EP1056687A1

EP1056687A1 - Method for purifying waste water by combining hydrogen-peroxide supported oxidation with a membrane separation method

Info

Publication number: EP1056687A1
Application number: EP99906196A
Authority: EP
Inventors: Jürgen PATZLAFF; Rüdiger KNAUF; Joachim Semel
Original assignee: Axiva GmbH
Current assignee: Siemens Axiva GmbH and Co KG
Priority date: 1998-02-20
Filing date: 1999-01-23
Publication date: 2000-12-06
Also published as: AU2621399A; DE19807155A1; JP2002503553A; WO1999042407A1; DE19807155C2; CA2321503A1

Abstract

The invention relates to a method for the oxidative treatment of water containing organic compounds, according to which the water is mixed (1) with a reagent containing hydrogen peroxide and iron or titanium ions in solution. Said method is characterized in that the mixture is subjected to a membrane separation stage (4), in which a permeate constituting the treated waste water (6) is extracted from the mixture.

Description

description

Process for the purification of waste water by combining oxidation supported by hydrogen peroxide with a membrane separation process

The invention relates to a process for the oxidative treatment of water containing organic compounds, in which the water is mixed with a reagent containing hydrogen peroxide and dissolved iron or titanium ions.

Methods of the type mentioned are known, for example from DE 4134003 A1 (D1) and from 4026831 A1 (D2). D1 discloses a process for the chemical-oxidative purification of waste water by means of Fenton's reagent by the action of hydrogen peroxide and iron (II) compounds in the acidic range and subsequent precipitation of the iron (III) compounds in the slightly acidic to alkaline range. The iron ions are precipitated, filtered off and dissolved in sulfuric acid. Before being returned to the process, Fe (III) is reduced electrochemically or chemically (SO ₂ , sulfites) to Fe (II).

In the process corresponding to D2, the Fentons oxidation is carried out by adding fresh Fe (III) salt.

Up to now, the widespread use of iron salts and the associated high costs for the disposal or treatment of the iron sludge resulting from the precipitation have stood in the way of a greater spread of Fenton's oxidation. Disposal is made even more difficult Treatment of this so-called Fenton sludge through its content of organic and inorganic contaminants.

Furthermore, EP 0 806 398 A1 (D3) discloses a method for cleaning waste water, in which a membrane filter device - preferably one

Nanofiltration device - used to separate the waste water constituents. The retentate stream is used to concentrate the waste water components in the 2

Cycle returned. The possibility of the wastewater constituents being broken down using Fenton's reagent is not mentioned.

In view of this prior art, the object of the invention was to improve or simplify the method mentioned at the outset.

It has now surprisingly been found that this object can be achieved by passing the mixture through a membrane separation stage in which a permeate is removed from the mixture as treated waste water.

This is not obvious insofar as the person skilled in the art would not expect that recycling the "used" Fentons reagent would permanently support the oxidative degradation of the organic compounds, and he also learned from D1 that before recycling the mixture a reduction in the Fe (III) to Fe (II) or precipitation is required.

The invention therefore relates to a process for the oxidative treatment of water containing organic compounds, in which the water is mixed with a reagent containing hydrogen peroxide and dissolved iron or titanium ions, characterized in that the mixture is passed through a membrane separation stage in which the Mixture deprives a permeate as treated wastewater.

Preferred embodiments result from the subclaims. It is also possible for one or more of the configurations disclosed in the subclaims to provide solutions to the underlying problem, either individually or in combination, and the individual features within the claim categories can also be combined as desired.

The oxidative treatment is preferably carried out at pH values from 2 to 3, at temperatures from 5 to 100 ° C., particularly preferably 40 to 60 ° C. and at pressures from 1 to 51 bar (abs).

A preferred embodiment is characterized in that the mixing of the water with the reagent is carried out in a container in that 3 withdraws the mixture from the container and feeds the membrane separation stage and that the retentate of the membrane separation stage is wholly or partly returned to the container.

