Solar composting greenhouse for organic waste treatment in fed-batch mode: Physicochemical and microbiological dynamics

Effect of different bulking agents on fed-batch composting and microbial community profile

The current study focused on analyzing the effect of different types of bulking agents and other factors on fed-batch composting and the structure of microbial communities. The results indicated that the introduction of bulking agents to fed-batch composting significantly improved composting efficiency as well as compost product quality. In particular, using green waste as a bulking agent, the compost products would achieve good performance in the following indicators: moisture (3.16%), weight loss rate (85.26%), and C/N ratio (13.98). The significant difference in moisture of compost products (p < 0.05) was observed in different sizes of bulking agent (green waste), which was because the voids in green waste significantly affected the capacity of the water to permeate. Meanwhile, controlling the size of green waste at 3-6 mm, the following indicators would show great performance from the compost products: moisture (3.12%), organic matter content (63.93%), and electrical conductivity (EC) (5.37 mS/cm). According to 16S rRNA sequencing, the relative abundance (RA) of thermophilic microbes increased as reactor temperature rose in fed-batch composting, among which Firmicutes, Proteobacteria, Basidiomycota, and Rasamsonia were involved in cellulose and lignocellulose degradation.

Determining the extraction conditions and phytotoxicity threshold for compost maturity evaluation using the seed germination index method

The phytotoxicity of the compost aqueous extracts determines the maturity. To improve the accuracy of compost maturity evaluation using the seed germination index (GI) method, different extraction methods (different moisture content and extraction ratio) were designed to obtain samples with various phytotoxic level. This study analyzed the effects of different extraction condition of compost samples on GI, and established the relationship between phytotoxicity and GI. The results showed that the moisture content and extraction ratio of the compost significantly affected the GI. The extraction ratio for the compost with 60-70 % moisture content was 1:10 (ratio of compost mass to extract volume). However, commercial compost, which must have a moisture content of 30-45 %, had an extraction ratio of 1:30 (w:v). More importantly, compost extraction based on dry weight, with a moisture content of 10-15 %, more effectively reflected the phytotoxicity variations during composting. In such cases, the extraction ratio should be at least 1:30 (w:v) but not exceed 1:50 (w:v). The relationship between phytotoxicity and GI showed that dissolved organic carbon and dissolved nitrogen were the most important factors influencing GI, followed by NH4+, electrical conductivity, K, volatile fatty acids, Zn, and Cu. For composts with a GI greater than 70 %, the dissolved organic carbon, dissolved nitrogen, and NH4+ concentrations were below 257, 164, and 73 mg/L, respectively. These findings provide an optimized standard method for compost maturity evaluation using GI and a concentration threshold of key phytotoxicity is proposed to achieve accurate control of compost maturity.

Mitigation of phytotoxic effect of compost by application of optimized aqueous extraction protocols

The abuse of chemical fertilizers in recent decades has led the promotion of less harmful alternatives, such as compost or aqueous extracts obtained from it. Therefore, it is essential to develop liquid biofertilizers, which in addition of being stable and useful for fertigation and foliar application in intensive agriculture had a remarkable phytostimulant extracts. For this purpose, a collection of aqueous extracts was obtained by applying four different Compost Extraction Protocols (CEP1, CEP2, CEP3, CEP4) in terms of incubation time, temperature and agitation of compost samples from agri-food waste, olive mill waste, sewage sludge and vegetable waste. Subsequently, a physicochemical characterization of the obtained set was performed in which pH, electrical conductivity and Total Organic Carbon (TOC) were measured. In addition, a biological characterization was also carried out by calculating the Germination Index (GI) and determining the Biological Oxygen Demand (BOD₅). Furthermore, functional diversity was studied using the Biolog EcoPlates technique. The results obtained confirmed the great heterogeneity of the selected raw materials. However, it was observed that the less aggressive treatments in terms of temperature and incubation time, such as CEP1 (48 h, room temperature (RT)) or CEP4 (14 days, RT), provided aqueous compost extracts with better phytostimulant characteristics than the starting composts. It was even possible to find a compost extraction protocol that maximize the beneficial effects of compost. This was the case of CEP1, which improved the GI and reduced the phytotoxicity in most of the raw materials analyzed. Therefore, the use of this type of liquid organic amendment could mitigate the phytotoxic effect of several composts being a good alternative to the use of chemical fertilizers.

