A multi-stage process for treatment of CTMP effluent has been developed. It comprises primary settling and four biological stages. The concentration of hydrogen peroxide, a compound which is toxic to anaerobic bacteria, is reduced in the first biological stage by means of the biocatalytic action of biomass that is recycled from the following acidogenic and/or aerobic stages. The second stage is an acidogenic stage, in which volatile fatty acids are formed and remaining peroxide is decomposed. A mixture of aluminum, iron and calcium salts is added to the effluent in order to detoxify compounds which are toxic to methanogenic bacteria. The main part of the COD and BOD removal takes place in the third stage, the methanogenic stage, after which follows an aerobic stage for polishing and removal of bad-smelling compounds. The COD and BOD7 removals in the anaerobic part of the process are 60 and 90 %, respectively, and the methane yield is 0.20-0.25 Nm3/kg COD removed.

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