Use of digestate from a decentralized on-farm biogas plant as fertilizer in soils: An ecotoxicological study for future indicators in risk and life cycle assessment

Unlocking integrated waste biorefinery approach by predicting calorific value of waste biomass

The current study analyzed the high heating values (HHVs) of various waste biomass materials intending to the effective management and more sustainable consumption of waste as clean energy source. Various biomass waste samples including date leaves, date branches, coconut leaves, grass, cooked macaroni, salad, fruit and vegetable peels, vegetable scraps, cooked food waste, paper waste, tea waste, and cardboard were characterized for proximate analysis. The results revealed that all the waste biomass were rich in organic matter (OM). The total OM for all waste biomass ranged from 79.39% to 98.17%. Likewise, the results showed that all the waste biomass resulted in lower ash content and high fixed carbon content associated with high fuel quality. Based on proximate analysis, various empirical equations (HHV=28.296-0.2887(A)-656.2/VM, HHV=18.297-0.4128(A)+35.8/FC and HHV=22.3418-0.1136(FC)-0.3983(A)) have been tested to predict HHVs. It was observed that the heterogeneous nature of various biomass waste considerably affects the HHVs and hence has different fuel characteristics. Similarly, the HHVs of waste biomass were also determined experimentally using the bomb calorimeter, and it was observed that among all the selected waste biomass, the highest HHVs (21.19 MJ kg-1) resulted in cooked food waste followed by cooked macaroni (20.25 MJ kg-1). The comparison revealed that experimental HHVs for the selected waste biomass were slightly deviated from the predicted HHVs. Based on HHVs, various thermochemical and biochemical technologies were critically overviewed to assess the suitability of waste biomass to energy products. It has been emphasized that valorizing waste-to-energy technologies provides the dual benefits of sustainable management and production of cleaner energy to reduce fossil fuels dependency. However, the key bottleneck in commercializing waste-to-energy systems requires proper waste collection, sorting, and continuous feedstock supply. Moreover, related stakeholders should be involved in designing and executing the decision-making process to facilitate the global recognition of waste biorefinery concept.

Sustainable management and recycling of anaerobic digestate solid fraction by composting: A review

The aim of the study was to review and discuss the management and recycling of anaerobic digestate solid fraction by composting process in the context of circular bioeconomy and sustainable development. The conversion of the solid fraction into compost can be recognized as novel process-enhancing supplements for land reclamation. Moreover, the solid fraction of the digestate is a valuable substrate for compost production, both as a monosubstrate and as a valuable additive for other raw materials to enrich in organic matter. These results should serve as reference point to target adjusting screws for anaerobic digestate solid fraction by composting process improvement, its implementation in modern bioeconomy perspective as well as provide a guideline for effective waste management.

Developing a biogas centralised circular bioeconomy using agricultural residues - Challenges and opportunities

Anaerobic digestion (AD) can be used as a stand-alone process or integrated as part of a larger biorefining process to produce biofuels, biochemicals and fertiliser, and has the potential to play a central role in the emerging circular bioeconomy (CBE). Agricultural residues, such as animal slurry, straw, and grass silage, represent an important resource and have a huge potential to boost biogas and methane yields. Under the CBE concept, there is a need to assess the long-term impact and investigate the potential accumulation of specific unwanted substances. Thus, a comprehensive literature review to summarise the benefits and environmental impacts of using agricultural residues for AD is needed. This review analyses the benefits and potential adverse effects related to developing biogas-centred CBE. The identified potential risks/challenges for developing biogas CBE include GHG emission, nutrient management, pollutants, etc. In general, the environmental risks are highly dependent on the input feedstocks and resulting digestate. Integrated treatment processes should be developed as these could both minimise risks and improve the economic perspective.

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

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

To separate or not? A comparison of wastewater management systems for the new city district of Hiedanranta, Finland

In this study, life cycle assessment (LCA) and life cycle costing (LCC) methods were applied for the new city district of Hiedanranta, where source-separating sanitation systems are being considered. Two source-separating systems were compared to the conventional sanitation system with a centralized wastewater treatment plant (WWTP). With a separating system, three to 10 times more nitrogen could be recovered compared to the conventional system. If the nutrient potential of the reject water of the sludge digestion were to be utilized, the recovery rate would be even higher. For phosphorus, the recovered amount would be at the same level for all the alternatives. However, the plant availability of phosphorus is higher in separating systems. Based on the environmental impacts of separating systems with improved nutrient recovery, the climate and eutrophication impacts could be reduced, but the acidification impact may be higher. However, the actual climate benefits depend on how the avoided emissions will be realized, which is highly dependent on the policy and decision-making processes in the society. The life cycle costs of the alternative source-separating systems are higher at current prices. Source-separating sanitation produces new recycled nutrient products of human origin that contain fewer contaminants and could therefore be more easily accepted for end use when certain boundary conditions are met.

Ammonium and Phosphate Recovery from Biogas Slurry: Multivariate Statistical Analysis Approach

Livestock biogas slurry is an effluent containing nutrients such as ammonium and phosphate that are released by the industries. Therefore, recovery and reuse of ammonium and phosphorus is highly necessary. In recent years, many studies have been devoted to the use of different multivariate statistical analyses to investigate the interrelationship of one factor to another factor. The overall objective of this research study was to understand the significance of phosphate and ammonium recovery from biogas slurry using the multivariate statistical approach. This study was conducted using a range of salts that are commonly found in biogas slurry (ZnCl2, FeCl3, FeCl2, CuCl2, Na2CO3, and NaHCO3). Experiments with a biogas digester and aqueous solution were conducted at pH 9, with integration with NH4+, Mg2+, and PO43− molar ratios of 1.0, 1.2, and 1.8, respectively. The removal efficiency of ammonium and phosphate increased from 15.0% to 71.0% and 18.0% to 99.0%, respectively, by increasing the dose of respective ions K+, Zn2+, Fe3+, Fe2+, Cu2+, and CO32−. The elements were increased from 58.0 to 71.0 for HCO3−, with the concentration increasing from 30 mg L−1 to 240 mg L−1. Principal component, regression, path analysis, and Pearson correlation analyses were used to investigate the relationships of phosphate and ammonium recovery under different biochar, pyrolysis temperature, element concentration and removal efficiencies. Multivariate statistical analysis was also used to comprehensively evaluate the biochar and struvite effects on recovery of ammonium and phosphate from biogas slurry. The results showed that combined study of multivariate statistics suggested that all the indicators positively or negatively affected each other. Pearson correlation was insignificant in many ionic concentrations, as all were more than the significant 0.05. The study concluded that temperature, biochar type, and varying levels of components, such as K+, Zn2+, Fe3+, Fe2+, Cu2+, CO32−, and HCO3−, all had a substantial impact on P and NH4+ recovery. Temperature and varying amounts of metal salts enhanced the efficacy of ammonium and phosphate recovery. This research elucidated the methods by which biochar effectively reuses nitrogen and phosphate from biogas slurry, presenting a long-term agricultural solution.

