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Prevention of biofilm formation at pulp and paper mill

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Pulp and paper making industry

The main feedstock in the paper production is lignocellulosic material – cellulose pulp, agricultural residues, waste paper, and non-wooden materials [1].

Beside this, the paper production requires large amounts of fresh water with the generation of large quantity of heavily polluted effluent. Lot of efforts have

been made to reduce usage of fresh water and water system closure in the pulp and paper production in past years.

Pulp and paper industries face problems such as acute shortages of raw materials, corrosion in machines, recovery of chemicals from black

liquor, effluent treatment and slime formation in machines [1]. With increasing number of pulp and paper units, the problems are only going to

aggravate. Slime formation is perhaps the most troublesome of all the problems. Slime causes web breaks, shrinkage, increased production

costs, corrosion and foul odors. The presence of slime also affects the quality of the finished product by introducing defects such as slime spots,

discoloration, pin holes and poor sheet formation.

The average paper mill in Europe use ca. 50 m3 of water per ton of product. However, in modern, high class paper mills water consumption can

be only 10 m3 of water per ton of product. In average paper mill, paper production per day is around 1000 tons so the consumption of fresh

water is very high. Also other process water (nowadays usually recycled water) is needed around 100 000 m3 per day. Recyled water can be e.g.

white water (fiber containing water) or clarified water.

Profile of investigated mill

Average daily output of the considered factory is 300 t of coated and uncoated cardboard of different weight. Feedstock materials are waste

paper, fibers, CaCO3, additives, sludge from the effluent treatment plant (ETP). [1]

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Cardboard grades produced in the factory and their ratio is presented in Figure 1.

Figure 1. Grade and structure of cardboard [1]

The water system of the mill (presented in Fig.2) is partially closed. Average water consumption is 24m3 t-1. Primary recirculation of water is

provided on the step “sheet formation” (paper machine sieve section), secondary conducted on dissolved air flotation (DAF) unit (marked as

“save all – DAF” in Fig.2). Re-use and recirculation of water that is purified by the ETP (as a tertiary recirculation) has not hitherto been realized

(Nassar, 2003). [1]

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The largest part of the fresh water is used in sheet formation on the cardboard machine (200 m3 h−1) and the smallest quantity is used in stock

preparation (90 m3 h−1 for thickening). White water from sheet formation and spent water from the thickener are treated in the DAF unit. The

treated water is used for different operations in stock preparation for pulping and multi-level cleaning. Spent water from stock preparation (84

m3 h−1) and the paper machine (196 m3 h−1) are treated on ETP and discharged into the river. [1]

The ETP presents physico-chemical treatment that includes several successive processes, which provide for the removal of suspended solids,

colloids, floating matter, color and toxic substances by sedimentation enhanced by coagulants and flocculants. The overall objectives of the ETP

are to separate the wastes from the water for disposal elsewhere and to produce an effluent which can be discharged to a receiving water

body without pollution. Separated solid waste, which is not suitable for reuse immediately, is temporary disposed. Wastewater treatment in

the ETP includes the addition of chemicals to enhance the primary treatment: Al2(SO4)3 as coagulant and polyacrylamide (PAM) as a coagulation

aid (flocculant). The usual doses of Al2(SO4)3 are 400 mg L−1 and 3 mg L−1 for PAM. The type and doses of the chemicals were determined in a

preliminary investigation, when the company had a higher specific water consumption and a lower production capacity. The ETP was projected

for 300 m3 h−1 of wastewater and 1500 mg L−1 of suspended solids. Primary or mechanical wastewater treatment by gravity forces is carried out

for the removal of plastic and polystyrene pieces and suspended solids, such as fibers, bark particles and inorganic particles (fillers, lime

particles, etc.). Particles which settle on the bottom of the primary clarifier form a sludge, which has to be evacuated and potentially reused.

This is achieved by pumping, in circular clarifiers in combination with bottom scraping. The sludge is normally low in dry solids content,

approximately 2% and has to be dewatered, if final disposal will be solution. Sludge from the primary clarifier has suitable properties to be

reused in the production process, for middle layer of cardboard. [1]

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Figure 1. The water system in the cardboard mill – dotted lines are for the cleaner production measures [1]

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Characterization of wastewater from the mill is presented in Table 1. [1]

Biocides in pulp and paper industry

The tendency in modern pulp and paper mills is both to close the circulation of process waters and to get closer to neutral pH [3,4]. Both trends

favour the growth of micro-organisms. In some mills, the use of recycled fibres at moderate temperatures adds further to the presence and

activity of micro-organisms. Most micro-organisms in the process waters are harmless, but some cause problems as they colonise surfaces, so-

called biofouling, leading to the growth of biofilms (Figure 3)[5].

