Treatment of Petrochemical Industry Wastewater : A review

Tr Treatment of Petrochemical Industry Wastewater astewater : A review Treatm Treatment ent of Petrochem Petrochemical ical Industry Industry Wastewater Wastewater including Membrane Technology

Rimeli Roy Choudhury (14/ChE/2015) /5/2015

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Treatment of Petrochemical Industry aste!ater " # re$ie! Abstract Abstract:: The wastewater from a petrochemical complex consists of numerous types of

pollutants including hydrocarbons, in free and emulsified form, phenols including cresols and xylons, mercaptans, sulphides, ammonia and cyanide. In this review paper, various industrial wastewater treatment technologies which are currently available are discussed. An extensive list of various methods of removal of mercury, chromium, cadmium, sulphur, nitrogen and other heavy metals and COD from petrochemical industry wastewater has been discussed.

Introduction:

etrochemical Industry is one of the fastest growing core sectors of the economy. As a result, many petrochemical plants of different si!es and technologies co"exist at the present time. The petrochemical industry is highly technological and capital"intensive. Tech Technologies nologies for petrochemical industries have been developing very fast. Tremendous resources and effo effort rtss are are bein being g cont contin inuo uous usly ly spent spent on incr increas easin ing g si!e si!e and and yield yield of plan plants ts thro throug ugh h continuous upgrade of catalyst, reducing energy consumption and cost reduction through novel process rate, new chemistries or scale up approaches. The petrochemical industry is a complex and is an integrated industry that includes a large variety of processes and products. #ecause of a large number of processes, use of wide variety of raw materials, catalysts, additiv additives, es, chemical chemicals, s, presen presence ce of explos explosives ives and ha!ard ha!ardous ous materia materials, ls, the proble problem m of environmental pollution from petrochemical industries is also $uite complex. A wide wide varie variety ty of poll polluta utant ntss is disc discha harg rged ed into into wate waterr strea stream m and and emitt emitted ed into into the the environment. The $uantity and characteristics of wastewater generated from a petrochemical comple complex x is strongl strongly y depend dependent ent on indivi individua duall process process plants plants operati operating ng at the comple complex. x. %astewater generated from ethylene crac&er are inorganic sulphides, mercaptans, soluble

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Treatment of Petrochemical Industry aste!ater " # re$ie!

hydrocarbons, polymerised product, phenolic compounds, sulphide, cyanide, heavy oils, co&e, spent caustic, 'Ox, (Ox, hydrocarbons, particulates, water borne waste containing #OD, COD, suspended solid, oil and those from aromatic plants are dissolved organics, volatile organic compounds, heavy metals, hydrocarbons, particulates, ) *', 'Ox, (Ox, CO, water borne waste containing #OD, COD, suspended solid, oil + grease, toluene, ben!ene, xylenes, )Cl, chlorine, cadmium. These pollutants can lead to several direct effects on social and environmental health and almost appears in three dimensions of water, soil and vibrations. The most considerable is water and soil pollution which had the most effect on local ecosystems. As there are several pollutants present in the wastewater effluent from the petrochemical industry so several techni$ues have been developed to omit or reduce the contamination of these pollutants. Treatment of petrochemical waste water to minimi!e its environmental impact has caught the devotion of researchers over the last few decades towards the development of an environment"friendly cost effective continuous method. Amidst the growing stringent discharge rules all over the world, petrochemical industrial houses has to suffer due to the formation of verities of wastes formed inside the industry. oreover, the treatment methods prior to discharge should be cheaper because the recovery and discharge processes by separation and purification technology plays a ma-or role in hi&ing up the cost of a complete process. )ence, our aim is to find a sustainable green and clean technology under reduced conditions of energy, material and energy and cost consumption with a promise to achieve higher engineering flexibility to the plant and lowest environmental impacts. Thus old, inefficient, energy intensive technologies should be replaced with new, smaller, safer and modular designed e$uipment. arge amounts of nitrogen and sulphur presents in wastewater effluent coming from catalytic hydro"crac&ing unit of petrochemical industries, in the form of ammonia /() 01 and hydrogen sulphide /) *'1, respectively. )ydrogen sulphide, one of the main constituents of

