Sour Water COD: Why Sulphides and Emulsified Oil Must Be Removed Together

Two pollutants, one number

Sour water in a refinery, meaning the sulphide-bearing wastewater drawn from the process units, generally carries a high-COD problem. The COD is driven mainly by the combined action of oil and sulphides, and its consequences reach well beyond the water itself: it loads the downstream treatment train, disturbs the stripping units, and sets the size of everything that follows.

Treating it as a single pollution problem produces the wrong answer. The two contributors behave differently, respond to different separation mechanisms, and interact in a way that makes each one harder to remove while the other is present.

Sulphide chemistry and its share of the oxygen demand

Sulphides exist in three forms in water: H2S, HS- and S2-. They hydrolyse readily, and the hydrolysis of S2- produces OH-. Their solubility decreases especially above pH 7, which means that a pH excursion can move sulphide out of the aqueous phase in ways that upset both the analysis and the treatment. Conversely, sulphides are more soluble in the non-polar oil phase, because their molecules interact more strongly with oil than with water.

That last point is the one that matters most. Sulphides are strong reducing agents, and their oxidation markedly raises COD. The theoretical contribution ratio is about 1 mg sulphide to 1.6 mg COD. In a practical case, a sulphide concentration of 3520 mg/L contributed about 5632 mg/L of COD. Run the arithmetic and the implication is immediate: in that sample, sulphide accounted for the great majority of the oxygen demand, and no amount of oil removal alone would have brought the COD within range.

Why the oil matters more than the sulphide

The root cause of the modern problem is the increasing share of low-quality heavy crude in the refinery slate. Natural surfactants in that crude, asphaltenes and resins among them, together with refinery additives, form a stable emulsified oil system. Sulphides are mainly dissolved in or entrained by the oil phase, so oil, COD and sulphide problems are deeply coupled rather than merely coexisting.

That coupling is what defeats stripping. Emulsified oil and dissolved macromolecular oil, the C5+ fraction, are difficult to remove by conventional stripping, and this is the main reason the stripped water COD stays high, for example around 2500 mg/L even after the stripper has done its work. The stripper removes the volatile sulphide it can reach; it cannot remove oil that is dispersed in the water as droplets, nor oil that has dissolved.

What oil does to the units downstream

Emulsified oil is not only a loss of product oil. It damages equipment in three distinct ways.

• It disrupts stripper operation. Oil causes foaming, disturbs the gas-liquid balance, increases steam consumption and reduces capacity. A foaming stripper has to be cut back, which means the sour water backs up.
• It blocks critical equipment. Trays and reboilers clog with accumulated oil, reducing efficiency and shortening equipment life. Reboiler fouling is a maintenance outage, not a minor inefficiency.
• It contaminates by-products. Black sulphur is formed and liquid ammonia carries oil, degrading product quality. A by-product that fails specification is a waste stream with a disposal cost.

The case for removing oil first

Laboratory work and engineering practice point the same way: deep oil removal first removes sulphur and cuts COD at the same time. The mechanism is straightforward once the coupling is understood. If sulphide is carried in the oil phase, removing the oil removes the carrier.

A documented case combined an SiC filter with a high-efficiency coalescing oil separator to treat vacuum-tower overhead water with an oil content above 3000 mg/L. The outlet oil content was below 200 mg/L, giving oil removal above 97.9%, while sulphide and COD removal reached 72.1-80.7% and 89.2-95.88% respectively. The COD result is the interesting one: a purely physical oil removal step achieved most of the COD reduction without any oxidant, because the COD was attached to the oil.

Stripping after oil removal then shows four advantages: markedly lower COD in the stripped water; better quality of the sulphur and liquid ammonia by-products; higher mass-transfer efficiency in stripping with lower steam consumption; and a lower stripping load, which extends turnaround intervals and unit life. Each of those is a direct consequence of sending a cleaner feed to the tower.

Handling what remains after stripping

Oil removal does not close the problem. The dissolved macromolecular oil that survives stripping, the C5+ fraction, is not separable by physical means and not removable by stripping. For that residual, advanced treatment downstream of the stripper is the appropriate step; catalytic ozonation degrades the hard-to-remove organics further and brings the effluent within standard. The sequence is therefore staged by design: physical oil removal upstream, advanced oxidation for the residual COD downstream, with the stripper operating between them on a feed it can actually handle.

The economics support the staging. Where the calculation assumes 2% oil and 2000 mg/L sulphides in the sour water, the annual cost savings from reduced steam, recovered oil and avoided by-product downgrade are substantial. The numbers are site-specific, but the direction is not: the cost of removing oil upstream is almost always lower than the cost of carrying it through the stripper and paying for it in steam, fouling and off-spec product.

The deadlock and how it breaks

As refineries process more low-quality heavy crude, the high COD of sour water is essentially an oil-sulphur coupling problem rather than a single-pollutant problem. Efficient physical oil removal, such as SiC filtration plus coalescing separation, used as the pretreatment core removes most of the sulphides and the emulsified oil at the same time, markedly lowering COD and keeping the stripper running steadily. The residual dissolved oil then requires downstream advanced oxidation. That staged strategy is what breaks the deadlock and delivers environmental and economic gains together.