AD out in the cold

ANAEROBIC digestion at temperatures as low as 20°C has been demonstrated to produce methane yields that are comparable to more energy-intensive processes.

Treating municipal organic solid wastes economically is a challenge, with techniques such as incineration, gasification and pyrolysis often requiring high levels of energy. Anaerobic digestion is an alternative method, capable of removing high concentrations of organic waste and providing clean fuel and fertiliser from renewable feedstocks. Techniques currently used involve a consortium of microorganisms that undertake a series of biochemical transformations in the absence of oxygen. However, for optimal efficiency, bioreactors are operated at temperatures of 35–55°C, which requires substantial energy input in colder countries.

Now, researchers from Canada’s Concordia University, in collaboration with Bio-Terre Systems, have demonstrated the viability of anaerobic digestion in a lower-temperature environment. The technique involved the use of psychrophilic bacteria, which thrive in lower temperatures, and so bioreactors will require less heating for optimal performance.

Mohammad Saifur Rahaman, from Concordia's Department of Building, Civil and Environmental Engineering, said: “There is enormous potential here to reduce the amount of fuel that we use for solid waste treatment. What we've learned is that we can now use adapted psychrophilic bacteria to produce a level of methane comparable to those more common forms of digestion, while using less energy.

'Managing and treating food waste is a global challenge, particularly for cold countries like Canada where the temperature often falls below -20°C and energy demands related to heating are high.'

The researchers also used a unique process for feeding the bioreactor. It involved a semi-continuously-fed constant volume overflow approach, whereby the amount of food waste fed into the bottom opening necessitated the removal of an equal amount of treated effluent from the top.

The researchers performed various tests on the extracted material to determine its physicochemical characteristics as well as to monitor the biogas quality and quantity. Methane content was constantly in the range of 64–69%, showing the biogas to be of good quality.

The study’s lead author, Bio-Terre’s Rajinikanth Rajagopal said: 'There aren't many studies that look into developing new applications for treating food waste. We hope that this study will mark the beginning of a promising new research direction.'

Process Safety and Environmental Protection: http://doi.org/f9pvzb