ES2612380B1 - Wastewater treatment procedure: FLOX process - Google Patents

Abstract

Wastewater treatment procedure: flox process. # This procedure includes the simultaneous addition and at the same stage of a mixture of at least two oxidants between: ozone, oxygen and / or hydrogen peroxide, and a coagulant comprising Fe ions (III) in amounts suitable to produce coagulation and precipitation of ferric hydroxide at a pH between 3 and 7, it comprises a final stage of separation of the treated waters from the particles formed during the process and, optionally, the addition of a flocculant in the oxycoagulation stage or in a subsequent stage, a neutralization and / or precipitation stage, and a series of steps for the recycling of ferric hydroxide obtained as a byproduct.

Description

PROCEDURE FOR THE TREATMENT OF WASTEWATER: FLOX PROCESS

OBJECT OF THE INVENTION

The present invention relates to a process for the treatment of wastewater, and especially toxic or biorefractory wastewater with contaminants dissolved therein.

BACKGROUND OF THE INVENTION

At present, different procedures for the treatment of wastewater are known whose purpose is to eliminate the contaminating elements contained therein until recovering a quality that allows the

15 water reuse. The most used procedures for the treatment of this type of wastewater are carried out by physical-chemical, biological or incineration methods. The procedures that are most related to what is

The object of the present invention are those which include oxidative methods of pollutants, usually comprising the addition to the waters to be treated of an oxidant, generally ozone or hydrogen peroxide; and including the process, in earlier or later stages, the addition of coagulants such as aluminum salts, iron salts or lime, to achieve a reduction in the final concentration,

25 and the addition of flocculants in order to allow sedimentation and removal of the components in a final filtration stage. Oxidative methods-based procedures often use ozone as an oxidizing agent, since it is capable of participating in a large number of reactions with organic and inorganic compounds, providing a

30 decrease in color and turbidity of wastewater. It also acts as a disinfectant agent, given its biocidal properties. However, conventional ozonation processes consume at least stoichiometric amounts of ozone, which is an expensive oxidant, so they are only applied as a tertiary treatment.

or in dilute solutions, for example for the purification of mouth water. These 35 stoichiometric quantities are governed by the Chemical Oxygen Demand

(000) of each wastewater. On the other hand, ozone is not able to mineralize in itself

only to organic pollutants, so it is used by activating it, for example, with ultraviolet (UV) lamps to produce hydroxyl (OH) radicals. In those cases the costs soar and the treatment becomes in fact impracticable for very colored or turbid waters due to their low transparency to UV radiation.

Frequently these wastewater treatment procedures include, at a stage before or after ozonation, coagulation and f10culation processes in which colloidal particles and suspensions of very fine particles are combined into agglomerates that can be separated via sedimentation, notation, filtration, centrifugation or other separation methods. This separation is achieved by adding different chemical substances (coagulants and flocculants) that favor destabilization and agglomeration of individual colloidal particles. In coagulation the colloidal particles and suspensions of very fine solids are destabilized, to begin to agglomerate in microflocols, if the conditions are appropriate; while in the flocculation, the microflocles agglomerate forming macrophages, which can be filtered or separated by decantation. However, coagulation is not very efficient when it comes to eliminating contaminants dissolved in water (see example 2).

As background to the present invention, it is worth mentioning the existence of different documents describing treatments of contaminated water by oxidative processes with ozone or hydrogen peroxide and, in some cases, coagulation and flocculation processes before or after ozonation; The following documents being outstanding:

In US4156648 a method for the removal of suspended solids in wastewater is described, including a centrifugation process, a coagulation and flocculation process, a filtrate for the removal of colloidal particles, a pressurization and depressurization treatment and finally introducing ozone for the oxidation of organic matter.

In US5560831 a method for the purification of wastewater from the washing of agricultural products is described, initially including a series of coagulation stages and subsequently a final ozonation.