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petrochemical industry effluents is a toxic and corrosive gas that causes environmental and economic problems in a variety of sectors. 'ome researcher has proved that one of the best way to control and remove sulphide is the use of nitrate 23,*,04. resence of ammonia and its derivatives in water effluent from petrochemical industry are one of those reasons which are responsible for water pollution. 5arious researches have been done for biological settlement of wastewater contaminated by ammonia and its derivatives. There are a number of aerobic and anaerobic microorganisms are there which are able to express the en!yme urease /urea amidohydrolase1 which catalyses the hydrolysis of urea 26,74. Copper and chromium are another two most common metals found in wastewater discharge of petrochemical plants wastewater discharge from other industrial sites 284 where hexavalent chromium, Cr/5I1 present at concentrations ranging from tenths to hundreds of mg9 2:4. irbagheri et al.2;4 used ferrous sulfate and lime Ca/O)1* for p) ad-ustment and conversion of Cr/5I1 to Cr/III1 and Cr/III1 precipitation, respectively. <86:38:*== The largest industries which produce wastewater containing mercury and cadmium are vinyl chloride monomer and 5C producing petrochemical factories. ala&ahmad et al. has performed a lab"scale experiment with a 'e$uencing #atch >eactor /'#>1 to treat a synthetic petrochemical wastewater containing mercury and cadmium. 2<4. %astewater of petrochemical industries also contains high amounts of emulsified aliphatic or aromatic hydrocarbons. Taran has showed )aloarcula sp. I>?3 can degrade petrochemical wastewater and produce )# from it in different conditions 23=4. embrane technologies have became the most popular separation process for treatment of petrochemical industry wastewater. (ow"a"days it is also competing with traditional schemes 233"374. embrane separation processes have various advantages li&e a1 3==@ purity of product can be achieved, b1 low energy consumption, 01 compared with other conventional techni$ues, membranes can offer a simple, easy"to"operate, low"maintenance

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process option, c1 no external chemicals are re$uired to add for separation, d1 can operate at a moderate temperature and pressure. %ith all of these advantages there are also certain disadvantages of this membrane based separation processes Ca&e formation or fouling can be considered as a ma-or problem of membrane based separation processes which is the main reason for reduction of permeate flux. #ut this problem can also overcome by using cross flow arrangement and using different types of membrane modules. Brom this perspective of mindset, the sincere contribution towards environment and reduction of operating cost by process intensification has triggered our effort towards membrane based processes. ressure"driven >everse Osmosis being comparatively an innovative one, possess the ability to stand as a viable solution replacing conventional separation and purification techni$ues li&e distillation, evaporation ion exchange, absorption. #eing modular in design, membrane based plants are able to ensure the possibility of operation in a simpler plant with a re$uired number of active units which offers high flexibility to the plant. #y the virtue of high selectivity membranes are able to offer high degree of separation and purification /over <;@1 to the targeted molecules. embrane based processes are highly efficient to act as a perfect substitute to the conventional unit operation techni$ues li&e distillation, condensation or absorption in an eco"friendly way while involving less man power or electrical energy. Due to no involvement of phase changing phenomena energy and cost consumption can be efficiently reduced while implementing membrane technology at the industrial level for product purification. embrane based processes employing highly selective membranes offer a high degree of separation and purification with high permeate flux. embrane based reactors are easy to design and easier to scale up. roper utili!ation of raw materials by continuous recycling and recovery of byproducts could be efficiently performed using such technologies. Conse$uently they ensure a compact design while reducing the capital cost. 'o a properly designed membrane

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integrated hybrid treatment system employed for effective removal of wastes from petrochemical refinery effluents is expected to overcome all the technology barriers as discussed previously. Thus evidently membrane involved process schemes can meet all the aims of process intensification and sustainable industriali!ation. In this case our goal is to dedicate ourselves to some environment friendly, economically feasible continuous production scheme for the proper treatment of petrochemical waste water, eliminating the drawbac&s associated with conventional processes. In this paper, a brief discussion about the traditional treatment has been provided highlighting the ma-or drawbac&s associated with them.

