Introduction petrochemical industries (Malakahmad et al., 2011). Various

Introduction

Many researches and studies have
been carried out toward developing practical techniques and approaches to
decline the adverse impact of highly toxic generated oily wastewaters, and
therefore to resolve the scarcity of water resources in many regions around the
world.  Oily wastewater is wastewater
blended with oily materials under a wide range of concentrations. The soil,
water, air and human beings are threatened by oily wastewater because of its
extremely toxic contaminants and also hazardous nature of its oil contents
including radionuclides, persistent organic pollutants, particular hazardous
compounds (PHCs), polycyclic aromatic hydrocarbons (PAHs), inorganics and
dissolved minerals, hydrocarbons (i.e. oil, fats, grease), etc. (Jamaly
et al., 2015). Heavy metals (e.g. Cr, Cd, Hg, Ag,
etc.) in conjunction with particularly hazardous chemicals (PHCs) could be
observed in oily wastewater especially in petroleum refineries and
petrochemical industries (Malakahmad et al., 2011).

Various approaches have been studied
for treating oily wastewater including: physical treatment (e.g. membrane
filtration (polymeric/ceramic membranes (Salahi et
al., 2010)), reverse osmosis (Duong et al.,
2014), submerged membrane (Yuliwati et al.,
2012), hollow fiber, PVDF membrane (Zhang et
al., 2013), flotation, biological treatment (e.g. up-flow anaerobic
sludge bed reactor (Rastegar et al., 2011),
membrane bioreactor (Pendashteh et al., 2012),
sequence batch reactor (Malakahmad et al., 2011;
Shariati et al., 2011), biopolymeric flocculation (Ahmad et al., 2004), etc.), electrochemical
treatment  (e.g. electrocoagulation (Xu and Zhu, 2004), Fenton (Madani et al., 2015 ; Ishak and Malakahmad, 2013),
electrochemical oxidation, electroflotation (Ibrahim
et al., 2001), chemical coagulation (Madani
et al., 2015), or the like (Körbahti and Artut, 2010; Yan et al., 2011)),
chemical treatment (e.g. adsorption, chemical coagulation (Ngamlerdpokin et al., 2011), treatment using
ultrasound-dispersed nanoscale zero-valent iron particles, titanium dioxide,
etc.), as well as application of biosurfactants, destabilization of emulsions
through the use of zeolites and other natural minerals, treatment by vacuum
ultraviolet radiation, hybrid technologies, etc.

Although oil, fats, grease are of
biodegradable organics that could be readily be degrade through biological
methods and substrates, high attention is required for selecting proper
biological treatment due to the presence of toxic pollutants in oily
wastewaters. However, the general parameters which are considered as analytical
parameter to evaluate the efficiency of oily wastewater treatment are COD, BOD,
TSS, TOC, TDS, TPH, TFAs, VS, turbidity, oil/grease, salinity, and also other
significant factors based on the target wastewater are assessing PAHs &
boron concentration, FFA & FAME, as well as the concentration of nitrogen,
sulphate, phosphorus, phenol and phenolic or our target compounds. Also, there
are some other factors for assessment of aerobic processes (such as, MLSS,
MLVSS, SVI).

Selection of the appropriate treatment
system for any discharge greatly depends on its characteristics and discharging
criteria. To do so, various oily wastewater treatment technologies are
currently used; however, the percentage of oil or other contaminants removal
varies among them. The assessment of the most efficient technology depends on
several factors including (I) the influent quality, (II) the treatment cost,
(III) the environmental footprint, (IV) and energy consumption (Jamaly et al., 2015). So, considering most of the
previous approaches are either too expensive to be implemented on a commercial
scale or require large environmental footprints, novel sustainable methods for
oily wastewater treatment, that will be geared toward economic savings and
environmental preservation, is needed to be devised.

Although, based on the
characteristics of oily wastewater and target contaminant the design and amount
of operational would vary, investigating real oily wastewater under treating
process would generate more reliable and precise outcomes, because through
scrutinizing previous researches it could be inferred that there are
significant variations among results of experiments done on synthetic and real
oily wastewater. It has been declared that real oily wastewaters are majorly
contained some other particles such as colloids, solid particles and the like (Pendashteh et al., 2012; Madaeni et al., 2013).
Finally, it should be added that the BOD of oily wastewater like refinery
wastewater is mainly lower than municipal wastewater due to the existence of
some materials which could not be easily biodegraded (Al
Zarooni and Elshorbagy, 2006).

