Nowadays, the excess salt flow produced by in-land reverse osmosis systems has become a serious groundwater problem.
Therefore, the management technologies of this excess salt are strongly considered economically and environmentally. Especially in a country like Iran
ZLD implies complete evaporation of the reverse osmosis concentrate to salts. If we are talking about water that is not contaminated with heavy metals, it is advisable to soften it to remove hardness ions and then neutralize it. This will produce a concentrate consisting mainly of sodium or potassium chlorides and sulfates. The separation of these salts is carried out in evaporative crystallizers by changing the temperature and pressure. This process is well described in the literature. If we are talking about water contaminated with heavy metals, manganese or iron, then for their effective removal, preliminary precipitation can be carried out, for example, using a solution of calcium hydroxide.
Based on the experience of mine water treatment, non-ferrous metals (copper, nickel) can be purposefully removed into the concentrate without precipitation. The concentrate then goes to a two- or three-stage ion exchange, where separate sorption of valuable ions takes place. A similar method is described in one of my works (the text is in Russian, but the figures and tables are in English):
Article Specific Features of Using Reverse Osmosis for Mine Water Treatment
Since evaporation is one of the most expensive waste disposal methods, the amount of processed concentrate should be minimized as much as possible. This requires a two- or three- or even four-stage reverse osmosis.
Vasilii Kurdiumov's response is a case in point. From your question, I infer that you are processing brackish water and not seawater (please confirm). Luckily, for inland processes, the TDS is reasonably manageable. Brackish waters in the salinity range of 2,000 – 5,000 mg/L. Desalination at the salinity range of most surface and groundwater can be performed at low trans-membrane pressures (200 – 300 psi).
Inland brine disposal sites are limited to three general categories: 1. Deep Well Injection 2. Evaporation Ponds and 3. Solar Ponds. Other systems have been proposed. These include irrigation of salt tolerant plants and brine shrimp harvesting. These systems are limited and not practical to very large effluent volumes.
It is important to always keep in mind that desalination (as a technology) is to enhance water quality and not to market its byproducts. For example, the recovery of inorganic salts with some commercial value has been suggested in the past. However, with low sodium diets and the construction costs of chemical facilities may deem these suggestions impractical. ZLD has been used in certain situations where waste brine streams are small and land is hardly available for Evaporation Ponds and Solar Ponds. This technology is applied to wastewater disposal from power plants, oil refineries and certain mining operations. I suggest blending the effluent with another effluent (such as from Wastewater process plants to lower the concentration initially. The final stage may involves evaporation that creates a solid residue. A disadvantage is that energy requirements are excessive for handling large brine volumes.
Dear Sami Maalouf & Vasilii Kurdiumov
Thank you for the answers of both respected researchers
To clarify the issue, I will mention a few points:
1- In our Inland, we have used the usual methods mentioned by you before, but it has made the aquifers saltier.
2- Pistachio farms in Iran are very large and its outflow of excess brine has a high flow rate.
3- Due to the low energy consumption tariff in Iran, it is more important to prevent the salinization of groundwater.
4- The income from the sale of pistachios offsets the costs of the "ZLD" process.
5 - If the farmer cannot use his land for agriculture must migrate to find suitable water and land, in which case other aquifers will become more saline.
Therefore, despite the high price of "ZLD" systems, their use seems to be inevitable in some cases.
There's a book called, "Concentrate Management in Desalination: Case Studies" that I read a few years ago. the book covers a few pilot case studies that discuss ZLD. Let me know if you are familiar with it. I think I have a copy of it that I can look for if you would like to look at it. Meanwhile, thank you for clarifying the details of your project.
Sami Maalouf
thanks for your help
Yes, if possible, I will be happy, please send me the book.
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