treatment of petrochemical industry wastewater : a review

Treatment of Petrochemical Industry Wastewater : A review

Treatment of Petrochemical Industry Wastewater including Membrane Technology

Rimeli Roy Choudhury (14/ChE/2015)

5/5/2015

Treatment of Petrochemical Industry Wastewater : A review 2

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:

Petrochemical Industry is one of the fastest growing core sectors of the economy. As a

result, many petrochemical plants of different sizes and technologies co-exist at the present

time. The petrochemical industry is highly technological and capital-intensive. Technologies

for petrochemical industries have been developing very fast. Tremendous resources and

efforts are being continuously spent on increasing size and yield of plants through

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.

Because of a large number of processes, use of wide variety of raw materials, catalysts,

additives, chemicals, presence of explosives and hazardous materials, the problem of

environmental pollution from petrochemical industries is also quite complex.

A wide variety of pollutants is discharged into water stream and emitted into the

environment. The quantity and characteristics of wastewater generated from a petrochemical

complex is strongly dependent on individual process plants operating at the complex.

Wastewater generated from ethylene cracker are inorganic sulphides, mercaptans, soluble


hydrocarbons, polymerised product, phenolic compounds, sulphide, cyanide, heavy oils,

coke, spent caustic, SOx, NOx, hydrocarbons, particulates, water borne waste containing

BOD, COD, suspended solid, oil and those from aromatic plants are dissolved organics,

volatile organic compounds, heavy metals, hydrocarbons, particulates, H2S, SOx, NOx, CO,

water borne waste containing BOD, COD, suspended solid, oil & grease, toluene, benzene,

xylenes, HCl, 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 techniques have been developed to omit or reduce the

contamination of these pollutants. Treatment of petrochemical waste water to minimize 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.

Moreover, the treatment methods prior to discharge should be cheaper because the recovery

and discharge processes by separation and purification technology plays a major role in

hiking up the cost of a complete process. Hence, 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 equipment.

Large amounts of nitrogen and sulphur presents in wastewater effluent coming from

catalytic hydro-cracking unit of petrochemical industries, in the form of ammonia (NH3) and

hydrogen sulphide (H2S), respectively. Hydrogen sulphide, one of the main constituents of


petrochemical industry effluents is a toxic and corrosive gas that causes environmental and

economic problems in a variety of sectors. Some researcher has proved that one of the best

way to control and remove sulphide is the use of nitrate [1,2,3]. Presence of ammonia and its

derivatives in water effluent from petrochemical industry are one of those reasons which are

responsible for water pollution. Various 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 enzyme urease (urea

amidohydrolase) which catalyses the hydrolysis of urea [4,5]. Copper and chromium are

another two most common metals found in wastewater discharge of petrochemical plants

wastewater discharge from other industrial sites [6] where hexavalent chromium, Cr(VI)

present at concentrations ranging from tenths to hundreds of mg/L [7]. Mirbagheri et al.[8]

used ferrous sulfate and lime Ca(OH)2 for pH adjustment and conversion of Cr(VI) to

Cr(III) and Cr(III) precipitation, respectively. 9647167200

The largest industries which produce wastewater containing mercury and cadmium are vinyl

chloride monomer and PVC producing petrochemical factories. Malakahmad et al. has

performed a lab-scale experiment with a Sequencing Batch Reactor (SBR) to treat a

synthetic petrochemical wastewater containing mercury and cadmium. [9]. Wastewater of

petrochemical industries also contains high amounts of emulsified aliphatic or aromatic

hydrocarbons. Taran has showed Haloarcula sp. IRU1 can degrade petrochemical

wastewater and produce PHB from it in different conditions [10].

Membrane technologies have became the most popular separation process for treatment of

petrochemical industry wastewater. Now-a-days it is also competing with traditional

schemes [11-15]. Membrane separation processes have various advantages like a) 100%

purity of product can be achieved, b) low energy consumption, 3) compared with other

conventional techniques, membranes can offer a simple, easy-to-operate, low-maintenance


process option, c) no external chemicals are required to add for separation, d) can operate at

a moderate temperature and pressure. With all of these advantages there are also certain

disadvantages of this membrane based separation processes; Cake formation or fouling can

be considered as a major problem of membrane based separation processes which is the

main reason for reduction of permeate flux. But this problem can also overcome by using

cross flow arrangement and using different types of membrane modules.

