| Aerobic composting is one of the main methods to sanitize, minimize, and reutilize livestock manure. The process will be affected by the physicochemical properties of composting materials and the strategies of composting operation. Development of mathematical models that explore the theoretical correlation between biochemical reactions and material variations duing composting, which could help better explain composting mechanisms, has became a research hotspot. This study aimed to improve the particl-scale oxygen uptake rate (OUR) model and develop a particl-scale methane (CH4) emission model for pig manure-wheat straw aerobic composting. Based on the optimization of methods for characterizing manure particle structures and the development of a Fourier transform infrared (FT-IR) microspectroscopic method for aerobic layer thickness (Lp) of manure particles, this study modified the particl-scale OUR model with practical characteristics of manure particles, as well as surface convection between composting materials and gas (Qsurf); on this basis this study also developed the particl-scale CH4 emission model for pig manure-wheat straw composting. This research enabled better insights into microbiological kinetics during composting, and provided methodological support and a theoretical basis for design of composting strategies that could simultaneously reduce ventilation cost and CH4 emission.The main innovation achievements and conclusions were obtained as follows.1. To characterize manure particle size and shape, vacuum freeze drying-mechanical vibration and digital image analysis were used as the preferred pretreatment and measurement methods, respectively. The median diameter (D50) of manure particles was 501±16μm and the span of particle size distribution was 1.45±0.04. Manure particles had an irregular shape with an aspect ratio of 0.57±0.01 and a sphericity of 0.61±0.01. During pig manure-wheat straw aerobic composting, the particle structures and chemical compositions of pig manure varied significantly, and the variation in particle structures was closely related to the organic matter degradation of pig manure. During composting, the D50 of manure particles exponentially decreased while the shape parameters were almost contant; particle porosity (6) linearly increased; the degree of degradation of aliphatics and polysaccharides was high, chemical construction tended to be stable, and the degradation of hemicellulose was a main reason for the changes in particle porosity; the evolution of organic matter content in pig manure did not completely follow the first-order reaction kinetics, and the revised first-order reaction constant was proportional to the reciprocal of D50 and reversely proportional to θ.2. This study was a first attempt to characterize the aerobic layer thickness (Lp) of manure particles by FT-IR Microspectroscopy. Manure particles were freeze-dried under vacuum and then sectioned into 10 μm slices. The optimal FT-IR Microspectroscopy parameters included the spectral range of 4000-650 cm-1, the spectral resolution of 16 cm-1, the pixel dimension of 6.25 μm×6.25μm, and a mean of 16 scans that was used to obtain the second derivative with 9 smoothing points.2856 cm-1 and 1568 cm-1 were selected as the characteristic wavenumbers. Along the radial directions of the particls, the difference between the second-derivative spectra at these two wavenumbers was used for quantification of the Lp. An exponential increase of Lp was observed. The change could be explained by multiple dynamic factors, such as oxygen concentration, temperature, and microbiological activity. In addition, an intermediate area of 20μm was detected between the aerobic layer and the anaerobic core of the manure particles.3. The revised OUR model was able to effectively simulate the OUR evolution during pig manure-wheat straw aerobic composting. The model’s accuracy, in terms of lower maximum and mean deviations of OUR, was improved. Consideration of the Qsurf provided a more precise simulation of the composting temperature of the thermophilic phase.4. The particle-scale CH4 model could effectively predict CH4 emission from pig manure-wheat straw aerobic composting (determination coefficient 0.94, root-mean-square error 2888 ppmv), particularly in the self-heating and cooling phases. Pig manure was characterized by a high CH4 yield coefficient (0.6414 mol CH4 mo-1 Cman) and maximum CH4 oxidation rate (0.0205 mol CH4 kg-1 VSaero h-1). During the mesophilic phase, the CH4 generation rate increased significantly while the CH4 oxidation rate was almost zero, and CH4 emission mainly occurred in this period. The amount of oxidized CH4 accounted for 10.34% of the total CH4 generation, which was relevant to the oxygen permeability in composting particles. The optimal ventilation strategy for this study was an intermittent aeration (6 h on/15 min off) at a rate of 8.34 L min-1. |
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