Benefits to decomposition rates when using digestate as compost co-feedstock: Part I - Focus on physicochemical parameters

Nutrient recovery from anaerobic digestate: Fertilizer informatics for circular economy

This study explores the integration of fertilizer informatics into the circular economy, with a focus on enhancing nutrient recovery from anaerobic digestate. It utilizes advanced algorithms and data analytics to develop new nutrient management strategies essential for sustainable agriculture. This research provides a detailed assessment of current nutrient recovery technologies, evaluating their environmental impact, cost efficiency, and adaptability. Our findings highlight the importance of merging circular economy principles with fertilizer informatics, showcasing the potential for transforming waste into environmentally friendly fertilizers. This approach has significant implications for improving agricultural practices towards sustainability. The methodologies and insights presented are relevant for ongoing research in environmental stewardship and sustainable resource management. This study describes practical solutions and new perspectives, making it a valuable reference for future research.

Environmental implications, potential value, and future of food-waste anaerobic digestate management: A review

Globally, the valorisation of food waste into digestate through the process of anaerobic digestion is becoming increasingly popular. As a result, a large amount of food-waste digestate will need to be properly utilised. The utilisation of anaerobic digestion for fertiliser and alternative uses is essential to obtain a circular bioeconomy. The review aims to examine the environmental management of food-waste digestate, the value of digestate as a fertiliser and soil conditioner, and the emerging uses and improvements for post-anaerobic digestion reuse of digestate. Odour emissions, contaminants in food waste, emission and leaching of nutrients into the environment, and the regulations, policies, and voluntary initiatives of anaerobic digestion are evaluated in the review. Food-waste digestate can provide essential nutrients, carbon, and bio-stimulants to soils and increase yield. Recently, promising research has shown that digestates can be used in hydroponic systems and potentially replace the use of synthetic fertilisers. The integration of anaerobic digestion with emerging uses, such as extraction of value-added products, algae cultivation, biochar and hydrochar production, can further reduce inhibitory sources of digestate and provide additional economic opportunities for businesses. Moreover, the end-product digestate from these technologies can also be more suitable for use in soil application and hydroponic use.

Hydrodynamic zones and the influence of microorganisms on nitrogen transformation in the diverging area of branched rivers

Diverging area is widespread in river networks, and understanding its biogeochemical process characteristics is of great significance to river ecological restoration and environmental quality improvement. Microbial communities affected by hydrodynamics play an important role in biogeochemical processes, but their relationship in diverging area is little known. Here, the composition of microbial community and its feedback to hydrodynamics and nitrogen conversion in the diverging area of river networks were first studied by coupling ecological theory, biogeochemical theory, microbial DNA sequencing and mathematical model of water environment. The results showed that there were five hydrodynamic zones with significant velocity differences in the diverging area, namely low velocity zone, maximum velocity zone, stagnant zone, separation zone, and deflection zone. According to the flow velocity grouping, there were significant differences in the microbial diversity and abundance among low velocity group, maximum velocity group and stagnant group had significant differences (p < 0.05, stress = 0.1207). In the low velocity group, Firmicutes was the dominant phylum which had a highest abundance and may promot the conversion of organic nitrogen into ammonia nitrogen. In the maximum velocity group, Bdellovibrionota was the dominant phylum which had a highest abundance and may promot the conversion of nitrate and nitric oxide to nitrogen. In the stagnant zone, Methylomirabilota was the dominant phylum which had a highest abundance and may promot the conversion of nitrogen into nitrate and ammonium. In addition, dissolved oxygen was the most sensitive environmental factor for shaping microorganisms and nitrogen conversion in the diverging area of the river networks by canonical correlation analysis. The denitrifying bacteria Rhodocyclaceae, was shown to negatively correlated with the flow velocity. This research improves the scientific basis for the study of the ecosystem in river networks, which will guide the construction of river ecological projects.

