Grease Trap Management: The 25% Rule and Pumping Schedules

The 25% Rule: When to Pump

The 25% rule is the industry-standard and regulatory threshold: a grease trap must be pumped when the combined depth of floating grease and settled solids reaches 25% of the total liquid depth of the trap. This rule is codified in the EPA's Guidance for Federal Facilities on Grease Management and adopted by most local pretreatment programs.

How to measure: open the trap access cover and measure three depths using a measuring stick or dipstick:

1. Floating grease layer: Lower the stick slowly until you feel the grease/water interface. The grease layer is the depth from the surface to this interface.
2. Water depth: The total depth of liquid in the trap from bottom to surface.
3. Settled solids: Lower the stick to the bottom. Food particles and heavy solids settle on the bottom. The solids layer is the depth from the bottom to where the stick meets resistance.

25% Check: (Grease Layer + Solids Layer) / Water Depth ≥ 0.25 → Pump Now

Example: A trap with 24 inches of liquid depth. If the grease layer is 4 inches and the settled solids are 3 inches: (4 + 3) / 24 = 29%. This trap needs immediate pumping.

Why 25%? Above this ratio, the grease trap's separation efficiency drops dramatically. The effective water column available for grease flotation and settling is reduced, flow velocity through the trap increases, and FOG begins passing through to the sewer in increasing quantities.

FOG Accumulation by Kitchen Type

FOG accumulation rates vary enormously by kitchen type, menu, volume, and dishwashing practices. Industry data from grease trap service companies, PDI testing, and municipal pretreatment programs provides approximate ranges per meal served:

• Full-service restaurant (frying-heavy menu): 0.015–0.025 gallons FOG per meal served
• Full-service restaurant (moderate menu): 0.020–0.035 gallons FOG per meal served
• Fast food restaurant: 0.010–0.020 gallons FOG per meal
• Cafeteria/institutional: 0.005–0.015 gallons FOG per meal
• Bakery: 0.008–0.015 gallons FOG per meal equivalent
• Deli/sandwich shop: 0.003–0.008 gallons FOG per meal

These are rough estimates based on aggregate field data. The only accurate method is to measure the actual FOG accumulation in your specific trap over several pumping cycles. Record the grease layer depth and solids depth at consistent intervals (weekly is ideal), calculate the accumulation rate, and set the pumping schedule based on reaching 25% before the next pump-out.

Factors that increase accumulation: fried foods, heavy butter/oil use, manual pot washing with grease, garbage disposals (which add food solids), and high-volume operations. Factors that decrease accumulation: pre-scraping and dry-wiping dishes before washing, proper oil disposal (not down the drain), low-FOG menus.

Grease Trap Sizing Verification

Grease traps are sized by flow rate (GPM) per the Uniform Plumbing Code (UPC) Table 10-3 or the Plumbing and Drainage Institute (PDI) standard PDI-G101. The flow rate is determined by the number and type of fixtures connected to the trap.

Sizing methods:

UPC Method: Calculate the total drainage fixture units (DFU) connected to the trap, convert to GPM using UPC tables, then select the trap size. Standard sizes: 20, 30, 40, 50, 75, 100 GPM for in-floor interceptors; 10, 15, 20, 25, 35, 50 GPM for under-sink traps.

PDI Method: More detailed, accounts for number of compartment sinks, dishwashers, and other fixtures. A 3-compartment sink with 20" × 20" × 14" compartments has a flow rate of about 75 GPM per PDI calculations.

Signs of an undersized trap:

• Reaching the 25% threshold within 1–2 weeks despite proper kitchen practices
• FOG observed in downstream manholes or cleanouts
• Frequent sewer backups downstream of the trap
• Grease visible on the trap outlet during peak kitchen hours

If your trap reaches 25% faster than the pumping company can service it economically (say, less than every 2 weeks), the trap is likely undersized for the operation. Consult a plumbing engineer about upsizing to the next standard capacity.

Compliance, Records, and Best Practices

Most municipal pretreatment programs require food service establishments to maintain grease trap pumping records and make them available for inspection. Typical requirements:

• Pumping manifests: Date, time, hauler name and license number, volume pumped, disposal location. Retain for 3–5 years (varies by jurisdiction).
• Pumping frequency: Most programs require pumping at least every 90 days, regardless of accumulation level. Some high-FOG operations may require monthly or biweekly service.
• Inspection records: Grease and solids depths measured between pump-outs. Weekly or monthly internal inspections are recommended.

Best management practices that reduce FOG discharge and extend pumping intervals:

• Dry wipe: Wipe pots, pans, and dishes with paper towels before washing. This alone can reduce FOG accumulation 30–50%.
• Proper oil disposal: Used cooking oil goes in designated collection containers, never down the drain. Many rendering companies collect used oil for free or even pay for it.
• No garbage disposals on trapped lines: Food waste increases solids loading and can overwhelm the trap. Scrape food into trash, not the disposal.
• Strainer baskets: Install strainer baskets in all floor drains and sink drains connected to the trap. Empty daily.
• Biological treatments: Some operations use enzyme or bacterial additives to break down FOG in the trap. Results vary; these are a supplement to proper pumping, not a replacement.