A method for removing hardness from wastewater

Abstract

The present application relates to a kind of wastewater hardness removal methods, comprising: 1) the pH of sewage is adjusted to about 6.0-6.5 by adding acid, flocculant A is added in the wastewater to be treated to carry out reaction, after being stirred sufficiently, it is placed;2) after settling, sodium carbonate is added in supernatant and pH is adjusted to 8.8-10.5 by alkali, flocculant B is added, after being stirred sufficiently, flocculant C is added, stirring, standing, carry out secondary hardness removal reaction.The present application method treatment effect is good, reagent dosage is low, and on-site operation is stable, and after treatment, it can meet the requirements of subsequent wastewater reuse and double membrane treatment to hardness.

Description

TECHNICAL FIELD

[0001] The present application relates to a wastewater treatment method, in particular a method for removing hardness from circulating water blowdown water. BACKGROUND

[0002] Petrochemical enterprises are large water users, and it is the trend for sustainable development of enterprises to achieve zero discharge of wastewater as much as possible. With the in-depth development of water saving and emission reduction, the concentration multiple of circulating water is getting higher and higher, and generally reaches 5-6 times. The circulating water blowdown water after repeated multiple cooling tower circulation and concentration has the following characteristics: high electrical conductivity, high turbidity, high hardness, high alkalinity (except for circulating water treated by acid addition formula), high scale inhibitor and dispersant content, high total phosphorus content, and relatively high COD value and pH value. The treatment technology for removing calcium and total hardness of such water is one of the difficulties in achieving near zero discharge of wastewater.

[0003] The main methods for removing hardness from water are: chemical precipitation, ion exchange, membrane technology (nanofiltration or ultrafiltration + reverse osmosis membrane), electrodialysis and electrolytic removal of hardness, etc. Among them, the ion exchange method is mature in technology, but it will produce a large amount of waste acid and alkaline and high conductivity neutralization backwash water; membrane technology has strict requirements on the quality of the incoming water, the membrane is easy to be polluted, and the treatment cost is high, in addition, a large amount of multi-medium filtration backwash water, nanofiltration backwash water, UF backwash water, RO cleaning water and RO concentrated water will be produced; the energy consumption of electrodialysis technology is high, and it will produce high conductivity concentrated water, which is suitable for the removal of hardness of low hardness or drinking water; the treatment effect of electrolytic removal of hardness is unstable, and the electrode plate is easy to scale and hydrogen evolution reaction; while chemical precipitation is the most classic and cheapest method for removing hardness, which is suitable for the removal of hardness of almost all industrial wastewater, and its disadvantage is that the dosage of reagent is relatively large.

[0004] The principle of chemical removal of hardness is to adjust the pH value of the wastewater and add a certain amount of sodium carbonate to make the scale-forming ions (calcium ions and magnesium ions) in the water react with carbonate ions and hydroxide ions to precipitate calcium carbonate and magnesium hydroxide, thereby achieving the purpose of removing hardness.

[0005] Ca 2+ +CO3 2- →CaCO3

[0006] Mg 2+ +CO3 2- →MgCO3

[0007] Ca 2+ +OH - +HCO3 - →CaCO3

[0008] Mg 2+ +OH - →Mg(OH)2

[0009] With the general implementation of sewage treatment plant sewage pollution, the mainstream treatment method of circulating water sewage (including double membrane backwash water) is: high density sedimentation to remove turbidity, hardness and phosphorus + ozone BAF + double membrane (ultrafiltration + reverse osmosis membrane) treatment, double membrane effluent is used for circulating water makeup or as chemical water system ion exchange desalination water, and the boundary conditions of hardness removal are determined according to the requirements of each enterprise sewage into double membrane, generally requiring total hardness <80-150 mg / L.

[0010] The hardness removal of circulating water sewage is difficult, and the consumption of chemicals is large. When treating conventional wastewater, the pH value of high-density sedimentation tank for hardness removal is generally controlled at about 10.5-11.3. When treating circulating water sewage, the pH value of high-density sedimentation tank is often controlled at 11.0-11.3. When the operation is unstable or the circulating water is started after pre-membrane (the composition of pre-membrane agent is high-concentration organic phosphorus corrosion and scale inhibitor), the pH value of high-density sedimentation tank is adjusted to 11.0-11.3, and the sedimentation still does not occur, which causes huge consumption of chemicals and brings great trouble to on-site operation and management.

