In petroleum refining, a large amount of water is used in the refining process. This water is processed using wastewater treatment techniques. Since the amount of water used in refining is so vast, waste water treatment is an integral part of the refining process. There are four different types of waste water; cooling water, process water and steam, storm water and sanitary sewage water. These four types of water carry large amounts of pollutants as well. Pollutants like, liquid hydrocarbons, suspended and dissolved solids, mercaptans, phenols, amines, and cyanides. All of these pollutants are a direct result of the water’s use in the refinery.

There are two types of measurement systems that are used to measure the level of contamination in waste water. Those two types are the Biochemical Oxygen Demand and the Chemical oxygen demand. Biochemical Oxygen Demand measures the amount of oxygen consumed by microorganisms in decomposing organic matter, and the Chemical Oxygen Demand measures the total oxygen consumption by organic and inorganic chemicals present in water.

Refinery waste water however, can’t be treated in municipal waste water treatment plants. This is because the municipal water treatment plants are not capable of processing the contaminants that arise from water that is used in petroleum refining. It is also important to keep the different wastewater streams segregated as the different types of wastewater have different levels of different contaminants. This making the combining of the streams undesirable as the treatment of the water would be much more difficult, placing to high of a strain on the water treatment machinery.

References.

1. https://cms.psu.edu/section/content/default.asp?WCI=pgDisplay&WCU=CRSCNT&ENTRY_ID=F20C6357261A4AE2A750C141B721E8C1

There are a number of reasons why refineries treat their wastewater right on site instead of sending it off to a municipal treatment plant, but for the most part, it has to do with the contaminants present in the samples. Wastewater from processes such as desalting, distillation, thermal and catalytic cracking, and coking possibly contain benzene (toxic aromatic compound), H₂S, NH₃, heteroatoms, and acids. Requirements for wastewater in municipal treatment plants vary from state to state so refineries are just playing it safe by treating their water in-house. The refineries have the equipment and knowledge to detect the exact amount of contaminant species, therefore they should be the ones who treat it. On top of all this, you can’t combine the four different types of wastewater (cooling water, process water and steam, storm water, and sanitary sewage water) so transporting it to the municipal plant would require four different vehicles, a bunch of unneeded money spending, and communication of specifics. Even though industrial wastewater treatment plants are capable of removing chemicals and acids, it is not safe to use the facilities for water that has came in contact with petroleum fractions. The contaminants found in this water is basically human poison and is best left to the people who know where the all the water has came from.

Sources:

Petroleum Refining, by J. H. Gary, G. E. Handwerk, M. J. Kaiser, 5th Edition, CRC Press NY, 2007, Chapter 13, Supporting Processes, pp. 290-293.

http://en.wikipedia.org/wiki/Industrial_wastewater_treatment#Oils_and_grease_removal

http://10statesstandards.com/wastewaterstandards.html#52

Heavily polluted wastewater streams come in direct contact with petroleum fractions and require serious treatment processes for purification. Hydrocarbons such as aromatic compounds and heteroatoms can be found in wastewater streams from distillation and different forms of cracking. There is a set of quality parameters to measure the treatment required for the wastewater processes including amount of suspended solids, hydrocarbon content, nitrogen content, phenols content, and acidity. Municipal wastewater treatment cannot handle treating the pollutants because the different streams need to be kept separate to reduce the load on the treatment units. The wastewater treatment constitutes a very significant supporting process for safe operation. Most municipal wastewater treatment facilities can only handle cooling water and sanitary sewage water after it has had minor treatments. The amount of refinery equipment and treatment processes require expensive machinery as well as many units for the processes that municipals may not be able to afford. Primary treatments are physical whereas secondary treatments are biological processes. Primary treatment of sour water contaminated with oils and solid particles involve the stripped of dissolved H2S using steam, separators, and settling tanks. After primary treatments are conducted typically water can be sent to municipalities for further treatment processes because they are at a acceptable level for municipalities to handle. Secondary treatments utilize microorganisms to further remove organic contaminants.

The wastewater from refineries can not be treated in regular municipal wastewater treatment plants for a variety of reasons. Municipal wastewater treatment plants are designed to treat wastewater from residential houses. There can also be some rainwater runoff going it to these plants as well. These plants are not equipped to clean the wastewater that would be coming from a refinery. The waste water coming from a refinery is divided into four different types. The types of wastewater are Storm water, sanitary sewer water, cooling water, and the most polluted process water and steam. The process water and steam contains many forms of pollutants that include liquid hydrocarbons, suspended solids, dissolved solids, mercaptans, phenols, amines, and cyanides. These chemicals would not be able to be processed by a municipal wastewater treatment because of the toxicity of some of these compounds. These compounds have to be stripped from the sour water using steam to remove H2S, float/sink density separators, as well as settling tanks to separate heavier oils. After these processes are completed the water can be directed to a wastewater treatment plant where the water can be completely processed. There is also a secondary treatment process that uses microorganism to remove organic compounds from the waste water. This process produces a substance called biocoke.

Wastewater treatment is considered an important supporting process in petroleum refining. The four types of refinery wastewater include cooling water, process water and steam, storm water, and sanitary sewage water. The refinery stages that produce the most wastewater are desalting, distillation, thermal and catalytic cracking, and coking. The most polluted wastewater is the process water that comes into direct contact with petroleum fractions; thus it cannot be simply treated in municipal wastewater treatment plants. Storm water is considered contaminated as a result of incidental exposure to pollutant sources and accidental spills during refinery reactions. The refinery’s cooling water and sanitary sewage water will probably not require much treatment before sending it to public water facilities where municipal wastewater is treated. Different wastewater streams are usually not mixed even if it reduces the load on treatment units. This is because different wastewater streams have different components and toxicities. The wastewater’s contamination level depends on its usage in the petroleum refinery. Generally, the pollutants in the streams include hydrocarbons, particularly toxic aromatic compounds such as benzene. Also, the wastewater streams include other heteroatom compounds mercaptans, amines, phenols, and cyanides, dissolved gases (H₂S and NH₃), and acids (H₂SO₄ and HF). Such components require much more treatment than municipal wastewater. In addition, environmental policies such as the Clean Water Act and Safe Drinking Water Act require refineries to effectively treat their wastewater.

Source:

Course Website

Wastewater that comes into contact petroleum fractions cannot be treated at municipal water treatment facilities. This is because municipal water treatment does not have the equipment to clean the pollutants that are added to the water during petroleum processing. It is also not the job of the municipal water treatment to clean the many poisons that were added to the water during petroleum processing. Municipal water treatment facilities clean the water of sewage and storm run off. The kind of toxic chemicals found in the petroleum refining waste water contain poisonous compounds which under normal circumstances would never come into contact with any water stream. For instance, a multitude of hydrocarbons, aromatic compounds, such as benzene and heteroatom compounds like mercaptans, amines, phenols, cyanide’s. These polluted streams of water also contain dissolved gases like H2S and NH3, acids such as H2SO4 and HF. All of which a municipal waste water facilities is not capable of removing to a safe level for release back into the environment.