Air impingement drying of digestate

Drying behavior of solid digestate and reaction kinetics of ammonium degradation during laboratory-scale drying

Fundamental knowledge of the drying behavior and ammonia emission from digestate is required in order to properly design efficient drying processes. In this study, laboratory-scale drying experiments with two different digestates (D1 and D2) were conducted at four different drying temperatures (50, 60, 70, and 80 °C). The solid digestate D1 mainly consisted of plant silage (88.7%), while D2 comprised primarily of manure (55.9%). The drying experiments were performed in a controlled drying chamber using relatively small-sized samples of digestate (30 g for D1, 37 g for D2). These samples were characterized by an initial moisture content of about 74% wb (D1) and 78% wb (D2). This sample size mass was deliberately chosen to ensure homogeneous drying conditions within a thin layer and to facilitate precise laboratory quality measurements of parameters including water content, nitrogen contents (N-total, NH4-N), and pH values before and after the drying process. Both types of digestate exhibited a similar kinetics of drying and ammonium degradation. The first period of constant drying rate was followed by two periods with decreasing rates. Similarly, the ammonium degradation was characterized by three periods with decreasing reaction rates. Considering only the first reaction period herein, ammonium degradation was determined as a first-order reaction. The activation energy for this reaction period was obtained to be 20.00 kJ mol-1 for digestate D1 and 18.44 kJ mol-1 for digestate D2. The results of this study may serve as a basis for the conceptualization and design of optimized, continuous drying processes tailored to digestate materials.

A new cutting-edge review on the bioremediation of anaerobic digestate for environmental applications and cleaner bioenergy

Circular agriculture and economy systems have recently emerged around the world. It is a long-term environmental strategy that promotes economic growth and food security while reducing negative environmental consequences. Anaerobic digestion (AD) process has a high contribution and effective biodegradation route for bio-wastes valorization and reducing greenhouse gases (GHGs) emissions. However, the remaining massive digestate by-product contains non-fermented organic fractions, macro and/or micro-nutrients, heavy metals, and metalloids. Direct application of digestate in agriculture negatively affected the properties of the soil due to the high load of nutrients as well as the residuals of GHGs are emitted to the environment. Recycling and valorizing of anaerobic digestate is the main challenge for the sustainable biogas industry and nutrients recovery. To date, there is no global standard process for the safe digestate handling. This review described the biochemical composition and separation processes of anaerobic digestate. Further, advanced physical, chemical, and biological remediation's of the diverse digestate are comprehensively discussed. Moreover, recycling technologies such as phyco-remediation, bio-floc, and entomoremediation were reviewed as promising solutions to enhance energy and nutrient recovery, making the AD technology more sustainable with additional profits. Finally, this review gives an in-depth discussion of current biorefinery technologies, key roles of process parameters, and identifies challenges of nutrient recovery from digestate and prospects for future studies at large scale.

Exploring drying kinetics and fate of nutrients in thermal digestion of solid organic waste

Thermal digestion has emerged as a novel technique for the rapid treatment of solid organic waste (SOW). Dehydration mechanism and fate of nutrients during the thermal digestion of the SOW were explored. A series of experiments were carried out in a specially designed laboratory-scale dehydrator to determine its drying kinetics. The statistical analysis revealed that the diffusion model predicted the dehydration profile most accurately than other models. The effective moisture diffusivity coefficient depended on the temperature and varied from 2.81 × 10^-08 m²/s to 8.68 × 10^-08 m²/s at the tested temperature range. The activation energy required for complete dehydration was found to be 26.56 kJ/mol. The artificial neural network (ANN) model was found highly efficient (R² - 0.983) in predicting the total drying time required for attaining equilibrium moisture content. The total N decreased from 2.2% to 1.81% due to evaporation of ammonical nitrogen, while the availability of P and K was increased from 0.38% to 0.43% and 1.47% to 1.75%, respectively when the temperature was increased from 110 °C to 170 °C. The thermal dehydration technique was found effective in digesting the organics and improving the bioavailability of the nutrients, which favours for its re-utilization in agriculture.

Water Status and Predictive Models of Moisture Content during Drying of Soybean Dregs Based on LF-NMR

To explore the drying characteristics of soybean dregs and a nondestructive moisture content test method, in this study, soybean dregs were dried with hot air (80 °C), the moisture content was measured using the drying method, water status was analyzed using low-field nuclear magnetic resonance (LF-NMR) and the moisture content prediction models were built and validated. The results revealed that the moisture contents of the soybean dregs were 0.57 and 0.01 g/g(w.b.), respectively, after drying for 5 and 7 h. The effective moisture diffusivity increased with the decrease in moisture content; it ranged from 5.27 × 10^-9 to 6.96 × 10^-8 m²·s^-1. Soybean dregs contained bound water (T₂₁), immobilized water (T₂₂) and free water (T₂₃ and T₂₃'). With the proceeding of drying, all of the relaxation peaks shifted left until a new peak (T₂₃') appeared; then, the structure of soybean dregs changed, and the relaxation peaks reformed, and the peak shifted left again. The peak area may predict the moisture content of soybean dregs, and the gray values of images predict the moisture contents mainly composed of free water or immobilized water. The results may provide a reference for drying of soybean dregs and a new moisture detection method.

A Case Study of Turbulent Free Jet Flows Issuing from Rectangular Slots on Process Performances and Quality of Hot-Air-Dried Apple

This study deals with the improvement in drying process performances and the quality of the final product for industrial equipment in the food industry. Designers need to optimize the design parameters of devices to create synergies between the greater energy efficiency of the process and high-quality dried products. Air impingement drying was carried out on apple cylinders at 323 K and with air velocities ranging between 30 and 60 m s−1. The studied drying process presents a particular setup of jets as they are multiple rectangular slot jets issued from triangular nozzles. The effect of four design jet parameters (slot width, nozzle-to-surface height, nozzle-to-nozzle spacing, and airflow) on the drying process performances and the quality of the final product was analyzed and optimized using response surface methodology (RSM). A minimal influence of design jet parameters on the process performances was shown, while an important impact was observed on the quality of dried apple. The slot width and the nozzle-to-nozzle spacing had a significant effect on the textural and functional properties. Predictive models were established and good agreements were found between predictive and observed values. Sorption isotherms were properly modeled by the Guggenheim–Anderson–de Boer (GAB) model.

Production of high sensory quality Shiitake mushroom (Lentinus edodes) by pulsed air-impingement jet drying (AID) technique

The effect of pulsed air-impingement jet drying (AID) on the sensory qualities of Shiitake mushroom was comprehensively examined compared with hot air drying (HAD) and vacuum freeze-drying (VFD). AID considerably improved the characteristic flavors (onion-like odor and umami) of dried mushrooms by partially inhibiting enzymatic and Maillard reactions. The texture characteristics (rehydration and shrinkage) of AID mushrooms had no significant difference to VFD ones and were better than HAD ones. AID combined the advantages of HAD and VFD technologies and outperformed HAD and VFD in terms of overall quality, though the total content of free amino acids and soluble sugars of AID mushrooms dropped slightly. In summary, AID is a promising drying technology for obtaining high sensory quality Shiitake mushrooms compared to prevailing drying methods.