Oil Refinery Effluent: What You Need to Know

Petroleum wastewater that emanates from refineries usually contains large amounts of pollutants. The pollutants have high levels of refinery effluents such as oil and chemical products that could prove difficult to degrade. Many regulatory authorities, therefore, deny oil companies the permit to discharge petroleum wastewater in a bid to maintain the physical, biological, and chemical integrity of freshwater sources. However, several factors such as the low quality of crude and an increase in crude prices mean that wastewater pollutants from oil products have hit a new high. Refineries, therefore, have to employ advanced water treatment and recovery methods to handle the ever-increasing effluent rates. The oil refinery effluents are a major aquatic environmental source of pollution. Some of the common effluents include oil, grease, and toxic compounds. In the United States, the Environmental Protection Agency enacted the Petroleum Effluent Guidelines and Standards that provide how oil companies ought to manage their effluent. The paper will focus on the definition of wastewater components, treatment mechanisms for oil effluents, and future technological advancements in treatment. Petroleum or oil refinery is a complex procedure that consists of various operations at different levels. Refinery companies differ in size, products, and the crude used, and as such, water demands might vary from one company to the other. However, what remains apparent is that an oil refinery company is a major user of water compared to other companies in the region. As such, the release of refinery effluents to the water sources is a common phenomenon prompting the need to ensure constant treatment. The wastes produced range from chemical to inorganic wastes that have a toll on the aquatic environment. Treatment primarily focuses on the removal of unwanted particles, and chemicals, and in some instances, replacement with desirable chemicals. The Environmental Protection Agency (EPA) and other authorities have clearly outlined the treatment thresholds that each company must meet. Although the traditional treatment mechanisms have proved essential in cleaning the wastewater, new strategies with better efficiency have promised to revolutionize the management of oil refinery effluents. According to the United States Environmental Protection Agency, an effluent is a "wastewater-treated or untreated-that flows out of a treatment plant, sewer, or industrial outfall" (Tony, Purcell, & Zhao, 2012, p. 436). It, therefore, refers to the waste products discharged into the water surfaces. The oil refining procedure primarily processes crude oil into three different categories of products that have a high likelihood of causing effluents. The three major categories include the fuel products such as gasoline, jet fuels, refinery fuel, and petroleum gases amongst others. The nonfuel products include lubricants, waxes, naphtha solvents, and nonfuel coke amongst others. The third major categories of oil products with a high probability of pollution include petrochemicals. Examples include propane, ethane, butane, and BTEX compounds such as benzene and xylene (Yu, Han, & He, 2017, p. 1931). Before delving into the various types of wastewater, it is first vital to analyze the various refinery processes that lead to the addition of these effluents in the water. The first category is known as a topping which primarily involves the separation of crude oil components. The crude oil is separated into several hydrocarbon groups through the application of several processes including desalting, atmospheric distillation, and vacuum distillation amongst others. The next strategy used in the oil refinery industry is known as thermal and catalytic cracking which fundamentally involves the breakdown of large hydrocarbons formed in the topping process to smaller ones. Some of the processes that could be used to achieve this include thermal operations, delayed coking, visbreaking, catalytic cracking, and catalytic hydrocracking amongst others. Combining or rearranging hydrocarbons involves processes geared towards converting the hydrocarbons to form the desired end products through processes such as alkylation, polymerization, catalytic reformation, and isomerization amongst others. Removal of impurities through catalytic hydrotreating ensures the removal of impurities such as nitrogen, sulfur, and metal products from waste gas streams. As earlier noted, all these major refinery projects in one way or the other contribute to the formation of wastewater as a result of the deposition of the effluent products described above. The wastewater are classified according to the nature of the process or waste that led to its formation. One of the major examples is known as desalter water. Desalter is a process in the oil refinery that involves the removal of salt from the crude oil (Yu, Han, & He, F., 2017, p. 1931). Important to note is that the salt produced dissolves not in the crude oil but in the water. It is usually the first process of crude oil refining. Examples of salts found in crude oil include magnesium, sodium, and calcium. Therefore, the desalter water has high salt content since it emanates the washing of crude oil before the start of the toping operations. The second type of wastewater is known as sour water. Sour water is produced from the vacuum and atmospheric crude columns at the refineries. Some of the major components of sour water include ammonia and hydrogen sulfide. Normally, the two components need to be removed before utilizing the water elsewhere in the plant. The processes that lead to the formation of the sour water include steam stream stripping and fractionating operations. Another type of wastewater is referred to as “other process water” which emanates from actions such as catalyst regeneration, product washing, and other dehydrogenation actions. Spent caustic comes as a result of the extraction of acidic elements from the product streams. Tank bottoms are wastewaters of accumulation of water at the bottom of the tanks used in the storage of crude. Ballast water comes from product tankers while stormwater emanates from either process or non-process area runoff from the storm events. Treatment in oil refinery effluents primarily takes a three-staged step. The stages include primary and secondary oil and