Characterising and modelling free ammonia and ammonium inhibition in anaerobic systems

A novel strategy for integration of oxidation within advanced thermal hydrolysis of sludge

Due to its environmental impact, the growing production of sewage sludge is a prime concern for wastewater treatment plants. In this study, advanced thermal hydrolysis, the combination of thermal hydrolysis and oxygen, was examined to enhance biogas production and overcome the disadvantages of thermal hydrolysis, including sludge colour, high energy consumption, and high level of ammonia concentration in the treated sludge. A mixture of 55 % primary sludge and 45 % waste activated sludge was pre-treated using advanced thermal hydrolysis at 100, 115, 130, and 145 °C with a processing time varied from 5 to 30 min and oxygen pressure from 10 to 30 bar before anaerobic digestion. Advanced thermal hydrolysis process at 145 °C 15 min 20 bar O₂ is the condition that provided the highest biogas yield (439.6 mL/g VS added). At this treatment condition, the concentration of ammonia nitrogen and propionic acid in the treated sludge was sufficiently low (approximately 302 mg/L and 559.7 mg/L, respectively) to minimise adverse effects on anaerobic digestion.

Modifications to the anaerobic digestion model no. 1 (ADM1) for enhanced understanding and application of the anaerobic treatment processes - A comprehensive review

Anaerobic digestion (AD) is a promising method for the recovery of resources and energy from organic wastes. Correspondingly, AD modelling has also been developed in recent years. The International Water Association (IWA) Anaerobic Digestion Model No. 1 (ADM1) is currently the most commonly used structured AD model. However, as substrates become more complex and our understanding of the AD mechanism grows, both systematic and specific modifications have been applied to the ADM1. Modified models have provided a diverse range of application besides AD processes, such as fermentation and biogas upgrading processes. This paper reviews research on the modification of the ADM1, with a particular focus on processes, kinetics, stoichiometry and parameters, which are the major elements of the model. The paper begins with a brief introduction to the ADM1, followed by a summary of modifications, including extensions to the model structure, modifications to kinetics (including inhibition functions) and stoichiometry, as well as simplifications to the model. The paper also covers kinetic parameter estimation and validation of the model, as well as practical applications of the model to a variety of scenarios. The review highlights the need for improvements in simulating AD and biogas upgrading processes, as well as the lack of full-scale applications to other substrates besides sludge (such as food waste and agricultural waste). Future research directions are suggested for model development based on detailed understanding of the anaerobic treatment mechanisms, and the need to recover of valuable products.

Natural Ecuadorian zeolite: An effective ammonia adsorbent to enhance methane production from swine waste

Anaerobic digestion (AD) of swine waste allows obtaining renewable energy, biofertilizer and the reduction of environmental impacts. However, the low C:N ratio of pig manure generates high concentrations of ammonia nitrogen in the digestion process, reducing methane production. Zeolite is an effective ammonia adsorbent; thus, in this research the ammonia adsorption capacity of natural Ecuadorian zeolite was studied under different operating conditions. Subsequently, its effect on methane production from swine waste was evaluated using three doses of zeolite, 1.0, 4.0 and 8.0 g, in 1 L batch bioreactors. The results showed that the Ecuadorian natural zeolite has an adsorption capacity of around 19 mgNH₃-N gZ^-1 when using ammonium chloride solution and, an adsorption capacity between 37 and 65 mgNH₃-N gZ^-1 using swine waste. On the other hand, the addition of zeolite had a significant effect on methane production (p < 0.01). The zeolite doses that provided the highest methane production were 4.0 and 8.0 g L^-1, which led to values of 0.375 and 0.365 Nm³CH₄ kgVS^-1, compared to the values of 0.350 and 0.343 Nm³CH₄ kgVS^-1 that were obtained for the treatments without addition of zeolite and using a dose of 1.0 g L^-1, respectively. Addition of natural Ecuadorian zeolite meant not only a significant increase on methane production in the AD of swine waste, but also a better quality of the biogas with higher percentages of methane and lower concentrations of H₂S.

Accumulating ammoniacal nitrogen instead of melanoidins determines the anaerobic digestibility of thermally hydrolyzed waste activated sludge

Full-scale thermal hydrolysis processes (THP) showed an increase in nutrients release and formation of melanoidins, which are considered to negatively impact methanogenesis during mesophilic anaerobic digestion (AD). In this research, fractionation of THP-sludge was performed to elucidate the distribution of nutrients and the formed melanoidins over the liquid and solid sludge matrix. Degradation of the different fractions in subsequent AD was assessed, and the results were compared with non-pre-treated waste activated sludge (WAS). Results showed that the THP-formed soluble melanoidins were partially biodegradable under AD, especially the fraction with molecular weight under 1.1 kDa, which was related to protein-like substances. The use of THP in WAS increased the non-biodegradable soluble chemical oxygen demand (sCOD) after AD, from 1.1% to 4.9% of the total COD. The total ammoniacal nitrogen (TAN) concentration only slightly increased during THP without AD. However, after AD, TAN released was 34% higher in the THP-treated WAS compared to non-treated WAS, i.e., 36.7 ± 0.7 compared to 27.4 ± 0.4 mgTANreleased/gCODsubstrate, respectively. Results from modified specific methanogenic activities (mSMAs) tests showed that the organics solubilised during THP, were not inhibitory for acetotrophic methanogens. However, after AD of THP-treated sludge and WAS, the mSMA showed that all analysed samples presented strong inhibition on methanogenesis due to the presence of TAN and associated free ammonia nitrogen (FAN). In specific methanogenic activities (SMAs) tests with incremental concentration of TAN/FAN and melanoidins, TAN/FAN induced strong inhibition on methanogens, halving the SMA at around 2.5 gTAN/L and 100 mgFAN/L. Conversely, melanoidins did not show inhibition on the methanogens. Our present results revealed that when applying THP-AD in full-scale, the increase in TAN/FAN remarkably had a greater impact on AD than the formation of melanoidins.

Thermodynamic restrictions determine ammonia tolerance of methanogenic pathways in Methanosarcina barkeri

Ammonia is a ubiquitous potential inhibitor of anaerobic digestion processes, mainly exhibiting inhibition towards methanogenic activity. However, knowledge as to how ammonia affects the methanogens is still limited. In this study, we cultured a multitrophic methanogen, Methanosarcina barkeri DSM 800, with acetate, H₂/CO₂, and methanol to evaluate the influence of ammonia on different methanogenic pathways. Aceticlastic methanogenesis was more sensitive to increased ammonia concentrations than hydrogenotrophic and methylotrophic methanogenesis. Theoretical maximum NH₃ tolerances of M. barkeri fed with acetate, H₂/CO₂, and methanol were calculated to be 39.1 ± 9.0, 104.3 ± 7.4, and 85.7 ± 1.0 mg/L, respectively. The order of the ΔG range of M. barkeri under three methanogenic pathways reflected the order of ammonia tolerance of M. barkeri. Our results provide insights into the role of the thermodynamic potential of methanogenesis on the tolerance of ammonia stress; and shed light on the mechanism of ammonia inhibition on anaerobic digestion.

