PROCESS FOR PURIFYING SLUDGE CONTAINING MAINLY WATER

Abstract: The invention relates to a method for improving purification of sludge containing mainly water, so that before a mechanical dewatering stage two components are added to the sludge in order to improve its dewatering properties. First a water-containing coagulating mixture is added, which contains a ferric salt and an organic polymer having a molecular weight of under 1 000 000, and then a polyelectrolyte with a molecular weight of above 3 000 000 is added.

Description: Process for purifying sludge containing mainly water This invention relates to a process for purifying sludge containing mainly water, especially a waste water sludge of the forest industry, wherein the dewatering properties of the sludge are improved before carrying out a mechanical dewatering of the sludge. Besides the house hold also the industries and, above all, the forest industry are producing waste water. The paper production is carried out from a diluted water suspension wherein water is an auxiliary driving medium. Waste water contains varying amounts of dissolved and solid organic and inorganic matter originating from used raw materials. Waste water thus pollutes the water system to which it is drained. The solid, insoluble matter makes the water turbid and forms a layer of sludge on the bottom of the water system. Organic matter, whether dissolved or solid, is consuming the oxygen of the water system, dyeing the water and giving directly or indirectly a taste or a smell to it. One way to purify waste water is by flocculation. Floc- culation or coagulation can be enhanced by varying the so- called zetapotential of the particles contained in the water by adding suitable chemicals. Because these additions only influence on the active sites on the solid material, quite small amounts of the chemicals are usually needed, i.e. generally 0.1-1 mg/1. If the flocculant has a high molecular weight, it is simultaneously also functioning as a pro¬ tecting colloid. There is a myriad of flocculants on market. They are water- soluble but can get hydrolyzed when diluted and form insoluble solid or colloidal matters, such as e.g. A1(0H) 3 . Because flocculants alone or as combined with the particles generally have an amphoteric nature, i.e. contain groups of opposite charges, or are salts of bases and acids, they may and act both anionically and cationically, according to the pH value of the environment. When both of the amphoteric groups contribute as much in the molecule or flake conglo¬ merate, which requires a certain hydrogen ion concentra¬ tion, the isoelectric point of the matter or flake has been reached. The zetapotential is then near the value of ±0mV„ If the flocculant by itself forms flakes, such as e.g. Al(OH) 3 , Fe(OH) 3 and Si0 2 , the dissolved substances can be adsorbed to these flakes and thus get separated from the water to be purified. As an example the absorption of humus and coloring substances to the flakes of Al(OH) 3 can be mentioned. In the following some usual flocculants are listed: The resin glue is mainly Na-abietate. The contribution of it to the zetapotential is negative. Thus its action is anio- nic. The principal factors of alum are the hydrolysis products of the hexa-aquoaluminum ion with a charge of +3, and the mag¬ nitude and the sign of the charge depend on the pH value of the environment. The contribution of alum to the zetapoten¬ tial is positive. Thus its action is cationic. By using both of these and by adjusting the acidity it is possible to adjust the zetapotential near the value of ±OmV and thus to effect flocculation. The synthetic flocculants can be divided into following categories: Non-ionic polymers, e.g. polyvinyl alcohol, polyethylene oxide. Ionic polymers, the so-called polyelectrolytes: anionic polymers such as polyacrylate, polymethacrylate cationic polymers such as quaternary ammonium compounds copolymers of anionic and cationic groups, having a protein-like effect. Copolymers of ionic and non-ionic groups, e.g. partly saponified polyacrylamide. In the chemical treatment it is in the first place question of destabilization (also called coagulation) of a colloidal suspension, so that the separate particles adhere to each other and form so large solid particles that these can be separated from the water by conventional mechanical methods. Thus it is evident that also a part of the so-called dis¬ solved matter is removed in the chemical treatment. In the chemical treatment of water usually inorganic salts are used which form trivalent ions in the water solution. These include the aluminum and ferric salts. In chemically suitable conditions they form in water gelatinous hydroxide polymer precipitates, floes. When a cationic electrolyte is added in