Anaerobic digestate management, environmental impacts, and techno-economic challenges

Digestate is a nutrient-rich by-product from organic waste anaerobic digestion but can contribute to nutrient pollution without comprehensive management strategies. Some nutrient pollution impacts include harmful algal blooms, hypoxia, and eutrophication. This contribution explores current productive uses of digestate by analyzing its feedstocks, processing technologies, economics, product quality, impurities, incentive policies, and regulations. The analyzed studies found that feedstock, processing technology, and process operating conditions highly influence the digestate product characteristics. Also, incentive policies and regulations for managing organic waste by anaerobic digestion and producing digestate as a valuable product promote economic benefits. However, there are not many governmental and industry-led quality assurance certification systems for supporting commercializing digestate products. The sustainable and safe use of digestate in different applications needs further development of technologies and processes. Also, incentives for digestate use, quality regulation, and social awareness are essential to promote digestate product commercialization as part of the organic waste circular economy paradigm. Therefore, future studies about circular business models and standardized international regulations for digestate products are needed.

Can anaerobic digestion be a suitable end-of-life scenario for biodegradable plastics? A critical review of the current situation, hurdles, and challenges

Growing concern regarding non-biodegradable plastics and the impact of these materials on the environment has promoted interest in biodegradable plastics. The intensification of separate biowastes collection in most European countries has also contributed to the development of biodegradable plastics, and the subject of their end-of-life is becoming a key issue. To date, there has been relatively little research to evaluate the biodegradability of biodegradable plastics by anaerobic digestion (AD) compared to industrial and home composting. However, anaerobic digestion is a particularly promising strategy for treating biodegradable organic wastes in the context of circular waste management. This critical review aims to provide an in-depth update of anaerobic digestion of biodegradable plastics by providing a summary of the literature regarding process performances, parameters affecting biodegradability, the microorganisms involved, and some of the strategies (e.g., pretreatment, additives, and inoculum acclimation) used to enhance the degradation rate of biodegradable plastics. In addition, a critical section is dedicated to suggestions and recommendations for the development of biodegradable plastics sector and their treatment in anaerobic digestion.

Olive Mill Waste-Based Anaerobic Digestion as a Source of Local Renewable Energy and Nutrients

This study focused on what combination of anaerobic digestion (AD) temperature (ambient, mesophilic, and thermophilic) and olive mill waste (OMW) to dairy manure (DM) ratio mixture delivers the desired renewable energy and digestate qualities when using AD as olive mill waste treatment. OMW is widespread in the local environment in the North Sinai region, Egypt, which causes many environmental hazards if left without proper treatment. Three different mixtures consisting of OMW, dairy manure (DM), and inoculum (IN) were incubated under ambient, mesophilic, and thermophilic conditions for 45 days. The results showed that mixture B (2:1:2, OMW:DM:IN) at 55 °C produced more methane than at 35 °C and ambient temperature by 40% and 252%, respectively. Another aim of this study was to investigate the effects of the different concentrations of the digestate taken from each mixture on faba bean growth. The results showed that the maximum fresh weight values of the shoot system were observed at 10% and 15% for mixture B at ambient temperature. The best concentration value for the highest root elongation rate is a 5% addition of digestate mixture A at 55 °C, compared with other treatments.

It Works! Organic-Waste-Assisted Trichoderma spp. Solid-State Fermentation on Agricultural Digestate

This study aimed at valorizing digestate through Trichoderma spp. solid-state fermentation (SSF) to produce a potentially ameliorated fertilizer combined with fungal biomass as a value-added bioproduct. Plant-growth-promoting Trichoderma atroviride Ta13, T. reesei RUT-C30, T. asperellum R, and T. harzianum T-22 were tested on different SSF substrates: whole digestate (WD), digestate dried up with wood sawdust (SSF1), and digestate enriched with food waste and dried up with wood sawdust (SSF2). The fungal biomass was quantified by using a qPCR assay. The growth of the four Trichoderma spp. was only observed on the SSF2 substrate. The highest quantity of mycelium was produced by T. reesei RUT-30 (689.80 ± 80.53 mg/g substrate), followed by T. atroviride Ta13, and T. asperellum R (584.24 ± 13.36 and 444.79 ± 91.02 mg/g substrate). The germination of Lepidium sativum seeds was evaluated in order to assess the phytoxicity of the Trichoderma-enriched substrate. The treatments with 7.5% SSF2-R, 3.75% SSF2-T-22, and 1.8% SSF2-Ta13 equally enhanced the root elongation in comparison to the non-fermented SSF-2. This study demonstrated that digestate, mixed with agro-food waste, was able to support the cultivation of Trichoderma spp., paving the way to the valorization of fermented digestate as a proper biofertilizer.

A Glimpse of the World of Volatile Fatty Acids Production and Application: A review

Sustainable provision of chemicals and materials is undoubtedly a defining factor in guaranteeing economic, environmental, and social stability of future societies. Among the most sought-after chemical building blocks are volatile fatty acids (VFAs). VFAs such as acetic, propionic, and butyric acids have numerous industrial applications supporting from food and pharmaceuticals industries to wastewater treatment. The fact that VFAs can be produced synthetically from petrochemical derivatives and also through biological routes, for example, anaerobic digestion of organic mixed waste highlights their provision flexibility and sustainability. In this regard, this review presents a detailed overview of the applications associated with petrochemically and biologically generated VFAs, individually or in mixture, in industrial and laboratory scale, conventional and novel applications.

Agricultural application of digestates derived from agricultural and municipal organic wastes: a health risk-assessment for heavy metals

A Human-health Risk Assessment was performed for an agricultural site in North-East Italy undergone digestate application to (i) check the compliance of digestate land spreading with the Italian and European regulations on contaminated agricultural soils and (ii) evaluate how resulting risk estimations can be influenced by the applied modeling assumptions. The assessment estimated the risk related to adults and children intake of Heavy Metals (HM) contained in crops at concentrations estimated by a soil-plant transfer model based on the substance-specific soil-water partition coefficients. Eight different scenarios were investigated, according to different digestate type (from biowaste and agro-industrial byproducts), digestate application techniques and soil background concentrations. Non-risky situations resulted in all scenarios involving digestate application. The totality of calculated non-carcinogenic Hazard Indexes (HI) and carcinogenic total risk (RTOTC) resulted below 0.02 and 3E10^-9, respectively. In contrast with the definition, non-carcinogenic risks were associated with the considered soil background concentrations, with HI s up to 1.7 for child receptors, while carcinogenic risk was calculated below the concern threshold (i.e., RTOTC < 10^-5). Accordingly, this study highlighted (i) non-concerning situations related with lawful application of digestates and (ii) the need to improve the modeling of bioavailability to plant of HMs background content of soil.

