# Prospects of industrial membrane concentrates: treatment of landfill leachates by coupling reverse osmosis and wet air oxidation

Emilie Gout, Mathias Monnot, Olivier Boutin, Pierre Vanloot, Philippe Moulin

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## To cite this version:

Emilie Gout, Mathias Monnot, Olivier Boutin, Pierre Vanloot, Philippe Moulin. Prospects of industrial membrane concentrates: treatment of landfill leachates by coupling reverse osmosis and wet air oxidation. Environmental Science and Pollution Research, 2025, 32, pp.16570-16578. 10.1007/s11356-024-32461-4. hal-04593773

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# Prospects of industrial membrane concentrates: treatment of landfill leachates by coupling reverse osmosis and wet air oxidation

Emilie Gout<sup>a</sup>, Mathias Monnot<sup>a</sup>, Olivier Boutin<sup>a</sup>, Pierre Vanloot<sup>b</sup>, Philippe Moulin<sup>a*</sup>

<sup>a</sup> Aix Marseille Univ, CNRS, Centrale Marseille, M2P2, Marseille, France

<sup>b</sup> Aix Marseille Univ, Univ Avignon, CNRS, IRD, IMBE, Marseille, France

*Corresponding author: philippe.moulin@univ-amu.fr – Tel.: +33 6 67 14 14 18 – Aix-Marseille Univ., Laboratoire de Mécanique, Modélisation et Procédés Propres, Équipe Procédés Membranaire (EPM-M2P2-CNRS-UMR 7340), Europôle de l’Arbois, BP 80, Bat. Laennec, Hall C, 13545 Aix-en-Provence cedex 04, France

## Abstract

Stabilized landfill leachates are characterized by a high chemical oxygen demand and limited biodegradability. This study investigates the removal of the organic fraction (chemical oxygen demand and total organic carbon) using a treatment approach that couples membrane processes and wet air oxidation (operating at 200–300 °C and 18 MPa). The aim is to address the challenges posed by landfill leachate membrane concentrates considering variations throughout the year across three different seasons. The efficiency of the treatment path was assessed through the removal of the chemical oxygen demand and total organic carbon, with additional insights provided by fluorescence spectroscopy to evaluate the degradation of the organic compounds’ complexity. The most favorable results were achieved at the highest temperature (300 °C) with removals over 90% for total organic carbon and 87% for chemical oxygen demand, along with a complete elimination of fluorescence. The coupling of membrane processes and wet air oxidation has demonstrated its effectiveness in degrading the organic compounds of landfill leachates. The overall mass balance on the treatment path indicates the feasibility of discharging the treated effluent into the environment, meeting regulatory thresholds for total organic carbon and chemical oxygen concentrations.

## Keywords

Membrane process; Wet air oxidation process; Industrial effluent; Hybrid process; Excitation-emission; fluorescence matrix
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# 1 Introduction

Landfill leachates result from rainwater percolation and the degradation of waste in landfills. They present a complex composition, containing both organic and inorganic compounds, some of which can be refractory to conventional biological treatments. Over time, these leachates become less biodegradable, posing significant challenges for effective treatment. One commonly employed treatment process over the past three decades has been reverse osmosis (RO), known for its efficiency in producing large volumes of high-quality permeate suitable for environmentally compliant discharge. However, the resulting RO concentrates are present in small volumes and exhibit high levels of pollution (Renou et al. 2008). Consequently, additional treatment steps are required to enable their safe reuse or discharge into the environment. Given the increasing concerns related to water scarcity and the imperative to safeguard water resources, returning clean water to the environment is essential. The chosen process must be able to degrade organic compounds to meet stringent discharge standards. Moreover, to address the problematics posed by the membrane concentrate, the process must be adapted to higher pollutant concentrations while also striving for energy efficiency, as an alternative to current incineration practices.

Oxidation processes offer effective solutions for degrading various pollutants, including advanced oxidation processes, chemical oxidation and wet air oxidation (WAO). While advanced oxidation processes, which employ chemicals like ozone and hydrogen peroxide, can achieve reliable pollutant removals, their use is often restricted due to the production of byproducts and the usage of chemicals reagents. Among these options, WAO emerges as one of the most suitable processes (Andreozzi et al. 1999; Bhargava et al. 2007). It does not rely on chemical reagents and does not generate harmful byproducts or sludge. WAO involves an oxidation reaction that takes place in an aqueous phase, where dissolved oxygen reacts with dissolved organic matter. The reaction occurs under high temperature and pressure conditions, typically within the range 125–300 °C and 0.5–20 MPa. Also, it has been used at the industrial scale, treating sewage sludge and industrial waste liquors. During the process, the organic soluble fraction is converted into biodegradable compounds, such as carbon dioxide, water and short-chain organic compounds. Inorganic compounds like sulfur or phosphorus species and nitrogen-containing compounds are converted to sulfates, phosphates and ammonia, respectively.

