The Bayshore Regional Sewerage Authority operates the wastewater treatment facility in Union Beach, N.J. that currently serves the borough and seven other nearby towns. The authority was established in 1968 and completed installation of a 6.0 mgd sewage treatment system in 1974, which included a fluid bed incinerator. The plant has been expanded twice since that time and the facility now has a capacity of 13.2 mgd on an annual average. Peak thirty-day capacity is 16.0 mgd and instantaneous is 33 mgd. It currently operates at an average of 9.2 mgd.
Fluid bed incineration has been used as the sludge disposal method since the plant started up in 1973. The original incinerator was a Dorr-Oliver cold windbox type that was modified in 1982 to a warm windbox. The new unit is an Infilco Degremont (IDI) Thermylis hot windbox system.
System description
Sludge is dewatered using belt filter presses and pumped to the reactor via piston pumps through two feed ports. While the system has dual fuel capability, No. 2 fuel oil, rather than natural gas, is normally used as auxiliary fuel during startup and operation as needed because of lower fuel cost and because oil has a lower auto-ignition temperature. The freeboard operates at a design temperature of 1,550 degrees F.
The reactor has an expanded freeboard to allow deceleration of the larger particles to minimize sand carry over and maximize carbon burnout. The bed make-up sand may be fed to the reactor pneumatically during operation if required. Bed sand removal is via a pneumatic system. Sand can either be returned to the storage silo or sent to a truck for disposal.
The reactor is of the hot windbox type equipped with a refractory arch distributor. To minimize auxiliary fuel usage, fluidizing air is preheated to 1,237 degrees F in an external tube and shell heat exchanger utilizing the exhaust flue gas of the reactor as a heat source. The air pollution control system includes a venturi scrubber followed by a tray tower and a wet electrostatic precipitator. One tray of the tower has the capability of caustic addition for acid gas control, as does the WESP. Hot air at 500 degrees F is added to the stack gas prior to release to the atmosphere for plume suppression. Plume suppression air is preheated in a secondary heat exchanger using the exhaust flue gas from the primary heat exchanger.
There is approximately 590 pounds per hour of ash and sand carryover in the flue gas. The solids are primarily removed in the high pressure drop venturi scrubber where an ash-slurry is produced, which is pumped to out-door concrete ash decanting lagoons for de-watering. Dried ash at approximately 50% total solids is removed from the drying lagoon four or five times per year.
Performance
The unit satisfied its performance requirements on the first test. Emission requirements were met by a wide margin upon initial testing and continued in conformance during all subsequent testing. Stack testing was performed on February 23, 1996 to demonstrate compliance for the unit. The requirements included:
- The US EPA 503 regulations for carbon monoxide (CO), Total Hydrocarbons (THC), Total Suspended Particulate (TSP) and the following heavy metals: arsenic, cadmium, chromium, lead, and nickel;
- The National Emission Standard for Hazardous Air Pollutants (NESHAPS) requirements for beryllium and mercury;
- The New Source Performance Standards (NSPS) requirements for particulate matter;
- The New Jersey Air Pollution Control Regulations codified as N.J.A.C.7: 27-1 et. seq.; and
- Final Air Pollution Control Permits - Sludge Incinerator- DEPE log #1-91-1235, November 15,1991, which imposed limits on sulfur dioxide, hydrogen chloride, benzo(a)pyrene and benzene in addition to the requirements of the above regulations.
Incineration versus other sludge disposal methods
The original incinerator at the plant was a cold Dorr-Oliver windbox system installed in 1973 and converted to a warm windbox) in 1982. In 1989/1990, a second shell was installed over the original because of the significant hot spots and thinning that had occurred. The authority then commissioned a study on future options.
In March 1991, the authority received the results of this study discussing potential sludge disposal alternatives. Alternatives considered were composting, contract disposal and incineration. The contract disposal option was sending either dewatered sludge or liquid to existing offsite facilities, which either would incinerate the sludge or process it for land application. Composting and land application by the facility was also initially included in the study, but was eventually eliminated. It was not considered a practical alternative for this site due to the large amount of land needed, the general unavailability of suitable agricultural land within the Authority's service area or the surrounding County and the uncertainty of whether sufficient agricultural interest could be generated in the project. The Dalby report provided amortized project costs for all sludge disposal options studied.
Based on these findings, the study recommended incineration as the most desirable disposal method. The study was reviewed by the New Jersey Department of Environmental Protection and Energy, which determined in view of the costs and technical problems associated with the non-incineration sludge management alternatives considered. The DEPE has concluded that, "the continuation of incineration at BRSA is an acceptable sludge management method.”
The new Infilco Degremont Inc. hot windbox system was put into operation in November 1995 and the existing warm windbox system was placed on standby. Design capacity of both units was 2,250 pounds per hour of sludge at 25% dry solids.
Sludge dewatering currently is by belt press to about 22% dry-solids and the incinerator combusts a 70/30 mix of primary and activated solids. Sludge is primarily municipal. When the new incinerator was originally installed, industrial waste was 3 to 4% of total input, but currently is less than 1%. Scum and grease are accumulated in dewatering collection pits and fed to the incinerator periodically by mixing with the sludge prior to dewatering. With the hot windbox system, the preheat-burner could be relocated to the windbox, permitting more rapid return to full operation from either cold or hot starts with much less fuel requirement.
A typical cold start took 30 hours with the warm windbox, while only 16 hours are required with the hot windbox. A typical hot start required one hour for full operation with the warm windbox, while only half that time is required with the hot windbox. This is attributed not only to the difference in size of the heat exchangers, but also on the location of the preheat burner.
Polymer Cost: As anticipated, polymer represents the highest operating cost. In designing a plant, it is important to appropriately selecting the right dewatering equipment to minimize polymer consumption. With higher solid content in the warm windbox system (25%TS versus 22% in the hot windbox system), polymer cost was higher.
Fuel Cost: The hot windbox also resulted in a reduced demand for auxiliary fuel. Because of the concurrent reduction in sludge dryness, no direct comparison of auxiliary fuel usage can be made. On a theoretical basis, the decrease from 25 to 22% cake-dryness should have resulted in an increased auxiliary fuel demand of 24.4 gallons per ton with the warm windbox. Instead the fuel demand per ton decreased from an average of 83.2 gallons per ton with the warm windbox to 38.6 gallons per ton with the hot windbox, or a net savings of 69 gallons per dry ton.
Labor Cost: Personnel is also reduced from 4.25 man-hour per dry ton in 1995 to 2.68 man-hour per dry ton in 2003. This is mainly attributed to the new control system and to the better concept of the hot windbox system versus the cold windbox system as explained above. Cost of power is not available, because power usage in the incineration building is not separated from the overall usage of the treatment plant. Due to less stringent air emission requirements, caustic is not required in 1995. Sand cost represents the lowest operating cost at the plant.
Final conclusion
Operational experience has justified the recommendation of the NJ DEPE and the decision of the authority made in 1991 to continue incineration and to replace the Dorr-Oliver warm windbox system by the hot windbox system. The hot windbox has proven to be reliable and much more cost efficient than its counter-part warm windbox.
Fuel costs, operator man-hours, maintenance costs and emissions have been substantially reduced when compared with the previous cold windbox system. With improved air quality and lower sludge disposal costs, the installation has brought benefit to all interested parties—the wastewater treatment plant, the regulators, the taxpayers, and the surrounding community.
