WO2007088138A1

WO2007088138A1 - Method for treatment of sludge

Info

Publication number: WO2007088138A1
Application number: PCT/EP2007/050769
Authority: WO
Inventors: Stefan JÄFVERSTRÖM; Kjell Stendahl; Hans David Ulmert
Original assignee: Feralco AB
Current assignee: Feralco AB
Priority date: 2006-01-31
Filing date: 2007-01-26
Publication date: 2007-08-09
Anticipated expiration: 2008-07-31

Abstract

Method for treatment of sludge (11), which includes trivalent iron, whereby the sludge (11) is subjected to acid dissolution (A), and thereafter is subjected to at least one membrane filtration (B), whereby a permeate is obtained, including trivalent iron. The trivalent iron is reduced to divalent iron before a percipitation (C) of said divalent iron.

Description

TITLE: METHOD FOR TREATMENT OF SLUDGE

Technical Field

The invention relates to treatment of sludge from waterworks, and similar sludge from industrial processes, such as paper industry. More specifically, the invention relates to a method for recycling of iron from a sludge, which sludge includes trivalent iron.

Prior Art

When pure water is to be obtained from surface water, suspended matter and organic material must be separated. Such organic material is mostly a brownish substance, so called humus substances. These substances are formed during incomplete breakdown of dead vegetables and occur naturally in a varying amount in lakes and watercourses.

A separation of suspended matter lowers the turbidity of the water and a separation of humus reduces the discoloration of the water.

To be able to accomplish this separation it is common to add inorganic chemical coagulants, such as trivalent metallic salts of iron and aluminium. The metallic ions formed in this connection, during mild stirring, flocks of hydroxide, that encase and adsorb the suspended material and the in water solved organic substances.

After terminated flocculation the formed flocks are separated in different ways, such as flotation/sandfiltra- tion, sedimentation/sandfiltration, or merely sandfiltra- tion. The separated flocks are pumped as thin sludge out from the construction, directly back to the recipient or to a sludge lagun. Optionally the sludge is dewatered, for example in a centrifuge, to be deposited thereafter. In warm countries the sludge may be laid on drying beds, to be deposited thereafter. Another alternative method to take care of the thin sludge is to add acid, preferably sulphuric acid. When adding a sufficient amount acid the metallic hydroxide, which was obtained during the flocculation process, is dissolved in such way that metallic ions are obtained, mainly Fe³⁺ and Al³⁺. When the metallic hydroxide has been dissolved a sludge mixture with low pH is thus obtained, that includes suspended matter, organic substances and inorganic ions. This sludge mixture may then be filtrated in a membrane filtration process, in such way that a concentrate and a permeate are obtained. As a result, said permeate includes mainly the inorganic chemical coagulants in solution.

Different types of membranes are used in the different membrane processes. In a lot of processes membranes with large pores are used (for example micro filtration) , while the membranes in other processes have small pores (for example reversed osmosis) . Some processes are based on the fact that the membranes are charged (for example nano filtration) , while the possible charge of the membranes does not affect the main separation mechanism in other processes (for example micro filtration) . Thus, the sludge mixture is led to a first construction for membrane filtration, which may be a construction for ultra filtration or a construction for micro filtration. During ultra filtration (UF) the size of the particles mainly decides what will be separated and what will pass through the membrane. Thus, the sieving mechanism dominates, but diffusion and interaction between membrane and the dissolved substances are also of importance. The separation with micro filtration (MF) is totally based on a sieving mechanism, and the size of the pores is the decisive factor in respect of what will pass through the membrane .

Thus, the sludge mixture is pumped through a MF con- struction or a UF construction. The MF construction sepa- rates mainly suspended substances and colloids, but not dissolved organic substances, while the UF construction also separates bigger organic molecules.

