Fermentation Bagasse Pulp
Paper is primarily made from plant fibers. Bagasse, which is the fibrous residue left after extracting juice from sugarcane, is rich in hemicellulose, cellulose, and lignin. These components make it an excellent raw material for paper production. However, if bagasse is directly mechanically pulped, the presence of lignin can cause excessive energy consumption and high costs. Therefore, it is essential to remove or reduce the lignin content to improve efficiency.
Bagasse has a complex structure composed mainly of cellulose, hemicellulose, and lignin. Approximately 20.6% of its composition is hemicellulose, and about 18.6% is lignin. Cellulose makes up around 35.4% of bagasse and consists of glucose units linked by β-1,4-glycosidic bonds, forming a two-phase system with both crystalline and amorphous regions. The crystalline regions have hydroxyl groups that easily form hydrogen bonds with water molecules, making them resistant to breakdown. Hemicellulose is chemically similar to cellulose but has a lower degree of polymerization and contains various sugars like arabinose. Lignin, which accounts for about 95% of the molecular weight (around 10,000–50,000), is mainly found in the epidermis of the bagasse. It binds with hemicellulose to form a protective layer that prevents microorganisms and enzymes from accessing cellulose, making hydrolysis difficult. White rot fungi and soft rot fungi have been shown to effectively degrade lignin, helping to soften and break down the bagasse structure.
The bagasse starter contains a variety of fungi, bacteria, and actinomycetes that can decompose lignin and loosen the bagasse structure, aiding in fiber separation during refining. Additionally, biological pretreatment through fermentation significantly improves the mechanical properties of the pulp, such as tensile and tear strength. The process involves several key steps to ensure successful fermentation.
Step 1: Start-up
The functional microbes in the bagasse starter are most active at temperatures between 10°C and 15°C. Below this range, they become inactive or dormant. At 0°C, they stop working entirely. To speed up the process, especially in colder seasons, artificial warming methods like using warm water, heaters, greenhouses, or steam can be applied to raise the temperature to the ideal range. Once activated, the microbes will rapidly multiply, generating heat and creating a positive feedback loop that supports further growth.
Step 2: Material Ratio
The optimal carbon-to-nitrogen (C/N) ratio for microbial activity is between 20:1 and 30:1. Bagasse naturally has a suitable C/N ratio, so no additional adjustments are usually needed. One kilogram of starter can ferment 3–5 tons of bagasse, depending on conditions.
Step 3: Dilution Method
To enhance microbial activity, the bagasse is often diluted with rice bran (at a ratio of 1:5). Rice bran provides good nutrition and air permeability. If rice bran is unavailable, alternatives like cornmeal or wheat bran can be used. A small amount of water is added to moisten the mixture without causing clumping, ensuring even distribution across the pile.
Step 4: Moisture Control
The moisture level should be maintained at 60–65%. A simple test is to grab a handful of material; if no water drips off, it’s just right. Too dry or too wet conditions hinder microbial activity. If the material is too wet, spreading it out or adding absorbent materials like straw or sawdust can help. If too dry, water should be added gradually, preferably hot water to accelerate the start-up.
Step 5: Ventilation
The microbes involved are aerobic, meaning they require oxygen. Proper ventilation is crucial. Methods include turning the pile regularly, punching holes in the material, covering with breathable materials like straw, or using a blower to supply oxygen. This helps maintain an aerobic environment and prevents the formation of anaerobic zones that can lead to bad odors.
Step 6: pH Regulation
The ideal pH range for the microbes is 6–8. Most natural materials do not require adjustment, but if the pH is too high or low, lime or acetic acid can be used to balance it. Maintaining the correct pH ensures optimal microbial performance.
Step 7: Fermentation Rhythm Control
Fermentation speed can be controlled by adjusting temperature, nutrient availability, aeration, and the number of turns. Adding already fermented material can also speed up the process. If fermentation becomes too fast, measures like cooling or reducing aeration can slow it down.
After about a week, the temperature of the pile will drop to 40–50°C, indicating that fermentation is nearly complete. The material will be darker, odorless, and have a pleasant earthy smell. At this point, it can be spread out and cooled for further processing. If the temperature remains above 60°C for more than ten days, it may indicate over-fermentation, which could damage cellulose and hemicellulose. In such cases, cooling measures should be taken before proceeding to the next stage, such as drying or mechanical pulping.
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