Composting, one of nature's most basic processes, can be used to process more than green waste. For example, the Camden County, N.J., Municipal Utilities Authority is using in-vessel composting to dispose of sludge from this highly populated area in southern New Jersey. The operation also creates a new source of revenue for the county.
Originally, the authority had implemented static-pile, open-air composting when the federal Clean Water Act banned offshore dumping of sludge. However, nearby residents and businesses objected to the project's unfiltered odors.
Next, the authority incinerated some sludge locally and transported the rest to an open-air composting facility in southwest Philadelphia. Hauling the sludge to another location, however, was expensive costing up to $70 per ton. Finally, the authority settled on a $65 million, in-vessel facility which enclosed the compost to minimize odor.
Odors result from several factors at a facility, including the number of openings, how efficiently it operates, the amount of air needed and how that air is vented. The Camden County system's closed reactor, which reportedly can process up to 50 dry tons per day, uses negative pressure to contain odor.
A key element in the system is a series of 20 tunnel reactors (see diagram). Each tunnel is 63 feet long, 12 feet high and 18 feet wide. To maximize space, the design in-cluded a basement to house processing equipment such as conveyors, supply and exhaust blowers, HVAC equipment and ductwork as well as some odor control equipment.
The basement design supplies air from below, using one pair of vacuum and pressure blowers. To alleviate chronic clogging and access problems, the system allows for easy access to the conveyor belts and features a high degree of redundancy for back-up.
At the start of the 14-day composting process, conveyor belts move de-watered sludge to storage bins. From there, the sludge is transported to mixers that blend it with amendments such as wood chips, sawdust and recycled compost. Conveyors then move the mixture to the concrete reactor tunnels, where it heats up, becoming stabilized. Meanwhile, air is pumped in to stimulate aerobic action. The temperature in the vessels is approximately 60Degree celsius, and pressure and vacuum air is pumped into different zones to optimize decomposition.
Next, the compost is moved by conveyors to a separate curing area, where it's housed for 30 days. After curing, the compost is moved to a storage building that can hold up to three months' worth of material. This storage building is particularly important during the winter, when the New Jersey Department of Environmental Protection prohibits spreading compost on frozen ground.
To mitigate odor, the air from each phase of the composting process is "scrubbed" with water and several chemicals. Odor is caused by several chemical compounds, including ammonia, chlorine, di-methyl disulfide and organic compounds that contain sulfur.
Air treatment involves scrubbing process air from the reactor with a blend of water, sulfuric acid and surfactant to remove ammonia; and water, sulfuric acid and sodium hypochlorite to remove dimethyl disulfide. Next, acid is used to mitigate odor from non-polar, sulfur-containing organic compounds.
In the third stage, a final "polishing" scrub of water, sodium hydroxide and hydrogen peroxide is administered to remove chlorine and other residuals before the air is released into the atmosphere.
Odor also is controlled in the curing and storage areas, where a two-stage process is used. The first is similar to the air cleaning process used in the tunnel reactors; the second cleaning stage removes dimethyl disulfide further by using an oxidizing scrubber of water, sodium hydroxide and hydrogen peroxide.
After the material emerges from the plant, the authority can sell the compost product for gardening to help offset operating costs. The large number of tunnels reportedly allows the authority to produce a variety of compost mixes. Tests conducted by a local technical school and local gardens have indicated that the compost will be useful for professional landscapers, commercial nurseries and home gardeners.
After less than a year, the in-vessel composting operation has received the Outstanding Engineering Achievement Award from the New Jersey Society of Professional Engineers. The system was designed by PWT Waste Solutions Inc., Birmingham, Ala. Hill International, Willingboro, N.J., served as construction manager for the project. Other members of the project team include design engineer Greeley & Hansen, Philadelphia, and the R.J. Longo Construction Company, Denville, N.J.