DE102010001796A1 - wastewater treatment plant - Google Patents

Abstract

The invention relates to a wastewater purification plant for purifying wastewater (1) of a machine for producing and / or refining a paper, cardboard, tissue or other fibrous web with at least one biological cleaning stage (3, 4). The removal of colloidally dissolved contaminants is to be achieved in an efficient manner by at least part of the wastewater (1) after a biological purification stage (4) in an electrophysical purification stage (2) for coagulation and flocculation and then in a separation stage (5) Flake enlargement and flake separation is performed.

Description

The invention relates to a wastewater treatment plant for the purification of waste water of a machine for producing and / or finishing a paper, board, tissue or another fibrous web with at least one biological cleaning stage.

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 allow sticky deposits to form 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 contaminants can further lead to increased foam by lowering the surface tension, which in turn leads to paper defects and tears 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 the object has been achieved in that at least a portion of the wastewater is passed after a biological purification stage in an electrophysical purification stage for coagulation and flocculation and then in a deposition stage with flake enlargement and flocculation.

In the interest of a comprehensive cleaning as possible, the wastewater treatment plant should have several biological purification stages, of which preferably at least one anaerobic and at least one aerobic is executed.

In anaerobic purification stages, harmful or disruptive organic carbon compounds are removed by microorganisms without oxygen.

In aerobic, biological treatment stages, microorganisms degrade the organic substances in the wastewater and partially oxidize the inorganic substances, whereby air (oxygen) is pumped into the wastewater. For this purpose, various methods, such as the activated sludge process, the trickling filter method, the submersible method, the fixed bed reactor method u. a. developed.

This remains after this biological, aerobic purification stage, a biologically inert COD (chemical oxygen demand), which can be advantageously reduced by the fact that between the biological and / or between the biological and the electrophysical cleaning stage a cleaning stage with wet oxidation is present.

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 so-called 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. The broken-up molecules can then be broken down in the subsequent purification stage.

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 this case, the electrophysical purification stage should advantageously be arranged after the last biological purification stage, preferably after the purification stage with wet oxygenation.

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 purification stage allows in the invention a demand-oriented, targeted transfer and the discharge of most anionic contaminant as a solid.

As a result, the treated wastewater as a partial or complete fresh water replacement in the manufacturing process, d. H. be returned to the production machine of the fibrous web.

With regard to the control of the wastewater treatment plant, it is advantageous if the load level of the wastewater is measured and the intensity of the electrophysical cleaning is controlled at least partially in a way 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 (SCD), 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 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 attached drawing shows:

1 : An investment scheme of a wastewater treatment plant and

2 : A schematic cross section through an electrophysical cleaning stage.

In the wastewater treatment plant according to 1 becomes the sewage 1 a paper mill for COD reduction by an anaerobic 3 and a subsequent aerobic 4 Cleaning stage led. For cleaning the wastewater 1 by settling is before the biological treatment stages 3 . 4 a pre-clarifier 7 and after these a post-settling tank 8th , Possibly in connection with mechanical cleaning stages so the wastewater 1 already sufficiently freed from suspended solids.

In the subsequent purification stages 6 . 2 the inert, ie for the biological treatment methods inaccessible residual COD should be reduced. In the interest of a maximum reduction of the COD can this wastewater 1 from the post-clarifier 8th in a cleaning stage 6 be conducted with wet oxydation. Easily oxidizable substances are to be converted into anionically charged impurities, the precipitation and flocculation treatment in an electrophysical purification stage 2 are more accessible.

With the help of inert electrodes and an applied electrical potential difference are in the purification stage 6 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.

Contaminants consisting of long-chain molecules are broken in this oxidation process and thus the downstream electro-physical precipitation / flocculation 2 made accessible.

After the optional wet oxidation 6 goes through the sewage 1 a cleaning step 2 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 into one in 2 illustrated container 13 led, which is a sacrificial anode 11 , For example, made of aluminum and a particular inert cathode 12 owns which via a control unit 9 with a DC voltage source 14 are connected. The cathode 12 However, it can also consist of sacrificial anode material, which offers the opportunity for 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 can control the current through the control unit 9 be changed 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 2 subsequent deposition stage 5 it comes to flake enlargement and flocculation, in particular by means of flotation, sedimentation, filtration o. Ä. In this case, the particle agglomerates 10 , which with the help of the electro-physical cleaning stage 2 were generated by the sewage 1 separated and discharged.

The combination of biological 3 . 4 and electrophysical 2 Purification stage allows the partial or complete recycling of the purified wastewater 1 after the deposition stage 5 to the paper machine, without affecting the paper machine operation, the effectiveness of the process chemicals, etc. by the concentration of colloidally dissolved, anionic or non-ionic impurities. Thus, the proportion of continuously added fresh water as well as the discharged amount of waste water can be 1 reduce.

Claims (8)

Wastewater treatment plant for the purification of wastewater ( 1 ) a machine for producing and / or refining a paper, board, tissue or another fibrous web with at least one biological cleaning stage ( 3 . 4 ), characterized in that at least a part of the wastewater ( 1 ) after a biological purification step ( 4 ) in an electrophysical purification stage ( 2 ) for coagulation and flocculation and then in a deposition stage ( 5 ) with flake enlargement and flake separation. Wastewater treatment plant according to claim 1, characterized in that it comprises a plurality of biological purification stages ( 3 . 4 ), of which preferably at least one anaerobic ( 3 ) and at least one aerobic ( 4 ) is executed. Wastewater treatment plant according to one of the preceding claims, characterized in that between the biological ( 3 . 4 ) and / or between the biological ( 4 ) and the electrophysical ( 2 ) Purification stage a purification stage ( 6 ) is present with wet oxydation. Wastewater treatment plant according to claim 2 or 3, characterized in that the electrophysical purification stage ( 2 ) after the last biological purification stage ( 4 ), preferably after the purification step ( 6 ) with wet oxydation follows. Wastewater treatment plant according to one of the preceding claims, characterized in that the purified wastewater ( 1 ) is returned to the machine. 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 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 6, characterized in that the load level of the wastewater ( 1 ) is determined by measuring chemical oxygen demand (COD), dissolved carbon (DOC), turbidity, anionic charge character (SCD), evaporation residue, titrated cationic demand or similar. Method for controlling the wastewater treatment plant according to claim 6 or 8, characterized in that the intensity of the electro-physical cleaning is controlled by the electrical power consumption.

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