A review on recent disposal of hazardous sewage sludge via anaerobic digestion and novel composting

Elimination of antibiotic resistance genes and adaptive response of Firmicutes during chicken manure composting

Aerobic composting is an effective method for reducing the abundance of antibiotic resistance genes (ARGs), but its effectiveness is influenced by the complex conditions during the process. However, the impact of composting conditions on the fate of ARGs and the underlying mechanisms remains unclear. This study examined the profile of ARGs, their mobility potential, hosts, and the risk of antibiotic resistome under different chicken manure composting conditions. A total of 708 ARGs were identified, and composting reduced their relative abundance by 29.4 %-82.4 %. Composting amendments and aeration rates exhibited similar elimination efficiencies for ARGs at different levels. However, low initial moisture content (50 %) caused a rebound in ARG abundance during the maturation phase. ARGs were mainly located on plasmids. After composting, the percentage of plasmid-born ARGs decreased from 46.3 %-70.8 % to 28.4 %-49.0 %. ARGs co-localized with mobile genetic elements displayed similar trends. The tolerance of Firmicutes to low moisture content played a key role in the rebound of ARGs and variations in their mobility potential. Composting reduced antibiotic resistance and ARG mobility in pathogens. Conversely, low moisture content hindered this attenuation effect in Firmicutes, which increased antibiotic resistome risk. This study provides comprehensive insights into the fate of ARGs and highlights the environmental risks of ARGs during composting.

Enhancing sustainability in meat production through insect biorefinery

Valuable feed crops and fossil fuel energy are used to produce animal meat. To become sustainable, meat production methods must adapt to include non-food substrates and renewable fossil-fuel alternatives. We evaluated the potential of protein livestock feed and biodiesel production through insect biorefining. The bioconversion efficiency of organic waste into black soldier fly larvae (BSFL) biomass was 32.0-35.8% after 24 d. The protein and lipid composition of BSFL changed with the cultivation time. The substrate influenced lipid content, and low lipid content led to lower lipid accumulation in the BSFL. Nevertheless, the potential productivity of proteins (42,471-48,345 kg ha-1 y-1) and lipids (41,642-64,708 kg ha-1 y-1) from BSFL cultivation with organic waste was higher than that of conventional livestock feed/biodiesel feedstocks, such as maize or soybean. In conclusion, insect biorefineries using BSFL can contribute significantly to the establishment of sustainable meat production.

Comparison of fractionation methods to assess Ni impact on anaerobic digestion: Filtration versus DGT

Excess of trace elements (TE) significantly alters the performances of anaerobic digestors (AD). Due to interactions with organic matter in particular, only a small fraction of TE can effectively interact with the biomass. However, assessing the bioavailable fraction of TE remains an issue. This study focuses on two chemical methods to monitor the potentially bioavailable Ni, selected as a model TE. The determination of the dissolved fraction by microfiltration was selected because it is commonly used, while the determination of the labile fraction by DGT (Diffusive Gradients in Thin films) was tested as it might better assess the bioavailable fraction. Different levels of Ni and ligands (EDTA and extracellular polymeric substances) were added in lab-scale AD reactors to induce AD performance alteration by changes of Ni amount or speciation. The AD performances were evaluated through biogas production and methane content. The results show that monitoring dissolved Ni highlights the alteration of AD performance due to variations of Ni amount but failed to detect alteration by change of speciation. The monitoring of DGT-labile Ni highlights reactor performance alteration by both variation of Ni amount and speciation. DGT therefore appears as an interesting complementary tool to detect potential alterations induced by TE.

Microbial safety of black soldier fly larvae (Hermetia illucens) reared on food waste streams

Black soldier fly larvae (BSFL) can valorise different organic matter and yield a product of high nutritional value. The lack of knowledge about the microbial safety of BSFL grown on different organic waste streams influences the commercialisation of BSFL as stockfeed ingredient. This study evaluates the microbial safety of BSFL grown on five different commercial food waste streams collected from two commercial production facilities. The effect of larval instar and post-harvest treatments (blanching and drying) on the microbial quality of the larvae was also investigated. The results of this study showed that spore forming bacteria including B. cereus and C. perfringens appeared in higher concentration (1.3 log CFU/g - 6.6 log CFU/g) compared to yeast and moulds (1.3 log CFU/g - 4.4 log CFU/g) depending on the waste utilised. The microbial counts for the different pathogens that tested positive were higher for larvae reared with simpler homogenous waste streams (SW and BV diets). The results also showed that blanching reduced the counts of Y&M and C. perfringens compared to those observed in the unprocessed larvae. The thermal processing methods could ensure that BSFL met the microbial safety criteria determined for animal feeds by regulatory bodies. Overall, the results of this study revealed that BSFL reared on commercial waste streams can be safe against several pathogenic microbes including Listeria monocytogenes, Salmonella, Escherichia coli and Staphylococcus aureus. The 6th instar larvae (pre-pupae) have lower microbial counts than the 5th instar BSFL, attributed to the antimicrobial effect of some fatty acids present in the larvae. The outcomes of this study will assist in the development of guidelines for good manufacturing practices for commercial BSFL manufacturers, thereby promoting the quality and commercialisation of the product.

The fate and mobility of chromium, arsenic and zinc in municipal sewage sludge during the co-pyrolysis process with organic and inorganic chlorides

Co-pyrolysis is an efficient approach for municipal sewage sludge (SS) treatment, facilitating the production of biochar and promoting the stabilization and removal of heavy metals, particularly when combined with chlorinated materials. This study explores the impact of pyrolysis temperatures (400 °C and 600 °C) and chlorinated additives (polyvinyl chloride (PVC) as an organic chloride source and ferric chloride (FeCl3) as an inorganic chloride source) at 10% and 20% concentrations, on the yield, chemical speciation, leachability, and ecological risks of arsenic (As), chromium (Cr), and zinc (Zn) in biochar derived from SS. The results revealed that increasing the pyrolysis temperature from 400 to 600 °C significantly reduced biochar yield due to enhanced volatilization of organic components, as well as the removal of heavy metals in interaction with chlorinated materials. Chlorinated additives distinctly influenced heavy metal behavior. PVC treatments at 600 °C effectively reduced the total concentrations of As and Zn by 60% and 88.3%, respectively, while FeCl3 reduced Cr concentrations by up to 72.5%. Chemical speciation analysis showed that PVC treatments increased the residual fractions of As and Zn, reducing their bioavailability and environmental risk. In contrast, FeCl3 promoted the transformation of Cr into oxidizable fractions, enhancing its stability. TCLP results confirmed the effectiveness of both additives in reducing heavy metal leachability, with PVC at 600 °C demonstrating superior performance for As and Zn, and FeCl3 excelling in Cr stabilization. Ecological risk index assessments revealed that PVC treatments consistently resulted in lower RI values at both temperatures and concentrations, keeping them below the low-risk threshold. In contrast, FeCl3 treatments exhibited elevated risk levels, especially at higher concentrations and temperatures, reaching moderate to considerable risk categories. Overall, PVC treatment at 600 °C proved to be the most effective strategy for reducing As and Zn leachability and enhancing biochar stability. While FeCl3 demonstrated better performance in Cr stabilization, these findings highlight the importance of selecting appropriate chlorinated additives based on the target heavy metal for optimizing biochar production and minimizing environmental impacts effectively.

