Grease Traps

Grease traps remove fats, oils, and grease (FOG) from wastewater by providing retention time for gravity separation and flotation. Wastewater flows through baffled chambers where lighter FOG rises to the surface while heavier solids settle to the bottom, allowing clarified effluent to discharge. Properly sized units achieve 85-95% FOG removal efficiency when maintained regularly. The key limitation is that grease traps require frequent manual cleaning and pumping to prevent FOG accumulation from reducing capacity and causing bypass conditions that can lead to collection system blockages.

• Headworks Pretreatment: Installed upstream of primary clarifiers to protect downstream biological processes. Grease traps remove FOG (fats, oils, grease) that could coat clarifier mechanisms and reduce settling efficiency. Typically sized for 2-4 hours detention at peak flow, connecting directly to grit removal systems and feeding to primary treatment.

• Restaurant/Commercial Discharge Points: Municipal plants serving areas with significant food service establishments install large-capacity grease traps (500-2,000 gallon) at key collection points. These intercept FOG before it reaches the main plant, preventing operational disruptions and reducing chemical costs for downstream biological treatment.

• Pump Station Protection: Grease traps installed at major lift stations protect pumping equipment from FOG accumulation that can cause impeller fouling and reduced pump efficiency. Common in plants serving mixed residential/commercial areas where grease loadings exceed 100 mg/L.

• Sidestream Treatment: Some facilities install grease traps to treat high-strength waste streams from septage receiving or industrial discharge permits, preventing shock loads to the main treatment process.

Daily Operations: Operators monitor grease layer thickness using measuring sticks, checking for 6-12 inch maximum accumulation before pumping. Water temperature monitoring ensures 85-95°F range for optimal separation. Flow rate verification prevents hydraulic overloading that reduces separation efficiency. Visual inspection of outlet quality identifies breakthrough conditions requiring immediate attention.

​

Maintenance: Grease removal every 30-90 days using vacuum trucks, depending on loading rates and trap capacity. Monthly cleaning of inlet/outlet baffles prevents grease buildup and flow restrictions. Heating system maintenance includes quarterly coil cleaning and annual electrical inspections. PPE requirements include chemical-resistant gloves, eye protection, and respiratory protection due to H2S generation. Confined space entry procedures required for internal inspections.

​

Troubleshooting: Grease carryover indicates overloading, inadequate heating, or excessive detention time reduction. Odor complaints suggest inadequate ventilation or excessive organic loading requiring more frequent pumping. Heating system failures cause grease solidification and reduced separation efficiency. Typical service life ranges 15-20 years for concrete structures, 10-15 years for fiberglass units, with major component replacement

• Inlet Baffle System: Submerged inlet baffles (typically 18-24 inches below surface) force incoming flow downward, reducing turbulence and allowing FOG separation. Constructed from fiberglass or stainless steel, sized for 3-6 fps inlet velocity to prevent grease re-entrainment.

• Grease Storage Compartment: Upper chamber stores separated grease, typically sized for 30-90 days storage capacity based on loading rates of 0.1-0.5 lbs FOG per 1,000 gallons treated. Equipped with grease level sensors and removal ports for vacuum truck access.

• Outlet Weir Assembly: Adjustable outlet weirs maintain proper water levels and prevent grease carryover. Standard weir loading rates of 5,000-10,000 gpd per linear foot ensure adequate separation time. Typically constructed from stainless steel with manual or automatic adjustment mechanisms.

• Heating System: Electric or hot water heating coils maintain 85-95°F operating temperature to keep grease fluid for effective separation. Heating capacity typically sized at 50-75 BTU per gallon of trap volume for northern climates.

• Flow Rate Capacity: 5-500 GPM for municipal pretreatment applications. Size based on peak hourly flow from commercial/industrial dischargers, not daily average.

• Hydraulic Retention Time: 30-45 minutes minimum at design flow rate per local ordinances. Some jurisdictions require 60 minutes for high-strength waste.

