Search Results (82)

Anaerobic digestion (AD) is a sustainable and widely adopted technology for the treatment of organic solid wastes (OSWs). However, AD efficiency varies significantly across different substrates, primarily due to differences in the microbial community and metabolic pathways. This review provides a comprehensive summary of the AD processes for four types of typical OSWs (i.e., sewage sludge, food waste, livestock manure, and straw), with an emphasis on their universal characteristics across global contexts, focusing mainly on the electron transfer mechanisms, essential microbial communities, and key metabolic pathways. Special attention was given to the mechanisms by which substrate-specific structural differences influence anaerobic digestion efficiency, with a focused analysis and discussion on how different components affect microbial communities and metabolic pathways. This study concluded that the hydrogenotrophic methanogenesis pathway, TCA cycle, and the Wood–Ljungdahl pathway serve as critical breakthrough points for enhancing methane production potential. This research not only provides a theoretical foundation for optimizing AD efficiency, but also offers crucial scientific insights for resource recovery and energy utilization of OSWs, making significant contributions to advancing sustainable waste management practices. Full article

The increasing production of bioplastics worldwide requires sustainable end-of-life solutions to minimize the environmental burden. Anaerobic digestion (AD) has been recognized as a potential technology for valorizing waste and producing renewable energy. However, the inherent resistance of certain bioplastics to degradation under anaerobic conditions requires specific strategies for improvement. Thus, in this review, the anaerobic biodegradability of commonly used bioplastics such as polylactic acid (PLA), polyhydroxybutyrate (PHB), polybutylene adipate-co-terephthalate (PBAT), polybutylene succinate (PBS), polycaprolactone (PCL), and starch- and cellulose-based bioplastics are critically evaluated for various operational parameters, including the temperature, particle size, inoculum-to-substrate ratio (ISR) and polymer type. Special attention is given to process optimization strategies, including pretreatment techniques (mechanical, thermal, hydrothermal, chemical and enzymatic) and co-digestion with nutrient-rich organic substrates, such as food waste and sewage sludge. The combinations of these strategies used for improving hydrolysis kinetics, increasing the methane yield and stabilizing reactor performance are described. In addition, new technologies, such as hydrothermal pretreatment and microbial electrolysis cell-assisted AD, are also considered as prospective strategies for reducing the recalcitrant nature of some bioplastics. While various strategies have enhanced anaerobic degradability, a consistent performance across bioplastic types and operational settings remains a challenge. By integrating key recent findings and limitations alongside pretreatment and co-digestion strategies, this review offers new insights to facilitate the circular use of bioplastics in solid waste management systems. Full article

The conversion of food waste into caproate via anaerobic chain elongation has gained increasing attention. However, limitations such as reliance on external electron donors, low carbon conversion efficiency under high loads, and unclear microbial mechanisms hinder its application. Fe₃O₄ reportedly can act as an electron shuttle and mitigate product inhibition during anaerobic digestion of sludge. Thus, Fe₃O₄ addition could overcome the challenges from high loads under certain conditions. In this study, the experiments were conducted under batch and semi-continuous conditions. This study investigated the effects of organic loads on the hydrolysis, acidification, and chain elongation of fermentation. Furthermore, the influences of Fe₃O₄ on caproate production and microbial profile under varying substrate-to-inoculation ratios and dosages were examined. The key results harvested from the semi-continuous trial indicate that high organic loads severely inhibited caproate production. And in batch tests, at an F/M ratio of 1:2, increasing Fe₃O₄ dosage evidently enhanced caproate production by promoting lactate conversion to butyrate and carbon chain elongation. At an F/M ratio of 6:1, maximum caproate yield reached 0.45 g COD/g COD at Fe₃O₄ of 2.0 g/L. High organic load reduced the abundance of butyrate-producing bacteria (Latilactobacillus and Stenotrophomonas). Nevertheless, the addition of Fe₃O₄ increased the abundance of butyrate-producing and caproate-producing bacteria (Caproiciproducens). In conclusion, Fe₃O₄ at an optimal dosage evidently enhanced caproate production under high organic loads by stimulating microbial electron transport and enriching relevant microorganisms. Full article

