Polyaromatic Hydrocarbons Essay

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POLYAROMATIC HYDROCARBONS (PAHs) ARE PRIORITY HAZARDOUS

SUBSTANCES PRESENT IN WASTEWATER AND WATER. DISCUSS THEIR

SOURCES, DISTRIBUTION AND REMOVAL FOR EACH OF THESE TREATMENT

PROCESSES.

BY

SABINA YETUNDE IBINITIE GBADEGESIN

4154823

MODULE: WATER TREATMENT ENGINEERING (J14WTE)

WORD COUNT: 3876

12th DECEMBER, 2011



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CONTENT

LIST OF TABLES 3

LIST OF FIGURES 3

ABSTRACT 4

INTRODUCTION 5

Sources and distribution of PAHs 5

CHARACTERISATION OF THE PROBLEM 7

TREATMENT OPTIONS 8

Determination of PAHs in wastewater 8

Determination of PAHs in drinking water 9

Removal of PAHs from water and wastewater 10

Removal by adsorption 11

Removal by volatilisation 12

Removal by photolysis 13

Removal by microbial-/biodegradation/remediation 15

CURRENT STATUS 19

Use of a Conventional Activated Sludge Process (CASP) in a WWTP 19

with a Membrane Bioreactor (MBR)

Use of activated carbon or oxidation processes in drinking water treatment 20

CONCLUSION 21

Recommendation 21

BIBLIOGRAPHY 22



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LIST OF TABLES

Formula and Molecular Weight of the 16 PAHs in U.S.EPA priority pollutants List 6

Standards and Regulations for Polycyclic Aromatic Hydrocarbons in Water 7

Table showing the US EPA method detection limits for the determination of PAHs 9

in drinking water and waste water

Table showing some adsorbents and their characteristics for PAHs removal in water 12

LIST OF FIGURES

The chemical structures of the 16 PAHs listed in the U.S.EPA lists of priority 6

pollutants

Chromatographic determination of the 16 US EPA priority PAHs using HPLC 10

Flow chart of a Wastewater Treatment Plant (WWTP) at Fusina, Venice, Italy 11

A Photo-chemical Reactor 13

Scheme of PAHs photo-degradation pathways in the O2 /H2O system 14

Photolysis products of benzo(a)pyrene 15

Figure showing the proposed pathway for the microbial catabolism of PAHs 16

A block diagram showing a wastewater treatment plant that uses 17

biodegradation technology

A schematic of the in situ bioremediation of PAH-contaminated groundwater 18

A pilot scale MBR in parallel with a WWTP 19



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ABSTRACT

This paper discusses the prevalence of PAHs – a priority hazardous pollutant in the

aquatic environment with special reference to wastewater and water. The various

treatment processes such as adsorption, bioremediation, photo-chemical oxidation and

volatilisation were looked into. All the treatment options were seen capable of removing

certain PAHs to an extent from water and wastewater. The PAHs considered in this paper

are the 16 PAHs listed in the US EPA list of priority pollutants; they are naphthalene,

acenaphthylene, acenaphthene, flourene, phenanthrene, anthracene, fluoranthene,

pyrene, Benzo(a)anthracene, chrysene, Benzo(b)fluoranthene, Benzo(k)fluoranthene,

Benzo(a)pyrene, indeno(1,2,3-c,d)pyrene, Benzo(g,h,i)perylene and

dibenzo(a,h)anthracene. Adsorption process was seen capable of removing all of the 16

PAHs with a removal efficiency of over 95% when the adsorbent used is activated

carbon. Bioremediation was also capable of removing some of the PAHs (2-3 rings) with

an efficiency of about 94%; but had difficulty in removing the high molecular weight

PAHs (4 rings and above). Volatilisation was seen to contribute little or nothing to PAHs

removal from water as it had a removal efficiency of 1-2%; and only the low molecular

weight PAHs were able to volatilise easily from the water body. Photolysis was seen to

degrade the PAHs at a very fast rate with initial PAHs removal of 50% within 20seconds.