Ultrasonication as anaerobic digestion pretreatment to improve sewage sludge methane production: Performance and microbial characterization

A large amount of sludge is inevitably produced during sewage treatment. Ultrasonication (US) as anaerobic digestion (AD) pretreatment was implemented on different sludges and its effects on batch and semi-continuous AD performance were investigated. US was effective in sludge SCOD increase, size decrease, and CH4 production in the subsequent AD, and these effects were enhanced with an elevated specific energy input. As indicated by semi-continuous AD experiments, the mean daily CH4 production of US-pretreated A2O-, A2O-MBR-, and AO-AO-sludge were 176.9, 119.8, and 141.7 NmL/g-VSadded, which were 35.1%, 32.1% and 78.2% higher than methane production of their respective raw sludge. The US of A2O-sludge achieved preferable US effects and CH4 production due to its high organic content and weak sludge structure stability. In response to US-pretreated sludge, a more diverse microbial community was observed in AD. The US-AD system showed negative net energy; however, it exhibited other positive effects, e.g., lower required sludge retention time and less residual total solids for disposal. US is a feasible option prior to AD to improve anaerobic bioconversion and CH4 yield although further studies are necessary to advance it in practice.

Succession from acetoclastic to hydrogenotrophic microbial community during sewage sludge anaerobic digestion for bioenergy production

To assess microbial dynamics during anaerobic digestion (AD) of sewage sludge (SWS) from a municipal Wastewater Treatment Plant (WWTP), a Biochemical Methane Potential (BMP) assay at 37 °C under mono-digestion conditions was conducted. Utilizing the Illumina MiSeq platform, 16S ribosomal RNA (rRNA) gene sequencing unveiled a core bacterial community in the solid material, showcasing notable variations in profiles. The research investigates changes in microbial communities and metabolic pathways to understand their impact on the efficiency of the digestion process. Prior to AD, the relative abundance in SWS was as follows: Proteobacteria > Bacteroidota > Actinobacteriota. Post-AD, the relative abundance shifted to Firmicutes > Synergistota > Proteobacteria, with Sporanaerobacter and Clostridium emerging as dominant genera. Notably, the methanogenic community underwent a metabolic pathway shift from acetoclastic to hydrogenotrophic in the lab-scale reactors. At the genus level, Methanosaeta, Methanolinea, and Methanofastidiosum predominated initially, while post-AD, Methanobacterium, Methanosaeta, and Methanospirillum took precedence. This metabolic transition may be linked to the increased abundance of Firmicutes, particularly Clostridia, which harbor acetate-oxidizing bacteria facilitating the conversion of acetate to hydrogen.

Perspectives on innovative non-fertilizer applications of sewage sludge for mitigating environmental and health hazards

As waste production increases and resources become limited, sewage sludge presents a valuable resource with potential beyond traditional land use and incineration. This review emphasizes exploring innovative non-fertilizer applications of sewage sludges and advocates for viewing wastewater treatment plants as sources of valuable feedstock and carbon sequestration. Innovative uses include integrating sewage sludge into construction materials such as asphalt pavements, geopolymer, cementitious composites, and masonry blocks. These methods not only immobilize heavy metals and mitigate environmental hazards but also support carbon sequestration, contrasting with incineration and land application methods that release carbon into the atmosphere. The review also addresses emerging technologies like bio-adhesives, bio-binders for asphalt, hydrogels, bioplastics, and corrosion inhibitors. It highlights the recovery of valuable materials from sewage sludge, including phosphorus, oils, metals, cellulose, and polyhydroxyalkanoates as well as enzyme production. By focusing on these non-fertilizer applications, this review presents a compelling case for re-envisioning wastewater treatment plants as sources of valuable feedstock and carbon sequestration, supporting global efforts to manage waste effectively and enhance sustainability.

Unravelling the detoxification trail of potential toxic heavy metals: an insight into heavy metal auditing and ecological health upon valorisation by Lampito mauritii and Eudrilus eugeniae

Evidence on prospective remediation of municipal solid waste contaminated with toxic heavy metals by Eudrilus eugeniae (Eu) and Lampito mauritii (L) is very scarce and yet to be explored. In this study, heavy metal detoxification potential of E. eugeniae and L. mauritii in municipal solid waste (MSW) + cowdung (CD) (3:1)-based feedstocks were investigated against Eisenia fetida (E) (a well-known vermi-remediator) and aerobic composting. Excellent reduction (70.01-93.04%) of potentially toxic heavy metals (PTHMs) (Pb, Cr, Cd and Zn) were evident in both E. eugeniae and L. mauritii employed treatments. Moreover, the results on heavy metal budget quotient clearly demonstrated the unique detoxification route undertaken by E. eugeniae and L. mauritii via humic composite facilitated chelation over the nominal bioaccumulation pathway. The principal component analysis (PCA) confirmed the strong negative correlation between the heavy metal (HM) level in earthworm gut and MSW substrate, whereas a strong positive correlation between humic substances and HM remediation. Furthermore, analysis of ecological health parameters indicated substantial reduction of environmental risk and guaranteed negligible risk of PTHM if utilized as manure. Moreover, significant increment in total N content (3.2-3.8-fold), available P (4-5.9-fold), exchangeable K (3.66-fourfold) and enzyme activity along with significant reduction of TOC (~ 87%) confirmed E. eugeniae and L. mauritii could effectively stabilize MSW. Thus, the metal-binding potential of humic substances produced by earthworms during the detoxification of municipal solid waste (MSW), coupled with a metal budget analysis, has offered valuable insights into the usage of E. eugeniae and L. mauritii as effective contenders for sanitizing heavy metal-laden MSW.

Occurrence and dissipation mechanisms of organic contaminants during sewage sludge anaerobic digestion: A critical review

Sewage sludge, a complex mixture of contaminants and pathogenic agents, necessitates treatment or stabilization like anaerobic digestion (AD) before safe disposal. AD-derived products (solid digestate and liquid fraction) can be used as fertilizers. During AD, biogas is also produced, and used for energy purposes. All these fractions can be contaminated with various compounds, whose amount depends on the feedstocks used in AD (and their mutual proportions). This paper reviews studies on the distribution of organic contaminants across AD fractions (solid digestate, liquid fraction, and biogas), delving into the mechanisms behind contaminant dissipation and proposing future research directions. AD proves to be a relatively effective method for removing polychlorinated biphenyls, polycyclic aromatic hydrocarbons, pharmaceuticals, antibiotic resistance genes and hydrocarbons. Contaminants are predominantly removed through biodegradation, but many compounds, especially hydrophobic (e.g. per- and polyfluoroalkyl substances), are also sorbed onto digestate particles. The process of sorption is suggested to reduce the bioavailability of contaminants. As a result of sorption, contaminants accumulate in the largest amount in the solid digestate, whereas in smaller amounts in the other AD products. Polar pharmaceuticals (e.g. metformin) are particularly leached, while volatile methylsiloxanes and polycyclic aromatic hydrocarbons, characterized by a high Henry's law constant, are volatilized into the biogas. The removal of compounds can be affected by AD operational parameters, the type of sludge, physicochemical properties of contaminants, and the sludge pretreatment used.

Sewage sludge fertilization affects microbial community structure and its resistome in agricultural soils

Global sewage sludge production is rapidly increasing, and its safe disposal is becoming an increasingly serious issue. One of the main methods of municipal sewage sludge management is based on its agricultural use. The wastewater and sewage sludge contain numerous antibiotic resistance genes (ARGs), and its microbiome differs significantly from the soil microbial community. The aim of the study was to assess the changes occurring in the soil microbial community and resistome after the addition of sewage sludge from municipal wastewater treatment plant (WWTP) in central Poland, from which the sludge is used for fertilizing agricultural soils on a regular basis. This study used a high-throughput shotgun metagenomics approach to compare the microbial communities and ARGs present in two soils fertilized with sewage sludge. The two soils represented different land uses and different physicochemical and granulometric properties. Both soils were characterized by a similar taxonomic composition of the bacterial community, despite dissimilarities between soils properties. Five phyla predominated, viz. Planctomycetes, Actinobacteria, Proteobacteria, Chloroflexi and Firmicutes, and they were present in comparable proportions in both soils. Network analysis revealed that the application of sewage sludge resulted in substantial qualitative and quantitative changes in bacterial taxonomic profile, with most abundant phyla being considerably depleted and replaced by Proteobacteria and Spirochaetes. In addition, the ratio of oligotrophic to copiotrophic bacteria substantially decreased in both amended soils. Furthermore, fertilized soils demonstrated greater diversity and richness of ARGs compared to control soils. The increased abundance concerned mainly genes of resistance to antibiotics most commonly used in human and animal medicine. The level of heavy metals in sewage sludge was low and did not exceed the standards permitted in Poland for sludge used in agriculture, and their level in fertilized soils was still inconsiderable.

