Evaluation of continuous mesophilic, thermophilic and temperature phased anaerobic digestion of microwaved activated sludge

Superparamagnetic Iron Oxide Nanoparticles as Additives for Microwave-Based Sludge Prehydrolysis: A Perspective

Wastewater treatment plants are critical for environmental pollution control. The role that they play in protecting the environment and public health is unquestionable; however, they produce massive quantities of excess sludge as a byproduct. One pragmatic approach to utilizing excess sludge is generating methane via anaerobic digestion. For this, a prehydrolysis step can significantly improve digestion by increasing biogas quality and quantity while decreasing final sludge volumes. One of the many prehydrolysis approaches is to deliver heat into sludge via microwave irradiation. Microwave-absorbing additives can be used to further enhance thermal degradation processes. However, the implications of such an approach include potential release of said additive materials into the environment via digested sludge. In this perspective, we present and discuss the potential of superparamagnetic iron oxide nanoparticles (SPIONs) as recoverable, hyperreactive microwave absorbers for sludge prehydrolysis. Due to their size and characteristics, SPIONs pack spin electrons within a single domain that can respond to the magnetic field without remanence magnetism. SPIONs have properties of both paramagnetic and ferromagnetic materials with little to no magnetic hysteresis, which can enable their rapid recovery from slurries, even in complicated reactor installations. Further, SPIONs are excellent microwave absorbers, which result in high local heat gradients. This perspective introduces the vision that SPION properties can be tuned for desirable dielectric heating and magnetic responses while maintaining material integrity to accomplish repeated use for microwave-enhanced pretreatment.

Strategies for energy conversion from sludge to methane through pretreatment coupled anaerobic digestion: Potential energy loss or gain

Anaerobic digestion (AD) of wasted activated sludge from wastewater plants is recognized as an effective method to reclaim energy in the form of methane. AD performance has been enhanced by coupling various pretreatments that impact energy conversion from sludge. This paper mainly reviewed the development of pretreatments based on different technologies reported in recent years and evaluated their energy benefit. Significant increases in methane yield are generally obtained in AD with pretreatments demanding energy input, including thermal- and ultrasound-based methods. However, these energy-intense pretreatments usually gained negative energy benefit that the increase in methane yield consumed extra energy input. The unbalanced relationship counts against the goal of energy reclamation from sludge. Combined pretreatment consisting of multiple technologies normally outcompetes the single pretreatment, and the combination of energy-intense methods and chemicals potentially reduces energy input and simultaneously ensure high methane yield. For determining whether the energy reclamation from sludge via AD contribute to mitigating global warming, integrating greenhouse gas emission into the evaluation system of pretreated AD is further warranted.

Hydrophilic spongy biochar crosslinked with starch and polyvinyl alcohol biocarrier for nitrate, phosphorus, and cadmium removal in low carbon wastewater: Enhanced performance mechanism and detoxification

This study aims to develop a functional biocarrier with hydrophilic spongy biochar crosslinked with starch and polyvinyl alcohol (WSB/starch-PVA) for simultaneous removal of NO₃^--N, total phosphorus (TP) and Cd²⁺ in low carbon wastewater. Results showed that the WSB/starch-PVA bioreactor achieved the maximum NO₃^--N removal efficiency in subphase 1.2 with 98.07 % (3.64 mg L^-1h^-1) versus control (75.30 %, 2.81 mg L^-1h^-1), and removed 54.84 % and 73.97 % of TP and Cd²⁺. Material characterization suggested that functional groups (related to C, N and O) on biocarrier and biofilm, and biogenic co-precipitation facilitated TP and Cd²⁺ removal. The WSB made the biocarrier pores larger and regular, and decreased fluorescent soluble microbial products. The predicted metagenome further suggested that central citrate cycle, oxidative phosphorylation of bio-community, and NO₃^--N removal were enhanced. Functions for microbial induced co-precipitation, Cd²⁺ transport/efflux, antioxidants, and enhanced biofilm formation favored the NO₃^--N/TP removal and Cd²⁺ detoxification.

Anaerobic Digestion of Food Waste and Its Microbial Consortia: A Historical Review and Future Perspectives

Renewable energy source, such as food waste (FW), has drawn great attention globally due to the energy crisis and the environmental problem. Anaerobic digestion (AD) mediated by novel microbial consortia is widely used to convert FW to clean energy. Despite of the considerable progress on food waste and FWAD optimization condition in recent years, a comprehensive and predictive understanding of FWAD microbial consortia is absent and therefore represents a major research challenge in FWAD. The review begins with a global view on the FWAD status and is followed by an overview of the role of AD key conditions’ association with microbial community variation during the three main energy substances (hydrogen, organic acids, and methane) production by FWAD. The following topic is the historical understanding of the FWAD microorganism through the development of molecular biotechnology, from classic strain isolation to low-throughput sequencing technologies, to high-throughput sequencing technologies, and to the combination of high-throughput sequencing and isotope tracing. Finally, the integration of multi-omics for better understanding of the microbial community activity and the synthetic biology for the manipulation of the functioning microbial consortia during the FWAD process are proposed. Understanding microbial consortia in FWAD helps us to better manage the global renewable energy source.

Energy feasibility and life cycle assessment of sludge pretreatment methods for advanced anaerobic digestion

Energy sustainability is one of the critical parameters to be studied for the successful application of pretreatment processes. This study critically analyzes the energy efficiency of different energy-demanding sludge pretreatment techniques. Conventional thermal pretreatment of sludge (∼5% total solids, TS) produced 244 mL CH₄/gTS, which could result in a positive energy balance of 2.6 kJ/kg TS. However, microwave pretreatment could generate only 178 mL CH₄/gTS with a negative energy balance of -15.62 kJ/kg TS. In CAMBI process, the heat requirements can be compensated using exhaust gases and hot water from combined heat and power, and electricity requirements are managed by the use of cogeneration. The study concluded that <100 ℃ pretreatment effectively enhances the efficiency of anaerobic digestion and shows positive energy balance over microwave and ultrasonication. Moreover, microwave pretreatment has the highest global warming potential than thermal and ultrasonic pretreatments.

Application of autothermal thermophilic aerobic digestion as a sustainable recycling process of organic liquid waste: Recent advances and prospects

Autothermal thermophilic aerobic digestion (ATAD) has been used to stabilize organic waste since the 1960s and is considered sustainable technology. ATAD has several advantages, including high biodegradation efficiency, pathogen inactivation, and ease of operation. Although ATAD research has a long history, the number of studies on ATAD is much lower than those on similar aerobic processes, particularly composting. Previous review articles addressed the origin, design, operational experiences, metabolism, and the microorganisms at the thermophilic stage of ATAD. This article reviews the digestion systems, applications, and characteristics of ATAD; compares system performance and microbial community structure of ATAD with those of other biological processes such as composting, activated sludge, and anaerobic digestion; and discusses the physicochemical properties and factors of ATAD. The challenges, opportunities, and prospects for the application of ATAD are also discussed. This review suggests that ATAD is feasible for treating organic liquid waste (1-6% total solid content) in small-sized towns and can help establish a sustainable society.

