Solid Anaerobic Digestion Research Articles

Enhanced anaerobic digestion of freezing and thawing pretreated cow manure with increasing solid content: kinetics and microbial community dynamics

High solid anaerobic digestion has proved the mainstream technology for the treatment of organic wastes. However, the high molecular weight and complex lignocellulosic structure of cow manure (CM) make it indigestible and inefficient, leading to limit the hydrolysis step of anaerobic digestion at high solid content. To mitigate this bottleneck, an improved cost-effective freezing and thawing pretreatment technique was proposed in this study. The freezing and thawing pretreatment of raw CM without any dilution was done for 20 days. The maximum cumulative methane yield (487 mL CH4 g− 1VS) was achieved at a total solid (TS) of 5% followed by TS of 10% and 15%, which was 13%, 20% and 21% higher than obtained from untreated CM, respectively. The kinetic results revealed that the biodegradable materials could be utilized at increasing TS with decreasing hydrolysis rate. The pretreatment significantly enhanced the methylotrophic methanogenic pathway during high solid anaerobic digestion, which was contrary to the general concept that the process is usually dominated by acetoclastic and hydrogenotrophic methanogens. This study is very important to understand the effect of solid content but also important to understand the effect of freezing and thawing pretreatment on process parameters and microbial community dynamics in high solid anaerobic digestion.

Characterization of municipal solid waste with the perspective of biofuels and bioproducts recovery in Northeast Mexico

This work focuses on analyzing the physical composition of municipal solid waste (MSW) and the physicochemical characterization of the organic fraction of municipal solid waste (OFMSW) in a city in Northeast Mexico to propose an adequate treatment and valorization system. Diverse samples were analyzed over 5 months at the city of Saltillo landfill, where the daily discarded MSW was classified into household (HW), central market waste (CMW), and public areas and parks waste (PPW). For the HW and CMW, the fraction with the highest proportion was the organic residues from food products, with 22.15 and 25.78%, followed by other organic wastes (manure, yard waste, leaves, etc.) at 12.58 and 10.24%, respectively. Furthermore, the organic fraction was segregated from the rest of the MSW and classified into four subtypes, and their physical composition and physicochemical characteristics were determined. The results contribute to laying the foundations for the proper treatment of the OFMSW not just in the studied region but also in cities with similar conditions. Moreover, the OFMSW’s feasibility in treating via bioenergy technologies is revealed, and this research proposes a biorefinery treatment pathway through dark fermentation followed by high solids anaerobic digestion generating bioenergy and diverse bioproducts.

Start-up of a full-scale two-stage partial nitritation/anammox (PN/A) process treating reject water from high solid anaerobic sludge digestion (HSAD)

High solid anaerobic digestion (HSAD) achieves the benefits of high volumetric loading rates and lower reject water production, which, however, results in much more concentrated reject water with a remarkable increase in organics and nitrogen compared with that from conventional AD with low solid content. The high concentrations of ammonium (2000–3500 mg/L) and COD (3000–4000 mg/L) were reported to exert inhibition on anammox bacteria (AnAOB), posing challenges to the application of the partial nitritation/anammox (PN/A). To date, no cases of PN/A process start-up for sludge HSAD reject water were reported. This study demonstrated the start-up process of a 480 m3/d PN/A project without anammox sludge inoculation and treating HSAD reject water from a centralized dewatered sludge treatment plant. The project did not construct new infrastructures but utilized previously constructed tanks to upgrade the process from existing short-cut nitrification-denitrification to a two-stage PN/A process. Although no external anammox sludge inoculation was performed to save seeding sludge cost, the start-up was successfully achieved in about 9 months (273 days) based on a three-step method of “AnAOB enrichment - sludge acclimation - capacity doubling”. During start-up, the relative abundance of AnAOB (Candidatus_Kuenenia) increased from near zero to 12.0%. After start-up, the total inorganic nitrogen (TIN) removal load reached 0.74 kgN/(m3•d), with a total nitrogen removal efficiency of over 90%. Compared to the traditional nitrification-denitrification process, the PN/A process remarkably reduces the addition of organic chemicals and aeration energy consumption, saving approximately 4.2 million yuan (RMB) in operational costs annually. In summary, this research provides a full-scale reference for the start-up of the PN/A process treating sludge HSAD reject water.

