The effect of biomass density on cellulose solubilisation rates

Biological strategies for enhanced hydrolysis of lignocellulosic biomass during anaerobic digestion: Current status and future perspectives

Lignocellulosic biomass is the most abundant renewable bioresource on earth. In lignocellulosic biomass, the cellulose and hemicellulose are bound with lignin and other molecules to form a complex structure not easily accessible to microbial degradation. Anaerobic digestion (AD) of lignocellulosic biomass with a focus on improving hydrolysis, the rate limiting step in AD of lignocellulosic feedstocks, has received considerable attention. This review highlights challenges with AD of lignocellulosic biomass, factors contributing to its recalcitrance, and natural microbial ecosystems, such as the gastrointestinal tracts of herbivorous animals, capable of performing hydrolysis efficiently. Biological strategies that have been evaluated to enhance hydrolysis of lignocellulosic biomass include biological pretreatment, co-digestion, and inoculum selection. Strategies to further improve these approaches along with future research directions are outlined with a focus on linking studies of microbial communities involved in hydrolysis of lignocellulosics to process engineering.

Hydrolysis rate constants at 10-25 °C can be more than doubled by a short anaerobic pre-hydrolysis at 35 °C

Hydrolysis is the first step of the anaerobic digestion of complex wastewater and considered as the rate limiting step especially at low temperature. Low temperature (10-25 °C) hydrolysis was investigated with and without application of a short pre-hydrolysis at 35 °C. Batch experiments were executed using cellulose and tributyrin as model substrates for carbohydrates and lipids. The results showed that the low temperature anaerobic hydrolysis rate constants increased by a factor of 1.5-10, when the short anaerobic pre-hydrolysis at 35 °C was applied. After the pre-hydrolysis phase at 35 °C and decreasing the temperature, no lag phase was observed in any case. Without the pre-hydrolysis, the lag phase for cellulose hydrolysis at 35-10 °C was 4-30 days. Tributyrin hydrolysis showed no lag phase at any temperature. The hydrolysis efficiency of cellulose increased from 40 to 62%, and from 9.6 to 40% after 9.1 days at 15 and 10 °C, respectively, when the pre-hydrolysis at 35 °C was applied. The hydrolysis efficiency of tributyrin at low temperatures with the pre-hydrolysis at 35 °C was similar to those without the pre-hydrolysis. The hydrolytic activity of the supernatant collected from the digestate after batch digestion of cellulose and tributyrin at 35 °C was higher than that of the supernatants collected from the low temperature (≤25 °C) digestates.

Mitigation of Humic Acid Inhibition in Anaerobic Digestion of Cellulose by Addition of Various Salts

Humic compounds are inhibitory to the anaerobic hydrolysis of cellulosic biomass. In this study, the impact of salt addition to mitigate the inhibitory effects of humic compounds was investigated. The experiment was conducted using batch tests to monitor the anaerobic hydrolysis of cellulose in the presence of humic acid. Sodium, potassium, calcium, magnesium and iron salts were tested separately for their efficiency to mitigate humic acid inhibition. All experiments were done under mesophilic conditions (30 °C) and at pH 7. Methane production was monitored online, using the Automatic Methane Potential Test System. Methane production, soluble chemical oxygen demand and volatile fatty acid content of the samples were measured to calculate the hydrolysis efficiencies. Addition of magnesium, calcium and iron salts clearly mitigated the inhibitory effects of humic acid and hydrolysis efficiencies reached up to 75%, 65% and 72%, respectively, which were similar to control experiments. Conversely, potassium and sodium salts addition did not mitigate the inhibition and hydrolysis efficiencies were found to be less than 40%. Mitigation of humic acid inhibition via salt addition was also validated by inductively coupled plasma atomic emission spectroscopy analyses, which showed the binding capacity of different cations to humic acid.

