FI130301B

FI130301B - Monitoring the chemical load of wastewater in an industrial process

Info

Publication number: FI130301B
Application number: FI20155068A
Authority: FI
Inventors: Ari Poukka
Original assignee: Metsae Fibre Oy
Priority date: 2015-01-30
Filing date: 2015-01-30
Publication date: 2023-06-09
Also published as: WO2016120531A1; FI20155068A

Abstract

The invention relates to methods monitoring the chemical load of wastewater in manufacturing mills and industrial processes, such as in a pulp mill.. Such a process has a number of sub-processes, at least some of which have identifiable root causes for the chemical load they contribute with to the wastewater. The inventive method comprises the steps of: - selecting a number of root causes in said industrial process that causes a chemical load in said wastewater; - calculating the contribution of each of said root causes to the chemical load of said wastewater; - calculating an estimated combined chemical load for all sub-processes to said wastewater. The inventive concept also comprises a software product and an apparatus adapted to run such a software product for monitoring the chemical load of wastewater produced by an industrial process according to the inventive method.

Description

Title of the Invention

Monitoring the chemical load of wastewater in an industrial process

Field of the Invention

The present invention concerns methods for monitoring the chemical load of wastewater produced by an industrial process. More particularly, the inventive concept is aimed at improved methodologies to determine and control the chemical load of wastewater in manufacturing mills and process plants having outlets of wastewater.

Background of the Invention

Many process plants, like cellulose mills, have wastewater permissions with limits that are set by environmental authorities. The chemical or biological oxygen demand, adsorbable organic halogen compounds, and total organic carbon are important process control parameters in wastewater treatment. Industrial processes again contaminate waste water with inorganic, i.e. metallic residues, like iron, manganese, barium or phosphorus.

The chemical oxygen demand is monitored by collecting samples once every 24 hours, excluding or including weekends, which then are analyzed. The results are, due to the long delay in getting them, not very useful for controlling a process on a daily basis. Discharges and emissions are monitored by various measurements, such as the conductivity in the wastewater pipelines. Measured deviations in such measurements reveal to the operator that some process parameters need adjustment. Acquired in this traditional way, definite information of the environmental load of the process is at hand after 1-2 days after the measurements took place.

Apart from the long delays, one of the present problems are that one collected sample may

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O represent a whole day, i.e. 24 hours. Any sudden peaks in the environmental load may thus

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— easily go unnoticed, as will the reason for such peaks.

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O . . . . . .

O It is possible to use online measurement instruments for measuring the concentration of

I . . . . . . £ 25 chemicals, turbidity etc. in aqueous flows. Such instruments are expensive, and do not give any 23 reasons for any deviation, so control of the process requires further analysis. In a modern pulp

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O mill, there are in all some 1000 — 2000 measuring points and sensors, which data need to be

S collected, processed and interpreted. It is not conceivable that all necessary measurements affecting the COD can be carried out by online instruments. Managing all data is a hugely complex task, which has hitherto not been solved with respect to the problem of providing accurate and frequent measurements on the chemical load in the wastewater. Such compiled data is a prerequisite to find causes and consequences of chemical load levels and variations, and thus to develop the remedies.

There is thus room for considerable improvement for monitoring the chemical load of wastewater in an industrial process. The present invention aims to bring a new methodology to determine and control the COD (Chemical Oxygen Demand) load of wastewater, which will solve the problems related to process control in a totally new way.

