WO2011112140A1

WO2011112140A1 - Method and system for treatment of an effluent from a pulp-producing process

Info

Publication number: WO2011112140A1
Application number: PCT/SE2011/050248
Authority: WO
Inventors: Göran Olsson
Original assignee: Metso Paper Oy
Current assignee: Valmet Technologies Oy
Priority date: 2010-03-09
Filing date: 2011-03-04
Publication date: 2011-09-15
Anticipated expiration: 2012-09-09
Also published as: SE1000219A1

Abstract

The invention relates to a method for treatment of an effluent flow (E) from a pulp-producing process, comprising the steps of: passing the effluent flow through a separation step (1) to create a first fraction (F1) and a second fraction (F2), the first fraction being the fraction passing through the separation step and the second fraction being the fraction retained in the separation step; passing the first fraction (F1) to a biological treatment step (2) and passing the second fraction (F2) to a combustion step (3). The invention further relates to a system for carrying out such a method.

Description

Method and system for treatment of an effluent from a pulp-producing process

TECHNICAL FIELD

The present invention relates to a method and a system for handling the effluent from an industrial process, especially pulp -producing processes.

BACKGROUND

In effluents, or wastewater, from industrial processes, especially pulp- producing processes, the effluent is often contaminated with organic substances. While the low-molecular organic substances in the effluent may efficiently be degraded in biological treatment steps, the problem of the resistant high-molecular organic substances often remains.

The effluent load, i.e. the amount of harmful substances released to a recipient, is commonly measured in terms of COD (Chemical Oxygen Demand} or BOD (Biological Oxygen Demand). COD is based on the fact that nearly all organic substances can be fully oxidized to carbon dioxide in the presence of an oxidizing agent under acidic conditions. To simplify, it is an indirect measure of the amount of organic substances contained in a liquid, such as an effluent from an industrial process. Many governments impose strict regulations regarding the maximum chemical oxygen demand allowed in wastewater before it can be returned to the environment. Biological Oxygen Demand (BOD) is an analysis procedure for determining the uptake rate of dissolved oxygen by the biological organisms in a body of water. It is not a precise quantitative test, although it is widely used as an indication of the quality of water. When comparing the two methods, BOD is similar in function to COD, in that both measure the amount of organic compounds in water. However, COD is less specific, since it measures everything that can be chemically oxidized, rather than just levels of biologically active organic matter. In the lignocellulosie pulp industry, COD is the most common measurement method.

In WO 90/ 15028 it is described that high-molecular resistant substances from effluents of pulp bleach plants can be separated through membrane separations techniques, and the concentrate may be disposed of by, for example, incineration or dumping, both techniques described as causing serious environmental problems. As an inventive method it is proposed to treat the effluent in a biological purification stage prior to separation of resistant substances and then recycling the resistant substances to the biological purification step for a prolonged treatment.

An additional problem when using conventional combustion in the case of effluents from a chemical pulp-producing process is the requirement of a specific soda recovery boiler for carrying out the combustion. A soda recovery boiler takes care of the by-product black liquor produced during kraft cooking of cellulose-containing pulp and at the same time generates large amounts of useful energy as well as effectuates recycling of valuable chemicals. This double function of the recovery boiler requires a sophisticated structure and makes the operation of the boiler considerably more demanding than power plant boilers using "normal" fuels. The black liquor is a tar-like liquid comprising lignin, hemi-cellulose and wood extractives. Since the black liquor to be combusted in the soda recovery boiler contains a certain amount of highly corrosive substances, such as i.e. sodium (Na), chlorine (CI) and sulphur (S) the interior walls of the boiler has to be of a corrosion resistant material. The interior walls of a recovery boiler commonly consists of tubes functioning as heat exchangers, and since the demands on such tubes are very high, they tend to be very expensive. Moreover, the handling of sodium as a part of the recovery process also presents special requirements on the design of the boiler. A soda recovery boiler is thus very costly as an initial investment. For a mechanical pulp mill, a soda recovery boiler is not normally a part of the plant as is the case for a chemical pulp mill which is based on kraft cooking, i.e. a sulphate pulp mill. In some cases, where the mechanical pulping process is integrated with a sulphate pulp mill, a technique of cross- recovery may be utilized which means that the soda boiler of the sulphate mill also is used also for final evaporation and combustion of the effluent of the mechanical pulping process. However, for "stand-alone" mechanical pulp mills, this is not a possibility. It is technically possible to have a separate evaporation plant as well as a soda recovery boiler for a stand-alone mechanical pulp mill. However, as a soda recovery boiler is a very expensive investment, this is not economically a feasible option.

