Specific Methanogenic Activity Tests Research Articles

Long-term performance and activity study of a two-stage anaerobic EGSB reactors system treating complex and toxic industrial wastewater.

Anaerobic treatment of industrial wastewater using upflow anaerobic reactors is an extended trend due to its high efficiency and biogas production potential, but its implementation in some sectors is limited due to the complexity and toxicity of the wastewaters. In this study, a two-stage expanded granular sludge bed (EGSB) reactors system has been investigated at both bench and pilot scale for the treatment of complex and toxic real wastewater from a petrochemical industry. The effect of different operational parameters including organic loading rate (OLR), hydraulic retention time (HRT) and influent characteristics over COD removal and biogas production and composition have been studied. Additionally, biomass specific methanogenic activity (SMA) and wastewater toxicity have been evaluated after long-term operation. Optimum total HRT of 24 h has been determined resulting in total COD and SO4 2- removal of 56.30 ± 5.25% and 31.68 ± 14.71%, respectively, at pilot scale, and average biogas production of 93.47 ± 34.92 NL/day with 67.01 ± 10.23 %CH4 content and 5210.11 ± 6802.27 ppmv of H2S. SMA and toxicity tests have confirmed inhibitory and toxic effects of wastewater over anaerobic biomass with average maximum inhibition of 65.34% in the unacclimated anaerobic inoculum while chronic toxicity produced a decrease of an order of magnitude in SMA after 600 days of operation. This study demonstrates the feasibility of applying an anaerobic treatment to this wastewater using EGSB reactors between a 0.97-1.74 gCOD/L/day OLR range. Nonetheless, periodic reinoculation would be necessary for long-term operation due to chronic toxicity of the wastewater exerted on the anaerobic biomass. PRACTITIONER POINTS: A two-stage EGSB reactors system has been operated at bench and pilot scale to treat complex and toxic petrochemical wastewater. Optimal total HRT of 24 h resulted in average COD removal ranging from 40% to 60%. SMA and toxicity tests have been performed to study long-term acclimation, detecting an activity depletion of an order of magnitude.

Assessment of Specific Methanogenic Activity from Cow Dung

The specific methanogenic activity (SMA) is a test to measure the producing potential of an anaerobic bacteria until it’s allowing a relevant organic loading rates to be applied for a selectedsubstrate. Commonly, acetate is used as substrate for the SMA test. Anaerobic bacteria were mostly taken from an anaerobic digester and cow dung was also implemented as a source of an anaerobic bacteria. However, the results of SMA of cow dung was less reported. Therefore, this study is initiated to determine the potential of methane production from the cow dung by using the SMA test. Prior the SMA test, the cow dung was characterized for solids where the results showed that the cow dung is having 12.00 g L-1for total solid and 10.50 g L-1for volatile solid. The SMA test was conducted at mesophilic condition by using an automatic methane potential system test (AMPTS II) and the SMA of the cow dung was found as 0.04 in unit g COD-CH4g-1VS-1d-1. The significance of this research is to determine the anaerobic bacteria potential of cow dung for use in the anaerobic digestion process, which offers numerous advantages for manufacturing, particularly in industrial applications such as methane production (fuel).

Heterogeneous photocatalysis and an anaerobic biological process for leachate treatment

The research used a coupling heterogeneous photocatalysis (TiO2-UVsolar) and an anaerobic biological process (Specific Methanogenic Activity Test) to study the treatment of leachate from a landfill in Norte de Santander (Colombia). It developed photocatalysis in a Composite Parabolic Collector (CPC), H2O2 as oxidation assistant, and the combination of different TiO2 levels and pH modeled with a factorial design 32. In the biological test, for a maximum load of 4,500 mg. L-1 of Chemical Oxygen Demand (COD) leachate, 2.0 g. L-1 of Volatile Suspended Solids (VSS) inoculum, and a hydraulic retention time of 23 days, there were no significant COD removal percentages, evidencing the recalcitrant character of these leachates. In the Advanced Oxidation Process (AOP) there was mineralization in terms of DOC of 57%, which evidences the capacity of the process to support the pollutant load of the leachate. The combined process (AOP-Biological) allows additional mineralization in terms of DOC of 21% in the biological process, for a total contribution of the coupling of 78%, which shows the capacity of the AOP to convert toxic wastewater into one with characteristics more suitable for its subsequent degradation in anaerobic biological reactors.