In US5639379 a process for the removal of the color and odor of aqueous tributaries contaminated with textile dyes is described, the effluent being treated with permanganate, followed by a treatment with hydrogen peroxide, a coagulation and flocculation treatment maintaining a basic pH, a treatment with a water soluble polymer and the separation of the precipitated material.

Patent US5679257 describes an industrial wastewater treatment in which ozone is used for the elimination of contaminants, combined with the application of ultrasound to cause coagulation and precipitation thereof. In a second stage, azonization is combined with ultraviolet light.

In US5888403 a procedure and a system for water treatment is described, in which the water to be treated is introduced into a first ozone mixer and subjected to magnetic forces, the ozone being fed from a first ozonator to produce oxidation and coagulation of polluting substances, these being eliminated by a first filter. The resulting water is subjected to a second ozonation treatment and magnetic forces by introducing the resulting water into a reaction vessel containing active carbon, the substances coagulated by a second filter being removed.

In US6180014 a device and method for the treatment of water with ozone generated by water electrolysis is described. Ozone is produced "in situ" at high concentration through the interaction of electrolytically produced oxygen and ultraviolet light, being used to produce oxidation of pollutants. The by-products are subsequently removed by decantation, flocculation, coagulation or filtration.

Patent G82056962 describes a process for purification of wastewater containing coloring materials; said process comprising an oxidation phase in which hydrogen peroxide is used as an oxidizing agent and with coagulation, flocculation and decantation phases, before or after.

In the aforementioned background the use of oxidative processes and coagulation and flocculation processes is frequent but always in separate stages or phases.

On the other hand, it is worth mentioning the existence of the well-known Fenton process, which consists of the oxidation of organic molecules through the use of hydrogen peroxide and a salt of Fe (II), which is added, in small quantities, as a catalyst. This process is based on the generation of OH radicals by the catalytic decomposition of hydrogen peroxide in acidic medium. These radicals rapidly oxidize most organic substances until they are mineralized, transforming them into CO2 • water and inorganic species. Oxidation processes that are based on the in situ generation of OH radicals from clean oxidants such as those mentioned above are generically called advanced oxidation processes.

The aforementioned Fenton process presents a series of drawbacks for the treatment of wastewater, the following being noteworthy: the need to control an acidic pH (2-3), which then requires significant amounts of base for neutralization, and the use of quantities of Oxidant in excess, both aspects making the global process more expensive.

On the other hand, the presence of certain substances such as chlorides, bicarbonates and bisulfalos are a problem since they consume OH radicals, competing with the pollutants to oxidize. In addition, various suspended materials, such as organic micelles or cells, can disintegrate or break their membrane by reacting with oxidants, thereby increasing the organic load dissolved in water.

Other related processes, such as electrochemical oxidation, are relatively slow, have a high energy cost and need a high conductivity of the water to be treated, which remains in the already treated.

The use of other chemical oxidants, such as permanganate or chlorine, leaves by-products and residues, such as manganese dioxide or organochlorine compounds, which can be toxic or dangerous and often difficult to separate from treated water.

DESCRIPTION OF THE INVENTION

In accordance with the invention and in order to solve the exposed problem, this process includes the simultaneous addition and at the same stage of a mixture of at least two oxidants between ozone, oxygen and / or hydrogen peroxide, so that ozone and / or hydrogen peroxide are added in sub-stoichiometric quantities with respect to the DaO of the pollutants to be removed, and an iron-containing substance Fe (lIl) in adequate amounts to produce coagulation and precipitation of ferric hydroxide to a pH between 3 and 7. We will call oxycoagulation at this stage that integrates oxidation and coagulation at the same time.

Our investigations have revealed that the combination of a coagulation-flocculation process using an iron compound capable of supplying Fe (lll) ions to the contaminated water, with simultaneous dosing and at the same stage of two or more clean oxidants, such as cited above, provides a synergistic effect with obtaining results superior to those achieved by using both processes separately (see example 2)

This cooperative effect of the integration of both processes in a single stage can be explained by admitting that small amounts of oxidants are capable of creating radical oxidation and / or electrically charged intermediate products, of sufficient life to be trapped (by adsorption or occlusion) in the precipitate of ferric hydroxide that occurs in the short time of coagulation, usually a few minutes. It is important that the coagulant comprises Fe ions (II) given their ability to catalyze oxidation reactions with radical generation. Adsorbed radicals on the surface can help coagulation and f10culation of particles by coupling, reducing or eliminating the needs of flocculants.

