Evaluation of pressurised carbon dioxide pre-treatment aimed at improving the sanitisation and anaerobic digestibility of co-settled sewage sludge.

This work reports on the use of pressurized CO2 pretreatment to improve methane yield and pathogen indicator organism die-off in co-settled sewage sludge (SS). Four semi-continuous mesophilic anaerobic digesters were fed on co-settled SS to establish a baseline for performance and stability. One pair of digesters was then fed with co-settled SS pretreated by P CO2 at 2800 kPa for 23 h. The trial continued for 70 days during which specific biogas and methane production, volatile solids destruction and loss of viability of Escherichia coli was monitored in test and control digesters. The pretreatment had no positive influence on any of these parameters, which was further confirmed using batch biochemical methane potential tests and direct measurement of die-off of E. coli and Salmonella enterica in samples of different sizes treated in pressure vessels of different sizes and in matrices of nutrient medium and co-settled SS. Pressurised CO2 pretreatment was effective at killing fecal indicator bacteria in nutrient medium but ineffectual in SS, strongly suggesting that the nature of suspending matrix was a principle determining factor. Paper concludes that pressurized CO2 pretreatment is not a satisfactory approach to improve either biogas production or pathogen destruction in anaerobic digestion.

Anaerobic digestion of spring and winter wheat: Comparison of net energy yields.

Anaerobic digestion of wheat was investigated under batch conditions. The article compares the potential net energy yield between a winter wheat (sown in the autumn) and a spring wheat (sown in the spring) grown in the same year and harvested at the same growth stage in the same farm. The spring wheat had a slightly higher biochemical methane potential and required lower energy inputs in cultivation, but produced a lower dry biomass yield per hectare, which resulted in winter wheat providing the best overall net energy yield. The difference was small; both varieties gave a good net energy yield. Spring sowing may also offer the opportunity for growing an additional over-winter catch crop for spring harvest, thus increasing the overall biomass yield per hectare, with both crops being potential digester feedstocks.

Enhanced performance of hexavalent chromium reducing cathodes in the presence of Shewanella oneidensis MR-1 and lactate.

Biocathodes for the reduction of the highly toxic hexavalent chromium (Cr(VI)) were investigated using Shewanella oneidensis MR-1 (MR-1) as a biocatalyst and performance was assessed in terms of current production and Cr(VI) reduction. Potentiostatically controlled experiments (-500 mV vs Ag/AgCl) showed that a mediatorless MR-1 biocathode started up under aerated conditions in the presence of lactate, received 5.5 and 1.7 times more electrons for Cr(VI) reduction over a 4 h operating period than controls without lactate and with lactate but without MR-1, respectively. Cr(VI) reduction was also enhanced, with a decrease in concentration over the 4 h operating period of 9 mg/L Cr(VI), compared to only 1 and 3 mg/L, respectively, in the controls. Riboflavin, an electron shuttle mediator naturally produced by MR-1, was also found to have a positive impact in potentiostatically controlled cathodes. Additionally, a microbial fuel cell (MFC) with MR-1 and lactate present in both anode and cathode produced a maximum current density of 32.5 mA/m(2) (1000 Ω external load) after receiving a 10 mg/L Cr(VI) addition in the cathode, and cathodic efficiency increased steadily over an 8 day operation period with successive Cr(VI) additions. In conclusion, effective and continuous Cr(VI) reduction with associated current production were achieved when MR-1 and lactate were both present in the biocathodes.

Anaerobic digestion of two biodegradable municipal waste streams.

