60 minutes of the treatment process reached the level of 80%,. It was found that during the process that com¬bines ozonation with hydrogen peroxide (system O3+H2O2) that the effectiveness of oxidation of the contaminants denoted as TOC removal rate, the TOC removal rate follow¬ing an hour of ozonation process supported by H2O2 was The most effective combined process (O3+H2O2) With this dose of the hy¬drogen peroxide 40 mg/L


Figure 7. Turbidity removal efficiency expressed in nephelometric turbidity units (NTU).

Figure 7 Shows turbidity removal efficiency with ozone alone and ozone combined with hydrogen peroxide treatments with various concentrations of H2O2 (5, 10, 20, and 40 mg/L) during different reaction times (0, 10, 20, 30, 40 50 and 60 minutes). The curves present two kinetic periods. In the first period (from time zero up to about 11 minutes), the rate of destruction of organic pollutants and the extent of mineralization can be considerably increased which is marked by a rapid decrease in turbidity removal values; fast kinetics are followed by a second period marked by slow changes in rurbidity removal values, which corresponds to slow rates. This change from fast to slow is probably due to changes in ozone reaction mechanisms following changes in the nature of compounds with which ozone reacted (i.e., oxidation by-products). Ozone reacts very rapidly with the initial compounds but eventually slowly with the acids and the aldehydes. When ozone alone was used, this occurred almost simultaneously with the change in trend of the off-gas ozone concentration. A change in the trend of the off-gas ozone concentration occurred later when hydrogen peroxide was added. In contrast, a change in the trend of the off-gas ozone concentration occurred later (about 21 minutes) after the change in turbidity removal kinetics when hydrogen peroxide was added. The maximum removal efficiency of turbidity obtained by ozone alone was 75.60%, but by ozone combined with hydrogen peroxide at 40 mg/L it was 82.80%, which shows that H2O2 increased the turbidity removal rate by almost O3/H2O2 as compared to ozone alone due to higher hydroxyl radical production in the O3/H2O2 system due to higher hydroxyl radical production in the O3/H2O2 system without significant effect on pH changes.
Conclusions
Treatment of landfill leachates using ozonation (O3), O3/H2O2 isthe most effective process. The combination between O3 and H2O2, called peroxonation process, was reported to be more efficient than ozonation alone because hydrogen peroxide is intended to accelerate the decomposition of ozone in liquid and thus producing more hydroxyl radicals, and best results were obtained from O3/H2O2 at ozone dose 20 mg/L and 40 mg/L of H2O2 which is reactive after increasing treatment time to 60 minutes. Better results require elevated pH (pH>8) and low alkalinity from carbonate. The efficiency of the removal of organic matter and organic compounds from leachate was characterized through physicochemical parameters. The efficiency of COD removals with O3/H2O2 can be up to 92%, which is greater than O3 alone that reached 84%. The efficiency of TOC removal was 29.21–58.42 mg/L. The biodegradation indicated by BOD5/COD ratio increased from 0.17 to 0.74 after processes, and the efficiency of turbidity removal was 75.60%–82.80%, respectively. O3/H2O2 advantages are simple free radical production, high bactericide activity, less time consuming, and high removal efficiencies. O3/H2O2 disadvantages are low solubility of O3 in aqueous solutions, limited mass transfer, ineffectiveness at high toxic pollutants concentrations, and high energy consumption. High operating and capital costs of these techniques are not economically acceptable for the treatment of large scale effluents, depending on regulations and leachate characteristics, thus the combination of AOPs with a biological process would significantly decrease the overall cost of leachate treatment.
Researches considering the toxicity reduction were involved. However, the toxicity assessment of landfill leachate is very important, which determines the effect of the subsequent biological treatment or the influence on the environment. So, the toxicity reduction of AOPs should be evaluated in future researches.

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