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INTRODUCTION

Polycyclic (polynuclear) aromatic hydrocarbons (PAHs) belong to a class of complex

organic compounds whose molecules usually consist of at least two aromatic rings fused

together which are arranged in different configurations (Kanaly and Harayama, 2000).They are formed during the pyrolysis or incomplete combustion of organic substances,

either naturally or anthropogenically (Ravindra et al., 2008; Popp et al., 2000). They are

referred to as priority hazardous substances because some of them have proven to be

carcinogenic, mutagenic, recalcitrant and ubiquitous in nature (Crisafully et al., 2008;

Cao et al., 2005). PAHs are listed in the European Union and in the United States

Environmental Protection Agency (US EPA) priority list of pollutants. About a hundred

PAHs have been identified but only 16 of them have been included in the US EPA list of

priority pollutants (Cao et al., 2005), due to more information being known about them

concerning their probable carcinogenic effects on humans and mutagenic effects on

aquatic life; and their high level of concentration in the environment compared to the

other PAHs. The sixteen PAHs (Wang et al., 2006; Lataweic and Reid, 2010) are listed in

table 1 with their corresponding molecular weight; while their corresponding chemical

structures are shown in figure 1 (Dionex Corporation, 2009).

SOURCES AND DISTRIBUTION OF PAHs

Sources of PAHs can be categorised either as natural or anthropogenic sources. PAHs are

ubiquitous; they can be found in air, soil, sediments and water. Some of the natural

sources of PAHs include emissions from volcanic eruptions, burning vegetation and forest

fires; while anthropogenic sources include emissions from the incomplete combustion of

fossil fuels (i.e. coal, petroleum & natural gas), waste incinerators, industrial plants, car

exhaust, seepage of some petroleum products (from vehicle maintenance and fuel

stations) and tyre degradation. However, most of the sources of PAHs in our

environment are anthropogenic due to the level of human activities that take place which

outweigh the natural sources (Cao et al., 2005; Popp et al., 2000).

Some of the sources of PAHs in wastewater and water include produced water that is

obtained during the production of crude oil, storm run-off with PAHs from car exhausts,

and wastewater produced from petrochemical plants (Crissafully et al., 2008).

These PAHs-containing wastes become distributed by entering ground waters and

surface waters through seepage or percolation processes (Sponza and Oztekin, 2010).



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Table 1: Formula and Molecular Weight of the 16 PAHs in U.S.EPA priority pollutants List

S/N

PAHd Formulae MolecularWeight(gmol-1)

Reference

1 NaphthaleneAcenaphthyleneAcenaphthene

FlourenePhenanthreneAnthraceneFlouranthene

PyreneBenzo(a)anthraceneChrysene

Benzo(b)flourantheneBenzo(k)flouranthene

Benzo(a)pyreneIndeno(1,2,3-cd)pyreneBenzo(g,h,i)peryleneDibenzo(a,h)anthracene

C10H8 128 c2 C12H8 152 c

3 C12H10 154 c

4 C10H10 166 b, c

5 C14H10 178 c

6 C14H10 178 c

7 C16H10 202 a, c

8 C16H10 202 a, c

9 C18H12 228 a, c

10 C18H12 228 a, c

11 C20H12 252 a, c

12 C20H12 252 a, c

13 C20H12 252 a, c14 C22H12 276 c

15 C22H12 276 c

16 C22H14 278 a, c

a (Perez-Gregorio et al., 2010); b (Shemer and Linden, 2007) ; c (Sponza andOztekin, 2010); d (Wang et al ., 2006); e (Ferrarese et al ., 2008)

Figure 1: The chemical structures of the 16 PAHs listed in the U.S.EPA lists of priority

pollutants (Dionex Corporation, 2009).