 

 

 

1.   
Some of Applied Approaches for Treating Oily Wastewater

 

1.1.   
Coagulation

 

1.1.1      
Electrochemical Treatment

 

(I)     
Electrochemical Oxidation (direct/indirect)

Investigated parameters: current
density and reaction temperature in electrochemical reactor (Körbahti and Artut, 2010),
initial pH and cell voltage (Yan et al., 2011).

(II)    Electro-Fenton

In the
Electro-Fenton process, Fe(II) is oxidized by H2O2 to form Fe(III).
This lead to forming a hydroxyl radical (HO•) and a hydroxide ion (OH?) in the process as
well. In the next step Fe(III) is then reduced back to Fe(II) by another
molecule of H2O2, forming a radical of (HOO•)
and a proton (H+). The main effect of adding H2O2 is
to create two different oxygen-radical species, with water (H+ + OH?)
as a byproduct (Ishak
and Malakahmad, 2013).

 

Fe2+ + H2O2
? Fe3+ + HO• + OH?

(1)

Fe3+ + H2O2
? Fe2+ + HOO• + H+

(2)

In the
second reaction free radicals of HOO are produced. Hydroxyl radical (HO•)
is an authoritative, strong, and non-selective oxidant which can start the new
reactions rapidly.  Oxidation of an
organic compound by Fenton’s reagent can be done very quickly but it involved
with exothermic reactions that results in increasing the temperature of the
solutions. The main purpose of this process is to oxidation of pollutants to
primarily carbon dioxide and water (Kavitha, V., & Palanivelu, K., 2005). Generally, Fe(II) sulfate (FeSO4)
is used as catalyst in the reactions. In case of electro-Fenton process,
hydrogen peroxide is produced in situ from the electrochemical reduction of
oxygen. Also, Fenton’s reagent during the radical substitution reaction is used
in organic synthesis for the hydroxylation of aromatic hydrocarbon (Casado et al., 2005). For instance, classical conversion
of benzene (C6H6) into phenol (C6H5OH)
can be expressed as: (Casado
et al., 2005)

C6H6 + FeSO4
+ H2O2 ? C6H5OH

(3)

Meinero and Zerbinati
(2006) investigated
the oxidative and energetic efficiency of various electrochemical oxidation
processes. The electro-Fenton process was verified to have the best degradation
efficiency in terms of energy consumption: for that case the specific energy
consumption was 0.3 kWh/g of COD, corresponding to 41.8 kWh/m3.

Many
works classified electro-Fenton or the very Fenton process as advanced
oxidation process (AOP). Some of AOPs are, electro-Fenton process, TiO2/H2O2,
photocatalysis reactions, etc., that are chemical oxidation processes mainly
used as an attractive pretreatment method to improve the biodegradability of
various industrial discharges, that is able to generate and use hydroxyl free
radicals (•OH) as strong oxidant (Klamertha
et al., 2010; Sin et al., 2011).

The application of AOPs not only
reduces the COD load and contaminants levels in wastewater, but also generates
fewer toxic effluents. Besides, AOPs augment the biodegradability of wastewater
through forming intermediates similar to the metabolic pathway substances (Ollis, 2000). Advanced oxidation process (AOP)
which employ strong oxidant agents (ozone, hydrogen peroxide and UV, Fenton,
etc.), are able to remove organic and phenolic pollutants of the Olive Mill
Wastewater (OMW) (Madani
et al. 2015).

The
Fenton process could be enumerated as one of the promising alternative
oxidation methods because of its cost efficiency, operation simplicity, lack of
residue, and ability to treat a spectrum of substances. Fenton process, which
is in fact a synthesis of oxidation and coagulation reaction, reduces toxicity
and COD concentration using hydrogen peroxide and ferrous sulfate (Madani et al. 2015). To be specific, the oxidation
mechanism by the Fenton process is due to the generation of hydroxyl radical in
an acidic solution by the catalytic decomposition of hydrogen peroxide and in
presence of ferrous (II) ions (Ledakowicz
et al., 2001).