Control Techniques

The control technology is to be based upon the most exemplary combination of in"process and end"of"process treatment + control technologies. This level of technology is primarily based upon significant reductions in the COD, as wel l as the #OD. nd"of"pipe treatment in this case will be biological plus additional activated carbon treatment. The techni$ues that can be applied to new plants and to existing facilities will differ. In existing plants, the choice of control techni$ues is usually restricted to process integrated /in"plant1 control measures, in"plant treatment of segregated individual streams and end"of"pipe treatment. (ew plants provide better opportunities to improve environmental performance through the use of alternative technologies to prevent wastewater generation. An appropriate control strategy for waste water from the etrochemical industry can be summari!ed as /a1 Organic wastewater streams not containing heavy metals or toxic or non biodegradable organic compounds are potentially fit for combined biological wastewater treatment /sub-ect to an evaluation of biodegradability, inhibitory effects, sludge deterioration effects, volatilit y and residual pollutant levels in the effluent1.

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/b1%astewater streams containing heavy metals or toxic or non"biodegradable organic compounds /e.g. indicated by high AOE 9OE or high COD9#OD ratios1 are preferably treated or recovered separately. Individual waste streams containing toxic or inhibitory organic compounds or having low bio"degradability are treated separately e.g. by /chemical1 oxidation, adsorption, filtration, extraction, /steam1 stripping, hydrolysis /to improve biodegradability1 or anaerobic pretreatment. Technologies to treat chemical industry effluents

There are mainly four stages of petrochemical industry wastewater treatment. Birst is preliminary treatment which involves the removal of large particles as well as solids found in wastewater samples. 'econd is primary treatment, which involves the removal of organic and inorganic solids by means of a physical process, and the effluent produced is termed as primary effluent. The third treatment is called secondary treatment this is where suspended and residual organics and compounds are bro&en down. 'econdary treatment involves biological /bacterial1 degradation of undesired products. The fourth is tertiary treatment, normally a chemical process and very often including a residual disinfection.

Physico-chemical treatment

Oil F%ater 'eparatorFTreatment of oily effluent etrochemical industries report high levels of oil and grease in their effluents /with an Oil and grease concentration up to *==,=== mg9l1 238,3:4. Oil and grease presents in wastewater can be either of these forms free, dispersed or emulsified where free oil is characteri!ed with droplet si!es greater than 37= mm in si!e, dispersed oil has a si!e range of *=F37= mm and emulsified oil has droplets typically less than *= mm. Oil and grease concentrations in wastewater can be measured by different test procedures of the ?' nvironmental rotection Agency but they failed to determine the presence of specific compounds. Gravity

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separation and s&imming, dissolved air flotation, de"emulsification, coagulation and flocculation are the several conventional approaches of treating oily wastewaters. Gravity separation followed by s&imming is effective in removing free oil from wastewater whereas the AI oil F water separator is designed to separate the oil and suspended solids from their wastewater effluents. #ut this is not effective in removing smaller oil droplets and emulsions. rimary clarifier is used to remove the oil that adheres to the surface of solid particles. %astewater is usually pre"treated chemically to destabili!e the emulsified oil followed by gravity separation. The wastewater is also heated to reduce viscosity and density differences and to wea&en the interfacial films stabili!ing the oil phase which is followed by acidification and addition of cationic polymer9 alum for the neutrali!ation of negative charges on oil droplets. %hile waste water treatment, p) is &ept at some high value /al&aline regime1 to induce floc& formation of inorganic salts. The resulting floc& with the adsorbed oil is then separated, followed by sludge thic&ening and sludge dewatering.

CoagulationFflocculation Coagulation9flocculation is one of the most important processes in the primary purification of water and in petrochemical wastewater treatment 23;"*=4. This method is widely used as the primary purification processes mainly due to the ease of operation, high efficiency, cost effective. Also, it uses less energy than alternative treatment 2*="**4. It is also called clarification in which the velocity of the water is lowered below the suspension velocity and the suspended particles settle down due to gravity. 'ettled solids are removed as sludge, and floating solids are removed as scum. %astewater leaves the sedimentation tan& over an effluent weir to the next step of treatment. Bactors such as the type and dosage of

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coagulant9flocculant, p), mixing speed and time, temperature and retention time are the governing parameters to evaluate the efficiency of the process 2*84 . #oth inorganic and organic such as aluminum sulfate /alum1, ferrous sulfate, ferric chloride and ferric chloro" sulfate are widely used as coagulants in petrochemical industry wastewater treatment for removing a broad range of impurities from effluent, including organic matter, turbidity, colour, microorganism, colloidal particles and dissolved organic substances 23<,*=,*0,*74. Altaher et al. 2*:4 demonstrated in his paper that the p) plays a significant role in coagulation"flocculation process. The experiments conducted showed that increase in p) form acidic range to al&aline range promotes turbidity removal which also indicates that the p) played a significant role in imparting surface charge of organic and inorganic colloids. This treatment process can remove almost <=@ of the suspended solids from the wastewater but fails to remove organic, inorganic particles, heavy metals present in the wastewater.

