Ammonia stripping by in situ biogas self-circulation to upgrade continuous thermophilic and mesophilic digestion of hydrothermal high-solid sludge

High-solid anaerobic digestion of hydrothermal sewage sludge has been developed. In order to upgrade the process by focusing on ammonia inhibition, a simply-equipped stripping system without additional alkali or heat supply was introduced by in situ biogas self-circulation. As the determined limit of total ammonia nitrogen at 1500 mg/L and 1000 mg/L for the mesophilic (MAD) and thermophilic anaerobic digestion (TAD) respectively and stripping rate at 5 L/min, continuous MAD and TAD was conducted in parallel. The stripping system successfully polished up the ammonia inhibition, and methanogenic capability of the TAD was promoted to approximately 90.0 % of the potential. Intermittent stripping mode proved usable. More frequent stripping was inevitable for the TAD as compared to the MAD. Hydraulic retention time below 20 d resulted in failure of the stripping mode due to rapid ammonia generation. Overall, this technology was practical in upgrading high-solid sludge digestion by effective ammonia control.

Different rapid startups for high-solid anaerobic digestion treating pig manure: Metagenomic insights into antibiotic resistance genes fate and microbial metabolic pathway

High-solid anaerobic digestion (HSAD), as an emerging disposal technology for swine manure, was commonly hampered by the long lag phase and slow startup, resulting in poor performance. Rapid startups by different leachate reflux forms can solve the problem, but related study was scarcely reported. Therefore, metagenomic analysis was used to exploit the effects of different rapid startups on the biogas performance, antibiotic resistance genes (ARGs) removal and microbial metabolic pathway during HSAD. Compared anaerobic digestion with natural start (T1), three different rapid startups were set, including with autologous leachate reflux (T2), with water reflux (T3) and with exogenous leachate reflux (T4). The results showed that rapid startups (T2-T4) enhanced biogas yield and the cumulative methane yield was increased by 3.7-7.3 times compared with the control. Totally, 922 ARGs were found, most of which belonged to multidrug and MLS ARGs. About 56% of these ARGs could be reduced in T4, while just 32% of ARGs were reduced in T1. Antibiotic efflux pump is the main mechanism of microbial action, which could be decreased largely by these treatments. Moreover, all the rapid startups (T2-T4) made Methanosarcina content (9.59%-75.91%) higher than that in the natural startup of T1 (4.54%-40.27%). This is why these fast-startups helped methane production fast. Network analysis showed that microbial community and environmental factors (pH and VFAs) both contributed to the spread of ARGs. The reconstructed methane metabolic pathway by different identified genes showed that all methanogenesis pathways existed but acetate metabolic pathway was dominant. And the rapid startups made the abundance of acetate metabolic (M00357) higher than the natural startup.

Synergetic effects of biochar addition on mesophilic and high total solids anaerobic digestion of chicken manure

High solids anaerobic digestion (AD) of chicken manure (CM) is often challenging due to ammonia-N inhibition and accumulation of volatile fatty acids (VFAs). This study evaluated the effect of adding biochars from different feedstock to ameliorate semi-dry AD of fresh CM during batch fermentation. Experiments were performed in 300 mL at two total solid (TS) levels (12% and 15%) under mesophilic (36 ±1ᵒC) conditions for 55 d, using activated sludge as inoculum. Treatments included: fresh CM (at 12% or 15% TS) mixed separately with rice husks char (RB), wood char (WB) and bamboo char (BB) at biochar dosages of 2.5%, 5% and 10% of TS in the CM, inoculum only and inoculum plus CM without addition of char as the control. Results indicated that addition of biochar reduced the lag phases to 4-5.4 d and AD performances were significantly improved with total volatile solids removal of 53-67% and 62-71%, and cumulative methane of 277-380 mL/gVS (CH₄ content ≈ 51-63%) and 297-438 mL/gVS (CH₄ content ≈ 49-67%) at 12% and 15% TS, respectively. Biochar buffered over acidification and stabilized pH in the range of 6.5-7.8 but mild ammonia inhibition still occurred in all biochar treatments due to the high residual total ammonia-N (4.3 g-5.6 g/L). For all the investigated parameters, WB amended digesters exhibited the best results owing to its high specific surface area, porosity, cationic exchange capacity, and elemental composition which were superior to those of RB and BB. At 10% dosage of all tested biochars, the AD process was more stable and methane content neared optimal of >65% CH₄. Therefore, addition of biochar from lignocellulosic materials at a given threshold dosage could promote semi-dry and dry biogas production from chicken manure and thus add value to this waste which in most cases is improperly managed.