When the mixture is fed from the container into the membrane separation stage, the dwell time for protecting the membrane can be such that the

Has decomposed hydrogen peroxide before reaching the membrane separation stage. In order to prevent hydrogen peroxide from the reaction solution of the Fentons oxidation damaging a plastic membrane, the hydrogen peroxide can be catalytically decomposed after the container. Metal oxide alloys and noble metals on supports are particularly suitable as catalysts. If chemically resistant membranes are used, catalytic decomposition of H ₂ O _{2 can be} dispensed with.

The membranes used preferably have a separation limit which is greater than 100 and less than 1000 g / mol, preferably it is between 200 and 500 g / mol, measured by the method R. Rautenbach, A. Gröschl, published in Separation Potential on Nanofiltration Membranes Desalination 77 (1990). Suitable membrane materials are polymers or ceramic materials, preferably polysulfone, polyether sulfone, modified polyamides or aluminum oxide, zirconium oxide or silicon carbide.

In the process according to the invention, among other things, the Fe (II) and Fe (III) can be kept in solution and returned directly as a concentrate. In addition, the Fe (II) / Fe (III) can be continuously recovered from the waste water and recycled. The process according to the invention can preferably be carried out without subjecting the mixture to precipitation or reduction of the Fe (III) to Fe (II) prior to membrane separation.

The advantages of the method according to the invention are essentially the reduction of the wastewater-specific amount of metal salts and thus the metal sludge to be disposed of by recovering the iron, and the increase in the space-time yield of the chemical reactor by recycling not 4 implemented wastewater constituents, and the reliable reduction of problematic wastewater constituents by chemical conversion into biodegradable components.

Surprisingly, it was found that the continuously recycled oxidized or “used” iron salts permanently maintain the Fentons reaction and that the space-time yield can be increased by a factor of 10-15.

An exemplary embodiment of the invention is described in more detail below with reference to FIGS. 1 and 2. It is not intended to limit the invention in any way.

It shows

1: a process flow diagram of a first particular embodiment of the method according to the invention; 2 shows a process flow diagram of a second special embodiment of the method according to the invention.

According to the inventive method (Fig. 1) 1 wastewater c _AO χ, hydrogen peroxide c _H2 o2 and Fe (II) solution c _Fβ (iron salt) is mixed in a reactor and by means of a pump 2 via a catalyst 3 to a downstream membrane separation 4 Retention of iron ions supplied. After membrane separation 4, which enables an additional purification of the waste water, the retentate 5, which is enriched in iron ions and insufficiently oxidized organic compounds (waste water constituents), is returned to the reactor 1 for further oxidation. If necessary, in order to prevent accumulation of ions or compounds that are difficult to oxidize, a partial flow can be discharged at any point in the process cycle. A permeate 6 leaves the membrane separation 4 as treated waste water.

The direct feeding of the waste water into the reactor 1 for oxidation, as shown in Fig. 1, is particularly suitable for highly concentrated waste water. For diluted 5 Waste water is recommended as a further embodiment, the direct feeding of the waste water into the membrane separation.

In a further embodiment of the invention (FIG. 2), the hydrogen peroxide is mixed with the retentate in a mixer 7 and fed to the reactor 1.

In the following, the invention is further explained on the basis of tests carried out and numerical simulation.

Examples

Discontinuous attempt

An aqueous solution of 4-chlorophenol with a COD = 9.65 g / l (chemical oxygen demand) and an AOX = 1.66 g / l (adsorbable organic halogenated hydrocarbons) was used as model wastewater. The pH of this wastewater was adjusted to pH = 2 by concentrated sulfuric acid. After the iron salt (FeSO ₄ solid) had dissolved in the model wastewater, the H ₂ 0 _{2 was} added dropwise within 15 minutes, the temperature rising to 52 ° C. So much H ₂ O _{2 was} metered in that the proportion of active oxygen was 100%. This means that the amount of oxidizing agent H ₂ O ₂ is theoretically sufficient to completely degrade the initial COD. The amount of salt was chosen so that the ratio Fe: H ₂ O ₂ = 1:10. The mixture was then stirred for 90 minutes until the temperature had almost dropped to room temperature. Analysis of the treated waste water showed a COD = 2.41 g / l and an AOX = 0.13 g / l. This corresponds to a COD reduction of 75% and an AOX reduction of 92%.