Landfill avoidance for organic wastes is now a high priority worldwide. Two fractions of the municipal waste stream were considered with respect to their potential for diversion through controlled anaerobic digestion. The physical and chemical properties of source segregated domestic food waste (ss-FW) and of the mechanically-recovered organic fraction of municipal solid waste (mr-OFMSW) were analysed, and their methane yields determined in both batch and semi-continuous digestion. Methane potentials were compared with predicted values based on biochemical composition, elemental analysis and carbon mass balance, and the differences explained by compositional analysis of feedstocks and digestates. The ss-FW had a higher percentage biodegradability and higher energy potential on a dry weight basis due to the high proportion of proteins and fats in this waste, although the energy potential of the mr-OFMSW was slightly higher on a wet weight (WW) basis. The mr-OFMSW showed very stable digestion characteristics, whereas the ss-FW had a high digestate ammoniacal-N concentration and volatile fatty acid accumulation leading to some process instability. Digestates from semi-continuous trials with mr-OFMSW had high concentrations of potentially toxic elements (PTE) and a lower nutrient content than ss-FW digestate, making the former unsuitable for application to land used in food production.

Effectiveness of pressurised carbon dioxide for inactivation of Escherichia coli isolated from sewage sludge.

This research explored the possible application of pressurised carbon dioxide (P CO(2)), a promising non-thermal sterilisation technique, for the treatment of sewage sludge (SS) before anaerobic digestion to inactivate pathogenic microorganisms. Escherichia coli was selected as the test organism and was isolated from SS and maintained in pure culture. The growth curve of the isolated strain was determined by measuring the optical density (OD) in liquid culture medium and relating this information to the spread plate count so that a culture of known cell density could be grown for optimisation experiments. Inactivation of E. coli was enhanced by increase in pressure (1,500, 2,000 and 2,800 kPa) and treatment time (from 0.75 to 24 h). A short exposure time at high pressure was sufficient to provide a degree of inactivation which could also be achieved by longer exposure at lower pressure. Complete inactivation (8 log(10) reduction) was possible at all three pressures. scanning (SEM) and transmission (TEM) electron microscopy studies of E. coli treated with P CO(2) revealed that the cell walls were ruptured, and the cytoplasm was unevenly distributed and had lost its density, indicating the possible leakage of intracellular substances.

Determination of long chain fatty acids in anaerobic digesters using a rapid non-derivatisation GC-FID method.

A rapid non-derivatisation gas chromatographic (GC) method for quantification of palmitic, stearic and oleic acids was achieved using a flame ionisation detector and a highly polar capillary column at elevated temperature. These long chain fatty acids (LCFA) can accumulate in anaerobic digesters and a simple extraction method was also developed to permit a more rapid sample turn-around time, facilitating more frequent monitoring. The GC method was satisfactory in terms of peak separation, signal response, reproducibility and linearity range. The extraction method achieved recoveries of 103.8, 127.2 and 84.2% for palmitic, stearic and oleic acid respectively. The method was tested on digestate from mesophilic laboratory-scale digesters fed with source-segregated domestic food waste, and showed good repeatability between replicate samples. It was observed that the concentrations of stearic and palmitic acid in digesters routinely supplemented with trace elements were lower in proportion to the applied lipid loading than those without supplementation.

Dynamic changes in anaerobic digester metabolic pathways and microbial populations during acclimatisation to increasing ammonium concentrations.

Transitions in microbial community structure in response to increasing ammonia concentrations were determined by monitoring mesophilic anaerobic digesters seeded with a predominantly acetoclastic methanogenic community from a sewage sludge digester. Ammonia concentration was raised by switching the feed to source segregated domestic food waste and applying two organic loading rates (OLR) and hydraulic retention times (HRT) in paired digesters. One of each pair was dosed with trace elements (TE) known to be essential to the transition, with the other unsupplemented digester acting as a control. Samples taken during the trial were used to determine the metabolic pathway to methanogenesis using 14C labelled acetate. Partitioning of 14C between the product gases was interpreted via an equation to indicate the proportion produced by acetoclastic and hydrogenotrophic routes. Archaeal and selected bacterial groups were identified by 16S rRNA sequencing, to determine relative abundance and diversity. Acclimatisation for digesters with TE was relatively smooth, but OLR and HRT influenced both metabolic route and community structure. The 14C ratio could be used quantitatively and, when interpreted alongside archaeal community structure, showed that at longer HRT and lower loading Methanobacteriaceae were dominant and hydrogenotrophic activity accounted for 77% of methane production. At the higher OLR and shorter HRT, Methanosarcinaceae were dominant with the 14C ratio indicating simultaneous production of methane by acetoclastic and hydrogenotrophic pathways: the first reported observation of this in digestion under mesophilic conditions. Digesters without TE supplementation showed similar initial changes but, as expected failed to complete the transition to stable operation.