Sewage sludge management and enhanced energy recovery using anaerobic digestion: an insight

Sewage sludge (SS) is a potential source of bioenergy, yet its management is a global concern. Anaerobic digestion (AD) is applied to effectively valorize SS by reclaiming energy in the form of methane. However, the complex floc structure of SS hinders hydrolysis during AD process, thus resulting in lower process efficiency. To overcome the rate-limiting hydrolysis, various pre-treatment methods have been developed to enhance AD efficiency. This review aims to provide insights into recent advancements in pre-treatment technologies, including mechanical, chemical, thermal, and biological methods. Each technology was critically evaluated and compared, and its relative worth was summarized based on full-scale applicability, along with economic benefits, AD performance improvements, and impact on digested sludge. The paper illuminates the readers about existing research gaps, and the future research needed for successful implementation of these approaches at full scale.

Bacterial dispersal enhances the elimination of active fecal coliforms during vermicomposting of fruit and vegetable wastes: The overlooked role of earthworm mucus

Earthworms play a pivotal role in the elimination of fecal coliforms during vermicomposting of fruit and vegetable waste (FVWs). However, the specific mechanisms underlying the action of earthworm mucus remain unclear. This study investigated the mechanisms of fecal coliform reduction related to earthworm mucus during FVWs vermicomposting by comparing treatments with and without earthworms. The results show that the secretion of earthworm mucus decreased by 13.93 % during the startup phase, but significantly (P < 0.001) increased by 57.80 % during the degradation phase. Compared to the control without earthworms, vermicomposting led to a significant (P < 0.05) 1.22 -fold increase in the population of active bacteria, with a strong positive correlation between mucus characteristics and dominant bacterial phyla. As the dominant fecal coliforms, Escherichia coli and Klebsiella pneumoniae significantly (P < 0.05) declined by 86.20 % and 93.38 %, respectively, in the vermi-reactor relative to the control. Bacterial dispersal limitation served as a key factor constraining the elimination of E. coli (r = 0.73, P < 0.01) and K. pneumoniae (r = 0.77, P < 0.001) during vermicomposting. This study suggests that earthworm mucus increases the active bacterial abundance and cooperation by weakening the bacterial dispersal limitation, thus intensifying competition and antagonism between fecal coliforms and other bacteria.

From organic fertilizer to the soils: What happens to the microplastics? A critical review

In recent, soil microplastic pollution arising from organic fertilizers has been of a great increasing concern. In response to this concern, this review presents a comprehensive analysis of the occurrence and evolution of microplastics in organic fertilizers, their ingress into the soil, and the subsequent impacts. Organic fertilizers are primarily derived from solid organic waste generated by anthropocentric activities including urban (daily-life, municipal wastes and sludge), agricultural (manure, straw), and industrial (like food industrial waste etc.) processes. In order to produce organic fertilizer, the organic solid wastes are generally treated by aerobic composting or anaerobic digestion. Currently, microplastics have been widely detected in the raw materials and products of organic fertilizer. During the process of converting organic solid waste materials into fertilizer, intense oxidation, hydrolysis, and microbial actions significantly alter the physical, chemical, and surface biofilm properties of the plastics. After the organic fertilizer application, the abundances of microplastics significantly increased in the soil. Additionally, the degradation of these microplastics often promotes the adsorption of organic pollutants and affects their retention time in the soil. These microplastics, covered by biofilms, also significantly alter soil ecology due to the unique properties of the biofilm. Furthermore, the biofilms also play a role in the degradation of microplastics in the soil environment. This review offers a new perspective on the soil environmental processes involving microplastics from organic fertilizer sources and highlights the challenges associated with further research on organic fertilizers and microplastics.

Applying new regional background concentration criteria to assess heavy metal contamination in deep-sea sediments at an ocean dumping site, Republic of Korea

It is crucial to establish appropriate background concentrations to discern heavy metal pollution in the marine environment. In this study, we analyzed heavy metals in deep-sea sediment cores to determine regional background concentrations at the East-Sea Byeong Ocean dumping site. The vertical profiles of heavy metals were categorized into three groups based on their contamination characteristics, and regional background levels for 12 metals were determined using pre-1900 averages. The enrichment factor, contamination factor, and pollution load index, calculated using regional background concentrations, indicated significant contamination by Cr, Co, Cu, Zn, Cd, Hg, and Pb during the ocean dumping period. These results differ from those obtained using global average concentrations. This underscores the importance of considering regional characteristics to minimize the risk of misinterpreting anthropogenic impacts. The approach based on local information is considered useful when sediment quality guidelines are absent or inapplicable.

Influences of straw alkaline pretreatment on biogas production and digestate characteristics: artificial neural network and multivariate statistical techniques

Anaerobic digestion is one of the best options for producing valuable end products (biogas and biofertilizer). The aim of this study was to investigate the influences of thermoalkaline pretreatment of wheat straw on biogas production and digestate characteristics from codigestion with waste-activated sludge. Different alkaline conditions (NaOH, KOH and Na2CO3) and pretreatment durations (1, 3 and 5 h) were used for straw pretreatment. Batch anaerobic codigestion of sludge and pretreated straw was conducted under different pretreatment conditions. A feedforward neural network (FFNN) model, logistic model and statistical analysis were applied to the experimental data to predict biogas and investigate the significance and relationships among the variables. NaOH pretreatment for 5 h showed the best treatment conditions: biogas yield was 6.59 times higher than that without treatment. Moreover, the proportions of total solids, total volatile solids, chemical oxygen demand and microbial count removed reached 63.52%, 74.60%, 78.15% and 82.22%, respectively. The methane content was 67.50%, indicating that the biogas had a high quality. The thermoalkaline pretreatment significantly affected biogas production and digestate characteristics, allowing it to be used as a biofertilizer. Experimental data were successfully modelled for predicting biogas production using the applied models. The R2 values reached 0.985 and 0.999 for the logistic and FFNN models, respectively.

Near-infrared spectroscopy (NIRS) for monitoring the nutritional composition of black soldier fly larvae (BSFL) and frass

BACKGROUND

The demand for protein obtained from animal sources is growing rapidly, as is the necessity for sustainable animal feeds. The use of black soldier fly larvae (BSFL) reared on organic side streams as sustainable animal feed has been receiving attention lately. This study assessed the ability of near-infrared spectroscopy (NIRS) combined with chemometrics to evaluate the nutritional profile of BSFL instars (fifth and sixth) and frass obtained from two different diets, namely soy waste and customised bread-vegetable diet. Partial least squares (PLS) regression with leave one out cross-validation was used to develop models between the NIR spectral data and the reference analytical methods.