water separation, biological treatment, and tertiary treatment. Oil removal occurs in two steps to guarantee the removal of oil from the wastewater before it is ultimately fed into the biological system. The process of oil removal is carried out using an API separator followed by dissolving in an air flotation unit. The wastewater is thereafter taken to the secondary oil/water separation unit and then taken into the equalization system. The biological system primarily consists of a clarifier or an aeration tank. From the clarifier, the effluent can be taken to tertiary treatment if it is necessary. In some instances, pretreatment of the wastewater can occur before the actual treatment which consists of three major steps occur. The primary treatment of wastewater is fundamentally a physical process. It involves a form of gravity separation that seeks to remove settleable and floating materials from wastewater. Some of the basic processes that occur in this stage include oil/water separation and the removal of solids that could be in the water. After this, a process follows that involves the secondary oil/water/solid separation where two different strategies can be used including Induced Air Flow (IAF) or Deduced Air Flow (DAF). The API is the method of choice for separating oil from wastewater. The API separators utilize the principle of difference in the specific gravity by allowing denser materials to settle below liquids that are less dense. The hydrocarbons floating on the surface are removed through skimming while the sludge that stays at the bottom is periodically removed through an outlet (Tony, Purcell, & Zhao, 2012, p. 437). A typical API separator allows for the collection of wastewater for the process of pretreatment. A diffusion barrier is subsequently formed, allowing the wastewater to flow toward the outlet while the lighter oil fractions are skimmed off. In the removal of heavier solids, the process can employ scrappers and flights. In preventing the oil from escaping through the outlets, it is critical to seek the intervention of the underflow baffle plates. However, The API has its own shortcomings with regard to its suitability to act in the water/oil separation. First, the API separator cannot remove dissolved or emulsified oil that is present in most cases. The API separators sometimes have to contend with high pH that can stabilize emulsions. Therefore, it may call on the use of spent caustic streams that act either to neutralize or reduce the pH inside the API separators (El-Naas, Alhaija, & Al-Zuhair, 2014, p.57). Important to note is that the API separator plays a significant role in a three-phase separation that involves oil, solids, and water. In the primary stage, other refineries can employ other strategies such as parallel plate separators (PPI) and corrugated plate interceptors (CPI). These machines are smaller compared to the API and therefore require less space. Whereas the PPI and CPI are effective in the two-phase separation involving water and oil, they are ineffective in the third phase involving the solids. The secondary treatment involves the use of biological means in the treatment of wastewater. The process mainly focuses on the dissolved organic compounds in the wastewater. Biological treatment fundamentally occurs in two categories. The first step is the suspended growth process and the second is the attached growth process. The suspended growth process utilizes microorganisms mixed with the organic compounds in the liquids in a suspension. The microorganisms thereby utilize the organic components as food to foster their growth and their clumping into biomass. One of the methods used in the suspended growth process is activated sludge. In this method, the microorganisms are introduced into the organic waste hence using these materials as a source of carbon and energy for microbial growth and survival. In the attached growth process, instead of suspending the microorganisms, they are attached or connected to an inert packing material (Hedaoo, 2012, p. 431). The packing material can be a host of items including gravel, plastics, and rocks amongst others. Once the wastewaters come into contact with the microorganisms, they act upon it leading to the production of biomass and carbon dioxide. The biomass film continues to grow until it sloughs off on its own after reaching a certain threshold of a thickness (El-Naas, Alhaija, & Al-Zuhair, 2014, p. 60). Tertiary treatment is not compulsory but could be used if the aim of the process is to eliminate contaminants such as chemical oxygen demand (COD), total suspended solids (TSS), dissolved and suspended metals, and trace organics. Some of the techniques used in this system include sand flirtation, activated carbon, and chemical oxidation. The effluent that emanates from the biological treatment primarily contains between 25 and 80 mg/l of suspended solids. However, refineries in many places require to levels as low as 15mg/l on a consistent basis. As such, these calls for the use of sand filters in a bid to reduce the solid particles present in the wastewater. The effluent is passed through a filter media in a bid to restrict even the smaller particles that could have been missed in the previous stages. The filter primarily contains anthracite embedded over sand. Therefore, the relatively larger particles are held by the anthracite with the smaller ones getting trapped by the sand particles. Alternatively, tertiary treatments can involve the use of activated carbon where the organic constituents from the wastewater are exposed to a process known as carbon adsorption. The wastewater passes through a series of granular activated carbon (GAC), and the carbon thereby adsorbs all the organics (El-Naas, Alhaija, Al-Zuhair, 2014, p. 62). The last method that could be used in the tertiary method involves chemical oxidation that majorly targets non-biodegradable and traces organic compounds. However, it is not a conventional method to use. Some of the reagents used include hydrogen peroxide, chlorine dioxide, and ozone. Although the strategies discussed above have been used by oil refineries to treat their effluents to the required thresholds by the government, new technologies are emerging and promising to improve wastewater from oil refineries. Some of the technologies that are coming up include the use of basic media, microfiltration, microfiltration