Summary of the Invention

The inventive concept is particularly well aimed to improve methodologies to determine and control the COD (Chemical Oxygen Demand) load of wastewater in cellulose pulp manufacturing mills and other process industries having outlets of wastewater to the watercourse. The chemical oxygen consumption or demand in pulp mill wastewater consists of lignin and carbohydrates or methanol and other oxygen-consuming substances. In environmental chemistry, the chemical oxygen demand (COD) test is commonly used to indirectly measure the amount of organic compounds in water. COD determines the amount of organic pollutants found in wastewater, thus making it a useful measure of water quality. COD is expressed in milligrams per liter (mg/l) also referred to as ppm (parts per million), and indicates the mass of oxygen consumed per liter of solution. — According to one aspect of the invention, a method for calculating the chemical load of wastewater produced by an industrial process is provided. The industrial process may have a number of sub-processes, at least some of which have identifiable root causes for the chemical 0 load they contribute with to the wastewater. For each root case, the variables affecting the

N chemical load it produces have been identified and guantized. The resulting model may be based

O 25 — on statistics from the past. Current input values are retrieved by online measurements, whereby a & chemical load index can be calculated. The method comprises the steps of: i © - selecting a number of root causes in said industrial process that causes a chemical load

S in said wastewater;

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> - calculating the contribution of each of said root causes to the chemical load of said wastewater;

- calculating an estimated combined chemical load for all sub-processes to said wastewater.

The inventive method thus yields a model for calculating an index, the value of which represents the environmental load of the wastewater generated by a mill or a plant. Such an index facilitates real-time process monitoring and controlling, and if used e.g. in a pulp mill, it maps the COD of the wastewater by mathematical functions that represent the impacts of root causes of the wastewater load. A suitable quantity for such an index may be kgCOD/t (kilograms COD/ton of cellulose), or just a relative number from 0 — 100, for example. Evaluating each root cause and adding their contributions represents a new way to predict the amount of COD load in the — wastewater. The inventive approach greatly improves the capability of the mill or the plant to tune its processes towards a minimum of wastewater load.

The inventive method is based on calculations and knowledge in the field of statistics, chemistry, physics and process engineering knowhow in interpreting wastewater information. The wastewater load is according to the invention determined by continuously measuring a much — reduced set of parameters indicating the chemical load each root cause produces in a plant or mill. By calculating in a commensurate way the load caused by each root cause in the various sub-processes and by adding the loads of each sub-process, the inventive method produces an accurate estimate of the total chemical load to the wastewater. Causes for anomalities in the chemical load may thus be possible to detect even before they impact the chemical load of the wastewater, rendering the inventive method great advantages over known methods.

The COD may be determined in intervals of 10 minutes, which is a suitable interval for a pulp © mill. Other processes may have different optimal intervals. This corresponds to 144 > measurements in 24 hours, to be compared to the once-a-day sample taking with all the 5 drawbacks as mentioned above. -

O 25 — According to some embodiments of the invention, the method may comprise the further steps of:

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N - continuously measuring the chemical load of said wastewater; 3 2 - comparing simultaneously measured and calculated chemical loads, and

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- based on the comparison, storing in a memory time-stamped data regarding any differences as statistical and/or feedback information.

The industrial process may be then controlled in response to the measurement, in order to collect information and to further optimize the actual chemical load of the wastewater.

According to some embodiments, the method further comprises the steps of: - treating wastewater influent from said industrial process in a biological treatment plant; - measuring the chemical load of said wastewater effluent from said biological treatment; - comparing the measured chemical load with a predetermined acceptance value, and - based on the comparison, storing in a memory time-stamped data regarding any differences as feedback information for optimizing said biological treatment.

The invention yields a model that represents the environmental load of the wastewater generated by a mill or a plant. According to some embodiments, the environmental load is the chemical oxygen demand (COD) of the wastewater. Being able to monitor peaks and minimum points of the COD load gives the information needed for improvements. The difference in COD between a laboratory control sample and the result produced by the inventive model gives a measure to focus, develop, calibrate and learn about the processes. Detected variations may also report a new or so far unknown COD load. The underlying principles and methodology of the present invention may be used in a variety of wastewater treatment processes, for example in processes — where BOD (Biological Oxygen Demand), AOX (Adsorbable Organic Halogen Compounds)

LO and/or TOC (Total Organic Carbon) need to be measured, the wastewater treated and the

S chemical load discharged to watercourse minimized. Industrial processes, which contaminate 5 process water with inorganic i.e. metallic residues, like iron, manganese barium or phosphorus, 2 may egually well be mapped of their root causes for the contamination, the amounts being

E 25 quantized and calculated in a manner consistent with and without departing from the present a © invention. ©

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O According to some embodiments, the industrial process is a process for producing pulp from

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N wood. The root causes may be identified and included for calculating the chemical oxygen demand in at least the following sub-processes of pulp-making: debarking, cooking, bleaching, drying, evaporation, causticising and recovery boiler.

