November 15, 2022     Posted by :

The wastewater is the drainage of the primary reverse osmosis desalination unit – concentrated brine. The RO process produces concentrated water, which contains various organic and inorganic pollutants that may pollute the soil, surface water, and ocean if discharged directly; if discharged into the municipal wastewater treatment system, the excessive total dissolved solids are also very detrimental to the growth of activated sludge.

The highly concentrated RO concentrated water and the chemicals introduced by the cleaning agent and scale inhibitor are also bound to have adverse effects when discharged directly into the environment.

Therefore, it is of great importance to find a cost-effective RO concentrated water treatment method to protect the environment.

Overview

Reverse osmosis membrane separation technology, 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, easy operation, and management, etc., widely used in many industries in recent years.

However, the current design yield of reverse osmosis technology is generally 75%, and the actual yield is even lower, producing about 30% of concentrated brine.

If the raw water is very poor quality brackish groundwater or seawater, the amount of concentrated water produced will be even greater, possibly up to 50%.

Currently, many reverse osmosis processes produce concentrated water that is discharged directly without treatment, resulting in a waste of water and energy, as well as pollution of the surrounding environment.

For reverse osmosis concentrated water, the current research mainly focuses on three purposes:

Reduction – optimize the design of the reverse osmosis process to reduce the amount of concentrated water;

Harmless – to explore economic and effective treatment methods to reduce the harm caused by the direct discharge of reverse osmosis concentrated water to the environment.

Harmless – in view of the direct discharge of reverse osmosis concentrated water may cause harm to the environment, to explore effective treatment means to reduce the harm;

Resource – to explore ways to reuse the reverse osmosis concentrated water, turning waste into treasure.

In fact, the reuse of reverse osmosis concentrated water needs to consider a variety of factors, these three purposes are not isolated but need to be integrated, and complementary to each other.

For the purpose of discharge

※ Discharge by separate treatment

The main problem of reverse osmosis concentrated water is high calcium and magnesium plasma content and high hardness. Generally speaking, after a simple softening treatment, it can achieve the standard discharge.

Softening mainly adopts the method of adding alkaline substances such as lime and soda ash, using them to react with calcium and magnesium in the concentrated water to generate carbonate precipitation, which is removed from the water body to reduce the hardness of concentrated water and reduces its harm to the environment.

The following is the chemical reaction equation

Discharge by separate treatment

Mixing with other wastewater for joint treatment

For most manufacturing companies, in addition to the reverse osmosis concentrated water produced by the water production plant, various other wastewaters are generated. For example, production wastewater discharged from the production plant and domestic wastewater from the plant.

It is often not economical to treat each type of wastewater separately. Therefore, most companies choose to divert some special wastewater and mix all kinds of wastewater together for treatment.

The water quality condition of reverse osmosis concentrated water is high hardness and high salt content, while important pollution indexes like turbidity and COD are low.

Mixing reverse osmosis concentrated water into other wastewater can play a certain role in dilution adjustment, and then reduce the pollution load of the inlet water of the mixed wastewater treatment system.

And some enterprises contain a lot of alkaline substances such as sodium carbonate and sodium hydroxide in production wastewater, which can react with calcium and magnesium in reverse osmosis concentrated water to generate hydroxide or carbonate precipitation and reduce water hardness.

For example, a cogeneration plant of the aluminum industry has modified its reverse osmosis concentrated water treatment system.

The waste alkaline water produced in the production process was mixed with the reverse osmosis concentrated water to remove calcium and magnesium hardness and then the concentrated water was reused for production, reducing production water costs by about US280,000/year.

Aiming at a reduction

The reduction is for the reverse osmosis system itself. If the reverse osmosis system is designed properly and the feed water quality condition of the enterprise is stable, the amount of concentrated water produced by the system can be controlled in an optimal ratio.

When designing a reverse osmosis machine, there are two ways to improve the recovery rate of the system, i.e., to reduce the production of concentrated water. One is to increase the length of water flow through the reverse osmosis membrane module, and the other is to concentrate water return.

Increase the length of water flow through the reverse osmosis membrane element.

As the water flows through the reverse osmosis membrane element, freshwater continuously passes through the membrane, achieving the separation of concentrated water and fresh water. Theoretically, the longer the membrane element through which water flows, the greater the yield of fresh water and the higher the recovery rate.

Due to convenience and standardization issues, the length specifications of various membrane elements in the market have been determined, but they can be connected in series to form membrane modules according to the process requirements.

And due to the decreasing flow and pressure, the membrane modules cannot be too long and need to be segmented, i.e., multiple membrane modules in series.

Therefore, the number of segments can be appropriately increased in the reverse osmosis design in order to reduce the amount of concentrated water generated and increase the system recovery rate, while ensuring the effluent quality and system stability.

However, the relative 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 machine input and energy cost of the system operation will increase.

Concentrated water return

Concentrated water return

Concentrated water reflux is to return part of the concentrated water produced by the reverse osmosis machine to the front of the high-pressure pump, mix it with the feed water, and then enter the membrane module again for reverse osmosis treatment.