From 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. Pressure-driven Reverse Osmosis being comparatively an

innovative one, possess the ability to stand as a viable solution replacing conventional

separation and purification techniques like distillation, evaporation ion exchange, absorption.

Being modular in design, membrane based plants are able to ensure the possibility of

operation in a simpler plant with a required number of active units which offers high

flexibility to the plant. By the virtue of high selectivity membranes are able to offer high

degree of separation and purification (over 98%) to the targeted molecules. Membrane based

processes are highly efficient to act as a perfect substitute to the conventional unit operation

techniques like 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. Membrane based

processes employing highly selective membranes offer a high degree of separation and

purification with high permeate flux. Membrane based reactors are easy to design and easier

to scale up. Proper utilization of raw materials by continuous recycling and recovery of

byproducts could be efficiently performed using such technologies. Consequently they

ensure a compact design while reducing the capital cost. So a properly designed membrane


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 industrialization. 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

drawbacks associated with conventional processes. In this paper, a brief discussion about the

traditional treatment has been provided highlighting the major drawbacks 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 well as the BOD. End-of-pipe treatment in

this case will be biological plus additional activated carbon treatment. The techniques that

can be applied to new plants and to existing facilities will differ. In existing plants, the

choice of control techniques is usually restricted to process integrated (in-plant) control

measures, in-plant treatment of segregated individual streams and end-of-pipe treatment.

New 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 Petrochemical industry can be summarized as:

(a) Organic wastewater streams not containing heavy metals or toxic or non

biodegradable organic compounds are potentially fit for combined biological wastewater

treatment (subject to an evaluation of biodegradability, inhibitory effects, sludge

deterioration effects, volatility and residual pollutant levels in the effluent).


(b) Wastewater streams containing heavy metals or toxic or non-biodegradable organic

compounds (e.g. indicated by high AOX /EOX or high COD/BOD ratios) 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 (chemical)

oxidation, adsorption, filtration, extraction, (steam) stripping, hydrolysis (to improve

biodegradability) or anaerobic pretreatment.

Technologies to treat chemical industry effluents

There are mainly four stages of petrochemical industry wastewater treatment. First is

preliminary treatment which involves the removal of large particles as well as solids found

in wastewater samples. Second 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 broken down. Secondary treatment involves

biological (bacterial) degradation of undesired products. The fourth is tertiary treatment,

normally a chemical process and very often including a residual disinfection.

Physico-chemical treatment

Oil –Water Separator–Treatment of oily effluent

Petrochemical industries report high levels of oil and grease in their effluents (with an Oil

and grease concentration up to 200,000 mg/l) [16,17]. Oil and grease presents in wastewater

can be either of these forms: free, dispersed or emulsified where free oil is characterized

with droplet sizes greater than 150 mm in size, dispersed oil has a size range of 20–150 mm

and emulsified oil has droplets typically less than 20 mm. Oil and grease concentrations in

wastewater can be measured by different test procedures of the US Environmental


Protection Agency but they failed to determine the presence of specific compounds. Gravity

separation and skimming, dissolved air flotation, de-emulsification, coagulation and

flocculation are the several conventional approaches of treating oily wastewaters. Gravity

separation followed by skimming is effective in removing free oil from wastewater whereas

the API oil – water separator is designed to separate the oil and suspended solids from their

wastewater effluents. But this is not effective in removing smaller oil droplets and

emulsions. Primary clarifier is used to remove the oil that adheres to the surface of solid

particles.

Wastewater is usually pre-treated chemically to destabilize the emulsified oil followed by

gravity separation. The wastewater is also heated to reduce viscosity and density differences

and to weaken the interfacial films stabilizing the oil phase which is followed by

acidification and addition of cationic polymer/ alum for the neutralization of negative

charges on oil droplets. While waste water treatment, pH is kept at some high value

(alkaline regime) to induce flock formation of inorganic salts. The resulting flock with the

adsorbed oil is then separated, followed by sludge thickening and sludge dewatering.

Coagulation–flocculation

Coagulation/flocculation is one of the most important processes in the primary purification

of water and in petrochemical wastewater treatment [18-20]. 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 [20-22]. 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. Settled solids are removed as sludge, and

floating solids are removed as scum. Wastewater leaves the sedimentation tank over an


effluent weir to the next step of treatment. Factors such as the type and dosage of

coagulant/flocculant, pH, mixing speed and time, temperature and retention time are the

governing parameters to evaluate the efficiency of the process [26] . Both inorganic and

organic such as aluminum sulfate (alum), 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 [19,20,23,25].