Supplementation of ammonium bicarbonates for the treatment of fruit cordial wastewater by anaerobic digestion process.

Lack of nitrogenous substrate and buffering capacity have been identified as causing failure in previous work on the treatment of fruit cordial wastewater using anaerobic continuous stirred tank reactors. In this study, ammonium bicarbonate was proposed to be used as the substrate for nitrogenous and buffering resources. In order to determine the toxicity effect of the ammonium salts on the anaerobic system, a series of concentration from 0 to 40 mg L(-1) was tested. Biogas production was used as the indicator for NH4+ toxicity. The results showed no indication that methanogen was affected by the additional ammonium salt within the dosing regime. Application of the specific mathematical function (G=G(m)(k)(/t)) to describe the kinetic of biogas production, suggested that the optimal concentration of ammonium bicarbonate that can be used is 10 mg L(-1). This study also shows that the dosage regime up to 40 mg L(-1) can be used to supplement the lack of nitrogenous and buffering capacity for the anaerobic digestion process of the fruit cordial wastewater using CSTR.

Impact of low loading on digestion of the mechanically-separated organic fraction of municipal solid waste.

Changing waste management practice, introduction of new technologies, and population demographics and behaviour will impact on both quantity and composition of future waste streams. Laboratory-scale anaerobic digestion of the mechanically-separated organic fraction of municipal solid waste (ms-OFMSW) was carried out at relatively low organic loading rates (OLR), and results analysed using an energy modelling tool. Thermophilic operation with water addition and liquor recycle was compared to co-digestion with dilution water replaced by sewage sludge digestate (SSD); thermophilic and mesophilic mono-digestion were also tested at low OLR. All thermophilic conditions showed stable operation, with specific methane production (SMP) from 0.203 to 0.296 m3 CH4 kg-1 volatile solids (VS). SSD addition increased biogas production by ~20% and there was evidence of further hydrolysis and degradation of the SSD. Long-term operation at 1 kg VS m-3 day-1 had no adverse effect except in mesophilic conditions where SMP was lower at 0.256 m3 CH4 kg-1 VS and stability was reduced, especially during OLR increases. This was probably due to low total ammonia nitrogen, which stabilised at ~0.2 g N kg-1 and limited the buffering capacity. Energy analysis showed thermophilic operation at OLR 2 g VS L-1 day-1 gave 42% of the theoretical methane potential and 38% of the higher heating value, reducing to 37% and 34% respectively in mesophilic conditions. Scenario modelling indicated that under low ms-OFMSW load even an energy-depleted co-substrate such as SSD could contribute to the energy balance, and would be a better diluent than water due to its nutrient and buffering capacity.

A Rapid, Sensitive, Low-Cost Assay for Detecting Hydrogenotrophic Methanogens in Anaerobic Digesters Using Loop-Mediated Isothermal Amplification.

Understanding how the presence, absence, and abundance of different microbial genera supply specific metabolic functions for anaerobic digestion (AD) and how these impact on gas production is critical for a long-term understanding and optimization of the AD process. The strictly anaerobic methanogenic archaea are essential for methane production within AD microbial communities. Methanogens are a phylogenetically diverse group that can be classified into three metabolically distinct lineages based on the substrates they use to produce methane. While process optimization based on physicochemical parameters is well established in AD, measurements that could allow manipulation of the underlying microbial community are seldom used as they tend to be non-specific, expensive, or time-consuming, or a combination of all three. Loop-mediated isothermal amplification (LAMP) assays combine a simple, rapid, low-cost detection technique with high sensitivity and specificity. Here, we describe the optimization of LAMP assays for the detection of four different genera of hydrogenotrophic methanogens: Methanoculleus, Methanothermobacter, Methanococcus, and Methanobrevibacter spp. By targeting archaeal elongation factor 2 (aEF2), these LAMP assays provide a rapid, low-cost, presence/absence indication of hydrogenotrophic methanogens that could be used as a real-time measure of process conditions. The assays were shown to be sensitive to 1 pg of DNA from most tested methanogen species, providing a route to a quantitative measure through simple serial dilution of samples. The LAMP assays described here offer a simple, fast, and affordable method for the specific detection of four different genera of hydrogenotrophic methanogens. Our results indicate that this approach could be developed into a quantitative measure that could provide rapid, low-cost insight into the functioning and optimization of AD and related systems.