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Figure 3. Biofilms in paper mill environment [5]

The main micro-organisms present in the pulp and paper processes are: bacteria, fungi, yeast and algae. Typical composition of the paper

industry microflora is presented in Figure 4. Average values for aerobic colony forming microbes are 100 – 300 x 106 col/ml of process water)

and for anaerobic colony forming microbes 15 – 950 x 106 col/ml of process water)[6]. Due to the diversity in the microenvironment conditions,

the density of population can vary greatly, although the normal range is between 104 to 108 micro-organisms/ml. On the other hand, it is

important to consider that the dominant species vary, even in the same factory, depending on the conditions of the environment.

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Figure 4. Typical microflora composition in paper industry process [6]

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Control of microbial growth in the paper making process is most difficult on the wet end of the paper machine [1]. It is here where the paper is

actually formed and the prevention of microbial and other deposits has a direct impact on the quality of the finished product. Various measures

have been used to control the slime formation as discussed below.

Biocide should be added to the wet end (fresh water) of the machine, the primary white water circuit, where good dispersion can take place

[7]. The levels required will depend on the cleanliness of the system, the quality of the raw material and the degree of microbial control

required. A typical regime may involve initially dosing at a level of 50g per tonne of finished paper product, reducing to 10-20g per tonne after

2-3 weeks. When used in combination with other biocides, the concentrations of certain biocides can be further reduced.

The main objective of the microbiological control programme in paper and board mills is the control of the growth of biofilms in the surface of

the machine and pipelines. Microbes can be attached to steel surfaces either straight (primany biofilm formation) and start growth there or

they can be attached to the surface of the already formed biofilm by other microbes (secondary biofilm formation).

Conventionally, the papermaker has used biocides to control growth of micro-organisms. They can be classified in three different groups:

agents that inactivate the enzymes of the cell membrane, electrophyllic agents and agents that act through oxidative decomposition of micro-

organisms [6]. However, increased public consciousness regarding environmental issues has led to strict regulation on the use of biocides

(including slimicides) in western countries. This has led to a limited number of biocides being available for the papermaker to use.

Other method for microbe control in pulp and paper mills is the control of the contamination sources, as for example, the treatment of fresh

water and additives, control of the residence time in the storage tanks, and control of the stagnant flow areas. Also simple mechanical cleaning

processes of pipelines and rough surfaces and using detergents during annual service breaks has important role in biofilm prevention.

A biocide is a chemical substance or microorganism which can deter, render harmless, or exert a controlling effect on any harmful organism by

chemical or biological means [8]. Biocides are commonly used in medicine, agriculture, forestry, and industry. Biocidal substances and products

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are also employed as anti-fouling agents or disinfectants under other circumstances: chlorine, for example, is used as a short-life biocide in

industrial water treatment but as a disinfectant in swimming pools. Many biocides are synthetic, but a class of natural biocides, derived from

e.g. bacteria and plants, also exists.

Biocides can be added to other materials (typically liquids) to protect them against biological infestation and growth [8]. For example, certain

types of quaternary ammonium compounds (quats) are added to pool water or industrial water systems to act as an algaecide, protecting the

water from infestation and growth of algae. It is often impractical to store and use poisonous chlorine gas for water treatment, so alternative

methods of adding chlorine are used. These include hypochlorite solutions, which gradually release chlorine into the water, and compounds

like sodium dichloro-s-triazinetrione (dihydrate or anhydrous), sometimes referred to as "dichlor", and trichloro-s-triazinetrione, sometimes

referred to as "trichlor". These compounds are stable while solids and may be used in powdered, granular, or tablet form. When added in small

amounts to pool water or industrial water systems, the chlorine atoms hydrolyze from the rest of the molecule forming hypochlorous acid

(HOCl) which acts as a general biocide killing germs, micro-organisms, algae, and so on. Halogenated hydantoin compounds are also used as

biocides.

A new innovation is the use of copper and its alloys (brasses, bronzes, cupronickel, copper-nickel-zinc, and others) as biocidal surfaces to

destroy a wide range of microorganisms (E. coli O157:H7, methicillin-resistant Staphylococcus aureus (MRSA), Staphylococcus, Clostridium

difficile, influenza A virus, adenovirus, and fungi) [9-11]. The United States Environmental Protection Agency has approved the registration of

355 different antimicrobial copper alloys that kill E. coli O157:H7, methicillin-resistant Staphylococcus aureus (MRSA), Staphylococcus,

Enterobacter aerogenes, and Pseudomonas aeruginosa in less than 2 hours of contact.

