Legal requirements for digestate as fertilizer in Austria and the European Union compared to actual technical parameters

Sustainable valorization of slaughterhouse waste through anaerobic digestion: A circular economy perspective

Slaughterhouse waste (SHW) poses significant environmental challenges due to its complex composition. In response, a novel review exploration of anaerobic digestion (AD) as a means of valorising SHW within the context of the circular economy (CE) is presented. The physicochemical properties of individual SHW, representing key parameters for the correct management of the AD process, are scrutinized. These parameters are further connected with identifying suitable pretreatment methods to enhance biogas production. Subsequently, the review examines the diverse technologies employed in the AD of SHW, considering the complexities of mono- or co-digestion. Various AD systems are evaluated for their effectiveness in harnessing the substantial biogas production potential from SHW, encompassing key parameters, reactor configurations, and operational conditions that influence the AD process. Moreover, the review interestingly extends its scope to the recovery and management of digestate, the by-product of AD. Along with the digestate composition, strategies for various utilization of this by-product are discussed. This investigation thus underscores, within the principles of the CE, the dual sustainable benefits of SHW processing via AD in biogas production and utilization of the resultant nutrient-rich digestate in various sectors.

Impact of natural degradation of the invasive alga Rugulopteryx okamurae on anaerobic digestion: Heavy metal pollution and kinetic performance

This study shows, for the first time, how the natural biodegradation of the Phaeophyceae Rugulopteryx okamurae (R.o.) affects its methane yield, by biochemical methane potential assays, and the methane production kinetics. Additionally, a mechanical (zeolite-assisted milling) and a thermal (120 °C, 45 min) pretreatments were assessed. The highest methane yield was obtained from the mechanically pretreated fresh ashore biomass (219 (15) NLCH4 kgVS-1), which presents the use of zeolite during milling as an economical alternative for heavy metal toxicity reduction. Moreover, no significant differences were observed between the other tests (with the exception of the lowest value obtained for the mechanically pretreated fresh R.o.). Low methane yields were linked to the heavy metal content. However, an increase of 28.5 % and 20.0 % in the k value was found for the untreated fresh R.o. biomass and fresh ashore biomass, respectively, when subjected to thermal pretreatment. Finally, an enhancement of 80.5 % in the maximum methane production rate was obtained for the fresh ashore biomass milled with zeolite compared to the untreated fresh ashore biomass.

Assessment of economic viability and production costs for the innovative microfiltered digestate

This paper assesses the economic viability of the production process of an innovative form of digestate known as "microfiltered digestate", suitable for use as a fertilizer in driplines for permanent crops. A Break-Even Analysis was performed to determine the Break-Even Point which identifies the minimum quantity of products necessary to cover production costs at a certain selling price. Our results show that the production of microfiltered digestate provides positive economic viability for anaerobic digestion managers, providing them with an attractive market outlet and, at the same time, a new form of income. The experimental study was stated in Sicily, but it is replicable in any territorial context with anaerobic digestion plants. The results are in line with the principles of circular economy and are very current with reference to the use of organic fertilizers instead of chemical ones.

Carbon footprint of dairy manure management chains in response to nutrient recovery by aerobic pre-treatment

