Technical Path

Reverse osmosis membrane separation technology has many advantages, including 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, etc. In recent years, it has been widely used in many industries. However, the general design water production rate of current reverse osmosis technology is 75%, and the actual water production rate is even lower, about 30% brine will be 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, the concentrate produced by many reverse osmosis processes is discharged directly without treatment, resulting in waste of water resources and energy, and pollution of the surrounding environment.

For reverse osmosis concentrate, current research mainly focuses on three goals: reduction—optimizing reverse osmosis process design to reduce concentrate production; harmlessness—exploring economical and 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 goals are not isolated but need to be considered comprehensively and complement each other.

※ Separate Treatment and Discharge

The main problems of reverse osmosis concentrate are high calcium and magnesium ion content and high hardness. Generally, simple softening treatment can achieve compliance discharge. 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, which are removed from the water body, reducing the hardness of the concentrate and reducing its harm to the environment.

The following are the chemical reaction equations:

※ 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 wastewater, etc. It is often not economical to treat each type of wastewater separately. Therefore, most enterprises choose to separate some special wastewater and treat various wastewaters 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 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 recovery rate, that is, to reduce the production of concentrate. 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.

While the water flows through the reverse osmosis membrane element, fresh water continuously passes through the membrane, achieving separation of concentrate and fresh water. Theoretically, the longer the water flows through the membrane element, the greater the fresh water 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, that is, 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, that is, 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 to improve 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 pollutant concentration 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 room, 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 filter backwashing modification, which can greatly reduce the amount of concentrate discharged and reduce production costs.

根据反渗透的原理和国内外众多实际运行案例，即使反渗透系统设计合理，使得回收率达到最佳，系统产生的浓水比例也得至少占到进水量的25%左右。对于钢铁、化工等行业的用水大户来说，每小时产生的浓水量可以达到上百吨。若作为废水处理后排放，会浪费大量的能源和水资源。所以找到合适的浓水回用途径，实现废水替代部分新水具有重要的现实意义和环境综合效益。

从国内外一些工程案例来看，反渗透浓水的回用的方式有很多。大多都需要根据企业自身的生产特点。浓水可以回用于企业内其他合适的车间。也可以代替原先使用的自来水，用作厂区如冲洗、清扫等。

近几年，为响应国家节能减排，构建循环经济社会的号召，反渗透浓水回用项目在国内遍地开花，有大量的工程实例可供借鉴。

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某钢厂用反渗透技术将钢铁生产过程中的冷却水、生活废水等处理成脱盐水，并考虑将浓水用来反冲洗反渗透预处理系统中的多介质过滤器。企业一次投资5万元，每年可节约50万m3用水，经济效益显著。

某热电站将反渗透浓水用于锅炉冲灰，使灰渣得到了更好的沉淀效果，并且使循环水补水量减少了110t/h，加药量降低了10%。

某啤酒厂，将原本需要处理后排放的反渗透浓水作为锅炉的水膜除尘器用水，可以实现年节约用水9万吨，节省水费和排污费共20万元/年。

国内某集团公司，将纯水生产中反渗透和前级超滤设备排放的浓水回收利用，作为工艺厂房排气洗涤塔的稀释喷淋水补水。经过改造后，系统节水25m3/h，每年可节约21万吨自来水，半年即可收回投资，节能减排效益非常可观。

总之，大量的工程实例告诉我们，充分考虑企业自身状况，使反渗透浓水在企业内部进行消化，是一种有效处理反渗透浓水，合理利用水资源的手段。

水处理工艺的选择是水处理处理工程成败的关键，处理工艺是否合理直接关系到水处理设施的处理效果、运转的稳定性、投资、运行成本和管理水平等。因此水处理工艺的选择首先应结合工厂的实际情况，综合考虑厂内各种作用因素，慎重选择适合本厂的水处理工艺，以达到水处理设施的最佳处理效果及最好的经济、社会和环境效益。

根据工厂的实际情况，选择工艺流程应满足处理要求的同时适应工厂的实际需要，目前工厂内排放废水为一级浓水，一级浓水属于结垢型水质；由于4倍浓缩的缘故，其含盐量高、硬度高、水质稳定、基本不发生pH的显著波动，属于高矿化度咸水。根据现有RO工艺的运行管理经验，确定一级RO浓水回收工艺仍为RO工艺，即浓水反渗透工艺处理一级反渗透排放浓水方案。

本项目原水水温随季节性波动较大，为保证浓水处理反渗透装置长期稳定运行，反渗透装置前设置原水加热系统—板式换热器。热源：低压蒸汽。

主体工艺采用浓水一级反渗透+浓水反渗透+三效蒸发工艺（单独设计）。