Search

Phosphorus recovery through struvite precipitation has gained interest due to the potential use of struvite as a fertiliser, with fluidised bed reactors being a popular technology for carrying out the process. Struvite precipitation requires a magnesium source and an alkaline reagent. This research uses a low-grade magnesium oxide (LG-MgO) industrial by-product with a 56 wt% of MgO as magnesium source and an alkaline reagent to lower operating costs and value-add an industrial by-product. LG-MgO is poorly soluble in water, but its solubility increases significantly when dissolved in anaerobic digestion supernatants due to its circumneutral pH and high buffer capacity. Phosphorus precipitation was carried out in a laboratory-scale fluidised bed reactor where three operating variables (i.e. P:Mg molar ratio, feed inlet position, and recirculation flow rate) were studied to determine the LG-MgO impact on precipitate struvite content. Experimental results showed a high struvite content in all precipitates, close to the values reported for pure magnesium sources. The P:Mg molar ratio influenced precipitate composition. The percentage of struvite in the precipitate were 75–82 wt%, 85–88 wt%, and 75–76 wt% for the P:Mg ratio of 1:0.5, 1:1 and 1:3, respectively. The feed inlet position (side or bottom) also had an impact on precipitate struvite content when the P:Mg molar ratio was 1:3, but not for the other molar ratios. The recirculation flow rate did not have a significant impact on precipitate struvite content. • LG-MgO is suitable Mg source and alkaline reagent for struvite precipitation. • The precipitates obtained had struvite as the main mineral phase (75 – 88 wt%). • The struvite content of the precipitate was influenced by the P:Mg molar ratio. • The recirculation flow rate did not affect the struvite content of the precipitate. • The feed position influenced struvite content when the P:Mg molar ratio was 1:3. [ABSTRACT FROM AUTHOR]

This research focuses on the production of a liquid stream rich in volatile fatty acids (VFAs) and low ammoniacal nitrogen content (<0.1 g N/L) from biowaste. The liquid stream was obtained by combining (i) mixed culture acidogenic fermentation to maximise VFA production and (ii) gas-permeable membrane (GPM) contactor to recover ammoniacal nitrogen. Three batch fermentation tests of biowaste collected in a full-scale mechanical-biological treatment plant provided high and stable VFA concentrations (37–39 g COD VFA /L). VFAs represented 73–81 % of the soluble chemical oxygen demand (sCOD) concentration, with a predominance of acetic, propionic and butyric acids. A highly specialized microbial community was observed in all batch tests, with Bacteroidota and Firmicutes as predominant phyla (>90 % of relative abundance). The GPM contactor recovered more than 99 % of the ammoniacal nitrogen in the fermentation liquid without VFA losses. The suitability of the produced fermentation liquid with a high C/N ratio for downstream applications was evaluated using biomethane potential tests (BMP) at different total ammonium nitrogen (TAN) concentrations (0.76–3.15 g N/L) and circumneutral pH. Despite achieving similar ultimate methane yields (279–314 NmL CH 4 /g COD feed), lower TAN concentrations in the biowaste fermentation liquid improved anaerobic biodegradation kinetics, enhancing its potential applicability for methane production. [Display omitted] • VFA-rich fermentation liquids (up to 39 g CODVFA/L) enriched in acetic, propionic and butyric acids were produced using OFMSW. • Members of Bacteroidota and Firmicutes predominated in the microbial community, with a major role for three members of the family Prevotellaceae. • 99 % of TAN in the fermentation liquid (4.1–5.0 g N/L) was recovered using a gas permeable membrane. • Higher TAN content in the fermentation liquid reduced methane production rates (48.4–24.5 NmL CH4/g CODfeed d at 0.76–3.15 g N/L, respectively). [ABSTRACT FROM AUTHOR]

• Grüneck WWTP implemented several strategies to achieve 88% energy self-sufficiency. • Upgrading to rotary lobe blowers reduced net energy consumption by 3.0 kWh PE−1 a−1. • Food waste co-digestion (5.5 t d−1) increased energy production by 5.6 kWh PE−1 a−1. • Co-digestion causes downstream impacts (e.g. reduced dewaterability). • Solar dryer reduced biosolids transportation costs. Population growth, tightening effluent discharge requirements and increasing energy costs are driving the wastewater treatment sector to improve energy efficiency and strive towards energy self-sufficiency. Despite many strategies being proposed for improving energy self-sufficiency at wastewater treatment plants (WWTPs), limited case studies have been conducted. This full-scale case study at Grüneck WWTP evaluates the effectiveness of two different strategies and quantifies their plant-wide impact. Grüneck WWTP increased energy self-sufficiency by 24% (from 64 to 88%) through reducing energy consumption with aeration upgrades (8% increase) and increasing energy production with food waste co-digestion (16% increase). The plant-wide analysis indicated that the aeration upgrades did not affect effluent quality; however co-digesting food waste at 20% additional organic load caused some minor downstream impacts including reduced dewaterability, fluctuating biogas quality and solids accumulation. A solar dryer was installed to manage the increased biosolids production resulting from co-digestion. The dryer reduced biosolids transportation costs by 30% with minimal increase in total plant energy (below 2%). Payback periods for the co-digestion facility and blower upgrade were 10 and 17 months, respectively. The solar dryer, however, has a payback period of 30 years. Findings from this case study provide practical knowledge of the trade-offs for different strategies commonly employed to improve energy self-sufficiency at WWTPs. The results provide evidence that there is significant incentive for similar plants to improve energy self-sufficiency through co-digestion and aeration upgrades. [ABSTRACT FROM AUTHOR]

