Discussion on resource recovery and utilization of reverse osmosis concentrated water

1. Basic information

The wastewater is concentrated brine discharged from the primary reverse osmosis desalination unit. Concentrated water will be produced during the preparation of pure water by reverse osmosis (RO) process, which contains various organic and inorganic pollutants. If directly discharged, it may pollute soil, surface water, ocean, etc; If discharged into the municipal sewage treatment system, too high total dissolved solids are also very unfavorable to the growth of activated sludge. Highly concentrated ro concentrated water and chemicals introduced by cleaning agent and scale inhibitor will be directly discharged into the environment, which will inevitably have adverse effects. Therefore, it is of great significance to find an economical and efficient ro concentrated water treatment method for environmental protection.

2. Technical path

2.1 overview

Reverse osmosis membrane separation technology has been widely used in many industries in recent years because it has the advantages of no phase change of materials, low relative energy consumption, good desalination effect, mature and reliable treatment process, simple equipment, high degree of automation and easy operation and management. However, at present, the general design water production rate of reverse osmosis technology is 75%, and the actual water production rate is lower, which will produce about 30% concentrated brine. If the raw water is underground brackish water or seawater with very poor water quality, the production of concentrated water will be greater, which may reach 50%. At present, the concentrated water produced by many reverse osmosis processes is directly discharged without treatment, resulting in a waste of water and energy resources and pollution to the surrounding environment.

For reverse osmosis concentrated water, the current research mainly focuses on three purposes: reduction - optimizing the reverse osmosis process design and reducing the production of concentrated water; Harmlessness - in view of the situation that the direct discharge of reverse osmosis concentrated water may cause harm to the environment, explore economic and effective treatment methods to reduce the harm; Recycling -- explore the way of reuse of reverse osmosis concentrated water and turn waste into treasure.

In fact, many factors need to be considered for the reuse of reverse osmosis concentrated water. These three purposes are not isolated, but need comprehensive consideration and complement each other.

2.2. For the purpose of emission

※ separate treatment and discharge

The main problem of reverse osmosis concentrated water is the high content of calcium and magnesium plasma and high hardness. Generally speaking, standard discharge can be achieved after simple softening treatment. Softening mainly adopts the method of adding alkaline substances such as lime and soda ash, which react with calcium, magnesium and other substances in concentrated water to generate carbonate precipitation, which is removed from the water body to reduce the hardness of concentrated water and its harm to the environment.

The following is its chemical reaction equation:

※ mixed with other wastewater for common treatment

For the vast majority of production enterprises, in addition to the reverse osmosis concentrated water produced by the water making workshop, other kinds of wastewater will also be produced. For example, the production wastewater discharged from the production workshop and the domestic wastewater in the plant area. It is often not economical to treat each kind of wastewater separately. Therefore, most enterprises choose to divert some special wastewater and treat all kinds of wastewater together after mixing.

The water quality of reverse osmosis concentrated water is high hardness and high salt content, while important pollution indicators such as turbidity and COD are very low. Mixing reverse osmosis concentrated water into other wastewater can play a certain role in dilution regulation, and then reduce the influent pollution load of the mixed wastewater treatment system.

The production wastewater of some enterprises contains a large number of alkaline substances such as sodium carbonate and sodium hydroxide, which can react with calcium and magnesium in reverse osmosis concentrated water to produce hydroxide or carbonate precipitation and reduce the hardness of water body.

For example, a thermal power plant of an aluminum industry transformed its reverse osmosis concentrated water treatment system. The waste alkali water produced in the production process is mixed with reverse osmosis concentrated water, and the concentrated water is reused for production after removing calcium and magnesium hardness, reducing the cost of production water by about 1.76 million yuan / year.

2.3. For the purpose of reduction

Reduction is aimed at the reverse osmosis system itself. If the reverse osmosis system is designed reasonably and the inlet water quality of the enterprise is stable, the production of concentrated water in the system can be controlled in an optimal proportion. In the design of reverse osmosis system, there are two methods to improve the recovery rate of the system, that is, to reduce the output of concentrated water. One is to increase the length of water flow through reverse osmosis membrane module, and the other is concentrated water reflux.

Increase the length of the reverse osmosis membrane module through which the water flows.

While the water flows in the reverse osmosis membrane element, the fresh water continuously passes through the membrane to realize the separation of concentrated water and fresh water. Theoretically, the longer the membrane element, the greater the output of fresh water and the higher the recovery rate. Due to the problems of convenience and standardization, the length specifications of various membrane components in the market have been determined, but the membrane components can be formed in series according to the process requirements. Due to the decrease of flow and pressure, the membrane module can not be too long, and it 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 production of concentrated water and improve the system recovery rate, the number of sections can be appropriately increased in the reverse osmosis design.

However, the lengthening of the membrane system needs to increase 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 energy consumption cost of equipment input and operation of the system will increase.

