WO1992003385A1

WO1992003385A1 - Process for treatment of waste waters or equivalent impure waters as well as a flocculating adjuvant used in the process

Info

Publication number: WO1992003385A1
Application number: PCT/FI1991/000257
Authority: WO
Inventors: Pertti Kalevi Hynninen; Ilkka Seppo Olavi Nevalainen; Pertti Juhani Vuoriranta
Original assignee: Enviro Data Oy
Priority date: 1990-08-17
Filing date: 1991-08-19
Publication date: 1992-03-05
Also published as: FI90856C; FI904070L; FI90856B; FI904070A0; AU8314391A

Abstract

The invention concerns a process for treatment of, in particular, waste waters from the pulp and paper industry. The invention also relates to a flocculating adjuvant used in the process. According to the invention, in the primary clarification stage, the waste water is admixed with biosludge obtained from an activated sludge process or an aerated stabilization basin in order to achieve flocculation. At least a part, preferably at least about 50 %, in particular at least about 60 % of the microorganism cells of the biosludge have been crushed prior to the introduction into the waste water. In order to flocculate and precipitate the organic substance contained in the biosludge and in the waste water, the pH is adjusted to a value between 2 and 3. By means of the process, it is possible to substitute at least a part of the flocculants normally used in the clarification stage of waste water purification.

Description

Process for treatment of waste waters or equivalent impure waters as well as a flocculating adjuvant used in the proces

The present invention concerns a process in accordance with the preamble of claim 1 for the treatment of waste waters or equivalent impure waters.

According to a method of the present kind, the water which is to be treated is admixed in a mixing zone with a componen which provides flocculation of the organic substance. The mixture obtained from the mixing process is then introduced into a separation zone for separation of the flocculating substance from the liquid, and, finally, the treated effluen is conducted to a further treatment, if any.

The process in accordance with the invention is suitable for the treatment of waste waters stemming from, in particular, the pulp and paper industry, but it can also be used for purification of other industrial waste waters, such as waste waters from the oil refining industry and the polymer industry as well as fcr purification of municipal waste waters.

The invention also relates to a flocculating adjuvant for treatment of waste waters cr equivalent impure waters.

During removal cf organic substances from waste waters .of the pulp and paper industry by the activated sludge process, from 0.4 to 0.6 kg of biological sludge (in the following "biosludge") are formed for each kg of removed BOD7 (BOD = biological oxygen demand) . This organic substance contains both living and dead microorganisms. Further, it contains finely divided organic and inorganic solids which have been conducted to the biological purification plant together with the waste water. In the pulp and paper industry, the biomass amounts to from about 50 to about 80 % of the weight of the activated sludge. Large amounts of activated sludge are today being produced in Finland; annually the sludge mass is in the range of from 30,000 to 40,000 tons calculated as dry substance. The amount is expected to increase to about 80,000 to 100,000 tons annually towards the end of this decennium.

There is no economically interesting use for the biological sludge. Nowadays, it is either dried and combusted together with the fibrous sludge or it is transported to a land filling area and dumped. The microorganisms contained in the activated sludge are not either being utilized. In some cases, phosphorous and nitrogeneous compounds bound to the sludge have been recovered. These substances have been re- utilized in biological purification processes.

The present invention is based on the idea of using the activated sludge as a flocculating adjuvant in the primary clarification stage of waste water purification. Presently, the activated sludge is often removed from the process together with sludge from the primary clarifier by introducing the biosludge together with the water into the primary clarification. A part of the biosludge may, however, float to the surface if the conditions of the waste^' water are anaerobic. The biosludge may also float to the surface because of other reasons. Generally, the solids removal in the primary clarification stage is therefore impaired.

Measurements and experiments carried out in connection with the present invention have shown that by breaking down at least a part of the microorganism cells of the biosludge and by adjusting the pH during the treatment of the waste water to, at the most, 3, preferably to a value in the range from 2 to 3, it is possible to obtain precipitating floes when the treated biosludge is added to waste waters.

In particular, the process in accordance with the invention is characterized by what is stated in the characterizing part of claim 1. The flocculating adjuvant is characterized by what is state in the characterizing part of claim 7.

