The important parameters of composting are temperature, pH, moisture content, and oxygentransfer, which is regulated by aeration, free airspace, and agitation. The main properties offeed materials include C/N ratio, ratio size, rigidity, and nutrient and lignin compost.
Carbon and nitrogen are the two most important elements in the composting process, as one or the other is normally a limiting factor [18]. Carbon serves primarily as an energy source for
microorganisms, while a small fraction of the carbon is incorporated in their cells. Nitrogen is critical for microbial population growth, as it is a constituent of protein which forms over 50% of dry bacterial cell mass. If nitrogen is limiting, microbial populations will remain small and it will take longer to decompose the available carbon. Excess nitrogen, beyond the microbial requirements, is often lost from the system as ammonia gas or other mobile nitrogen and can cause odours or other environmental problems. While the typically recommended C:N ratios for composting municipal solid waste (MSW) are 25:1 to 40:1 by weight, these ratios may
need to be altered to compensate for varying degrees of biological availability. Moisture management requires a balance between these two functions: microbial activity and oxygen supply [18]. Moisture is essential to the decomposition process, as most of the decom-position occurs in thin liquid films on the surfaces of particles. Excess moisture will fill many of the pores between particles with water, limiting oxygen transport. A minimum moisture content of 50 to 55% is usually recommended for high rate composting of MSW. The heat and airflow generated during composting evaporate significant amounts of water and tend to dry the material out. During the active composting phase, additional water usually needs to be added to
prevent premature drying and incomplete stabilization. MSW compost mixtures usually start at about 52% moisture and dry to about 37% moisture prior to final screening and marketing. Oxygen and temperature fluctuate in response to microbial activity, which consumes oxygen and generates heat [18]. Oxygen and temperature are linked by a common mechanism of control: aeration. Aeration both resupplies oxygen as it is depleted and carries away excess94 Compostable Polymer Materials heat. Inadequate oxygen levels lead to the growth of anaerobic microorganisms which can produce odorous compounds. Oxygen concentrations in the large pores must normally be at least
12–14% (ideally 16–17%) to allow adequate diffusion into large particles and water filled pores. Most MSW composting systems used a forced aeration system with blowers and distribution pipes to supply oxygen during the initial phases of active composting. Temperatures of 45 to 59oC provide the highest rate of decomposition, with temperatures above 59oC reducing the rate of decomposition due to a reduction in microbial diversity [18]. Since temperatures in excess of 55oC for several days are usually required for pathogen control,the ideal temperature operating range is relatively narrow. Composting systems attempt to control temperatures to a narrow range near 55 to 60oC in order to compromise between reaction rate, pathogen reduction, and odour generation. To maintain these temperature ranges, heat gains
from microbial activity need to be balanced against heat losses, which occur primarily through evaporation of moisture and heating the aeration rate. Temperature, like oxygen supply, is usually managed by an aeration system: the same air which supplies oxygen can carry away excess heat.