Mixing effects on decentralized high-solid digester for horticultural waste: Startup, operation and sensitive microorganisms

This work studied the use of a horizontal high-solid digester for the decentralized anaerobic treatment of horticultural waste (fallen leaves), where the effect of intermittent mixing by a modified double helical ribbon impeller was investigated. Before experimental verification, the flow pattern and theoretical mixing time were first characterized by CFD simulation. Subsequently, three mixing time intervals (i.e., 3 min/3 hr; 18 min/3 hr; 108 min/3 hr) and one control setup (i.e., without mixing) were compared for their performance during start-up and semi-continuous operation. It was found that minimal mixing was necessary for an efficient digester's start-up but increased mixing intensity for semi-continuous operation. The results were further interpreted by correlating the digester performance and microbial communities. Those microorganisms sensitive to increased mixing intensity were highlighted and analysed.

Food waste treating by biochar-assisted high-solid anaerobic digestion coupled with steam gasification: Enhanced bioenergy generation and porous biochar production

A food waste treating system was proposed in this study by combining biochar-assisted high-solid anaerobic digestion and subsequent steam gasification of the digestate. The effect of solid level, biochar dosage in anaerobic digestion on the properties of biogas, syngas, and final biochar products were investigated. Results showed that at a high total solid level and biochar dosage of 25 g/L and 50 g/L, the accumulative methane yield reached 110.3 mL CH₄/g VS and 126.7 mL CH₄/g VS, respectively. From steam gasification of different digestates under 850 °C for 15 min, a maximum of 34.92 mmol/g for the hydrogen yield and 11.44 MJ/m³ for the higher heating value could be obtained for the syngas. Furthermore, the by-product produced from steam gasification was a nutrient-enriched porous biochar, which was suitable to be used as compost. This study demonstrated a pathway for food waste treating to produce methane-enriched biogas, hydrogen-enriched syngas, and nutrient-enriched biochar.

Biochar enhanced high-solid mesophilic anaerobic digestion of food waste: Cell viability and methanogenic pathways

The underlying mechanisms of biochar enhance high-solid anaerobic digestion (HSAD) of food waste were investigated with a focus on the cell viability, microbial community, and methanogenic pathways. This study assessed the effects of different dosages of biochar in HSAD. Optimal biochar dosage was found to be 25 g/L, which produced accumulative methane yields of up to 251 mL CH₄/g VS significantly promote volatile fatty acid degradations, especially in butyric acid concentrations. Effects of biochar with a dosage of 25 g/L on the cell viability showed that viable cells based on cell membrane integrity increased from 2.9% to 6.4%. Meanwhile, intact and highly active cells with high DNA content were probably involved in direct interspecies electron transfer (DIET) via membrane-bound electron transport proteins. Further analysis demonstrated that Syntrophomonas and methanogens Methanosarcina &Methanocelleus were selectively enriched by biochar, which resulted in the methanogenic pathways shifting from acetoclastic/hydrogenotrophic methanogenic pathways to more metabolically diverse methanogenic pathways. Accordingly, biochar-mediated DIET was possibly established between Syntrophomonas and Methanosarcina species due to those viable cells. In conclusion, biochar is a feasible additive in enhancing HSAD methanogenic performance.

Revealing hydrodynamics and energy efficiency of mixing for high-solid anaerobic digestion of waste activated sludge

Anaerobic digestion is a feasible and promising technique to deal with emerging waste activated sludge issues. In this work, the hydrodynamics and digestion performance of horizontal anaerobic systems equipped with double-bladed impeller and ribbon impeller were investigated. Simulation using computational fluid dynamics technique visually showcased the favorable mixing status implementing ribbon impeller. The mixing modes were considered as the major motivation for the difference of mixing efficiencies. Tracing experiment indicated that the minimum thorough mixing time with ribbon impeller was 20 min at a rotation speed of 50 rpm, whereas it was 360 min for the double-bladed impeller under similar conditions. The superior mixing performance of ribbon impeller resulted in better anaerobic digestion and energy efficiency outputs. The digester employing ribbon impeller obtained an ultimate biogas yield of 340.38 ± 15.91 mL/g VS (corresponding methane yield of 210.34 ± 7.55 mL/g VS) and produced a surplus energy of 16.23 ± 0.76 MJ/(m³·d). This study thus ascertained that ribbon impeller was proficient for high-solid anaerobic digestion and it will prominently benefit future system designs.

Improved anaerobic digestion efficiency of high-solid sewage sludge by enhanced direct interspecies electron transfer with activated carbon mediator

High-solid anaerobic digestion (AD) faces the problems of easy acidification and low methane production efficiency. In this study, activated carbon (AC)-enhanced direct interspecies electron transfer (DIET) was investigated to overcome such problems. Results showed the conversion of volatile fatty acids (VFAs) into methane rate was increased with AC addition, which led improved methane production efficiency. The methane yields from the early AD stage improved by 124.0-146.3% with AC addition. The T₈₀ shortened by 8-9 days with AC addition. The relative abundances of Geobacter, Syntrophomonas and Methanosaeta that associated with DIET improved for 63.65%, 256.3% and 4.35% by AC addition, which reflected the enhanced DIET with AC addition. The redox activity of AC might be responsible for the enhanced DIET. This study would advance the understanding of DIET and provide a potential solution to the problems existed in high-solid AD.

