Treatment and reuse of wastewater in oil refineries

• 1. Contaminants in wastewater from petrochemical industries and oil refineries                                                                   
• 2. Wastewater treatment plants for oil refineries 
• 3. Use of dissolved air flotation (DAF) systems
• 4. Conclusion

Petrochemical plants and oil refineries consume significant amounts of water. These are the main uses of water in these facilities:

• Cooling of equipment and processes.
• Steam generation.
• Injection into reservoirs.
• Washing of tanks and equipment.
• Treatment of crude oil and other materials during the refining process.

The petrochemical industry is very strict regarding the environmental controls, standards, and protocols it follows, as the wastewater it produces is highly polluting. For this reason, oil refineries have on-site water treatment plants that mitigate the environmental impact of their wastewater. These plants use various physical, chemical, biological, and filtration processes, among others, to remove contaminants from wastewater before it is discharged or reused elsewhere in the refinery.

Furthermore, the water shortages around some oil refineries require measures to reuse as much of the wastewater as possible.

1. Contaminants in wastewater from petrochemical industries and oil refineries

Oil refineries produce large volumes of wastewater during their operations. These effluents contain various types of pollutants, including organic compounds, heavy metals, and other toxic substances.

Different types of effluents are produced in an oil refinery:

• Desalination water (crude oil washing)
• Sour water (from crude oil processes)
• Process water (product washing, catalyst regeneration)
• Spent caustics (difficult to treat)
• Tank bottoms (sediments)
• Blowdown water (from cooling towers)
• Treatment by-products (sludge, osmosis reject, resin regeneration)
• Rainwater (which can be reused)
• Ballast water (requires treatment to prevent diseases and invasive species)

The main sources of wastewater in petroleum refineries are as follows:

• Refrigeration of equipment and machinery.
• Washing of tanks and other equipment.
• Processing of crude oil and other materials.
• Wastewater may also contain by-products of the refining process, such as sludge, which require further treatment and disposal.

Oil refinery effluents contain significant concentrations of hydrocarbons, oils, and grease, which produce high COD levels and, consequently, an O₂ deficiency in the water.

Among the compounds present in this wastewater, the following stand out:

• Benzene, toluene, and xylene are volatile organic compounds (VOCs) used in refining processes and can be found in wastewater from cooling and processing equipment. These compounds are carcinogenic and can cause respiratory problems, among other health issues, in humans.
•  Polycyclic aromatic hydrocarbons (PAHs), which are organic compounds formed during the refining process and found in wastewater from refrigeration and processing equipment. (PAHs), which are organic compounds formed during the refining process and found in wastewater from refrigeration and processing equipment. They are carcinogenic and can harm aquatic life by altering the reproductive and immune systems of fish and other organisms.
• Heavy metals such as lead, mercury, cadmium and chromium such as lead, mercury, cadmium and chromium, which are found in wastewater from equipment cleaning and process water. They can accumulate in aquatic organisms and cause neurological and developmental problems in humans.
• Ammonia and nitrogen compounds, which are present in wastewater from refrigeration and processing equipment, can harm aquatic life by reducing oxygen levels in the water.
• Sulfides , present in wastewater from refining processes, can be toxic to aquatic life, causing death and reducing growth rates.
• Chlorinated compounds , such as polychlorinated biphenyls (PCBs), dioxins, and furans, are present in wastewater from refrigeration and processing equipment. These are highly toxic compounds and can cause cancer and other health problems in humans and animals.

2. Wastewater treatment plants for oil refineries

Oil refineries They require complex processes to treat their effluents. The goal is to reduce pollutant loads to the levels required by law, or to reuse the treated water within the industry itself, as service water or even as process water.

A wastewater treatment plant for an oil refinery typically consists of several stages:

• Preliminary treatment: In this phase, solids and oversized wastewater residues such as rocks, gravel and large pieces of equipment are removed from the wastewater. Screens, desanders and sedimentation tanks are often used for this purpose.                                                     
• Primary treatment: this phase consists of removing suspended solids and part of the organic matter from the wastewater. For this purpose, hydrocarbon separators, dissolved air flotation (DAF) systems, or sedimentation tanks, where the solids float to the surface to be separated as sludge, are usually used.
• Secondary treatment: consists of removing dissolved organic matter and other pollutants from wastewater. Biological treatment processes, such as activated sludge, trickling filters, or MBR systems, which use microorganisms to break down pollutants, are generally used for this purpose.                                                          

• Tertiary treatment: in this phase an advanced treatment of the effluent is carried out, with the objective of achieving the necessary quality to discharge or to be able to reuse the water. This is where the most sophisticated technologies are applied, such as filtration at different levels (micro/ ultra/ nano), including reverse osmosis, ion exchange resins, or evaporation for the most demanding waters.       
• Effluent sterilization: prior to final discharge, effluents are usually subjected to sterilization treatment, either by oxidizers or UV radiation.
• Sludge treatment: Sludge generated in the various stages of preliminary and primary treatment is generally treated separately from the wastewater. This sludge can be thickened and dewatered, followed by further treatment such as digestion, composting or incineration. Sludge from biological treatment is usually treated separately (digestion, mechanical or thermal drying).

PROCESS DIAGRAM FOR THE TREATMENT OF EFFLUENTS FROM THE PETROCHEMICAL INDUSTRY

Oil refinery wastewater treatment line with water recovery

Below is a treatment line for the reuse of wastewater generated by a petrochemical industry. This solution minimizes both raw water consumption and environmental impact.

FLOW DIAGRAM

The system has been designed for a flow rate of 100 m³/h, from which water is intended to be recovered for general services and industrial processes.

Dissolved air flotation (DAF) is essential for the separation of suspended solids (TSS), oils, and grease, which pose a risk to the proper functioning of membrane water recovery systems (UF/NF/RO).

