DE102010001801A1 - wastewater treatment plant - Google Patents

Abstract

The invention relates to a wastewater treatment plant for cleaning wastewater (1) of a machine for producing a paper, cardboard, tissue or other fibrous web (2) which has at least one press section (8) for dewatering the fibrous web (2). The load on the water cycle of the machine with colloidally dissolved substances is to be reduced in an efficient way by at least part of the wastewater (1) from the press section (8) going to an electrophysical cleaning stage (3) for coagulation and flocculation and then to a separation stage ( 4) with flake enlargement and flake separation.

Description

The invention relates to a wastewater treatment plant for the purification of waste water of a machine for producing a paper, board, tissue or another fibrous web, which has at least one press section for dewatering the 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. Chemical additives (eg oils, solvents, resin glues, synthetic sizing agents, adhesives, wet strength agents, retention aids, starches, biocide formulations, dispersants, bleach chemicals, cleaning agents, dyes, complexing agents and solubilizers), which are specifically supplied to the process, are also supported 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 growing concentration of contaminants leads to a reduced efficiency of the mostly cationic functional chemicals (eg fixatives, retention polymers). The saturation of the process water with colloidally dissolved, anionic impurities, which occurs at high process temperatures, leads to precipitations and deposits in the cooler zones. 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 and can have a speed-reducing effect.

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, d. H. 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, this object is achieved in that at least a portion of the wastewater of the press section is guided into an electrophysical cleaning stage for coagulation and flocculation and then into a separation stage with flake enlargement and flake separation.

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 load on the water circuits of the machine as a whole.

Furthermore, the electrophysical cleaning of the wastewater from the press section is 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.

However, in the interests of the greatest possible purification, it may be advantageous if waste water from the press section and from the sheet forming area of the production machine is guided into 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 wastewater from the press section is led into the electrophysical cleaning stage.

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

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 micro-ligation, sedimentation o. Ä. Is performed. 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 to be electrochemically and / or UV-supported. In electrochemical wet oxidation, radicals or strong oxidants (ozone, OH ^- , O ⁺ ) are produced by the decomposition of water by electrolytic means.

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 deposition stage it comes to flake enlargement and flocculation, in particular by means of flotation, sedimentation, filtration o. Ä. Accordingly, the particle agglomerates, which were generated by means of electro-physical precipitation / flocculation in the previous step, here from purified filtrate separated.

In particular, the electrophysical cleaning stage allows in the invention demand-oriented, targeted transfer and discharge of the mostly 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 Störfrachtniveaus.

The measurement of the load level of the waste water, d. H. Concentration of impurities should be determined by measuring chemical oxygen demand (COD), dissolved carbon (DOC), turbidity, anionic charge character (SOD), evaporation residue, titrated cationic demand or similar.

In this case, the intensity of the electro-physical cleaning on the electrical power consumption, d. H. be controlled / regulated by the amount of voltage applied to the electrodes and / or the current.

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 installation diagram of a wastewater treatment plant for a paper machine.

The paper machine is used to produce a fibrous web 2 specifically a paper web and starts with a headbox, the pulp suspension 17 on an endlessly circulating sieve or between two surrounding sieves of a leaf forming area 7 brings.

In the leaf formation area 7 is separated on the sieves a significant amount of water and as white water I in the white water tank 13 collected. This leads to sheet formation, which is the subsequent dewatering of the fibrous web 2 in the press section 8th allowed.

In the press section 8th we use the fibrous web 2 passed together with at least one water-absorbing, endless circulating belt through at least one press nip. This from the fibrous web 2 Pressed white water II is in the white water tank 14 guided.

After the press section 8th goes through the fibrous web 2 even more treatment stages, such as a drying section 9 for drying the fibrous web 2 , optionally also a coating device and usually also a smoothing device 10 for smoothing the fibrous web 2 before it can be wound up.

The pulp suspension fed to the headbox 17 is in the upstream stock preparation 11 made of pulp and / or waste paper with the addition of water, wherein the pulp suspension 17 in the stock preparation 11 several treatment stage, such as bleaching, grinding, etc. goes through.

After the stock preparation 11 the pulp suspension arrives 17 in the constant part 12 , in which additives are added and the consistency of the pulp 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, with the white water I from the white water tank 13 realized.

The part of the wastewater not necessary for the dilution 1 of the sheet formation area 7 comes from the white water tank 13 to the white water tank 14 of white water II, ie of waste water 1 the press section 8th ,

The white water container 14 includes all wastewater 1 the press section 8th and a part of the sewage 1 of the sheet formation area 7 ,

Because the sewage 1 the press section 8th significantly more colloidally dissolved impurities than the other wastewater 1 This is subject 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 wastewater 1 of the white water tank 14 is for Entstoffung and pre-cleaning in a disc filter 6 guided. During the fiber and fillers containing catch 16 the window filter 6 in the stock preparation 11 led to reuse, a part of the wastewater passes 1 of the disc filter 6 (Clear filtrate) in a wastewater container 15 and the other part in an electrophysical cleaning step 3 for coagulation and flocculation.

