One-stage and two-stage anaerobic digestion of lipid-extracted algae

Waste-grown microalgae are a potentially important biomass for wastewater treatment. The lipid accumulated in microalgae could be utilized as feedstocks for biodiesel production. The algal residues, as major by-products derived from lipid extraction, mainly consist of carbohydrate and protein, making anaerobic digestion an efficient way to recover energy. The conversion of lipid-extracted algal residues into methane plays dual role in renewable energy production and sustainable development of microalgal biodiesel industry. Therefore, an anaerobic fermentation process for investigation of the methane production potential of algal residues was conducted in this paper. The effect of inoculum to substrate ratios (ISRs) on the methane production by anaerobic digestion of Chlorella sp. residue in a single stage was evaluated. The maximum methane yield of 195.6 ml CH₄/g volatile solid (VS) was obtained at an ISR of 1:1. The stability and progress of the reaction from algal residues to methane were monitored by measuring the pH, volatile fatty acids (VFAs), total ammoniacal nitrogen (TAN), and methane volume. Based on the results of one-stage experiments, two-stage technology was proposed and was found to be more suitable for high organic load. The optimum conditions for acidogenesis and methanogenesis are indicated in this paper.     

Estimating changes of isotopic fractionation based on chemical kinetics and microbial dynamics during anaerobic methane oxidation: apparent zero- and first-order kinetics at high and low initial methane concentrations

Changes in natural isotopic composition may be used to reveal metabolic pathways of substrate transformation by microbial communities (Vavilin in Ecol Model 240:84–92, 2012b). Anaerobic oxidation of methane (AOM) by sulfate has been described using a mathematical model based on chemical kinetics, microbial dynamics and equations for ¹³C isotope accumulation in products as well as their redistribution between substrate and products. Experimental data for two batch cultures that originated from microbial mats covering methane seep chimneys in the Black Sea, previously obtained by Seifert et al. (Org Geochem 37:1411–1419, 2006) and Holler et al. (Env Microbiol Reports 1(5):370–376, 2009), were used to model AOM. During long-time incubation, changes of isotope signatures in CH₄ showed that in the Seifert et al. batch tests (low methane concentration), in contrast to the Holler et al. batch tests (high methane concentration), methane production occurred along with methane oxidation. In accordance with the model, apparent zero and first-order kinetics of methane oxidation were valid for the Holler et al. and Seifert et al. batch tests, respectively. The observed change of $ \delta {}^{13}{\text{CH}}_{4} $ was explained by microbial kinetics reflecting that the rate is lower for heavy substrate microbial utilization when compared to light substrate microbial utilization. The model showed that small amounts of methanogenesis will change the carbon isotopic composition of methane because biogenic methane has a distinct isotopic composition and due to the large difference between the maximum specific rates of methane oxidation and production. The estimated biomass doubling time of methane-oxidizers for high and low methane concentration was 408/126 days and 4640/1160 days, respectively, depending on the value of the half-saturation constant K _S (5 and 20 mM).     

Improvement of Biomethane Production from Sewage Sludge in Co-digestion with Glycerol and Waste Frying Oil,Using a Design of Experiments

A central composite design circumscribed method was used to define the experimental conditions that improve the methane production rate (k_CH4, liters of methane per kilogram of VS of waste added and per day) and the cumulative methane production (cMP, liters of methane per kilogram of VS of waste added) of the co-digestion of sewage sludge (SS) with crude glycerol (cGly) and waste frying oil (WFO). Three factors were selected, i.e., SS concentration, global co-substrate concentration, and mass fraction of cGly (x_cGly) in a mixture of cGly and WFO (in chemical oxygen demand, COD). SS digestion without co-substrate reached a cMP of (294?±?6) L·kg^?1 and a k_CH4 of (64?±?1) L·kg^?1·d^?1, at standard temperature and pressure conditions and expressed relatively to the initial volatile solids. After statistical analysis, SS and co-substrate concentrations of 4.6 g·L^?1 and 8.8 g·L^?1 (in COD), respectively, with x_cGly of 0.8, were defined to simultaneously boost cMP (91 % more) and k_CH4 (3-fold increase). Application of these conditions would yield 214 MWh more in electricity per 1000 m³ of SS digested.     