There is thus a need for an improved method and a system which overcomes or at least reduces the disadvantages of the previously known methods and systems.

SUMMARY

A general object of the invention is to provide an improved method for handling an effluent from an industrial process. A specific object is to provide a method which can meet the increased requirements on minimizing the effluent load, still having relatively low investments costs as well as low maintenance costs. Another object is to provide a system which meets the increased requirements mentioned above.

These objects are achieved in accordance with the attached claims.

Briefly, the present invention is based on the recognition that a separation into two fractions enables an efficient subsequent handling of the effluent from an industrial process. Especially, it is recognized that a certain way of separation into the two fractions is needed to enable a more simple and cost- efficient handling of the effluent. In accordance with the invention is thus provided a method for treatment of an effluent flow from an industrial process, comprising the steps of:

a) passing the effluent flow through a separation step to create a first fraction and a second fraction, the first fraction being the fraction passing through the separation step and the second fraction being the fraction retained in the separation step;

b) passing the first fraction to a biological treatment step;

c) passing the second fraction to a combustion step

The separation step comprises separation through a membrane filter and the first, fraction is a penneate and the second fraction is a concentrate.

According to one embodiment, the combustion step is being preceded by an evaporation step in which a main part of the liquid content of the second fraction is evaporated before combustion.

The membrane filter may be a filter adapted to separate relatively easily biologically degradable low-molecular substances into the first fraction and relatively difficultly biologically degradable high-molecular substances into the second fraction.

According to one embodiment, the separation is carried out such that the second fraction contains the main part of substances amounting to the chemical oxygen demand of the effluent flow.

The separation step is adapted to be carried out such that only a minimum amount of sodium is retained within the second fraction, the concentrate. This may be accomplished by using a membrane filter with a cut-off within the interval of 5000-100000g/moI preferably 10000-20000 g/moL Since sodium- containing substances melt, at the high temperatures during incineration and thus create problems sticking to the inside of a combustion device, removing the main part of the sodium enables the combustion step to be carried out. in a relatively simple incinerator. Such an incinerator is very Inexpensive in comparison to a conventional soda recovery boiler, and thus provides a method for the handling of effluent which is far more economic.

The invention is especially useful in connection with a pulp -producing process not being a chemical pulping process, e.g. for producing a thermomechanical pulp (TMP). The invention is also useful when the pulp produced is a chemitheraiomechanical pulp (CTMP) or a chemimechanical pulp (CMP). The pulp-producing process may further be a process producing a neutral sulfite semi-chemical pulp (NSSC). The invention is especially useful when such a process is carried out as a stand-alone mill, i.e. not integrated with a chemical pulp mill with a soda recovery boiler. In the terminology commonly used in pulping technology, all the pulping processes mentioned above are referred to as high-yield processes, since their yield in comparison to chemical pulping processes is considerable higher. TMP, CTMP and CMP are commonly denoted mechanical pulping processes and NSSC comes close to or in the same region of such processes in yield, despite not being a mechanical pulping process. in accordance with the invention is further provided a system for carrying out the inventive method. The system for treatment of effluent from a pulp- producing process comprises a separation device, a biological treatment device and a combustion device, where the separation device is adapted to separate the effluent in a first fraction and a second fraction and the biological treatment device is adapted to treat the first fraction and the combustion device is adapted to treat the second fraction.

The system comprises a membrane filter as the separation device, with a cutoff of the membrane chosen such that a minimum amount of sodium is retained on the membrane filter. A relatively simple incinerator, without, specific means for handling sodium, is used as the combustion device.

The proposed method and system for treatment of effluent, leads to a number of advantages, including:

● possibility to meet the tougher requirements on effluent load. * possibility to meet the tougher requirements on effluent load with low investment costs as well as low operational costs.

