Wastewater is the brine from a first-stage reverse osmosis desalination unit. The reverse osmosis (RO) process for preparing pure water produces concentrate, which contains various organic and inorganic pollutants. Direct discharge may pollute soil, surface water, and oceans; if discharged into the municipal sewage treatment system, the high total dissolved solids are also very detrimental to the growth of activated sludge. The highly concentrated RO concentrate and chemicals introduced by cleaning agents and antiscalants, if directly discharged into the environment, will inevitably have adverse effects. Therefore, finding an economical and efficient method for treating RO concentrate is of great significance for environmental protection.
Reverse osmosis membrane separation technology has been widely used in many industries in recent years due to its advantages of no phase change of materials, relatively low energy consumption, good desalination effect, mature and reliable treatment process, simple equipment, high degree of automation, easy operation and management. However, the general design water production rate of reverse osmosis technology is currently 75%, and the actual water production rate is even lower, with about 30% brine produced. If the raw water is very poor quality underground brackish water or seawater, the concentrate production will be even greater, possibly reaching 50%. Currently, much of the concentrate produced by reverse osmosis processes is discharged without treatment, resulting in waste of water resources and energy, and pollution of the surrounding environment.
Current research on reverse osmosis concentrate focuses on three main objectives: reduction—optimizing reverse osmosis process design to reduce concentrate production; harmlessness—exploring cost-effective treatment methods to mitigate the potential environmental hazards of direct discharge of reverse osmosis concentrate; and resource utilization—exploring ways to reuse reverse osmosis concentrate, turning waste into treasure.
In fact, the reuse of reverse osmosis concentrate needs to consider multiple factors. These three objectives are not isolated but need to be considered comprehensively and complement each other.
Separate Treatment and Discharge
The main problem with reverse osmosis concentrate is the high content of calcium and magnesium ions and high hardness. Generally, simple softening treatment can achieve compliance with discharge standards. Softening mainly uses the method of adding alkaline substances such as lime and soda ash, which react with calcium and magnesium substances in the concentrate to form carbonate precipitates, removing them from the water and reducing the hardness of the concentrate, reducing its harm to the environment.
Mixed with Other Wastewater for Treatment
For most production enterprises, in addition to the reverse osmosis concentrate produced by the water production workshop, various other wastewaters are also produced. For example, production wastewater discharged from the production workshop, factory area domestic sewage, etc. Separate treatment of each type of wastewater is often not economical. Therefore, most enterprises choose to separate some special wastewater and treat all types of wastewater together after mixing.
The water quality of reverse osmosis concentrate is characterized by high hardness and high salt content, while important pollution indicators such as turbidity and COD are very low. Mixing reverse osmosis concentrate with other wastewater can play a certain dilution and adjustment role, thereby reducing the influent pollution load of the mixed wastewater treatment system.
Some enterprises' production wastewater contains a large amount of alkaline substances such as sodium carbonate and sodium hydroxide, which can react with calcium and magnesium in the reverse osmosis concentrate to form hydroxide or carbonate precipitates, reducing water hardness.
For example, a certain aluminum plant's thermal power plant has modified its reverse osmosis concentrate treatment system. By mixing the waste alkaline water produced in the production process with the reverse osmosis concentrate, the calcium and magnesium hardness is removed, and the concentrate is reused in production, reducing the cost of production water by about 1.76 million yuan/year.
Reduction is aimed at the reverse osmosis system itself. If the reverse osmosis system is designed reasonably and the water quality of the enterprise's influent is stable, the amount of concentrate produced by the system can be controlled at an optimal ratio. When designing a reverse osmosis system, there are two methods to improve the system's recovery rate, i.e., reduce the amount of concentrate produced. One is to increase the length of the water flow through the reverse osmosis membrane module, and the other is concentrate reflux.
Increase the length of the water flow through the reverse osmosis membrane module.
As the water flows through the reverse osmosis membrane element, freshwater continuously permeates the membrane, achieving separation of concentrate and freshwater. Theoretically, the longer the water flows through the membrane element, the greater the freshwater yield and the higher the recovery rate. However, due to convenience, standardization, and other issues, the length specifications of various membrane elements on the market have been determined, but they can be connected in series into membrane modules according to process requirements. Due to the decrease in flow rate and pressure, the membrane module cannot be too long and needs to be segmented, i.e., multiple membrane modules are connected in series.
Therefore, on the premise of ensuring the effluent quality and system stability, in order to reduce the amount of concentrate produced and improve the system recovery rate, the number of stages can be appropriately increased during reverse osmosis design.
However, relatively speaking, the lengthening of the membrane system requires an increase in the driving force of the membrane, i.e., the power of the pump needs to be increased or the number of pumps needs to be increased. Therefore, the equipment investment and operating energy consumption cost of the system will increase.
Concentrate reflux: Concentrate reflux is to return part of the concentrate produced by the reverse osmosis system to the high-pressure pump and mix it with the influent before entering the membrane module again for reverse osmosis treatment. This is also an effective means of improving the recovery rate of the reverse osmosis system. It is particularly suitable for systems with small water production and water flow that cannot flow through a 12m long membrane module.
