In the United States, studies of windrow and aerated static pile composting were conducted in the mid-1970s in Beltsville, Maryland, and in Carson, California. As of 2003, there were more than 200 wastewater sludge composting facilities in the United States. Most of the composting facilities are aerated static pile systems, and some are in-vessel systems. Windrow composting is rarely used because of the potential odor generation and the large area requirement.
The city of Eustis, Florida, uses the windrow process for composting aerobically digested sludge. The dewatered sludge cake of 12 to 14% solids mixed with wood chips at the ratio of 1 : 2 by volume is formed into windrow piles by a front-end loader on paved surface. The pile is mixed using a windrow machine every day for three to five days until the temperature reaches 55°C. The temperature is maintained between 55 and 65°C for 15 days, during which time the pile is turned about five times. The composting process takes about three weeks. The compost is then moved to a curing area and cured for three months. The compost is turned once every month during the curing process. The cured compost meets the class A requirements and is sold to distributors. The city of Austin, Texas, uses the windrow process for composting anaerobically digested sludge. The dewatered sludge cake of 15 to 25% solids is mixed with bulking agents, including yard waste, for composting. The compost is sold to the general public through registered vendors for use as a soil conditioner for residential lawns and flower gardens.
The Reedy Creek Improvement District (RCID) in Florida selected the aerated static pile system to compost Walt Disney World's wastewater sludge (Harkness, 1994). Aerobically digested sludge is dewatered to 20 to 22% solids and is mixed with wood chips at the ratio of 1 : 1 by weight (1 : 2 by volume). The mixture is then placed on a base of perforated aeration piping covered with wood chips and then covered with unscreened compost. Figure 7.21 shows the piles and the air piping system. Air is drawn through the pile 24 hours after creating the pile. Aeration is controlled by a timer to maintain the temperature between 60 and 65°C and the oxygen level at a minimum of 13%. Drawn air is discharged through a biofilter before discharging to the atmosphere. The composting process lasts about four weeks, after which the material is screened (see Figure 7.22) and cured for an additional four weeks. Cured compost has a moisture content of 35 to 40%, total nitrogen of 1.4 to 2.2%, and total phosphorus of 0.3 to 0.4%. The average heavy metal contents in mg/kg of dry solids are cadmium 15, copper 290, nickel 18, lead 15, and zinc 200. Fecal coliform is 5 MPN. The compost meets the class A requirements and is sold to distributors. RCID had earlier used an in-vessel composting system in two vertical plug flow reactors. Although the system produced stable compost, the facility
experienced several problems during the operation. RCID conducted a cost analysis of three in-vessel systems and the aerated static pile system. Based on the analysis, the aerated static pile system was found to be the most cost-effective.
The wastewater treatment plants in Bristol, Tennessee, and Bristol, Virginia, have a shared in-vessel composting facility (Clifton et al., 1997). The combined raw primary and waste activated sludge at an average 4% solids is mixed with sawdust at the ratio 1 : 1 by volume and is dewatered with a cationic polymer in belt filter presses to 35% dry solids. There are two enclosed in-vessel units of 1400 m3 each, one for the bioreaction process and the other for the curing. The dewatered mixture of sludge and sawdust from the storage silo is transferred by a conveyor to the top of the bioreactor, where a rotating finned disk layers the mixture over the previous loadings. Air is introduced at the bottom of the reactor. The temperature is maintained at 55°C for a minimum of three consecutive days. The bottom layer is removed from the reactor after 15 day of detention time and transferred to the top of the second reactor for curing. After 15 days of curing, the compost is stored in rows on a paved concrete surface for two to three months. The compost from the facility meets the class A requirements and is distributed to the public.
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