Inhibition by ammoniacal nitrogen, consisting of free ammonia (NH 3 ) and ammonium ion (NH 4 + ), has been widely investigated for anaerobic digestion. However, despite the large amount of research on the subject, ammoniacal nitrogen inhibition still threatens many anaerobic digesters. This paper presents (i) a method to reliably characterise ammoniacal nitrogen inhibition and (ii) a robust inhibition modelling approach. Results showed that NH 3 and NH 4 + inhibition need to be jointly determined, which can only be done by performing inhibition tests at various total ammoniacal nitrogen (TAN) concentrations and pH values. These test conditions were reliably achieved using the salts NH 4 HCO 3 and NH 4 Cl without pH adjustment, rather than by using NH 4 Cl with pH adjustment. The use of only salts showed a lower pH change during the inhibition test (∼1.5 days), thereby decreasing the uncertainty in TAN speciation and strengthening the test and model outputs. A threshold inhibition function satisfactorily described (R 2  > 0.99) the joint inhibition of NH 3 and NH 4 + on three distinct inocula, and provided a better description of the inhibition testing results than a non-competitive inhibition function (R 2 ∼0.70). The key advantage of the proposed threshold inhibition function is its capacity to identify the inhibition lower limit (concentration where inhibition starts; KI min ) and upper limit (concentration where inhibition is complete; KI max ). The threshold inhibition function also identifies the 50% inhibition concentration (KI 50 ) at the midpoint of KI min and KI max . Finally, experimental and model results show that at pH 7.3–7.7 and TAN concentrations above 2000 mgN·L −1 , both NH 3 and NH 4 + contribute significantly to overall inhibition. [ABSTRACT FROM AUTHOR]

Highlights • Humic acid (HA) inhibition resilience differed with the inoculum source. • Experimental results below 5 gHA L−1 support a threshold inhibition mechanism. • HA < 5 g L−1 led to a time lag, but cellulose hydrolysis rate was unaltered after it. • HA did not affect protein digestion up to 5 gHA L−1. • Methanogenesis was inhibited at 20 gHA L−1. Abstract There is increasing evidence that humic acid (HA) is hampering the performance of anaerobic digesters treating animal manures and thermally-hydrolysed waste activated sludge. In the present study, HA inhibition and inhibition resilience was examined for hydrolysis (carbohydrate and protein) and acetotrophic methanogenesis with four distinct full-scale anaerobic inocula. The aim was to further understand HA inhibition and to explore potential relationships between microbial factors and inhibition resilience. For two of the four tested inocula, cellulose degradation showed a start-up delay that lengthened as HA concentration increased from 0 to 2 g L−1. This inhibition was reversible because, after the initial delay, subsequent hydrolysis rates and methane yields were not significantly influenced by HA concentration. Cellulose hydrolysis results at HA concentrations below 2 g L−1 support a threshold inhibition mechanism, i.e. HA complexes with hydrolytic enzymes preventing them from binding with cellulose, but once all the HA had been complexed, enzymes subsequently released are free to bind with cellulose. Inocula with higher cellulose hydrolytic activity were less affected by HA inhibition, suggesting a potential link between HA inhibition resilience and microbial activity. However, above 5 gHA L−1, cellulose hydrolysis rates decreased with increasing HA concentration; indicating that the mechanisms of inhibition may change depending on some threshold HA concentration. Protein hydrolysis and acetotrophic methanogenesis were less susceptible to HA inhibition than cellulose hydrolysis, since signs of inhibition were only observed above 5 gHA L−1. Acetotrophic methanogenesis was partially inhibited at 10 gHA L−1 and completely inhibited at 20 gHA L−1. These results further support that HA inhibition is selective towards particular enzymes. [ABSTRACT FROM AUTHOR]

The impact of the starting inoculum on long-term anaerobic digestion performance, process functionality and microbial community composition remains unclear. To understand the impact of starting inoculum, active microbial communities from four different full-scale anaerobic digesters were each used to inoculate four continuous lab-scale anaerobic digesters, which were operated identically for 295 days. Digesters were operated at 15 days solid retention time, an organic loading rate of 1 g COD L r −1 d −1 (75:25 - cellulose:casein) and 37 °C. Results showed that long-term process performance, metabolic rates (hydrolytic, acetogenic, and methanogenic) and microbial community are independent of the inoculum source. Digesters process performance converged after 80 days, while metabolic rates and microbial communities converged after 120–145 days. The convergence of the different microbial communities towards a core-community proves that the deterministic factors (process operational conditions) were a stronger driver than the initial microbial community composition. Indeed, the core-community represented 72% of the relative abundance among the four digesters. Moreover, a number of positive correlations were observed between higher metabolic rates and the relative abundance of specific microbial groups. These correlations showed that both substrate consumers and suppliers trigger higher metabolic rates, expanding the knowledge of the nexus between microorganisms and functionality. Overall, these results support that deterministic factors control microbial communities in bioreactors independently of the inoculum source. Hence, it seems plausible that a desired microbial composition and functionality can be achieved by tuning process operational conditions. [ABSTRACT FROM AUTHOR]

Biochemical methane potential (BMP) test is a key analytical technique to assess the implementation and optimisation of anaerobic biotechnologies. However, this technique is characterised by long testing times (from 20 to >100 days), which is not suitable for waste utilities, consulting companies or plants operators whose decision-making processes cannot be held for such a long time. This study develops a statistically robust mathematical strategy using sensitivity functions for early prediction of BMP first-order model parameters, i.e. methane yield (B 0 ) and kinetic constant rate ( k ). The minimum testing time for early parameter estimation showed a potential correlation with the k value, where (i) slowly biodegradable substrates (k ≤ 0.1 d −1 ) have a minimum testing times of ≥15 days, (ii) moderately biodegradable substrates (0.1 < k < 0.2 d −1 ) have a minimum testing times between 8 and 15 days, and (iii) rapidly biodegradable substrates (k ≥ 0.2 d −1 ) have testing times lower than 7 days. [ABSTRACT FROM AUTHOR]

Batch solid-phase anaerobic digestion is a technology for sustainable on-farm treatment of solid residues, but is an emerging technology that is yet to be optimised with respect to start-up and inoculation. In the present study, spent bedding from two piggeries (site A and B) were batch digested at total solids (TS) concentration of 5, 10 and 20% at mesophilic (37 °C) and thermophilic (55 °C) temperatures, without adding an external inoculum. The results showed that the indigenous microbial community present in spent bedding was able to recover the full methane potential of the bedding (140 ± 5 and 227 ± 6 L CH 4 kgVS fed −1 for site A and B, respectively), but longer treatment times were required than for digestion with an added external inoculum. Nonetheless, at high solid loadings (i.e. TS level > 10%), the digestion performance was affected by chemical inhibition due to ammonia and/or humic acid. Thermophilic temperatures did not influence digestion performance but did increase start-up failure risk. Further, inoculation of residues from the batch digestion to subsequent batch enhanced start-up and achieved full methane potential recovery of the bedding. Inoculation with liquid residue (leachate) was preferred over a solid residue, to preserve treatment capacity for fresh substrate. Overall, the study highlighted that indigenous microbial community in the solid manure residue was capable of recovering full methane potential and that solid-phase digestion was ultimately limited by chemical inhibition rather than lack of suitable microbial community. [ABSTRACT FROM AUTHOR]

