Microbial community dynamics and biogas production from manure fractions in sludge bed anaerobic digestion.

AIMS: To elucidate how granular sludge inoculum and particle-rich organic loading affect the structure of the microbial communities and process performance in upflow anaerobic sludge bed (UASB) reactors. METHODS AND RESULTS: We investigated four reactors run on dairy manure filtrate and four on pig manure supernatant for three months achieving similar methane yields. The reactors fed with less particle rich pig manure stabilized faster and had highest capacity. Microbial community dynamics analysed by a PCR/denaturing gradient gel electrophoresis approach showed that influent was a major determinant for the composition of the reactor communities. Comparisons of pre- and non-adapted inoculum in the reactors run on pig manure supernatant showed that the community structure of the nonadapted inoculum adapted in approximately two months. Microbiota variance partitioning analysis revealed that running time, organic loading rate and inoculum together explained 26 and 31% of the variance in bacterial and archaeal communities respectively. CONCLUSIONS: The microbial communities of UASBs adapted to the reactor conditions in treatment of particle rich manure fractions, obtaining high capacity, especially on pig manure supernatant. SIGNIFICANCE AND IMPACT OF THE STUDY: These findings provide relevant insight into the microbial community dynamics in startup and operation of sludge bed reactors for methane production from slurry fractions, a major potential source of biogas.

Anaerobic degradation of carbon capture reclaimer MEA waste.

The anaerobic biodegradation of reclaimer MEA (monoethanolamine) waste (MEAw) with easily degradable co-substrates was investigated in a laboratory-scale bioreactor at room temperature during a 160 d experimental run. The reactor that was constructed with three phases to facilitate attached biofilm and suspended biomass retention for degradation of the complex and challenging MEAw performed well. A feed strategy of step-wise increasing organic loading rate (OLR) by either increasing feed MEAw concentration or the hydraulic loading rate was applied. The system performance was evaluated by chemical oxygen demand (COD) removal efficiency, methane yield, MEA removal, and the accumulation of ammonia and volatile fatty acid (VFA). The total COD removal efficiency initially was 93% when the feed was mainly easily degradable co-substrate. The total removal dropped to 75% at the end when MEAw constituted 60% of the feed COD. Ion chromatography results show that the MEA and some unidentified feed chemicals were almost completely consumed. The main products of MEAw degradation were ammonia, VFAs and biogas. The ammonia nitrogen concentration reached about 2.0 g/L, which may explain the observed inhibition of acetoclastic methanogenesis leading to acetate accumulation. Methane accounted for up to 80% of the biogas generated. The highest methane yield was 0.34 L/g-COD while the yield was 0.16 L/g-COD at the highest load. This study shows that more than 80% reclaimer MEAw COD degradation with a co-substrate can be maintained in a hybrid anaerobic bioreactor operated in a wide loading range.

H2 consumption by anaerobic non-methanogenic mixed cultures.

The impact of H2 consumption, by homoacetogens, on the overall hydrogen yield in mixed culture fermentation is the focus of this study. Batch reactors were used to test the ability of anaerobic digested sludge, heat treated to eliminate methanogens, to produce and consume molecular hydrogen. The measured average hydrogen production rate from 4.2 g glucose/L was 44 +/- 4 mmol H2/L sludge . d, while the H2 consumption rate varied much more, in the range from 4 to 62 mmol H2/L sludge . d. Hydrogen consumption rate depends on acetic acid concentration, headspace H2 partial pressure and mass transfer rates. Different but relatively long lag-phases for hydrogen consumption were observed. It is concluded that homoacetogenesis can have a significant negative effect on bio-H2 production by mixed cultures, limiting the possibilities for sustainable solutions.

Apparent hydrogen consumption in acid reactors: observations and implications.

Investigations of hydrogen production by dark fermentation have received increasing attention as a green fuel production process. Research focus is mainly on yields and rates of hydrogen production under different operation conditions. The importance of hydrogen consumption is addressed here, based on results from lab-scale reactors. Experiments were run using mixed cultures and a variety of operating conditions: HRT 6-40 hours; temperature 25-55 degrees C. Initial hydrogen yields between 0.8-1.5 mol H(2)/mol glucose and approximately 50% H(2) in headspace was observed, followed by a decrease in hydrogen production as the culture matures, resulting in hydrogen yields down to 0.02 mol H(2)/mol glucose. It is concluded that hydrogen or "hydrogen equivalents" consumption is significant, especially in reactors with high biomass concentration and/or high sludge age. Sustainable H(2) production by dark fermentation alone is therefore not likely to be developed. The results suggest that it is possible to control and avoid significant H(2) production in dark fermentation. Minimizing H(2) production can be useful in preparation of organic feed for other bio-fuel production processes, such as methanogenic processes and bio-electrochemical H(2) production.

Enhancing hydrolysis with microaeration.

Effects of microaeration on hydrolysis of primary sludge are investigated in 500 ml batch reactors at 377 degrees C. Two experiments, one with a microaerobic inoculum and one with a combination of a microaerobic and an anaerobic inoculum, are carried out to also investigate the role of the inoculum. Assuming an acidogenic, methanogenic and aerobic biomass yield of 0.1, 0.05 and 0.45 mgC/mgC, respectively, a 50-60% hydrolysis increase, during the 4 day experiment, is observed with a ratio between aerobic and anaerobic metabolism in the range 0.5-0.7. The extra hydrolysed products are oxidized to carbon dioxide and incorporated into new biomass. The oxygen utilization to carbon dioxide production ratio was - 1:1 on a mol basis. Effects of the oxygen supplied on the hydrolysis of carbohydrates, proteins and lipids are analyzed based on measurements and balances of dissolved carbon, nitrogen and COD. The total observed hydrolysis increase can be accounted for by increased hydrolysis of carbohydrates and proteins. Lipids are only hydrolysed when anaerobic inoculum is added, but no effect of oxygen availability is detected.