Advances in the Application of the Non-Conventional Yeast Pichia kudriavzevii in Food and Biotechnology Industries

Pichia kudriavzevii is an emerging non-conventional yeast which has attracted increased attention for its application in food and biotechnology areas. It is widespread in various habitats and often occurs in the spontaneous fermentation process of traditional fermented foods and beverages. The contributions of P. kudriavzevii in degrading organic acid, releasing various hydrolase and flavor compounds, and displaying probiotic properties make it a promising starter culture in the food and feed industry. Moreover, its inherent characteristics, including high tolerance to extreme pH, high temperature, hyperosmotic stress and fermentation inhibitors, allow it the potential to address technical challenges in industrial applications. With the development of advanced genetic engineering tools and system biology techniques, P. kudriavzevii is becoming one of the most promising non-conventional yeasts. This paper systematically reviews the recent progress in the application of P. kudriavzevii to food fermentation, the feed industry, chemical biosynthesis, biocontrol and environmental engineering. In addition, safety issues and current challenges to its use are discussed.

Microbial cooperation promotes humification to reduce antibiotic resistance genes abundance in food waste composting

Antibiotic resistance genes (ARGs) fate in a full-scale Food waste (FW) facility was investigated. Results showed that with the changes in ARGs, microbial networks could be naturally divided into two clusters, named as the ARGs increasing group (AI group) and the ARGs decreasing group (AD group). The significant difference between two groups (i.e. stronger microbial competition in the AI group and stronger microbial cooperation in the AD group) implied that the variation in ARGs over time were caused by a switch between competition and cooperation. These results indicated that microbial competition might increase ARGs abundance, while cooperation might reduce it. Meanwhile, structural-equation-model (SEM model) showed that humification indexes (e.g. GI value) was an indicator for characterizing microbial interactions and ARGs. The results of the linear model further confirmed that mature compost (GI values > 92.6 %) could reduce the risk of ARGs.

Optimization of vegetable waste composting and the exploration of microbial mechanisms related to fungal communities during composting

The application of additives to regulate the microbial functional composition during composting has attracted much research attention. However, little is known about the succession and role of the fungal community in the laboratory-scale composting of vegetable waste supplemented with pig manure and microbial agents. The purpose of this study was to identify effective additives for improving vegetable waste composting performance and product quality, and to analyze the microbial community succession during composting. The results showed that the combined addition of pig manure and microbial agents (T2 treatment) accelerated the pile temperature increase, enhanced total organic carbon degradation (23.36%), and promoted the maturation of the compost. Furthermore, the T2 treatment increased the activities of most enzymes, reshaped the microbial community, and reduced the relative abundance of potential animal (1.60%) and plant (0.095%) pathogens. The relative abundance of Firmicutes (71.23%) increased with the combined addition of pig manure and microbial agents in the thermophilic stage. In the middle and late stages, Saccharomonospora, Aspergillus, and Thermomyces, which were related to C/N and total phosphorus, were enriched in the T2 treatment. Network analysis demonstrated that the complexity and stability of the fungal network were more evidently increased in the T2 treatment, and Saccharomonospora, Aspergillus, and Microascus were identified as keystone taxa. The keystone taxa associated with extracellular enzymes contributed significantly to compost maturation. These results provide a reference for the application of additives to improve compost safety in pilot-scale composting.

Effects of Regular Water Replenishment on Enzyme Activities and Fungal Metabolic Function of Sheep Manure Composting on the Qinghai-Tibet Plateau

The dry climate characteristics of the Qinghai-Tibet Plateau will seriously affect microbial metabolism during composting. In this study, we aimed to investigate the effects of regular water supplementation on the fungal and enzymatic activities of sheep manure composting in the Qinghai-Tibet Plateau. The experiment set up the treatments of water replenishment once every 7 days(T2) and 3.5 days (T3) days, and no water supplementation was used as the control (T1). The results showed that regular water supplementation increased the activities of various enzymes during composting, and the activities of protease, cellulase, peroxidase and polyphenol oxidase in T3 were higher than those in T2. Regular water supplementation increased the relative abundance of Remersonia and Mycothermus, which were significantly positively correlated with the germination index, and degradation of organic components. Regular water supplementation could enrich fungi carbohydrate, protein, and nucleotide metabolisms, and T3 had a better effect. A redundancy analysis showed that environmental factors could significantly affect the fungal community; among them, moisture content (76.9%, p = 0.002) was the greatest contributor. In conclusion, regular water supplementation can improve the key enzyme activities and fungal metabolic function of sheep manure composting, and water replenishment once every 3.5 days had the best effect.