This is also an effective means to improve the recovery rate of reverse osmosis machines. It is especially suitable for systems with low water production, where the water flow cannot pass through the 12m long membrane module.

However, as the concentration of contaminants in the feed water increases due to the return flow of concentrated water, the risk of reverse osmosis machine scaling increases further, and therefore, the operational control and management of the reverse osmosis machine must be strengthened.

If the feed water quality of the reverse osmosis machine used by the production company is stable and better than the design value, and the processing capacity of the machine still has spare capacity, the method can also be considered for the system modification.

For example, the feed water TDS of a chemical company’s reverse osmosis machine was about 40% smaller than the design value. In order to improve the recovery rate, it was modified so that part of the reverse osmosis concentrated water was returned and mixed with the raw water in proportion before reverse osmosis treatment.

In actual operation, through strict control of feed water salt content, system recovery rate, operating temperature, and other parameters, the discharge of concentrated water is greatly reduced while ensuring stable operation.

There are also some plants with low salt content of reverse osmosis concentrated water, which can be partially reused for filter backwashing, which can greatly reduce the discharge of concentrated water and lower production costs.

Reuse as the purpose

According to the principle of reverse osmosis and many actual operation cases at home and abroad, even if the reverse osmosis machine is reasonably designed to make the recovery rate reach the best, the proportion of concentrated water produced by the system must account for at least about 25% of the incoming water volume.

For large water users in industries such as the steel and chemical industries, the concentrated water produced per hour can reach hundreds of tons. If discharged as wastewater, it would waste a lot of energy and water resources.

Therefore, it is of great practical significance and comprehensive environmental benefits to find a suitable way to reuse concentrated water and to achieve partial replacement of new water by wastewater.

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 the enterprise’s own production characteristics.

The concentrated water can be reused in other suitable workshops in the enterprise. It can also replace the originally used tap water and be used for plant areas such as flushing, cleaning, etc.

In recent years, in response to the national call for energy conservation and emission reduction, and to build a circular economy, reverse osmosis concentrated water reuse projects have blossomed in China, and there are a large number of engineering examples to learn from.

A factory uses the pressure of reverse osmosis concentrated water to store it in a high-level water tank and then uses the water for condensation flushing, floor flushing, barrel exterior cleaning, and workshop cleaning as needed.

Three months to achieve savings of US$4000.00 in water costs, coal savings of $9500.00, and less than a year to recover the investment costs.

A steel plant uses reverse osmosis technology to treat cooling water and domestic wastewater from the steel production process into desalinated water and considers the concentrated water for backwashing the multi-media filter in the reverse osmosis pretreatment system.

The enterprise invests $8000.00 at a time and can save 500,000 m3 of water per year, with significant economic benefits.

A thermal power station used reverse osmosis concentrated water for boiler ash flushing so that the ash slag got a better precipitation effect, the circulating water makeup water was reduced by 110t/h, and the dosage was reduced by 10%.

In a brewery, the reverse osmosis concentrated water, which originally needed to be discharged after treatment, was used as water for the boiler’s water membrane dust collector, which could achieve an annual water saving of 90,000 tons and save a total of $35,000/year in water and sewage charges.

In a domestic group company, the concentrated water discharged from reverse osmosis and pre-stage ultrafiltration equipment in pure water production was recycled and used as dilution spray water make-up water for the exhaust scrubber tower of the process plant.

After the transformation, the system saves 25m3/h of water, which can save 210,000 tons of tap water per year, and the investment can be recovered in half a year, so the energy saving and emission reduction benefits are very considerable.

In conclusion, a large number of engineering examples tell us that it is an effective means to deal with reverse osmosis concentrated water and make reasonable use of water resources by fully considering the enterprise’s own condition and making the reverse osmosis concentrated water digested within the enterprise.

Reuse as the purpose

The choice of the water treatment process is the key to the success or failure of the water treatment project, whether the treatment process is reasonable directly related to the treatment effect of water treatment facilities, the stability of operation, investment, operating costs, and management level.

Therefore, the choice of water treatment process should first be combined with the actual situation of the plant, taking into account the various factors at play in the plant, carefully select the water treatment process suitable for the plant, in order to achieve the best treatment effect of water treatment facilities and the best economic, social and environmental benefits.

According to the actual situation of the plant, the process should be selected to meet the treatment requirements while adapting to the actual needs of the plant. At present, the discharged wastewater in the plant is first-class concentrated water, and the first-class concentrated water belongs to the scaling type water quality;

Due to the 4 times concentration, it has high salt content, high hardness, stable water quality, basically no significant fluctuation of pH, and belongs to high mineralization brackish 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 the RO process, i.e. concentrated water reverse osmosis process to treat the primary reverse osmosis discharge concentrated water program.

The raw water temperature of this project fluctuates greatly with seasonality, in order to ensure the long-term stable operation of the concentrated water treatment reverse osmosis unit, the raw water heating system – plate heat exchanger is set up in front of the reverse osmosis unit. Heat source: low-pressure steam.

The main process uses concentrated water primary reverse osmosis + concentrated water reverse osmosis + three-effect evaporation process (separate design).




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