Causes of High COD in Refinery Sour Water and the Coordinated Treatment Path

Sour water (sulphide-containing wastewater) in refineries generally faces a high-COD problem driven mainly by the combined action of oil and sulphides, which severely impacts downstream water treatment and stripping units.

1. Core causes of the high-COD problem

1.1 Solubility characteristics and contribution of sulphides:

Sulphides (H₂S, HS⁻, S²⁻) hydrolyse readily in aqueous solution (for example S²⁻ hydrolysis produces OH⁻), and their solubility decreases especially above pH 7.

Conversely, sulphides are more soluble in the non-polar oil phase because their molecules interact more strongly with oil.

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.

1.2 The key role of oil:

The root cause is the increasing share of low-quality heavy crude in the refinery slate. Its natural surfactants (asphaltenes, resins and the like) together with additives form a stable emulsified oil system.

Sulphides are mainly dissolved in or entrained by the oil phase, so the oil, COD and sulphide problems are deeply coupled.

Emulsified oil and dissolved macromolecular oil (C5+) are difficult to remove by conventional stripping, and this is the main reason why the stripped water COD stays high (for example around 2500 mg/L).

2. Harm caused by oil in sour water

Emulsified oil not only means loss of product oil but also severely impacts downstream units:

1. It disrupts stripper operation: oil causes foaming, disturbs the gas-liquid balance, increases steam consumption and reduces capacity.

2. It blocks critical equipment: trays and reboilers clog with accumulated oil, reducing efficiency and shortening equipment life.

3. It contaminates by-products: black sulphur is formed and liquid ammonia carries oil, degrading product quality.

3. The solution: efficient oil removal as the core

Laboratory and engineering practice show that deep oil removal first can remove sulphur and cut COD at the same time:

Case: a process combining an SiC filter and a high-efficiency coalescing oil separator treated vacuum-tower overhead water with an oil content above 3000 mg/L; the outlet oil content was below 200 mg/L (oil removal above 97.9%), and sulphide and COD removal reached 72.1-80.7% and 89.2-95.88% respectively.

Four advantages of stripping after oil removal:

1. Markedly lower COD in the stripped water;

2. Better quality of the sulphur and liquid ammonia by-products;

3. Higher mass-transfer efficiency in stripping and lower steam consumption;

4. Lower stripping load and longer turnaround intervals and unit life.

4. Targeted recommendations

4.1 Deep upstream oil and sulphur removal:

For emulsified oil and oil-phase sulphides, pre-treat with the combined SiC filter plus high-efficiency coalescing oil separator process to remove oil, sulphur and COD together efficiently.

4.2 Downstream advanced treatment of refractory COD:

For the dissolved macromolecular oil (C5+) remaining after stripping, apply advanced treatment such as catalytic ozonation downstream of the stripper to degrade the hard-to-remove organics further and ensure the effluent meets standards.

Conclusion

As refineries process more low-quality heavy crude, the high COD of sour water is essentially an oil-sulphur coupling problem. Using efficient physical oil removal (such as SiC filtration plus coalescing) as the pre-treatment core removes most of the sulphides and emulsified oil at the same time, markedly lowering COD and keeping the stripper running steadily; the residual dissolved oil then requires downstream advanced oxidation. This staged treatment strategy is the key to breaking the sour water deadlock and achieving both environmental and economic gains.