In the practice of wastewater treatment system design, process selection is often constrained by factors such as operational costs, performance, and physical footprint. Processes employing colloidal gaseous aphron (CGA) technology have proven to be highly effective for the clarification and thickening of activated sludge mixed liquor, waste-activated sludge (WAS), and anaerobically digested sludge within a small footprint. Technically, an aphron is defined as a gas or liquid phase encapsulated by a surfactant film. Since their initial identification and formulation, aphrons have been used extensively in a number of chemical processes, including gas and oil drilling and food processing waste-treatment applications. The generation and use of CGAs for thickening of WAS and other wastewater applications has been commercialized with the development of the Suspended Air^® flotation (SAF^®) process. The purpose of this paper is to (1) provide background on CGAs flotation technology, (2) identify applications of CGA in wastewater treatment, (3) discuss thickening of WAS with CGA, and (4) present findings from three case studies where WAS thickening with a legacy dissolved air flotation (DAF) process was replaced with a CGA process to increase capacity or address operational challenges, typically within the same flotation tank footprint. The case studies demonstrate the versatility of the SAF^® CGA process for (a) its ability to process the most challenging feedstock, including stored WAS; (b) enhancement of the digestion process and elimination of digester foaming; and (c) high capacity and ease of operation, reducing operation needs. Full article

The promulgation of the Biogas Act in South Korea has increased the number of organic waste treatment facilities and the amount of food waste digestion sludge (FWDS), a byproduct of the biogas process. FWDS recovery involves various challenges, which leads to the accumulation or improper disposal of sludge. Hence, FWDS needs to be treated in environmentally sound and safe ways. In this study, anaerobic digestion sludges were mixed with unused forest biomass to produce fuel. The results showed that pellets produced via mixing of FWDS with unused forest biomass had improved durability, bulk density, and fine particle performance compared to surface-carbonized wood pellets. Carbonized pellets manufactured with 30% FWDS had a moisture content of 11.746% and met all biosolid waste fuel (SRF) standards, except for moisture content. Carbonized pellets prepared with 15% FWDS met the L2 wood pellet standards for ash content (less than 3.0%) and bulk density (greater than 550 kg/m³), as well as all other standard values in both the industrial wood pellet quality standards and bio-SRF criteria. This study confirmed the potential and suitability of digestion sludge and unused forest biomass for fuel utilization by addressing their respective limitations. Full article

The application of municipal sewage sludge is often limited by concerns over heavy metal (HM) safety. This study explored the reduction of HM content in hydrothermal sewage sludge (HTS) through co-pyrolysis with food waste digestate (FD), aiming to lower ecological risks in the produced biochar. Results indicated that FD addition effectively lowered HM concentrations in biochar, mainly via dilution effect. Moreover, increased pyrolysis temperatures and FD addition promoted the stabilization of Cr, Ni, Cu, Zn, As, Cd, and Pb. Notably, a 50% FD mix significantly increased the proportion of HMs in the residual fraction of Ni (75.66%), Cu (71.66%), Zn (98.13%), and Cd (58.14%) compared to solo pyrolysis at 700 °C. Consequently, the potential ecological risk index significantly dropped from 47.86 to 26.29. Biochar created under optimal conditions (700 °C with a 50% FD ratio) showcased improved application prospects due to reduced bioavailability, thus diminishing HM-related ecological dangers. Full article

Organic waste has the potential to produce methane gas as a substitute for petrol-based fuels, while reducing landfilling and possible environmental pollution. Generally, anaerobic digestion (AD) is used only in wastewater treatment plants as a tertiary stage of sewage sludge treatment, generating a fraction of the energy that such process plants require. In this study, four different wastes—food waste (FW), dairy industry waste (DIW), brewery waste (BW), and cardboard waste (CBW)—were tested for biogas production. The biochemical methane potential (BMP) of each sample was evaluated using an automatic methane potential system (AMPTS). Operating parameters such as pH, temperature, total solids, and volatile solids were measured. Experiments on the anaerobic digestion of the samples were monitored under mesophilic conditions (temperature 37 °C, retention time 30 days). Specific methane yields (SMYs), as well as the theoretical methane potential (BMPth), were used to calculate the biodegradability of the substrates, obtaining the highest biodegradability for BW at 95.1% and producing 462.3 ± 1.25 NmL CH₄/g volatile solids (VS), followed by FW at an inoculum-to-substrate ratio (ISR) of 2 at 84% generating 391.3 NmLCH₄/g VS. The BMP test of the dairy industry waste at an inoculum-to-substrate ratio of 1 was heavily inhibited by bacteria overloading of the easily degradable organic matter, obtaining a total methane production of 106.3 NmL CH₄/g VS and a biodegradability index of 24.8%. The kinetic modeling study demonstrated that the best-fitting model was the modified Gompertz model, presenting the highest coefficient of determination (R²) values, the lowest root means square error (RMSE) values for five of the substrates, and the best specific biogas yield estimation with a percentage difference ranging from 0.3 to 3.6%. Full article