The filtration, by a MF/UF membrane filtration con- struction, thus results in a concentrate, including mainly suspended matter and organic compounds, that can not pass through the filter, and a permeate, including mainly water with inorganic ions, such as Fe³⁺ and Al³⁺, which pass through said filter. In this manner up to 90 % of the used amount of aluminium and iron ions in the flocculation process may be recovered in said permeate. The permeate may therefore be used as chemical coagulant in both wastewater treatment plants and waterworks. However, the permeate will also include dissolved organic substances with low molecu- lar weight and such heavy metals that, just as aluminium and iron ions, have been dissolved during the acid treatment. This is a disadvantage. Both heavy metals and organic substances will therefore accumulate in the system and constantly increase in respect of concentration, which may result in a deteriorating quality on the treated water.

Since water is classified as a foodstuff, also the public health board and the public may raise objections against that not a totally "clean" product is used as chemical coagulants in waterworks. However, the same problem do not arise if the same obtained permeate is used as chemical coagulant during treatment of wastewater, which is not used as drinking-water.

To increase the amount of aluminium and iron ions in said permeate a concentration process may be performed in a nano filtration construction (NF) , or in a reversed osmosis construction (RO) . During NF substances are separated according to two separation processes. Uncharged substances are separated in respect of size, while possible retention of ions depend on the electrical interaction between ion and membrane. Thus, if the permeate is filtered with a NF construction the trivalent ions, i.e. Fe³⁺ and Al³⁺, will be retained in the concentrate, while ions with lower charge in some extent will pass through the membrane and thus retrieved in the permeate. If an additional concentration process is performed by a RO construction also ions with lower charge will be retained in the concentrate, while the permeate is almost free from ions. The obtained concentrate, both from a NF construction and from a RO construction, may be re-used as chemical coagulant, but with the same reservation that was brought forward in accordance with UF/MF permeate.

To be able to re-use the from membrane processes recovered iron and/or aluminium ions as chemical coagulants in waterworks, an additional purification in respect of organic substances and heavy metals has to be performed.

US 5,674,402 describes a process wherein the concentration is obtained by precipitation of Al, in form of alunite, which means that alunite on one hand has to be reprocessed, by dissolving the alunite in acid, to obtain a water soluble chemical coagulant, and on the other hand has to be calcinated, to get rid of co-precipitated organic matter. Furthermore, precipitation of alunite does not give a product that is free from heavy metals, which results in that it may be difficult to fulfil the demands on chemical substances in drink-water by this process.

Summary of the invention

Accordingly, the present invention preferably seeks to mitigate, alleviate or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination and solves at least the above mentioned problems by providing a method and a chemical coagulant according to the appended patent claims.

An aspect of the present invention is to provide a method for recycling iron from a sludge, which sludge has been obtained from waterworks, or similar sludge from industrial processes, such as paper industry.

This is accomplished by reducing trivalent iron to divalent iron in an acid dissolution, after which dissolution a filtration separates said divalent iron from organic substances present in said sludge.

To fulfil these aspects a method and a chemical coagulant have obtained the characterising features in accordance with the claims.

Brief Description of the Drawings

To explain the invention in further detail illustrative embodiments thereof will be described below, with reference to enclosed drawings, in which; Fig. 1 is a flow chart of an embodiment of the present invention.

Detailed Description of Preferred Embodiments

In a first embodiment of the present invention, in accordance with Fig. 1, a sludge 11, containing iron hydroxide (Fe(OH)₃) formed during flocculation in flocculation basins in waterworks, or similar sludge from industrial processes, such as paper industry, is led to an acid dissolution step A. This iron hydroxide is formed from trivalent iron (Fe³⁺) that is added in said flocculation basin as a chemical coagulant. In this acid dissolution step an acid 12 is added to the sludge. This acid may for example be sulphuric acid, but other acids, which are known to the person skilled in the art may also be used. In this acid dissolution step A the iron hydroxide