* possibility of providing an efficient and more inexpensive method and system for handling the effluent of a "stand-alone" mechanical pulp mill.

* possibility to design and operate pulp-producing processes with lower pulp yield, i.e making pulps with higher strength or higher brightness.

The last listed advantage is due to the effect that a better effluent treatment enables allowing the pulp-producing process to generate a higher amount of COD, without this affecting the quality of purified effluent let out from the pulp mill.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, may best be understood by reference to the following description and appended drawing, in which:

Fig. 1 is a schematic figure illustrating the system and the method according to one embodiment of the invention;

DETAILED DESCRIPTION

Both the method and the system will be described in connection with figure 1, and reference numbers in the figure may thus relate to method steps as well as system components.

Fig. 1 schematically illustrates a system 500 for treatment of effluent according to one embodiment of the invention and the method is described with reference to the system. It should be noted that the method could be carried out also in different system than the one, although very schematically, illustrated in the figure. In a first step 1, an effluent E from an industrial process, e.g. a pulp- producing process, is treated in a separation device 100 separating the effluent flow E into a first fraction Fl and a second fraction F2, In a preferred embodiment the separation device 100 is a membrane filter 101 capable of separating the effluent flow E into two fractions where the first fraction Fl contains mainly low-molecular organic substances which are relatively easily biodegradable while the second fraction F2 contains mainly high -molecular organic substances, which are known to be relatively difficultly biodegradable. There is no exact definition of the term "relatively easily biodegradable", but in effluent handling the term "easily biodegradable" is generally and commonly used to denote such substances that in a short time may be degraded by means of biological processes.

The separation device .100 may however be any device capable of separating the flow in such fractions. The device may also be adapted to make the separation in such a way that the main part of the substances amounting to the chemical oxygen demand of the effluent flow E, is contained in the second fraction.

In a second step 2, the first fraction Fl is conveyed to and treated in a biological treatment device 200. The biological treatment device may be of a conventional design. In one embodiment, microorganisms degrade the organic material in an anaerobic treatment (i.e. without the presence of oxygen), Alternatively, the biological treatment may be canted out by an activated sludge in which microorganisms in the presence of oxygen biologically oxidize the organic material. The oxygen may be added in the form of an air flow. In the illustrated embodiment, such an optional flow of air 201 that may be added to the device, is indicated by the dashed arrow in the figure. Alternatively, pure oxygen may be added to the device. A purified flow of water 202 leaving the biological treatment device 200 may be returned to the pulp process or let out to the surrounding environment. The first fraction is thus efficiently treated to fulfill the requirements regarding harmful substances let out to the environment. In case the separation is carried out such that a main part of the substances amounting to the chemical oxygen demand of the effluent flow E, is contained in the second fraction, the biological treatment device 200 in the second step 2 may be comparatively small.

In a third step 3, which may be carried out simultaneously with the second step 2, the second fraction F2 is conveyed to and treated in a combustion device 300. Optionally, the combustion device may be preceded by an evaporation device 310, in order to evaporate, in an evaporation step 3a, at least a part of an amount of water that still may be contained in the second fraction F2. In the figure, the evaporation device 310 is illustrated in dashed lines, to emphasize that this is an optional stage, which may be dispensed with. In the combustion device 300 a remainder 312 of the second fraction F2 after removal of an amount of water 311, in case an evaporation device 310 is used, or, if no evaporation is carried out, the entire fraction F2, is combusted. The high -molecular organic substances contained in the fraction F2 are thus transferred into energy usable in the industrial process which initiated the effluent, or in other processes in need of energy.

The combustion device 300 is a simple incinerator. The word incinerator is generally, and specifically in this context, used to differentiate such a combustion device from a more sophisticated soda recovery boiler. It is to be understood that an incinerator is merely a destruction device with no means for recovery or other handling of substances contained in the effluent, e.g. sodium.

Sodium is generally a problem in incineration at high temperatures, since the sodium- containing compounds melt and then tend to stick to the interior walls of the incinerator creating problems, e.g. increased corrosion and an increased need for maintenance. In a soda recovery boiler, the sodium, in form of sodium carbonate and sodium sulphide is taken out as a melt in the bottom, which requires a fairly complicated structure of the boiler. Further, to obtain sodium sulphide instead of sodium sulphate, the combustion has to be carried out. in a reducing environment, i.e. with an oxygen deficit.

