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Life cycle assessment (LCA) is a commonly used method for assessing environmental impacts of systems, but cannot produce absolute values, i.e. a comparison with existing calculated values, which represents limits of what can be emitted into the environment. Therefore, absolute environmental sustainability assessments have been developed to assess impacts against the planetary boundaries (PBs) of the safe operating space for humanity. Since PB-LCAs are novel, it is useful to analyze both results from this method and conventional LCAs, something which has not been done before. This study applied both methods to two full-scale sanitation systems in the city of Helsingborg, Sweden. The current conventional system for handling wastewater with active sludge and food waste to biogas production was compared with the novel project H+ source separation system with three pipes (food waste, grey and black water) with increased resource recovery through anaerobic digestion, ammonia stripping, struvite precipitation and pelletization. The Planetary Boundaries LCA (PB-LCA) results showed that both systems exceeded eight of the assigned shares of PBs, including climate change and biogeochemical flows of nutrients. Traditional LCA (ReCiPe impact assessment) showed net savings for the H+ system in a few categories and considerable reductions in several impacts, e.g., global warming potential (GWP), stratospheric ozone depletion, terrestrial acidification, and water consumption. In PB-LCA the H+ system gave additional impacts in both assessments for a few categories, mostly due to high consumption of chemicals in the ammonium stripping process used for nutrient recovery. In conclusion, the combined assessments highlight hot-spots for process optimization in the H+ system. From a methodological standpoint, PB-LCA still needs improvements to better reflect avoided burdens and results from traditional LCA should be fully transparent and analyzed carefully. The assessment methods complement each other and can be combined to better represent environmental performances of systems. [Display omitted] • Planetary Boundaries LCA was compared to the traditional LCA for sanitation systems. • Source separation in 3 streams in Sweden performed better in 10 categories in Recipe. • Eight of the Boundaries were exceeded but the avoided burdens are not represented. • Electricity and chemicals played major roles in the systems assessed. • Improvements are needed in PB-LCA to also reflect the benefits of the systems. [ABSTRACT FROM AUTHOR]

This study will serve as a detailed reference for the recycling of anaerobically digested sludge into lightweight aggregates. [Display omitted] • The lightweight aggregate mixture containing a large amount of anaerobic digestion sludge was reduced in weight through the loss of organic matter. • Viscous behavior, gas generation, air permeability, etc. have to be considered in relation to the swelling and weight reduction of artificial lightweight aggregate. • Artificial lightweight aggregate containing 50 wt% of anaerobic digested sludge was successfully obtained. In this study, in an effort to manufacture lightweight aggregates recycled from anaerobic digested sewage sludge (ADSS), the firing process characteristics of artificial lightweight aggregates containing a large amount of ADSS were identified and the manufacturing process was optimized and modeled using machine learning techniques. Samples were prepared by rapid firing and according to the orthogonal arrangement experimental design, the particle density and water absorption rate were measured, and a pore analysis was conducted by 3D-CT and the mercury intrusion method. Random forest, SVR, and XGBoost techniques were used as machine learning techniques to analyze the data. As a result of the rapid firing test, samples with an ADSS content of 40 wt% or more were found to have a low density and were found not to show bloating. For aggregates containing more than 40% ADSS, a loss of organic matter played a greater role in the decreased density than bloating due to viscous behavior. Data expansion using an orthogonal arrangement table and modeling by the SVR method for the ADSS 50 mixture found an RMSE value of 0.013 with significant predicted values of the physical properties also obtained. Aggregates with a content of 50 wt% of anaerobically digested sludge were successfully prepared on a pilot scale, and this aggregate mix met EN-13055 standards. [ABSTRACT FROM AUTHOR]

Fulvic acids (FAs) belong to inert organic matters in sewage sludge and their influences are often overlooked during convectional anaerobic digestion (AD). Currently, thermal hydrolysis (TH) has been widely applied on sludge pretreatment before AD processes, which makes FAs undergo drastic evolutions and aggressive to sludge AD. Results in the present study indicated that FAs concentration in the liquid was elevated by over incredible 150 folds during sludge TH, from 3.4 mg/L in raw sludge to 590.0 mg/L in hydrolyzed sludge at 180 °C. Moreover, during sludge TH, the chemical structures of FAs, including aromatic condensation degree, elemental composition and functional group, also underwent significant changes, which enhanced FAs electron transfer capability, reduced their biodegradability and promoted their roles on sludge AD. Furthermore, fortunately, the evolutions of FAs were favorable to sludge AD in general. Methane production could be promoted by about 20% under FAs concentration of 0.6 g/L, and the FAs extracted from hydrolyzed sludge presented higher promotion performances than that of the raw FAs, in which 180 °C FAs were particularly conspicuous. Furthermore, FAs evolutions would present differential influences on each phase of sludge AD, promotional to acidogenesis and methanogenesis but inhibitory to hydrolysis. Pearson correlation analysis indicated FAs influences on sludge AD, particularly the hydrolysis phase, were not only related to their concentration, but also chemical structure. The findings of this study demonstrated that FAs influences should not be negligible anymore during sludge AD with TH pretreatment. Meanwhile, since FAs promotion on sludge AD was closely related to their concentration and chemical structure, it would be significant to take FAs evolutions as auxiliary indexes for the regulations of sludge TH. [Display omitted] • Total contents of FAs increased significantly during sludge THP. • FAs concentration in the liquid phase increased by incredible two orders of magnitude with THP. • Chemical structure of FAs also underwent drastic changes during sludge THP. • FAs evolutions promoted acidogenesis and methanogenesis, but inhibited hydrolysis. • Evolutions of FAs during THP made their influences on sludge AD non-ignorable anymore. [ABSTRACT FROM AUTHOR]

The effects of microbial electrolysis cells (MECs) at three applied voltages (0.8, 1.3, and 1.6 V) on simultaneously enhancing methanization and reducing hydrogen sulfide (H 2 S) production in the anaerobic digestion (AD) of sewage sludge were studied. The results showed that the MECs at 1.3 V and 1.6 V simultaneously enhanced the methane production by 57.02 and 12.70% and organic matter removal by 38.77 and 11.13%, and reduced H 2 S production by 94.8 and 98.2%, respectively. MECs at 1.3 V and 1.6 V created a micro-aerobic conditions for the digesters with oxidation-reduction potential as −178∼-232 mv, which enhanced methanization and reduced H 2 S production. Sulfur reduction, H 2 S and elemental sulfur oxidation occurred simultaneously in the ADs at 1.3 V and 1.6 V. The relative abundances of sulfur-oxidizing bacteria increased from 0.11% to 0.42% and those of sulfur-reducing bacteria decreased from 1.24% to 0.33% when the applied voltage of MEC increased from 0 V to 1.6 V. Hydrogen produced by electrolysis enhanced the abundance of Methanobacterium and changed the methanogenesis pathway. [Display omitted] • The effects of high applied voltages on MEC-AD of sewage sludge were studied. • Methanization enhancement and H 2 S production reduction was simultaneously realized. • High voltages (1.3 V and 1.6 V) created micro-aerobic condition for the AD. • High voltages increased the abundances of SOB and decreased those of SRB. • High voltages enhanced the hydrogenotrophic methanogenesis. [ABSTRACT FROM AUTHOR]

Biomethane production by anaerobic digestion can significantly contribute to the decarbonation and defossilization of the European energy mix. Although previous estimates have been provided about biomethane potential production in several European countries, they suffer from a coarse evaluation of substrate availability and a lack of spatial resolution. This paper provides a spatially-explicit assessment of the potential for biomethane production in France at a fine spatial resolution (NUTS 4 region) from agricultural biomass (livestock manure, crop residues and cover crops) and sewage sludge. This assessment is based on (i) a fine estimation of substrate availability, particularly for cover crop production, by combining a crop model with spatially-explicit soil and climate databases; (ii) the use of an anaerobic digestion model that predicts biomethane production from agricultural biomass; and (iii) a procedure for optimizing biomethane production while respecting the stoichiometric constraints of the substrate mixture. We found a significant potential for annual biomethane production of 64.1 TWh, mainly from cattle manure, a value that could reach 108.7 TWh through the generalization of cover crops within arable cropping systems. Our results also highlighted the importance of detailed substrate availability data to account for their large spatial variations. Accordingly, we showed that biomethane production was highly variable across the French regions and that crop residues could only partly be used for anaerobic digestion – especially in arable regions – because they lead to excessively dry substrate mixtures. This study helped to identify the most suitable regions for biomethane production and to provide accurate estimates for biomethane development in France. • High-resolution estimate of agricultural biomethane production in France • Biogas production could reach 64.1 TWh/year under current substrate availability • Cover crop generalization could increase biogas production to 108.7 TWh/year • 24–40% of crop residues could not be used due to their stoichiometric composition • Biogas production was highly clustered in both crop- and livestock-oriented regions [ABSTRACT FROM AUTHOR]