How anaerobic sludge microbiome respond to different concentrations of nitrite, nitrate, and ammonium ions: a comparative analysis

High concentrations of ammonium, nitrite, and nitrate always induce inhibition in anaerobic wastewater treatment. Due to the complexity and vulnerability of the microbial community (especially methanogens) in anaerobic sludge, little is understood about its underlying microbial mechanism under such inhibition. In this study, the shifts of microbial communities in anaerobic sludge under increasing levels of nitrite, nitrate, and ammonium ions were compared. Results show that although half maximal inhibitory concentrations (methanogenesis) were different for nitrite, nitrate, and ammonium ions with EC₅₀ values of 12, 30, and 3000 mg N/L, respectively, bacteria genera Kosmotoga and Brooklawnia dominated in all of the three high-stress inhibitory systems. Network analysis and redundancy analysis (RDA) of the microbial community showed the treatments with nitrate and nitrite ions decreased the modularity of anaerobic microorganisms. RDA showed that specific methanogenic activity was positively related to coenzyme F₄₂₀ under nitrite inhibition (r_p = 0.833, p < 0.05) and closely correlated with viability under nitrate inhibition. Gram-positive and nonmotile Brooklawnia genus showed a negative correlation with physiological characteristics in the ammonia treatments, suggesting its high resistance to ammonia.

Current challenges of high-solid anaerobic digestion and possible measures for its effective applications: a review

The attention that high solids anaerobic digestion process (HS-AD) has received over the years, as a waste management and energy recovery process when compared to low solids anaerobic digestion process, can be attributed to its associated benefits including water conservation and smaller digester foot print. However, high solid content of the feedstock involved in the digestion process poses a barrier to the process stability and performance if it is not well managed. In this review, various limitations to effective performance of the HS-AD process, as well as, the possible measures highlighted in various research studies were garnered to serve as a guide for effective industrial application of this technology. A proposed design concept for overcoming substrate and product inhibition thereby improving methane yield and process stability was recommended for optimum performance of the HS-AD process.

Distinguishing responses of acetoclastic and hydrogenotrophic methanogens to ammonia stress in mesophilic mixed cultures

A shift from the acetoclastic to the hydrogenotrophic pathway in methanogenesis under ammonia inhibition is a common observation in anaerobic digestion. However, there are still considerable knowledge gaps concerning the differential ammonia tolerance of acetoclastic and hydrogenotrophic methanogens (AMs and HMs), their responses to different ammonia species (NH₄⁺, NH₃), and their recoverability after ammonia inhibition. With the successful enrichment of mesophilic AMs and HMs cultures, this study aimed at addressing the above knowledge gaps through batch inhibition/recovery tests and kinetic modeling under varying total ammonia (TAN, 0.2-10 g N/L) and pH (7.0-8.5) conditions. The results showed that the tolerance level of HMs to free ammonia (FAN, IC₅₀=1345 mg N/L) and NH₄⁺ (IC₅₀=6050 mg N/L) was nearly 11 times and 3 times those of AMs (NH₃, IC₅₀=123 mg N/L; NH₄⁺, IC₅₀=2133 mg N/L), respectively. Consistent with general belief, the AMs were more impacted by FAN. However, the HMs were more adversely affected by NH₄⁺ when the pH was ≤8.0. A low TAN (1.0-4.0 g N/L) could cause irreversible inhibition of the AMs due to significant cell death, whereas the activity of HMs could be fully or even over recovered from severe ammonia stress (FAN≤ 0.9 g N/L or TAN≤10 g N/L; pH ≤8.0). The different tolerance responses of AMs and HMs might be associated with the cell morphology, multiple energy-converting systems, and Gibbs free energy from substrate-level phosphorylation.

Elucidating the role of solids content in low-temperature thermal hydrolysis and anaerobic digestion of sewage sludge

The role of total solids content in low-temperature thermal hydrolysis and anaerobic digestion of sewage sludge was investigated. Increasing total solids from 2% to 6% improved thermal hydrolysis and anaerobic digestion performance, while increasing it further to 12% decreased methane production. Maximum sludge solubility (22.9% ± 0.6%) and methane production (320 ± 7 mL/g volatile solids) were achieved at 6% solids. The increase in solids content from 2% to 6% improved heating efficiency and volatile fraction content, which facilitated sludge solubilization and enrichment of methanogens. However, further increases in solids content resulted in a stable floc structure with excess ammonia nitrogen and volatile fatty acids, which limited the release of substrates and reduced the abundance of acidifying bacteria and methanogens, ultimately leading to reduced methane production. An in-depth understanding of the role of solids content opens up new avenues for improved low-temperature thermal hydrolysis of sludge.

Pore-Scale Heterogeneities Improve the Degradation of a Self-Inhibiting Substrate: Insights from Reactive Transport Modeling

In situ bioremediation is a common remediation strategy for many groundwater contaminants. It was traditionally believed that (in the absence of mixing-limitations) a better in situ bioremediation is obtained in a more homogeneous medium where the even distribution of both substrate and bacteria facilitates the access of a larger portion of the bacterial community to a higher amount of substrate. Such conclusions were driven with the typical assumption of disregarding substrate inhibitory effects on the metabolic activity of enzymes at high concentration levels. To investigate the influence of pore matrix heterogeneities on substrate inhibition, we use a numerical approach to solve reactive transport processes in the presence of pore-scale heterogeneities. To this end, a rigorous reactive pore network model is developed and used to model the reactive transport of a self-inhibiting substrate under both transient and steady-state conditions through media with various, spatially correlated, pore-size distributions. For the first time, we explore on the basis of a pore-scale model approach the link between pore-size heterogeneities and substrate inhibition. Our results show that for a self-inhibiting substrate, (1) pore-scale heterogeneities can consistently promote degradation rates at toxic levels, (2) the effect reverses when the concentrations fall to levels essential for microbial growth, and (3) an engineered combination of homogeneous and heterogeneous media can increase the overall efficiency of bioremediation.

The inhibitory effects and underlying mechanism of high ammonia stress on sulfide-driven denitrification process

Sulfide-driven denitrification (SD) process has been widely studied for treating wastewater containing sulfate and ammonia in recent years. But influence of high ammonia stress on the SD process and microbial community remained unclear. In this work, a series of tests were conducted to investigate effects of different ammonia stress (200-3000 mg-total ammonia nitrogen (TAN)/L) on denitrification efficiency, byproduct accumulation and microbial community of the SD process. According to our results, the SD process was severely inhibited, and 32.67 ± 5.15 mg/L NO₂^--N was accumulated when ammonia stress reached 3000 mg TAN/L. But the inhibited SD process could recover in about 40 days when ammonia stress was decreased to 200 mg TAN/L. After analyzing the microbial community, Thiobacillus sp. (Thiobacillus sp. 65-29, Thiobacillus sp. SCN 64-317, Thiobacillus sp. 63-78 and Thiobacillus denitrificans) was confirmed as dominant bacteria responsible for the SD process. Further, expression of narG, napA, nirK and nirS were inhibited under high ammonia stress, thus making the SD process stuck in NO₃^- and NO₂^- reduction step. This study reveals the inhibitory effects of high ammonia stress on the SD process and its possible underlying mechanism with discussion in gene level.

