Liquefaction of lignocellulosic biomass for methane production: A review.

Hydrothermal pretreatment (HTP) (Hot water extraction (HWE) and steam pretreatment) and pyrolysis have the potential to liquefy lignocellulosic biomass. HTP produces hydrolysate, consisting mainly of solubilized hemicellulose, while pyrolysis produces aqueous pyrolysis liquid (APL). The liquid products, either as main products or by-product, can be used as anaerobic digestion (AD) feeds, overcoming shortcomings of solid-state AD (SS-AD). This paper reviews HWE, steam pretreatment, and pyrolysis pretreatment methods used to liquefy lignocellulosic biomass, AD of liquefied products, effects of inhibition from intermediate by-products such as furan and phenolic compounds, and pretreatment tuning to increase methane yield. HTP, focusing on methane production, produces less inhibitory compounds when carried out at moderate temperatures. APL is a challenging feed for AD due to its complexity, including various inhibitory substances. Pre-treatment of biomass before pyrolysis, adaptation of microorganism to inhibitors, and additives, such as biochar, may help the AD cultures cope with inhibitors in APL.

Settling velocity and size distribution measurement of anaerobic granular sludge using microscopic image analysis.

Settling velocity and size distribution of anaerobic granular sludge samples were studied using microscopic image analysis and settling column experiments. Five granule samples were considered in this study. Three samples were collected at the Top, Middle and Bottom sections of a lab scale upflow anaerobic sludge bed reactor (UASB). Two other granule samples were obtained from industries. This paper aims to establish a method that uses microscopic image analysis and shape factor as a tool to determine the size distribution and settling velocity of anaerobic granules. Image analysis technique was used to calculate the shape factor and equivalent diameter of granules. The equivalent diameter was then used to calculate the theoretical settling velocities based on Allen's formula and estimate size distributions. The results showed that there was a good agreement between the theoretical and experimental mean settling velocity values. Both measured and calculated settling velocities increased with increasing Reynolds number (Re). However, the agreement between measured and calculated values was found to be weaker at higher Re values. Size distribution analyses of the granules have revealed that there was significant difference in the size distribution of granule samples collected at different heights of the lab scale reactor. Overall, granules from the bottom section of the reactor had larger diameter, settling velocity and shape factor than those at the middle and top section granules. Whereas granules collected from the top section exhibited the smallest granular diameter, settling velocity and shape factor.

Mapping anaerobic sludge bed community adaptations to manure supernatant in biogas reactors.

In upflow anaerobic sludge bed (UASB) reactors, biomass present as granules allows for long solids retention time. Here, granules from a process treating pulp and paper industrial wastewater were successfully applied as inoculum in UASB reactors treating pig manure supernatant, despite high particle content and high ammonium concentrations in the influent. We did a detailed characterization of archaeal and bacterial communities associated with the inoculum and with the aggregated and dispersed fractions of the influent and the reactors after one year of operation. The granular communities underwent major changes and adapted to the highly distinct conditions without disintegration of the granules. Although the granules persisted in the reactors, non-granular aggregates accumulated, and partly replaced the granules. Particles introduced to the reactors by the pig manure influent apparently contributed both as food and biofilm growth support. Archaeal communities in the dispersed reactor phase were similar to those dispersed in the influents, implying successful retention and little loss of archaeal biomass due to detachment or disintegration of granules and other aggregates. Unique bacterial communities developed in the dispersed fraction of the reactors despite of low hydraulic retention times. They probably consisted of fast growing organisms consuming readily degradable organic matter.

Anaerobic digestion of pig manure supernatant at high ammonia concentrations characterized by high abundances of Methanosaeta and non-euryarchaeotal archaea.

We examined the effect of ammonium and temperature on methane production in high rate upflow anaerobic sludge bed reactors treating pig manure supernatant. We operated four reactors at two ammonium concentrations ('low' at 1.9, 'high' at 3.7 g L-1, termed LA and HA reactors, respectively) and at variable temperatures over 358 days. Archaeal and bacterial communities were characterized by Illumina sequencing of 16S rRNA amplicons. Ammonium was a major selective factor for bacterial and archaeal community structure. After ~200 days of adaptation to high ammonium levels, acetate and propionate removal and methane production improved substantially in HA reactors. Aceticlastic Methanosaeta was abundant and positively correlated to methane yield in the HA reactors, whereas Methanosarcina was more abundant in LA reactors. Furthermore, a group of monophyletic OTUs that was related to Thaumarchaeota in phylogenetic analysis was highly abundant in the archaeal communities, particularly in the HA reactors. The most abundant bacterial OTU in LA reactors, representing Syntrophomonadaceae, was also positively correlated to methane yield in the HA reactors, indicating its importance in methane production under ammonia stress. In conclusion, efficient methane production, involving aceticlastic methanogenesis by Methanosaeta took place in the reactors at free ammonia concentrations as high as 1 g L-1.

High rate manure supernatant digestion.

The study shows that high rate anaerobic digestion may be an efficient way to obtain sustainable energy recovery from slurries such as pig manure. High process capacity and robustness to 5% daily load increases are observed in the 370 mL sludge bed AD reactors investigated. The supernatant from partly settled, stored pig manure was fed at rates giving hydraulic retention times, HRT, gradually decreased from 42 to 1.7 h imposing a maximum organic load of 400 g COD L(-1) reactor d(-1). The reactors reached a biogas production rate of 97 g COD L(-1) reactor d(-1) at the highest load at which process stress signs were apparent. The yield was ∼0.47 g COD methane g(-1) CODT feed at HRT above 17 h, gradually decreasing to 0.24 at the lowest HRT (0.166 NL CH4 g(-1) CODT feed decreasing to 0.086). Reactor pH was innately stable at 8.0 ± 0.1 at all HRTs with alkalinity between 9 and 11 g L(-1). The first stress symptom occurred as reduced methane yield when HRT dropped below 17 h. When HRT dropped below 4 h the propionate removal stopped. The yield from acetate removal was constant at 0.17 g COD acetate removed per g CODT substrate. This robust methanogenesis implies that pig manure supernatant, and probably other similar slurries, can be digested for methane production in compact and effective sludge bed reactors. Denaturing gradient gel electrophoresis (DGGE) analysis indicated a relatively fast adaptation of the microbial communities to manure and implies that non-adapted granular sludge can be used to start such sludge bed bioreactors.

Effects of psychrophilic storage on manures as substrate for anaerobic digestion.

The idea that storage can enhance manure quality as substrate for anaerobic digestion (AD) to recover more methane is evaluated by studying storage time and temperature effects on manure composition. Volatile fatty acids (VFA) and total dissolved organics (CODs) were measured in full scale pig manure storage for a year and in multiple flasks at fixed temperatures, mainly relevant for colder climates. The CODs generation, influenced by the source of the pig manure, was highest initially (0.3 g COD L(-1)d(-1)) gradually dropping for 3 months towards a level of COD loss by methane production at 15°C. Methane emission was low (<0.01 g COD L(-1)d(-1)) after a brief initial peak. Significant CODs generation was obtained during the warmer season (T > 10°C) in the full scale storage and almost no generation at lower temperatures (4-6°C). CODs consisted mainly of VFA, especially acetate. All VFAs were present at almost constant ratios. The naturally separated manure middle layer without sediment and coarser particles is suitable for sludge bed AD and improved further during an optimal storage time of 1-3 month(s). This implies that high rate AD can be integrated with regular manure slurry handling systems to obtain efficient biogas generation.