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

Modelling and techno-economic assessment of possible pathways from sewage sludge to green energy in India

Efficient domestic wastewater management is essential for mitigating the impact of wastewater on human health and the environment. Wastewater management with conventional technologies generates sewage sludge. The present study considered a modelling approach to evaluate various processing pathways to produce energy from the sewage sludge. Anaerobic digestion, gasification, pyrolysis, and hydrothermal liquefaction are analysed in terms of their energy generation potentials with the Aspen Plus software. A techno-economic assessment is performed to assess the economic viability of each pathway. It reveals that gasification appears as the most promising method to produce electricity, with 0.76 kWh/kgdrysludge, followed by anaerobic digestion (0.53 kWh/kgdrysludge), pyrolysis (0.34 kWh/kgdrysludge), and hydrothermal liquefaction (0.13 kWh/kgdrysludge). In contrast, the techno-economic analysis underscores the viability of anaerobic digestion with levelized cost of electricity as 0.02 $/kWh followed by gasification (0.11 $/kWh), pyrolysis (0.14 $/kWh), and hydrothermal liquefaction (2.21 $/kWh). At the same time, if the products or electricity from the processing unit is sold, equivalent results prevail. The present study is a comprehensive assessment of sludge management for researchers and policymakers. The result of the study can also assist policymakers and industry stakeholders in deciding on alternative options for energy recovery and revenue generation from sewage sludge.

A review of volatile fatty acids production from organic wastes: Intensification techniques and separation methods

High value-added products from organic waste fermentation have garnered increasing concern in modern society. VFAs are short-chain fatty acids, produced as intermediate products during the anaerobic fermentation of organic matter. VFAs can serve as an essential organic carbon source to produce substitutable fuels, microbial fats and oils, and synthetic biodegradable plastics et al. Extracting VFAs from the fermentation broths is a challenging task as the composition of suspensions is rather complex. In this paper, a comprehensive review of methods for VFAs production, extraction and separation are provided. Firstly, the methods to enhance VFAs production and significant operating parameters are briefly reviewed. Secondly, the evaluation and detailed discussion of various VFAs extraction and separation technologies, including membrane separation, complex extraction, and adsorption methods, are presented, highlighting their specific advantages and limitations. Finally, the challenges encountered by different separation technologies and novel approaches to enhance process performance are highlighted, providing theoretical guidance for recycling VFAs from organic wastes efficiently.

Sustainable strategies: Nature-based solutions to tackle antibiotic resistance gene proliferation and improve agricultural productivity and soil quality

The issue of antibiotic resistance is now recognized by the World Health Organisation (WHO) as one of the major problems in human health. Although its effects are evident in the healthcare settings, the root cause should be traced back to the One Health link, extending from animals to the environment. In fact, the use of organic fertilizers in agroecosystems represents one, if not the primary, cause of the introduction of antibiotics and antibiotic-resistant bacteria into the soil. Since the concentrations of antibiotics introduced into the soil are residual, the agroecosystem has become a perfect environment for the selection and proliferation of antibiotic resistance genes (ARGs). The continuous influx of these emerging contaminants (i.e., antibiotics) into the agroecosystem results in the selection and accumulation of ARGs in soil bacteria, occasionally giving rise to multi-resistant bacteria. These bacteria may harbour ARGs related to various antibiotics on their plasmids. In this context, these bacteria can potentially enter the human sphere when individuals consume food from contaminated agroecosystems, leading to the acquisition of multi-resistant bacteria. Once introduced into the nosocomial environment, these bacteria pose a significant threat to human health. In this review, we analyse how the use of digestate as an organic fertilizer can mitigate the spread of ARGs in agroecosystems. Furthermore, we highlight how, according to European guidelines, digestate can be considered a Nature-Based Solution (NBS). This NBS not only has the ability to mitigate the spread of ARGs in agroecosystems but also offers the opportunity to further improve Microbial-Based Solutions (MBS), with the aim of enhancing soil quality and productivity.

Food waste-based bio-fertilizers production by bio-based fermenters and their potential impact on the environment

Increasing food waste is creating a global waste (and management) crisis. Globally, ∼1.6 billion tons of food is wasted annually, worth ∼$1.2 trillion. By reducing this waste or by turning it into valuable products, numerous economic advantages can be realized, including improved food security, lower production costs, biodegradable products, environmental sustainability, and cleaner solutions to the growing world's waste and garbage management. The appropriate handling of these detrimental materials can significantly reduce the risks to human health. Food waste is available in biodegradable forms and, with the potential to speed up microbial metabolism effectively, has immense potential in improving bio-based fertilizer generation. Synthetic inorganic fertilizers severely affect human health, the environment, and soil fertility, thus requiring immediate consideration. To address these problems, agricultural farming is moving towards manufacturing bio-based fertilizers via utilizing natural bioresources. Food waste-based bio-fertilizers could help increase yields, nutrients, and organic matter and mitigate synthetic fertilizers' adverse effects. These are presented and discussed in the review.