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CHARACTERISATION OF THE PROBLEM

PAHs are released into water through atmospheric deposition, industrial wastewater

effluents and petroleum spills. Generally, PAHs are hydrophobic; they have a low

solubility in water which decreases with their increasing molecular weight. Thishydrophobic property makes their affinity for suspended particles in water high. It

becomes deposited on suspended solids, sediments, aquatic organisms and sea beds;

and their concentration level in oils is estimated to be much higher (Anyakora and Coker,

2006). This accumulation potential of PAHs in water bodies has brought about great

environmental concerns as a result of the carcinogenic and recalcitrant nature of the

PAHs (Haritash and Kaushik, 2009). There is no recommended safe level of PAHs

concentration in water; this is due to its carcinogenic nature which is highly toxic to both

humans and aquatic life (US Department of Health and Human Services, Public Health

Service, Agency for Toxic Substances and Disease Registry, 1995). Benzo(a)pyrene is

the PAH that has been thoroughly studied the most; and the European Union gave its

limit alone in drinking water to be 10ng/L. The guideline value (GV) in drinking water

quality for Benzo(a)pyrene has been estimated to be 0.7mg/L and this corresponds to an

excess lifetime cancer risk of 10-5 (WHO, 1998). The Maximum Contaminant Level of

some PAHs in water according to the US EPA is given in table 2 below.

Table 2: Standards and Regulations for Polycyclic Aromatic Hydrocarbons in Water

Agency Maximum

ContaminantLevel (MCL)(mg/L)

PAHs

U.S. Environmental Protection Agency

0.0001

0.0002

0.0003

0.0004

MCL forBenz(a)anthracene

MCL forBenzo(a)pyrene,Benzo(b)fluoranthene,Benzo(k)fluoranthene,Chrysene

MCL for

Dibenzo(a,h)anthracene

MCL for Indeno(1,2,3-cd)pyrene

Source: (Agency for Toxic Substances and Disease Registry (ATSDR), 2009)



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TREATMENT OPTIONS

The removal of PAHs from wastewater and water is twofold. The PAHs have to be

detected in the samples first before they are removed. Several methods have been

employed in the removal of PAHs from wastewater and water such as microbial/bio

degradation, adsorption, chemical degradation or photolysis, volatilization; all these

treatment options may be categorized under biological, physical or chemical removal

processes (Haritash and Kaushik, 2009). Before PAHs removal from water can be

considered, its presence and concentration has to be determined first so that an

appropriate removal technology may be used for the water treatment.

DETERMINATION OF PAHs IN WASTEWATER

The method used in the determination of PAHs in municipal and industrial wastewater is

the US EPA method 610. This method detects the 16 PAHs mentioned in the US EPA

priority pollutants list. The procedure can be described thus; a one liter volume of the

sample of wastewater is measured and extracted with methylene chloride by using the

Continuous Liquid-Liquid Extraction (CLLE) process (Brown et al., 1999; US EPA, 1996).

The extract, i.e. the wastewater sample together with the methylene chloride is then

dried and separated. Two methods can be used to carry out this separation;

1. High Performance Liquid Chromatography (HPLC) separation with dual

wavelength Ultraviolet (UV) detector or Fluorescence Detector (FLD) for the PAHs

identification and measurement.

2. Gas Chromatography (GC) separation with Flame Ionization Detector (FID) for

identifying and measuring the PAHs.

Once the PAHs have been identified and measured by the procedure above, the results

have to be confirmed by an additional appropriate qualitative technique. The US EPA

method 625 efficiently provides conditions necessary for confirmation of the results by

making use of a Gas Chromatograph/Mass Spectrometer (GC/MS) on the extract.

An illustration of the representation of the method using HPLC is as shown in figure two;in this case, the conventional US EPA method 610 was not entirely applied in the

determination process. HPLC analysis of the actual wastewater sample was carried out

using a photodiode array detector (PAD) for the PAHs identification and measurement.