Concentrated water reflux:

Concentrated water reflux is to return part of the concentrated water produced by the reverse osmosis system to the front of the high-pressure pump, mix with the inlet water, and then enter the membrane module for reverse osmosis treatment. This is also an effective means to improve the recovery rate of reverse osmosis system. Especially when the water yield of the system is small and the water cannot flow through the 12m long membrane module, it is very suitable.

However, due to the reflux of concentrated water, the concentration of pollutants at the inlet will increase, and the risk of scaling in the reverse osmosis system will further increase. Therefore, the operation control and management of the reverse osmosis system must be strengthened. If the influent quality of the reverse osmosis system used by the production enterprise is stable and better than the design value, and the treatment capacity of the system still has spare power, this method can also be considered to transform the system.

For example, the inlet TDS of the reverse osmosis system of a chemical company is about 40% lower than the design value. In order to improve the recovery rate, it is reformed to make part of the reverse osmosis concentrated water reflux and mix it with the raw water in proportion before reverse osmosis treatment. In the actual operation, by strictly controlling the parameters such as influent salt content, system recovery rate and operating temperature, the discharge of concentrated water is greatly reduced while ensuring stable operation.

In some factories, the salt content of reverse osmosis concentrated water is low, and some can be recycled for backwashing of filters, which can greatly reduce the discharge of concentrated water and reduce the production cost.

2.4. For reuse

According to the principle of reverse osmosis and many practical operation 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 account for at least 25% of the water inflow. For large water users in steel, chemical and other industries, the 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, it is of great practical significance and comprehensive environmental benefits to find a suitable way for the reuse of concentrated water and realize the substitution of wastewater for some new water.

From some engineering cases at home and abroad, there are many ways to reuse reverse osmosis concentrated water. Most of them need to be based on their own production characteristics. The concentrated water can be reused in other suitable workshops in the enterprise. It can also replace the original tap water and be used in the plant, such as flushing, cleaning, etc.

In recent years, in response to the call of national energy conservation and emission reduction and building a circular economy society, reverse osmosis concentrated water reuse projects have blossomed everywhere in China, and a large number of engineering examples can be used for reference.

A factory uses the pressure of reverse osmosis concentrated water to store it in a high-level water tank, and then uses the water to condense, flush the car, flush the floor, clean the outer wall of the wine barrel and clean the workshop as needed. In three months, 25000 yuan of water and 58000 yuan of coal will be saved, and the investment cost will be recovered in less than a year.

A steel plant uses reverse osmosis technology to treat the cooling water and domestic wastewater in the process of steel production into demineralized water, and considers using concentrated water to backwash the multi-media filter in the reverse osmosis pretreatment system. With a one-time investment of 50000 yuan, the enterprise can save 500000 m3 of water every year, with remarkable economic benefits.

In a thermal power station, reverse osmosis concentrated water is used for Boiler Ash Flushing, which makes the ash and slag get better sedimentation effect, reduces the make-up water of circulating water by 110t / h and the dosage by 10%.

A brewery uses the reverse osmosis concentrated water originally discharged after treatment as the water for the water film dust collector of the boiler, which can save 90000 tons of water per year and 200000 yuan / year of water and sewage charges.

A domestic group company recycles the concentrated water discharged from reverse osmosis and pre stage ultrafiltration equipment in pure water production as dilution spray water make-up water for exhaust scrubber of process plant. After transformation, the system saves 25m3 / h of water and 210000 tons of tap water can be saved every year. The investment can be recovered in half a year, and the benefits of energy conservation and emission reduction are very considerable.

In short, a large number of engineering examples tell us that fully considering the enterprise's own situation and digesting the reverse osmosis concentrated water within the enterprise is a means to effectively treat the reverse osmosis concentrated water and make rational use of water resources.

3. Determination of process flow

The selection of water treatment process is the key to the success or failure of water treatment project. Whether the treatment process is reasonable is directly related to the treatment effect, operation stability, investment, operation cost and management level of water treatment facilities. Therefore, the selection of water treatment process should first be combined with the actual situation of the plant, comprehensively consider various functional factors in the plant, and carefully select the water treatment process suitable for the plant, so as to achieve the best treatment effect of water treatment facilities and the best economic, social and environmental benefits.

4. Idea of process selection

According to the actual situation of the plant, the selected process flow shall meet the treatment requirements and meet the actual needs of the plant. At present, the wastewater discharged from the plant is grade I 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 fluctuation of pH. it belongs to high salinity salt water. According to the operation and management experience of the existing RO process, it is determined that the primary ro concentrated water recovery process is still RO process, that is, the concentrated water reverse osmosis process is used to treat the concentrated water discharged by primary reverse osmosis.

The raw water temperature of the project fluctuates greatly with seasons. In order to ensure the long-term stable operation of the concentrated water treatment reverse osmosis device, the raw water heating system plate heat exchanger is set in front of 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 (designed separately).