The fate of antibiotic resistance genes during anaerobic digestion of sewage sludge with ultrasonic pretreatment

This study investigated the effect of ultrasonic (US) pretreatment at three different contact times (30, 45, and 60 min) with a power of 240 W and frequency of 40 kHz on the fate of antibiotic resistance genes (ARGs), mobile genetic elements (MGEs), and enteric pathogens during anaerobic digestion (AD) of sludge. By using real time-qPCR, three MGEs (int1, int2, and tnpA) and seven ARGs (sul1, sul2, tetW, tetA, tetO, ermF, and aac(6')-lb) were quantified that have serious human health impacts and represent the most widely used antibiotics (tetracycline, sulfonamide, macrolide, and aminoglycoside). Results indicated that US pretreatment under different contact times improved the removal of ARGs and MGEs. Compared to 30 and 45 min of US pretreatment, 60 min of US pretreatment resulted in a higher reduction of ARGs with total ARG reduction of 41.70 ± 1.13%. Furthermore, the relative abundance of ARGs and MGEs after US pretreatment was reduced more effectively in anaerobic reactors than in a control AD without US pretreatment. The total ARGs and MGEs removal efficiency of control AD was 44.07 ± 0.72% and 63.69 ± 1.43%, and if US pretreatment at different times were applied, the total ARGs and MGEs removal efficiency of the whole pretreatment AD process improved to 59.71 ± 2.76-68.54 ± 1.58% and 69.82 ± 2.15-76.84 ± 0.22%. The highest removal of total ARGs (68.54 ± 1.58%) and MGEs (76.84 ± 0.22%) was achieved after AD with US pretreatment at 45 min. However, US pretreatment and AD with US pretreatment were not effective in inactivation of enteric pathogens (total coliforms and E. coli), suggesting that posttreatment is needed prior to land application of sludge to reduce the level of enteric pathogens. There was no detection of the studied ARGs and MGEs in the enteric pathogens after US pretreatment in subsequent AD. According to this study, long contact times of US pretreatment can mitigate ARGs and MGEs in AD processes, offering valuable insight into improving environmental safety and sustainable waste management. Additionally, the study highlights the need to investigate posttreatment techniques for reducing enteric pathogens in AD effluent, a crucial consideration for agricultural use and environmental protection.

Presence and role of viruses in anaerobic digestion of food waste under environmental variability

BACKGROUND

The interaction among microorganisms in the anaerobic digestion of food waste (ADFW) reactors lead to the degradation of organics and the recycling of energy. Viruses are an important component of the microorganisms involved in ADFW, but are rarely investigated. Furthermore, little is known about how viruses affect methanogenesis.

RESULTS

Thousands of viral sequences were recovered from five full-scale ADFW reactors. Gene-sharing networks indicated that the ADFW samples contained substantial numbers of unexplored anaerobic-specific viruses. Moreover, the viral communities in five full-scale reactors exhibited both commonalities and heterogeneities. The lab-scale dynamic analysis of typical ADFW scenarios suggested that the viruses had similar kinetic characteristics to their prokaryotic hosts. By associating with putative hosts, a majority of the bacteria and archaea phyla were found to be infected by viruses. Viruses may influence prokaryotic ecological niches, and thus methanogenesis, by infecting key functional microorganisms, such as sulfate-reducing bacteria (SRB), syntrophic acetate-oxidizing bacteria (SAOB), and methanogens. Metabolic predictions for the viruses suggested that they may collaborate with hosts at key steps of sulfur and long-chain fatty acid (LCFA) metabolism and could be involved in typical methanogenesis pathways to participate in methane production.

CONCLUSIONS

Our results expanded the diversity of viruses in ADFW systems and suggested two ways that viral manipulated ADFW biochemical processes. Video Abstract.

Maximizing the potential of leachate from sewage sludge as a sustainable nutrients source to alleviate the fertilizer crisis

Leachate from separate digesters in biological wastewater treatment plants contains valuable biogenic compounds that can serve as fertilizer nutrients. In this study, a method was developed to utilize leachate from sewage sludge dewatering as a raw material for the preparation of a plant conditioner, providing water, nutrients, and growth-stimulating amino acids. A chemical conditioning procedure (65% HNO₃) was used to prepare the leachate solution for fertilization. The feasibility of producing an amino acid-based fertilizer using shrimp shells and inorganic acids (96% H₂SO₄ and 85% H₃PO₄) was also demonstrated. Microbiological analysis confirmed the safety of the formulations, and chelation of micronutrients with available amino acids was proven (up to 100% chelating degree). The bioavailability of all nutrients was confirmed through extraction tests (extraction in neutral ammonium citrate). Germination tests showed similar fresh plant masses to those with commercial preparations, demonstrating the effectiveness of the developed technology. This approach aligns with circular economy principles and sustainable development and contributes to mitigating the impacts of climate change.

Response of soil bacterial populations to application of biosolids under short-term flooding

Biosolids are applied to agricultural land as a soil conditioner and source of crop nutrients. However, there is concern that bacteria from biosolids may become established in soils, particularly if that soil becomes water-logged. This study examined the microbial community of arable soils cultivated with barley under different applications of biosolids (0, 24t/ha, 48t/ha) in laboratory mesocosms which simulated a 10-day flood. Nutrients (P and N) and organic matter in the soil increased with application rate, but plant growth was not affected by biosolid application. The biosolids contained 10× more genetic material than the soil, with much lower bacterial diversity, yet application did not significantly change the taxonomy of the soil microbiome, with minor changes related to increased nutrients and SOM. Anaerobic conditions developed rapidly during flooding, causing shifts in the native soil microbiome. Some bacterial taxa that were highly abundant in biosolids had slightly increased relative abundance in amended soils during the flood. After flooding, soil bacterial populations returned to their pre-flood profiles, implying that the native microbial community is resilient to transient changes. The short-term changes in the microbiome of biosolid-amended soils during flooding do not appear to increase the environmental risk posed by biosolid application.