Microbial community acclimation enhances waste hydrolysis rates under elevated ammonia and salinity conditions

Hydrolysis rates under potentially inhibitory concentrations of ammonia and salinity were investigated for two model feedstocks (manure and food waste). Rates were determined under a range of ammonia and salinity concentrations (1.0-10.0 g TAN [total ammonia nitrogen] L(-1) and 3.9-20.0 g sodium L(-1)) with unacclimated and acclimated microbial inocula. Microbial community changes as a function of acclimation and feedstock were also investigated. Using unacclimated inocula, hydrolysis was found to be severely inhibited for elevated ammonia and salinity (~4 to 10-fold, respectively) or hydrolysis was not detected. However, for inocula acclimated over 2-4 months, statistically significant inhibition generally was not detectable. Molecular analyses demonstrated that microbial community composition changed during acclimation, and bacterial communities under elevated ammonia were distinct from communities under elevated salinity. Feedstock source also had a major influence on bacterial community structure.

Increased temperature in the thermophilic stage in temperature phased anaerobic digestion (TPAD) improves degradability of waste activated sludge

Two-stage temperature phased anaerobic digestion (TPAD) is an increasingly popular method to improve stabilisation of sewage waste activated sludge, which normally has inherently poor and slow degradation. However, there has been limited systematic analysis of the impact of the initial thermophilic stage (temperature, pH and retention time) on performance in the main mesophilic stage. In this study, we demonstrate a novel two-stage batch test method for TPAD processes, and use it to optimize operating conditions of the thermophilic stage in terms of degradation extent and methane production. The method determines overall degradability and apparent hydrolysis coefficient in both stages. The overall process was more effective with short pre-treatment retention times (1-2 days) and neutral pH compared to longer retention time (4 days) and low pH (4-5). Degradabilities and apparent hydrolysis coefficients were 0.3-0.5 (fraction degradable) and 0.1-0.4d(-1), respectively, with a margin of error in each measurement of approximately 20% relative (95% confidence). Pre-treatment temperature had a strong impact on the whole process, increasing overall degradability from 0.3 to 0.5 as temperature increased from 50 to 65 °C, with apparent hydrolysis coefficient increasing from 0.1 to 0.4d(-1).

Performance of a three-stage membrane bioprocess treating the organic fraction of municipal solid waste and evolution of its archaeal and bacterial ecology

A novel three-stage bioprocess achieved 75% volatile solids (VS) removal at an organic loading rate (OLR) of 4g VSL(-1)day, a solids retention time (SRT) of 66days, a hydraulic retention time (HRT) of 20days, at a temperature of 35 degrees C. The bioprocess consisted of an anaerobic hydrolytic reactor (HR) where the solids and liquid fractions of the Organic Fraction of the Municipal Solid Waste (OFMSW) were separated with a mesh. The leachate was pumped to a Submerged Anaerobic Membrane Bioreactor (SAMBR) and the treated permeate was polished in an Aerobic Membrane Bioreactor (AMBR). Denaturing Gradient Gel Electrophoresis (DGGE) and DNA sequencing analyses indicated that the increase in methane content in the HR caused by the excess sludge recycle from the SAMBR was associated with an increase in the number of hydrogenotrophic species, mainly Methanobrevibacter sp., Methanobacterium formicicum and Methanosarcina sp. At 20 degrees C VS removal dropped to 50% in the HR and some DGGE bands disappeared when compared to 35 degrees C samples, while some bands such as the one corresponding to Ruminococcus flavefaciens were reduced in intensity. The species associated with the COD-polishing properties of the AMBR correspond to the genera Pseudomonas, Hyphomonas and Hyphomicrobiaceae. These results highlight the positive effect of recycling the excess sludge from the SAMBR to re-inoculate the HR with hydrogenotrophic species.

