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Contenuto archiviato il 2022-12-23

Microbiology in paper making

Obiettivo

A.BACKGROUND

The European pulp, paper and board industry is continuously seeking to improve its economic and environmental performance. The efficient use of wood fibres (virgin or recycled), efficient water re-use, and control of emissions to environmental media are some of the key factors in this respect. In these contexts, the awareness of microbiological issues can be crucial to production performance. For instance, paper manufacturing today is characterised by increased closure of the water system, conversion to neutral/alkaline systems, changes in chemical additives and increased recycled fibre use, all of which may exascerbate microbiological problems and decrease mill performance.

A well known and long standing problem in paper manufacture is the proliferation of biological slimes on machinery. The slime seeds process water, clogs wires and may even contaminate the paper product itself - sometimes to the point of discolouration. The slime has an enormous influence on the runnability of the paper machine and often leads to unplanned shut downs. Adequate control of microbial-induced slime deposits has therefore become an essential part of modern pulp and paper manufacture.

Process water contamination, slime deposits and biocide treatment

The growth of micro-organisms is greatly promoted by modern processes and the water supply conditions. However, the contamination levels in process water do not necessarily correlate with the slime and deposit problems encountered. These problems depend upon several factors of which mill conditions and the diversity of microbial populations are important. The microbial viability in process water can be reduced efficiently by adopting adequate treatment programmes. However, since the different types of micro-organisms have varying degrees of resistance to microbiocides, there is no single treatment which can solve all the problems occuring in the pulp and paper industry. In addition, many of these substances may act selectively on the micro-flora thereby enabling more troublesome bacteria to flourish.

The main drawback of biocide treatment is that efficacy can only be accomplished against the micro-organisms in the water-phase, not micro-organisms in slime deposits. This high selectivity of the treatments adopted today may lead to other types of biological deposits such as pink slime. Pink slime micro-organisms which develop in remote hidden places in the paper machines also have the ability to form resistance to minimal levels of chlorine. The micro-organisms in pink slime represent less than 10% of the total slime forming population. However, they are more troublesome than this percentage would indicate because their growth contributes to more complex, viscous and rubbery type of slime formations.

Many different micro-organisms are commonly found in the process water, but only a few types may survive the manufacturing process. The remaining bacteria in finished products are essentialy those capable of forming endospores. In the spore state, bacteria are resistant to drastic changes in their environment, such as heat, cold, drying or toxic chemicals. They are even capable of surviving passage through the dryer section (110ø C) of the paper machine. Even if the number of micro-organisms is not reglemented, the finished products, for specific uses, may require high microbiological purity.

Non-biocide-based technologies

Non-biocide-based technologies for biological control, are receiving increased interest. Such techniques include those directed at deposit control such as enzyme treatment or the use of dispersing agents. These substances neither kill nor inhibit the growth of bacteria and they may even serve as nutrient sources for micro-organisms in process water circuits. Since the contamination of paper is related to the overall bacterial load in process water, any increase in bacterial levels may add to paper contamination. Dispersant activity in papermaking systems is poorly understood, partly because of lack of screening methods for the assessment of their efficacy. Furthermore, little is known of their long-term influence on the microflora.

Alkaline paper manufacture

In alkaline paper machine systems, slime problems are different than those generally experienced in acid paper manufacture - the dominating system in the past. Micro-organisms are usually associated with slime deposit. But the use of starch, fillers, size hydrolyzate, and in some cases, aluminium hydroxide gel, can form the deposit-and-food matrix which takes up, holds, and supports micro-organisms. The growth conditions of bacteria in alkaline systems are more favourable than acid system (optimum pH, temperature and nutrient balance). Besides, many of the conventional biocides are inefficient, since they have been developed for acid conditions.

Offensive odours in paper products

In recent years, an increasing number of paper mills have experienced considerable financial losses due to offensive odours in paper products. Such odours are due to the presence of volatile fatty acids (VFAs). Unless introduced to a mill system from external sources (i.e. chemical additives), VFAs accumulate in mill systems as a result of the metabolic activities of many species of facultative anaerobic bacteria. Several factors and conditions in mills can contribute to the propagation of these anaerobic bacteria. With mills closing their water systems, VFAs problems are occurring more often than before with their accumulation in the process streams and ultimately, in the product, which results either in an unmarketable product or complaints from the customers.

