Carbon capture and storage (CCS) to remove CO2 is becoming an important technology to combat climate change. But how to get rid of the waste from the removal process? Bacteria might be the answer.
Carbon capture and storage (CCS) is becoming an important technology to combat climate change. In CCS, carbon dioxide (CO2) from power plants and industry is captured, compressed and stored in underground formations.
A full scale capture plant could remove as much as 1 million tonnes of CO2 per year, as it is already happening in BD3 SaskPower in Canada. At such scales, the amount of produced waste from the removal process becomes significant. However, an economic and efficient waste disposal system is not yet established.
Bacteria are commonly used to treat wastewater. Could it be possible to treat the waste from CSS too? This is what we investigated.
The CO2-removal process
CO2 can be captured by absorption to different chemicals, such as nitrogen (N)-containing amines. In this process, the flue gas from the power plants or industry is led through an absorber column where the gas meets the liquid amine solvent.
The CO2 in the gas is captured by the amine (absorbed) and the CO2-rich solvent is led to the second column, called the stripper column. Here, the CO2 is released and separated from the amine, making the solvent ready for reuse.
Are bacteria the answer for waste treatment?
During the absorption and stripping process, the solvent is reclaimed for reuse, but it will gradually be degraded due to side-reactions and heating.
Therefore, it is necessary to purify the solvent from time to time by removing the unwanted by-products as waste. This is essential for maintaining the CO2 capture capacity, as the accumulation of degradation products leads to operational problems.
The process waste contains mainly undegraded amine as well as other nitrogen compounds. We wanted to investigate if this waste could be biodegraded in a process called biological nitrogen removal (BNR). This is a two-step process called nitrification and denitrification, eventually resulting in non-toxic dinitrogen (N2) gas, as found at 78% in the air.
In the BNR process, two types of bacteria are used: Nitrifying bacteria and denitrifying bacteria. The nitrifying bacteria are more sensitive and require oxygen, while the denitrifying bacteria work under so called anoxic conditions utilizing nitrate for their respiration.
The most common amine at use for CCS so far, Monoethanolamine (MEA), and its reclaimer waste were successfully treated over a long-term period in the complete BNR process described above.
Other types of amines
For screening purposes, the biodegradability of different amines were measured with and without oxygen and results revealed big differences. For assessing the anoxic biodegradability, a new rapid and simple method was developed, applying syringes filled with biofilm carriers.
These feasibility studies were done in moving bed biofilm reactors (MBBRs). While the organic matter could be utilized by the denitrifiers, the nitrifying bacteria were inhibited at the same concentrations.
This highlights the importance of removing the organic matter within the anoxic denitrification step, before the waste reaches the aerobic nitrification step. This anoxic-aerobic process configuration of BNR is called pre-denitrification. Furthermore, we identified loading and biofilm composition as important limitation factors of the process.
Yes, bacteria can do the job
In total, this work shows that amine-based CCS waste can successfully be treated biologically with BNR processes.
This blog post is written by Ingrid Henry. She completed her PhD-degree at the Department of Biotechnology in December 2016.