High salt wastewater in the realization of "zero discharge" path



01 What is high salt wastewater?

Source and water quality characteristics of high salt wastewater

In China, there are three main sources of high-salt wastewater:

1. Concentrated brine produced in the process of seawater desalination

There are two main ways to deal with high-salt wastewater generated by seawater desalination: one is to use waste recycling to generate economic benefits and achieve true "zero emission"; The second is to directly discharge high-salt wastewater into sewage treatment systems, rivers, lakes or oceans.

However, due to the lack of technical and economic costs in most coastal areas, the second treatment method is generally chosen for production.

2. High salt wastewater directly discharged in the industrial production process

Generally speaking, inorganic salts in high-salt wastewater mainly come from production wastewater and domestic sewage (potassium ions, calcium ions, sodium ions, chloride ions, sulfate ions, etc.), and some of the organic substances it contains are mainly glycerol and low carbon chain compounds.

It is worth mentioning that most industrial wastewater in addition to containing the above potassium sodium calcium and other inorganic salt ions, the inorganic salt ions contained in industrial wastewater in different fields are very different, and even some high-salt wastewater also contains some heavy metal elements.

3. Salt water generated by recycling of industrial production wastewater

Such as iron and steel enterprises, coal chemical industry, petroleum and other large displacement of industrial industries, in order to save energy and reduce emissions, in the production process need to recover most of the water reuse, in the reuse process will also have a certain concentration of salt water generated.

If this part of concentrated brine is not treated and then discharged, it will cause great environmental pollution. After treatment, different industrial wastewater will produce high content of wastewater, such as calcium, magnesium, potassium, sodium, chloride ions, carbonate ions and so on.

02 What are the treatment methods of high salt wastewater?

Traditional biological treatment methods are difficult to play

At present, there are dozens of treatment methods for high-salt wastewater, including thermal method, membrane method, ion exchange method, hydrate method, solvent extraction method and freezing method.

Thermal and membrane desalination technologies are the main technologies used in large-scale industrial applications at present.

The thermal method can be divided into multi-stage flash distillation (MSF), multi-effect evaporation (MED) and pressure vapor distillation (VC). In the 1990s, the seawater desalination technology was mainly multi-stage flash evaporation, especially in the Middle East countries, but MSF was greatly challenged by multi-effect evaporation and membrane technology in the later period [3].

Membrane desalination technology represented by RO technology, because it does not require a lot of heat energy, it is suitable for large, medium and small-scale brine desalination.

For the zero discharge treatment of high salt wastewater, direct evaporation and crystallization can achieve the purpose of zero discharge, but it costs a lot of energy and wastes resources.

The use of membrane technology can further concentrate high-salt wastewater into ultra-high salt wastewater, and the freshwater part can be directly reused. The concentrated ultra-high salt wastewater can be evaporated and crystallized to achieve zero discharge, which greatly reduces energy consumption and rationally utilizes part of water resources.

However, membrane technology has certain requirements for the quality of incoming water. Therefore, the high-salt wastewater must be pre-treated (agent softening, filtration, ion exchange, etc.), so that the membrane pollution can be effectively reduced, the service life of the membrane, and the effluent quality can be improved.

03 Key technologies for zero discharge of high salt wastewater

3 stages: pretreatment, film treatment, evaporation crystallization

Combined with the above analysis, the key technologies of zero discharge of high-salt wastewater can be divided into three stages: pretreatment stage, membrane treatment stage and evaporation stage.

1. Pretreatment

Hardness is divided into total hardness, temporary hardness and permanent hardness.

The total hardness refers to the total amount of Ca2+ and Mg2+ in the water.

Temporary hardness is also known as carbonate hardness, the main chemical composition is Ca(HCO3)2, Mg(HCO3)2. Because the salts decompose into precipitates after heating from water, it is called temporary hardness.

