Woody biomass as a potential feedstock for fermentative gaseous biofuel production.

Biogas and biohydrogen are compatible gaseous biofuels that can be blended with natural gas for reticulated fuel supply to reduce greenhouse gas emissions. Sustainably grown woody biomass is emerging as a potential feedstock in the production of biofuels. Woody biomass is widely available, uses non-arable land for plantation, does not require synthetic fertilisers to grow and acts as a carbon sink. The cellulose and hemicellulose fractions of wood are renewable sources of sugars that can be used for fermentative production of gaseous biofuels. However, widespread use of wood as a gaseous biofuel feedstock is constrained due to the recalcitrant nature of wood to enzymatic hydrolysis. Pretreatment makes cellulose and hemicellulose accessible to microbial enzymes to produce fermentable sugars. Here we review wood composition, its structure and different pretreatment techniques in the context of their effects on deconstruction of wood to improve hydrolysis and fermentative gaseous fuel production. The anaerobic digestion of pretreated wood for biogas and dark fermentation for biohydrogen production are discussed with reference to gas yields. Key advancements in lab-scale research are described for pretreatments and for pure, co- and mixed culture fermentations. Limitations to yield improvements are identified and future perspectives and prospects of gaseous biofuel production from woody biomass are discussed, with reference to new developments in engineered biocatalysts and process integration.

Mixed culture polyhydroxyalkanoate (PHA) synthesis from nutrient rich wet oxidation liquors.

Organic waste residues can be hydrothermally treated to produce organic acid rich liquors. These hydrothermal liquors are a potential feedstock for polyhydroxyalkanoate (PHA) production. We investigated the effect of dissolved oxygen concentration and substrate feeding regimes on PHA accumulation and yield using two hydrothermal liquors derived from a mixture of primary and secondary municipal wastewater treatment sludge and food waste. The enriched culture accumulated a maximum of 41% PHA of cell dry weight within 7 h; which is among the highest reported for N and P rich hydrothermal liquors. Recovered PHA was 77% polyhydroxybutyrate and 23% polyhydroxyvalerate by mass. The families Rhodocyclaceae (84%) and Saprospiraceae (20.5%) were the dominant Proteobacteria (73%) in the enriched culture. The third most abundant bacterial genus, Bdellovibrio, includes species of known predators of PHA producers which may lead to suboptimal PHA accumulation. The PHA yield was directly proportional to DO concentration for ammonia stripped liquor (ASL) and inversely proportional to DO concentration for low strength liquor (LSL). The highest yield of 0.50 Cmol PHA/Cmol substrate was obtained for ASL at 25% DO saturation. A progressively increasing substrate feeding regime resulted in increased PHA yields. These findings demonstrate that substrate feeding regime and oxygen concentration can be used to control the PHA yield and accumulation rate thereby enhancing PHA production viability from nutrient rich biomass streams.

Application of hydrothermal treatment to affect the fermentability of Pinus radiata pulp mill effluent sludge.

A hybrid technique incorporating a wet oxidation stage and secondary fermentation step was used to process Pinus radiata pulp mill effluent sludge. The effect of hydrothermal oxidation at high temperature and pressure on the hydrolysis of constituents of the waste stream was studied. Biochemical acidogenic potential assays were conducted to assess acid production resulting from anaerobic hydrolysis of the wet oxidised hydrolysate under acidogenic conditions. Significant degradation of the lignin, hemicellulose, suspended solids, carbohydrates and extractives were observed with wet oxidation. In contrast, cellulose showed resistance to degradation under the experimental conditions. Extensive degradation of biologically inhibitory compounds by wet oxidation did not show a beneficial impact on the acidogenic or methanogenic potential compared to untreated samples.

Transformation and removal of wood extractives from pulp mill sludge using wet oxidation and thermal hydrolysis.

In order to remove wood extractive compounds from pulp mill sludge and thereby enhancing anaerobic digestibility, samples were subjected to either oxidative hydrothermal treatment (wet oxidation) or non-oxidative hydrothermal treatment (thermal hydrolysis). Treatments were carried out at 220 °C with initial pressure of 20 bar. More than 90% destruction of extractive compounds was observed after 20 min of wet oxidation. Wet oxidation eliminated 95.7% of phenolics, 98.6% fatty acids, 99.8% resin acids and 100% of phytosterols in 120 min. Acetic acid concentration increased by approximately 2 g/l after 120 min of wet oxidation. This has potential for rendering sludge more amenable to anaerobic digestion. In contrast thermal hydrolysis was found to be ineffective in degrading extractive compounds. Wet oxidation is considered to be an effective process for removal of recalcitrant and inhibitive compounds through hydrothermal pre-treatment of pulp mill sludge.

