FIGURE 9.2 Static pile composting systems: (a) single static pile; (b) extended aerated pile.
The process does not require digestion or stabilization of sludge prior to composting, although there may be increased odor production issues to deal with when composting raw sludges. Composting projects are frequently designed based on 20% solids, but many operating projects are starting with 12 to 18% solids and as a result end up using more bulking agent to absorb moisture to get to approximately 40% solids in the mix of sludge and bulking agent. The end product is useful as a soil conditioner (and is sold for that purpose in many locations) and has good storage characteristics.
The major process requirements include: oxygen at 10 to 15%, a carbon-to-nitrogen ratio of 26:1 to 30:1, volatile solids over 30%, water content 50 to 60%, and pH 6 to 11. High concentrations of metals, salts, or toxic substances may affect the process as well as the end use of the final product. Ambient site temperatures and precipitation can have a direct influence on the operation. Most municipal sludges are too wet and too dense to be effectively composted alone, so the use of a bulking agent is necessary. Bulking agents that have been used successfully include wood chips, bark, leaves, corncobs, paper, straw, peanut and rice hulls, shredded tires, sawdust, dried sludge, and finished compost. Wood chips have been the most common agent and are often separated from the finished compost mixture and used again. The amount of bulking agent required is a function of sludge moisture content. The mixture of sludge and bulking agent should have a moisture content between 50 and 60% for effective composting. Sludges with 15 to 25% solids might require a ratio of between 2:1 and a 3:1 of wood chips to sludge to attain the desired moisture content in the mixture (USDA/USEPA, 1980).
Mixing of the sludge and the bulking agent can be accomplished with a frontend loader for small operations. Pugmill mixers, rototillers, and special composting machines are more effective and better suited for larger operations (USEPA, 1984). Similar equipment is also used to build, turn, and tear down the piles or windrows. Vibratory-deck, rotary, and trommel screens have all been used when separation and recovery of the bulking agent are process requirements. The pad area for either windrow or aerated pile composting should be paved. Concrete has been the most successful paving material. Asphalt may be suitable, but it may soften at higher composting temperatures and may itself be susceptible to composting reactions.
Outdoor composting operations have been somewhat successful in Maine and in other locations with severe winter conditions. The labor and other operational requirements are more costly for such conditions. Covering the composting pads with a simple shed roof will provide greater control and flexibility and is recommended for sites that will be exposed to subfreezing temperatures and significant precipitation. If odor control is a concern, it may be necessary to add walls to the structure and include odor control devices in the ventilation system.
For static pile systems, the aeration piping shown in Figure 9.2 is typically surrounded by a base of wood chips or unscreened compost about 12 to 18 in. (30 to 45 cm) deep. This base ensures uniform air distribution and also absorbs excess moisture. In some cases, permanent air ducts are cast into the concrete base pad. The mixture of sludge and bulking agent is then placed on the porous base material. Experience has shown that the total pile height should not exceed 13 ft (4 m) to avoid aeration problems. Typically, the height is limited by the capabilities of most front-end loaders. A blanket of screened or unscreened compost is used to cover the pile for thermal insulation and to adsorb odors. About 18 in. (45 cm) of unscreened or about 10 in. (25 cm) of screened compost is used. Where the extended pile configuration is used, an insulating layer only 3 in. (8 cm) thick is applied to the side that will support the next composting addition. Wood chips or other coarse material are not recommended, as the loose structure will promote heat loss and odors.
The configuration shown in Figure 9.2 draws air into the pile and exhausts it through a filter pile of screened compost. This pile should contain about 35 ft3 (1 m3) of screened compost for every 3.3 ton (3 mt) of sludge dry solids in the compost pile. To be effective, this filter pile must remain dry; when the moisture content reaches 70%, the pile should be replaced.
Several systems, both experimental and operational, use positive pressure to blow air through the compost pile (Kuter et al., 1985; Miller and Finstein, 1985). The blowers in this case are controlled by heat sensors in the pile. The advantages claimed for this approach include more rapid composting (12 vs. 21 d), a higher level of volatile solids stabilization, and a drier final product. The major concern is odors, as the air is exhausted directly to the atmosphere in an outdoor operation. Positive aeration, if not carefully controlled, can result in desiccation of the lower part of the pile and therefore incomplete pathogen stabilization. The approach seems best suited to larger operations with enclosed facilities, in which the increased control will permit realization of the potential for improved efficiency.
The time and temperature requirements for either pile or windrow composting depend on the desired level of pathogen reduction. If "significant" reduction is acceptable, then the requirement is a minimum of 5 d at 105°F (40°C) with 4 hr at 130°F (55°C) or higher. If "further" reduction is necessary, then 130°F (55°C) for 3 d for the pile method or 130°F (55°C) for 15 d with five turnings for the windrow method is required. In both cases, the minimum composting time is 21 d, and the curing time in a stockpile, after separation of the bulking agent, is another 21 d.
