The rapid expansion of desalination through reverse osmosis has significantly transformed global water resource management, but it has also created a growing challenge related to the management of end-of-life membranes. These industrial wastes, mainly composed of engineering polymers such as polyamide, polysulfone, and polypropylene, exhibit physicochemical properties that, while initially optimized for filtration, may become an asset within an energy recovery framework [1-2].
The average lifespan of reverse osmosis membranes ranges from five to ten years, generating substantial waste streams worldwide [1]. As an order of magnitude, a desalination plant with a capacity of 100,000 m³/day can generate between 2,000 and 5,000 discarded membrane modules per replacement cycle, corresponding to approximately 50 to 150 tons of complex plastic waste every 5 to 7 years. Globally, the annual waste generation is estimated between 30,000 and 50,000 tons of end-of-life membranes, with an annual growth rate exceeding 8% due to the expansion of desalination.
RDF from Desalination Membranes – Key Parameters
The analysis of Refuse Derived Fuel (RDF) technical requirements highlights specific thresholds that any waste stream must meet to be valorized in cement kilns. The lower heating value (LHV) is a fundamental parameter, with a minimum requirement of 3,500 kcal/kg [5]. By comparison, end-of-life tires typically exhibit LHV values between 6,500 and 7,500 kcal/kg, industrial plastics between 5,000 and 8,000 kcal/kg, while conventional municipal RDF ranges from 2,500 to 4,000 kcal/kg [5-6].
In this context, desalination membranes, due to their polymeric composition, can reach estimated LHV values between 4,000 and 6,000 kcal/kg after drying and shredding, making them energetically attractive. For instance, one ton of recovered membranes could theoretically substitute approximately 0.6 to 0.8 tons of coal in a cement kiln, depending on operating conditions. In natural gas equivalent, this corresponds approximately to 500 to 700 Nm³ of natural gas, assuming an average calorific value of about 8500–9000 kcal/Nm³.
However, LHV is not the only determining parameter. Moisture content must be kept below 15%, as higher values reduce energy efficiency and disrupt combustion stability [5]. Used membranes generally exhibit low moisture content (typically below 5% after storage), which provides a clear advantage compared to sludge, which may contain up to 70–80% moisture before drying. Nevertheless, in cases of prolonged outdoor storage or contamination, a drying step may be required to ensure stable energy performance.
Ash content is another critical constraint, with a maximum allowable limit of 20% [5]. Ash directly affects clinker composition and may require adjustments in raw material inputs. Membranes, being largely organic, typically exhibit intrinsic ash content below 10%. However, accumulated deposits during operation (salts, silica, carbonates, metal oxides) may increase this value to 15–25% in some cases. For example, membranes used in seawater desalination can accumulate 5–10% mineral salts by mass, requiring pre-washing to meet acceptable thresholds.
The most critical parameter remains chlorine content, with a maximum limit of 0.5% in RDF used in cement kilns [6-7]. Chlorine is responsible for internal recycling phenomena within kilns, leading to deposits, fouling, and operational disruptions [7]. Membranes may contain residual chlorine from chemical cleaning processes or accumulated salts. For instance, membranes regularly treated with sodium hypochlorite may contain chlorine residues ranging from 0.2 to 1% by mass if no washing is performed. This implies that direct incorporation into RDF may exceed acceptable limits. A common strategy is therefore to limit the membrane fraction to 5-15% of the RDF blend or to apply washing processes capable of reducing chlorine content by 50–80%.
Regarding sulfur, the recommended limit is around 1.5% [6]. Membranes generally contain low sulfur levels (typically below 0.5%), unless specific contamination occurs. By comparison, tires may contain 1–2% sulfur, while some industrial sludges may exceed 3%. Thus, membranes do not represent a major constraint in terms of sulfur and may even contribute to diluting high-sulfur waste streams.
Valorization of Desalination Membranes
The integration of membranes into RDF should therefore be considered within a co-processing approach, combining them with other waste streams. For example, a typical formulation could include 10% membranes, 40% plastics, 20% biomass, and 30% textile waste, resulting in an overall LHV exceeding 4,000 kcal/kg while maintaining chlorine content below 0.5%. This approach allows dilution of problematic elements and optimization of fuel properties [5-6]. In practice, European cement plants already use complex RDF mixtures with thermal substitution rates reaching 60–80%.