Need of adjustment of methanogenic activities from solids digester sludge: Modelling dilution effects on micro-organisms biomass concentration

Anaerobic digestion is a widespread technology used for organic-based solid waste management. Specific methanogenic activity tests are simple and cost-effective tools for sludge characterization and system diagnosis. However, in solid digesters, substrate and organic inert material dilute micro-organisms, enlarging the activity tests and distorting experimental results. To correct this situation, correction factors, defined as the ratio of micro-organisms concentration to total volatile solids content, are considered. Due to the impossibility to have a quick measurement of the mass of micro-organisms in the digester content, correction factors were evaluated based on the simulation of a simple model. To verify the importance of the correction, hydrogenotrophic and acetoclastic methanogenic activities were studied, involving a stage for sludge acclimatization and subsequent processing in a continuous digester. This situation was compared with a wastewater up-flow anaerobic reactor where no dilution effects are presented. A correcting factor of 0.79 was obtained for the acclimatization period, whereas correcting factors of 0.25–0.30 were estimated for the two periods of the digester. Tendencies shown for raw activities differed from those observed after using correcting factors to adjust activity values for periods 1 and 2 of the digester; also, the gap between the up-flow anaerobic reactor (without solids dilution effects) and the digester activities was reduced from sixfold to double, evidencing the relevance of this correction tool. Additionally, correcting factors also enabled a reasonable calculation of the inoculum size during the design of the activity tests.

Effect of microaerophilic treatment on swine wastewater (SWW) treatment: Engineering and microbiological aspects

The microaerobic process on swine wastewater (SWW) treatment was investigated, evaluating its effect on organic matter hydrolysis and removal, biogas production, operational stability, and microbial community structure. UASB reactors operating under higher organic loading rates (OLRs) and lower hydraulic retention times (HRTs) than those found in the SWW treatment literature were also assessed. The microaerophilic reactor R2 presented a higher total and particulate organic matter removals and operational stability than the anaerobic reactor R1, reaching CODP removals of 79.4 ± 4.6%. In the specific methanogenic activity (SMA) tests, the microaerobic sludge (R2) showed hydrolytic and acetogenic/methanogenic activity superior to inoculum and anaerobic sludge (R1). The microbiological evaluation of R2 revealed the high presence of hydrolytic microorganisms, therefore justifying the higher hydrolytic activity found in the SMA tests and higher particulate organic matter removal found in the microaerobic reactor.

Treatment of Agricultural Wastewater with Chlorpyrifos by Coupling of Heterogeneous Photocatalysis and Anaerobic Biological Process

Treatment of wastewater with Chlorpyrifos content using heterogeneous photocatalysis was evaluated, for which a compound parabolic collector (CPC) was used, under different combinations of pH and TiO2 concentration designed using the response surface methodology (RSM), and an anaerobic biological process based on the specific methanogenic activity test (SMA), using a flocculent sludge and an initial concentration of 2.0 g VSS/L. Initially, water resulting from the triple washing of equipment for manual application of Chlorpyrifos from the El Zulia Irrigation District and the municipality of Bucarasica were characterized. In the biological process, COD and TOC removals of 46.4% and 86.6% were obtained; while by applying preliminary photocatalytic processes, under optimal conditions of TiO2 (100 mg/L), pH (3 units) and retention time (3 h), better biodegradability efficiencies (72.2% and 53.0%) were achieved. The results revealed that the combined use of advanced oxidation processes with biological treatment technologies is technically viable, as it increases the efficiency of removing contaminants from agricultural effluents with Chlorpyrifos.

