Treatment of high-salt wastewater

Thread Content

This post was last edited by cdpulin on 2012-2-20 at 11:11. Industrial wastewater, especially chemical wastewater, not only has a high COD value but also features another problematic parameter: salt content. In addition to B/C and C/N, the salinity level is another key factor that restricts the treatment of such wastewater. Such wastewater has a high salt content and a high organic load, so the difficulty of treating it is evident. Generally, when biologically treating wastewater, the salt content should not exceed 1%; some sources indicate 0.6%, which corresponds to a salt content of 6000 mg/L. However, in practice, good results can also be achieved when the salt content is as high as 1.4%, or 14000 mg/L. Even with high salt content, the desired treatment effects are often not achieved, even in the presence of salt-tolerant bacteria. When dealing with such wastewater, several suggestions are put forward for reference only. As follows: 1. When the organic load of wastewater is high but the salt content is not particularly high, tap water or other water sources can be used for dilution first, followed by treatment for regeneration. This requires an abundant supply of tap water or water for dilution. Considering engineering examples where biochemical treatment is possible with a salt content of 0.6%–1.4%, it is feasible to dilute the solution with water at the initial stage of commissioning before sending it to the biochemical treatment tank. As debugging progresses and the treatment efficiency improves, the salt content is gradually increased, thereby reducing the amount of dilution water added. When carrying out this step, it is necessary to continuously monitor the properties of the sludge as well as the performance of the effluent, in order to find the optimal operating conditions. Once instability in the sludge is detected, water should be added to dilute it. 2. Another one is the membrane treatment process, which is well-known to everyone. This method is essentially a process of concentration, where the salts in the water are accumulated and then removed. However, reverse osmosis membranes cannot concentrate the salts in water by a factor of two; that is, water with 1% salt content cannot be turned into concentrated water with 50% salt content (this is just an example, not a reflection of reality). The reverse osmosis process is used in equipment with high water quality requirements, but its reuse rate cannot be 100%. The concentrated water produced after one round of reverse osmosis treatment can be fed back into the reverse osmosis system, but this increases the cost of treatment. It also leads to frequent scaling and other issues that block the membrane elements, thereby shortening the lifespan of the membranes. Some people say that desalination isn’t necessarily that serious; but have you considered that the salt composition in seawater varies, which means the pre-treatment processes are different as well, and the membrane elements used also differ slightly. Cost determines the processing technique to be used. In the reverse osmosis process, wastewater with a high organic load can also be treated. However, unlike pure water production and reclaimed water reuse, in this case as well a large amount of concentrated liquid is generated; this concentrated liquid has a higher organic load and higher salt content, making it more difficult to treat. 3. Incineration is also a method for dealing with high organic loads. As the name implies, burning means using fire. Depending on the amount of water, different incinerators are used, with the ultimate goal of converting the organic substances in the water into inorganic ones. In the incinerator, the waste liquid is sprayed into the furnace in a mist form; with high temperatures and an adequate supply of oxygen, these three conditions being met, the removal of organic pollutants can be achieved. What remains are inorganic substances, which can be removed through cleaning or water flushing. This method does not allow for water recovery, but it can effectively address organic pollution. 4. Multi-effect evaporation can also effectively remove salts and organic substances from water. Multiple-effect evaporation involves the use of multiple evaporators in series, with the temperature of each evaporator decreasing sequentially. The principle is simple: steam is used to heat the original liquid, causing the water to evaporate; what remains is a residue with a high concentration of organic substances and salts. The liquid remains and flows into the next evaporator; therefore, as water evaporates, the viscosity of the remaining liquid increases, as does the salt content. Therefore, the pure liquid needs to be pumped into the evaporator. The evaporated water enters the condenser, where the condensed water is reused. Salts and organic substances will precipitate, and eventually be discharged in a semi-solid form. (This article does not discuss the design of multi-effect evaporation; it only provides a general overview of the treatment methods.) With the current shortage of water resources, it is desirable to reuse water after it has been treated for sewage/wastewater purposes. Therefore, biological treatment, reverse osmosis, and multi-effect evaporation are all treatment methods that are given priority. Burning, personally, I think it’s an option to consider first when there’s really no other way. Cost is a reality that no enterprise can avoid. The costs associated with treating wastewater are higher than the benefits, which is one of the reasons why environmental protection is challenging nowadays. The cost of biochemical treatment is relatively low, whereas the costs of reverse osmosis and multi-effect evaporation are relatively high. For mature processes, optimization is possible; through intensive treatment, energy consumption can be reduced as well as processing costs. Setting aside biochemical treatment, let’s discuss the operational cost analysis of reverse osmosis and multi-effect evaporation. First is the selection of the process; while meeting the water demand, efforts should be made to simplify the treatment process, either by using reverse osmosis or multi-effect evaporation. Secondly, for combined processes, it is necessary to find a balance that meets the water demand while reducing energy consumption. For example, 100 tons of raw water containing 10% salt need to be treated, with no requirement for a reuse rate. Pure reverse osmosis alone cannot achieve desalination; a combination of reverse osmosis and multi-effect evaporation can be used, or multi-effect evaporation alone can also be employed. Which one should I choose? In one case, reverse osmosis + multi-effect evaporation is used. After reverse osmosis, assuming that the concentrated water amounts to 50 tons with a salt content of 20%, multi-effect evaporation is then applied; in this case only single-effect evaporation is possible. Theoretically, 50 tons of steam are required per day. The cost consists of the electricity costs for reverse osmosis, the electricity costs for multi-effect evaporation, labor costs, chemical costs, steam costs, and equipment costs. In one case, multi-effect evaporation is sufficient. Raw water with 10% salt content can be processed using 5 effect distillation; theoretically, 20 tons of steam are required per day. The cost consists of the electricity cost for multi-effect distillation, the steam cost, labor costs, and equipment costs. In that case, which one has a lower cost?