The calibration curves were obtained by preparing standards with known concentration

so that each of the PAH compounds were quantified in actual wastewater samples and

simulated wastewater samples (Tikilili and Nkhalambayausi-Chirwa, 2011).



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DETERMINATION OF PAHs IN DRINKING WATER

The method used for the determination of PAHs in drinking water is the US EPA method

550.1. It also detects the 16 PAHs in the US EPA list of priority pollutants. The method is

quite similar to that used for PAHs determination in wastewater; In this case, the PAHs

are extracted when 1Liter of the drinking water sample is passed through a cartridge

which contains a solid inorganic matrix that is chemically coated with an organic phase.

This is known as Liquid-Solid Extraction (LSE). Methylene chloride is added to the extract

and this is dried and concentrated to a volume of 0.5ml by adding 3ml of acetonitrile to

it. HPLC separates the extract and UV & FLD are used to identify and measure the PAHs

(US EPA, 1990).

The US EPA specifies the Method Detection Limits (MDL)for the determination of PAHs in

wastewater and water and they are as shown in the table 3 below.

Table 3: Table showing the US EPA method detection limits for the determination of

PAHs in drinking water and waste water

S/N PAH MDL inDrinking

Water(µg/L)a

MDL inWastewater

(µg/L)b

1 NaphthaleneAcenaphthyleneAcenaphthene

FlourenePhenanthreneAnthraceneFlouranthenePyrene

Benzo(a)anthraceneChryseneBenzo(b)flourantheneBenzo(k)flourantheneBenzo(a)pyreneIndeno(1,2,3-cd)pyreneBenzo(g,h,i)peryleneDibenzo(a,h)anthracene

2.20 1.8

2 1.41 2.3

3 2.04 1.8

4 0.126 0.215 0.150 0.64

6 0.140 0.66

7 0.009 0.21

8 0.126 0.27

9 0.004 0.013

10 0.160 0.15

11 0.006 0.018

12 0.003 0.017

13 0.016 0.023

14 0.036 0.043

15 0.020 0.076

16 0.035 0.030aUS EPA Method 550.1: Determination of Polycyclic Aromatic Hydrocarbons in

Drinking Water by Liquid-Solid Extraction and HPLC with Coupled Ultraviolet and

Fluorescence Detection, 1990bUS EPA Methods for Organic Chemical Analysis of Municipal and Industrial

Wastewater: Method 610 – Polynuclear Aromatic Hydrocarbons, 1996



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Figure 2: Chromatographic determination of the 16 US EPA priority PAHs using HPLC

(Tikilili and Nkhalambayausi-Chirwa, 2011).

REMOVAL OF PAHs FROM WATER AND WASTEWATER

The figure three on the next page shows a flowchart for a wastewater treatment plant in

Venice, Italy in which aqueous and sludge samples were collected from it for PAHs

determination and removal. The US EPA 610 method as described above was used and a

Solid Phase Extraction (SPE) procedure was applied so that all of the 16 PAHs were

recovered simultaneously with yields greater than 70% (Busetti et al., 2006).



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Figure 3: Flow chart of a Wastewater Treatment Plant (WWTP) at Fusina, Venice, Italy

(Busetti et al ., 2006).

REMOVAL BY ADSORPTION

In water and waste water treatment, adsorption may be defined as the process in which

pollutants or contaminants leave solution and stick on the surface of a solid by means of

physical or chemical bonding (Metcalf and Eddy, 1979). The pollutant molecules are

referred to as the adsorbate while the solid which attracts these pollutants are referred

to as the adsorbent. Adsorption has been seen to effectively remove organic pollutants,

especially PAHs from water and waste water (Yuan et al., 2010). Table four lists some

example adsorbents that have been used to remove PAHs from aquatic environments. In

wastewater treatment plants (WWTP), the physical adsorption process is used. In water

treatment processes, PAHs removal by adsorption has been carried out in coagulation,

flocculation, sedimentation, filtration and Activated Sludge Process (ASP) with activated

