Anaerobic treatment of thermomechanical pulping whitewater at 35–70°C

Abstract

Anaerobic treatment of thermomechanical pulping whitewater was studied in semicontinuously fed batch digesters at 35, 55 and 65°C, and in upflow anaerobic sludge blanket (UASB) reactors at 55 and 70°C. Also post-treatment of the 55°C UASB effluent by thermophilic activated sludge process and by acid precipitation was investigated. The batch digester studies demonstrated treatability of whitewater at all applied temperatures. In UASB reactors up to 65–75% COD removal was obtained at 55°C at loading rates of 14–22 kg COD m −3d −1. About 60% COD removal was maintained at 55°C in the UASB reactor at a loading rate as high as 80 kg COD m −3d −1, which corresponded with a hydraulic retention time of 55 min. In the UASB reactor at 70°C, loading rates of about 13 kg COD m −3d −3 were reached with about 60% COD removal. Both the batch and UASB studies showed increased effluent residual COD at increasing treatment temperatures. Compounds other than volatile fatty acids and carbohydrates accounted for most of the COD increment. Activated sludge process and acid precipitation further removed the COD present in the anaerobically treated whitewater.