Altaher et al. [27] demonstrated in his paper that the pH plays a significant role in

coagulation-flocculation process. The experiments conducted showed that increase in pH

form acidic range to alkaline range promotes turbidity removal which also indicates that the

pH played a significant role in imparting surface charge of organic and inorganic colloids.

This treatment process can remove almost 90% of the suspended solids from the wastewater

but fails to remove organic, inorganic particles, heavy metals present in the wastewater.

Adsorption techniques 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 “petrochemical” waste water. Granular

activated carbon zeolites, silica-aluminas and silicas are the most popular adsorbent

mediums due to their high surface area to volume ratio. Zeolites have some peculiar

characteristics, which include i) high selectivity due to a strictly defined chemical

composition and porous texture; ii) tunable hydrophilicity; iii) proven stability under harsh


conditions; and iv) in most cases, excellent regenerability [28]. Zeolite can remove heavy-

metal-cation by applying cation – exchange technique [29], have a wide spectrum of

amorphous molecular sieve materials which make them markedly different from natural

zeolites. Due to their i) wide pore openings, ii) high specific surface areas and iii) large

specific pore volumes, silica-aluminas and silicas have drawn attention for their adsorption

of major amounts of non-dissociated contaminants characterized by bulky molecules (which

are unable to diffuse through zeolite micropores) that have been dissolved or even dispersed

in water as oily droplets. Many research studies have been done where non-conventional

adsorbents, such as agricultural and industrial solids wastes are used for the removal of

heavy metals [30-32]. There are other materials which have also been used to remove heavy

metals from wastewater, such as peat, wool, silk, and water hyacinth. Many researchers

have worked on preparation of activated carbon from cheaper and readily available

materials [31,32].

Maretto et al.[33] used two different microporous materials, a natural zeolite called

clinoptilolite and a polymeric chelating resin named Purolite_ Resin S910, to remove

dissolved heavy metals, and a mesoporous siliceous material to uptake hydrocarbons from

wastewater. The batch experiments indicated a good adsorption rate and a percentage of

heavy metal (Pb2+, Cd2+ and Ni2+) and hydrocarbon removal (benzene and toluene) that was

always greater than 90%. They developed a new adsorption model to better describe the

adsorption mechanism of heavy metals and a two-step mechanism for hydrocarbons. Here

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.

But with all the advantages described above adsorption technique also posses certain

disadvantages like i) most of the adsorbent are temperature sensitive; ii) with time their


adsorption ability may deteriorate; in that case adsorbents need to be changed after a certain

time.

Fixed bio film reactor

The fixed bio film reactor is nothing but a trickling filter that consists of a bed of highly

permeable media on whose surface a mixed population of microorganisms is developed as a

slime layer. Wastewater passes through the filter which causes the development of a

gelatinous coating of bacteria, protozoa and other organisms on the media. The continual

increase in the thickness 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 nozzles, trickling filters should be preceded by primary sedimentation tanks

equipped with scum collecting devices. Trickling filters should be followed by secondary

sedimentation tanks to remove the sloughed solids and to produce a relatively clear effluent.

With the advantages of its simple design, trouble free, ease of maintenance and control

nature (as compare to activated sludge process) trickling filter also has some disadvantages

such as excessive organic loading without a corresponding higher recirculation rate

clogging of under drain system, non-uniform media size or breaking up of media.

Electrosorption

Electrosorption is nothing but the absorption on surface of an electrode. After the

polarization 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


electrode. Electrosorption has attracted a wide interest in the adsorption processes for

treatment of wastewater due to its environmental friendly and less power consuming nature.

But 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.

Membrane technology

Application of membrane based separation processes such as microfiltration (MF),

ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) for treating oily

wastewater are increasing day by day. There are three broad categories of oily wastes – free-

floating oil, unstable oil/water emulsions, and highly stable oil/water emulsions of which

membranes are most useful with stable emulsions, particularly water soluble oily wastes

[34]. Mechanical separation devices can remove the free oil by using gravitational force as

the driving force whereas unstable oil/water emulsions can be mechanically or chemically

broken and then gravity separated. Cheryan et al. [35] reported a study where a semi-batch

type recycle membrane unit was employed. A constant level was maintained in the process

tank adding wastewater feed at a rate equal to the rate of withdrawal of clean permeate and

retantate stream containing oil and grease was recycled back to the process tank. When the

oils and grease and other suspended matter reached a certain predetermined concentration in

the tank, the feed was stopped and the retentate allowed to concentrate which finally gave a

result of final concentrate volume that was only 3-5% of initial volume of oily wastewater.