By minimizing the amount of sodium in the concentrate to be incinerated, it is possible to use a less complicated device in the combustion step 3, which is thus a much more inexpensive apparatus. A relatively simple incinerator, without any means for handling the problems connected with sodium, may in the inventive method and system be used to combust the concentrate, fraction F2.

A membrane 101 of the separation device 100 should thus be chosen such that the membrane 101 lets through the low-molecular organic substances and a main part of the sodium ions into the perrneate, such that the amount of sodium retained in the concentrate, fraction F2, is kept to a minimum. A measure of the cut-off is commonly given in g/moL indirectly corresponding to the molecular weight which will be retained on the membrane. However, if the filter cut-off is chosen too wide to ensure that all sodium- containing substances pass the membrane, the risk of high -molecular organic substances passing is increased. There is thus a balance between separating the high- molecular substances not suitable for biological treatment and retaining only a minimum of sodium in the concentrate. A cut-off of 5000- lOOOOOg/mol, preferably 10000-20000g/mol seems to enable an adequate balance.

As an example of a separation device suitable for performing the separation step, reference is made to the published PCT application WO 92/03216, in which a device for membrane separation is described.

Although the invention has been described with reference to a specific illustrated embodiment, it is emphasized that it also covers equivalents to the disclosed features, as well as changes and variants obvious to a man skilled in the art. Thus, the scope of the invention is only limited by the appended claims.

Claims

1. A method for treatment of an effluent flow (E) from a pulp -producing process, comprising the steps of:

passing the effluent flow through a separation step (1) to create a first fraction (Fl) and a second fraction (F2), the first fraction being the fraction passing through the separation step and the second fraction being the fraction retained in the separation step; passing the first fraction (Fl) to a biological treatment step (2) for biological degradation;

passing the second fraction (F2) to a combustion step {3} for combustion, the separation step (1) comprising separation through a membrane filter, the first fraction (Fl) being a permeate and the second fraction (F2) being a concentrate

characterized by the separation step (1) being carried out such that only a minimum amount of sodium is retained within the second fraction (F2) and the combustion step (3) being carried out in an incinerator.

2. A method according to claim 1 , characterized by the combustion step (3) being preceded by an evaporation step (3a) in which a main part of the liquid content of the second fraction (F2) is evaporated before combustion.

3. A method according to claim 1 or 2, characterized by through the membrane filter separating relatively easily biologically degradable low- molecular substances into the first fraction (Fl) and relatively difficuidy biologically degradable high-molecular substances into the second fraction (F2).

4. A method according to claim 3, characterized by the separation step (1) being carried out such that the second fraction (F2) contains the main part of substances amounting to the chemical oxygen demand of the effluent flow (E).

5. A method according to any of the preceding claims, characterized by using a membrane filter with a cut-off within the interval of 5000-lOOOOOg/mol, preferably 10000-20000 g/mol.

6. A method according to any of the preceding claims, characterized by the pulp-producing process belonging to the group of mechanical or semi- chemical high-yield pulp-producing processes.

7. A method according to claim 6, characterized by the pulp producing process being a thermomechanical pulping process.

8. A method according to claim 6, characterized by the pulp-producing process being a chemithermomechanical or a chemimechanical pulping process,

9. A method according to claim 6, characterized by the pulp-producing process being a neutral sulfite semi-chemical pulping process.

10. A system (500) for treatment of effluent from a pulp-producing process comprising

a separation device (100), a biological treatment, device (200) and a combustion device (300), where the separation device is adapted to separate the effluent in a first fraction (Fl) and a second fraction (F2) and the biological treatment device is adapted to treat the first fraction Fl such that, a main part of the contents of the first fraction is biologically degraded and the combustion device is adapted to treat the second fraction (F2) such that a main part of the contents of the second fraction is combusted,

characterized in that the separation device (100) is a membrane filter with a cut-off chosen such that only a minimum amount of sodium is retained within the second fraction (F2) and that the combustion device (300) is an incinerator.