Mechanical-biological treatment plants treat municipal solid waste to recover recyclable materials, nutrients and energy. Waste paper and cardboard (WP), the second main compound in municipal solid waste (∼30% in weight basis), is typically used for biogas generation. However, its recovery is gaining attention as it can be used to produce add-value products like bioethanol and residual derived fuel. Nevertheless, WP suppression or replacement will impact anaerobic digestion in terms of biogas production, process stability and digestate management. Two lab-scale reactors were used to assess the impact of WP in anaerobic digestion performance. A control reactor was only fed with biowaste (BioW), while a second reactor was fed with two different mixtures of BioW and WP, i.e. 85/15% and 70/30% (weight basis). Results indicate that either replacing half of the WP by BioW or removing half of the WP has little impact on the methane production. When removing half of the WP, methane production could be sustained by a larger waste biodegradability. The replacement of all WP by BioW increased the reactor methane production (∼37%), while removing all WP would have reduced the methane production about 15%. Finally, replacing WP loading rate by BioW led to a system less tolerant to instability periods and with poorer digestate quality. [ABSTRACT FROM AUTHOR]

A leachbed is a relatively simple anaerobic digester suitable for high-solids residues and on-farm applications. However, performance characteristics and optimal configuration of leachbeds are not well-understood. In this study, two 200 L pilot-scale leachbeds fed with spent straw bedding from pigs/swine (methane potential, B 0 = 195–218 L CH 4 kg −1 VS fed ) were used to assess the effects of leachate recirculation mode (trickling vs. flood-and-drain) on the digestion performance. Results showed comparable substrate solubilisation extents (30–45% of total chemical oxygen demand fed) and methane conversion (50% of the B 0 ) for the trickling and flood-and-drain modes, indicating that digestion performance was insensitive to the mode of leachate flow. However, the flood-and-drain leachbed mobilised more particulates into the leachate than the trickling leachbed, an undesirable outcome, because these particulates were mostly non-biodegradable. Inoculation with solid residues from a previous leachbed (inoculum-to-substrate ratio of 0.22 on a VS basis) hastened the leachbed start-up, but methane recovery remained at 50% of the B 0 regardless of the leachate recirculation mode. Post-digestion testing indicated that the leachbeds may have been limited by microbial activity/inhibition. The high residual methane potential of leachate from the trickling (residual B o = 732 ± 7 L CH 4 kg −1 VS fed ) and flood-and-drain leachbeds (582 ± 8 L CH 4 kg −1 VS fed ) indicated an opportunity for further processing of leachate via a separate methanogenic step. Overall, a trickling leachbed appeared to be more favourable than the flood-and-drain leachbed for treating spent bedding at farm-scale due to easier operation. [ABSTRACT FROM AUTHOR]

There is a growing trend to consider organic wastes as potential sources of renewable energy and value-add products. Fermentation products have emerged as attractive value-add option due to relative easy production and broad application range. However, pre-fermentation and extraction of soluble products may impact down-stream treatment processes, particularly energy recovery by anaerobic digestion. This paper investigates primary sludge pre-fermentation at different temperatures (20, 37, 55, and 70 °C), treatment times (12, 24, 48, and 72 h), and oxygen availability (semi-aerobic, anaerobic); and its impact on anaerobic digestion. Pre-fermentation at 20 and 37 °C succeeded for VFA production with acetate and propionate being major products. Pre-fermentation at 37, 55, and 70 °C resulted in higher solubilisation yield but it reduced sludge methane potential by 20%. Under semi-aerobic conditions, pre-fermentation allowed both VFA recovery (43 g COD VFA kg −1 VS) and improved methane potential. The latter phenomenon was linked to fungi that colonised the sludge top layer during pre-fermentation. [ABSTRACT FROM AUTHOR]

The present study evaluates the effect of moisture in low-temperature and ultrasound pretreatment on lignocellulosic substrates anaerobic biodegradability, where brewer’s spent grain was used as model substrate. Besides moisture content, low-temperature pretreatment was also evaluated in terms of temperature (60–80 °C) and exposure time (12–72 h). Likewise, ultrasonication was also evaluated in terms of specific energy (1000–50,000 kJ kgTS −1 ). In addition, the effect of substrate particle size reduction by milling pretreatment was also considered. The results clearly demonstrated that substrate moisture (total solid concentration) is a significant parameter for pretreatment performance, although it has been rarely considered in pretreatment optimisation. Specifically, moisture optimisation increased the methane yield of brewer’s spent grain by 6% for low-temperature pretreatment (60 °C), and by 14% for ultrasound pretreatment (1000 kJ kgTS −1 ) towards the control (without pretreatment). In both pretreatments, the experimental optimum total solid concentration was 100 gTS kg −1 . Thus, lowering substrate moisture, a strategy suggested attaining energetic pretreatment feasibility, needs to be analysed as another pretreatment variable since it might have limited correlation. Finally, a preliminary energetic balance of the pretreatments under study showed that the extra methane production could not cover the energetic pretreatment expenses. [ABSTRACT FROM AUTHOR]