Data related to anaerobic digestion of bioplastics: Images and properties of digested bioplastics and digestate, synthetic food waste recipe and packaging information.

The data presented in this article are related to the research article entitled 'Degradation of some EN13432 compliant plastics in simulated mesophilic anaerobic digestion of food waste' (W. Zhang, S. Heaven, C. Banks, 2018). Zhang et al., 2018. They include quantification of residual materials from preparation of a synthetic food waste feedstock; photographic images of the physical appearance of the test plastics after prolonged exposure to microbial degradation in a continuously-operated anaerobic digestion trial; microscopic images of selected plastics after anaerobic biodegradation; test data and results for a Biochemical Methane Potential assay for the plastics; analytical data for potentially toxic elements in the plastics; and values for residual biogas potential of the digestate. Additional data on experimental methods is given, including a recipe for a synthetic food waste specifically designed for use in anaerobic digestion simulation studies; and details on adjustment of calculations after amendment of the digestate sampling methodology used in the main study.

A pilot-scale comparison of mesophilic and thermophilic digestion of source segregated domestic food waste.

Source segregated food waste was collected from domestic properties and its composition determined together with the average weight produced per household, which was 2.91 kg per week. The waste was fed over a trial period lasting 58 weeks to an identical pair of 1.5 m(3) anaerobic digesters, one at a mesophilic (36.5 degrees C) and the other at a thermophilic temperature (56 degrees C). The digesters were monitored daily for gas production, solids destruction and regularly for digestate characteristics including alkalinity, pH, volatile fatty acid (VFA) and ammonia concentrations. Both digesters showed high VFA and ammonia concentrations but in the mesophilic digester the pH remained stable at around 7.4, buffered by a high alkalinity of 13,000 mg l(-1); whereas in the thermophilic digester VFA levels reached 45,000 mg l(-1) causing a drop in pH and digester instability. In the mesophilic digester volatile solids (VS) destruction and specific gas yield were favourable, with 67% of the organic solids being converted to biogas at a methane content of 58% giving a biogas yield of 0.63 m(3) kg(-1) VS(added). Digestion under thermophilic conditions showed potentially better VS destruction at 70% VS and a biogas yield of 0.67 m(3) kg(-1) VS(added), but the shifts in alkalinity and the high VFA concentrations required a reduced loading to be applied. The maximum beneficial loading that could be achieved in the mesophilic digester was 4.0 kg VS m(-3) d(-1).

Continuous operation of thermophilic food waste digestion with side-stream ammonia stripping.

Digesters fed on food waste (high nitrogen content) were operated successfully over an extended period using sidestream biogas stripping to control total ammonia nitrogen (TAN) below inhibitory concentrations. This is the first time biogas stripping has been used to achieve stable thermophilic operation with undiluted substrate of this type. Stripping columns operated batch-wise treated the equivalent of 1.7-4.1% of digester contents daily at pH >10 and 70°C, with no detrimental effect on digestion. TKN removal was 54%, with potential to recover 3.5kgNtonne-1 substrate. When stripping was stopped in one digester TAN increased, accompanied by rising propionic acid concentrations with progressive instability observed from 2.5gNL-1. Eventual failure as TAN approached 5gNL-1 was due to rapid acetic acid accumulation, resulting in a fall in pH to below 6.5. The pattern of VFA accumulation indicated failure of both acetoclastic methanogenesis and acetate oxidation.

In situ biogas stripping of ammonia from a digester using a gas mixing system.

