[Stratification of bulk density and its dynamics in the process of co-composting]

Co-composting of sewage sludge and pig manure was studied. At the stages of temperature starting and high temperature, the bulk density were 0.82 g x cm(-3) and 0.66 g x cm(-3), respectively, but oxygen in the pile at each stage was enough for the microorganism to live on. At the stage of cooling, the bulk density was 0.58 g x cm(-3), which made aeration better. The bulk density was 0.54 g x cm(-3) at the stage of maturing when aeration is best. The stratified effect of bulk density occurred, near the door of the pile at the stages of starting and high temperature, almost all parts of the pile at the stage of cooling, however, it weakened at the stage of the maturing. The depth had remarkable influence upon the bulk density of compost, by the axis of the pile at the stage of the starting, near the inner wall at the stage of high temperature, for the whole pile at the stage of cooling, and near the door and axis at the stage of maturing. The door had significant effect on the bulk density for both stages of starting and maturing. The bulk density increased with the depth of the pile at every stage of composting. Dynamics of bulk density met with two-order kinetic equation.