The Effects of Soil Application of Digestate Enriched with P, K, Mg and B on Yield and Processing Value of Sugar Beets

The aim of this research was to find out if the supplementation of digestate, a by-product of the anaerobic digestion of sugar beet pulp, with phosphorus, potassium, magnesium and boron can improve digestate performance as a soil amendment. The materials of this study were: digestate and sugar beet roots (Beta vulgaris cv. Fighter). A field trial was carried out on sugar beet growth under soil application conditions of solid and liquid digestate fractions with or without supplementation with P, K, Mg and B. It was shown that the root yield obtained from the plots amended with digestate supplemented with P, K, Mg and B was higher compared to the yield of other treatments. Soil amendment with digestate supplemented with P, K, Mg and B affected quality parameters of sugar beet roots. An increase in the following parameters under the effects of enriched digestate application was found: sucrose content, dry residue, pomace content, inverted sugars, α-amino and amide nitrogen fractions, as well as sodium and potassium content. A reduction in the content of conductometric ash was noted but this difference was not proven. The enrichment of digestate with P, K, Mg and B resulted in the beneficial modification of beet roots’ processing parameters with the exception of the predicted content of sugar in molasses. In the case of the liquid fraction and its supplementation with P, K, Mg and B, six among eleven technological quality parameters were increased.

Thermal hydrolyzed food waste liquor as liquid organic fertilizer

Thermal hydrolysis (TH) is an efficient technology for food waste (FW) management. This study investigated the nutrients released from FW under various TH temperature (140, 160, 180, 200 and 220 °C) and evaluated the feasibility of the hydrolyzed liquor (HL) as liquid organic fertilizer. The phytotoxicity and biotoxicity of HL was analyzed using wheat seed and Pseudomonas putida. Results revealed that TH could effectively solubilize FW and release nutrients (N, P and K) and organic substances. The highest content of total nitrogen (TN, 1685 mgN/L) and phosphorus (TP, 235 mgP/L) in the HL was obtained under 180 °C. The K⁺ was 278-293 mg/L regardless of treatment temperature. Secondary nutrients (Ca and Mg) and micro metals (Fe, Cu, Zn, Al, Co and Mn) were all detected at relatively high level, while heavy metals (As and Cd) were generally lower than 0.5 mg/L. Twenty types of free amino acid were identified and the maximum total concentration was 4965.13 mg/L. 2% HL displayed higher germination index (>80%) and enhanced root and shoot lengths. No biotoxicity was observed as confirmed by the bioassay. This study proposes a feasible method to solubilize food waste and produce liquid organic fertilizer.

A review about methods for supporting failure risks analysis in eco-assessment

This paper critically reviewed 106 scientific papers proposing methods to enrich eco-assessment with failure determination and risk assessment. The provided research perspective is new and significantly different from the reviews in the literature which are mostly limited to analyse the environmental impacts of uncertainties and off-design functioning rather than the failures. The analysis, based on the contributions of the literature over more than 20 years, was carried out manually and allowed to identify and classify the application fields, the types of identifiable failures and the approaches used for their determination, for the analysis of their risk of occurrence and for their eco-assessment. The different classifications have also been intersected with each other and all the proposed approaches have been discussed in detail, highlighting the advantages and disadvantages in relation to eco-assessment. From the study emerged a growing and heterogeneous interest on the subject by the scientific community, and a certain independence of the analysed methods with respect to traditional approaches of both failure risk analysis and eco-assessment. Great attention of the methods about product functioning has been highlighted, in addition to the use of tests, simulations, FMEA (failure mode and effect analysis)-based approaches and knowledge databases to determine the failures, while statistical methods are preferred to support risks analysis and LCA (life cycle assessment) for environmental impact calculation. If, in the coming years, this argument also spreads in industry, the results provided by this review could be exploited as a first framework for practitioners.

Crop performance and soil fertility improvement using organic fertilizer produced from valorization of Carica papaya fruit peel

In recent times, research attention is focusing on harnessing agricultural wastes for the production of value-added products. In this study, the valorization of Carica papaya (Pawpaw) fruit peels was evaluated for the production of quality organic fertilizer via anaerobic digestion (AD) while the effects of the fertilizer on maize crop were also assessed. Pawpaw peel was first pretreated by thermo-alkaline methods before AD and analyses were carried out using standard methods. The resulting digestate was rich in nutrients and was dewatered to form solid organic fertilizer rich in microbes and soil nutrients. When applied to maize plants, organic fertilizer showed a better effect on plant traits than NPK 15–15–15 fertilizer and without fertilizer application. These were more pronounced at mid to high organic fertilizer applications (30-to-60-kg nitrogen/hectare (kg N/ha)) rate. Comparison between the values obtained from the field experiments reveals that the organic fertilizer showed better performance in all parameters such as the number of leaves, leaf area, plant height, stem girth, total shoot, and root biomass, and length of the root. However, the chemical fertilizer outperformed all the organic fertilizer applied rates in the average highest size of the corn ear by 1.4%. After harvesting, nutrient elements were found to have bioaccumulated in plant organs (leaves, stem, and root) with the highest values being 29.7 mg/L for nitrogen in the leaf and this value was reported from the experiment with 50 kg N/ha. For phosphorus and potassium, the highest concentrations of 7.05 and 8.4 mg/L were recorded in the plant’ stem of the experiment with 50 kg N/ha. All the treated soils recorded an increase in values of all nutrient elements over the control with the highest values recorded in the experiment with 60 kg N/ha. In soil with 60 kg N/ha, the nitrogen, phosphorus, and potassium increased by 28, 40, and 22% respectively over the chemical fertilizer applied experiment while different levels of increases were also recorded for all other macro and microelements in all the experiments. Thus, agricultural practices by using anaerobic digestates as organic fertilizers is a sustainable method to overcome the dependence on inorganic fertilizers high rate.

A technical review of bioenergy and resource recovery from municipal solid waste

Population growth, rapid urbanization, industrialization and economic development have led to the magnified municipal solid waste generation at an alarming rate on a global scale. Municipal solid waste seems to be an economically viable and attractive resource to produce green fuels through different waste-to-energy conversion routes. This paper reviews the different waste-to-energy technologies as well as thermochemical and biological conversion technologies for the valorization of municipal solid waste and diversion for recycling. The key waste-to-energy technologies discussed in this review include conventional thermal incineration and the modern hydrothermal incineration. The thermochemical treatments (e.g. pyrolysis, liquefaction and gasification) and biological treatments (e.g. anaerobic digestion and composting) are also elaborated for the transformation of solid wastes to biofuel products. The current status of municipal solid waste management for effective disposal and diversion along with the opportunities and challenges has been comprehensively reviewed. The merits and technical challenges of the waste-to-energy technologies are systematically discussed to promote the diversion of solid wastes from landfill disposal to biorefineries.