A novel strategy for enhancing high solid anaerobic digestion of fecal slag and food waste using percolate recirculation and dosage of nano zero-valent iron

To speed up reaching UN Sustainable Development Goal 6 for safe sanitation by 2030, integrating high-solid anaerobic digestion (HSAD) into decentralized systems could recycle fecal slag (FS) and food waste (FW), aiding a circular economy and toilet revolution. In this study, a percolate recirculation system and conductive material were used to improve mass transfer, stability, and enhance methane production in HSAD of FS and FW. This setup consists of a percolate tank and a digester tank, where nano-zero valent iron (nZVI) was dosed in the percolate tank (PnZVI in P) and the digester tank (PnZVI in D) and compared with a control with no additive (PControl). The highest cumulative methane yield of 519.43 mL/gVS was achieved in PnZVI in D, which was 4.52 and 3.59 times higher than that of PControl (144.59 mL/gVS) and PnZVI in P (114.96 mL/gVS). This finding demonstrates that the dosing strategy of PnZVI in D facilitated effective interaction among organic matter, microbial communities, and nZVI, resulting in organics removal efficiencies of 67.42 % (total solid) and 77.22 % (volatile solid). Moreover, microbial community analysis supported the efficacy of the PnZVI in D strategy, revealing the enrichment of Clostridium sensu stricto 1 (46.91 %), which potentially engaged in interspecies electron transport (Interspecies hydrogen transfer (IHT) and direct interspecies electron transfer (DIET)) with Methanobacterium (81.19 %) and Methanosarcina (17.11 %). These interactions contribute to enhanced methane yield and stability maintenance in the HSAD system with percolate recirculation. The findings of this study demonstrate that the implementation of HSAD of FS and FW, coupled with percolate recirculation and the addition of nZVI, holds promise for enabling sustainable sanitation practices in developing regions. Moreover, this approach not only facilitates resource recovery but also eliminates the requirement for water.

Influence of residual anaerobic granular sludge (AnGS) from anaerobically digested molasses wastewater in aerobic granular sludge reactor

This study investigated the impact of residual anaerobic granular sludge (AnGS) from anaerobic digesters treating molasses wastewater on ammonium reduction in a downstream aerobic granular sludge (AGS) reactor. Two conditions were tested: raw (high AnGS concentration) and settled (low AnGS concentration) anaerobically digested molasses wastewaters were fed into the AGS reactor. With the introduction of raw wastewater, enhanced nitrite accumulation at 30 % and improved total inorganic nitrogen (TIN) removal at 11 % were observed compared to 1 % nitrite accumulation and 8 % TIN removal with the introduction of settled wastewater. However, AnGS adversely affected other aspects of reactor performance, increasing effluent solid content and decreasing soluble chemical oxygen demand removal efficiency from 20 % in the low AnGS condition to 11 % in the high AnGS condition. Despite the observed retention of AnGS in the reactor, no significant bioaugmentation effects on the microbial community of the AGS were observed. Aerobic granular sludge was consistently observed in both conditions. The study suggests that AnGS may act as a nucleus for granule formation, helping to maintain granule stability in a disturbed environment. This study offers a systematic understanding of the impact of AnGS on subsequent nitrogen removal process using AGS, aiding in the decision making in the treatment of high solid anaerobic digestate.

Synergistic integration of hydrothermal pretreatment and co-digestion for enhanced biogas production from empty fruit bunches in high solids anaerobic digestion

