Tanks for Oily Wastewater Treatment - Separation, Equalisation and Polishing

Oily wastewater is the stream where an oil facility meets its discharge consent. Refinery and terminal effluent carries free oil, emulsified oil, suspended solids and variable flow - and a treatment system that works beautifully at steady state will fail on the first upset if it has not been designed for the variation. Tanks are where most of that resilience lives.

Engineered by Shijiazhuang Zhengzhong Technology Co., Ltd. (Center Enamel), tanks for oily wastewater treatment are specified around the actual flow and composition profile, with gravity separation, equalisation and sludge handling designed as one system rather than as separate units.

1. How Is Oily Wastewater Treated?

In stages: gravity separation for free oil, equalisation to smooth the flow, chemical or mechanical treatment for emulsified oil, and biological or filtration polishing where the consent requires it. Tanks carry the first three stages.

· Gravity Separation: Free oil rises and is skimmed; heavy solids settle.

· Equalisation: A tank smooths flow and composition so downstream units see steady conditions.

· Emulsion Treatment: Chemical dosing, heating or flotation breaks emulsified oil that gravity cannot separate.

· Sludge Handling: Settled solids are collected, thickened and removed.

· Polishing: Filtration or biological treatment removes the residual oil and dissolved contamination.

2. Why Is Equalisation So Important?

Because treatment units are designed for a steady load. Without equalisation, a slug of high- strength effluent punches straight through the separator and the consent is breached before anyone can respond.

· Flow Smoothing: Peaks are absorbed so downstream hydraulic loading stays constant.

· Composition Averaging: Mixing reduces the concentration spikes that would otherwise break through.

· Protection for Downstream Units: Flotation and biological treatment perform best at steady load.

· Mixing Provision: Equalisation basins are mixed so contents stay homogeneous and solids do not settle out.

· Operational Margin: Operators gain time to respond to an upset rather than reacting to a breach.

3. How Are Separated Oil and Sludge Handled?

Skimmed oil goes back to recovery; sludge is collected, thickened and removed. Both need a physical route out of the tank that works without dismantling anything.

· Skimming: A skimmer or adjustable weir removes the floating oil layer and routes it to recovery.

· Sludge Collection: Settled solids are collected in a hopper or sump and drawn off.

· Thickening: Where volumes justify it, sludge is thickened before disposal to reduce cost.

· Draw-Off Design: Draw-offs are located so they remove the target layer without disturbing the others.

· Cleaning Access: Tanks are provided with access for periodic desludging and inspection.

Engineering Assurance and Project Support

Every welded oil tank delivered by Shijiazhuang Zhengzhong Technology Co., Ltd. (Center Enamel) is designed to API 650 with shell courses sized by the one-foot method and checked for wind, seismic and hydrostatic load cases, or to EN 14015 and the Eurocodes where the site is European. Shell plate is shot-blasted to Sa 2.5 and coated to a documented dry film thickness, longitudinal and annular plate welds are examined by radiography or ultrasonic testing to the acceptance level written into the purchase order, and every tank is hydrostatically tested and dimensionally surveyed before hand-over. Treatment tanks are supplied as welded steel or coated steel basins with internals, skimmers, mixers, sludge hoppers and draw-offs engineered together, and delivered with hydraulic calculations and coating documentation.

"Oily water treatment fails on the upset, not on the average day. Equalisation is what buys you the time to respond."

Frequently Asked Questions (FAQ)

What tanks are needed for oily wastewater treatment?

Typically a gravity separator for free oil, an equalisation basin to smooth flow and composition, a flotation or chemical treatment stage for emulsified oil, and tanks for sludge collection and thickening. The exact set depends on the effluent characteristics and the discharge consent, but equalisation is almost always the difference between a system that survives upsets and one that does not.

Why does a slug of oily water break through a separator?

Because gravity separators are designed for a steady hydraulic load and a specific residence time. A sudden increase in flow reduces residence time and raises turbulence, so oil droplets do not have time or quiescence to rise. An equalisation basin upstream absorbs that slug and lets the separator see the conditions it was designed for.

How is the separated oil recovered?

The floating oil layer is removed by a skimmer or an adjustable weir and routed back to recovery or slop oil storage. The detail that matters is that the skimmer removes the oil layer without drawing significant water with it, and that the recovered stream has somewhere to go that preserves its value.

Can the tanks be customised for our effluent?

Yes. Capacity and geometry, separator internals, equalisation volume and mixing, flotation or dosing provision, skimmer and sludge draw-off arrangement, coating selection, access and instrumentation are all engineered to your effluent characteristics and consent limits.