Anaerobic digestion (AD), or biogas technology, is an optimal method for municipal organic waste (MOW) treatment, recovering both material and energy. This study takes a life cycle assessment perspective and examines the economic and environmental impacts of a BIO facility in Minamisanriku Town, Japan, which has utilized MOW (kitchen/food waste and surplus sludge from sewage) as local biomass resources since 2012. Stakeholder interviews were conducted to gather data on material flows and impacts. Scenario analysis considered various conditions, such as pre- and post-operation of the BIO facility, the use and non-use of digestate as liquid fertilizer, and the facility’s 100% operational efficiency. The results indicate that full operation of the BIO facility and marketing of value-added products, such as branded rice grown using liquid fertilizer, could significantly reduce greenhouse gas (GHG) emissions, lower integrated environmental costs, improve the regional economy, and increase net income. In the business as usual (BAU) scenario with a 56% operation rate of the BIO facility, there is an over 10% improvement in economic and environmental impacts compared to the pre-operation baseline. This study underscores the importance of maximizing biomass utilization to develop value-added uses by enhancing, extending, and expending stakeholder collaboration. Full article

Reducing waste production and improving waste treatment are key objectives in the EU’s Circular Economy Action Plan. Anaerobic digestion of food waste is a promising method, but safely disposing of its by-products, which contain valuable nutrients like nitrogen, phosphorus, and organic matter, remains a challenge. These nutrients suggest potential use in agriculture to enhance sustainability, yet their effects on plant growth need thorough understanding. This study investigated the impact of liquid digestates from anaerobic digestion of food waste, combined with fish sludge (from recirculated aquaculture systems), on plant growth (Lepidium sativum and Triticum aestivum) through direct soil tests. The content of biogenic elements in the digestates did not differ and was 0.32% for N, <0.05% for P, and 0.15% for K. Two tests were conducted, both using artificial soil prepared to OECD 207 standards: the Phytotoxkit™ test (ISO 18763:2016) and a pot experiment (OECD 208). Results showed that digestates initially delayed germination and hindered early plant growth, an effect that diminished over time. This may be due to the high organic matter content of digestates, similar to standard fertilizers like manure or compost. Pre-incubating digestates in soil before application, similar to common agricultural practices with soil amendments, is suggested as a potential solution. Full article

To face the ongoing issues related to global warming, a circular economy approach should be pursued, rethinking the waste management system and the recovery of organic waste. The main organic waste streams are Food Waste (FW) and municipal Sewage Sludge (SS). In the spirit of circularity, a commingled treatment of FW and SS could be a viable solution. To this end, the present work aims to review the technical and environmental aspects of the co-treatment of FW and SS through biological and thermal processes. Firstly, a detailed characterization of the two substrates is presented as well as the current and future treatment technologies. Then, the technical feasibility and the environmental impacts of conventional biological co-treatments of FW and SS (i.e., composting, anaerobic digestion, and a combination of them), as well as innovative thermal ones (i.e., incineration, gasification, pyrolysis, and hydrothermal carbonization), is summarized. The outcomes of this work could contribute to achieving a more sustainable way to approach organic waste treatment and to help policy-making authorities move toward sustainable planning. Full article

Struvite (MgNH₄PO₄·6H₂O) is a precipitation product that can be obtained in municipal wastewater treatment plants (WWTPs) and represents a promising fertilizer and technical solution for phosphorus recovery. Struvite can be recovered from the wastewater stream, during sludge digestion or by accelerated acid leaching from sludge followed by solid–liquid separation and struvite precipitation from the liquid phase. Moreover, struvite can be precipitated from industrial effluents or agricultural wastes. The resulting products are of different purity. Antibiotic residues are a relevant class of contaminants as already traces can induce or promote the development of antibiotic resistance in the environment. The aim of the current study was a screening of struvite raw materials precipitated by different processes in German WWTPs for their contamination by selected antibiotics out of the classes of sulfonamides (SAs), fluoroquinolones (FQs) and tetracyclines (TCs). Slightly higher antibiotic residues were detected when struvite was precipitated from the solid phase with a maximum of 133 µg TCs, 484 µg FQs and 8 µg SAs compared to 8 µg TCs, 86 µg FQs and 9 µg SAs per kg dry weight (DW) when struvite was obtained from the liquid phase. FQs were most frequently found in low but quantifiable concentrations in almost all struvite raw materials. Yet, the contamination level of struvite from WWTPs can generally be regarded as low compared to sewage sludge. Products received from effluents from the food industry were found to be almost free of antibiotic residues. Full article