Recycling Biogas Digestate from Energy Crops: Effects on Soil Properties and Crop Productivity

Digestate from biogas production can be recycled to the soil as conditioner/fertilizer improving the environmental sustainability of the energy supply chain. In a three-year maize-triticale rotation, we investigated the short-term effects of digestate on soil physical, chemical, and microbiological properties and evaluated its effectiveness in complementing the mineral fertilizers. Digestate soil treatments consisted of combined applications of the whole digestate and its mechanically separated solid fraction. Digestate increased soil total organic C, total N and K contents. Soil bulk density was not affected by treatments, while aggregate stability showed a transient improvement due to digestate treatments. A decrement of the transmission pores proportion and an increment of fissures was observed in digestate treated soils. Soil microbial community was only transiently affected by digestate treatments and no soil contamination from Clostridiaceae-related bacteria were observed. Digestate can significantly impair seed germination when applied at low dilution ratios. Crop yield under digestate treatment was similar to ordinary mineral-based fertilization. Overall, our experiment proved that the agronomic recycling of digestate from biogas production maintained a fair crop yield and soil quality. Digestate was confirmed as a valid resource for sustainable management of soil fertility under energy-crop farming, by combining a good attitude as a fertilizer with the ability to compensate for soil organic C loss.

Effects of woody biochar on dry thermophilic anaerobic digestion of organic fraction of municipal solid waste

This study presents the results of semi-pilot scale anaerobic digestion tests conducted under dry thermophilic conditions with the addition of biochar (6% on fresh mass basis of inoculum), derived from an industrial gasification plant, for determining biogas and biomethane production from organic fraction of municipal solid waste. By using two types of inocula (from a full-scale dry anaerobic digestion plant and from lab-scale biomethanation tests), the obtained experimental results did not show significant increase in methane yield related to the presence of biochar (330.40 NL CH₄ kgVS^-1 using plant inoculum; 335.41 NL CH₄ kgVS^-1 using plant inoculum with biochar, 311.78 NL CH₄ kgVS^-1 using lab-inoculum and 366.43 NL CH₄ kgVS^-1 using lab-inoculum with biochar), but led to significant changes in the microbial community composition. These results are likely related with the specific biochar physical-chemical features and low adsorption potential. Resulting digestate quality was also investigated: biochar-enriched digestates were characterized by increased biological stability (809 ± 264 mg O₂ kgVS^-1 h^-1 vs. 554 ± 76 mg O₂ kgVS^-1 h^-1 for biochar-free and biochar-enriched digestates, respectively), lower heavy metals concentrations (with the exception of Cd), but higher polycyclic aromatic hydrocarbons content, with a reported maximum concentration of 8.9 mgPAH kgTS^-1 for biochar-enriched digestate derived from AD test with lab-inoculum, which could trigger non-compliance with regulation limits for agricultural reuse of digestates. However, phytotoxicity assessments showed a decreased toxicity of biochar-containing digestates when compared to biochar-free digestates.

Removal of pollutants from biogas slurry and CO2 capture in biogas by microalgae-based technology: a systematic review

Recent research interest has focused on microalgae cultivation for biogas slurry purification and biogas upgrading due to the requirement of high efficiency for nutrient uptake and CO₂ capture, with economic feasibility and environmental benefits. Numerous studies have suggested that biogas slurry purification and biogas upgrading can occur simultaneously via microalgae-based technology. However, there is no comprehensive review on this technology with respect to the nutrient removal from biogas slurry and biogas upgrading. This article summarizes microalgal cultivation with biogas slurry and biogas from anaerobic digestion. The parameters, techniques, and modes of microalgae cultivation have been discussed in detail to achieve high efficiency in biogas slurry purification and biogas upgrading. In addition, the evaluation of energy efficiency and safety has also been explored. Compared with mono-cultivation of microalgae and co-cultivation of microalgae and bacteria, microalgae-fungi symbiosis has demonstrated greater development prospect and higher energy efficiency and the energy consumption for pollutants and CO₂ removal were 14.2-39.0% · USD^-1 and 19.9-23.3% · USD^-1, respectively. Further, a sustainable recycling scheme is proposed for the purification of biogas slurry from anaerobic digestion process and biogas upgrading via microalgae-based technology.

Techno-Economic Assessment of Three Modes of Large-Scale Crop Residue Utilization Projects in China

In China, the non-exploitation of bioenergy poses major problems and challenges. To solve bioenergy problems, considerable efforts have been made to expedite the construction of large-scale crop residue utilization projects. In this study, three principal supported modes of large-scale crop residue utilization projects were taken as empirical cases in Hubei province bioenergy planning. In terms of the overall benefit and sustainable development, a third-grade evaluation index system was established. The analysis was carried out using the analytical hierarchy process, principal component projection, and grey relational analysis. The conclusion indicates that according to the evaluation values, the sequence from best to worst was crop residue biogas project, crop residue briquette fuel project, and crop residue gasification project. Nevertheless, there was no remarkable difference in the overall evaluation values. The biogas project had certain advantages in terms of the production cost, soil improvement, and expenditure saving, whereas the gasification project was comparatively insufficient in environmental efficiency, product benefit, by-product disposal, and technical rationality. According to actual evaluation results, the unilateral determination approach of the single weight index can be seen as being overcome through the unified adaptation of the evaluation methods. The research results can serve as a reference for making investment decisions to build large-scale crop residue utilization projects.

Environmental and Economic Analysis of an Anaerobic Co-Digestion Power Plant Integrated with a Compost Plant

Italian power generation through anaerobic digestion (AD) has grown significantly between 2009 and 2016, becoming an important renewable energy resource for the country, also thanks to the generous incentives for produced electricity available in the last years. This work focuses on the economic and environmental issues of AD technology and proposes a techno-economic analysis of investment profitability without government support. In particular, the analysis focuses on an AD power plant fed by zootechnical wastewater and agro-industrial residues coupled to a cogeneration (CHP) system and a digestate-composting plant that produces soil fertilizers. We aim to determine the economic profitability of such AD power plants fed by inner-farm biomass wastes, exploiting digestate as fertilizer, using the cogenerated heat and taking into account the externalities (environmental benefits). Environmental analysis was carried out via a life cycle analysis (LCA), and encompassing the production of biogas, heat/electricity and compost in the downstream process. The un-released environmental emissions were converted into economic benefits by means of a stepwise approach. The results indicate that integrating a compost plant with a biogas plant can significantly increase the carbon credits of the process. The results were evaluated by means of a sensitivity analysis, and they report an IRR in the range of 6%–9% according to the Italian legislative support mechanisms, and possibilities to increase revenues with the use of digestate as fertilizer. The results significantly improve when externalities are included.