using reverse osmosis, microfiltration with nanofiltration, and ion exchange softening. Some of the important factors to not include the fact that the particular technology to be applied is often site-specific and therefore evaluation should happen on a case-by-case basis. Some of the factors used to assess the suitability of technology include operability, flexibility, capital and operating cost, space required, and the ability to provide required water specifications amongst others. None of these technologies are currently used in a wide scale. However, the refining industry has started looking at the options especially due to the increase in the cost of water. Therefore, due to the cost pressures and the regulatory demands imposed on the industry, oil refinery companies must ensure that they consider these methods. The first upcoming technology is the use of basic media which is alternatively known as sand infiltration. The method mainly serves in the removal of suspended and gross solids that exist in the refinery effluent. The water is pushed through a vessel that contains both sand and/or anthracite. In a bid to improve the efficiency of particle removal, the addition of cationic and anionic polymers could be an essential exercise. This type of treatment is majorly effective in the treatment of particles that are larger than 5 micrometers in size. The media should be periodically back-washed to enhance the efficiency of the system. The second technology that is emerging in the field of effluent treatment is known as microfiltration or ultrafiltration. Just like media filtration, it is also used in the removal of suspended materials in the refinery effluent. Microfiltration is relatively efficient compared to media filtration because it removes particles greater than 0.1 micrometers. The use of the ultrafiltration method, however, removes even smaller particles that are approximately 0.01 micrometers (Munirasu, Haija, Banat, 2016, p. 183). Microfiltration and ultrafiltration methods are membrane separation methods driven by pressure thus allowing for the separation of particulate matter from the soluble components. Materials used in these two processes must be hydrophobic and polymeric at the same time. Some of the materials used in microfiltration and ultrafiltration include polyethersulfone, polysulphone, and polypropylene amongst others. Due to their hydrophobic nature, they are prone to organic fouling by grease and oil. Microfiltration and ultrafiltration can be used with reverse osmosis (RO) essential in the removal of metals and dissolved salts in the refinery effluent thereby producing water that could be readily reused in the refinery process. The reason why RO would be effective in the removal of effluent particles is that the membrane typically has membranes with small pores of less than 0.001 micrometers. The membrane therefore selectively allows the passage of pure water at a rejection rate of 99% or higher (Munirasu, Haija, & Banat, 2016, p. 185). The RO membranes are therefore increasingly susceptible to fouling by hydrocarbons and grease. Another related technology that is coming up is the use of microfiltration/ultrafiltration with nanofiltration. Nanofiltration (NF) involves a pressure membrane used in the removal of specific dissolved organic compounds and in softening water. NF and RO share many similarities in terms of their functioning and design. The only difference is that salt rejection in Ro is much higher than in NF. The importance of NF is that it enables the removal of both dissolved and suspended particles. The only downside is that it is relatively ineffective in salt rejection. The last prospective piece of technology that will become essential in the treatment of oil refinery wastewater in the near future is the use of ion exchange. It functions appropriately in the removal of dissolved organic compounds existing in the refinery effluent. It is a different process to the other methods previously discussed because it is not exclusively a filtration process. The water is passed through a packed bed containing both cation and anion resins thereby enabling the exchange between undesirable salts such as magnesium and calcium with desirable ones such as the hydronium ion (Tony, Purcell, & Zhao, 2012, p. 435). Important to note that the process is reversible and the ion exchanger can always be loaded with the desirable ions. As noted before, most of these new technologies are not in use. Moving from the traditional means of effluent treatment to these new technologies will provide both advantages and disadvantages in equal measure. With regards to the environmental impact, the new treatment mechanism will provide better cleaning of the aquatic environment given that they remove even the tiniest of organic particles. Strategies such as ion exchange and nanofiltration will remove the organic compounds and salts using ways that limit the accumulation of a larger amount of waste thereby improving the environment. However, the only disadvantage that comes with some of the new treatment methods such as media filtration and microfiltration is that they are not effective in removing inorganic compounds such as metals and salts thereby failing to treat the effluent wastewater effectively when used alone. For instance, the use of media filtration as a standalone treatment mechanism cannot be regarded as a viable option in wastewater management (Yu, Han, & He, 2017, p. 1932). Tradition systems follow the primary, secondary, and tertiary steps thereby guaranteeing environmental safety at all levels. However, they are ineffective in dealing with smaller particles which ultimately find their way into the treated water. In terms of expenses and expenditures, running both the modern and the traditional treatment methods is a costly affair. However, the new mechanisms will potentially require more monetary input to start up and enhance maintenance. For example, the use of ion exchange or nanofiltration requires chemical reagents that could cost a refinery company more than what they could spend if they used the traditional means which partly rely on biological means that are easily accessible. Therefore, the traditional means are less costly to start up and can easily be managed. However, the new methods are expensive but provide the user with value for money due to the increased efficiency. 

