WO2011098167A1 - Wastewater purifying system - Google Patents

Abstract

The invention relates to a wastewater purifying system for purifying wastewater (1) of a machine for producing a paper, cardboard, tissue, or other fibrous material web (2), said machine having at least one press section (8) for dewatering the fibrous material web (2). The aim of the invention is to reduce the pollution of the water circuit of the machine by colloidally dissolved substances in an efficient manner by feeding at least some of the wastewater (1) of the press section (8) into an electrophysical purification stage (3) for coagulation and flocculation and subsequently into a separation stage (4) with floc enlargement and floc separation.

Description

 wastewater treatment plant

The invention relates to a wastewater treatment plant for the purification of wastewater of a machine for producing a paper, board, tissue or another fibrous web, which at least one press section for dewatering the

Has fibrous web.

The increasing use of rejects and waste paper in papermaking and the increased reduction in the use of fresh water have led to an increase in harmful or disturbing substances in the water cycles. Also, chemical additives (e.g., oils, solvents, rosin size, sizing agents, adhesives, wet strength agents, retention aids, starch, biocide formulations,

Dispersants, bleaching chemicals, cleaning agents, dyes, complexing agents and solubilizers), which are specifically supplied to the process, contribute by enrichment in the circuits to an increase in the concentration of colloidally dissolved impurities directly or from the interaction with each other. Other sources include extracts from the pulps, lignin and lignin derivatives,

Hemicelluloses and carbohydrates.

The increasing concentration of contaminants results in reduced efficiency of mostly cationic functional chemicals (e.g., fixatives, retention polymers). The saturation of the process water with colloidally dissolved, anionic impurities, which occurs at high process temperatures, increases in the cooler zones

Precipitates and deposits. Even low temperature gradients are sufficient to cause sticky deposits on hydrophobic or particularly adhesive surfaces (screen material, felt material, roll surfaces, low-flow zones). These can affect the process sensitively by forming holes in the paper, breaks and cleaning stops.

The high loads can also lead to increased microbiological activity in laid paper, as it contains easily metabolizable substances. Paper properties such as whiteness, opacity, color and strength are also affected by the presence of colloidal impurities.

In addition, an increased tendency to odor in the paper may occur. In addition, the dewatering and drying process in papermaking can lead to a significant deterioration in runability due to the fate of these contaminants in the finest capillaries

reduce the speed.

 The impurities can also lead to increased foam by lowering the surface tension, which has a negative impact on the paper quality or makes the increased use of antifoaming agents required.

The accumulation of contaminants in the entire water cycle system depends on the amount of raw materials supplied, the process temperature, the

Extractability, the water circulation rate, the amount discharged with the wastewater, the discharge of impurities with the paper produced and the supply of fresh water.

In particular, for the narrowing of the water cycles, i. the reduced supply of fresh water and the correspondingly reduced removal of wastewater sets up an increased concentration of colloidally dissolved impurities in the circulating waters. In addition to the high concentration level, dynamic fluctuations of the contaminant loads also represent a limitation for a targeted chemical-technological management of the process. This leads to continuous incorrect dosing of functional chemicals with the effects described above.

The object of the invention is therefore to enable the removal of colloidally dissolved impurities in an efficient manner.

According to the invention the object has been achieved in that at least a portion of the wastewater of the press section in an electrophysical cleaning step for coagulation and flocculation and then in a deposition step with

Flake enlargement and flocculation is performed. By focusing on the treatment of the usually most polluted wastewater of the production machine, the cost of the electro-physical cleaning can be kept within reasonable limits. Nevertheless, this cleaning causes a significant minimization of the burden of

Water cycles of the machine as a whole.

Furthermore, the electrophysical purification of the wastewater is the

Press section very efficient to reduce concentration peaks in terms of loading with colloidally dissolved impurities in the circuits.

To minimize the effort, it may be advantageous if only wastewater of the press section, is performed in the electrophysical cleaning stage.

In the interest of a comprehensive cleaning as possible, it may be advantageous if wastewater of the press section and from the sheet forming area of the

 Manufacturing machine is performed in the electrophysical cleaning stage.