In the context of both synthetic and industrial effluents, the coupling of membrane processes and wet air oxidation has been scarcely reviewed. However, noteworthy examples can be found in the Table 1, providing insights into the diverse applications and methodologies explored in these publications. The chosen research topic of this paper centers on the treatment of real industrial effluents, specifically landfill leachates, which are already concentrated at the industrial scale with a high flowrate, to address the associated challenges (variability, complexity). The literature review revealed that the few studies available on this subject demonstrated highly promising performances. However, mechanisms, operational conditions, and performance details are not always described. The scarcity of comprehensive documentation regarding the underlying mechanisms and specific operational conditions motivates this study to provide a more detailed understanding of the coupling of membrane processes and wet air oxidation for the treatment of landfill leachates.

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<table><caption>**Table 1** Coupling of membrane processes and WAO in literature</caption><thead><tr><th>Authors</th><th>Application</th><th>Membrane process</th><th>WAO operating conditions</th><th>Removals</th></tr></thead><tbody><tr><td>(Foussard et al. 1989)</td><td>Textile industry colored effluent</td><td>UF</td><td>240–265 °C, 2–2.3 MPa, 20 min</td><td>80% reduction in pollution</td></tr><tr><td>(Lei et al. 1998)</td><td>Textile industry membrane concentrates</td><td>Not specified</td><td>110 °C, hydrogen peroxide</td><td>80% COD reduction, 90% color reduction</td></tr><tr><td>(Verenich et al. 2000)</td><td>Paper mill effluents concentrated by UF</td><td>UF</td><td>180 °C (max), 120 min, catalysts</td><td>1–30% COD reduction</td></tr><tr><td>(Dhale and Mahajani 2000)</td><td>Textile dyeing baths</td><td>NF</td><td>190–225 °C, 0.69–1.38 MPa, 120 min</td><td>99% color elimination</td></tr><tr><td>(Zhang et al. 2020)</td><td>Synthetic effluent with fatty acids</td><td>NF + Hydrothermal Oxidation</td><td>380 °C, 25 MPa, 50% excess O<sub>2</sub>, recirculation</td><td>Almost 100% COD degraded</td></tr><tr><td>(Pinchai et al. 2019)</td><td>Synthetic effluents (greywater, bilge water, pharmaceuticals)</td><td>UF</td><td>300 °C, 15 MPa, 60 min, agitation</td><td>83–96% COD reduction</td></tr></tbody></table>

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This study aims to evaluate the efficiency of a treatment path coupling RO and WAO for the treatment of a real industrial effluent: landfill leachates. The assessment of degradation was based on conventional parameters such as chemical oxygen demand (COD) and total organic carbon (TOC). Additionally, fluorescence spectroscopy was studied to provide insights into the complexity of the pollution both before and after treatment.

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# 2 Material and methods

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## 2.1 Effluent and wet air oxidation

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Landfill leachates (Cannet-des-Maures, France) were collected three times during in 2020 to account for seasonal variations (January, May, October, Table 2). On-site treatment at an industrial scale was performed using RO. Detailed information regarding the inlet and outlet characteristics of the RO process can be found in previous publications (Gout et al. 2022, 2023).

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<table><caption>**Table 2** Characteristics of landfill leachates RO concentrates (Gout et al. 2022, 2023)</caption><thead><tr><th>Parameters</th><th>RO recovery rate*</th><th>RO concentration factor*</th><th>pH</th><th>Conductivity (mS·cm<sup>-1</sup>)</th><th>TOC (mgC·L<sup>-1</sup>)</th><th>COD (mgO<sub>2</sub>·L<sup>-1</sup>)</th></tr></thead><tbody><tr><td>January</td><td>95%</td><td>1.16</td><td>7.8</td><td>14.7</td><td>1,081</td><td>1,874</td></tr><tr><td>May</td><td>97%</td><td>1.26</td><td>7.7</td><td>22.5</td><td>2,213</td><td>3,324</td></tr><tr><td>October</td><td>99%</td><td>1.66</td><td>7.6</td><td>40.2</td><td>2,094</td><td>8,250</td></tr></tbody></table>

