| As an aquatic plant and a resource for energy production, water hyacinth (Eichhornia crassipes) has many advantages, such as fast growth, rapid reproduction and environmental benefits for extracting nutrients and heave metals from contaminated water. Furthermore, unlike land energy crops, water hyacinth doesn’t occupy land due to growth in water and doesn’t need fertilization because it can take up nutrients from water and purifies the water at the same time. Therefore, water hyacinth is a potential and competitive bioenergy feedstock in the time of bioenergy.A set of experiments in both lab-scale and pilot-scale were conducted to investigate the characteristics of water hyacinth anaerobic fermentation, including the influences of water source, harvest frequency, temperature, inoculum, addition of trace metals and pre-acidification on water hyacinth anaerobic digestion. The anaerobic digestion characteristics of root, stem and leaf of water hyacinth were also discussed. Microbial diversities in anaerobic fermentation slurry were studied with PCR-denaturing gradient gel electrophoresis (DGGE). Because water hyacinth has very low density, it is difficult to digest water hyacinth directly because of floating scum, pipe clogging and longer hydraulic retention time. To solve these difficulties, a novel technology (water hyacinth is squeezed into juice and solid residue, and then they are used as feedstock for anaerobic digestion, respectively.) was established. In the last part of the paper, environmental impacts of electricity production from water hyacinth, including solid-liquid separation, anaerobic digestion, composting of digestion residue and utilization of digestrate and compost, were assessed with life cycle assessment (LCA) via a case study in the experimental station of Jiangsu Academy of Agricultural Sciences, located in Changzhou city, Jiangsu province.The results showed as follows:1. To investigate the influences of N and P in water, water hyacinth from Baishan Bay of the Dianchi Lake, Caohai of the Dianchi Lake, Taihu Lake and Pond 2 of Jiangsu Academy of Agricultural Sciences was collected, and mesophilic batch anaerobic experiments were performed. The results indicated that there was a significant correlation between methane productivity and N, P in water where water hyacinth grew. N and P were the highest in Caohai, and its water hyacinth had the highest CH4 yield of 264 mL/gVS, which was 70% of the theoretical methane production. Water hyacinth from Baishan Bay, which contained the lowest N and P, produced the lowest CH4 of 172 mL/gVS, only 45% of its theoretical methane production. The concentrations of N and P in Taihu Lake were close to those in Pond 2, and water hyacinths from the two sites had similar methane yield of 229 mL CH4/gVS and 235 mL CH4/gVS, and 68% and 66% of the theoretical methane production, respectively. Cellulose, hemicellulose and crude protein were the major components of water hyacinth converted into biogas. The degradation rate of the three components in water hyacinth from Baishan Bay were the lowest. Acetate and propionate were the main volatile fatty acids (VFA) produced in anaerobic digestion of water hyacinth. Water hyacinth from Caohai produced the highest VFA, up to 2466 mg/L, while the water hyacinth from Baishan Bay yielded the lowest VFA, only 915 mg/L. Our results suggest that N and P in water contribute to the differences in structure (like ratio of root to shoot) and composition of water hyacinth, and eventually cause the differences in biodegradability and biogas productivity of water hyacinth.2. To assess the influence of root-shoot ratio of water hyacinth on biogas production, anaerobic digestion characteristics of root, stem and leaf of water hyacinth were compared. Results revealed that the composition and biodegradability were significant different among root, stem and leaf of water hyacinth. Leaf, which had the highest contents of total organic carbon (TOC) and protein, produced the highest methane with a yield of 246 mL CH4/gVS, while stem produced 177 mL CH4/gVS, and root had the lowest production of 110 mL CH4 /gVS. The anaerobic biodegradability of root was the lowest since it had the most lignin. Either VFA or SCOD production was the lowest in root. The degradation rates of cellulose and semi-cellulose in stem and leaf were two times more than those in roots. PCR-DGGE results suggested that the bacteria community structure was quite different in the anaerobic sludge from root, stem and leaf. The sludge from the leaves displayed the highest diversity of bacteria, whereas that from roots had the lowest diversity. There was little similarity in bacteria total number among different organs of water hyacinth, but the archaea population was relatively similar in all organs. Whether bacteria or archaea, the microbial community