Humification in extracellular polymeric substances (EPS) dominates methane release and EPS reconstruction during the sludge stabilization of high-solid anaerobic digestion

High-solid anaerobic digestion (HAD) can directly treat dewatered sewage sludge (total solid content ≥15%) with superior volume efficiency. Sludge stabilization during HAD is expected to achieve by throughout organic degradation and conversion towards methane-rich biogas release and humic formation. Sewage sludge is the combination of microbial zoogleas and theirs adsorption of organic and inorganic matter, in which the extracellular polymeric substances (EPS) account 60-80% of total sludge organic matter, inevitably participating most extracellular metabolic pathways. The interactions between EPS transformation and genetically annotated metabolic pathways were found in this research. In brief, noticing the highly cross-linked structures in EPS with major active components of humic substances (HS) and protein (PN), as PN hydrolysis and decomposition in EPS were enhanced in the high-solid anaerobic condition, the exposure of aromatic groups and sites in HS were considerable. HS release was the main factor shifting the electron exchange capacity and activity, which aided in energy metabolism of sludge microorganisms involved in redox reactions, especially the methanogenesis, thus in turn facilitating the PN degradation; Then, the screened humic groups and active protein derives might act as the beneficial precursors to regenerate neo-humic structures, whose significant bridging effect and signal role on stimulating amino acid biosynthesis, member transport and metallic complexation could further contribute to proteolytic condensation and EPS reconstruction. Hence, the in-depth sludge stabilization mechanism during HAD process was established for developing enlightening strategies.

Enhancement of sustainable flux by optimizing filtration mode of a high-solid anaerobic membrane bioreactor during long-term continuous treatment of food waste

Membrane fouling or flux limitation is the major bottleneck that hinders anaerobic membrane bioreactor (AnMBR) application. An AnMBR with a working volume of 15 L was operated for 180 days to investigate the maximum sustainable flux at different high solid concentrations during the anaerobic treatment of food waste. A total of eight filtration-to-relaxation (F/R) ratios were incorporated, with a fixed filtration time of 3 min and varied relaxation times (decreased from 12 to 1 min). Besides, a total of five instantaneous fluxes were applied: 12, 14, 16, 18 and 20 L/m²/h (LMH). Results showed that sustainable flux was greatly enhanced by filtration mode optimization. The optimal F/R ratios were 3:1, 3:1, 3:1 and 3:6 at mixed liquor total solid (MLTS) concentrations of 10, 15, 20 and 25 g/L, respectively. The corresponding sustainable flux values were 13.2 ± 0.3, 10.1 ± 0.4, 9.3 ± 0.2 and 4.0 ± 0.3 LMH, respectively. These values were 29%, 35%, 52% and 21% higher than the critical flux determined by the flux-stepping technique. The results of this study were used to perform a mathematical simulation. The obtained regression equation between the maximum sustainable flux and MLTS concentration can be used to predict the sustainable flux at other MLTS concentrations. This work provides valuable insight into the design and operation of high-solid AnMBRs, and is expected to contribute to further advances in the application of AnMBRs in industry.

Effect of aromatic repolymerization of humic acid-like fraction on digestate phytotoxicity reduction during high-solid anaerobic digestion for stabilization treatment of sewage sludge

Humification plays an important role in the sewage sludge stabilization treatment and probably influences the end-product ecotoxicity, which are not fully understood for high-solid anaerobic digestion (AD) process. This study monitored the aromaticity degree of humic acid-like fraction (HA) and detected digestate phytotoxicity during a 48-d high-solid AD. Significant repolymerization of aromatic structures in HA. Electrical conductivity (EC) of digestate changed negatively with the aromaticity degree in HA, and the digestate with high EC had low seed germination index (SGI). A positive correlation between SGI and the aromaticity degree of HA was found. Aromatic repolymerization of HA supplies more aromatic electron-rich sites to form neo-humic polymers and to determine the distribution of organic and/or inorganic phytotoxic compounds; thus, aromatic repolymerization of HA may control the digestate phytotoxicity by reducing excessive salinity. The results showed the effect of HA repolymerization on digestate stabilization and phytotoxicity reduction during the high-solid AD process. The inadequate phytotoxicity reduction efficiency (SGI ≤ 17%) may be ascribed to unaccomplished HA repolymerization, creating a demand for humification intensification studies.