Cation exchange membrane behaviour of extracellular polymeric substances (EPS) in salt adapted granular sludge

This paper aims to elucidate the role of extracellular polymeric substances (EPS) in regulating anion and cation concentrations and toxicity towards microorganisms in anaerobic granular sludges adapted to low (0.22 M of Na+) and high salinity (0.87 M of Na+). The ion exchange properties of EPS were studied with a novel approach, where EPS were entangled with an inert binder (PVDF-HFP) to form a membrane and characterized in an electrodialysis cell. With a mixture of NaCl and KCl salts the EPS membrane was shown to act as a cation exchange membrane (CEM) with a current efficiency of ∼80%, meaning that EPS do not behave as ideal CEM. Surprisingly, the membrane had selectivity for transport of K+ compared to Na+ with a separation factor (SK+/Na+) of 1.3. These properties were compared to a layer prepared from a model compound of EPS (alginate) and a commercial CEM. The alginate layer had a similar current efficiency (∼80%.), but even higher SK+/Na+ of 1.9, while the commercial CEM did not show selectivity towards K+ or Na+, but exhibited the highest current efficiency of 92%. The selectivity of EPS and alginate towards K+ transport has interesting potential applications for ion separation from water streams and should be further investigated. The anion repelling and cation binding properties of EPS in hydrated and dehydrated granules were further confirmed with microscopy (SEM-EDX, epifluorescence) and ion chromatography (ICP-OES, IC) techniques. Results of specific methanogenic activity (SMA) tests conducted with 0.22 and 0.87 M Na+ adapted granular sludges and with various monovalent salts suggested that ions which are preferentially transported by EPS are also more toxic towards methanogenic cells.

Activity of preserved anaerobic sludge

There is a need for a broad study addressing different preservation conditions of anaerobic sludge and its activity after a prolonged storage. This study compared four different preservation methods of mesophilic anaerobic sludge for a period of up to 12 months: storage at 23 ± 2 °C, +4 °C, ‒20 °C, and freeze-dried. Anaerobic sludge was sampled from upper and bottom ports of an up flow anaerobic sludge blanket (UASB) reactor fed with microalgae and sodium acetate at organic loading rate of 5.4 gCOD/L·d. Specific methanogenic activity (SMA) tests were performed on the sludge samples after 2.5, 6, and 12 months of storage. Results demonstrated a statistically significant decrease in the SMA of the bottom port preserved sludge, but not of the upper port sludge, regardless of the method used for preservation. A varying susceptibility to the storage of the two types of the anaerobic sludge can be explained by the content of the methanogenic microorganisms, with bottom port sludge having a higher amount of the methane producing species. Interestingly, lyophilized samples were able to produce similar amounts of biogas when compared to the other three storage conditions, with the only difference of having a longer re-activation period.

Effects of storage temperature on CH4 emissions from cattle manure and subsequent biogas production potential

CH4 is one of the main greenhouse gases (GHGs) generated from agricultural sector, and a significant amount of it is emitted during the storage of livestock manure. To mitigate the CH4 emissions, strong acid addition to the manure was attempted, which is only applicable to slurry-type manure. On the other hand, lowering the storage temperature could be an effective method to reduce the CH4 emissions, particularly applicable to solid-type manure. In this study, cattle manure (CM) with a high-solid content (TS > 30%) was stored at different temperatures (15–35 °C) for 80 d. The highest CH4 emissions of 375.1 kg CO2 eq./ton VS was observed at 35 °C, and this was reduced to less than half at ≤20 °C. Like the difference in CH4 emissions, the degradation of organic matter showed a similar trend. The maximum VS reduction of 29% was observed at 35 °C, while only 8% reduction was observed at 15 °C. Results from microbial community analyses and specific methanogenic activity tests indicated that hydrogenotrophic methanogens were the dominant indigenous CH4-producers, and the abundance of psychrophilic methanogens increased with decreasing temperature. The conservation of organic matter at low temperature led to an increase in biogas production potential from 25 to 43 L CH4/kg CM. It was calculated that the GHGs emissions from electricity consumption for cooling CM below 25 °C can be offset by mitigating CH4 emissions during storage but increasing in subsequent biogas production potential of CM. Compared at 35 °C, 91.6 kg CO2 eq./ton CM of GHGs reduction can be attained at 15 °C.