Previous studies have suggested the use of digester biogas mixing systems for in situ ammonia removal from anaerobic digestates. The feasibility of this was tested at moderate and complete gas mixing rates at mesophilic and thermophilic temperatures in a 75-L digester. Experimental results showed that at gas mixing rates typical of full-scale commercial digesters the reduction in total ammonia nitrogen concentrations would be insufficient to allow stable acetoclastic methanogenesis in mesophilic conditions, or to prevent total inhibition of methanogenic activity in thermophilic food waste digestion. Simulation based on batch column stripping experiments at 55°C at gas violent flow rates of 0.032 m3 m-2 min-1 indicated that ammonia concentrations could be reduced below inhibitory values in thermophilic food waste digestion for organic loading rates of up to 6 kg VS m-3 day-1. These mixing rates are far in excess of those used in full-scale gas-mixed digesters and may not be operationally or commercially feasible.

Effect of mean cell residence time on transmembrane flux, mixed-liquor characteristics and overall performance of a submerged anaerobic membrane bioreactor.

Kinetic control of Mean Cell Residence Time (MCRT) was shown to have a significant impact on membrane flux under steady-state conditions. Two laboratory-scale flat-plate submerged anaerobic membrane bioreactors were operated for 245 days on a low-to-intermediate strength substrate with high suspended solids. Transmembrane pressure was maintained at 2.2 kPa throughout four experimental phases, while MCRT in one reactor was progressively reduced. This allowed very accurate measurement of sustainable membrane flux rates at different MCRTs, and hence the degree of membrane fouling. Performance data were gathered on chemical oxygen demand (COD) removal efficiency, and a COD mass balance was constructed accounting for carbon converted into new biomass and that lost in the effluent as dissolved methane. Measurements of growth yield at each MCRT were made, with physical characterisation of each mixed liquor based on capillary suction time. The results showed membrane flux and MLSS filterability was highest at short MCRT, although specific methane production (SMP) was lower since a proportion of COD removal was accounted for by higher biomass yield. There was no advantage in operating at an MCRT <25 days. When considering the most suitable MCRT there is thus a trade-off between membrane performance, SMP and waste sludge yield.

Biogas production from undiluted chicken manure and maize silage: A study of ammonia inhibition in high solids anaerobic digestion.

The feasibility of co-digestion of chicken manure (CM) and maize silage (MS) without water dilution was investigated in 5-L digesters. Specific methane production (SMP) of 0.309LCH4g(-1) volatile solids (VS) was achieved but only at lower %CM. Above a critical threshold for total ammonia nitrogen (TAN), estimated at 7gNL(-1), VFA accumulated with a characteristic increase in acetic acid followed by its reduction and an increase in propionic acid. During this transition the predominant methanogenic pathway was hydrogenotrophic. Methanogenesis was completely inhibited at TAN of 9gNL(-1). The low digestibility of the mixed feedstock led to a rise in digestate TS and a reduction in SMP over the 297-day experimental period. Methanogenesis appeared to be failing in one digester but was recovered by reducing the %CM. Co-digestion was feasible with CM ⩽20% of feedstock VS, and the main limiting factor was ammonia inhibition.

Development and testing of a fully gravitational submerged anaerobic membrane bioreactor for wastewater treatment.

A gravity-operated submerged anaerobic membrane bioreactor (SAnMBR) was set up in order to test its principle of operation as an alternative to conventional pumped permeation of the membrane. This operating mode allowed the membrane flux rate to be measured accurately whilst maintaining a constant transmembrane pressure (TMP), and allowed small transient variations in the flux rate to be observed. The reactor was operated at 36°C for a period of 115 days using a nutrient-balanced synthetic substrate with a high suspended solids concentration. Membrane cleaning was in situ by a gas scouring system using recirculation of headspace biogas. With an initial TMP of 7.0 kPa, the membrane flux slowly decreased due to membrane fouling and had not reached a constant value by day 71. The results indicated that the system was still acclimatizing up to 50 days after start-up; but from that point onwards, performance parameters became much more stable. A constant flux of 2.2 L m(-2) h(-1) was achieved over the last 45 days after the TMP was reduced to 2.3 kPa. The stable flux was maintained over this period and the loading raised to 1 g COD L(-1) d(-1) by increasing the influent strength. Under these conditions, the average chemical oxygen demand removal efficiency was 96% and the specific methane potential was 0.31 L CH4 g(-1) COD removed.