However, this procedure can be complemented by the addition of a flocculant, either in the initial stage of oxycoagulation or in a subsequent stage of flocculation and sedimentation in a decanter.

The introduction of at least one oxidant in the gas phase can also produce the flotation of the polluting substances or their flocs, facilitating their separation from the treated water

In case one of the oxidants used is oxygen, this can be pure or in the form of a gas containing it, for example air, which substantially lowers the cost of the process compared to those that use pure gases.

On the other hand, by not using stoichiometric amounts of expensive oxidants (ozone, hydrogen peroxide), the costs are reduced compared to other advanced oxidation processes.

On the other hand, the amounts of iron suitable for oxycoagulation are not catalytic as in Fenton processes, but clearly higher, of the order of 1 gram / liter. According to the invention, the amount of ferric ions Fe (lll) that are supplied to the wastewater is greater than 0.1 grams / Iiter and preferably is between 0.2 and 2 grams / liter, although the optimal dose depends on each wastewater and should be studied on a case-by-case basis (see example 1).

The process also contemplates, if the case requires, a neutralization and / or precipitation step by the addition of an alkaline compound, such as a hydroxide and / or a magnesium, calcium, sodium or potassium carbonate.

The process culminates with a final stage of separation of the ferric hydroxide precipitate from the purified waters by a conventional system, such as decantation, flotation, centrifugation or filtration. Saying

By-product can be recycled to the process after a heat treatment to oxidize contaminants entrained with it and subsequent acid redisolution.

According to the invention, the iron can be added either in the form of iron compounds, for example Fe (111) salts such as sulfates or chlorides, or in the form of metallic iron or alloys containing it. The latter materials can be dissolved in the water to be treated by oxidizing attack in acidic medium or by anodic dissolution8. The addition of iron in any of these forms allows obtaining satisfactory results.

The process of the invention can be carried out at any temperature between DOC. and ggoC, preferably between 10 ° C and 50 ° C. However, its remarkable efficiency at room temperature and at atmospheric pressure, allows to significantly reduce its costs.

This procedure has a number of important advantages over those currently used, the following being notable:

•

It can develop at room temperature (as opposed to incineration) and at atmospheric pressure (as opposed to wet oxidation -wet oxidation, in English- or supercritical oxidation) so it does not require an energy input higher than that required to generate the ozone, in the case of using this oxidant.

•

It does not need a pH as acidic as the Fenton process, so that the consumption of acids, and bases for subsequent neutralization, are much lower.

•

Unlike the Fenton process and other advanced oxidation processes, an excess of chlorides, bicarbonates, bisulfates or other radical-trapping substances (scavengers) or the presence of suspended solids is not a problem for their development.

•

Nor does the conductivity of the waters (as opposed to electrochemical processes), nor its color or transparency (as opposed to photochemical processes, with visible light or UV lamps) influence it.

•

It does not need special materials (such as certain electrodes in electrochemical processes).

•

Quickly and efficiently removes total organic carbon (TOC), chemical oxygen demand (000), suspended materials, color, odor, turbidity, ammoniacal nitrogen and toxicity. The treated waters are also disinfected by their germicidal action.

•

The only notable investment cost is the ozonator, in case ozone is used as an oxidant.

•

The operating costs are significantly lower than those of incineration (due to the large amount of energy needed to evaporate all the water to be treated) or those of advanced oxidation processes, which consume higher amounts of expensive oxidants.

•

The oxidizers used are not toxic and are clean since their only reaction by-products are oxygen and water.

DESCRIPTION OF THE FIGURES.

Figure 1 shows a comparative graph of the results obtained in the treatment of water contaminated with phenol by a conventional ozonation process and in several experiments performed according to the procedure object of this invention.