Changes in bacterial diversity, co-occurrence pattern, and potential pathogens following digestate fertilization: Extending pathogen management to field for anaerobic digestion of livestock manure

Digestate can spread pathogens into agroecosystem, posing serious threats to public health. However, the effect of digestate fertilization on digestate- or soil-borne pathogens has not been fully explored. Herein, two settings of microcosm experiment were performed with arable soil and digestate collected at two sites (Beilangzhong or Shunyi) to dissect the succession of the total and potential pathogenic bacterial communities following digestate fertilization. Each experimental setting consisted of three treatments, including digestate aerobically incubated in sterilized soil, and soil amended with sterilized or non-sterilized digestate. Digestate-borne potential pathogenic bacteria were enriched after the aerobic incubation, with Streptococcus sobrinus in the Beilangzhong setting, and Escherichia coli and Enterococcus faecium in the Shunyi setting. Potential soil-borne pathogenic bacteria, such as Acinetobacter lowffii and Pseudomonas fluorescens, were stimulated by the sterilized digestate in the Shunyi setting. Interestingly, S. sobrinus, E. coli, and Ent. faecium did not increase when digestate was amended into the non-sterilized soil, suggesting that soil microorganisms can inhibit the resurgence of these digestate-borne pathogens. A large-scale survey further revealed that organic fertilization exerted a site-dependent effect on different species of potential pathogen, but it did not enrich the total relative abundance of potential pathogenic bacteria in soils. Collectively, these results highlight that pathogen management of anaerobic digestion of livestock manure needs to be extended from anaerobic reactor to field.

Implications for assessing sludge hygienization: Differential responses of the bacterial community, human pathogenic bacteria, and fecal indicator bacteria to sludge pretreatment-anaerobic digestion

Sewage sludge is the byproduct of wastewater treatment plants, which host enormous diversity of microbes including potential pathogens. However, there are still challenges in assessing hygienization during sludge stabilization due to the complex relationships between dominant microbes and human pathogenic bacteria (HPB), and the accuracy of fecal indicator bacteria (FIB) is also disputed. Here, the responses of the bacterial community, HPB, and FIB to sludge pretreatment-anaerobic digestion (AD) were comprehensively compared using culture-based and 16S rRNA gene molecular analysis methodologies. Bacterial and HPB communities differed in response to sludge pretreatment-AD. AD drove the variation of bacterial community, but led to the convergence of HPB communities in pretreated sludge, indicating the existence of ecological niches that favors HPB dissemination in digesters. The correlation analysis indicated that FIB was suitable for characterizing general pathogen removal instead of showing the real pattern of HPB (i.e., each HPB), implying the need for comprehensive assessment approaches. Moreover, AD-related parameters including pH, total solids destruction, and methane yield were found to play important role in assessing pathogen inactivation given their correlation. This work provides theoretical basis for the selection of appropriate sludge stabilization approaches and future supervision of biosolids biosafety, which finally benefits human health.

Current and prognostic overview on the strategic exploitation of anaerobic digestion and digestate: A review

The depletion of fossil fuels and increasing demand for energy are encountered by generating renewable biogas. Anaerobic digestion (AD) produces not only biogas, also other value-added products from the digestate using various organic, municipal and industrial wastes which have several benefits like remediating waste, reduces greenhouse gas emissions, renewable energy generation and securing socio-economic status of bio-based industries. This review work critically analyzes the biorefinery approaches on AD process for the production of biogas and digestate, and their direct and indirect utilization. The left-out residue obtained from AD is called 'digestate' which enriched with organic matter, nitrogen, heavy metals and other valuable micronutrients. However, the direct disposal of digestate to the land as fertilizer/landfills creates various environmental issues. Keeping this view, the digestate should be upgraded or transformed into high valued products such as biofertilizer, pyrochar, biodiesel, syngas and soil conditioner that can aid to enrich the soil nutrients and ensures the safe environment as well. In this context, the present review focused to illustrate the current techniques and different strategic exploitations on AD proper management of digestate products for storage and further applications. Such a technology transfer provides a proven strategic mechanism towards the enhancement of the sustainability of bio-based industries, attaining the energy demand, safest waste management, protection of environment and reduces the socio-economic issues of the industrial sector.

The role of microbial ecology in improving the performance of anaerobic digestion of sewage sludge

The use of next-generation diagnostic tools to optimise the anaerobic digestion of municipal sewage sludge has the potential to increase renewable natural gas recovery, improve the reuse of biosolid fertilisers and help operators expand circular economies globally. This review aims to provide perspectives on the role of microbial ecology in improving digester performance in wastewater treatment plants, highlighting that a systems biology approach is fundamental for monitoring mesophilic anaerobic sewage sludge in continuously stirred reactor tanks. We further highlight the potential applications arising from investigations into sludge ecology. The principal limitation for improvements in methane recoveries or in process stability of anaerobic digestion, especially after pre-treatment or during co-digestion, are ecological knowledge gaps related to the front-end metabolism (hydrolysis and fermentation). Operational problems such as stable biological foaming are a key problem, for which ecological markers are a suitable approach. However, no biomarkers exist yet to assist in monitoring and management of clade-specific foaming potentials along with other risks, such as pollutants and pathogens. Fundamental ecological principles apply to anaerobic digestion, which presents opportunities to predict and manipulate reactor functions. The path ahead for mapping ecological markers on process endpoints and risk factors of anaerobic digestion will involve numerical ecology, an expanding field that employs metrics derived from alpha, beta, phylogenetic, taxonomic, and functional diversity, as well as from phenotypes or life strategies derived from genetic potentials. In contrast to addressing operational issues (as noted above), which are effectively addressed by whole population or individual biomarkers, broad improvement and optimisation of function will require enhancement of hydrolysis and acidogenic processes. This will require a discovery-based approach, which will involve integrative research involving the proteome and metabolome. This will utilise, but overcome current limitations of DNA-centric approaches, and likely have broad application outside the specific field of anaerobic digestion.

A review of advances in valorization and post-treatment of anaerobic digestion liquid fraction effluent

Traditionally, digestate is considered a waste, which is used as fertiliser in the agriculture industry. Recent studies focus on increasing the profitability of digestate by extracting reusable nutrients to promote biogas plants cost-effectiveness, sustainable management and circular economy. This review focuses on the post-treatment and valorization of liquor which is produced by solid-liquid fractioning of digestate. Nutrient recovery and removal from liquor are possible through mechanical, physicochemical and biological procedures. The processes discussed involve complex procedures that differ in economic value, feasibility, legislative restrictions and performance. The parameters that should be considered to employ these techniques are influenced by liquor characteristics, topography, climate conditions and available resources. These are key parameters to keep in mind during designing and manufacturing a biogas plant. In the following chapters, a discussion on available liquor treatment methods takes place. The present study examines the critical aspects of the available liquor treatment methods.

