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The treatment processes can be divided into the following four categories and subcategories:
(1) biological treatment process;
(a) aerobic treatment
Aerobic treatment is one of the common technologies applied which include the activated sludge (AS) process, extended aeration activated sludge process, as with granular activated carbon, and membrane bioreactors.
(b) anaerobic treatment
Anaerobic treatment has been done by using continuous stirred tank reactors (anaerobic digestion), fluidized bed reactors, and up-flow anaerobic sludge reactors, etc.
(2) advanced treatments;
(a) membrane technology
Implementation of membranes in water treatment is greatly increasing. It is well-known that low-pressure membranes are capable of removing microbial constituents without increasing disinfection byproducts.
(b) activated carbon
Adsorption using activated carbon (AC) is well-suited to remove OCs due to its high surface area (over 1000 m2/g) and the combination of a well-developed pore structure and surface chemistry properties.
(c) membrane distillation
Membrane distillation is a very important separation technology with interesting properties. Presently membrane distillation is used for the production of demineralized water.
(3) advanced oxidation processes
(a) ozone/hydrogen peroxide treatment
Ozone is a very strong oxidizing agent that either decomposes in water to form hydroxyl radicals which are stronger oxidizing agents than ozone itself, thus inducing the so-called indirect oxidation, or attacks selectively certain functional groups of organic molecules through an electrophilic mechanism
(b) Fenton oxidation
Fenton’s reagent involves the reaction of hydrogen peroxide with ferrous or ferric ions via a free radical chain reaction which produces hydroxyl radicals. Since iron is an abundant element, this process is the most viable for wastewater treatment.
Photocatalysis is the acceleration of a photochemical transformation by the action of catalyst such as TiO2 or Fenton’s reagent. Photocatalysis is the best suited process for effluents having a high COD and for complete transformation of highly refractory organic contaminants to reach biological treatment level.
(d) electrochemical oxidation/degradation
Electrochemical method is based on in situ production of hydroxyl radical (•OH) as the main oxidant, which is the second strongest oxidizing agent known after fluorine, having such a high standard reduction potential (E° (•OH/H2O) = 2.8 V vs SHE) that it is able to nonselectively react with most organic contaminants via hydroxylation or dehydrogenation until their total mineralization.
(e) ultrasound irradiation
Ultrasound irradiation is a relatively very recent technique which has been applied for wastewater treatment. Many estrogenic compounds have been removed by ultrasonic irradiation from contaminated waters,
(f) wet air oxidation
Wet air oxidation is a thermochemical process where hydroxyl radicals and other active oxygen species are formed at elevated temperatures (200–320 °C) and pressures (2–20 MPa). Recent research has shown the applicability of this process to remove COD to a great extent.
(4) hybrid technologie
Gupta V K, Ali I, Saleh T A, et al. Chemical treatment technologies for waste-water recycling—an overview[J]. Rsc Advances, 2012, 2(16): 6380-6388.
Gadipelly C, Pérez-González A, Yadav G D, et al. Pharmaceutical industry wastewater: review of the technologies for water treatment and reuse[J]. Industrial & Engineering Chemistry Research, 2014, 53(29): 11571-11592.