Adsorption techni$ues to treat wastewater Adsorption is a natural process by which molecules of a dissolved compound adsorbs to the surface of an adsorbent solid. This adsorption method becomes economically unviable for the removal of heavy metals at lower concentrations and thus it appears to be very promising for the remediation and recovery of Hpetrochemical waste water. Granular activated carbon !eolites, silica"aluminas and silicas are the most popular adsorbent mediums due to their high surface area to volume ratio. Jeolites have some peculiar characteristics, which include i1 high selectivity due to a strictly defined chemical composition and porous texture ii1 tunable hydrophilicity iii1 proven stability under harsh conditions and iv1 in most cases, excellent regenerability 2*;4. Jeolite can remove heavy"

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metal"cation by applying cation F exchange techni$ue 2*<4, have a wide spectrum of amorphous molecular sieve materials which ma&e them mar&edly different from natural !eolites. Due to their i1 wide pore openings, ii1 high specific surface areas and iii1 large specific pore volumes, silica"aluminas and silicas have drawn attention for their adsorption of ma-or amounts of non"dissociated contaminants characteri!ed by bul&y molecules /which are unable to diffuse through !eolite micropores1 that have been dissolved or even dispersed in water as oily droplets. any research studies have been done where non"conventional adsorbents, such as agricultural and industrial solids wastes are used for the removal of heavy metals 20="0*4. There are other materials which have also been used to remove heavy metals from wastewater, such as peat, wool, sil&, and water hyacinth. any researchers have wor&ed on preparation of activated carbon from cheaper and readily available materials 203,0*4. aretto et al.2004 used two different microporous materials, a natural !eolite called clinoptilolite and a polymeric chelating resin named uroliteK >esin '<3=, to

remove

dissolved heavy metals, and a mesoporous siliceous material to upta&e hydrocarbons from wastewater. The batch experiments indicated a good adsorption rate and a percentage of heavy metal /b*L, Cd *L and (i *L1 and hydrocarbon removal /ben!ene and toluene1 that was always greater than <=@. They developed a new adsorption model to better describe the adsorption mechanism of heavy metals and a two"step mechanism for hydrocarbons. )ere both of the materials seemed to maintain good adsorption capabilities. It was also showed in this experiment that increase in ionic strength tends to decrease the adsorption performance of the microporous material and the presence of organic interfering contaminants. #ut with all the advantages described above adsorption techni$ue also posses certain disadvantages li&e i1 most of the adsorbent are temperature sensitive ii1 with time their

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adsorption ability may deteriorate in that case adsorbents need to be changed after a certain time.

Bixed bio film reactor The fixed bio film reactor is nothing but a tric&ling filter that consists of a bed of highly permeable media on whose surface a mixed population of microorganisms is developed as a slime layer. %astewater passes through the filter which causes the development of a gelatinous coating of bacteria, proto!oa and other organisms on the media. The continual increase in the thic&ness of the slime layer with time which in turns produce anaerobic end products next to the media surface, and the maintenance of a hydraulic load to the filter, eventually causes sloughing of the slime layer to start to form. To prevent clogging of the distribution no!!les, tric&ling filters should be preceded by primary sedimentation tan&s e$uipped with scum collecting devices. Tric&ling filters should be followed by secondary sedimentation tan&s to remove the sloughed solids and to produce a relatively clear effluent. %ith the advantages of its simple design, trouble free, ease of maintenance and control nature /as compare to activated sludge process1 tric&ling filter also has some disadvantages such as

excessive organic loading without a corresponding higher recirculation rate

clogging of under drain system, non"uniform media si!e or brea&ing up of media.

lectrosorption lectrosorption is nothing but the absorption on surface of an electrode. After the polari!ation of the electrodes, the polar molecules or ions can be removed from the electrolyte solution by the imposed electric field and adsorbed onto the surface of the