A system design requires a mass balance approach to manage the input and output of solid material (sludge and bulking agent) and to account for the changes in moisture content and volatile solids. A continuing materials balance is also essential for proper operation of the system. The pad area for a composting operation can be determined using Equation 9.5:
A = Pad area for active compost piles (m2; ft2). S = Total volume of sludge produced in 4 wk (m3; ft3). R = Ratio of bulking agent volume to sludge volume. H = Height of pile, not including cover or base material (m; ft).
A design using odor-control filter piles should allow an additional 10% of the area calculated above for that purpose. Equation 9.5 assumes a 21-d composting period but provides an additional 7 d of capacity to allow for low temperature, excessive precipitation, and malfunctions. If enclosed facilities are used or if positive pile aeration is planned, proportional reductions in the design area are possible.
The area calculated using Equation 9.5 assumes that mixing of sludge and bulking material will occur directly on the composting pad. Systems designed for a sludge capacity of more than 15 dry ton per day should provide additional area for a pugmill or drum mixer.
In many locations the finished compost from the suction-type aeration will still be very moist, so spreading and additional drying are typically included. The processing area for this drying and screening procedure to separate the bulking agent is typically equal in size to the composting area for a site in cool, humid climates. This can be reduced in more arid climates and where positive-draft aeration is used.
An area capable of accommodating 30 d of compost production is recommended as the minimum for all final curing locations. Additional storage area may be necessary, depending on the end use of the compost. Winter storage may be required — for example, if the compost is used only during the growing season.
Access roads, turnaround space, and a wash rack for vehicles are all required. If runoff from the site and leachate from the aeration system cannot be returned to the sewage treatment plant, then a runoff collection pond must also be included. Detention time in the pond might be 15 to 20 d, with the effluent applied to the land as described in Chapter 8. Most composting operations also have a buffer zone around the site for odor control and visual esthetics; the size will depend on local conditions and regulatory requirements.
The aeration rate for the suction-type aerated pile is typically 8 ft3/min (14 m3/hr) per ton sludge dry solids. Positive-pressure aeration, at higher rates, is sometimes used during the latter part of the composting period to increase drying (Miller and Finstein, 1985). Kuter et al. (1985) used temperature-controlled positive-pressure aeration at rates ranging from 47 to 200 ft3/min (80 to 340 m3/hr) per ton sludge dry solids and achieved a stable compost in 17 d or less. These high aeration rates result in lower temperatures in the pile (below 113°F [45°C]). The direction of air flow can be reversed during the latter stages to elevate the pile temperature above the required 131°F (55°C). The temperatures in the final curing pile should be high enough to ensure the required pathogen kill so the composting operation can be optimized for stabilization of volatile solids.
Monitoring is essential in any composting operation to ensure efficient operations as well as the quality of the final product. Critical parameters to be determined include:
• Moisture content in sludge and bulking material to ensure proper operations
• Metals and toxics in sludge to ensure product quality and compost reactions
• Pathogens as required by regulations
• pH in sludge, particularly if lime or similar chemicals are used
• Temperature taken daily until the required number of days above 130°F (55°C) is reached; weekly thereafter at multiple sites to ensure that the entire mass is subjected to appropriate temperatures
• Oxygen, initially, to set blower operation
Determine the area required for a conventional extended-pile composting operation for the wastewater treatment system described in Example 9.3 (1500 m3
sludge production per year at 7% solids). Assume that a site is available next to the treatment plant so runoff and drainage can be returned to the treatment system.
1. Use wood chips as a bulking agent. At 7% solids, the sludge is still "wet," so a mixing ratio of at least 5 parts of wood chips to 1 part sludge will be needed. Assume top of compost at 2 m. Thus:
2. Use Equation 9.5 to calculate the composting area: A = 1.1 S (R + 1)/H
3. Filter piles for aeration = 10% of A = 0.1 x 381 = 38.1 m2.
4. Processing and screening area = A = 381 m2.
5. Curing area: Assume 150 m2.
6. Wood chip and compost storage: Assume 200 m2.
7. Roads and miscellaneous: Allow 20% of total. Total A = 381 + 38.1 + 381 + 150 + 200 = 1150 m2 Roads = (0.2)(1150) = 230 m2
8. Total area including roads = 1380 m2.
A buffer zone might also be necessary, depending on site conditions. The area calculated here is significantly less than the area calculated in Example 9.2 for freeze drying beds. This is because composting can continue on a year-round basis, but the freezing beds must be large enough to contain the entire annual sludge production.
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