Permanent hardness, also known as non-carbonate hardness, mainly refers to CaSO4, MgSO4, CaCl2, MgCl2, Ca(NO3)2, Mg(NO3)2 and other salts in water. This hardness cannot be removed by heating, so it is called permanent hardness.

Hardness is an important indicator of water quality, removing the hardness of water is called the softening of water. At present, water softening mainly includes precipitation softening method, enhanced crystallization technology, adsorption and ion exchange method.

Chemical softening

Chemical softening method mainly includes traditional chemical softening method and biodegradable urea carbonate precipitation method.

The traditional chemical softening method is divided into lime softening method, lime - gypsum softening method and lime - soda ash softening method. The disadvantage of this kind of method is that it may cause secondary pollution, and the cost of the drug is higher, and the cost will be increased.

Biodegradation urea carbonate precipitation method is mainly the use of biological enzymes to decompose urea and a series of biochemical reactions after the formation of carbonate precipitation, and then through filtration removal.

The disadvantage of this method is that the concentration of ammonium ions generated in the reaction process is high, and the subsequent treatment cost is also increased.

Enhanced crystallization technique

The use of fluidized bed to remove the hardness of water first began in the 1990s, the basic principle of fluidized bed is to use gas or liquid to make solid particles in a state of suspension. A researcher used aeration into sewage to increase the pH value of sewage to strengthen crystallization, and the results showed that the removal rates of phosphate, Mg2+ and Ca2+ reached 65%, 51% and 34%, respectively.

Nowadays, solid particles such as granular calcite (CaCO3) and quartz sand are mainly added in fluidized bed reactors, which has the advantage of not only effectively removing calcium and magnesium ions, but also recycling the resulting sediments containing calcium and magnesium.

Adsorption and ion exchange method

The ion exchange dehardening method is mainly used to remove Mg2+ and Ca2+ from the water in full or in part before membrane treatment.

Since the 20th century, the study of low-cost and renewable adsorbents has been the focus of adsorption and ion exchange research.

Some people abroad have used alginate to adsorb Mg2+ and Ca2+ ions in water and achieved good results, and it has been popularized. This non-toxic polysaccharide alginate is extracted from brown algae.

At the same time, some people also use chemically modified sugarcane honey and mercerized cellulose to remove Mg2+ and Ca2+ in water, and the removal effect is also significant.

Ion exchange resin is another material to remove hardness, it is a polymer with a corresponding functional group. The raw water is passed into the ion exchange resin adsorption column, and the Mg2+ and Ca2+ in the water will exchange with the cation on the resin to achieve the purpose of removing the hardness of the water.

At present, scholars are developing multiple types of resins. Among them, the United States Orica Watercare company developed a magnetic weak acid cation exchange resin, which is used to remove the hardness effect is very good.

2. Membrane technology

In the 1980s, reverse osmosis, ion exchange, microfiltration, ultrafiltration, nanofiltration and other membranes gradually entered the stage of promotion and application. The emergence and application of membrane technology has comprehensively improved the technology of water treatment.

So far, with the comprehensive development of membrane technology, many new technologies have been derived. Among them, the new polyvinylidene fluoride (PVDF) hollow fiber hydrophobic membrane can achieve 99.9% desalination efficiency, and the effluent COD can be guaranteed between 30 and 40mg/L.

Similarly, a new membrane separation technology - vacuum membrane distillation, which is used in high-concentration solution reconcentration, removal of Mg2+, Ca2+ and other aspects.

The advanced treatment technologies for low hardness water mainly include RO/ electro-deionization (EDI), reverse electrodialysis (EDR), electrodialysis (ED) and reverse electrodeionization (EDIR).

It is worth mentioning that RO/ electric deionization (EDI) (also known as filled bed electrodialysis) soft water technology refers to the water treatment process of removing calcium and magnesium ions in water under the action of external direct current electric field, which has the characteristics of deep removal of hard, continuous water production, and no regeneration agents.

