Leachate landfill treatment using plasma and Ozone

doi:10.21203/rs.3.rs-2841624/v1

Plasma discharge in liquid was used, and a combination system with plasma and ozone was tested looking for an effective reduction of organic material, cell growth, and discoloring process of leachate landfill. The treatments were divided into four groups and operated on three samples for leachate dilution using distilled water. The diluted samples were submitted to Ozone, plasma, plasma followed by Ozone, and Ozone, followed by plasma treatments. Microbiological and chemical oxygen demand (COD) analyses were performed for the initial leachate landfill and the products obtained after the treatments. The microbiological analyses showed that adding Ozone was most effective in reducing the bacterial load while using the plasma positively reduced COD. The best treatment strategy was first the application of plasma, followed by the addition of Ozone, which resulted in a 71% reduction of COD and an 81% decrease in bacteria growth. In comparison to results reported in the literature, the benchtop tests performed in this work demonstrated the effectiveness of the developed strategy.

Ozone

Innatura leachate landfill treatment

Plasma

Plasma Reactor

Plasma and Ozone treatment to reduce leachate landfill contamination
Degradation of organic matter in leachate landfill
Plasma reactor coupled with Ozone to reduce cell growth

The combined impacts of excessive energy use, population growth, and high consumption levels have generated urban wastes that threaten the environment if they are not correctly managed (Nanda and Berruti 2021). The global increases in industrialization and populations have seriously affected water quality, often causing problems with potable water supply (Ma et al. 2021).

In rural areas, domestic waste, animal manure generated from livestock production, and biomass residues from agricultural production are generally treated using anaerobic systems to reduce waste and recycle resources. The anaerobic treatment of solid organic waste generates a leachate landfill rich in organic carbon, nitrogen, and phosphorus. The release of this untreated material constitutes a threat to ecosystems. In addition, the biogas generated by anaerobic digestion is challenging to use directly as a fuel for vehicles, due to its high content of impurities (Zhang et al. 2022).

The degradation of organic matter in landfills produces a dark and turbid fluid that can cause many environmental problems. The formation of this liquid is favored, for example, by high precipitation that increases the humidity of the waste, leading to the formation of a leachate landfill with complex composition and, frequently, hazardous contaminants that are difficult to remove of recalcitrant organic peroxy disulfate (Gu et al. 2018). Currently, the most common treatments applied to landfill leachate combine biochemical processes and membranes for nanofiltration and reverse osmosis (Zhang et al. 2020).

There is increasing interest in the use of plasma discharge technologies for the treatment of various contaminated media. One such technique is dielectric barrier discharge (DBD) plasma, which shows promise for application in wastewater treatment, offering the advantages of homogeneous and high-density plasma discharge (Y. Chen et al. 2022); (Ansari et al. 2020).

Most of the described studied processes in literature only use plasma, without any additional requirement for Ozone or just Ozone. Reddy et al. 2013 reported an oxidation process based on dielectric barrier discharge at the gas-water interface, applied to the oxidative decomposition of methylene blue, representing the contamination of wastewater with dyes. The authors highlighted the advantage of plasma treatment compared to conventional physical pollutant removal and mineralization methods, where pollutants may only be transferred from one form to another. The investigation was made to evaluate the methylene blue degradation related to the applied voltage, gas flow, dye concentration, and addition of Na₂SO₄ and Fe²⁺, with the determination of H₂O₂ formation and pH changes. High degradation was achieved, with a yield of up to 67 g/kWh. The hydrogen peroxide formed during the reaction acted as a powerful oxidant while adding Fe²⁺ improved process performance, possibly due to Fenton-type reactions. It was also observed that degradation of the dye followed first-order reaction kinetics.

Ievolino G and collaborators used dielectric barrier discharge plasma to remove pollutants, including methylene blue, phenol, paracetamol, caffeine, and ceftriaxone from water (Iervolino, Vaiano, and Palma 2019). They used short treatment times with oxygen gas generated in the plasma discharge at a high voltage of 20 kV. On the other hand, Kostov K. and collaborators reported the characteristics of a dielectric barrier discharge (Kostov et al. 2009). They related the reactor geometry to the type of high-voltage discharge source, where the minimum electrode distance between 1.0 and 3.0 mm is essential for the design of DBD reactors. The present study analyzed different processes in the treatment of leachate landfill, with the plasma DBD first used to break chemical bonds by means of oxidative processes, followed by intensified reduction processes to provide clarification and odor reduction of the liquid obtained after the plasma treatment.