Impact of solids retention time on the biological performance of an AnMBR treating lipid-rich synthetic dairy wastewater

ABSTRACT In this study, the impact of applied solids retention time (SRT) on the biological performance of an anaerobic membrane bioreactor (AnMBR) treating synthetic dairy wastewater with high lipid content was assessed. Two side-stream AnMBR systems were operated at an SRT of 20 and 40 days (R20 and R40, respectively), equipped with an inside-out tubular membrane operated in cross-flow mode under full-scale operational conditions, i.e. crossflow velocity, transmembrane pressure, membrane flux. Successful operation was achieved and removal efficiencies of both reactors were up to 99% applying an organic loading rate (OLR) of 4.7 g COD L−1 d−1. No precipitation of lipids was observed throughout the operational period, keeping the lipids available for the anaerobic degradation. Long chain fatty acid (LCFA) accumulation was very modest and amounted 148 and 115 mg LCFA-COD per gram of volatile suspended solids (VSS) for R20 and R40, respectively. At an SRT of 40 days, a slightly better biological conversion was obtained. Periodically performed specific methanogenic activity (SMA) tests showed stabilization of the SMA for R40 sludge, whereas for R20 sludge the SMA continued to decrease. This study revealed a more stable reactor performance operating the AnMBR at an SRT of 40 days compared to 20 days.

Evaluation of a zeolite/anaerobic buffled reactor hybrid system for treatment of low bio-degradable effluents.

The main objective of this work was to reduce the inhibitory effects of high contents of organics, ammonia, and heavy metals in an anaerobic buffled reactor (ABR), and to prevent the sludge wash-out using zeolites as media. In this work, a pilot scale of ABR with 8 compartments and a working volume of 14.4 L was used, and the last four ABR compartments were filled with a zeolite. The bioreactor was operated at HRTs of 3, 4, and 5 days, zeolite filling ratios of 10, 20, and 30%, and influent chemical oxygen demand (COD) concentrations of 10,000, 20,000, and 30,000 mg/L. The results obtained showed that the maximum removal efficiencies of COD and BOD5 reached 78 and 68%, respectively. The maximum removal was observed at a HRT of 5 days, a 30% medium filling ratio, and a COD of 10,000 mg/L. Increasing the filling ratio in the reactor increased the removal efficiencies of COD and BOD5 but increasing the concentration of the influent COD and decreasing HRT reduced the removal efficiency of the reactor. The initial BOD5/COD ratio was equal to 0.36, which increased by 46% when the medium filling ratio was elevated to 30%. The maximum biogas yield was 0.23 L/g of CODRemoved, and the specific methanogenic activity test verified the toxicity effect of the leachate on the gas-producer organisms. The results of scanning electronic microscopy and EDS showed that the zeolite medium immobilized the microorganisms and a biofilm was formed. Also the zeolite, as a well-known ion exchanger, decreased the concentrations of the major inhibitors (ammonia and heavy metals) and improved the reactor efficiency.