Effect of Biogas Digestate and Mineral Fertilisation on the Soil Properties and Yield and Nutritional Value of Switchgrass Forage

The aim of this study was to assess the effect of a three-year application of digestate from an agricultural biogas plant on the physicochemical properties of highly acidic pHKCl 4.4 ± 0.23, silty loam soils with low macronutrient content and on the yield and nutritional value of switchgrass (Panicum virgatum L.) biomass harvested for green fodder. The experiment included the following treatments: (1) O (control)—no fertilisation, (2) NPK—mineral fertilisation with (in kg ha−1) 150 N, 53.0 P and 105 K, (3) biogas digestate at 30 m3 ha−1 and (4) biogas digestate at 60 m3 ha−1. The higher application rate of biogas digestate significantly reduced soil acidity to pHKCl 4.9 ± 0.18 and improved its sorption properties. It also increased the soil organic matter content from 5.6 ± 0.21 to 6.4 ± 0.22 g Corg kg−1 and of K and Zn. The higher level of biogas digestate significantly increased switchgrass yield to 5.15 ± 0.26 t ha−1. The lower application rate of biogas digestate resulted in forage yield of 4.30 ± 0.20 t ha−1 comparable to that obtained after mineral fertilisation (4.33 ± 0.22 t ha−1). Following application of mineral fertilisers and the higher level of biogas digestate, the number of panicles per plant (150 ± 2.49–157 ± 0.6.17), panicle height (107 ± 1.98–114 ± 2.08), crude ash content (61.2 ± 0.43–65.5 ± 0.38) and protein content (106 ± 0.59–92 ± 1.11) in the switchgrass biomass from the first cut were higher than in the case of unfertilised soil (110 ± 3.81, 93 ± 1.32, 55.5 ± 0.40, 80.3 ± 0.37). The use of mineral fertilisers and biogas digestate increased the content of protein, P and Mg in biomass from the second cut. The results suggest that the use of digestate improved the physicochemical properties of highly acidic soil and increased the yield of switchgrass forage without diminishing its nutritional value.

Influence of Anaerobic Mesophilic and Thermophilic Digestion on Cytotoxicity of Swine Wastewaters

Recycling wastewater from animal production for fertilizers using anaerobic digestion (AD) is a common method to recover the nutrients in the digestate. However, the digestate toxicity is not well understood because AD is mainly designed for chemical oxygen demand reduction. This study determined the toxicity during AD and the controlling factors with the goal to improve digestate safety during farmer handling to reuse the nutrients. Thermophilic and mesophilic AD of two swine wastewater sources were studied. Mammalian cell cytotoxicity revealed that the effluent after thermophilic digestion was at least 69% more toxic than the mesophilic effluent, owing to higher ammonia and total organic carbon in the former. Ammonia accounted for >55% total cytotoxicity, and the organics of the thermophilic digestate were twice more toxic than those in the mesophilic digestate. Despite less toxicity contribution than the ammonia, the organics did demonstrate significant adverse effects on the thiol-mediated cellular protection mechanism. For swine wastewater nutrient recovery, converting ammonia to less toxic nitrogen forms could lower the toxic hazard of the AD digestate. With much less ammonia, the organics would be the remaining decisive factor for toxicity, which is favorably reduced using thermophilic AD over mesophilic. If the ammonia is not reduced, mesophilic AD would generate a less toxic digestate.

Phosphorus recovered from digestate by hydrothermal processes with struvite crystallization and its potential as a fertilizer

Phosphorus (P) recovery from digestate has attracted considerable interest. In this study, hydrothermal processes in combination with struvite crystallization were performed to promote P solubilization and capture from digestate; its potential as a phosphate-based fertilizer was also investigated. Hydrothermal treatment with HCl and H₂O₂ showed good results for the solubilization of organic and slightly soluble P, and achieved the lowest input energy need (768 kWhkg^-1_P). Struvite crystallization reached 99.3% (Mg²⁺:PO₄^3-1.84:1, pH 9.98). X-ray diffractometry and energy dispersive X-ray spectrometer mapping demonstrated the main precipitate component was struvite. For the fertilization of maize, P utilization from struvite was 19.0%. Light microscope analysis revealed that appropriate amounts of struvite may have an influence on the growth of the primary root. Overall, 16.6% of total P was recovered after P was solubilized, captured and made available.

Organic contaminants of emerging concern in Norwegian digestates from biogas production

The aim of this study was to analyze a variety of environmental organic contaminants of emerging concern (CEC) and their metabolites in representative digestate samples from Norwegian biogas production plants. Biogas digestates can be a valuable source for soil amendments and/or fertilizers in commercial agriculture. It is important to assess whether the digestates contain harmful contaminants in order to avoid unintended exposure of human consumers. In total 19 biogas digestates from 12 biogas production plants in Norway were collected and analyzed. Furthermore, process related parameters such as pretreatment of substrates, additives, flocculation and temperature conditions were considered for interpretation of the results. The CEC levels found in the digestates were shown to be dependent on the original composition of the substrate, dry-matter content, and conditioning of the substrate. The sunscreen octocrylene (147 μg L-1) and acetaminophen (paracetamol; 58.6 μg L-1) were found at the highest concentrations in liquid digestates, whereas octocrylene (>600 ng g-1, on a wet weight basis = ww) and the flame retardant TCPP (tris(1-chloro-2-propyl) phosphate, >500 ng g-1 ww) were found at the highest levels in solid digestates, exceeding even the upper limit of quantification (uLOQ) threshold. The highest levels of total CECs were measured in solid digestates (1411 ng g-1 ww) compared to liquid digestates (354 μg L-1 equals 354 ng g-1). The occurrence of CECs in digestate samples, even after extensive and optimized anaerobic digestion, indicates that the operational conditions of the treatment process should be adjusted in order to minimize CEC contamination.

Semi-continuous anaerobic co-digestion of flotation sludge from broiler chicken slaughter and sweet potato: Nutrients and energy recovery

Energy production based on the proper allocation of environmental liabilities is in line with the concept of sustainability. Flotation sludge (S) is a type of waste derived from the physical treatment of the wastewater generated in significant quantities during chicken slaughter in Brazil. If not treated, this wastewater may contribute to pollution, but further treatment provides clean energy and nutrient recycling. The present study aimed at evaluating the reduction of (S) organic load by means of mono and co-digestion with sweet potatoes (P) while promoting its conversion into energy (methane) and nutrients (digestate). Semi-continuous reactors (60 L capacity) were used with a hydraulic retention time of 25 days. The reactors were fed daily with 2.4 L consisting of 60% digestate recirculation, 40% non-chlorinated water and 4.5% total solids (TS). Using nine reactors and six progressive periods, eleven conditions were evaluated with three replicates each. The percentages of (P) and (S) varied from 0 to 100. The best observed condition in terms of energy recovery and TS removal was 60% of P + 40% of S (p ≤ 0.05), as it presented values of at least an increase of 92% in total biogas volume, an increase of 123% in specific methane production, an increase of 98% in specific methane yield and an increase of 44% in TS removal efficiency compared to mono-digestions. The fertilizer potential of the digestate generated in the different conditions was calculated and evaluated according to the area of (P) production. The results varied from 3.6 to 10.8 ha of (P) using 100 m³ of digestate. A multivariate analysis showed that higher amounts of (P) in substrate composition favor energy recycling while higher concentrations of (S) enhance the production of a digestate with valuable agronomic characteristics.