Because biocides are intended to kill living organisms, many biocidal products pose significant risk to human health and welfare [8]. Great care

is required when handling biocides and appropriate protective clothing and equipment should be used. The use of biocides can also have

significant adverse effects on the natural environment. Anti-fouling paints, especially those utilising organic tin compounds such as TBT, have

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been shown to have severe and long-lasting impacts on marine eco-systems and such materials are now banned in many countries for

commercial and recreational vessels (though sometimes still used for naval vessels). Disposal of used or unwanted biocides must be

undertaken carefully to avoid serious and potentially long-lasting damage to the environment.

In many cases the traditional biocidal programs are unable to perform under conditions of alkaline pH (and the shift in microbiological

population related to alkaline pH), increased resistance or immunity developed by the microbial flora, and the increased organic loading.

Additionally increasing requirements with regards to handling and safety make many conventional programs less favorable. To compensate

many manufacturers are resorting to adding more chlorine oxidizer, alternating biocides, or using duel treatment in order to get the fast-acting

broad-spectrum microbiological slime control necessary to maintain maximum runnability and prevent stains and slime breaks. Often these

practices increase problems such as cost, system corrosion, product handling, worker exposure, and higher discharge of biocide and reaction

byproducts into the waste water or the environment.

Alternatives to biocides

Large amounts of exopolysaccharides (EPS) are produced in biofilm [3]. EPS is the main chemical component of biological slime, and functions

as a binder to keep the biofilm together as well as a shield for protection. Nutrients are caught and filtered by the EPS surrounding the cells.

Different strains of bacteria produce different amounts of EPS, and any one bacterium can produce various sorts of EPS at different times. For

example, it has been shown how a strain of Pseudomonas uses one sort of EPS to attach to a surface, and another to later detach from the

same surface. So, to control the production of EPS is the same as controlling the consistency and the growth of the biofilm.

It is for this reason that biocide-free slime control appears to be an attractive part of the present and future, but actually it is an almost twenty-

year-old concept, which is today well proven and effective when the right product is used [3].

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BIM Kemi AB has marketed biocide-free slime control since 1981, and its own product, Bimogard, since 1991. This paper will describe practical

experiences of the mode of action of this slime control.

Bimogard is a multi-functional multi-component product consisting of chemically modified ligno-sulphonates and is fortified with various

surfactants. It is not toxic to micro-organisms at the concentrations used in the mills.

Table 2 shows the effect of Bimogard on the amount of EPS after introduction to a mill previously using biocides. The mill uses 100 % DIP

(deinked pulp). DIP, on the contrary to TMP (thermomechanical pulp), does not itself contain measurable amounts of EPS.

Table 2. The amount of EPS decreases after introduction of biocide-free slime control in a paper mill using 100 % DIP [3]

EPS (mg/g)

Sample The day before Bimogard introduction

2 weeks after Bimogard introduction

4 weeks after Bimogard introduction

4 months after Bimogard introduction

Deposit from section under forming roll

13,0 6,1 5,5 0,0

Backwater 6,3 3,1 1,2 0,0

Process-water from wire pit

3,8 1,3 0,0 0,0

Fresh water 0,0 0,0 0,0 0,0

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Almost all vegetative bacteria die off in the drying section of the paper machine, and are not transported to the paper or carton. However,

most gram-positive bacteria that can form so-called endospores survive in the drying section and are encapsulated in the paper or the carton. It

is of great interest in certain mills producing carton for food or hygiene tissue to minimise endospore content. Table 3 compares the endospore

formation in the presence or absence of the slime control agent.

Table 3. Endospore formation of Bacillus subtilis (colony forming unit/ml) [3]

Bimogard concentration

Time (h) 0 ppm 500 ppm 1600 ppm

9 1,6 x 104 - -

13 1,5 x 105 - -

17 1,6 x 105 1,6 x 105 -

21 3,4 x 105 3,0 x 105 -

24 5,3 x 105 4,9 x 105 -

Figure 5 shows laboratory tests on growth curves for the spore forming bacteria Bacillus subtilis with or without Bimogard. As can be seen,

depending on the dosage, the treatment delays or totally inhibits the growth of Bacillus subtilis.

