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Formation of fouling deposits is a serious problem facing paper mills. Despite the search for alternative methods, chemical biocides still represent the chief countermeasure to control microbial growth and general fouling buid-up in pulp and paper mills. The purpose of this work was to determine the effect of two biocides (carbamate and glutaraldehyde) on both planktonic cells and fouling layers of a paper machine system. A flow system was used for the study of fouling accumulation in an industrial white water circuit. Both biocides proved to be more effective in reducing the microbial loading of the white water circuit than the deposit accumulated on the stainless steel surfaces. Carbamate, in contrast to glutaraldehyde, had the ability of promoting cell agglomeration since the microbial loading decreased much more when the white water, treated with carbamate, was filtered through a filter-linen. The retention of suspended cells in the cellulose fibres acquires major importance since it is obtained by using an already existing physical process (filtration), which strongly enhances the overall microbial reduction obtained with the addition of the carbamate, without increasing the economic costs. These results also suggest that the use of conventional retention agents in pulp and paper processes can be efficient in controlling unwanted microbial effects., Junta Nacional de Investigação Científica e Tecnológica (JNICT) - PRAXIS XXI, 2/2.1/BIO/37/94, BD/3242/94. Instituto de Biotecnologia e Química Fina (IBQF).

The effectiveness of a non-oxidising biocide (a carbamate-based solution) as a retention agent for cells of Pseudomonas fluorescens in a paper pulp suspension was studied, as a way to reduce biofouling in pulp and paper production processes. The results indicated that the addition of the biocide not only changed the pH of the paper pulp suspension, but also shifted the surface charges of the bacterial cells from negative to neutral or positive values, depending on the pH value and the biocide concentration Therefore, since the paper pulp fibres are negatively charged, adhesion of bacterial cells to the fibres is promoted. Hence, it was possible to obtain an increase in cell retention on the cellulose fibres from 45% to 75%, within less than 5 min of contact between the carbamate and the pulp suspension. This effect increased with the concentration of carbamate solution in the fibre suspension (100-300 mgl-1), and was always higher than in the absence of biocide.