carbon or sand. The sludge in the ASP is referred to as an organic adsorbent. Table fourshows some adsorbents that have been used to remove certain PAHs in certain studies;

from the table, it can be seen that activated carbon has the highest efficiency for

removing PAHs. Although, the certain PAH studied was naphthalene, but in practice, it

has been seen to remove all of the 16 PAHs listed in the US EPA list of priority pollutants

at above 95% efficiency. Other adsorbents, such as porous carbon, leonardite and wood

fibre; which are generally referred to as low cost adsorbents, also have the ability to

remove certain PAHs from water but are not as efficient as activated carbon. Also, they

have not been tested on a large scale (Zeledon-Toruno et al., 2007).



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Table 4: Table showing some adsorbents and their characteristics for PAHs removal in

water

Type of Adsorbent Advantages & Disadvantages

PAHsRemoved

(Studied)

RemovalEfficiency &

Scale-up

Environ-mental

conside-rations

Activated Carbon(from carbonaceous

sources)(a)

Advantage:It has a highadsorptioncoefficient due to

the intra – particle diffusionwithin its pores.Disadvantage:It is expensive toproduce.

Naphthalene

NaphthaleneFluorenePyrene

FluoranthenePhenanthrene

Over 95% of the PAH wasremoved.It can be

used in largescale but itscost is alimitingfactor.

Probability of CO2 emissionduring itsproduction

which is aGHG (b).

Derived PorousCarbon fromPetroleum Coke(A bye-product of

bitumenUpgrading

process)(a)

Advantage:Cheap sourceand low cost of

production.Disadvantage:Slow process; It

takes long hoursto reachadsorptionequilibrium.

Over 90% of the PAHswere

removed.It can bescaled up.

Thepetroleumcoke source

containsabout 8%Sulphur which

is undesirablein theenvironment.

Leonardite(Immature Coal)(c)

Advantage:

Low cost andavailability.Disadvantage:Low Flourene

removalefficiency.

Flourene

PyreneBenzo(k)-flourantheneBenzo(a)-

pyreneBenzo(g,h,i)-perylene

Over 82% of

the PAHswereremoved.Scale up is

still underconsiderationbut has beensuccessful ina series of batch

experiments.

Leonardite is

regarded as apromisingcandidate forpollution

remediationin both soiland groundwater.

(a)(Yuan et al ., 2010); (b) GHG – Green House Gas which causes global warming;(c)(Zeledon-Toruno et al., 2007)

REMOVAL BY VOLATILIZATION

Volatilization in this context can be defined as the movement of PAHs from water to the

atmosphere. It is useful in removing PAHs with two to three rings from aquatic

environment, e.g. naphthalene and anthracene. This is due to the relative volatility of

the PAHs in water and their low molecular weight compared to the other PAHs. The rate

of volatilization of PAHs from water bodies is dependent on its distribution coefficient and



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environmental factors such as temperature, water turbulence and wind. Although, this

process employs little or no procedure, it is not efficient. It contributes to only 1-2% of

the PAHs removal and there is a probability of the volatilized PAHs returning to the water

via atmospheric deposition. Volatilization does not and is not being expected to play a

significant role in the reduction of carcinogenic PAHs in aquatic environments

(Southworth, 1979).

REMOVAL BY PHOTOLYSIS

Photolysis of PAHs in water involves photo-chemical transformations whereby the PAHs

are decomposed through photo-chemical oxidation processes. It is the most commonly

employed mechanism for the removal of PAHs from water (Zeledon-Toruno et al., 2007).

The photo-oxidation of these PAHs in water goes through a complex mechanism which

usually involves oxygen (i.e. singlet oxygen, ozone) or HO-radical ((Nagpal 1993);

(Miller and Olejnik, 2001)). Singlet oxygen photo-oxidation happens to be the most

dominant process for PAHs degradation in water.

Figure 4: A Photo-chemical Reactor (Miller and Olejnik, 2001).