Mechanisms of manganese removal from wastewaters in constructed wetlands comprising water hyacinth (Eichhornia crassipes (Mart.) Solms) grown under different nutrient conditions.

This article discusses key mechanisms involved in removing 1 mg/L Mn from synthetic wastewaters in constructed wetlands comprising water hyacinth (Eichhornia crassipes (Mart.) Solms) grown under different nutrient levels of 1-fold (28 mg/L and 7.7 mg/L of total nitrogen and total phosphorus, respectively), 2-fold, 1/4-fold, and 1/8-fold. A mass balance was carried out to evaluate the key removal mechanisms. Phytoremediation mainly due to phytoextraction substantially contributed to manganese removal. However, chemical precipitation was absent, suggesting that manganese has a higher solubility in the given average pH (6.2 to 7.1) conditions in constructed wetlands. Bacterial mediated immobilization mechanisms also did not contribute to manganese removal. Sediments constituted a minor sink to manganese, implying that manganese has a poor adsorption potential. Constructed wetlands comprising water hyacinth are effective at removing manganese from wastewaters despite the fact that the plants are grown under higher or lower nutrient conditions.

Contribution of water hyacinth (Eichhornia crassipes (Mart.) Solms) grown under different nutrient conditions to Fe-removal mechanisms in constructed wetlands.

Severe contamination of water resources including groundwater with iron (Fe) due to various anthropogenic activities has been a major environmental problem in industrial areas of Sri Lanka. Hence, the use of the obnoxious weed, water hyacinth (Eichhornia crassipes (Mart.) Solms) in constructed wetlands (floating aquatic macrophyte-based plant treatment systems) to phytoremediate Fe-rich wastewaters seems to be an appealing option. Although several studies have documented that hyacinths are good metal-accumulating plants none of these studies have documented the ability of this plant grown under different nutrient conditions to remove heavy metals from wastewaters. This paper, therefore, reports the phytoremediation efficiencies of water hyacinth grown under different nutrient conditions for Fe-rich wastewaters in batch-type constructed wetlands. This study was conducted for 15 weeks after 1-week acclimatization by culturing young water hyacinth plants (average height of 20+/-2cm) in 590L capacity fiberglass tanks under different nutrient concentrations of 1-fold [28 and 7.7mg/L of total nitrogen (TN) and total phosphorous (TP), respectively], 2-fold, 1/2-fold, 1/4-fold and 1/8-fold with synthetic wastewaters containing 9.27Femg/L. Another set-up of hyacinths containing only Fe as a heavy metal but without any nutrients (i.e., 0-fold) was also studied. A mass balance was carried out to investigate the phytoremediation efficiencies and to determine the different mechanisms governing Fe removal from the wastewaters. Fe removal was largely due to phytoremediation mainly through the process of rhizofiltration and chemical precipitation of Fe2O3 and FeOH3 followed by flocculation and sedimentation. However, chemical precipitation was more significant especially during the first 3 weeks of the study. Plants grown in the 0-fold set-up showed the highest phytoremediation efficiency of 47% during optimum growth at the 6th week with a highest accumulation of 6707Femg/kg dry weight. Active effluxing of Fe back to the wastewater at intermittent periods and with time was a key mechanism of avoiding Fe phytotoxicity in water hyacinth cultured in all set-ups. Our study elucidated that water hyacinth grown under nutrient-poor conditions are ideal to remove Fe from wastewaters with a hydraulic retention time of approximately 6 weeks.

Biogas production from water hyacinth (Eichhornia crassipes (Mart.) Solms) grown under different nitrogen concentrations.