In the second stage, the oxidatively treated model wastewater was treated by the membrane separation process nanofiltration. Like reverse osmosis and ultrafiltration, nanofiltration is a pressure-driven membrane process for working up aqueous solutions down to the molecular range and can be used with regard to operating pressure (5-30 bar) and Place a 6 molecular separation limit (200-500 g / mol) between these processes. To characterize the principle suitability of the method, two different nanofiitration membranes were first examined in a flat channel test cell with an effective membrane area of A _m β _m = 80 cm ² .

The experiment showed that a selective separation of the catalytically active iron ions from the wastewater is possible with reasonable permeate flows. The analysis of the wastewater after the membrane stage resulted in the use of membrane A (approx. 300 g / mol) approx. One COD = 0.3 g / l and one AOX = 0.03 g / l and for membrane B ( Separation limit approx. 200 g / mol) a COD = 0.2 g / l and an AOX = 0.03 g / l. This corresponds to COD reductions of around 90% in the membrane stage or 98% for the overall process. The AOX reduction is about 75% for the membrane stage and 98% for the overall process. The iron content after the membrane is very low at 4 mg / l (B) and 8 mg / l (A). The iron retention is higher than 99% in both cases.

Continuous attempt

The test arrangement essentially consisted of a tempered, double-walled glass container (3 l), a nanofiltration apparatus with a flat channel test cell (effective membrane area A _mem = 80 cm ² ) and a static mixer (FIG. 2).

Analogous to the discontinuous test, model wastewater from an aqueous solution of 4-chlorophenol (6.25 g / l) with a COD = 10 g / l and an AOX = 1.72 g / l was used. At the start of the continuous tests, 2 liters of the model wastewater that had already largely reacted were submitted. This waste water was adjusted to pH = 2 by concentrated sulfuric acid. After the iron salt (10.58 g / l FeSO ₄ ) had dissolved in the model wastewater, 42.5 g H ₂ O ₂ solution (50% by weight) was slowly added, the temperature rising to 40 ° C. as a result of the oxidation reaction. The amount of H ₂ O ₂ corresponds to 100% of the active oxygen, ie the H ₂ O ₂ is theoretically sufficient to completely degrade the initial COD. The amount of salt was chosen 7 that the ratio Fe: H ₂ O ₂ = 1:10. The analysis of the treated wastewater showed a COD = 1.3 g / l and an AOX = 0.027 g / l. This corresponds to an initial COD reduction of 87% and an AOX reduction of 98.4% (batch reactor).

After the test set-up was put into operation with the amount of wastewater presented and the nanofiltration membrane C (separation limit approx. 200-300 g / mol) was conditioned for several hours, the permeate flow was 39.5 l / m ² h. The experiments were then continued by continuously adding model wastewater and H ₂ O ₂ . The metering was carried out at constant volume, ie the amount of waste water corresponding to the permeate flow was determined

(0.32-0.24 l / h), a side stream was not removed. A total of 5.6 liters of model wastewater were processed using the method according to the invention.

The results of the experiment (Table 1) show that by using the method according to the invention, efficient treatment of waste water with water constituents that are difficult to biodegrade or toxic to bacteria with AOX elimination rates over 99.5% and COD elimination rates over 98% are possible.

The steady-state degradation rates in the circuit are, as expected, well below the values of the freight-related degradation rates. While the latter are essentially determined by the initial state of the fully reacted wastewater, in the steady state the selective separation of the nanofiltration leads to an increase in the AOX and COD concentration in the circuit and thus to a higher reaction rate in the system and a higher space-time yield Reactor.

The iron retention capacity of the membrane means that the wastewater-specific amount of iron salt used can be reduced between 90% and 98% depending on the selectivity. The iron retention capacity of the membranes was 95% on average in the tests.

Exemplary numerical simulation 8th

An exemplary estimate of the expected performance characteristics of the method according to the invention can be carried out numerically, assuming idealizing conditions. 3 and 4 show results of the simulation calculations of an oxidation reaction in a continuous stirred tank and a process according to the invention consisting of a stirred tank and membrane separation (process according to FIG. 1 without catalytic decomposition). The system of equations was drawn up from mass and mass balances for the sub-balance areas and solved numerically using the MathCad 5.0 program. This system of equations can be used to estimate the influence of certain operating parameters and the potential of the method. The following simplifications were made:

continuous operation, without catalytic decomposition and side stream 2nd order reaction rate approach for AOX oxidation isothermal conditions - constant retention to describe the local

Permeate compositions within the membrane stage

3 shows the AOX concentrations (educt of the oxidation) to be achieved in steady-state operation as a function of the residence time.