Optimisation of biomass and lipid production by adjusting the interspecific competition mode of Dunaliella salina and Nannochloropsis gaditana in mixed culture

The high cost of algal cultivation has been a barrier associated with the commercialisation of algal biodiesel. Therefore, this study aimed to enhance lipid production by optimising the nutrient supply to benefit the coexistence of Dunaliella salina and Nannochloropsis gaditana. The effects on biomass and lipid production of using different proportions of D. salina and N. gaditana, urea and NaHCO₃ were optimised by response surface method with a 17-run Box–Behnken design. The optimal conditions for the algal growth are 58 % of D. salina in the mixture at OD₆₈₀, 150 μL day^?1 urea (0.0044 g day^?1) and no addition of NaHCO₃. The biomass concentration and lipid production reached 1.00 and 0.383 g L^?1, respectively, which are exceeded by the amount before optimisation, indicating the efficiency of the model obtained by response surface method.     

Effect of Harvest Time and Frequency on Biomass Quality and Biomethane Potential of Common Reed (<Emphasis Type="Italic">Phragmites australis</Emphasis>) Under Paludiculture Conditions

This study examined the effect of harvest time (from May to September) and dry matter partitioning on biomethane potential and methane yield per unit area of Phragmites australis cultivation under paludiculture conditions. The experimental site is part of a larger experimental platform (San Niccolò, Pisa) located within the Massaciuccoli Lake Basin in Central Italy (Tuscany, IT). The study also took into account the double cut strategy by evaluating the regrowth from June to September. Biomethane potentials ranged from 384 to 315 and from 412 to 283 NL CH₄ kg VS^?1 (normal liters of methane per kg of volatile solids) for leaves and stems, respectively. About digestion kinetics, maximum daily production rate (R _max) was significantly affected by harvest time and not by plant partitioning. Along the harvest season, biomethane yield per unit area was mostly driven by the biomass yield showing an increasing trend from May (1659 Nm³ ha^?1) to September (3817 Nm³ ha^?1). The highest value was obtained with the double harvest option (4383 Nm³ ha^?1), although it was not statistically different from the single harvest carried out in September. Owing to its remarkably lower yields, P. australis cannot be considered along the same lines as crops conventionally used for biogas production, but it may represent an interesting option for paludiculture cropping systems by coupling peatland restoration with bioenergy production. September harvest management seemed the most feasible option, although further investigation on crop lifespan is needed.     

Kinetic study of Lactococcus lactis strains (SLT6 and SLT10) growth on papain-hydrolysed whey

The effect of controlled whey hydrolysis by papain on growth of two lactic acid bacteria isolated from artisanal Leben: Lactococcus lactis var. diacetylactis (SLT6 and SLT10) was investigated. The higher biomass and maximum specific growth rate (μ _max) were obtained after 30 min of hydrolysis. HPLC analysis of peptides showed that whey hydrolysis reduced the amount of peptides of MW > 400 Da and increased those peptides of MW < 400 Da. The two studied strains exhibited different peptide requirements. The pH-controlled batch cultures in 30 min hydrolysed whey followed the Monod kinetic for growth and for lactate production. The values of the key kinetic constants were: maximum specific growth rate (μ _max), 1.08 and 0.56 h^?1; yield biomass on lactose (Y _x/s), 0.20 and 0.18 g g^?1 and saturation constant K _s, 4.2 and 2.8 g L^?1 for SLT6 and SLT10, respectively. When compared with batch experimental data, the model provided good predictions for growth, lactose utilisation and lactate production profiles.     

