Chopping down trees to process into paper is degrading the world's forests. Since wheat straw is plentiful and easily renewable, it is an alternative raw material for paper.
Unfortunately, current processing techniques for wheat straw generate a large amount of corrosive and toxic wastewater. Bacterial remediation is an attractive possibility for treating this wastewater in an environmentally-friendly fashion.
The challenge is that one would need especially hardy bacteria to withstand the corrosive and toxic environment. Ping Xu and coworkers (Shandong University and Shanghai Jiao Tong University, both in China) have fortuitously discovered bacteria that neutralize and partially detoxify wastewater from wheat straw processing.
The wastewater.
In order to process trees or wheat straw into paper, the lignin polymers must be extracted from the celluose polymers. Alkaline (high pH) conditions are commonly used in such extractions, which means that the wastewater is highly corrosive.
Historically, paper mills have burned their wastewater to reduce the corrosivity and to generate energy. This is unfortunately a nonideal option for wastewater derived from wheat straw processing, because unlike wood, the extra chemicals produced during wheat straw processing render it more difficult to treat in a cost-effective and non-polluting manner.
Treating the wastewater with bacteria is an attractive option. However, the alkaline conditions and the pollutant content of the wastewater defeat most bacteria.
Ping Xu and coworkers have accidentally (luckily!) found bacteria that not only survive a trip in the toxic wastewater, but also neutralize and partially detoxify it. They then set out to elucidate how the bacteria do their job.
Detoxifying and neutralizing bacteria.
The scientists found eleven species total in two groups of bacteria (the Bacillus and Halomonas families) that lowered the pH of the wastewater (by secreting acids) and partially detoxified the wastewater.
The latter was judged by the "chemical oxygen demand" of the wastewater, the mass of oxygen required to fully oxidize the chemicals within it to carbon dioxide, ammonia, and water. This is a common technique to indirectly measure water pollutant levels.
Some of the bacterial species were more effective than others. The scientists reasoned that if they combined all eleven bacterial species into a group, they could take advantage of the capabilities of all the species.
Working together for optimum benefit.
The group of 11 bacteria, when combined, decreased the pH of the wastewater from 11 (alkaline) to 7.9 (very close to neutral). The chemical oxygen demand was reduced by 27%.
This remediaton occurred in two stages. In the first stage (68 hours), during which the Halomonas bacteria were most active, the bacteria secreted 16.7 grams per liter of lactic, formic, and acetic acids (reducing the pH from 11 to 7.9). The chemical oxygen demand of the wastewater was reduced by 13%.
In the second stage (112 hours), during which most of the bacterial species were equally active, the pH remained the same, while the chemical oxygen demand was reduced to a total of 27% lower than original levels. Thus, both the pH and the toxicity of the wastewater were considerably reduced.
Importantly, this process is capable of being run continuously, for at least a month at a time. This feature is critical for efficient and cost-effective industrial applications.
Environmentally-friendly paper production.
Ping Xu and coworkers have found bacteria that neutralize and partially detoxify wastewater generated from wheat straw processing into paper, meaning that wheat straw is a more viable alternative to paper. In addition to reducing pollution and saving the world's forests, their research also adds value to wheat straw, which is generally considered to be a waste product.
for more information:
Yang, C.; Cao, G.; Li, Y.; Zhang, X.; Ren, H.; Wang, X.; Feng, J.;
Zhao, L.; Xu, P.
A constructed alkaline consortium and its dynamics in treating alkaline
black liquor with very high pollution load.
PLoS ONE 2008, 3, e3777.