CN104370394B - A kind of processing method of surface water desalter by-product brine waste - Google Patents

Abstract

The invention provides a treatment method for by-product saline wastewater of a surface water desalination device. The saline wastewater is firstly pretreated by soda ash-caustic soda-flocculation sedimentation method to remove hardness, silicon and organic matter; then use bipolar membrane electrodialyzer filled with monovalent selective ion exchange membrane for resource treatment, and the containing The salt in the salt wastewater is converted into the corresponding acid and alkali; the acid and alkali obtained by electrodialysis can be used for the regeneration and pretreatment of the ion exchange resin in the desalination device, and the light brine obtained by electrodialysis can be used as the feed water of the desalination device. The method has high treatment capacity, low treatment cost, significantly reduces water consumption of a desalination device, and is suitable for industrialization.

Description

A treatment method for by-product saline wastewater from a surface water desalination device

technical field

The invention relates to the field of waste water treatment and comprehensive utilization, in particular to a treatment method for by-product saline waste water from surface water desalination.

Background technique

With the improvement of people's living standards and the development of industrial production, the demand for fresh water resources is increasing day by day. Among them, surface water desalination has become one of the important sources of fresh water. However, no matter membrane method, thermal method or ion exchange method is used to desalinate surface water, by-product saline wastewater will be unavoidable. As we all know, the direct discharge of high-salt wastewater will not only lead to soil compaction, water salinization, damage to crops, and deterioration of the ecological environment, but also waste related inputs such as surface water desalination and treatment, and increase water production costs.

Therefore, realizing the resource utilization of saline wastewater is one of the problems to be solved urgently. By increasing the water recovery rate of reverse osmosis and the concentration ratio of electrodialysis to reduce the output of by-product high-salt wastewater, and then use the obtained high-concentration brine directly for the chlor-alkali industry or to produce salt through crystallization, but this method consumes a lot of energy And it has strict requirements on the follow-up processing, resulting in limited application.

Conventional bipolar membrane electrodialysis is usually used in the prior art for the treatment of saline wastewater. American Badruzzaman (Journal of Membrane Science 326 (2009) 392) used the conventional bipolar membrane electrodialysis method to treat the salty wastewater by-product of reverse osmosis surface water desalination. Bipolar membrane electrodialysis is used to treat and recycle high-salt wastewater; this method uses constant (electric) current operation with a small current density, and the resulting acid and alkali concentrations are less than 0.2mol/L. It is not difficult to see that the above method has the following technical defects: (1) The cost of the pretreatment process is high, and the frequent regeneration of the resin requires a higher concentration of acid and alkali, which has hidden dangers of environmental pollution; (2) The intermittent treatment process and low current density (3) The desalination rate of saline wastewater is low, which cannot be directly reused or discharged, and the obtained acid and alkali concentrations are low, and the reuse value is limited.

Contents of the invention

The invention provides a method for treating saline waste water produced by surface water desalination suitable for industrial scale production. This method overcomes the problems of high processing cost and low processing capacity in the prior art; it not only turns waste (salt) into treasure (acid, alkali), but also the treated wastewater can be returned to the desalination process, reducing the water consumption of the desalination process and comprehensive production Reduce water costs, avoid environmental pollution caused by direct discharge of high-salt wastewater, and realize resource utilization of saline wastewater. The processing method of saline waste water of the present invention is realized like this:

A method for treating by-product saline wastewater of a surface water desalination device, the method sequentially includes the following steps:

a. Pretreatment, first using soda ash and/or sodium hydroxide as a precipitating agent, using a fluidized pellet reactor to decalcify the salty wastewater; then adding a flocculant to adjust the pH value to 10-11, Flocculation and sedimentation remove the hardness, silicon and organic matter of the salty wastewater;

b. electrodialysis treatment, carrying out bipolar membrane electrodialysis treatment to the effluent of the pretreatment; the electrodialysis treatment converts the salt in the effluent water into acid and alkali, and the light brine obtained by the electrodialysis treatment returns to the The desalination device.

In specific implementation, the desalination method adopted by the desalination device is: reverse osmosis method, electrodialysis method, ion exchange method or thermal method.

In specific implementation, during the decalcification described in step a, the filler of the fluidized pellet reactor is garnet sand with a particle size of 100 μm-300 μm; the filling height of the filler in the case of no flow and The ratio of the reactor is 1:5-1:4, and the aspect ratio of the reactor is 80:1-120:1. During the flocculation and precipitation described in step a, a flocculant and a coagulant aid are first added to the effluent from the decalcification treatment, and then sodium hydroxide is added to adjust the pH value to 10.5-11; the flocculant is aluminum chloride, polymeric chlorine aluminum chloride or aluminum ferric sulfate, and the coagulant aid is cationic or nonionic polyacrylamide.