Comparison of liquid and dewatered digestate as inoculum for anaerobic digestion of organic solid wastes

In this study, the application of liquid and dewatered digestate was compared for the inoculation of the anaerobic digestion (AD) process. In addition to the specific methanogenic activity and biohydrogen potential tests, biochemical methane potential assay was also conducted using four different types of municipal and industrial waste streams (primary sludge, thickened waste activated sludge, source separated organics, and cattle manure). The specific methanogenic activity of the digestate cake (5.0 ± 0.5 mL-CH4/g-VSS.d) was higher than that of the liquid digestate (3.4 ± 0.2 mL-CH4/g-VSS.d) for the food to microorganism ratio of 0.5. The BMP results also revealed that regardless of the type of the substrate used, the application of the digestate cake as inoculum achieved statistically significantly higher methane production rate compared to the utilization of liquid digestate, most likely due to the lower concentration of dissolved contents (i.e., ammonia, soluble organic matter, heavy metals, etc.) in the diluted digestate cake. Despite the increased process rate, no statistically significant effect of the type of the inoculum was observed on the ultimate methane yield. The biohydrogen potential test revealed the similar performance of the digestate- and digestate cake-inoculated digesters in terms of biohydrogen and volatile fatty acids production. The findings of this study suggest that the digestate cake can be used as an effective alternative to the liquid digestate for the inoculation of full-scale anaerobic digesters, reducing the transportation volume by up to 90%.

Effects of pig slurry acidification on methane emissions during storage and subsequent biogas production

In addition to undesirable odorous gases, substantial amounts of greenhouse gases (GHG), particularly methane (CH4), are generated during the storage of livestock manure. To reduce the CH4 emissions, first, pig slurry (PS) was stored for 40 d at 30 °C after adjusting the pH at 5.0–7.0 using H2SO4 solution. In the control (non-acidified PS), 3.7 kg CO2 eq./ton PS of CH4 emissions was detected, which was reduced to 1.8, 0.9, 0.4, 0.2, and 0.1 kg CO2 eq./ton PS at pH 7.0, 6.5, 6.0, 5.5, and 5.0, respectively. Methanosarcina was found to be the dominant genus (67% of the total archaeal sequence) in the control, whose dominance was reduced as storage pH decreased. The results of ribonucleic acid analysis and specific methanogenic activity test further confirmed the inhibition of indigenous methanogens by acidification. Later, the biochemical CH4 potential of stored PS was tested. Compared to the control (10.6 L CH4/L PS), the acidified PS showed higher CH4 yields of 12.7–14.6 L CH4/L PS, presumably by keeping degradable organic matters in PS under acidic condition. Among different acidification pHs tested, the maximum amount of GHG reduction was achieved at pH 6.0 by reducing CH4 emission to +0.4 kg CO2 eq./ton PS during storage while increasing biogas production potential equivalent to 48.3 kWh/ton PS (−22.5 kg CO2 eq./ton PS), resulting in a further reduction of (−)9.6 kg CO2 eq./ton PS compared to the control.

Impact of trace element supplementation on mesophilic anaerobic digestion of food waste using Fe-rich inoculum

Trace elements (TEs) play an indispensable role in enhancing the stability of anaerobic digestion (AD) of food waste (FW). Significant research on AD of FW with TE supplementation has been conducted with low Fe content inoculum. However, the use of Fe-rich inoculum is inevitable due to chemical phosphorous removal from wastewater in North America. We conducted comprehensive mesophilic batch tests to investigate the effect of TEs (Fe, Ni, Co, Se, and Mo) on FW digestion inoculated with Fe-rich sludge (≥ 1000mg FeL-1). This paper presents the impact of supplementing various concentrations of TEs on specific methanogenic activity (SMA), maximum specific methane production rate (SMPRmax), and apparent hydrolysis rate constant (Kh). The addition of TEs adversely impacted methanogenic activity by 20 to 58% in the SMA tests. The effects of individual and mixed supplementation of TEs on the SMPRmax and Kh during FW digestion were negligible; exceptions include Fe, Mo, and Co. Final soluble TE concentrations were 10-29% of the initial soluble TEs. The high Fe concentration in the inoculum reduces the bioavailable fraction of added TEs via coprecipitation. Contrary with many literature reports indicating the need to supplement TE to improve FW digestion efficiency, with Fe-rich sludges, FW digestion does not require TE supplementation.