The energy crisis, depletion of fossil fuels, and global waste issue highlight the need for sustainable and eco-friendly energy processes. In this study, the anaerobic co-digestion of sewage sludge (DS), milk sludge (MS), and food waste (FW) with glycerol (GL) from biodiesel production was investigated. Binary co-digestion of MS-GL, DS-GL, and FW-GL, as well as ternary co-digestion of MS-FW-GL and DS-FW-GL, were examined to determine the optimal combinations for biogas production and process stability. Adding 5% GL (v/v) increased methane yields by 88.78%, 405.82%, and 40.03% for binary mixtures and 55.57% and 298.53% for ternary mixtures, respectively. In addition, the ternary mixtures resulted in the accumulation of volatile fatty acids at 2136.96 mg/L and 1843.62 mg/L, respectively, causing a reduction in methane yield compared to that in binary mixtures. GL added to binary and ternary mixtures delayed optimal methanogenic activities, with higher hydrolysis rates and shorter lag times observed in single substrate-contained and binary mixture reactors. Ternary mixtures showed lower hydrolysis rates and longer lag times, indicating that methanogens required more time to adjust to the increased organic content. Co-digestion procedures using different substrate ratios should consider the risk of organic overloads, which can lead to system instability or failure. [Display omitted] • AD of sewage sludge (DS), milk sludge (MS), and food waste (FW) with glycerol (GL) was investigated. • MS-GL co-digestion produced the highest methane yield of 0.496 L/g VS. • Ternary mixtures of MS-FW-GL and DS-FW-GL reduced methane yield. • Adding GL to the AD process delayed the lag phase with lower hydrolysis rates. • GL is a promising co-substrate for AD, but organic overload must be controlled. [ABSTRACT FROM AUTHOR]

• A three-dimensional electrode system was constructed. • FMBC acts as a third electrode in the electrochemical system. • Three-dimensional system promotes organic matter leaching and methane production. Electrochemical pretreatment (EPT) of sewage sludge demonstrates an aggressive effect on extracellular polymeric substance destruction and sludge solubilization. While the low efficiency of methane production does not match the initial dissolution of the sludge after EPT. Here we proposed a novel method, using Fe-Mn-modified biochar (FMBC) strengthening electrochemical pretreatment, aiming at improving the methanogenic efficiency. The strengthening mechanism of FMBC was studied according to the physicochemical parameters of sludge at different concentrations and the change of microbial community. Experimentally, with the addition of 2 g/L FMBC, the methane production increased by 7.96% and 10.06% compared with electrochemical pretreatment alone and the control group. The sludge organic matter solubilization varied from 344.6 mg/L in E0 to 383.4 mg/L in E2 after pretreatment. Simultaneously, analysis of microbial communities showed that the addition of 2 g/L FMBC obviously increased the relative abundance of Methanofastidiosum and Methanosaeta from 5.5% and 20.9% in CK to 16% and 28.7% in E2, respectively. The addition of FMBC also intensified the acetoclastic methanogenesis and methylotrophic methanogenesis. This study provides new ideas for the enhancement of electrochemical pretreatment of sludge. [ABSTRACT FROM AUTHOR]

• Feasibility of anaerobic co-digestion of sewage sludge and wine vinasse. • Comparison of single-stage versus two-stage anaerobic co-digestion. • Single-stage mesophilic anaerobic co-digestion process generates more CH 4. • Energy recovery in single-stage and two-stage processes is similar. • Acidogenic hydrolytic stage in two-stage process doesńt improve energy balance. The present work consisted of the energetic valorization of sludge from wastewater treatment plants and discharges from the wine sector by means of anaerobic co-digestion. The valorization was approached through the application of two different processes: a) single-stage mesophilic anaerobic digestion process (35 °C) and, b) Temperature - Phased Anaerobic Digestion (TPAD) with separation of microorganisms: thermophilic acidogenic (55 °C) and mesophilic methanogenic (35 °C) phases. Mesophilic methanogenic biodegradability tests (BMP) were developed to determine the biogas (methane) production yield in single-stage process and sequential thermophilic hydrogen (BHP) and BMP were developed to study the biogas (hydrogen and methane) production yield in anaerobic co-digestion processes in TPAD process. The results obtained showed that the single-stage mesophilic anaerobic co-digestion process was more effective for methane generation with yields of 55.39 mL CH 4 /g VS added for the sludge and vinasse mixture, compared to 45.79 mL CH 4 /g VS added for the TPAD process. [ABSTRACT FROM AUTHOR]

Currently, the phenomenon of direct interspecies electron transfer (DIET) is of great interest in the technology of anaerobic digestion (AD) due to potential performance benefits. However, the conditions for the occurrence of DIET and its limits on improving AD under conditions close to real have not been studied enough. This research is concentrated on the effect of conductive carbon cloth (R3), in comparison with a dielectric fiberglass cloth (R2) and control (R1), on the AD performance in large (90 L) thermophilic reactors, fed with a mixture of simulated organic fraction of municipal solid waste and sewage sludge. While organic loading rate (OLR) was gradually increased from 2.4 to 8.66 kg VS/(m3 day), a statistically significant (p < 0.05) difference in biogas production was observed between R1 and both R2 and R3. However, at a maximum OLR of 12.12 kg VS/(m3 day) in R3, an increase in biogas production (p < 0.05) was observed both compared to R1 (by 8.97%) and R2 (by 4.24%). The content of volatile fatty acids in R3 as a whole was the lowest, especially at the maximum OLR. Biofilm on carbon cloth was rich in syntrophic microorganisms of the genera Tepidanaerobacter , as well as Defluviitoga , capable of DIET in mixed cultures with Methanothrix , which was the most abundant methanogen in biofilm. Suspended Bifidobacterium , Fervidobacterium and Anaerobaculum were negatively affected, while Defluviitoga , Methanothermobacter and Methanosarcina, on the contrary, were positively affected by the increase in OLR and showed, respectively, a negative and positive correlation (p < 0.05) with the main AD performance parameters. [Display omitted] • Statistically significant improvement of carbon cloth-assisted AD of complex waste. • Tepidanaerobacter and Defluviitoga promote DIET in biofilm on carbon cloth. • Statistically significant correlation of microbial groups with key AD parameters. • Effective stimulation of DIET on a larger scale AD reactor at high organic loads. [ABSTRACT FROM AUTHOR]

This research investigated the impact of polyethylene terephthalate (PET) microplastics on the anaerobic co-digestion of sewage sludge and food waste. The performance of anaerobic digester was investigated with different particle sizes (30 µm and 250 µm) PET at concentrations of 0, 0.45, 1.35, and 2.70 mg/g total solid (TS). Results suggested that the CH 4 production at 2.70 mg/g TS level decreased by 21.63% (30 µm) and 15.87% (250 µm) compared with the control group, respectively. PET microplastics exerted negative influence through leaching toxic diisobutyl phthalate (DIBP) and dibutyl phthalate (DBP). Microbial community analysis and redundancy analysis showed that DIBP and DBP reduced the abundances of key hydrolysis bacteria (Bacteroides vadin HA17) and acidification bacteria (Clostridium and Sphaerochaeta). Structural equation model analysis showed that DBP and DIBP were the key factors that reduced the methane production, and they had stronger impact on daily methane production in the 30 µm group compared with the 250 µm group. [Display omitted] • PET microplastics had negative effect on methane production. • PET microplastics-30 µm inhibited the methane production more obviously. • PET microplastics inhibited the hydrolysis and acidification in AD systems. • Leached DBP and DIBP reduced key acidogens and methanogens. [ABSTRACT FROM AUTHOR]