Organic micropollutants, heavy metals and pathogens in anaerobic digestate based on food waste

Anaerobic digestate based on food waste is increasingly used as fertilizer in food production. This study examined the characteristics of anaerobic digestate based on food waste from three biogas plants in Sweden. The characterization included measurements of heavy metals (n = 7), chemicals of emerging concern (CECs), such as currently used drugs and pesticides (n = 133), and an extended range of food-borne pathogens, including two notable sporeformers and some widespread antibiotic-resistant bacteria. The amounts of Escherichia coli, enterococci, and Salmonella and the concentrations of the target heavy metals were all below the maximum accepted levels at all three locations studied. However, the spore-forming Bacillus cereus was found to be present at high levels in samples from all three biogas plants. Among the 133 CECs investigated, 48 were detected at least once, and the highest concentrations were found for pyroxidine, nicotine, caffeine, theobromine, and nicotine. The biofertilizers from the different biogas plants had similar CEC profiles, which indicate similarities in household waste composition and thorough mixing in the biogas plants. If this profile is found to be spatially and temporally consistent, it can help regulators to establish priority lists of CECs of top concern. Assuming increasing use of biofertilizers for food production in the future, it would be beneficial to have concentration limits for CECs Risk estimation based on risk quotients (RQs) indicated generally low environmental risks associated with application of biofertilizer to soils for food crop production. However, the toxicity of CEC mixtures needs to be considered when estimating the risks from application of biofertilizers on agricultural land or in other production systems.

Fate of Clostridia and other spore-forming Firmicute bacteria during feedstock anaerobic digestion and aerobic composting

Pathogenic spore-forming Firmicutes are commonly present in animal and human wastes that are used as fertilizers in crop production. Pre-treatments of organic waste prior to land application offer the potential to abate enteric microorganisms, and therefore reduce the risk of contamination of crops or adjacent water resources with pathogens carried in these materials. The inactivation and reduction of gram-positive spore formers such as Clostridium spp., Clostridioides spp. and Bacillus spp. from animal and human waste can be challenging given the recalcitrance of the spores these bacteria produce. Given the significance of these organisms to human and animal health, information concerning spore-forming bacteria inactivation during anaerobic digestion (AD) and aerobic composting (AC) is required as the basis for recommending safe organic waste management practices. In this review, an assessment of the inactivation of spore-forming Firmicutes during AD and AC was conducted to provide guidance for practical management of organic matrices of animal or human origin. Temperature and pH may be the main factors contributing to the inactivation of spore-forming Firmicutes during batch lab-scale AD (log reduction <0.5-5 log). In continuous digesters, wet AD systems do not effectively inactivate spore-forming Firmicutes even under thermopholic conditions (log reduction -1.09 - 0.98), but dry AD systems could be a feasible management practice to inactivate spore-forming Firmicutes from organic materials with high solid content (log reduction 1.77-3.1). In contrast, composting is an effective treatment to abate spore-forming Firmicutes (log reduction 1.7-6.5) when thermophilic conditions last at least six consecutive days. Temperature, moisture content and composting scale are the key operating conditions influencing the inactivation of spore-forming Firmicutes during composting. Where possible, undertaking AD with subsequent composting to ensure the biosafety of digestate before its downstream processing and recycling is recommended to abate recalcitrant bacteria in digestate.

Anaerobic treatment of swine manure under mesophilic and thermophilic temperatures: Fate of veterinary drugs and resistance genes

The effect of anaerobic treatment of swine manure at 35 °C (mesophilic) and 55 °C (thermophilic) on methane production, microbial community and contaminants of emerging concern was investigated. Pasteurization pretreatment and post treatment was also investigated in combination with anaerobic treatment at 35 °C. Specific methane production (SMP), 26 pharmaceutical compounds (PhACs) and five antibiotic resistance genes (ARGs) (qnrS, tetW, ermB, sul1 and bla_TEM) were evaluated. Mesophilic treatment resulted in the highest SMP regardless of whether pasteurization was applied. Marbofloxacin was the most abundant antibiotic in swine manure. In general, all groups of PhACs showed higher removals under thermophilic temperatures as compared to mesophilic. In general, pasteurization pretreatment followed by mesophilic anaerobic digestion provided the highest removals of ARGs. Finally, the genera Streptococcus, Clostridium and Pseudomonas which contain pathogenic species, were present in the swine manure. Streptococcus, which was the most abundant, was decreased during all the treatments, while the others only decreased under certain treatments.

Tracking Klebsiella pneumoniae carbapenemase gene as an indicator of antimicrobial resistance dissemination from a hospital to surface water via a municipal wastewater treatment plant

Antibiotic-resistant bacteria originating from hospitals are ultimately discharged to municipal wastewater treatment plants (WWTP), which may serve as important reservoirs for the spread of antibiotic resistant genes. This study traced and quantified the presence of a rare but clinically relevant antimicrobial resistance gene; Klebsiella pneumoniae carbapenamase (KPC)-and the viable organisms (KPCO) which carried this gene in hospital, non-hospital wastewater discharges, various compartments within a municipal WWTP, receiving water and sediment samples. High concentration of the gene, blaKPC harbored in viable and multispecies KPCO was detected in the hospital wastewater and in the forepart stages of the WWTP, but was not detected in the final effluent following UV disinfection. KPCO were not detected in multiple non-hospital sources of wastewater discharges tested. The treatment train used in the sampled WWTP was found to help remove and reduce KPCO load. Using whole-genome sequencing, a KPC-producing Klebsiella oxytoca strain identical to strains seen in the patients and hospital environment was isolated from the downstream receiving water on one sampling event. KPCO were also found to persist in the biosolids throughout the WWTP, but were not detected in the processed compost-products made from WWTP-biosolids. This study systematically demonstrates dissemination of KPCO from hospital point source to environment via municipal WWTP. Understanding hospitals as the origin and source of spread of some of the most clinically urgent antimicrobial-resistant organisms may help direct interventions that target rate at which antibiotic resistant bacteria evolve and spread via enhancement of wastewater treatment and mitigation of dissemination at source.

