Introduction: Why oil and grease (FOG) matter

Fats, oils, and grease (FOG) from residential, commercial, and industrial sources can significantly disrupt
wastewater collection and treatment systems. Excess FOG causes blockages, overloads primary treatment,
interferes with biological processes, and increases operational costs. This guide explains how FOG impacts
each treatment stage, outlines diagnostics, provides actionable control strategies, and offers data-driven
examples you can adapt to your facility programs.

FOG sources and characteristics

• Residential kitchens: Cooking oils, meat fats, dairy, and food scraps discharged via sinks and dishwashers.
• Food service establishments (FSEs): Fryers, grills, dishwashing, and floor drains—often the largest contributors.
• Industrial dischargers: Food processing, biodiesel, cosmetics, and mechanical shops with oily rinses.
• Collection system inputs: Illicit connections, poor grease trap maintenance, and sewer deposition/mobilization.
• Chemical profile: Triglycerides and free fatty acids that solidify at lower temperatures and emulsify under shear.

Impacts of FOG across the treatment plant

• Collection system: Grease congeals and adheres to pipe walls, forming blockages and causing sanitary sewer overflows (SSOs).
• Headworks (screens/grit): FOG mats foul screens, reduce capture efficiency, and increase ragging and maintenance.
• Primary clarification: FOG forms scum layers that demand increased skimming, reduce hydraulic capacity, and cause odors.
• Biological treatment (aeration/BNR): High FOG loads inhibit oxygen transfer, cause filamentous growth, and upset MLSS stability.
• Secondary clarification: Poor floc quality and scum carryover increase TSS and effluent variability.
• Digestion and solids handling: FOG can lead to foaming in anaerobic digesters, unstable gas production, and dewatering problems.
• Odor and corrosion: FOG-rich scum supports septicity and sulfur compounds that accelerate corrosion.

Diagnostics and monitoring

1. Influent/FOG profiling: Composite sampling at influent and strategic sub-sewersheds; assess diurnal peaks (e.g., meal times).
2. FOG measurement: Hexane extractable material (HEM) or n-hexane extractable materials; monitor mg/L trends versus permits/targets.
3. Scum and solids tracking: Quantify scum removed at primary clarifiers and secondary basins; correlate with plant upsets.
4. Microbiology indicators: Microscopic exams for filaments, foam-formers (Nocardia-type), and floc integrity.
5. Operational KPIs: MLSS/MCRT, SVI, DO profiles, OUR, and aeration energy vs. load during FOG events.
6. Collection system surveys: CCTV inspections in known hotspots near FSE clusters; map blockages and cleaning frequency.

Example plant data and operational thresholds

Collection system controls

• Grease trap/Interceptor program: Permit FSEs, require properly sized interceptors, and enforce maintenance (e.g., 25% rule for pump-out).
• Best management practices (BMPs) outreach: Educate on scraping plates, dry wiping pans, and avoiding sink disposal of oils.
• Hotspot cleaning schedules: Jetting and vacuum combo trucks on FSE-dense corridors; document debris volumes and intervals.
• Source control inspections: Routine FSE audits with records of manifest/pump-outs and photo documentation.
• Targeted sampling: Upstream/downstream of FSE clusters to verify program effectiveness and adjust enforcement.

Headworks and primary treatment strategies

• Screening optimization: Maintain clean screens; adjust rake speeds during peak FOG; consider finer screening where feasible.
• Grit and scum management: Dedicated scum beach/skimmers; automate scum removal frequency based on level sensors.
• Chemical aids: Add coagulants or polymers at primary basins to enhance FOG capture (pilot to avoid downstream impacts).
• Flow equalization: Buffer diurnal FOG peaks with EQ basins to protect downstream processes.
• Temperature management: Prevent cold zones that congeal FOG; insulate piping and minimize heat loss where practical.

Biological treatment tactics

• Maintain DO and mixing: Increase aeration during FOG spikes; verify diffuser performance; avoid surface crusts.
• Control sludge age (MCRT): Adjust wasting to sustain robust, diverse biomass capable of lipid breakdown.
• Bioaugmentation (as needed): Trial lipid-degrading consortia; validate with side-stream testing and microscopic assessment.
• Nutrient balance: Ensure adequate nitrogen/phosphorus to avoid filamentous dominance under high organic loading.
• Secondary scum control: Skimmers and scum boxes with routine removal; avoid recycle of scum to aeration without conditioning.

Digestion and solids handling

• Feed rate control: Smooth digester loading; avoid slugs of FOG-rich scum; blend with primary sludge.
• Antifoam management: Use mechanical foam breakers first; dose antifoam only as operationally necessary.
• Thermal/chemical pretreatment: Consider thermal hydrolysis or enzymatic aids for high-FOG sludge streams.
• Dewatering optimization: Polymer selection and dose optimization; clean belts/centrifuges frequently under FOG conditions.

Industrial pretreatment and enforcement

• Local limits for FOG: Establish mg/L discharge limits aligned with plant capacity and historical upset thresholds.
• Pretreatment requirements: DAF units, API separators, and coalescing plate interceptors for oily industries.
• Monitoring and penalties: Sampling frequency escalation, corrective action plans, and fee structures for violations.
• Collaborative improvement: Technical assistance, BMP training, and performance benchmarking for dischargers.

Case studies and outcomes

Standard operating procedures (SOP) excerpts

Collection system FOG SOP

1. Identify hotspots: GIS map FSE clusters; review SSOs and blockage history.
2. Schedule cleaning: Jet/vac every 4–8 weeks; adjust frequency based on debris volumes.
3. Recordkeeping: Log nozzle settings, lengths cleaned, and loads removed.
4. Inspect traps: Verify interceptor maintenance records and issue notices for non-compliance.

Primary scum handling SOP

1. Skimmer checks: Inspect skimmer blades and scum beach daily; adjust to avoid overflows.
2. Scum pumping: Pump to dedicated scum holding; avoid direct recycle to aeration.
3. Chemical trials: Jar test polymers/coagulants; monitor effluent TSS impacts.

BNR resilience SOP

1. DO setpoints: Maintain 1.5–2.5 mg/L; increase air during FOG spikes.
2. Microscopy: Weekly checks; respond to foam-formers with wasting and selector optimization.
3. Wasting control: Target MCRT suited to load; avoid under-wasting during high FOG events.

Performance monitoring and KPIs

Communication and public outreach

Help us protect the sewer system: never pour fats, oils, or grease down the drain. Wipe pans with paper towels,
use a container to dispose of cooled grease in the trash, and maintain grease traps if you operate a restaurant.
Together we can reduce blockages, protect the environment, and keep utility rates lower.

Conclusion

Oil and grease impose measurable risks on wastewater plants—from SSOs and scum overloads to aeration inefficiency,
poor settling, and digester foaming. A robust program that combines source control, optimized headworks and primary
capture, resilient biological operations, and targeted pretreatment enforcement will stabilize performance, cut costs,
and improve effluent quality. Use the diagnostics, SOPs, and KPIs here to build a sustainable FOG control strategy.