The root causes for the debarking process stem from residues and dissolved compositions from the bark, these and other parameters which may be quantized by measuring or taken into 5 account. These include tree species, the amount of bark, the degree of barking performed, the amount of water to be compressed from the bark, the time of the year, etc.

Root causes for the bleaching process may include the production volume of bleaching, the amount of lignin to be dissolved, the amount of dissolving carbohydrates, wash losses, extractable compositions dissolving during bleaching, and oxidizing chemicals affecting COD — reduction.

The root causes for the evaporation process may include data of a unit to be washed, the COD- content of washing water, the amount of wash water, and the concentration (mg/l) (converted from a conductivity value in mS/m). The conductivity is the measured parameter, which is then converted to concentration in order to be usable in COD calculations. The root causes for the — causticising process may include the concentration of the wash water, the amount of wastewater, and the conductivity (mS/m) conversion to concentration (mg/l). The root causes for the recovery boiler process may include contamination of a secondary condensate that is fed to the wastewater, the amount of a secondary condensate that is fed to the wastewater, and the concentration in mg/l (as converted from conductivity mS/m). The boundaries between various — sub-processes in a plant are not always unambiguous, in the sense of vocabulary used and because of technical differences between implemented processes in each plant. The meaning of principal process step names used herein, like fiber line and recovery, are thus not meant to be 0 limiting. For example recovery, interpreted broadly in the literature, may include evaporation

N and causticising and condensates and secondary condensates processing.

O 25 & A recovery boiler is a vessel used for recovery and reformation of white liguor from black liguor. = For the purpose of the invention, it is important only to identify and measure such definite x process steps which have a bearing on the amount of COD produced.

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S According to embodiments, the total chemical oxygen demand for pulp-making process is calculated by adding together the calculated chemical oxygen demands for each sub-process.

According to one aspect of the invention, a software product is produced for monitoring the chemical load of wastewater produced by an industrial process according to the inventive method.

According to a further aspect of the invention, an apparatus is provided that is adapted to run a software product enabling the apparatus to monitor the chemical load of wastewater produced by an industrial process according to the inventive method. Such an apparatus may be a general- purpose computer, or a dedicated digital controller capable of running the software.

According to still an aspect of the invention, an industrial plant is provided where at least one of its main processes is producing a chemical load of wastewater, and wherein the process is monitored according to the inventive method.

The inventive concept brings indeed considerable advantages to process control in a complex plant. The load imposed by wastewater on the environment is reduced e.g. by the following: - utilization of process data is to identify flaws in the processes is brought to an entirely new level. The wastewater and the COD sources becomes much better known. Once errors are identified, they are easy to correct, and/or it becomes possible to develop the process in a better direction. - by creating a wastewater index system based on process data, information about root causes and how to calculate the COD is easily shared and distributed throughout the organization and mills. Best practices are thus easily shared and applied. - most importantly, the biological wastewater treatment which has to take place

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O before the wastewater is discharged to the watercourse, is easier to optimize. Once

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— the incoming wastewater is well known in detail, the biological treatment process

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Oo and the nutrients and other ingredients needed may be optimized for the influent

O r 25 waste load. That increases the environmental efficiency of the plant. a a

S The various embodiments of the invention are in the following described more closely by means

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LO of examples and by referring to the attached drawings.

