The present situation of water treatment facilities in the forest industry of Chile is briefly described. Some data of the Chilean pulp mill production and the present level of water pollution are shown. This paper is the result of five years of applied research conducted to study and evaluate liquid effluent treatment from pulp, paper and board mills. We describe laboratory results corresponding to three industrial mills that we have studied. The first case presented in this paper corresponds to a bleached Kraft pulp mill with a fiber line consisting in a MCC digester, a delignification unit and four steps in bleaching. In this mill the effluents are treated in an extended lagoon and the suspended solids are settled in a primary treatment. Lab scale lagoons were installed to developed start up strategies and to determine optimal operational efficiencies. The second case that we present is a paper mill, with primary treatment in an air flotation unit used for separating suspended solids. A technical economical analysis was necessary to burn solid sludge in order to improve energy balance. Finally, the third case we have studied corresponds to a hardboard mill effluent. In this case, high COD, mainly as colloidal solids, characterize this effluent. Laboratory experiences were made using a system of two sequential aerated lagoons.

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