Depending on the load of the wastewater, it may be necessary that all the wastewater of the press section is passed into the electrophysical cleaning stage. Often, however, it is sufficient if only a part of the waste water of the

Press section is led into the electrophysical cleaning stage.

As a result, at least a portion of the wastewater purified in the electrophysical cleaning stage may be used as a partial or complete fresh water replacement in the manufacturing process, i. to the production machine of the fibrous web

to be led back.

In the field of the manufacturing machine, this purified wastewater can be used in particular for water spray devices, such as spray tubes or for chemical dilution. The remaining part of the waste water purified in the electrophysical purification stage should be fed into the stock preparation preceding the production machine or the constant part.

 In stock preparation, a pulp suspension of pulp and / or waste paper is made with the addition of water and additives.

 In the constant part, the highly consistent pulp suspension of the stock preparation, which is essentially formed of processed fibers and fillers, is diluted with water and then fed to the headbox of the production machine. To minimize the contaminant load, it is advantageous if the wastewater before the electrophysical purification stage by a filter, in particular a

Disc filter, a microflotation, sedimentation or similar to be led. However, it is also possible to use other purification stages, in particular other mechanical purification stages.

It is also advantageous if a cleaning stage with wet oxydation is present before the electrophysical cleaning stage, preferably between the filter and the electrophysical cleaning stage. Wet oxidation is understood to mean the chemical oxidation of organic compounds in a liquid phase.

 In this case, the wet oxidation by the introduction of ozone or oxygen (at high pressure and high temperature) can be brought about. However, it is considerably more efficient and less costly if the wet-oxidation purification stage is designed electrochemically and / or UV-supported.

In electrochemical wet oxidation, radicals or strong

Oxidizing agents (ozone, OH, O +) by decomposing water

produced electrolytic ways.

By applying a DC voltage, the water is decomposed, producing at the anode OH radicals and above a certain current ozone and at the cathode hydrogen. For this purpose, inert electrodes, preferably diamond electrodes should be used.

 In UV-based wet oxidation, the radicals or oxidants are generated by the irradiation of the waste water with UV radiation.

To intensify the COD reduction, it may be advantageous to use the electrochemical and the UV-supported wet oxidation in combination.

As a result, in particular, colloidally dissolved impurities in the wastewater are oxidized by this wet oxydation and their long-chain molecules are broken up.

In the purification step with electrophysical precipitation for coagulation and flocculation of colloidally dissolved impurities, the water to be treated is passed through a reactor equipped with sacrificial electrodes in which various electrochemical reactions take place by applying an electric current. This produces metal hydroxide flakes and various highly reactive radicals. The metal hydroxide flakes have a high adsorption capacity and can thus bind finely divided particles to themselves. It also leads to mit precipitation and inclusion precipitation reactions in which the colloidally dissolved substances are precipitated. In addition to the precipitation reactions, there are also oxidative effects, mainly by highly reactive oxygen radicals, which leads to further degradation of organic compounds.

In the subsequent to the electrophysical purification stage

Abscheidungsstufe it comes to flake enlargement and flocculation, in particular by means of flotation, sedimentation, filtration or the like. Accordingly, the particle agglomerates, with the help of the electro-physical

Precipitation / flocculation were generated in the previous step, here separated from the purified filtrate. In particular, the electrophysical cleaning stage allows in the invention, a demand-oriented, targeted transfer and the discharge of the most

anionic contaminant as a solid. With regard to the control / regulation of the sewage treatment plant, it is advantageous if the load level of the wastewater is measured and the intensity of the electro-physical cleaning is at least partially controlled / regulated in a manner that balances the stress level over time.

 On the one hand, this involves reducing the overall load level of the waters with colloidally dissolved impurities, so that certain water qualities are achieved, which reduce the use of fresh water.

Another often much more important object of the invention is the timely

Elimination or reduction of peak loads with the aim of setting a constant level of disturbance.

The measurement of the load level of the effluent, i. The concentration of impurities should be determined by measuring the chemical oxygen demand (COD), the dissolved carbon (DOC), the turbidity, the anionic charge character (SCD), the evaporation residue, the titrated cationic demand or similar. respectively.