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Definitions

COD

COD is a form of chemical load in wastewater and other liquids. The chemical load a process causes can thus be calculated on the basis of the amount of waste water (I/s) and the concentration of the chemical load (mg/l). COD in cellulose wastewater consists mainly of lignin and carbohydrates. The basis for a COD test is that nearly all organic compounds can be fully oxidized to carbon dioxide with a strong oxidizing agent under acidic conditions. According to the International

Standard ISO 6060 for determination of the chemical oxygen demand, COD is the “mass concentration of oxygen equivalent to the amount of dichromate consumed by dissolved and suspended matter when a water sample is treated with that oxidant under defined conditions”. ISO 15705 is a more recent (2002) standard for the determination of a chemical demand index based on spectrophotometry. Other relevant standards include SFS 5504 and DIN 38409.

Wastewater

Although self-explanatory to a great extent, it should be understood that each phase of a plant produces their own fractions of wastewater, which eventually are brought together to the wastewater discharge or outlet and must be treated and cleaned in various ways to minimize the load on the environment. In a pulp mill, water is often re-used and circulated between different units for a number of reasons, the obvious one being that water is used as a washing and transport medium in the plant.

Kappa number

The Kappa number, having a value in the range of 1 to 100, is defined as in the

Lo standard ISO 302:2004. The Kappa number indicates the demand of chemicals to > achieve a certain grade of bleaching. It is thus an estimate of the amount of lignin in 5 25 the pulp given by K = c"1, where K=the Kappa number, c is a process-dependent

O constant = 6,57 and I = the content of lignin in %.

I Bleach yield

N The pulp yield in bleaching depends mainly of the amount of dissolved lignin and

S dissolved carbohydrates and some extractives. The pulp yield in the bleaching is = 30 usually 94 -96 %.

N Pulp mill

A pulp mill consists of a debarking unit, a fiber line, and of recovery and waste water treatment plants.

Fiber line

A fiber line consist typically of the phases of cooking, oxygen stage, pulp washing, bleaching, drying.

Recovery

A recovery plant consists at least of a recovery boiler, an evaporation unit and a causticising unit. Broadly defined recovery also includes condensates processing, such as secondary condensates.

Reduction

In every plant, stage or phase producing wastewater and thus having a COD calculated by any of the formulas given below, a correction term must be used to account for reductions in the COD values. This may or may not be related to chemical reduction (oxidizing) resulting in a lower COD e.g. in bleaching, but reductions also result from process exits to burning and recirculation or re-use of wastewater. The efficiency of a biological treatment process of wastewater is measured by means of the reduction it causes in the COD value of the wastewater.

Brief Description of the Drawings

Fig. 1 illustrates the difference in sampling intervals between prior art and the present invention;

S Fig. 2 shows a comparison of averaged COD values taken over an extended period of time;

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O Fig. 3 is a schematic view of a pulp mill and its wastewater flow. a = 25 Infig. Iis shown and denoted by reference number 1 sampling and measuring of COD values in a © wastewater according to prior art, one sample per 24 hours. The curve marked with reference © . . .

B number 2 shows the corresponding COD values when calculated every 10 minutes according to

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S the present invention. The variations are considerable, and one sample every 24 hours leaves

N these mostly undetected. Even if detected, any deviations will be brought to the operator” s knowledge only days after they occurred, whereby the cause mostly has disappeared or is difficult to determine, leaving the lessons unlearnt from these events.

According to Figure 2, where is shown a similar graph 3 over as in figure 1of COD values calculated according to the invention, but stretched over a period of 10 months, it can be shown that the difference D between the line 4 showing the average of curve 3 over these 10 months represents, in this exemplary case, an average COD load of 8 tCOD/d (tonnes/day) more than the conventionally measured average shown by line 5. This amount of 8 tCOD/d is thus not explainable by conventional sampling techniques, based on one sample/24 hours plus laboratory — analyses. The monetary savings, if a COD error of this magnitude is eliminated in a pulp mill, and the pulp-making process is optimized to follow the environmental restrictions and producing pulp with a desired Kappa number, are millions of euros on an annual basis.