Biodegradation kinetics of ammonium enriched food waste digestate compost with biochar amendment

High concentration of NH₄⁺-N in food waste digestate (FWD) produced from biological treatment of food waste is considered as a major threat on the composting process resulting in production of immature compost. Hence, a laboratory batch composting study was conducted to examine the feasibility of using biochar as a physical additive to ameliorate the inhibitory effect of NH₄⁺-N and to mitigate the nitrogen loss during FWD composting. FWD was co-composted with tobacco biochar at a dosage of 0%, 2.5%, 5% or 10% (dw) in bench-scale composters with a controlled aeration system. The addition of 10% biochar enhanced the degradation rate resulting in 44% higher carbon decomposition than the control. Besides, 10% biochar amendment reduced NH₃ and N loss by 58% and 5%, respectively and significantly reduced NH₄⁺-N content to HKORC limit of < 700 mg/kg dw within 5 days showing the beneficiary impact of biochar addition.

Food waste digestate composting: Feedstock optimization with sawdust and mature compost

Direct land application of food waste digestate (FWD) leads to 60-70% of nitrogen loss through NH₃ volatilization due to its innate characteristics like high ammonium nitrogen (NH₄⁺-N) (~6000 mg/kg dry matter) and high moisture content (~75%). Hence, bio stabilization of FWD through composting is a promising solution to curb the environmental and occupational hazards. Hence the aim of this study was to assess the feasibility of using sawdust and/or mature compost as a bulking agent to achieve effective composting. The results showed that mixing of FWD with sawdust alone or together with mature compost could produce quality compost with reduced NH₄⁺-N (<700 mg/kg dry matter) and increased seed germination index (>80%) within 2 weeks of co-composting. Composting FWD with both sawdust and mature compost effectively reduced ~ 83% of NH₃ volatilization demonstrating that this approach can effectively produce mature nitrogen enriched FWD compost.

Sustainable management and recycling of food waste anaerobic digestate: A review

Anaerobic digestion (AD) is a widely used technology to valorise food waste for biogas production yet a considerable amount of digestate remains under-utilised. Sustainable management and recycling of the nutrient-rich food waste anaerobic digestate (FWD) is highly desirable for closing resource loop and actualising circular economy. This work reviews the distinct properties of FWD and the existing treatment technologies. FWD shows great prospects as a nutrient source for microalgal cultivation and biofuel production. Emerging technologies such as thermal conversion (e.g., pyrolysis and hydrothermal treatment) of FWD into value-added products such as functionalised biochar/hydrochar with diverse applications would be attractive and warrant further research investigation. Integrated AD with subsequent valorisation facilities is highly encouraged to achieve complete utilisation of resources and reduce carbon emissions.

Degradation of bioplastics in organic waste by mesophilic anaerobic digestion, composting and soil incubation

The aim of the study was to assess the effects of high concentrations (10 % w/w, data projected for 2030) of commercial bioplastics, i.e. starch based shopping bags (SBSB) and polylactic acid (PLA) tableware, in the organic fraction of municipal solid wastes (MSW) on compost quality obtained by pilot-scale dry mesophilic anaerobic digestion and subsequent composting of the digestate. After the biological processes, 48.1 % total solids (TS) of SBSB and 15 % TS of PLA degraded, resulting in a high bioplastics content (about 18 % TS) in compost. Subsequent compost incubation in soils indicated that bioplastics degraded by pseudo-zero order kinetics (0.014 and 0.010 mg C cm^-2 d^-1 for SBSB and PLA, respectively), i.e. complete degradation was expected in 1.6 years (SBSB) and 7.2 years (PLA), confirming the intrinsic biodegradability of bioplastics. Nevertheless, enhancing the rate and amount of bioplastics degradation during waste management represents a goal to decrease the amount of bioplastics reaching the environment.

A review on nitrogen dynamics and mitigation strategies of food waste digestate composting

Food waste digestate is a by-product of the anaerobic digestion of food waste. Presence of high ammonium nitrogen content significantly increase the nitrogen loss upon direct application on soil or by conventional composting. In this review, a comprehensive discussion regarding the effective management of food waste digestate is outlined, in which global food waste digestate production, characteristics, and composting are discussed. The nitrogen dynamics cycle considering high ammonium nitrogen content in the digestate is also evaluated, including ammonification, nitrification, denitrification, and other possible mechanisms based on the current literature. Mitigation strategies for reducing nitrogen loss via C/N ratio adjustment and the addition of physical, chemical, and microbial amendments were evaluated and estimated for 15 countries based on the available data on food waste anaerobic digestion plants. Reduced nitrogen loss and high quality compost could be produced from food waste digestate by adapting mitigation strategies.

