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

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.