API 421, formally titled “Management of Water Discharges: Design and Operation of Oil-Water Separators,” is the industry’s primary reference for sizing and designing gravity-based oil-water separators. It establishes the baseline criteria for achieving predictable oil droplet removal under controlled flow conditions.
Key specifications include:
- API 421 1.2 (pg. 2): Conventional API separators only remove droplets down to ~150 microns — far larger than the droplets found in most real-world wastewater.
- API 421 2.1.4.1 (pg. 7): Maximum allowable velocity of <3 ft/min to maintain laminar flow and prevent turbulence.
- API 421 4.1.3 (pg. 25): Enhanced gravity or coalescing designs should target 60-micron droplet capture.
Here’s the catch:
Most API concrete tanks fail to meet these requirements under real loading conditions and cannot consistently maintain laminar flow or handle solids. Modern enhanced gravity systems — like Mercer’s Compliance Master™ — are engineered specifically to meet API 421 performance targets in a compact, maintainable footprint.