Metabolic Regulation of Mesophilic Methanosarcina barkeri to Ammonium Inhibition

Undesirable ammonium concentrations can lead to unstable anaerobic digestion processes, and Methanosarcina spp. are the representative methanogens under inhibition. However, no known work seems to exist for directly exploring the detailed metabolic regulation of pure cultured representative Methanosarcina spp. to ammonium inhibition. We used transcriptomics and proteomics to profile the metabolic regulation of Methanosarcina barkeri to 1, 4, and 7 g N/L of total ammoniacal nitrogen (TAN), where free ammonia concentrations were between 1.5 and 36.1 mg N/L. At the initial stages of ammonium inhibition, the genes participating in the acquisition and assimilation of reduced nitrogen sources showed significant upregulation where the minimal fold change of gene transcription was about 2. Apart from nitrogen metabolism, the transcription of some genes in methanogenesis also significantly increased at the initial stages. For example, the genes encoding alternative heterodisulfide reductase subunits (HdrAB), energy-converting hydrogenase subunit (EchC), and methanophenazine-dependent hydrogenase subunits (VhtAC) were significantly upregulated by at least 2.05 times. For the element translocation at the initial stages, the genes participating in the uptake of ferrous iron, potassium ion, and molybdate were significantly upregulated with a minimal fold change of 2.10. As the cultivation proceeded, the gene encoding the cell division protein subunit (FtsH) was significantly upregulated by 13.0 times at 7 g N/L of TAN; meanwhile, an increment in OD₆₀₀ was observed at the terminal sampling point of 7 g N/L of TAN. The present study explored the metabolic regulation of M. barkeri in stress response, protein synthesis, signal transduction, nitrogen metabolism, methanogenesis, and element translocation. The results would contribute to the understanding of the metabolic effects of ammonium inhibition on methanogens and have significant practical implication in inhibited anaerobic digestion.

Integrated multi-omics analyses reveal the key microbial phylotypes affecting anaerobic digestion performance under ammonia stress

Ammonia inhibition is one of the most common causes of instability during the operation of commercial biogas plants. Here, the sensitivity of different functional bacteria to ammonia stress, the ability of functional bacteria to adapt to ammonia stress, and the key phylotypes affecting anaerobic digestion (AD) performance were studied by evaluating the process performance, active microbiome, and protein expression patterns during endogenous ammonia accumulation using integrated metagenomics and metaproteomics analyses. Acetate metabolism was most sensitive to ammonia stress, and the expression activity of methyl-CoM reductase of Methanothrix was inhibited at relatively low ammonia concentrations, which resulted in the accumulation of acetate and other short-chain volatile fatty acids (VFAs) through feedback effects. As the AD process progressed, the abundance of active Methanosarcina with high ammonia tolerance increased, and the activity of their enzymes related to acetoclastic methanation was significantly up-regulated, which resulted in the complete restoration of acetate metabolism and AD performance. Thus, microbial communities can cope with acetate metabolic repression through self-optimization. In contrast, when the total ammonia nitrogen (TAN) and free ammonia nitrogen (FAN) increased to 4846.95 mg N/L and 337.46 mg N/L, respectively, propionate (and no other VFAs) accumulated in the digester, which was accompanied by a decrease in methane yield by more than 65%. At this time, the abundance of active syntrophic propionate-oxidizing bacteria (SPOB) decreased by 52%, and the expression of key enzymes related to propionate degradation was significantly down-regulated. The proportion of down-regulated differentially expressed proteins in the dominant Pelotomaculum was as high as 94%, indicating the severe suppression of the growth of these functional bacteria as well as their inability to easily acclimate to ammonia stress. Thus, SPOB appeared to be the key microbial phylotypes affecting AD performance under ammonia stress. Ammonia inhibited the methylmalonyl-CoA pathway of Pelotomaculum by inhibiting the expression of succinyl-CoA synthase, which resulted in the suppression of syntrophic propionate oxidation. The results of this study provide new insights into the microbial mechanism of ammonia inhibition and identify the key phylotypes affecting AD performance under ammonia stress. Our findings also shed light on the microbial regulatory targets of nitrogen-rich waste anaerobic digesters.

Sustainable additives for the regulation of NH3 concentration and emissions during the production of biomethane and biohydrogen: A review

This study reviews the recent advances and innovations in the application of additives to improve biomethane and biohydrogen production. Biochar, nanostructured materials, novel biopolymers, zeolites, and clays are described in terms of chemical composition, properties and impact on anaerobic digestion, dark fermentation, and photofermentation. These additives can have both a simple physical effect of microbial adhesion and growth, and a more complex biochemical impact on the regulation of key parameters for CH₄ and H₂ production: in this study, these effects in different experimental conditions are reviewed and described. The considered parameters include pH, volatile fatty acids (VFA), C:N ratio, and NH₃; additionally, the global impact on the total production yield of biogas and bioH₂ is reviewed. A special focus is given to NH₃, due to its strong inhibition effect towards methanogens, and its contribution to digestate quality, leaching, and emissions into the atmosphere.

Optimising the Flux Enhancer Dosing Strategy in a Pilot-Scale Anaerobic Membrane Bioreactor by Mathematical Modelling

Flux enhancers (FEs) have been successfully applied for fouling mitigation in membrane bioreactors. However, more research is needed to compare and optimise different dosing strategies to improve the filtration performance, while minimising the use of FEs and preventing overdosing. Therefore, the goal of this research is to develop an optimised control strategy for FE dosing into an AnMBR by developing a comprehensive integrated mathematical model. The integrated model includes filtration, flocculation, and biochemical processes to predict the effect of FE dosing on sludge filterability and membrane fouling rate in an AnMBR. The biochemical model was based on an ADM1, modified to include FEs and colloidal material. We developed an empirical model for the FE-induced flocculation of colloidal material. Various alternate filtration models from the literature and our own empirical models were implemented, calibrated, and validated; the best alternatives were selected based on model accuracy and capacity of the model to predict the effect of varying sludge characteristics on the corresponding output, that is fouling rate or sludge filterability. The results showed that fouling rate and sludge filterability were satisfactorily predicted by the selected filtration models. The best integrated model was successfully applied in the simulation environment to compare three feedback and two feedforward control tools to manipulate FE dosing to an AnMBR. The modelling results revealed that the most appropriate control tool was a feedback sludge filterability controller that dosed FEs continuously, referred to as ∆R20_10. Compared to the other control tools, application of the ∆R20_10 controller resulted in a more stable sludge filterability and steady fouling rate, when the AnMBR was subject to specific disturbances. The simulation environment developed in this research was shown to be a useful tool to test strategies for dosing flux enhancer into AnMBRs.

Metal-based flocculation to harvest microalgae: a look beyond separation efficiency

Metal-based flocculants are commonly used for biomass harvesting in microalgae-based bio-refineries. Besides the high separation efficiency, additional aspects should be considered, related to the toxicity of metals for the algal biomass. Partitioning tests for commonly used flocculants (i.e., FeCl₃ and Al₂(SO₄)₃) showed that metals were mostly transferred to the solid phase with more than 95% of dosed metal ending up into the biomass, and low metal concentrations in the liquid effluent (lower than 0.4 mg L^-1 for both metals), thus allowing for water reuse. Photosynthesis inhibition was tested on microalgae and microalgae-bacteria cultures, using a standardized photo-respirometry protocol in which typical concentrations used during coagulation-flocculation were assessed. Modelling dose-response curves, concentrations corresponding to 50% inhibition (IC₅₀) were obtained, describing short-term effects. The obtained IC₅₀ ranged from 13.7 to 28.3 mg Al L^-1 for Al, and from 127.9 to 195.8 mg Fe L^-1 for Fe, showing a higher toxicity for the Al-based flocculant. The recovery of photosynthesis inhibition was also quantified, to evaluate the possibility of reusing/recycling the harvested biomass. The results highlighted that the residual photosynthetic activities, evaluated after 1 h and 24 h of exposure to metals were partially recovered, especially for Al, passing from 67.3% to 94.6% activity, respectively, while long-term Fe effects were stronger (passing from 64.9% to 77.6% activity). A non-toxic flocculant (cationic starch) was finally tested, excluding potential effects due to biomass aggregation, as the reduction of photosynthetic activity only reached 3.4%, compared to control. Relevant modifications to the light availability and the optical properties of algal suspensions were assessed, identifying a strong effect of iron which caused an increase of the light absorbance up to approximately 40% at high Fe concentrations. Possible implications of dosing metallic flocculants in MBWWT processes are discussed, and suggestions are given to perform inhibition tests on flocculating chemicals.

