## 3 Treatment of Hydrocarbon-Containing Wastewater  
It should be noted that concentrations of petroleum hydrocarbons in wastewater may decrease due to natural processes of decomposition and chemical oxidation, evaporation and biological degradation by native microflora. In natural environments, however, these processes are relatively slow. To enhance the oil contaminant removal from wastewater, the mechanical, chemical, physicochemical and biological methods, as well as their combinations, are employed, providing the required purification rate at reasonable costs. Importantly, the choice of a treatment method in each case is determined by the source of wastewater, the diversity and levels of contaminants, and the subsequent intended use of treated effluents. Recent developments in treatment technologies for petrochemical wastewater and produced water from oil and gas industries are reviewed by [PERSON] et al. [5]. Treatment of petroleum wastewater usually involves two stages, firstly, physical (mechanical) pre-treatment to remove free oil & grease fractions and suspended particles. Secondly, an advanced treatment, usually involving a combination of different physicochemical and biological methods, to decrease the pollutant level to acceptable discharge values.  
The sedimentation treatment, which is used to separate bulk free oil from water, is mechanically achieved by gravity in API (American Petroleum Institute) or CPI (Corrugated Plate Interceptor) separators and dissolved air floatation (DAF) units. During wastewater sedimentation, insoluble solids are also removed, which prevents clogging and wearing of devices used in subsequent treatment stages. Mechanical treatment techniques, while allowing the reuse of roughly purified water in the operation cycle, are inefficient in the recovery of finely dispersed oil, dissolved organics, metals and colloids. The mechanical step is followed by the physiochemical step, in which small-sized suspended solids and dispersed oil are further reduced by agglomeration into large-sized particles to ease the removal by filtration, sedimentation or floatation [3]. However, settling of petroleum products as insoluble non-utilizable sediments (sludges) performed using expensive chemicals (particularly coagulants and flocculants) entails costly disposal of the resulting sludge and may lead to secondary contamination of the treated water [9].  
Several techniques that are used as advance treatment methods for reducing the effluent contamination level to allowable limits for discharge into water bodies include membrane filtration and electrodialysis [29; 30], adsorption [31], biological systems [5], advanced oxidation [5] and electrochemical processes [32]. Of these, sorption methods are advantageous because their performance and cost efficiency are compatible with other techniques for petroleum removal, and sorbents are reusable after regeneration. Sorbents are porous materials, both natural (activated carbon, sawdust, peat, perlite, clay, sand) and synthetic (polyurethane foam, ceramic materials, synthetic fiber) with high affinity for the targeted pollutants, good adsorption capacity and hydrophilic-lipophilic properties. The choice of the material relies on its ecological safety and availability in the region [31; 33].  
Currently used biological methods are activated sludge systems [34; 35], aerated lagoons and wetlands [36; 37]. Laboratory-proved technologies include aerobic [38; 39; 40; 41], anaerobic [42; 43; 44] and hybrid [15; 45; 46; 47] bioreactors. However, because of heavy petroleum hydrocarbon load, high salinity, low nutrient concentrations and toxicants present, the current activated sludge bioprocesses often suffer from operation problems, such as low removal rate and high suspended solid concentration [48]. To overcome these problems, advanced bioreactor systems and selected cultures with enhanced degradative capabilities and tolerance to environmental stresses can be used.