Anaerobic treatment of evaporator condensates from the chemical pulp industry
This paper describes the design and operation of two different full-scale anaerobic wastewater treatment plants treating evaporator condensates from the chemical pulping industry. The cases described comprise effluent treatment plants at a kraft based pulp mill and a sulphite based pulp mill. In both cases the IC expanded sludge bed reactor is the heart of the installation. As the internal circulation does not affect the upflow velocity in the upper part of the IC reactor and because of the two-stage separation system optimal sludge retention is ensured. Special attention is given to the typical characteristics of both types of condensates. The potential inhibiting effects of some components as also solutions to avoid inhibition and the necessity to add nutrients and essential (trace) metals are discussed. Kraft mill condensate is characterised by high concentrations of methanol (approx. 2500 mg/l). When operating the IC reactor at volumetric loading rates of 10-18 kg/m3.d removal efficiencies were as high as 80-85 % for COD and more than 99 % for methanol. Sulphite mill condensate is characterised by high concentrations of acetic acid (approx. 2000 mg/l). Operating at volumetric loading rates between 5 and 22 kg/m3.d the IC reactors realised removal efficiencies of more than 90 % on COD and more than 97 % on acetic acid.