Aerobic pre-treatment of liquid dairy manure has previously been reported as an effective nutrient export and emissions mitigation approach. The first objective of this study was to experimentally determine the optimal intermittent aeration ratio for nutrient recovery from liquid dairy manure through an on-site pilot-scale reactor to partially reduce the required energy for the aerobic process. The second objective was to theoretically investigate the total carbon footprints of direct manure spreading on croplands and permanent manure storage in open anaerobic lagoons in response to nutrient removal by the optimal determined intermittent aerobic treatment ratio. Four scenarios (S) were included; S1 was the traditional scenario of manure spread on croplands without the aerobic pre-treatment, S2 was the modified scenario of manure spread on croplands that included the aerobic pre-treatment, S3 was the traditional scenario of manure storage in lagoons, and S4 was the modified scenario of manure storage in lagoons that included the aerobic pre-treatment. The results showed that comparable nutrient removal efficiencies could be obtained with a 5:1 intermittent aeration ratio. Total nitrogen (TN) and total phosphorus (TP) were recovered were 41.5 ± 1.3% and 37.0 ± 4.0%, respectively, in ammonium sulfate and phosphorus-rich sludge, while 55.3 ± 1.4% of the chemical oxygen demand (COD) was removed. The estimated total carbon footprint for S1, S2, S3, and S4 were 24.4, 37.9, 45.3, and 45.9 kg _CO2-eqton^-1, respectively. However, the total carbon footprint of S2' and S4', which used renewable-based energy to run the reactor instead of fossil-based energy used in S2 and S4, were estimated to 29.5 and 37.5 kg _CO2-eqton^-1, respectively. Clearly, applying the aerobic pre-treatment increased the total carbon footprint of all cases except S4', in which the total carbon footprint was mitigated by -17.2%. Accordingly, the aerobic pre-treatment is only recommended in the case of S4' from a carbon footprint point of view although it is an effective nutrient recovery technology.

High Purity Struvite Recovery from Hydrothermally-Treated Sludge Supernatant Using Magnetic Zirconia Adsorbent

Recovery of phosphorus from sludge will help to alleviate the phosphorus resource crisis. However, the release of phosphorus from sludge is accompanied by the leaching of large amounts of coexisting ions, i.e., Fe, Al, Ca, and organic matter, which decreases the purity of sludge-derived products. In this study, an adsorption-desorption process using magnetic zirconia (MZ) as the adsorbent is proposed to obtain a high purity recovery product. The process involves selective adsorption of phosphate from the hydrothermally treated sludge supernatant (HTSS) using MZ, followed by desorption and precipitation to obtain the final product: struvite. The results indicated that at a dosage of 15 g/L, more than 95% of phosphorus in the HTSS could be adsorbed by MZ. Coexisting ions (Ca²⁺, Mg²⁺, Fe³⁺, Al³⁺, SO₄^2-, NO₃^-, Cl^-, etc.) and organic matter (substances similar to fulvic and humic acid) in the HTSS had a limited inhibitory effect on phosphate adsorption. Using a binary desorption agent (0.1 mol/L NaOH + 1 mol/L NaCl), 90% of the adsorbed phosphorus could be desorbed. Though adsorption-desorption treatment, struvite purity of the precipitated product increased from 41.3% to 91.2%. Additionally, MZ showed good reusability, maintaining a >75% capacity after five cycles. X-ray photoelectron spectroscopy (XPS) indicated that MZ adsorbed phosphate mainly by inner-sphere complexation. This study provided a feasible approach for the recovery of phosphorus from sludge with high purity.

Systems analysis of digestate primary processing techniques

In this paper, we performed technology assessment and systems analysis of primary digestate processing techniques to provide a comprehensive analysis of their environmental and cost performance. We compiled more than 100 observations from large-scale biogas plants and considered digestate based on manure, crops and agro-wastes, and food waste under the geographical contexts of Sweden and Belgium. Centrifuge, screw press, and rotary drum were identified as suitable primary processing techniques. We analyzed the climate impact, energy use, and operational cost of digestate management under these scenarios: no processing, partial processing (solid-liquid separation) and full processing (solid-liquid separation followed by ammonia stripping). As expected, the suitable digestate processing varied with the context, transport was often the most critical cost factor, and emissions from storage reduced the climate savings from the use of biofertilizers. However, treating liquid fraction became a main contributor to cost and climate impact under the Belgian conditions. Consequently, the possibility for local application of liquid fraction as biofertilizer could prevent costs and impacts associated with its further treatment. The main novelty of this work is in its integrative and comprehensive approach toward the choices and impacts of primary processing of digestate. We tried to bridge many individual case studies, drew from experiences of biogas plants in different geographical contexts, assessed suitable processing techniques for different digestate types, and analyzed the environmental impacts and cost of digestate management from a life cycle perspective. We believe that such integrated approaches would help decision-making for increased sustainability of the biogas sector.

