Sludge reduction by in-plant process modification: New York City's experiences

Abstract

The City of New York treats about 4.62 X 1012 m3 (1.3 billion gal/day of wastewater) at 12 wastewater treatment plants. Two additional plants, presently under construction, will add another 870 550 m3/d (230 million gal/day) capacity, and essentially end the need for dry-weather bypassing of wastewater. Of the 12 existing treatment plants, nine that have been upgraded since 1972 use the step aeration process and remove at least 85% of suspended solids (SS) and 5-day biochemical oxygen demand (BOD5). The Newtown Creek, Owls Head, and Coney Island plants have not been upgraded and use modified aeration treatment with average SS and BOD removals of between 60 and 70%. A schematic layout for a typical upgraded wastewater treatment plant is presented in Figure 1. Sludge treatment common to all plants involves gravity thickening and anaerobic digestion in the high rate mode. The raw sludge is digested for stabilization and odor elimination because the plants are located in densely populated areas. After digestion, the sludge produced (8400 m3/d) is stored onsite. The sludge is disposed at sea by a fleet of four City-owned self-propelled sludge vessels, two with a capacity of 2800 m3 (100 000 cu ft) and two that hold 1400 m3 (50 000 cu ft) each. The marine sludge disposal operation costs about $4.1 million annually. However, the reliability of the operation is tenuous because of vessel breakdowns, adverse weather con ditions, or cancelled trips caused by crew shortages. Each cancelled trip requires either the services of contracted private vessels or continued in-plant storage of the digested sludge. If the capacity of the plant storage is exceeded, the sludge is forced to back-up in the system with the possibility of process deterioration. In addition, a new sludge dumping site, as much as 170 km (106 miles) off shore may be required. This has prompted consideration of methods to reduce sludge volume. Because the digested sludge concentration averaged only 3%, any slight increase in solids concentration would represent a tremendous volume reduction; however, this decision came during a financial crisis. A proposal would be acceptable only if it met the test of low capital and operating costs, minimum manpower expenditure, and ease of operation and control. This could be accomplished with spare tanks and traditionally disregarded unit processes. The sludge volume reduction tech niques used include digested sludge recirculation, decanting, elutriation-rethickening, and thermophilic digestion, processes used singly and in combination.