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Yet, the bacteria remain as vegetative cells and are thus killed in the drying section. The data in Table 2 and Figure 5 have been confirmed in

mill cases.

Figure 5. Bimogard delays the growth curve of Bacillus subtilis. The bacteria remain vegetative; they do not form spores [3]

Both EPS-production and spore formation are bacterial defence mechanisms which can be triggered by dehydration, starvation, heat and

toxins. As mentioned above, Bimogard is not toxic to micro-organisms and therefore does not trigger these mechanisms, unlike biocides.

Further, this biocide-free slime control is dosed continuously, which is not economically feasible when using biocides. Yet, the incoming flow of

bacteria is continuous, and so should the slime control be.

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The bacteria count is usually 105/ml to 107/ml in mills using Bimogard. Most of these bacteria are harmless and die in the drying section

without causing slime-problems.

Bimogard is presently run on some 30 paper machines in Scandinavia using a variety of different pulps for production of tissue, newsprint and

carton seen in table 4. In Sweden, this has decreased the use of toxic slimicides with 20 – 25 %.

Also, this slime control has been successfully used in low as well as neutral to high pH conditions, and at small, old as well as large, modern,

fast-moving paper machines. Bimogard is also used at intermittent board machines in pulp mills.

Table4. Different pulp and paper qualities in mills presently using Bimogard.

PAPER GRADE TYPE OF PULP

Newsprint DIP, TMP

Newsprint DIP, TMP

Newsprint DIP, TMP

newsprint DIP, TMP

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Wood containing printing paper Sulphite, SGW-pulp

Folding box board SGW-pulp, sulphate

Printing & writing paper Sulphite

Printing & writing paper Sulphite

Tissue Sulphate

Tissue DIP

Tissue DIP

Tissue DIP

Tissue Sulphate

Tissue Sulphite, sulphate

Printing & writing paper Sulphite, sulphate

Printing & writing paper Sulphite, sulphate

Greaseproof Sulphate

Newsprint DIP, TMP

White-lined chipboard Waste paper

Tissue Sulphate

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Human dermatological tests performed in Canada on tissue produced in the presence of Bimogard show that it is not a skin irritant. Unlike

biocides, it does not pose a hazard to the staff in the mill.

The cost for using Bimogard for slime control is the same or, most often, less than using biocides. Bimogard has also many other advantages:

· Keeps surfaces clean and smooth thereby preventing biofouling.

· Decreases the production of EPS ("slime"), thereby preventing growth of biofilm.

· Delays the growth of bacteria and decreases the formation of spores, thereby preventing bacteria in ready-made paper or carton.

· Is pH-stable

· Is temperature-stable

· Is environmentally friendly.

· Does not endanger the health of the staff.

· Is proven to be a satisfactory or even better alternative to biocides.

· Is cost effective.

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Other biocides

BCDMH, Bromo chloro dimethylhydantoin, is a new oxidizing biocide registered for use in the Canadian Pulp and Paper Industry [12]. BCDMH is

an organic, bromine releasing biocide. It is a solid material which is safer to handle than the traditional oxidizing biocides - chlorine and sodium

hypochlorite. BCDMH dissolves in water and rapidly hydrolyses to the active hypobromous and hypochlorous species. Hypochlorous acid in

turn converts bromide back to hypobromous ion. BCDMH is a broad spectrum and fast-acting biocide which is effective over the pH range

found in paper mill systems. BCMH has advantages over non-oxidizing biocides in alkaline waters high in organics and nitrogen compounds.

Evidence of the safety of application is that BCDMH can be used in swimming pools and has NSF approval as treatment for potable water. The

Swedish National Chemical Inspectorate (1994) has approved BCDMH as one of only 11 biocide actives approved for the pulp and paper

industry.

While BCDMH may not be the ideal biocide it is recommended as a cost effective biocide which overcomes many of the deficiencies of the non-

oxidizing product currently used in the pulp and paper industry.

Often also so called biodispersants are used. The main characteristic of the dispersant is its capacity to reduce the size of dispersed particles

and reduce the possibility of the formation of microbiological deposits. Enzymes are also used nowadays for biofilm treatment at mills.

Other promising “green” biocides are e.g. hydrogen peroxide and per acetic acid [13]. Based on the CV runs it was seen that hydrogen peroxide

and peracetic acid could react with potential treatment to produce radical reactions in the synthetic paper mill water. Other tested biocides did

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not react during the treatment. Hydrogen peroxide and peracetic acid could be used together with potential treatment to enhance radical

forming reactions in paper mill circulating waters.