th." presence of a base so that charged polymerized hydroxide molecules Me x (0E) y - ~ are formed, the colloid adsorbs these on its surface and the zetapotential is lowered. As a zetapotential making the collision and adhering of the particles possible the values in the range of -5 to +5mV are generally considered. Alum flocculates best within the pH range of 4.5 to 7.5. Because it is by nature an acidic material in a water solution, an addition of a base is often needed in order to keep the pH value within the optimal range. Sodium aluminate is basic by nature. The combined use of alum and aluminate can thus give a good result without any adjustment of the pH with other chemicals. When alum is added to water with a sufficient basicity aluminum hydroxide is immediately formed. The molecules form quickly polymers with linear chains and of an order of magnitude lOnm. This is observed as a turbidity. The hydro¬ xide polymers are immediately absorbed on the negatively charged surface of the colloidal particle, and cover it. If there are no impurity particles present, the hydroxide po¬ lymers form floes with each other. This is of course inef¬ ficient in respect of the water purification. The precipitates formed with the aid of alum and iron salts, in the following floes, can be small and they disintegrate quite easily. The activated silicic acid is reinforcing them; the same result is sought by the addition of long- chain polyelectrolyte chemicals, flocculating aids. Gene- rally the main purpose of this, however, is to gather up the widely scattered and small floes and to reduce their speci¬ fic surface, thus increasing the sedimentation rate of the solid particles. The destabilization of the colloids can also be carried out by adding cationic polyelectrolytes to the water to be treated. These bring the system to the isoelectrical point without any pH change. Albeit such polymers are as coagu¬ lants 10 to 15 times more effective as alum, they still are clearly more expensive for this purpose. Therefore they are mainly used in very small quantities e.g. to enhance primary flocculation especially with alum. The polyelectrolytes which are used as an flocculating aid, are generally classified according to their origin (natural or synthetic)- and their ionic nature (cationic, anionic, non-ionic or copolymers) . To mention examples, starch is natural and cationic polyacrylic acid as well as anionic polyacrylic amide are synthetic. The molecular weights vary between 15 000 to 15 000 000 and the lengths between about 5 000 to 20 OOOnm. FI Patent Application No. 890533 presents a process for treatment of waste water by adding a) an inorganic pre¬ cipitant, b) an anionic polymer and c) a cationic polymer to the waste water. The treatment, i.e. the order of addition represents in this patent application a part of the inven¬ tion. FI Patent Application No. 830492 presents an invention for forming a filter cake with such chemicals which enhance separation of solid and liquid phases. According to the application, a polyelectrolyte having a high molecular weight is first added, and finally an inorganic matter or a polyelectrolyte having a small molecular weight. The present invention expressly concerns an improvement by which the separation of solid matter from a sludge is made more effective, especially from the waste water sludge of the forest industry, by a combination of a coagulant of a suitable type and polymeric chemicals. In the forest industry, as a result of the biological puri¬ fication of waste water a sludge surplus is formed which is usually treated together with fibre or bark sludges. The most common way is to mix the sludges together, condition with an organic polyelectrolyte, remove water mechanically e.g. with a filter belt press and dispose of the dried (= water has been mechanically removed) sludge by burning. The separation of water from the biosludge is very diffi¬ cult, and often the situation is hampered by the fact that the portion of the biosludge in the mixture to be treated is varying. In that case the importance of the onditioning chemical which is added before the mechanical dewatering stage is further emphasized. In Finland it is also common to use a (poly)ferric sulphate or polyaluminium chloride together with a polyelectrolyte, so that this inorganic coagulant is added first, and at a later stage the polyelectrolyte. By this way a better drying result can be obtained and save in the chemical costs. There are also publications concerning the addition of ferric sulphate and a cationic polyelectrolyte at different moments in the conditioning of forest industry sludges [Tappi J. 1985, Chem. Eng. 1985] . According to the patents JP-893088253 and JP-02180700 the treatment of an organic sludge with ferric or aluminum