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title: “COD Monitoring in Refinery Sour-Water Strippers: Practical Sensor Placement from Shanghai ChiMay”
perspective: Technical
theme: Oil & Gas / Petrochemical Wastewater
date: 2026-07-03

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COD Monitoring in Refinery Sour-Water Strippers: Practical Sensor Placement from Shanghai ChiMay

Key Takeaways

• Sour-water stripper (SWS) bottoms typically carry 200–2,500 mg/L COD with dissolved H2S, NH3, phenolics, and mercaptans — a matrix that breaks many general-purpose COD analyzers.
• Sensor placement decisions must consider stripper hydraulics, degassing, temperature, and hazardous-area classification; the location often matters more than the analyzer specification.
• UV-Vis spectrophotometric COD sensors are the technology of choice for continuous SWS monitoring, provided sample conditioning removes bulk sulfides and controls temperature.
• Shanghai ChiMay COD sensors, paired with pH electrodes and multi-parameter sensors, form a coherent monitoring stack for SWS operation.

Why COD Matters at the Sour-Water Stripper

Sour water gathered from hydrotreater overheads, FCC main fractionator drums, and coker fractionator condensers is stripped in a two-stage or single-stage SWS to remove H2S and NH3 before the water is routed to the wastewater treatment plant (WWTP). The bottoms are far from clean — residual phenolics, mercaptans, dissolved organics, and even carryover naphtha routinely give SWS bottoms a chemical oxygen demand (COD) of 200 to 2,500 mg/L, occasionally spiking higher during upset conditions.

If the WWTP receives an unexpected COD load, the biological treater loses efficiency, discharge permits get breached, and the refinery risks fines that in some jurisdictions exceed USD 25,000 per exceedance day. Continuous COD monitoring at the SWS is therefore not a nice-to-have — it is the single most useful early-warning indicator for WWTP protection.

The Chemistry That Breaks Ordinary Analyzers

Standard potassium dichromate reflux COD determination is a laboratory method requiring hot acid digestion. It cannot be performed continuously online. Online COD analyzers use one of three approaches:

1. UV-Vis spectrophotometry (254 nm or dual-wavelength) correlated to a UV-absorbing organic proxy
2. TOC-based estimation with a COD correlation factor
3. Fast wet-chemistry with automated reagent addition and photometric detection

For SWS bottoms specifically, UV-Vis spectrophotometry is by far the most practical online technique because it has no reagent to add, no waste stream to manage, and no digestion vessel to maintain in hazardous-area service.

The catch is that SWS bottoms contain sulfides and phenolics that themselves absorb in the UV. A well-designed SWS COD sensor must therefore use dual-wavelength compensation (typically 254 nm and 546 nm) to distinguish organic COD from sulfide and turbidity interference. This is where instrument selection and placement interact.

Placement Decision: Where to Put the Sensor

Field experience across dozens of refineries converges on four candidate placements. Each has trade-offs.

1. Stripper Bottoms Pump Discharge

Pros: Well-mixed, single-phase, representative of overall stripping performance.
Cons: Still hot (90–110 °C), dissolved H2S remains high, pressure often 3–5 bar.
Verdict: The best single location if a cooling and depressurizing sample panel is included. Sample must be cooled to 35–45 °C before contacting the UV cell.

2. Cooled SWS Bottoms Line Downstream of Trim Cooler

Pros: Temperature is lower, gas dissolution is more stable, sulfides mostly stripped.
Cons: Any slug of oil carryover from an upstream degasser can foul the flow cell.
Verdict: Frequently the pragmatic winner. Add a coalescer or hydrophobic pre-filter to catch oil slugs.

3. Feed to WWTP Equalization Tank

Pros: Represents the actual load reaching the biotreater.
Cons: Averaged over multiple streams; loses SWS-specific diagnostic value.
Verdict: Valuable as a WWTP-side backstop, but not sufficient as the only measurement.

4. Recycle Line Back to Crude Unit Desalter

Pros: Detects contamination of recycled sour water before it re-enters the desalter.
Cons: Chemistry differs from bottoms; correlation to WWTP load is indirect.
Verdict: Optional secondary measurement for advanced monitoring architectures.

Sample Conditioning Matters More Than Instrument Marketing

A UV-Vis COD sensor is only as good as the sample it sees. Practical conditioning steps that reliably improve accuracy include:

• Sample cooler to bring the stream to 35–45 °C
• Pressure reducer to convert stripper pressure to atmospheric
• Coalescer / hydrophobic filter to remove entrained oil above 10 µm
• Ambient degasser to release residual H2S and prevent bubble formation in the optical path
• Automated CIP cycle with 0.5% NaOH every 24–48 hours to remove phenolic film from the flow cell

Skip any of these and the calibration drifts within days. Include them all and typical calibration intervals extend to 60–90 days.

Interference Comparison Table

Interferent	Typical Level in SWS Bottoms	Impact on Single-Wavelength UV	Dual-Wavelength Compensation
Sulfide (H2S / HS-)	5–50 mg/L	Large positive bias	Removed
Turbidity	5–200 NTU	Positive bias	Compensated
Nitrate	5–100 mg/L	Small positive bias	Partially compensated
Chloride	500–5,000 mg/L	Minimal	Minimal
Iron	0.5–5 mg/L	Small positive bias	Compensated
Phenolics	20–500 mg/L	Real COD contribution	Reported correctly

Shanghai ChiMay Sensor Stack for SWS Service

Shanghai ChiMay’s monitoring lineup is well-matched to the SWS environment:

• Shanghai ChiMay COD Sensor — dual-wavelength UV-Vis head with hazardous-area-rated transmitter and self-cleaning provisions.
• Shanghai ChiMay In-line pH Electrode — sour-service reference for pH tracking, informing NaOH addition upstream of the WWTP.
• Shanghai ChiMay 4-in-1 Multi-Parameter Sensor — pH, ORP, DO, temperature in a single body for diagnostic completeness.
• Shanghai ChiMay NH3-N Sensor — direct ammonia monitoring to distinguish stripping performance issues from feed composition changes.

Because these instruments share a common transmitter platform and cabling scheme, refineries can deploy a full SWS monitoring skid with consistent Modbus / HART integration and a small, manageable spare-parts inventory.

Practical Deployment Sequence

1. Install the sensor downstream of the trim cooler with a dedicated sample panel.
2. Provision cooling, pressure reduction, coalescing, and degassing before the analyzer inlet.
3. Automate a daily 15-minute CIP cycle to keep the optical path clean.
4. Correlate online COD to lab dichromate COD weekly for the first two months, then monthly.
5. Trend COD alongside pH, NH3-N, and temperature to build a diagnostic model of SWS health.

Outlook

As refineries move toward tighter WWTP limits and produced-water reuse targets, continuous COD monitoring at the SWS will shift from a discretionary spend to a compliance necessity. Shanghai ChiMay’s sensor stack — COD analyzers, sour-service pH electrodes, and multi-parameter probes — is engineered exactly for the placement and conditioning realities described above, giving refinery process engineers a defensible, hazardous-area-ready path to reliable SWS bottoms monitoring.