• Aqueous product generated from HTL of single and mixed feedstocks without pretreatment produced a methane inhibition ranging from 30% to 57%. • Anaerobic digestion of GAC treated aqueous product generated from HTL of single feedstocks produced a biomethane yield of 313 mL/gCOD (winery rose lees), 182 mL/gCOD (brewery trub) and 26 mL/gCOD (primary sludge). • Anaerobic digestion of GAC treated aqueous product generated from HTL of mixed feedstocks produced a greater biomethane yield of 318 mL/gCOD (winery rose lees/primary sludge) and 225 mL/gCOD (brewery trub/primary sludge). • Energy recovery of methane produced from anaerobic digestion of aqueous product from mixed feedstocks (80.64%) was greater than single feedstocks (6.6%). Beverage waste and sewage sludge are two of the most generated waste-based biomasses. Recently, the production of biocrude through the hydrothermal liquefaction (HTL) of both wastes independently or jointly resulted in the generation of an aqueous byproduct (ACP) of high dissolved organic carbon. The high dissolved organic carbon means that ACP can be digested anaerobically to recover more bioenergy products like methane. This study investigated the anaerobic digestion (AD) of ACP produced from co-HTL of beverage waste [winery rose lees (RL)/brewery trub (BT)] and primary sludge (PS) with reference to the quantity and energy content of methane produced as well as the suitability of digestate as an organic fertilizer. Results indicated that ACP generated from the HTL of combined RL and PS (RLPS) produced the highest specific methane yield of 317 mL/gCOD and energy recovery of 80.64% while ACP generated from PS produced the lowest specific methane yield of 26 mL/gCOD and energy recovery of 6.62%. Results also indicated the N/P ratio of the anaerobic digestate makes it suitable for fertilizer application although it needs to be supplemented by K, Ca, and Mg. The results of this study proved that ACP produced during the HTL processing of mixed sewage sludge and beverage waste generates higher methane yield than what could be generated if both wastes are not mixed. This study emphasized that efficient process optimization and energy recovery can be obtained in AD of HTL ACP using a mixed feedstock rather than single feedstocks. [ABSTRACT FROM AUTHOR]

The present investigation explores the feasibility of generating biogas from water hyacinth (WH) through a pretreatment process. The WH samples were subjected to a high concentration of H 2 SO 4 pretreatment to enhance biogas production. The H 2 SO 4 pretreatment aids in breaking down the lignocellulosic materials found in the WH. Additionally, it helps modify the cellulose, hemicellulose, and lignin, which assists in the anaerobic digestion process. The samples underwent pretreatment with 5% v/v H 2 SO 4 for 60 min. Biogas production was conducted for both untreated and pretreated samples. Furthermore, sewage sludge and cow dung were used as inoculants to promote fermentation in the absence of oxygen. The results of this study demonstrate that the pretreatment of water hyacinth with 5% v/v H 2 SO 4 for 60 min considerably enhances biogas production through the anaerobic co-digestion process. The maximum biogas production was recorded by T. Control-1, with a production rate of 155 mL on the 15th day compared to all other controls. All the pretreated samples showed the highest biogas production on the 15th day, which is comparatively five days earlier than the untreated samples. In terms of CH 4 production, the maximum yield was observed between the 25th and 27th days. These findings suggest that water hyacinth is a viable source of biogas production, and the pretreatment method significantly improves biogas yield. This study presents a practical and innovative approach to biogas production from water hyacinth and highlights the potential for further research in this area. • A high concentration of H 2 SO 4 pretreatment was used to enhance biogas production to break down the lignocellulosic materials. • Sewage sludge and cow dung were used as inoculants to promote fermentation in the absence of oxygen. • The pretreatment of water hyacinth with 5% v/v H2SO4 for 60 min significantly enhances biogas production. • Water hyacinth is a viable source of biogas production, and the pretreatment method significantly improves biogas yield. [ABSTRACT FROM AUTHOR]

• Increased loading was possible both with and without thermal hydrolysis treatment. • The pre-treatment had more pronounced effect on sewage sludge than on food waste. • Dewatering and pre-treatment were effective ways to increase retention time. • Pre-treatment increased the proportion of Clostridia in industrial-scale digesters. • Firmicutes:Bacteriodetes ratio positively correlated with methane yield. This study examined the impact of thermal hydrolysis process (THP) pre-treatment on anaerobic co-digestion of wastewater sludge and household waste and assessed whether THP was vital to achieve higher process capacity. Performance data were collected for both industrial- and laboratory-scale digesters and response in microbial community structure was evaluated by Illumina sequencing. Implementation of THP at the industrial-scale plant increased methane yield by 15% and enhanced substrate degradability. Possibility to extend the sludge retention time due to a higher solid content of the substrate, sanitisation of the digestate and improved fertiliser quality of the digestate were other industrial-scale benefits of THP installation. Continuously-fed laboratory-scale digesters were fed THP-treated or untreated substrate at an organic loading rate (OLR) of 5 g volatile solid (VS)/L/day, a feeding rate necessary at the corresponding industrial-scale plant to meet the estimated population increase within the municipality. The results indicated that the plant could have increased the capacity with unimpaired stability independently of THP installation, even though the retention time was significantly shortened during operation with untreated substrate. Microbial community analyses revealed increased contribution of the Clostridia class after THP installation in industrial-scale digesters and positive correlation between Firmicutes:Bacteriodetes and methane yield in all digesters. Differentiated profiles in laboratory-scale digesters indicated that a temperature increase from 37 to 42 °C in association with THP installation and altered substrate composition were strong determining factors shaping the microbial community. Overall, these findings can assist industrial-scale plants in choosing management strategies aimed at improving the efficiency of anaerobic digestion processes. [ABSTRACT FROM AUTHOR]

The anaerobic co-digestion of fruit and vegetable waste (FVW) and municipal sewage sludge (MSS) is investigated under mesophilic conditions. This was done at a constant hydraulic retention time (HRT) similar to that typically used at waste water treatment plant digesters, 20 days. The effects on digester performance of the FVW:MSS ratio and the organic loading rate (OLR) were examined. Initially the digester was fed with MSS from wastewater treatment plants (WWTP) with an average OLR of 1.03 kgVS (m3 d)−1. The co-digestion of MSS and FVW was performed at various ratios of FVW in the mixture, while increasing the OLRs, from 1.03 to 4.78 kgVS (m3 d)−1. The experimental specific methane production (SMP) was 0.303 m3 (kgVS)−1 for MSS and 0.403 m3 (kgVS)−1 for FVW as single substrate. This value varied for co-digestion with a maximum of 0.445 m3 (kgVS)−1 for a FVW:MSS ratio of 40:60. Alkalinity and pH values remain relatively constant regardless the different FVW:MSS ratios fed. As this ratio increases, the removal of the volatile solids (VS) increased from 38.7% to 82%. The average methane content of digester biogas was about 62–64%. [ABSTRACT FROM AUTHOR]

• Thermal alkaline pretreatment promoted methane yield of sludge. • Protein-like organics mainly located in SPOM and REOM fractions. • Biodegradability is well correlated with the ratio of I RB to I PB. • Bioaccessibility of readily degradable organic matters was enhanced by LAT. Thermal hydrolysis has proven to be a successful approach to make sewage sludge more amenable to anaerobic digestion. Three heat pretreatment scenarios were compared in this study, i.e. thermal alkaline treatment (LAT, 0.1 M NaOH, 80 °C), low temperature thermal treatment (LT, 80 °C) and high temperature thermal treatment (HT, 170 °C). The biodegradability of pretreated sludge was testified by using biochemical methane potential (BMP) test, meanwhile, the repartition and complexities of organic matters in sludge subjected to various pretreatments were characterized by a revised chemical extraction protocol combined with 3D fluorescence spectroscopy. The cumulative methane yield of sewage sludge was significantly increased by LAT (+135%), LT (+95%) and HT (+112%) as compared to the control. Nevertheless, results show that the solubilization degree of sludge was insignificantly correlated to BMP values, meanwhile high correlation values were observed for the soluble polysaccharide concentration in hydrolysate. The degradation rates of bioaccessible fraction of soluble particulate organic matter (SPOM) and readily extractible organic matter (REOM) were improved after thermal pretreatments at varied levels, which indicates that the chemical accessibility is positively correlated with the bioaccessibility. Furthermore, the biodegradable index F digestion was proposed to evaluate the biodegradability of organic matter, which is helpful for the optimization of various pretreatment strategies. [ABSTRACT FROM AUTHOR]

Highlights • Metagenomic approach revealed the effects of antibiotics on anaerobic digestion. • Antibiotics impacted archaeal community and genes encoding fatty acid degradation. • All methanogenesis pathways were influenced by antibiotics pressure. • Antibiotics increased the antimicrobial resistant risks of digested sludge. • Positive association among antibiotics, ARG and specific microbe were found. Abstract Continuous stirred-tank digesters with tetracyclines and sulfonamides were operated to investigate the impacts of antibiotic pressure on sludge anaerobic digestion. The versatile methanogen Methanosarcinales and strictly hydrogenotrophic methanogen Methanobacteriales increased and decreased by 21.1% and 10.9% under antibiotic pressure, respectively. KEGG analysis revealed that hydrogenotrophic and acetoclastic methanogenesis pathways were all affected. The decrease in abundance of function genes involved in lipid metabolism, carbohydrate metabolism, and fatty acid degradation, would lead to a reduction in methane production by 25%. Network analysis indicated positive associations among tetracycline residuals, abundance of resistance genes (ARGs), and specific member of potential hosts. Over 1000 ARG subtypes were widely detected in sludge, including macrolide (28%), tetracycline (24%), fluoroquinolone (20%), and peptide (20%) resistance genes. AD process exposed to long-term antibiotic would increase the diversity and abundance of ARG, enhance the association of ARG with specific microbes, and select bacteria able to perform chemotaxis mechanism. [ABSTRACT FROM AUTHOR]

