METHOD FOR REMOVING CALCIUM IONS FROM HIGH CONCENTRATION ORGANIC WASTEWATER

Abstract

A method for removing calcium ions from high concentration organic wastewater is provided. The method comprises the steps of: (1) introducing high concentration organic wastewater containing Ca.sup.2+, inorganic carbon and a seed crystal into a reactor with a molar ratio of Ca.sup.2+ to inorganic carbon of 1:(3.2-6.2); (2) adjusting the hydrogen ion activity α(H.sup.+) and ionic strength of the solution in the reactor; (3) sequentially stirring and precipitating in the reactor to convert Ca.sup.2+ in the high concentration organic wastewater into calcium carbonate which is then precipitated for calcium removal.

Claims

1. A method for removing calcium ions from high concentration organic wastewater, wherein the method comprises the steps of: (1) introducing high concentration organic wastewater containing Ca.sup.2+, inorganic carbon and a seed crystal into a reactor with a molar ratio of Ca.sup.+ to inorganic carbon of 1:(3.2-6.2); (2) adjusting a hydrogen ion activity α(H.sup.+) and ionic strength of the solution in the reactor; (3) sequentially stirring and precipitating in the reactor to convert Ca.sup.2+ in the high concentration organic wastewater into calcium carbonate which is then precipitated for calcium removal.

2. The method for removing calcium ions from high concentration organic wastewater according to claim 1, wherein a COD and Ca.sup.2+ concentration of the high concentration organic wastewater in step (1) is more than 2000 mg/L and more than 500 mg/L, respectively.

3. The method for removing calcium ions from high concentration organic wastewater according to claim 1, wherein the inorganic carbon in step (1) is a kind of liquid or soluble solid that can directly release the inorganic carbon.

4. The method for removing calcium ions from high concentration organic wastewater according to claim 1, wherein the seed crystal in step (1) is biomass with negative potential on a surface.

5. The method for removing calcium ions from high concentration organic wastewater according to claim 4, wherein a particle size of the seed crystal is 0.2 μm-2 mm.

6. The method for removing calcium ions from high concentration organic wastewater according to claim 1, wherein a reaction temperature of the reactor in step (1) is 10-40° C., and a hydraulic retention time is 6-32 h.

7. The method for removing calcium ions from high concentration organic wastewater according to claim 1, wherein the α(H.sup.+) in step (2) is adjusted to 3.16×10.sup.−12-6.31×10.sup.−9 mol/L.

8. The method for removing calcium ions from high concentration organic wastewater according to claim 1, wherein the ionic strength in step (2) is in a range of 0.3-2 mol/L.

9. The method for removing calcium ions from high concentration organic wastewater according to claim 1, wherein the stirring in step (3) is at a speed of 100-600 r/min for 10-20 h.

10. The method for removing calcium ions from high concentration organic wastewater according to claim 1, wherein a precipitating time in step (3) is 6-12 h.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 shows the Ca.sup.2+ removal rate under different anaerobic digestate/leachate ratios in Example 1;

[0029] FIG. 2 shows the Ca.sup.2+ removal rate under different initial α(H.sup.+) in Example 1.

DETAILED DESCRIPTION OF EMBODIMENTS

[0030] The present invention will be described in detail below with reference to the drawings and specific examples.

Example 1

[0031] (1) The incineration plant storage pit leachate was fed with Ca.sup.2+ concentration of 4200 mg/L, COD of 61200 mg/L and pH of 4.12 into the sequencing batch reactor. The anaerobic digestate (with Ca.sup.2+ concentration of 85 mg/L, COD of 2700 mg/L, and the inorganic carbon IC of 2010 mg/L) after the anaerobic treatment was used as inorganic carbon source, with a molar ratio of Ca.sup.2+ to inorganic carbon of 1:3.2. The anaerobic digestate was precipitated for 24 h to remove anaerobic sludge. The mixing ratio of anaerobic digestate/leachate was shown in FIG. 1, and the stirring speed was 180 rpm. After 20 h, the concentration of inorganic carbon that could be used for crystallization increased as the mixing ratio of biogas slurry/leachate increased, and the removal rate of Ca.sup.2+ increased monotonically. However, the highest removal rate was only 30% under the inhibition of high concentration organic matters.

[0032] (2) α(H.sup.+) was adjusted to above 10.sup.−8 mol/L using 50% NaOH solution, and at the same time, the ionic strength in the reactor was controlled to be less than 2 mol/L.

[0033] (3) The same stirring speed was 180 rpm. After 20 h, as shown in FIG. 2, the removal rate of Ca.sup.2+ could reach up to 99%. However, if the initial α(H.sup.+) of the reactor was adjusted to 3.98×10.sup.−9 mol/L, the removal rate of Ca.sup.2+ would be 72%.