Despite the importance of quantifying inhibitory capacity of compounds in anaerobic digestion, there is currently no well-defined method to assess it. Experimental methods in literature are frequently time-consuming and resource intensive. As a result, detailed inhibition testing rarely forms part of anaerobic digestion studies, despite the importance and utility of this information. This study develops and validates a simple and rapid inhibition test protocol, based on relative inhibition of acetoclastic methanogens. The inhibition potential of a compound is determined from the reduction in specific methanogenic activity as inhibitor concentration is increased. The method was successfully performed on two inoculums from different source environments and with both biostatic and biocidal inhibitors. Optimisation work indicated that: (i) sodium acetate is a preferred carbon source compared to acetic acid; (ii) an inoculum to acetate ratio of 5 g VS g −1 acetate is preferred, and (iii) that the inoculum concentration should be normalised to 10 g L −1 VS to reduce mass transfer problems and promote consistency. A key advantage over existing methods is that the sampling strategy has been optimised to three events over 1.5 days while effectively controlling the relative analytical error. [ABSTRACT FROM AUTHOR]

The feasibility of coupling anaerobic digestion and struvite precipitation in the same reactor was evaluated to enhance manure anaerobic digestion methane yields through ammonia inhibition mitigation. Five different magnesium sources were tested as a struvite (ammonia sequestration agent) precursor, i.e. MgCl 2 , Mg(OH) 2 , two industrial by-products rich in MgO but with different reactivity, and a stabilizing agent. The latter was formulated in advance with the low reactivity industrial by-product and phosphoric acid. The effect of each magnesium source on anaerobic digestion as well as its struvite precipitation capacity was evaluated through biomethane potential tests. Results indicated that all magnesium sources were able to reduce ammonia concentration to different extents. However, the stabilizing agent was the unique magnesium source that did not inhibit the anaerobic digestion process. The avoidance of adding a phosphate source directly into the digester medium and the high newberyite content were the advantages of the stabilizing agent over the other magnesium sources. Finally, a series of experiments indicated that if anaerobic digestion and struvite precipitation are combined in a single reactor, stabilizing agent addition should be carried out through several small additions rather than few large additions. [ABSTRACT FROM AUTHOR]

This paper investigates anaerobic co-digestion of pig manure and algae ( Scenedesmus sp.) with and without extraction of intracellular algal co-products, with views towards the development of a biorefinery concept for lipid, protein and/or biogas production. Protein and/or lipids were extracted from Scenedesmus sp. using free nitrous acid pre-treatments and solvent-based Soxhlet extraction, respectively. Processing increased algae methane yield between 29% and 37% compared to raw algae (VS basis), but reduced the amount of algae available for digestion. Co-digestion experiments showed a synergy between pig manure and raw algae that increased raw algae methane yield from 0.163 to 0.245 m 3 CH 4 kg −1 VS. No such synergy was observed when algal residues were co-digested with pig manure. Finally, experimental results were used to develop a high-level concept for an integrated biorefinery processing pig manure and onsite cultivated algae, evaluating methane production and co-product recovery per mass of pig manure entering the refinery. [ABSTRACT FROM AUTHOR]

Some existing anaerobic digesters treating sewage sludge have a non-used capacity. The use of this extra capacity by introducing additional wastes to conduct the co-digestion could enhance biogas production and plant economic feasibility. Fruit wastes from the food industry could be proper co-substrates due to their high biodegradability, but the harvesting seasons require the use of different kind of fruits causing many transitory conditions throughout the year. Two lab-scale continuous anaerobic digesters treating sewage sludge were operated, one as a reference reactor and the other one as a co-digester. The transitory state was evaluated when fruit waste supply was started, when the co-substrate was changed (peach, banana and apple waste) and when fruit waste supply was stopped. In the transition from mono- to co-digestion, volatile fatty acids concentration rose from 0.07 to 1.70 g L −1 due to the OLR increase, but this situation was recovered in only 5 days. The introduction of different kind of fruit wastes resulted in an alteration of alkalinity, without affecting volatile fatty acids concentration, and in an increase of methane production between 110% and 180% depending on the characteristics of the co-substrate. Finally, when co-digestion was stopped, the parameters converged, at different rates, to the values recorded in the reference digester. It could be concluded that the change of one co-substrate by another one of the same type did not lead to system instability. [ABSTRACT FROM AUTHOR]

Anaerobic codigestion (AcoD) is a proven option to significantly boost biogas production while utilizing existing digesters and infrastructure. The aim of the present research was to conduct an exhaustive study regarding anaerobic codigestion of mixed sewage sludge and crude glycerol considering impacts on organic load, hydraulic load, process performance and microbial community. The methane potential of crude glycerol varied from 370 mL CH 4 ·g −1 VS to 483 mL CH 4 ·g −1 VS for different samples tested. The half maximal inhibitory concentration of crude glycerol was 1.01 g VS L −1 , and the primary mechanism of inhibition was through overload from rapid fermentation rather than the presence of toxic compounds in the crude glycerol. In continuous operation over 200 days, feeding glycerol at up to 2% v/v, increased organic load by up to 70% and resulted in a 50% increase in methane production. Glycerol dosing resulted in no change in apparent dewaterability, with both codigestion and control reactors returning values of 22%–24%. Members of the phylum Thermotogae emerged as a niche population during AcoD of sewage sludge and glycerol; however there was no gross change in microbial community structure and only minimal changes in diversity. AcoD did not result in synergisms between sewage sludge and crude glycerol. Actually, at dose rate up to 2% v/v glycerol dosing is still an effective strategy to increase the organic loading rate of continuous anaerobic digesters with minimal impact of the hydraulic retention time. Nonetheless, the dose rate must be managed to: (i) prevent process inhibition and (ii) ensure sufficient degradation time to produce a stable biosolids product. [ABSTRACT FROM AUTHOR]

Anaerobic co-digestion has been widely investigated, but there is limited analysis of interaction between substrates. The objective of this work was to assess the role of carbohydrates, protein and lipids in co-digestion behaviour separately, and together. Two sets of batch tests were done, each set consisting of the mono-digestion of three substrates, and the co-digestion of seven mixtures. The first was done with pure substrates – cellulose, casein and olive oil – while in the second slaughterhouse waste – paunch, blood and fat – were used as carbohydrate, protein and lipid sources, respectively. Synergistic effects were mainly improvement of process kinetics without a significant change in biodegradability. Kinetics improvement was linked to the mitigation of inhibitory compounds, particularly fats dilution. The exception was co-digestion of paunch with lipids, which resulted in an improved final yield with model based analysis indicating the presence of paunch improved degradability of the fatty feed. [ABSTRACT FROM AUTHOR]