Responses of mesophilic anaerobic sludge microbiota to thermophilic conditions: Implications for start-up and operation of thermophilic THP-AD systems

Anaerobic digestion (AD) has been widely employed for wastewater and organic waste treatment, in which methanogenesis is highly driven by close microbial interactions among intricate microbial communities. However, the ecological processes underpinning the community assembly that support methanogenesis in such engineered ecosystems remain largely unknown, especially when exposed to challenging circumstances (e.g., high temperature, ammonium content). Here, eight AD bioreactors were seeded with four different inocula (two from full-scale mesophilic AD systems and the other two from lab-scale mesophilic AD systems), and were operated under thermophilic conditions (55 °C) for treating thermal hydrolysis process (THP) pre-treated waste activated sludge to investigate how mesophilic community responds to thermophilic conditions during the long-term cultivation. Results showed that the inocula collected from the full-scale systems were more resilient than that from the lab-scale systems, which may be primarily attributed to indigenous robust methanogens. As a result, the former efficiently generated methane which was predominantly contributed by Methanothermobacter and Methanosarcina (healthy AD ecosystem), while methanogenic activity was remarkably prohibited in the latter (dysfunctional AD ecosystem). Thermophilic environment was a strong selection force, resulting in the convergence of microbial communities in both the healthy and dysfunctional AD ecosystems. Deterministic processes predominated the community assembly regardless of AD ecosystem function, but stronger influences of stochastic processes were observed in dysfunctional AD ecosystems, which was likely attributable from the stronger effect of immigrants from the feedstock. As indicated by molecular ecological network analysis, the microbial network structures in the healthy AD ecosystems were more stable than those in the dysfunctional AD ecosystems. Although keystone taxa were different among the bioreactors, most of which played vital roles in organic hydrolysis/fermentation. To sum up, this study greatly improved our understanding of the relationships between microbiological traits and AD ecosystem function under thermophilic conditions, which could provide useful information to guide thermophilic AD (e.g., THP-AD) start-up and health diagnosis during operation.

Enhanced methane yield through sludge two-phase anaerobic digestion process with the addition of calcium hypochlorite

This study investigated the effects of calcium hypochlorite (Ca(ClO)₂) on biomethane generation from sludge two-phase anaerobic digestion system. In first (acidogenic) phase, volatile fatty acids (VFAs) were largely generated when pretreated by Ca(ClO)₂, while the methane yield was severely inhibited. In second (methanogenic) phase, the methane yield was observably enhanced by Ca(ClO)₂. Further calculation showed that the total methane yield from the two phases was firstly promoted from 156.0 ± 4.5 to 269.9 ± 5.2 mL when Ca(ClO)₂ dosage enhanced from 0 to 1.6 g/L, which then reduced to 235.4 ± 5.5 mL when Ca(ClO)₂ content reached 2.0 g/L. Mechanism analysis showed that the suppression of Ca(ClO)₂ on coenzyme F₄₂₀ activity was relieved in methanogenic phase, and the abundances of functional microbes in methanogenic phase were enriched when added with Ca(ClO)₂. The Ca(ClO)₂-based method well realized the balance between efficacy and economy, possessing outstanding potential for large-scale applications.

Inactivation and risk control of pathogenic microorganisms in municipal sludge treatment: A review

The rapid global spread of coronavirus disease 2019 (COVID-19) has promoted concern over human pathogens and their significant threats to public health security. The monitoring and control of human pathogens in public sanitation and health facilities are of great importance. Excessive sludge is an inevitable byproduct of sewage that contains human and animal feces in wastewater treatment plants (WWTPs). It is an important sink of different pollutants and pathogens, and the proper treatment and disposal of sludge are important to minimize potential risks to the environment and public health. However, there is a lack of comprehensive analysis of the diversity, exposure risks, assessment methods and inactivation techniques of pathogenic microorganisms in sludge. Based on this consideration, this review summarizes the control performance of pathogenic microorganisms such as enterovirus, Salmonella spp., and Escherichia coli by different sludge treatment technologies, including composting, anaerobic digestion, aerobic digestion, and microwave irradiation, and the mechanisms of pathogenic microorganism inactivation in sludge treatment processes are discussed. Additionally, this study reviews the diversity, detection methods, and exposure risks of pathogenic microorganisms in sludge. This review advances the quantitative assessment of pathogenic microorganism risks involved in sludge reuse and is practically valuable to optimize the treatment and disposal of sludge for pathogenic microorganism control.

Influence of operating conditions on the persistence of E. coli, enterococci, Clostridium perfringens and Clostridioides difficile in semi-continuous mesophilic anaerobic reactors

This study examined the combined effect of hydraulic retention time (HRT), organic loading rate (OLR) and heat pretreatment of manure (70 °C, 1 h) on the fate of E. coli, enterococci, C. perfringens, C. difficile, and on chemical parameters (volatile fatty acids and ammonia) that may inactivate pathogens. Semi-continuous mesophilic anaerobic reactors were fed with pig manure and horse feed. The operating conditions were 2, 3, 4 COD.L^-1.d^-1 (OLR), 24, 35, 46 days (HRT) and use or not of a thermal pretreatment. The levels of the chemical parameters did not reach concentrations capable of inactivating the four bacteria. Anaerobic digestion led to a Log₁₀ removal > 3 (E. coli), 0.9-2.1 (enterococci), 0.1-0.6 (C. perfringens) and 0-1 (C. difficile). Increasing HRT only reduced the concentration of E. coli in the digestate. Increasing OLR reduced the Log₁₀ removal of enterococci and C. difficile. The heat pretreatment led to non-detection of E. coli in the digestate, reduced the concentration of C. perfringens by 0.8-1.3 Log₁₀ and increased the concentration of C. difficile by 0.04-0.7 Log₁₀. Enterococci, not detected in the heated manure, were present in the digestate. The distribution of genes encoding virulence factors of C. difficile (tcdA and tcdB) and C. perfringens (cpa, cpb2 and cpb) was not impacted by anaerobic digestion or by the heat pretreatment. Enterococci, C. perfringens, C. difficile were present in the digestate at relatively stable concentrations regardless of the operating conditions, indicating that even with heat pretreatment, the biosafety of digestate cannot be guaranteed in mesophilic conditions.

