WO1990015028A1

WO1990015028A1 - A method for biological treatment of wastewater

Info

Publication number: WO1990015028A1
Application number: PCT/SE1990/000388
Authority: WO
Inventors: Mats Almemark; Bengt Boman; Björn FROSTELL
Original assignee: Ab Institutet För Vatten- Och Luftvårdsforskning
Priority date: 1989-06-05
Filing date: 1990-06-05
Publication date: 1990-12-13
Also published as: AU5926090A; SE9000379L; SE9000379D0

Abstract

Wastewater which is contaminated with resistant organic substances, particularly effluent from a pulp bleach plant, is treated in a biological purification stage (2). The resistant substances are separated from the treated wastewater, preferably by membrane filtration (13), and are retained in the biological purification stage together with the microorganisms which are active in the biodegradation process, such that they are caused to have a retention time in the biological purification stage which is substantially longer than that of the wastewater.

Description

A method for biological treatment of wastewater

This invention concerns a method for biological treatment of wastewater contaminated with organic resistant substances, especially industrial wastewater, such as effluents from i 5 chemical, metallurgical and engineering industries. A pre¬ ferred, but not exclusive, field of application of the inven¬ tion is in the treatment of effluents from wood-processing industries, such as bleach plant effluents, contaminated with organic resistant substances, particularly substances contain

10 ing organic chlorine compounds.

In this context, the term "resistant substances" means firstly aromatic substances originating from wood, coal or petroleum, and also resin acids and terpenes, and secondly organochlorine, organonitrogen and organometallic compounds,

15 and is to be construed as comprising one or more of these substances and compounds.

In accordance with a different definition of the term "resistant substances", which is also applicable to the invention, such substances are organic substances, par-

20 ticularly high-molecular substances, which have a ratio of BOD₂₀ (Biochemical Oxygen Demand, 28 days) to C0D_Cr (Chemical Oxygen Demand, dichro ate oxidation) less than 0.3.

In recent years it has become possible to achieve a substantial reduction of the emission of environmentally

25 harmful substances with the effluents from pulp mills as far as fibres and easily degradable substances are concerned. The less easily degradable or resistant substances which are normally contained in high concentrations in the effluents from pulp mills still present a difficult problem, however,

30 partly because of their toxic nature and partly because it is difficult to separate such substances from the wastewater and

« to dispose of or utilize them in an economically feasible way It is known to separate high-molecular resistant substan- ces from effluents from pulp bleach plants through membrane

35 separation techniques (reverse osmosis, ultrafiltration, microfiltration) . Through such separation techniques, a concentrate of the resistant substances is obtained which can be disposed of by, for example, incineration or dumping, or can be subjected to further processing with a view to produc- ing useful products. Incineration and dumping cause serious environmental problems, and utilization of the concentrate for the production of useful products is not a satisfactory way of taking care of the harmful substances. An object of the invention is to provide an economically and environmentally advantageous method for the treatment of wastewater contaminated with substances of the character referred to, especially pulp bleach plant effluents contamina¬ ted with clorinated high-molecular substances. The invention concerns a method for the treatment of wastewater, especially pulp bleach plant effluents, which is contaminated with organic resistant substances, especially substances containing organochlorine compounds, which method comprises the steps of passing the wastewater through a biological purification stage for subjecting the wastewater to a biodegradation process, and separating the resistant sub¬ stances to a treatment for rendering them harmless.

In the process according to the invention, the above- mentioned object is achieved by subjecting the wastewater to said treatment in the biological purification stage prior to the separation of the resistant substances; separating the resistant substances from a stream of the wastewater compris¬ ing at least the major portion of the wastewater stream discharged from the biological purification stage and re- cycling them to the biological purification stage; and impart¬ ing to the resistant substances and the microorganisms active in said biodegradation process a retention time in the bio¬ logical purification stage which is substantially longer than that of the wastewater. The separation of the resistant substances from the strea of wastewater suitably is effected by subjecting the stream t membrane filtration.

Advantageously, the microorganisms which are active in th treatment of the wastewater in the biological purification stage are separated from the wastewater simultaneously with the resistant substances and recycled to the biological purification stage together with them.