[0011] CN110590040A discloses a wastewater treatment method and system, which adopts double-alkali hardness removal + nanofiltration membrane + reverse osmosis membrane. Calcium hydroxide and sodium carbonate are added for hardness removal in double-alkali hardness removal, the dosage of calcium hydroxide is 500-800 mg / L, and the dosage of sodium carbonate is 600-900 mg / L. The dosages are large, and the effluent needs to be treated by nanofiltration membrane before entering the reverse osmosis membrane.

[0012] CN113087197A discloses a circulating water sewage treatment and recycling method, which includes coagulation sedimentation + resin hardness removal + nanofiltration + reverse osmosis membrane. Hardness removal is mainly achieved by ion exchange resin and nanofiltration membrane. The resin hardness removal produces a large amount of acid-base neutralization wastewater, and the nanofiltration membrane can remove almost all divalent anions and cations such as Ca 2+ , Mg 2+ , CO3 2- , SO4 2- , etc. The disadvantages are that the membrane is easily contaminated, the cleaning also produces a large amount of acid-base wastewater, and the treatment cost is much higher than that of chemical hardness removal.

[0013] CN112079518A discloses a circulating water sewage treatment method, which adopts hardness removal filtration + ozone oxidation + denitrification treatment + ultrafiltration + reverse osmosis series of deep desalination treatment, and the reverse osmosis product water is used for circulating water makeup. The conventional double-alkali hardness removal method is used for chemical hardness removal at the front end. In order to overcome the inhibition of scale inhibitor and dispersant in circulating water on hardness removal, the dosages of double-alkali agents are large, the mass concentration of hardness removal agent is 15%-30%, and the pH value control range is high, i.e. pH value 10.5-11.5, preferably 10.8-11.5. SUMMARY

[0014] The present application aims to overcome the defects of the prior art, and provides a method capable of efficiently and economically removing hardness in wastewater, especially in circulating water blowdown water.

[0015] A wastewater hardness removal method, comprising:

[0016] 1) adding acid to adjust the pH value of the wastewater to about 6.0-6.5, adding flocculant A to the wastewater to be treated for reaction, fully stirring, and then standing;

[0017] 2) after settling, adding sodium carbonate and adjusting the pH value to 8.8-10.5 with alkali in the supernatant, adding flocculant B, fully stirring, then adding flocculant C, stirring, standing, and performing secondary hardness removal reaction.

[0018] The specific steps are as follows:

[0019] First step: adjusting the pH value by adding acid, the acid can be hydrochloric acid, sulfuric acid, nitric acid, etc., the pH value of the wastewater is adjusted to about 6.0-6.5, flocculant A is added to the wastewater to be treated for reaction, fast stirring for 2-8 min at a speed of 300-600 rpm, slow stirring for 6-15 min at a speed of 20-100 rpm, standing for 15-60 min;

[0020] Second step: after settling, the supernatant is removed, sodium carbonate is added to the supernatant, and the pH value is adjusted to 8.8-10.5 with alkali such as NaOH or Ca(OH)2, flocculant B is added, fast stirring for 2-8 min at a speed of 300-600 rpm, flocculant C is added, slow stirring for 6-15 min at a speed of 20-100 rpm, standing for 15-60 min, and performing secondary hardness removal reaction.

[0021] The specific total amount of sodium carbonate and alkali needs to be obtained by experiment according to the calcium hardness, total hardness and alkalinity of the actual water quality, and the addition amount is different for different water qualities, and the general addition amount is: sodium carbonate 50-300 mg / L, alkali 50-300 mg / L.

[0022] The flocculant A described in the present application can be an inorganic small molecule compound, preferably a potassium salt, an aluminum salt and an iron salt, such as one or more of aluminum sulfate, potassium aluminum sulfate, aluminum chloride, iron sulfate, ferric chloride, aluminum chloride, aluminum ferric sulfate, etc.

[0023] The flocculant B can be an inorganic high molecular compound, preferably a polymeric aluminum salt and an iron salt, such as one or more of polymeric aluminum sulfate, polymeric aluminum chloride, polymeric ferric sulfate chloride, polymeric silicon aluminum sulfate, polymeric ferric sulfate, polymeric ferric chloride, polymeric aluminum chloride, etc.

[0024] The flocculant C is one or more of organic polymers, such as anionic polyacrylamide, cationic polyacrylamide and non-ionic polyacrylamide, with a molecular weight of 4 million to 30 million, preferably anionic polyacrylamide with a molecular weight of 8 million to 24 million.

[0025] The method of the present application is suitable for treating wastewater with calcium hardness (in terms of CaCO3) of 200-1600 mg / L, total hardness (in terms of CaCO3) of 300-2500 mg / L and total alkalinity (in terms of CaCO3) of 200-1000 mg / L.