Figure 2 shows a comparative graph of the results obtained from reduction of IOC and or of contaminated water by conventional coagulation treatments (A), ozonation (B), coagulation with pre-ozonation

(C) and by the method of the invention (O) respectively.

PREFERRED EMBODIMENT OF THE INVENTION

EXAMPLE 1

In a series of experiments, aqueous solutions of 100 ppm of phenal were treated by the process of this invention, using a combination of 0 3, O2 and FeCb at pH 6, controlled by the addition of sodium hydroxide, and varying the mole ratio Fe ( IH) / phenol between 1 and 4. Fe (III) was added at the beginning and the tests were performed at room temperature. During the experiments samples were taken that were filtered before analyzing their Cal. In Figure 1 they can

observe the results obtained for the different mole ratios Fe (III) / Phenol.

In this figure, the results obtained through a conventional simple ozonation process have also been represented for illustrative purposes and in order to be able to carry out a comparative study.

In the mentioned figure 1 it can be seen how the results obtained by the process of the invention considerably improve the degree and speed of mineralization with respect to those obtained with the conventional simple ozonation process. The rapid decreases in COT during the first 5 minutes of the process, which approximate 70% in all cases, are especially notable. During that time a total of 8.7 millimoles of ozone was introduced into the system (a part of which did not react since a residual ozone concentration was detected in the output gas exceeding 12 g / m3), while having take into account the stoichiometric equation

C6 H50H + 14 0 3 -7 6 CO .. + 3 H20 + 14 O .. the reaction of at least 10.4 mmol of ozone would have been needed to achieve the mineralization obtained, which illustrates the consumption of a sub-stoichiometric amount of Ozone in the oxy-coagulation.

Also in the mentioned figure it is also observed that increasing the Fe (l) Ifenol molar ratio results in an increasing mineralization for the 1, 1.5 and 2 ratios; and that the degree of mineralization does not increase significantly as the iron dose increases above the molar ratio 2.

COD eliminations were even greater than those of COT. For example, after 60 minutes of reaction, 92% and 95% of COD were removed in experiments with molar ratios 1 and 2 respectively.

EXAMPLE 2

One of the experiments carried out according to the process of the invention, specifically with 2mol Fe (III) / mol phenol, was carried out at 19 ° C, with a generation 0 3 of 5 gl h And a regulated pH between 6.0 and 6, 8 by the addition of NaOH, achieving in 60 minutes of treatment the values of the Chemical Oxygen Demand (COD) and Total Organic Carbon (TOC) detailed below:

t (min.) O 60

pH 6.3 6.7 0 3 residual g / m3 0.0 13.1 COT% 100.0 19.2 000% 100.0 5.0

The graph of Figure 2 compares the results of COI and DaO reduction obtained in the previous experiment (D) with the results obtained in 5 similar conditions by conventional coagulation treatments (A),

ozonation (B), and coagulation with pre-ozonation (C).

In the mentioned figure 2 it can be seen that it is the oxycoagulation process, corresponding to the process of the invention, that provides better results (D), both in the reduction of COT and 000.

10 EXAMPLE 3 In another embodiment, industrial wastewater has been treated with the following characteristics of origin:

fifteen

pH = 8 Total Organic Carbon (TOC) = 72000mg / L

Chlorides = 24600 mg / L

Chemical Oxygen Demand (DOO) = 174000mg / L

Ammoniacal N = 500mg / L

Biological Oxygen Demand (BOD) = 200 mg / L

The treatment was carried out on 750 mL of diluted solution of said 20 waters in a 1/10 ratio, with a mixture of oxygen and ozone.

The maximum gas flow used was 200Llh. With the application to the ozone generator of a current of 1 amp, a resulting flow of 13g / h of ozone is achieved.

Fe (ll!) In the form of hydrated iron sulfate was used for its treatment. 50g were weighed and dissolved in 150mL of hot millipore quality water and with the addition of sulfuric acid to pH O.