The interaction between lipids and ammoniacal nitrogen mitigates inhibition in mesophilic anaerobic digestion

Ammoniacal nitrogen and long chain fatty acids (LCFA) are common inhibitors of the anaerobic digestion process. However, the interaction between these inhibitors has received little attention. Understanding the interaction between these inhibitors is important to optimise the operation of anaerobic digesters treating slaughterhouse waste or using fat, oil and grease (FOG) as co-substrate among others. To study the interaction between ammoniacal nitrogen and LCFA inhibition, 20 different conditions were trialled in mesophilic batch tests. Experimental conditions included 5 mixtures between slaughterhouse wastewater and LCFA (100:0, 75:25, 50:50, 20:80, 0:100 on a VS basis), each one tested at 4 different ammoniacal nitrogen concentrations (0, 1, 3, 6 gN_added·L^-1). Experimental and modelling results showed that ammoniacal nitrogen inhibition was less severe in LCFA-rich mixtures, indicating that LCFA mitigated ammoniacal nitrogen inhibition to a certain extent. However, the positive interaction between inhibitors did not only depend on the LCFA concentration. A protective LCFA coat that limited the diffusion of free ammonia into the cell and/or provided a localised lower pH in the vicinity of the microbial cell could explain the experimental results. However, ammoniacal nitrogen and LCFA inhibition comprise up to 6 different but interrelated inhibitors (i.e. NH₃, NH₄⁺, LCFA, VFA, H₂ and pH) and therefore the specific mechanism could not be elucidated. Nonetheless, these results suggest that LCFA do not exacerbate TAN-related inhibition and that LCFA-rich substrates can be utilised as co-substrates in mesophilic N-rich digesters.

Considering syntrophic acetate oxidation and ionic strength improves the performance of models for food waste anaerobic digestion

Current mechanistic anaerobic digestion (AD) models cannot accurately represent the underlying processes occurring during food waste (FW) AD. This work presents an update of the Anaerobic Digestion Model no. 1 (ADM1) to provide accurate estimations of free ammonia concentrations and related inhibition thresholds, and model syntrophic acetate oxidation as acetate-consuming pathway. A modified Davies equation predicted NH₃ concentrations and pH more accurately, and better estimated associated inhibitory limits. Sensitivity analysis results showed the importance of accurate disintegration kinetics and volumetric mass transfer coefficients, as well as volatile fatty acids (VFAs) and hydrogen uptake rates. In contrast to the default ADM1, the modified ADM1 could represent methane production and VFA profiles simultaneously (particularly relevant for propionate uptake). The modified ADM1 was also able to predict the predominant acetate-consuming and methane-producing microbial clades. Modelling results using data from reactors dosed with granular activated carbon showed that this additive improves hydrogen uptake.

Elucidating the kinetics of ammonia inhibition to anaerobic digestion through extended batch experiments and stimulation-inhibition modeling

Ammonia can be accumulated to a level inhibitory to methanogenesis. There are large discrepancies in the reported inhibition thresholds. Through extended batch digestion experiments (up to 110 d) at 6 ammonia concentrations (0.70-13 g N/L), this study discovered sequential occurrence of adaptable and unadaptable inhibition that reveals the discrepancies. Lag phase length representing adaptable inhibition increased exponentially with ammonia concentration. The kinetics of specific biogas yield that reveals unadaptable inhibition was best simulated with the Han & Levenspiel model. The 50% unadaptable inhibition thresholds were 10.7 g N/L with active inoculum and 6.8 g/L with stressed inoculum. The digesters with stressed inoculum had faster adaptation to adaptable inhibition though less resistance to unadaptable inhibition. The inhibition sequence was evidenced by microbial population shifts and confirmed by earlier studies employing short (2-65 d) and long (80-198 d) batch experiments. Distinguishing adaptable from unadaptable inhibition provides precise guidance for mitigating ammonia inhibition.

Exploring the combined effect of total ammonia nitrogen, pH and temperature on anaerobic digestion of chicken manure using response surface methodology and two kinetic models

Ammonia inhibition is the most challenging issue in the anaerobic digestion (AD) of nitrogen-rich substrates. Total ammonia nitrogen (TAN) concentration, temperature and pH are the main operational parameters affecting the chemical equilibrium between free ammonium nitrogen (NH₃; FAN) and ammonium ions (NH₄⁺). However, it is still unclear how these parameters together affect digestion performance by influencing this equilibrium. To determine the effect and linkages of these parameters, a Box-Behnken design-response surface methodology (RSM), correlation analysis and two kinetic models were carried out. The results revealed that the linear effect of TAN, the interaction effect between TAN and temperature, and temperature and pH were significant, however, the quadratic effect of TAN, temperature and pH were also significant. Furthermore, TAN and temperature were positively correlated with M_max, k and R_max. Findings from this study could provide a theoretical basis to develop the way of relieving ammonia inhibition.

Methane production from acetate, formate and H2/CO2 under high ammonia level: Modified ADM1 simulation and microbial characterization

This study evaluated the methanogenic performance of typical substrates (acetate, formate, H₂/CO₂, and glucose) under low and high ammonia levels and the Anaerobic Digestion Model No.1 (ADM1) was extended and modified for better simulation and understanding of the process. Formate-utilizing and hydrogen-utilizing methanogenesis showed stronger ammonia resistance than acetate-utilizing methanogenesis (13-23% vs. 34% decrease in methane production (MP)). Model extension, based on foundational experiments fed with three typical precursors (R² > 0.92), was then validated with glucose degradation experiments, and satisfactory predictions of MP and total volatile fatty acids were obtained (R² > 0.91). Based on the modified ADM1, the carbon fluxes of glucose degradation were determined, and formate-utilizing methanogenesis showed its importance with a 28-34% contribution of the total methanation, becoming the dominant pathway under high ammonia level. Formate-utilizing methanogenesis also had a thermodynamic advantage among the three pathways. 16S rRNA sequencing suggested a homology between the hydrogen-utilizing and formate-utilizing methanogens. Methanobacterium and Methanobrevibacter were found to be key methanogens, and their enrichment under high ammonia level confirmed the stronger ammonia tolerance of formate-utilizing and hydrogen-utilizing methanogenesis. The microbial characterization and modified ADM1 simulations supported each other.

Shift in methanogenic community in protein degradation using different inocula

Anaerobic digestion (AD) of protein-rich wastes is problematic due to production of ammonia and hydrogen sulfide. In this work, eight inocula were used in batch AD of solutions of gelatin and gluten at 3 g COD substrate/1g VSS inoculum. AD plants from which inocula originated were treating food waste or food wastewater, wastewater sludge, or a combination of them. Inocula were evaluated by fitting methane production data using the modified Gompertz model. Sequencing of 16 s rRNA of microorganisms showed that Methanoculleus was dominant in inocula from plants that were treating food waste, and Methanosaeta was dominant in the others. The maximum methane production rate varied by a factor of three for each substrate: 2.734-7.438 mLCH₄ gCOD^-1 d^-1 for gelatin, and 1.950 to 5.532 mLCH₄ gCOD^-1 d^-1 for gluten. This study demonstrates that inoculum must be chosen appropriately when treating proteinaceous waste by AD.