As has been stated above, the invention yields a model that represents the environmental load of — the wastewater generated by a mill or a plant. The model maps the chemical load of the wastewater by mathematical functions, which are representing the contributions of the root causes to the wastewater load. Measuring the chemical load caused by each root cause and summing the contributions in the model describes in real time the formation of the chemical load of the wastewater. In the case of a pulp mill, the chemical oxygen demand COD is the key — variable, and the root causes for the COD load in each of the various steps of pulp making are mapped and quantized. In the following, exemplary process steps from a pulp mill, their root causes and the COD functions derived therefrom is presented.

The process environment in the pulp mill is first analyzed with process measurement equipment,

N in order to establish the COD of every part of it. Preferably, an index representative of calculated

N 25 COD values is created and consists of a number of functions parameters, to make the result of

S the COD calculations comparable over time and also between plants. Such an index may be = based on data retrieved and stored over time, to provide a robust statistical base for interpreting

E measured and processed wastewater information. 3

O In order to produce calculable results from the various phases of pulp making, all COD = 30 contributions and their reductions are preferably provided in a form that can be simply added

N together. Here, the unit for an end result of any COD function described below is thus mg/l. If not, the unit needs to be converted. This is the case in measuring the conductivity (mg/l) of a wastewater fraction, where the resulting unit mS/m (milliSievert/meter) is converted to concentration (mg/l) according to pre-calculated conversion tables for each stage and chemical compositions.

The measurement equipment needed to implement the inventive method is the same or almost — the same that are present in the plants anyway. They include flow (amount) measurement equipment, temperature, position and surface level sensors, pH and conductivity measurement devices, to name a few. Implementation of the present invention does thus not require big investments on equipment.

Debarking, COD:

The COD load which is dissolved from debarking depends on: " the tree species = the amount of bark " the degree of barking performed = the amount of water to be compressed from the bark = the time of the year

The total COD load derived from barking may thus be covered by adding two functions: (1) + (2), where

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2

N (1) = flaevark (tree species, amount of bark measured, time of the year)

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S 20 (2) = PDaebark (tree species, amount of bark measured, dry substance of the bark, time = of the year) a 00 . . . .

S Obviously, most of the bark is separated, pressed and burnt, so the COD will result from residues

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2 and dissolved compositions from the bark that is eventually forming a wastewater fraction.

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Cooking of cellulose is typically done in a closed-loop system, which residues are either burnt or passed on as wash losses to the bleaching phase. The COD of cooking is therefore included in the bleaching COD calculations described below.

Fiber Line Overflow, COD: = Filtrate tanks may overflow as a result of unexpected conditions in the process. Surface sensors will in many cases detect the condition and its duration, which obviously affect also the estimate COD value of the actual wastewater fraction.

Bleaching, COD:

The COD load which is caused by bleaching depends on: - the production " the production volume of the bleaching is calibrated as the “official” production volume. Bleach production is an important calculation parameter, and is traditionally dependent on flow metering and consistency measurements. However, these methods are not accurate enough. A more accurate figure can be obtained by matching the production with later stages in the line, e.g. as measured from the baling line. This value can then be used for continuous calibration of the online measurements. - the amount of lignin to be dissolved, which depends on = 20 " the kappa value

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5 = the production volume a - the amount of dissolving carbohydrates (2- 3 %), which among other things

Tr a depends on co © 3 " the kappa value

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N 25 " the wood species - wash losses, which depend on:

" the amount of wash liquid to be used " the kappa reduction in the oxygen phase " the purity of the secondary condensate - extractable compositions dissolving during bleaching, which depends on " the tree species = the time of the year - oxidizing chemicals affecting COD reduction.

