abstract
With the final aim of reducing the energy consumption and increase the methane production at Swedish
pulp and paper mills, the methane potential of 62 wastewater effluents from 10 processes at seven pulp
and/or paper mills (A–G) was determined in anaerobic batch digestion assays. This mapping is a first step
towards an energy efficient and more sustainable utilization of the effluents by anaerobic digestion, and
will be followed up by tests in lab-scale and pilot-scale reactors. Five of the mills produce kraft pulp (KP),
one thermo-mechanical pulp (TMP), two chemical thermo-mechanical pulp (CTMP) and two neutral sul-
fite semi-chemical (NSSC) pulp. Both elementary and total chlorine free (ECF and TCF, respectively)
bleaching processes were included. The effluents included material from wood rooms, cooking and oxygen
delignification, bleaching (often both acid- and alkali effluents), drying and paper/board machinery as
well as total effluents before and after sedimentation.
The results from the screening showed a large variation in methane yields (percent of theoretical methane
potential assuming 940 NmL CH4 per g TOC) among the effluents. For the KP-mills, methane yields
above 50% were obtained for the cooking effluents from mills D and F, paper machine wastewater from
mill D, condensate streams from mills B, E and F and the composite pre-sedimentation effluent from mill
D. The acidic ECF-effluents were shown to be the most toxic to the AD-flora and also seemed to have a
negative effect on the yields of composite effluents downstream while three of the alkaline ECF-bleaching
effluents gave positive methane yields. ECF bleaching streams gave higher methane yields when hardwood
was processed. All TCF-bleaching effluents at the KP mills gave similar degradation patterns with
final yields of 10–15% of the theoretical methane potential for four of the five effluents. The composite
effluents from the two NSSC-processes gave methane yields of 60% of the theoretical potential.
The TMP mill (A) gave the best average yield with all six effluents ranging 40–65% of the theoretical
potential. The three samples from the CTMP process at mill B showed potentials around 40% while three
of the six effluents at mill G (CTMP) yielded 45–50%.