In this section we describe the factors that need to be considered in the design and operation of composting facilities. These factors, the factors influencing composting described earlier, and the theoretical aspects of composting described in another section should be considered to meet the requirements of each composting system.
Energy Balance Haug (1993) has shown that the organic decomposition in a composting operation is self-sustaining when the ratio W is less than 10, where mass of water in initial compost mixture
mass of degradable organics in the mixture
The degradability of the mixture can be adjusted by the addition of a bulking agent or amendment that contain high concentrations of degradable organic material. The bulking agents and amendments are usually dry, and they increase the volatile fraction and decrease the moisture fraction of the mixture, thereby reducing the ratio, W.
The ratio W can also be defined as an energy balance equation:
W = weight of water evaporated weight of volatile solids lost
Organic decomposition produces water and generates heat. To keep this ratio below 10, it is important that sufficient moisture is removed from the mixture by evaporation. However, the composting process temperature should be maintained for proper decomposition. The temperature in the mixture will not rise if rate of heat loss exceeds the rate of heat generation.
Detention Time When the aerated static pile composting system was developed by the U.S. Department of Agriculture in the 1970s, it established the active composting period to be 21 days followed by 30 days of curing period without aeration for the composting mix of dewatered wastewater sludge and wood chips. Several states have established this detention time in their regulations. However, in-vessel suppliers have established their own active composting periods. Some systems recommend as little as 14 days of active composting, although most horizontal agitated systems use 21 days.
Even after a long curing period, with the compost considered stable, the organics in stored compost can continue to decompose, especially if it not screened because of the biodegradable organics that remain in the bulking agent. The stability of the compost can be measured either by the carbon dioxide production or oxygen consumption. Compost with a respiration rate of 3 mg CO2 per gram of organic carbon per day or a consumption rate of 1 mg O2 per gram of organic carbon per day is considered stable. From the aerobic decomposition equation shown earlier in this chapter, this oxygen consumption rate is equivalent to 1.4 CO2 per gram of organic carbon.
Temperature Control and Aeration Temperature in a composting pile may exceed 70°C if not sufficiently controlled. A temperature in excess of 70°C is detrimental to microbial activity. Sufficient aeration should be provided to control the temperature rise. As the airflow rate is increased in a system, the temperature in the pile decreases. This also increases the rate of water vapor removal. In an agitated composting system, agitation also releases heat and water vapor.
An aeration rate of 34 m3/Mg • h (1100 ft3/dry ton-hr) provides adequate drying, oxygen for organic decomposition, and high-enough temperatures for pathogen destruction. During the mesophilic stage of composting, a higher aeration rate is required to prevent excessive temperature buildup. Therefore, aeration capacities in the range 30 to 160 m3/Mg • h (1000 to 5000 ft3/dry ton-hr) should be provided.
Centrifugal blowers can provide the necessary pressure for aeration of the compost mixture and also push the air through the odor biofilter. Aeration blowers can be controlled by a timer to turn them on and off. The percentage of time the blowers are on is set based on temperature readings. The bowers can also be turned on and off automatically based on temperature readings and temperature feedback control. In an aerated static pile system, air is either exhausted or pulled through the pile. When air is exhausted through the pile, water vapor is driven to the surface and promotes drying and avoids accumulation of condensation in the pile and aeration piping. When air is pulled thorough the pile, moist air drawn through the pile condenses in cooler areas of the pile. When enough condensate accumulates, it will leach materials from the sludge. Condensate also collects in the aeration piping, which should be removed by water traps.
This condensate, the leachate, and any contaminated rainfall runoff should be collected and discharged to the treatment plant influent stream for treatment because the contaminated water can become a source of odors if allowed to accumulate in puddles around the piles. The main advantage of aeration by drawing air through the pile instead of exhausting is that the odorous drawn air can be pushed directly through a biofilter for odor control.
Screening Bulking agents such as wood chips should be recovered from the compost by screening. Fine materials such as sawdust cannot be recovered effectively. Recycling of the bulking agent can save 50 to 80% of the cost of new bulking agent. Drying of the compost is critical to screening because screens do perform well on compost with more than 50% moisture. Compost is dried by providing adequate aeration or agitation to drive off moisture. Two types of screen are generally available: vibrating deck screens and rotating screens. Depending on the number of screens in a deck, vibrating deck screens can separate materials into several classifications of sizes. However, rotating screens with cleaning brushes can screen compost with higher moisture content.
Site Considerations Windrows and aerated static piles are normally constructed on concrete or asphalt paved surfaces. Leachate and stormwater runoff from the composting surface are collected and discharged to the influent facilities of the treatment plant. Therefore, proximity to a treatment plant should be one consideration in selecting a site. Other factors in selecting a site include availability of land area and availability of buffer zone because of potential odor problems. Climatic considerations rarely create a problem, as composting is gaining popularity even in the colder countries of northern Europe. Increasingly newer aerated static pile composting facilities, including bulking agent and cured compost storage, are covered with a roof to keep precipitation out, thereby eliminating stormwater runoff from the facilities.
Odor Control Odor control is the primary environmental consideration in the operation of a composting facility because organic decomposition produces odorous volatile compounds. Odor sources include dewatered sludge storage facilities, mixing and turning of compost mixture, buildings, surface emission from active piles, leachate puddles around piles, and blower exhaust. Process and operational improvements can reduce odors considerably. Good odor control starts with prompt mixing of sludge and bulking agents. In addition, lumps of materials or puddles of liquid should not be allowed to remain in the mixing area. Condensate, leachate, and runoff from the piles should be collected and treated as quickly as possible. The compost should be adequately cured before it is removed, and any unstable material should be recycled back into the composting process for further treatment.
Composting odor is the result of a mixture of compounds that include mercaptans, organic sulfides, ammonia, organic nitrogen, fatty acids, and ketones. Containment and treatment of exhaust are trends in the newer composting facilities. Biofilters and wet scrubbers are two odor control technologies that are typically used. Well-stabilized and cured compost with bulking agents is used in building the biofilter beds or piles (see Figure 7.5). In a biofilter, odor compounds are oxidized by microorganisms. Biofilters are relatively inexpensive because compost is readily available. They are also easy and inexpensive to operate. Fabricated biofilters are also available commercially and rely on a fixed film developed on a structural packing medium for treatment. Wet scrubbers with as many as three stages, although more expensive to build and operate, are very effective in removing odorous compounds from the exhaust.
Safety and Health Issues Composting facilities use vehicles and conveyers for material handling, which are potential causes for injuries to operators. Another cause for danger to operators is poorly ventilated buildings where composting process is performed and materials are stored. Although Salmonella, fecal coliforms, total coliforms, and viruses increase in numbers during the initial stage of composting process, they are essentially destroyed within two weeks. However, exposure to pathogens can occur during the composting process by body contact of materials and inhalation of aerosols containing microorganisms. Airborne microorganisms include bacteria, fungi, and acti-nomycetes and microbial toxins. Fungus is also present in high concentrations in wood chips. Buildings that enclose composting process and stored materials should be ventilated a minimum of six air changes per hour for the comfort and safety of operators. When handling materials and when operating conveyors and screens, operators should wear dust masks. Good housekeeping, including dust control, should also be practiced for health reasons.
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