The principle of calculating the COD load of the bleaching process is as follows: (3) foo kappa (tree species, kappa measurement, production volume (t/d)) * £ (100 - reduction A (%))/100 (4) fDoyieta (tree species, bleach yield (%), production volume (t/d) * £ (100 — reduction B (%))/100 (5) fDowash 1oss (dry solid content of black liquor (%), kappa measurement of the oxygen stage before and after, purity of the secondary condensate (mg/l), production volume (t/d) * £ (100- reduction C (%))/100 (6) frokappa (tree species, kappa measurement, production volume (t/d)) * f (100- reduction D (%))/100

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> (7) feoyiera (tree species, bleach yield (%), production volume (t/d)) * £ (100- 5 reduction E (%))/100

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- 20 (8) Fsecondary condensate in bleaching (purity of the secondary condensate (mg/l), measured & amount (I/s)) * (100 — reduction F (%))/100 co 3

O (9) friber line filtrate tank overflow (Measurement of the tank surface (%), concentration > (mg/l), amount (1/s))

(10) foyer filtrate tank overflow (Measurement of the tank surface (%), concentration (mg/l), amount (I/s))

Bleaching consist of acid stages DO and alkaline stages EO. The total bleaching COD = (3) + (4) + (5) + (6) + (7) + (8) + (9) + (10).

Evaporation Unit, COD:

The COD load which is caused in the evaporation unit depends on: = the unit A — J to be washed = the COD-content of wash water fed to the waste water channel = the amount of wash water fed to the waste water channel " the concentration (mg/l) (converted from conductivity value in mS/m) = valve data (the time a valve is on or off) (11) Fl Evaporation (wash unit A wash data, concentration, valve data, amount) + (12) Zevaporation (wash unit B wash data, concentration, valve data, amount) (13) f3Evaporation (wash unit C wash data, concentration, valve data, amount) (14) fAEvaporation (wash unit D wash data, concentration, valve data, amount) (15) fSEvaporation (wash unit E wash data, concentration, valve data, amount) (16) förvaporation (wash unit F wash data, concentration, valve data, amount) (17) fTEvaporation (wash unit G-J wash data, concentration, valve data, amount)

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> Total Evaporation Unit COD = 5 20 (18) X {Evaporation (Wash unit £ wash data, concentration, valve data, amount), where 2 k=A-J. z Causticising, COD: a & The COD load which is caused by the causticising depends on: = the concentration of the wash water fed to the wastewater

N 25 = the amount of wastewater = the concentration (mg/l) (converted from conductivity value in mS/m)

Total Causticising COD = (19) F Causticising (White liquor filters wash data, concentration, valve data, amount)

Recovery, COD:

Occasionally part of the secondary condensate is fed to the wastewater. Ouality control of secondary condensates are performed by conductivity measurements. The COD load of secondary condensate that is fed to the wastewater channel depends on: " contamination " amount "concentration (mg/l) (converted from conductivity value in mS/m) (20) Fl Recovery (concentration of secondary condensate 1, valve data, amount) (21) Drecovery (concentration of secondary condensate 2, valve data, amount) (22) recovery (concentration of secondary condensate 3, valve data, amount) (23) fArecovery (concentration of contaminated condensate, valve data, amount)

Total Recovery COD = (20) + (21) + (22) + (23).

Other factors COD:

A pulp mill may also have other specific root causes of chemical load in wastewater.

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N The COD to wastewater treatment:

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2 The total COD load transferred to wastewater treatment (reduction) may be given as follows: =

N 20 (24) CODpebarking + CODCooking + CODsleaching + CODprying + COD txtraction + CODCausticising +

S CODRecovery + CODotners. The order of these factors are not necessarily reflecting the order they

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= are performed in a pulp mill, as there usually are many branches and return paths in various

N process steps. Cooking, Bleaching and Drying may be referred to as the fiber line of a pulp mill.

The COD value may also be affected by chemical reactions in the mixture and by COD reductions during settling.