Understanding methane emission from stored animal manure: A review to guide model development

National inventories of methane (CH₄ ) emission from manure management are based on guidelines from the Intergovernmental Panel on Climate Change using country-specific emission factors. These calculations must be simple and, consequently, the effects of management practices and environmental conditions are only crudely represented in the calculations. The intention of this review is to develop a detailed understanding necessary for developing accurate models for calculating CH₄ emission from liquid manure, with particular focus on the microbiological conversion of organic matter to CH₄ . Themes discussed are (a) the liquid manure environment; (b) methane production processes from a modeling perspective; (c) development and adaptation of methanogenic communities; (d) mass and electron conservation; (e) steps limiting CH₄ production; (f) inhibition of methanogens; (g) temperature effects on CH₄ production; and (h) limits of existing estimation approaches. We conclude that a model must include calculation of microbial response to variations in manure temperature, substrate availability and age, and management system, because these variables substantially affect CH₄ production. Methane production can be reduced by manipulating key variables through management procedures, and the effects may be taken into account by including a microbial component in the model. When developing new calculation procedures, it is important to include reasonably accurate algorithms of microbial adaptation. This review presents concepts for these calculations and ideas for how these may be carried out. A need for better quantification of hydrolysis kinetics is identified, and the importance of short- and long-term microbial adaptation is highlighted.

A mechanistic model of methane emission from animal slurry with a focus on microbial groups

Liquid manure (slurry) from livestock releases methane (CH4) that contributes significantly to global warming. Existing models for slurry CH4 production-used for mitigation and inventories-include effects of organic matter loading, temperature, and retention time but cannot predict important effects of management, or adequately capture essential temperature-driven dynamics. Here we present a new model that includes multiple methanogenic groups whose relative abundance shifts in response to changes in temperature or other environmental conditions. By default, the temperature responses of five groups correspond to those of four methanogenic species and one uncultured methanogen, although any number of groups could be defined. We argue that this simple mechanistic approach is able to describe both short- and long-term responses to temperature where other existing approaches fall short. The model is available in the open-source R package ABM (https://github.com/sashahafner/ABM) as a single flexible function that can include effects of slurry management (e.g., removal frequency and treatment methods) and changes in environmental conditions over time. Model simulations suggest that the reduction of CH4 emission by frequent emptying of slurry pits is due to washout of active methanogens. Application of the model to represent a full-scale slurry storage tank showed it can reproduce important trends, including a delayed response to temperature changes. However, the magnitude of predicted emission is uncertain, primarily as a result of sensitivity to the hydrolysis rate constant, due to a wide range in reported values. Results indicated that with additional work-particularly on the magnitude of hydrolysis rate-the model could be a tool for estimation of CH4 emissions for inventories.

Combined Pretreatment by Ultrasound and Struvite Precipitation of Raw Substrates: A Strategy to Overcome C/N Ratio Unbalance in Nitrogen-Rich Anaerobic Co-Digestion Systems

The present study aimed to optimize the struvite chemical precipitation process in nitrogen-rich anaerobic co-digestion systems. Struvite precipitation experiments were carried out using a mixture of cattle slurry liquid fraction and sewage sludge, with and without ultrasound pretreatment. Marine salt or MgCl2 were used as magnesium source in NH4+:Mg2+ stoichiometric proportions of 1:1.5 and 1:3. Under the tested conditions, ammonium nitrogen and orthophosphate were removed from the mixed liquor with a maximum observed efficiency of 43% and 92%, respectively, when the ultrasound treatment was applied prior to struvite precipitation, using MgCl2 as source of magnesium (NH4+:Mg2+ of 1:3). The operating time was 40 min. Different pretreatments were tested prior to the biomethanization experiments, struvite precipitation, ultrasound and a combination of both pretreatments. The application of ultrasound (with an energy input of 218 kJ L−1) and struvite precipitation (NH4+:Mg2+ of 1:3) increased the methane content in the biogas by 82% and reduced hydraulic retention time by 28%, when compared to the anaerobic co-digestion assays without pretreatment. The hydrolytic pretreatment increased the bioavailability of nitrogen by 5%, thus enhancing the removal efficiency of ammonium nitrogen by 20%. Consequently, an increase in the carbon to nitrogen ratio was observed, favoring the methanogenesis process.

Assessment of sludge management strategies in wastewater treatment systems using a plant-wide approach

The objective of this paper is to use plant-wide modeling to assess the net impacts of varying sludge management strategies. Special emphasis is placed on effluent quality, operational cost and potential resource recovery (energy, nutrients). The study is particularly focused on a centralized bio-solids beneficiation facility (BBF), which enables larger, more capital intensive sludge management strategies. Potential barriers include the ability to process reject streams from multiple donor plants in the host plant. Cape Flats (CF) wastewater treatment works (WWTW) (Cape Town, South Africa) was used as a relevant test case since it is currently assessing to process sludge cake from three nearby facilities (Athlone, Mitchells Plain and Wildevoelvlei). A plant-wide model based on the Benchmark Simulation Model no 2 (BSM2) extended with phosphorus transformations was adapted to the CF design / operational conditions. Flow diagram and model parameters were adjusted to reproduce the influent, effluent and process characteristics. Historical data between January 2014 and December 2019 was used to compare full-scale measurements and predictions. Next, different process intensification / mitigation technologies were evaluated using multiple criteria. Simulation values for COD, TSS, VSS/TSS ratio, TN, TP, NH₄⁺/NH₃, H_xPO4^3-x, NO_x alkalinity and pH fall within the interquartile ranges of measured data. The effects of the 2017 severe drought on influent variations and biological phosphorus removal are successfully reproduced for the entire period with dynamic simulations. Indeed, 80% of all dynamically simulated values are included within the plant measurement uncertainty ranges. Sludge management analysis reveals that flow diagrams with thermal hydrolysis pre-treatment (THP) result in a better energy balance in spite of having higher heat demands. The flow diagram with THP is able to i) increase biodegradability/solubility, ii) handle higher sludge loads, iii) change methanogenic microbial population and iv) generate lower solids volumes to dispose by improving sludge dewaterability. The study also reveals the importance of including struvite precipitation and harvesting (SPH) technology, and the effect that pH in the AD and the use of chemicals (NaOH, MgO) may have on phosphorus recovery. Model-based results indicate that the current aerobic volume in the water line (if properly aerated) would be able to handle the returns from the sludge line and the contribution of a granular partial nitritation/Anammox (PN/ANX) reactor on the overall nitrogen removal would be marginal. However autotrophic N denitrification generates a much lower sludge production and therefore increases AD treatment capacity. The study shows for the very first time in Africa how the use of a (calibrated) plant-wide model could assist water utilities to decide between competing plant layouts when upgrading a WWTW.

Optimization of hydrothermal pretreatment conditions for mesophilic and thermophilic anaerobic digestion of high-solid sludge

Hydrothermal pretreatment (HTP) conditions were optimized for continuous mesophilic (MAD) and thermophilic (TAD) anaerobic digestion of high-solid sludge (10-11% total solids). COD solubilization increased with prolonged HTP durations, and became not significant after 210 min. According to the methane production rate and energy consumption, the optimal HTP temperature was determined at 160 °C. Regarding continuous operation without HTP, TAD achieved higher methane yield and volatile solids (VS) reduction, at 0.12 L/g VS_added and 23.9%, respectively. After HTP, methane yield and VS reduction in MAD and TAD were increased by 400% and 191% (MAD), 67% and 72% (TAD), respectively. TAD was limited due to the inhibition from about 2800 mg/L of NH₄⁺-N concentration. The methanogenic activity of MAD was enhanced, whereas TAD displayed a reduced value owing to ammonia inhibition. Ultimately, MAD with HTP and TAD without HTP achieved the higher energy balance, 5.25 and 3.27 kJ/g VS, respectively.