Abstract High amounts of organic matter and protein in slaughterhouse wastes make them a viable choice for anaerobic digestion and biogas production. The purpose of this study was to investigate potentials of biogas production from poultry slaughterhouse residues (waste and sludge) in a batch-fed anaerobic co-digestion process. In the first phase of the study, laboratory-scale experiments were conducted using 1-liter reactors at 34 °C with a retention time of 50 days in order to optimize digestion parameters; i.e. inoculum-substrate ratio (ISR) and total solids (TS). Based on these experiments, the ISR of 4 and the TS of 5% were determined as optimal settings for digestion over a retention time of 50 days. The highest biogas and methane yields in these experiments were respectively reported by 0.631 and 0.462 m3/kg - VS added , achieved with 66% removal of volatile solids (VS). The optimized parameters obtained from the first phase were also used for verification at a larger scale (20 liters), wherein biogas and methane yields were found to be 0.574 and 0.402 m3/kg - VS added ; respectively. Moreover, results showed that optimal anaerobic digestion of slaughterhouse wastes in 20-liter digester could lead to 63% removal of VS and 88% reduction of chemical oxygen demand (COD) over a retention period of 42 days. The findings also indicated that decreasing ISR and increasing TS could bring about reduced pH as well as accumulation of fatty acids and ammonia, which could severely undermine yields of methanogenesis process. Graphical abstract fx1 Highlights • Anaerobic digestion of slaughterhouse wastes is a viable eco-friendly solution. • Co-digestion of slaughterhouse waste with the sludge can increase biogas production. • Decreasing the ISR and increasing the TS undermine the yield of methane production. [ABSTRACT FROM AUTHOR]

In the present study, anaerobic co-digestion process was carried out with 23 mixed substrates proportion (MSP) of fruit waste (FRW), municipal wastewater treatment plant aerated biosolid (MPABS) and dairy effluent treatment plant returned biosolid (DPRBS). During co-digestion process, the effect of MSP on carbon/nitrogen (C/N) ratio and hydrogen production was investigated. The results revealed that MSP17 (70 FRW:20 MPABS:10 DPRBS) has yielded maximal hydrogen production of 295 mL with C/N ratio of 30, followed by MSP9 (70 FRW:30 DPRBS) exhibiting 253 mL of hydrogen production with C/N ratio of 29 and MSP2 (90 FRW:10 MPABS) attained 223 mL of hydrogen production with C/N ratio of 27. Then, SEM analysis of digested substrate sample was also performed in which flocs observed to be small and loose in structure in co-digested samples and intact form in non co-digested samples. Hence, this study results can be used for a sustainable approach by utilizing the FRW and biosolids for hydrogen production. [ABSTRACT FROM AUTHOR]

• An improved AD configuration with THP post-treatment was developed. • High concentration of SCOD could be dissolved during THP post-treatment. • AD-THP performed much better than the THP pretreatment system. • The dewatering of post-THP sludge was excellent without addition of polymer. • The pH changed in opposite directions during pre- and post-THP treatment. Thermal hydrolysis process (THP) is widely used as the pretreatment for sludge anaerobic digestion (AD), but with limited improvement of biogas production, dewaterability, and high energy consumption. In this study, an improved AD configuration based on post-THP treatment (AD-THP) was developed to overcome these limitations. The digested sludge was thermally hydrolyzed and then dewatered directly, the centrate was recirculated for re-digestion with raw sludge. Batch and semi-continuous experiments were conducted to investigate the differences between pre- and post-THP treatment and compare the process performances of three configurations, including conventional anaerobic digestion (CAD), thermal hydrolysis pretreatment (THP-AD), and post-treatment (AD-THP). Results showed that post-THP could further dissolve much SCOD from the digested sludge by disintegrating the recalcitrant organic matter of raw sludge and EPS generated during digestion, resulting in further improvement of methane production. The SCOD solubilization during THP post-treatment was depended on the degradation extent of digested sludge. The main difference between pre- and post-treatment was that the pH changed towards the opposite directions due to the different properties of raw and digested sludge. The maximum specific methane production and volatile solid reduction of the AD-THP system were 344.25 mL CH 4 /g VS add and 62% during the CSTR experiments, respectively, significantly higher than those of THP-AD and CAD configurations. In addition, the dewaterability of THP treated sludge was always significantly better than the digested sludge, whether for THP pretreatment or post-treatment. The improved AD-THP configuration could be a promising alternative for the upgrading of sludge treatment. [ABSTRACT FROM AUTHOR]

Hydrothermal carbonization (HTC) has recently emerged as a promising technology for sustainable sludge minimization and solid products valorization. Nevertheless, a not negligible amount of process water remains as by-product, containing organic compounds which solubilize during the process (10–63 g COD L−1). Its valorization through anaerobic digestion (AD) has been assessed, but no systematic review has been reported to date. The main parameters, which influence the valorization of HTC process water through AD, are initial feedstocks characteristics, nutrients concentration, HTC process temperature and residence time, inoculum acclimation to toxic compounds and its buffer capacity. Experiences reported in the literature proved that HTC applied to sewage sludge at 180–200 °C at residence times in the range 60–90 min could be a good compromise between hydrochar production and valorization, hydrochar dewaterability, biogas production from AD of process water and energy consumption. [ABSTRACT FROM AUTHOR]

Addition of food waste (FW) as a co-substrate in anaerobic digesters of wastewater treatment plants is a desirable strategy towards achievement of the potential of wastewater treatment plants to become energy-neutral, diverting at the same time organic waste from landfills. Because substrate type is a driver of variations in phylogenetic structure of digester microbiomes, it is critical to understand how microbial communities respond to changes in substrate composition and concentration. In this work, high throughput sequencing was used to monitor the dynamics of microbiome changes in four parallel laboratory-scale anaerobic digesters treating sewage sludge during acclimation to an increasing amount of food waste. A co-occurrence network was constructed using data from 49 metagenomes sampled over the 161 days of the digesters' operation. More than half of the nodes in the network were clustered in two major modules, i.e. groups of highly interconnected taxa that had much fewer connections with taxa outside the group. The dynamics of co-occurrence networks evidenced shifts that occurred within microbial communities due to the addition of food waste in the co-digestion process. A diverse and reproducible group of hydrolytic and fermentative bacteria, syntrophic bacteria and methanogenic archaea appeared to grow in a concerted fashion to allow stable performance of anaerobic co-digestion at high FW. [ABSTRACT FROM AUTHOR]

Abstract The problem of waste recycling becomes more serious every year. As the population grows, the amount of organic bio-waste is increased proportionally. Even the subsequent energy utilization of such bio-waste leaves a huge amount of unprocessed organic mass, as in the case of anaerobic digestion. In this paper, the results of experimental studies of the torrefaction of sewage sludge and animal waste (mixture of horse manure and crop waste) after anaerobic digestion are presented. The aim of this work was to investigate the possibility of applying torrefaction to improve the thermotechnical properties of bio-waste products. The research results showed that the torrefaction allows to improve heating value and hygroscopicity, but with increasing of process temperature these values grow insignificantly. In addition, the conclusion about the ash content value of the bio-waste in which it is effectually to apply torrefaction treatment was made. [ABSTRACT FROM AUTHOR]