This study investigates the co-digestion of hydrothermally pretreated empty fruit bunches (EFB) at 190 °C for 5 min (HTP190-EFB) with decanter cake (DC) to improve biogas production in high solid anaerobic digestion (HSAD). The HTP190-EFB exhibited a 67.98 % reduction in total solids, along with the production of 0.89 g/L of sugar, 2.39 g/L of VFA, and 0.56 g/L of furfural in the liquid fraction. Co-digestion of HTP190-EFB with DC at mixing ratios of 5, 10, and 15 %w/v demonstrated improved methane yields and process stability compared to mono-digestion of HTP190-EFB. The highest methane yield of 372.69 mL CH4/g-VS was achieved in the co-digestion with 5 %w/v DC, representing a 15 % increase compared to digestion of HTP190-EFB (324.30 mL CH4/g-VS) alone. Synergistic effects were quantified, with the highest synergistic methane yield of 77.65 mL CH4/g-VS observed in the co-digestion with 5 %w/v DC. Microbial community analysis revealed that co-digestion of hydrothermally pretreated EFB with decanter cake promoted the growth of Clostridium sp., Lactobacillus sp., Fibrobacter sp., Methanoculleus sp., and Methanosarcina sp., contributing to enhanced biogas production compared to mono-digestion of pretreated EFB. Energy balance analysis revealed that co-digestion of HTP190-EFB with DC resulted in a total net energy of 599.95 kW, 52 % higher than mono-digestion of HTP190-EFB (394.62 kW). Economic analysis showed a shorter return on investment for the co-digestion system (0.86 years) compared to the mono-digestion of HTP190-EFB (1.02 years) and raw EFB (2.69 years). The co-digestion of HTP190-EFB with 5 %w/v DC offers a promising approach to optimize methane yield, process stability, and economic feasibility, supporting the palm oil industry for producing renewable energy and sustainable waste management.

Comparative evaluation of high solids anaerobic digestion of rice husk and rice straw: Impact of substrate characteristics and solids concentration on biogas yield and digestate quality

Open burning of crop residues in agricultural fields causes soil infertility and contributes to global warming. Solid-state or dry anaerobic digestion (AD) is an emerging process for converting high solids (90 ̶ 95%) biomass such as rice husk (RH) and rice straw (RS) into energy and resources without the need for large volmes of water. This paper describes laboratory experiments investigating the dry AD of RH and RS across a range of total solids (TS) concentrations from 20 to 60%. Conducted in 500 mL bioreactors under batch mode at mesophilic temperatures, the experiments had a batch retention time of 45 days. An increase in the F/I ratio from 8.37 to 25.01 correlated with a rise in biogas generation from 2.9 to 4.6 L in RH reactors. The highest biogas yield from RS (0.49 L/g VS) and RH (0.17 L/g VS) was obtained at 20 and 50% TS, respectively. From the mass balance, 68.7 g of biomanure and 4.6 g of biogas were produced from the 50H reactor, while the 20S reactor yielded 20.9 g of biomanure and 8.4 g of biogas. From the modified Gompertz model a shorter lag phase of 1.9 days was observed in reactor 50H compared to the 4.6 days observed in reactor 20S. HIGHLIGHTS Dry AD of rice straw and husk at variable total solids (20–60% TS) Maximum biogas yield of 0.49 L/g VSreduced was observed at 20% TS from rice straw Maximum biogas yield of 0.17 L/g VSreduced was observed at 50% TS from rice husk Rice husk yielded concentrated digestate than rice straw with good nutrient quality Shorter lag phase was observed for 20S and 50H reactors

Enhancing Lettuce Yield through Innovative Foliar Spray of Biopolymers Derived from Municipal Biowastes.

Municipal waste biomass could be valorized as an alternative feedstock to produce compounds beneficial for agricultural applications. The foliar spray application of biostimulants emerges as a promising and innovative technique due to its environmental safety and ability to enhance crop yields. In recent years, the exploitation of biopolymers obtained through alkaline hydrolysis of the solid anaerobic digestate from municipal biowastes has attracted researchers' interest. The aim of this study is to investigate the effects on lettuce growth of a product obtained through alkaline hydrolysis from municipal biowaste, Biopolymers (BPs), and of a derivate subjected to a further oxidation process, Biopolymers Oxidate (BPs OX). The effects of the treatments at various concentrations were evaluated by monitoring plant growth and observing the trends in the activities of the main enzymes involved in the nitrogen metabolic pathway of lettuce. Results suggest that the best treatments in terms of fresh weight were achieved by using BPs at 10 mg/L and BPs OX at 100 mg/L, increasing yield by around 28% and 34%, respectively. The innovative aspect of this work was to make easier for farmers the biopolymers application by testing a foliar spray methodology for BPs and BPs OX, which has never been tested before in any crop.