Within the scope of this application, the term "biosludge" means a sludge which conteins proteins and stems from a waste water purification plant, such as an activated sludge plant or an aerated stabilisation basin. It also contains living and dead micrcorganisms.

The terms "flocculation" and "precipitation" are inter¬ changeably used to indicate processes in which dissolved substances are brought into solid state, thus separating from the solution. The solid substance formed during precipitation may coagulate forming flings and floes which, depending on the process conditions, either are deposited on the bottom or - in flotation - float to the surface. Thus, the terms "flocculation" and "precipitation" cover the processes in which flings and floes are formed and are coagulated.

The flocculating adjuvant is added to the waste water which is to be treated in the primary clarification stage of an activated sludge process or in an equivalent preliminary treatment of a waste water purification process intended for removing dissolved matter, solids and colouring com¬ pounds from the waste waters. The biosludge is added in order to improve the separation efficiency of the process, but it may even in some cases completely substitute the flocculating agent normally used.

In the process according to the invention, the pH may be adjusted with a suitable acid, in particular a mineral acid, such as hydrochloric acid, nitric acid or sulphuric acid. It is, however, possible to use acidic waste water fractions, such as ferrous sulphate containing waste waters. Since the pH of these liquids often is rather low, even less than 1, they are in particular suitable for adjusting the pH of waste waters from the alkaline stage of pulp bleaching to a desired value. According to the invention, the pH should be kept below about 3 to provide precipitation. As the examples below will show, the process does not achieve the desired result at a pH value of about 4. Due to economical reasons it is, on the other hand, seldom sensible to lower the pH below 2, even if the process works in that pH range as well.

The biosludge is added to the waste water in a mixing zone, the waste water being mixed, e.g., in successive mixing containers. In a preferred embodiment of the invention, the mixture formed by the treated sludge and the waste water is mixed in two stages, first under strong agitation and then under slow agitation. The two stages are performed in different mixing vessels.

At least a part of the microorganism cells, preferably at least about 50 %, and in particular about 60 % to 90 % of the microorganism cells of the biosludge adjuvant are crushed by a suitable crushing treatment before the adjuvant is admixed with the waste water. According to one preferred embodiment, the biosludge is treated in a sonicator by ultrasonic sound. According to another preferred embodiment, the microorganism cells are crushed by subjecting the sludge to an increased pressure which is rapidly relieved (explosion technique).

After the crushing treatment, the uncrushed solid substance may be removed by filtering or centrifugation. The consistency of the biosludge added to the waste water is then about 0.2 %, but it can be raised to about 2 to 3 % by concentrating the sludge.

The amount of flocculant needed for providing precipitation depends on the amount of dissolved and suspended compounds of the waste water. Normally, the amount of dry substance of the treated biosludge adjuvant added amounts to from about 0.2 to 1.5 g per litre of waste water. Under the acidic conditions mentioned above (pH below about 3) the biosludge, which primarily comprises dissolved substances, will coagulate, thus flocculating and precipitating the organic substance contained in the waste water.

In addition to the treated biosludge, flocculants know per se may be used in the process according to the invention. These flocculants include, e.g., ferrous and ferric sulphate and aluminium sulphate. They can be added, for instance, in the form of aqueous solutions containing ferrous, ferric and/or aluminium ions. One suitable solution is the afore¬ mentioned ferrous waste water of the chemical industry.

In the process according to the invention it is possible to use, in addition to the treated biosludge, another flocculating adjuvant together with the flocculant, if any. This flocculating adjuvant known per se can, depending on the composition of the waste water, be comprised of differen cationic polymers, anionic polymers and polyelectrolytes. It can be added to the waste water before the addition of the biosludge or, alternatively, after the addition thereof, or simultaneously with it.

The flocculating substance and the effluent are separated by conducting the mixture of the waste water and sludge, for instance, to the equalization basin or to the aeration basin of an activated sludge plant, wherein the flocculating substance is separated by methods known per se.