Biomimetic economy: human ecological-economic systems emulating natural ecological systems

Purpose

The purpose of this paper is to examine the use of biomimicry to inspire sustainable development in economic systems. The research purpose is to explore the link between ecological systems and economic systems to highlight applied environmental solutions. The goal is to propose some driver to develop sustainable business practices inspired on the principles of biomimicry.

Design/methodology/approach

This paper provides a theoretical approach that builds the basis for a better understanding of the relationship between nature and sustainable economic decisions. The premise is that in the field of sustainable development, strategies based on “learning from nature” are useful. Furthermore, the concept of biomimicry provides principles and tools specifically aimed at design practice.

Findings

The complexity of economic systems has shown that high levels of abstraction are required when conceptualising problems and explanations related with nature-inspired solutions. Stakeholder engagement and transdisciplinary collaboration are required to face long-term environmental challenges. Moreover, the exploratory analysis applied in this paper appeared suitable to compile existing literature.

Practical implications

The study provides some general guidelines and empirical approach through case studies that could help decision makers convert nature-inspired alternatives into valuable strategic business opportunities. Although presented practical cases are framed in the local sphere (i.e. the Basque Country), they can serve as references in other international contexts.

Social implications

New business models should recognize the positive synchronization between well-managed social, environmental and economic systems.

Originality/value

The proposed ideas deepen the understanding on the sustainable development and the link between ecological and economic systems. In fact, the concept of biomimetic economy has not been dealt with or developed in depth in previous academic works, nor has it been published thoroughly in the field of research.

Statistical analysis for the quality assessment of digestates from separately collected organic fraction of municipal solid waste (OFMSW) and agro-industrial feedstock. Should input feedstock to anaerobic digestion determine the legal status of digestate?

Management options for digestate produced by anaerobic digestion plants influence the environmental and economic sustainability of the biogas sector. Further, digestate can be both used or disposed of according to its legal classification: that is, waste or by-product, or product (by using End of Waste procedure). Currently, legal digestate status is decided by EU member states on a case-by-case basis, according to specific positive lists of input feedstocks and quality requirements in terms of physical properties and chemical concentrations. Biased exclusion of input feedstock can force digestate to a specific waste classification and undergo post-treatment and disposal options that can negatively affect the profitability of biogas installations. This is the case of the Italian regulation, where the positive list of input feedstock excludes a priori separately collected organic fractions of municipal solid waste (OFMSW), while including agro-industrial residues (AGRO). This study determined the differences between the two digestate typologies (OFMSW versus AGRO) through statistical analysis, implemented on a dataset, designed to gather data about digestate's physical-chemical parameters from relevant scientific literature and unpublished private databases. The datasets consisted of 190 entries, derived from more than 2,000 samples. Further, the study provided a compliance assessment between the resulting parameter means and the current regulation limits. Upper confidence limits for the means (level of significance α = 0.05) calculated for both digestate typologies were found to be compliant with the legal requirements. Therefore, no statistical ratio seems to support the difference in the legislative approach as proposed by Italian law-makers. OFMSW resulted significantly different from AGRO for VS (650.1 g/kg TS vs. 843.8 g/kg TS, respectively), N-NH4 (81.9 g/kg TS vs. 46.19 g/kg TS), N-TOT (109.7 g/kg TS vs. 65.32 g/kg TS), P-TOT (7.22 g/kg TS vs. 21.9 g/kg TS), Pb (18.6 mg/kg TS vs. 4.66 mg/kg TS), Ni (11.03 mg/kg TS vs. 8.20 mg/kg TS), Cr-TOT (12.74 mg/kg TS vs. 8.74 mg/kg TS) and Hg (0.08 mg/kg TS vs. 0.05 mg/kg TS). However, the statistical analysis must be implemented on a wider set of parameters not covered by this study (e.g. ecotoxicological features).

Assessment of the ecotoxicity of phytotreatment substrate soil as landfill cover material for in-situ leachate management

Phytotreatment capping in closed landfills is a promising, cost-effective, in situ option for sustainable leachate treatment and might be synergistically coupled with energy crops to produce renewable energy (e.g.: biodiesel or bioethanol). This study proposes to use 0.30 m of soil as growing substrate for plants cultivated on the temporary cover of closed landfills. Once the leachate phytotreatment process is no longer required, 0.70 m of the same soil would be added to attain the final top cover configuration. This solution would entail saving the costs of excavation and backfilling. However, worsening of the initial soil quality due to potential contaminant transfer from the liquid to the solid matrix must be avoided because EU legislation (such as that in Italy) fixes concentration limits for contaminants in soil. In this research, samples of soil used as substrate in a lab-scale leachate phytotreatment test with sunflowers were analysed to provide chemical characterization before, during, and at the end of the experiment. The results showed that the phytotreatment activity did not increase initial contaminant concentrations. These results are reinforced by those from ecotoxicological bioassays in which Eisenia fetida (earthworms), Lepidium sativum (cress), Folsomia candida (collembola), and Caenorhabditis elegans and Steinernema carpocapsae (nematodes) were used. It was observed that, by the end of the experiment, the substrate soil did not affect the earthworms, collembola and nematode behaviour, or the growth of cress.

The Future Agricultural Biogas Plant in Germany: A Vision

After nearly two decades of subsidized and energy crop-oriented development, agricultural biogas production in Germany is standing at a crossroads. Fundamental challenges need to be met. In this article we sketch a vision of a future agricultural biogas plant that is an integral part of the circular bioeconomy and works mainly on the base of residues. It is flexible with regard to feedstocks, digester operation, microbial communities and biogas output. It is modular in design and its operation is knowledge-based, information-driven and largely automated. It will be competitive with fossil energies and other renewable energies, profitable for farmers and plant operators and favorable for the national economy. In this paper we discuss the required contribution of research to achieve these aims.

Nutrient recovery technologies integrated with energy recovery by waste biomass anaerobic digestion

Anaerobic digestion widely considered as a promising waste biomass disposal treatment approach, is attracting increasing interest in all corners of the globe. However, due to the specific features of different types of waste biomass, the bioenergy conversion efficiency of this process is not ideal. Another problematic aspect of anaerobic digestion is that the nutrient rich effluent sometimes needs to be treated before discharge. This review presents the recent achievements of waste biomass digestion from the perspective of energy recovery and nutrient recovery. In this work, the anaerobic treatment characteristics of common types of waste biomass are summarized and compared. With a focus of nutrient recovery and post treatment issues, the challenges and technical hurdles encountered in the anaerobic digestion of waste biomass are critically reviewed. Finally, an integrated system of anaerobic digestion, anaerobic ammonia oxidation (anammox) and phosphorus recovery is proposed for efficient energy and nutrient recovery from waste biomass.