Corrosion

In this context attention should also be paid to the sulphate-reducing bacteria (SRB) which play an important and sometimes a decisive role in microbial induced corrosion (MIC). Slime deposits may provide an anaerobic zone enabling the sulphate-reducing bacteria to grow and metabolise even when the overall water contains oxygen close to saturation. Sulphate-reducing bacteria produce hydrogen sulphide, which can deteriorate concrete or corrode metals.

Water re-use

Water re-use involves either reclaiming water without making changes to its quality or improving the quality before using it in other process operations. Ample sources of fresh water allowed early paper makers considerable flexibility in dealing with potential problems in the paper manufacturing. In largely open water systems, soluble chemicals could be dealt with through discharge and subsequent external water treatment. Nevertheless, abundant, inexpensive, high quality fresh water has become scarce in many regions in Europe. Even more critical to mill operations are environmental regulations that severely limit wastewater discharge. Emission of warm/hot water also means a loss of energy. Changes made in response to this environmental pressure can impact the micro-organisms colonising the paper machines.

General notes

Various techniques to reduce microbial burden, apart from conventional biocides, have been tested. For instance, in recycled fibre production, addition of hydrogen peroxide (HP) both in wet end (upgrading of recycled fibres through hot dispersion) and on the paper machine (size press) have been evaluated. The HP treatment prior to hot dispersion turned out to be useless, since the fibres are easily reinfected by contaminated process water. The virgin pulp fibres after the cooking or bleaching stage are considered free of microbiological contamination, though may immediately become contaminated when process water is used for stock preparation. This underlines the importance of localising the critical points and taking the necessary measures to prevent such problems.

Good Manufacturing Practice (GMP) has recently been introduced in the paper making industry as a tool for papermakers to assume their responsibility towards the consumer and to comply with user specifications. Therefore, all the elements of the process will have to be examined, the points at which problems might occur identified, and the means to control them specified. GMP takes for granted that the tools necessary to control the micro-organisms are at hand.

Moreover, it should be pointed out that the Council of Europe is preparing resolutions covering paper and board materials. These will be determinant in the definition of suitable microbiological control programmes and paper hygiene.

Links to other European activities

The proposed COST Action is likely to benefit with links to COST Actions in the field of Biotechnology (action under planning) and Towards Zero Liquid Effluent in Papermaking (Cost Action E 14 with start-up mid 1998). The Action will benefit from results obtained in COST Action E 1 Paper Recyclability, which will finish by 1998. The work of the Action will be complementary and will not overlap with the other actions.

B.OBJECTIVES AND BENEFITS

The main objective of the action is to provide pulp, paper and board industries with tools to manage problems arising from microbiological activity.

Detailed objectives are:

-to understand the conditions which encourage undesired microbiological growth in production
-to understand the factors which influence the performance of microbiological control programmes
-to investigate biocide-free and environmentally-sound strategies for microbiological control

-to improve diagnostic methods to monitor microbiological activity
-to improve process water system management and/or design, in order to reduce the risk of microbiological activity
-to investigate the influence of micro-organisms in wet end chemistry
-to control microbiological production of malodorous compounds suspected to influence final product quality.

A key tool for reaching the objective of this COST Action will be to encourage universities, research institutes, and other research providers to coordinate their resources toward the identified needs. Among the mechanisms to be used are the promotion of seminars and workshops with presence of industrials.

The expected benefits include:

-Step-wise improvement in the management of problems experienced in industry associated with microbial activity such as machine runnability, product hygiene, malodorous compounds, catalase activity and corrosion
-Better competence in the pulp, paper and board industry for buying chemicals for microbiological control

-Reduced impacts on the environment by adoption of Best Practicable Environmental Option (BPEO) techniques, e.g. water management, biocide-free control etc.
-Control of odorous volatile fatty acids in finished products, to improve quality and to decrease the risk for customer complaints
-Adoption of rapid detection techniques for process management, process control and good manufacturing practice
-Tools to comply with future EC guidelines regarding water discharges and product hygiene.

