Science: Converting organic waste into valuable chemicals

By changing the pH, the composition of microbiota present changed dramatically. This resulted in a fermentation that favored the formation of (iso)-butyric acid, an important building block used by the chemical industry. On November 3^rd, he successfully defended his PhD thesis: Open Culture Chain Elongation for Branched Carboxylate Formation.

Recycling

From waste to value’ is an important recycling principle that contributes to a more sustainable society. ETE’s research therefore has a strong focus on recovery of energy and chemicals from waste. Often, microorganisms play a key role. For example, to recycle organic waste, microorganisms present break down biomass and convert it into other chemicals. Most often, this fermentation results in the formation of acetate and eventually biogas, or methane. Depending on the different microbiota species present, also more valuable compounds may be formed, such as fatty acids and alcohols. These chemicals have many applications in industry and agriculture as feed additives, solvents, lubricants, bioplastics, fuels etc. Industrial use of these fermentation products is highly sustainable since it reduces the pressure on other resources. For example, many fatty acids originate from fossil oil or palm oil. Especially palm oil production takes a heavy toll on pristine tropical rainforests, that are cleared to make room for oil palm plantations. So, if the right microbiota species are present, fermentation may thus turn organic waste into a resource for high-quality chemicals.

Size matters

The dominant component formed in a ‘standard’ fermentation is acetate, a
short chain fatty acid. In addition, small alcohols, like methanol and ethanol can be formed. In his research, De Leeuw focused on exploring how to manipulate the microbiota species composition to stimulate the formation of high-grade chemicals. During his experiments, he used so-called open-culture bioreactors, that are open to influx of micro-organisms from the surroundings. By manipulating reactor conditions, like temperature, pH, nutrients, and flow rate, he tried to favor microbiota that produce bigger molecules with higher value, like longer-chain fatty acids. With the right microorganisms present, longer-chain fatty acids can be the dominant fermentation products. And for these compounds, size matters: the longer they are, the fattier and oilier their properties and consequently the better their industrial applications. ‘For that reason, I tried to select for these species that formed longer, more useful fatty acid molecules’, de Leeuw explains. ‘I tried to achieve that by, what we call, chain elongation fermentation, where a short molecule, like acetate, can be merged with for example methanol, to form a larger reaction product.’   

Dramatic shift

Eventually, de Leeuw’s discovered that pH was the key to the formation of the desired, bigger molecules (fig. 1): ‘A pH change towards either pH 5.2 or pH 6.7 resulted in a dramatic shift in microbiota composition and the products they formed’, de Leeuw says. ‘At pH 5.2 a Clostridium bacteria species dominated, forming a lot of the fatty acid iso-butyric acid. At pH 6.7 an Eubacterium was mostly present and formed just butyric acid.’ Both compounds have valuable industrial applications. In addition, de Leeuw found small amounts of caproate, another long-chain fatty acid with many possible applications. These microbiota used methanol and acetate to synthesize the longer chemicals. This methanol-based chain elongation required the addition of extra methanol to speed up the process, but according to de Leeuw, that is not a problem: ‘Methanol is a simple molecule than can easily be made by certain organic waste fermentations or even from CO₂ gas streams.’ So, this process is even more sustainable since it could eventually use a greenhouse gas in the reaction.

Fig. 1. Isobutyrate formation in an open- culture bio-reactor at different pH values. 

Main challenge

De Leeuw’s research convincingly showed the proof of principle, with a
high reaction efficiency: about 70-90 percent of all carbon present was
eventually converted into (iso)butyric acid. The process can now be further tested for the industrial production of these components. According to de Leeuw, the first important test is to make the system work with real organic waste streams in a pilot reactor. The second challenge is to recover the products formed from the watery environment in an efficient and cost-effective way. De Leeuw: ‘This down-stream processing is vital for an economically sustainable operation of the process.’

Clogged hoses

Looking back at his four years of research, de Leeuw thinks that perseverance and being attentive were essential. ‘Many reactor experiments take months and the main challenge is to have the system operate in a measurements.’ These stable conditions were often opposed by clogged hoses, leakages, and failing stable way’, he says. ‘Only during a steady operation, you can take samples and perform trustworthy equipment. De Leeuw: ‘It was crucial to detect those failures early enough to prevent lost time.’ 

publication

de Leeuw, K., de Smit S.M., van Oossanen, S., Moerland, M.J., Buisman C.J.N., & Strik, D.P.B.T.B. 2020. Methanol-Based Chain Elongation with Acetate to n-Butyrate and Isobutyrate at Varying Selectivities Dependent on pH. ACS Sustainable Chem. Eng. 8, 22: 8184–8194. https://pubs.acs.org/doi/abs/10.1021/acssuschemeng.0c00907