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electrode. lectrosorption has attracted a wide interest in the adsorption processes for treatment of wastewater due to its environmental friendly and less power consuming nature. #ut it has been limited by the performance of electrode material. Activated carbon fibre cloth with high specific surface area and high conductivity is considered to be the most effective material which can be used as electrode materials.

embrane technology Application of membrane based separation processes such as microfiltration /B1, ultrafiltration /?B1, nanofiltration /(B1 and reverse osmosis />O1 for treating oily wastewater are increasing day by day. There are three broad categories of oily wastes F free" floating oil, unstable oil9water emulsions, and highly stable oil9water emulsions of which membranes are most useful with stable emulsions, particularly water soluble oily wastes 2064. echanical separation devices can remove the free oil by using gravitational force as the driving force whereas unstable oil9water emulsions can be mechanically or chemically bro&en and then gravity separated. Cheryan et al. 2074 reported a study where a semi"batch type recycle membrane unit was employed. A constant level was maintained in the process tan& adding wastewater feed at a rate e$ual to the rate of withdrawal of clean permeate and retantate stream containing oil and grease was recycled bac& to the process tan&. %hen the oils and grease and other suspended matter reached a certain predetermined concentration in the tan&, the feed was stopped and the retentate allowed to concentrate which finally gave a result of final concentrate volume that was only 0"7@ of initial volume of oily wastewater.

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Big. 3. 'chematic of typical membrane system for treatment of oily wastes /adapted from Cheryan 2*641

embranes have several advantages, among them /31 %idely applicable across a wide range of industries /*1 Do not involve phase change 01 The membrane is a positive barrier to re-ected components. 'eparation process can have a higher degree of purity /<<@1 than other processes /61 (o separation agent is re$uired, ma&ing subse$uent oil recovery easier /71 embranes can be used in"process to allow recycling of selected waste streams within a plant /81 nergy costs are lower compared to thermal treatments /:1 The plant can be highly automated and does not re$uire highly s&illed operators. embrane processes have some limitations /i1 'cale"up is almost linear above a certain si!e. Thus capital costs for very large effluent volumes can be high /ii1 Bouling is the most important problem in case of membrane separation processes. Due to fouling the flux decreases with time /iii1 Clogging is another important phenomena occurs in membrane separation process which not only decrease the permeate flux but is also a reason behind membrane degradation during use. Thus membranes are re$uired to be replaced fre$uently, which can increase operating costs significantly. 'everal researches has been done to mitigate this problem according to which the use of vibratory or centrifugal devices to enhance shear at the membrane surface to decrease concentration polari!ation, modification of membrane surfaces to increase hydrophilicity, and pre"treatment of feed are the most effective techni$ues to be followed.

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208,0:4. In spite of the above disadvantages, membrane processing of oily wastewaters, sometimes in con-unction with other methods for treating the residuals is widely used for the treatment of wastewater all over the world. Bratila"Apachitei et al. 20;4 has reused petrochemical effluent as cooling water after treating it by a scheme comprising of ultrafiltration and reverse osmosis. They have used >O membrane to decrease the salinity to an allowable level for reuse as cooling water followed by a dead"end ?B membrane which was used to reduce the turbidity of the wastewater. They performed ?B test at constant transmembrane pressure /=.* bar1 using hollow fibre polyethersulphone membranes coated with poly"vinyl"pirrolidone. To compare the performance characteristics two membranes with different molecular si!es/7= and 37= &Da1 were ta&en and performed separately where the 37= &Da membrane showed a very fast flux decline /i.e. *=@ in * min1 re$uiring fre$uent bac&washing /#%1, whereas 7= &Da membrane showed a relatively slow flux decline i.e. *=@ flux in *= min. As a gradual change from complete to intermediate bloc&ing and ca&e filtration was observed in both cases, analysis of the bloc&ing mechanisms failed to explain the rapid drop in flux for the 37= &Da membrane as compared with the 7= &Da membrane. #ut a field emission scanning electron microscopy /B'1 analysis of both ?B membranes suggested that the highly interconnected pore system of the 7= &Da membrane is mainly responsible for filtration performance which in turns result in a M0D"bridge"typeN surface morphology. On the other side Teodosiu et al. 20<4 also wor&ed on to evaluate the possibilities of using ?B as a pre"treatment for >O, in a double membrane filtration scheme where the two ?B membrane provided by the

same manufacturer, made of polyetherosulphone 9

polyvinylpirollidone1 with the same molecular weight cut"off of 37=,=== Da but with different coatings have been used. They showed that the low fouling membrane is easy to

