WASTEWATER TREATMENT PROCESS FOR REMOVING CHEMICAL OXYGEN DEMAND

Abstract

A process for removing chemical oxygen demand, which realizes the deep COD treatment by the combination of metal salt and hydrogen peroxide and then by an ozone containing gas with hydrogen peroxide or ultraviolet radiation with hydrogen peroxide. It features using less metal salt and hydrogen peroxide, having less ozone gas residual and being more suitable for industrialization.

Claims

1. A process for treating wastewater having a chemical oxygen demand of from 100 to 500 mg/L, comprising: (a) contacting the wastewater with a composition comprising at least one metal salt and hydrogen peroxide, at a dosage of metal salt of from 0.003 to 0.009 mole of metal salt per liter wastewater and a molar ratio of metal salt to hydrogen peroxide of from 1.0:1 to 1.5:1, to obtain a mixture having a pH of from 3 to 6; (b) contacting a base compound with the mixture obtained at step (a) to form a metal hydroxide precipitate and a liquid medium; (c) separating the liquid medium and precipitate; and (d) contacting the liquid medium with an ozone containing gas and hydrogen peroxide or with ultraviolet radiation and hydrogen peroxide.

2. The process according to claim 1, wherein step (a) further comprises contacting the wastewater with an acid compound to adjust the pH of the mixture obtained in step (a).

3. The process according to claim 1, wherein the metal salt comprises at least one metal element selected from the group consisting of Fe, Co, Ni, Mg, Zn, W, and Cu.

4. The process according to claim 1, wherein the step (a) reduces the chemical oxygen demand of the wastewater by from 20% to 60%.

5. The process according to claim 1, wherein the wastewater exhibits a total organic carbon content and step (a) reduces the total organic carbon content of the wastewater by from 20% to 60%.

6. The process according to claim 1, wherein pH value at the end of step (b) is from 7.5 to 8.5.

7. The process according to claim 1, wherein step (d) comprises contacting the liquid medium with an ozone containing gas and hydrogen peroxide at molar ratio of ozone to hydrogen peroxide of from 0.5:1 to 3:1.

8. The process according to claim 1, wherein step (d) comprises contacting the liquid medium with ultraviolet radiation and hydrogen peroxide at a molar ratio of hydrogen peroxide to chemical oxygen demand of from 1:1 to 3:1.

9. The process according to claim 1, wherein at the end of step (d) the liquid medium exhibits a chemical oxygen demand of from 20 to 50 mg/L.

10. The process according to claim 1, wherein at the end of step (d) the liquid medium exhibits a total organic carbon value of from 5 to 30 mg/L.

11. A composition, comprising: wastewater having a chemical oxygen demand of from 100 to 500 mg/L, at least one metal salt, and hydrogen peroxide, wherein the metal salt and hydrogen peroxide are present in a molar ratio of metal salt to hydrogen peroxide of from 1.0:1 to 1.5:1.

12. The process according to claim 3, wherein the metal salt comprises at least one metal element selected from the group consisting of Fe, Mg, and Zn.

13. The process of claim 12, wherein the metal salt comprises Fe.

Description

DETAILS OF THE INVENTION

[0034] The present invention provides a process for treating wastewater comprising at least chemical oxygen demand (COD) comprised from 100 to 500 mg/L, comprising at least the following steps:

(a) contacting at least the wastewater with a composition comprising at least one metal salt and hydrogen peroxide to obtain a mixture having a pH comprised from 3 to 6, the dosage of metal salt being comprised from 0.003 to 0.009 mol per liter wastewater, the molar ratio of metal salt to hydrogen peroxide being comprised from 1.0:1 to 1.5:1;
(b) reacting a base compound with the mixture obtained at step (a) to form a metal hydroxide precipitation and a liquid medium;
(c) separating off the liquid medium; and
(d) contacting the liquid medium with an ozone containing gas with hydrogen peroxide or ultraviolet radiation with hydrogen peroxide.

[0035] The COD in wastewater is comprised from 100 to 500 mg/L and more preferably from 250 to 350 mg/L.

[0036] The TOC in wastewater could be preferably from 30 to 150 mg/L and more preferably from 70 to 100 mg/L.

[0037] Step (a) The wastewater before treating preferably has a pH comprised from 7.0 to 9.0 and more preferably from 7.5 to 8.5. Notably pH is equal to 7.5, 7.6, 7.7, 7.8, 7.9, 8.0, 8.1, 8.2, 8.3, 8.4, 8.5 or any range obtained between these values.