• Grease Removal Efficiency: 90-95% removal of fats, oils, and grease (FOG) at design flow rate. Performance degrades significantly above 110% of rated capacity.

• Temperature Range: Designed for 40-180°F influent temperatures. Higher temperatures reduce grease solidification and capture efficiency.

• Baffle Configuration: Minimum 18-inch inlet/outlet separation. Inlet invert typically 6 inches below outlet invert to promote quiescent conditions.

• Pumping Requirements: Grease removal pumps sized for 2-5 GPM at 20-50 feet TDH. Solids handling pumps require 2-inch minimum discharge for FOG transport.

• Structural Loading: Designed for 300 psf live load minimum for vehicular traffic areas. Buried units require H-20 loading per AASHTO standards.

• Access Requirements: 24-inch minimum manholes for maintenance access. Larger units require multiple access points spaced maximum 20 feet apart.

• 1. What is the required grease trap capacity based on actual discharge patterns? Municipal engineers must determine peak hourly flow rates from each commercial discharger. Undersizing by 20% can reduce efficiency to 70-80%, leading to downstream collection system problems and regulatory violations. Requires detailed flow monitoring data and grease interceptor sizing calculations per PDI-G101 standards.

• 2. Should the system use gravity separation or enhanced removal technology? Standard gravity units work for flows under 100 GPM with moderate FOG concentrations (<500 mg/L). Enhanced systems with air flotation or coalescing media needed for higher flows or concentrations. Wrong choice results in either over-investment or poor performance requiring costly retrofits.

• 3. What pumping and conveyance method will handle collected grease? Vacuum truck removal requires 4-inch minimum connections and adequate site access. Pump-out systems need FOG-compatible pumps (progressive cavity or air-operated diaphragm) and heated piping for viscous waste. Poor planning leads to maintenance access problems and operational failures.

• 4. How will the system integrate with existing pretreatment monitoring requirements? Sampling ports, flow measurement, and temperature monitoring must comply with local industrial pretreatment programs. Missing requirements result in permit violations and enforcement actions.

• Division 44 - Pollution Control Equipment Section 44 21 00 - Grease Interceptors

• Secondary: Division 40 - Process Integration (40 30 00 - Industrial Process Piping) for associated pumping and conveyance systems.

• Material/Equipment Verification: PDI/IAPMO certification required; Verify capacity ratings match actual flow calculations; Confirm inlet/outlet sizing and invert elevations

• Installation Requirements: Adequate access for pumping trucks (typically 100-foot hose reach); Proper structural support for large units; Bypass provisions during maintenance

• Field Challenges: Coordinating with kitchen equipment installation; Maintaining proper slope to units; Ventilation requirements often overlooked

• Coordination Issues: Early coordination with food service contractors critical; Lead times typically 4-6 weeks for standard units

• Zurn - Z1170 series cast iron interceptors (500-2000 GPM municipal applications)

• Canplas - BigMax series fiberglass units popular in Canadian municipalities

• Josam - J-1000 series with flow control fittings for larger municipal facilities

• Highland Tank - Fiberglass interceptors with 1000-5000 gallon capacities for municipal pump stations

• All offer IAPMO/PDI certifications required by most municipal specs.

• Biological Grease Treatment Units - Higher upfront cost ($15K-30K vs $3K-8K) but reduce pumping frequency by 75%. Best for high-volume facilities.

• Grease Recovery Devices - Automated skimming systems ($20K-50K) that recover saleable grease. Economic payback depends on local grease markets.

• Hydromechanical Interceptors - Compact units using baffles/weirs instead of retention time. Lower capacity but faster installation in retrofit situations.

Oversizing by 25% reduces maintenance frequency significantly - worth the upfront cost in municipal applications. Establish relationships with certified pumping contractors early; many municipalities struggle with FOG waste disposal logistics. Consider remote monitoring systems for larger installations - saves costly emergency calls. Always specify removable covers rather than bolted access - operators will thank you during routine maintenance.