From a technical standpoint, valorization involves several steps: dismantling membrane modules, separating components, shredding, washing, and homogenization. A standard spiral-wound membrane module (approximately 1 meter in length) weighs between 12 and 20 kg, with more than 80% of the material potentially recoverable for energy purposes. Shredding typically produces particles smaller than 30 mm, compatible with kiln feeding systems. Washing processes can reduce salt content by more than 70%, significantly improving RDF quality [3-4].
Furthermore, studies have demonstrated that end-of-life membranes can be transformed or reused in other applications (ultrafiltration, nanofiltration), reinforcing their potential within a circular economy framework [2,4]. However, these reuse pathways remain limited in scale, justifying the need for energy recovery solutions to absorb larger volumes.
Environmental Benefits
From an environmental perspective, the use of RDF in cement kilns offers significant benefits. Substituting one ton of coal with RDF can avoid approximately 0.8 to 1 ton of CO₂ emissions. Thus, the valorization of 10,000 tons of membranes could potentially avoid up to 8,000 tons of CO₂ emissions, while reducing landfill disposal. In natural gas equivalent, and on a comparable energy basis, such valorization could also substitute several million cubic meters of natural gas (on the order of 5 to 7 million Nm³), thereby contributing to the reduction of emissions associated with the combustion of gaseous fossil fuels The high temperatures in cement kilns (above 1,400°C) ensure complete destruction of organic compounds, while mineral residues are incorporated into clinker [5].
However, this approach also raises several strategic questions. The standardization of membrane waste streams remains a major challenge, with composition variability reaching ±30% depending on sources. Investments required for pre-treatment facilities (shredding, washing, drying) may range between €1 and €5 million for a capacity of 10,000 tons/year. Logistics costs, particularly transportation, may represent 20–30% of total valorization costs. Finally, regulatory frameworks must evolve to support and control this emerging sector.
In countries where desalination is rapidly expanding, this convergence between waste management and the cement industry represents a strategic opportunity. With multiple large desalination plants, the potential waste stream could reach several thousand tons of membranes per year in the medium term. These membranes could constitute a complementary resource for alternative fuels, provided that key thresholds for chlorine (≤ 0.5%), sulfur (≤ 1.5%), moisture (≤ 15%), and ash (≤ 20%) are respected [5-7].
Conclusion
The valorization of used desalination membranes into RDF represents an innovative and promising pathway, but requires an integrated approach combining technical control, regulatory frameworks, and industrial cooperation. It fully aligns with circular economy and energy transition objectives, transforming a complex waste stream into a strategic resource for the cement industry.
References
[1] Rodriguez-Gonzalez, L., Martinez-Mateo, L., & Pérez, J., 2020. Recycling of reverse osmosis membranes: A review. Journal of Membrane Science, 595, 117513. https://doi.org/10.1016/j.memsci.2019.117513
[2] Khaless, K., Lejarazu-Larrañaga, A., & Ibañez, R., 2021. Recycling of spent reverse osmosis membranes: A review. Minerals, 11(6), 637. https://doi.org/10.3390/min11060637
[3] Lawler, W., Bradford-Hartke, Z., Cran, M., Duke, M., & Leslie, G., 2012. Reverse osmosis membrane reuse: Opportunities and challenges. Desalination, 299, 103–112. https://doi.org/10.1016/j.desal.2012.05.006
[4] Morón-López, J., et al., 2023. End-of-life management of reverse osmosis membranes: Reuse and recycling options. Desalination, 545, 116163. https://doi.org/10.1016/j.desal.2022.116163
[5] Genon, G., & Brizio, E., 2008. Perspectives and limits for cement kilns as a destination for RDF. Waste Management, 28(11), 2375–2385. https://doi.org/10.1016/j.wasman.2007.09.019
[6] Rahman, M., Rasul, M., Khan, M., & Sharma, S., 2015. Recent development on the uses of alternative fuels in cement manufacturing process. Fuel, 145, 84–99. https://doi.org/10.1016/j.fuel.2014.12.029
[7] Werther, J., & Ogada, T., 1999. Sewage sludge combustion. Progress in Energy and Combustion Science, 25(1), 55–116. https://doi.org/10.1016/S0360-1285(98)00020-3