Solid Digestate—Mathematical Modeling of Combustion Process

The combustion of waste biomass is becoming a key challenge for the energy sector, especially in times of growing energy demand and increasing environmental awareness of society. Processing such fuels is usually demanding. Mathematical modeling makes it possible to adapt the course of the process. The best option is to use appropriate fuel mixtures. The aim of this work was to develop a simple model that would allow the pre-selection of the mixture of two types of digestate. Residues from methane fermentation of maize and apple pomace were used in the study. The paper presents both a mathematical model and laboratory tests. The practical part was to check the validity of the model estimates. The model works on the basic data from physicochemical analysis. Laboratory tests included combustion of the samples of different mixtures. We used image analysis of thermograms to determine changes during combustion, which allowed us to determine when and where the main combustion occurs. Results show that a simple mathematical model is able to estimate the course of the combustion of a fuel particle.

Hydrochar and hydrochar co-compost from OFMSW digestate for soil application: 2. agro-environmental properties

The work concerns the study of the hydrochar from digestate and hydrochar co-compost characterization as amendments. The processes for hydrochar and co-compost production were described in Part 1 of this work (Scrinzi et al., 2022). The amendment properties of hydrochar (produced at 180-200-220 °C for 3 h) and co-composts (25%, 50%, and 75% hydrochar percentage of digestate substitution) were assessed by phytotoxicity, plant growth bioassay, and soil effect. Different seeds species (Lepidium sativum, Cucumis sativus, and Sorghum bicolor sp.) were dosed at increased concentrations using both wet raw amendments and their water extracts. The chemical characterization showed phytotoxic compounds content depending on both the initial feedstock (digestate) and the HTC process; at the same time, the analysis highlighted the reduction of these compounds by composting (organic acid, polyphenols, salt concentration). The dose-response was analyzed by the Cedergreen-Streibig-Ritz model and the half-maximal effective concentration (EC50) was calculated based on this equation. The soil properties and GHG emissions measurements (CH₄, CO₂, N₂O, and NH₃) highlighted the effect on N dynamics and on soil respiration induced by substrates. The HC200 soil application determined a significant impact on CO₂ and N₂O emission and NH₃ volatilization (10.82 mol CO₂/m²; 51.45 mmol N₂O/m²; 112 mol NH₃/m²) and a significant reduction of total N and TOC (46% of TKN and 49% of TOC). The co-compost (75%) showed specific effects after soil application compared to other samples an increase of available P (48%), a greater content of nitrogen (1626 mg/kg dry basis), and a reduction of organic carbon (17%). Our results demonstrate the good quality of co-compost and at the same time the validity of this post-treatment for addressing many issues related to hydrochar use in the soil as an amendment, confirming the suitability of HTC process integration for digestate treatment in anaerobic digestion plants.

Selecting the optimal nutrients recovery application for a biogas slurry based on its characteristics and the local environmental conditions: A critical review

Digestate (effluent of biogas plants) became the main bottleneck for biogas industry expansion because it often exceeds the capacity of surrounding croplands as fertilizer. Nutrients recovery from digestate is a promising solution for closing nutrients cycles and generating high value-added byproducts. In fact, numerous nutrients recovery technologies were reported and utilized for that purpose. However, each technology has optimum working conditions, while digestates have different characteristics due to the different substrates, digestion conditions, and handling methods. On the other hand, no protocol has been reported yet for selecting the optimal nutrients recovery technology or sequenced technologies for different digestates regarding their characteristics and the surrounding environmental conditions. In this study, an interactive flowchart was suggested and discussed for selecting the most appropriate technology or sequential techniques among the different alternatives. The whole digestate utilization technologies, solid-liquid separation technologies, liquid and solid processing technologies were included.