[0034] According to this estimate, the fouling time of the anaerobic reactor for leachate treatment could be delayed by more than 3 times. Moreover, after the reactor stayed for 24 h, α(H.sup.+) would be reduced to about 3.16×10.sup.−8 mol/L along with the simultaneous fermentation process, combined with the buffer capacity of the subsequent anaerobic reactor, so that the anaerobic methanogenesis microbes could be in the optimal range, and the microbial activity could be ensured. In addition, the daily operation of the conventional anaerobic reactor required internal circulation to dilute the concentration load of the influent water and maintained a certain rising flow rate to form granular sludge. Therefore, this application example did not need to add additional pumps and energy to provide an inorganic carbon source.

Example 2

[0035] The sequencing batch reactor was the same as in Example 1. For the summer incineration plant storage pit leachate (with Ca.sup.2+ concentration of 3700 mg/L, COD of 78300 mg/L, pH of 6.83), the molar ratio of Ca.sup.2+ to inorganic carbon was adjusted to 1:6.2, and the intensity was 200 rpm for stirring. After 2 h, the removal rate of Ca.sup.2+ was up to 56%.

Example 3

[0036] The sequencing batch reactor was the same as in Example 1. For the papermaking black water (with Ca.sup.2+ concentration of 1000 mg/L, COD of 24500 mg/L, pH of 6.83), Na.sub.2CO.sub.3 was used as the inorganic carbon source, the molar ratio of Ca.sup.2+ to inorganic carbon was adjusted to 1:6.2, the intensity was 200 rpm for stirring, and the initial α(H.sup.+) of the reactor was adjusted to 3.98×10.sup.−11 mol/L. After 20 h, the removal rate of Ca.sup.2+ was up to 99%.

Example 4

[0037] A method for removing calcium ions from high concentration organic wastewater, the method comprises the steps of:

[0038] (1) introducing high concentration organic wastewater containing Ca.sup.2+, such as the leachate from the storage pit of the municipal solid waste incineration plant, an inorganic carbon source (sodium bicarbonate), and a seed crystal (calcite) with a particle size of 0.2-10 μm into a reactor to keep a molar ratio of Ca.sup.2+ to inorganic carbon of 1:3.2; and reaction at 10° C., with the hydraulic retention time of 32 h.

[0039] (2) adjusting α(H.sup.+) to 6.31×10.sup.−9 mol/L by using NaOH, with the ionic strength of 0.3-1 mol/L;

[0040] (3) stirring at 100 rpm for 20 h and precipitating for 6 h sequentially in the reactor to convert Ca.sup.2+ in the high concentration organic wastewater into a calcium carbonate crystal which is then precipitated for removal.

[0041] The invention optimized the solution and hydraulic conditions of the reactor via the reactor, which increased the supersaturation of calcium carbonate, accelerated the exchange of water molecules on the crystal surface with Ca.sup.2+ and inorganic carbon, and promoted the nucleation and crystal growth of calcium carbonate crystals under the inhibition of high concentration organic matters, resulting in 21% Ca.sup.2+ removal.

Example 5

[0042] A method for removing calcium ions from high concentration organic wastewater, the method comprises the steps of:

[0043] (1) introducing high concentration organic wastewater containing Ca.sup.2+ such as municipal solid waste landfill leachate, an inorganic carbon source (sodium bicarbonate), and a seed crystal (such as quartz sand) with a particle size of 10 μm-200 mm into a reactor with a molar ratio of Ca.sup.2+ to inorganic carbon of 1:6.2; and reacting at 40° C., with the hydraulic retention time of 6 h.

[0044] (2) adjusting α(H.sup.+) to 3.16×10.sup.−12 mol/L by using NaOH, with the ionic strength of 1-2 mol/L;

[0045] (3) stirring at 600 rpm for 10 h and precipitating for 12 h sequentially in the reactor to convert Ca.sup.2+ in the high concentration organic wastewater into a calcium carbonate crystal which is then precipitated for removal.

[0046] The invention optimized the solution and hydraulic conditions of the reactor, which increased the supersaturation of calcium carbonate, accelerated the exchange of water molecules on the crystal surface with Ca.sup.2+ and inorganic carbon, and promoted the nucleation and crystal growth of calcium carbonate crystals under the inhibition of high concentration organic matters, resulting in 99.5% Ca.sup.2+ removal.

[0047] The above examples are only used to illustrate the technical solution of the present invention, but not to limit the present invention. Changes, substitutions, modifications, and simplifications made by an ordinary person skilled in the art within the essential scope of the present invention are equivalent transformations, which does not depart from the purpose of the present invention, and should also belong to the protection scope of the claims of the present invention.