As we can see, there are different filtration stages. The selection of the appropriate membranes will depend on the size and characteristics of the contaminants present in the effluent. It is typically recommended to include a microfiltration stage before ultrafiltration and reverse osmosis, in order to separate the remains of dissolved solids that could foul the membranes.

EXPECTED PARAMETERS IN PETROCHEMICAL INDUSTRY DISCHARGE TREATMENT LINE WITH WATER RECOVERY

The following table shows, as a guide, the values presented by the different contaminants at each stage of the treatment process.

Parameter	Unit	1	2	3	4	5	6	7	8	9	10	11	12	13	14
Flow rate	m3/h	100	98	95	95	90	5	10	105	25	130	125	10	100	15
pH	-	6,5-8	6,5-8	7-7,5	6,5-7	6,5-7	7,5	8	7-8	7-8	7-8	7-8	7-8	6-7	7
TSS	mg/l	600	450	≤50	≤100	≤40	-	200	≤50	≤10	≤5	≤1	≤1	≤0,1	≤5
Turbidity	N.T.U.	≥500	≤300	≤50	≤100	≤20	-	40	≤30	≤5	≤2	≤0,1	≤0,1	≤0,1	≤0,5
Oils and fats	mg/l	450	≤100	≤10	≤5	≤2	-	≤10	≤2	≤1	≤1	≤0,1	≤0,1	≤0,1	≤0,1
COD	mg/l	≤10000	≤8000	≤5000	≤1000	≤500	20	≤500	≤500	≤10	300	≤100	≤10	≤1	≤5
NTK	mg/l	20	20	20	10	10	≤10	≤10	≤10	≤5	≤10	≤10	≤10	≤5	≤20
Conductiv.	µS/cm	3000	3000	3000	3000	3000	6000	3000	4000	1500	3000	3000	3000	≤200	≤15000
Phenols	mg/l	60	≤50	≤30	10	10	-	50	15	≤1	≤10	≤1	≤5	≤0,1	≤10
Ca++	mg/l	90	90	90	90	90	180	90	100	50	90	90	90	≤5	≤400
Mg++	mg/l	40	40	40	40	40	80	80	50	20	40	40	40	≤2	≤200
HCO3–	mg/l	150	150	150	150	150	300	150	200	100	150	150	150	≤20	≤600
SO4-2	mg/l	300	300	300	300	300	600	300	400	200	300	300	300	≤20	≤1000
Cl-	mg/l0	500	500	500	500	500	1000	500	400	250	500	500	500	≤50	≤3000

3. USE OF DISSOLVED AIR FLOTATION (DAF) SYSTEMS

Dissolved air flotation (DAF) is a process commonly used in petroleum refineries, petrochemical and chemical plants for the separation of contaminants present in their effluents.

Acting as a primary clarification unit, DAF systems offer the following performance:

• 90-95% elimination in TSS.
• 95-99% oil and grease removal.
• Excellent results in reducing Chemical Oxygen Demand (COD) present in grease, oil, biological sludge, colored organic matter and colloidal material.

Dissolved air flotation can also be applied to:

• Sludge thickening.
• Secondary clarification after biological treatment. In this case, the DAF system is used to remove any residual solids, oils, and grease that were not removed during the primary and secondary treatment phases.

For a DAF system to operate efficiently, wastewater must undergo a coagulation-flocculation process before entering the dissolved air flotation equipment. If this physical-chemical prerequisite process is not carried out, or if it is not properly designed, the performance of the DAF unit will be negatively affected.

The treatment stage of wastewater from an oil refinery by dissolved air flotation follows these steps:

• Pretreatment: Refinery wastewater first undergoes preliminary treatment to remove large solids. The wastewater is then screened to remove any large particles. It then typically passes through an API or CPI pretreatment system, where most of the easily separable hydrocarbons and oils present are separated by density.
• Chemical conditioning: To improve the performance of the DAF system, a coagulation-flocculation process is carried out to bind contaminants, and chemical agents are used to optimize pH levels. Depending on the nature of the contaminants present in the wastewater, various chemicals can be used, such as ferric chloride, aluminum sulfate, or polymers. This helps bind oils, greases, and other materials dispersed in the water, which are in colloidal or finely peptized form.            
• DAF Operation: The wastewater is then pumped into a DAF system, where compressed air is injected into the tank, causing microbubbles to form that adhere to the flocculated particles, which float to the surface and form a sludge layer. 
• Sludge removal: The sludge layer on the surface of the DAF tank is skimmed off by a skimmer and collected in a separate sludge tank for further treatment. The clarified water is then conveyed to the next stage of the treatment process.

TYPES OF DAF SYSTEMS

Sigmadaf manufactures various models of DAF systems for the oil sector.

The FPHF and FPBC models can operate as a high-head unit thanks to a lower flotation surface area. They include a lamella pack to improve the separation process. The FPBC can treat 10 to 250 m3/h, while the FPHF operates in the 300 to 1,000 m3/h range.

The FPAC model is a lamella-free unit for treating high solids loadings. It is ideal for treating water with high solids concentrations. These models can be built in concrete tanks if the treatment capacity exceeds the limits of units constructed of stainless steel.

4. Conclusion

Wastewater from petrochemical industries contains a high concentration of organic matter and traces of hydrocarbons, oils, and grease. The growing trend toward water reuse in refineries requires the design of efficient and reliable wastewater treatment plants.

A significant portion of these contaminants are in colloidal form (TSS), which requires chemical coagulation/flocculation conditioning, with corresponding pH adjustment. The DAF system, and in particular, the equipment provided by SIGMADAF, are specifically designed to facilitate the separation and sweeping of these agents, with a level of efficiency that minimizes the risk of leaks .