Goodbye to "Using a Cannon to Swat a Mosquito": A "Green Shortcut" for Sour Water Treatment in Refineries
In the production process of the atmospheric and vacuum distillation units of a refinery, after crude oil passes through core processes such as distillation and fractionation, a special and troublesome process wastewater is produced — sour water. Because this water contains small amounts of sulphides, ammonia nitrogen and trace oil substances, according to the industry's traditional treatment flow, regardless of water quality it is sent directly to the huge sulphur recovery unit for unified "high-specification, full-process" deep purification.
However, over long practice we have found that the sour water produced by the atmospheric and vacuum distillation units of some refineries is in fact far "cleaner" than traditionally assumed. Continuing along the old path not only keeps the operating costs of the sulphur recovery unit high but also causes serious misallocation of energy and process resources, running counter to the refining industry's trend towards refined management and energy saving and consumption reduction.
SINOKLE has accurately identified this industry pain point and created a tailor-made, "point-to-point" precise treatment path for a large refinery, completely abandoning the extensive "using a cannon to swat a mosquito" treatment mode and achieving both environmental protection and economic benefits in sour water treatment.
1. Project Background: High-Quality Sour Water That Has Been "Collaterally Damaged"
In the traditional process design of refineries, environmental compliance and production safety are always given top priority. On this principle, the sour water produced by atmospheric and vacuum distillation units is usually sent wholesale to the sulphur recovery unit for deep purification because it contains potential pollutants such as sulphides. Although this treatment path maximises environmental compliance and is mature and reliable, it has long concealed a core pain point that the industry has overlooked — a serious mismatch between water quality and treatment process, causing large amounts of high-quality resources to be "treated wastefully".
To accurately understand the actual quality of the sour water from this refinery's atmospheric and vacuum distillation units, SINOKLE assembled a professional technical team and carried out three months of continuous monitoring and sampling analysis of this sour water. It was finally determined that the water quality shows a typical "two lows and one high" profile — "lightweight" wastewater that requires no high-specification treatment process at all:
• Low sulphide content: continuous monitoring showed that the sulphide content of this sour water is stable at around 30 ppm, far below the treatment load of conventional high-concentration sour wastewater (usually ≥100 ppm), so deep removal through the high-temperature, high-pressure process of the sulphur recovery unit is unnecessary.
• Low ammonia nitrogen content: the ammonia nitrogen content is maintained at around 20 ppm, a relatively mild pollutant concentration that meets the basic acceptance standard of a conventional wastewater treatment plant, so there is no need for additional complex denitrification for deep treatment.
• Relatively high oil content: this is the core difficulty of treating this sour water. The dispersed and emulsified oil droplets in the water are fine; if sent directly to the wastewater plant they would shock the biological treatment system, reducing treatment efficiency and causing non-compliant effluent, so targeted oil-removal pre-treatment is required.
2. The Awkward Status Quo: A Resource-Waste Dilemma of "High Spec, Low Use"
Forcing this "lightweight", low-concentration sour water into a sulphur recovery unit designed to treat high-concentration, highly toxic pollutants is like using an anti-aircraft gun to shoot a mosquito. It not only causes serious waste of resources but also imposes multiple operational burdens on the refinery:
On the one hand, treatment costs remain high. The sulphur recovery unit was designed to treat high-concentration sulphide wastewater, and its operation consumes large amounts of steam, electricity and chemical agents to keep high-temperature reaction and catalytic conversion stages running stably. Even when low-concentration wastewater is treated, this consumption of energy and chemicals cannot be reduced, sharply pushing up the unit treatment cost and imposing a heavy economic burden on the refinery over the long run.
On the other hand, high-quality resources are lost for nothing. After deep treatment in the sulphur recovery unit, the purified water produced is of extremely high quality, with all indicators far exceeding the acceptance standards of an ordinary wastewater plant and even meeting some process reuse requirements. However, because the refinery lacks the corresponding reuse channels, this high-quality purified water cannot realise its due value and can only be sent to the wastewater plant for secondary treatment, wasting the earlier treatment energy and increasing the pressure on the wastewater plant.
