The treatment of pulp and industrial paper effluents uses energy. Aeration is the single most energy demanding process in pulp and wastewater treatment, and it is therefore desirable to affect when energy efficiency is wanted. The sludge from pulp and paper treatment plants has until recently been regarded as a problem. Now, the prospect of using the sludge as feedstock in biogas production through anaerobic digestion is gaining interest.

Two possible cases for the treatment of pulp and paper effluents have been evaluated to find the most energy efficient process. In Case 1, the process effluent is treated with a conventional activated sludge process. The sludge is anaerobically digested to produce biogas. In Case 2, the process is extended with a chemical treatment stage that is added prior to the biological activated sludge stage, the purpose of which is to reduce surface-active wood extractives to obtain a more efficient oxygen transfer. The total COD reduction is the same as in case 1. The sludge is anaerobically digested. The decreased energy need for aeration, in combination with a higher potential for energy recovery in the form of biogas, leads to a positive energy balance for case 2.