2.1 Plasma reactor description

A schematic representation of the construction of the plasma reactor is shown in Figure 1. Pointing reactor inlets and the location of the rod for electrical contact. The reactor components are as follows: (1) reactor body, which may be constructed of glass or metal; (2) reactor cover (glass or metal); (3) inlets for injection of ozone or reagents for the oxidation and reduction processes; (4) inlet for the electrical contact rod; (5) electrical contact rod; (6) screw for the positive connection of the electrical circuit; (7) set of 8 electrodes; (8) screw for the earth connection of the electrical circuit; (9) earthed metallic base.

The better distance between the liquid and the set of electrodes is shown in Figure 2. From this figure, the best setup achieved to maintain the plasma discharge was 10mm.

2.2 Experimental procedures

The experimental procedures included leachate landfill collection, microbiological analysis, and COD analysis.

2.2.1 Leachate landfill collection

The Natura leachate landfill used in the experiments was collected at a solid waste treatment facility in São José dos Campos in October 2021. Table 1 shows the dilutions and treatments employed. The samples were prepared by dilution of the leachate landfill in distilled water, resulting in 3 dilution groups and 5 treatment groups: (a) no treatment (control), (b) plasma, (c) ozone, (d) ozone followed by plasma, and (e) plasma followed by ozone.

Table 1. Dilution groups.
Group	H₂O + leachate (mL)	H₂O + leachate (%)	Treatment
1	150 + 150	50 + 50	(a), (b), (c), (d), (e)
2	210 + 90	70 + 30
3	270 + 30	90 + 10

2.2.2 Treatment

After dilutions, the samples of the leachate landfill were treated using the plasma reactor described in Figure 1, coupled with an ozone generator (model O&L 3.0 RD, Ozone & Life, São José dos Campos, Brazil. The protocol adopted was as follows: The pH was evaluated before and after the treatment. The leachate landfill presented pH 8 before and after water dilution. A 300 mL volume of solution was used for each treatment. Each treatment was performed for 30min. Subsequently, an aliquot of 50 mL was collected for analyses of cell viability and chemical oxygen demand (COD).

2.2.3 Ozonation process

A commercial generator produced ozone (model O&L 3.0 RD, Ozone & Life, Brazil). The ozone dosage calculation was made following the previous description from (van Leeuwen 2015). The ozone system was coupled with a plasma system, and the ozone concentration delivered was 50mg/L. The ozone flux inserted into the leachate landfill was 0.5L/min, and the total treatment was 30 minutes. Therefore, the ozone mass generated per minute was 50mg/L x 0.5L/min = 25mg/min. In addition, the total ozone mass in 30 minutes was 25mg/min x 30 min = 750mg of ozone.

2.2.4 Microbial growth analysis

After the treatments, the samples were submitted to microbiological analysis to determine the reduction of microorganisms, enabling evaluation of the efficiency of the treatment in terms of decreased microbial growth. A 2 mL volume of leachate landfill from each step was transferred to a 24-well plate, followed by adding 3 mL of Luria Bertani (L.B.) microbial growth medium (Sigma-Aldrich). The plates with the samples were left for 24 h in an incubator at 37 °C. Another plate similar to the first was prepared, but with 4 h of growth. The plates were analyzed using a Synergy HT Multi-Detection microplate reader (BioTek, Winooski, VT, USA) set at 590 nm. The data obtained were plotted to determine the growth reduction/death of the microorganisms for each run and was reported as an optical density (O.D.) treatment process.

2.2.5 COD analysis

The effectiveness of the leachate landfill treatments was evaluated by determination of the chemical oxygen demand (COD), which considers the amount of dissolved oxygen (D.O.) consumed in an acidic environment that leads to the degradation of organic matter. This analysis used the dilution with 10% leachate landfill and 90% H₂O.

The procedure used for COD analyses is described hereafter: 50mL of the leachate sample was transferred to a clean glass bottle and labeled with sample identification details. The pH of the leachate sample was measured using a pH meter and recorded the value. The procedure used in this paper was the Micro-Digestion Method: In this method, a known volume of the leachate (50mL) sample is mixed with a specific amount of potassium dichromate and sulfuric acid in a digestion tube. Using a digestion block, the mixture is digested for 2 hours at 148°C. The residual potassium dichromate is titrated with a standardized ferrous ammonium sulfate solution, and the COD is calculated from the amount of ferrous ammonium sulfate used in the titration.