Background: Triclosan (TCS) is an antimicrobial agent widely used in health care and consumer products. This compound is present in sludge of wastewater treatment plants (WWTPs), and because of its bactericidal characteristics, it can inhibit the methanogenic activity in anaerobic digestion (AD) technology. The aim of this study was to evaluate the toxic effects of TCS on the methanogenic activity. Results: Batch anaerobic reactors were used with TCS concentrations of 7.8, 15.7, 23.5, and 31.4 mg/L. These assays consisted in three successive feedings (I, II, and III), wherein the sludge was exposed to each TCS concentration and volatile fatty acid (VFA) substrate. For evaluation of the residual sludge activity during feeding III, only VFA was used. The results showed that the increase in TCS concentrations correlated with the reduction in methane (CH4) production. In this case, the minimum values were achieved for TCS concentration of 31.4 mg/L with CH4 levels between 101.9 and 245.3 during feedings I, II, and III. Regarding the effect of TCS on VFA consumption, an inhibitory effect was detected for TCS concentrations of 23.5 and 31.4 mg/L, with concentrations of acetic, butyric, and propionic acids at the end of the assay (37 d) between 153.6 and 206.8, 62.5 and 60.1, and 93.4 and 110 mg/L, respectively. Regarding the removal of TCS during AD, these values were above 47%. Conclusion: TCS is an inhibitor of methanogenic activitywith a decrease between 63 and 70% during the different feedings. The CH4 production was not recovered during feeding III, with inhibition percentages of 21-72%. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A united microbial electrolysis cells-anaerobic system was designed. • The use of microbial electrolysis cells improved dewaterability of digestate. • Electrical regulation induced 2.8-fold increase in biomethane production. • The united system internal mechanism was speculated. • Syntrophic interaction of microorganisms promoted system performance. Abstract Microbial electrolysis cell (MEC) was integrated into conventional anaerobic digestion (AD) system (i.e. MEC-AD) to electrochemically regulate the co-fermentation of food waste (FW) and sewage sludge (SS). Two anaerobic systems (i.e. MEC-AD, and single AD) were operated in parallel to explore the potential stimulation of electrical regulation in metabolic behaviors of FW and SS and subsequent biomethane production. The highest accumulative methane yield was achieved at an applied voltage of 0.4 V and the FW and SS ratio of 0.2:0.8, increasing by 2.8-fold than those in AD. The combined MEC-AD system mitigated N 2 O emission and considerably improved ammonia removal and the dewaterability of digestate, in contrast to AD. Scanning electron microscope (SEM) visualized the presence of a large number of rod-like and cocci-like electroactive microbes on the electrode surface. Electrical regulation stimulated the self-growth and proliferation of typical Methanobacterium and Methanosaeta , accordingly contributing to biomethane production greatly. [ABSTRACT FROM AUTHOR]

Abstract Microalgal-based wastewater treatment plants are conceived as low cost and low energy consuming systems. The operation of these plants involves the management of primary sludge and microalgal biomass. The aim of this study is to analyse the anaerobic co-digestion of both by-products in terms of biogas production and contaminants of emerging concern removal. The co-digestion of microalgae and primary sludge (25/75% on a volatile solids basis) was investigated in continuous reactors and compared to microalgae mono-digestion at a hydraulic retention time of 20 days. Results showed how the co-digestion enhanced the anaerobic digestion of microalgal biomass, since primary sludge is a more readily biodegradable substrate, which increased the methane production by 65% and reduced the risk of ammonia toxicity. Regarding the contaminants, musk fragrances (galaxolide and tonalide) and triclosan showed the highest abundance on primary sludge (0.5–25 μg/g TS), whereas caffeine, methyl dihydrojasmonate and triphenyl phosphate were barely detected in both substrates (<0.1 μg/g TS). The removal of these contaminants was compound-depending and ranged from no removal to up to 90%. On the whole, microalgae mono-digestion resulted in a higher removal of selected contaminants than the co-digestion with primary sludge. Graphical abstract Unlabelled Image Highlights • Co-digestion of microalgae and primary sludge was evaluated in continuous reactors. • Occurrence and fate of the most common emerging organic contaminants was evaluated. • Methane yield increased by 65% with co-digestion versus microalgae mono-digestion. • An energy assessment shows that co-digestion generated 4.5-fold the energy consumed. • Removal of emerging organic contaminants ranged from no removal to 90%. [ABSTRACT FROM AUTHOR]

The cost-effective treatment of activated sludge that is generated by refining petroleum is a challenging industrial problem. In this study, semi-continuous stirred tank reactors (CSTRs) containing petroleum refinery excess activated sludge (PREAS) were used to comparatively investigate hydrolysis and acidification rates, after the addition of heneicosane (C21H44) (R1) and 1-phenylnaphthalene (C16H12) (R2) to different and individual reactors. Operation of the reactors using a sludge retention time (SRT) of 6 days and a pH of 5.0, resulted in the maintenance of stable biological activity as determined by soluble chemical oxygen demand (SCOD), volatile fatty acids (VFAs) production and oil removal efficiency. The optimum conditions for hydrogen production include a SRT of 8 days, at pH 6.5. Under these conditions, hydrogen production rates in the control containing only PREAS were 1567 mL/L (R0), compared with 1365 mL/L in R1 and 1454 mL/L-PREAS in R2. Coprothermobacter, Fervidobacterium, Caldisericum and Tepidiphilus were the dominant bacterial genera that have the potential to degrade petroleum compounds and generate VFAs. This study has shown that high concentrations of heneicosane and 1-phenylnaphthalene did not inhibit the hydrolytic acidification of PREAS. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Pre-treatment methods were investigated and evaluated in terms of BMP and biodegradation. • Ultrasonic pre-treatment had the best result among the single technologies and physical technology reflects better results than chemical technologies. • Ultrasonication-ozone pre-treatment showed better effect than single methods. • Ultrasonication-ozone pre-treatment has the potential for economic and energy conservation. Abstract The aim of the study is to enhance hydrolysis of dewatered sewage sludge to tackle the problem of low biological methane potential (BMP) and low efficiency of dry anaerobic digestion. Different pre-treatment i.e. physical (ultrasonication), chemical (acid, ozone) and combined (ultrasonication-ozone) methods were investigated and evaluated in terms of BMP and biodegradation. Ultrasonic pre-treatment had the best result among the single technologies, the BMP increased by 104.7%, while total solid (TS), volatile solid (VS) and chemical oxygen demand (COD) reduction were improved by 30.1%, 36.9% and 33.9%, respectively, over control. Combined pre-treatment (ultrasonication-ozone) showed more significant enhancement than single methods as evidenced by 138.2% higher BMP and 53.7%, 63.7% and 57.3% more reduction in TS, VS, COD, respectively, over control. The BMP increment positively correlated either with energy input, concentration or dose of pre-treatment applied. Among the tested methods, the physical pre-treatments out-compete chemical ones. Ultrasonic combined with ozone pre-treatment technology has good energy and economic feasibility. [ABSTRACT FROM AUTHOR]

Immobilization of bacterial cells on carriers ensures the stability and intensity of biochemical transformations. The choice of the right carrier is often decisive to the success of the biotechnological process. So far, a relatively small group of materials has been tested in the anaerobic digestion (AD) process and the experiments produced different results. In this study, lignin grafted with polyvinylpyrrolidone (PVP) was used for the first time as an innovative microbial carrier in the AD process due to its particularly positive properties such as porosity, thermal stability, and availability. PVP improved cell adhesion to the carrier surface. Waste wafers (WAF) and sewage sludge (SS) were tested as separate samples. The process was carried out in the batch mode under mesophilic conditions. Monitoring of key process parameters such as pH, the volatile fatty acids (VFA), and the VFA-to-total alkalinity (VFA/TA) ratio proved the stability of digestion both in variants with and without the carrier. Results of microbiological and biochemical analyses proved that the addition of lignin-PVP material considerably increased the proliferation of eubacteria in the wafer sample (by 77%) and increased the enzymatic activity, especially in the SS sample (by 30%). Scanning electron microscope observations revealed the presence of microbial colonies on the lignin surface. Improvement of microbiological and biochemical parameters resulted in a natural increase in the amount of biogas/methane produced, that is, an increase of 33.9% in the WAF sample (1,201.45 m3/Mg volatile solids [VS] of biogas, including 685.53 m3/Mg VS of methane) and an increase of 46.8% in the SS sample (746.82 m3/Mg VS of biogas, including 379.60 m3 Mg/VS of methane). As lignin grafted with PVP was proven to have positive effect on the condition of bacterial flora and AD efficiency, the material can be used as a microbial carrier. [ABSTRACT FROM AUTHOR]

• Amino acids were decomposed evidently by THP at 120 °C (8.9%) and 160 °C (26.7%). • Amino acids degradation in AD was promoted largely via decomposition during THP. • Varied THP temperature changed dominating bacteria utilizing amino acids in AD. • 120 °C played little role in promoting amino acids metabolic pathways during AD. • 160 °C effectively enhanced amino acids metabolism via Stickland pathway during AD. Three semi-continuous anaerobic digesters with raw sludge (R1) or thermally hydrolyzed sludge (120 °C (R2) or 160 °C (R3)) were operated to investigate the effects of thermal hydrolysis on the metabolism of amino acids during high solid anaerobic digestion with sewage sludge. Thermal hydrolysis pretreatment (THP) decomposed protein in terms of hydrolytic amino acid in raw sludge by 8.90% and 26.69% under 120 °C and 160 °C, respectively. The decomposition of amino acids during THP was the main contributor to the final enhanced amino acids degradation in sewage sludge with THP after anaerobic digestion. The dominating bacterial genera related to amino acids degradation in R2 and R3 shifted to Fastidiosipila and Proteiniphilum / Tissierella , respectively, from Gelria in R1, influencing the utilization of amino acids, especially glutamic acid, glycine and proline. Different from 120 °C, THP at 160 °C played an important role in promoting amino acid metabolism during AD through the Stickland pathway by the bacteria belonged to order Clostridiales. [ABSTRACT FROM AUTHOR]