Microbial interaction promote the degradation rate of organic matter in thermophilic period

Composting is an efficient, microbe-driven method for the biodegradation of solid organic substrates. In such a complex engineering ecosystem, microbial interaction is more important to function than relative abundance and alpha diversity. However, microbial interaction and its driving force in the composting process has been rarely reported. Thus, we combined network analysis and positive cohesion to analyze the relationship between cooperation among bacteria taxa and the degradation of organic matter in ten industrial-scale food waste composting piles. The results showed that although the complexity of network and microbial diversity were inhibited by high temperature, microbial cooperation was stimulated in the thermophilic period. The positive cohesion, which reflected the degree of microbial cooperation, tended to be positively correlated with the degradation rate of organic matter, functional genera, and genes associated with organic matter degradation. Thus, microbial cooperation was a key factor in the promotion of the degradation of organic matter. From the insight microbial community, Thermobifida was the genera with high abundance, high occurrence frequency, and high contributions to microbial structure. Additionally, it was not only highly associated with the degree of cooperation but was also highly linked with the functional genera in the composting, implying that it might play an important role in regulating cooperation to promote the functional genera. Our research provides a deep understanding of the interaction among bacteria taxa during the composting process. Focusing on the abundance of Thermobifida might be an efficient way to improve composting quality by enhancing the cooperation of microbes.

Black soldier fly larvae for treatment and segregation of commingled municipal solid waste at different environmental conditions

The commingled nature of the municipal solid waste in many developing nations is one of the primary reasons behind the failure to its successful management. Although there are technologies to effectively treat and process well-segregated and classified waste, they are ineffective in managing the commingled waste. Commingled waste has neither calorific value enough to support waste-to-energy operations nor is it suitable for producing quality compost to generate market value. In this article, a novel Black Soldier Fly Larva (BSFL) -based technology has been proposed and tested to auto-segregate and treat the biodegradable fraction of the Commingled Municipal Solid Waste (CMSW). Several BSFL feeding experiments on five different CMSW compositions were conducted at various temperatures and relative humidity conditions. BSFL could selectively consume the biodegradable fraction of the CMSW to convert it into its body mass and separate itself from the rest of the waste, which mainly consisted of inert and recyclable waste fractions that can be further treated using appropriate waste treatment technology. The mature larvae or the pre-pupae grown at the expense of the biodegradable waste fraction can find several commercial uses. The larvae's consumption rate and weight gain were dependent on the environmental conditions; 30 °C and relative humidity of 65-75% provided the highest consumption rate and most significant weight gain. The batch experiments in the laboratory proved that BSFL could be promising for the treatment of CMSW. The experimental data presented may help design a process for further scaling up an effective treatment method for CMSW, which might benefit many developing nations in managing their waste effectively and economically.

Selection of sensitive seeds for evaluation of compost maturity with the seed germination index

The seed germination index (GI) is a widely used indicator of compost maturity and is a required index in many national standards. However, seeds of different species vary markedly in sensitivity to the biological toxicity of compost. Therefore, suitable seed selection is essential for evaluation of compost maturity with the GI. This study systematically investigated the germination percentage and root length for seeds of 17 species incubated in deionized water for 48 h at 25 °C in the dark. Based on the germination percentage, seeds of eight species (white radish, cucumber, fruit radish, edible rape, round radish, hybrid cucumber, cress, and Chinese cabbage) were selected for determination of the GI of chicken manure composted with differing proportions of tobacco powder and mushroom substrate. The GI of hybrid cucumber seeds showed a significant positive correlation with temperature and pH, and a significant negative correlation with E₄/E₆ ratio and ammonium-nitrogen content. The change in GI of hybrid cucumber seeds during composting and the GI value at the completion of composting were consistent with other maturity indicators. Among the tested seeds, the biological toxicity of the compost was best characterized by hybrid cucumber seeds, which thus represented a sensitive and reliable seed suitable for evaluation of compost maturity with the GI.

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.