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Lignocellulosic materials (LMs) are the most abundant residues on the planet and have a huge potential for methane production. Several strategies have been tested to enhance the methane potential of LMs, with a particular emphasis on environmentally friendly and economically convenient pretreatments. This chapter revisits the potential of two chemical, i.e. organosolv and N-methylmorpholine N-oxide (NMMO)-driven, and one physical, i.e. ultrasounds, pretreatments. Organosolv pretreatment enables to obtain a pure lignin fraction from LMs, leaving most of the fermentable sugars in the solid matrix. The result is a lignin-poor material with an increased porosity and a higher bioavailability of the sugar fraction. Another advantage is the cost-effectiveness and the easy recovery of the chemicals involved. NMMO pretreatment focuses on the cellulosic component of the biomass, aiming to reduce its crystallinity and to increase the porosity of the substrate. The main advantage of NMMO lies in its high recovery percentage, which reaches up to 99%. Ultrasound pretreatment involves ultrasonic waves that allow fractionating LMs, breaking the linkages between lignin, cellulose and hemicellulose, generally leaving cellulose and most of the hemicellulose in the solid fraction and dissolving the lignin in the liquor. Ultrasound pretreatment does not require chemicals and can be easily combined with other pretreatment methods to enhance its effectiveness.

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This research investigated the effects of two different chemical pretreatments on the methane yields of the lignocellulosic material rice straw, which is one of the most abundant agricultural wastes worldwide. Initially, the raw agricultural residue was pretreated with the organic solvent N-methylmorpholine-N-oxide (NMMO) at 120°C for 3 h. Consequently, batch bio-methane production (BMP) tests were performed under mesophilic conditions for 40 days. In a similar experimental run, the effect of organosolv pretreatment on the rice straw was investigated. 50% ethanol was used as organic solvent and the pretreatment was carried out at 150 and 180°C for 30 min. Furthermore, this study focused in improving the rice straw biodegradability through the addition of trace elements. Iron, cobalt, nickel and selenium were added to the rice straw at different concentrations and in different combinations. Both NMMO and organosolv pretreatments were particularly effective in enhancing the biodegradability of the rice straw and, consequently, increasing the biogas yields up to 82 and 42%, respectively, compared to those of the untreated material. On the other hand, the addition of trace elements had only a minor effect (i.e. up to 8%) on the enhancement of the biogas yield from the AD of rice straw.

This study investigated the effects of two different chemical pretreatments on the methane yields of three lignocellulosic materials (LMs), namely hazelnut skin and cocoa shells, generated during the industrial processing of the raw agricultural products, and rice straw, which is one of the most abundant agricultural wastes worldwide. Initially, the three LMs were separately pretreated with an organic solvent, called N-Methylmorpholine-N-oxide (NMMO), at 120°C for 3 h. Consequently, batch bio-methane production (BMP) tests were performed under mesophilic (i.e. 37 ± 2°C) conditions for 40 days. In a similar experimental run, the effects of organosolv pretreatment on the three LMs were investigated. 50% ethanol was used as organic solvent and the pretreatment was carried out at 150 and 180°C for 30 min. Both NMMO and organosolv pretreatments were particularly effective for rice straw, enhancing the biodegradability of the LM and, consequently, increasing the biogas yields compared to those of the untreated material.

A local sensitivity analysis was performed for a S0-driven two-step denitrification model, accounting for NO2 - accumulation, biomass growth and S0 solubilization. The model sensitivity was aimed at verifying the model stability, understanding the identifiability of the model structure and evaluating the model parameters to be further optimized. The sensitivity analysis identified the mass specific area of the sulfur particles (a*) and hydrolysis kinetic constant (k1) as the dominant parameters. Additionally, the maximum growth rate of the denitrifying biomass on NO3 - (μmax 2,3) and NO2 - (μmax 2,4) were detected as the most sensitive kinetic parameters. Further calibration would be performed for the sensitive model parameters to optimize the quality of the model.