(Fe(OH)₃) is dissolved, i.e. free iron ions are formed. The acid environment in the acid dissolution step A reduces the trivalent iron ions (Fe³⁺) to divalent iron ions (Fe²⁺) , because of the organic content in the sludge. This reduction occurs mainly through reaction (s) between the organic substances, present in the sludge, and the trivalent iron at acid conditions. This reduction is also influenced by the temperature, and is promoted by elevated temperatures. Elevated temperatures, in this context, means temperatures in the interval 60 to 150 °C, such as 90 to 130 °C, for example 100 to 120 °C . The present inventors have found that the divalent iron ions (Fe²⁺) formed in this acid dissolution step do not react with the organic substances, which will be shown to be very favourable later in the process, as seen below. Therefore, by in this step maximizing the reduction of trivalent iron to divalent iron a condition is obtained in which the separation between iron and organic substances is optimized.

In another embodiment of the present invention a reduction agent 13 is added to further improve the transformation of trivalent iron ions to divalent iron ions. In one embodiment of the present invention this reduction agent 13 is at least one reduction agent selected from the group comprising sodium bisulphite and metallic iron powder.

Hereafter, the acidified sludge, containing the divalent iron ions (Fe ⁺) formed in the acid dissolution step, is transported to a membrane filtration step B. Said membrane filtration step B may be an ultra filtration step, a nano filtration step, or a combination of an ultra filtration step and a nano filtration step. In this membrane filtration step B organic substances are separated as an organic concentrate 14, while the permeate comprises the divalent iron, which was formed in the acid dissolution step A. The permeate leaving the membrane filtration step B is of course still of a low pH. In one embodiment said permeate has a pH of 2. Since the separation between iron (divalent iron) and organic substances were optimized in the dissolution step A, the separation of iron (divalent iron) and organic substances is also optimized in this filtration step B. Said organic concentrate 14 may be used or deposited 15.

In another embodiment of the present invention the further improvement of the reduction of the trivalent iron by addition of a reduction agent 13 is performed after said membrane filtration step B. The only limitation in this respect is to perform the reduction before a precipitation step C, further described below.

In this context, the separation is obtained by a driving force, in form of a difference in chemical potential over the membrane. This driving force may be an applied pressure, a difference in concentration or in temperature, or a difference in electric potential. The separation mechanism is based on a solution theory, in which the solubility of the dissolved substances and the diffusivity in the membrane are decisive.

Subsequent to the membrane filtration step B said permeate is exposed to a precipitation step C, wherein divalent iron is precipitated as iron hydroxide (Fe(OH)₂). This precipitation is accomplished by adding a suitable alkali 16, whereupon the pH naturally is increased. This increase of pH makes the pH rise from 2 to neutral. The increase of pH may of course vary somewhat, but normally it results in a pH of 7 to 8. This alkali 16 may be selected from the group comprising potassium hydroxide, sodium hydroxide, sodium carbonate, magnesium hydroxide, potassium carbonate, magnesium carbonate, and/or magnesium oxide.

In a separation step D the iron hydroxide (Fe(OH)₂) is separated from the solution (i.e. the permeate from the membrane filtration step, which has been exposed to the precipitation step C) , since this solution still contains a degree of organic substances. This separation may be performed by any kind of filtration, which filtration is obvious to the person skilled in the art. Examples of such separation methods are filtration, sedimentation, flotation/sandfiltration, sedimentation/sandfiltration, or merely sandfiltration . Therefore, a filtrate 17 is obtained in said separation step D. In one embodiment of the present invention this filtrate 17 is exposed to a nanofiltration step G, from which water 18 of neutral water and an organic concentrate. This organic concentrate may be combined with the organic concentrate 14 obtained in the membrane filtration step B. Said organic concentrate, or combined organic concentrate, 14 may then be used or deposited 15. The iron hydroxide (Fe (OH) 2) is then exposed to an oxidation step E. In this oxidation step E said divalent iron hydroxide (Fe (OH) 2) is oxidized to the trivalent form of iron hydroxide (Fe(OH)₃). This oxidation is accomplished by adding a suitable oxidation agent 19. In one embodiment of the present invention said oxidation agent is oxygen in air or concentrated oxygen. In other embodiments of the present invention said oxidation agent may be selected from the group consisting of ozone and hydrogen peroxide, or active chlorine, such as chlorine gas, chlorate, and sodium hypochlorite. Here oxidising compounds including active oxygen are preferred, since chlorine together with organic substances may form toxic and carcinogenic organochlorines .