Thus, in accordance with the invention, the resistant substances are treated together with the other organic sub- stances which are degradable in the biological purification stage. Surprisingly, it has been found that the biological purification stage is capable of efficiently degrading the resistant substances as well, in spite of the fact that the separation of the resistant substances and the recycling of them to the biological purification stage means that the concentration of these substances is substantially higher in the biological purification stage than in the incoming waste¬ water. It is essential, however, that the retention time of the microorganisms in the biological purification stage, the so-called sludge age, and the retention time of the resistant substances, is substantially longer, at least 5 times and preferably 20-50 times longer, than the retention time of the wastewater (the hydraulic retention time) . The invention may be applied both to anaerobic purifi¬ cation processes (to each stage in the case of multistage anaerobic purification processes) and to aerobic and combined anaerobic/aerobic purification processes in which micro¬ organisms are utilized for the degradation of the resistant substances. In the case of anaerobic purification, which is believed to become the predominant field of application of th invention, the treatment of the wastewater and the resistant substances may take place in a closed vessel (reactor) having means for collecting the gas, chiefly methane and carbon dioxide, which is produced. Aerobic treatment according to th invention may be carried out in an open vessel or reactor to which oxygen is supplied in any suitable manner, e.g. by injection of air or pure oxygen. The gas which is produced, chiefly carbon dioxide, is allowed to escape to the atmo- sphere.

An embodiment of the method according to the invention will now be described with reference to the accompanying diagrammatic drawing in which a system for anaerobic treatmen of wastewater is illustrated. A stream 1 of wastewater having a COD (Chemical Oxygen

Demand) value of 100-100000 mg/1, preferably 1000-50000 mg/1, is continuously fed to a reactor in the shape of a closed vessel 2 having a suitably positioned inlet 3 for wastewater to be treated, a suitably positioned outlet 4 for solid and liquid products and a suitably positioned outlet 5 for the discharge and collection of gaseous products. If required, th vessel 2 may be provided with additional inlets and outlets. Within the vessel 2 the supplied wastewater accommodates space which is limited downwardly by the bottom 6 of the vessel, laterally by the sidewalls 7 of the vessel and upward ly by a controlled free water surface 8. The level of the surface 8 may vary in the course of the treatment but is controlled so as to have a predetermined average value. The vessel 2 is sufficiently large to provide for a wastewater retention time in the vessel, calculated as the ratio of the average volume in ³ of the liquid contained in the vessel to the volume flow in m³/h of the incoming waste¬ water, in the range of 1-200 hours, preferably 5-50 hours. Th temperature in the vessel 2 is controlled and kept at a constant value in the range of 5-70°C, suitably 20-60°C and preferably 25-35°C. The contents of the vessel 2 are agitated in any suitable manner, such as by a rotary stirrer 9 as show in the drawing. During the start-up of the installation, microorganisms, so-called inoculum, taken from an existing purification syste for similar wastewater to be treated or from any other suit¬ able source, are supplied to the vessel 2. The composition of the incoming wastewater 1 is controlled such that a balanced mixture of organic material and macro- and micronutrients is fed to the vessel 2. More particularly, the relationship of COD/N/P to one another should be controlled so as to be in th range of 300/5/1 to 1500/5/1 in the case of anaerobic puri¬ fication. The pH in the vessel 2 is adjusted by means of a suitable neutralising agent, such as sodium hydroxide, sodium bicarbonate or lime, such that the pH in the vessel is in the range of 6.0-8.0, preferably in the range of 6.5-7.5.

Organic material in the wastewater contained in the vesse 2 is degraded under action of microorganisms which utilize th organic material as a source of carbon and energy. In the present exemplary case, which concerns anaerobic purification the chief products are methane, carbon dioxide and new micro¬ organisms. Gaseous products escape from the system through th outlet 5 and are collected. Contained in the vessel 2 is a mixture of mainly water, microorganisms and undegraded mate¬ rial supplied with the wastewater.

Through the outlet 4 the contents 10 of the vessel 2 are withdrawn in any suitable manner, e.g. by gravity flow or pumping, at a rate which on the average corresponds to the incoming liquid flow through the inlet 3.

Following withdrawal from the wessel 2 the contents 10 of the vessel are divided in any suitable manner, e.g. by pumping, into two part streams, a main stream 11 and a bleed stream 12. The bleed stream 12 is controlled in respect of it magnitude such that it corresponds to 0.2-20 percent, pre¬ ferably 1-10 percent, of the wastewater stream 1 fed to the system.

As indicated above, the main stream 11 and the bleed stream 12 may be withdrawn from the vessel 2 through differen outlets if required. The bleed stream 12 is transferred to a unit generally designated by 20 for further processing therei or disposal.

The main stream 11 is passed to a chemical/physical separation unit 13, namely, a membrane filtration unit, provided with an inlet 14, an outlet 15 for a stream 16 of liquid enriched with the microorganisms and resistant substan ces, and an outlet 17 for a stream 18 of liquid which is depleted of the microorganisms and the resistant substances. The stream 16 is recirculated to the vessel 2 so that the retention time of both the microorganisms and the resistant substances in the vessel 2 is increased. The stream 18 of liquid is discharged or subjected to further processing in a unit which is generally designated by 19 and constitutes an aerobic purification stage.