Where (1) Sampling port; (2) UV Lamp; (3) Gas Inlet; (4) Quartz well; (5) Thermostatic

bath; (6) Magnetic stirrer.



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Figure four shows a photo-chemical reactor which was used for the experimental study

of the photolysis of certain three PAHs; they are benzo(a)pyrene, chrysene and fluorene.

The reactor was a glass type of 1dm3 volume. It was glass to allow for the emission and

absorption of UV light. The influence of the initial PAHs concentration was such that the

higher it was, more UV light was absorbed by the PAHs which in turn increased the

reaction rates for the photo-chemical reactions that occurred. The quartz well was to

allow for the insertion of a low and medium pressure UV lamp; the thermostatic bath

was to maintain a constant temperature throughout the procedure. The magnetic stirrer

brought about agitation of the reaction mixture. The complex mechanism by which these

reactions occur is as shown in figure four below.

Figure 5: Scheme of PAHs photo-degradation pathways in the O2 /H2O system (Miller and

Olejnik, 2001).

Figure five above shows the series of possible reactions that may occur to PAHs in water

with oxygen and UV radiation. In this complex mechanism, intermediates of the photo-

degradation may compete for oxygen, UV light or sometimes react with each other;

thus, there is no simple kinetic model available for the depiction of the PAHs degradation

by photolysis in water. Basically, there are three different mechanisms by which PAHs

may decompose; ozonolysis (reaction with ozone, O3), ultra-violet radiation (hv ) or



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reaction with a hydroxyl radical (●OH) (Ledakowicz et al., 2001). The PAHs with three

condensed rings decompose via reactions with a radical cation; examples of such PAHs

are fluorene, anthracene, naphthalene, acenaphthene and phenanthrene. The PAHs with

four rings decompose via radical reactions with oxygen taking part in the reaction in the

initial period; examples of such PAHs are Benzo(a)anthracene, Benzo(a)pyrene and

chrysene (Miller and Olejnik, 2001). The products obtained from the photolysis of

Benzo(a)pyrene is shown in figure six below.

Figure 6: Photolysis products of Benzo(a)pyrene (Nagpal, 1993).

Due to the complexity of this method, it has not been tried out on a large scale in water

or wastewater treatment plants. It has only been performed on a laboratory scale with

batch experiments for removal of selected PAHs from water. In a study carried out by

(Miller and Olejnik, 2001) on the photolysis of Benzo(a)pyrene, chrysene and flourene in

water, it was observed that 50% of the initial concentration of Benzo(a)pyrene was

removed in 20 seconds. The degradation of Benzo(a)pyrene proved to be the most

effective out of the three PAHs. 50% of the initial concentration of chrysene decomposed

within a minute while that of flourene spent up to 7 minutes before 50% decomposition

was achieved.

REMOVAL BY MICROBIAL-/BIO- DEGRADATION/REMEDIATION

With reference to PAHs removal from water, biodegradation/bioremediation may be

defined as the process by which PAHs are broken down into less complex metabolites via

biotransformation and into inorganic minerals, H2O, CH4 (anaerobic) or CO2 (aerobic) via

mineralization. Microbial degradation is the major degradation process which PAHs



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undergo and it may also be referred to as bioremediation; this process transforms the

PAHs into less hazardous forms with little or no chemical input. It also involves less time

& energy and it is less expensive when compared to the other methods already

discussed. Also, the PAHs are less likely to be transferred from one phase to the other

during this process compared to the other treatment/removal processes (Haritash and

Kaushik, 2009).

The rate at which the degradation of the PAHs occur depends upon the environmental

conditions (temperature, oxygen), the chemical structure of the PAHs and the type &

population of the micro-organisms taking part in the degradation process. The main

micro-organisms which take part in the degradation of PAHs include bacteria, fungi and

algae. Enzymes are also involved in the degradation of PAHs in water (Haritash and

Kaushik, 2009).