Abstract Phosphorus recovery is usually based on the chemical precipitation of struvite from the supernatant comming from anaerobic digestion of surplus sludge of wastewater treatment plants. However, the recovery of phosphorus from the mainstream would be very beneficial to the operation of the plant. This work represents a step forward in this challenge. The configuration of a sequencing batch reactor performing Enhanced Biological Phosphorus Removal (EBPR) was modified to include the extraction of a P-rich supernatant after a settling period. The liquid extracted after the anaerobic phase has the highest phosphorus concentration in all the reactor cycle and is therefore suitable for implementing P-recovery strategies. This is the first experimental system operated under this configuration and in a long-term basis (more than one year) under different influent COD/P ratios with good performance and stability. This work shows experimentally that up to 60% of the P could be extracted without a deleterious effect on EBPR. The system could even operate at high COD/P ratio (7.5) with high activity. Finally, anaerobic supernatant extraction was linked to anaerobic sludge purging and it was demonstrated that higher methane production (24.3%) was obtained compared to the default case with the sludge purged at the end of the aerobic phase. Highlights • Long-term assessment of the EBPR2 configuration for P-recovery from mainstream line. • 60% P-recovery with anaerobic supernatant extraction without adverse effect on EBPR. • Long-term good performance under low influent COD/P ratio with high EBPR activity. • 24.3% higher methane production with sludge purged at anaerobic phase vs aerobic. [ABSTRACT FROM AUTHOR]

• Co-digestion performances improved since 25% of fish or beyond 50% of grass added. • Optimal Digestion Indicator was built to assess the optimal methane production. • Mono-Digestion Indicator was built to assess the expected methane production. • 75% (gCOD) of fish in co-digestion increased final methane production by 1.9 times. • 75% (gCOD) of grass in co-digestion increased final methane production by 1.7 times. Anaerobic co-digestion (AcoD) is a promising strategy to increase the methane production of anaerobic digestion plants treating wastewater sludge (WAS). In this work the degradability of six different mixtures of WAS with fish waste (FW) or garden-grass (GG) was evaluated and compared to the three mono-digestions. Degradation performances and methanogenic pathways, determined with the isotopic signatures of biogas, were compared across time. Fish and grass mono-digestion provided a higher final methane production than WAS mono-digestion. In co-digestion the addition of 25% of fish was enough to increase the final methane production from WAS while 50% of grass was necessary. To determine the optimal blend of WAS co-digestion two indicators were specifically designed, representing the maximum potential production (ODI) and the expected production in mono-digestion conditions (MDI). The comparison between these indicators and the experimental results showed that the most productive blend was composed of 75% of co-substrate, fish or grass, with WAS. Indeed, the final methane production was increased by 1.9 times with fish and by 1.7 times with grass associated to an increase of the methane production rate by 1.5 times. Even if the same succession of methanogenic pathways across time was observed for the different mixtures, their relative proportions were different. Sewage sludge degradation was mostly achieved through hydrogenotrophic pathway while acetoclastic pathway was dominant for fish and grass degradation. These results were confirmed by the identification of Archaea with 16S sequencing. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Nanoferrosonication (NFS) pretreatment was studied. • NFS was evaluated to improve the solubilization and anaerobic digestion of sludge. • Anaerobic tests were conducted at 15,000 and 25,000 kJ/kg TS with 2 and 7 mgFe0/g VS. • The biogas yield increased from 106 to 308 mL/g VS with 7 mgFe0/g VS + 15,000 kJ/kg TS. • The results demonstrated that NFS was effective in intensifying the process. Abstract In this work, the effect of coupling ultrasonic pretreatment with dosing of zero-valent iron nanoparticles (nanoferrosonication, "NFS") to improve the anaerobic digestion of sewage sludge was studied. Biochemical methane potential tests were conducted at 15,000 and 25,000 kJ/kg TS and their combinations with 2 and 7 mgFe0/g VS. The biogas yield increased from 106 (control) to 143 (25,000 kJ/kg TS) and 308 mL/g VS with NFS (7 mgFe0/g VS + 15,000 kJ/kg TS). The methane content increased from 55.6 to 66%, and the maximum VS removal was 11.5% at 7 mgFe0/g VS + 15,000 kJ/kg TS. The results demonstrated that NFS was effective in intensifying the process. [ABSTRACT FROM AUTHOR]

Due to the current need for alternative energy sources associated with the practice of actions to improve the environment, residual biomass has gained ground and importance in the energy sector, because of its high availability and cost-benefit, contributing to sustainable development. This study aimed to evaluate the process of anaerobic digestion using cassava wastewater, a pollutant and toxic residue of cassava processing, and sludge from a sewage treatment plant, to determine the best conditions for the production of biogas and to identify its chemical composition, specifically the levels of methane and carbon dioxide. To accomplish this, six anaerobic digestion media were produced and placed into 100-mL penicillin bottles. The experiments were carried out in triplicate, differentiated by the substrate/inoculum concentration (on a mass basis) and by the type of inoculum used (primary or secondary sludge). The best result was achieved using the following media: 80% cassava wastewater and 20% primary sludge, 4:1, which produced the highest methane content in the biogas over the shortest period of time, reaching 81.41% mol/mol of methane in 48 days of fermentation. In this way, biogas produced by cassava wastewater, both in large starch-producing factories and in smaller flour factories, can be used as a source of renewable energy, reducing production costs and providing an environmentally correct destination for this waste. [ABSTRACT FROM AUTHOR]

Abstract Mesophilic batch anaerobic digesters fed by different substrates were set up to identify the role of substrate rheology in anaerobic digestion performance while operating below the toxic level. Five substrates of different rheological behaviour but at the same amount of organic matters were prepared by addition of different amount of an inert material (0, 0.03, 0.07, 0.11, and 0.20 g) per g of waste activated sludge (WAS). To gain a comprehensive insight, the interactive relationship between substrate rheology, physico-chemical properties and biogas production as well as digestate dewaterability was investigated. The results proved that better access of microorganisms to organic matters improved the digester performance and led to 19.29% and 12.5% increase in biogas yield and VS removal efficiency, respectively. Moreover, the statistical analysis showed that consistency index and loss modulus of sludge could be employed as promising indications for biogas yield while yield stress could predict dewaterability of digestate as far as the other physico-chemical properties remained unchanged. During digestion measurement of consistency index and loss modulus of digestate could be performed as a reliable tool to monitor biogas production. Graphical abstract Image 1 Highlights • Improvement of anaerobic digester performance by increasing substrate fluidity. • Biogas production increased linearly by reduction of consistency index of substrate. • Biogas production increased linearly by reduction of loss modulus of substrate. • Dewaterability improved linearly by reduction of yield stress of sludge. • Rheological measurements can be used as a tool to monitor anaerobic digestion process. [ABSTRACT FROM AUTHOR]

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

Abstract This study investigated changes in microbial community structure and composition in response to operational disturbance and subsequent process recovery by inoculum addition. Amplicon sequencing of 16S rRNA and mcrA marker genes on the Illumina Miseq platform was used for microbial community analysis. The results show that imbalance among core microbial groups caused volatile fatty acid accumulation and subsequent deteriorated biogas production (decreased by 45% of daily volume) and methane content (<49%). Operational disturbance led to the enrichment of hydrolytic and fermentative bacteria (accounted for >57% of the total abundance) and reduction of acetogenic and methanogenic microbes (they accounted for <9% and <3% of the total abundance, respectively). Acetogens and methanogens were replenished by inoculum addition to recover digester performance. Although digester performances were similar in stable (prior to disturbance) and post recovery phases, the microbial community did not return to the original state, suggesting the existence of functional redundancy in the community. Graphical abstract Image 1 Highlights • Acetogens and methanogens are most sensitive to operational disturbance. • Acetogens and methanogens from inoculum addition facilitate digester recovery. • Microbial communities do not return to the original state after recovery. • Different microbial community structures can sustain similar level of performance. • Thus, the same function may be achieved by more than one group of microorganisms. [ABSTRACT FROM AUTHOR]