Exogenous enzyme amendment accelerates maturity and changes microflora succession in horse and wildlife animal manure co-composting

Composting has been a rational method to dispose of horse or wildlife animal manures, especially in the developed cities with horse clubs and wildlife parks. However, few studies have focused on the mechanism and improvement methods for composting the horse or wildlife animal manures. In this study, we investigated the effect of exogenous compound enzymes on thermophilic composting, which could potentially support the management of horse and wildlife animal manures. With the presence of exogenous enzymes, the duration of high temperature (> 60 °C) was significantly prolonged (p < 0.05), and the germination index was significantly improved (p < 0.05). More-efficient improvement of composting maturity was associated with the addition of that exogenous enzyme that might influence microflora succession and the interaction among microorganic communities, especially fungal, during the composting process. Furthermore, redundancy and canonical correspondence analyses indicated that the C/N ratio, temperature, and germination index were significant variations to influence bacterial communities (p < 0.05). The dominant Flavobacterium, Thermopolyspora, Thermomonospora, and Chaetomium and Saccobolus could play an essential role in carbohydrate and phytotoxin degradation, while Thermobispora and norank_f_Limnochordaceae could lead to temperature rising.

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.

Composting of anaerobic sludge from the co-digestion of used disposable nappies and expired food products

Anaerobic sludge originating from the co-digestion of used disposable nappies and expired food products treated in a pilot two-stage system was examined as feed material for a continuous pilot-scale composter (capacity: 300 L feed per week). The feed materials and final compost products were analyzed and evaluated for their suitability as compost materials. Ιn terms of stability, the compost products were identified as stable through static respiratory index measurement (0.11-0.24 g O₂/(kg Volatile Solids h)), heavy metals concentrations were within acceptable limits (i.e. concentration of Cu, Cd, Zn, Pb, Cr, As lower than 1 mg/kg dry mass) as well as polycyclic aromatic hydrocarbons (0.06-0.34 mg/kg dry mass lower than 6 mg/kg dry mass). During composting, significant losses of nitrogen from the digestate and the urea added for C/N correction were observed (51-75%), indicating that the adjustment of C/N ratio through the addition of chemicals is not efficient in composting processes with forced aeration and the pre-existing nitrogen in digestate was susceptible to air-stripping. The continuous composting process implemented proved capable of producing mature compost with a retention time of 14 d. The final products were within acceptable limits for all the parameters examined, except for the presence of pathogens (Salmonella and Enterococcus) which were not eliminated, even though the composter reached 56 °C for 3-4 days at the thermophilic stage. The characteristics of the anaerobic sludge samples examined indicate that direct land application of the anaerobic effluent should be considered as an option.

From anaerobic to aerobic treatment: upcycling of digestate as a moisturizing agent for in-vessel composting process

In Tunisia, there are crucial challenges facing both urban and rural areas, the most prominent of which are the production of organic waste, the need for waste treatment, the demand for water and energy and the need for a circular economy. To this end, the study was designed to develop a technical concept on closed cycle ‘biowaste to bioenergy’ treating, basically food waste (FW) through combined biological processes. In this approach, the generated digestate from FW anaerobic reactors was used successfully as a moisturizing agent for FW in-vessel composting. Four types of digestate were examined to be used as moisturizing agent (MA). The selection of the appropriate MA was achieved based on technical criteria; moisture content (MC), C:N ratio and heavy metals concentrations. The findings showed that the digestate obtained from anaerobic co-digestion of food waste and wheat straw (D1) was the most efficient AD-effluent to be added. In terms of composting process performance, the thermophilic phase of the amended reactor (A1) lasted 16 days and reached higher temperatures of about 72 °C, while the unamended one (A1) was characterized by a thermophilic temperature of around 66 °C indicating that the end products were of a pathogen-free compost. When it comes to the physico-chemical factors examined demonstrating that the biological conditions were sufficiently developed. The findings showed overall decreasing profiles during the composting period for moisture, C:N ratio as well as nitrification index (NI). From the quality-point of view, it was found that heavy metal concentrations had lower limits than those values set by German standards. Moreover, all the compost samples appeared to be stable and classified as class IV and V end product.