Microwave Irradiation in Technologies of Wastewater and Wastewater Sludge Treatment: A Review

Every year, the human impact on the world’s water sources becomes more pronounced. One of the triggers to this increase is the use of ineffective wastewater and sludge treatment systems. Recently, the number of studies of microwave processing in handling liquid municipal and industrial waste has increased. This paper discusses heat treatment, change in properties, decomposition of substances, removal of metals, demulsification, pyrolysis, biogas processing, disinfection, and other topics. The findings of European, Chinese, Russian, and other authors are summarised and presented in this review. In addition, the most notable Russian patents for microwave installations/devices and reactors suitable for a wide variety of applications are discussed. In this article, the authors look at microwave wastewater and sludge treatment from the perspective of practical application in various fields of human economic activity.

A Review of Pretreatment Methods to Enhance Solids Reduction during Anaerobic Digestion of Municipal Wastewater Sludges and the Resulting Digester Performance: Implications to Future Urban Biorefineries

The rapid increase in the population is expected to result in the approaching of design capacity for many US wastewater treatment plants (WWTPs) over the next decade. WWTPs treat both municipal and industrial wastewater influents, resulting in the production of biosolids after digestion. Biogas, a potential recovered alternative energy source, is also produced as an output from successful anaerobic digestion. More than 7M of dry tons/year of biosolids produced in the US are most often disposed in either landfills or land-applied (~80%). These options are becoming more challenging to implement due to increases in transportation costs and tipping fees, decreases in the availability of landfill/landfarm space, and most importantly, increased regulations. This situation is strongly encouraging WWTPs to find alternatives for the disposal of biosolids. Developing alternative management/disposal options for biosolids are evolving. One of the most attractive alternative option from a sustainability perspective are biorefineries (converts waste to commercial products), which are a fast-growing option given the push toward circular urban source economies (little to no waste generation). Anaerobic digestion has been widely applied in WWTPs to reduce the volume of activated sludge due to its low energy requirements, effective handling of fluctuations due to organic loading rate, relative flexibility with temperature and pH changes, and since biogas is produced that can be transformed into energy. Various pretreatment methods for waste sludges prior to digestion that have been studied to reduce solids production and increase the energetic content of the biogas are presented and discussed. Solids handling and management, which comprises ~60% of the operational cost of a WWTP, is estimated to save more than $100 M annually by achieving at least 20% reduction in the annual production of biosolids within the US. This review incorporates an assessment of various pretreatment methods to optimize the anaerobic digestion of waste sludges with a focus on maximizing both biosolids reduction and biogas quality.

Impacts of Temperature and Solids Retention Time, and Possible Mechanisms of Biological Hydrolysis Pretreatment on Anaerobic Digestion

Anaerobic digestion (AD) has benefits in sludge management, energy recovery, and pathogen reduction. In order to better understand the mechanisms of biological hydrolysis (BH) pretreatment on AD, biochemical methane potential (BMP) and continuous stirred-tank reactor (CSTR) tests were utilized to compare untreated municipal combined sludge with pilot-scale BH pretreated sludge. During the BH process, there was 15%, 30%, and 33% (w/w) volatile solids (VS) reduction after BH at 42 °C (BH42) for 24, 48, and 72 h, respectively; under BH61 (42 °C for 36 h and 61 °C for 6 h), and there was 10% and 30% (w/w) overall VS reduction after 36-h and 42-h hydrolysis, respectively. BMP results showed that BH42-pretreated sludge had 22.6% enhancement of methane yield compared to untreated sludge, and BH61 pretreated sludge had 29.4% enhancement of methane yield. Both temperature and solids’ retention time (SRT) contributed to the enhanced AD performance within 36 h, while temperature played more important roles after 36-h BH pretreatment. CSTR tests confirmed the acceleration of anaerobic digestion by BH pretreatment, and higher enhancement was observed when SRT of anaerobic digestion was shorter than 16 days. Through a literature review of BH-related studies, the possible mechanisms were highlighted for further optimization on the scale-up systems in order to reduce carbon footprint and operating expenditure for wastewater treatment plants.

Annual microbial community dynamics in a full-scale anaerobic sludge digester from a wastewater treatment plant in Colombia

Anaerobic digestion is a microbe-driven process widely applied to treat activated sludge from municipal wastewater treatment plants. It is one of the most efficient solutions for sludge reduction along with biogas production. However, the knowledge of the microbial consortium involved in this process is still unknown in full-scale anaerobic digesters from Latin America. This study aimed to elucidate the dynamics of the microbial community of a full-scale anaerobic digester for a year using 16S rDNA amplicon sequencing with the Illumina Miseq platform. The results showed fluctuations in the frequencies of dominant phyla with a decrease of Proteobacteria and Bacteroidetes after a temporary suspension of anaerobic digester. The core community was affiliated with bacterial phyla Firmicutes, Actinobacteria, Proteobacteria, and Chloroflexi. The core community was represented by 154 OTUs that accounted for 74% of all the processed reads. The Anaerolineaceae family, within Chloroflexi phylum, was the most frequently observed taxonomic group in all samples analyzed. Despite the microbial fluctuations, the biogas production was stable over the studied year (average 66% methane production), which might indicate a functional redundancy in the microbial consortium.

Immobilized-microbial bioaugmentation protects aerobic denitrification from heavy metal shock in an activated-sludge reactor

The inhibition of denitrification by heavy metals is a problem in nitrogen wastewater treatment, but the solutions are rarely studied. In this study, Pseudomonas brassicacearum LZ-4, immobilized in sodium alginate-kaolin, was applied in an activated-sludge reactor to protect denitrifiers from hexavalent chromium (Cr(VI)). Q-PCR result showed that the strain LZ-4 was incorporated into activated sludge under the help of immobilization. In the non-bioaugmentation system, the removal efficiency of nitrate was decreased by 86.07% by 30 mg/L Cr(VI). Whereas, denitrification was protected and 95% of nitrate was removed continuously in immobilized-cell bioaugmentation system. Miseq sequencing data showed that bioaugmentation decreased the impact of Cr(VI) on microbial communities and increased the abundance of denitrifiers. Based on the results of biomass and extracellular polymers, activated sludge was protected from Cr(VI) toxicity. This discovery will provide a feasible technique for nitrogen wastewater treatment in the presence of distressing heavy metals.

The performance evaluation and kinetics response of advanced anaerobic digestion for sewage sludge under different SRT during semi-continuous operation

Sludge retention time (SRT) is vital for advanced anaerobic digestion (AD) to realize energy self-sufficient. However, the criteria on reasonable SRT has not been fully understood. This study investigated the performance and kinetics response of AD under different SRT in semi-continuous AD with microwave (MW) pretreatment, according to the long-term operation and methane production during one feeding interval. Results showed that modified Gompertz model better described the kinetics than first-order model. At short SRT (15 d), pretreatment coupled with two-stage AD preserved methane production with the high attainable methane potential (B₀) of 257.98 mL/g VS and hydrolysis rate constant (k_hyd) of 0.075 h^-1. But the acceptable decrease of methane production rate seems to be unavoidable, which was possibly derived from the evolution of methanogenesis pathway. This study emphasized the importance of improved methane production rate in semi-continuous AD under short SRT rather than methane production potential obtained from batch experiment.