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clean by bac&washing or enhanced bac&washing, having a better flux restoration and a higher efficiency as production and operation and application of the polymer coating for the low fouling membrane, although decreases permeability, has a positive effect concerning membrane A"B performance. xperimentally they have proved that ultrafiltration offers almost complete removal of suspended solids and colloids /<;@ as turbidity1 and partial removal of organic compounds attached to suspended solids /0=@ as COD1 and thus ultrafiltration can be considered to be a good pre"treatment for a reverse osmosis process, which has to remove further dissolved inorganic and organic compounds, in order to achieve the re$uirements for recycling 26=4.

Biological treatment of petrochemical industry wastewater

Aerobic treatment In the wastewater treatment sector, biological processes deal primary with organic impurities. Aerobic degradation is a simple, inexpensive and environment friendly way to degrade wastes. arameters which effect the aerobic treatment are temperature, moisture, p), nutrients and aeration rate that the bacterial culture is exposed to, with temperature and aeration being two of the most critical parameters that determine the degradation rates by the microorganism. 'oluble organic sources of biochemical oxygen demand /#OD1 can be removed by any viable microbial process, aerobic, anaerobic or anoxic of which the aerobic microbial reactions almost 3= times faster than anaerobic microbial reactions. ThatNs why aerobic reactors can be built relatively small and open to the atmosphere, yielding the most economical means of #OD reduction. %ith the advantages aerobic bioprocess also have certain disadvantages. The ma-or disadvantage of aerobic bioprocesses over anaerobic processes for wastewater treatment, is the large amount of sludge production due to

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accumulation of biomass /as biomass yield for aerobic microorganisms is relatively high, almost 6 times greater than the yield for anaerobic organisms1.

embrane bioreactors embrane bioreactors is a combination of the activated sludge process and a membrane separation process. A simplified #> diagram is shown in Bigure *.

Bigure * Diagram showing the basic configuration of a membrane bioreactor 2*:4

A decrease in sludge production, improved effluent $uality and efficient treatment of wastewaters with varying contamination pea&s are the different advantages #>s offered over traditional activated sludge process. 'ome disadvantages of this system include this system needs fre$uent membrane monitoring and maintenance, operates at relatively high running costs and there is a limitation of the pressures, temperatures and p) the system which are considered as the basic disadvantages of the system. Due to membrane fouling proper designing of these &ind of reactor is very difficult. And because of these reasons #>s are not being as widely used in large scale wastewater treatments in comparison to traditional activated sludge plants 263,6*4.

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haing et al.2604 treatment petrochemical industry wastewater from a petrochemical using a novel submerged membrane distillation bioreactor /D#>1 proved that it is feasible to treat and reuse the wastewater using submersed D#> technology. #ut the pitfalls are same as that in #> including flux declination is also plays a ma-or role due to inorganic fouling of the membranes.

Sequencing batch reactor

Conventional methods to remove heavy metals petrochemical industry wastewater usually involve physico"chemical treatments such as precipitation, ion exchange, electron" deposition 2664. There are some ma-or problems associated with these methods such as they are more costly compared to biological treatment methods and can themselves produce other waste problems which limited their industrial applications 267,684. Among the available treatment methods, se$uencing batch reactors /'#>s1

has caught attention due to some

reasons such as reduced chemicals re$uirement for the overall treatment process, low operating costs, eco"friendly and cost"effective alternative of conventional techni$ues and, efficient at lower levels of contamination 26:4. Other than these the main advantage of '#>s is that they can accommodate large fluctuations in the incoming wastewater flow and composition without failing which may not get from conventional activated"sludge processes, in which an increase in the incoming flow rate results in a lower residence time of the wastewater in the aeration tan& and of the sludge in the clarifier, with potential failure of one of them or both. ven the wastewater residence time in '#>s can be extended until the microbial population has recovered and completed the degradation process and settling time also can be varied to allow complete settling before discharging. A '#> is an activated sludge process periodically operated, fill"and"draw reactor 26;4 which has five discrete

