SE466850B - SETTING TO EXCEED UNUSEDICATED SUBSTANCES FROM WASTEWATER - Google Patents

Abstract

A method of precipitating out undesirable substances from sewage, which is sewage which is derived from the production of mechanical cellulose pulp, preferably chemimechanical pulp, so-called CTMP, and which has been biologically purified and has been detoxified using complex-forming chemicals, such as aluminium sulphate and/or iron chloride, by means of lowering the pH to less than 4.5, preferably to a pH less than 3.

Description

15 20 25 30 35 466 850 2 the fermentation step. Through the hydrolysis step described above with the addition of aerobic sludge, the hydrogen peroxide could be decomposed with good results. in the process known from SE 451 262, the pH is adjusted after fiber separation and heat exchange to a value of 6.0-6.5, whereby nutrients are also added, and then the hydrolysis and acid fermentation described above and the methane fermentation are carried out with sludge recycling such as described above.

Another method of carrying out anaerobic treatment of wastewater from pulp production, which has previously been considered to involve complications due to the substances contained in the wastewater which have a toxic effect on the anaerobic bacteria, in particular methanogenic bacteria during the methane fermentation, e.g. peroxides derived from the bleaching process, are described in SE 440 498, where the peroxide content is reduced by an enzyme step introduced as a pretreatment, where the enzymes are generated in a subsequent hydrolysis step in a two-step anaerobic purification process, consisting of a hydrolysis step and a methane fermentation step.

The enzymes can also be generated in an aerobic step arranged after the anaerobic purification. The sludge from the hydrolysis step and / or the aerobic step is returned to the enzyme step. The sludge recycling is controlled by measuring the redox potential, whereby a control of the enzyme activity is also achieved.

Another method for removing toxic substances in wastewater from a process for the production of mechanical cellulose pulp, in particular chemical mechanical pulp (so-called CTMP), means that so-called water purification chemicals. such as aluminum sulphate and / or ferric chloride, are used for complex binding of the toxic substances instead of being used in the usual way for precipitating organic substances. This complexation takes place at a high pH value, which exceeds pH 7, which is too high for any precipitation to take place. An unused chemical effect should therefore exist, as the water containing these chemicals leaves the treatment plant. The object of the present invention is to make optimal use of the added so-called the water purification chemicals, in this case acting as detoxification chemicals, also for precipitating undesirable oxygen-consuming substances such as dissolved organic material, in addition to the already used effect of detoxifying the water by binding in complex form of toxic substances.

This object is achieved by lowering the pH value of the wastewater coming out of the biological treatment to a value below pH = 4.5; preferably to a value below pH = 3, whereby undesirable substances precipitate and the COD value is reduced. The lowering of the pH value takes place by adding acid, whereby in the present case, where the plant is a so-called sulphate factory, the so-called residual acid, which is obtained as a by-product in the production of chlorine dioxide, and consists of sulfuric acid and sodium sulphate.

The measure of the reduction of undesirable substances is in the usual way the so-called The COD value (KS value), ie the consumption of oxygen in chemical processes in water, which value is determined by standard methods.

In the accompanying drawings, Fig. 1 shows a block diagram of the biological treatment plant. used in the present invention, Figs. 2 a-c show the effects obtained when precipitated with aluminum sulphate, iron chloride resp. pH adjustment, Fig. 3 shows the effects of pH adjustment and precipitation of wastewater after CTMP purification at different times and Fig. 4 shows the results obtained when the CTMP wastewater after purification is mixed with wastewater from the dry cleaner's, whereupon pH was lowered.

The following abbreviations are used in the accompanying drawings: AVR = aluminum sulphate JKL = ferric chloride COD = chemical oxygen demand, KS CTMP = chemical mechanical pulp. The block diagram in Fig. 1 thus shows the process used in the invention for purifying wastewater from the production of mechanical cellulose pulp, in particular chemical mechanical pulp. s.k. CTMP, whereby the effluent from the CTMP process is first led to a pre-sedimentation to remove coarser particles, then the water is passed through a heat exchanger to recover the heat content, and then chemicals are added for pH control to a value above pH = 7 and for complex bonding of toxic substances, and also nutrients. The added chemicals consist of aluminum sulphate, iron chloride, lime, NaOH, urea and phosphoric acid, which are in themselves conventional additives in such processes. Thereby a detoxification of the wastewater is obtained by complexing the toxic substances. However, no conventional precipitation by the addition of aluminum sulphate and ferric chloride is obtained, as the pH value at this stage is too high, namely exceeding pH = 7, while optimum values for aluminum sulphate precipitation are pH = 4.5 and for ferric chloride pH = 3. 5.

After this detoxification, anaerobic purification takes place in the usual manner in hydrolysis and methane fermentation steps with accompanying sedimentation, after which an aerobic step is carried out and a purified wastewater is taken out.

However, the purified water resulting from the purification process described above should still contain an unused capacity in the form of a precipitating ability of the added chemicals. This precipitating ability can be realized by lowering the pH to a value below the optimum pH for the added water purification chemicals, namely 4.5 for the aluminum sulphate, the result being further improved by a lowering below the optimum value for the iron chloride, i.e. pH = 3 , 5. It has been found that the precipitation of undesirable substances, measured by the COD value, is further improved by a further lowering of the pH value, e.g. to pH = 2, and t.o.m. at a value as low as pH = 1.5. These results are verified by the embodiments described below, with reference to the experiments reported in the accompanying diagrams.

