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Process Optimization of Composting Systems
Naoto Shimizu
1. INTRODUCTION TO PROCESS OPTIMIZATION OF COMPOSTING SYSTEMS
Food supply is a primary issue for people around the world. Increasing demand for food has been anticipated by the increased intake of meat, fat, processed foods, sugar and salt nutrition transition. The livestock (cattle, swine, chicken) sector is a substantial source of nutrients for human consumption. In Japan, total production of animal waste in 2015 was 83 million tons. There is a need to develop management systems that use cattle manure effectively and without causing adverse environmental effect.
Problems associated with waste from animal husbandry are, safety, financial and environmental. Huge amounts of solid wastes from animal husbandry result in odor problems that can lead to complaints from neighbors and other people. Composting is a simple and energy efficient way to solve this problem. The purposes of composting are:
• Elimination of pathogens and weeds
• Microbial stabilization
• Reduction of volume and moisture
• Removal and control of odors
• Ease of storage, transport and use
Many studies have addressed the basic requirements for composting (Kimura and Shimizu, 1981a,b; Bach et al., 1987; Wu et al., 1990). Composting system technology is required to support production in agricultural ecosystems. However, the main problem is the practical application of these technologies. We begin with an introduction to the composting process (2) and sensor fro systems operation (3), then define with function and mechanism of aeration (4), the results is indicated the results of bin composting (5) and is discussed with the early stage composting by packed bed-type reactor (6) and adiabatic-type reactor (7). Because composting systems are not uniform in degradation and material temperature, information on the degradation of materials within forced aeration composting is very useful for practical operation.
2. THE COMPOSTING PROCESS
Composting is the aerobic (oxygen-requiring) decomposition of organic materials by microorganisms under controlled conditions. During composting, microorganisms consume oxygen (O2) while feeding on organic matter. Active composting generates considerable heat, large quantities of carbon dioxide (CO2) and release water vapor into the air. CO2 and water (vapor) losses can amount to half the weight of the initial waste materials (Fig. 1). Thus, composting reduces both the volume and mass of the raw materials while transforming them into valuable soil conditioner. Factors affecting the composting process are oxygen, aeration, nutrients (carbon:nitrogen (C:N) ratio), moisture content, porosity, structure, texture, particle size, pH and temperature (Table 1).
Fig. 1. Principles of the composting process. The carbon, chemical energy, organic matter and water in finished compost is less than that in the raw materials. The volume of the finished compost is 50% or less than that of the raw material.
Table 1. Recommended conditions for rapid composting.
Condition | Reasonable range* | Preferred range |
Carbon to nitrogen (C:N) ratio | 20:1–40:1 | 25:1–30:1 |
Moisture content | 40–60%** | 50–60% |
Oxygen concentration | Greater than 5% | Much greater than 5% |
Particle size (diameter in meters) | 3.2 × 10−3−1.3 × 10−2 | Varies** |
pH | 5.5–9.0 | 6.5–8.0 |
Temperature | 43–66 | 54–60 |
2.1 Temperature and the Physical Properties of Compost Material
Fig. 2. Thermophilic composting process by aerobic degradation.
Temperature increase within composting materials is a result of heat balance during composting (Kimura and Shimizu, 2002, Fig. 2a). Temperature is one of the most important variables in the composting process (Schulze, 1962). Composting at temperatures below 20°C has been demonstrated to significantly slow and even stop the composting process. Therefore, temperature can be an indicator of activity in the biological process of composting. In the aerobic decomposition of biomass, the desired products are water, CO2 and heat byproducts of composting. Mesophilic organisms which function best within the range of 24 to 40°C, initiate the composting process (Fig. 2b). As microbial activity increases soon after the formation of a composting pile, temperatures within piles of sufficient volume and density also increase. Thermophilic microorganisms take over at temperatures above 40°C. The temperature in the compost matrix typically increases rapidly to 54 to 65°C within 24 to 74 h in an adiabatic-type reactor at the laboratory scale (Kimura and Shimizu, 1981a). In thermophilic composting, any soluble sugars in the original mixture are almost immediately used up by bacteria and other microorganisms. Other components such as protein, fat, and cellulose get broken down by heat-tolerant microbes. Nitrogen is readily available when it is in the proteinaceous, peptide, or amino acid forms. Lignins (large polymers that cement cellulose fibers together in wood) are among the slowest compounds to decompose because of their complex structure that is highly resistant to enzyme attack.
Porosity, structure and texture relate to the physical properties of a material such as particle size, shape, and consistency, affect the composting process by their influence on aeration. They can be modified by the selection of raw materials and grinding or mixing. Materials added to adjust these properties are referred to as amendments or bulking agents. For composting applications, an acceptable porosity and structure can be achieved in most of the raw materials, if the moisture content is less than 65% (w/v). However, some situations profit from special attention to porosity, structure, or texture. Composting piles are susceptible to settling, so large par...