Hygienic assessment of digestate from a high solids anaerobic co-digestion of sewage sludge with biowaste by testing Salmonella Typhimurium, Escherichia coli and SARS-CoV-2

Anaerobic digestion is a consolidated technology to convert sewage sludge and other organic wastes into biogas and a nutrient-rich fertilizer (i.e. digestate). The origin of sewage sludge does not exclude the potential presence of pathogens (e.g. Salmonella spp. and SARS-CoV-2) in mature digestate that hence could represent a source of sanitary concerns when it is spread on soil for agriculture purpose. Therefore, an experimental study aimed at proving the sanitizing effect of a full scale thermophilic high solids anaerobic digestion process was conducted by monitoring the hygienic characteristics of mature digestate. Although Salmonella spp. was detected in the sewage sludge fed to the full scale plant, the anaerobic digestion treatment demonstrated sanitization capacity since the monitored pathogens were never found in the mature digestate over the entire duration of the monitoring survey. Furthermore, tests on the regrowth of Salmonella Typhimurium and Escherichia coli, artificially inoculated on mature digestate, were also conducted under both anaerobic and aerobic conditions with the aim to assess the effectiveness of mature digestate as microbial growth medium. Concentrations of Salmonella Typhimurium and Escherichia coli were drastically reduced after a short time of incubation under anaerobic process and the two microorganisms already resulted undetectable after 24-48 h, whereas, under aerobic conditions, two microorganisms' concentrations were stably high for longer than 10 days. The combination of no free oxygen, high temperature, anaerobic metabolites (e.g. total ammonium nitrogen, and volatile fatty acids) production, bacteria competition and lack of nutritional elements in mature digestate considerably reduced in 24-48 h the sanitary risks associated to accidently contaminated digestate. Furthermore, a SARS-CoV-2 monitoring survey on mature digestate during 13 months, resulted in the absence of the virus RNA in the analyzed digestate.

Emerging electrochemistry-based process for sludge treatment and resources recovery: A review

The electrochemical process is gaining widespread interest as an emerging alternative for sludge treatment. Its potentials for sludge stabilization and resources recovery have been well proven to date. Despite the high effectiveness of the electrochemical process having been highlighted in several studies, concerns about the electrochemical sludge treatment, including energy consumption, scale-up feasibility, and electrode stability, have not yet been addressed. The present paper critically reviews the versatile uses of the electrochemical processes for sludge treatment and resource recovery, from the fundamentals to the practical applications. Particularly considered are the enhancement of the digestion of the anaerobic sludge and dewaterability, removal of pathogens and heavy metals, and control of sludge malodor. In addition, the opportunities and challenges of the sludge-based resource recovery (i.e., nitrogen, phosphorus, and volatile fatty acids) are discussed. Insights into the working mechanisms (e.g., electroporation, electrokinetics and electrooxidation) of electrochemical processes are reviewed, and perspectives and future research directions are proposed. This work is expected to provide an in-depth understanding and broaden the potential applications of electrochemical processes for sludge treatment and resource recovery.

Challenges of pathogen inactivation in animal manure through anaerobic digestion: a short review

Animal manure is the main source of bioenergy production by anaerobic digestion (AD). However, the pathogenic bacteria in manure may pose a high risk to human health by contaminating the environment if not effectively inactivated during AD. Worldwide, more than 20,000 biogas plants are running for the treatment of animal manure. AD has been playing the important role in establishing a circular economy in the agricultural sector and may contribute to the United Nations sustainable development goal (UN SDG). Nevertheless, whether AD is a reliable approach for pathogens inactivation has been challenged. A comprehensive understanding of the coping mechanisms of pathogens with adverse conditions and the challenges of establishing the AD process to inactivate effectively pathogens are yet to be analyzed. In this review, the diversity and resistance of pathogens in animal manure are summarized. The efficiencies and the difficulties of their inactivations in AD are also analyzed. In particular, three forms of pathogens i.e. sporing-forming pathogens, viable but non-culturable (VBNC) pathogens, and persistent pathogens are discussed. The factors influencing the pathogens’ inactivation and AD efficiencies are analyzed. The trade-off between energy production and pathogens inactivation in an AD system was consequently pointed out. This review concluded that the development of anaerobic processes should meet the goals of high efficient bioenergy production and deep hygienization.

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.

Benefits and risks of agricultural reuse of digestates from plastic tubular digesters in Colombia

The aim of this study is to characterize the digestates from three plastic tubular digesters implemented in Colombia fed with: i) cattle manure; ii) cattle manure mixed with cheese whey; iii) pig manure. All the digesters worked under psychrophilic conditions. Physico-chemical characteristics, heavy metals, pathogens, and agronomic quality were investigated. All the digestates were characterized by physico-chemical characteristics and nutrients concentration suitable for their reuse as biofertilizer. However, these digestates may only partially replace a mineral fertilizer due to the high nutrients dilution. Heavy metals were under the detection limit of the analytical method (Pb, Hg, Ni, Mo, Cd, Chromium VI) or present at low concentration (Cu, Zn, As, Se) in all the digestates. Biodegradable organic matter and pathogens (coliform, helminths and Salmonella spp.) analysis proved that all the digestates should be post-treated before soil application in order to prevent environmental and health risks, and also to reduce residual phytotoxicity effects. The digestate from pig manure had a higher nutrient percentage (0.2, 0.6 and 0.05 % w/w of total N, P₂O₅ and K₂O, respectively), but also higher residual phytotoxicity than the other digestates. Co-digestion seemed not to significantly improve the digestate fertilizing potential. Finally, further studies should address how to improve fertilizing potential of digestates from plastic tubular digesters, avoiding environmental and health risks.