Fig. 1. Schematic of typical membrane system for treatment of oily wastes (adapted from Cheryan [24])

Membranes have several advantages, among them: (1) Widely applicable across a wide

range of industries; (2) Do not involve phase change; 3) The membrane is a positive barrier

to rejected components. Separation process can have a higher degree of purity (99%) than

other processes; (4) No separation agent is required, making subsequent oil recovery easier;

(5) Membranes can be used in-process to allow recycling of selected waste streams within a

plant; (6) Energy costs are lower compared to thermal treatments; (7) The plant can be

highly automated and does not require highly skilled operators. Membrane processes have

some limitations: (i) Scale-up is almost linear above a certain size. Thus capital costs for

very large effluent volumes can be high; (ii) Fouling is the most important problem in case

of membrane separation processes. Due to fouling the flux decreases with time; (iii)

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 required to be replaced frequently, which can increase operating

costs significantly. Several 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 polarization, modification of membrane surfaces to increase

hydrophilicity, and pre-treatment of feed are the most effective techniques to be followed.


[36,37]. In spite of the above disadvantages, membrane processing of oily wastewaters,

sometimes in conjunction with other methods for treating the residuals is widely used for the

treatment of wastewater all over the world.

Fratila-Apachitei et al. [38] has reused petrochemical effluent as cooling water after treating

it by a scheme comprising of ultrafiltration and reverse osmosis. They have used RO

membrane to decrease the salinity to an allowable level for reuse as cooling water followed

by a dead-end UF membrane which was used to reduce the turbidity of the wastewater.

They performed UF test at constant transmembrane pressure (0.2 bar) using hollow fibre

polyethersulphone membranes coated with poly-vinyl-pirrolidone. To compare the

performance characteristics two membranes with different molecular sizes(50 and 150 kDa)

were taken and performed separately where the 150 kDa membrane showed a very fast flux

decline (i.e. 20% in 2 min) requiring frequent backwashing (BW), whereas 50 kDa

membrane showed a relatively slow flux decline i.e. 20% flux in 20 min. As a gradual

change from complete to intermediate blocking and cake filtration was observed in both

cases, analysis of the blocking mechanisms failed to explain the rapid drop in flux for the

150 kDa membrane as compared with the 50 kDa membrane. But a field emission scanning

electron microscopy (FESEM) analysis of both UF membranes suggested that the highly

interconnected pore system of the 50 kDa membrane is mainly responsible for filtration

performance which in turns result in a ‘3D-bridge-type’ surface morphology.

On the other side Teodosiu et al. [39] also worked on to evaluate the possibilities of using

UF as a pre-treatment for RO, in a double membrane filtration scheme where the two UF

membrane provided by the same manufacturer, made of polyetherosulphone /

polyvinylpirollidone) with the same molecular weight cut-off of 150,000 Da but with

different coatings have been used. They showed that the low fouling membrane is easy to


clean by backwashing or enhanced backwashing, 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-LF performance. Experimentally they have proved that ultrafiltration offers

almost complete removal of suspended solids and colloids (98% as turbidity) and partial

removal of organic compounds attached to suspended solids (30% as COD) 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 requirements for recycling [40].

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. Parameters which effect the aerobic treatment are temperature, moisture,

pH, 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. Soluble organic sources of biochemical oxygen demand (BOD) can be

removed by any viable microbial process, aerobic, anaerobic or anoxic of which the aerobic

microbial reactions almost 10 times faster than anaerobic microbial reactions. That’s why

aerobic reactors can be built relatively small and open to the atmosphere, yielding the most

economical means of BOD reduction. With the advantages aerobic bioprocess also have

certain disadvantages. The major disadvantage of aerobic bioprocesses over anaerobic

processes for wastewater treatment, is the large amount of sludge production due to


accumulation of biomass (as biomass yield for aerobic microorganisms is relatively high,

almost 4 times greater than the yield for anaerobic organisms).

Membrane bioreactors

Membrane bioreactors is a combination of the activated sludge process and a membrane

separation process. A simplified MBR diagram is shown in Figure 2.