Considerable benefits are obtained by means of the invention. Thus, a considerable reduction of the solid matter of waste water is achieved. Even many finely divided substances in the waste water, which normally are not easily precipitated, now flocculate and precipitate. In connection withthe pulping industry, it should be noted that the process is suitable for the treatment of waste waters from mechanical pulping as well as from chemical pulping. Barking and bleaching waste waters may be treated. Since certain organic compounds (generally macromolecular compounds) such as acids and chlorinated compounds are precipitated in connection with the process, it is possible also to treat waste waters of oil refining processes and from the preparation of poly¬ mers. By means of the invention it is possible considerably 5 to decrease the amount of conventional flocculants needed. They can, in some cases, even be substituted completely by the biosludge in accordance with the invention.

In the following, the invention will be examined in more o detail with reference to the attached drawing and by means of two working examples. The figure in the drawing depicts a simplified process scheme of the invention.

In the process according to the invention, biosludge 5 comprising surplus sludge of the activated sludge process is crushed in crushing unit 1 and used in primary clarification as a flocculating adjuvant. The crushing process is carried out, for instance, by subjecting the sludge to high pressure which is rapidly relieved (the 0 explosion technique) or by sonication. The waste water which is to be treated is admixed in a first mixing vessel 2 with the crushed biosludge. The sludge is fed to the treated waste water in an amount corresponding to 0.2 to 1.5 g solid substance (calculated as evaporation residue) per 5 litre of water conducted to the clarification. The pH of the waste water is adjusted by adding an acidic waste water fraction or by adding a mineral acid (sulphuric acid or a similar acid) such that the pH lies in the range from 2 to 3. In the first mixing vessel, the pH of the mixture is 0 measured and adjusted by means of control device 3, comprising means for indicating, registering and controlling the pH (QIRC) and means for indicating, registering and controlling the amount of acid feed (FIRC) .

5 The treated waste water is agitated, first rapidly for 1 to 2 minutes in the first mixing vessel 2, and then slowly for 6 to 10 minutes in a second mixing vessel 4. The biosludge will then coagulate and the sludge and the organic substances of the waste water will begin to flocculate and precipitate. The mixture of adjuvant and waste water is fed to a clarifier or to a flotation apparatus 6. If the equipment used for separating the water from the floes contain construction parts made from concrete or other parts which are susceptible to acid induced corrosion, the pH of the mixture should be raised to from about .8 to about 5.3 by adding, for instance, lime (an aqueous solution of calcium hydroxide), which is introduced in the middle vessel 5. The pH of the mixture conducted to the clarifier is controlled in the above-described way by using control means 3.

For separation of the mixture in the clarification stage a clarifier 6 conventionally dimensioned for clarification of waste waters in the pulp and paper industry, will be needed (surface load 0.6...1.2 m³/m², delay time 3 to 5 hours) .

When the flotation technique is being employed, the surface load may be from about 3 to about 5 m³/m²h and the volume of water known as dispersion water may be from 10 to 20 % of the amount of treated water.

The waste water is conducted from clarification 6 to the biological purification stage. The sludge is fed to the sludg treatment and treated together with other sludges of the waste water treatment by methods known per se.

The following examples will illustrate the invention without restricting the invention to the details of the examples.

Example 1

The waste water used in this example was comprised of waste water from a chemimechanical pulping mill (CTMP), which was introduced into a primary clarifier. During pulping, spruce had been used as raw material of the pulp. The biosludge used in the example consisted of biological sludge from the activated sludge process of the same ^'mill. The biological sludge was sonicated and the solid material left was removed by centrifugation. A standard amount of biological sludge was added to the waste water. The pH of different samples was adjusted by dilute sulphuric acid to different values. After the addition of the acid, the samples were agitated at constant speed for about 10 minutes and the samples were then allowed to settle for about 30 minutes. Samples of the aqueous phases were then taken and the solids content was determined by using a GF/A filter. The results are indicated in the following table 1.

Table 1. The influence of pH change on solids removal when crushed biosludge is being used as flocculating ad uvent

The reduction of the solids content has been calculated on basis of the solids content of the original waste water.