[0026] The method of the present application is particularly suitable for treating wastewater with high hardness, high total hardness and high alkalinity, such as wastewater with calcium hardness of 800-1600 mg / L, total hardness of 1000-2500 mg / L and total alkalinity of 600-1000 mg / L.

[0027] The method of the present application is particularly suitable for treating circulating water blowdown wastewater, which can be circulating water blowdown wastewater containing scale and corrosion inhibitors, and the wastewater can contain total phosphorus (in terms of P) of more than 0.5 mg / L, such as 0.6-3 mg / L, and organic phosphorus (in terms of P) of more than 0.2 mg / L, such as 0.3-3 mg / L.

[0028] The method of the present application has good treatment effect, low reagent dosage, stable operation and can meet the requirements of subsequent wastewater reuse and double membrane treatment for hardness.

[0029] According to common knowledge of those skilled in the art, the pH value of circulating water blowdown wastewater after multiple cycles of concentration is generally 8.4-9.5 for a circulating water system operated at natural pH. The present inventors have unexpectedly found in a large number of experimental studies on hardness removal from circulating water blowdown wastewater that the pH value of the wastewater is first lowered and flocculant A is added for preliminary hardness removal, then sodium carbonate and alkali are added to raise the pH value and further remove hardness, and then flocculants B and C are added for flocculation and sedimentation, and the final hardness removal effect is much better than that of conventional one-step hardness removal.

[0030] We have also found in experiments that although flocculants A, B and C are conventional flocculants, the order of addition is very important. Flocculant A can only be added for primary hardness removal and sedimentation, and the treatment effect is similar to that of conventional one-step hardness removal if flocculant B is used instead.

[0031] The two-step hardness removal method of the application can better remove the hardness in the circulating water blowdown water, and finally remove the calcium hardness and total hardness in the wastewater under the condition of pH value of 8.8-10.5. Especially for the wastewater with calcium hardness as the main total hardness, the total hardness of the wastewater can be reduced to below 80 mg / L, the calcium hardness can be reduced to below 50 mg / L, and the addition amount of sodium hydroxide is greatly reduced by adjusting the final pH value to about 8.8-9.2.

[0032] It is well known that the amount of acid and base consumed for adjusting the pH value under neutral conditions is much less than that under strong acidic or strong alkaline conditions without considering the buffering capacity of the water body, and the inventors of the application have also confirmed this in experiments: the amount of base required for adjusting the pH value from about 6.2 to about 8.8 is much less than the amount of base consumed for adjusting the pH value of the wastewater from 10.3-10.5 to 11.0-11.5, which is also the reason why the consumption of the agent of the application is lower than that of the conventional two-alkali hardness removal method. DETAILED DESCRIPTION

[0033] Example 1

[0034] The circulating water blowdown water of a certain refining and chemical enterprise in the south: because the wastewater needs to be treated by double membrane, the calcium hardness needs to be reduced to below 100 mg / L. First step: add dilute hydrochloric acid to adjust the pH value of the wastewater to 6.1-6.2, add aluminum sulfate 50 mg / L, fast stirring for 5 min, slow stirring for 10 min, and standing for 30 min, and then separating the precipitate; second step: add Na2CO3 200 mg / L to the supernatant, and continue to adjust the pH value to 9.2 by adding NaOH (the amount of NaOH is about 120 mg / L), add polyaluminum chloride 30 mg / L, fast stirring for 5 min, add 1 mg / L anionic polyacrylamide, slow stirring for 10 min, and standing for 30 min, and then take a sample for analysis. The hardness removal results are as follows: calcium hardness 73 mg / L, total hardness 186 mg / L, and magnesium hardness 113 mg / L.

[0035] Table 1 Water quality of the circulating water blowdown water of a certain refining and chemical enterprise in the south

[0036]

[0037]

[0038] Example 2

[0039] A certain refining enterprise circulating water sewage: pH 8.9, calcium hardness 751.5 mg / L (as CaCO3, the same below), total hardness 892.7 mg / L (as CaCO3, the same below), magnesium hardness 141.2 mg / L, alkalinity 541.2 mg / L (as CaCO3, the same below), because the total hardness of sewage needs to be reduced to below 120 mg / L before entering the double membrane treatment. First step: add dilute hydrochloric acid, adjust the pH of the sewage to 6.1-6.2, add aluminum sulfate 50 mg / L, fast stirring for 5 min, slow stirring for 10 min, standing for 30 min, separating the precipitate; second step: add Na2CO3 350 mg / L to the supernatant, continue to adjust the pH to 10.5 with NaOH (NaOH dosage about 200 mg / L), add polyaluminum chloride 20 mg / L, fast stirring for 5 min, add 1 mg / L anionic polyacrylamide, slow stirring for 10 min, standing for 30 min, then take sample analysis, hardness removal results: calcium hardness 78.5 mg / L, total hardness 112.6 mg / L, magnesium hardness 34.1 mg / L.