The potential of microalgal biomass production for biotechnological purposes using wastewater resources

The utilization of microalgae for wastewater treatment represents an attractive opportunity for wastewater valorization through the use of the produced biomass. Five strains of microalgae were isolated from municipal wastewater and grown in autoclaved and non-autoclaved effluent at 30 °C and 150 μmol photons m^?2 s^?1 to study biomass production, nutrient removal, and the biochemical composition of the biomass. All strains reached high biomass productivity (35.6 to 54.2 mg dry weight L^?1 day^?1) within 4 days of batch culturing. In this period, ammonium-N and phosphate were reduced by more than 60 and 90 %, respectively. The high growth rate (0.57 to 1.06 day^?1) ensured a rapid removal of nutrients and thereby a short retention time. By the fourth day of cultivation, the algal biomass contained 32 % protein, but only 11 % lipids and 18 % carbohydrates. It was found that the biomass was a suitable raw material for biogas production by anaerobic digestion. Biodigestion of obtained biomass was simulated by employing the Aspen HYSYS modeling software, resulting in methane yields comparable to those found in the literature. The elemental analysis of the algal biomass showed very low concentrations of pollutants, demonstrating the potential of use of the digestate from biodigestion as a bio-fertilizer.     

Effect of co-substrate feeding on methane yield of anaerobic digestion of <Emphasis Type="Italic">Chlorella vulgaris</Emphasis>

Microalgal production has many advantages over the use of terrestrial plants; therefore, increases in the use of microalgae for energy production can be expected. Algal biomass can be processed anaerobically to methane; however, the unfavorable C/N ratio of the substrate may have an inhibitory effect. The impact of the application of used cooking oil, maize silage, and mill residue on anaerobic co-digestion of the microalgal Chlorella vulgaris was studied in semi-continuous, laboratory-scale digestion. During the full period of the trial involving anaerobic digestion of algae in the case of mono-digestion and co-digestion with used cooking oil, maize silage, and mill residue, the volumetric methane yields were 0.38?±?0.07, 1.56?±?0.26, 1.19?±?0.18, and 1.16?±?0.13 L L^?1, respectively. Trials were carried out to determine the long-term effect of the total solid (TS) content of substrates (co-digestion of C. vulgaris and used cooking oil at 3.8 and 7.2 % of TS, respectively). Both designs could be increased to 5.5 g VS L^?1 d^?1, but a higher TS% resulted in increased methane production and a longer period of decline in the methane yield due to washout. The sharp decrease in methane content at the end of 90 days was accompanied by a reorganization of the methanogenic archaeal community.     

Population dynamics of methanogens and methane formation associated with different loading rates of organic acids along with ammonia: redundancy analysis

In anaerobic processes, the population dynamics of methanogens in the methanogenic stage were monitored along with hydraulic retention times (HRTs) shift. Decreasing HRTs increased the loading rates of volatile fatty acids (VFAs) and ammonia. Methanomicrobiales (MMB) began to be dominant at longer than 12.5 days HRT, Methanosarcinales (MSL) were dominant at 8, 10, and 12.5 days HRT, and Methanobacteriales (MBT) were dominant at shorter than 6 days HRT. Increased loading rates of VFAs and ammonia increased MBT, decreased MMB, and had no significant effect on MSL. Maximal daily methane production was observed at 1.57 L/L when MSL copy numbers also reached 3.60 × 10⁷ copy/mL as a peak, which were expressed as positive correlation between DMA and MSL. No sooner had methane yield (MY) increased from 1.15 to 1.32 L/g VS_removed along with HRT reduction from 25 to 22.5 days, then MY gradually decreased from 1.32 to 0.04 L/g VS_removed.     

