Pulp and paper mill wastewater can become biogas

Samples from 70 wastewater streams at seven mills show that the potential to extract biogas from the material is at least 70 million normal cubic metres of methane per year. (One normal cubic metre (Nm3) of gas equals one cubic metre of gas at normal air pressure and 0°C.)

"We hope to achieve 100 million Nm3, which we probably will when we have fine-tuned the processes. This volume would mean an increased biogas production of roughly 65 per cent, compared to Sweden's total production for 2012," says Bo Svensson, professor at WES, and project manager together with Dr Annika Björn and Dr Jörgen Ejlertsson.

For various types of mills, the lab tests identified the point that has the greatest potential for extraction of biogas. For instance there is good potential in streams from bleaching plants, paper machines and wastewater from presedimentation in mechanical pulp processes. At the sulphite mills there is good potential in the fibre sludge that comes from presedimentation.

The results from the two first years of research are so promising that the trials will now move from the lab to the mills.

Today most mills clean their wastewater with oxygen-thirsty aerobic technologies that also consume lots of energy. Two anaerobic, oxygen-free technologies that have proven to work well in the lab will now be scaled up. These are UASB (upflow anaerobic sludge blanket) and CSTR (completely stirred tank reactors) with recirculation sludge. In both cases you make use of the biogas that forms in the process.

There will also be trials to actively recirculate sludge from the mills' sedimentation pools to the reactors. This would increase the extraction of biogas and reduce the costs of dewatering and aeration of the ponds.

The research, conducted in collaboration with Scandinavian Biogas Fuels AB, Pöyry Sweden AB and a number of pulp and paper mills, has already generated considerable interest, both nationally and internationally. In particular researchers from Finland have got in touch.

"We're one step ahead of the rest of the world -- we have extensive knowledge of what is required to ensure that the microorganisms in a reactor develop well," explains Prof Svensson.