Grease Separator Calculator

Grease separators (grease interceptors or traps) remove fats, oils, and grease (FOG) from commercial kitchen wastewater before entering sanitary sewers, preventing blockages, overflows, and infrastructure damage. Municipal sewer authorities mandate grease separation per EN 1825 (European standard) and local plumbing codes (IPC, UPC). Proper sizing based on flow rate, retention time, and grease loading ensures effective separation while preventing bypass, odor problems, and maintenance issues. FOG accumulation in municipal sewers causes costly emergency repairs and capacity expansion—proper grease management protects public infrastructure and avoids enforcement penalties.

Separation Principles: Gravity separation exploits density differences between FOG (specific gravity 0.85-0.92) and water (1.00). Grease floats forming scum layer; solids settle as sludge; clarified middle layer flows to sewer. Effective separation requires sufficient retention time (3-10 minutes per EN 1825) for grease droplets to rise before wastewater exits. Turbulence, short-circuiting, and excessive velocities (>0.05 m/s) prevent proper separation, allowing FOG bypass—occurs when separators are undersized, improperly installed, or overloaded beyond design flow.

EN 1825 Classification and Sizing: Class I separators (nominal size NS ≥ 50 kg grease capacity) serve large commercial kitchens and food processing plants with high grease loads. Class II separators (NS < 50 kg) suit smaller establishments like cafes and convenience stores. Sizing accounts for peak wastewater flow rate (L/s), density factor (fat/oil content), and installation type (in-ground, above-ground). Commercial kitchens generate 15-40 L/min peak flow depending on size and equipment. Fast-food restaurants with continuous fryers produce higher concentrations requiring larger separators or pre-treatment.

Retention Time and Hydraulic Design: Retention time = separator volume ÷ flow rate. EN 1825 requires minimum 3 minutes for Class I; many jurisdictions mandate 5-10 minutes. Example: 1,000-liter separator at 5 L/s provides 3.3 minutes retention (marginally adequate); 1,500 liters provides 5 minutes with better performance. Baffles force wastewater through the clarified middle zone, preventing scum/sludge exit while minimizing turbulence. Dishwasher discharge (180-190°F) emulsifies grease—pre-cooling or separate routing improves performance.

Maintenance and Operational Requirements: Most jurisdictions require pumping when grease scum and settled solids occupy 25-30% of volume (leaving 70-75% clear capacity). High-volume kitchens need weekly/bi-weekly pumping; low-volume facilities require quarterly/semi-annual schedules. Neglected separators overflow grease creating violations, odors, and eventual blockage. Proper pumping removes all contents followed by refilling with clean water to re-establish grease/water interface. Installation requires venting, access manholes (24-30 inch diameter), inlet/outlet baffles, and sampling ports.

Advanced Systems and Compliance: Automatic grease recovery devices (AGRD) skim FOG mechanically achieving 95%+ grease removal efficiency (GRE) versus 80-90% for passive separators. Compact footprint suits retrofits and tight spaces. Regulatory compliance involves plan approval, installation inspection, operational permits, and FOG monitoring (<100 mg/L discharge limit). Violations trigger warnings to fines and permit suspension. FOG control ordinances require employee training, passive collection, and maintenance contracts. Recovered grease can be processed into biodiesel, animal feed, or industrial lubricants—large separators generate 500-5,000 liters annually with economic value.

Standards Reference: EN 1825 (Grease Separators), IPC (International Plumbing Code), UPC (Uniform Plumbing Code), local FOG control ordinances and sewer use regulations.