The COD going into watercourse:

The level of COD of the wastewater can be reduced by: " biological treatment " settling sedimentation " reduction (chemical reduction)

Different methods affect the COD load of the various wastewater fractions individually. For example, the COD effect of biological cleaning of washing water coming from bleaching is greater than for a fraction from the EO stage of the bleaching.

When the COD components of each fraction are identified and their COD reductions are known, the amount of COD which is passed on to the environment may be calculated: (25) Total COD = debarking COD * (100 - debarking COD -reduction)/100 + bleaching DO-COD * (100 - DO-COD -reduction)/100 +

LO bleaching EOP- COD” (100 — EOP-COD -reduction)/100 +

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N fiber line COD * (100 — fiber line COD -reduction)/100 +

O o 20 evaporation COD * (100 — evaporation COD -reduction)/100 +

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I causticising COD * (100 — causticising COD -reduction)/100 + a & recovery COD" (100 — secondary condensate COD -reduction)/100 +

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O other COD(100 - other COD -reduction)/100.

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A numerical example taken from a pulp mill is shown in table 1. For each unit, predicted COD values in tCOD/d and kgCOD/t (kilograms COD/ton of cellulose) are given, as well as their reduction percentages.

The total COD is given by adding the reduced COD values of each unit together. tCOD/d kgCOD/t

Debarking clarifier

Belt filter press]

Belt filter press 2

Bio compressor | 838 ~~ [01 ~~ Joo

Integrate [85 ~~ 108 fos 00000

Bleaching, DO

Bleaching, EOP

Extractor Recovery 83.8 0.1

Plant

Dryer [88 ~~ Joo ~~ Joo

Total COD Jo ~~ AN

Table 1

In Fig. 3 is shown a schematic view of a pulp mill 30 and its wastewater flow. In the figure is shown the main units of a pulp mill, i.e. a debarking unit 31, a fiber line 32 and a recovery plant or unit 33. The sub-processes contained in each main unit are also mentioned. The debarking unit feeds the fiber line with raw material at 28 and the fiber line the recovery plant at 29. Other units include belt filter presses 34 and 35, a furnace 36 and a biological wastewater treatment plant 37.

The belt filter presses 34, 35 feed one or several furnaces with e.g. dried bark from unit 31 or concentrates from wastewater treatment plant 37, etc. The heat from the furnace is according to normal pulp mill energy solutions used to produce steam to generate electricity in a turbine or for

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S process heating.

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O 15 Line 39 is the wastewater collection line with influx of wastewater fractions from all units or

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0 sub-processes, and an output to the treatment unit 37. From the treatment unit, the wastewater is

E discharged into watercourse 38. The figures in the wastewater fraction lines denote typical 2 amounts of tCOD/d emerging into the wastewater from that particular sub-process. For example,

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O debarking produces 8,6 tCOD/d, bleaching 39 tCOD/d etc.

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At point 40, the combined COD load of the wastewater can be continuously or otherwise measured, e.g. for process feedback and archiving purposes. When compared with the calculated

COD the difference data may be stored in a memory of a computer with a time-stamp.

At point 41, the effluent from the treatment plant 37 can be measured. The remaining chemical load should, for example, not exceed any restrictions or acceptance values set by the authorities.

Also here the measured COD load may be time-stamped and stored in a memory for reporting purposes, and/or for getting feedback information for optimizing the biological treatment.

The effluent from the treatment plant 37 may be further subject to a tertiary treatment based on chemicals. Such additional reduction in the COD values may naturally be part of the overall

COD reduction performance of the industrial plant in question.

It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment.

As used herein, a plurality of items, units, phases, and/or materials may be presented in a

LO common list for convenience. However, these lists should be construed as though each member > of the list is individually identified as a separate and unigue member. Thus, no individual 5 member of such list should be construed as a de facto equivalent of any other member of the

S 25 — same list solely based on their presentation in a common group without indications to the z contrary. In addition, various embodiments and example of the present invention may be referred

N to herein along with alternatives for the various components thereof. It is understood that such

S embodiments, examples, and alternatives are not to be construed as de facto equivalents of one = another, but are to be considered as separate and autonomous representations of the present

N 30 invention.

