Conditions for cost-efficient reuse of biological sludge for paper and board manufacturing

• Reuse of biological sludge for paper production reduces paper strength.
• Negative effect of 1% bio-sludge can be compensated for by 0.5 g/m2 surface starch.
• There is an optimum sludge reuse ratio for a minimum production cost.
• Decision to reuse sludge depends on external treatment cost and market conditions.

Integrated water treatment plants in recycled paper and board mills produce significant amounts of excess biological sludge. Disposing of it is becoming increasingly expensive, as land filling is no longer an option. One possible solution is to reuse it internally for manufacturing paper. Recycling it in the process would reduce both the cost associated with sludge treatment and the amount of recovered paper to be purchased. However, this strategy deteriorates the mechanical properties of the paper to some extent. This detrimental effect can be compensated for by surface sizing. This paper examines the conditions in which it may be profitable for a mill to replace a proportion of recovered paper with biological sludge, depending on the cost of sludge disposal, surface sizing and the recovered paper itself. The effect of biological sludge reuse on the drainability of the papermaking suspension is studied as well. The consequences in terms of microbiological contamination of the paper and board are also reported. In the present situation, bio-sludge reuse is not cost-efficient. However that may change in the near future, as the cost of sludge disposal and the price of starch increase.

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