chemicals together with an amphoteric polymer (= a mixture of different types) will improve the separation of water from the sludge and make it more easy to remove the sludge cake from the filter cloth. The chemical consumption is also decreased. Mixtures of an inorganic coagulant and an organic polymer have been used, for instance, to removing turbidity of waste water, but not together with a polyelectro. ' .yte which is added separately [US-4 902 779, US-5 035 808] . Neither are there any applications to be found on the treatment of the forest industry sludges with a system of a coagulant mixture + a polyelectrolyte. The goal of the invention is to provide a process for purification of a sludge containing mainly water, especially a forest industry waste water sludge, by enhancing the dewa¬ tering properties of the sludge, which will further facili- tate and increase the efficiency of the subsequent mechani¬ cal dewatering. The main characteristics of the invention appear from the appended patent claims. According to the invention, it has surprisingly been obser- ved that when in the first addition step (a) instead of a ferric salt a mixture is used wherein to the ferric salt an organic polymer with a molecular weight under 1 million is added, the amount of the organic polyelectrolyte having a molecular weight above 3 million and which is added in the second addition step (b) , can be reduced without impairing the dewatering properties of the sludge. It has according to the invention also been observed that if the amount of this polyelectrolyte is not reduced, the dewatering properties of the sludge are improved. The ferric salt of the coagulant mixture which is used ac¬ cording to the invention, is preferably ferric sulphate and/or polyferric sulphate. This coagulant mixture is pre¬ ferably formed of a mixture of a water solution of the ferric salt and a water solution of an organic polymer. The water solution of the ferric salt contains preferably about 12 percent by weight ferric salt. The coagulating mixture contains ferric salt and an organic polymer preferably at a ratio of 1:0.01 to 1:0.33. Said organic polymer is preferably poly(dimethyl diallyl ammonium chloride) and/or polyepiamine. Said polyelectrolyte is preferably cationic, anionic or non- ionic polyacrylamide and/or an polyamine. In the following the invention is described in more detail with Examples with reference to the appended drawings, wherein Fig. 1 is a graphical presentation of the correlation between the dry solids and CST, and Figs. 2-4 are graphical presentations of the results obtained with the reference tests described in the Examples. The water separability from the sludge is described by the obtainable dry solids content of the cake, also with the CST (capillary suction time) value: the lower the CST value is, the more easily the water is separable from the sludge. It has to be noted that low CST values are also reached with overdoses of the polyelectrolyte, but then the water sepa¬ ration is not necessarily good. The correlation between the dry solids content and CST is represented by the curve of in Fig. 1. There is a correlation between CST and the dry solids con¬ tent: when CST decreases the dry solids content increases. Thus the CST is a very rapid method for a rough comparison of chemicals and estimation of chemical dosage. The precentages given in the following Examples are by weight. Example 1 In this example a sludge mixture from a forest industrial plant was used which contained 40% biosludge calculated from the dry solids content and the dry solids content of which was 1,4 %. At the first stage a coagulating mixture accor¬ ding to the invention was used instead of polyferric sulp- hate (reference) and at the later addition stage the amount of the polyelectrolyte was reduced. The used polyelectrolyte was a cationic polyacrylamide (trade name Millfloe Q39) . The coagulating mixtures were mixtures of polyferric sulp- hate PFS (12% water solution) , and polydimetby1 diallyl ammonium chloride (40% water solution) , in short the polymer or p-DMDAC The ' weight ratios of the polyferric suphate and the polymer in the coagulating mixture were the following: Coagulating mixture 1 Coagulating mixture 2 The reference test was carried out by using only PFS at the first stage. The coagulant dosage was 113mg/g of dry solids of the sludge mixture in all chemical adding methods. The results are presented in Fig. 2 so that the reference test is (a) , the coagulating mixture 1 is (b) , and the coa¬ gulating mixture 2 is (c) . According to the results the amount of the polyelectrolyte to be added at the second stage per dry solids (the horizontal axis of Fig. 2) can clearly be reduced without impairing the final result when, at the first stage, the coagulating