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WAO experiments were performed on the three seasonalities of RO leachate concentrates. These experiments were carried out under at high pressure and high temperature conditions in a 1 L laboratory-scale continuously stirred reactor. Detailed information and specifications on the equipment can be found in previous publications (Gout et al. 2022, 2023). Landfill leachate concentrates (300 mL) were introduced into the reactor, then heated to

<footnote>(* mean value for COD and TOC)</footnote>
the working temperature (200, 250 or 300 °C) under calculated nitrogen pressure (Lefèvre et al. 2011). After the heating period, a sample was withdrawn to measure the thermal pollution removal. The duration of the heating period varied from 40 and 90 min, depending on the temperature. At this point of the experiment, air injection into the reactor as initiated to reach the working pressure (18 MPa), allowing a 70% excess of oxygen. The performance of WAO was monitored through concentration changes of COD (WTW kits) and TOC, (Shimadzu TOCL-L<sub>CSH/CSN</sub> Total Organic Carbon Analyzer) and removal rates (%) over a 6-hour experiment.

<figure><img src="image_3.png" /></figure>

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Fig. 1 Schematic diagram of WAO batch reactor. 1 – stirrer; 2 – gas purge; 3 – nitrogen and air inlet; 4 – heating

furnace; 5 – gas purge; 6 – high pressure pump; 7 – Rupture disk; 8 – autoclave; 9 – liquid sampling

## 2.2 Fluorescence monitoring

Membrane concentrates and WAO outlets samples were filtered through 0.45 µm filters for TOC measurement.

Depending on their respective concentrations, samples were diluted to a uniform TOC concentration of 5 mgC·L⁻¹ prior fluorimetry analysis. Fluorescence excitation-emission matrices (EEMs) were recorded in liquid phase using a 1 cm quartz cell (Perkin Elmer FL 6500, xenon source 40 kW, 100 Hz). EEM spectra were acquired by varying emission wavelengths from 200 to 395 nm in 5 nm increments, while emission wavelengths were scanned from 280 to 550 nm in 0.1 nm increments. Both excitation and emission slit widths were set to 5 nm and the scanning monochromator speed was set to 1,200 nm·min⁻¹. The 3D EEMs were built from the 2D spectra measured at the forty different excitation wavelengths. 3D EEM processing was performed using Excel, first by the subtraction of ultrapure water EEM and scattering, while also accounting dilution factors (Tucker et al. 1992; Guo et al. 2010). Subsequently, the spectra were divided into five regions following the classification proposed by Chen et al. (2003) where regions I and II are associated with protein-like compounds (tyrosine and tryptophan, respectively), region III is associated with fulvic acid-like compounds, region IV is associated with
soluble microbial by-products and region V is associated with the presence of humic acid-like compounds and their hydrolysates.

The integration of fluorescence regions reflects the relative presence of specific fluorophores in the different regions of the EEM spectra. For this integration method, the EEM spectra were divided into five fluorophores regions. The volume ($\Phi_i$) beneath each region “i” of the EEM was calculated. To normalize ex/em area volumes ($\Phi_{i,n}$) equations (1) and (2) were applied (Chen et al. 2003; Hong et al. 2012; Jacquin et al. 2017):

$$ \Phi_i = \sum_{ex} \sum_{em} I \cdot (\lambda_{ex} \cdot \lambda_{em}) \Delta\lambda_{ex} \cdot \Delta\lambda_{em} \quad (1) $$

$$ \Phi_{i,n} = MF_i \cdot \Phi_i \quad (2) $$

where $\Delta\lambda_{ex}$ represents the excitation wavelength interval (5 nm), $\Delta\lambda_{em}$ is the emission wavelength interval (0.5 nm) and $I(\lambda_{ex}, \lambda_{em})$ is the fluorescence intensity at each excitation-emission pair. Also $MF_i$ is a correction factor to account for secondary and tertiary responses at longer wavelengths equivalent to the inverse of the fractional projected excitation-emission area (Chen et al. 2003).

# 3 Results and discussion

## 3.1 Effect of temperature and seasonality on WAO

The impact of the temperature should be considered in conjunction with the duration of the heating period, which was set at 40, 60 and 90 min for temperatures of 200, 250 and 300 °C, respectively. While the influence of the-heating is not always considered in literature, it can significantly contribute to the removal of certain thermosensitive compounds. COD removals rates (Fig. 2a) after it averaged at least 24% and were quite consistent across all three seasonal variations. As the temperature increased, thermal degradations increased, reaching an average of 49%. In terms of TOC removals (Fig. 2b), thermal degradations averaged 19% at 200 °C and increased to 38% at 300 °C. Lei et al. (1998) also highlighted an effect of the heating period on textile industry effluent with a TOC removal of 34%, along with other studies (Zalouk et al. 2009; Carrier et al. 2009). The injection of the oxidant was proven effective, especially at higher temperature. After 90 min, removal rates averaged 64% and 74% for COD and TOC, respectively.