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periods in each operation cycle fill, react, settle, draw, and idle 2664. >eactions start during fill with the reactor nearly empty except for a layer of acclimated sludge on the bottom and the reactor is then filled up with the wastewater and the aeration and agitation are started and complete during react. After react, the mixed li$uor suspended solids /''1 are allowed to separate by sedimentation during settle in a defined time period the treated effluent is withdrawn during draw and the time period between the end of the draw and the beginning of the new fill is &nown as idle 26<4. >esearchers have been wor&ing on it and a number of papers also have been published which provide good description and evaluation of the '#> systems in treatment of heavy metals 266,7=F7*4. ala&ahmad et al 2704 treated synthetic refinery wastewater containing )g *L and Cd*L, in a '#> after acclimated the system for 8= days. The '#> was first introduced to mercury and cadmium in low concentrations which then was increased gradually to <.=0P=.=* mg9 )g and 37.7*P=.=* mg9 Cd until day 33=. The study revealed that the COD removal efficiency ranged from 88 to ;;@ before addition of heavy metals due to appropriate acclimati!ation of the biomass during start"up period and ade$uate retention of 5'' concentration which contributed to high COD removal efficiency. 5'' concentration /population of microorganisms1 which showed an appreciable growth during reactor start"up and reached to 3;:= mg9, was affected by heavy metals concentration increment in each step and finally its concentration has fallen to 73= mg9. )eavy metals added to the '#> decrease the settleability of the sludge . The study also showed that at maximum concentrations of the heavy metals, the '#> was able to remove :8F<=@ of )g *L and <8F<;@ of Cd *L. %ith all the advantages there are certain drawbac&s associated with this method such as i1 a higher level of sophistication is re$uired /compared to conventional systems1, especially for larger systems, of timing units and controls ii1 higher level of maintenance /compared to

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conventional systems1 associated with more sophisticated controls, automated switches, and automated valves iii1 potential of discharging floating or settled sludge during the D>A% or decant phase with some '#> configurations iv1 potential plugging of aeration devices during selected operating cycles, depending on the aeration system used by the manufacturer v1 potential re$uirement for e$uali!ation after the '#>, depending on the downstream processes.

Anaerobic treatment Anaerobic reactor differs from the aerobic reactors primarily because the former must be closed in order to exclude oxygen from the system while oxygen plays a ma-or role in case or aerobic reactor. To remove the ga!es /mainly methane and carbon dioxide1 produced during anaerobiosis an anaerobic reactor must provide with an appropriate vent or a collection system. Anaerobic microbial processes have several important advantages over aerobic microbial processes li&e /31 lower production rate of sludge, /*1 operable at higher influent #OD and toxics levels, /01 no cost associated with delivering oxygen to the reactor, and /61 production of a useful by"product, methane /biogas1. According to Qerushalmi et al. 2764, addition of a co"substrate increases the biogas potential due to a well"e$uilibrated medium and the accumulation of limiting nutrients. anure is considered to be a superb co" substrate, due to its ability of providing buffering and many nutrients important for microbial development /'ambusiti et al.2774, Qang and iu 27841. 'iddi$ue et al. 27:4 operated anaerobic co"digestion /ACD1 of petrochemical wastewater /%%1 and activated manure /A1 in a continuous stirred tan& reactor where he achieved an ;=@ methane yield of 33.3 m 0 m"0 d"3 with <;.7: P =.7@ elimination of chemical oxygen demand at five daysR hydraulic retention time using a ratio of 7=@ %%97=@ A. Although anaerobic digestion

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provides numerous advantages, it is not extensively applied in the petrochemical industries due to slow reaction, longer hydraulic retention time and lac& of process stability, higher capital and operating expenses than aerobic processes because the anaerobic systems must be closed and heated.