In addition, large volumes of low-concentration wastewater entering the sulphur recovery unit occupy the unit's effective treatment capacity, so that high-concentration, difficult wastewater cannot be treated in time, affecting the overall efficiency of the plant's wastewater treatment system. Based on the above pain points, we reach a clear conclusion: this low-concentration sour water must be "liberated" from the lengthy treatment chain of the sulphur unit, and a targeted on-site pre-treatment process should be adopted so that it can be sent directly once it meets the wastewater plant's acceptance standards, achieving "precise treatment and full use of resources".
3. The Way Forward: SINOKLE's Tailored "Filtration + Coalescing Oil Removal" Solution
Faced with the special water quality of this sour water — "high oil content and many impurities, but low sulphide and ammonia nitrogen content and low toxicity" — SINOKLE abandoned the one-size-fits-all traditional approach and, adhering to the principle of "precise targeting, simple and efficient", designed a tailored on-site pre-treatment solution of "filtration + coalescing oil removal".
The core logic of this solution is: focus on the core difficulty (oil removal), remove the interference factors (suspended solids and mechanical impurities), preserve the water quality advantages (low toxicity and low pollution), and use the most economical and efficient process to achieve compliant wastewater discharge, completely resolving the "high spec, low use" dilemma.
(1) Brief description of the process flow
Taking into account the site conditions at the refinery, the water quality characteristics and the wastewater plant's acceptance standards, SINOKLE optimised and designed a simple and efficient treatment flow that is fully automatically controlled without requiring large amounts of additional manual operation. The specific flow is as follows:
Sour water feed → regulating buffer tank (homogenising flow and quality) → filtration unit (removing suspended solids and mechanical impurities) → coalescing oil separator (core oil removal stage) → effluent testing → sent directly to the wastewater plant once compliant
Note: the addition of the regulating buffer tank is mainly to cope with the instability of sour water quality and quantity caused by fluctuations in refinery production; by homogenising flow and quality it prevents shock to downstream treatment units and safeguards the stable operation of the entire pre-treatment system.
(2) Analysis of the core technical highlights
The solution appears simple, but in fact every stage is precisely designed to fit the treatment needs of this sour water; the coordinated combination of the filtration unit and the coalescing oil removal unit forms the core competitiveness of the solution:
1. Pre-filtration: building the "first line of defence" for pre-treatment. First, a precision filtration unit with high-efficiency filter media precisely intercepts suspended particles, mechanical impurities and a small amount of large oil droplets in the water, with a filtration precision down to below 10 μm. This stage not only creates a good water quality environment for subsequent coalescing oil removal but also effectively prevents fine impurities from clogging the internal components of the coalescing oil separator, extending equipment service life and reducing maintenance costs. At the same time, the filtration unit uses a backwash design and can automatically backwash at regular intervals, without frequent replacement of filter media, reducing O&M workload.
2. The core weapon — coalescing oil removal: precisely solving the oily water problem. This is the essence of the solution and the key to resolving the core difficulty of this sour water. For the relatively high content of dispersed and emulsified oil in the water (particle size usually between 0.1 and 10 μm, difficult to separate naturally), SINOKLE's independently developed coalescing oil separator uses specially modified oleophilic and hydrophobic material as the coalescing medium. Its working principle is: when oily wastewater flows through the coalescing medium, the tiny oil droplets are adsorbed and gathered by the medium and gradually coalesce into larger droplets (up to over 100 μm in size); using the specific gravity difference between oil and water, the droplets float rapidly to the oil collection trough at the top of the equipment and are discharged through the automatic oil discharge system, achieving efficient oil–water separation.