The explanation for the reduction of the COD value, which is achieved by reducing the pH below those for aluminum sulphate resp. iron chloride optimal pH values may be that the lignin remaining in the wastewater functions as a polymer, ie that individual lignin molecules are combined into larger aggregates. which theory is supported by the fact that it is known that lignin dissolved in the sulphate process can precipitate if the pH value is greatly reduced. Furthermore, it is known that at low pH values, small particles in the wastewater become positively charged through the excess hydrogen ion, which also promotes aggregate formation. However, other mechanisms are also conceivable, and the invention should not be construed as limited to the above theoretical discussion.

The invention is described in more detail below with reference to the accompanying drawings, and the following exemplary embodiments, which only illustrate the invention without limiting its scope.

Example 1 The diagrams shown in Fig. 2 show the effects of chemical contents resp. pH lowering during treatment of CTMP wastewater after biological purification and detoxification in accordance with the block diagram shown in Fig. 1.

Fig. 2a shows the relationship between the aluminum sulphate content and the COD content reached after the precipitation, at pH = 4.5, Fig. 2b shows the relationship between the iron chloride content and COD reduction at pH = 3.5 and Fig. 2c shows that a considerable reduction of dissolved organic material, measured as COD expressed in mg / l, is obtained by lowering the pH from the starting value in the purified wastewater from the CTMP process. ie pH = 8.3, where COD = about 3800 mg / l. to a pH = 2.0, where COD = about 1300 mg / l, ie a value below the optimum for aluminum sulphate resp. ferric chloride, which as mentioned above, may possibly be due to e.g. lignin dissolved in water forms precipitable aggregates.

From the results reported in Figs. 2 a-c, it can also be concluded that the precipitations made with aluminum sulphate and / or ferric chloride are only effects of the pH adjustment.

Example 2 The pH value of the purified water coming out of the biological purification of CTMP wastewater was lowered to pH = 3, whereby a precipitate formed. The COD value of the purified water was about 5300 mg / l at the time of the experiment, and was reduced by the precipitation caused by the pH lowering to a value of about 2400 mg / l, i.e. a COD reduction of more than 50%.

Then, 0.5 g / l of aluminum sulphate with pH control was added to the "pH-precipitated" water to the optimum value for aluminum sulphate precipitation of 4.5, and iron chloride in an amount of 0.5 ml / l at a pH of 3.5, after which the COD value was determined for the respective samples. It turned out that the difference in the COD values was only marginal, which shows that the effects obtained in Example 1 and reported in Figs. 2a and 2b, are only due to the pH decrease.

This experiment was repeated at two additional time points, yielding similar results (see Fig. 3). Example 3 To further demonstrate the effectiveness of the pH lowering according to this invention, the water coming out of the purification of the CTMP wastewater was mixed with wastewater from the dryer, which is normally led to a sedimentation basin, this basin being able to be used both for the purifying water and the - coming purified the water from the CTMP plant.

The two waters were mixed in this example in relation to the flows. in this case 120 ml of CTMP water, which can be considered as a "precipitating chemical" in this case, and 700 ml of purification water, after which the pH was lowered to 3.0 and 1.5, respectively. At a starting value of about 1900 mg / l COD, a pH drop of about 1900 mg / l COD gives a reduction in COD of about 25%, which corresponds to a reduced emission of COD. A starting value of about 1700 mg / l COD decreases to about 1000 mg / l when lowering the pH to 3.0 and a pH drop to 1.5 lowers the COD content to about 700 mg / l l, ie a total reduction of about 60%, which corresponds to a reduction in the emission of COD of about 22 tons per day, and if the initial value is about 1000 mg / l, a reduction to about 550 and 350, respectively, is obtained, then pH is reduced to 3.0 and 1.5, respectively, ie a total of about 65%, which corresponds to a daily reduction in COD emissions of 13. The values obtained are reported as block diagrams in Fig. 4. Wastewater from the plant's bleaching plant can also be treated on this way.

With the flows that this factory has, the emissions of COD could thus be reduced by about 10-20 tonnes per day, out of a total emission of about 60-70 tonnes per day.

From the above exemplary embodiments it should thus be seen that a lowering of the pH of the purified water, which is obtained after biological purification and detoxification by means of chemicals such as e.g. aluminum sulphate and / or ferric chloride of wastewater from the production of so-called CTMP cellulose pulp, causes a significant reduction of undesirable substances, as shown by the resulting reduction in COD values. The method according to the invention thus obtains an optimal utilization of the added chemicals, which improves the economy of the process and to a considerable extent reduces the emissions of environmental pollutants.

Claims (4)

10 15 20 25 466 ass 8 Patent claim A method of precipitating undesirable substances from waste water from the production of mechanical cellulose pulp, preferably chemical mechanical cellulose pulp, so-called CTMP pulp, wherein the waste water is added with chemicals for pH regulation to a value above 7 and complexing detoxification chemicals for disposal. of toxic substances, and is then biologically purified in one or more anaerobic and / or aerobic steps, is known from the fact that the detoxification chemicals consist of aluminum sulphate and / or ferric chloride and that the pH value of the biological purification resulting from the poisoned water is lowered to a value below the optimum pH value for precipitation by the detoxification chemicals of 4.5, preferably below 3.0, whereby undesirable substances in the biologically purified and detoxified water are precipitated and COD the value is reduced. 2. A method according to claim 1, characterized in that before lowering the pH value, the biologically purified and detoxified water is mixed with fibrous wastewater. 3. A method according to claim 2, characterized in that the biologically purified and detoxified water is mixed with fibrous water. 4. A method according to claims 1-3, characterized in that the pH lowering is carried out by adding acid, preferably so-called residual acid.