This study focuses on the biorecovery of iron and other metals, simultaneously with nitrogen removal from wastewaters, by means of an innovative bioprocess which is ferrous iron-mediated autotrophic denitrification. Biological oxidation with metal recovery is a promising and cost-effective alternative to the conventional chemical processes of neutralization followed by precipitation/sedimentation for metal-containing water treatment. Nowadays, the need of metals is increasing but the natural high-grade ores are rapidly decreasing (Hoque et al., 2011). In addition, heavy metal pollution is one of the biggest environmental issues (Fu et al., 2011) mainly due to the discharge of contaminated industrial and mining wastewaters into the hydrosphere (Jadhav et al., 2013). As an example, an environmental catastrophe recently occurred and is still taking place in Brazil’s mineral-rich state of Minas Gerais, due to the collapse of two mining dams having serious consequences for the population such as the contamination of the water supplies. Environmental regulations have become more stringent and require eco-friendly technologies, with low cost and low energy requirements while giving high removal yields (Hoque et al., 2011; Jadhav et al., 2013). Therefore, biorecovery of metals from wastewater is an economic and profitable solution for the high demand of metals and the reduction of the mining impact. Among metals, iron is one of the most important and the processing of iron for industrial purposes accounts for 95% of worldwide metal production (Ilbert et al., 2013). The discharge of nitrogen compounds into the water resources is another serious environmental problem. Metal and nitrogen contamination can be simultaneously present in mining processing waters and acid mine drainage (AMD). Denitrification can be used for nitrogen removal, as it converts nitrate to nitrogen gas (Papirio et al., 2014) and, recently, chemoautotrophic denitrifiers capable of coupling ferrous iron oxidation to nitrate reduction have gained scientific interest (Di Capua et al., 2015). Fe(II) oxidation results in the production of ferric iron precipitates or co-precipitates with other metals that can be subsequently removed and recovered. Within this study, the feasibility of maintaining denitrification with different microbial cultures was initially investigated by using batch bioassays. A Thiobacillus denitrificans-dominated mixed culture and an activated sludge inoculum were used as their capability in reducing nitrate with Fe(II) as electron donor had been previously demonstrated (Straub et al., 1996; Nielsen and Nielsen, 1998). For both inocula, Fe/NO3- ratio was maintained at 5, with feed Fe(II) and NO3- ranging between 8-16 and 2-5 mM, respectively. Thiosulfate (S2O32-) in concentration of 0.5 mM was occasionally used as an additional electron donor to stimulate the process. Besides the influence of different inocula, experiments were carried out in order to evaluate the optimal values of pH and concentration of EDTA, used to keep Fe(II) in solution. Batch bioassays were performed at room temperature (22 ± 2°C) at feed pH of 7 and 8 using 125 mL serum bottles, flushed with He, aseptically sealed and placed on a gyratory shaker. Microcosms were prepared in triplicate and, in addition, more bottles were prepared for electron-donor free and abiotic controls. The results showed that the process was strongly pH dependent with the highest efficiency achieved at pH 7. An initial chemical Fe(II) oxidation of approximately 30% was observed in all the tests in the first 6 h of incubation. When Thiobacillus denitrificans-dominated cultures were used, denitrification coupled to Fe(II) oxidation was maintained only when thiosulfate was supplemented as an additional electron donor. After 7 d, ferrous iron oxidation was complete with a maximum nitrate removal of 70% demonstrating that S2O32- supplementation was only needed in order to start-up the process. Fe(II) oxidation rate averagely remained at 1mM/d. In the bottles seeded with the activated sludge inoculum, nitrate reduction occurred with Fe(II) as sole electron donor. Under these conditions, denitrification was slower resulting in 50% of Fe(II) oxidation and nitrate reduction in 10 d of incubation. Finally, the microbial activity of the two previously described mixed cultures will be compared with that of a pure culture of Pseudogulbenkiania sp. strain 2002. Subsequently, the most performing microbial incubation will be seeded in two moving bed biological reactors (MBBRs) in order to optimize the operating parameters of the process under continuous-flow and steady-state conditions.

This study developed an integrated bioprocess comprising a continuous stirred tank reactor (CSTR) and a gravity settler for precipitative removal of Fe(III) from hydrometallurgical processing at ambient temperature. CSTR was fed with the raffinate solution (pH 1.4) from Zijinshan copper heap bioleaching plant containing nutrients and trace elements. In the CSTR, Fe(II) was biologically oxidized and pH was controlled by using KOH and CaCO3. The solution redox potential and pH was maintained at 580-600 mV (vs Ag/AgCl) and 2.5-3.0, respectively. The oxidation rate of Fe(II) was 1.0 g/L.h. The sludge in the gravity settler was recycled to CSTR as jarosite seeds and carrier material for microbial cells. Iron and sulfate removal efficiencies were 52 and 21%, and 86 and 32% at pH 2.6 and 3.0, respectively. The precipitate was characterized by X-ray diffraction (XRD) and elemental analysis, indicating the formation of mainly jarosite in the gravity settler.