The solution was treated at room temperature (26 ° C). By continuously introducing ozone into the system, foams were produced which were controlled by reducing the gas flow rate, but by adding Fe (III) to the solution, the 30 foams increased while the pH decreased. By adding diluted NaOH the pH was increased to 6 and oxycoagulation occurred. Later,

by adding H2S04, the pH was reduced to 4 and the foams decreased. The gas flow rate was slowly increased for 15 minutes to the point where all the ozone could pass through the reactor. The pH continued to drop spontaneously, but was controlled between 3 and 4 during the rest of the experiment. In total, 8 mL of Fe (II I) solution was fed into the system.

The measures of Total Organic Carbon (TOC) in samples obtained during treatment were the following:

t (m in.) O 5 40

COT 7000 2700 2600 % COT 100 39 37

10 As can be seen, again the elimination of most of the COT occurred during the first 5 minutes, when oxycoagulation occurred. The energy consumption of the ozonator during these 5 minutes was less than 0.05 Kwh / L, which means a very low cost. A major reduction of ammoniacal nitrogen was also obtained.

EXAMPLE 4 In a final embodiment, 30 liters of a solution containing 30 grams of aniline were treated at an initial pH of 3 in a stirred reactor

20 with 0.37 mol / hour of hydrogen peroxide dosed regularly at a temperature of 40 ° C. The stoichiometric amount to completely oxidize the aniline would have been more than 5 moles of said peroxide. Simultaneously, Fe (III) ions were introduced into the system by dissolving metallic iron and oxygen through a diffuser. The pH increased spontaneously during the process, but was maintained

25 between 3 and 4 by adding small volumes of sulfuric acid. At the same time a precipitate formed containing ferric hydroxide and coagulated organic substances together with it. After one hour the Total Organic Carbon (TOC) of the solution had decreased by 61%, and after 2 hours by 83%.

Once the nature of the invention has been sufficiently described, it is stated for the appropriate purposes that it may be introduced

modifications deemed appropriate as long as they do not involve an alteration of the essential features of the invention that are claimed below.

Claims (7)

1.-Procedure for wastewater treatment; characterized in that it comprises the simultaneous addition and at the same stage of a mixture of at least two oxidants between: ozone, oxygen and / or hydrogen peroxide, so that the amounts of ozone and / or peroxide are sub-stoichiometric with respect to the pollutants to be removed, and an iron ion supply substance Fe (llI) in amounts between 0.1g / LY 2g / L to produce coagulation and precipitation of ferric hydroxide at a pH between 3 and 7, because it comprises a final stage of separation 10 of the treated waters of the particles formed during the process, and because it optionally comprises: - the addition of a fluid in said oxycoagulation stage or in a subsequent stage of foculation, - a neutralization and / or precipitation stage, 15 -and a series of steps for the recycling of ferric hydroxide comprising a heat treatment to incinerate the contaminants precipitated with it and a subsequent acid redisolution of the resulting solid. 2. Method according to claim 1, characterized in that the optional neutralization and / or precipitation step includes the addition of a basic compound, such as a hydroxide and / or a magnesium, calcium, sodium and / or potassium carbonate. 3. Method according to claim 1, characterized in that the amount of iron ions Fe (lIl) that are supplied to the wastewater is greater than 0.1 grams / Iiter. 4. Process according to claims 1 and 3, characterized in that the amount of iron ions Fe (II) that are supplied to the wastewater is between 0.2 and 2 grams / liter. 5. Method according to any preceding claim, characterized in that it is carried out at a temperature between 0 ° C and 99 ° C, preferably between 10 ° C and 50 ° C. 6. Method according to claim 1, characterized in that the addition of Fe ions (lIl) is carried out in the form of iron compounds. 7. Method according to claim 1 and 6, characterized in that the 5 iron compounds are salts of Fe (11 1) such as sulfates or chlorides. 8. Method according to claim 1, characterized in that the addition of iron is carried out in the form of metallic iron or iron alloys that dissolve in the water to be treated by oxidative attack in acidic medium or by dissolution 10 anódic8.