## 2 Sources and Impacts of Hydrocarbon-Containing Wastewater
These petroleum compounds consist of three main hydrocarbon groups--paraffins (straight chained _n_-alkanes from C\({}_{1}\) to C\({}_{40}\) and branched isoalkanes), naphtenes or cycloparaffins (naphthene rings contain typically 5 or 6 carbon atoms; more condensedicyclonapthtenes C\({}_{8}\) and C\({}_{9}\) are present in addition to monocyclonapthtenes) and aromatics (having the benzene ring comprising alternate double and single bonds with adjacent carbon atoms; monocyclic and polycyclic aromatics). In addition, naphthenic acids (a mixture of alkyl-substituted acyclic, monocyclic and polycyclic carboxylic acids with an aliphatic chain of 9 to 20 carbon atoms), which are known to cause toxic effects and are difficult to remove from refinery wastewater [15]. Other pollutants found in wastewater include hydrogen sulphide, ammonia, phenols, benzene, cyanides, and suspended solids containing metals and inorganic compounds (e.g., halides, sulphates, phosphates, sulphides) [11]. Petroleum refinery effluents are priority pollutants due to their high PAH contents, which are toxic and tend to be more persistent in the environment [3]. Another major source of oily wastewater is the metallurgical industry. Most of the oily waste comes from metal-working or metal-forming operations, such as coke quenching, steel rolling, solvent extraction and electroplating [13]. Oil-in-water and water-in-oil emulsions of different compositions, ranging from trace of oil in water to trace of water in oil, are used as cooling and lubricating agents; they also provide corrosion protection for machined parts and machining tools. Such spent oily water emulsions can be highly viscous and often severely hamper wastewater treatment plant capabilities, thus causing increased maintenance costs and energy consumption. Also, solvent extraction and electro-deposition processes used in hydrometallurgy consume myriads of highly toxic organic substances as extractants, diluents, matrix modifiers, flocculants, brightening agents, acid fog inhibitors. A general feature of metallurgical process-related effluents is the complex composition and relatively high toxicity. The primary components of metallurgical oily wastewaters are emulsified oil, emulsifiers, degreasing agents, surfactants, solvents, suspended solids, metals and acids/alkalines [13]. For example, the wastewater from rare earth metallurgy contains radioactive metals, such as thorium, uranium, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Type of the Effluent** & **Toxic Organic Components** & **Toxic Inorganic Components** & **References** \\ \hline Metallurgy wastewater & \begin{tabular}{c} BTEX, phenols, quinolines, PAHs, \\ hydrazines and inimie-carbohydrazides, \\ thiophene \\ \end{tabular} & \begin{tabular}{c} Heavy metals, acids/alkalines, \\ radioactive metals, ammonia, \\ cyanide \\ \end{tabular} & [13] \\ \hline Urban runoff & \
\begin{tabular}{c} PCBs, di-(2-ethylhexyl)phthalate \\ (DEHP), linear alkyl benzene \\ sulphonates (LAS), nonylphenol \\ ethoxylates (NPE), dioxins (PCDD), \\ diphenyl ethers (PBDEs), \\ pharmaceuticals, personal care \\ compounds, flame-retadants, biocides \\ and pesticides \\ \end{tabular} &
\begin{tabular}{c} Heavy metals, salts, acids \\ \end{tabular} & [2,14] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Cont. radium, due to the paragenetic relation of these elements with rare earth metals. To treat these oily wastewaters, the hybrid processes consolidating the advantages of two or more technologies are often employed because of high metal concentrations and the recalcitrant nature of organic fractions. Large amounts of oily wastewater are produced by the transportation industry, namely, road, rail, marine and air transport. The emergence of various kinds of automobiles or vehicles led to the considerable increase in the use of motor fuels and oils, which are the important sources of hydrocarbon pollution in wastewater. Furthermore, the car wash, engine wash, paint spraying workshop or petrol station wastes also contain residual hydrocarbons.