Struvite precipitation and pig manure anaerobic digestion were coupled in the same reactor in order to mitigate the inhibitory effect of free ammonia and avoid precipitator costs. The stabilizing agent used to facilitate struvite precipitation was formulated with low-grade magnesium oxide by-product; an approach that would notably reduce struvite processing costs. The interaction between pig manure and stabilizing agent was analyzed in batch experiments, on a wide range of stabilizing agent additions from 5 to 100 kg m-3. The monitoring of the pH and ammonia removal during 24 h showed the high capacity of the stabilizing agent to remove ammonia; removal efficiencies above 80% were obtained from 40 kg m-3. However, a long-term anaerobic digester operation was required to assess the feasibility of the process and to ensure that the stabilizing agent does not introduce any harmful compound for the anaerobic biomass. In this vein, the addition of 5 and 30 kg m-3 of the stabilizing agent in a pig manure continuous digester resulted in a 25% (0.17 m³ kg-1) and a 40% (0.19 m³ kg-1) increase in methane production per mass of volatile solid, respectively, when compared with the reference digester (0.13 m³ kg-1). Moreover, the stability of the process during four hydraulic retention times guarantees that the stabilizing agent did not exert a negative effect on the consortium of microorganisms. Finally, scanning electron microscopy and X-ray diffraction analysis confirmed the presence of struvite as well as two precipitation mechanisms, struvite precipitation on the stabilizing agent surface and in the bulk solution. [ABSTRACT FROM AUTHOR]

Highlights: [•] The capability of anaerobic biomass to stand temperature oscillations was studied. [•] Anaerobic biomass was able to tolerate oscillations from 37 to 45°C. [•] Methanogens could not tolerate oscillations from 37 to 55°C longer than 2h. [•] Propionate is suggested as key parameter when recovering an inhibited digester. [ABSTRACT FROM AUTHOR]

Abstract: Seven mixed sewage sludges from different wastewater treatment plants, which have an anaerobic digester in operation, were evaluated in order to clarify the literature uncertainty with regard to the sewage sludge characterisation and biodegradability. Moreover, a methodology is provided to determine the Anaerobic Digestion Model No. 1 parameters, coefficients and initial state variables as well as a discussion about the accuracy of the first order solubilisation constant, which was obtained through biomethane potential test. The results of the biomethane potential tests showed ultimate methane potentials from 188 to 214 mL CH4 g−1 CODfed, COD removals between 58 and 65% and two homogeneous groups for the first order solubilisation constant: (i) the lowest rate group from 0.23 to 0.35 day−1 and (ii) the highest rate group from 0.27 to 0.43 day−1. However, no statistically significant relationship between the ultimate methane potential or the disintegration constant and the sewage sludge characterisation was found. Next, a methodology based on the sludge characterisation before and after the biomethane potential test was developed to calculate the biodegradable fraction, the composite concentration and stoichiometric coefficients and the soluble COD of the sewage sludge; required parameters for the implementation of the Anaerobic Digestion Model No. 1. The comparison of the experimental and the simulation results proved the consistency of the developed methodology. Nevertheless, an underestimation of the first order solubilisation constant was detected when the experimental results were simulated with the solubilisation constant obtained from the linear regression experimental data fitting. The latter phenomenon could be related to the accumulation of intermediary compounds during the biomethane potential assay. [Copyright &y& Elsevier]

Highlights: [•] Successful thermophilic anaerobic co-digestion between pig manure and glycerol. [•] Glycerol limiting concentration was 3%, on wet-basis. [•] Glycerol provided easy biodegradable matter which led to high biogas flow rates. [•] Larger residence time was required to use co-digestion digestate as soil fertiliser. [Copyright &y& Elsevier]

Abstract: The anaerobic digestion of raw sewage sludge was evaluated in terms of process efficiency and sludge hygienization. Four different scenarios were analyzed, i.e. mesophilic anaerobic digestion, thermophilic anaerobic digestion and mesophilic anaerobic digestion followed by a 60 °C or by an 80 °C hygienization treatment. Digester performance (organic matter removal, process stability and biogas yield) and the hygienization efficiency (reduction of Escherichia coli, somatic coliphages and F-specific RNA phages) were the main examined factors. Moreover, a preliminary economical feasibility study of each option was carried out throughout an energy balance (heat and electricity). The obtained results showed that both thermophilic anaerobic digestion and mesophilic anaerobic digestion followed by a hygienization step were able to produce an effluent sludge that fulfills the American and the European legislation for land application. However, higher removal efficiencies of indicators were obtained when a hygienization post-treatment was present. Regarding the energy balance, it should be noted that all scenarios have a significant energy surplus. Particularly, positive heat balances will be obtained for the thermophilic anaerobic digestion and for the mesophilic anaerobic digestion followed by 60 °C hygienization post-treatment if an additional fresh-sludge/digested sludge heat exchanger is installed for energy recovery. [Copyright &y& Elsevier]

Crude glycerol derived from biodiesel production is characterized by its high concentration of organic carbon and its solubility in water; properties that make it a suitable co-substrate to improve the efficiency of a manure digester. An increase of about 400% in biogas production was obtained under mesophilic conditions when pig manure was co-digested with 4% of glycerol, on a wet-basis, compared to mono-digestion. The increase in biogas production was mainly a consequence of the increase in organic loading rate. However, the differences could also be related to the synergy between both substrates and the carbon-to-nitrogen ratio. Moreover, the analysis of the macro-compounds, protein, lipids, carbohydrates and fibers, showed lower removal efficiencies in the co-digester as the microorganisms obtained nutrients from the soluble carbohydrates provided by the glycerol. The digestate stability, evaluated through a respirometric assay, showed that co-substrate addition does not exert a negative impact on digestate quality. [ABSTRACT FROM AUTHOR]

