Julkaisut

The aim was to investigate how the collected data from microscopic images of biosludge could be utilized to improve activated sludge process control. Microscopic images and biosludge data were collected from five pulp and paper mills located in Northen Europe. The results of the image analysis demonstrated a correlation between biosludge structural quality and image data. Flocs characterized by higher porosity levels and a wider distribution of porosity indicated higher sludge volume index (SVI) values. The quantity of filaments was also found to affect SVI and increase the risk of sludge bulking when the proportion of filaments in the total sludge area exceeded 10%.  

Computer vision analysis provides quantitative data on biosludge properties, offering a more objective basis for assessing sludge quality than traditional manual inspection methods. By monitoring changes in biosludge structure with computer vision technology, early deterioration of biosludge quality in the activated sludge process can be detected before final effluent quality is compromised. On the other hand, the slow improvements in biosludge quality can also be registered to get confirmation, that the set process conditions and adjustments are favoring the growth of well-settling biosludge. The computer vision analysis of biosludge microscopic images opens an opportunity to maintain stable and efficient wastewater treatment process performance. Based on the results it can be stated that computer visions analysis emerges as a valuable tool for collecting informative data on biosludge properties, suggesting its wide adoption in pulp and paper industry.

In this study, a novel approach was taken to evaluate the porosity of flocs in the activated sludge process using computer vision. The aim was to investigate the correlation between floc porosity and biosludge settling properties in 5 pulp and paper mill activated sludge processes, located in Northern Europe.

The results of the image analysis revealed a correlation between floc porosity and biosludge settling properties. Flocs with a higher level of porosity and wider porosity distribution resulted in decreased settling properties, as indicated by higher sludge volume index (SVI) values. In addition, the correlation between average floc porosity and SVI values was consistent across all the mills studied, with an R² value greater than 0.5.

Traditionally, the number of filamentous bacteria and prevalence of protozoa have been used to monitor and evaluate biosludge characteristics. By monitoring changes in floc porosity in parallel with number of filamentous bacteria using computer vision technology, the early deterioration of the activated sludge process could be identified. This would provide more time to react and implement measures to ensure stable and efficient wastewater treatment performance. Therefore, the evaluation of floc porosity should be considered as a novel parameter in the assessment of activated sludge processes.

The characteristics of biosludge from pulp mill activated sludge process were studied with computer vision for a time period of 11 months. Work included computer vision model training, collecting microscope images of biosludge and image analyses. Image analyses demonstrated that changes in biosludge characteristics can be detected and closely monitored with computer vision. For example, the proportion of filamentous bacteria in the biosludge area varied between 1 and 10 %. Also, the average floc size increase and the floc density improvement during evaluation period were well observed.

Increasingly stringent environmental regulations set constant pressure for pulp and paper mills to improve their wastewater treatment performance and final effluent quality. Due to high capital, energy and operational costs related with new treatment processes, more attention is given to emerging alternatives that aim to improve existing treatment process performance. Among the recently introduced solutions is the utilization of smart technologies in process control. This conference paper presents an implementation project of a cloud-based application, which was designed for improving activated sludge process control by optimizing chemical dosing, return sludge and excess sludge flowrates. After establishing a two-way data pipeline between mill’s database and cloud-infrastructure, mill operators started to receive calculated values for optimal process adjustments. The gained benefits from both operational and environmental point of views are demonstrated with three pulp and paper industry wastewater treatment plants. As the accuracy of process adjustments and their timing increased, mills reported improvements in sludge settling properties and effluent quality. Due to increased process control the final COD, TSS, nitrogen and phosphorus loads were decreased by 9 – 32 %, 48 – 68 %, 12 – 29 % and 43 – 51 %.

Increasingly stringent environmental regulations set constant pressure for pulp and paper mills to improve their wastewater treatment performance and final effluent quality. Due to high capital, energy and operational costs related with new treatment processes, more attention is given to emerging alternatives that aim to improve existing treatment process performance. Among the recently introduced solutions is the utilization of smart technologies in process control. This conference paper presents an implementation project of a cloud-based application, which was designed for improving activated sludge process control by optimizing chemical dosing, return sludge and excess sludge flowrates. After establishing a two-way data pipeline between mill’s database and cloud-infrastructure, mill operators started to receive calculated values for optimal process adjustments. The gained benefits from both operational and environmental point of views are demonstrated with three pulp and paper industry wastewater treatment plants. As the accuracy of process adjustments and their timing increased, mills reported improvements in sludge settling properties and effluent quality. Due to increased process control the final COD, TSS, nitrogen and phosphorus loads were decreased by 9 – 32 %, 48 – 68 %, 12 – 29 % and 43 – 51 %.