[0040] Example 3

[0041] A certain coal chemical industry enterprise in Inner Mongolia circulating water sewage: pH 8.6, total hardness 419.4 mg / L, calcium hardness 382.2 mg / L, magnesium hardness 37.9 mg / L, alkalinity 346.9 mg / L, the total hardness of sewage needs to be reduced to below 80 mg / L before entering the double membrane treatment: first step: adjust the pH of the sewage to 6.2-6.3 with dilute hydrochloric acid, add aluminum chloride iron 40 mg / L, fast stirring for 5 min, slow stirring for 10 min, standing for 30 min, separating the precipitate, second step: add Na2CO3 250 mg / L to the supernatant, continue to adjust the pH to 9.5-9.6 with NaOH (NaOH dosage about 150 mg / L) add polyaluminum sulfate 20 mg / L, fast stirring for 5 min, add 1 mg / L anionic polyacrylamide, slow stirring for 10 min, standing for 30 min, then take sample analysis, hardness removal results: total hardness 78.6 mg / L, calcium hardness 54.5 mg / L, magnesium hardness 24.1 mg / L.

[0042] Comparative Example 1

[0043] Compared with Example 3, the conventional double alkali hardness removal method is used.

[0044] The circulating water sewage of a coal chemical enterprise in Inner Mongolia is added with Na2CO3 250 mg / L, and the pH value is adjusted to 9.5-9.6 by NaOH (the amount of NaOH is about 120 mg / L), fast stirring for 5 min, adding aluminum iron chloride 40 mg / L, polyaluminum sulfate 20 mg / L, fast stirring for 5 min again, adding 1 mg / L anionic polyacrylamide, slow stirring for 10 min, and standing for 30 min, and then sampling and analyzing, the hardness removal result is: total hardness 311.9 mg / L, calcium hardness 273.8 mg / L, and magnesium hardness 37.1 mg / L.

[0045] Comparative Example 2

[0046] Compared with Example 3, the order of adding the medicaments is different.

[0047] The pH value of the sewage is adjusted to 6.2-6.3, polyaluminum sulfate 20 mg / L is added, fast stirring for 5 min again, slow stirring for 10 min, standing for 30 min, and then separating the precipitate, in the second step, Na2CO3 250 mg / L is added, the pH value is adjusted to 9.5-9.6 by NaOH (the amount of NaOH is about 150 mg / L), aluminum iron chloride 40 mg / L is added, fast stirring for 5 min, 1 mg / L anionic polyacrylamide is added, slow stirring for 10 min, and standing for 30 min, and then sampling and analyzing, the hardness removal result is: total hardness 313.1 mg / L, calcium hardness 278.9 mg / L, and magnesium hardness 34.2 mg / L.

[0048] Example 4

[0049] The circulating water sewage of a 100 million tons / year ethylene production enterprise: pH value 8.9, calcium hardness 941.8 mg / L, total hardness 1092.1 mg / L, magnesium hardness 150.3 mg / L, and alkalinity 771.9 mg / L (calculated as CaCO3, the same below), which requires to be treated to total hardness <120 mg / L. In the first step, the pH value of the sewage is adjusted to 6.2-6.4, aluminum sulfate 20 mg / L and ferric chloride 20 mg / L are added, fast stirring for 5 min, slow stirring for 10 min, standing for 30 min, and then separating the precipitate, in the second step, the supernatant is added with Na2CO3 250 mg / L, the pH value is adjusted to about 10.5 by NaOH (the amount of NaOH is about 250 mg / L), polyaluminum chloride 20 mg / L is added, fast stirring for 5 min, 1.5 mg / L anionic polyacrylamide is added, slow stirring for 10 min, and standing for 30 min, and then sampling and analyzing, the hardness removal result is: calcium hardness 61.5 mg / L, total hardness 108.6 mg / L, and magnesium hardness 47.1 mg / L.

[0050] Comparative Example 3 of Example 4

[0051] Compared with Example 4, the conventional double-alkali hardness removal method is adopted.