[0038] In present invention, an acid compound could be optionally employed in step (a) to adjust the pH value. The sequence for adding the metal salt, hydrogen peroxide and acid is not particularly limited. They can be added in to wastewater simultaneously or respectively. In a preferred embodiment, metal salt and hydrogen peroxide are added first and acid is then added slowly to adjust the pH value. It is possible to add in the the mixture of step (a) a salt, for example, acid salt such as NaHCO.sub.3, NaHS, NaHSO.sub.4, NaH.sub.2PO.sub.4 and Na.sub.2HPO.sub.4.

[0039] The acid compound employed in step (a) could be organic, inorganic acid. It could be notably inorganic acid, such as mineral acids: hydrochloric acid (HCl), nitric acid (HNO.sub.3), phosphoric acid (H.sub.3PO.sub.4), sulfuric acid (H.sub.2SO.sub.4), boric acid (H.sub.3BO.sub.3), hydrofluoric acid (HF), hydrobromic acid (HBr), perchloric acid (HClO.sub.4), hydroiodic acid (HI). Among these, hydrochloric acid (HCl) or sulfuric acid (H.sub.2SO.sub.4) is more preferable.

[0040] The pH of the mixture may be preferably from 4.5 to 5.5. Notably pH is equal to 4.5, 4.6, 4.7, 4.8, 4.9, 5.0, 5.1, 5.2, 5.3, 5.4, 5.5 or any range obtained between these values.

[0041] The metal salt of present invention comprises at least one transition metal element or at least one element of group IIA of the Periodic Table. Preferably, the metal salt may comprise at least one metal element chosen in the group consisting of Fe, Co, Ni, Mg, Zn, W and Cu and more preferably chosen in the group consisting of Fe, Mg and Zn and most preferably Fe.

[0042] Examples of metal salt notably are: [0043] iron(II) salts, such as iron(II) sulfate(FeSO.sub.4), iron(II) chloride(FeCl.sub.2), iron(II) bromide(FeBr.sub.2), iron(II) fluoride(FeF.sub.2), iron(II) oxalate(FeC.sub.2O.sub.4) and iron(II) perchlorate(Fe(ClO.sub.4).sub.2). [0044] magnesium salts, such as magnesium sulfate(MgSO.sub.4), magnesium chloride(MgCl.sub.2). [0045] zinc salts, such as zinc sulfate(ZnSO.sub.4), zinc chloride(ZnCl.sub.2).

[0046] The dosage of metal salt in step (a) is comprised from 0.003 to 0.009 mol per liter wastewater and could be preferably from 0.003 to 0.006 mol per liter wastewater.

[0047] The molar ratio of metal salt to hydrogen peroxide in step (a) could be equal to 1.0:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1 or any range obtained between these values.

[0048] The removal rate of COD by step (a) could be comprised from 20% to 60% and preferably from 40% to 50%.

[0049] The removal rate of TOC by step (a) could be comprised from 20% to 60% and preferably from 40% to 50%.

[0050] The reaction temperature of step (a) may be comprised from 10 to 100 C. and preferably from 10 to 40 C. It is preferable that the reaction of step (a) occurs at room temperature.

[0051] The reaction time of step (a) may be comprised from 0.5 to 3 hours and preferably from 0.5 to 1 hour.

[0052] Step (b) The base compound employed in the process could be an organic, inorganic base. It could notably be an inorganic base, such as sodium hydroxide, potassium hydroxide. It is also possible to use a salt, such as sodium carbonate, sodium bicarbonate, potassium carbonate and potassium bicarbonate.

[0053] The concentration of base compound used to precipitate the metal hydroxide is not particularly limited. People having ordinary skill in the art could adjust the mixture to precipitate the metal hydroxide by using bases of different concentration.

[0054] Optionally, a flocculating agent could be used in this step to increase the flocculating efficiency. As used herein, flocculating agent refers to chemical additives that cause suspended solids to form aggregates called flocs. It should be understood any agent which could increase the flocculating efficiency in this invention could be used. Flocculating agent is particularly polyacrylamides (PAM)-soluble polyelectrolytes bearing negative (anionic) or positive (cationic) charge along the chain.

[0055] In a preferred embodiment, pH value at the end of step (b) could be comprised from 7 to 13. Preferably, pH value at the end of step (b) may be comprised from 7.5 to 8.5.

[0056] Step (c) The method for separating off the liquid medium is not particularly limited and can use several known separation techniques to separate precipitation from mixture obtained at step (b), such as for instance filtrating or centrifuging. Filtration may be made at positive pressure, such as comprised from 0.3 to 0.6 MPa, or under vacuum, such as comprised from 100 to 900 mbar.