Long-term experiences of simultaneous precipitation (SP) of phosphorus using ferric sulphate in the secondary treatment of softwood ECF kraft pulp mill wastewater were examined from a time period of 7 years. Phosphorus reduction in an activated sludge process (ASP) was improved from monthly average level 43% - 67% to 72% - 84% by adding ferric iron in the aeration basin. Due to periodic changes in wastewater qualities, the demand for iron addition to wastewater varied between 0 and 10 mgFe/L, depending mainly on influent P/BOD₇ ratio.

Introduction of SP has not caused any notable disadvantages to daily wastewater treatment plant operation, excluding minor increase of total sludge production. Experiences and conclusions from long-term operation were similar to our preliminary findings from pilot-scale and short-term full-scale studies. Thus, it can be verified, that SP offers an alternative to decrease phosphorus discharges from pulp mill ASP instead of more expensive tertiary treatments.

Modern pulp and paper mill wastewater treatment plants, which usually include activated sludge process, are at least moderately well instrumented and equipped with digital control systems. Vast amounts of process data are collected and stored to mill archives. However, without the right tools to translate collected data to meaningful insights, only a small fraction of collected data can be used as such in daily activated sludge process operation. Activated sludge process is typically adjusted based on laboratory and onsite measurements from wastewater treatment plant. From an operational point of view, the challenge in operating activated sludge process comes with long retention times and slowly changing process, which makes daily process variations and disturbances difficult to observe before they affect the final effluent quality. Often these adverse effects are seen as suspended solids overflow from secondary clarifiers. At that time the input for process disturbance may have affected the process already for several days. Since these disturbances may cause 5-20 % of COD and up to 60 % of phosphorous annual loads to waterways from pulp and paper mills, one way to meet with the tightening environmental regulations set for final effluent quality would be to improve activated sludge process stability. Successful activated sludge process operation is as much about preventing process upsets as it is about maintaining a stable process. Potential option to increase process awareness is to apply sophisticated data analytics to support process monitoring and adjustments.

Utilization of predictive analytics in activated sludge process control is an example of algorithmic approach that utilizes mill process data archives, real-time data and data-driven soft sensors to determine an event before it has occurred. With predictive analytics, the mindset in activated sludge process control is switched from reactive to proactive. When the emerging upsets are observed at their early stages, corrective measures can be implemented before final effluent quality is compromised. Also, the recovery times from process disturbances are reduced, when root causes are identified quickly. The applicability of Toihan’s predictive algorithms to improve activated sludge process awareness and performance was tested at a Finnish ECF kraft pulp mill. Mill personnel controlled activated sludge process with remote support by Toihan, who provided recommendations for optimal chemical dosing, return sludge flow and biosludge withdrawal according to predictive analytics. The results gained during the 4 months trial period were encouraging. The reduction improvements measured from final effluent quality, when compared to reference effluent loads from previous years, were: COD 20 %, phosphorus 12 %, nitrogen 31 % and AOX 24 %.

Predictive analytics provides a tool to understand and control the cumulative nature of activated sludge process. However, the effective use of predictive analytics requires a comprehensive process knowledge for user to be able to interpret past events and trends that set the basis for future predictions. Because predictive algorithms provide a set of predicted outcomes, human interaction is required to translate produced information to effective process interventions. Toihan has built a supervisory monitoring and control support service concept, PAULA-WWTP®, that enables Toihan experts to provide remote support for mill operators. Acronym PAULA stands for “Predictive analytics for user level assistance”. Once the data link between PAULA-WWTP® and mill process data is established, the service elements can easily be added or removed depending on the current situation in activated sludge process. The service concept is built on modern cloud-based data architecture, which makes it adaptable to existing data interfaces in pulp and paper mills without geographical restrictions.