Effects of CO on hydrogenotrophic methanogenesis under thermophilic and extreme-thermophilic conditions: Microbial community and biomethanation pathways

Coke oven gas is considered as a potential hydrogen source for biogas bio-upgrading. In this study, the effects of CO on biomethanation performance and microbial community structure of hydrogenotrophic mixed cultures were investigated under thermophilic (55 °C) and extreme-thermophilic (70 °C) conditions. 5% (v/v) CO did not inhibit hydrogenotrophic methanogenesis during semi-continuous operation, and 83–97% CO conversion to CH4 was achieved. Methanothermobacter thermoautotrophicus was the dominant methanogen at both temperatures and was the main functional archaea associated with CO biomethanation. Specific methanogenic activity test results showed that long-term 5% CO acclimation shortened the lag phase from 5 h to 1 h at 55 °C and 15 h to 3 h at 70 °C. CO2 was the preferred carbon source over CO for hydrogenotrophic methanogens and CO consumption only started when CO2 was completely depleted. M. thermoautotrophicus dominated mixed cultures showed a great potential in simultaneous hydrogenotrophic methanogenesis and CO biomethanation.

Enhanced methanogenic degradation of cellulose-containing sewage via fungi-methanogens syntrophic association in an anaerobic membrane bioreactor

An anaerobic membrane bioreactor was configured for methanogenic degradation of cellulose-containing sewage. The degradation performance and microbial changes were evaluated under five hydraulic retention times (HRTs). The results indicated the methane production was largely enhanced with 92.6% efficiency of chemical oxygen demand (COD) converting to methane and 80% proportion of methane in produced biogas, meanwhile the biomass yield presented the fewest at the shortest HRT 8h. Enhanced methane production with decreased biomass yield was attributed to an association between fungi and methanogens. Microbial analysis showed fungi Basidiomycota and methanogen Methanoregula apparently established the association, especially Basidiomycota reaching 93% relative abundance at HRT 8h. Specific methanogenic activity (SMA) and biochemical methane potential (BMP) tests suggested the association was derived from H2 production by fungi and H2 consumption by methanogens, during the process of cellulose degradation. The methanogenic degradation of cellulose-containing sewage was markedly promoted via the fungi-methanogens syntrophic association.

Simultaneous wastewater treatment and biogas production using integrated anaerobic baffled reactor granular activated carbon from baker’s yeast wastewater

ABSTRACTIn this study, simultaneous degradation of organic matter and color removal from food processing industries wastewater using an integrated anaerobic baffled reactor granular activated carbon (IABRGAC) was investigated. Theretofore, effective parameters such as hydraulic retention time (HRT) and granular activated carbon (GAC) filling ratio were studied. The bioreactor was operated at 3, 4 and 5 d of HRT and GAC filling ratio of 20%, 35% and 50%. To analyze and optimize the independent operating variables, response surface methodology was applied. Operating condition was optimized for HRT (4 d) and GAC filling ratio (50%). Better COD (94.6%) and BOD (93.7%) removal efficiency occurred with loading COD of 15,000 mg/L, with diminished wastewater color around 54% and turbidity to 54 NTU. In addition, methane production, methane yielding rate (Ym) and specific methanogenic activity (SMA) test in an integrated system were investigated. The system IABRGAC was able to generate a volumetric rate about 0.31 and 0.44 L/g CODremoved d at the experimental condition. The Ym was between 0.31 and 0.44 L/g CODremoved.d and SMA was between 0.13 and 0.38 g COD/g volatile suspended solid. Based on results it can be concluded that the IABRGAC to be a successful pretreatment for highstrength wastewater before discharging the final effluent to sewerage and aerobic treating processes.