Crop-based composting of lignocellulosic digestates: Focus on bacterial and fungal diversity

In this study, organic matter degradation and microbial diversity were assessed during the composting of lignocellulose-rich digestates. Digestates were collected based on each crop type during anaerobic co-digestion of cow manure and barley, triticale, wheat and rye. Bacterial and fungal diversity in digestate composting systems were determined by 16S and 18S rRNA gene amplicon sequencing, respectively. Crop-based composting of anaerobic digestates showed similar process trends in terms of pH, temperature, moisture content (MC) and C:N ratio. The properties of final compost products were in accordance with the national legislations regarding soil applications, except MC, which were therefore air-dried before being amended to soil. Most abundant bacterial genera were represented by Luteimonas, Bacillus, Ochrobactrum and Thermobifida. Meanwhile, Thermomyces, Aspergillus, Galactomyces and Neurospora were detected as the predominant fungal genera in all compost samples.

Odor generation patterns during different operational composting stages of anaerobically digested sewage sludge

This study aimed to evaluate the global patterns of odor generation and odorant composition for different operational stages of anaerobically digested sewage sludge (ADS) composting at pilot scale. To this end, gas emissions were sampled and analyzed during storage, forced aeration treatment (active phase), turning process and curing. For each operational stage, odors were monitored by measuring the odor emission rates (OER in OU_E h^-1 kg^-1ADS) through dynamic olfactometry and computing the odor activity values (OAVs) of compounds quantified by analytical methods (i.e., GC/MS). Ammonia and volatile sulfur compounds (VSCs) were the most abundant air pollutants, representing 55.5% and 20.6% of the cumulative mass emitted, respectively. The first eight days of aerobic treatment and the first turning of the compostable mixture were the critical steps for odor generation with OER ranging from 30 to 317 OU_E h^-1 kg^-1ADS. Particularly, the first turning process was responsible for strong odor episodes that were emitted in a short process time (295 OU_E h^-1 kg^-1ADS). Based on the OAVs approach, dimethyl disulfide, dimethyl sulfide, and methanethiol were the predominant odorants along these early operational stages. Odor potential and composition shifted for the middle and later active phase, second turning, and curing stage where OER fluctuated from 0.18 to 12.6 OU_E h^-1 kg^-1ADS, and hydrogen sulfide showed the most substantial odor contribution. A principal component analysis explaining 77% of the variability in odor concentration and OAVs datasets eased the recognition of these odor patterns.

Characterization of digestate composting stability using fluorescence EEM spectroscopy combining with PARAFAC

A laboratory scale experiment of digestate composting was carried out using a reactor system. In this study, conventional physicochemical and biological analyses were carried out and fluorescence excitation-emission matrix (EEM) spectroscopy combined with parallel factor analysis (PARAFAC) was used to assess the maturity and stability during digestate composting. A four-component model was obtained and three components, i.e. fulvic-like (C1 and C3), protein-like (C2), and humic-like (C4) components, were identified. Furthermore, the ratios of each two components were calculated and the relationships with other parameters were established using Pearson correlation analysis. The results showed that the main humification process during digestate composting was the accumulation of fulvic-like substances and that secondary formation occurred at the late stage of digestate composting. Moreover, the EEM-PARAFAC technique could be used as a sensitive and efficient tool for assessing the dynamic changes of digestate composting. The ratio C4/(C1 + C3) is the most suitable indicator in evaluating the stability of digestate composting.

Effects of digestion time in anaerobic digestion on subsequent digestate composting

Digestion time (DT) in anaerobic digestion (AD) on performance of subsequent digestate composting regarding compost maturity and greenhouse gas (GHG) emission was investigated. Digestates for composting were obtained after anaerobically digested mixture of dairy manure, corn stalks, and tomato residues (48:32:20, volatile solids based) with DT of 15, 30, and 45 days, respectively. Digestates were composted with corn stalks (85:15, wet weight based). Results showed approximately 30% and 70% of biochemical methane potential (342.0 L/kg VS_feedstock) were obtained when DT of 15 and 30 days. Digestate co-composting with cornstalks could be initiated effectively and reduced GHG emissions by 18.9-29.0% compared to compost with raw materials. DT of 30 and 45 days digestate composting cause benefit on germination index. DT of 45 days had the highest net power production in combine AD and composting system. DT of 30 days digestate composting was optimum choice for compost maturity and GHG emissions.