An activated sludge process (ASP) in a Finnish ECF kraft pulp mill was controlled with data-driven soft sensors for 120 days to gain user experience and to test their applicability in reducing COD and phosphorus loads (kg/ADt) to environment. The mill process data from 23 data positions was collected and transferred to a mathematical activated sludge model that worked as a platform for soft sensors. The soft sensors utilized mill-specific algorithms to calculate recommendations for chemical dosing, return sludge flow and biosludge withdrawal from the activated sludge process. The pulp mill activated sludge process was adjusted by mill operators once a week according to given control parameters. Wastewater load to waterways during the trial period was compared to the wastewater loads from previous years. With improved activated sludge process control the COD and phosphorus loads to waterways were approximately 20 % and 12 % lower than previous three years.

This conference presentation presents results from a laboratory-scale study in which the precipitation of dissolved lignin, BOD5 and COD with ferrous sulphate from ECF kraft pulp bleaching filtrates in alkaline conditions were examined. Both calcium hydroxide and sodium hydroxide were used as alkalization chemicals. The reduction of dissolved lignin was between 45-60% when precipitation was done with ferrous sulphate and calcium hydroxide. In contrast, the reduction of BOD5 remained below 10%. These results indicate that ferrous iron removed mainly non- or slowly biodegradable COD. It was also clearly seen that the addition of calcium improved the removal of COD in all cases compared to usage of ferrous sulphate and sodium hydroxide.

Slowly or non-biodegradable organic compounds, such as lignin, have a great impact on the effluent color and chemical oxygen demand (COD) discharges at many mill-sites refining wood chemically or semi-chemically. For example, in a modern kraft pulp mill, the quantity of the high molecular weight (HMW) compounds in bleaching filtrates varies typically between 15…20 kgCOD/ADt, depending on wood raw material, brownstock washing efficiency and bleaching conditions. The removal of HMW COD at a wastewater treatment plant (WWTP), on the other hand, is strongly affected by the surface charges of colloids. In a conventional pulp WWTP including secondary biological treatment, the acting adsorbent media is mainly biosludge. However, the negative surface charge of lignin released during the alkaline pulping process usually hinders the adsorption to biosludge and therefore lignin based compounds causes rather high COD  concentrations in the final effluent of WWTP.

It is known that polyvalent metals such as Fe3+, Al3+ or Ca2+ can be used to precipitate lignin from black liquor and pulp mill effluents. In this study, the utilization of different solid waste or side streams containing calcium from integrated pulp and paper mill were studied in laboratory scale to remove dissolved COD from pulp mill wastewaters in highly alkaline conditions. The results from precipitation studies with chemically activated fly ash from a solid fuel boiler, waste lime from causticizing and coating sludge from a paper mill were very promising. The highest reductions of COD concentrations were measured from alkaline bleaching filtrates. It was also observed that the adsorbable organic halogen (AOX) and the color were significantly lowered during the alkaline COD reduction process (ACRP) as introduced in this study.

Based on the laboratory scale studies and the COD mass balance calculations, the effects of ACRP as a pretreatment unit prior to activated sludge process was estimated. In this specific case, the total COD reduction in WWTP was found to increase from 65% to 80%, when ACRP was addressed to alkaline bleaching filtrates. ACRP would also reduce the need for wastewater neutralization caused by the acidity of current wastewaters.

The biodegradability of total suspended solids, dissolved high molecular weight (HMW) solids and dissolved low molecular weight (LMW) solids of kraft pulp bleaching filtrates (soft- and hardwood), BCTMP effluents, paper mill effluents and debarking effluents were studied. Fractionation of organic material was carried out by microfiltration (pore size 1.5 μm) and ultrafiltration (MWCO 2 kDa). Biodegradability of untreated, micro-filtered and ultra-filtered samples were estimated by measuring chemical oxygen demand (COD) and biochemical oxygen demand (BOD). BCTMP effluent had the highest BOD7/COD-ratios in total, dissolved HMW and dissolved LMW fractions. Effluent from a paper machine using bleached kraft pulp also had relatively high BOD/COD ratios. In ECF kraft pulping the raw wood and bleaching stage had a major effect on BOD/COD ratios in all fractions. According to this study, the most suitable bleaching filtrate for a biological treatment was acidic filtrate (D0) with softwood as raw material. The lowest BOD/COD ratios were measured from alkaline bleaching filtrate (softwood) and debarking effluent.

In this presentation, the suitability of different kinds of biological treatment or utilization processes for the examined wastewater streams are also discussed. It is shown for example that COD fractionation based on molecular weight and biodegradability is a valuable and useful procedure while considering or developing new process units to improve an existing wastewater treatment plant.