Figure 7: Figure showing the proposed pathway for the microbial catabolism of PAHs

(Haritash and Kaushik, 2009).

The PAHs biodegradation mechanisms by algae, fungi, bacteria and enzymes are as

depicted in the figure seven above. From the schematic, it can be observed that the

degradation may occur using a combination of algae, fungi and bacteria. The

degradation can also occur using any of the micro-organisms individually. Making use of



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the micro-organisms in combination is a more efficient process for the PAHs removal

when compared to using them individually (Haritash and Kaushik, 2009).

The biotransformation of the PAHs into less complex metabolites is as shown in figure

seven. The mechanism uses a combination of fungi, bacteria, O2 and Cyt-P450 Mono-

oxygenase (an enzyme) to degrade the PAH into an arene-oxide which may be further

decomposed by enzymatic action using epoxide-hydrolase to a trans-diol or the arene-

oxide further degrades by non-enzymatic action to a phenol which decomposes to give

less hazardous metabolites such as glucoside, xyloside etc. The mineralization of the

PAHs may also go through different routes as shown in figure seven. The first uses a

combination of fungi (white rot), enzymes (Laccase, Lignin/Mn peroxidase) and H2O2 to

degrade the PAHs into PAH-Quinones which further goes through ring fission to produce

CO2. The second may be described as an aerobic mechanism; it makes use of bacteria

and enzyme (dioxygenase) in the presence of oxygen to produce CO2 while in the third

scenario, the PAHs undergo mineralization via anaerobic mechanism.

The addition of Membrane Bio-Reactors (MBR) to municipal wastewater systems has

been seen to increase the removal of PAHs with efficiency of 40% to 60% (Fatone et al.,

2011). The application of biodegradation technology for the removal of PAHs from water

can be seen in figure eight below where a membrane bio-reactor (which is the anaerobic

digester) has been included in the water treatment plant.

Figure 8: A block diagram showing a wastewater treatment plant that uses

biodegradation technology (Fatone et al., 2011).



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The application of bioremediation technology to the removal of PAHs in ground water can

be seen in figure nine below. Here, the biodegradation is carried out in-situ (i.e. within

the water aquifer) and the abstraction and injection wells are used to circulate the

oxygen source, inoculum and the nutrients round the aquifer; and it was seen to reduce

the PAHs concentration in groundwater from 11µgL-1 to 0.7 µgL-1 which is approximately

94% removal efficiency (Bamforth and Singleton, 2005).

Figure 9: A schematic of the in situ bioremediation of PAH-contaminated groundwater

(Bamforth and Singleton, 2005).

Bioremediation has been seen to be the most significant process of all the removal

technologies for PAHs from water (Gok and Sponza, 2010) but it is not advisable to carry

out biodegradation on PAH-contaminated water when the significant amount of PAHs

present in the water are those containing more than four rings; this is because there is

little or no removal of these PAHs as a result of their high molecular weight and the time

taken for a reasonable reduction of these PAHs is so long that it is not economically

viable (Bamforth and Singleton, 2005). Hence, this procedure is expensive for the

removal of high molecular weight PAHs.



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CURRENT STATUS

USE OF A CONVENTIONAL ACTIVATED SLUDGE PROCESS (CASP) IN A WWTP WITH A

MEMBRANE BIOREACTOR (MBR).

At the moment, there are a lot of speculations as to which is the best removal

technology to employ; but it seems the ones gaining prevalence are the adsorption and

biological methods. This may be due to the high PAHs (for high and low molecular

weight) removal efficiency of the physical adsorption method. At the moment,

volatilisation is the least employed of all the methods due to the very low efficiency of

the process.

Currently, the conventional activated sludge process is used in parallel with a membrane

bioreactor which employs the method of biodegradation as shown in figure ten below.

Figure 10: A pilot scale MBR in parallel with a WWTP (Fatone et al., 2011).

