Assessment of Processes to Increase the Useful Life and the Reuse of Reverse Osmosis Elements in Cape Verde and Macaronesia
2. Technical Actions to Be Carried Out
2.1. Re-Use of Reverse Osmosis Elements
- Brackish water reverse osmosis membranes can also have a second life in tertiary treatment plants that do not require a very demanding quality of permeate for irrigation, especially if they are membranes with low fouling or with a large spacer, since the water coming from the treatment plant to the tertiary treatment plant is normally dirtier than the water from a brackish well installation, which is the most common way of collecting this type of water in Cape Verde in particular and in Macaronesia in general. Therefore, the membranes of the brackish water plant may have suffered salt precipitation, especially in the last positions of the second or third stage, but the best ones can be selected, and after a chemical cleaning they can be proposed for reuse in a tertiary treatment plant, where the problem, rather than salt precipitation, may be the fouling of the water. The membranes will eventually become clogged, but only after a second life and after they have been exploited 100%. Furthermore, the water from the reverse osmosis tertiary treatment plant will be used for irrigation or returned to the sea, such that it will not be used for drinking water because the law does not allow it; unlike in other countries such as Singapore, we do not run the risk that the reused membranes may have some kind of leak that could worsen the quality of the water and make it unsuitable for human consumption [15,18,19,20].
- There are some industrial processes that use special reverse osmosis membranes which end up very dirty in less than a month, so many of them must be replaced in a short time. By studying each industrial application that requires osmosis, depending on the salinity and pressures, it is possible to propose the reuse of second-hand membranes from other desalination plants in these processes to exploit them further before throwing them away. Other industrial processes include coffee concentrate, for example, where special reverse osmosis membranes are used and end up black in less than a month, meaning that many must be replaced in a short time. By studying each industrial application that requires osmosis, depending on salinity and pressures, it is possible to propose the reuse of second-hand membranes from other desalination plants in these processes to exploit them even more before throwing them away [15,21,22,23].
- There is also the possibility of leachate from landfill waste and other waste, which is sometimes treated by reverse osmosis to concentrate it; this process makes the membranes very dirty, and it is very difficult to find one that has a sufficiently long life to be profitable. In these cases, the option of reusing seawater membranes can be considered, if the salinity of the leachate is close to that of seawater, which is very common, or membranes from tertiary or brackish water plants if the salinity is lower. In this way, these reverse osmosis elements would be used in a final leachate concentration process before disposing of the membranes after they have been used 100%. Regarding leachate treatment, there are leachate evaporation systems in the complexes, although it has been accepted in WWTPs, which may be currently treating the surplus. However, it has always been thought, after consultation with experts in waste management, that the leachate could be used as a fertilizer after treatment. As salinity is one of the big problems, we think that treatment with membranes could be very interesting. There is bibliography on the subject, and it is believed that it could be an interesting line of work, even starting with a project [24,25,26,27].
- The membranes in a brackish water plant are more likely to be extended by chemical cleaning, but in these plants which usually have several stages, the membranes can also be combined between the two stages before disposal. Similarly, brackish water plants normally have a minimum of two or three stages to achieve conversions close to 75%, and this can help us to reuse older membranes in the third or sacrificial stage where they are more likely to precipitate salts earlier and be used to produce some more water until they are finally exploited to 100%.
2.2. Recycling of Reverse Osmosis Components
3.1. Results of the Qualitative and Quantitative Diagnostic Study of Reverse Osmosis Membranes
3.2. Results of the Reuse of Reverse Osmosis Membranes: Oxidation Process
- Membrane oxidation has already been carried out and has worked successfully at the Barranco Seco WWTP of Emalsa (Las Palmas de Gran Canaria), so it has been considered for the reuse of reverse osmosis elements, and its technical and economic feasibility will be studied later to carry it out.
- The oxidation of reverse osmosis membranes must be carried out under the following dosage and exposure level (ppm × h) of free chlorine: 6200 (NF)—30,000 (UF). Before oxidation process in the pilot plant, it has been studied and already identified that the process of passive transformation at the laboratory scale is feasible, and the level of exposure of the membranes (ppm × h) to achieve a satisfactory transformation of the properties of deteriorated reverse osmosis membranes to properties of ultrafiltration has also been identified.