Before adding new facilities to a wastewater treatment plant, the efficiency of the existing process should be optimized. Even though process control is well-established nowadays in pulp and paper production, it has not reached the same level in wastewater treatment process control. Increased process control could prevent injurious effects of any unusual events or process upsets that may occur in the wastewater treatment plant, if the process was adjusted accordingly. This motivates wastewater treatment experts to develop improved control systems for the pulp and paper industry. 

The efficiency of BOD removal in the biological treatment of kraft pulp mill wastewaters is usually very high while the removal of total COD is very depended on the wastewater treatment process (WWTP) because of high concentration on non-biodegradable or slowly biodegradable organic compounds such as lignin or lignin derivatives. Nowadays wastewaters from pulp mills are relatively often treated with various modifications of low loaded activated sludge process. It is vital to have large aeration basins with abundant of biomass when treating wastewaters containing lots of non- or slowly biodegradable organics, because the removal of these compounds is mainly based on adsorption to biosludge. The disadvantages of low loaded activated sludge process are high energy consumption and the release of nutrients as the biosludge is already digested aerobically during the process. Due to this, there is a great need for continuous monitoring of lignin or lignin derivatives concentrations in different parts of the WWTP to ensure sufficient and energy efficient removal of total COD.

Earlier studies have shown that refractometer is a reliable measurement when measuring total dissolved solids (TDS) in the area of brown stock washing, where the concentrations of dissolved materials are very high. Latest studies have also shown that accuracy of refractometer measurement is adequate for measuring low TDS concentration from 0.5 to 0.1 % in wastewaters. The aim of this study was to define whether it would be possible to identify the concentration changes of lignin or lignin derivatives in the water by the use of an on-line refractometer and whether these results could be utilized for evaluating the performance of the WWTP. Firstly, it was studied in a laboratory scale how a new precision refractometer reacts to different lignin concentration changes in the water. Secondly, on-line refractometer was tested in a laboratory scale activated sludge process to see, whether the measurement could be utilized to monitor the removal of COD in different parts of aeration basin.

The results indicate that new precision refractometer can be used for the detection of very small changes in TDS at low concentrations (about 50 ppm). Good correlations between refractive index and lignin concentrations were found in laboratory tests. Online measurement could be used to estimate the concentration of lignin or lignin derivatives in different waste streams of WWTP. Results from laboratory scale activated sludge process studies suggest that the measurement could be used within aeration basin too. With online refractometer it was possible to detect the removal of organic matter via biodegradation and adsorption of organic compounds in different parts of aeration. In addition, TDS concentrations in latter parts of aeration basin correlated fairly well with residual dissolved COD and dissolved organic carbon (DOC).

Adsorption of non- or slowly biodegradable organic compounds such as lignin or its derivatives on biosludge may weaken its dewatering properties and suitability for anaerobic digestion. These compounds accumulate in biosludge of an activated sludge process (ASP) within a kraft pulp mill as the sludge retention time (SRT) increases. The specific adsorption characteristics of influent COD and BOD were studied in laboratory-scale with biosludge and ferric sulphate. The accumulation of non- or slowly biodegradable compounds in biosludge was estimated by dynamic modelling using specific influent COD characteristics and the process data from an ECF kraft pulp mill. Approximately 20-30% of influent COD consisted of adsorbable organic compounds that had no effect on BOD5. Due to this fraction, the non- or slowly biodegradable organic content of mixed liquor volatile suspended solids (MLVSS) in biosludge was 45-70% with SRT varying between 25 and 60 days.

This study investigated the possibility of controlling an activated sludge plant by using continuous dissolved dry solids (DDS) measurements and conductivity analysers. In addition, the correlations of the results attained were compared to typical wastewater sum parameters, such as COD or TOC. The tests were performed by installing five (5) refractometers and five (5) conductivity analysers in a wastewater treatment plant (WWTP) and by collecting data and hand samples from a Finnish integrated pulp and paper mill. The results indicate that new precision refractometers can be used in a WWTP for the detection of very small changes in the DDS at low concentrations (about 50 mg/L). The results also indicate a strong correlation between the measured DDS and the COD and TOC values, suggesting great potential for their use in monitoring influent load and the quality of effluent before it is introduced into the local water system. Conductivity measurement also works well, but since the purpose of an activated sludge plant is to remove organic material from wastewater, this measurement method is not very effective in monitoring effluent load in the local water system. However, more research is needed to gain a better understanding of the use of these on-line measurements in monitoring the operation of WWTPs.