In specific implementation, in step b, the bipolar membrane electrodialysis is a three-chamber type, which is a feed liquid chamber, an acid chamber and an alkali chamber in turn; the cation exchange membrane and anion exchange membrane used are monovalent selective ion exchange membranes. The current density of the bipolar membrane electrodialysis is 10-100mA/cm ² , the liquid flow rate in the electrodialyzer is 10-100L/h, and the thickness of the separator is 0.5-1.2mm.

The present invention first uses soda ash and/or sodium hydroxide as a precipitant, and uses a fluidized pellet reactor to decalcify the salty wastewater; Organic matter existing in the water; after flocculation and precipitation, the hardness (calculated by the total amount of calcium and magnesium ions) in the treated raw water system is not greater than 150mg/L, and the silicon content (calculated by silicon dioxide) is not greater than 30mg/L; enter the bipolar There is no need to undergo any concentration operation before the membrane electrodialyzer, and the initial conductivity of the raw water is about 8-30ms/cm.

The dehardening operation of concentrated brine is aimed at the calcium ions and magnesium ions in the system. According to the ion product constant, it can be known that calcium removal relies on the addition of carbonate ions to form calcium carbonate precipitation, while magnesium removal relies on hydroxide ions. The addition of magnesium hydroxide precipitates. Therefore, if there is more calcium in the system and the total alkalinity is low (low carbonate or carbonate ions), sodium carbonate (that is, soda ash) needs to be added; and the removal of magnesium in the system depends on adding sodium hydroxide, specifically The amount of addition depends on the specific composition and content of the hardness in the raw water system and is determined according to the stoichiometric ratio.

The present invention adopts the bipolar membrane electrodialysis method to carry out resource treatment on the above-mentioned pretreated high-salt wastewater, converts salt into acid and alkali, and is used for the regeneration of ion exchange resin in the desalination section and the pretreatment of resource recovery of high-salt wastewater Sections, etc.; the light brine obtained from the treatment is directly used as water for the desalination section.

Saline wastewater does not need to undergo any concentration operation before entering the bipolar membrane electrodialyzer. Generally, its initial conductivity is about 8-30ms/cm. When the saline wastewater enters the bipolar membrane electrodialyzer, a two-way arrangement is adopted, one for opening and one for preparation. The conversion of the two channels is determined according to the ion content of the influent water of the desalination section, that is, the upper limit of the ion content of the influent water of the desalination section of the surface water is subject to, for example, 1000mg/L. The concentration of acid production and base production should be determined according to the requirements of recycling, generally not more than 1mol/L; and when the acid and base chambers after taking out part of the acid and base are replenished with pure water, the concentration of acid and base should not be lower than 0.02mol/L.

In the process route of the conventional bipolar membrane electrodialysis method, pretreatment usually adopts the traditional lime method or ion exchange method, and then uses intermittent bipolar membrane electrodialysis to recycle high-salt wastewater, using a constant (electric ) flow operation, resulting in lower concentrations of acid and base. Compared with the prior art, the saline wastewater resource treatment method of the present invention overcomes the defects of the prior art: (1) the pretreatment cost is greatly reduced, (2) the operation is simple and can be applied to large-scale production, (3) The obtained acid and alkali concentrations are directly satisfied for the regeneration of the ion exchange resin or the cleaning of the membrane, and have high utilization value. (4) The light brine generated is directly returned to the desalinated water device, realizing "zero discharge" of wastewater and avoiding pollution to the environment .

The salty wastewater resource treatment method of the present invention has low treatment cost and high treatment capacity, not only can significantly reduce the water consumption of the desalination process, but also can turn waste (salt) into treasure (acid, alkali), thereby greatly reducing the comprehensive cost of the desalination process. Reduce the cost of water production, and effectively avoid the environmental pollution caused by the direct discharge of high-salt wastewater.

Description of drawings

Fig. 1 is a process flow diagram of a method for treating by-product saline wastewater of a surface water desalination device according to the present invention.

detailed description

The technical solution of the present invention will be further described in detail below in conjunction with the examples and accompanying drawings, and the scope of protection of the present invention is not limited to the following specific embodiments.