The anaerobic degradability of thermoplastic starch: polyvinyl alcohol blends: potential biodegradable food packaging materials

A systematic study on the anaerobic degradability of a series of starch:polyvinyl alcohol (TPS:PVOH) blends was performed to determine their fate upon disposal in either anaerobic digesters or bioreactor landfills. The aims of the study were to measure the rate and extent of solubilisation of the plastics. The extent of substrate solubilisation on a COD basis reached 60% for a 90:10 (w/w) blend of TPS:PVOH, 40% for 75:25, 30% for 50:50 and 15% for PVOH only. The rate of substrate solubilisation was most rapid for the 90:10 blend (0.041 h(-1)) and decreased with the amount of starch in the blend in the following order 0.034 h(-1)(75:25); 0.023 h(-1)(50:50). The total solids that remained after 900 h were 10 wt.% (90:10); 23 wt.% (75:25); 55 wt.% (50:50); 90 wt.% (0:100). Starch containing substrates produced a higher concentration of volatile fatty acids (VFAs) and biogas, compared to the 0:100 substrate. The major outcome was that PVOH inhibited the degradation of the starch from the blend.

Cellulose hydrolysis by a methanogenic culture enriched from landfill waste in a semi-continuous reactor

This study investigates the hydrolysis of cellulose by a mixed culture enriched from landfill waste in a continuous reactor operated under prolonged residence times to accommodate methanogenic conditions. Chemostat studies of hydrolysis under balance methanogenic conditions are rarely reported, despite the importance of hydrolysis under these conditions in waste management and renewable energy industries. Continuous digestion was studied in a 1.25l digester, fed with a 1% (w/v) slurry of 50mum cellulose in sterilized leachate drawn from a 220l digester operated on a feedstock of mixed municipal solid waste. Unsterilized leachate was used as the inoculum. Stable and rapid hydrolytic conditions were established at residence time of 2.5, 3.5 and 5d with a 1st order hydrolysis rate 0.45+/-0.07d(-1) and high methane yields ranging from 57% to 62% of solubilised cellulose on a COD basis. Biomass yields were between 32% and 35% of solubilised cellulose on a COD basis, over three times that observed with fermentative cultures. This is attributed to the diversity of the microbial population which fully converted solubilised COD to methane, as evident by VFA yields of less than 8% on a COD basis.

Application of flowcell technology for monitoring biofilm development and cellulose degradation in leachate and rumen systems

In this study, a flat plate flowcell was modified to provide a reactor system that could maintain anaerobic, cellulolytic biofilms while providing the data needed to carry out a chemical oxygen demand mass balance to determine the cellulose digestion rates. The results showed that biofilms could be observed to grow and develop on cellulose particle surfaces from both anaerobic digester leachate and rumen fluid inocula. The observations suggest that the architecture of rumen and leachate derived biofilms may be significantly different with rumen derived organisms forming stable, dense biofilms while the leachate derived organisms formed less tenacious surface attachments. This experiment has indicated the utility of flowcells in the study of anaerobic biofilms.

Effect of leachate inoculum on biopretreatment of municipal solid waste by a combined hydrolytic-aerobic process

The biopretreatment of municipal solid waste by a combined hydrolytic-aerobic process has become of great interest for biostabilization or biodrying. The study aimed at investigating the effect of leachate inoculum on the biopretreatment. During the hydrolytic stage, the inoculum addition at the ratio of leachate to waste (LWR) of 5% and 7% stimulated the start-up of hydrolysis and enhanced the hydrolysis rate by 27.4% and 24.2%, respectively; whereas the inoculation at LWR of 1% had almost no effect on the hydrolysis rate and the inoculation at LWR of 10% reduced the hydrolysis rate by 12.7%. During the subsequent aerobic stage, the inoculations at LWRs greater than 5% decreased organics degradation rate. As a whole, compared with none inoculation, the total degradation rates of organics for inoculating trials at LWRs of 5%, 7% and 10% decreased by 14.5%, 14.3% and 32.7%, respectively. Correspondingly, their net water removal rates were reduced by 4.4%, 5.8% and 19.0%. The inoculation at LWR of 1% could not significantly affect the biopretreatment. The inoculum addition at LWR of 5% and 7% could shorten hydrolytic stage and thus accelerate the whole combined process. Moreover, the inoculations at LWRs greater than 5% were favorable for lignocelluloses degradation.