Firstly, in this study the possibility to control activated sludge plant by using continuous dissolved dry solids (DDS) measurements and conductivity analysers were investigated. In addition, the correlation of the attained results were compared to typical wastewater sum parameters like COD or TOC. The tests were performed by installing five (5) refractometers and five (5) conductivity analysers in the wastewater treatment plant (WWTP) and collecting data and hand samples from a Finnish Kraft pulp mill. The results indicate that new precision refractometers can be used for the detection of very small changes in the DDS at low concentrations (about 50 ppm) in WWTP. The results also indicates that measured DDS had a good correlation to COD- and TOC-values giving a great potential to use it as a “police meter” for wastewater quality before it is introduced to local water course. However, more tests will be needed to get better understanding of the phenomena around WWTP using these on-line measurements.

Secondly, biochemical dynamics of purifying process and the use of aeration capacity were studied by measuring concentration profiles with respect to time in a plug flow aeration basin. The measured parameters were: dissolved oxygen concentration (DO), redox potential (ORP) and oxygen uptake rate (OUR) in sludge as well as residual COD, TOC and total phosphorus in water phase. OUR, DO, ORP and total phosphorus proflies showed that the major part of biochemical acticity occurred during the first hours of aeration. Only COD and TOC profiles continued after slow decline during the whole aeration time which indicates that there was a limiting factor, probably the defience of phosphorus, in the process. This kind of aeration profiling can be used when optimizing an excisting ASP.

Simultaneous precipitation (SP) using iron sulfates in the secondary treatment of elemental chlorine-free (ECF) kraft pulp mill wastewater was studied. In short-term pilot plant studies phosphorus reduction was improved from 58% up to 81% by adding 10 mgFe/L ferrous sulfate in the activated sludge (AS) process. The phosphorus reduction in full-scale experiment was about 80% when less than 10 mgFe/L was fed in with the influent of the AS plant. The reduction of phosphorus decreased with the dose of iron during the three month experiment. No notable change in either chemical oxygen demand (COD) or adsorbable organic halogen (AOX) reduction nor in the properties of sludge settling were observed during this SP experiment. Furthermore, no problems in sludge treatment due to increased iron concentrations in the waste sludge were reported. SP offers a simple way to decrease phosphorus discharges from AS plants instead of reliance on more expensive tertiary treatments.

Environmental protection forces industry to regard its processes and mass balances as a unity. This complete outlook on an industrial plant demands industrial symbiosis and to close loops of the waste streams. Olive oil production has its challenges in seasonal function, which has often made the industrial symbiosis thinking not attractive. Olives are natural organic products, which can be seen as renewable energy resources. Process industry should think about the big picture, the process as a whole.

This study offers a new perspective to examine olive oil production and its waste streams. The quantity of dry solid waste is about 1.4 tons / tons olive oil produced. The energy content of this solid side stream could cover the whole energy demand for the olive oil produces including the drying of the solid waste itself and the treatment of wastewater (3-phase). In this study, the energy balance of the whole olive oil process is also compared with the kraft pulp mill energy balance. A major side stream of the pulping process, black liquor, is used as an energy source for the pulping process. In Finland for example circa 34 % of renewable energy production per year is coming from black liquor. Similarities between pomace and black liquor are discussed and the combined olive oil and energy production is introduced.

Concentration profiles of different wastewater qualities with respect to time were measured in plug flow aeration basins. Three activated sludge plants within ECF kraft pulp mills were studied, two of which were integrated pulp and paper mills. Concentrations at different stages of aeration were also studied in two pilot-scale activated sludge plants. The aeration time in the examined activated sludge processes was between 14-20 hours and sludge retention time varied from 17 to 30 days. In addition to wastewater concentration profiles, dissolved oxygen, oxygen uptake rate, pH and temperature profiles were also measured on site.