[0057] Step (d) The said ozone (O.sub.3) containing gas may comprise at least 2 wt % ozone with respect to total weight of gas supplied to the liquid medium. Preferably, the gas may comprise 2 wt % to 20 wt % of ozone with respect to total weight of gas supplied to the liquid medium and more preferably 3 wt % to 8 wt %. The ozone containing gas may also comprise some inert gases such as He, Ne or Ar.

[0058] Dosage of ozone gas in this step depends on wastewater source. In a specific embodiment, 1.0-5.0 kg O.sub.3 might be required in order to remove 1 kg COD.

[0059] O.sub.3:H.sub.2O.sub.2 mol ratio in step (d) may be comprised from 0.5:1 to 3:1, preferred from 1:1 to 2:1.

[0060] O.sub.3 reactor can be designed as plug flow or completed stirred reactor (CSTR), O.sub.3 can be added by diffuser disc or Jet aeration or Venturi injection. H.sub.2O.sub.2 can be added before O.sub.3 injection point with static mixer.

[0061] Ultraviolet radiation could be realized by some well-known ultraviolet light equipment, such as ultraviolet light lamp. The UV dosage depends on the wastewater source. In a specific embodiment, it could be comprised from 20 to 500 KWH per stere liquid medium.

[0062] When ultraviolet radiation is used in step (d), H.sub.2O.sub.2 dosage depends on the COD in liquid medium. Specifically, H.sub.2O.sub.2:COD mol ratio could be comprised from 1:1 to 3:1 and preferably from 1.5:1 to 2.5:1.

[0063] The reaction time of step (d) may be comprised from 0.5 to 10 hours and preferably from 1 to 5 hours.

[0064] The COD value obtained at the end of step (d) may be comprised from 20 to 50 mg/L and preferably from 25 to 45 mg/L.

[0065] The TOC value obtained at the end of step (d) may be comprised from 5 to 30 mg/L and preferably from 10 to 15 mg/L.

[0066] The following examples are included to illustrate embodiments of the invention. Needless to say, the invention is not limited to the described examples.

EXPERIMENTAL PART

Example 1

[0067] To treat Reverse Osmosis (RO) concentrated effluent (reject effluent), COD=300 mg/L, TOC=100 mg/L.

[0068] Step (a): FeSO.sub.4.7H.sub.2O and H.sub.2O.sub.2 was added into wastewater simultaneously. pH of the mixture was adjusted to 5.0 by adding H.sub.2SO.sub.4. The reaction mixture is then stirred for 45 min at room temperature.

[0069] FeSO.sub.4.7H.sub.2O dosage:1.0 g/L (0.0036 mol/L)

[0070] H.sub.2O.sub.2 dosage: 0.1 g/L (0.0029 mol/L)

[0071] FeSO.sub.4.7H.sub.2O:H.sub.2O.sub.2 mol ratio: 1.24:1

[0072] Step (b): pH of the liquid medium was adjusted to 8.0 by adding NaOH and then flocculating agent was added (PAM, type: Kemira Superfloc C492PWG) 2 mg/L for flocculation (10 minutes).

[0073] Step (c): Then sludge was separated by filtration. The supernant COD decreased from 300 mg/L to 150 mg/L. TOC decreased from 100 mg/L to 50 mg/L.

[0074] Step (d): Retention time of O.sub.3/H.sub.2O.sub.2 treatment for liquid medium obtained at step (c) is 30 min. COD further decreased from 150 to 35 mg/L, TOC decreased from 50 to 15 mg/L. The initial pH was 8.0 and end pH was 7.5 without pH control.

[0075] O.sub.3 dosage: 0.35 g/L

[0076] O.sub.3:COD weight ratio: 3.5:1

[0077] O.sub.3:H.sub.2O.sub.2 mol ratio: 2:1

Example 2

[0078] The objective is to treat current outlet from biological wastewater treatment unit (WWTU) for water reuse. To treat COD from 350 mg/L to <50 mg/L.

[0079] Step (a): FeSO.sub.4.7H.sub.2O and H.sub.2O.sub.2 was added into wastewater simultaneously. The initial pH is 7.2. After FeSO.sub.4.7H.sub.2O and H.sub.2O.sub.2 is input, pH automatically decreased to 4.0. The reaction mixture is then stirred for 45 min at room temperature.

