Oil-Water Separators

Oil-Water Separators remove free and dispersed petroleum products from municipal wastewater streams through gravity separation and coalescence. These systems typically consist of multiple chambers where oil droplets rise to the surface while heavier water settles below, with baffles and coalescing media enhancing separation efficiency. Well-designed separators achieve 95-99% removal of free oil and grease when properly sized for detention times of 10-20 minutes at design flow rates. The primary limitation is their inability to effectively remove emulsified oils or dissolved hydrocarbons, requiring upstream pretreatment or downstream polishing for complete petroleum product removal in sensitive discharge applications.

• Maintenance Shop Washdown (2-8 MGD plants): Separates petroleum products from vehicle/equipment cleaning operations before discharge to plant headworks. Selected for 99% hydrocarbon removal efficiency and ability to handle intermittent high-strength loads (100-500 mg/L oil). Upstream: floor drains and wash bays. Downstream: plant influent or sanitary sewer.
• Emergency Generator Areas (All plant sizes): Captures diesel fuel spills and routine maintenance washdown from backup generators. Required by SPCC regulations for facilities with >1,320 gallons fuel storage. Handles storm runoff and occasional high-concentration events (1,000+ mg/L). Downstream connection typically to plant influent.
• Chemical Feed Building Sumps (5+ MGD plants): Treats floor drainage containing hydraulic fluids, lubricants, and polymer residuals. API separators sized for 3-5 gpm continuous flow with 15-minute retention time. Critical for preventing process upsets in biological treatment systems sensitive to petroleum compounds.

Daily Operations: Operators perform visual inspections for oil thickness (should be <1 inch), check effluent quality using field test kits (target <15 mg/L), and verify skimmer operation. Flow monitoring ensures hydraulic loading stays within 0.5-2.0 gpm/ft² design range. Oil removal frequency varies from daily (high-load applications) to weekly (maintenance areas).

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Maintenance: Monthly cleaning of baffles and skimming equipment prevents clogging. Quarterly sludge removal requires confined space entry procedures and Level B PPE due to hydrocarbon vapors. Annual inspection of structural integrity and coating systems. Skilled maintenance staff needed for mechanical skimmer repairs and pump maintenance.

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Troubleshooting: Effluent oil breakthrough indicates hydraulic overloading, damaged baffles, or excessive turbulence. Odor complaints suggest anaerobic conditions requiring increased cleaning frequency. Typical service life: 15-20 years for concrete units, 10-15 years for steel with proper coating maintenance. Early failure modes include baffle deterioration and skimmer mechanical problems.

• Inlet Baffle/Flow Distribution: Stainless steel or concrete structure directing flow downward to reduce turbulence. Sized for 2-4 ft/min horizontal velocity to prevent oil re-entrainment. Selection based on expected flow variations and upstream pumping conditions.
• Separation Chamber: Concrete or steel tank providing 15-45 minute retention at design flow (typically 5-50 gpm for municipal applications). Length-to-width ratio of 3:1 to 5:1 optimizes separation efficiency. Depth ranges 4-8 feet depending on oil specific gravity.
• Oil Skimming System: Rotating tube or belt skimmers for continuous removal, or manual skimming for smaller units. Tube skimmers handle 1-10 gph oil recovery rates. Selection depends on expected oil loading and automation requirements.
• Sludge Removal: Bottom scrapers or manual cleanout ports for settled solids removal. Hopper bottoms with 60-degree slopes prevent accumulation. Sizing based on expected solids loading (typically 10-30% of oil volume).