It is a fact that the rapid increase of biodiesel production over the last years has resulted in the generation of large and constant amounts of glycerine, which is causing an oversupply problem. Since glycerine is a biodegradable organic compound exempt of nitrogen, it can be applied as a co-substrate in the anaerobic digestion process of pig manure (PM). In order to analyze the feasibility of a mixture of pig manure and glycerine in anaerobic processes and to define the effect originated by the nitrogen limitation when large amounts of glycerine are added, several biodegradability batch tests were performed with different mixtures. These were named as: 100% PM, 80% PM, 60% PM, 40% PM and 20% PM, in pig manure wet weight-basis. Furthermore, a modified model based on anaerobic digestion model no.1 (ADM1) was used to simulate the methane production profiles for the mixtures tested. Specifically, both experimental and model results show the power of the co-digestion technology. In particular, the mixture of 80% PM produced the highest methane production with 215 mL CH4 g−1 COD, almost 125% more methane than when pig manure was mono-digested. In contrast, the one with 20% PM was clearly inhibited by the volatile fatty acid due to the low nitrogen concentration of the mixture. In addition, the specific methane production predicted by the model was in good agreement with the experimental results, although in some samples the shape of the profiles did not match perfectly. Moreover, the modified ADM1 appears to be a useful tool to predict the methane production and the limitations related to the lack/excess of nitrogen during the co-digestion process of pig manure and glycerine. [Copyright &y& Elsevier]

The anaerobic digestion of the organic fraction of municipal solid wastes is taking increasing importance in the recent years. The main problem of some anaerobic digestion process is the large quantity of ammonia that is released, especially when high solid digestion is implemented. A fraction of the supernatant is treated and the remaining is recirculated to maintain the reactor in the optimum solids concentration. The question arising is if this recirculation stream should also be treated to improve biogas production. However, when doing the latter the quantity of ammonia inside the reactor increases too which could lead to inhibit the reactor operation. In this paper it appears that not only free ammonia affects the methanogenic fermentation but also ammonium ion concentration. Biogas production profiles are estimated using the Gompertz model. On the other hand, inhibition constants are fitted using a non-competitive inhibition model equation Thus, 50% inhibition of biomethane production was observed at level of 215 and 468 mg NH3_N/L under mesophilic and thermophilic conditions. However, the methane generation under mesophilic and thermophilic conditions was reduced by 50% when ammonium ion reach concentrations of 3,860 and 5,600 mg NH4+_N/L respectively. Under mesophilic conditions, pH higher than 7 impacted the methanogens bacteria negatively. This threshold pH limit, is variable under thermophilic conditions, depending on the total ammonia concentration. [ABSTRACT FROM AUTHOR]

The application of anaerobic membrane bioreactors (AnMBR) for mainstream municipal sewage treatment is almost ready for full-scale implementation. However, some challenges still need to be addressed to make AnMBR technically and economically feasible. This article presents an updated review of five challenges that currently hinder the implementation of AnMBR technology for mainstream sewage treatment: (i) membrane fouling, (ii) process configuration, (iii) process temperature, (iv) sewage sulphate concentration, and (v) sewage low organics concentration. The gel layer appears to be the main responsible for membrane fouling and flux decline being molecules size and morphology critical properties for its formation. The review also discusses the advantages and disadvantages of five novel AnMBR configurations aiming to optimise fouling control. These include the integration of membrane technology with CSTR or upflow digesters, and the utilisation of scouring particles. Psychrophilic temperatures and high sulphate concentrations are two other limiting factors due to their impact on methane yields and membrane performance. Besides the methane dissolved in the effluent and the competition for organic matter between sulphate reducing bacteria and methanogens, the review examines the impact of temperature on microbial kinetics and community, and their combined effect on AnMBR performance. Finally, the review evaluates the possibility to pre-concentrate municipal sewage by forward osmosis. Sewage pre-concentration is an opportunity to reduce the volumetric flow rate and the dissolved methane losses. Overall, the resolution of these challenges requires a compromise solution considering membrane filtration, anaerobic digestion performance and economic feasibility. • Five main challenges were identified for mainstream AnMBR implementation. • Membrane fouling appears influenced by molecules size and morphology. • New AnMBR configurations are gaining attention to improve fouling control. • Psychrophilic temperatures and influent sulphate are limiting factors for AnMBRs. • Sewage pre-concentration could improve the applicability of AnMBR technology. [ABSTRACT FROM AUTHOR]

The relationship between anaerobic digestion operational conditions and (i) microbial community, (ii) acetoclastic methanogenic activity and (iii) free ammonia (NH 3 ) inhibition resilience was investigated. Thirteen inocula were obtained from full and pilot scale digesters fed with different substrates, digester configurations, operating temperatures and NH 3 concentrations (0.1–241 mgN·L −1 ). Substrate type and temperature were the primary factors influencing microbial community composition. Methanogenic activity ranged from 0.04 to 0.14 gCOD-CH 4 ·g −1 VS·day −1 , and was significantly correlated with archaeal relative abundance and archaeal community PC2. The variability of NH 3 resilience among inocula was moderate, with inhibition threshold values (KI 50 ) ranging between 32 and 175 mgNH 3 -N·L −1 . No microbial or operational factors correlated with NH 3 resilience. However, the slopes of inhibition threshold curves were influenced by some environmental factors, namely substrate type, digester temperature and NH 3 concentration. Overall, these results indicate that low and moderate background NH 3 concentrations is not a key determinant of microbial community nor NH 3 resilience. [ABSTRACT FROM AUTHOR]

The pre-treatment of the organic fraction of municipal solid waste is one of the main challenges in mechanical–biological treatment plants equipped with anaerobic digesters. The present study shows the successful revamping of the Sant Adrià del Besós plant pre-treatment line, where a prototypal optical sorter, a wet-crusher, and a hydrocyclone–decanter substituted the previous pulper and decanter system. The prototypal optical sorter, which uses near-infrared spectroscopy, was able to enhance the organic matter content from 40% to 60% in a more efficient and less energy demanding way than conventional systems. The new configuration not only improved digester feedstock and performance but also led to a significant reduction of the treatment and maintenance costs. The high methane yield of the digester, between 480 and 580 CH 4 m 3 t −1 VS, together with an energy efficiency index of 2.2 kW h produced /kW h consumed , demonstrated the viability of this novel configuration with respect to conventional ones. Thus, optical sorting arises as a new alternative for mechanical–biological treatment plants that needs to be constructed or revamped. [ABSTRACT FROM AUTHOR]