Effects of low- and high-temperature thermal-alkaline pretreatments on anaerobic digestion of waste activated sludge

To compare the effects of low- and high-temperature thermal-alkaline pretreatments (LTTAP, 60 ± 1 °C, pH 12.0 ± 0.1, 30 min and HTTAP, 160 ± 1 °C, pH 12.0 ± 0.1, 30 min, respectively) on anaerobic digestion (AD) of waste activated sludge, long-term and semi-continuous experiments were conducted in three laboratory continuous stirred tank reactors. The experimental results showed that the two pretreatments increased the methane yield of sludge from 89.20 ± 2.41 mL/g added volatile solids (VS) to 117.50 ± 5.27 mL/g added VS (LTTAP) and 156.40 ± 2.99 mL/g added VS (HTTAP). After AD, the reduction of sludge (volatile solid) increased from 32.91 ± 0.27% to 44.17 ± 1.53% (LTTAP), and 50.86 ± 1.18% (HTTAP), and the abundance of pathogenic bacteria decreased from 6.53% to 0.38% (LTTAP) and 0.14% (HTTAP). LTTAP enhanced both hydrogentrophic and acetoclastic methanogenis and HTTAP only enhanced acetoclastic methanogenis. Additionally, the energy efficiency of HTTAP and its subsequent AD was lower than that of LTTAP and its subsequent AD.

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.

Phase Separation in Anaerobic Digestion: A Potential for Easier Process Combination?

The flexibilization of bioenergy production has the potential to counteract partly other fluctuating renewable energy sources (such as wind and solar power). As a weather-independent energy source, anaerobic digestion (AD) can offer on-demand energy supply through biogas production. Separation of the stages in anaerobic digestion represents a promising strategy for the flexibilization of the fermentative part of biogas production. Segregation in two reactor systems facilitates monitoring and control of the provision of educts to the second methanogenic stage, thus controlling biogas production. Two-stage operation has proven to reach similar or even higher methane yields and biogas purities than single-stage operation in many different fields of application. It furthermore allows methanation of green hydrogen and an easier combination of material and energy use of many biogenic raw and residual biomass sources. A lot of research has been conducted in recent years regarding the process phase separation in multi-stage AD operation, which includes more than two stages. Reliable monitoring tools, coupled with effluent recirculation, bioaugmentation and simulation have the potential to overcome the current drawbacks of a sophisticated and unstable operation. This review aims to summarize recent developments, new perspectives for coupling processes for energy and material use and a system integration of AD for power-to-gas applications. Thereby, cell physiological and engineering aspects as well as the basic economic feasibility are discussed. As conclusion, monitoring and control concepts as well as suitable separation technologies and finally the data basis for techno-economic and ecologic assessments have to be improved.

Comparative evaluation on municipal sewage sludge utilization processes for sustainable management in Tibet

In recent years, a sharp increase in the amount of municipal sewage sludge (MSS) in Tibet has posed serious threats to the fragile ecological environment. Tibetan sludge, with a high content of volatile and low heavy metals, has re-utilization advantages, and thus, the selection of appropriate utilization processes for Tibetan MSS is of great importance. In this study, not only the processes themselves, but also other factors including legislations and environmental pollution were investigated. This study introduced the current waste management legislation situation (especially for MSS) in China and Tibet, China. Moreover, a series of SWOT (strength & weakness and opportunity & threats) analyses were conducted to compare anaerobic digestion (AD), incineration, pyrolysis (PY), gasification, and anaerobic digestion coupled with pyrolysis. The results showed that anaerobic digestion coupled with pyrolysis was the optimal treatment option, because anaerobic digestion was suitable for the low oxygen content in Tibet. Although only 50-60% of the organic matter in MSS could be degraded by anaerobic digestion, the residual organic matter (energy) could be further decomposed by pyrolysis, converting it into pyrolytic gas, bio-oil, and biochar, as valuable products. Sludge digestate pyrolysis could reduce environmental risks, save energy, recover materials, and produce high value-added materials. Moreover, it provides a "zero waste" solution for sludge disposal and promotes a "Circular Economy." The challenges and obstacles of MSS anaerobic digestion coupled with pyrolysis in Tibet were also investigated. This study provides an important technical reference for the comprehensive utilization of Tibetan MSS.

Conventional and Innovative Hygienization of Feedstock for Biogas Production: Resistance of Indicator Bacteria to Thermal Pasteurization, Pulsed Electric Field Treatment, and Anaerobic Digestion

Animal by-products (ABP) can be valorized via anaerobic digestion (AD) for biogas energy generation. The digestate issued from AD process is usually used to fertilize farming land for agricultural activities, which may cause potential sanitary risk to the environment. The European Union (EU) requires that certain ABP be thermally pasteurized in order to minimize this sanitary risk. This process is called hygienization, which can be replaced by alternative nonthermal technologies like pulsed electric field (PEF). In the present study, Enterococcus faecalis ATCC 19433 and Escherichia coli ATCC 25922 were used as indicator bacteria. Their resistance to thermal pasteurization and PEF treatment were characterized. Results show that Ent. faecalis and E. coli are reduced by 5 log10 in less than 1 min during thermal pasteurization at 70 °C. The critical electric field strength was estimated at 18 kV∙cm−1 for Ent. faecalis and 1 kV∙cm−1 for E. coli. “G+” bacteria Ent. faecalis are generally more resistant than “G−” bacteria E. coli. AD process also plays an important role in pathogens inactivation, whose performance depends on the microorganisms considered, digestion temperature, residence time, and type of feedstock. Thermophilic digestion is usually more efficient in pathogens removal than mesophilic digestion.