However, due to concentrate reflux, the concentration of pollutants at the inlet will increase, and the risk of scaling in the reverse osmosis system will further increase. Therefore, it is necessary to strengthen the operation control and management of the reverse osmosis system. If the production enterprise uses a reverse osmosis system with stable influent water quality and better than the design value, and the system's processing capacity still has spare capacity, this method can also be considered for system modification.
For example, the influent TDS of a certain chemical company's reverse osmosis system is about 40% less than the design value. In order to improve the recovery rate, it was modified to allow part of the reverse osmosis concentrate to be refluxed and mixed with the raw water in a certain proportion before reverse osmosis treatment. In actual operation, by strictly controlling parameters such as influent salinity, system recovery rate, and operating temperature, the discharge of concentrate was greatly reduced while ensuring stable operation.
Some factories have lower salinity in their reverse osmosis concentrate and can be partially refluxed and partially reused for backwashing of filters, which can greatly reduce the amount of concentrate discharged and reduce production costs.
Based on the principles of reverse osmosis and numerous practical operating cases at home and abroad, even if the reverse osmosis system is reasonably designed to achieve the best recovery rate, the proportion of concentrated water produced by the system must still account for at least 25% of the influent. For large water users in industries such as steel and chemicals, the amount of concentrated water produced per hour can reach hundreds of tons. If it is discharged as wastewater after treatment, it will waste a lot of energy and water resources. Therefore, finding a suitable reuse path for concentrated water and replacing some new water with wastewater has important practical significance and comprehensive environmental benefits.
From some domestic and foreign engineering cases, there are many ways to reuse reverse osmosis concentrated water. Most of them need to be based on the specific production characteristics of the enterprise. Concentrated water can be reused in other suitable workshops within the enterprise. It can also replace the tap water originally used for factory areas such as washing and cleaning.
In recent years, in response to the national call for energy conservation and emission reduction and the construction of a circular economy society, reverse osmosis concentrated water reuse projects have blossomed all over the country, and there are many engineering examples to learn from.
A certain factory uses the pressure of reverse osmosis concentrated water to store it in a high-level water tank, and then uses the water for condenser flushing, ground flushing, cleaning the outer walls of wine barrels, and cleaning the workshop as needed. In three months, it saved 25,000 yuan in water fees and 58,000 yuan in coal fees, and the investment cost can be recovered in less than a year.
A certain steel plant uses reverse osmosis technology to treat cooling water and domestic sewage in the steel production process into desalted water, and considers using concentrated water to backwash the multi-media filter in the pretreatment system of reverse osmosis. The enterprise invests 50,000 yuan at a time, and can save 500,000 cubic meters of water annually, with significant economic benefits.
A certain thermal power plant uses reverse osmosis concentrated water for boiler ash removal, which improves the sedimentation effect of ash and reduces the supplementary water volume of circulating water by 110t/h and the amount of chemicals added by 10%.
A certain brewery uses the reverse osmosis concentrated water that originally needed to be treated and discharged as water for the water film dust remover of the boiler, which can save 90,000 tons of water annually, saving a total of 200,000 yuan/year in water fees and sewage discharge fees.
A certain group company in China recycles and utilizes the concentrated water discharged from the reverse osmosis and pre-stage ultrafiltration equipment in pure water production as supplementary water for the dilution spray of the exhaust gas washing tower in the process workshop. After the transformation, the system saves 25m3/h of water, saving 210,000 tons of tap water annually, and the investment can be recovered in half a year, with very significant energy-saving and emission-reduction benefits.
In short, a large number of engineering examples tell us that fully considering the company's own situation and digesting reverse osmosis concentrated water within the company is an effective way to treat reverse osmosis concentrated water and rationally utilize water resources.
The selection of water treatment technology is the key to the success or failure of water treatment projects. Whether the treatment technology is reasonable directly affects the treatment effect, operational stability, investment, operating cost, and management level of the water treatment facilities. Therefore, the selection of water treatment technology should first be combined with the actual situation of the factory, comprehensively consider various influencing factors within the factory, carefully select the water treatment technology suitable for the factory, so as to achieve the best treatment effect and the best economic, social, and environmental benefits of the water treatment facilities.
According to the actual situation of the factory, the selection of the process flow should meet the treatment requirements while adapting to the actual needs of the factory. Currently, the wastewater discharged from the factory is primary concentrated water, which belongs to scaling water quality; due to 4 times concentration, it has high salt content, high hardness, stable water quality, and basically no significant fluctuations in pH, which belongs to high mineralization brine. Based on the existing operating management experience of the RO process, it is determined that the primary RO concentrated water recovery process is still the RO process, that is, the concentrated water reverse osmosis process treats the primary reverse osmosis discharge concentrated water scheme.
The raw water temperature of this project fluctuates greatly with the seasons. To ensure the long-term stable operation of the concentrated water treatment reverse osmosis device, a raw water heating system—plate heat exchanger—is installed before the reverse osmosis device. Heat source: low-pressure steam.
The main process adopts concentrated water primary reverse osmosis + concentrated water reverse osmosis + three-effect evaporation process (separate design).