ADM1 simulations of hydrogen production.

Hydrogen can be produced by fermentation of organic wastes as a renewable CO2 emission free fuel. The production potential as a function of feed composition is investigated using the ADM1 and experimental data from the literature. Lactate and ethanol are included in the model as intermediates to simulate the bio-hydrogen production processes more closely. Simulated effects of carbohydrate to protein ratio in the feed on pH, H2, biomass and fatty acid production using standard model parameters compare quite well with experimental results. The overall hydrogen and biomass production corresponds well with measurements for some feeds and less for others. The maximum theoretical yield is significantly higher than the simulated and measured values and is highest when the feed consists of only carbohydrates. The analysis suggests that the modified ADM1 is capable of simulating the main mechanisms involved in biological hydrogen production processes, implying that the model can be used to identify, and find strategies to influence limiting factors in bio-hydrogen production processes. Model weaknesses regarding the acidogenesis processes are observed and areas for further improvements discussed.

Can constructed wetlands reduce the diffuse phosphorus loads to eutrophic water in cold temperate regions?

Construction of wetlands is a possible supplement to best management practices (BMP) at the field level to mitigate phosphorus (P) pollution from agricultural areas. In this paper, annual results from 17 intensively studied wetlands in the cold temperate or boreal climatic zone are reported and analyzed. Surface areas varied from 0.007 to 8.7% of the catchment area. The average total phosphorus (TP) retention varied from 1 to 88%, and the dissolved reactive phosphorus (DRP) retention from -19 to 89%. Retention varied substantially from site to site, indicating the existence of site-specific factors in the catchment and wetlands that influenced the P removal. Factors important for P retention in wetlands were evaluated through multiple statistical analyses by dividing P into two fractions: particulate phosphorus (PP) and DRP. Both relative (%) PP and DRP retention increased with wetland surface area. However, PP retention was not as sensitive as DRP in terms of wetland size and retention: specific PP retention (gram P retention per m(2) and year) decreased as wetland area (A(w)) increased, suggesting the existence of a site-specific optimal wetland to catchment area (A(c)) ratio. Particulate P retention decreased with increasing DRP to TP ratio, while the opposite was found for DRP. Dissolved reactive P retention was higher in new than in old wetlands, while increasing age did not influence PP retention negatively. Effective BMP in the catchment is important to keep the P loss low, because the outlet concentration of P from wetlands is often positively correlated to the input concentration. However, wetlands act as the last buffer in a catchment, since the retention often increases as the P concentration in streams increases.

Modelling production of extracellular polymeric substances in a Pseudomonas aeruginosa chemostat culture.

Formation of extracellular polymeric substances (EPSs) by mucoid Pseudomonas aeruginosa was investigated using literature data from chemostat cultures. The data were used to calibrate a product formation regime allowing substrate sufficient and endogenous EPS formation. Yield coefficients for both formation conditions were elucidated based on metabolic pathway analysis. Growth and non-growth related specific formation rates of 0.18 g CEPS/g Ccell/h and 0.03 1/h were estimated, respectively. The exogenous and endogenous EPS yield was found to be 0.77 g CEPS/g Cglu and 0.79 g CEPS/g Ccell, respectively. Being structurally equivalent to comprehensive maintenance models, this model allows for non-growth related product formation, showing the same quality of fit as previous models restricted to exogenous EPS precursors.

Quantification of biofilm accumulation by an optical approach.

Methods for non-invasive, in situ, measurements of biofilm optical density and biofilm optical thickness were evaluated based on Pseudomonas aeruginosa experiments. Biofilm optical density, measured as intensity reduction of a light beam transmitted through the biofilm, correlates with biofilm mass, measured as total carbon and as cell mass. The method is more sensitive and less labor intensive than other commonly used methods for determining extent of biofilm mass accumulation. Biofilm optical thickness, measured by light microscopy, is translated into physical thickness based on biofilm refraction measurements. Biofilm refractive index was found to be close to the refractive index of water. The P. aeruginosa biofilms studied reached a pseudo steady state in less than a week, with stable liquid phase substrate, cell and TOC concentrations and average biofilm thickness. True steady state was, however, not reached as both biofilm density and roughness were still increasing after 3 weeks.

Activity of Pseudomonas aeruginosa in biofilms: Steady state.

Aerobic glucose metabolism by Pseudomonas aeruginosa in steady-state biofilms at various substrate loading rates and reactor dilution rates was investigated. Variables monitored were substrate (glucose), biofilm cellular density, biofilm extracellular polymeric substance (EPS) density, and suspended cellular and EPS concentrations. A mathematical model developed to describe the system was compared to experimental data. Intrinsic yield and rate coefficients included in the model were obtained from suspended continuous culture studies of glucose metabolism by P. aeruginosa. Experimental data compared well with the mathematical model, suggesting that P. aeruginosa does not behave differently in steady-state biofilm cultures, where diffusional resistance is negligible, than in suspended cultures. This implies that kinetic and stoichiometric coefficients for P. aeruginosa derived in suspended continuous culture can be used to describe steady-state biofilm processes.