[0052] The pH of the wastewater is adjusted to about 10.5 by NaOH, and Na2CO3 250 mg / L is added, fast stirring for 5 min, then adding polyaluminum sulfate 20 mg / L, fast stirring for 5 min again, adding anionic polyacrylamide 1.0 mg / L, slow stirring for 10 min, and sampling after standing for 30 min. The hardness removal results are as follows: calcium hardness 321.6 mg / L, total hardness 413.1 mg / L.

[0053] Comparative Example 4

[0054] In comparison with Example 4, the conventional dual-alkali hardness removal method (increasing pH) is used.

[0055] The pH of the wastewater is adjusted to about 11.5-11.7 by NaOH, and Na2CO3 300 mg / L is added, fast stirring for 5 min, then adding polyaluminum sulfate 20 mg / L, fast stirring for 5 min again, adding anionic polyacrylamide 1.0 mg / L, slow stirring for 10 min, and sampling after standing for 30 min. The hardness removal results are as follows: calcium hardness 92.6 mg / L, total hardness 146.1 mg / L, magnesium hardness 63.5 mg / L.

[0056] Example 5

[0057] A simulated water with total hardness 419.4 mg / L, calcium hardness 382.2 mg / L, magnesium hardness 37.9 mg / L, and alkalinity 346.9 mg / L is prepared by Beijing tap water in the laboratory, and the hardness removal experiment is carried out according to the method of Example 3. The final pH is 9.5-9.6, and the hardness removal results are as follows: total hardness 91.9 mg / L, calcium hardness 63.8 mg / L, and magnesium hardness 28.1 mg / L.

[0058] Comparative Example 5

[0059] The wastewater is treated in the same way as in Example 5, and the conventional dual-alkali hardness removal method of Comparative Example 1 is used: adjusting the pH to 9.5-9.6, and the treatment results are as follows: total hardness 81.7 mg / L, calcium hardness 58.6 mg / L, and magnesium hardness 23.1 mg / L.

[0060] As can be seen from the examples and comparative examples of the present application, the two-step hardness removal method of the present application can better remove the hardness in the blowdown water of the circulating water, and the calcium hardness and total hardness in the wastewater can be removed under the condition of pH 10.3-10.5. Especially for wastewater in which the total hardness is mainly calcium hardness, the total hardness in the wastewater can be reduced to below 80 mg / L and the calcium hardness can be reduced to below 50 mg / L by adjusting the final pH to about 8.8-9.2, which greatly reduces the dosage of sodium hydroxide.

Claims

1. A wastewater hardness removal method, comprising: 1) adjusting the pH of the wastewater to 6.0-6.5 by adding acid, adding inorganic small molecule flocculant A to the wastewater to be treated, and reacting, fully stirring, and then standing, wherein the flocculant A is selected from one or more of aluminum sulfate, potassium aluminum sulfate, aluminum chloride, ferric sulfate, ferric chloride, aluminum ferric chloride, and aluminum ferric sulfate; 2) after settling, adding sodium carbonate to the supernatant and adjusting the pH to 8.8-10.5 with a base, adding inorganic high molecular flocculant B, fully stirring, adding organic polymer flocculant C, stirring, standing, and performing a second hardness removal reaction, wherein the flocculant B is selected from polymeric aluminum and iron salts, and the flocculant C is selected from one or more of anionic polyacrylamide, cationic polyacrylamide, and non-ionic polyacrylamide; The wastewater is circulating water blowdown wastewater.

2. The method of claim 1, wherein, After adding the flocculant A to the wastewater to be treated, fast stirring is performed for 2-8 min at a speed of 300-600 rpm, slow stirring is performed for 6-15 min at a speed of 20-100 rpm, and then standing is performed for 15-60 min.

3. The method of claim 1, wherein, After adding the flocculant B, fast stirring is performed for 2-8 min at a speed of 300-600 rpm.

4. The method of claim 1, wherein, After adding the flocculant C, slow stirring is performed for 6-15 min at a speed of 20-100 rpm.

5. The method of claim 1, wherein, The flocculant B is selected from one or more of polyaluminum sulfate, polyaluminum chloride, polyferric sulfate chloride, polysilicon aluminum sulfate, polyferric sulfate, polyferric chloride, and polyaluminum chloride.

6. The method of claim 1, wherein, The molecular weight of the flocculant C is 4 million-30 million.

7. The method of claim 1, wherein, The flocculant C is selected from anionic polyacrylamide with a molecular weight of 8 million-24 million.

8. The method of claim 1, wherein, The wastewater is high-calcium hardness, high-total hardness, and high-total alkalinity wastewater, with a calcium hardness range of 200-1600 mg / L, a total hardness range of 300-2500 mg / L, and a total alkalinity range of 200-1000 mg / L.

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