This is possible because the sewage 1 because of the disc filter 6 already sufficiently freed of suspended solids.

In the interest of maximum reduction of COD, the wastewater 1 as shown in the figure, from the filter 6 via a cleaning stage 5 with wet oxidation for the electrophysical cleaning stage 3 be directed.

Easily oxidizable substances are to be converted into anionically charged impurities, the precipitation and flocculation treatment in an electrophysical purification stage 3 are more accessible.

With the help of inert electrodes and an applied electrical potential difference are in the purification stage 5 Produced with wet oxydation ozone and highly reactive OH ^- radicals, which act as an oxidizing agent. In the course of this process, colloidally dissolved impurities are oxidized in the water.

Disturbances consisting of long-chain molecules are broken up during this oxidation process and thus made accessible to the downstream electro-physical precipitation / flocculation.

After the optional wet oxidation 5 goes through the sewage 1 a cleaning step 3 for coagulation and flocculation.

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

For this purpose, the wastewater 1 led into a container which has a sacrificial anode, for example made 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 offers the possibility of pole reversal.

As already described, the electric current flow leads to an 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 comes not only to a reduction but also in terms of time to a homogenization of the residual COD.

In the refer to the electrophysical cleaning stage 3 subsequent deposition stage 4 it comes to flake enlargement and flocculation, in particular by means of flotation, sedimentation, filtration o. Ä.

In this case, the particle agglomerates, which by means of the electro-physical cleaning stage 3 were generated by the sewage 1 separated and discharged. The cleaned wastewater 1 is then in the wastewater tank 15 led, from where it into the stock preparation 11 and directed to the manufacturing machine.

According to the requirements and the burden of the sewage 1 can this after the filter 6 also entirely through the electrophysical cleaning stage 3 be directed. But it is also possible only wastewater 1 from the press section 8th through the electrophysical cleaning stage 3 to lead or this cleaning step 3 temporarily switch off.

By the invention, the proportion of continuously added fresh water as well as the discharged amount of waste water 1 significantly reduce.

Claims (13)

Wastewater treatment plant for the purification of wastewater ( 1 ) a machine for producing a paper, board, tissue or another fibrous web ( 2 ), which at least one press section ( 8th ) for dewatering the fibrous web ( 2 ), characterized in that at least a part of the wastewater ( 1 ) of the press section ( 8th ) in an electrophysical purification stage ( 3 ) for coagulation and flocculation and then in a deposition stage ( 4 ) with flake enlargement and flake separation. Wastewater treatment plant according to claim 1, characterized in that only wastewater ( 1 ) of the press section ( 8th ) into the electrophysical purification stage ( 3 ) to be led. Wastewater treatment plant according to claim 1, characterized in that wastewater ( 1 ) of the press section ( 8th ) and from the sheet formation area ( 7 ) of the manufacturing machine into the electrophysical cleaning stage ( 3 ) to be led. Wastewater treatment plant according to one of the preceding claims, characterized in that the entire wastewater ( 1 ) of the press section ( 8th ) into the electrophysical purification stage ( 3 ) to be led. Wastewater treatment plant according to one of claims 1 to 3, characterized in that only a part of the waste water ( 1 ) of the press section ( 8th ) into the electrophysical purification stage ( 3 ) to be led. Wastewater treatment plant according to one of the preceding claims, characterized in that at least a part of the in the electrophysical cleaning stage ( 3 ) purified wastewater ( 1 ) is guided back into the manufacturing machine. Wastewater treatment plant according to claim 6, characterized in that in the electrophysical cleaning stage ( 3 ) purified wastewater ( 1 ) is used in the production machine for water spray devices or for chemical dilution. Wastewater treatment plant according to one of the preceding claims, characterized in that at least a part of the in the electrophysical cleaning stage ( 3 ) purified wastewater ( 1 ) into the stock preparation upstream of the production machine ( 11 ) or the constant part ( 12 ) to be led. Wastewater treatment plant according to one of the preceding claims, characterized in that the wastewater ( 1 ) before the electrophysical cleaning step ( 3 ) through a filter ( 6 ), preferably a disc filter is guided. Wastewater treatment plant according to one of the preceding claims, characterized in that prior to the electrophysical purification stage ( 3 ), preferably between the filter ( 6 ) and the electrophysical purification stage ( 3 ) a cleaning step ( 5 ) is present with wet oxydation. Method for controlling the wastewater treatment plant according to one of the preceding claims, characterized in that the load level of the wastewater ( 1 ) and the intensity of the electrophysical cleaning ( 3 ) is at least partially controlled in a manner that balances the stress level over time. Method for controlling the wastewater treatment plant according to claim 11, characterized in that the load level of the wastewater ( 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. Method for controlling the wastewater treatment plant according to claim 11 or 12, characterized in that the intensity of the electrophysical cleaning ( 3 ) is controlled via the electrical power consumption.

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