Enhanced thermophilic high-solids anaerobic digestion of organic fraction of municipal solid waste with spatial separation from conductive materials in a single reactor volume

Despite benefits such as lower water and working volume requirements, thermophilic high solids anaerobic digestion (THSAD) often fails due to the rapid build-up of volatile fatty acids (VFAs) and the associated drop in pH. Use of conductive materials (CM) can promote THSAD through stimulation of direct interspecies electron transfer (DIET), while the need for their constant dosing due to poor separation from effluent impairs economic feasibility. This study used an approach of spatially separating magnetite and granular activated carbon (GAC) from the organic fraction of municipal solid waste (OFMSW) in a single reactor for THSAD. GAC and magnetite addition could both mitigate the severe inhibition of methanogenesis after VFAs build-up to ∼28–30 g/L, while negligible methane production was observed in the control group. The highest methane yield (286 mL CH4/g volatile solids (VS)) was achieved in magnetite-added reactors, while the highest maximum CH4 production rates (26.38 mL CH4/g VS/d) and lowest lag-phase (2.83 days) were obtained in GAC-added reactors. The enrichment of GAC and magnetite biofilms with various syntrophic and potentially electroactive microbial groups (Ruminiclostridium 1, Clostridia MBA03, Defluviitoga, Lentimicrobiaceae) in different relative abundances indicates the existence of specific preferences of these groups for the nature of CM. According to predicted basic metabolic functions, CM can enhance cellular processes and signals, lipid transport and metabolism, and methane metabolism, resulting in improved methane production. Rearrangement of metabolic pathways, formation of pili-like structures, enrichment of biofilms with electroactive groups and a significant improvement in THSAD performance was attributed to the enhancement of the DIET pathway. Promising results obtained in this work due to the spatial separation of the bulk OFMSW and CM can be useful for modeling larger-scale THSAD systems with better recovery of CM and cost-effectiveness.

Enhancing the efficiency of high solid anaerobic digestion of empty fruit bunches under thermophilic conditions by particle size reduction and co-digestion with palm oil mill effluent

This study investigates the impact of thermophilic high solid anaerobic digestion (HS-AD) on biogas production from empty fruit bunches (EFB), focusing on the effects of total solids (TS) loading (5–40 %), particle size reduction (0.5, 3.25, and 6 cm), and co-digestion with palm oil mill effluent (POME) (10–30 % VS basis). The HS-AD at a 15–20 % TS loading has a methane yield of 103.4–105.3 mL CH4/g-VS with 24.6–25.1 % biodegradability. Particle size reduction to 0.5 cm enhanced methane yield by 54–61 % and improved hydrolysis rates by 45 % compared to the untreated EFB (6 cm) at a 15–20 % TS loading. Co-digestion of EFB with POME at a ratio of 31:1 based on VS basis led to a synergistic effect of 17.77 mL CH4/g-VS, increasing methane yield by 24–46.5 % and improving process stability, as evidenced by a 22.8–38.1 % reduction in volatile fatty acids (VFAs) accumulation. Microbial community analysis showed a 2-fold increase in the relative abundance of hydrogenotrophic methanogens (Methanothermobacter sp. and Methanoculleus sp.) during co-digestion, while the abundance of key cellulolytic bacteria (Clostridium sp. and Fibrobacter sp.) increased by 1.5-fold. The optimized HS-AD process achieved a maximum methane yield of 287.77 mL CH4/g-VS and a biodegradability of 61.2 % under thermophilic conditions, with a 20 % POME co-digestion addition (31:1 VS ratio) and 0.5 cm particle size. These findings demonstrate the potential of thermophilic HS-AD for the sustainable management of EFB and highlight the importance of process optimization and co-digestion strategies for enhanced biogas production from EFB.

Screening transparent model fluids to emulate the rheological properties of high solid anaerobic digestate of food waste

Due to the opacity of high solid anaerobic digestate, optical visualization techniques (e.g., particle image velocimetry) cannot be used directly for measuring the flow field inside high solid anaerobic digestion (HSAD) systems. This study mainly focused on selecting suitable surrogate fluids for optical visualization techniques by comparing the flow curves, thixotropic properties, viscoelastic behavior and creep recovery properties of HSAD digestate with a total solid from 10 % to 15 % (w/w) and transparent model fluids. Results indicated that the Bingham model outperformed the power-law model and Herschel-Bulkley model in describing the flow curves of HSAD digestate, xanthan gum and laponite. The digestate and xanthan gum had a similar thixotropic kinetic coefficient of 0.003 and 0.004, respectively. Besides, the digestate and xanthan gum had a similar modulus transformation trend with a power function whose power exponents were 0.17 and 0.19, respectively. In general, xanthan gum can be a suitable substitute for HSAD digestate with a total solid from 10 % to 15 %. The results can guide the implementation of optical visualization techniques to understand the flow pattern within HSAD systems, and improve the mass and heat transfer efficiency during food waste disposal processes.