RESULTS

Calibration models with good [coefficient of determination in calibration (Rcal 2 ): 0.90; ratio of performance to deviation (RPD) value: 3.6] and moderate (Rcal 2 : 0.76; RPD value: 2.1) prediction accuracy was observed for acid detergent fibre (ADF) and total carbon (TC), respectively. However, calibration models with moderate accuracy were observed for the prediction of crude protein (CP) (Rcal 2 : 0.63; RPD value: 1.4), crude fat (CF) (Rcal 2 : 0.70; RPD value: 1.6), neutral detergent fibre (NDF) (Rcal 2 : 0.60; RPD value: 1.6), starch (Rcal 2 : 0.52; RPD value: 1.4), and sugars (Rcal 2 : 0.52; RPD value: 1.4) owing to the narrow or uneven distribution of data over the range evaluated.

CONCLUSION

The near-infrared (NIR) calibration models showed a good to moderate prediction accuracy for the prediction of ADF and TC content for two different BSFL instars and frass reared on two different diets. However, calibration models developed for predicting CP, CF, starch, sugars and NDF resulted in models with limited prediction accuracy. © 2023 Society of Chemical Industry.

Multiple-layer statistical methodology for developing data-driven models of anaerobic digestion process

When modelling anaerobic digestion, ineffective data handling and inadequate designation of modelling parameters can undermine the model reliability. In this study, a multilayer statistical technique, which employed a machine learning technique using regression models, was introduced to systematically support the development of anaerobic digestion models. Layer-by-layer statistical techniques including cubic smoothing splines (missing data reconstruction), principal component analysis (identifying correlated parameters), analysis of variance (analysing differences among datasets), and linear regression (developing data-driven models) were used to develop and validate anaerobic digestion models. Experimental data collected from the long-term operation of lab-scale (operated for 350 days), pilot-scale (operated for 150 days), and full-scale reactors (operated for 750 days) were used to demonstrate the modelling process. The multivariate models based on a data-driven modelling technique were developed by subjecting the experimental and monitored data to a modelling process. The developed models could predict the biogas production and effluent chemical oxygen demand during anaerobic digestion. Statistical analyses verified the modelling hypotheses, evaded invalid model development, and ensured data integrity and parameter validity. Multiple linear regression of principal components demonstrated that the performance of biogas production using food waste was influenced by the variances of the nitrogen and organic concentrations, but not by the chemical oxygen demand to total nitrogen (C/N) ratio. In the validation process, the model developed with lab-scale reactor data showed relatively high accuracy with R2, SSE, and RMSE values of 0.86, 34.45, and 0.72.

Black soldier fly (Diptera: Stratiomyidae) reduction of different sludges, subsequent safety, and research gaps

One of the many waste components that end up in landfills is sludge, an organic waste that Black Soldier Flies (BSF) may be capable of reducing or removing along with potential pathogens from the environment. Throughout this review, knowledge gaps were evaluated as well as indications that BSF can positively impact the environment and economy when it comes to reducing sludge. This review first aimed to establish consistency across assessed papers (n = 41). Second, previous research on the efficiency and capability of sludge to be reduced by BSF both in isolation and with other substrates (co-digestion) was assessed. Subsequently, this review also evaluated the resulting safety of both the BSF and remaining sludge after consumption. Through this review, reduction difficulties and gaps in research and industry were evaluated. Unfortunately, complications come from the lack of policy for industry status as well as the need for further research. Therefore, further research is required on the consumption of different sludges. In particular, sludge should be tested with co-substrates to decrease waste removal problems. Sludge lacks crucial nutrients and contains extracellular polymeric substances preventing BSFL consumption. This review could potentially be helpful in future research to formulate a methodology for better BSFL consumption and production of insect biomass by sludge pre-treatment. This could lead to the implementation of the new policy in sludge management.

Advances in sewage sludge application and treatment: Process integration of plasma pyrolysis and anaerobic digestion with the resource recovery

Sewage sludge (SS) is an environmental issue due to its high organic content and ability to release hazardous substances. Most of the treatments available are biological, thermal hydrolysis, mechanical (ultrasound, high pressure, and lysis), chemical with oxidation (mainly ozonation), and alkali pre-treatments. Other treatment methods include landfill, wet oxidation, composting, drying, stabilization, incineration, pyrolysis, carbonization, liquefaction, gasification, and torrefaction. Some of these SS disposal methods damage the ecosystem and underutilize the potential resource value of SS. These challenges must be overcome with an innovative technique for the improvement of SS's nutritional value, energy content, and usability. This review proposes plasma pyrolysis and anaerobic digestion (AD) as promising SS treatment technologies. Plasma pyrolysis pre-treats SS to make it digestible by AD bacteria and immobilizes the heavy metals. The addition of Char to the upstream AD process increases the quantity and quality of biogas produced while enhancing the nutrients in the digestate. These two processes are integrated at high temperatures, thus creating concerns about their energy demand. These challenges are offset by the generated energy that can run the treatment plant or be sold to the grid, generating additional cash. Plasma pyrolysis wastes can also be converted into biochar, organic fertilizer, or soil conditioner. These combined technologies' financial sustainability depends on the treatment facility's circumstances and location. Plasma pyrolysis and AD can treat SS sustainably and provide nutrients and resources. This paper explains the co-process treatment route's techno-economic prospects, challenges, and recommendations for the future application of SS valorization and resource recovery.

Near Infrared Spectroscopy for Prediction of Yeast and Mould Counts in Black Soldier Fly Larvae, Feed and Frass: A Proof of Concept

The use of black soldier fly larvae (BSFL) grown on different organic waste streams as a source of feed ingredient is becoming very popular in several regions across the globe. However, information about the easy-to-use methods to monitor the safety of BSFL is a major step limiting the commercialization of this source of protein. This study investigated the ability of near infrared (NIR) spectroscopy combined with chemometrics to predict yeast and mould counts (YMC) in the feed, larvae, and the residual frass. Partial least squares (PLS) regression was employed to predict the YMC in the feed, frass, and BSFL samples analyzed using NIR spectroscopy. The coefficient of determination in cross validation (R2CV) and the standard error in cross validation (SECV) obtained for the prediction of YMC for feed were (R2cv: 0.98 and SECV: 0.20), frass (R2cv: 0.81 and SECV: 0.90), larvae (R2cv: 0.91 and SECV: 0.27), and the combined set (R2cv: 0.74 and SECV: 0.82). However, the standard error of prediction (SEP) was considered moderate (range from 0.45 to 1.03). This study suggested that NIR spectroscopy could be utilized in commercial BSFL production facilities to monitor YMC in the feed and assist in the selection of suitable processing methods and control systems for either feed or larvae quality control.

Pharmaceutical active compounds in sewage sludge: Degradation improvement and conversion into an organic amendment by bioaugmentation-composting processes

Around 143,000 chemicals find their fate in wastewater treatment plants in the European Union. Low efficiency on their removal at lab-based studies and even poorer performance at large scale experiments have been reported. Here, a coupled biological technology (bioaugmentation and composting) is proposed and proved for pharmaceutical active compounds degradation and toxicity reduction. The optimization was conducted through in situ inoculation of Penicillium oxalicum XD 3.1 and an enriched consortium (obtained from non-digested sewage sludge), into pilot scale piles of sewage sludge under real conditions. This bioaugmentation-composting system allowed a better performance of micropollutants degradation (21 % from the total pharmaceuticals detected at the beginning of the experiment) than a traditional composting process. Particularly, inoculation with P. oxalicum allowed the degradation of some recalcitrant compounds like carbamazepine, cotinine and methadone, and also produced better stabilization features in the mature compost (significant passivation of copper and zinc, higher macronutrients value, adequate physicochemical conditions for soil direct application and less toxic effect on germination) compared to the control and the enriched culture. These findings provide a feasible, alternative strategy to obtain a safer mature compost and a better removal of micropollutants performance at large scale.