Simultaneous Synergy in CH4 Yield and Kinetics: Criteria for Selecting the Best Mixtures during Co-Digestion of Wastewater and Manure from a Bovine Slaughterhouse

Usually, slaughterhouse wastewater has been considered as a single substrate whose anaerobic digestion can lead to inhibition problems and low biodegradability. However, the bovine slaughter process generates different wastewater streams with particular physicochemical characteristics: slaughter wastewater (SWW), offal wastewater (OWW) and paunch wastewater (PWW). Therefore, this research aims to assess the anaerobic co-digestion (AcoD) of SWW, OWW, PWW and bovine manure (BM) through biochemical methane potential tests in order to reduce inhibition risk and increase biodegradability. A model-based methodology was developed to assess the synergistic effects considering CH4 yield and kinetics simultaneously. The AcoD of PWW and BM with OWW and SWW enhanced the extent of degradation (0.64–0.77) above both PWW (0.34) and BM (0.46) mono-digestion. SWW Mono-digestion showed inhibition risk by NH3, which was reduced by AcoD with PWW and OWW. The combination of low CH4 potential streams (PWW and BM) with high potential streams (OWW and SWW) presented stronger synergistic effects than BM-PWW and SWW-OWW mixtures. Likewise, the multicomponent mixtures performed overall better than binary mixtures. Furthermore, the methodology developed allowed to select the best mixtures, which also demonstrated energy and economic advantages compared to mono-digestions.

Effect of proton pump inhibitor on microbial community, function, and kinetics in anaerobic digestion with ammonia stress

The proton pump is a convincing mechanism for ammonia inhibition in anaerobic digestion, which explained how the ammonia accumulated intercellularly due to diffusion of free ammonia. Proton pump inhibitor (PPI) was dosed for mitigating the accumulation in anaerobic digestion with ammonia stress, with respect to kinetics. Results show PPI inhibited β-oxidation of fatty acids by targeting ATPase in anaerobic digestion with ammonia stress. Alternatively, PPI stimulated syntrophic acetate oxidization. Random forest located key genera as syntrophic consortia. Methane increased 18.72 ± 7.39% with 20 mg/L PPI at the first peak, consistent with microbial results. The deterministic Gompertz kinetics and stochastic Gaussian processes contributed 97.63 ± 8.93% and 2.37 ± 8.93% in accumulated methane production, respectively. Thus, the use of PPI for anaerobic digestion allowed mitigate ammonia inhibition based on the mechanism of proton pump, facilitate intercellularly ammonia accumulation, stimulate syntrophic consortia, and eliminate uncertainty of process failure, which resulted in efficient methane production under ammonia stress.

Zeolite favours propionate syntrophic degradation during anaerobic digestion of food waste under low ammonia stress

Zeolite addition has been widely suggested for its ability to overcome ammonia stress occurring during anaerobic digestion. However little is known regarding the underlying mechanisms of mitigation and especially how zeolite influences the microbial structuration. The aim of this study was to bring new contributions on the effect of zeolite on the microbial community arrangement under a low ammonia stress. Replicated batch experiments were conducted. The microbial population was characterised with 16S sequencing. Methanogenic pathways were identified with methane isotopic fractionation. In presence of ammonia, zeolite mitigated the decrease of biogas production rate. Zeolite induced the development of Izimaplasmatales order and preserved Peptococcaceae family members, known as propionate degraders. Moreover methane isotopic fractionation showed that hydrogenotrophic methanogenesis was maintained in presence of zeolite under ammonia low stress. Our results put forward the benefit of zeolite to improve the bacteria-archaea syntrophy needed for propionate degradation and methane production under a low ammonia stress.

Ammonia stress decreased biomarker genes of acetoclastic methanogenesis and second peak of production rates during anaerobic digestion of swine manure

Research shows that anaerobic digestion could acclimate to ammonia stress; however, the acclimation remained unaddressed. In this study, evolution of microbial community, functional gene, and pathway was linked with apparent kinetic and performance in unacclimated inoculum under ammonia stress, to deepen understanding of the acclimation. The second peak in production rate demonstrated crucial kinetic changes under ammonia stress. The methane loss was mainly protein in residual COD. Metagenomic showed initial inhibition in all methane metabolism pathways under ammonia stress, and recovery in acetate uptake was the key to ammonia acclimation. The acclimation was found in alternative pathway of Acetyl-CoA (CH₃CO-S-CoA) synthesis from acetate, accompanying by syntrophic methanogenesis. Ammonia inhibited acetoclastic methanogenesis by competing CH₃-CO-P_i with pta and formed speculative sediment CH₃-CO-PO₄[NH₄]₂. Biomarker of methanogenesis kinetic was suggested as mcr, hdr, and mch. The biomarker could indicate acclimation stages to ammonia, empowering anaerobic digestion by early warning of methane loss.

Comprehensive analysis of the microbial communities and operational parameters of two full-scale anaerobic digestion plants treating food waste in South Korea: Seasonal variation and effect of ammonia

There are about ninety full-scale anaerobic digestion (AD) plants in South Korea that treat food waste (FW); however, the key diff ;erences in the microbial communities in different seasons and the effects of ammonia in AD remain poorly understood. In this study, the seasonal changes in microbial communities associated with operational parameters of two full-scale ADs (C and W plants) treating FW were analyzed. The organic loading rate (OLR) variability had an influence on the seasonal CH₄ yield; the W plant had a lower CH₄ yield with an unstable AD performance while the C plant had a higher CH₄ yield with a stable AD performance. It was mainly due to the substantially different NH₄⁺ concentration; the W plant had a NH₄⁺ concentration nearly 1.6 times higher compared to the C plant. The high NH₄⁺ presence in the W plant led to the dominance of class Clostridia, and methanogenesis was mostly done by hydrogenotrophs (Methanomassiliicoccus luminyensis). Additionally, the members belonging to Clostridia and Bacteroidia were found at both plants in each season (share ≥0.5%) implying their indispensable role during the anaerobic digestion of FW.

Long-term performance and microbial community characteristics of pilot-scale anaerobic reactors for thermal hydrolyzed sludge digestion under mesophilic and thermophilic conditions

Anaerobic digestion (AD) with thermal hydrolysis pretreatment has been attracted widespread attention in recent years due to its high efficiency. However, few studies focus on systematical comparison of the downstream AD processes for thermal hydrolyzed sludge and their corresponding microbial community compositions, especially on those at pilot scale and above. Thus, this study systematically compared the long-term performance and microbial communities of two pilot-scale anaerobic reactors for thermal hydrolyzed sludge digestion under mesophilic and thermophilic conditions. The results presented that mesophilic anaerobic digestion (MAD) showed a better performance of methane production than thermophilic anaerobic digestion (TAD). The hydraulic retention time (HRT) needed to be longer than 12 days in MAD while 20 days in TAD to achieve the relatively high methane production, which could be explained by that the ammonia nitrogen accumulation especially the free ammonia determined in TAD was higher than that in MAD at all HRTs, emerging an inhibition of methane yield in TAD. High-throughput Illumina sequencing results demonstrated a more diverse microbial community in MAD than that in TAD. TAD was mediated by a suite of thermophiles, such as Coprothermobacter and Methanothermobacter, while taxa harbored in MAD mostly belonged to Bacteroidetes and relatively broad types of methanogens. In addition, hydrogenotrophic methanogens were the predominant of archaea communities in both digesters probably due to the relatively high concentration of ammonia nitrogen.