Abstract: Agro-residues account for a large proportion of the wastes generated around the world. There is thus a need for a model to simulate the anaerobic digestion processes used in their treatment. We have developed model based on ADM1, to be applied to agro-wastes. We examined and tested the biodegradability of apple, pear, orange, rape, sunflower, pig manure and glycerol wastes to be used as the basis for feeding the model. Moreover, the fractions of particulate COD (X c) were calculated, and the disintegration constant was obtained from biodegradability profiles, considering disintegration to be the limiting process. The other kinetic and stoichiometric parameters were taken from the ADM1 model. The model operating under mono-substrate and co-substrate conditions was then validated with batch tests. At the same time the model was validated on a continuous anaerobic reactor operating with pig manure at lab scale. In both cases the correlation between the model and the experimental results was satisfactory. We conclude that the anaerobic digestion model is a reliable tool for the design and operation of plants in which agro-wastes are treated. [Copyright &y& Elsevier]

[Display omitted] • Fractional calculus is a promising tool for engineering and biological applications. • Fractional calculus is a robust generalization of first-order models. • Fractional calculus accurately estimates methane yield and degradation rate. • The model adapts to diverse scenarios, capturing varied degradation kinetics. This study presents a fractional calculus model as a generalized kinetic model for estimating the maximum methane yield and degradation kinetics in biomethane potential (BMP) assays, a key analytical method in anaerobic digestion research and application. The fractional model outperformed common first-order kinetic models by yielding superior data fitting and properly managing substrate heterogeneity. The fractional model showed robust performance in mono-digestion, co-digestion and pre-treatment BMP assays with or without presence of large tailing or sigmoidal patterns in the BMP curve. The main advantage of the fractional model over other models is its ability to capture the complexities of the methane production process without losing model accuracy. Assessment of the mathematical model revealed that for fractional orders greater than 0.8 the Mittag-Leffler sequence could be transformed into a more computationally efficient exponential function. [ABSTRACT FROM AUTHOR]

This review aims at providing a unified methodology for free ammonia nitrogen (FAN) calculation in anaerobic digesters, also identifying the factors causing the huge disparity in FAN inhibitory limits. Results show that assuming ideal equilibria overestimates the FAN concentrations up to 37% when compared to MINTEQA2 Equilibrium Speciation Model, used as reference. The Davies equation led to major improvements. Measuring the concentrations of NH 4 +, Na+ and K+ was enough to achieve major corrections. The best compromise between complexity and accuracy was achieved with a novel modified Davies equation, with systematic differences in FAN concentrations of 2% when compared to MINTEQA2. Applying this modified Davies equation, data from the literature (1590 data points from over 50 scientific studies) were used to recalculate FAN inhibitory limits using a clustering approach. This procedure allowed to link inhibition resilience with operational conditions and microbial communities, providing also generalized values of inhibitory constants. The results showed that pH and temperature are the main factors affecting FAN inhibition, with thermophilic systems having a higher resilience towards FAN inhibition. The clustering results showed that Methanosaeta -dominated reactors have the lowest resilience towards FAN, verifying the relatively low inhibition limits for acetoclastic archaea. Mixotrophic Methanosarcina dominated at intermediate FAN concentrations, being more resistant than Methanosaeta but less resilient than hydrogenotrophic archaea. Methanoculleus appeared as the most resilient methanogen. This article provides general guidelines for accurate FAN calculation, explaining also how FAN resilience relates to the operational conditions and the microbial communities, underlying the importance of microbial adaptation. Image 1 • The ideal equilibrium equation overestimates the FAN concentration up to 37%. • The Davies equation considering few ions leads to a mayor improvement in most cases. • A more accurate modified Davies equation is proposed. • pH and temperature as main factors determining the resilience to FAN inhibition. • Methanosaeta replaced by hydrogenotrophic archaea at high FAN concentrations. [ABSTRACT FROM AUTHOR]

Anaerobic digestion stands as a key technology in the emerging green energy economy. Mg 2+ has been identified as an important element to improve digesters methane production; however the inhibition risk that high Mg 2+ concentrations can cause to the AD process must also be considered when dosing Mg reagents and wastes containing Mg 2+ . Despite its importance, Mg 2+ stimulation and inhibition mechanisms as well as threshold values are scarce in the literature. This research paper investigates the impact (stimulation and inhibition) of Mg 2+ on pig manure anaerobic digestion. Mathematical modelling was used to better understand the interaction between substrate, inoculum and magnesium, where Mg 2+ inhibition was modelled by a n-component non-competitive inhibition function. Modelling was done on absolute curves rather than specific methane productions curves (new approach) to account for the lower background methane production of the inoculum as the Mg 2+ concentration increased. Results showed that no stimulation or inhibition occurred between 40 (native concentration) and 400 mg Mg 2+ L −1 , while minor and major inhibition were observed at 750 and 1000 mg Mg 2+ L −1 , and at 2000 and 4000 mg Mg 2+ L −1 , respectively. Mg 2+ half maximal inhibition concentration was estimated at 2140 mg Mg 2+ L −1 with an inhibition order of 2. The latter indicates that Mg 2+ inhibition is a progressive rather than a steep inhibition mechanism. [ABSTRACT FROM AUTHOR]

Anaerobic digestion is a worldwide technology for the treatment of organic waste streams with clear environmental benefits including generation of methane as renewable energy. However, the need to improve process feasibility of existing applications as well as to expand anaerobic digestion to a range of new substrates has raised interest on several intensifications techniques. Among them, the supplementation of inorganic and biological additives has shown good results at improving digesters performance. This manuscript presents a comprehensive review about recent advances in the utilization of inorganic and biological additives. On the one hand, reviewed inorganic additives comprise: (i) macro- (e.g. P, N and S) and micro- (e.g. Fe, Ni, Mo, Co, W and Se) nutrients supplements, (ii) ashes from waste incineration, (iii) compounds able to mitigate ammonia inhibition, and (iv) substances with high biomass immobilization capacity. Among them, iron (Fe 0 and Fe(III)) has shown particularly promising results, which have been mainly related to their action as electron donor/acceptor and cofactor of key enzymatic activities. On the other hand, reviewed biological additives include: (i) the dosage of microbial inocula with high hydrolytic or methanogenic activity (bioaugmentation), and (ii) the addition of enzymes able to facilitate particulate organic matter solubilization. [ABSTRACT FROM AUTHOR]