Other techniques in biofilm treatment

Research work for exploring possibilities of photocatalytic TiO2 coating for reducing biofilms on non-living surfaces has been investigated [14].

The model organism, Deinococcus geothermalis, known to initiate growth of durable, colored biofilms on machine surfaces in the paper

industry, was allowed to form biofilms on stainless steel, glass and TiO2 film coated glass or titanium. When biofilms on photocatalytic TiO2

surfaces, submerged in water, were exposed to 20 W h m−2 of 360 nm light, both kinds of adhesion threads were completely destroyed and the

D. geothermalis cells were extensively removed (from >107 down to below 106 cells cm−2). TiO2 films prepared by the sol-gel technique were

slightly more effective than those prepared by the ALD technique. Doping of the TiO2 with sulfur did not enhance its biofilm-destroying

capacity. The results show that photocatalytic TiO2 surfaces have potential as a self-cleaning technology for warm water using industries.

Also antifouling potential of electric polarization combined and not combined with biocides was studied in nonsaline warm water with high

organic content [15]. When D. geothermalis biofilms grown for 24 h in simulated paper machine water were exposed to cathodic or

cathodically weighted pulsed polarization at least 60% (P < 0.05) of the biofilms were removed from stainless steel (AISI 316L). Biofilm removal

by 25 ppm (effective substances 5–25 ppm) of oxidizing biocides (bromochloro-5,5-dimethylhydantoin, 2,2-dibromo-2-cyanoacetamide,

peracetic acid) increased to 70% when combined with cathodically weighted pulsed polarization. Using a novel instrument that allows real-time

detection of reactive oxygen species (ROS) we showed that the polarization program effective in antifouling generated ROS in a pulsed manner

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on the steel surface. We thus suggest that the observed added value of oxidative biocides combined with polarization depended on ROS. This

suggestion was supported by the finding that a reductive biocide, methylene bisthiocyanate, counteracted the antifouling effect of polarization.

Electrochemically generated biocides can be a green alternative for organic biocides in use today [16]. Electrochemically generated biocides are

created by electrolysis of diluted salt solutions in an electrolysis cell. The oxidation-reduction potential was shown to be an important factor

that defines the biocidal activity of electrolyzed anode water. In paper and board machines broke towers are usually the places where

anaerobic conditions develop. Anaerobic bacteria are more sensitive to oxidizing compounds than aerobic ones and hence electrochemically

generated biocides are powerful against anaerobes. Bacterial spores are highly resistant to many traditional organic biocides. Oxidizing biocides

such as hypochlorite are proven to be more effective against spores. Electrochemically generated hypochlorite had a bleaching effect and that

was apparent as an ISO brightness increase of approximately 1 unit.

Especially electrochemical oxidation has been applied successfully to degrade different organic pollutants, like phenols or industrial

wastewaters, like textile wastewaters and olive mill wastewaters [17].

Inactivation of microbes by electrochemical oxidation may happen in two different ways. The first is the direct anodic oxidation. Bacteria cells

are destroyed at the electrodes surface by hydroxyl radicals generated from water by electrolysis. Hydroxyl radical is the most powerful oxidant

and it can cause oxidative stress against cell of bacteria. The second way is indirect electrochemical oxidation when microbes are destroyed by

electrochemically generated ozone or hydrogen peroxide.

Electrochemically generated biocides showed an effective way to control microbial problems at a paper mill [16]. They can be added to water

or pulp and they have hardly any negative effect on the process or end-product. For paper makers this study is of great interest because onsite-

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generated biocides are low cost solutions based on actual biocide need. Onsite-generated biocides also eliminate the storage and

transportation of biocides and provides a basis for building a new control program. These electrochemically generated biocides are

environmentally friendly, nontoxic, not required to have special handling, hypoallergenic, safely disposed in municipal sewage systems, fast

acting, powerful biocide agents, used during all stages of disinfection and cleaning, applied in liquid, aerosol or frozen forms, chemical residue

free, generated on-site or in concentrated amounts for imminent use, eliminating handling and storage issues and produced from municipal tap

water and salt.

Some comparison of different techniques in biofilm prevention at paper mill is presented in Table 5.