Figure 2: Diagram showing the basic configuration of a membrane bioreactor [27]

A decrease in sludge production, improved effluent quality and efficient treatment of

wastewaters with varying contamination peaks are the different advantages MBRs offered

over traditional activated sludge process. Some disadvantages of this system include this

system needs frequent membrane monitoring and maintenance, operates at relatively high

running costs and there is a limitation of the pressures, temperatures and pH the system

which are considered as the basic disadvantages of the system. Due to membrane fouling

proper designing of these kind of reactor is very difficult. And because of these reasons

MBRs are not being as widely used in large scale wastewater treatments in comparison to

traditional activated sludge plants [41,42].


Khaing et al.[43] treatment petrochemical industry wastewater from a petrochemical using a

novel submerged membrane distillation bioreactor (MDBR) proved that it is feasible to treat

and reuse the wastewater using submersed MDBR technology. But the pitfalls are same as

that in MBR including flux declination is also plays a major 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 [44]. There are some major 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 [45,46]. Among the available

treatment methods, sequencing batch reactors (SBRs) has caught attention due to some

reasons such as reduced chemicals requirement for the overall treatment process, low

operating costs, eco-friendly and cost-effective alternative of conventional techniques and,

efficient at lower levels of contamination [47]. Other than these the main advantage of SBRs

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 tank and of the sludge in the clarifier, with potential failure of

one of them or both. Even the wastewater residence time in SBRs 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 SBR is an activated

sludge process periodically operated, fill-and-draw reactor [48] which has five discrete


periods in each operation cycle: fill, react, settle, draw, and idle [44]. Reactions 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 liquor suspended solids (MLSS) 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 known as idle [49]. Researchers have been working on it and a

number of papers also have been published which provide good description and evaluation

of the SBR systems in treatment of heavy metals [44,50–52].

Malakahmad et al [53] treated synthetic refinery wastewater containing Hg2+ and Cd2+, in a

SBR after acclimated the system for 60 days. The SBR was first introduced to mercury and

cadmium in low concentrations which then was increased gradually to 9.03±0.02 mg/L Hg

and 15.52±0.02 mg/L Cd until day 110. The study revealed that the COD removal efficiency

ranged from 66 to 88% before addition of heavy metals due to appropriate acclimatization

of the biomass during start-up period and adequate retention of MLVSS concentration

which contributed to high COD removal efficiency. MLVSS concentration (population of

microorganisms) which showed an appreciable growth during reactor start-up and reached

to 1870 mg/L, was affected by heavy metals concentration increment in each step and

finally its concentration has fallen to 510 mg/L. Heavy metals added to the SBR decrease

the settleability of the sludge . The study also showed that at maximum concentrations of the

heavy metals, the SBR was able to remove 76–90% of Hg2+ and 96–98% of Cd2+.

With all the advantages there are certain drawbacks associated with this method such as: i) a

higher level of sophistication is required (compared to conventional systems), especially for

larger systems, of timing units and controls; ii) higher level of maintenance (compared to


conventional systems) associated with more sophisticated controls, automated switches, and

automated valves; iii) potential of discharging floating or settled sludge during the DRAW

or decant phase with some SBR configurations; iv) potential plugging of aeration devices

during selected operating cycles, depending on the aeration system used by the

manufacturer; v) potential requirement for equalization after the SBR, 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 major role in case

or aerobic reactor. To remove the gazes (mainly methane and carbon dioxide) 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 like (1) lower production rate of sludge, (2) operable at higher

influent BOD and toxics levels, (3) no cost associated with delivering oxygen to the reactor,

and (4) production of a useful by-product, methane (biogas). According to Yerushalmi et al.

[54], addition of a co-substrate increases the biogas potential due to a well-equilibrated

medium and the accumulation of limiting nutrients. Manure is considered to be a superb co-

substrate, due to its ability of providing buffering and many nutrients important for

microbial development (Sambusiti et al.[55], Yang and Liu [56]). Siddique et al. [57]

operated anaerobic co-digestion (ACD) of petrochemical wastewater (PWW) and activated

manure (AM) in a continuous stirred tank reactor where he achieved an 80% methane yield

of 11.1 m3 m-3 d-1 with 98.57 ± 0.5% elimination of chemical oxygen demand at five days'

hydraulic retention time using a ratio of 50% PWW/50% AM. Although anaerobic digestion


provides numerous advantages, it is not extensively applied in the petrochemical industries

due to slow reaction, longer hydraulic retention time and lack 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 normalized hydrogen

electrode. The chemical oxidation processes can be classified in two classes: - Classical

Chemical Treatments and Advanced Oxidation Processes (AOPs).