Thereafter, the iron hydroxide (Fe(OH)₃) is exposed to an acid dissolution step F. In this acid dissolution step F an acid 20 is added. From said acid dissolution step F trivalent iron ions (Fe³⁺) 20 are obtained. Said trivalent iron ions 21 may thereafter be reused as chemical coagulant in waterworks, or similar sludge from industrial processes, such as paper industry. Although the present invention has been described above with reference to specific embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the invention is limited only by the accompanying claims and, other embodiments than the specific above are equally possible within the scope of these appended claims.

In the claims, the term "comprises/comprising" does not exclude the presence of other elements or steps. Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly advantageously be combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. In addition, singular references do not exclude a plurality. The terms "a", "an", "first", "second" etc do not preclude a plurality. Reference signs in the claims are provided merely as a clarifying example and shall not be construed as limiting the scope of the claims in any way.

Claims

1. Method for treatment of sludge (11), which includes trivalent iron, whereby the sludge (11) is subjected to acid dissolution (A), characterized in reducing said trivalent iron to divalent iron in said acid dissolution (A) before a precipitation (C) of said divalent iron, and filtrating said sludge (11) in a membrane filtration (B) before said precipitation (C) of said divalent iron.

2. Method according to claim 1, further comprising an addition of a reducing agent (13) before or after the membrane filtration (B) .

3. Method according to claim 1 or 2, wherein said divalent iron are precipitated in form of iron hydroxide.

4. Method according to claim 1, 2, or 3, wherein said precipitation is obtained by adding alkali (16) .

5. Method according to claim 4, wherein said alkali (16) may be one or more alkali selected from the group comprising potassium hydroxide, sodium hydroxide, sodium carbonate, magnesium hydroxide, potassium carbonate, magnesium carbonate, and/or magnesium oxide.

6. Method according to claim 1, 2, 3, 4, or 5, further comprising separation (D) of said precipitated divalent iron.

7. Method according to claim 6, wherein said separation is selected from the group comprising filtration, sedimentation, flotation/sandfiltration, sedimentation/sandfiltration, or sandfiltration .

8. Method according to claim 6 or 7, further comprising filtration (G) of a filtrate obtained in said separation (D) , whereby a first organic concentrate and water (18) is obtained.

9. Method according to claim 1, wherein a second organic concentrate is obtained from said membrane filtration (B) .

10. Method according to claim 8 or 9, wherein said first and/or second organic concentrate (14) is deposited.

11. Method according to any of claims 1 to 8, further comprising oxidation (E) of said divalent iron to trivalent iron.

12. Method according to claim 11, wherein said oxidation (E) is performed by adding at least one oxidation agent (19) selected from the group comprising oxygen in air, concentrated oxygen, ozone, hydrogen peroxide, chlorine gas, chlorate, and sodium hypochlorite.

13. Method according to claim 11 or 12, further comprising an acid dissolution (F) , wherein trivalent iron ions (21) are obtained by dissolving the oxidized trivalent iron .

14. Method according to claim 13, further comprising recirculation of said trivalent iron ions (21) as chemical coagulant in a flocculation basin.

15. Method according to claim 1, wherein said reduction is obtained through reaction between said trivalent iron and organic substances, present in said sludge (11) .

16. Method according to claim 1, wherein said reduction of trivalent iron to divalent iron is obtained by adding a reduction agent (13) .

17. Method according to claim 16, wherein said reduction agent (13) is at least one reduction agent selected from the group comprising sodium bisulphite or metallic iron powder.

18. Chemical coagulant, in form of trivalent iron ions (21), obtained from the method according to claim 13 or 14.