High-solid anaerobic digestion of corn straw for methane production and pretreatment of bio-briquette

In this study, an integrated process was developed to produce methane and high-quality bio-briquette (BB) using corn straw (CS) through high-solid anaerobic digestion (HS-AD). CS was anaerobic digested by using a leach bed reactor at four leachate recirculation strategies. After digesting for 28 days, highest methane yield of 179.6 mL/g-VS, which was corresponded to energy production of 5.55 MJ/kg-CS, was obtained at a higher initial recirculation rate of 32 L-leachate per day. Compared with bio-briquette manufactured from raw CS and lignite, the compressive, immersion and falling strength properties of bio-briquette made from AD-treated CS (solid digestate) and lignite were significantly improved. A preferred BB can be obtained with side compressive strength of 863.8 ± 10.8 N and calorific value of 20.21 MJ/kg-BB. The finding of this study indicated that the integrated process could be an alternative way to produce methane and high-quality BB with CS.

Influence of NaOH and thermal pretreatment on dewatered activated sludge solubilisation and subsequent anaerobic digestion: Focused on high-solid state

In this study, the influence of NaOH and thermal pretreatment of dewatered activated sludge (DAS) on the high-solid solubilisation and anaerobic digestion was separately investigated by monitoring common parameters. The results indicated that COD, proteins and carbohydrates were efficiently solubilised in both NaOH and thermal pretreated DAS samples. For NaOH pretreatment, the concentrations of volatile fatty acids (VFAs) and total ammonium nitrogen (TAN) firstly increased followed by decreasing with NaOH dose increasing. However, they decreased with the severity of thermal pretreatment. During the batch digestion experiments (at 37°C), for 80mg NaOHg(-1) total solid (TS) DAS pretreatment it resulted in a 6.99% decrease in cumulative methane yield (CMY) compared to untreated DAS. While for 80, 100, 120°C and 20mg NaOH pretreatment, CMY increased by 15%, 42%, 71% and 35%, respectively, in comparison to untreated DAS.

Rheology evolution of sludge through high-solid anaerobic digestion

The main purpose of this study was to investigate the rheology evolution of sludge through high-solid anaerobic digestion (AD) and its dependency on sludge retention time (SRT) and temperature of AD reactor. The operation performance of high-solid AD reactors were also studied. The results showed that sludge became much more flowable after high-solid AD. It was found that the sludge from reactors with long SRT exhibited low levels of shear stress, viscosity, yield stress, consistency index, and high value of flow behaviour index. While the flowability of sludge from thermophilic AD reactors were better than that of sludge from mesophilic AD reactors though the solid content of the formers were higher than that of the latters, which could be attributed to the fact that the formers had more amount of free and interstitial moisture. It might be feasible to use sludge rheology as an AD process controlling parameter.

New insight into chemical changes of dissolved organic matter during anaerobic digestion of dewatered sewage sludge using EEM-PARAFAC and two-dimensional FTIR correlation spectroscopy

Dissolved organic matter (DOM) is a key component in reaction network of anaerobic digestion. In this study, fluorescent excitation-emission matrix-parallel factor (EEM-PARAFAC) analysis and two-dimensional (2D) FTIR correlation spectroscopy were firstly used to explore chemical changes of soluble intermediates in high-solid biogas reactor. EEM-PARAFAC showed that fluorescent components (tyrosine-like, tryptophan-like and humic-like groups) in DOM over time increased gradually, implying that these groups were reluctant to biodegrade (acidogenesis). The resistance to biodegradation presented the following order: humic-like group>tyrosine-like group>tryptophan-like group. 2D FTIR correlation spectroscopy indicated that the DOM change sequence with time followed the order: protein-like groups>structural carbohydrates and carboxylic acids>polysaccharides-like groups. Fluorescence intensities from EEM-PARAFAC and main bands of FTIR spectra correlated significantly with other chemical parameters, e.g. biogas production and dissolved organic carbon content. These findings supply novel realization for degradation degree and order of individual DOMs during anaerobic digestion for dewatered sewage sludge.

Liquid mixing and solid segregation in high-solid anaerobic digesters

An experimental procedure (Residence Time Distribution technique) was used to characterize the macro-mixing of both liquid and solid phases of a laboratory-scale dry anaerobic digester using appropriate tracers. The effects of the waste origin and total solid content were studied. An increase in TS content from 22% to 30% TS (w/w) induced a macro-mixing mode closer to a theoretical Plug Flow Reactor. The segregation of particles having different densities was investigated regarding the RTD of the solid phase. Segregation of dense particles occurred at low TS content. By using different TS content and waste origins, it was also determined that the yield stress was a key parameter in the mechanism of segregation. At high yield stress, dense particles were more stable and thus less subjected to settling. As a consequence, operating at high TS content may permit to prevent the sedimentation of the denser particles.