- The positive results of reusing reverse osmosis membranes by oxidizing them and transforming them into nanofiltration, ultrafiltration or microfiltration membranes are basically the following:
- The reuse of a reverse osmosis membrane has a lower environmental impact than the production of a new one. Water consumption during the membrane transformation process for membrane reuse is 20 times lower than the consumption to produce new membranes.
- The carbon footprint of reused membranes in other processes is in the order of 40 to 60 times lower than the production of commercial membranes, and the price of reused membranes is 10 times lower than the price of new membranes. Therefore, in only 1 or 2 years of operation of reused membranes, the transformation process to reuse membranes is already beneficial both economically and environmentally.
- The membrane oxidation process can be either actively in a pilot plant with pressure tubes or passively in a tank with several submerged membranes. From an economic and environmental point of view, passive processing of reverse osmosis membranes is better .
3.3. Results of Reverse Osmosis Membrane Recycling: Recycling and Pyrolysis Process
- The basic components of the gas are CO, CO2, H2, CH4 and more volatile compounds from the cracking of organic molecules, together with those already existing in the waste. This gas is very similar to the synthesis gas obtained in gasification, but there is a greater presence of tars, waxes, etc., to the detriment of gases, since pyrolysis works at lower temperatures than gasification. The gaseous products are a highly energetic fuel with a calorific value of approximately 50 MJ/m3.
- The liquid residue is basically composed of long-chain hydrocarbons such as tars, oils, phenols and waxes formed by condensation at room temperature.
- The solid waste consists of all non-combustible materials, which are either unprocessed or originate from molecular condensation with a high content of carbon, heavy metals and other inert components of these wastes.
- The cost of the passive transformation would be between 28 and 38 EUR/module of ultrafiltration.
- The cost for landfill disposal is in between 1 and 1.6 EUR/module.
- This means a cost of 0.76–1.03 EUR/m2 for the transformed UF membranes.
- The cost of the commercial UF membranes is around 17.2 EUR/m2.
Institutional Review Board Statement
Data Availability Statement
Conflicts of Interest
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|Size||<0.001 µm||0.001–0.01 µm||0.01–0.1 µm||0.1–1 µm||1–10 µm|
|Separation Materials||Ion, Low Molecule Weight Organics||High Molecular Weight Polymer, Colloid||Colloid, Clay (Bacteria)||Clay (Coliform)||Clay (Cryptosporidium)|
|Water Treatments||Reverse Osmosis (RO)||Nanofiltration (NF)||Ultrafiltration (UF)||Microfiltration (MF)||Microfiltration (MF)|
|Island||Number of Membranes in Use per Year||Annual Membrane Replacement||Weight (kg) Replacement||Volume (m3) Replacement|
|Lanzarote (Canary Islands)||8000||1600||32,000||67|
|Fuerteventura (Canary Islands)||8650||1730||34,600||73|
|Gran Canaria (Canary Islands)||23,500||4700||94,000||197|
|Tenerife (Canary Islands)||9350||1870||37,400||79|
|El Hierro (Canary Islands)||350||70||1400||3|
|La Gomera (Canary Islands)||150||30||600||1|
|Porto Santo (Madeira)||450||45||900||2|
|Praia-Palmarejo (Cape Verde)||700||100||2000||4|
|St Vincent (Cape Verde)||350||50||1000||2|
|Salt (Cape Verde)||350||50||1000||2|
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Tavares, T.; Tavares, J.; León-Zerpa, F.A.; Peñate-Suárez, B.; Ramos-Martín, A. Assessment of Processes to Increase the Useful Life and the Reuse of Reverse Osmosis Elements in Cape Verde and Macaronesia. Membranes 2022, 12, 613. https://doi.org/10.3390/membranes12060613
Tavares T, Tavares J, León-Zerpa FA, Peñate-Suárez B, Ramos-Martín A. Assessment of Processes to Increase the Useful Life and the Reuse of Reverse Osmosis Elements in Cape Verde and Macaronesia. Membranes. 2022; 12(6):613. https://doi.org/10.3390/membranes12060613Chicago/Turabian Style
Tavares, Tomás, Jorge Tavares, Federico A. León-Zerpa, Baltasar Peñate-Suárez, and Alejandro Ramos-Martín. 2022. "Assessment of Processes to Increase the Useful Life and the Reuse of Reverse Osmosis Elements in Cape Verde and Macaronesia" Membranes 12, no. 6: 613. https://doi.org/10.3390/membranes12060613