Volatile fatty acids (VFAs) are intermediate products in anaerobic digestion. The effect of substrate loading or inoculum to substrate ratio (ISR), the addition of methanogen inhibitor, O₂ presence, control the reactor's pH, and inoculum adaptation on the VFAs production from food waste through acidogenesis process was investigated in this study. Addition of 2-bromoethane sulfonic (BES) as methanogen inhibitor suppressed VFA consumption by methanogens at ISR 1:1. At higher substrate loading (ISR 1:3), methane production can be suppressed even without the addition of BES. However, at high substrate loading, controlling the pH during acidogenesis is important to achieve high VFAs yield. Acclimatization of inoculum is also one of the strategies to achieve high VFA yield. The highest VFAs yield obtained in this work was 0.8 g VFA/g VS _added at ISR 1:3, controlled pH at 6, with the presence of initial O₂ (headspace unflushed).

Second-generation feedstocks are a potential source of fermentable sugars; they offer great advantages in reducing the cost of production, greenhouse gas emissions, and competition with food and agricultural lands. Fermentation processes for production of different biofuels from these feedstocks differ but the prefermentation processes are similar. Effective fermentation processes are a prerequisite for biofuel production in large capacity as well as making its production economically competitive with the price of crude oil. This chapter addresses various fermentation processes for biofuels; major pathways, fermenting microorganisms, challenges associated with the specific fermentation processes, and possible solutions. In addition, process optimization strategies and bioreactor design suitability for effective fermentation processes are described. © 2019 Angelo Basile and Francesco Dalena Published by Elsevier Inc. All rights reserved.

Dry anaerobic co-digestion of citrus wastes (CW) with chicken feather (CF), wheat straw (WS) and manure bedded with straw (MS) was investigated in batch and continuous processes. Experiments were designed with different mixing ratios considering the inhibitory effect of CW, C/N ratio, and total solid content of individual feedstocks. Best mixing ratio (CF:CW:WS:MS) of 1:1:6:0, enhanced methane yield by 14% compared to the expected yield calculated according to the methane yields obtained from the individual fractions. The process performance of this mixture was then investigated in continuous plug flow reactors at different organic loading rates (OLR) with feedstock total solid contents of 21% TS (RTS21) and 32% TS (RTS32). At OLR of 2 gVS/L/d, a methane yield of 362 NmlCH4/gVSadded was obtained from RTS21, which is 13.5% higher than the yield obtained from RTS32 (319 NmlCH4/gVSadded). However, it was not possible to achieve a stable process when the OLR was further increased to 3.8 gVS/L/d; there were increased total VFAs concentrations and a decline in the biogas production.

Biogas technology has been used quite extensively to generate renewable energy from organic wastes while also recycling nutrients in the wastes and reducing harmful emissions. However, the challenges of low biogas yield from recalcitrant and inhibitory solid wastes together with high construction and operation costs of bioreactors have impeded optimal performance through this process. Additionally, solid organic wastes with total solids (TS) contents greater than 20 % are produced daily in enormous amounts; treating these wastes in conventional wet anaerobic digestion processes has required the addition of water, leading to large reactor volumes, high energy costs for heating, and costly dewatering processes for the digestate residue. In this study, the challenges mentioned above were addressed by using biological pretreatment and dry anaerobic digestion processes. Pretreatment, non-pretreatment strategies, biogas bioreactors design were also studied since this will aid optimizing its economy.

The suitability of a novel textile bioreactor for biogas production was accessed in dry digestion process (dry-AD) treating manure bedded with straw (22 – 30 %TS of feedstock). The 90-L textile bioreactor was robust and simple to operate; it can be accessed easily by developing countries where required expertise may not be available. Methane yield from the manure with straw was 290 NmlCH₄/gVS using acclimatised bacteria; the digestate residue was affirmed suitable as bio-fertiliser. The efficiency of continuous plug flow reactor for dry anaerobic digestion of manure bedded with straw was also investigated at 22 %TS. Organic loading rates up to 4.2 gVS/L/d with retention time of 40 days gave better process stability.