Ammonia stress reduces antibiotic efflux but enriches horizontal gene transfer of antibiotic resistance genes in anaerobic digestion

The dynamics of antibiotic resistance genes (ARGs) response to ammonia stress were evaluated using metagenomics and quantitative PCR (qPCR) in anaerobic digestion (AD). Ammonia stress reduced ARGs associated with antibiotic efflux, especially the major facilitator superfamily (MFS) of tet(L), due to free ammonia (FA) that changed the proton gradient of efflux system. Nonetheless, ARGs of antibiotic target alteration, especially ermB, were enriched under ammonia stress, which could be attributed to the initiation of the internal enhancer of the transferability of the broad host range plasmid, pAMbeta1. Statistical analysis elucidated the significant changes of ARGs are directly attributed to the mobile genetic elements (MGEs), but the little affected ARGs are mainly determined by the functional microbes reflected by nitrogen cycling genes (NCyc). This study deciphered the profiles of ARGs response to ammonia stress in AD, which indicated the importance of alleviation of ammonia inhibition for the mitigation of ARGs dissemination.

Enhanced anaerobic treatment of swine wastewater with exogenous granular sludge: Performance and mechanism

Anaerobic biotechnology has been widely used to the treatment of swine wastewater, but its organic loading rate is far lower than the expected. In this study, the fatigue effect was observed for indigenous anaerobic sludge (IAS) of anaerobic digestion system treating swine wastewater. On the contrary, the enhancement effect was demonstrated for exogenous granular sludge (EGS) originated from the internal circulation reactor treating pulping wastewater. The results showed the anaerobic digestion of swine wastewater with acclimatized EGS was much better than with IAS, 10th-day COD removal efficiency of 85% and 37% respectively. The better performance of acclimatized EGS was attributed to the more efficient degradation of volatile fatty acids (VFAs) as well as a stronger tolerance to the ammonia inhibition of swine wastewater. Revealed by molecular techniques, the acclimatized EGS contained more abundant syntrophic bacteria and methanogens than IAS. These functional microbes colonized in the acclimatized EGS could overcome the fatigue effect of IAS which contained a similar microbial community to pig gastrointestinal tract microbes. This study provides a feasible and promising way to enhance the efficiency of anaerobic digestion of swine wastewater.

Biochar triggering multipath methanogenesis and subdued propionic acid accumulation during semi-continuous anaerobic digestion

The semi-continuous anaerobic digestion (AD) performances of dry chicken manure (DCM) were investigated at the temperature of 35 ± 1 °C with and without biochar. The average specific methane productions of 0.18 L/g VS_added and 0.17 L/g VS_added were achieved without biochar at the organic loading rate (OLR) of 3.125 and 6.25 g VS/L/d, respectively. An increase of 12% in methane production was obtained in the presence of biochar at the two operational OLRs. Accumulation of propionic acid was observed associating with AD of DCM, which was substantially alleviated by biochar supplement. The buffer capacity of biochar was supposed to develop through strengthening the buffer system established by NH₄⁺ and volatile fatty acids. Methanosarcina that can utilize multiple nutrients for methanogenesis was the dominant archaea in the presence of biochar, while the strictly aceticlastic Methanosaeta was dominant in control digester. These results suggest that biochar enhanced methanogenesis through intensifying its available pathway.

Assessing practical identifiability during calibration and cross-validation of a structured model for high-solids anaerobic digestion

High-solids anaerobic digestion (HS-AD) of the organic fraction of municipal solid waste (OFMSW) is operated at a total solid (TS) content ≥ 10% to enhance the waste treatment economy, though it might be associated to free ammonia (NH₃) inhibition. This study aimed to calibrate and cross-validate a HS-AD model for homogenized reactors in order to assess the effects of high NH₃ levels in HS-AD of OFMSW, but also to evaluate the suitability of the reversible non-competitive inhibition function to reproduce the effect of NH₃ on the main acetogenic and methanogenic populations. The practical identifiability of structural/biochemical parameters (i.e. 35) and initial conditions (i.e. 32) was evaluated using batch experiments at different TS and/or inoculum-to-substrate ratios. Variance-based global sensitivity analysis and approximate Bayesian computation were used for parameter optimization. The experimental data in this study permitted to estimate up to 8 biochemical parameters, whereas the rest of parameters and biomass contents were poorly identifiable. The study also showed the relatively high levels of NH₃ (i.e. up to 2.3 g N/L) and ionic strength (i.e. up to 0.9 M) when increasing TS in HS-AD of OFMSW. However, the NH₃ non-competitive function was unable to capture the acetogenic/methanogenic inhibition. Therefore, the calibration emphasized the need for target-oriented experimental data to enhance the practical identifiability and the predictive capabilities of structured HS-AD models, but also the need for further testing the NH₃ inhibition function used in these simulations.

Saline wastewater treatment with purple phototrophic bacteria

Biological removal of organics, nitrogen and from saline wastewaters is adversely impacted by high salinity, which can be a major concern for treatment of industrial or domestic saline wastewater. In anaerobic treatment systems, sulfidogensis, especially when treating sulfate-rich saline wastewaters (e.g. seawater has 930 mgSO₄-S L^-1, or 2800 mg L^-1 as SO₄^2-) can cause additional biological, operational, and safety issues, due to H₂S toxicity. Here, the use of anaerobic purple phototrophic bacteria (PPB) is tested as mediator to treat high salinity domestic wastewater (NaCl), and marine wastewater (Red Sea Salt - high sulfate, potassium, etc.) in a continuous anaerobic infra-red photo bioreactor, operated over 372d. Saline adapted PPB simultaneously removed COD, nitrogen and phosphorus with biomass yields of 0.8 gCOD gCOD^-1. Batch activity tests found a broad optimum peak for saline adapted PPB between 30 and 70 mS cm^-1, and 50% reduced activity at 140 mS cm^-1 (3.5x seawater). For marine wastewater, high sulfate influent concentrations (770 mgSO₄-S L^-1) did not result in substantial H₂S production (<1.6 mgS L^-1) over 80 d. When irradiation was removed, sulfide rapidly rose to >90 mgS L^-1 and the process failed. The results indicate rapid adaptation to high-salt conditions (both NaCl and marine), and the capacity for PPB to form a combined wastewater treatment/resource recovery process, particularly for salty industrial wastewater.

Individual Based Model Links Thermodynamics, Chemical Speciation and Environmental Conditions to Microbial Growth

Individual based Models (IbM) must transition from research tools to engineering tools. To make the transition we must aspire to develop large, three dimensional and physically and biologically credible models. Biological credibility can be promoted by grounding, as far as possible, the biology in thermodynamics. Thermodynamic principles are known to have predictive power in microbial ecology. However, this in turn requires a model that incorporates pH and chemical speciation. Physical credibility implies plausible mechanics and a connection with the wider environment. Here, we propose a step toward that ideal by presenting an individual based model connecting thermodynamics, pH and chemical speciation and environmental conditions to microbial growth for 5·105 individuals. We have showcased the model in two scenarios: a two functional group nitrification model and a three functional group anaerobic community. In the former, pH and connection to the environment had an important effect on the outcomes simulated. Whilst in the latter pH was less important but the spatial arrangements and community productivity (that is, methane production) were highly dependent on thermodynamic and reactor coupling. We conclude that if IbM are to attain their potential as tools to evaluate the emergent properties of engineered biological systems it will be necessary to combine the chemical, physical, mechanical and biological along the lines we have proposed. We have still fallen short of our ideals because we cannot (yet) calculate specific uptake rates and must develop the capacity for longer runs in larger models. However, we believe such advances are attainable. Ideally in a common, fast and modular platform. For future innovations in IbM will only be of use if they can be coupled with all the previous advances.