Example 1

A company adopts ion exchange method to desalinate surface water to produce water for production. The content of heavy metal ions and suspended solids in the by-product saline wastewater is low. The typical main composition of the water sample is: Na ⁺ is 158.48±50mmol/L, Ca ²⁺ is 21.764±15mmol/L, Mg ²⁺ is 7.83±5mmol /L, SO ₄ ^2- is 2.92±2mmol/L, NO _3- is 1.22±1mmol ^/ L, ^Cl- is 222.50±70mmol/L, silicon content (calculated as SiO ₂ ) is 1.15±1mmol/L; alkalinity (HCO ₃ ^- and CO ₃ ^2- ) is 1.84±1.5mmol/L; UV ₂₅₄ is 1.857; system pH is 7.10; conductivity is 20±5ms/cm. The following methods are used to treat and recycle the saline wastewater:

The first step is preprocessing. For the water sample with the above average composition, firstly, the wastewater is pumped into the fluidized pellet reactor (the filler is garnet sand, the particle size is controlled to be 150 μm-200 μm, and the ratio of the filling height to the reactor under no flow condition is 1:5 The length-to-diameter ratio of the reactor is 100:1, and soda ash-caustic soda is added according to the stoichiometric ratio to carry out the decalcification operation; then 100 mg/L of flocculant aluminum chloride and 85 mg/L of coagulant non-ionic polyacrylamide are added to the above effluent water L, after stirring evenly, add an appropriate amount of sodium hydroxide to adjust the pH of the system to be 10.80 and let it stand for deep removal of hardness, silicon reduction and removal of residual organic matter in the water system. Get the upper strata water sample analysis and know that the total amount of Ca ²⁺ and Mg ²⁺ is 62.61mg/L, silicon content (calculated as SiO ₂ ) is 12.64mg/L, UV ₂₅₄ is 1.116.

The second step is bipolar membrane electrodialysis. Each electrodialyzer contains 60 units, and each unit includes a bipolar membrane (Fuma-Tech product of Germany), a monovalent selective cation exchange membrane (Asahi Nitrogen Membrane CSO) and a monovalent selective anion exchange membrane (Asahi Nitrogen Membrane ASV ). The membrane area is 100mm×200mm, and the separator is made of elasticized polypropylene separator with a thickness of 1mm. The liquid flow rate in the electrodialyzer is controlled at 80L/h. Bipolar membrane electrodialysis was operated in constant (electrical) current mode, and the initial current density was set at 30 mA/cm ² . When the by-product saline wastewater enters the bipolar membrane electrodialyzer, a two-way arrangement is adopted, one for opening and one for preparation. The conversion of the two channels is determined according to the highest ion content of the influent of the surface water desalination process, that is, when the ion content of the influent water of this channel is less than 1000mg/L, the channel is closed and the other channel is activated instantaneously. The concentrations of acid and base produced are determined according to the highest concentration of acid and base required by the surface water desalination process (the concentration of acid and base used for regeneration of ion exchange resin is 1mol/L). Instantaneously replenish part of the acid and alkali chambers after taking out the acid and alkali with pure water. And the resistance of the alkali chamber will not be too large, reducing energy consumption).

According to the analysis, the total amount of Na ⁺ and K ⁺ in the obtained hydrochloric acid is less than 0.02mol/L, the total amount of Ca2 ⁺ and ^Mg2+ is less than 0.2mmol/L, and the silicon content (calculated as _SiO2 ) is less than 0.1mg/L. The ion content in the treated saline wastewater is controlled to be less than 1000mg/L, and it can be directly returned to the desalinated water production section because it has undergone advanced pretreatments such as hardness removal, silicon removal, and organic matter removal before entering the bipolar membrane electrodialysis. The average current efficiency of bipolar membrane electrodialysis is 68%, and the energy consumption is 3.2kwh/kg (calculated as sodium hydroxide).

Example 2

A company uses reverse osmosis to desalinate surface water to produce water for production. The content of heavy metal ions and suspended solids in the by-product saline wastewater is low. The typical main composition of the water sample is: Na ⁺ is 65±15mmol/L, Ca ²⁺ is 2.4±1mmol/L, Mg ²⁺ is 0.5±0.3 mmol/L, SO ₄ ^2- is 3.3±2mmol/L, NO _3- is 2.9±1mmol ^/ L, ^Cl- is 72.7±20mmol/L, silicon content (calculated as SiO ₂ ) is 1.5±0.5mmol/L; Alkalinity (HCO ₃ ^- and CO ₃ ^2- ) is 4.5±1.5mmol/L; UV ₂₅₄ is 1.729; system pH is 7.10; conductivity is 10.5±2ms/cm. The following methods are used to treat and recycle the saline wastewater:

The first step is preprocessing. For the water sample with the above average composition, firstly, the waste water is pumped into the fluidized pellet reactor (the filler is garnet sand, the particle size is controlled to be 220μm-260μm, and the ratio of the filling height to the reactor in the case of no flow is 1:4 The length-to-diameter ratio of the reactor is 120:1, and soda ash-caustic soda is added according to the stoichiometric ratio to carry out the decalcification operation; then 100 mg/L of polyaluminum chloride as a flocculant and 85 mg/L of anionic polyacrylamide as a coagulant are added to the above effluent water L, then add an appropriate amount of sodium hydroxide to adjust the pH of the system to 10.65, then let it stand still, deeply remove hardness, reduce silicon, and remove residual organic matter in the water system. Take the upper water sample for analysis, and the total amount of Ca ²⁺ and Mg ²⁺ is 23.39mg /L, the silicon content (calculated as SiO ₂ ) is 5.57mg/L, and the UV ₂₅₄ is 1.077.