Application of electron beam water radiolysis for sewage sludge treatment-a review

A review of the applicability of electron beam water radiolysis for sewage sludge treatment is presented. Electron beam treatment has been proven to be a successful approach to the disinfection of both wastewater and sewage sludge. Nevertheless, before 2000, there were concerns about the perceived high capital costs of the accelerator and with public acceptance of the usage of radiation for water treatment purposes. Nowadays, with increased knowledge and technological development, it may be not only possible but also desirable to use electron beam technology for risk-free sewage sludge treatment, disposal and bio-friendly fertiliser production. Despite the developing interest in this method, there has been no attempt to perform a review of the pertinent literature relating to this technology. It appears that understanding of the mechanism and primary parameters of disinfection is key to optimising the process. This paper aims to reliably characterise the sewage sludge electron beam treatment process to elucidate its major issues and make recommendations for further development and research. Graphical abstract.

Effect of Pasteurisation on Methane Yield from Food Waste and Other Substrates in Anaerobic Digestion

The effect of pasteurisation and co-pasteurisation on biochemical methane potential values in anaerobic digestion (AD) was studied. Pasteurisation prior to digestion in a biogas plant is a common hygienisation method for organic materials which contain or have been in contact with animal by-products. Tests were carried out on food waste, slaughterhouse waste, animal blood, cattle slurry, potato waste, card packaging and the organic fraction of municipal solid waste (OFMSW); pasteurisation at 70 °C for 1 h was applied. Pasteurisation had increased the methane yields of blood (+15%) and potato waste (+12%) only, which both had a low content of structural carbohydrates (hemi-cellulose and cellulose) but a particularly high content of either non-structural carbohydrates such as starch (potato waste) or proteins (blood). With food waste, card packaging and cattle slurry, pasteurisation had no observable impact on the methane yield. Slaughterhouse waste and OFMSW yielded less methane after pasteurisation in the experiments (but statistical significance of the difference between pasteurised and unpasteurised slaughterhouse waste or OFMSW was not confirmed in this work). It is concluded that pasteurisation can positively impact the methane yield of some specific substrates, such as potato waste, where heat-treatment may induce gelatinisation with release of the starch molecules. For most substrates, however, pasteurisation at 70 °C is unlikely to increase the methane yield. It is unlikely to improve biodegradability of lignified materials, and it may reduce the methane yield from substrates which contain high contents of volatile components. Furthermore, in this experimental study, the obtained methane yield was unaffected by whether the substrates were pasteurised individually and then co-digested or co-pasteurised as a mixture before batch digestion.

What do we know about the SARS-CoV-2 coronavirus in the environment?

In view of the current situation regarding the Covid-19 disease, a discussion is proposed on the need for research focusing on the presence and evolution of the SARS-CoV-2 virus in water, soils and other environmental compartments, reached through wastewater and sewage sludge spreading. Also, the evaluation of current treatments for wastewater and sewage sludge, as well as the eventual development of new specific techniques, based on sorption, nanotechnology, etc., would be of great interest for controlling the environmental dissemination of these viruses in the current and eventual future outbreaks.

Dynamics of Clostridium genus and hard-cheese spoiling Clostridium species in anaerobic digesters treating agricultural biomass

Biogas plants are a widespread renewable energy technology. However, the use of digestate for agronomic purposes has often been a matter of concern. It is controversial whether biogas plants might harbor some pathogenic clostridial species, which represent a biological risk. Moreover, the inhabitance of Clostridium hard-cheese spoiling species in anaerobic digesters can be problematic for hard-cheese manufacturing industries, due to the issue of cheese blowing defects. This study investigated the effect of mesophilic anaerobic digestion processes on the Clostridium consortia distribution over time. Specifically, three lab-scale CSTRs treating agricultural biomass were characterized by considering both the whole microbial community and the cultivable clostridial spores. It is assessed an overall reduction of the Clostridium genus during the anaerobic digestion process. Moreover, it was evidenced a slight, but steady decrease of the cultivable clostridial spores, mainly represented by two pathogenic species, C. perfringens and C. bifermentans, and one hard-cheese spoiling species, C. butyricum. Thus, it is revealed an overall reduction of the clostridial population abundance after the mesophilic anaerobic digestion treatment of agricultural biomass.

Effect of ultrasound irradiation combined with ozone pretreatment on the anaerobic digestion for the biosludge exposed to trace-level levofloxacin: Degradation, microbial community and ARGs analysis

Anaerobic digestion, the principal method of stabilizing biosolids in wastewater treatment plants (WWTPs), can efficiently and largely attenuate the antibiotic resistances in biosludge. This study aims to investigate the effect of oxidative pretreatment with ultrasound irradiation combined with ozone (US/O₃) on the mesophilic and thermophilic anaerobic digestion (MAD and TAD) for the biosludge bearing trace fluoroquinolones contaminants-levofloxacin (LEVO) which was widely used in recent years. During the oxidation, the trace-level LEVO was almost completely degraded. The methanogenic activity in US/O₃ pretreated TAD dosed 0.1 mg/L LEVO was much higher than those in single MAD and TAD, therefore leading to a remarkable increase in biogas production. The identification of levofloxacin intermediates during chemical degradation was analyzed using LCMS technique and the reaction pathway based on them was proposed. Hydroxyl radicals provided by US/O₃ contributed to oxidative ring opening of LEVO as well as degradation of other biomacromolecules in the biosludge. Besides, the quinoline resistance genes-qnrA and qnrS declined significantly by 1-2 orders of magnitude in US/O₃-pretreated TAD, indicating that the active radicals produced by US/O₃ oxidized and degraded LEVO and therefore inactivated the antibiotic resistant bacteria or genes in the biosolids. Meanwhile, the composition and structure of the microbial community altered and the diversity and richness of total bacterial and potential human pathogens decreased, the pattern of which was correlated with LEVO-resistant genes. Among the well-known AD-related phylum including Bacteroidetes, Firmicutes, Methanobacteria as well as Thermotogae which has been previously detected in TAD and performed organic hydrolysis and degradation, the potential LEVO-resistant bacteria were probably affiliated to Actinobacteria, Bacteroidetes, Proteobacteria, Thermotogae. This study revealed the contribution of US/O₃ pretreatment to the anaerobic digestion in terms of ARGs reduction for trace-LEVO- exposed biosludge and could provide useful guidance for controlling the dissemination of ARB and ARGs in sewage sludge.