Furthermore, the described features, units or characteristics may be combined in any suitable manner in one or more embodiments. In the description, numerous specific details are provided, such as examples of amounts, calculations, operations, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention.

Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

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Claims

1. A method for monitoring and controlling the chemical load of wastewater produced by an industrial pulp-making process, said process having a number of sub- — processes, at least some of which have identifiable root causes for the chemical load they contribute with to the wastewater, characterized by that the method comprises a software running in a computer or controller, said software causing the computer or controller to perform the steps of: - selecting a number of root causes in each of said sub- processes which have measured parameters having an impact on the chemical oxygen demand (COD) value in said wastewater, said sub-processes being the processes of debarking, cooking, bleaching, drying, evaporation, causticising and recovery; - calculating, based on said measured parameters, the COD contribution of each of said root causes in each selected sub-process to the chemical load of said wastewater; - calculating an estimated combined COD chemical load for all selected sub- processes to said wastewater; and - using said estimate as an input value for said monitoring and controlling of the chemical load in wastewater produced by said industrial pulp-making process. N O

N 2. A method according to claim 1, characterized by comprising the further LÖ O steps of: ™ r - continuously measuring the COD chemical load of said wastewater; = © 25 - comparing simultaneously measured and calculated chemical loads, O 0 and LO O N - based on the comparison, storing in a memory time-stamped data regarding any differences as statistical and/or feedback information.

3. A method according to claim 1 or 2, characterized by comprising the further steps of: - treating wastewater influent from said industrial pulp-making process in a biological treatment plant; - measuring the chemical load of said wastewater effluent from said biological treatment; - comparing the measured chemical load with a predetermined acceptance value, and - based on the comparison, storing in a memory time-stamped data regarding any differences as feedback information for optimizing said biological treatment.

4. A method according to any of claims 1 -3, characterized by that said measured parameters for the debarking process root causes include residues and dissolved compositions and are guantized as: the tree species, the amount of bark, the — degree of barking performed, the amount of water to be compressed from the bark, the time of the year.

5. A method according to any of claims 1 -4, characterized by that said measured parameters for the bleaching process root causes include: the production volume of bleaching, the amount of lignin to be dissolved, the amount of dissolving carbohydrates, wash losses, extractable compositions dissolving during bleaching, N and oxidizing chemicals affecting COD reduction. O

N 6. A method according to any of claims 1-5, characterized by that said S measured parameters for the evaporation process root causes include: data of a unit 2 to be washed, the COD-content of washing water, the amount of wash water, and the = 25 conductivity (mS/m) conversion to concentration (mg/l). 3

3 7. A method according to any of claims 1 —6, characterized by that said = measured parameters for the causticising process root causes include: the N concentration of the wash water, the amount of wastewater, and the conductivity (mS/m) conversion to concentration (mg/l).

8. A method according to any of claims 1 —7, characterized by that said measured parameters for a recovery boiler process root causes include: contamination of a secondary condensate that is fed to the wastewater, the amount of a secondary condensate, and the concentration (mg/l) (converted from conductivity value in mS/m).

9. A method according to any of claims 1 —8, characterized by that the total chemical oxygen demand for said pulp-making process is calculated by adding together the calculated chemical oxygen demands for each sub-process.

10. The use of the method according to any of claims 1 — 9 in a pulp mill.

11. A software product for monitoring the chemical load of wastewater produced by an industrial pulp-making process according to the method of any of claims 1 — 9.

12. An apparatus adapted to run a software product according to claim 11 for monitoring the chemical load of wastewater produced by an industrial pulp-making process according to the method of any of claims 1 — 9. N N O N LÖ <Q ™ I a a co © O LO LO O N