Design and optimization of a novel intestinal reactor to efficiently improve organic waste uniformity and solid content in high solid organic waste anaerobic digestion

High solid anaerobic digestion (HSAD) technology, a major treatment method for solid organic waste. Existing HSAD reactors comprise primarily vertical and horizontal reactors. Vertical reactors use high-pressure pneumatic stirring, which has high energy consumption and poor stirring performance. Horizontal reactors use long-uniaxial mechanical stirring, which occupies large land areas and has high strength requirements for stirring shaft material. In addition, during the operation of the two reactor types, the continuous liquefaction of the solid waste produces solid-liquid stratification, thus affecting mixing effects and decreasing the reactor volume rate. To solve the above problems, we designed a new simulated intestinal anaerobic digestion reactor with a natural solid-liquid separation system, short-axis mechanical stirring, and a stirring blade optimized through CFD. Use of the solid-liquid separation system increased the solid content in the reactor by 44%–55%. The operation results indicated that the homogenization of the solid waste with the optimal stirring blade was more than 50% greater than that of a conventional vertical reactor. The intestinal reactor solves the problems of poor uniformity and solid content decrease during anaerobic digestion in traditional HSAD, decreases the strength requirements for stirring shaft material, and has potential to increase urban waste treatment capacity.

Machine learning for high solid anaerobic digestion: Performance prediction and optimization

Biogas production through anaerobic digestion (AD) is one of the complex non-linear biological processes, wherein understanding its dynamics plays a crucial role towards process control and optimization. In this work, a machine learning based biogas predictive model was developed for high solid systems using algorithms, including SVM, ET, DT, GPR, and KNN and two different datasets (Dataset-1:10, Dataset-2:5 inputs). Support Vector Machine had the highest accuracy (R2) of all the algorithms at 91 % (Dataset-1) and 87 % (Dataset-2), respectively. The statistical analysis showed that there was no significant difference (p = 0.377) across the datasets, wherein with less inputs, accurate results could be predicted. In case of biogas yield, the critical factors which affect the model predictions include loading rate and retention time. The developed high solid machine learning model shows the possibility of integrating Artificial Intelligence to optimize and control AD process, thus contributing to a generic model for enhancing the overall performance of the biogas plant.

Nutrient utilization and growth of tomato crops fertilized with solid anaerobic digestate

Anaerobic digestate is a valuable resource for horticultural production, as it contains nutrients and fibers that can be used in plant growing medium. However, compared with hydroponic production based on mineral fertilizers, obtaining accurate nutrient supply at each stage of the growth cycle may be challenging. In an experiment using container-grown tomato crops, we evaluated different fertilization regimes with solid anaerobic digestate (SAD). Four different treatments were compared, two involving different proportions (19 % and 37 % by volume) of SADs in the peat-based growing medium, one treatment where growing medium with 37 % SAD was inoculated with active nitrifying bacteria, and one treatment where 15 % (v/v) of the peat in growing medium with 37 % SAD was replaced with sawdust to control nitrogen (N) availability during cultivation. A mineral-fertilized treatment (N-P-K 5–1-5) with approximately similar N amount as in the treatment with 37 % SAD was used as reference. Nutrient availability, nutrient uptake efficiency, crop performance (plant growth, biomass accumulation), and plant stress (chlorophyll fluorescence) were monitored during cultivation. The concentration of ammonium was initially high (190–416 mg/L substrate) in the growing media fertilized with anaerobic digestate, while the concentration of nitrate was low. Readily available ammonium concentration decreased rapidly during cultivation, to around 50 % after 10 days and to almost 100 % by the end of the cultivation. Available nitrate concentration was initially low (0–8 mg/L in the different treatments) and decreased to zero within a week, but increased slightly from day 40 of cultivation. Nutrient use efficiency was generally higher (15–50 % for different nutrients) in the treatment with 19 % digestate. Inclusion of sawdust in the growing medium decreased nutrient use efficiency by 30–50 %. Compared with the mineral-fertilizer reference, biomass production was lower in all treatments fertilized with digestate, with 37 % and 19 % SAD resulting in 62 % and 47 % of total biomass obtained in the reference, and similar reductions in yield of harvestable fruits. Chlorophyll fluorescence measurements indicated elevated plant stress in the treatments fertilized with SAD. Addition of sawdust or nitrifying bacteria did not help to control nitrogen availability during cultivation. Therefore, anaerobic digestate fertilizers need further optimization before they can be a competitive alternative to mineral fertilizers.