mixture according to the invention is used. On the other hand, when only PFS is used, the dewatering result is uneven and occasionally very poor. Example 2 In this example the starting situation was that the sludge mixture with a dry solids content of 1,5 % was from another forest industrial plant and contained 40 % surplus sludge from an active sludge plant and 60% fibre slvάje. In the reference test polyferric sulphate (PFS) was used at the first stage as a water solution having a Fe-content of 12%. For the tests the coagulant mixtures 3 and 4 were prepared, which were composed of the above mentioned PFS and the po¬ lymeric p-DMDAC according to Example 1. The weight ratios of the coagulant mixtures were as follows: Coagulant mixture 3 Coagulant mixture 4 When conditioning with any of the three chemical addition ways the same polyelectrolyte dosage, which was lmg/g, was used after the above mentioned dosage. The polyelectrolyte was a cationic polyacrylamide as a 0.5% water solution. The trade name of the polyelectrolyte is Fennopol K1351 and its molecular weight was over 3x10 6 . The results are presented in Fig. 3, where the result of the reference test is (A) , the result obtained with the result coagulant mixture 3 is (B) , and the result vith the coagu- lant mixture 4 is (C) . From the Figure it car. oe seen, that when the coagulant mixtures 3 and 4 are used, a higher sludge dry solids content is obtained than with only PFS, when the polyelectrolyte dosage with all three is the same. An essential improvement can be obtained especially with relatively high PFS dosages. Example 3 In this example a sludge mixture of a third plant was used, which sludge contained less biosludge than in the previous examples, only 4% of the dry solids. As a reference coagulant PFS (12% water solution) was used and as the coagulants according to the invention the follo¬ wing mixtures of PFS and p-DMDAC (40% watev solution) with the following weight ratios: PFS The polymer Coagulant mixture 5 0,18 Coagulant mixture 6 1 0,25 Coagulant mixture 7 1 0,33 The dosage of the coagulant mixture was 10-15 mg/g of the dry solids. As the polyelectrolyte after the dosage of the coagulant a 0.5% water solution of polyacrylamide was used. The trade name of the polyelectrolyte is Fennopol K504. The results are presented in Fig. 4, wherein the reference test is (I) , so that the result I. was obtained with the dosage of lOmg/g and the result I b was obtained with the dosage of 15mg/g. The result of the coagulant mixture 5 is (II) , wherein the dosage was lOmg/g. With the coagulant mixture 6 the results III. and III b were obtained, corres¬ ponding the dosages of lOmg/g and 14mg/g. The coagulant mixture 7 is (IV) and the dosage was 10 mg/g. From the Figure it can be seen that increasing the amount of both PFS and the organic polymer mixed to PFS essentially improves the dewatering properties of the sludge, i.e. the CST value is reduced. The amount of the polyelectrolyte can thus be reduced considerably.

Claims: Claims 1. A process for purifying sludge containing mainly water, comprising a stage, at which a mechanical dewatering of the sludge is carried out, characterized in that before the mentioned dewatering stage the sludge is treated in order to enhance its dewatering properties in the following order: a) with a water-containing coagulating mixture containing a ferric salt and an organic polymer having coagulating pro¬ perties by itself and having a molecular weight of under 1 000 000, and b) with a polyelectrolyte with a molecular weight of over 3 000 000. 2. Process according to claim 1, characterized in that the sludge is a waste water sludge of forest industry. 3. Process according to claim 1, characterized in that the ferric salt of the coagulating mixture is ferric sulphate and/or polyferric sulphate. 4. Process according to claim 1, characterized in that said organic polymer is poly(dimethyl diallyl ammonium chloride) and/or polyepiamine. 5. Process according to claim 1, characterized in that the polyelectrolyte is a cationic, anionic tai non-ionic po¬ lyacrylamide and/or polyamine. 6. Process according to claim 1, characterized in that the coagulating mixture ratio of ferric salt : organic polymer = 1:0.01-1:0.33. 7. Process according to claim 1, characterized in that the water-containing coagulating mixture is composed of a mix- ture of a water solution of the ferric salt and a water solution of said organic polymer. 8. Process according to claim 7, characterized in that the water solution of the ferric salt contains about 12 weight percent of the ferric salt.

Inventors: JOKINEN SIMO, LAINE EIJA, JOKINEN, SIMO, LAINE, EIJA
Applicants: JOKINEN SIMO, KEMIRA OY, LAINE EIJA
Application year: 1993
Main classification: C02F11
IPC codes: C02F1, C02F11