• Flow Rate: 0.1-25 MGD for municipal applications, with typical plant washwater flows of 50-500 GPM during backwash cycles. Size for peak instantaneous flow, not average daily flow.
• Hydraulic Loading Rate: 2-8 GPM/ft² surface area for gravity separators; 15-25 GPM/ft² for coalescing plate separators. Conservative loading extends retention time and improves separation efficiency.
• Retention Time: 10-30 minutes minimum for gravity separation; 3-8 minutes for enhanced coalescing systems. Longer retention improves oil removal but increases footprint and cost.
• Oil Droplet Size: Design for 20-150 micron removal capability. Smaller droplets require coalescing media or dissolved air flotation enhancement.
• Effluent Oil Content: Target <15 mg/L for discharge to sanitary sewer; <5 mg/L for direct surface water discharge per local NPDES permits.
• Operating Pressure: Atmospheric for gravity separators; 15-30 PSI for pressurized coalescing units.
• Temperature Range: 35-120°F operating range; higher temperatures improve separation efficiency but may require cooling.
• Solids Loading: <100 mg/L TSS recommended; higher solids concentrations interfere with oil separation and require pretreatment.

• What is the peak instantaneous flow rate and oil concentration? Undersizing for 2-hour filter backwash events (typically 200-800 GPM) results in carryover and permit violations. Need detailed backwash schedules and oil usage inventory from maintenance records.
• Should you specify gravity separation or enhanced coalescing technology? Gravity units cost $15-25K but require 15-20 mg/L influent oil for effective removal. Coalescing plate or DAF systems cost $35-75K but handle 5-10 mg/L influent reliably. Wrong choice leads to chronic permit exceedances.
• What level of automation and monitoring is required? Basic manual skimming costs $8-15K additional; automated oil removal and level controls add $20-35K but prevent operator oversight issues. Consider staffing patterns and operator experience levels.
• How will you handle solids accumulation and disposal? Separators accumulate 0.5-2 ft³ sludge per million gallons treated. Without proper cleanout access and disposal planning, units become ineffective within 6-12 months. Factor $2-5K annual disposal costs and quarterly maintenance requirements.

• Primary: Division 46 23 13 - Packaged Water Treatment Equipment
• Secondary: Division 40 30 00 - Oil-Water Separators (if using specialized environmental specifications)
• Municipal applications typically fall under water treatment equipment specifications rather than industrial process equipment sections

• Material/Equipment Verification: Verify NSF-61 certification for potable water contact, Confirm structural calculations for burial depth and traffic loading, Review corrosion resistance ratings for specific wastewater chemistry
• Installation Requirements: Excavation shoring often required for deeper units, Crane access critical for large fiberglass or concrete units, Electrical rough-in coordination for automated skimming systems
• Field Challenges: High groundwater requiring dewatering systems, Utility conflicts in congested areas, Access limitations for maintenance equipment
• Coordination Issues: HVAC/plumbing trades for heated buildings integration, Lead times: 12-16 weeks for custom fiberglass units, 8-12 weeks for standard steel

• Hydro International - Downstream Defender series for stormwater applications, widely specified in municipal MS4 compliance projects.
• WesTech Engineering - Dissolved Air Flotation systems including their SUPRACELL units, common in water treatment plant washwater applications.
• Highland Tank - Fiberglass and steel API separators, frequently used for vehicle maintenance facility drainage.
• Xerxes Corporation - Underground fiberglass separators popular for municipal fleet maintenance facilities.

• Coalescing Plate Separators - 20-30% higher efficiency than API separators, preferred for tighter discharge limits. Cost premium of 15-25%.
• Dissolved Air Flotation (DAF) - Best for emulsified oils and fine solids removal. 2-3x cost of gravity separators but handles variable loading better.
• Media Filtration - Walnut shell or activated carbon for polishing. Used downstream of primary separation, adds $50-100/GPM installed cost.

Establish service agreements during procurement - many manufacturers provide excellent field support but response times vary significantly by region. Consider oversizing inlet chambers by 25% to handle unexpected debris loading, particularly common in municipal fleet facilities. Specify removable covers rather than permanent concrete lids to reduce long-term maintenance costs. WesTech and Hydro International typically offer the most responsive technical support for troubleshooting operational issues.