Dissecting methanogenesis for temperature-phased anaerobic digestion: Impact of temperature on community structure, correlation, and fate of methanogens

This study investigated the relationship between the temperature (35, 42, and 55 °C) used in temperature-phased anaerobic digestion (TPAD) and fate of methanogens between the two anaerobic digestion (AD) phases. Methanogens were profiled by using next generation sequencing (NGS) and droplet digital PCR approaches. The results showed that optimal combined temperatures for methane production were 55 °C during biological hydrolysis (BH) and 35 or 42 °C during AD. BH exhibited much lower archaeal population and was more susceptible to changes in temperature, compared to the AD phase. Additionally, we demonstrated, for the first time, that the BH step could affect the subsequent AD phase by altering AD methanogen composition and improve the stability of the process by enriching the rapidly growing Methanosarcina in the BH-AD process. These results are significant for understanding the mechanisms and stability of methane production in TPAD systems.

Free nitrous acid pre-treatment enhances anaerobic digestion of waste activated sludge and rheological properties of digested sludge: A pilot-scale study

In this study, the effects of free nitrous acid (FNA) pre-treatment on the rheological properties of digested sludge were investigated at a pilot-scale, along with the improvement in volatile solids (VS) destruction and biogas production. Two pilot-scale anaerobic sludge digesters were operated for one year, one receiving thickened waste activated sludge (TWAS) without pre-treatment (control) and one receiving TWAS pre-treated for 24 h at an FNA concentration of 4.9-6.1 mgN/L (nitrite = 250 mgN/L, pH = 5.0, T = 22-30 °C). The results confirmed the enhancing effect of FNA pre-treatment on methane production (37 ± 1%), consistent with previous laboratory studies. Equally importantly, FNA pre-treatment substantially reduced the shear viscosity of TWAS by 51 ± 8% at 100 s^-1 and 49 ± 7% at 250 s^-1, likely due to the solubilization of the TWAS (11.1 ± 0.8%). Similarly, FNA pre-treatment also reduced these viscosity parameters of the digested sludge by 80 ± 4% and 78 ± 4%, respectively, caused by both enhanced VS destruction and disintegration of the digested sludge. The dewaterability of digested sludge, assessed by dewatered solids content, capillary suction time and specific resistance to filtration, was not improved by FNA pre-treatment. The polymer requirement for dewatering was reduced by 24 ± 0.6% due to the lower solids concentration in the digested sludge achieved with FNA pre-treatment. The changes to sludge rheological properties revealed in this study further enhances the business case for the FNA pre-treatment technology.

Molecular weight distribution of pretreated thickened waste activated sludge and fat, oil, and grease

Co-digestion samples containing thickened waste activated sludge and fat, oil and grease were subjected to three different pretreatment methods, i.e., microwave at 175 °C, hyper-thermophilic stage at 70 °C, and conventional heat at 70 °C. The soluble matter extracted from the un-pretreated and pretreated samples were subjected to an ultrafiltration (UF) process using four different membrane sizes (300, 100, 10, and 1 kDa) for molecular weight distribution analysis. Every pretreatment method had a different effect on the solubilization and redistribution of the soluble matter (SCOD and TVFA). For example while MW pretreatment resulted in a significant increase in the SCOD at the lowest molecular weight (< 1 kDa) and at the highest molecular weight (> 300 kDa), Hyper pretreatment caused the majority of the SCOD ( ̴ 62.7% of total SCOD) to be concentrated at the smaller molecular weight range (< 10 kDa). The MW and hyper-thermophilic pretreatments were much more effective in increasing samples solubilization and biogas production compared to the conventional heat pretreatment. The hyper-thermophilic samples had the maximum improvement in cumulative biogas production from all the molecular weights compared to MW- and Heat-pretreated samples; Hyper-pretreated samples achieved 86.5% higher cumulative biogas production compared to the control.

Feeding control of anaerobic co-digestion of waste activated sludge and corn silage performed by rule-based PID control with ADM1

Anaerobic co-digestion (AcoD) of corn silage (CS) and waste activated sludge (WAS) co-substrates, compared with anaerobic digestion (AD) of mono-substrate WAS, was simulated under mesophilic conditions with the adapted IWA Anaerobic Digestion Model No. 1 (ADM1), and a rule-based PID control system for control of the AcoD of CS and WAS, through control of their ratios in the feed, was developed, implemented with the model as a test platform. Tests on AcoD of co-substrates were conducted at the COD mass-based feeding ratios of CS to WAS 1:2.5, 1:2.0 and 1:1.2. The maximal biogas production was 0.94 m³/kgVS·d at the feeding ratio 1:1.2. The ADM1 was adapted, and the high-sensitivity kinetic parameters were calibrated and optimised using the data from the tests of steady state mono-digestion of WAS and AcoD of CS and WAS. The simulated data of biogas and methane production, methane content, VFA and pH agreed well with the test data. The rule-based PID control was developed with an additional expert system, in which the lower level controller operated the level of VFA/TIC and the upper level controller manipulated the setpoints of methane production. The feeding ratio of CS to WAS was used as a manipulated variable. With the constraint boundaries, the test on the control system showed that it could keep methane production stable to the setpoint and maximise methane production while resisting the disturbances to AcoD by adjusting the feeding ratios of CS to WAS.

Biogas Production from Vegetable and Fruit Markets Waste—Compositional and Batch Characterizations

This study presents a complete characterization of the residual materials found in fruit and vegetable markets and their adaptability to be treated by anaerobic digestion with the aim of generating biogas as a new and renewable energy source. It has been determined that these substrates are perfectly suitable to be treated by anaerobic digestion, being rich in simple carbohydrates, with a high content of moisture and solids (total and volatile), which makes it a substrate of easy solubilization and with a great amount of matter directly accessible to the microorganisms responsible for anaerobic degradation. The process develops smoothly, with a slight release of acidic elements, but without impact by the development of the buffer effect by ammonia. In addition, a phenomenon of digestion is observed in two phases, indicating that despite the particulateing of the substrate, it manages to digest the organic matter directly accessible and the inaccessible. In numerical terms, 100 g of residue V produce 913.282 NmL of biogas, of which 289.333 NmL correspond to methane. The disintegration constant is 0.200 days−1, with 16,045% of the substrate degraded. As an innovation, the hydrogen generated in the process has been used as an indicator of the stability and development of the process. Accompanied by a statistical analysis and mathematical adjustments, it is possible to characterize in depth the process and its evolution, determining that the degradation is fast, with a rapid and stable hydrolysis.