In this paper, the COD was used as an indicative parameter of organic pollution and did not provide information on the specific organic compounds present in the leachate sample. The COD test is also not an alternative to the more comprehensive analysis required to understand the composition of the leachate fully.

The appropriate equation to calculate the COD value of the leachate sample after using the Micro-Digestion Method is:

equation 1

Where:

V1 = volume of ferrous ammonium sulfate solution used for blank titration (mL)

V2 = volume of ferrous ammonium sulfate solution used for sample titration (mL)

N = normality of ferrous ammonium sulfate solution

V = volume of leachate sample used for the digestion (mL)

8000 = milligrams of oxygen per milliequivalent of ferrous ammonium sulfate

The equation 1 was used to conduct a blank titration using the same amount of reagents and equipment but without adding any leachate sample. Then, the titration on the leachate sample and the volume of ferrous ammonium sulfate solution used in each case was recorded. These values were used to calculate the COD of the leachate sample using the equation described above.

3.1 - Decoloring comparison

Figure 3.a shows photographs of the samples from dilution (150 mL H₂O + 150 mL leachate landfill), without and with the different treatments: (a) innatura leachate landfill and H₂O; (b) ozone treatment; (c) plasma treatment; (d) ozone + plasma treatment; (e) plasma + ozone treatment. It can be seen that only the treatments, including the presence of ozone, led to the discoloration of the leachate landfill. The plasma treatment presented ozone produces superoxide radicals(O2-) as demonstrated in previous work from our research group, but it, in this case, for a dilution of 50%v/v; in the present work, the reactive oxygen species from the air environment was not sufficient to promote leachate landfill discoloration (de Oliveira et al. 2019). The most favorable treatment was to apply plasma first, followed by the application of ozone, due to the early opening and functionalization of chains with C=C double bonds. Previous work found that chemical oxidation applied to wastewater from soil remediation did not improve the biological degradability of organic constituents (Gulyas, von Bismarck, and Hemmerling 1995). However, (Y. Chen et al. 2022) reported that ozone could promote discoloration of wastewater containing the Orange II azo dye (AOII), with 97% removal efficiency reached in 2 min. On the other hand, (Xu et al. 2019) described that 254 nm U.V. light could be used to degrade photo-inert dimethyl phthalate in water.

3.1 Microbial growth

Figure 4 shows three plots of cell growth according to treatment, measured on Synergy HT Multi-Detection microplate reader (BioTek, Winooski, VT, USA) set at 590 nm. The optical density (O.D.) values were plotted for each dilution and treatment, as described in Table 1.

Figure 4(a) shows a plot of cell growth for leachate landfill dilution of 50%v/v. It can be seen that the most effective treatment was obtained when ozone was added after the application of plasma, resulting in an optical density of 0.0715±0.0007, compared to a value of 0.3905±0.0955 for the innatura leachate. This result was the same for all dilutions, showing that procedure dilution did not interfere with protocol treatment.

Figure 4. (b) Shows a plot of cell growth for the samples diluted using 30% leachate and 70% H₂O and submitted to the different treatments. In this dilution, only the treatments employing ozone provided discoloration of the leachate landfill, and the cell growth results for the samples with ozone were the same regardless of the sequence of it was plasma first or ozone. For this dilution, the plasma or ozone provides the worst environment for cell growth as a positive result.

Figure 4(c) shows a plot of cell growth for the samples corresponding to the dilution (leachate landfill: H₂O ratio of 10:90 v/v). It can be seen that only the treatments using ozone, alone or in combination with plasma, provided discoloration of the leachate, confirming ozone properties independent of plasma species. However, discoloration effectiveness was better carried out for plasma followed ozone treatment, presenting a growth of 0.0715±0.0007. The effect was attributed to plasma species and reactive oxygen species from the air environment produced in plasma discharge that decreased, cell growth, discoloration, and a part of this result as just a perception evaluation of the odorless was achieved just for this sequence.

Analyzing the treatments and dilutions based on cell growth using optical density (O.D.) values, it is possible to conclude that: The treatment using just ozone was less effective for the 50:50 v/v dilution, presenting an optical density value of 0.0900±0.028, suggesting that dilution favored the treatment. The most effective treatment was achieved by adding plasma followed by applying ozone or vice versa for all dilutions except for 50/50v/v.