Example 2

Softwood pulp prepared by the kraft process to Kappa-number 31 was bleached on laboratory scale by using the bleaching sequence C/D E1DE2D. The aim was to attain brightness ISO 90. The waste water of the alkaline stage (stage E_]_ ) , the acidic waste water (stage C/D, wherein C stands for chlorin and D for chlorine dioxide) and biosludge which had been sonicated, crushed and filtered were mixed, the sludge bein introduced together with the acidic acidic waste water into the alkaline waste water under agitation. The mixture was mixed for 5 minutes and then it was allowed to settle for about 1 hour. A sample was taken of the water and the colour was determined at constant pH. The results are given in the enclosed table 2. The volumetric ratios of the alkaline water to acidic water to biosludge were 1:2:0.2.

Table 2. Colour reduction of alkaline and acidic waste water by the addition of crushed biosludge

Colour Colour reduction mg Pt/1 %

Acidic waste water 320

Alkaline waste water 6600 Biological sludge 1500

Treated waste water 690 71

In other purification experiments we have observed that, e.g., the addition of crushed biological sludge to primary clarification does not, determined by COD, increase the amount of dissolved organic substances. The purification result during precipitation of the solid substances is not substantially impaired even if the uncrushed part of the biological sludge is not removed by centrifugation.

Claims

Claims :

1. A process for treatment of waste waters or equivalent impure waters, according to which process - the water which is to be treated is admixed in a mixing zone (2, 4) with a component which provides flocculat¬ ion of organic substances contained in the water,

- the mixture obtained from the mixing process is then introduced into a separation zone for separation of the flocculating and precipitating substance from the liquid phase, and

- the treated effluent is removed from the separation zone and routed to additional treatment, if any, c h a r a c t e r i z e d in that - the flocculating and precipitating agent is at least partially comprised of a protein containing biosludge which has been subjected to a crushing treatment, and

- the pH in the mixing zone (2, 4) is adjusted to 3 or less.

2. The process as claimed in claim 1, c h a r a c t e r ¬ i z e d in that the biosludge is obtained from an activated sludge process or an aerated stabilization basin and at least 50 %, preferably about 60 to 90 % of the microorganism cells of the biosludge have been crushed.

3. The process as claimed in claim 1 or 2, c h a r a c t ¬ e r i z e d by adjusting the pH to a value in the range from about 2 to about 3.

4. The process as claimed in any of claims 1 to 3, c h a r a c t e r i z e d by using an acid waste water fraction or mineral^* acid for pH adjustment.

5. The process as claimed in any of the foregoing claims, c h a r a c t e r i z e d by adding in the mixing zone (2, 4) from about 0.2 to 1.5 g biosludge per litre of water calculated as solid substance.

6. The process as claimed in any of the foregoing claims, c h a r a c t e r i z e d by mixing the mixture consisting of sludge and water in two stages: first rapidly and then slowly.

7. Flocculating adjuvant for, in particular, the treatment of waste waters or equivalent impure waters, c h a r a c t ¬ e r i z e d in that it consists essentially of biosludge which has been treated in such a way that at least a part of the microorganism cells of the biosludge has been crushed.

8. The flocculating adjuvent as claimed in claim 7, c h a r a c t e r i z e d in that at least about 50 %, preferably at least 60 % of the microorganism cells of the biosludge have been crushed.

9. The flocculating adjuvant as claimed in claims 7 or 8, c h a r a c t e r i z e d in that the biosludge is obtained from an activated sludge process or from an aerated stabilization basin.

10. The flocculating adjuvant as claimed in any one of claims 7 to 9, c h a r a c t e r i z e d in that the consistency of the biological sludge is from about 0.2 to 3 %.

11. The flocculating adjuvant as claimed in any one of claims 7 to 10, c h a r a c t e r i z e d in that the biosludge has been sonicated to crush the microorganism cells.

12. The flocculating adjuvant as claimed in any one of claims 7 to 10, c h a r a c t e r i z e d in that the microorganism cells of the biological sludge have been crushed by subjecting the sludge to an elevated pressure, which has been rapidly relieved.

13. The flocculating adjuvant as claimed in any one of claims 7 to 12, c h a r a c t e r i z e d in that at least some of the uncrushed microorganism ceils have been removed.