Assessment of the Possibilities for the Use of Selected Waste in Terms of Biogas Yield and Further Use of Its Digestate in Agriculture

The utilization of municipal waste and sewage sludge as a source of energy is technically very difficult due to high variability of their physical and chemical properties. The aim of this study was to evaluate the efficiency of the conversion of biomass contained in the whitewater fraction of municipal waste and sewage sludge by means of methanogenesis. The second objective was to assess the chemical composition of the digestate in the context of its use for fertilizer purposes. The whitewater fraction of municipal waste and sewage sludge was subjected to methanogenesis under static experimental conditions, according to DIM DIN 38414 methodology. The methanogenesis of concentrated substrates used in agricultural biogas plants was taken as a reference to evaluate the efficiency of the process. The organic fraction of the municipal waste was characterized by approximately 30% lower value of the soluble COD, with a comparable level of total COD compared to other materials. The total biogas yield, i.e., 404 dm³ per 1 kg of dry weight of the batch, was measured in the facility with sewage sludge. In COD value, this is 0.232 dm³·g O₂ COD. In the case of corn, these values were, respectively, 324 dm³ and 0.193, and for the organic sub-sieve fraction of municipal waste, 287 dm³·kg⁻¹ dw or 0.178 dm³·g O₂ COD, respectively. The type of fermented material did not affect the intensity of biogas production. The maximum level of biogas production occurred between the 13th and 15th day of the process. The digestate obtained in the process of methanogenesis of corn silage and the organic fraction of municipal waste was characterized by good parameters in terms of possible use for fertilization purposes.

Best Available Technology for P-Recycling from Sewage Sludge—An Overview of Sewage Sludge Composting in Austria

In order to close the phosphorus cycle in the long term, efficient recycling processes are necessary to ensure that this critical nutrient can be returned to arable land. Sewage sludge recycling is of particular importance due to the relatively high phosphorus content of sewage sludge. In this article, the current recycling paths of Austrian sewage sludge are highlighted, focusing on the advantages and limitations of sewage sludge composting. In addition to nutrient contents, pollutant loads were also analyzed in order to also discuss the limitations of this recycling pathway. Therefore, data from Austrian composting plants with focus on sewage sludge are used. The results show that the currently relevant pollutants (heavy metals) are predominantly below the limits prescribed for recycling and spreading on arable land. However, in order to decide on a recycling path at an early stage, a pollutant monitoring system must be in place. Due to pollution, mono-incineration with subsequent phosphorus recovery is also currently being discussed in Austria. Mono-incineration can represent an important component of sewage sludge disposal, because some sewage sludges are not suitable for composting due to potential environmental hazards. Therefore, it is important that evidence-based limit values and measures for the reduction in pollutants for input sources are determined.

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.

Impact of digestate addition on the biostabilization of undersized fraction from municipal solid waste

Biostabilization is a commonly applied method in mechanical-biological treatment (MBT) plants to process municipal solid waste. In many ways, e.g. by applying additives to waste, MBT plant operators strive to enhance the effectiveness of biostabilization, which leads to reducing the time and energy outlays necessary for the process, as well as to minimizing the amount of final stabilized waste directed to landfills. This paper deals with the impact of digestate waste from agricultural biogas plants used as additive to the biostabilization process of undersized fraction from municipal solid waste (UFMSW) on the intensive phase of the process and properties of stabilized waste. The aim of this study was to assess whether, and if so to what extent, the application of digestate waste affects the process. Five different input compositions were tested (without digestate and with the addition of digestate at: 2.5; 5; 7.5 and 10 wt%). Waste treatment time was 2 weeks. Changes in moisture content, organic matter (OM), respiration activity (AT4), bulk density, air-filled porosity, heavy metal content, pH, carbon to nitrogen ratio, as well as composition of process gases emitted were evaluated. Additionally, microorganisms (including pathogens) inhabiting the processed waste in the aspect of waste sanitation were analyzed. It was found that the addition of digestate at 2.5, 5 and 7.5 wt% extended the duration of the thermophilic phase and decreased the CO₂ content in process gases. The addition of digestate at 2.5 wt% and 5 wt%, decreased also OM by approx. 25% of the initial value and AT4 by approx. 30%. It was also proved that the addition of digestate favors the limited sanitation of UFMSW. As a result of the research, it was found that the addition of digestate at 2.5 wt% and 5 wt% is sufficient to accelerate the aerobic biological degradation of UFMSW.