Sewage sludge pretreatment: current status and future prospects

Sewage sludge is regarded by wastewater treatment plants as problematic, from a financial and managerial point of view. Thus, a variety of disposal routes are used, but the most popular is methane fermentation. The proportion of macromolecular compounds in sewage sludges varies, and substrates treated in methane fermentation provide different amounts of biogas with various quality and quantity. Depending on the equipment and financial capabilities for methane fermentation, different methods of sewage sludge pretreatment are available. This review presents the challenges associated with the recalcitrant structure of sewage sludge and the presence of process inhibitors. We also examined the diverse methods of sewage sludge pretreatment that increase methane yield. Moreover, in the field of biological sewage sludge treatment, three future study propositions are proposed: improved pretreatment of sewage sludge using biological methods, assess the changes in microbial consortia caused with pretreatment methods, and verification of microbial impact on biomass degradation.

Additive facilitated co-composting of lignocellulosic biomass waste, approach towards minimizing greenhouse gas emissions: An up to date review

Although the composting of lignocellulosic biomass is an emerging waste-to-wealth approach towards organic waste management and circular economy, it still has some environmental loopholes that must be addressed to make it more sustainable and reliable. The significant difficulties encountered when composting lignocellulosic waste biomass are consequently discussed in this study, as well as the advances in science that have been achieved throughout time to handle these problems in a sustainable manner. It discusses an important global concern, the emission of greenhouse gases during the composting process which limits its applicability on a broader scale. Furthermore, it discusses in detail, how different organic minerals and biological additives modify the physiochemical and biological characteristics of compost, aiming at developing eco-friendly compost with minimum odor, greenhouse gases emission and an optimum C/N ratio. It brings novel insights by demonstrating the effect of additives on the microbial enzymes and their pathways involved in the degradation of lignocellulosic biomass. This review also highlights the limitations of the application of additives in composting and suggests possible ways to overcome these limitations in the future for the sustainable and eco-friendly management of agricultural waste. The present review concludes that the use of additives in the co-composting of lignocellulosic biomass can be a viable remedy for the ongoing issues with the management of lignocellulosic waste.

Evaluation of the influence of rice husk amendment on compost quality in the composting of sewage sludge

This study aimed to evaluate the influence of rice husk addition on compost quality and maturity in sewage sludge composting using a pilot scale aerated in-vessel reactor. During the composting process, changes in compost quality and physicochemical factors including pH, temperature, moisture content, electrical conductivity, total organic carbon (TOC), total nitrogen (TN), and carbon to nitrogen ratio (C/N) were monitored. In the pile containing 25% rice husk, the lowest losses occurred with 52.49% for TOC and 23.24% for TN, while C/N ratio in the final compost was 18.82, achieving mature and quality compost. The moisture contents of the final composts were found as 50.72% in the control group while it was 31.73% and 28.18% in the reactors containing 10% and 25% rice husk, respectively. These results suggested that rice husk addition was beneficial for reducing moisture content and balancing the C/N ratio in sewage sludge composting.

Synergistic effect of biochar amendment in milk processing industry sludge and cattle dung during the vermiremediation

The effective and sustainable management of fast growing and large quantities of industrial waste is a serious issue. The purpose of the present study was to assess the synergistic effect of biochar (BC) amended in milk processing industry sludge (MS) mixed with cattle dung (CD) in different ratios through vermiremediation. The MS₂₅ and MS₂₅BC₁₀ (25:75 + 10 % BC) showed the least mortality and greatest earthworm growth and development. The final product from all feed mixtures recorded a decrease in pH, total organic carbon and C/N ratio. Other parameters viz., electrical conductivity, total available phosphorus, total Kjeldahl nitrogen, total sodium, total potassium and ash content was observed to be increased after vermicomposting. Significantly lower heavy metal content was found in all biochar amended feed mixtures than in mixtures without biochar. The germination index of Trigonella foenum-graecum showed a value ranging from 89.14 to 131.46 % for mixtures without BC and 115.18-153.47 % for biochar amended mixtures.

Modelling and optimization of sewage sludge composting using biomass ash via deep neural network and genetic algorithm

In this study, the use of Deep Cascade Forward Neural Network (DCFNN) was investigated to model both linear and non-linear chaotic relationships in co-composting of dewatered sewage sludge and biomass fly ash (BFA). Model results were evaluated in comparison with RSM, Feed Forward Neural Network (FFNN) and Feed Back Neural Network (FBNN), and Cascade Forward Neural Network (CFNN). DCFNN produced predictive results with MAPE values less than 1% for all datasets in all experimental designs except one with 1.99%. Furthermore, the decision variables were optimized by Genetic Algorithm (GA). The desirability level obtained from the optimization results was found to be 100% in a few designs and above 95% in all other designs. The results showed that DCFNN is a reliable and consistent tool for modeling composting process parameters, also GA is a satisfactory tool for determining which outputs the input parameters will produce in an experimental setup.

Mechanical Properties and Water Resistance of Magnesium Oxychloride Cement–Solidified Residual Sludge

As a solid waste, the amount of residual sludge produced by the municipal wastewater treatment process is escalating. How to dispose it properly is attracting much attention in society. Herein, solidifying residual sludge using magnesium oxychloride cement (MOC) is promising for converting it into building materials. Various factors of mass ratio (RW/S) of liquid to solid, molar ratio (Rn) of MgO to MgCl2 in MOC, mass ratio (Rm) of residual sludge to MOC, the mass concentration of Na2SiO3 (DNa2SiO3), and dosage of fly ash (DF) influenced the unconfined compression strength (RC) of the as–obtained MOC–solidified residual sludge, and it was characterized using SEM and XRD analysis. The results show that the value of RC for MOC–residual sludge solidified blocks increased initially and then decreased as Rn and Rm increased, respectively, for 60–day curing. At 10–day curing, equilibrium RC was reached at all RW/S values except 1.38, and at 60–day curing, RC decreased with RW/S increasing. The maximum RC of 60 days of 20.90 MPa was obtained at RW/S = 0.90, Rn = 5.0, and Rm = 1.00. Furthermore, adding Na2SiO3 or fly ash in the solidifying process could improve RC. The water resistance test showed that SM13 and NF5 samples exhibited good alkaline resistance after immersion for 7 and 14 days in an aqueous solution with pH = 7.0–11.0. The water resistance of MOC–residual sludge solidified blocks decreased with increase in immersion duration in aqueous solutions. The fly ash could also help improve water resistance of MOC–solidified residual sludge in neutral and basic aqueous solutions. This work provides an important theoretical basis and possibility for the efficient disposal and comprehensive utilization of residual sludge through solidification/stabilization technology using MOC from the perspective of mechanics and water resistance.

