Mill wastewater becomes biogas

February 10, 2014, Linköping, Sweden – Wastewater from pulp
and paper mills contains large volumes of organic material that can be
converted into biogas, according to findings by researchers from Water and
Environmental Studies (WES) at Linköping University. The Swedish Energy Agency,
Linköping University and participating pulp and paper mills are funding the
continuation of this research with SEK 14.8 m over a two-year period.

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.