Hermetia illucens larvae can use organic wastes as a substrate, which makes them an interesting potential feed. However, waste may contain heavy metals, which are limited in feed. Here, we investigated the ability of H. illucens to grow on organic wastes and measured their heavy metal bioaccumulation. The larvae were fed with food waste, biogas digestates, and sewage sludge. When the first adult fly was visible, the tests were stopped and the larvae immediately processed. The samples (wastes before use, larvae after feeding) were analysed for mineral nutrient and heavy metal content using AAS and ICP-OES, respectively. The results show that the weight of the larvae fed with food waste increased sevenfold, which was broadly in line with expectations. Those fed with sewage sludge and digestate from biogas station increased threefold. While the larvae fed with sewage sludge exceeded the limits for heavy metals, particularly Cd and Pb, in feedstock, those fed with biogas digestate and food waste did not. These findings add to the literature showing the suitability of different wastes as H. illucens feed, and the importance of excluding waste contaminated with heavy metals from larvae intended for use as animal feed, or else diverting these larvae to non-feed uses. Full article

Acidogenic fermentation is an emerging biotechnology that allows for the utilization of food waste as a feedstock to produce high-value products such as short-chain fatty acids (SCFAs), effectively offering a tangible solution for food waste management as well as resource recovery. The objectives of the current study were to identify the ideal inoculum, waste-activated sludge (WAS) or anaerobic digester sludge (AD), for the acidogenic fermentation of food waste at room temperature, as well as to evaluate the impact of heat pretreatment of these inoculums on fermentation performance. The maximum hydrolysis yield of 399 g sCOD/kg VS _added was obtained when untreated AD was used as the inoculum, whereas the pretreated AD inoculum provided the highest SCFA yield and conversion efficiency of 238 g sCOD_SCFA/kg VS _added and 71%, respectively. Heat pretreatment had a detrimental impact on the WAS inoculum, leading to lower hydrolysis and SCFA yields, but exerted a positive influence on the AD inoculum. The microbial community showed that heat pretreatment negatively impacted the abundance of non-spore-forming hydrolytic and acidogenic microorganisms. Overall, this study demonstrates the critical role of inoculum type and heat pretreatment in optimizing the acidogenic fermentation process, laying the groundwork for future refinements in SCFA production from food waste through inoculum design. Full article

As methane fermentation is inhibited by ammonia derived from organic waste, anaerobic microbial communities tolerant to enriched wastewater with high concentrations of ammonia and salt must be obtained for methane fermentation. Therefore, acclimation cultures were prepared in bottles for 60–80 weeks with artificial wastewater medium added every 2 weeks, using three types of sludge from wastewater treatment plants in food factories. These cultures were maintained without substantially decreasing methanogenesis and gradually increasing NH₄-N and salt concentrations to 5 and 34 g/L, respectively, via the accumulation of ammonia and salt through anaerobic digestion and direct addition. The culture did not show the severe inhibition of methanogenesis or the accumulation of volatile fatty acids (VFAs) such as acetic and propionic acids. The analysis of bacterial consortia in the acclimated sludge based on the 16S rRNA sequence showed that hydrogenotrophic methanogenic bacteria of the genus Methanoculleus were dominant among archaea, whereas bacteria from the orders Clostridiales and Bacteroidales were dominant among eubacteria. Further, VFA-assimilating bacteria, including synthetic acetate-oxidizing bacteria coupled with hydrogenotrophic Methanoculleus to convert methane from acetate, were present to prevent the excessive accumulation of VFAs in the acclimation culture. The proposed acclimation process can enhance the anaerobic digestion of wastewater for methane production. Full article

Traditionally, anaerobic digestion has been applied to mixed sludge, combining primary sludge (PS) with secondary sludge. However, recent research has unveiled the advantages of dedicated PS digestion due to its higher energy content. Anaerobic digestion (AD) of primary sewage sludge can offer a sustainable solution for managing sewage sludge while generating renewable energy. The present study provides a comprehensive examination of the current state of knowledge regarding the anaerobic digestion of PS. Co-digestion of PS with organic substrates, including food waste and agro-industrial residues, emerges as a promising approach to boost biogas production. Additionally, the utilization of additives such as glucose and clay minerals has shown potential in improving methane yield. Critical factors affecting AD, such as pretreatment methods, carbon-to-nitrogen (C/N) ratio, temperature, pH, volatile fatty acids (VFAs) levels, organic loading rates (OLR), inoculum-to-substrate ratio (ISR), and the role of additives, have been meticulously studied. Finally, this review consolidates existing knowledge to advance our understanding of primary sewage sludge anaerobic digestion, fostering more efficient and sustainable practices in sludge management and renewable energy generation. Full article