In addition, the entire pre-treatment system uses PLC automatic control, monitoring influent quality, effluent quality and equipment operating status in real time. When water quality anomalies occur, the system automatically alarms and adjusts operating parameters to ensure stable, compliant effluent. The equipment also has a small footprint and can be flexibly arranged according to the refinery site without large-scale modification of existing facilities, reducing project construction costs and construction time.
(3) Verification of treatment performance
After the solution was implemented, a one-month continuous trial run showed that all treatment indicators met expectations and fully satisfied the plant's wastewater treatment acceptance standards. The specific results are as follows:
• Oil content: about 80–120 ppm before treatment, stable below 10 ppm after treatment, with a removal rate above 92%;
• Suspended solids: about 50–80 ppm before treatment, stable below 20 ppm after treatment, with a removal rate above 75%;
• Sulphides and ammonia nitrogen: after treatment they are stable below 5 ppm and below 15 ppm respectively, both meeting the wastewater plant's acceptance standards.
During the trial run the system operated stably with no equipment failures and a 100% compliance rate for effluent, enabling 100% of the sour water to bypass the sulphur recovery unit and enter the plant's terminal wastewater treatment plant directly, completely breaking the limitations of the traditional treatment model.
4. Benefit Analysis: A Double Win on Both the Environmental and Economic Ledgers
The application of SINOKLE's "filtration + coalescing oil removal" solution is not a simple process change but a quality-and-efficiency upgrade based on refined management at the refinery. It resolves the pain points of the traditional treatment model and achieves a triple improvement in economic, environmental and operational benefits — truly "environmental compliance, lower costs, higher efficiency".
(1) Economic benefits: "reducing the burden" on the sulphur unit and lowering operating costs
1. Directly reducing treatment costs: separating low-concentration sour water from the energy-intensive sulphur recovery unit and achieving compliant discharge through a low-cost pre-treatment process saves substantial steam, electricity and chemical agent costs each year. Calculations show that this project saves the refinery nearly one million yuan in operating costs per year, with an investment payback period of only 1.5 years — a very significant economic benefit.
2. Freeing up unit capacity: the sulphur recovery unit is freed from the burden of treating low-concentration wastewater to focus on higher-concentration, harder-to-treat sour wastewater, improving the unit's treatment efficiency and load factor, indirectly raising the overall treatment capacity of the plant's wastewater system and supporting the refinery's expansion of production scale.
(2) Environmental benefits: implementing the "dual carbon" strategy and achieving cascade utilisation of resources
1. Reducing ineffective energy consumption: avoiding the waste of steam, electricity and other energy caused by "high spec, low use" reduces the refinery's total carbon emissions, in line with the energy-saving and consumption-reduction requirements of the national "dual carbon" strategy, and helps the refinery achieve green, low-carbon development.
2. Optimising cascade utilisation of water resources: the high-quality purified water produced by the sulphur recovery unit is no longer sent to the wastewater plant for nothing but can be used preferentially for low-grade water uses such as refinery greening, floor washing and circulating water make-up, achieving cascade utilisation of water resources, raising the water utilisation rate and reducing fresh water consumption.
(3) Operational benefits: improving system stability and reducing O&M risk
The new pre-treatment unit not only achieves precise treatment of sour water but also acts as a regulating buffer. Through homogenising flow and quality, it effectively avoids the shock to the biological system of the wastewater plant caused by high-oil water entering directly, safeguarding stable operation of the biological system and reducing the risk of non-compliant effluent caused by water quality fluctuations. At the same time, the highly automated pre-treatment system requires little O&M work, lowering manual O&M costs and operational risk and further improving the stability and reliability of the plant's wastewater treatment system.
The successful application of the "filtration + coalescing oil removal" solution to atmospheric and vacuum distillation sour water treatment at this refinery solves the most practical problem with the most suitable process, completely abandoning the extensive "using a cannon to swat a mosquito" treatment mode. The solution not only opens a "green channel" for sour water to go directly to the wastewater plant, lowering operating costs and raising environmental performance, but also provides a replicable, scalable refined solution for sour water treatment in the refining industry.