Nanofiltration (NF), ultrafiltration (UF), Microfiltration (MF)

Because the nanofiltration operation interval is between ultrafiltration membrane and reverse osmosis, it can trap nano-scale (0.001 micron) substances, so it is called "nanofiltration". Its molecular weight of organic matter is about 200-800MW, the ability to retain dissolved salts is between 20%-98%, the removal rate of soluble univalent ions is lower than that of high-priced ions, nanofiltration is generally used to remove organic matter and pigment in surface water, hardness and radium in groundwater, and partially remove dissolved salts. Extraction and concentration of useful substances in food and pharmaceutical production.

The advantage is that the operating pressure is low and the passing amount is large. Nanofiltration technology has obvious advantages and unique energy-saving effects in the desalt purification of organic matter and water softening.

Ultrafiltration can intercept substances greater than 0.01 micron, allowing small molecular substances and dissolved solids (inorganic salts) to pass, remove macromolecular organic matter, colloids, proteins and microorganisms, etc. Ultrafiltration is the use of ultrafiltration membrane microporous processing, mainly used in drinking water, industrial wastewater treatment and high purity water preparation. Microfiltration also uses the hoof of the microfiltration membrane to trap viruses and particles between 0.1-1μm under pressure.

Microfiltration can trap particles greater than 0.1-1 microns, allowing macromolecules and dissolved solids (inorganic salts) to pass, but will trap suspended solids, bacteria, and large molecular weight colloids and other substances. The operating pressure of the microfiltration membrane is generally 0.3-7bar.

The separation mechanism of microfiltration membrane is mainly sieve interception, which has the advantages of low operating pressure and high membrane flux, but the general microfiltration membrane is easy to be polluted and has a low service life.

Ultrafiltration is used in medicine, chemical industry, water treatment and other fields. Microfiltration is mostly used for water supply pretreatment, and is also used in medicine, chemical industry, electronics and other fields. Ultrafiltration and microfiltration are also used in the treatment of high-salt wastewater, but are generally used as pretreatment.

Reverse osmosis (RO)

Reverse osmosis is also known as reverse osmosis, a membrane separation operation that uses the pressure difference as a driving force to separate the solvent from the solution.

At present, reverse osmosis technology has achieved good results in pre-desalting treatment. After reverse osmosis treatment, it can remove 99.5% of the magnesium and calcium components and 99% of the salt in the water. The load of ion exchange resin can be reduced by more than 90%, and the amount of regenerant of resin can also be reduced by 90%.

Therefore, it not only saves costs, but also benefits environmental protection. Reverse osmosis technology can also be used to remove particles, organic substances and colloids in water, which has a good effect on reducing the pollution of ion exchange resin and extending the service life.

In the case of membrane production technology is becoming more and more mature and the cost is gradually reduced, reverse osmosis also plays a great role in the treatment of high-salt wastewater. However, when the conductivity of high-salt wastewater is greater than 25000us/cm, the membrane flux will decay rapidly, and the scaling phenomenon of membrane parts is serious.

It is worth mentioning that in the reverse osmosis process with efficient crystallization technology, you can improve the amount of water treated by reverse osmosis, extend the service life of the membrane, and treat more high-salt wastewater.

Positive penetration (FO)

Because the operation principle of positive osmosis is different from that of traditional membranes, it has special advantages.

For example, the membrane device is simple in composition and easy to operate; The positive permeable membrane exerts low or even no pressure, saving energy consumption and reducing operating costs; Positive osmosis has a strong ability to separate pollutants and a high salt cutting rate. The pollution to the forward osmotic membrane is almost reversible, and the cleaning efficiency is relatively high.

Under ideal conditions, the forward permeable membrane needs to have an active layer with high retention rate, good hydrophilicity and high water flux, while the supporting layer should have the characteristics of thin thickness, low tortuous factor, high porosity and high mechanical strength. At the same time, it also needs to have strong anti-pollution ability and can be applied in many fields.