Leaf Litter Fuels Methanogenesis Throughout Decomposition in a Forested Peatland

Decomposing leaf litter is a large supply of energy and nutrients for soil microorganisms. How long decaying leaves continue to fuel anaerobic microbial activity in wetland ecosystems is poorly understood. Here, we compare leaf litter from 15 tree species with different growth forms (angiosperms and gymnosperms, deciduous, and longer life span), using litterbags positioned for up to 4 years in a forested peatland in New York State. Periodically, we incubated partially decayed residue per species with fresh soil to assess its ability to fuel microbial methane (CH₄) production and concomitant anaerobic carbon dioxide (CO₂) production. Decay rates varied by leaf type: deciduous angiosperm > evergreen gymnosperm > deciduous gymnosperm. Decay rates were slower in leaf litter with a large concentration of lignin. Soil with residue of leaves decomposed for 338 days had greater rates of CH₄ production (5.8 µmol g^?1 dry mass d^?1) than less decomposed (<0.42 µmol g^?1 dry mass d^?1) or more decomposed (2.1 µmol g^?1 dry mass d^?1) leaf residue. Species-driven differences in their ability to fuel CH₄ production were evident throughout the study, whereas concomitant rates of CO₂ production were more similar among species and declined with degree of decomposition. Methane production rates exhibited a positive correlation with pectin and the rate of pectin decomposition. This link between leaf litter decay rates, biochemical components in leaves, and microorganisms producing greenhouse gases should improve predictions of CH₄ production in wetlands.     

Impact of milling,enzyme addition,and steam explosion on the solid waste biomethanation of an olive oil production plant

Anaerobic digestion is a consolidated bioprocess which can be further enhanced by incorporating an upstream pretreatment unit. The olive oil production produces a large amount of solid waste which needs to be properly managed and disposed. Three different pretreatment techniques were evaluated in regard to their impact on the anaerobic biodegradability: manual milling of olive pomace (OP), enzyme maceration, direct enzyme addition, and thermal hydrolysis of two-phase olive mill waste. The Gompertz equation was used to obtain parameters for comparison purposes. A substrate/inoculum ratio 0.5 was found to be the best to be used in anaerobic batch test with olive pomace as substrate. Mechanical pretreatment of OP by milling increases the methane production rate while keeping the maximum methane yield. The enzymatic pretreatment showed different results depending on the chosen pretreatment strategies. After the enzymatic maceration pretreatment, a methane production of 274 ml CH₄ g VS _added ^?1 was achieved, which represents an improvement of 32 and 71 % compared to the blank and control, respectively. The direct enzyme addition pretreatment showed no improvement in both the rate and the maximum methane production. Steam explosion showed no improvement on the anaerobic degradability of two-phase olive mill waste; however, thermal hydrolysis with no rapid depressurization enhanced notoriously both the maximum rate (50 %) and methane yield (70 %).     

Comparative anaerobic treatment of wastewater from pharmaceutical,brewery, paper and amino acid producing industries

This study concerned the anaerobic treatment of five different industrial wastewaters with a diverse and complex chemical composition. The kinetics of biotransformation of this wastewater at different chemical oxygen demand (COD) were studied in a batch reactor. Wastewater from an amino acid producing industry (Fermex) and from a tank that received several types of wastewaters (collector) contained 0.83 g l⁻¹ and 0.085 g l⁻¹ sulfate, respectively. During the study period of 20 days, methane formation was observed in all types of wastewaters. Studies on COD biodegradation showed the reaction velocity was higher for Fermex wastewater and lower for collector wastewater, with values of 0.0022 h⁻¹ and 0.0011 h⁻¹, respectively. A lower methanogenic activity of 0.163 g CH₄ day⁻¹ g⁻¹ volatile suspended solids (VSS) and 0.20 g CH₄ day⁻¹ g⁻¹ VSS, respectively, was observed for paper producing and brewery wastewater. Adapted granular sludge showed the best biodegradation of COD during the 20-day period. The sulfate-reducing activity in pharmaceutical and collector wastewater was studied. A positive effect of sulfate-reducing activity on methanogenic activity was noted for both types of wastewaters, both of which contained sulfate ions. All reactions of methane generation for the tested industrial wastewaters were first-order. The results of this study suggest that the tested wastewaters are amenable to anaerobic treatment.