It is clear that with a comparable dwell time by adding a membrane stage (method according to the invention), the AOX discharge values can be reduced by a factor of 10-15 compared to a stirred tank. The return of unreacted AOX components to the reactor leads to an increase in the AOX concentration (w _A ox, _R βak process according to the invention) and thus to an increase in the average reaction rate in the reactor. This results in an additional increase in the efficiency of the method.

4 shows the transient concentration curve of the iron in the reactor for a stirred tank and for the process according to the invention with an average residence time of 1 h and various iron selectivities of the membrane. It is shown that with an iron retention capacity of 98%, a continuous addition of iron of <50 mg / l is sufficient to maintain the initial concentration of 2.12 g / l even in the stationary case. In the case of a continuous stirred tank, on the other hand, the amount of iron initially used is almost completely lost in the first 4 hours.

The tests carried out show that the use of the process according to the invention enables efficient treatment of waste water with water constituents which are difficult to biodegrade or are toxic to bacteria with AOX elimination rates of over 99.5% and COD injection rates of over 98%. As a result of the iron retention capacity of the membranes, the amount of iron salt used in the wastewater can be reduced between 90% and 98% depending on the selectivity. The test results achieved are confirmed by an exemplary simulation of the method according to the invention. On the basis of these calculations, it is clear that the space-time yield of a reactor can be increased by a factor of 10-15 by using the method according to the invention.

Table 1: Freights, concentrations and performance of the process according to the invention integral overall balance model concentration permeate degradation rate total freights wastewater in the system elimination

AOX [mg] 9632 249.3 25.4 97.1% 99.7% COD [mg] 56000 6 160 612 87.9% 98.9%

Total balance model concentration permeate degradation rate total concentration waste water in the system elimination quasi-steady state

AOX [mg / l] 1 720 155.8 6.34 90.6% 99.6% COD [mg / l] 10000 3850 153 61.5% 98.47%

Claims

10 claims

1. A process for the oxidative treatment of water containing organic compounds, in which the water is mixed with a reagent containing hydrogen peroxide and dissolved iron or titanium ions, characterized in that the mixture is passed through a membrane separation stage in which the mixture is a permeate as treated wastewater.

2. The method according to claim 1, characterized in that in the membrane trenπstufe polyvalent ions are retained in the mixture by the membrane and monovalent ions, water, and the oxidized and / or partially oxidized organic compounds penetrate through the membrane.

3. The method according to claim 1 or 2, characterized in that the membrane separation stage is a nanofiltration.

4. The method according to at least one of claims 1 to 3, characterized in that the membrane is made of a polymer or of a ceramic material, preferably of polysulfone, polyether sulfone, modified polyamide or of aluminum oxide, zirconium oxide or silicon carbide.

5. The method according to at least one of claims 1 to 4, characterized in that the separation limit of the membrane is greater than 100 and less than 1000 g / mol, preferably between 200 and 500 g / mol.

6. The method according to at least one of claims 1 to 5, characterized in that there is a pressure difference across the membrane from the range of 1 to 100 bar, preferably from the range of 10 to 50 bar.

7. The method according to at least one of claims 1 to 6, characterized in that the membrane separation is carried out at a temperature in the range of 5 to 60 C. 1 1

8. The method according to at least one of claims 1 to 7, characterized in that one carries out the mixing of the water with the reagent in a container, that the mixture is withdrawn from the container and fed to the membrane separation stage and that the retentate of the membrane separation stage entirely or partially returns to the container.

9. The method according to claim 8, characterized in that the residence time of the hydrogen peroxide when feeding the mixture from the container into the Membrantreπnstufe is dimensioned such that the hydrogen peroxide has decomposed before reaching the membrane separation stage.

10. The method according to claim 8 or 9, characterized in that the mixture is passed before the membrane separation stage for the decomposition of the hydrogen peroxide over a catalyst.