Insights into conditioning of landfill sludge by FeCl3 and lime

In this study, landfill sludge (LS) was excavated from a 10 year old full-scale sludge landfill and used to investigate effects of dosage on sludge dewaterability, rheological properties and extracellular polymeric substances (EPS) variations by FeCl₃-lime conditioning. LS had lower content of organic matters (0.28) and smaller particle size than excess sludge (ES), and greatly lower viscosity and high flowability. The suitable concentration of LS for conditioning (107.2-118.6 g/L) was much higher than that of ES (34 g/L) by rheological analysis. Both FeCl₃ and lime improved dewaterability of LS and caused decline of slime and loosely bound EPS (LB-EPS). FeCl₃ destroyed proteins in slime and LB-EPS owing to coagulation and acidification effects, weakened internal structure strength, and thus improved dewaterability. Lime addition caused alkaline hydrolysis of polysaccharides in slime and LB-EPS, reduced viscosity and flowability, and improved flowability and dewaterability for LS. The optimal dosage for dewatering using 57.6 mg lime/g dried solids (DS) and 53.6 mg FeCl₃/g DS was obtained by using an integrative response surface methodology (RSM) coupled nonlinear programming approach under water content constraint of 55%. The integrative optimization achieved 26.0% cost saving in comparison to RSM optimized condition.

Is anaerobic digestion a reliable barrier for deactivation of pathogens in biosludge?

As World Health Organization advocates, the global burden of sanitation related disease and access to safely managed sanitation and safely treated wastewater should be monitored strictly. However, the spread of pathogens through various agricultural applications or direct discharge of sewage sludge generated in municipal wastewater treatment plants poses a serious challenge on the environment and public health. Anaerobic digestion (AD), the principal method of stabilizing biosolids, can efficiently and largely deactivate viable pathogens, including parasite, virus, and the pathogens harboring antibiotic resistance genes. This review aims to provide a critical overview regarding the deactivation of sludge-associated pathogens by AD, through which a serious concern on the effectiveness and rationality of AD towards sludge pathogens control was raised. Meanwhile, the underlying deactivation mechanisms and affecting factors were all discussed, with the focus on pathogen-associated modeling, engineering design and technological aspects of AD. Lastly, a matric method incorporating the operating strategy of AD with the risk assessment was proposed for evaluating the reliability of AD-based pathogen deactivation, while the research agenda forward was also outlined.

A comprehensive dataset on anaerobic digestion of cattle manure, source separated organics, and municipal sludge using different inoculum sources

The dataset reported in this article provides quantitative data on anaerobic digestion of cattle manure, source separated organics (SSO), primary sludge (PS), and thickened waste activated sludge (TWAS) using different inoculum sources. The discussion and interpretation of the data are provided in another publication entitled “Comparison of liquid and dewatered digestate as inoculum for anaerobic digestion of organic solid wastes” [1]. The data presented in this article include 1) the gas chromatography (GC) procedure of determining the biogas composition, 2) the procedure of converting the daily biogas/methane production data from the experimental condition (mesophilic temperature of 38 °C and room pressure) to the standard temperature (0 °C) and pressure (1 atm) condition, 3) the specific methanogenic activity data, and 4) the methane daily production rate data, and 5) the organics biodegradation kinetic rates.

Microwave vs. alkaline-microwave pretreatment for enhancing Thickened Waste Activated Sludge and fat, oil, and grease solubilization, degradation and biogas production

The effects of microwave (MW) and combined alkaline-MW pretreatments on the co-digestion of TWAS:FOG mixtures with 20, 40 and 60% FOG were investigated. MW pretreatment at a high temperature of 175ᵒC was shown to be the most effective MW pretreatment option in solubilizing TWAS:FOG mixtures and boosting methane yield. MW pretreatment at 175ᵒC resulted in maximum solubilization (%) of 68.2% for the 20%FOG samples and a maximum methane yield that was 137% higher than the control for samples with 60%FOG. The combined alkaline-MW (NaOH-MW) pretreatment at pH 10 proved to be not an effective option for TWAS:FOG pretreatment before the anaerobic co-digestion. Despite the benefits of MW pretreatment on the TWAS:FOG samples, including a significant increase in solubilization, dewaterability improvement, high VS reductions, and high methane yield productions, the energy analysis resulted in negative net energy values for all MW-pretreated samples.

Metagenomic assessment of the microbial community and methanogenic pathways in biosolids from a municipal wastewater treatment plant in Medellín, Colombia

Abundance and diversity of microbial communities in biosolids are variable and poorly studied in the tropics, and it is known that rainfall is one of the events that could affect the phylogenetic and functional microbial structure. In the present study, using NGS technics, we studied the microbial diversity as well as the methanogenesis pathway in one of the largest WWTP in Colombia. Besides, we sampled and analyzed biosolids from rainy season and dry season. Phylogenetic classification showed a predominance of bacteria in both samples and difference in the dominant groups depending on the rainfall season. Whereas Pseudomonas was the dominant bacteria in the dry season, Coprothermobacter was in the rainy season. Archaea abundance was higher in the rainy season (11.5%) doubling dry season proportion. The bioreactor biogas production and total solids content showed similar results between rainy and dry season at the sampling dates. The most abundant Archaea related with methanogenesis was Methanosaeta, which is a methanogenic microorganism that exclusively uses acetate to produce methane. Moreover, annotation of the methanogenic pathway in the metagenome showed abundance in genes encoding Acetyl-CoA synthetases (ACSS), an enzyme that catalyzes acetate activation. Our results suggest that the microbial diversity was stable among the two time points tested, rainy season and dry season; and, although there were changes in the microbial abundance of dominant bacterial species, anaerobic digester performance is not affected.

A Review of Sludge-to-Energy Recovery Methods

The increasing volume of sewage sludge from wastewater treatment facilities is becoming a prominent concern globally. The disposal of this sludge is particularly challenging and poses severe environmental hazards due to the high content of organic, toxic and heavy metal pollutants among its constituents. This study presents a simple review of four sewage to energy recovery routes (anaerobic digestion, combustion, pyrolysis and gasification) with emphasis on recent developments in research, as well as benefits and limitations of the technology for ensuring cost and environmentally viable sewage to energy pathway. This study focusses on the review of various commercially viable sludge conversion processes and technologies used for energy recovery from sewage sludge. This was done via in-depth process descriptions gathered from literatures and simplified schematic depiction of such energy recovery processes when utilised for sludge. Specifically, the impact of fuel properties and its effect on the recovery process were discussed to indicate the current challenges and recent scientific research undertaken to resolve these challenges and improve the operational, environmental and cost competitiveness of these technologies.

