A new conversion process being developed at the University of Borås in Sweden suggests that it is possible to convert waste to both biogas and other valuable chemicals. PhD student Konstantinos Chandolias sees great opportunities for both industry and the environment with the new two step method.
The research project, which is being funded by the Swedish Research Council (Vetenskapsrådet) a public agency under the authority of the Ministry of Education and Research, aims to address the treatment of residual materials found in waste-based anaerobic digestion (AD) feedstock streams such as plastics and cellulosic fibres that can not be broken down by AD. Conventional pre-treatment methods to break down such materials are mainly based on chemicals and catalytic conversion to methane (CH4) is possible but an expensive process.
The method I’m working on is to feed bacteria from sewage sludge with synthesis gas. The synthesis gas is produced by heating the waste until it is gasified. The synthesis gas consists of hydrogen and toxic carbon monoxide (CO), and carbon monoxide is like candy for the bacteria. When the bacteria are fed with the synthesis gas, a fermentation process occurs and methane gas is formed, revealed Chandolias.
Two step process
By combining the two processes, first gasification and then fermentation, productivity is increased and it is possible to obtain different valuable materials.
In addition to methane and hydrogen, we can also extract valuable chemicals like organic acids, such as lactic- and acetic acid, and alcohols. At the same time, the amount of residual waste in the form of ash is less than, for example, conventional waste incineration, Chandolias explained, adding that it also reduces the problem of heavy metals in ash that can leach into the environment.
In his experiments, Chandolias has used waste from forestry, agriculture, households, mixed waste and sewage sludge.
These are wastes that there is a lot of, and I want to find a solution for waste that is hard to break down by digestion. For example, if it contains cellulose fibres, or poisonous substances, such as limonene found in citrus peels that kill the bacteria we want to use in the process, he said.
By using the two-stage process, the reactor can be fed with different types of waste, reducing the volume of waste by 80-90 percent, compared with the use of chemical pre-treatment.
Challenge to feed bacteria with gas
The challenges of industrial application with the new method lies with the large-scale feeding of the bacteria with the synthesis gas. The gas added to the reactor has a tendency to form bubbles that rise to the surface away from the bacteria. Furthermore, bacteria are also flushed out when changing the substrate.
Our solution is to use custom-designed membranes that look like tea bags. These are filled with bacteria. The membranes protect the bacteria from toxic substances and ensure that they remain inside the reactor. Within the industry, a number of pilot trials have been conducted, including on a larger scale. However, the concept is relatively new. We try to improve different parts of the process to make it attractive to industry, said Chandolias.
According to Chandolias the benefits of the new method are:
- It is cost effective
- It works on different types of waste
- It provides rapid reduction of the volume of waste
- It reduces the amount of ash and thus also the risk of leakage of heavy metals
- It offers end product control by varying the mixture of synthesis gas and waste to the bacteria