This paper reports the biogas production from water hyacinth (Eichhornia crassipes (Mart.) Solms) grown under different nitrogen concentrations of 1-fold [28 mg/L of total nitrogen (TN)], 2-fold, 1/2-fold, 1/4-fold and 1/8-fold and plants harvested from a polluted water body. This study was carried out for a period of 4 months at ambient mesophilic temperatures of 30.3-31.3 degrees C using six 3-barreled batch-fed reactors with the innermost barrel (45 L) being used as the digester. There was no marked variation in the C/N ratios of the plants cultured under different nitrogen concentrations. The addition of fresh cow dung having a low C/N of 8 resulted in a significant reduction in the C/N ratios of the water hyacinth substrates. However, gas production commenced 3 days after charging the reactors and gas production rates peaked in 4-7 days. The volatile solids (VS) degradation and gas production patterns manifested that in conventional single-stage batch digesters acidogenesis and methanogenesis of water hyacinth requires a retention time of around 27-30 days and 27-51 days, respectively. Substrates in the f-1 digester (i.e., the digester containing plants grown under 28 TN mg/L) having the lowest VS content of 45.3 g/L with a highest C/N ratio of 16 showed fairly higher gas production rates consistently (10-27 days) with higher gas yields containing around 50-65% of CH4 (27-51 days). Moreover the highest overall VS (81.7%) removal efficiencies were reported from the f-1 digester. Fairly higher gas production rates and gas yields with fairly higher CH4 contents were also noticed from the f-2 digester containing substrates having a C/N of 14 and f-out digester (containing the plants harvested from the polluted water body) having the lowest C/N ratio of 9.7 with a fairly high VS content of 56 g/L. CH4 production was comparatively low in the f-1/8, f-1/4 and f-1/2 digesters having VS rich substrates with varying C/N ratios. We conclude that water hyacinth could be utilized for biogas production irrespective of the fact that the plants are grown under higher or lower nitrogen concentrations and that there is no necessity for the C/N ratio to be within the optimum range of 20-32 required for anaerobic digestion. Further it is concluded that several biochemical characteristics of the substrates significantly influences biogas production besides the C/N ratio.

Removal of aluminium by constructed wetlands with water hyacinth (Eichhornia crassipes (Mart.) Solms) grown under different nutritional conditions.

This article reports the phytoremediation efficiencies of water hyacinth (Eichhornia crassipes (Mart.) Solms) grown under different nutritional conditions for Al rich wastewaters in batch type constructed wetlands (floating aquatic macrophyte-based plant treatment systems). This study was conducted for 15 weeks after 1 week acclimatization by culturing young water hyacinth (average height of 20 +/- 2 cm) in 590 L capacity fiberglass tanks under different nutrient concentrations of 2-fold [56 and 15.4 mg/L of total nitrogen (TN) and total phosphorous (TP), respectively], 1-fold, 1/2-fold, 1/4-fold and 1/8-fold with synthetic wastewaters containing 5.62 Al mg/L. A control set-up of hyacinths comprising only Al with no nutrients was also studied. A mass balance was carried out to investigate the phytoremediation efficiencies and to identify the different Al removal mechanisms from the wastewaters. Chemical precipitation of Al(OH)3 was a dominant contribution to Al removal at the beginning of the study, whereas adsorption of Al3+ to sediments was observed to be a predominant Al removal mechanism as the study progressed. Phytoremediation mainly due to rhizofiltration was also an important mechanism of Al removal especially during the first 4 weeks of the study in almost all the set-ups. However, chemical precipitation and sediment adsorption of Al3+ was a dominant contribution to Al removal in comparison with phytoremediation. Plants cultured in the control set-up showed the highest phytoremediation efficiency of 63% during the period of the 4th week. A similar scenario was evident in the 1/8-fold set-up. Hence we conclude that water hyacinth grown under lower nutritional conditions are more ideal to commence a batch type constructed wetland treating Al rich wastewaters with a hydraulic retention time of approximately 4 weeks, after which a complete harvesting is recommended.

Effects of substrate loading rate on biofilm structure.

The effects of substrate surface loading rate on biofilm growth and structure were investigated by chemical, biochemical and microscopic methods. Three tubular reactors were operated at equal C:N ratio of 0.1, with substrate loading rates of 1.2, 0.6 and 0.3g-C/m(2)/day. Substrate loading positively influenced the biofilm growth rate. Denser biofilms with lower porosities were formed at higher substrate loading. Slowly growing biofilms having porous structures were found to have higher specific activities. Nitrification was suppressed under the higher substrate loading conditions even at the equal C:N ratio of 0.1, thus proving that the spatial competition between nitrifiers and heterotrophs as one limiting criteria for stable nitrification. The spatial organization of the ammonia oxidizers was biofilm structure related. The strain variability of ammonia oxidizers was substrate loading dependent. These findings suggest that substrate loading is a key parameter in determining biofilm structure and function.