     

Methanomethylovorans uponensis sp. nov., a methylotrophic methanogen isolated from wetland sediment

A novel mesophilic, methylotrophic, methanogenic archaeon, designated strain EK1^T, was enriched and isolated from wetland sediment. Phylogenetic analysis showed that strain EK1^T was affiliated with the genus Methanomethylovorans within the family Methanosarcinaceae, and shared the highest 16S rRNA and methyl-coenzyme M reductase alpha-subunit gene sequence similarity with the type strain of Methanomethylovorans hollandica (98.8 and 92.6 %, respectively). The cells of strain EK1^T were observed to be Gram-negative, non-motile and irregular cocci that did not lyse in 0.1 % (w/v) sodium dodecyl sulfate. Methanol, mono-, di- and trimethylamine, dimethyl sulfide and methanethiol were found to be used as catabolic and methanogenic substrates, whereas H₂/CO₂, formate, 2-propanol and acetate were not. Growth was observed at 25–40 °C (optimum, 37 °C), at pH 5.5–7.5 (optimum, pH 6.0–6.5) and in the presence of 0–0.1 M NaCl (optimum, 0 M). Growth and methane production rates were stimulated in the presence of H₂/CO₂ although methane production and growth yields were not significantly affected; acetate, formate, 2-propanol and CO/CO₂/N₂ did not affect methane production. CoCl₂ (0.6–2.0 μM) and FeCl₂ (25 mg/l) stimulated growth, while yeast extract and peptone did not. The DNA–DNA hybridization experiment revealed a relatedness of <20 % between EK1^T and the type strains of the genus Methanomethylovorans. The DNA G+C content of strain EK1^T was determined to be 39.2 mol%. Based on the polyphasic taxonomic study, strain EK1^T represents a novel species belonging to the genus Methanomethylovorans, for which the name Methanomethylovorans uponensis sp. nov. is proposed. The type strain is strain EK1^T(=NBRC 109636^T = KCTC 4119^T = JCM 19217^T).     

Dry anaerobic ammonia–methane production from chicken manure

The effect of temperature on production of ammonia during dry anaerobic fermentation of chicken manure (CM), inoculated with thermophilic methanogenic sludge, was investigated in a batch condition for 8 days. Incubation temperature did not have a significant effect on the production of ammonia. Almost complete inhibition of production of methane occurred at 55 and 65°C while quite low yields of 8.45 and 6.34 ml g⁻¹ VS (volatile solids) were observed at 35 and 45°C due to a higher accumulation of ammonia. In order to improve the production of methane during dry anaerobic digestion of CM, stripping of ammonia was performed firstly on the CM previously fermented at 65°C for 8 days: the stripping for 1 day at 85°C and pH 10 removed 85.5% of ammonia. The first-batch fermentation of methane for 75 days was conducted next, using the ammonia-stripped CM inoculated with methanogenic sludge at different ratios, (CM: thermophilic sludge) of 1:2, 1:1, and 2:1 on volume per volume basis at both 35 and 55°C. Production of methane improved and was higher than that of the control (without stripping of ammonia) but the yield of 20.4 ml g⁻¹ VS was still low, so second stripping of ammonia was conducted, which resulted in 74.7% removal of ammonia. A great improvement in the production of methane of 103.5 ml g⁻¹ VS was achieved during the second batch for 55 days.     

The membraneless bioelectrochemical reactor stimulates hydrogen fermentation by inhibiting methanogenic archaea