The valorization of the anaerobic digestate from the organic fractions of municipal solid waste: Challenges and perspectives

The anaerobic digestion is a well-established process for the treatment of organic solid waste, pursuing its conversion into a methane rich gas destined to energy generation. Research has largely dealt with the enhancement of the overall bioconversion yields, providing several strategies to maximize the production of bio-methane from the anaerobic processing of a wide variety of substrates. Nevertheless, the valorization of the process effluents should be pursued as well, especially if the anaerobic digestion is regarded in the light of the circular economy principles. Aim of this work is in identifying the state of the art of the strategies to manage the digestate from the anaerobic processing of the organic fractions of municipal solid waste. Conventional approaches are described and novel solutions are figured out in order to highlight their potential scale up as well as to address future research perspectives.

A review on facilitating bio-wastes degradation and energy recovery efficiencies in anaerobic digestion systems with biochar amendment

In this review, progress in the potential mechanisms of biochar amendment for AD performance promotion was summarized. As adsorbents, biochar was beneficial for alleviating microbial toxicity, accelerating refractory substances degradation, and upgrading biogas quality. The buffering capacity of biochar balanced pH decreasing caused by volatile fatty acids accumulation. Moreover, biochar regulated microbial metabolism by boosting activities, mediating electron transfer between syntrophic partners, and enriching functional microbes. Recent studies also suggested biochar as potential useful additives for membrane fouling alleviation in anaerobic membrane bioreactors (AnMBR). By analyzing the reported performances based on different operation models or substrate types, debatable issues and associated research gaps of understanding the real role of biochar in AD were critically discussed. Accordingly, Future perspectives of developing biochar-amended AD technology for real-world applications were elucidated. Lastly, with biochar-amended AD as a core process, a novel integrated scheme was proposed towards high-efficient energy-resource recovery from various bio-wastes.

Performance evaluation of digestate spreading machines in vineyards and citrus orchards: preliminary trials

This research was carried out to evaluate a local biogas plant's solid fraction digestate spreading in a citrus orchard and vineyard. Three spreaders were tested: a broadcast manure spreader in the citrus orchard, and two cylindrical-shaped spreaders in the vineyard; the first one working in broadcast configuration, the second one in localised configuration. Experimental tests assessed effective work time, mean work speed, digestate flow rate and longitudinal and transverse spreading uniformity. In the citrus orchard, the digestate was mainly spread in the centre of the inter-row (around 66%), with low variability between inter-rows (coefficient of variation (CV) equal to 2.7%) and much higher variability within inter-rows (CV = 31.4%). The effective work time was about 28% of total field time and real work capacity was about 0.96 ha h⁻¹. In the vineyard, broadcast spreading released more on the right compared to the left (ratio 1.74) due to distributor disc rotation, whereas localised spreading was more uniform. Overall, variability between inter-rows had CV = 15.1% and within inter-rows CV = 33.3%. Real work capacity was about 0.16 ha h⁻¹ for broadcast spreading and 0.26 ha h⁻¹ for localised spreading. A preliminary economic evaluation, based on sub-contractor tariffs, produced the mean tariff for transaction and spreading costs of digestate in farms near the biogas plant.

What Could China Give to and Take from Other Countries in Terms of the Development of the Biogas Industry?

Anaerobic digestion is one of the most sustainable and promising technologies for the management of organic residues. China plays an important role in the world’s biogas industry and has accumulated rich and valuable experience, both positive and negative. The country has established relatively complete laws, policies and a subsidy system; its world-renowned standard system guarantees the implementation of biogas projects. Its prefabricated biogas industry has been developed, and several biogas-linked agricultural models have been disseminated. Nonetheless, the subsidy system in China’s biogas industry is inflexible and cannot lead to marketization, unlike that of its European counterpart. Moreover, the equipment and technology levels of China’s biogas industry are still lagging and underdeveloped. Mono-digestion, rather than co-digestion, dominates the biogas industry. In addition, biogas upgrading technology is immature, and digestate lacks planning and management. China’s government subsidy is reconsidered in this work, resulting in the recommendation that subsidy should be based on products (i.e., output-oriented) instead of only input subsidy for construction. The policy could focus on the revival of abandoned biogas plants as well.