Performance of semi-continuous anaerobic co-digestion of poultry manure with fruit and vegetable waste and analysis of digestate quality: A bench scale study

Poultry manure (PM) can contain ammonium and ammonia nitrogen, which may inhibit the anaerobic process. The aim of this work was to evaluate the performance of anaerobic digestion of PM co-digested with fruit and vegetable waste. Two semi-continuous bench scale (19L) stirred tank reactors were used. The operating conditions were: 34.5 °C, 2 gVS/L.d (organic load rate), 28 d of hydraulic retention time and 100 revolutions per m (1 h × 3 times by day) for the agitation. The reactors were fed PM and a mixture of PM and fruit and vegetable waste (FVW) at equal proportions (based on wet weight). The performance of the anaerobic process was assessed through biogas and methane yields, reduction of organic matter, release of nitrogen compounds and the monitoring of stability indicators (pH, volatile fatty acids (VFA), total (TA) and partial (PA) alkalinity). Moreover, the digestate quality was evaluated to determine potential risk and benefits from its application as biofertilizer. Toxicity was assessed using Daphnia magna immobilization tests. Results showed that biogas and methane yields from PM-FVW were 31% and 32% higher than PM alone, respectively. Values of organic matter, pH, alpha (PA/TA) and VFA revealed that stability was approached in PM and PM-FVW. The co-digestion of PM with FVW led to the highest methane and biogas yields, lower FAN and TAN concentrations, and a better digestate quality compared to mono-digestion of this manure.

Assessing the potential phytotoxicity of digestate from winery wastes

In this study, digestate from winery wastes was investigated focusing on phytotoxicity using macrophytes and evaluating the potential contribution of ammonium and copper. Spreading of digestate on soil could represent a suitable approach to recycle nutrients and organic matter, creating an on site circular economy. In this study, digestate quality was evaluated considering both chemical-physical characteristics and biological toxicity applying germination test. The effluent did not meet the entire amendment quality standard defined by Italian law (Decree 75/2010 germination index > 60% with solution of 30% v/v of digestate), but bio-stimulation was observed at low doses (3.15-6.25% v/v) for S. alba and S. saccharatum. The beneficial concentration agreed with Nitrate Directive dose and suggested that limited addition of digestate could have several positive effects on soil characteristics and on crop growth. Specific test using ammonium and copper solutions showed that these pollutants were not directly correlated to observed phytotoxicity.

Evaluation of benefits and risks associated with the agricultural use of organic wastes of pharmaceutical origin

Industrial fermentations for the production of pharmaceuticals generate large volumes of wastewater that can be biologically treated to recover plant nutrients through the application of pharmaceutical-derived wastes to the soil. Nevertheless, benefits and risks associated with their recovery are still unexplored. Thus, the aim of the present work was to characterize three potential organic residues (sludge, anaerobic digestate and compost) derived from the wastewater generated by the daptomycin production process. The main parameters evaluated were the physico-chemical properties, potential contaminants (heavy metals, pathogens and daptomycin residues), organic matter stabilization and the potential toxicity towards soil microorganisms and plants. The results showed that all the studied materials were characterized by high concentrations of plant macronutrients (N, P and K), making them suitable for agricultural reuse. Heavy metal contents and pathogens were under the limits established by European and Italian legislations, avoiding the risk of soil contamination. The compost showed the highest organic matter stabilization within the studied materials, whereas the sludge and the anaerobic digestate were characterized by large amounts of labile organic compounds. Although the pharmaceutical-derived fertilizers did not negatively affect the soil microorganisms, as demonstrated by the enzymatic activities, the sludge and the anaerobic digestate caused a moderate and strong phytotoxicity, respectively. The compost showed no toxic effect towards plant development and, moreover, it positively affected the germination and growth in lettuce and barley. The results obtained in the present study demonstrate that the valorization of pharmaceutical-derived materials through composting permits their agricultural reuse and also represents a suitable strategy to move towards a zero-waste production process for daptomycin.

A review on technological options of waste to energy for effective management of municipal solid waste

Approximately one-fourth population across the world rely on traditional fuels (kerosene, natural gas, biomass residue, firewood, coal, animal dung, etc.) for domestic use despite significant socioeconomic and technological development. Fossil fuel reserves are being exploited at a very fast rate to meet the increasing energy demands, so there is a need to find alternative sources of energy before all the fossil fuel reserves are depleted. Waste to energy (WTE) can be considered as a potential alternative source of energy, which is economically viable and environmentally sustainable. The present study reviewed the current global scenario of WTE technological options (incineration, pyrolysis, gasification, anaerobic digestion, and landfilling with gas recovery) for effective energy recovery and the challenges faced by developed and developing countries. This review will provide a framework for evaluating WTE technological options based on case studies of developed and developing countries. Unsanitary landfilling is the most commonly practiced waste disposal option in the developing countries. However, developed countries have realised the potential of WTE technologies for effective municipal solid waste management (MSWM). This review will help the policy makers and the implementing authorities involved in MSWM to understand the current status, challenges and barriers for effective management of municipal solid waste. This review concluded WTE as a potential renewable source of energy, which will partly meet the energy demand and ensure effective MSWM.

Anaerobic digestion of spent mushroom substrate under thermophilic conditions: performance and microbial community analysis

Spent mushroom substrate (SMS) is the residue of edible mushroom production occurring in huge amounts. The SMS residue can be digested for biogas production in the mesophilic anaerobic digestion. In the present study, performance of batch thermophilic anaerobic digestion (TAD) of SMS was investigated as well as the interconnected microbial population structure changes. The analyzed batch TAD process lasted for 12 days with the cumulative methane yields of 177.69 mL/g volatile solid (VS). Hydrolytic activities of soluble sugar, crude protein, and crude fat in SMS were conducted mainly in the initial phase, accompanied by the excessive accumulation of volatile fatty acids and low methane yield. Biogas production increased dramatically from days 4 to 6. The degradation rates of cellulose and hemicellulose were 47.53 and 55.08%, respectively. The high-throughput sequencing of 16S rRNA gene amplicons revealed that Proteobacteria (56.7%-62.8%) was the dominant phylum in different fermentative stages, which was highly specific compared with other anaerobic processes of lignocellulosic materials reported in the literature. Crenarchaeota was abundant in the archaea. The most dominant genera of archaea were retrieved as Methanothermobacter and Methanobacterium, but the latter decreased sharply with time. This study shows that TAD is a feasible method to handle the waste SMS.

