The feasibility of composting of pulp and paper industry sludges was studied. The aim was to study the progress of the composting process and the toxicity and applicability of the sludges in agriculture. Five different sludges were used: two biosludges and three biosludge and primary sludge mixtures. The sludges were composted in seven piles, two smaller of them founded with leachates collection systems. The piles were monitored throughout the composting period by physical measurements, chemical analyses and toxicity tests. Also the lignin concentrations were measured from the sludges and from the soil as well as the amount of mycelium.

The results showed that all sludges were easily composted and they will be suitable for the use as soil improvement in agricultural plant production. Toxicity of the sludges decreased during composting and all the sludges were proved to be non-toxic at the end of the composting. Heavy metal concentrations were low in all sludges and leachates. The leachates were also non-toxic and the concentrations of nutrients were low.

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