Abstract: Process water from pulp manufacture is purified in a separator device by mechanical or a combination of mechanical and chemical methods and is thereafter evaporated, at least part of the concentrate resulting from the evaporation being recycled and mixed with the process water fed into the separator device. In a preferred embodiment, a precipitant is added to the mixed water comprising the process water and the concentrate. The part of the concentrate not recycled and mixed with the process water is supplied, optionally after further evaporation, with an acid to a pH of 1-5, whereupon resulting flocs and precipitates are removed in a separator device.

Abstract: Wood-resin-containing process water or wastewater from the pulp industry is purified by adding, as flocculants, a high-molecular-weight polyethylene oxide and a water-soluble, non-ionic cellulose ether, suitably having a flocculation temperature of 35.degree.-80.degree. C. The added amount per liter of water is preferably 2-20 mg of the high-molecular-weight polyethylene oxide and preferably 5-30 mg of the non-ionic cellulose ether.