While removals are high, remaining molecules after 360 min of oxidation are usually low molecular weight compounds (e.g. carboxylic acids) which are challenging to oxidize, limiting WAO, though biodegradable (Shende and Mahajani 1994; Mishra et al. 1995; Bhargava et al. 2007). Also, it is noteworthy that removals (%) achieved after the heating period vary with temperature and seasonality. However, removals during and at the end of WAO are very similar for the three seasonalities at each temperature. Best COD and TOC removals (above 90%) were achieved at 300 °C, independently of the seasonality and are coherent with literature (Goi et al. 2009; García et al. 2020).
<figure><img src="image_4.png" /><figcaption>**Fig. 2** Variation of (a) ● COD and (b) ▲ TOC removals as function of seasonality for different temperatures and different times of sampling</figcaption></figure>

## 3.2 Fluorescence spectroscopy monitoring

### 3.2.1 Characteristics of landfill leachates

Landfill leachates present a diverse range of organic and inorganic compounds, the composition of which varies with their age, resulting in different spectral properties. The EEM spectra of landfill leachates concentrates before and after WAO are shown in Fig. 3. In the raw leachates, a characteristic presence of fulvic-like and humic-like substances (region III and V, respectively) is observed, known to be refractory to biological degradation. Notably, the fluorescence intensity of these compound families varies with the seasonalities of the leachates and is directly proportional to the concentration under the experimental conditions employed. The normalized percentages of each fluorescent region with the fluorescence volume are presented in Fig. 4 following the cartography of Chen et al. (2003). The fluorescence repartition across the spectra appeared consistent for all three seasonalities with a substantial presence of fulvic-like substances (49% in region III). Furthermore, the EEMs revealed the complex nature of the humic-like compounds, with the predominant peak intensity occurring at the longer excitation and emission wavelengths. This observation suggests the possible polycondensation of aromatic molecules and a heightened degree of the π-system conjugation or humification degree (Senesi et al. 1991; Kang et al. 2002).

<figure><img src="image_5.png" /></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 4 Fluorescence characteristics of landfill leachate concentrates before WAO</figcaption></figure>

### 3.2.2 Evolution of EEMs through WAO

The results of TOC and COD removals showed that WAO was efficient to degrade the organic fraction of leachates as high removal rates were achieved. While the concentration of these parameters decreased during the WAO process, there was an initial increase of fluorescence intensity observed after the heating period (t = 0 min). For instance, when examining the leachates from January treated at 200 °C (Fig. 5a), the quantum yield remained relatively stable throughout WAO. However, TOC and COD removals rates reached 57 and 56%, respectively. This suggests that while a fraction of the organic compounds was degraded, complex refractory aggregates persisted. By raising the WAO temperature to 300 °C (Fig. 5b), it took approximately 90 min to observe changes in fluorescence distribution and an overall decrease in fluorescence intensity. The modification of the distribution of fluorescence has been measured by other authors in the chemical oxidation of dye (Zhu et al. 2020). For the two more concentrated leachates (May, October, Fig. 5c-d-e-f), the increase of fluorescence intensity was still present after the heating (t = 0 min). At 200 °C, the WAO led to a decrease in fluorescence intensity, though by raising the temperature to 300 °C, 99% of the fluorescence intensity was removed. It is worth noting that leachates often contain metal complexes with organic matter, forming large molecules with limited fluorescence. When heated, these metallic complexes could undergo thermal degradation, potentially breaking down into smaller, more fluorescent aggregates. Additionally, the three studied leachates were stabilized and contained refractory COD, which could undergo partial degradation during the heating period. Consequently, both TOC and COD concentrations classically decreased as compounds underwent rearrangement and/or produced CO₂.

### 3.3 Global mass balance of the treatment path

The objective of this treatment path is to efficiently manage industrial membrane concentrates. The significant volumes of permeate generated by the RO can be safely discharged into the environment, in compliance with current regulations, regardless of the seasonal variations. To evaluate the performance of the industrial RO and the semi-industrial-scale WAO, a comprehensive analysis of the global mass balances was conducted. French environmental regulations governing water discharge from sanitary landfills to the environment specify a maximum COD of 300 mgO₂·L⁻¹ for daily flow rates less than 100 kg·L⁻¹, or 125 mg·L⁻¹ for higher daily flow rates, along with a maximum TOC of 70 mg·L⁻¹.
<figure><img src="image_7.png" /><figcaption>Fig. 5 Variation of fluorescence throughout WAO for landfill leachates of (a), (b) January, (c), (d) May, and (e), (f) October at 200 and 300 °C</figcaption></figure>