Recycling industrial biosludge for buffel grass production in Qatar: Impact on soil, leachate and plant characteristics

The agricultural industry in Qatar is highly dependent on using soil enhancing materials due to challenging soil and climatic conditions. Hence, this work investigated the potential of industrial biosludge from the wastewater treatment plant (WWTP) of a Gas-to-Liquids (GTL) plant to enhance an arid soil compared to fertilizer and compost. A fodder crop, buffel grass (Cenchrus ciliaris), was grown in semi-controlled pots containing a typical Qatari agricultural soil and admixtures over a 12-month period. The treatments included soil plus five biosludge percentage contents: 0.75, 1.5, 3, 6 and 12%. These were compared with soil only, soil plus 20-20-20 NPK fertilizer and soil plus 3% compost controls. Analyses of soil physical and chemical properties, the resulting leachate, and plant growth characteristics were conducted at set periods. The results indicate that up to 3% biosludge content led to better plant growth compared to the controls, with the optimum at 1.5% biosludge content for all growth characteristics studied. Biosludge addition to soil increased the volume of different pore types, especially micropores, which enhanced water retention and influenced plant growth. Regression modelling identified leachate Si and Fe concentrations, and biomass K content as the most influential variables for fresh biomass weight, plant height and the number of tillers, respectively. Biosludge addition to the soil around the optimum level did not cause detrimental changes to the resulting leachate and plant biomass. The findings of this work could lead to minimization of biosludge landfilling and allow for savings in fertilizers and irrigation water in arid regions.

The microbiome driving anaerobic digestion and microbial analysis

The microbiome residing in anaerobic digesters drives the anaerobic digestion (AD) process to convert various feedstocks to biogas as a renewable source of energy. This microbiome has been investigated in numerous studies in the last century. The early studies used cultivation-based methods and analysis to identify the four guilds (or functional groups) of microorganisms. Molecular biology techniques overcame the limitations of cultivation-based methods and allowed the identification of unculturable microorganisms, revealing the high diversity of microorganisms involved in AD. In the past decade, omics technologies, including metataxonomics, metagenomics, metatranscriptomics, metaproteomics, and metametabolomics, have been or start to be used in comprehensive analysis and studies of biogas-producing microbiomes. In this chapter, we reviewed the utilities and limitations of these analysis methods, techniques, and technologies when they were used in studies of biogas-producing microbiomes, as well as the new information on diversity, composition, metabolism, and syntrophic interactions of biogas-producing microbiomes. We also discussed the current knowledge gaps and the research needed to further improve AD efficiency and stability.

Occurrence, interactive effects and ecological risk of diclofenac in environmental compartments and biota - a review

Diclofenac, a nonsteroidal anti-inflammatory drug has turned into a contaminant of emerging concern; hence, it was included in the previous Watch List of the EU Water Framework Directive. This review paper aims to highlight the metabolism of diclofenac at different trophic levels, its occurrence, ecological risks, and interactive effects in the water cycle and biota over the past two decades. Increased exposure to diclofenac not only raises health concerns for vultures, aquatic organisms, and higher plants but also causes serious threats to mammals. The ubiquitous nature of diclofenac in surface water (river, lake canal, estuary, and sea) is compared with drinking water, groundwater, and wastewater effluent in the environment. This comprehensive survey from previous studies suggests the fate of diclofenac in wastewater treatment plants (WWTPs) and may predict its persistence in the environment. This review offers evidence of fragmentary available data for the water environment, soil, sediment, and biota worldwide and supports the need for further data to address the risks associated with the presence of diclofenac in the environment. Finally, we suggest that the presence of diclofenac and its metabolites in the environment may represent a high risk because of their synergistic interactions with existing contaminants, leading to the development of drug-resistant strains and the formation of newly emerging pollutants.

Anaerobic digestion of agricultural manure and biomass - Critical indicators of risk and knowledge gaps

Anaerobic digestion (AD) has been identified as a potential green technology to treat food and municipal waste, agricultural residues, including farmyard manure and slurry (FYM&S), to produce biogas. FYM&S and digestate can act as soil conditioners and provide valuable nutrients to plants; however, it may also contain harmful pathogens. This study looks at the critical indicators in determining the microbial inactivation potential of AD and the possible implications for human and environmental health of spreading the resulting digestate on agricultural land. In addition, available strategies for risk assessment in the context of EU and Irish legislation are assessed. Storage time and process parameters (including temperature, pH, organic loading rate, hydraulic retention time), feedstock recipe (carbon-nitrogen ratio) to the AD plant (both mesophilic and thermophilic) were all assessed to significantly influence pathogen inactivation. However, complete inactivation of all pathogens is unlikely. There are limited studies evaluating risks from FYM&S as a feedstock in AD and the spreading of resulting digestate. The lack of process standardisation and varying feedstocks between AD farms means risk must be evaluated on a case by case basis and calls for a more unified risk assessment methodology. In addition, there is a need for the enhancement of AD farm-based modelling techniques and datasets to help in advancing knowledge in this area.