Chemical oxidation Chemical Oxidation is a process by which electrons are transferred from one substance to another. which leads to a potential expressed in volts referred to a normali!ed hydrogen electrode. The chemical oxidation processes can be classified in two classes " Classical Chemical Treatments and Advanced Oxidation rocesses /AOs1. Classical chemical treatment Classical chemical treatments involves addition of an oxidant agent to the water containing the contaminant to oxidi!e it. 'ome widely used 27;4classical oxidants are chlorine, potassium permanganate, oxygen, hydrogen peroxide, o!on!tion etc. Chlorine is considered to be a good chemical oxidi!er for water evaporation because it destroys microorganisms. Though it is a strong and cheap oxidant, very simple to feed into the system 27;4. It also has some disadvantages li&e i1 its little selectivity that high amounts of chlorine are re$uired and ii1 it usually produces carcinogenic organochloride byproducts. )ydrogen peroxide is a multipurpose oxidant can be applied directly or with a catalyst. Berrous sul2hate, Al0L, Cu*L or other iron salts are generally used as catalyst. Its basic advantages are /i1 low cost /ii1 it has high oxidi!ing power, /iii1 easy to handle1, /iv1water" soluble /v1 it does not produce toxins or colour in by products vi1 it can also been used in presence of ultraviolet. O!onation is a strong oxidant that presents the advantage of both hydrogen peroxide and oxygen. It does not introduce Hstrange ions in the medium and has low solubility in water at standard temperature and pressure 27;4 . O!one plays a ma-or role

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many applications, li&e the elimination of colour, disinfection, elimination of smell and taste, elimination of magnesium and organic compounds etc. As the p) increases, the rate of decomposition of o!one in water also increases. The ma-or drawbac&s of this oxidi!er is that it has to be produced on site and needs installation in an o!one production system in the place of use due to which the cost of this oxidi!er is extremely high. Advanced Oxidation rocesses /AOs1 Among various AOs li&e ?59O 0 process, ?59)*O*, O09)*O*, Be0L9?5"vis process, ?59TiO* /)eterogeneous photocatalysis1, the Benton reagent /)*O*9 Be*L1 is the most effective methods of organic pollutant oxidation. Benton process is widely used as a suitable treatment method for highly concentrated wastewaters due to its effectiveness in producing hydroxyl radicals 27<,8=4. Application of traditional Benton process is limited by its acidic p) re$uirements, the formation of iron sludge and high cost of hydrogen peroxide 27<,834. #ut nowadays /AOs1 based on BentonNs reaction chemistry have received much attention for wastewaters remediation 2834. AO is the electro Benton /"Benton1 process 28*4, the most popular electro"chemical advanced oxidation process which can proceed by the following chain reactions 28*"804 )*O* L Be*L S Be0L L O) L O) "

/31

)*O S )L L O) L e " Be0L L e" S Be*L

/*1 /01

Davarne-ad et al. conducted an experiment where he compared aluminum and iron plate electrodes on COD and colour removal from etrochemical wastewaters and also evaluated the effects of reaction time, current density, p), ) *O*9Be*L molar ratio, and ) *O*

of

petrochemical wastewater /%1/ml9l1 on the performance of the process. The results revealed that COD and colour removal efficiencies of iron electrode were /8:.0@ and

22


:3.7;@, respectively1 which were more than those of aluminum electrode /70.<6@ and 8:.07@, respectively1. )owever, some disadvantages are also there in using the Benton reagent which are i1 the production of a substantial amount of Be /O)1 0 precipitate and ii1 additional water pollution caused by the homogeneous catalyst that added as an iron salt, cannot be retained in the process 27;4. A number of researchers have investigated the application of iron oxides such as hematite, ferrihydrite, semicrystalline iron oxide and crystalline goethite 27;4 where they have observed a greatly accelerated decomposition of hydrogen peroxide but variable amounts of contaminant were lost.

Conclusion

As the petrochemical industries effluents consist of different types of wastes it cannot be treated by using only one conventional techni$ue. 'everal physicochemical options and biological wastewater treatment processes are showed here which are technologically and economically feasible and have been widely utilised in the successful treatment of industrial wastewaters. AI F oil separator is an excellent techni$ue for oil removal from industrial wastewaters whereas both aerobic and anaerobic treatment systems are feasible to treat wastewater from all types of industrial effluents. 'o a combination using an anaerobic process followed by an aerobic treatment system is a better option but those hybrid systems produce a high removal of toxic pollutants. A membrane based integrated system followed by a coagulation9flocculation process can be applied where the membrane modules are in cross flow mode to increase the effectivity of the process an ultrafiltration /?B1 membrane is installed prior to reverse osmosis />O1 as a pretreatment where ?B

will remove

emulsions, colloids, macromolecules or proteins /si!e under 3== nm1 and />O1 will separate dissolved salts and small organics /si!e under 3 nm1.

23


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Treatment of Petrochemical Industry Wastewater : A review

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