Enhancing High Solid Anaerobic Digestion of Kitchen Waste with Red Mud Addition: Performance and Microbial Community

Enhancing High Solid Anaerobic Digestion of Kitchen Waste with Red Mud Addition: Performance and Microbial Community

A model-based approach for evaluating the effects of sludge rheology on methane production during high solid anaerobic digestion: Focusing on mass transfer resistance

Mass transfer process is vital for biochemical reactions, which is related to sludge rheology. But this inherent interaction has not yet been established for sewage sludge anaerobic digestion (AD). This work investigated the role of sludge rheology in bio-kinetics of AD. AD of sodium acetate and microcrystalline cellulose under different total solids (TS of 6.29%, 7.63%, 9.02%, 11.06%) of digested sludge was set up to identify the role of sludge rheology. Results showed that rheological properties, e.g., viscosity and viscoelasticity, increased with the increase of TS content. These resulted in negative effect on methane production of sodium acetate with lag phase increased from 0.93 d to 1.57 d, but almost no effect was observed for microcrystalline cellulose. An extended ADM1 model was developed, which revealed that enhanced sludge rheological properties increased mass diffusion resistance and reduced uptake rate of acetate. This effect can be described explicitly by mass transfer coefficient (kla_Sac). However, from the original ADM1, this effect was indistinctly lumped in apparent half-saturation coefficient (ksac). Based on the role of sludge rheology in mass transfer and bio-kinetics, it is suggested that modeling the methane production based on TS contents and sludge rheology is possible, which should be considered in giving an insight into the micro-scale phenomena (e.g. mass transport, bio-kinetics) in high solid AD.

Composite nanofiber membranes to enhance the performance of high solids anaerobic digestion of organic rural household waste resources

This study investigated the effect of using nanofiber membrane composites containing Prussian blue-like compound nanoparticles (PNPs) with a stainless steel mesh as the carrier (NMCs) to relieve ammonia nitrogen inhibition of rural organic household waste during high-solid (total solid content of 15 %) anaerobic digestion (AD) and increase methane production. NMCs with different PNP contents were added to high-solid AD. When NMC with 15 % PNPs was added, the concentrations of volatile fatty acids and ammonia nitrogen were lower than those of the blank control group, and the methane yield was the highest (301.31 mL/g volatile solids [VS]), at 2.8 times higher than that of the blank control group. The R2 value of the modified Gompertz equation was 0.997. During digestion, the activities of extracellular protease and coenzyme F420 were higher than those of the control group. NMCs were successfully recovered at the end of the experiment, and confocal scanning three-dimensional electron micrographs indicated that NMC with 15 % PNPs of NMCs had stronger biological activity; microbial community structure analysis indicated increased abundance of hydrogenotrophic methanogens in this group. The study provides a new solution for improving the efficiency of biogas production and the recycling of exogenously added materials.

Advanced steam explosion as thermal hydrolysis process for high solids anaerobic digestion and enhanced methane yield: Proof of concept