The second step is bipolar membrane electrodialysis. Each electrodialyzer contains 60 units, and each unit includes a bipolar membrane (product of Hebei Guangya Co., Ltd.), a monovalent selective cation exchange membrane (Asahi Nitrogen Membrane CSO) and a monovalent selective anion exchange membrane (Asahi Nitrogen Membrane ASV). The membrane area is 100mm×200mm, and the separator is made of elasticized polypropylene separator with a thickness of 0.8mm. The liquid flow rate in the electrodialyzer is controlled at 80L/h. The bipolar membrane electrodialysis was operated in constant (current) current mode, and the upper limit of the initial current density was set at 15 mA/cm ² . When the saline wastewater enters the bipolar membrane electrodialyzer, a two-way arrangement is adopted, one for opening and one for preparation. The conversion of the two channels is determined according to the highest ion content of the influent of the surface water desalination process, that is, when the ion content of one channel is less than 800 mg/L, the channel is closed and the other channel is activated instantaneously. The concentration of acid and base produced is determined according to the highest concentration of acid and base required by the desalination process (acid and base concentration for pretreatment or cleaning membrane fouling is 0.5mol/L). Instantaneously replenish part of the acid and alkali chambers after taking out the acid and alkali with pure water. At this time, the concentration of the acid chamber and alkali chamber is about 0.02mol/L.

According to the analysis, the total amount of Na ⁺ and K ⁺ in the obtained hydrochloric acid is less than 0.02mol/L, the total amount of Ca2 ⁺ and ^Mg2+ is less than 0.1mmol/L, and the silicon content (calculated as _SiO2 ) is less than 0.1mg/L. The ion content in the treated saline wastewater is controlled to be less than 800mg/L, and it can be directly returned to the desalinated water production section because it has undergone pretreatments such as hardness removal, silicon removal, and organic matter removal before entering the bipolar membrane electrodialysis. The average current efficiency of bipolar membrane electrodialysis is 81%, and the energy consumption is 2.6kwh/kg (calculated as sodium hydroxide).

Claims (4)

1. A treatment method for by-product saline waste water from a surface water desalination device, the method may further comprise the steps: a. Pretreatment, first using soda ash and/or sodium hydroxide as a precipitating agent, using a fluidized pellet reactor to decalcify the salty wastewater; then adding a flocculant to adjust the pH value to 10-11, Flocculation and sedimentation remove the hardness, silicon and organic matter of the salty wastewater; b. electrodialysis treatment, carrying out bipolar membrane electrodialysis treatment to the effluent of the pretreatment; the electrodialysis treatment converts the salt in the effluent water into acid and alkali, and the light brine obtained by the electrodialysis treatment returns to the The desalination device; In step b, the bipolar membrane electrodialysis is a three-chamber type, followed by a feed liquid chamber, an acid chamber and an alkali chamber; the cation exchange membrane and the anion exchange membrane used are monovalent selective ion exchange membranes; the bipolar membrane The current density of the electrodialysis is 10-100mA/cm ² , the liquid flow rate in the electrodialyzer is 10-100L/h, and the thickness of the separator is 0.5-1.2mm. 2. The processing method according to claim 1, characterized in that: The desalination method adopted by the desalination device is: reverse osmosis method, electrodialysis method, ion exchange method or thermal method. 3. The processing method according to claim 1, characterized in that: During the decalcification described in step a, the filler of the fluidized pellet reactor is garnet sand with a particle size of 100 μm-300 μm; the ratio of the filling height of the filler to the reactor under no-flow conditions is 1:5-1:4, the aspect ratio of the reactor is 80:1-120:1. 4. The processing method according to claim 1, characterized in that: During the flocculation and precipitation described in step a, a flocculant and a coagulant aid are first added to the effluent from the decalcification treatment, and then sodium hydroxide is added to adjust the pH value to 10.5-11; the flocculant is aluminum chloride, polymeric chlorine aluminum chloride or aluminum ferric sulfate, and the coagulant aid is cationic or nonionic polyacrylamide.