Incinerators

Incinerators thermally destroy biosolids and other organic waste materials from municipal wastewater treatment plants, reducing solids volume by 85-95% while generating heat energy that can be recovered for facility operations. These systems operate at temperatures between 1,400-1,800°F in controlled combustion chambers, converting dewatered sludge cake (typically 20-25% solids) into sterile ash. Modern fluidized bed incinerators can achieve thermal destruction efficiencies exceeding 99.9% for volatile solids while producing 3,000-4,000 BTU/lb of dry solids. The primary limitation is high capital cost ($15-25 million for mid-sized facilities) and stringent air emission controls required for compliance with EPA regulations.

• Biosolids Disposal (5-100 MGD plants): Multiple hearth or fluidized bed incinerators reduce dewatered biosolids (20-25% solids) to sterile ash, achieving 90%+ volume reduction. Located downstream of belt filter presses or centrifuges, upstream of ash handling systems. Selected when land application is restricted or hauling costs exceed $150/dry ton.
• Grit and Screenings Destruction (10+ MGD plants): Small auxiliary incinerators handle organic-laden grit and screenings at 1,000-5,000 lbs/day. Positioned after grit classifiers and fine screens, eliminating odorous waste streams. Chosen over landfilling when tipping fees exceed $80/ton or hauling distances exceed 50 miles.
• Emergency Biosolids Management: Backup incineration capacity during digester outages or when contracted haulers cannot accept material. Typically sized for 50-75% of normal biosolids production to maintain plant operations during extended maintenance periods.

Daily Operations: Operators monitor combustion temperature (±50°F setpoint), oxygen levels (6-12%), and feed rates every 2 hours. Adjustments include rabble arm speed (0.5-2 RPM), auxiliary fuel flow (10-30% of heat input), and scrubber pH (6.5-8.0). Stack opacity readings required every 15 minutes during startup/shutdown sequences.

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Maintenance: Refractory inspection every 2,000 operating hours requires confined space entry with full respiratory protection. Rabble arm replacement every 8,000-12,000 hours needs millwright skills and overhead crane operation. Weekly bearing lubrication and monthly baghouse pulse cleaning prevent unscheduled outages. Scrubber pump rebuilds every 6 months due to abrasive slurry conditions.

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Troubleshooting: High CO emissions (>100 ppm) indicate incomplete combustion from low temperature or excess moisture. Baghouse pressure drop >8 inches WC signals filter blinding or pulse system failure. Refractory spalling creates hot spots visible through thermal imaging.

• Combustion Chamber: Refractory-lined steel shell operating at 1,400-1,800°F. Multiple hearth units feature 8-12 hearths with 10-25 ft diameter; fluidized beds use 8-16 ft diameter vessels. Sized at 15-25 lbs biosolids/ft²/day for multiple hearth, 40-60 lbs/ft²/day for fluidized bed.
• Air Pollution Control: Wet scrubbers remove 95%+ of particulates and acid gases, followed by baghouse filters achieving <0.02 gr/dscf emissions. Sized for 6-10 ft/sec gas velocity with 3-4 second residence time.
• Heat Recovery System: Waste heat boilers capture 60-70% of combustion energy, generating 150-400 psig steam. Heat exchangers preheat combustion air to 400-600°F, improving fuel efficiency by 15-20%.
• Ash Handling: Pneumatic or mechanical conveyors transport 8-12% ash residue to storage silos. Sized for 2-4 hours ash storage capacity with dust collection systems maintaining workplace exposure below 0.1 mg/m³.

• Sludge Feed Rate: 500-15,000 lb/day dry solids for municipal plants (0.5-50 MGD). Typical loading: 8-12 lb dry solids/ft² hearth area/day for multiple hearth, 15-25 lb/ft²/day for fluidized bed systems.
• Moisture Content: Dewatered sludge at 15-25% solids typical. Higher solids reduce auxiliary fuel requirements - each 1% solids increase saves ~50 Btu/lb wet sludge.
• Operating Temperature: 1400-1600°F combustion zone for pathogen destruction and odor control. Lower temperatures risk incomplete combustion and regulatory non-compliance.
• Retention Time: Minimum 0.75 seconds at 1400°F per EPA requirements. Multiple hearth units typically provide 45-75 minutes total, fluidized bed 10-30 seconds at temperature.
• Auxiliary Fuel: Natural gas consumption 2,000-4,000 Btu/lb dry solids, depending on sludge characteristics and moisture content. Startup requires 15-25 MMBtu/hr for 2-4 hours.
• Air Requirements: 4-6 lb air per lb dry solids burned. Underfire air 2-3 lb/lb solids, overfire air 1-2 lb/lb solids for complete combustion.
• Ash Production: 15-25% of dry solids input as ash requiring disposal.

• What sludge production rate and characteristics justify incineration versus land application? Plants generating >2,000 lb/day dry solids with high metals content or biosolids marketing restrictions typically favor incineration. Wrong decision results in $2-5M capital cost for unnecessary equipment or ongoing $200-400/dry ton disposal costs.
• Multiple hearth versus fluidized bed technology selection? Multiple hearth suits variable loading (50-110% design) and higher ash content (>20%), while fluidized bed requires consistent feed and lower ash. Mismatched technology causes 20-30% efficiency loss and maintenance issues.
• What air pollution control level meets local regulations? Basic cyclone removes 80-90% particulates, scrubber systems achieve 95-99% removal at 2-3x operating cost. Underspecified systems risk regulatory violations and retrofits costing $1-3M.
• Standby/redundancy requirements for continuous operation? Single units require 2-4 week annual outages. Dual smaller units (60% capacity each) provide operational flexibility but increase capital costs 40-60%. Critical for plants without sludge storage alternatives.

• Division 40 - Process Integration
• Section 40 73 63 - Sludge Incineration Systems
• Primary specification covering complete incineration systems including furnaces, ash handling, and basic air pollution control. May reference Division 23 (HVAC) for combustion air systems and Division 44 for advanced emission control equipment.

• Material/Equipment Verification: Refractory specifications and thermal cycling ratings, Emission control equipment compliance certificates, Heat exchanger materials and corrosion allowances
• Installation Requirements: Heavy crane access for hearth installation (18-24 month lead times), Specialized refractory contractors, Extensive utility connections (electrical, natural gas, compressed air)
• Field Challenges: Refractory curing schedules (2-3 weeks), Alignment tolerances for rotating equipment, Emission stack height and permitting coordination
• Coordination Issues: Early utility design for high electrical loads, HVAC integration for building heating systems

• Andritz - PARAT multiple hearth incinerators, widely used in 5-50 dry ton/day municipal applications
• Metso Outotec - fluidized bed systems like the Lurgi CFB for larger facilities (20+ dry ton/day)
• Wheelabrator - multiple hearth and fluidized bed units, strong municipal presence
• Takuma - Japanese technology with US installations, known for advanced emission controls and energy recovery systems

• Belt filter presses + lime stabilization - 40-60% lower capital cost, suitable for smaller facilities (<10 dry ton/day) with available land
• Centrifuge dewatering + composting - lowest operating cost but requires 6-12 months processing time and odor management
• Thermal drying + beneficial reuse - emerging option for Class A biosolids, 20-30% higher capital than incineration but generates revenue stream through fertilizer sales

Establish strong manufacturer service relationships early - most municipal facilities lack in-house expertise for major repairs. Andritz and Metso offer comprehensive service contracts worth considering. Plan refractory replacement during scheduled outages; budget 8-12% of capital cost annually for refractory maintenance. Energy recovery systems pay for themselves in 3-5 years in northern climates through building heating offsets and reduced natural gas consumption.