Modified anaerobic digestion model No.1 (ADM1) for modeling anaerobic digestion process at different ammonium concentrations

Anaerobic digestion (AD) is an established method for sustainable energy production. Anaerobic digestion model No.1 (ADM1) was used to simulate methane production (MP) and volatile fatty acid (VFA) concentrations at different ammonium concentrations. In accordance with the incomplete description of several biochemical reactions and the omission of several reaction processes, ADM1 was modified with the consideration of acetic acid inhibition and valeric acid existence. ADM1_ac (ADM1 added acetic acid inhibition) could obtain better simulation accuracy of MP (goodness-of-fit value = 0.945), and VFA concentrations (goodness-of-fit values > 0.39) were all higher than ADM1_original, but cannot explain the valeric acid production. ADM1_va (ADM1 added valeric acid existence) could achieve better simulation of valeric acid (achieving a breakthrough of zero), nevertheless the accuracy of propionic and butyric acids was poorer than ADM1_ac with differences between experimental and simulation values were 5%-10% lower. With both factors coordinated, MP and VFA concentrations could be simulated accurately by ADM1_ac_va (ADM1 added acetic acid inhibition and valeric acid existence), with the highest goodness-of-fit values (>0.85). The results of a verification experiment with ADM1_ac_va simulation further indicated that acetic acid inhibition and valeric acid as new component were both important in ADM1. PRACTITIONER POINTS: ADM1_ac could simulate MP and acetate, propionate and butyrate concentrations better. ADM1_va could explain the valerate production during AD of glucose. ADM1_ac_va could simulate AD process quite accurately, with the highest goodness-of-fit values (>0.85). Acetate inhibition and valerate existence were both important and should be considered in ADM1.

Performance of Anaerobic Digestion of Chicken Manure Under Gradually Elevated Organic Loading Rates

Poultry manure is the main source of agricultural and rural non-point source pollution, and its effective disposal through anaerobic digestion (AD) is of great significance; meanwhile, the high nitrogen content of chicken manure makes it a typical feedstock for anaerobic digestion. The performance of chicken-manure-based AD at gradient organic loading rates (OLRs) in a continuous stirred tank reactor (CSTR) was investigated herein. The whole AD process was divided into five stages according to different OLRs, and it lasted for 150 days. The results showed that the biogas yield increased with increasing OLR, which was based on the volatile solids (VS), before reaching up to 11.5 g VS/(L·d), while the methane content was kept relatively stable and maintained at approximately 60%. However, when the VS was further increased to 11.5 g VS/(L·d), the total ammonia nitrogen (TAN), pH, and alkalinity (CaCO₃) rose to 2560 mg·L^-1, 8.2, and 15,000 mg·L^-1, respectively, while the volumetric biogas production rate (VBPR), methane content, and VS removal efficiency decreased to 0.30 L·(L·d)^-1, 45%, and 40%, respectively. Therefore, the AD performance immediately deteriorated and ammonia inhibition occurred. Further analysis demonstrated that the microbial biomass yield and concentrations dropped dramatically in this period. These results indicated that the AD stayed steady when the OLR was lower than 11.5 g VS/(L·d); this also provides valuable information for improving the efficiency and stability of AD of a nitrogen-rich substrate.

Single and combined inhibition of Methanosaeta concilii by ammonia, sodium ion and hydrogen sulfide

Single and combined inhibition of lag time λ and specific methanogenic activity R_CH4 of Methanosaeta concilii by NH₃, Na⁺ and H₂S were investigated using inhibition tests with a single inhibitor and a 3³ full-factorial experiment of NH₃, Na⁺ and H₂S concentrations (1.5 ≤ total ammonia nitrogen (TAN)/L ≤ 4.5 g, 1 ≤ Na⁺/L ≤ 4.3 g, 14.2 ≤ total hydrogen sulfide sulfur (THSS)/L ≤ 836 mg). All three inhibitors significantly increased λ and reduced R_CH4 of M. concilii. The half-maximal inhibitory concentrations of NH₃, Na⁺ and H₂S for M. concilii were 6.4 g TAN/L, 5.2 g Na⁺/L and 1.6 g THSS/L. Partial cubic models adequately approximated the corresponding response surfaces of λ and R_CH4 from the 3³ full-factorial experiment. The inhibitors inhibited R_CH4 synergistically, but inhibited λ in a complex manner. The combination of NH₃ and Na⁺ showed the strongest synergistic inhibition of both λ and R_CH4.

The roles of free ammonia (FA) in biological wastewater treatment processes: A review

Free ammonia (FA) can pose inhibitory and/or biocidal effects on a variety of microorganisms involved in different biological wastewater treatment process, which is widely presented in wastewater treatment plants (WWTPs) due to the high levels of ammonium in the systems. This review article gives the up-to-date status on several essential roles of FA in biological wastewater treatment processes: the impacts of FA, mechanisms of FA roles, modeling of FA impacts, and implications of FA for wastewater treatment. Specifically, the impacts of FA on both wastewater and sludge treatment lines were firstly summarized, including nitrification, denitrification, anaerobic ammonium oxidation (Anammox), enhanced biological phosphorus removal and anaerobic processes. The involved mechanisms were then analyzed, which indicated FA inhibition can slow specific microbial activities or even reconfigure the microbial community structure, likely due to negative impacts of FA on intracellular pH, specific enzymes and extracellular polymeric substances (EPS), thus causing cell inactivation/lysis. Mathematical models describing the impact of FA on both wastewater and sludge treatment processes were also explored to facilitate process optimization. Finally, the key implications of FA were identified, that is FA can be leveraged to substantially enhance the biodegradability of secondary sludge, which would further improve biological nutrient removal and enhance renewable energy production.

Ammonia Inhibition of Anaerobic Volatile Fatty Acid Degrading Microbial Communities

Ammonia inhibition is an important reason for reactor failures and economic losses in anaerobic digestion. Its impact on acetic acid degradation is well-studied, while its effect on propionic and butyric acid degradation has received little attention and is consequently not considered in the Anaerobic Digestion Model No. 1 (ADM1). To compare ammonia inhibition of the degradation of these three volatile fatty acids (VFAs), we fed a mixture of them as sole carbon source to three continuous stirred tank reactors (CSTRs) and increased ammonium bicarbonate concentrations in the influent from 52 to 277 mM. The use of this synthetic substrate allowed for the determination of degradation efficiencies for the individual acids. While butyric acid degradation was hardly affected by the increase of ammonia concentration, propionic acid degradation turned out to be even more inhibited than acetic acid degradation with degradation efficiencies dropping to 31 and 65% for propionic and acetic acid, respectively. The inhibited reactors acclimatized and approximated pre-disturbance degradation efficiencies toward the end of the experiment, which was accompanied by strong microbial community shifts, as observed by amplicon sequencing of 16S rRNA genes and terminal restriction fragment length polymorphism (T-RFLP) of mcrA genes. The acetoclastic methanogen Methanosaeta was completely replaced by Methanosarcina. The propionic acid degrading genus Syntrophobacter was replaced by yet unknown propionic acid degraders. The butyric acid degrading genus Syntrophomonas and hydrogenotrophic Methanomicrobiaceae were hardly affected. We hypothesized that the ammonia sensitivity of the initially dominating taxa Methanosaeta and Syntrophobacter led to a stronger inhibition of the acetic and propionic acid degradation compared to butyric acid degradation and hydrogenotrophic methanogenesis, which were facilitated by the ammonia tolerant taxa Syntrophomonas and Methanomicrobiaceae. We implemented this hypothesis into a multi-taxa extension of ADM1, which was able to simulate the dynamics of both microbial community composition and VFA concentration in the experiment. It is thus plausible that the effect of ammonia on VFA degradation strongly depends on the ammonia sensitivity of the dominating taxa, for syntrophic propionate degraders as much as for acetoclastic methanogens.