Classical chemical treatment: Classical chemical treatments involves addition of an oxidant

agent to the water containing the contaminant to oxidize it. Some widely used [58]classical

oxidants are chlorine, potassium permanganate, oxygen, hydrogen peroxide, ozonztion etc.

Chlorine is considered to be a good chemical oxidizer for water evaporation because it

destroys microorganisms. Though it is a strong and cheap oxidant, very simple to feed into

the system [58]. It also has some disadvantages like i) its little selectivity that high amounts

of chlorine are required and ii) it usually produces carcinogenic organochloride byproducts.

Hydrogen peroxide is a multipurpose oxidant can be applied directly or with a catalyst.

Ferrous sul[hate, Al3+, Cu2+ or other iron salts are generally used as catalyst. Its basic

advantages are: (i) low cost (ii) it has high oxidizing power, (iii) easy to handle), (iv)water-

soluble (v) it does not produce toxins or colour in by products vi) it can also been used in

presence of ultraviolet. Ozonation is a strong oxidant that presents the advantage of both

hydrogen peroxide and oxygen. It does not introduce “strange ions” in the medium and has

low solubility in water at standard temperature and pressure [58] . Ozone plays a major role


many applications, like the elimination of colour, disinfection, elimination of smell and taste,

elimination of magnesium and organic compounds etc. As the pH increases, the rate of

decomposition of ozone in water also increases. The major drawbacks of this oxidizer is that

it has to be produced on site and needs installation in an ozone production system in the

place of use due to which the cost of this oxidizer is extremely high.

Advanced Oxidation Processes (AOPs): Among various AOPs like UV/O3 process,

UV/H2O2, O3/H2O2, Fe3+/UV-vis process, UV/TiO2 (Heterogeneous photocatalysis), the

Fenton reagent (H2O2/ Fe2+) is the most effective methods of organic pollutant oxidation.

Fenton process is widely used as a suitable treatment method for highly concentrated

wastewaters due to its effectiveness in producing hydroxyl radicals [59,60]. Application of

traditional Fenton process is limited by its acidic pH requirements, the formation of iron

sludge and high cost of hydrogen peroxide [59,61]. But nowadays (EAOPs) based on

Fenton’s reaction chemistry have received much attention for wastewaters remediation [61].

EAOP is the electro Fenton (E-Fenton) process [62], the most popular electro-chemical

advanced oxidation process which can proceed by the following chain reactions [62-63]:

H2O2 + Fe2+ → Fe3+ + OH• + OH- (1)

H2O → H+ + OH• + e- (2)

Fe3+ + e- → Fe2+ (3)

Davarnejad et al. conducted an experiment where he compared aluminum and iron plate

electrodes on COD and colour removal from Petrochemical wastewaters and also evaluated

the effects of reaction time, current density, pH, H2O2/Fe2+ molar ratio, and H2O2 of

petrochemical wastewater (PW)(ml/l) on the performance of the process. The results

revealed that COD and colour removal efficiencies of iron electrode were (67.3% and


71.58%, respectively) which were more than those of aluminum electrode (53.94% and

67.35%, respectively). However, some disadvantages are also there in using the Fenton

reagent which are i) the production of a substantial amount of Fe (OH)3 precipitate and ii)

additional water pollution caused by the homogeneous catalyst that added as an iron salt,

cannot be retained in the process [58]. A number of researchers have investigated the

application of iron oxides such as hematite, ferrihydrite, semicrystalline iron oxide and

crystalline goethite [58] 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 technique. Several 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. API – oil separator is an excellent technique for oil removal from industrial

wastewaters whereas both aerobic and anaerobic treatment systems are feasible to treat

wastewater from all types of industrial effluents. So 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 coagulation/flocculation process can be applied where the membrane modules are in

cross flow mode to increase the effectivity of the process; an ultrafiltration (UF) membrane

is installed prior to reverse osmosis (RO) as a pretreatment where UF will remove

emulsions, colloids, macromolecules or proteins (size under 100 nm) and (RO) will separate

dissolved salts and small organics (size under 1 nm).


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