Conclusion 

In conclusion, an oil refinery is a process that leads to the release of effluents of different kinds into the water thereby interfering with the aquatic environment. The effluents consist of several chemical and organic matters that lead to the formation of wastewater. In the spirit of preservation of the environment, oil refinery companies engage in the treatment of wastewater to enable recycling and preservation of water sources. The traditional method of treatment takes primary, secondary, and tertiary approaches. The primary stage involves the separation of oil from water with the second involving the use of biological means of treatment. The tertiary method applies chemical reactions only when necessary. The new methods that are coming up seek to provide more efficiency by targeting minute particles and chemical components. The combination of traditional and new sources promises a better future in oil refinery effluent treatment. 

References  

El-Naas, M.H., Alhaija, M.A. and Al-Zuhair, S., 2014. Evaluation of a three-step process for the treatment of petroleum refinery wastewater.  Journal of Environmental Chemical Engineering ,  2 (1), pp.56-62. 

Hedaoo, M.N., Bhole, A.G., Ingole, N.W. and Hung, Y.T., 2012. Biological wastewater treatment. In the Handbook Of Environment And Waste Management: Air and Water Pollution Control  (pp. 431-473). 

Munirasu, S., Haija, M.A. and Banat, F., 2016. Use of membrane technology for oil field and refinery produced water treatment—A review.  Process Safety and Environmental Protection ,  100 , pp.183-202. 

Tony, M.A., Purcell, P.J. and Zhao, Y., 2012. Oil refinery wastewater treatment using physicochemical, Fenton and Photo-Fenton oxidation processes.  Journal of Environmental Science and Health, Part a ,  47 (3), pp.435-440. 

Yu, L., Han, M. and He, F., 2017. A review of treating oily wastewater.  Arabian journal of chemistry ,  10 , pp.S1913-S1922.