[0080] FeSO.sub.4.7H.sub.2O dosage: 1.5 g/L (0.0054 mol/L)

[0081] H.sub.2O.sub.2 dosage: 0.16 g/L (0.0047 mol/L)

[0082] FeSO.sub.4.7H.sub.2O:H.sub.2O.sub.2 mol ratio: 1.15:1

[0083] Step (b): pH of the liquid medium was adjusted to 8.5 by adding NaOH and then flocculating agent was added (PAM, type: Kemira Superfloc C492PWG) 2 mg/L for flocculation (10 minutes).

[0084] Step (c): Then sludge was separated by filtration. COD decreased from 350 mg/L to 180 mg/L. TOC decreased from 120 mg/L to 60 mg/L.

[0085] Step (d): UV/H.sub.2O.sub.2 treatment. The Lab reactor (volume 5.0 L) included two parts: 1) photo reactor with UV lamp inside; 2) main reactor. A recycle pump is used to build a loop between main reactor and photo reactor. After 2 hours reaction, COD decreased from 180 to 30 mg/L.

[0086] Reaction condition: UV power 100 W, reaction time 2 hours, UV dosage=100 W*2 h/5.0 L=20 KWh/m.sup.3, H.sub.2O.sub.2:COD mol ratio=2.0:1, H.sub.2O.sub.2 dosage:380 mg/L.

Example 3

[0087] The experiments were performed by the same way of step (a) in EXAMPLE 1. Results with different reaction parameters are expressed in Table 1.

[0088] Different FeSO.sub.4.7H.sub.2O:H.sub.2O.sub.2 mol ratios (1.0, 1.2, 1.5) were tried with same FeSO.sub.4.7H.sub.2O dosage (0.0036 mol/L) and initial pH (5.0). It is shown that FeSO.sub.4.7H.sub.2O:H.sub.2O.sub.2 mol ratio of 1.2 has better performance.

[0089] Different FeSO.sub.4.7H.sub.2O dosages were tried with same FeSO.sub.4.7H.sub.2O:H.sub.2O.sub.2 mol ratio (1.2) and initial pH (5.0). It is shown that FeSO.sub.4.7H.sub.2O dosage of 0.0054 mol/L has better performance.

[0090] Different pH was tried with same FeSO.sub.4.7H.sub.2O dosage (0.0054 mol/L) and FeSO.sub.4.7H.sub.2O:H.sub.2O.sub.2 mol ratios. It is shown that pH of 4.5 has better performance.

TABLE-US-00001 TABLE 1 FeSO.sub.47H.sub.2O FeSO.sub.47H.sub.2O:H.sub.2O.sub.2 Inlet Outlet COD Trial dosage ratio Intial COD COD removal unit mol/L mol:mol pH mg/L mg/L % 1 0.0036 1.0 5 300 168 44.3 2 0.0036 1.2 5 300 160 46.7 3 0.0036 1.5 5 300 186 38.0 4 0.0045 1.2 5 300 150 50.0 5 0.0054 1.2 5 300 145 51.7 6 0.0054 1.2 4.5 300 142 52.7 7 0.0054 1.2 5.5 300 151 49.7 8 0.0054 1.2 6 300 178 40.7

Example 4

[0091] The experiments were performed by the same way of step (d) in EXAMPLE 1. Results with different parameters are expressed in Table 2.

[0092] Different pH was tried with same O.sub.3 dosage 0.35 g/L and same H.sub.2O.sub.2 dosage (O.sub.3:H.sub.2O.sub.2 mol ratio=2.0). It is shown COD removal efficiency increases when pH is increased.

[0093] Different O.sub.3:H.sub.2O.sub.2 mol ratios were tried with same pH value and O.sub.3 dosage. It is shown O.sub.3:H.sub.2O.sub.2 mol ratio of 2.0 has better performance.

TABLE-US-00002 TABLE 2 kg O.sub.3/kg O.sub.3 O.sub.3:H.sub.2O.sub.2 Inlet Outlet COD removed Intial dosage mol ratio COD COD removal COD Trial pH g/L mol:mol mg/L mg/L % kg/kg 1 6 0.35 2 160 70 56.3 3.9 2 7 0.35 2 160 50 68.8 3.2 3 7.5 0.35 2 160 45 71.9 3.0 4 8.5 0.35 2 160 35 78.1 2.8 5 9 0.35 2 160 32 80.0 2.7 6 8.5 0.35 1 160 38 76.2 2.9 7 8.5 0.35 3 160 45 71.8 3.0 8 8.5 0.35 3.5 160 57 64.3 3.4 9 8.5 0.35 4 160 69 56.8 3.8