Promoting the Circular Economy on an Island: Anaerobic Co-Digestion of Local Organic Substrates as a Possible Renewable Energy Source

The local waste co-digestion is an interesting option to tackle in reduced and isolated areas like the islands. The islands have limited territory and scarce fuel production. Moreover, organic waste can create serious environmental problems in soil, water and air. Anaerobic co-digestion (AcoD) is a technology fulfilling the concept of waste-to-energy (WtE) based on local resources. The valorisation of organic waste through AcoD on an island would prevent environmental impacts, while being a source of renewable energy. In this study, cow manure (outdoor and indoor), pig slurry, bird manure, kitchen waste, sewage sludge and oily lacteous waste produced on Island Terceira (Portugal) were tested in mesophilic -35 °C- Biochemical Methane Potential (BMP) co-digestion assays. The goals were to analyse the recalcitrant and high potential produced waste and to estimate the energetic supply source on the island. The cow manure and pig slurry were used as inocula and specific methanogenic activities (SMAs) were carried out. The results showed that both substrates have a significant methanogenic activity-SMA 0.11 g-COD/(g-VSS.d) and 0.085 g-COD/(g-VSS.d), respectively. All the studied combinations were feasible in AcoD, showing TS removals in the range of 19-37%; COD removals in the range 67-78% and specific methane yields from 0.14 to 0.22 L/gCOD removed, but some differences were found. The modified Gompertz model fitted the AcoD assays (R² 0.982-0.998). The maximum biogas production rate, Rmax. was highest in the AcoD of Cow+Pig+Oily and in the Cow+Pig+Sludge with 0.017 and 0.014 L/g-VS_added.day, respectively, and the lowest in Cow+Pig+Bird with 0.010 L/g-VS_added. In our AcoD studies, the bird manure limited the performance of the process, since it was recalcitrant to anaerobic degradation. On the other hand, the oily lacteous waste showed a great potential in the anaerobic digestion. The estimated biogas production, from the best-studied condition, could cover the 11.4% of the energy supply of the inhabitants. These preliminary results would prevent the environmental impact of organic waste on the island and promote the use of local waste in a circular economy scenario.

Thermophilic-mesophilic biodegradation: An optimized dual-stage biodegradation technique for expeditious stabilization of sewage sludge

The present study investigated the stabilization of fresh sewage sludge through a dual-stage biodegradation process; rotary drum composting in series with vermicomposting. After thermophilic exposure in a rotary drum composter, the partially degraded feedstock was separated into S1 without vermiculture, S2 and S3 with Eudrilus eugeniae and Eisenia fetida vermi-monocultures, respectively. The S3-derived vermicompost exhibited an 80% and 88% reduction in CO₂ and ammonium-nitrogen evolution rates, respectively, demonstrating the expedient stabilization of sludge. The robust, more than 85% seed germination index supported S2 and S3 derived vermicompost viability. A significant decrease in heavy metals was evinced with S2 and S3-derived vermicompost; the S1-derived end product exhibited higher Zn, Cr, and Pb levels in the absence of vermicomposting. Furthermore, soil amended with 20% vermicompost from S3 displayed 50% more plant growth than S1. Thus, the optimized thermophilic-mesophilic dual-biodegradation technique stabilizes sewage sludge quickly, has a lot of potential in sludge management facilities around the world, and produces a marketable end product.

Utilization of nanoparticles for biogas production focusing on process stability and effluent quality

Abstract

One of the most important techniques for converting complex organic waste into renewable energy in the form of biogas and effluent is anaerobic digestion. Several issues have been raised related to the effectiveness of the anaerobic digestion process in recent years. Hence nanoparticles (NPs) have been used widely in anaerobic digestion process for converting organic wastes into useful biogas and effluent in an effective way. This review addresses the knowledge gaps and summarizes recent researchers’ findings concentrating on the stability and effluent quality of the cattle manure anaerobic digestion process using single and combinations nanoparticle. In summary, the utilization of NPs have beneficial effects on CH4 production, process optimization, and effluent quality. Their function, as key nutrient providers, aid in the synthesis of key enzymes and co-enzymes, and thus stimulate anaerobic microorganism activities when present at an optimum concentration (e.g., Fe NPs 100 mg/L; Ni NPs 2 mg/L; Co NPs 1 mg/L). Furthermore, utilizing Fe NPs at concentrations higher than 100 mg/L is more effective at reducing H2S production than increasing CH4, whereas Ni NPs and Co NPs at concentrations greater than 2 mg/L and 1 mg/L, respectively, reduce CH4 production. Effluent with Fe and Ni NPs showed stronger fertilizer values more than Co NPs. Fe/Ni/Co NP combinations are more efficient in enhancing CH4 production than single NPs. Therefore, it is possible to utilize NPs combinations as additives to improve the effectiveness of anaerobic digestion.

Article highlights

Influence of sludge treatment methods on behaviors of microplastics adsorbed cadmium and its driving factors

The complicated contamination of microplastics (MPs) and heavy metals in sludge has garnered substantial attention in recent years; however, research on the behavior of MPs loading of heavy metals in sludge after sludge treatment methods is limited. Four representative sludge treatment methods were selected herein: anaerobic digestion, thermal drying, thermal hydrolysis (TH), and aerobic composting. Before and after sludge treatment, the chemical bonding of MPs, cadmium (Cd) adsorption properties, and metabolic changes in the microbial community succession was analyzed, and the factors influencing differences in Cd sorption by sludge MPs were explored. The results revealed that Cd adsorption by MPs occurs as multilayer physical adsorption that can be well fitted by Freundlich isotherms. Compared with the other three treatments, TH led to the most significant effect on the chemical bonding properties of the MPs, with a more than two-fold increase in C-O single bonds and CO double bonds, as well as adsorption of the highest amount of Cd at 767 μg/g. In addition, sludge conductivity and water content also affected Cd sorption capacity, with correlation coefficients of 0.405 and -0.384. Pedobacter, Flavobacterium, Lysobacter, and Sphingobacterium in the sludge presented a high degree of coupling with adsorption capacity, it was inferred that the above dominant species of bacteria may affect the adsorption of Cd by microplastics through the production of extracellular enzyme forms.

A critical review on utilization of sewage sludge as environmental functional materials

Sewage sludge (SS) is increasingly used as an environment functional material to reduce or control pollution and improve plant growth because of the large amounts of carbon and essential plant nutrients in it. To achieve the best application results, it is essential to comprehensively review recent progress in SS utilization. This review aims to fill the gaps in knowledge by describing the properties of SS, and its usage as adsorbents, catalysts and fertilizers, and certain application mechanisms. Although SS generates several benefits for the environment and humans, many challenges still exist to limit the application, including the risks posed by potentially toxic substances (e.g., heavy metals) in SS. Therefore, future research directions are discussed and how to make SS applications more feasible in terms of technology and economy.

Study of Solidifying Surplus Sludge as Building Material Using Ordinary Portland Cement

In an attempt to effectively utilize a multitude of surplus sludge from sewage treatment plants, ordinary Portland cement was used to solidify the dry surplus sludge as a building material. The dry surplus sludge and cement were mixed at different proportions with a certain dosage of water and then cured for 3–60 days at room temperature. The unconfined compression strength (RC) of solidified blocks was investigated with respect to the effects of the ratio of liquid to solid (Rl/S), surplus sludge dosage (DS), the dosage of sodium silicate (DNa2SiO3), and the proportion of fly ash (WF). The fabricated solidified blocks were characterized by scanning electron microscopy (SEM), Fourier transform-infrared spectroscopy (FT-IR), and X-ray Diffraction Analysis (XRD). The results demonstrated that RC at 60 days reduced obviously with the increase in Rl/s when Ds was given, whereas RC reduced with DS increased to 15.0 wt% from 5.0 wt% for solidified blocks. When DS was 5.0 wt%, RC of 28 days was reduced from 20.87 MPa to 14.50 MPa, with an increase in Rl/s from 0.35 to 0.55. For the given Rl/s, such as Rl/s = 0.35, RC at 60 days was 23.75 MPa, 2.80 MPa, and 2.50 MPa when DS were 5.0 wt%, 10.0 wt%, and 15.0 wt%, respectively, which were relatively lower in comparison to that of Portland cement solidified blocks without surplus sludge (51.40 MPa). In addition, the addition of Na2SiO3 and fly ash was favorable in terms of improving the RC for solidified blocks. RC of 60 days increased initially and then reduced with the increase in DNa2SiO3 from 0.0 wt% to 9.0 wt% at Rl/s = 0.45 and DS = 5.0 wt%. At DNa2SiO3 = 7.5 wt%, Rl/s = 0.45, and DS = 5.0 wt%, the highest RC value of 34.70 MPa was achieved after being cured for 60 days. Furthermore, RC of 60 days increased initially and then reduced with WF increasing from 0.0 wt% to 25.0 wt%, and the highest RC value of 34.35 MPa was achieved at WF = 10.0 wt%, Rl/s = 0.45, and DS = 5.0 wt%. At the ratio of DNa2SiO3 = 7.50 wt%, Rl/S = 0.35, WF = 20 wt%, DS = 15.0 wt% and M = 1.00, RC of 28 days reached 26.70 MPa. With these values, the utilization of sludge utilized (DS = 15.0 wt%) was increased by double compared with DS = 5.0 wt% (20.87 MPa). To investigate the effect of environmental temperature on the mechanical properties and mass of solidified blocks, the freeze-thaw cycling experiment was carried out. The RC of 28 days and the mass of the solidified block reduced with the number of freeze-thaw cycles, increasing for solidified blocks with DS of 5.0 wt%, 10.0 wt%, and 15.0 wt%, manifesting a decrease of 25.60%, 32.30%, and 40.60% for RC and 3.40%, 4.10%, and 4.90% for mass, respectively. This work provides sufficient evidence that surplus sludge has a huge potential application for building materials from the perspective of improving their mechanical properties. It provides an important theoretical basis for the disposal as well as efficient utilization of sludge.