Influence of aqueous environment on agglomeration and dissolution of thiol-functionalised mesoporous silica-coated magnetite nanoparticles.

The purpose of the present research work is to investigate the stability and dissolution of magnetite (Fe3O4) nanoparticles (NPs) and thiol-functionalised mesoporous silica-coated magnetite NPs (TF-SCMNPs). The state of NPs in an aqueous environment was investigated under different pH conditions. Changes in the NPs' mean diameter due to aggregation were measured over a specific time. The effects of contact time and pH on the dissolution of NPs were also investigated. In order to avoid possible aggregation, Fe3O4 NPs were coated with silica and functionalised further with thiol organic groups. These methods imparted excellent stability to magnetite NPs in an aqueous medium over a wide range of pH values with reasonable hydrodynamic size. The organic group bound magnetite NPs allowed these particles to circulate over a long time in the aqueous system, and particle aggregation and sedimentation did not occur. The trend of decreasing zeta potential was observed after grafting thiol onto the surface of the SCMNPs. The results also revealed that silica exhibited a noteworthy efficient in eliminating the pH dependence and enhancing the NP stability of SCMNPs and SH-SCMNPs in aqueous medium. On the other hand, the dissolution of Fe3O4 NPs was found to be detrimental at pH 2.0 and 4.0 or had a long contact time.

Comparison of mesophilic and thermophilic anaerobic digestion of sugar beet pulp: performance, dewaterability and foam control.

Digestion of sugar beet pulp was assessed in relation to biogas and methane production, foaming potential, and digestate dewaterability. Four 4-litre working volume digesters were operated mesophilically (37±0.5 °C) and four thermophilically (55±0.5 °C) over three hydraulic retention times. Digesters were operated in duplicate at organic loading rates (OLR) of 4 and 5 g volatile solids l(-1) day(-1) without water addition. Thermophilic digestion gave higher biogas and methane productivity than mesophilic and was able to operate at the higher OLR, where mesophilic digestion showed signs of instability. Digestate dewaterability was assessed using capillary suction time and frozen image centrifugation. The occurrence of, or potential for, stable foam formation was assessed using a foaming potential test. Thermophilic operation allowed higher loadings to be applied without loss of performance, and gave a digestate with superior dewatering characteristics and very little foaming potential.

Improving the performance of enzymes in hydrolysis of high solids paper pulp derived from MSW.

BACKGROUND: The research aimed to improve the overall conversion efficiency of the CTec® family of enzymes by identifying factors that lead to inhibition and seeking methods to overcome these through process modification and manipulation. The starting material was pulp derived from municipal solid waste and processed in an industrial-scale washing plant. RESULTS: Analysis of the pulp by acid hydrolysis showed a ratio of 55 : 12 : 6 : 24 : 3 of glucan : xylan : araban/galactan/mannan : lignin : ash. At high total solids content (>18.5% TS) single-stage enzyme hydrolysis gave a maximum glucan conversion of 68%. It was found that two-stage hydrolysis could give higher conversion if sugar inhibition was removed by an intermediate fermentation step between hydrolysis stages. This, however, was not as effective as direct removal of the sugar products, including xylose, by washing of the residual pulp at pH 5. This improved the water availability and allowed reactivation of the pulp-bound enzymes. Inhibition of enzyme activity could further be alleviated by replenishment of ß-glucosidase which was shown to be removed during the wash step. CONCLUSIONS: The two-stage hydrolysis process developed could give an overall glucan conversion of 88%, with an average glucose concentration close to 8% in 4 days, thus providing an ideal starting point for ethanol fermentation with a likely yield of 4 wt%. This is a significant improvement over a single-step process. This hydrolysis configuration also provides the potential to recover the sugars associated with residual solids which are diluted when washing hydrolysed pulp.