The method according to the invention has been carried o in a laboratory system as shown in the drawing, the stream 18 being subjected to further processing in an activated sludge unit. In carrying out the method in this system a negatively charged tubular membrane (Nitto TR-7410-P2) was used in the separation unit 13 for the separation of the resistant sub¬ stances and the microorganisms active in the degradation of such substances. Tests have been carried out on wastewater from the production of bleached sulphate pulp and made up by mixture of four parts of acid liquor, namely C + D bleaching stage liquor, and one part of alkaline liquor, namely EO bleaching stage liquor.

In operation under constant conditions as set forth in Table 1 and, able 2, the results set forth in Table 3 were achieved:

Table 1 - Operating conditions in example carried out:

Parameter Unit Value Volume load Kg COD m d 1.5

Hydraulic retention time days 1.8

Sludge retention time (sludge age) days 50

Retention time of retained high-molecular fraction days 50 Bleed stream (Stream 12:Stream 1) - 1:30

Temperature in the reactor °C 35 pH in the reactor - 6.5

Redox potential in the reactor mV -500

In accordance with the general rule that an anaerobic purification stage should in actual practic always be succeded by an aerobic final purification stage, which serves at least to oxidize and remove the reducing and evil-smelling sulphur compounds formed during the anaerobic process, a conventional activated sludge final purification stage (with complete mixing) was added to the downstream end of the anaerobic purification system. The operating conditions of the activate sludge stage were as set forth in Table 2:

Table 2 - Operating conditions for further processing of the effluent stream from the anaerobic purification stage:

Parameter Unit Value

Volume load Kg COD/m^"3d^"1 1.7

Sludge content kg TSS m^"3 2-5

Hydraulic retention time days 0.6 Temperature in the aeration tank °C «20 pH in the aeration tank °C 8.5

Under the conditions set forth above the following result were achieved: Table 3 - Achieved results:

Para- Untreated Following anaerobic Following aerobic Bleed stream Degradation meter water purification according final purification 16 of con¬ of retained dis to method as described centrate solved organic substance concentr. concentr. reduct. concentr. reduct. concentr. reduction mg/l mg/l % mg/l X mg/l %

COO 2700 1000 63 500 81 15000-2000 40-50

AOX 150 40 73 30 80 800-1000

TSS 30 <10 -<10 .

AOX represents adsorbable bonded halogen (chlorine); the value of AOX is a common measure of the amoun of chlorinated organic material in wastewater. TSS represents the total amount of suspended solids.

The results set forth in Table 3 have to be judged in the light if the facts that the technique employed today in the

Swedish wood-processing industry (aeration tank) , results in COD and AOX reduction of 25-30 percent and that modern activated sludge installations in the Finnish wood-processing industry achieve reductions of 40-50 percent in respect of CO and AOX. In accordance with the present invention a COD reduction of 81 percent and a AOX reduction of 80 percent hav been achieved, and a very large proportion of the reduction i due not just to separation but also to degradation. Thus, the method according to the invention results in a degree of degradation which is substantially higher than that which is achieved with the present-day technique for treatment of wastewater containing resistant substances.

After the purification effect which can be attributed to plain physical separation of high-molecular dissolved materia through the membrane filtration has been taken into account, there remains a further purification effect which can only b attributed to a degradation of high-molecular organic mate¬ rial, both chlorinated and non-chlorinated material, by the anaerobic system to a degree which has not so far been known to be achievable.

A confirmation of this can be seen in the fact that the equilibrium concentration of COD in the vessel 2 was about 15000-20000 mg/l. The concentration should have been about 50000 mg/l if only the existing concentration ratio (about 30) across the membrane had determined the level. The difference can only be explained as an effect of biological degradation of the fraction of dissolved organic substances which was retained by the membrane and was therefore caused to stay in the reactor for 50 days instead of the 1.8 days the water phase was retained.

An additional indication of the unique capability of the system to biodegrade resistant organic material was the heavy production of biogas, about 3-4 1/day. The methane content of the produced gas corresponded to about 25 percent of the maximum theoretical yield, i.e. 25 percent of the COD reduc¬ tion which was measured across the anaerobic reactor corre- sponded to methane in the produced biogas.

In the tests accounted for above, the resistant substance were separated by a tubular membrane through which the waste¬ water was passed from the inside and outwardly. When using a tubular membrane it is also possible, however, and in certain circumstances indeed preferable, to cause the wastewater to flow through the membrane from the outside and inwardly. As will be appreciated it is within the scope of the invention to use other types of membranes than tubular mem¬ branes. For example, so-called dynamic membranes (membranes i which the membrane material is dissolved or suspended in a liquid which is circulated through a porous substrate) may be used.