Effects of Co-Digestion Sludge Application on Soil Productivity

Anaerobic digestion and agricultural use of sewage sludge are effective methods to treat and dispose of sewage sludge, respectively. Then, the anaerobic digested sewage sludge is applied in agricultural land and the improvement of soil properties can be expected. In this study, with the purpose of evaluating the potential of co-digestion sludge for agricultural use, plot experiments with two vegetable species (radish and Chinese cabbage) and three application dosages were carried out in a short term of six months. Focus was on soil physical properties, soil nutrient change and plant growth responses during the whole process. Results showed that application of co-digestion sludge had little effect on soil physical properties, including the bulk density, porosity, capillary porosity and non-capillary porosity. However, after the application of co-digestion sludge, the maximum increase in content of organic matter, total nitrogen, hydrolysable nitrogen, total phosphorus and available phosphorus in soil reached 51%, 125%, 212%, 15% and 87%, respectively, which supplied the available nutrients quickly and continuously. The application of co-digestion sludge promoted the growth of radish and Chinese cabbage, which was observed through increase of the leaf, root biomass and plants height. Consequently, co-digestion sludge has a good application prospect for improving soil productivity as fertilizer.

Effects of lignocellulosic biomass type on nutrient recovery and heavy metal removal from digested sludge by hydrothermal treatment

Sludge is a nutrient-rich organic waste generated from wastewater treatment plants. However, the application of sludge as a nutrient source is limited by its high contents of water and pollutants. In this study, the effects of biomass type on nutrient recovery and heavy metal removal from digested sludge by hydrothermal treatment (HTT) were investigated. Blending biomass with digested sludge for HTT at 180-240 °C increased the recovery of nitrogen in the treated solids. At the HTT temperature of 240 °C, HTT with hardwood sawdust led to the highest nitrogen recovery of 70.6%, compared to the lowest nitrogen recovery of 36.5% without biomass. Blending biomass slightly decreased the recovery of phosphorus compared to those without biomass. Nevertheless, the lowest phosphorus recovery of 91.3% with the use of hardwood sawdust at the HTT temperature of 240 °C was only ∼7.0% less than that without biomass. Blending biomass reduced the contents of macro-metals such as Ca, Fe, Mg and Al in treated solids but the metal contents varied with different biomasses. Regarding the heavy metals, the use of rice husk did not decrease the contents of Ni and Co while blending bagasse did not decrease the content of Cr at HTT temperatures of 210 °C and 240 °C compared to the use of other biomasses. The different effects of biomass type on nutrient recovery and heavy metals were likely related to the types and abundances of organic acids such as acetic acid, oxygen-containing functional groups such as C-OH and COOH, oxide minerals such as silica from biomasses and the overall effects of these factors. This study provides very useful information in selection of lignocellulosic biomass for HTT of sludge for nutrient recovery and heavy metal removal.

Review on fate and bioavailability of heavy metals during anaerobic digestion and composting of animal manure

Anaerobic digestion and composting are attracting increasing attention due to the increased production of animal manure. It is essential to know about the fate and bioavailability of heavy metals (HMs) for further utilisation of animal manure. This review has systematically summarised the migration of HMs and the transformation of several typical HMs (Cu, Zn, Cd, As, and Pb) during anaerobic digestion and composting. The results showed that organic matter degradation increased the HMs content in biogas residue and compost (with the exception of As in compost). HMs migrated into biogas residue during anaerobic digestion through various mechanisms. Most of HMs in biogas residue and compost exceeded relevant standards. Then, anaerobic digestion increased the bioavailable fractions proportion in Zn and Cd, decreased the F4 proportion, and raised them more than moderate environmental risks. As (III) was the main species in the digester, which extremely increased As toxicity. The increase of F3 proportion in Cu and Pb was due to sulphide formation in biogas residue. Whereas, the high humus content in compost greatly increased the F3 proportion in Cu. The F1 proportion in Zn decreased, but the plant availability of Zn in compost did not reduce significantly. Cd and As mainly converted the bioavailable fractions into stable fractions during composting, but As (V) toxicity needs to be concerned. Moreover, additives are only suitable for animal manure treated with slightly HM contaminated. Therefore, it is necessary to combine more comprehensive methods to improve the manure treatment and make product utilisation safer.

Correlating black soldier fly larvae growths with soluble nutrients derived from thermally pre-treated waste activated sludge

Black soldier fly larvae (BSFL) have been deployed to valorize various organic wastes. Nonetheless, its growth rate whilst being offered with waste activated sludge (WAS) is not promising, likely by virtue of the presence of extracellular polymeric substances' structure in WAS. In this work, the WAS were first thermally pre-treated under different treatment temperatures and durations before being administered as the feeding substrates for BSFL. The results showed the thermal pre-treatment could improve WAS palatability and subsequently, enhance the growth of BSFL especially after the pre-treatments at 75 °C and above. The highest larva weight gained was recorded at 2.16 mg/larva for the WAS sample being pre-treated at 90 °C and 16 h. Furthermore, the samples pre-treated above 75 °C also achieved higher degradation rates, indicating that the 75 °C was a threshold temperature to effectively hydrolyze the WAS. The changes of WAS characteristics, namely, (i) soluble chemical oxygen demand (SCOD), (ii) soluble carbohydrate, (iii) soluble protein, (iv) humic substances and (v) total soluble protein and humic substances, after the thermal pre-treatments were also studied in correlating with the BSFL growth. Accordingly, a model was successfully developed with the highest R2 value attained at 0.95, evidencing the SCOD was the most suitable WAS characteristic to accurately predict the BSFL growth behavior.

