In the past 15-20 years, biogas has arisen as a possible solution to a number of infrastructure problems associated with waste. It’s not only waste from sewage treatment plants that can be used as raw material in the digestion chambers: the waste from abattoirs, illegal alcohol, excess, faulty or out-of-date goods from the food industry, and many other forms of excess organic material can be used. Much of what in the past created major handling problems can now be used to form useful biogas.
Increasing demandDemand for biogas is increasing, and the waste-treatment plants in Sweden still have considerable capacity available in the reactors. These reactors were built to provide clean waste water in a more efficient and environmentally sensitive manner, and the biogas was initially simply a nice extra. An important part of the research at BRC, therefore, is focussed on increasing production in biogas reactors that are already in operation. Increasing this production will be an important factor in achieving the objective of producing 15 TWh of energy from biogas in 2030.
The BRC researchers, led by Annika Björn, Bo Svensson and Sepehr Shakeri Yekta from the Department of Thematic Studies – Environmental Change, have also carried out experiments adding extra nutrients to the reactor to encourage the small methane-forming organisms in their work, and experiments mixing new substrates and raw materials into the sludge from waste-water treatment that is normally digested.
Difficult to digest fatOne substrate that has long been under scrutiny is waste fat, oils and other sticky waste from the fat filters used in restaurants. These materials cause problems in handling the waste, but at the same time contain huge amounts of energy. The energy content can be 300 times higher than it is in the waste-water sludge that is digested at sewage treatment plants. It is, however, difficult to digest fat in the reactors. There is a tendency that long-chain fatty acids (LCFAs) form, and these can clog just about anything and disrupt the processes.
In a project that required the patience of Job, researchers from the BRC have mixed waste-water sludge from the Henriksdal sewage treatment plant in Stockholm with different amounts of fat from filters in restaurants at LiU, in order to study in detail how the small methane-forming organisms react.
“We wanted to increase our understanding of the organisms that break down fat,” says Annika Björn.
It became apparent that a central organism in this process is the Syntrophomonas bacterium.
“The concentration of Syntrophomonas is important in getting a good result. But this bacterium grows rather slowly and needs a somewhat longer retention time in the reactor to work well. The time spent in the reactor must be increased, and thus we also suggest that material is recirculated in order to increase the efficiency of the process. The retention time for the bacteria can in this way be increased,” says Annika Björn, senior lecturer at the Department of Thematic Studies – Environmental Change, who has led the research at BRC.
“By acting strategically and feeding the reactor with exactly the right amount of fat to keep the concentration of Syntrophomonas above a certain threshold, it is possible to control the production and considerably increase the amount of biogas produced. The process at the same time means that we exploit what otherwise is a challenging substrate to deal with,” says Annika Björn.
Microbial community adaption influences long-chain fatty acid conversion during anaerobic codigestion of fats, oils, and grease with municipal sludge. Ryan M Ziels, Anna Karlsson, David Beck, Jörgen Ejlertsson, Sepehr Shakeri Yekta, Annika Björn, David Stensel and Bo Svensson from the University of Washington, Scandinavian Biogas Fuels AB and the Department of Thematic Studies – Environmental Change, Linköping University, Water Research 2016. DOI 10.1016/j.watres.2016.07.043