Ultrasonically enhanced anaerobic digestion of thickened waste activated sludge using fluidized bed reactors

This study aimed at assessing the impact of ultrasonication on the anaerobic digestibility of thickened waste activated sludge (TWAS) in an anaerobic fluidized bed reactor (AnFBR). Two lab-scale AnFBRs treating TWAS were studied to explore the impact of ultrasonication (US) in the dispersing and reuse of scum for methane production. Our current work applied ultrasound energy at 120–600kJ/d for 2s per 30s corresponding to ultrasonication densities of 6–15.7MJ/kg dry-scum-d. At an organic loading rate (OLR) of 5.1kg COD/m3-d and US energy of 11.9MJ/kg dry-scum-d, scum decreased by 82% from 20.2gm/d to 3.7gm/d, and COD and VSS destruction efficiencies were 65% and 63%, respectively roughly 20% higher than the control reactor without US. Scum reduction varied linearly with US energy about 25kJ/g TS was required to break the scum. The energy balance also indicated that the aforementioned US energy of 11.9MJ/kg dry-scum-d was optimum because of energy neutrality. Specific methanogenic activity (SMA) tests showed that the activity-based sludge retention time (SRT) is higher for the ultrasonicated AnFBR (U-AnFBR) (7.1days) compared to AnFBR (5.1days). Furthermore, a higher rate of maximum specific biogas production (Rm) was observed in the U-AnFBR of 26.7ml/gmVSS-hr as compared to 15.7ml/gmVSS-hr for the control AnFBR, with respective suspended biomass activities of 2ml/gmVSS-hr and 4.1ml/gmVSS-hr i.e. confirming that US improved methanogenic activity in the reactor and reduced the detachment of active methanogens.

Evaluation of the effects of low energetic microwave irradiation on anaerobic digestion

The present study investigates the effects of microwave irradiation on the performance of anaerobic digestion processes. A first set of experiments is performed to distinguish the upper limit of the applied energy levels. Secondly, the effects of these treatments on the performance of the digestion process are evaluated in 3 experimental setups: (i) monitoring the acetic acid degradation, (ii) performing a biological methane potential (BMP) assay and (iii) conducting a specific methanogenic activity (SMA) test. The solubilisation experiment reveals a limited degree of disintegration of anaerobic biomass up to a microwave treatment of 10000 kJ/kg TS. Above this threshold value the soluble COD level started to rise, with up to 350% at 30000 kJ/kg TS regardless of the microwave output power. Because solubilisation of the biomass increases the easily degradable content, this would lead to false observations regarding increased activity. Therefore, solubilisation is minimized by limiting the microwave treatment to a maximum of 6000 kJ/kg TS during the second part of the experiments. Monitoring the degradation of acetic acid after a low intensity microwave treatment, reveals that microwave irradiation shortens the lag phase, e.g., from 21 to 3 h after a microwave treatment of 1000 kJ/kg TS at 100 W. However most treatments also result in a decrease of the maximum degradation and of the degradation rate of acetic acid. BMP assays are performed to evaluate the activity and performance of the entire anaerobic community. Every treatment results in a decreased biogas production potential and decreased biogas production rate. Moreover, each treatment induced an increase of the lag phase. The SMA experiments show no influence of the microwave irradiation in terms of biogas or methane production.

Effect of humic acids on the activity of pure and mixed methanogenic cultures

The impact of humic acid (HA) on methanogenic activity was investigated. Methanogenic crushed granular sludge and pure cultures of mesophilic methanogens were incubated in batch cultures with HA. Initial methane production rates and substrate consumption rates were quantified. In the presence of 1 kg m−3 HA, the methane production rate of all hydrogenotrophic methanogens was inhibited by more than 75%, except Methanospirillum hungatei that was not inhibited up to 5 kg m−3 HA. The acetoclastic Methanosarcina barkeri was completely inhibited by HA ≥1 kg m−3. However, Methanosaeta concilii was only slightly affected by HA up to 3 kg m−3. When methanogenic granular sludge was incubated with HA, the specific methanogenic activity (SMA) tests showed less inhibition, when compared to the pure cultures of methanogens. The SMA test with H2/CO2, formate and acetate showed reduced initial methane production rate of 42%, 23% and 40%, respectively. Differences in HA susceptibility were explained by differences in cell wall structure.