Insights into anaerobic transformation of key dissolved organic matters produced by thermal hydrolysis sludge pretreatment

The detailed dissolved organic matters (DOMs) profile by thermal hydrolysis pretreatment and their transformation during anaerobic digestion (AD) were investigated. Among the temperature tested, 172 °C treatment showed the best sludge solubilization and the maximum methane production. The study revealed that high temperature sludge pretreatment mainly improved the release of low molecular weight (LMW) proteins, LMW neutrals and LMW polysaccharides. Notably, the effluent from thermal treated sludge digesters contained more DOMs residues. The predominant residual DOMs were humic substances, LMW proteins and LMW neutrals. At the molecular level, over 50% of the residual LMW components were slowly biodegradable or nonbiodegradable steroid-like compounds and aromatics. Further profiling of the higher MW compounds detected the recalcitrant or inhibitory compounds, e.g. benzenoids, flavonoids, pyridines and their derivatives. It is recommended that polishing step should be considered to further reduce the refractory residues in AD liquor.

Enhanced dewaterability of waste activated sludge with Fe(II)-activated hypochlorite treatment

A novel method was explored to improve the waste-activated sludge (WAS) dewaterability using Fe(II) combined with calcium hypochlorite, and the possible mechanisms were investigated simultaneously. Capillary suction time (CST), specific resistance to filtration (SRF) of sludge, and water content (W_C) of dewatered sludge cake were selected as the factors to evaluate the sludge dewaterability. The maximum reduction of W_C (30.76%) was achieved under the optimal conditions of Ca(ClO)₂ 0.04 g/g total suspended solids (TSS), FeSO₄·7H₂O 0.097 g/g TSS, and pH 7.3, while the reduction of CST and SRF reached 91.24 and 99.47%, respectively. Three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy of extracellular polymeric substances (EPSs) showed that sludge dewaterability was mainly related to the degradation of tryptophan and tyrosine protein-like substances in sludge EPS. Economic analysis suggested Fe(II) combined with calcium hypochlorite treatment had greater potential on enhancing WAS dewaterability compared with the traditional sludge treatment.

Improving biogas production from anaerobic co-digestion of Thickened Waste Activated Sludge (TWAS) and fat, oil and grease (FOG) using a dual-stage hyper-thermophilic/thermophilic semi-continuous reactor

This paper investigates the feasibility and advantages of using a dual-stage hyper-thermophilic/thermophilic semi-continuous reactor system for the co-digestion of Thickened Waste Activated Sludge (TWAS) and Fat, Oil and Grease (FOG) to produce biogas in high quantity and quality. The performance of the dual-stage hyper-thermophilic (70°C)/thermophilic (55°C) anaerobic co-digestion system is evaluated and compared to the performance of a single-stage thermophilic (55°C) reactor that was used to co-digest the same FOG-TWAS mixtures. Both co-digestion reactors were compared to a control reactor (the control reactor was a single-stage thermophilic reactor that only digested TWAS). The effect of FOG% in the co-digestion mixture (based on total volatile solids) and the reactor hydraulic retention time (HRT) on the biogas/methane production and the reactors' performance were thoroughly investigated. The FOG% that led to the maximum methane yield with a stable reactor performance was determined for both reactors. The maximum FOG% obtained for the single-stage thermophilic reactor at 15 days HRT was found to be 65%. This 65% FOG resulted in 88.3% higher methane yield compared to the control reactor. However, the dual-stage hyper-thermophilic/thermophilic co-digestion reactor proved to be more efficient than the single-stage thermophilic co-digestion reactor, as it was able to digest up to 70% FOG with a stable reactor performance. The 70% FOG in the co-digestion mixture resulted in 148.2% higher methane yield compared to the control at 15 days HRT. 70% FOG (based on total volatile solids) is so far the highest FOG% that has been proved to be useful and safe for semi-continuous reactor application in the open literature. Finally, the dual-stage hyper-thermophilic/thermophilic co-digestion reactor also proved to be efficient and stable in co-digesting 40% FOG mixtures at lower HRTs (i.e., 9 and 12 days) and still produce high methane yields and Class A effluents.

The hydrolytic stage in high solids temperature phased anaerobic digestion improves the downstream methane production rate

The role of the hydrolytic stage in high solids temperature phased anaerobic digestion was investigated with a mixture of cattle slurry and maize silage with variable ratios (100, 70 and 30% volatile solids coming from cattle slurry). It was incubated for 48 h at 37, 55, 65 and 72 °C. Soluble chemical oxygen demand and biochemical methane potential were measured at 0, 24 and 48 h. Higher temperatures improved the amount of solubilized COD, which confirmed previously reported results. Nevertheless, solubilization mostly took place during the first 24 h. The rate of methane production in post-hydrolysis BMPs increased after 48 h hydrolysis time, but not after 24 h. The first order kinetic constant rose by 40% on average. No correlation was observed between soluble COD and downstream methane production rate, indicating a possible modification of the physical structure of the particulate solids during the hydrolytic stage.

Thermophilic Alkaline Fermentation Followed by Mesophilic Anaerobic Digestion for Efficient Hydrogen and Methane Production from Waste-Activated Sludge: Dynamics of Bacterial Pathogens as Revealed by the Combination of Metagenomic and Quantitative PCR Analyses

Thermophilic alkaline fermentation followed by mesophilic anaerobic digestion (TM) for hydrogen and methane production from waste-activated sludge (WAS) was investigated. The TM process was also compared to a process with mesophilic alkaline fermentation followed by a mesophilic anaerobic digestion (MM) and one-stage mesophilic anaerobic digestion (M) process. The results showed that both hydrogen yield (74.5 ml H₂/g volatile solids [VS]) and methane yield (150.7 ml CH₄/g VS) in the TM process were higher than those (6.7 ml H₂/g VS and 127.8 ml CH₄/g VS, respectively) in the MM process. The lowest methane yield (101.2 ml CH₄/g VS) was obtained with the M process. Taxonomic results obtained from metagenomic analysis showed that different microbial community compositions were established in the hydrogen reactors of the TM and MM processes, which also significantly changed the microbial community compositions in the following methane reactors compared to that with the M process. The dynamics of bacterial pathogens were also evaluated. For the TM process, the reduced diversity and total abundance of bacterial pathogens in WAS were observed in the hydrogen reactor and were further reduced in the methane reactor, as revealed by metagenomic analysis. The results also showed not all bacterial pathogens were reduced in the reactors. For example, Collinsella aerofaciens was enriched in the hydrogen reactor, which was also confirmed by quantitative PCR (qPCR) analysis. The study further showed that qPCR was more sensitive for detecting bacterial pathogens than metagenomic analysis. Although there were some differences in the relative abundances of bacterial pathogens calculated by metagenomic and qPCR approaches, both approaches demonstrated that the TM process was more efficient for the removal of bacterial pathogens than the MM and M processes.IMPORTANCE This study developed an efficient process for bioenergy (H₂ and CH₄) production from WAS and elucidates the dynamics of bacterial pathogens in the process, which is important for the utilization and safe application of WAS. The study also made an attempt to combine metagenomic and qPCR analyses to reveal the dynamics of bacterial pathogens in anaerobic processes, which could overcome the limitations of each method and provide new insights regarding bacterial pathogens in environmental samples.