The membraneless bioelectrochemical reactor (Ml-BER) is useful for dark hydrogen fermentation. The effect of the electrochemical reaction on microorganisms in the Ml-BER was investigated using glucose as the substrate and compared with organisms in a membraneless non-bioelectrochemical reactor (Ml-NBER) and bioelectrochemical reactor (BER) with a proton exchange membrane. The potentials on the working electrode of the Ml-BER and BER with membrane were regulated to ?0.9 V (versus Ag/AgCl) to avoid water electrolysis with a carbon electrode. The Ml-BER showed suppressed methane production (19.8?±?9.1 mg-C·L^?1·day^?1) and increased hydrogen production (12.6?±?3.1 mg-H·L^?1·day^?1) at pH_out 6.2?±?0.1, and the major intermediate was butyrate (24.9?±?2.4 mM), suggesting efficient hydrogen fermentation. In contrast, the Ml-NBER showed high methane production (239.3?±?17.9 mg-C·L^?1·day^?1) and low hydrogen production (0.2?±?0.0 mg-H·L^?1·day^?1) at pH_out 6.3?±?0.1. In the cathodic chamber of the BER with membrane, methane production was high (276.3?±?20.4 mg-C·L^?1·day^?1) (pH_out, 7.2?±?0.1). In the anodic chamber of the BER with membrane (anode-BER), gas production was low because of high lactate production (43.6?±?1.7 mM) at pH_out 5.0?±?0.1. Methanogenic archaea were not detected in the Ml-BER and anode-BER. However, Methanosarcina sp. and Methanobacterium sp. were found in Ml-NBER. Prokaryotic copy numbers in the Ml-BER and Ml-NBER were similar, as were the bacterial community structures. Thus, the electrochemical reaction in the Ml-BER affected hydrogenotrophic and acetoclastic methanogens, but not the bacterial community.     

Atmospheric methane removal by methane-oxidizing bacteria immobilized on porous building materials

Biological treatment using methane-oxidizing bacteria (MOB) immobilized on six porous carrier materials have been used to mitigate methane emission. Experiments were performed with different MOB inoculated in building materials at high (~20 % (v/v)) and low (~100 ppmv) methane mixing ratios. Methylocystis parvus in autoclaved aerated concrete (AAC) exhibited the highest methane removal rate at high (28.5?±?3.8 μg CH₄ g^?1 building material h^?1) and low (1.7?±?0.4 μg CH₄ g^?1 building material h^?1) methane mixing ratio. Due to the higher volume of pores with diameter >5 μm compared to other materials tested, AAC was able to adsorb more bacteria which might explain for the higher methane removal observed. The total methane and carbon dioxide-carbon in the headspace was decreased for 65.2?±?10.9 % when M. parvus in Ytong was incubated for 100 h. This study showed that immobilized MOB on building materials could be used to remove methane from the air and also act as carbon sink.     

Tobermolite effects on methane removal activity and microbial community of a lab-scale soil biocover

Three identical lab-scale biocovers were packed with an engineered soil (BC 1), tobermolite only (BC 2), and a mixture of the soil and tobermolite (BC 3), and were operated at an inlet load of 338–400 g-CH₄ m^?2 d^?1 and a space velocity of 0.12 h^?1. The methane removal capacity was 293 ± 47 g-CH₄ m^?2 d^?1 in steady state in the BC 3, which was significantly higher than those in the BC 1 and BC 2 (106 ± 24 and 114 ± 48 g-CH₄ m^?2 d^?1, respectively). Quantitative PCR indicated that bacterial and methanotrophic densities (6.62–6.78 × 10⁷ 16S rDNA gene copy number g-dry sample^?1 and 1.37–2.23 × 10⁷ pmoA gene copy number g-dry sample^?1 in the BC 1 and BC 3, respectively) were significantly higher than those in the BC 2. Ribosomal tag pyrosequencing showed that methanotrophs comprised approximately 60 % of the bacterial community in the BC 2 and BC 3, while they only comprised 43 % in the BC 1. The engineered soil favored the growth of total bacteria including methanotrophs, while the presence of tobermolite enhanced the relative abundance of methanotrophs, resulting in an improved habitat for methanotrophs as well as greater methane mitigation performance in the mixture. Moreover, a batch experiment indicated that the soil and tobermolite mixture could display a stable methane oxidation level over wide temperature (20–40 °C, at least 38 μmol g-dry sample^?1 h^?1) and pH (5–8, at least 61 μmol g-dry sample^?1 h^?1) ranges. In conclusion, the soil and tobermolite mixture is promising for methane mitigation.     