Highlights • The effects of NZVI (5–9 mg/g VS) on the AD of SS were studied. • High values of ETS activity (20.1–37.1 µgINT red /g VS ·h) were reached with NZVI. • NZVI presented higher effects in early stages of AD and promoted the acetogenesis. • NZVI decreased the ORP to −300 mV and increased the VFA's concentration. • The maximum methane yield (238 mLCH 4 /g VS) was achieved with 9 mg/g VS. Abstract This work studied the effects on the anaerobic digestion of sewage sludge by zero valent iron nanoparticles (NZVI) dosage. Biochemical methane potential tests were carried out with 5–9 mg/g VS (99.7%, 40–60 nm). The biogas yield increased from 132 (control) to 310 mL/g VS with 9 mg/g VS. The methane content increased from 63.2% (control) to 77.6% with NZVI, which corresponded to a maximum yield of 238 mLCH 4 /g VS with 9 mg/g VS. The maximum VS reduction was 19.6%. The highest INT-ETS activity (20.1–37.1 µgINT red /g VS ·h) corresponding to the maximum values of sCOD was reached within the first days. NZVI decreased the ORP to −300 mV and increased the VFA's concentration (+2000 mg/L). The ORP-VFA-pH analysis showed that NZVI promoted the acidogenesis-acetogenesis without acidification. That is, NZVI was effective in intensifying the performance and stability of the process. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Effects of THP on sludge properties and anaerobic biodegradability were evaluated. • THP affected sludge viscosity, sludge solubilization, and biogas production. • Optimal biomethane production was achieved at a THP temperature of 180 °C. • At > 180 °C, THP negatively affected anaerobic biodegradability and CH 4 production. Abstract The influence of thermal hydrolysis pretreatment (THP) on physicochemical properties (pH, total solids, volatile solids, chemical oxygen demand, total nitrogen, ammonium nitrogen, volatile fatty acids, viscosity, and cell morphology) and anaerobic biodegradability of highly concentrated waste activated sludge (WAS) with TS content ranging from 1 to 7% was evaluated at different temperatures ranging from 100 to 220 °C. The biomethane potential (BMP) of the WAS was systematically analyzed and evaluated. Images of its cellular structure were also analyzed. The results indicated that THP is a useful method for solubilizing volatile solids and enhancing CH 4 production regardless of the TS content of the WAS feed. The ultimate CH 4 production determined from the BMP analysis was 313–348 L CH 4 /kg VS (72.6–74.1% CH 4) at the optimum THP temperature of 180 °C. The results showed that THP could improve both the capacity and efficiency of anaerobic digestion, even at a high TS content, and could achieve the dual purpose of sludge reduction and higher energy recovery. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • SRT significantly influenced the ARGs fate in different configurations. • The role of SRT on the ARGs fate varied from ARG types and reactor configurations. • Shorter SRT availed the ARGs reduction for CK and two-stage digestion. • One-stage digestion of pretreated sludge need longer SRT for the ARGs reduction. • Co-selection from heavy metals contributed the most to ARGs fate response to SRT. Abstract In this study, three anaerobic digestion experiments were established to investigate the effects of solids retention times (SRT) on the fate of antibiotic resistance genes (ARGs) including anaerobic digestion of sewage sludge (CK), one-stage anaerobic digestion of microwave pretreatment sludge (MW) and two-stage anaerobic digestion of microwave pretreatment sludge (Acid stage and CH 4 stage). The response of ARGs to the SRT varied significantly from ARG types and reactor configurations. Shorter SRT could avail the ARGs reduction for CK and two-stage digestion, while MW need longer SRT for the ARGs reduction. Concerning the variance of microbial community caused by reactor configurations, the role of SRT was limited. The partial redundancy analysis and structural equation models analysis indicated that the role of SRT on the ARGs fate could be attributed the most to the co-selection from heavy metals. [ABSTRACT FROM AUTHOR]

Highlights • P/V (W/m3) does not capture AD's varying sensitivity to specific power input. • Proposal for an alternative specific power input criterion of P/M S (W/kgTS). • Optimal P/M S increases exponentially with solids concentration. Abstract The adoption of process intensification to anaerobic digestion can present significant complications for digester mixing and process performance. This work investigated how increasing the solids concentration of the digester sludge influenced the process at various mixing energy inputs. Based on the results, adequate mixing is defined qualitatively as the input of sufficient energy to mobilize the reactor contents without producing significant regions of inhibitory shear force. However, the quantitative definition is dependent on the solids concentration of the sludge. But, the existing design criterion of specific mixing power input based on fluid volume (W/m3) does not represent it well. Therefore, a new design criterion of specific mixing power input based on total solids in the sludge (W/kgTS) is proposed to achieve maximum biogas production using optimum power input. The relationship has its limitations, but it represents a significant step forward in the design and operation of improved digester mixing systems. [ABSTRACT FROM AUTHOR]

Abstract Mixing strategies of high solids anaerobic digesters using food waste (FW), codigested with sewage treatment plant sludge (STP) was evaluated for green and clean biogas production. Improper anaerobic digestion (AD) mixing leads to poor biogas yield at high organic loading rate (OLR). The present study evaluated the novel method of biogas recirculation using new 'O' shaped diffuser design for the AD mixing and compared the performance with the conventional mode of impeller driven mixing and unmixed mode for the optimum FW to STP ratio of 3:1 at high OLR The biogas recirculation frequency of 15 min/h produced maximum biogas (0.28–0.86 L/gVS r). An enhanced biogas yield was observed at intermittant biogas recirculation intensity of 2000 mL/min (0.25–0.92 L/gVS r). This was attributed to the synergistic effect of CO 2 acidification and high VFA production at OLR of 6 gVS/Ld. Consequently, led to benefit of reduced digester pH (from 8.3 to 6.6), NH 3 control (2380 mg/L), in-situ CH 4 enrichment (88%) and sheath protected microbial morphology than the other mixing intensities. It is constructed that an optimum intermittent biogas recirculation could be used as an alternative mixing method for the large scale AD, for enhanced biogas yield, reduced impeller power consumption, and improved energy efficiency. Graphical abstract Image Highlights • High R2 observed between stoichiometric and experimental methane potential. • The optimum mixing ratio for anaerobic co-digestion was 3:1. • Intense mixing depicted broken microbial morphology, with high power dissipation. • Intermittent biogas recirculation, feeding and waiting cycle produced high biogas. • Benefit of anaerobic process, pH, NH 3 control and high CH 4 , at low gas mixing. [ABSTRACT FROM AUTHOR]

Abstract Energy and exergy analyses of an integrated system based on anaerobic digestion (AD) of sewage sludge from wastewater treatment plant (WWTP) for multi-generation are investigated in this study. The multigeneration system is operated by the biogas produced from digestion process. The useful outputs of this system are power, freshwater, heat, and hydrogen while there are some heat recoveries within the system for improving efficiency. An open-air Brayton cycle, as well as organic Rankine cycle (ORC) with R-245fa as working fluid, are employed for power generation. Also, desalination is performed using the waste heat of power generation unit through a parallel/cross multi-effect desalination (MED) system for water purification. Moreover, a proton exchange membrane (PEM) electrolyzer is used for electrochemical hydrogen production option in the case of excess electricity generation. The heating process is performed via the rejected heat of the ORC's working fluid. The production rates for products including the power, freshwater, hydrogen, and hot water are obtained as 1102 kW, 0.94 kg/s, 0.347 kg/h, and 1.82 kg/s, respectively, in the base case conditions. Besides, the overall energy and exergy efficiencies of 63.6% and 40% are obtained for the developed system, respectively. Highlights • A novel integrated system based on anaerobic digestion of biomass. • Electricity, heat, freshwater, and hydrogen production for multigeneration. • Energy and exergy analyses of system components and overall system. • The more CH 4 molar fraction in biogas, the higher energy and exergy efficiencies. [ABSTRACT FROM AUTHOR]

Highlights • The methane yield of HTCWW achieved 286 mL CH 4 /g COD when HTC temperature was 170 °C. • The products of HTCWW leading to different methane yield were well characterized. • Different microbial communities were established due to different organic compositions. Abstract The conversion of dewatered sewage sludge (DSS) into bio-oil and hydro-char through hydrothermal conversion (HTC) has been well studied, while the treatment of wastewater (HTCWW) was overlooked. The present study aimed to investigate the organic compositions of HTCWW obtained under different temperatures and residence time (170–320 °C, 0.5–6.0 h) and their potentials use for methane production through anaerobic digestion (AD). Results showed that the methane yield achieved to 286 mL CH 4 /g COD when HTC temperature was 170 °C while it was decreased to only 136 mL CH 4 /g COD at 320 °C. The methane yield of HTCWW was also decreased with the increase of HTC residence time. Proteins and volatile fatty acids (VFAs) were the main products of HTCWW but more recalcitrant degradable compounds e.g. humic acid-like, melanoidins, nitrogen heterocycle, and phenols were formed through 3D-EEM and GC–MS analysis, which might result in the lower methane yield of HTCWW at increased temperatures. Furthermore, microbial analysis showed that different microbial community compositions for anaerobic degradation of the HTCWW samples were established due to the different organic compositions. Therefore, the integration of HTC and AD can be a potential complementary process for energy recovery from DSS. [ABSTRACT FROM AUTHOR]