Anaerobic co-digestion of recalcitrant agricultural wastes: Characterizing of biochemical parameters of digestate and its impacts on soil ecosystem

Anaerobic digestion (AD) of organic wastes is a promising alternative to landfilling for reducing Greenhouse Gas Emission (GHG) and it is encouraged by current regulation in Europe. Biogas-AD produced, represents a useful source of green energy, while its by-product (digestate) is a waste, that needs to be safely disposal. The sustainability of anaerobic digestion plants partly depends on the management of their digestion residues. This study has been focused on the environmental and economic benefits of co-digest recalcitrant agricultural wastes such olive wastes and citrus pulp, in combination with livestock wastes, straw and cheese whey for biogas production. The aim of this work was to investigate the effects of two different bioenergy by-products on soil carbon stock, enzymes involved in nutrient cycling and microbial content. The two digestates were obtained from two plants differently fed: the first plant (Uliva) was powered with 60% of recalcitrant agricultural wastes, and 40% of livestock manure milk serum and maize silage. The second one (Fattoria) was fed with 40% of recalcitrant agricultural wastes and 60% of livestock manure, milk serum and maize silage. Each digestate, separated in liquid and solid fractions, was added to the soil at different concentrations. Our results evidenced that mixing and type of input feedstock affected the composition of digestates. Three months after treatments, our results showed that changes in soil chemical and biochemical characteristics depended on the source of digestate, the type of fraction and the concentration used. The mainly affected soil parameters were: Soil Organic Matter (SOM), Microbial Biomass Carbon (MBC), Fluorescein Diacetate Hydrolysis (FDA), Water Soluble Phenol (WSP) and Catalase (CAT) that can be used to assess the digestate agronomical feasibility. These results show that the agronomic quality of a digestate is strictly dependent on percentage and type of feedstocks that will be used to power the digester.

Fate of copper, nickel and zinc after biogas digestate application to three different soil types

Soil application of organic residues from anaerobic digestion of municipal food waste and/or sewage sludge may introduce considerable amounts of heavy metals into the environment. In a column leaching experiment, mobility and release of Cu, Ni and Zn were investigated in three contrasting soils (sand, silt, loam) fertilized with biogas digestates of different origin. The effect of commercial digestates, based on food waste and sewage sludge, was compared to that of experimental digestates based on animal manure and whey permeate with or without fish ensilage, as well as untreated manure, mineral fertilizer and an untreated control. Manure and digestates were added to the columns as fresh material at equal amounts of available nitrogen. The experiment simulated high-intensity rainfall over a period of 7 days. In general, soil treated with the commercial digestates with higher original metal content showed less environmental impact in terms of Ni, Cu and Zn leaching than that treated with experimental digestates with lower original metal content and less than when animal manure or mineral fertilizer was applied. Although effects of digestate application on metal mobility in soils were seen in conditions of extreme precipitation, the leached concentrations of metals were below limitations published by the WHO but still significantly higher than that measured for control soils.

Ecological risk assessment of agricultural soils for the definition of soil screening values: A comparison between substance-based and matrix-based approaches

The Italian legislation on contaminated soils does not include the Ecological Risk Assessment (ERA) and this deficiency has important consequences for the sustainable management of agricultural soils. The present research compares the results of two ERA procedures applied to agriculture (i) one based on the "substance-based" approach and (ii) a second based on the "matrix-based" approach. In the former the soil screening values (SVs) for individual substances were derived according to institutional foreign guidelines. In the latter, the SVs characterizing the whole-matrix were derived originally by the authors by means of experimental activity. The results indicate that the "matrix-based" approach can be efficiently implemented in the Italian legislation for the ERA of agricultural soils. This method, if compared to the institutionalized "substance based" approach is (i) comparable in economic terms and in testing time, (ii) is site specific and assesses the real effect of the investigated soil on a battery of bioassays, (iii) accounts for phenomena that may radically modify the exposure of the organisms to the totality of contaminants and (iv) can be considered sufficiently conservative.

Comparative characterization of digestate versus pig slurry and cow manure - Chemical composition and effects on soil microbial activity

The growing number of biogas plants in Europe has resulted in increased production of nutrient-rich digestate with great potential as fertilizer for arable land. The nutrient composition of digestate varies with the substrate treated in the biogas plant and may contain compounds that stimulate or inhibit soil microbial activity. This study compared 20 digestates (D) with 10 pig slurries (PS) and 10 cow manures (CM) regarding their chemical content and their effect on soil microbial activities, i.e. potential ammonia oxidation rate (PAO) and soil respiration. The results showed no significant differences within the D group when divided based on substrate type. i.e. manure dominated vs. other organic waste materials in any of the tests. In general D contained significantly higher concentrations of ammonium while the concentrations of total carbon and volatile fatty acids were higher in PS and CM than in D. The D showed both stimulating and inhibiting effects on PAO, while all CM and all PS except one showed inhibiting effects on PAO. However, PAO activity was negatively correlated with the content of volatile fatty acids in the residues indicating that these compounds may be the cause of the inhibition. The maximum respiration activity (h_peakmax) was lower and the time point for the maximum respiration activity (t_peakmax) occurred earlier for D compared with CM and PS. This earlier peak time could be indicative of a high proportion of easily degradable carbon in D compared with PS and CM. However, the utilization rate of carbon, i.e. the proportion of added organic C converted to CO₂-C during 12days, did not differ significantly between D, PS and CM, indicating that overall carbon quality in the different fertilizers was still roughly comparable. In short, our results suggest that digestates were different compared with PS and CM but without posing a higher risk with respect to their impact on soil microbial activity.

Effect of aerobic pre-treatment on hydrogen and methane production in a two-stage anaerobic digestion process using food waste with different compositions

Aerobic pre-treatment was applied prior to two-stage anaerobic digestion process. Three different food wastes samples, namely carbohydrate rich, protein rich and lipid rich, were prepared as substrates. Effect of aerobic pre-treatment on hydrogen and methane production was studied. Pre-aeration of substrates showed no positive impact on hydrogen production in the first stage. All three categories of pre-aerated food wastes produced less hydrogen compared to samples without pre-aeration. In the second stage, methane production increased for aerated protein rich and carbohydrate rich samples. In addition, the lag phase for carbohydrate rich substrate was shorter for aerated samples. Aerated protein rich substrate yielded the best results among substrates for methane production, with a cumulative production of approximately 351ml/gVS. With regard to non-aerated substrates, lipid rich was the best substrate for CH₄ production (263ml/gVS). Pre-aerated P substrate was the best in terms of total energy generation which amounted to 9.64kJ/gVS. This study revealed aerobic pre-treatment to be a promising option for use in achieving enhanced substrate conversion efficiencies and CH₄ production in a two-stage AD process, particularly when the substrate contains high amounts of proteins.