Sewage sludge management is one of the major issues faced by wastewater treatment plant (WWTP) operators. Though anaerobic digestion (AD) is one of the most preferred sludge management techniques, the complex microbial floc structure of the sludge creates physical and chemical barriers that limit the rate of microbial degradation resulting in a longer hydraulic retention time (HRT) of 25–30 days. The thermal hydrolysis process (THP) can be used as a pretreatment technique to enhance the rate of biodegradation which results in reduced retention time and increased methane yields. This work investigated the effects of the THP on the semi-continuous anaerobic digestion of high solids sludge (15% total solids, TS) obtained from a municipal wastewater treatment plant (WWTP) operating at higher solids retention time (SRT) of 40 days. The thermal hydrolysis process achieved the effective release of organics into the liquid phase resulting in 37% chemical oxygen demand (COD) solubilization. Two semi-continuous digesters, one fed with untreated sludge (control digester, 5% TS) to mimic the conventional anaerobic digester and the other fed with THP sludge (160 °C, 30 min, 6 bar, 10% TS) were operated under a mesophilic regime and a hydraulic retention time (HRT) of 15 days. The methane yield from the THP digester (0.366 ± 0.010 m3/kg VSadded, 70 ± 1.13% CH4) was 47% higher than the control digester (0.248 ± 0.013 m3/kg VSadded). The volatile solids removal (VSR) was 51 ± 0.6% in the THP digester, i.e., 43% higher over control digester (35.7 ± 1.3%). The fecal coliforms concentration in the digestate of control and THP digesters were 9200 and 200 MPN/g, respectively. Thus, the THP sludge meets the Class A biosolids criteria (<1000MPN/g). THP integrated anaerobic digestion achieved higher solids reduction with enhanced energy recovery, high solids loading with shorter retention time, and generated Class A biosolids.

Effect of municipal biowaste derived biostimulant on nitrogen fate in the plant-soil system during lettuce cultivation

A main concern of agriculture is to improve plant nutrient efficiency to enhance crop yield and quality, and at the same time to decrease the environmental impact caused by the lixiviation of excess N fertilizer application. The aim of this study was to evaluate the potential use of biopolymers (BPs), obtained by alkaline hydrolysis of the solid anaerobic digestate of municipal biowastes, in order to face up these main concerns of agriculture. The experimental trials involved the application of BPs (at 50 and 150 kg/ha) alone or mixed with different amounts (100%, 60% and 0%) of mineral fertilizer (MF). Three different controls were routinely included in the experimental trials (MF 100%, 60% and 0%). The effect of BPs on lettuce was evaluated by monitoring growth parameters (fresh and dry weights of shoot and root, nitrogen use efficiency), and the N-flux in plant-soil system, taking into account the nitrate leached due to over irrigation events. The activities of enzymes involved in the nitrogen uptake (nitrate reductase, glutamate synthase and glutamine synthase), and the nitrogen form accumulated in the plant tissues (total N, protein and NO3−) were evaluated. The results show that the application to the soil of 150 kg/ha BPs allows to increase lettuce growth and nitrogen use efficiency, trough stimulation of N-metabolism and accumulation of proteins, and hence to reduce the use of MF by 40%, thus decreasing the nitrate leaching. These findings suggest that the use of BPs as biostimulant greatly contributes to reduce the consumption of mineral fertilizers, and to mitigate the environmental impact caused by nutrients leaching, according to European common agricultural policy, that encourages R&D of new bioproducts for sustainable eco-friendly agriculture.

Enhancement of System and Environmental Performance of High Solids Anaerobic Digestion of Lignocellulosic Banana Waste by Biochar Addition

Banana waste, a lignocellulosic waste material, is generated in large quantities around the world. High Solids Anaerobic Digestion (HS-AD) of lignocellulosic waste can recover energy and reduce its environmental impacts. However, high carbon/nitrogen ratios and low water content in HS-AD can potentially cause system acidification and/or failure. This study investigated the addition of biochar to enhance the performance of HS-AD of mixed banana waste (peel, stem, and leaf). Biochemical methane potential assays with varying biochar dosages (2.5–30%) showed that 10% biochar addition increased methane yields by 7% compared with unamended controls. Semi-continuous HS-AD studies, without and with 10% biochar addition, were conducted at varying solids retention times (42, 35, and 28 days) for long-term performance evaluation. Biochar addition reduced volatile fatty acid accumulation, improved system stability, and increased methane production by 20–47%. The nutrient content of digestate from HS-AD of banana waste indicated its potential use as a bio-fertilizer. Life cycle assessment results showed that biochar addition to HS-AD resulted in greater environmental benefits in most categories compared with the unamended control, including eutrophication, ecotoxicity, and fossil fuel depletion when biochar was available within a radius of 8830 km.