In order that the flux through the membrane separation unit, i.e. the volume of wastewater flowing through each unit of surface area of the membrane material per unit of time, ma be high even when the concentration factor is high, it is preferred to cause the wastewater to flow along the membrane surface at a certain velocity (shear velocity) , preferably at least about 3 m/s. The shear velocity, which thus is the relative velocity o the vastewater and the membrane along the surface of the membrane, may be achieved in different ways, e.g. by impartin to the wastewater in the separation unit a velocity component along the surface of the membrane by means of a power-driven mechanical stirrer (impeller) or by rotation the membrane. A mechanical device for producing the shear velocity may also be used for effecting a mechanical cleaning of the membrane.

In the embodiment of the method which has been described by way of example, the separation of the resistant substances takes place in a single stage, the microorganisms and the resistant substances being separated simultaneously. However, it is possible within the scope of the invention to carry out the separation in several successive stages, e.g. in a first stage corresponding to the illustrated and described stage, and one or more additional sequential stages following the first stage. The separation in the additional stage or stages may take place by reverse osmosis.

It is also possible to carry out the purification in one or more additional anaerobic purification stages. The separa¬ tion of the resistant substances may then be carried out in each such stage.

Claims

1. A method for the treatment of wastewater contaminated with resistant substances, especially industrial wastewater, such as effluents from chemical, metallurgical or engineering industries, preferably effluents from wood-processing industries, such as bleach plant effluents, which method comprises the steps of passing the wastewater through a biological purification stage for subjecting the wastewater to a biodegradation process, and separating the resistant sub- stances from the wastewater and subjecting them to a treatment for rendering them harmless, characterised in that the wastewater is subjected to said biodegradation process in the biological purification stage prior to said separation of the resistant substances; the resistant substances are separated from a stream of the wastewater comprising at least.the major portion of the stream of wastewater discharged from the biological puri¬ fication stage and are recycled to the biological purification stage; and the resistant substances and the microorganisms parti¬ cipating in said biodegradation process are caused to have a substantially longer retention time in the biological puri¬ fication stage than the wastewater.

2. A method according to claim 1, characterised in that the separation of the resistant substances is carried out in one or more stages.

3. A method according to claim 1 or 2, characterised in that the separation of the resistant substances is carried out by membrane separation of the said stream of wastewater.

4. A method according to claim 3, characterised in that the separation is carried out by means of a charged separation membrane.

5. A method according to claim 3 or 4, characterised by conferring a relative shear velocity on the wastewater and th separation membrane by means of a mechanical drive mechanism, said shear velocity preferably being at least about 3 m/s.

6. A method according to claim 5, characterised in that the mechanical drive mechanism is a rotary mechanism and is also used for mechanical cleaning of the membrane.

7. A method according to any one of claims 3 to 6, characterised in that the membrane separation is carried out using one or more tubular membranes, through which the waste¬ water is passed from the outside and inwardly.

8. A method according to any one of claims 1 to 7, characterised in that the microorganisms used for effecting the said biodegradation of the wastewater in the biological purification stage are separated from the said stream of wastewater simultaneously with the resistant substances and are recycled to the biological purification stage together with them.

9. A method according to any one of claims 1 to 8, characterised said biodegradation is carried out in one or more anaerobic purification stages.

10. A method accordig to any one of claims 1 to 9, characterised in that said biodegradation is carried out in a plurality of anaerobic purification stages and in that for each purification stage a separation of the resistant substan ces and a recycling of them to the purification stage are carried out.

11. A method according to claim 9 or 10, characterised in that an aerobic purification stage is provided downstream of the anaerobic purification stage, or the last anaerobic purification stage.

12. A method according to claim 11, characterised in that excess sludge containing aerobic microorganisms is recycled from the aerobic purification stage to the anaerobic puri¬ fication stage or stages.

13. A method according to any one of claims 1 to 12, characterised in that the retention time of the microorganism and the resistant substances (the sludge age) in the bio- logical purification stage or stages is at least 5 times and preferably from 20 to 50 times the retention time of the wastewater (the hydraulic retention time) .

14. A method according to claim 13, characterised in tha the retention time of the wastewater in the biological puri- 12 fication stage or stages is from 1 to 200 hours, preferably from 5 to 50 hours.

15. A method according to any one of claims 1 to 14, characterised in that when the wastewater to be treated is fe to the biological purification stage, or the first puri¬ fication stage, its total COD-value is from 100 to 100000 mg/l, preferably from 1000 to 50000 mg/l.

16. A method according to any one of claims 1 to 15, characterised in that the temperature of the wastewater in th biological purification stage or stages is 5-70°C, suitably 20 60°C, preferably 25-35°C.