Most of the dissolved chemical oxygen demand (COD), biochemical oxygen demand (BOD) and adsorbable organic halogen (AOX) decrease took place almost immediately when influent and return sludge was mixed due to biosorption. Also, oxygen and nutrient uptake was high at the beginning of aeration and decreased gradually in a few hours after which only minor changes in concentrations or oxygen uptake occurred. The concentrations of total dissolved nitrogen, phosphorus and AOX can even rise when aeration time is extended probably due to the mineralization of biomass. As a conclusions, the aeration capacities in the activated sludge plants studied here allow modifications like sequencing aeration for chlorate reduction or biological phosphorus removal.

The mass balance of phosphorus in secondary treatment i.e. the activated sludge process (ASP) is quite simple: the influent mass flow equals that of the effluent and waste sludge (WS) together in steady state situation. Hence to reduce effluent load one can either decrease the influent load or increase the phosphorus outflow within WS. This presentation concentrates on the latter case in a situation where excess phosphorus for biochemical need in ASP is present. Phosphorus outflow within waste sludge can be increased by increasing the flow of WS itself i.e. reducing sludge retention time (SRT). Reducing SRT on the other hand has its limits in pulp mill ASP because reduction of organic compounds measured by COD or TOC will decrease when SRT is decreased below 15 days. The scarcity of other nutrients like nitrogen can also limit the uptake of phosphorus by hindering biomass synthesis in ASP.

Effluent phosphorus load can also be reduced by increasing the phosphorus concentration in WS. This can be done by enhanced biological phosphorus reduction (BPR) or by chemically using simultaneous precipitation. BPR using variations between anaerobic and aerobic phases may be applied especially when phosphorus concentration is elevated for example due to opening phosphorus circulations in the pulp mill. On the other hand the simultaneous precipitation of phosphorus can be carried out quite easily even in an old or heavily loaded ASP by feeding iron salts in to the process and this way immobilize dissolved phosphorus from wastewater into activated sludge. However, the phosphorus reduction achieved with pulp mills’ wastewaters has not been as high as that with municipal wastewaters by this method.

Conference Report: IWA Forest Industry Wastewater Conference January 8-11, 2012 Concepcion, Chile Reported by Michael Paice, Chairman, IWA Forest Industry Specialist Group

IWA Best Oral and Poster Awards

The best oral presentation award was given to Sakari Toivakainen (with co-authors T. Laukkanen, H,. Sahid and O. Dahl) from Aalto University, Helsinki, Finland for the paper “Managing effluent phosphorus by optimizing the activated sludge process in ECF kraft pulp mill.” The best poster award was given to Soledad Chamorro, student of Prof Gladys Vidal, University of Concepcion, Chile for the poster “Characterizing kraft mill effluents from Pinus radiata and Eucalyptus globulus by a combination of yeast bioassays and GC-MS chemical analysis”.

Simultaneous precipitation is widely employed in Finland or the treatment of municipal wastewater, but the method has not spread to the forest industry. This article sets out research results according to which simultaneous precipitation is a noteworthy method also for managing the quantities of phosphorus in the forest industry.

The stability of chlorate in pulp mill wastewaters was studied both in mill and laboratory scale. Chlorate, chlorite and chloride concentrations were compared at different process phases from three mills. The laboratory-scale experiment was carried out with pulp mill wastewater and biosludge using parallel batch activated sludge process units. The study confirmed that chlorate can be removed from ECF kraft pulp mill wastewaters under long enough anoxic conditions in the equalisation basin or at the beginning of the activated sludge process. A further observation was that chlorate can be removed with a semi-anoxic phase. According to laboratory-scale experiments the efficiency of chlorate removal in semi-anoxic conditions was found to be equal to that in the anoxic phase, in which the aeration of sludge was completely hindered.

The approach should be systematic while exploring the possibilities to increase efficiency of biological wastewater treatment process. Various process enhancing measures can be tested simultaneously in laboratory conditions in brief period of time. Once optimal parameters are found they can be transferred from laboratory to pilot scale activated sludge plant where their affects to actual activated sludge process can be observed more closely. If the impacts of the enhancement in pilot scale experiment are as presumed, the experiment can be repeated in a full scale activated sludge plant.

Results from recent experiments at the pulp mill show that laboratory and pilot scale are quite similar with the full scale activated sludge process. Additionally, it should be mentioned that the utilized full scale activated sludge plant was oversized for its purpose and made it easier to simulate in pilot scale. Generally, the organic matter of pulp mill wastewater is divided into two phases: easily and slowly (or non) degradable phases. Due to this for example COD reduction seems to be quite independent from the scale of the treatment system.