Gel Piling Manual Machine Drawing

Triga

Agitated bed

Fairfield-Hardy

Source: G. Tchobanoglous, and F. Kreith, Solid Waste Handbook, 2nd ed., McGraw-Hill, New York, 2002.

Source: G. Tchobanoglous, and F. Kreith, Solid Waste Handbook, 2nd ed., McGraw-Hill, New York, 2002.

Windrow Composting In the windrow process, after processing the material to be composted is placed in long windrows (see Figure 3.20). The windrows are 3 to 6 feet high (1-2m) and 6 to 15 feet wide (2-5 m) at the base. The windrow process is conducted normally in uncovered pads and relies on natural ventilation with frequent mechanical mixing of the piles to maintain aerobic conditions. The windrow process can be accelerated if the compost is turned over every four or five days, until the temperature drops from about 1500 or 140 °F (66 ° or 60 °C) to about 100 °F (38 °C) or less. Under typical operating conditions, the windrows are turned every other day. The turning is accomplished with specialized equipment (see Figure 3.21) and serves to aerate the pile and allow moisture to escape. To meet the EPA pathogen reduction requirements, the windrows have to be turned five times in 15 days, maintaining a temperature of 55 °C. The complete compost process may require two to six months.

Because anaerobic conditions can develop within the windrow between turnings, putrescible compounds can be formed that can cause offensive odors, especially when the windrows are turned. In many locations, negative aeration is provided to limit the formation of odorous compounds. Where air is provided mechanically, the process is known as aerated windrow composting.12 Odors will result if the compost is not kept aerobic. It may be necessary to enclose the operation and provide fans and collectors of the odorous air, forcing it through a scrubber or other treatment device for discharge up a stack to the atmosphere.

Aerated Static Pile Composting In the aerated static pile process, the material to be composted is placed in a pile and oxygen is provided by mechanical aeration systems. Most states require paved surfaces for the pile construction areas to permit capture and control runoff and allow operation during wet weather. The most common aeration system involves the use of a grid of subsurface piping (see Figure 3.22). Aeration piping often consists of flexible plastic drainage tubing assembled on the composting pad. Because the drainage-type aeration piping

FIGURE 3.21 View of machine used to aerate compost placed in windrows (Courtesy H. Leverenz.)
Compost Pad

is inexpensive, it is often used only once. Before constructing the static pile, a layer of wood chips is placed over the aeration pipes or grid to provide uniform air distribution. The static pile is then built up to 8 to 12 feet (2.6-3.9 m) using a front-end loader. A cover layer of screened or unscreened compost is placed over the sludge to be composted. Typically, oxygen is provided by pulling air through the pile with an exhaust fan. Air that has passed through the compost pile is vented to the atmosphere though a compost filter for odor control.

In-Vessel Composting Systems In-vessel composting is accomplished inside an enclosed container or vessel. Every imaginable type of vessel has been used as a reactor in these systems, including vertical towers, horizontal rectangular and circular tanks, and circular rotating tanks. In-vessel composting systems can be divided into two major categories: plug flow and dynamic (agitated bed). In plug flow systems, the relationship between particles in the composting mass stays the same throughout the process, and the system operates on the basis of a first-in, first-out principle. In a dynamic system, the composting material is mixed mechanically during the processing.

Mechanical systems are designed to minimize odors and process time by controlling environmental conditions such as air flow, temperature, and oxygen concentration. The popularity of in-vessel composting systems has increased in recent years. Reasons cited for this increased use are process and odor control, faster throughput, lower labor costs, and smaller area requirements. The detention time for in-vessel systems varies from 1 to 2 weeks, but virtually all systems employ a 4- to 12-week curing period after the active composting period.

Other Composting Technologies Naturizer composting uses sorting, grinding and mixing, primary and secondary composting including three grinding operations, aeration, and screening. Digested wastewater sludge, raw-wastewater sludge, water, or segregated wet garbage is added at the first grinding for dust and moisture control. The total operation takes place in one building in about six days.

The Dano composting (stabilizer) plant consists of sorting, crushing, biosta-bilization three to five days in a revolving drum to which air and moisture are added, grinding, air separation of nonorganics, and final composting in open windrows. Temperatures of 140 °F (60 °C) are reached in the drum. Composting can be completed in 14 days by turning the windrows after the fourth, eighth, and twelfth days. Longer periods are required if the windrows are not kept small, turned, and mixed frequently and if grinding is not thorough. In a more recent version, the drum treatment is for 8 hours followed by screening, final composting in covered aerated piles for about three weeks, and then three weeks of aging in static piles.

The Fairfield - Hardy process handles garbage and trash and wastewater sludge. The steps in the process are (1) sorting—manual and mechanical to separate salvageable materials; (2) coarse shredding; (3) pulping; (4) wastewater sludge addition, if desired; (5) dewatering to about 50 percent moisture; (6) three-to five-day digestion with mixing and forced air aeration, temperature ranges from 140 ° to 170 °F (60 °-76 °C); (7) air curing in covered windrows; and (8) pelletizing, drying, and bagging. Compost from the digester is reported to have heat values of 4,000 Btu/lb and, when pelletized and dried, 6,450 Btu/lb.

The Bangalore process is used primarily in India. Layers of unshredded solid waste and night soil are placed in a shallow trench; the top is covered with soil. The duration of the treatment is 120 to 150 days.

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