The operating parameters on the industrial RO enable concentration factors of 1.15 times in January, 1.26 times in May and 1.66 times in October. This resulted in conversion rates of 13%, 21% and 40%, respectively, with retention rates consistently higher than 92 and 95%, for TOC and COD, respectively. The simplified global mass balances, leachate flow rates entering RO were presumed identical (4 m³·h⁻¹), concentrate flow rates in January, May and October were 3.5, 3.2 and 2.4 m³·L⁻¹, respectively. Considering both the efficiency of RO and WAO, experiments carried out at the highest temperature (300 °C) resulted in a blend with TOC concentrations ranging from 10 to 59 mgC·L⁻¹, fully compliant with regulatory thresholds. However, the COD concentrations exceeded regulatory threshold. Interestingly, fluorescence spectroscopy revealed a significant reduction, and in some
cases, complete elimination of the aromaticity of compounds or fluorescence. The residual compounds in the blend are probably less complex, with a smaller molecular weight.

<figure><img src="image_8.png" /><figcaption>**Fig. 6** Global mass balance of the coupling of RO and WAO at 300 °C and 18 MPa for landfill leachates of (a) January, (b) May and (c) October</figcaption></figure>

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To validate the discharge to the environment, considering only TOC and COD concentrations, the maximum RO concentrate flow rates that could be treated by WAO were calculated (Fig. 6). The most challenging and concentrated leachate (October, Fig. 6c) can meet COD and TOC regulatory thresholds with a concentrate flow rate of 0.13 m³·h⁻¹, accounting for 5.4% of the RO system's concentrate outlet. The remaining untreated flow rate could be directed back into the on-site leachate tank. For the less concentrated leachates of January and May, concentrate flow rates of 0.16 and 0.11 m³·h⁻¹, respectively, could be treated. Although COD and TOC concentrations are lower for January and May, retention rates by RO were not as high as in October. Consequently, the permeate quality was lower, impacting the quality of the final blend of the treatment path. Importantly, it is noteworthy that the cost of the process would not be influenced by the high temperature (300 °C), as the exothermicity of the oxidation could provide a sufficient energy to heat up the bubble column. The process could be self-sustained and require an external heat input at the beginning of the WAO. This autothermic nature could contribute to the economic viability of the process, minimizing energy-related expenses.

## 4 Conclusion

The degradation of the complex composition of landfill leachate RO concentrates was carried out using WAO. The influence of temperature and pollution concentration were assessed through COD, TOC and fluorescence spectroscopy. Significant removal rates of organic matter were achieved, especially at higher temperatures (300 °C). Leachates with the highest initial concentration (October) demonstrated best removals in terms of quantity, yet the removals (%) by WAO were similar regardless of seasonality. A minimum duration of 4 h was required to attain COD and TOC removals exceeding 80%. Monitoring through fluorescence spectroscopy provided insights into the complexity and degradation of pollutants during WAO. Despite an increase of fluorescence observed at the end of the heating period, fluorescence completely disappeared after 6 h of WAO at 300 °C, suggesting that aromatic pollutants were likely greatly degraded. Additional research is needed to evaluate the treatment path's cycle analysis and cost-effectiveness, considering both environmental and economic factors, as well as the validation of the industrial scale WAO (bubble column). Overall, this study provides data to enhance the understanding of WAO concerning landfill leachates, a current issue due to their complex composition and the environmental challenges posed by their treatment. This data contributes to the development of effective strategies to address present challenges related to the management of landfill leachates. These insights carry substantial importance for both researchers and professionals in wastewater treatment, and perhaps lead to a more sustainable treatment than current incineration. References

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295 Ethical Approval

296 Not applicable.

297 Consent to Participate

298 Not applicable.

299 Consent for Publication

300 Not applicable.

301 CRediT authorship contribution statement

302 **Emilie Gout:** Investigation, Visualization, Writing - Original draft preparation, **Mathias Monnot:**

303 Conceptualization, Supervision, Writing – Review & Editing, **Olivier Boutin:** Writing – Review & Editing,

304 **Pierre Vanloot:** Methodology, Software, Writing – original draft, **Philippe Moulin:** Conceptualization,

305 Supervision, Writing – Review & Editing.

306 Funding

307 This work was supported by the French National Research Agency (ANR) for the Project TEMPO under the reference number ANR-19-CE04-0002-01.

309 Competing Interests

310 The authors declare no competing interests.