Role of composted tannery solid waste and its autochthonous microbes in enhancing phytoextraction of toxic metals and stress abatement in sunflower

The excessive concentration of multiple heavy metals in the tannery solid waste (TSW) needs integrated process solutions for its decontamination. This study is aimed at deriving TSW compost and autochthonous microbe synergies for improving phytoextraction potential of sunflower. In-vessel composting of TSW was carried out by using fruit waste as an inoculum to achieve the optimized conditions. Autochthonous strains of Trichoderma viride and Bacilllus sp. isolated from TSW were utilized individually as well as in combination with TSWC amendments of 2.5, 5 and 10% (w/w) prepared in our pilot scale experiment. Analyses of TSW compost based on FTIR and SEM illustrated the wide range of functionality and porosity along the mesh of fungal hyphae and inorganic moieties present on the compost surface. Plant biomass and TMs uptake (Cr 540 mg kg^-1 > Cd 330 mg kg^-1 > Pb 285 mg kg^-1) were significantly pronounced in shoots of sunflower under combined treatments at 10% TSWC amended soils. However, in seeds, TMs were found below detection limit (BDL) through atomic absorption spectrophotometry. Biochemical assays of sunflower including total chlorophyll content (18%), total soluble protein (45%), superoxide dismutase (80%) and catalase (75%) activities were also increased significantly at higher level of amendment in combination with microbes than in the control. Despite being high in TMs, high biomass in sunflower and associated elevation in biochemical products demonstrate the potential of TSW for valorization.Novelty statement: This study identifies the cost-effective management of multi metal contaminated tannery solid waste through deriving its compost along with autochthonous microbes as phytoextraction assistants by yielding higher plant biomass. This study suggests the use of composted TSW inoculated with selected autochthonous fungi and bacteria for enhancing sunflower's biomass and enhancing the bioavailable fractions of toxic metals for phytoextraction.

C/N-Dependent Element Bioconversion Efficiency and Antimicrobial Protein Expression in Food Waste Treatment by Black Soldier Fly Larvae

The black soldier fly (BSF), Hermetia illucens, has emerged as a promising species for waste bioconversion and source of antimicrobial proteins (AMPs). However, there is a scarcity of research on the element transformation efficiency and molecular characterization of AMPs derived from waste management. Here, food waste treatment was performed using BSF larvae (BSFL) in a C/N ratio of 21:1−10:1, with a focus on the C/N-dependent element bioconversion, AMP antimicrobial activity, and transcriptome profiling. The C-larvae transformation rates were found to be similar among C/Ns (27.0−35.5%, p = 0.109), while the N-larvae rates were different (p = 0.001), with C/N 21:1−16:1 (63.5−75.0%) being higher than C/N 14:1−10:1 (35.0−45.7%). The C/N ratio did not alter the antimicrobial spectrum of AMPs, but did affect the activities, with C/N 21:1 being significantly lower than C/N 18:1−10:1. The lysozyme genes were found to be significantly more highly expressed than the cecropin, defensin, and attacin genes in the AMP gene family. Out of 51 lysozyme genes, C/N 18:1 and C/N 16:1 up-regulated (p < 0.05) 14 and 12 genes compared with C/N 21:1, respectively, corresponding to the higher activity of AMPs. Overall, the element bioconversion efficiency and AMP expression can be enhanced through C/N ratio manipulation, and the C/N-dependent transcriptome regulation is the driving force of the AMP difference.

Effects of three major nutrient contents, compost thickness and treatment time on larval weight, process performance and residue component in black soldier fly larvae (Hermetia illucens) composting

The cellulose content in vegetable waste (VW) is high and cannot be directly digested by black soldier fly larvae (BSFL). In this study, in order to treat VW using BSFL composting, kitchen waste (KW) is used as the only nutritional supplement for VW to analyze the effects of the different contents of crude protein (CP), crude fat (EE), carbohydrate (3C), compost thickness (CT), and treatment time on the larval weight (LW), survival rate (SR), dry matter reduction rate (DMR), bioconversion rate (BCR), physical and chemical properties of BSFL sand and changes in the microbial community. Our results showed that when the average 3C content increased by 40%, the average LW increased by 47.6%, and the SR, DMR, BCR, and organic matter (OM) content increased by 16.82%, 8.5%, 4.77%, and 3.86%, respectively. In contrast, when the average compost thickness increased by 5 cm, the average weight of BSFL decreased by 22.64%, while the SR of larvae, DMR, BCR, OM, and total nutrients (TN + P₂O₅ + K₂O) decreased by 5%, 5.2%, 4.42%, 9.6%, and 0.78%, respectively. Germination test showed that BSFL sand alone could not be used as soilless culture substrate. After BSFL treatment, we found that the dominant phyla in BSFL sand were Firmicutes (95.77%), Proteobacteria (2.54%), Actinobacteria (0.74%), and Chloroflexi (0.6%). Our results indicate that BSFL composting is an effective method of treating VW, and 3C content and CT have a significant effect on BSFL composting.

Analysis of organic matter conversion behavior and kinetics during thermal hydrolysis of sludge and its anaerobic digestion performance

In thermal hydrolysis (TH) of waste activated sludge (WAS), the material transformation of a specific temperature heating for a set duration is generally examined. However, this study looked at the material changes of TH as the temperature rose (90-210 °C) and the kinetic derivation of soluble chemical oxygen demand (SCOD), protein, and carbohydrate using the Coats-Redfern model. It was found that the proportion of soluble protein and soluble carbohydrate in the organic components and their contents reached the maximum (17.39 and 8.10 g L^-1 respectively) at 180 °C. Differently, volatile fatty acid (VFA), amino acids, and ammonia nitrogen increased with the TH temperature and reached a maximum at 210 °C. The fitting equations of non-isothermal dynamics at the medium- and low-temperature stages (90-180 °C) at n = 1, 0.5, and 2 were studied. When n = 1, the activation energies of COD, protein, and carbohydrate were 33.32, 23.34, and 36.15 kJ mol^-1, respectively. And the kinetic analysis results were in good agreement with the experimental results (the maximum rate of increase in protein and carbohydrate was at 135-150 °C and 150-180 °C, respectively). Moreover, the pattern of anaerobic digestion performance of WAS was comparable to the trend of protein and carbohydrate in TH, the highest cumulative methane production was 159.68 mL·g^-1VS for the TH sludge at 180 °C. This study provided a theoretical foundation for the use of thermal hydrolysis in engineering.

Treatment processes to eliminate potential environmental hazards and restore agronomic value of sewage sludge: A review

Land application of sewage sludge is increasingly used as an alternative to landfilling and incineration owing to a considerable content of carbon and essential plant nutrients in sewage sludge. However, the presence of chemical and biological contaminants in sewage sludge poses potential dangers; therefore, sewage sludge must be suitably treated before being applied to soils. The most common methods include anaerobic digestion, aerobic composting, lime stabilization, incineration, and pyrolysis. These methods aim at stabilizing sewage sludge, to eliminate its potential environmental pollution and restore its agronomic value. To achieve best results on land, a comprehensive understanding of the transformation of organic matter, nutrients, and contaminants during these sewage-sludge treatments is essential; however, this information is still lacking. This review aims to fill this knowledge gap by presenting various approaches to treat sewage sludge, transformation processes of some major nutrients and pollutants during treatment, and potential impacts on soils. Despite these treatments, overtime there are still some potential risks of land application of treated sewage sludge. Potentially toxic substances remain the main concern regarding the reuse of treated sewage sludge on land. Therefore, further treatment may be applied, and long-term field studies are warranted, to prevent possible adverse effects of treated sewage sludge on the ecosystem and human health and enable its land application.

Recent Progress and Prospects in Catalytic Water Treatment

Presently, conventional technologies in water treatment are not efficient enough to completely mineralize refractory water contaminants. In this context, the implementation of catalytic processes could be an alternative. Despite the advantages provided in terms of kinetics of transformation, selectivity, and energy saving, numerous attempts have not yet led to implementation at an industrial scale. This review examines investigations at different scales for which controversies and limitations must be solved to bridge the gap between fundamentals and practical developments. Particular attention has been paid to the development of solar-driven catalytic technologies and some other emerging processes, such as microwave assisted catalysis, plasma-catalytic processes, or biocatalytic remediation, taking into account their specific advantages and the drawbacks. Challenges for which a better understanding related to the complexity of the systems and the coexistence of various solid-liquid-gas interfaces have been identified.