In this case, the intensity of the electrophysical cleaning via the electrical power consumption, i. over the height of the voltage applied to the electrodes

Voltage and / or the amperage can be controlled / regulated. The invention should advantageously be used in particular in partially or completely closed processes with high levels of contaminant load and / or large fluctuations and / or small amounts of sewage or fresh water, since in these cases the effects of the concentration of colloidally dissolved impurities on the process stability are particularly disturbing are. The invention will be explained in more detail with reference to an exemplary embodiment. In the accompanying drawing, the figure shows an investment scheme of a

Wastewater treatment plant for a paper machine. The paper machine is used to produce a fibrous web 2 concretely a paper web and starts with a headbox, the pulp suspension 17 on an endlessly rotating sieve or between two rotating sieves of a

Sheet forming area 7 brings.

 In the sheet forming area 7, a considerable amount of water is separated via the sieves and collected as white water I in the white water tank 13. This results in sheet formation, which allows the subsequent dewatering of the fibrous web 2 in the press section 8.

In the press section 8 we led the fibrous web 2 together with at least one water-absorbing, endlessly circulating belt through at least one press nip. The hereby pressed from the fibrous web 2 white water II is fed into the white water tank 14.

After the press section 8, the fibrous web 2 passes through several more

Treatment steps, such as a drying section 9 for drying the fibrous web 2, optionally a coating device and usually also a smoothing device 10 for smoothing the fibrous web 2 before it can then be wound up. The headbox supplied pulp suspension 17 is formed in the upstream stock preparation 1 1 of pulp and / or waste paper with the addition of water, wherein the pulp suspension 17 in the stock preparation 1 1 several treatment stage, such as bleaching, grinding, etc. goes through.

After stock preparation 1 1, the pulp suspension 17 enters the

Constant part 12, in which additives are added and the consistency of the

Fibrous suspension 17 changed so, is usually lowered by dilution, that this can be performed to the headbox of the paper machine. This dilution in the constant part 12 is here, as usually, realized with the white water I from the white water tank 13.

The part of the effluent 1 not required for the dilution

Sheet forming area 7 passes from the white water tank 13 to

White water tank 14 of the white water II, i. of the waste water 1 of the press section 8.

The white water tank 14 thus comprises the entire waste water 1 of the press section 8 and a part of the waste water 1 of the sheet forming area 7.

Since the waste water 1 of the press section 8 contains significantly more colloidally dissolved impurities than the other waste water 1, this is subjected to a special treatment.

 In this way, peak loads can be reduced and because of the largely closed water cycle also generally the amount of stress can be substantially reduced.

The waste water 1 of the white water tank 14 is led to Entstoffung and pre-cleaning in a disc filter 6. While the fiber and filler-containing catcher material 16 of the disk filter 6 is fed into the stock preparation 1 1 for reuse, enters a portion of the effluent 1 of the disc filter 6 (clear filtrate) in a waste water tank 15 and the other part in an electrophysical

Purification level 3 for coagulation and flocculation.

This is possible because the wastewater 1 already because of the disc filter 6

sufficiently freed of suspended solids.

In the interest of a maximum reduction of the COD, the wastewater 1, as shown in the figure, are passed from the filter 6 via a cleaning stage 5 with wet oxygenation to the electrophysical cleaning stage 3. Easily oxidizable substances are to be converted into anionically charged impurities which are more accessible to the precipitation and flocculation treatment in an electrophysical purification stage 3. With the help of inert electrodes and an applied electric potential difference, ozone and highly reactive OH radicals are generated in the purification stage 5 with wet oxidation, which radicals act as oxidizing agents. In the course of this

Process are colloidally dissolved impurities oxidized in the water.

Contaminants consisting of long-chain molecules are used in this

Oxidation process broken up and made accessible to the downstream electro-physical precipitation / flocculation.

After the optional wet oxidation 5, the wastewater 1 passes through a

 Purification level 3 for coagulation and flocculation.

In this case, the colloidally dissolved impurities (anionically charged interfering substances, such as hemicelluloses, resins, polysaccharides, etc., which occur in particular in waste water from papermaking) are to be converted into larger agglomerates.