The forward osmosis membranes used in early studies were mainly reverse osmosis membranes and modified nanofiltration membranes. With the deepening of research, it is found that the concentration polarization of reverse osmosis is very large due to its thick porous supporting layer, resulting in a rapid decrease in water flux.

Membrane distillation

Membrane distillation technology is a membrane separation technology which combines distillation and membrane method.

The separation principle of vacuum membrane distillation is that one side is pumped into a vacuum state to achieve mass transfer of steam with the pressure difference at both ends, and other substances in the solution are trapped through the membrane, and the liquid is condensed after distillation to achieve separation or concentration.

The process of vacuum membrane distillation is that the operating temperature can be lower than other membrane distillation processes, and the permeability can be larger, so that it is convenient to use cheap heat sources such as geothermal, solar energy and waste heat.

In recent years, there have been more and more researches on the treatment of concentrated brine by vacuum membrane distillation.

Some scholars have studied the vacuum membrane distillation of RO seawater desalination concentrated brine by using polyethylene and polypropylene microporous membranes respectively. According to the research, the maximum retention rate of the membrane can be as high as 99.999%, so the concentration of RO desalination brine can be realized efficiently through this technology.

This technology uses the pressure difference on both sides of the membrane to generate the driving force, and has the advantages of low mass transfer resistance, high heat utilization efficiency, high separation efficiency, large membrane flux, and no evaporation of permeants. But at the same time, this process also has scaling problems and membrane pollution problems when dealing with concentrated brine.

3. Final evaporation technology

The discharge of brine with higher concentration will have adverse effects on the environment. There are two main reasons for this effect. First, the brine concentration is higher; Second, the composition of salt water is more.

On the one hand, the desired effect of evaporation technology is to compress the volume of higher concentration brine and crystallize the salt inside; On the other hand, it is the formation of a circular industrial economy, which provides the precipitated salt to the manufacturers who use it as a raw material, and realizes the purpose of "zero emission".

Natural evaporation

The principle of natural evaporation is to use sunlight to remove the water in the higher concentration of brine in the pool, so as to achieve the saturated crystallization point of the higher concentration of brine, so that the salt precipitation, this device is called "evaporation pond".

The energy of the device comes from sunlight, so the device is suitable for use in dry places with less annual rainfall and abundant solar radiation.

The facility has the following advantages, because the energy comes from sunlight, so the heat source has no life loss limit, daily maintenance is easier, the cost of handling higher concentration of salt water is also relatively low, and can resist the impact of load.

The disadvantage is that the "evaporation pond" device is a non-airtight device, and the volatile components in the concentrated brine can easily cause air pollution into the atmosphere.

At the same time, the seepage prevention works on the side and the bottom of the evaporation pond are also very important. If it is not properly treated, it will easily cause malignant pollution to rock and soil mass and underground water source.

Under normal circumstances, "evaporation pond" covers a large area, and the use of land resources in places will cause certain waste; During the operation of the "evaporation pond", it is difficult to use the fresh water resources evaporated, resulting in a certain waste.

Hot process

The thermal zero emission technology developed based on the thermal brine desalination system, due to its low energy consumption, multi-effect evaporation is one of the three most commonly used brine desalination technologies in contemporary times. Based on the foundation of this technology, the theory of multi-effect evaporation - evaporation crystallization has been derived, and has been more and more widely used.

Foreign scholars have explored the "no emission" system using evaporation crystallization. The evaporating steam is used to heat the water into the evaporator, and its efficiency is much higher than that of conventional evaporation crystallization facilities.

Multi-effect evaporation (MEE) is generally controlled in 3 to 6 effect evaporation, too little is not enough to save energy, too much temperature difference is not enough, and the system is too long and easy to cause problems. The first stage evaporator uses steam to heat, and the later evaporator successively uses the secondary steam generated by the previous evaporator as a heat source to achieve the reuse of multiple heat energy, that is, multi-effect evaporation.

Multi-effect evaporation is more serious than multi-stage flash evaporation.