Comparison of RNA- and DNA-based bacterial communities in a lab-scale methane-degrading biocover

Methanotrophs must become established and active in a landfill biocover for successful methane oxidation. A lab-scale biocover with a soil mixture was operated for removal of methane and nonmethane volatile organic compounds, such as dimethyl sulfide (DMS), benzene (B), and toluene (T). The methane elimination capacity was 211?±?40 g?m^?2 d^?1 at inlet loads of 330–516 g?m^?2 d^?1. DMS, B, and T were completely removed at the bottom layer (40–50 cm) with inlet loads of 221.6?±?92.2, 99.6?±?19.5, and 23.4?±?4.9 mg m^?2 d^?1, respectively. The bacterial community was examined based on DNA and RNA using ribosomal tag pyrosequencing. Interestingly, methanotrophs comprised 80 % of the active community (RNA) while 29 % of the counterpart (DNA). Types I and II methanotrophs equally contributed to methane oxidation, and Methylobacter, Methylocaldum, and Methylocystis were dominant in both communities. The DNA vs. RNA comparison suggests that DNA-based analysis alone can lead to a significant underestimation of active members.     

Batch anaerobic co-digestion of cow manure and waste milk in two-stage process for hydrogen and methane productions

Anaerobic co-digestion of cow manure (CM) and waste milk (WM), produced by sick cows during treatment with antibiotics, was evaluated in two-stage process under thermophilic condition (55 °C) to determine the effect of WM addition on hydrogen (H₂) and methane (CH₄) production potentials, volatile solids (VS) removal, and energy recovery. Six CM to WM VS ratios of 100:0, 90:10, 70:30, 50:50, 30:70, and 10:90 were examined using 1-L batch digesters. The WM VS ratio of 30 % was found to be the minimum limit for significant increases in specific H₂ and CH₄ yields, and VS removal as compared to digestion of manure alone (P < 0.05). The highest specific H₂ and CH₄ yields, VS removal and energy yield were 38.2 mL/g VS, 627.6 mL/g VS, 78.4 % and 25,459.8 kJ/kg VS, respectively, in CM:WM 30:70. Lag phases to H₂ and CH₄ productions were observed in CM–WM mixtures, increased with increasing the amount of WM in the feedstock and were greater than 72 h in CM:WM 50:50 and 30:70. The digestion system failed in CM:WM 10:90. The results suggest that CM:WM 30:70 was optimum, however, due to limited amount of WM usually generated and long lag phase at this ratio which may make the process uneconomical, CM:WM 70:30 is recommended in practice.     

Vegetation controls methane emissions in a coastal brackish fen

Climate change and associated sea level rise will likely affect coastal ecosystems and lead to more frequent inundations. Plants are an important control for methane (CH₄) emissions in peatlands because the metabolism of the living plant can either enhance or attenuate CH₄ emissions and plant litter supplies an easily available carbon source for methanogenesis. Here we compare the contribution of various dominant plant species to methane emissions in a degraded, rewetted coastal brackish fen at the southern Baltic Sea coast in Northeast Germany. We analyse one year of bi-weekly static closed chamber data gathered at measurement spots that were located in different mono-dominant vegetation stands (Bolboschoenus maritimus (L.) Palla, Schoenoplectus tabernaemontani (C.C.Gmel.) Palla, Carex acutiformis Ehrh.). Furthermore, data on water level, water temperature, conductivity (sulphate), and several peat characteristics were recorded. Generally, the annual methane emissions were low with an average across vegetation stands of 14 kg CH₄ ha^?1 a^?1, which we related to high decomposition of peat after drainage and to relatively low water levels in summer. Nevertheless, methane emissions varied between different vegetation types with significantly higher methane fluxes (31.8 ± 5.7 kg CH₄ ha^?1 a^?1) from Bolboschoenus maritimus stands compared to Carex acutiformis and Schoenoplectus tabernaemontani stands (4.3 ± 1.2 and 5.7 ± 2.4 kg CH₄ ha^?1 a^?1, respectively). None of the environmental variables that have been recorded can explain this difference. Thus, vegetation composition seems to be an important driver for methane emissions in coastal brackish fens and may therefore be crucial with regard to recreation measures.