Water Filtration Solutions
Spent Caustic Treatment water treatment system in operation
Application 05

Spent Caustic Treatment

Safe oxidation & neutralization of high-strength spent caustic

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OverviewProcess & ParametersChallenges & SolutionsPerformanceFiltrationSchematicSpecificationsLayoutDeliverables
Process Overview

What this application solves

Spent caustic from LPG, gasoline and olefins treating is one of the most hazardous refinery streams — high pH, high COD, sulfides, mercaptans, phenolics and odor. It cannot be sent directly to biological treatment. WFS designs dedicated pre-treatment that destroys sulfides and reduces COD before polishing.

Depending on the caustic type (sulfidic, naphthenic, cresylic), WFS applies wet air oxidation, chemical oxidation, neutralization with acid-gas stripping, or acidification with hydrocarbon recovery.

Application Parameters

Typical influent characteristics

ParameterTypical range
pH12 – 14
Sulfides1,000 – 40,000 mg/L
COD20,000 – 200,000 mg/L
Phenols1,000 – 20,000 mg/L
Caustic (NaOH)2 – 12 wt%

These ranges frame the design basis. WFS confirms the exact values for your site through laboratory characterization and 24-hour composite sampling, capturing variability across normal, upset and turnaround operation before any equipment is sized.

pH
12 – 14
Sulfides
1,000 – 40,000 mg/L
COD
20,000 – 200,000 mg/L
Phenols
1,000 – 20,000 mg/L
Challenges & Solutions

The problems — and how WFS solves them

Challenges faced
  • Extremely high COD, sulfide and phenol loads
  • Severe odor (H₂S / mercaptans) and corrosivity
  • Hazard and safety risk in handling and reaction
  • Wide variability between caustic types
  • Protecting the biological plant from shock & toxicity
Solutions needed
  • Wet Air Oxidation (WAO) for high-strength sulfidic caustic
  • Catalyzed chemical oxidation (air / peroxide) for lower strength
  • Neutralization with controlled acid-gas stripping & scrubbing
  • Acidification with hydrocarbon / cresylic acid recovery
  • Buffered, metered feed of treated stream to the ETP
Performance Parameters

Targets the system is designed to meet

Performance parameterDesign target
Sulfide< 1 mg/L (oxidized)
COD reduction70 – 99%
Effluentbiodegradable, ETP-compatible
Odor / H₂Sscrubbed to safe level

Guaranteed, tested performance

Every WFS system is commissioned against a documented performance test. We design to the discharge consent, reuse specification or zero-liquid-discharge target — and verify it on site before handover, then sustain it through O&M.

Filtration Approach

Filtration approach & system required

Oxidation is the core: it converts toxic sulfides to harmless sulfate/thiosulfate and breaks down organics. The route (WAO vs catalytic vs neutralization) is matched to strength, sulfur form and recovery value.

Off-gas is fully scrubbed and the treated liquid is buffered and dosed slowly into the ETP so the biology is never shocked.

Filtration system required
  • Blanketed storage / buffer tanks
  • Wet air oxidation or catalytic oxidation reactor
  • Neutralization & acid-gas stripping unit
  • Off-gas scrubber package
  • Recovery / phase-separation skid (optional)
  • Corrosion-resistant pumps & instrumentation
Treatment Train

Modular process-train schematic

Treatment Train Schematic Influent Treated
  1. 1Storage & Buffering
    Agitated, blanketed tank

    Stabilize variable feed

  2. 2Oxidation
    Wet air / catalytic oxidation

    Destroy sulfides, cut COD

  3. 3Neutralization
    Acid dosing + stripping

    Adjust pH, release/strip gas

  4. 4Gas Scrubbing
    Caustic / amine scrubber

    Capture H₂S & odor

  5. 5Recovery (optional)
    Phase separation

    Recover cresylic acid / HC

  6. 6Equalized Discharge
    Metered feed to ETP

    Protect biological plant

System Specifications

Indicative system specification

Capacity range
1 – 50 m³/h
Reactor
WAO up to ~250 °C / high pressure
Materials
SS316L / duplex / lined alloy
Automation
Interlocked safety control
Off-gas
Scrubbed & monitored
Codes
ASME VIII, API, PED, EHS
Plant Layout

Layout & general arrangement

A segregated, bunded hazardous-area block: blanketed storage, the oxidation reactor with heat-recovery exchangers, neutralization and a dedicated scrubber stack. Spacing and classification follow hazardous-area and toxic-gas standards.

Safety systems — gas detection, relief, emergency quench and showers — are integral. The treated-stream tie-in to the ETP is metered and interlocked.

Inlet ▸General Arrangement — Plot Plan▸ Outlet
  1. 01
    Storage & Buffering
  2. 02
    Oxidation
  3. 03
    Neutralization
  4. 04
    Gas Scrubbing
  5. 05
    Recovery (optional)
  6. 06
    Equalized Discharge
Utilities · Chemical Dosing · MCC / Control Room · Sludge Handling · Laboratory
Project Deliverables

What WFS delivers

  1. 1Spent caustic characterization by type
  2. 2Technology selection (WAO / oxidation / neutralization)
  3. 3HAZOP-reviewed process & safety design
  4. 4Reactor, scrubber & neutralization fabrication
  5. 5Installation, hazardous-area E&I & interlocks
  6. 6Commissioning, performance & emissions testing
  7. 7O&M, safety procedures & operator training
Delivery Model

From study to continuous O&M

  1. 01
    Study & Assess
  2. 02
    Characterize Water
  3. 03
    Custom Design
  4. 04
    Pilot & Trial
  5. 05
    Detailed Engineering
  6. 06
    Equipment Design
  7. 07
    Fabricate
  8. 08
    Deliver to Site
  9. 09
    Construct
  10. 10
    Electrical & Automation
  11. 11
    Continuous O&M

Discuss your spent caustic treatment project

Share your stream data and target specification — WFS will characterize the water and propose a custom train.

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