Recalcitrant structure of chicken feather wastes was altered using Bacillus sp. C4 (Bacillus pumilus) and this pretreatment improved methane yield by 124 % compared to the untreated. Considering the fact that easily degraded feedstocks are not highly available and some problems associated with pretreatments: dry anaerobic co-digestion of citrus wastes with chicken feathers, wheat straw and manure bedded with straw, was investigated at 20 %TS in batch process. The best mixing ratio enhanced methane yield by 14 % compared to the expected yield from individual fractions. Process performance at different organic loading rates (OLR) was then investigated in continuous plug flow reactors at 21 %TS and 32 %TS of feedstock. Stability of the process decline as OLR was increased to 3.8 gVS/l/d resulting in high total volatile fatty acids (VFA), VFA/alkalinity ratio and reduction in methane yield.

The aim of this work was to study dry anaerobic digestion (dry-AD) of manure bedded with straw using textile-based bioreactor in repeated batches. The 90-L reactor filled with the feedstocks (22-30% total solid) and inoculum without any further treatment, while the biogas produced were collected and analyzed. The digestate residue was also analyzed to check its suitability as bio-fertilizer. Methane yield after acclimatization increased from 183 to 290NmlCH4/gVS, degradation time decreased from 136 to 92days and the digestate composition point to suitable bio-fertilizer. The results then used to carry out economical evaluation, which shows dry-AD in textile bioreactors is a profitable method of handling the waste with maximum payback period of 5years, net present value from $7,000 to $9,800,000 (small to large bioreactors) with internal rate of return from 56.6 to 19.3%.

In this work, a plug flow reactor was developed for continuous dry digestion processes and its efficiency was investigated using untreated manure bedded with straw at 22% total solids content. This newly developed reactor worked successfully for 230days at increasing organic loading rates of 2.8, 4.2 and 6gVS/L/d and retention times of 60, 40 and 28days, respectively. Organic loading rates up to 4.2gVS/L/d gave a better process stability, with methane yields up to 0.163LCH4/gVSadded/d which is 56% of the theoretical yield. Further increase of organic loading rate to 6gVS/L/d caused process instability with lower volatile solid removal efficiency and cellulose degradation.[on SciFinder (R)]

Biogas or biomethane is traditionally produced via anaerobic digestion, or recently bythermochemical or a combination of thermochemical and biological processes viasyngas (CO and H2) fermentation. However, many of the substrates feedstocks haverecalcitrant structure and difficult to digest (e.g., lignocelluloses or keratins), or theyhave toxic compounds (such as fruit flavors or high ammonia content), or not digestibleat all (e.g., plastics). To overcome these challenges, innovative strategies for enhancedand economically favorable biogas production were proposed in this review. Thestrategies considered are commonly known physical pretreatment, rapid decompression,autohydrolysis, acid- or alkali pretreatments, solvents (e.g. for lignin or cellulose)pretreatments or leaching, supercritical, oxidative or biological pretreatments, as well ascombined gasification and fermentation, integrated biogas production and

Chicken feathers are available in large quantities around the world causing environmental challenges. The feathers are composed of keratin that is a recalcitrant protein and is hard to degrade. In this work, chicken feathers were aerobically pretreated for 2-8 days at total solid concentrations of 5, 10, and 20 % by Bacillus sp. C4, a bacterium that produces both α- and β-keratinases. Then, the liquid fraction (feather hydrolysate) as well as the total broth (liquid and solid fraction of pretreated feathers) was used as substrates for biogas production using anaerobic sludge or bacteria granules as inoculum. The biological pretreatment of feather waste was productive; about 75 % of feather was converted to soluble crude protein after 8 days of degradation at initial feather concentration of 5 %. Bacteria granules performed better during anaerobic digestion of untreated feathers, resulting in approximately two times more methane yield (i.e., 199 mlCH4/gVS compared to 105 mlCH4/gVS when sludge was used). Pretreatment improved methane yield by 292 and 105 % when sludge and granules were used on the hydrolysate. Bacteria granules worked effectively on the total broth, yielded 445 mlCH4/gVS methane, which is 124 % more than that obtained with the same type of inoculum from untreated feather.[on SciFinder (R)]