Anaerobic Conversion Of Aquatic Plants By Rumen Microorganisms

Recently aquatic plants are extensively applied for ecological restoration of eutrophic lakes or rivers. However, the rapid growth of aquatic plants causes difficulty in controling, and could become a new pollution source if effective post-treatment is missing. On the other hand, as a renewable lignocellulosic resource the aquatic plants can be converted into volatile fatty acids (VFAs) and/or biogas (i.e., H₂ and CH₄) through anaerobic digestion.In this work, an investigation into aquatic plants degradation by rumen microorganisms was perfonned and pretreatment techniques of aquatic plants were optimized. Also, a novel method for determining aquatic plant degradation process based on near infrared reflectance spectroscopy (NIRS) was developed. Main contents and results are as follows:1. The anaerobic degradation of cannas, a typical aquatic plant, was investigated and characterized using Fourier transferred infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and gel penetration chromatography (GPC). Experimental results indicate that the hydrolysis was the limiting step for the anaerobic digestion of aquatic plants by rumens. FTIR was able to semi-quantitively and qualitatively describe the degradation of cellulose, hemicellulose and lignin in aquatic plants. XPS and GPC analyses show that part of lignin fraction was decomposed to soluble components in the initial phase of the anaerobic digestion.2. Fractional factorial design (FFD) and response surface methods (RSM) were employed to select appropriate pH and substrate concentration to obtain high degradation efficiency and VFA yield. Results indicate that both substrate concentration and pH had a significant influence on VFA production. Furthermore, a high degradation efficiency could be obtained under optimized conditions of pH 6.6 and substrate concentration of 8.2 g VS/L, while a higher VFAs yield achieved at pH 6.7 and substrate concentration of 6.9 g VS/L.3. Steam explosion pretreatment was used to improve the ruminal degradation of Bulrush and optimized with RSM. Furthermore, the influences of steam explosion on Bulrush were also explored using UV spectroscopy, XPS, thermogravimetric analysis and other chemical analysis. A maximum R_max of 0.485 g COD/L/d could be obtained under the optimized conditions of a moisture of 18%, steam pressure of 1.6 MPa and retention time of 8.0 min, while a maximum P_s of 0.432 g COD/g could be obtained at a moisture of 20%, steam expressure of 1.8 MPa and retention time of 8.0 min. Spectral analyses show that steam explosion disrupted the rigid structure of plant cell wall to improve the specific surface area which was beneficial for the ruminal hydrolyzation of aquatic plants.4. The ruminal degradation of cattails in the presence of heavy metals was investigated. When the concentrations of Cd (II), Cu (II) and Cr (VI) were 1.6, 2.4 and 4.0 mg/L, respectively, a high degradation efficiency of cattails was obtained. A higher dosage of heavy metal inhibited the microbial activity, especially for the methanogenic microorganisms. The toxic order of these three metals at a high dosage was in the order of Cd (II) > Cu (II) > Cr (VI).5. An important quatic plant, cannas, was investigated for its anaerobic acidogenisis by rumen microorganisms. Experimental results indicate that Tween 80 concentration had a significant influence on cannas degradation. This might be attributed to the fact that Tween 80 at an appropriate concentration could improve the activity of the hydrolytic enzymes and weaken the negative effect of lignin fraction.6. A new NIRS-based method to predict the Klason lignin, total solids (TS), volatile solids (VS) and biodegradable fraction (BF) of aquatic plants was established. The prediction models were developed using partial least squares after pretreatment at a proper spectral range. Calibration models for the NIRS measurement yielded high determination coefficients (R²) of 0.959, 0.859, 0.893 and 0.821, respectively. Experimental results show that the established method was able to rapidly and effectively predict the composition and the biodegradability of aquatic plants.