Effect of aeration pathways on spatial homogeneity during in-vessel composting.

摘要

During reactor composting significant profiles of temperature and moisture develop resulting in sub-optimal conditions for biodegradation, lower reactor utilization and higher cost of composting. In order to improve reactor utilization, pathogen removal, and degradation rates, reactors have to be designed for homogeneity during composting. In this research experimental and modeling approaches were used to quantify the effect of controllable physical factors such as depth of bed, initial moisture level and compactability on homogeneity in reactor composting.; The effects of compactability and initial moisture content on air permeability of biosolids compost media were evaluated in the laboratory. An airflow model was used to describe the behavior of the compost with changes in moisture and compaction. Model parameters were evaluated at four moisture levels in the range of 42-57% (wb).; A two dimensional finite difference model was implemented to solve the momentum, heat transfer, mass transfer and degradation equations. The model was used to predict spatial variations in airflow, temperature, moisture and oxygen concentrations in the composting reactor. Amount of degradation, spatial homogeneity and cost of aeration were evaluated for numerous scenarios of operation, namely: different (1) initial moisture levels, (2) depth of bed, (3) ambient air temperature, and (4) material degradability, and the effects of (5) cooling air recirculation, and (6) blockage of plenum due to hard pan formation. Results indicate that cost of aeration were lowest at 55% moisture level and bed depth of 3.5 m. Energy requirement per unit substrate degraded decreased as the ambient temperature increased.; A field study was performed in a full scale commercial composting reactor (60 ft x 20 ft x 10 ft) and the model was parameterized and validated. Spatial and temporal changes in physical properties such as temperature, moisture, volatile matter, compactability, and particle size distribution were measured during the 21 day composting process in the full scale reactor. Significant profiles of temperature and moisture existing were verified under two conditions of initial moisture levels and ambient temperatures. It was found that lower initial moisture levels (about 50%) resulted in lesser compaction and lesser profiles of temperature, moisture and degradation.