Two-step heating mode with the same energy consumption as conventional heating for enhancing methane production during anaerobic digestion of swine wastewater

With high percentage of washing water, swine wastewater is characterized by large volume and low concentration of total solids. Thus, in treating swine wastewater, it is relatively difficult to heat digesters, resulting in low methane production at low ambient temperatures (ATs). To increase methane production from swine wastewater, this study proposed a novel "two-step heating (TSH)" mode with the same energy consumption as a one-step process for anaerobic digestion. Compared with the traditionally heated digesters (one-step heating), the digestion temperature in TSH digesters increased by 3.50-10.50 °C under the assumption of no heat dissipation and by 3.30-9.25 °C in the actual experiments. Although methane production of the TSH digesters improved by 15% in our experiments, the improvement was far less than theoretically estimated. This was mainly caused by short hydraulic retention time and sludge washout in the digesters. Moreover, the acetoclastic methanogenesis, accomplished by genus Methanosaeta, was the major methanogenesis pathway at low temperatures in both the TSH and conventional heating modes. However, the relative abundance of syntrophic bacteria and hydrogenotrophic methanogens in TSH mode were both higher than in the digesters operation in conventional heating mode when the atmospheric temperature was below 10 °C.

Microbial characteristics in anaerobic digestion process of food waste for methane production-A review

Food waste (FW) is rich in starch, fat, protein and cellulose. It is easy to decay and brings environmental pollution and other social problems. FW shows a high potential to produce methane by anaerobic digestion (AD) due to its high organic content. However, many inhibitors, such as accumulation of ammonia and volatile fatty acids (VFAs), usually result in inefficient performances and even process failure. Microorganisms play an important role in the process of hydrolysis, acidogenesis, acetogenesis and methanogenesis. This review provided a critical summary of microbial characteristics to obtain connects of microbial community structure with operational conditions at various states of AD, such as mesophilic and thermophilic, wet and dry, success and failure, pretreated or not, lab-scale and full-scale. This article emphasizes that it is necessary to analyze changes and mechanisms of microbial communities in unbalanced system and seek efficiency dynamic succession rules of the dominant microorganisms.

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

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

Advanced anaerobic digestion of municipal sludge using a novel and energy-efficient radio frequency pretreatment system

Microwave (MW) sludge pretreatment systems are usually limited to a frequency of 2.45 GHz and the heating frequency is constrained by commercially available hardware. Studies using MW heating at this frequency have reported negative net energy balance (output energy as methane minus input electrical energy). This necessitates further research into more efficient thermal pretreatment technologies. In this research, a novel and highly efficient radio frequency (RF) pretreatment system at a frequency of 13.56 MHz was designed, implemented, and tested for the first time. The system was custom-designed based on the dielectric characteristics of thickened waste activated sludge (TWAS) to achieve a very efficient and uniform heating system. The effects of three factors including pretreatment method (RF vs. MW), final temperature (60, 90 and 120 °C), and stationary (holding) time (0, 1 and 2 h) on sludge solubilization and performance of mesophilic batch anaerobic digestion were evaluated simultaneously. Energy measurements were also made to compare the efficiency of the custom-designed RF and conventional MW heating systems. The differences in sludge disintegration (solubilization) using the RF and MW pretreatment systems were negligible (P > 0.05). No statistically significant difference was also observed between the two pretreatment systems in terms of mesophilic biogas production rate and extent (P > 0.05). The energy efficiency of the RF pretreatment system was measured between 67.3 and 95.5% for the temperature range of 25-120 °C which was significantly higher than that of the MW system efficiency which varied from 37 to 43%. Overall, the average input energy of the RF system was less than half of the energy consumed during the operation of the MW system to achieve a same target temperature. Considering the results of this research, the RF heating at a frequency of 13.56 MHz is suggested as an effective and energy-efficient technique for thermal hydrolysis of TWAS.

Thermophilic and hyper-thermophilic co-digestion of waste activated sludge and fat, oil and grease: Evaluating and modeling methane production

Renewable energy and clean environment are two crucial requirements for our modern world. Low cost, energy production and limited environmental impact make anaerobic digestion (AD) a promising technology for stabilizing organic waste and in particular, sewage waste. The anaerobic co-digestion of thickened waste activated sludge (TWAS) and sewage treatment plant trapped fat, oil and grease (FOG) using different FOG-TWAS mixtures (20, 40, 60 and 80% of FOG based on total volatile solids (TVS)) were investigated in this study using both thermophilic (55 ± 1 °C) and two stages hyper-thermophilic/thermophilic (70 ± 1 °C and 55 ± 1 °C) anaerobic co-digestion. The hyper-thermophilic co-digestion approach as a part of the co-digestion process has been shown to be very useful in improving the methane production. During hyper-thermophilic biochemical methane potential (BMP) assay testing the sample with 60% FOG (based on TVS) has been shown to significantly increase the maximum methane production to 673.1 ± 14.0 ml of methane as compared to 316.4 ± 14.3 ml of methane for the control sample. This represents a 112.7% increase in methane production compared to the control sample considered in this paper. These results signify the importance of hyper-thermophilic digestion to the co-digestion of TWAS-FOG field.

Mesophilic and thermophilic alkaline fermentation of waste activated sludge for hydrogen production: Focusing on homoacetogenesis

The present study compared the mesophilic and thermophilic alkaline fermentation of waste activated sludge (WAS) for hydrogen production with focus on homoacetogenesis, which mediated the consumption of H2 and CO2 for acetate production. Batch experiments showed that hydrogen yield of WAS increased from 19.2 mL H2/gVSS at 37 °C and pH 10-80.1 mL H2/gVSS at 55 °C and pH 10. However, the production of volatile fatty acids (mainly acetate) was higher at 37 °C and pH 10 by comparison with 55 °C and pH 10. Hydrogen consumption due to homoacetogenesis was observed at 37 °C and pH 10 but not 55 °C and pH 10. Higher expression levels of genes relating with homoacetogenesis and lower expression levels of genes relating with hydrogen production were found at 37 °C and pH 10 compared to 55 °C and pH 10. The continuous experiment demonstrated the steady-state hydrogen yield of WAS was comparable to that obtained from batch experiments at 55 °C and pH 10, and homoacetogenesis was still inhibited. However, the steady-state hydrogen yield of WAS (6.5 mL H2/gVSS) was much lower than that (19.2 mL H2/gVSS) obtained from batch experiments at 37 °C and pH 10 due to the gradual enrichment of homoacetogens as demonstrated by qPCR analysis. The high-throughput sequencing analysis of 16S rRNA genes showed that the abundance of genus Clostridium, containing several homoacetogens, was 5 times higher at 37 °C and pH 10 than 55 °C and pH 10.