population in root sludge was quite different from that in stem and leaf.3. To investigate the influence of harvest frequency on biogas yield of water hyacinth, the anaerobic digestions with the water hyacinths harvested once a year (WH1) and harvested regularly (WH5, harvested 5 times) in a year were compared in batch experiments. Harvest frequency affected the biomass and composition of water hyacinth. Compared with WH5, the contents of lignin in WH1 were higher, crude protein, total nitrogen, total phosphorus, total potassium were lower. The biogas productions of WH1 and WH5 were 231 mL/gTS (266 mL/gVS) and 336 mL/gTS (517mg/gVS), respectively. It indicates that regular harvest not only increases water hyacinth biomass, which is beneficial to removal of N, P from water, but also promotes anaerobic biogas production.4. The effects of temperature, inoculums, trace metals (Fe2+, Co2+, Ni2+) and acidification pretreatment were examined. The results indicated that methane production increased with an increase in inoculating ratio and the optimal ratio was 1:1. Methane formation was faster at 55℃than 35℃, but total methane production and the rate of substrate biodegradability were similar at both temperatures. The addition of trace metals of Fe2+, Co2+, Ni2+, could increase the total methane production in the initial 7 days of fermentation, and the effect on the total biogas production was Fe2+>Co2+>Ni2+ Acidification pretreatment could increase biogas production, and the biogas production from water hyacinth with a three-day pre-acidification increased by 10.4% of fresh water hyacinth.5. To evaluate water hyacinth and water hyacinth juice as feedstock of anaerobic digestion, the anaerobic digestions of these two feedstocks were compared in two CSTR reactors in laboratory. The results showed that, when water hyacinth was used as feedstock, the suitable organic loading rate (OLR) was 2.0 kgVS/m3/d, the hydraulic retention time (HRT) was 27 d, the volume biogas yield was 0.61 m3/m3/d, the biogas production rate was 267 mL/gVS, average methane content was 58%. However, when the juice of water hyacinth was fed as substrate for biogas, the volume biogas yield and the OLR were 1.4 m3/m3/d and 6 kg COD/m3/d, respectively. HRT could be reduced to 2.4 d and the reduction of COD and MLVSS were over 85% and 88%, respectively. PCR-DGGE analysis displayed that OLR had little effect on anaerobic microbial diversity during the anaerobic process of water hyacinth juice, but affected their population structure, especially the structure of methanogenic bacteria. This study has demonstrated that the juice of water hyacinth is an ideal substrate for methane production, and CSTR anaerobic digestion system is suitable for its effective conversion.6. Biogas production of the solid residue from squeezing water hyacinth was estimated in both lab-scale and pilot-scale level. The results of a batch experiment in lab-scale showed that water hyacinth residue could produce biogas of 398 mL/gTS (445 mL/gVS), which was equal to the biogas yield produced by fresh water hyacinth, methane content reached 59% of biogas. The results of three batches at pilot scale revealed that 75% of biogas production potential could be obtained at normal temperature after 30 d of anaerobic fermentation, and the reactor start-up was quick, the highest volume biogas yield could reach 0.6 m3/m3/d, and the average volume biogas yield was 0.25 m3/m3/d.7. Life cycle assessment was employed to establish the inventory of emission and to assess life cycle environmental impacts on electricity generation from water hyacinth. The impact indexes of global warming, acidification and eutrophication were 2.1×10-3, 4.89×10-2, and 1.98×10-1, respectively. In comparison with conventional fossil fuels and energy crop, Electricity generation from water hyacinth emitted much fewer pollutants like CO2, SOx, NOx. Life-cycle efficiency for producing 1 MWh power via water hyacinth conversion system was 0.09, which was lower than that via energy crop conversion system. The water hyacinth conversion system had little adverse effect on environment. In the course of water hyacinth anaerobic digestion for electricity power, consumption of fossil energy and ammonia volatilization during composting and field utilization of anaerobic digestion residue and digestrate were the two key factors affecting life-cycle efficiency and environmental impact indices.In conclusion, water hyacinth is an ideal feedstock for bioenergy. It is proven to be feasible that water hyacinth is squeezed into juice and residue, and then the juice and residue are anaerobic digested respectively for producing electricity power. This is a more economical, more efficient, low environmental impact, novel way for biomass energy production in the future. |
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