| Degrading kinetics of aerobic composting process is one of the fundamental factors for composting simulation including modeling of biological heat production, biological water production as well as biological oxygen demand. In other words, mathematical modelings of heat production, water production and biological oxygen demand are often based on degrading kinetics. In addition, the changes of organic maters during composting process makes a significant influence on degradation yields for heat, water as well as carbon oxygen. Therefore, studies of degrading kinetics are of great interest. In order to get a better understanding on degradation kinetics of composting, the following studies were performed with experimental analysis and mathematical simulation.1. Analysis the influence of free airspace to substrate degradationTo study the effects of different initial free air space (FAS) on composting degradation of pig slurry-wheat straw with lab-scale reactor system, three levels of FAS (56.70%,62.67%,68.36%) were obtained by mixing pig slurry and wheat straw with the same mass ratio but in different wheat straw sizes (<lcm,2-5cm,-7cm). Oxygen concentration and temperature were monitored by sensors during the whole composting period, while moisture and volatile solid (VS) content were measured by chemical analysis method at the beginning and the end of composting. In addition, a Monod model was employed to simulate VS content during composting period. According to the results of composting test, changes of oxygen concentration, temperature, moisture and VS based on different initial FAS levels were significantly different, which indicated that the effects of FAS had influence on substrate degrading and those effects should be took into account for degrading simulation.GUIs were developed to facilitate FAS study based on the model developed.2. Degrading analysis of different types of substrate during composting of swine manure in a laboratory-scale reactorIn order to study the degrading characteristic of volatile solid content, soluble carbohydrate content, cellulose content, crude fat content and crude protein content for processing of swine manure composting, three parallel composting experiments were conducted with laboratory scale composting reactors. Same amount of swine manure was mixed with equal volume of wheat straw or plastic bulking agents. Temperature, oxygen concentration, moisture content, volatile solid content, soluble carbohydrate content, cellulose content, crude fat content and crude protein content were monitored and recorded respectively. Mathematical models were employed to detect changes of free air space and its effect on substrate decomposition. According to the results of composting experiments, temperature of composting pile using wheat straw as bulking agents raise faster than the one with plastic bulking agents at the beginning of the composting. Meanwhile wheat straw bulking agents showed better performance on water-holding capacity. However, plastic bulking agents was better at keeping free air space of the composting pile. Degrading data of soluble carbohydrate content, cellulose content, crude fat content and crude protein content were recorded for next-step studies.3. Thermal and moisture balancing models based on substrate degradation kineticsIn the study regarding thermal balance, simulation models for organic matter content and composting temperature were developed based on the first order assumption with respect to the quantity of biodegradable volatile solid (BVS) and the law of thermal conservation, respectively, during composting. Different kinds of organic matter were mainly degraded in different composting stages and this was considered importantly for constructing models. For the modeling of organic matter degradation, temperature, moisture content, oxygen content and free air space were considered as key influence factors to degeneration rate. Regarding the model for thermal balance, forced convention and conduction were considered as main heat removal forms. In order to verify the models constructed, aerobic co-composting experiment of pig slurry and wheat straw was conducted. Temperature, oxygen content, moisture content and organic matter concentration were monitored during composting. Degrading kinetics constituted important role for thermal-balance modeling.Simulation models regarding moisture content were developed respectively by law of conversation of mass and first order assumption with respect to the moisture content. Approaches by law of conversation of mass to moisture content prediction was based on an analysis of biologically produced water, exit gaseous moisture content and inlet gaseous moisture content. The approach for moisture content prediction based on first order moisture evaporation kinetics was developed by an analysis of composting substrate moisture content and aeration rate. An aerobic composting process of pig slurry and wheat straw was conducted in order to verify the models constructed. The experimental results showed that modeling results gave good accuracy, and the relative standard errors were8.01%and8.86%for the models developed, respectively.4. An integrated mathematical model of composting process developed to test the feasibility of integrating degradable-substrate distinction into composting modelsDifferential equations regarding to heat balances, moisture content, oxygen concentrations, substrate decomposition and microbe growth were developed. Degradable substrates were divided into soluble substrate and insoluble substrate. Consequently, heat conversion factor was described as a state variable related to soluble substrate content. The model was validated against observed data obtained from lab-scale composting experiments with swine manure and wheat straw, regarding temperature, water and oxygen contents as well as substrate variations. Based on the experimental results, distinct substrate degradation model could be integrated with other physical and biological models. Further studies should be continued to build up a more generalized and structured substrate degradation model. |
PDF Full Text Request