Thermophilic sludge digestion improves energy balance and nutrient recovery potential in full-scale municipal wastewater treatment plants

The conventional treatment of municipal wastewater by means of activated sludge is typically energy demanding. Here, the potential benefits of: (1) the optimization of mesophilic digestion; and (2) transitioning to thermophilic sludge digestion in three wastewater treatment plants (Tilburg-Noord, Land van Cuijk and Bath) in the Netherlands is evaluated, including a full-scale trial validation in Bath. In Tilburg-Noord, thermophilic sludge digestion covered the energy requirements of the plant (102%), whereas 111% of sludge operational treatment costs could be covered in Bath. Thermophilic sludge digestion also resulted in a strong increase in nutrient release. The potential for nutrient recovery was evaluated via: (1) stripping/absorption of ammonium; (2) autotrophic removal of ammonium via partial nitritation/anammox; and (3) struvite precipitation. This research shows that optimization of sludge digestion may lead to a strong increase in energy recovery, sludge treatment costs reduction, and the potential for advanced nutrient management in full-scale sewage treatment plants.

Biochemical methane potential tests of different autoclaved and microwaved lignocellulosic organic fractions of municipal solid waste

The aim of this research was to enhance the anaerobic biodegradability and methane production of two synthetic Organic Fractions of Municipal Solid Waste with different lignocellulosic contents by assessing microwave and autoclave pre-treatments. Biochemical Methane Potential assays were performed for 21days. Changes in the soluble fractions of the organic matter (measured by soluble chemical oxygen demand, carbohydrates and proteins), the first order hydrolysis constant kh and the cumulated methane production at 21days were used to evaluate the efficiency of microwaving and autoclaving pretreatments on substrates solubilization and anaerobic digestion. Microwave treatment led to a methane production increase of 8.5% for both the tested organic fractions while autoclave treatment had an increase ranging from 1.0% to 4.4%. Results showed an increase of the soluble fraction after pre-treatments for both the synthetic organic fractions. Soluble chemical oxygen demand observed significant increases for pretreated substrates (up to 219.8%). In this regard, the mediocre results of methane's production led to the conclusion that autoclaving and microwaving resulted in the hydrolysis of a significant fraction of non-biodegradable organic substances recalcitrant to anaerobic digestion.

Conventional heating vs. microwave sludge pretreatment comparison under identical heating/cooling profiles for thermophilic advanced anaerobic digestion

This research evaluates whether there is any advantage of selecting one of the thermal methods of sludge pretreatment, conventional heating (CH) and microwave hydrolysis (MW), over another to enhance municipal sludge disintegration and performance of thermophilic anaerobic digestion (AD). For this purpose, a custom-built CH system simulating MW hydrolysis under identical heating and cooling profiles was used. The effects of three main pretreatment parameters including pretreatment method (CH and MW), heating ramp rate (3, 6 and 11°C/min) and final temperature (80, 120 and 160°C) on sludge solubilization and performance of thermophilic batch AD were evaluated. The effects of CH and MW hydrolysis were observed to be similar for sludge disintegration and digester performance (p-value>0.05), while the effects of final temperature and heating ramp rate were proven to be different (p-value<0.05). According to the results, it is essential to apply MW and CH pretreatments under identical experimental condition for an unbiased comparison which supports the findings of the author's earlier study under mesophilic condition. Failing to address this issue explains the significant inconsistency observed among the findings of the previous CH vs. MW comparison studies that were unable to implement identical thermal profiles (between CH and MW) during sludge pretreatment. In comparison with mesophilic AD, thermophilic AD revealed lower biodegradation rate constant at the highest pretreatment temperature tested (160°C), suggesting its higher sensitivity to the inhibitory effects of thermal pretreatment at the elevated temperatures.

Rheological properties of sewage sludge during enhanced anaerobic digestion with microwave-H2O2 pretreatment

The rheological behavior of sludge is of serious concern in anaerobic digestion. This study investigated the rheological properties of sewage sludge during enhanced anaerobic digestion with microwave-H2O2 pretreatment (MW-H2O2). The results showed that MW-H2O2 pretreatment resulted in the improvement of sludge flowability and weakening of its viscoelastic properties. Further positive effects on the rheological properties of digested sludge during anaerobic digestion were observed. The flowability was improved with a low level of apparent viscosity. The decrease of the consistency index and increase of the flow behavior index indicated that the strength of the inner structures and non-Newtonian flow characteristics of digested sludge weakened. Both the storage modulus (G') and loss modulus (G″) decreased, indicating that the viscoelastic behavior became weak. These effects were possibly attributed to the changes of the digested sludge micro-structures, such as extracellular polymeric substances (EPS). This study concluded that anaerobic digestion for treating sewage sludge combined with pretreatment is a more favorable option than single anaerobic digestion from the perspective of rheology.

Improved biogas production from food waste by co-digestion with de-oiled grease trap waste

The objective of this study was to assess the feasibility of co-digesting food waste (FW) and de-oiled grease trap waste (GTW) to improve the biogas production. A lab-scale mesophilic digester (MD), a temperature-phased anaerobic digester (TPAD) and a TPAD with recycling (TPAD-R) were synchronously operated under mono-digestion (FW) and co-digestion (FW+de-oiled GTW). Co-digestion increased the biogas yield by 19% in the MD and TPAD-R, with a biogas yield of 0.60L/g VS added. Specific methanogenic activity in the TPAD-R was much higher than that in the MD. In addition to methane, hydrogen at a yield of approximately 1mol/mol hexose was produced in the TPAD-R. Alkalinity was consumed more in the co-digestion than in mono-digestion. Co-digestion resulted in more lipid accumulation in each digester. The MD favored the degradation of lipid and conversion of long-chain fatty acids more than the TPAD and TPAD-R.

EPS solubilization and waste activated sludge acidification enhanced by alkaline-assisted bi-frequency ultrasonic pretreatment revealed by 3D-EEM fluorescence

The effect of alkaline-assisted bi-frequency (28 + 40 kHz) ultrasonic pretreatment on extracellular polymeric substances (EPS) solubilization and waste activated sludge (WAS) acidification was investigated.