 For this purpose, the waste water 1 is guided into a container which has a sacrificial anode, for example of aluminum and a particular inert cathode, which are connected via a control unit with a DC voltage source. However, the cathode can also consist of sacrificial anode material, which provides the opportunity for

Pole reversal offers.

As already described, the electrical current flow leads to a

Electrophysical precipitation for the purpose of coagulation and flocculation of colloidally dissolved impurities.

Depending on the measured COD value of the wastewater 1, the current can be changed via a control unit so that it is not just one

Reduction but temporally also to a homogenization of the residual COD comes. In the subsequent to the electrophysical cleaning stage 3

Separation stage 4 can be used for flake enlargement and flake separation, in particular by means of flotation, sedimentation, filtration or the like.

In this case, the particle agglomerates which were generated with the aid of the electro-physical purification stage 3 are separated from the wastewater 1 and removed.

 The thus purified wastewater 1 is then passed into the Abwasserbehalter 15, from where it is passed into the stock preparation 1 1 and the manufacturing machine.

According to the claims and the load of the waste water 1, this can also be passed completely through the electrophysical cleaning stage 3 after the filter 6. But it is also possible only wastewater 1 from the press section 8 through the electrophysical cleaning stage 3 to lead or this

Deactivate cleaning stage 3 temporarily. By the invention, the proportion of fresh water continuously added and also the discharged amount of wastewater 1 can be significantly reduced.

Claims

Claims 1. Wastewater treatment plant for the purification of waste water (1) of a machine for producing a paper, board, tissue or another fibrous web (2), which has at least one press section (8) for dewatering the fibrous web (2), characterized in that at least a part of the waste water (1) of the press section (8) in an electrophysical cleaning step (3) for coagulation and flocculation and then in a deposition stage (4) with flake enlargement and flocculation is performed. 2. Wastewater treatment plant according to claim 1, characterized in that only waste water (1) of the press section (8) in the electrophysical cleaning stage (3) is guided. 3. Wastewater treatment plant according to claim 1, characterized in that Wastewater (1) of the press section (8) and from the sheet forming area (7) of the manufacturing machine in the electrophysical cleaning stage (3) is guided. 4. Wastewater treatment plant according to one of the preceding claims, characterized in that the entire waste water (1) of the press section (8) in the electrophysical cleaning stage (3) is guided. 5. wastewater treatment plant according to one of claims 1 to 3, characterized  characterized in that only a part of the waste water (1) of the press section (8) in the electrophysical cleaning stage (3) is guided. 6. Wastewater treatment plant according to one of the preceding claims, characterized in that at least part of the in the electrophysical Purification stage (3) of purified waste water (1) is fed back into the manufacturing machine. 7. wastewater treatment plant according to claim 6, characterized in that in the electrophysical cleaning stage (3) purified wastewater (1) in the manufacturing machine for Wassersprühvorrichtungen or for  Chemical dilution is used. 8. Wastewater treatment plant according to one of the preceding claims, characterized in that at least part of the in the electrophysical  Purification stage (3) of purified waste water (1) in the, the preparation machine upstream stock preparation (1 1) or the constant part (12) is guided. 9. wastewater treatment plant according to one of the preceding claims, characterized in that the wastewater (1) before the electrophysical  Cleaning stage (3) through a filter (6), preferably a disc filter is performed. 10. Wastewater treatment plant according to one of the preceding claims, characterized in that before the electrophysical cleaning stage (3), preferably between the filter (6) and the electrophysical  Cleaning stage (3) a cleaning stage (5) with wet oxidation is present. 1 1. Method for controlling the wastewater treatment plant according to one of  preceding claims, characterized in that the  Exposure level of wastewater (1) measured and the intensity of  electrophysical cleaning (3) at least partially in one  Load level is controlled in a time-uniform manner. 12.A method for controlling the wastewater treatment plant according to claim 1 1,  characterized in that the load level of the waste water (1) is determined by measuring the chemical oxygen demand (COD), the dissolved carbon (DOC), the turbidity, the anionic charge character (SCD), the Evaporation residue, the titrated cationic demand or similar is determined. .Method for controlling the wastewater treatment plant according to claim 1 1 or 12, characterized in that the intensity of the electro-physical cleaning (3) is controlled by the electrical power consumption.