The MBR technology involves biosorption (i.e. adsorption onto micro-organisms) and

biodegradation/bioremediation. This method aims to strike a balance between cost and

removal efficiency by taking advantage of the low cost of the biodegradation process and

the ability to remove both low and high molecular weight PAHs by the adsorption

process. The use of the MBR has been seen to enhance the bioremediation process of

PAHs in water. Using this technology, the 16 PAHs listed in the US EPA priority list of

pollutants were seen to be removed at an efficiency of about 60% with the WWTP in

question not even having primary sedimentation. The use of membrane bioreactors

technology is rapidly growing in water treatment plants and one of such plants is already

operating at full scale in Italy while the rest are still at the pilot stage (Fatone et al.,

2011). The outcome of PAHs in activated sludge systems have been studied and it was



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seen that under aerobic conditions, the PAHs were biodegraded with a 67% efficiency;

the low molecular weight PAHs (2-3 rings) were seen to biodegrade faster than the high

molecular weight PAHs (4-6 rings) (Fatone et al., 2011).

USE OF ACTIVATED CARBON OR OXIDATION PROCESSES IN DRINKING WATER

TREATMENT.

Recently, studies have shown that the conventional water treatment methods are not

able to efficiently remove PAHs from drinking water. Current methods are the use of

activated carbon for adsorption of the PAHs or advanced oxidation (photo-chemical

oxidation) processes before the chlorination stage. In a recent procedure carried out in

Hangzhou, China, the advanced oxidation technique was carried out on a drinking water

treatment plants and the following PAHs; flourene, benzo(a)pyrene, indeno(1,2,3-

c,d)pyrene, benzo(b)fluoranthene, benzo(k)fluoranthene and benzo(g,h,i)perylene were

successfully removed at an efficiency of 43.5% (Chen and Zhu, 2011).

The removal of 5-6 ring PAHs was also looked into in this study and it was observed that

the coagulation dosing technological process was able to remove the PAHs at a high

efficiency with the highest PAH removal efficiency being 78.3% for indeno(1,2,3-

c,d)pyrene (Chen and Zhu, 2011).



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CONCLUSION

The analysis procedures for the determination of PAHs and the different removal options

from water and wastewater have been looked into. All methods available have been seen

to be able to remove certain PAHs at certain removal efficiencies from both water and

wastewater. Of all the available technologies being employed for the removal of PAHs

from water, The adsorption has been seen to efficiently remove low as well as high

molecular weight PAHs especially when activated carbon is the adsorbent used; it has

also been seen to be employed in activated sludge process in wastewater treatment

plants as well as coagulation, flocculation, sedimentation and filtration in water and

wastewater treatment processes. Bioremediation also gives a high efficient PAHs

removal but finds it difficult to biodegrade PAHs with 4 rings (high molecular weight) and

above; and it is mainly employed in wastewater treatment processes. Not much interest

has been shown in the volatilization process; this is mainly due to the fact that it does

not contribute to the removal of PAHs from water, it has a PAHs removal efficiency of 1-

2%. The photo-chemical degradation/oxidation process involves a very complex

mechanism whose kinetic model is yet to be fully understood; it is mainly employed in

drinking water treatment processes and due to its complexity, is yet to be tried out on a

large scale. The adsorption and bioremediation processes are already well established

with some of their procedures already being employed in water treatment plants.

Current methods employed involve the combined usage of adsorption and

bioremediation to maximize PAHs removal at a lower cost; this is also a good innovation

as studies have shown that PAHs may already be developing some resistance to

bioremediation.

RECOMMENDATION

The adsorption process should be looked into critically and a means should be devised of

optimizing low cost adsorbents as a way of reducing the cost implications which may

arise as a result of using activated carbon. Photolysis presents a promising viable

technology on a large scale if its mechanism can be properly understood as itsdegradation process is very fast; removing about 50% of the initial PAHs concentration

in 20seconds.



Page 22 of 25

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