Table 5. Different techniques for biofilm prevention

Technique Cost/ton of paper Easy of use at mill Effectiveness Environmental risk

Biocide treatment 1- 4 euros Easy and fast High High

Physical cleaning 1-2 euros Challenging and slow High No risk

Biodispersants 1-4 euros Easy and slow Low Low

Electrochemical

treatment

5-10 euros Challenging and slow High Low

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Task

The methodology for an optimum control system takes into account the following aspects: identification of the type of microorganisms that

cause the problem; minimization of the contamination sources; control of the additive slurries, keeping a clean system by means of an

adequate maintenance shut-down programme, prevention of the growth of anaerobic bacteria, especially of spore-forming species and

selection of the most suitable control programme.

This pulp and paper mill is using BCDMH (Bromo chloro dimethylhydantoin) as a biocide on their processes for biofilm prevention. Now

environmental authorities are asking if there is possible to find out more environmentally friendly method for biofilm control. As a process

engineer of this mill, your task is now to evaluate all possible alternatives and select the best method in considering technological,

environmental and economical aspects. Options are as follows: 1. current biocide/some other biocide, 2. “green biocides”, like enzymes and

biodispersants or 3. electrochemical techniques. Please discuss following points in your short report to authorities:

• Which method would be the best regarding Green Chemistry principles?

• Which method would be economically the best? How about technologically (in practise) in real mill environment?

• Which method would have least environmental effects in surroundings (lakes, rivers, pulp and paper mill wastewater treatment plant

etc.) and smallest health risks to persons working at mill?

• Decide if it is reasonable to stop using biocides for biofilm control at this mill?

• Do you have any other suggestion/method for biofilm treatment in this mill?

And finally: what method should be used in the future at this mill?

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References

[ 1] Zarkovic, D.B, Rajakovic-Ognjanovic, V.N., Rajakovic, L. Conservation of resources in the pulp and paper industry derived from cleaner production approach, Resources, Conservation and Recycling 55 (2011) 1139– 1145.

[2] Afshar, P., Brown, M., Austin, P., Wang H., Breikin, T., Maciejowski, J., Sequential modelling of thermal energy: New potential for energy optimization in papermaking, Applied Energy 89 (2012) 97–105.

[3] http://www.tappsa.co.za/archive/Journal_papers/Biocide-free_slime_control/biocide- free_slime_control.html

[4] Blanco, M.A., Negro, C., Gaspar, L., Tijero, J., Slime problems in the paper and board industry, Appl. Microbiol. Biotechnol. 46 (1996) 203-208.

[5] Kolari, M., Attachment Mechanisms and Properties of Bacterial Biofilms on Non-Living Surfaces, PhD thesis, University of Helsinki, Finland, 2003.

[6] Blanco, M.A., Negro, C., Tijero, J., Paper recycling: An introduction to problems and their solutions, European Commission, COST E1, Belgium, 1998, p. 204.

[7] http://www2.basf.us/biocides/pdfs/PIB_Brochure.pdf

[8] http://en.wikipedia.org/wiki/Biocide

[9] http://en.wikipedia.org/wiki/Antimicrobial_properties_of_copper

[10] http://en.wikipedia.org/wiki/Antimicrobial_copper_touch_surfaces

[11] http://www.coppertouchsurfaces.org/antimicrobial/bacteria/index.html

[12] http://www.tappi.org/Downloads/unsorted/UNTITLED---ts02143pdf.aspx

[13] Särkkä, H., Kolari, M., Pulliainen, M., Sillanpää M., Potential generation of oxidizing radicals in synthetic paper mill water by electrochemical treatment combined with biocides, Current Organic Chemistry, in press.

[14] Raulio, M., Pore, V., Areva, S., Ritala, M., Leskelä, M., Linden, M., Rosenholm, J.B., Lounatmaa, K., Destruction of Deinococcus geothermalis biofilm by photocatalytic ALD and sol-gel TiO2 surfaces, J. Ind. Microbiol. Biotechnol. 33 (2006) 261-268.

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[15] Peltola, M., Kuosmanen, T., Sinkko, H., Vesalainen, N., Pulliainen, M., Korhonen, P., Partti-Pellinen, K., Räsänen, J.P., Rintala, J., Kolari, M., Rita, H., and Salkinoja-Salonen, M., Effects of polarization in the presence and absence of biocides on biofilms in a simulated paper machine water, J. Ind. Microbiol. Biotechnol. 38 (2011) 1719-1727.

[16] Ullah, S., Biocides in papermaking chemistry, MSc thesis, University of Jyväskylä, Finland, 2011

[17] Särkkä, H., Vepsäläinen, M., Pulliainen, M. and Sillanpää, M., Electrochemical Inactivation of Paper Mill Bacteria with Mixed Metal Oxide Electrode, J. Haz Mat. 156 (2008) 208-213.