Abstract The mesophilic anaerobic co-digestion of the liquid fraction from hydrothermal carbonization (LFHTC) of dewatered waste activated sludge with primary sewage sludge (PSS) has been studied. Mixtures of different composition (25, 50 and 75% of LFHTC on a chemical oxygen demand (COD) basis), as well as the individual substrates, have been tested using two inocula (flocculent (FS) and granular (GS) sludges). Methane production decreased as the LFHTC/PSS ratio increased, which can be related to the presence of recalcitrant compounds in the LFHTC, such as alkenes, phenolics, and other oxygen- and nitrogen-bearing aromatics hard-to-degrade through anaerobic digestion. Methane yield reached 248 ± 11 mL CH 4 STP/g COD added with the GS inoculum and 25% LFHTC. A 74 and a 30% increase of methane production was achieved in the 25% LFHTC runs respect to the obtained in the similar experiments with 100% LFHTC, using the FS and GS inocula, respectively. In those late runs, the COD was reduced more than 86%, with a negligible concentration of total volatile fatty acids. With both inocula, total Kjeldahl nitrogen hydrolysis increased as the LFHTC to PSS mixture ratio decreased, reaching values higher than 79% at the end of the experiments. Methane yield values fitted well the first-order, Cone and Weibull kinetic models for both inocula. Significant differences in the kinetic constant values, ranging from 0.100 to 0.168 d−1 and 0.059–0.068 d−1, were found with the FS and GS inocula, respectively. The results obtained support the potential integration of HTC of dewatered waste activated sludge in wastewater treatment plants. Graphical abstract Image 1 Highlights • Valorization of waste activated sludge by HTC + anaerobic co-digestion is studied. • Experiments with three LFHTC/PSS ratios and two inocula were performed. • High COD removal and CH 4 yield with negligible volatile acid concentration were reached. • Anaerobic co-digestion of LFHTC to PSS mixture ratios of 25% is a feasible option. • Cumulative methane yield were adequately fitted by first-order, Cone and Weibull equations. [ABSTRACT FROM AUTHOR]

Quorum sensing (QS) plays a key role in activating bacterial functions through small molecules called autoinducers. In this study, the QS of Gram-negative bacteria in waste sewage sludge (WSS) was downregulated by adding the quorum quenching enzyme, AiiM lactonase, which cleaved the acyl-homoserine lactone (AHL) autoinducer signals from Gram-negative bacteria, and subsequently methane production was inhibited by over 400%. The pH was lowered after 2 days in the anaerobic fermentation whereas protease activity at the hydrolysis step was almost the same with or without AiiM. The production of acetic acid significantly increased during the fermentation in the presence of AiiM. The bacterial community at day 2 indicated that the population of Gram-positive bacteria increased in the presence of AiiM, and the percentage of Gram-negative bacteria decreased in the WSS containing AiiM. The change in the bacterial community in the presence of AiiM may be due to the different antimicrobial agents produced in the WSS because some of the Gram-positive bacteria were killed by adding the solid-phase extraction (SPE) fraction from the WSS without AiiM. In contrast, the SPE fraction with AiiM had reduced bactericidal activity against Gram-negative bacteria. Thus, bacterial signaling between Gram-negative bacteria is critical for methane production by the microbial consortia. [ABSTRACT FROM AUTHOR]

Abstract The effects of microwave (MW) final temperature (80 and 160 °C) and holding time (1 and 30 minutes) were evaluated by ten anaerobic digesters (AD). MW was applied to dewatered secondary sludge with total solids of 10.5 ± 0.5% (by weight) to minimize energy input. Mixed sludge was digested under thermophilic and mesophilic conditions at sludge retention times (SRTs) of 20, 12, and 6 days. MW final temperature had significantly higher effect on solubilization than holding time. At MW of 80/160 °C, longer holding times further enhanced solubilization but increased volatile fatty acid accumulation at thermophilic SRT of 6 days. While all mesophilic digesters met the Class B requirement in terms of pathogens removal, all thermophilic digesters produced Class A biosolids. MW-ADs were able to tolerate 15–20% higher loading rates compared to controls. Up to 21% higher organics removals were achieved by MW. MW at 160 °C generated the highest amount of extra heat that could be used within/outside of plant. However energy recovered from the enhanced methane production was not sufficient to cover the electrical energy consumed by MW. Therefore, a custom-designed electromagnetic heating unit based on dielectric properties of sludge is needed to minimize energy input and maximize energy recovery for full-scale applications. Highlights • Waste sludge was first dewatered and then pretreated for a more focused heating. • MW temperature had significantly higher impact on solubilization than holding time. • MW holding time enhanced solubilization but caused VFA accumulation/high energy input. • High-temperature MW pretreatment produced the high amount of extra heat with CHP unit. • Extra methane was not sufficient to cover the energy consumed by MW pretreatment. [ABSTRACT FROM AUTHOR]

The biochemical methane potential (BMP) of primary and biological sludge varies in a wide range, mostly depending on location, sewer characteristics, wastewater treatment plant design and operating conditions. BMP tests are useful to verify the performance of a full scale digester, but they are not yet a common procedure in the operation of most Italian facilities because of cost and test duration. Changes in the composition of sewage sludge can lead to a high variation of biogas production. Aimed at developing BMP predictive models based on low cost and fast analyses, this study investigated the chemical composition of 20 sludge samples by means of principal component and multiple linear regression analyses. Three preliminary predictive models were developed based on soluble organic nitrogen, volatile solids, carbohydrates, proteins, lipids and an operational parameter, the sludge retention time: the explained variance and the standard errors of prediction of BMP are in the range 77-81% and 21-34 NmLCH4⋅gVS-1, respectively. Models were evaluated on five additional samples: errors ranged 2-15% for four samples and about 54% for one sample, collected from a peculiar facility. Further data and variables describing the operation mode of the waterline would certainly improve the reliability and robustness of the models. [ABSTRACT FROM AUTHOR]

Abstract Low organic matter content and high heavy metal levels severely inhibit the anaerobic digestion (AD) of sewage sludge. In this study, the effect of added manganese oxide-modified biochar composite (MBC) on methane production and heavy metal fractionation during sewage sludge AD was examined. The MBC could increase the buffering capacity, enhance the methane production and degradation of intermediate acids, buffer the pH of the culture, and stabilize the sewage sludge AD process. The application of MBC positively impacted methane production and the cumulative methane yield increased up to 121.97%, as compared with the control. The MBC addition can improve metal stabilization in the digestate. An optimum MBC dose of 2.36 g was recommended, which would produce up to 121.1 L/kg volatile solids of methane. After the AD process, even though most of the metals accumulated in the residual solids, they could be transformation from the bio-available fractions to a more stable fraction. The total organic- and sulfide-bound and residual fraction content at a 3 g dose of MBC that is 0.12 g/g dry matter were 51.06% and 35.11% higher than the control, respectively. The results indicated that the application of MBC could improve the performance of AD and promote stabilization of heavy metals in sewage sludge post the AD process. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

Highlights • Process performance with shortening HRTs was simulated by kinetic model. • Microorganisms were washed out at HRT of 3 days. • Acidogenesis and methanogenesis became the rate limiting step. • VS removal correlated with the methanogenic communities. Abstract Requirement of a long hydraulic retention time (HRT) for efficient degradation restrains the anaerobic digestion of hydrothermal pretreated sludge. Shortening the HRT can increase the treatment capacity of a plant but may also induce digester instability. This study investigated the impact of HRT on process performance and microbial community by consecutively operating a reactor for 145 days. The HRT was gradually decreased from 20 to 10, 5, and 3 days. The methane yield declined from 0.28 to 0.12 L/g-VS in with this shortening, and acetate concentration increased from 38 to 376 mg/L. Methanoculleus (58%) dominated methanogens at a 20 days HRT. However, the methanogenic structure shifted toward an increased level of Methanospirillum , representing 95% of the total archaea at a 3 days HRT. Microorganisms were almost washed out at the end of experiment. Conclusively, shortening HRTs is a feasible strategy to increase treatment capacity and produce more biogas at existing plants. [ABSTRACT FROM AUTHOR]