The Gamma model is a novel approach to characterise the complex degradation dynamics taking place during anaerobic digestion. This three parameters model results from combining the first-order kinetic model and the Gamma distribution function. In contrast to conventional models, where the kinetic constant is considered invariant, the Gamma model allows analysing the variability of the kinetic constant using a probability density function. The kinetic constant of mono-digestion and co-digestion batch tests of different wastes were modelled using the Gamma model and two common first-order models: one-step one-fraction model and one-step two-fraction model. The Gamma distribution function approximates three distinct probability density functions, i.e. exponential, log-normal, and delta Dirac. Specifically, (i) cattle paunch and pig manure approximated a log-normal distribution; (ii) cattle manure and microalgae approximated an exponential distribution, and (iii) primary sludge and cellulose approximated a delta Dirac distribution. The Gamma model was able to characterise two distinct waste activated sludge, one approximated to a log-normal distribution and the other to an exponential distribution. The same cellulose was tested with two different inocula; in both tests, the Gamma distribution function approximated a delta Dirac function but with a different kinetic value. The potential and consistency of Gamma model were also evident when analysing pig manure and microalgae co-digestion batch tests since (i) the mean k of the co-digestion tests were within the values of the mono-digestion tests, and (ii) the profile of the density function transitioned from log-normal to exponential distribution as the percentage of microalgae in the mixture increased. [Display omitted] • Gamma model as a new way to explore anaerobic digestion kinetics. • Gamma model allows mathematising the hidden variability of kinetic constants. • Each substrate displayed a characteristic distribution function. • Gamma model could model kinetic changes related to anaerobic co-digestion. [ABSTRACT FROM AUTHOR]

Biogas upgrading using bottom ash from the incineration of municipal solid waste (MSW) is especially interesting for integrated treatment facilities as it would allow closing a complete loop in MSW management. The aim of this research was to assess the interaction between bottom ash (BA) and biogas from anaerobic digestion with the outlook of upgrading in terms of CO 2 and H 2 S removal, with particle size and bottom ash aging as the main variables. The finest fraction of fresh bottom ash (FBA) presented the best performance, with a sorption capacity of 30–50 kg CO 2 ·t −1 dry FBA. The sorption capacity of weathered bottom ash (WBA) was lower than the recorded for the fresh material. From the results obtained it can be stated that the CO 2 absorption capacity of bottom ash mainly relies in the finest fraction because of its larger surface area and higher CaO content. In what respect to H 2 S sorption capacity, WBA presented a better performance, with values ranging 3.1–5.2 kg H 2 S∙t −1 dry WBA. In addition to the energetic enhancement, the proposed technology also allows an immediate stabilization and reuse of bottom ash as the leaching of heavy metals and metalloids remained under regulatory limits. [ABSTRACT FROM AUTHOR]

Waste activated sludge is slower to biodegrade under anaerobic conditions than is primary sludge due to the glycan strands present in microbial cell walls. The use of pre-treatments may help to disrupt cell membranes and improve waste activated sludge biodegradability. In the present study, the effect of ultrasound, low-temperature thermal and alkali pre-treatments on the rheology, hygienization and biodegradability of waste activated sludge was evaluated. The optimum condition of each pre-treatment was selected based on rheological criteria (reduction of steady state viscosity) and hygienization levels (reduction of Escherichia coli, somatic coliphages and spores of sulfite-reducing clostridia). The three pre-treatments were able to reduce the viscosity of the sludge, and this reduction was greater with increasing treatment intensity. However, only the alkali and thermal conditioning allowed the hygienization of the sludge, whereas the ultrasonication did not exhibit any notorious effect on microbial indicators populations. The selected optimum conditions were as follows: 27,000 kJ/kg TS for the ultrasound, 80 °C during 15 min for the thermal and 157 g NaOH/kg TS for the alkali. Afterward, the specific methane production was evaluated through biomethane potential tests at the specified optimum conditions. The alkali pre-treatment exhibited the greatest methane production increase (34%) followed by the ultrasonication (13%), whereas the thermal pre-treatment presented a methane potential similar to the untreated sludge. Finally, an assessment of the different treatment scenarios was conducted considering the results together with an energy balance, which revealed that the ultrasound and alkali treatments entailed higher costs. [ABSTRACT FROM AUTHOR]

Abstract: Anaerobic digestion is a commercial reality for several kinds of waste. Nonetheless, anaerobic digestion of single substrates presents some drawbacks linked to substrate characteristics. Anaerobic co-digestion, the simultaneous digestion of two or more substrates, is a feasible option to overcome the drawbacks of mono-digestion and to improve plant׳s economic feasibility. At present, since 50% of the publication has been published in the last two years, anaerobic co-digestion can be considered the most relevant topic within anaerobic digestion research. The aim of this paper is to present a review of the achievements and perspectives of anaerobic co-digestion within the period 2010–2013, which represents a continuation of the previous review made by the authors [3]. In the present review, the publications have been classified as for the main substrate, i.e., animal manures, sewage sludge and biowaste. Animal manures stand as the most reported substrate, agro-industrial waste and the organic fraction of the municipal solid waste being the most reported co-substrate. Special emphasis has been made to the effect of the co-digestion over digestate quality, since land application seems to be the best option for digestate recycling. Traditionally, anaerobic co-digestion between sewage sludge and the organic fraction of the municipal solid waste has been the most reported co-digestion mixture. However, between 2010 and 2013 the publications dealing with fats, oils and greases and algae as sludge co-substrate have increased. This is because both co-substrates can be obtained at the same wastewater treatment plant. In contrast, biowaste as a main substrate has not been as studied as manures or sewage sludge. Finally, three interdisciplinary sections have been written for addressing novelty aspects in anaerobic co-digestion, i.e., pre-treatments, microbial dynamics and modeling. However, much effort needs to be done in these later aspects to better understand and predict anaerobic co-digestion. [Copyright &y& Elsevier]