Sour Water COD: Why De-Oiling First Is Cheaper Than Chasing the Stripper

As refiners blend more inferior heavy crude, the sour water system absorbs the consequences. Sulfur-bearing wastewater from the atmospheric and vacuum units arrives with a COD load that keeps climbing, and because oil and sulfide reinforce each other in the same stream, conventional treatment struggles to keep up. The result is a recurring shock to the stripping section and to everything downstream of it.

Understanding where the number comes from is the first step toward controlling it, because the COD in sour water is not a single-pollutant problem.

Why Sour Water COD Is a Coupled Problem

The reduction contribution of sulfide

Sulfide in water exists as H2S, HS- and S2-, and each behaves differently depending on pH. Above pH 7 its solubility in the aqueous phase falls somewhat, but because sulfide interacts more strongly with the oil phase, it migrates readily into non-polar oil. As a strong reductant, its oxidation registers heavily in the COD measurement, and the arithmetic is unforgiving: each 1 mg of sulfide contributes roughly 1.6 mg of COD. In one observed case, at a sulfide concentration of 3520 mg/L, the sulfide alone accounted for about 5632 mg/L of COD.

The emulsification synergy of oil

Inferior crude brings a rich supply of macromolecular natural surfactants such as asphaltenes and resins, plus processing aids, and those materials readily build a very stable emulsified oil system in sour water. Most of the sulfide dissolves into or is entrained by that oil phase, so oil, sulfide and COD stop being separate problems and become one. Critically, the emulsified oil and the dissolved macromolecular fraction, the C5+ material, are extremely difficult to strip out in a conventional stripper. That is the main reason purified water COD stays high month after month.

What Oil-Bearing Sour Water Does to the Units

The damage is systematic rather than local.

• Vapour-liquid balance. Oil promotes foaming in the tower, which degrades two-phase mass transfer, sharply increases steam consumption and reduces capacity.
• Equipment blockage. Oil accumulates and cokes on trays and reboiler surfaces, narrowing flow paths, weakening heat transfer and separation, and shortening equipment life.
• Product quality. By-product flows are obstructed, with effects such as black sulfur formation or oil contamination in liquid ammonia, both of which devalue the product.

The Case for Front-End Deep De-Oiling

Engineering practice points to one conclusion: removing oil aggressively before the stripper delivers desulfurisation and COD reduction at the same time. In a refinery case where overhead condensate entered with oil content above 3000 mg/L, a combination of fine filtration and a high-efficiency coalescing de-oiler produced a rapid and measurable result.

Effluent oil content dropped below 200 mg/L, an oil removal efficiency above 97.9 percent. The effect on the other pollutants was the more interesting finding: sulfide removal reached 72.1 to 80.7 percent, and COD removal ranged from 89.2 percent up to 95.88 percent, achieved as a consequence of taking the oil out rather than as a separate treatment objective.

Four Reasons to Move De-Oiling Upstream

• Total COD in the purified water falls sharply, easing the load on everything that follows.
• By-product sulfur and liquid ammonia come out cleaner and more marketable.
• Mass transfer in the stripping section improves, which lowers steam consumption.
• Unit load drops, extending the maintenance interval and service life of the stripper and its internals.

What the Removal Figures Really Say

The numbers from the refinery case deserve a second look, because they show that de-oiling is doing more than oil removal. Sulfide removal of 72.1 to 80.7 percent and COD removal as high as 95.88 percent were achieved without a dedicated desulfurisation step. What happened is that the oil phase, which carried much of the sulfide with it, was taken out first, and the pollutants that depended on it travelled with it.

That is the argument for sequencing the train around the oil rather than around individual pollutants. Treat the carrier and the load goes with it; chase each pollutant separately and the plant pays to process the same stream several times over.

A Graded Strategy Rather Than a Single Fix

The workable configuration treats the problem in two places. At the front end, a fine filter followed by a high-efficiency coalescing de-oiler targets the emulsified oil and the oil-phase sulfide that a stripper cannot handle, achieving combined oil, sulfur and COD removal at the source. At the back end, after stripping, residual dissolved macromolecular oil and refractory organics need advanced oxidation, and technologies such as ozone catalytic oxidation are appropriate for breaking those chains down so that final effluent meets consent.

Neither half works as well alone. Front-end de-oiling removes the bulk of the load cheaply; end-of-pipe oxidation polishes what physics cannot reach. Run together, they keep a stripping unit operating in the condition it was designed for, and that is what allows a refinery to hold compliance while still processing the crude slate the market is actually supplying.