3.2 COD analysis

Table 2 shows the COD results used to evaluate organic pollution presented in leachate after dilution of 10%/90% v/v and treatments, leading to the lowest oxygen demand. The better result was achieved using plasma followed by ozone addition, which provided 74% removal of COD, compared to the untreated dilution. The transformation pathways involve interactions of inorganic anions with organic molecules, causing degradation and decreasing organic pollution. The degradation of organic molecules typically depends on the formation of free radicals associated with the type and nature of the inorganic compounds. Inorganic anions may induce not only the oxidation reactions of organic molecules but also to produce reactive oxygen species (ROS), such as HO•, O₂•−, and others, as well as reactive nitrogen species (RNS), such as NO•, NO₂•, and ONOO−(J. Chen et al. 2022)].

Table 2. Results of COD analysis in leachate landfill.
COD analysis
treatments	10% leachate + 90% H₂O	Ozone	Plasma	Ozone + Plasma	Plasma + Ozone
COD (mg/L)	4600	3400	1200	2000	1200

Table 3 summarizes the results for COD, microbiological analysis, pH, color, and as a part, odor, obtained for the 10/90v/v leachate dilution, submitted to the different treatments. It can be seen that the best treatment was applying plasma followed by adding ozone, which resulted in the lowest COD 1200 mg/L, associated with the lowest O.D., transparent color, and absence of odor.

Table 3. Comparison of COD, microbiological analysis, pH, color, and odor results for the 10/90v/v leachate dilution submitted to the different treatments.
	10% leachate + 90% H₂O	Ozone	Plasma	Ozone + Plasma	Plasma + Ozone
COD (mg/L)	4600	3400	1200	2000	1200
Microbiological analysis	0.370	0.072	0.362	0.072	0.072
pH	8	8	8	8	8
Color	dark	transparent	transparent	transparent	transparent
Odor	intense	little	little	little	without

3.3 Calculation of energy consumption

The energy consumption of the plasma reactor was calculated using the formula P = VI, where P = 220 V, and I = 0.15 A, resulting in a value of 33 W. Considering that for each treatment, the reactor was used during 8 hours daily, for 30 days, totaling 240 hours, the monthly energy consumption was 33 W x 240 h = 7.2 kWh, The cost was calculated based on the value per kWh charged by the energy supplier in the city of São José dos Campos (EDP São Paulo Distribuição de Energia S.A.) in January 2022. Table 4 shows the values (in Brazilian Real) corresponding to the energy consumption of the employed reactor. The value per liter of treated leachate landfill was calculated to be R$ 0.71/L of leachate landfill (US$ 0.13/L of leachate landfill) for an estimated volume of leachate landfill of 30 mL leachate landfill per batch and 360 batches per month.

A DBD plasma reactor was successfully developed, adjusting the electrode spacing and the distance between the electrodes and the liquid surface until a continuous discharge was obtained using 10 mm between the electrodes and the leachate landfill. The optimal arrangement was using an electrode in the shape of a cross, with 8 points of electrical discharge providing a greater contact area with the leachate landfill.

The experimental findings showed that the isolated plasma and ozone treatments did not generally provide satisfactory results, while considerable improvements were observed when the two treatments were combined. It was also noted that the order of the treatments influenced the efficacy of the leachate treatment, with the application of first ozone, and then plasma providing inferior results compared to the use of first plasma and then ozone. Under the best conditions, with 30 min exposure to each treatment, it was possible to reduce COD by around 76% and bacterial cell growth by 81%, leaving the leachate landfill transparent and odorless. Analysis of energy consumption showed that the treatment process was economically viable, with an estimated electricity cost of R$7.66 or US$ 1.45 for the use of the reactor for 8 hours daily for 30 days.

Author Contributions: Faria, A. C. ; conceived of the presented idea. Faria, A.C..; Sousa, L..; Vieira, A.A.; and Vieira, L.; developed the theory and performed the computations. Pradella,J.G.C; and. Vieira, L. verified the analytical methods. Pradella, J.G.C.; encouraged Faria, A.C. ; to investigate Microbiological and chemical oxygen demand (COD). All authors discussed the results and contributed to the final manuscript.

Funding: The following Brazilian research agencies supported this work: São Paulo State Research Foundation (FAPESP, grant #11/50773-0), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES, Finance Code #001), and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, grant #317253/2021-2).

Data Availability Statement: The data presented in this study are available on request from the corresponding author.

Conflicts of Interest: The authors declare no conflict of interest.

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Leachate landfill treatment using plasma and Ozone

Status:

Version 1

Abstract

Figures

Highlights

1. Introduction

2. Materials and Methods

3. Results and Discussion

4. Conclusions

Declarations

References

Status:

Version 1