C.SCIENTIFIC PROGRAM

The action is intended to create and organise the cooperation in ongoing and future European work in microbiology in the paper industry. An interdisciplinary approach is needed for the subject and the scientists and professionals who work under this action will originate from many different backgrounds, such as researchers within microbiology, mycology, chemistry, wet end chemistry, water technology and paper technology in universities and research institutes. The work will be carried out in close contact with industry.

The overall scientific programme will be divided into three scientific areas which also have strong interactions. The following list indicates the most obvious specific research tasks in each of the scientific areas (proposed Working Groups).

Scientific area 1:Understanding the importance of microbial activity in pulp, paper and board manufacturing
-Investigation to determine conditions which encourage microbiological problems in mills

A. Elaboration of an inquiry destinated to the European mills

F. Elaboration of an inquiry destinated to the chemical suppliers
F.Compilation of the overall problems encountered

-Development of detection techniques for mill perturbation management
Scientific area 2:Control of microbiological activity
-Factors affecting the performance of microbiological control programmes
F.Mill design, choice of materials
F.Process water system design
F.Nature of deposits
F.Process water quality and microbial loading

-Alternative treatment programmes

F.Localisation of the critical points for microbiological activity
F.Definition of the best treatment programme
F.Development of methods to characterise the efficacy of new treatment agents
F.Definition of the efficacy of new treatment agents

- Good House Keeping
Scientific area 3:Understanding the effects of increased water re-use
F. Consequences of water re-use on microbial activity
F.Increased concentration of nutrients
F. Increased temperature
F. Decreased dissolved oxygen content
F.Impaired product quality

Evolution of microflora
F.Change in microbial population (type, number, habit)
F.Increased opportunity for generation of malodourous compounds
-Development of appropriate water management practices that enable water close-up without excessive microbial activity requiring reliance on biocides
The proposed Action will establish a forum where information on microbiological aspects in papermaking is stimulated and shared, to assist the European paper industry in its efforts to improve its competitiveness and product quality. The specialists in this field are spread over a large number of research organisations and the teams at each organisation is normally small. For this reason a concerted action is particularly relevant.

D.ORGANISATION AND TIMETABLE
The cooperative process will be led by a Management Committee. Under the Management Committee, three Working Groups are proposed to be established, one for each scientific area. This Organisation could be summarised in the following way:
WG1: Working Group 1:Understanding the importance of microbial activity in pulp, paper and board manufacturing
WG2: Working Group 2:Control of microbiological activity
WG3: Working Group 3:Understanding the effects of increased water re-use

The work of the Action is divided into three phases:
Phase 1: Organising the Action, state of the art (12 months). The work will start with a Management Committee meeting in which the objectives are refined, the working programme is scheduled and the Working Groups are established. The Working Groups will meet and compile the ongoing national research and state of the art in the field. The results of the work will be presented in a joint workshop at the end of the phase.
Phase 2: Work in the network of Working Groups (24 months). The working groups will arrange meetings and seminars in order to report and disseminate the information gained from the results of national research work and discuss research problems and findings. Additionally, short-term scientific visits will be organised. In addition, information exchanges will be favoured. The working groups will report to the Management Committee. An evaluation will be made at mid-term of the Action. A combined workshop is to be arranged at the end of the phase.
Phase 3: Conclusions, state of the art review and proposals for further work (12 months). The working groups will arrange meetings and seminars in order to report work progress and discuss research problems and findings. The results of the Action will be concluded. A final report will be adopted, produced and disseminated.

E.ECONOMIC DIMENSION

The following COST countries have actively participated in the preparation of the Action or otherwise indicated their interest:

- Finland
- France
- Hungary
- Portugal
- Slovenia
- Spain
- Sweden
- United Kingdom

On the basis of national estimates provided by the representatives of these countries and taking into account the coordination costs to be covered over the COST budget of the European Commission, the overall cost of the activities to be carried out under the Action has been estimated, on the basis of prices in 1998, at roughly EUR 12 million.
This estimate is valid under the assumption that all the countries mentioned above, but no other countries, will participate in the Action. Any departure from this will change the total cost accordingly.
This estimated overall cost will probably increase in the following years due to the increasing interest on this topic.

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