Discussion on Misconceptions Regarding Membrane Concentration of High-Salt Wastewater Followed by Evaporation for Desalination

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Basic knowledge to keep in mind: the higher the salt concentration in wastewater, the fewer evaporation cycles there are, the lower the efficiency of steam utilization, and the greater the energy consumption； The lower the salt content, the higher the efficiency factor, the better the utilization of steam, and the lower the energy consumption. Generally, 3%-4% can achieve 8 effects, 4-5% can achieve 7 effects, 6-8% can achieve 6 effects, 8-12% can achieve 5 effects, 12-15% can achieve 4 effects, 15% can achieve 3 effects, and above 20% only 1 effect is possible. On the forum, I saw many cases of wastewater being concentrated using membrane methods, and then evaporated after concentration. After the wastewater is concentrated, the water volume decreases while the salt content increases; evaporation desalination can then be used, and this is an approach that many people adopt. It sounds like it makes sense, but it’s a small misconception. Where is the misconception? Basically, few people think about this issue, so let me explain it to you all. For example, 10% saline wastewater is concentrated to 20% using membrane technology, and then evaporation is applied; if the total volume of water is 100 tons, the volume after concentration is 50 tons. As the salt concentration increases from 10% to 20%, evaporation is used for concentration; at 20%, this essentially involves single-effect evaporation. Theoretically, 50 tons of steam are required per day. Such high steam consumption is associated with additional costs related to membrane concentration processes. Moreover, the heat generated by those 50 tons of steam needs to be cooled, which requires even more circulating cooling water, as well as a higher installed power capacity. If we don’t concentrate the waste for evaporation – with 10% salt content and 100 tons of wastewater – and use 5-effect evaporation, what would be the cost in that case? Theoretically, it’s 20 tons of steam. By making a comparison, we can discern some patterns: the cost of membrane concentration for 50 tons of steam, membrane concentration equipment capable of handling 100 tons, the costs associated with membrane concentration for 100 tons, the cooling required to handle 50 tons of steam, and the electricity costs related to forced circulation for 50 tons ； The cost of direct 5-effect evaporation without membrane concentration: 20 tons of steam are required; no membrane concentration equipment is needed, cooling for the heat generated by 20 tons of steam, and electricity costs for forced circulation – the investment in equipment is roughly equivalent to these costs. Let’s compare and think about it together. High-salt wastewater does not necessarily have to be concentrated first before undergoing evaporation treatment. It doesn’t mean much. In other words, concentration makes sense only within a certain range; in many cases it is actually unnecessary. For example, increasing it from 3% to 8% makes sense when considering everything together. If further concentration is carried out, it may seem interesting on the surface, but in reality it has no meaning at all; because concentrating using membrane methods isn’t a way to save money – it actually costs more money. A friend of mine asked me about wastewater of this kind: 20 tons of wastewater with a 20% concentration, and it can only be treated using a single-stage process. 20 tons of steam are needed per day. So what would be the result if it is diluted by half and then evaporated? It’s 40 tons of wastewater at 10% concentration; in this case, 5-effect evaporation can be used, resulting in a steam consumption of 8 tons per day. There is such a large difference in steam usage. Why can the cost be reduced even after diluting wastewater with a higher concentration? Every thing has its own characteristics, and both positive and negative aspects need to be considered. So many times, one needs to think flexibly, understand thoroughly, and use the real brain to think in order to acquire true knowledge. Let’s work together. My views on high-salt wastewater – I’m just sharing my experience; it’s entirely original, based solely on experience and knowledge