Fenton’s
reagent (a solution of hydrogen peroxide (H2O2) and an iron catalyst (like FeSO4,
iron electrode, FeSO4.7H2O (ferrous sulfate
heptahydrate), etc.)) is used to oxidize contaminants or organic compounds in
wastewaters such as trichloroethylene (TCE), tetrachloroethylene (perchloroethylene, PCE), and
refinery wastewater to augment biodegradability. The Fenton reaction is shown
in Eqs. (4) to (13). At acidic pH it leads to the production of ferric ion and
hydroxyl radical (Ishak
and Malakahmad, 2013):

H2O2 + Fe2+ ? Fe3++ •OH + OH-

(4)

Fe3+ + H2O2 ? Fe-OOH2+ + H+ ? •H2O
+ H+

(5)

Hydroxyl radicals may be scavenged
by reaction with another Fe2+ or with H2O2:

•OH + Fe2+ ? OH? + Fe3+ 

(6)

•OH
+ H2O2 ? H­O2 • + H2O

(7)

Hydroxyl
radicals may react with organic and starting a chain reaction:

•OH
+ RH ? H2O
+ R• (RH=organic substrate)

(8)

R•
+ O2 ? ROO• ? products of
degradation

(9)

Ferrous ion and radicals are produced during the reactions:

H2O2 + Fe3+ ? H+ + FeOOH2+

(10)

FeOOH2+ ? HO2• + Fe2+

(11)

HO2• + Fe2+ ? HO2? + Fe3+

(12)

HO2• + Fe3+ ? O2 + Fe2++
H+

(13)

Ishak and Malakahmad
(2013) showed
that Fenton process is able to augment the biodegradability of refinery
wastewater as a pretreatment for recalcitrant contaminants. Studied operational
parameters were reaction time (20 – 120 min), H2O2/COD (2 – 12) and
H2O2/Fe2+ (5 – 30) molar ratios. They
determined that BOD5/COD as an index of biodegradability of
wastewater increased from 0.27 to 0.43 under optimum conditions of operational
parameters, including reaction time (71 min), H2O2/COD (2.8) and H2O2/Fe2+
(4) molar ratios: the process was optimized using response surface methodology
based on a five-level central composite design.

In addition to low biodegradability of petroleum refinery
wastewater, the higher concentration of COD in characterized refinery
wastewater is because of presence of some compounds such as phenols and
sulfide. So, such wastewater with low BOD and high COD is consider as low
biodegradability wastewater (Metcalf and Eddy,
2003). Moreover, considering high concentration of some contaminants
including oil and grease; Benzene and Toluene as PHCs; Ethylbenzene and Xylene
as aromatic hydrocarbons, it could be implied that the petroleum wastewater or
other oily wastewaters containing biorecalcitrant contamination or heavy metals
requires pretreatment before application of any biological decontamination (Ishak and Malakahmad, 2013).

According to Ishak and Malakahmad
(2013), although the range of time factor in Fenton process was from 20
to 120 min, the results revealed that in the first 20 minutes of the Fenton
reaction, more than 90% of COD and BOD removal was achieved. Also, BOD5/COD
ratio of 0.40 was attained within 20 minutes. This finding, shows very short
period of time required for a significant biodegradability improvement and
pollution reduction in a Fenton process which is of special interest in the
industrial application of Fenton’s reagent: hydroxyl free radicals
bear a the short half-life, so the extension of reaction time
does not improve degradation.

Even though by increasing of H2O2
concentration better organic degradation will be attained due to more
generation of more hydroxyl radicals (Kang and
Hwang, 2000), at a certain limit, the complete organic removal could not
be obtained even with higher than stoichiometric quantity of H2O2/COD
and this eventually led to reducing the removal efficiency. Generally, it means
biodegradability declined after increasing H2O2/COD molar
ratio to more than 2 (Ishak and Malakahmad, 2013).

Regarding the third studied influential factor, i.e. H2O2/Fe2+,
it has been verified that both peroxide dose and iron concentration (Fe2+)
are influential factors in the Fenton reaction for better degradation
efficiency  and reaction kinetics,
respectively (Kavitha and Palanivelu, 2005;
Siedlecka and Stepnowski, 2005). In that experiment, decrease of H2O2/Fe2+
molar ratio (i.e. higher concentration of Fe2+) caused more
biodegradability and higher removal of the target compound and formation of
early intermediates, i.e. generating more hydroxyl radicals for the degradation
process (Catalkaya and Kargi, 2007; Ishak and
Malakahmad, 2013). Excessive amount of Fe2+ competes with the
organic carbon for hydroxyl radicals when high Fe3+ concentration is
used.

(III) 
Electrocoagulation

Some
electrode materials like iron, aluminum, boron-doped diamond, platinum-iridium,
and titanium-rubidium have been testified for treating varied types of oily
wastewater so far. In electrocoagulation processes more current density (mA/cm2),
electrolysis time, salinity cause more removal of turbidity, COD, TSS,
contaminants (such as sulphate, phenol, etc.). However, several investigation
have tried to evaluate the the optimum amount of operational parameters
including current density, electrode
materials, the distance and arrangement of electrodes, reaction temperature,
initial pH, voltage, effluent concentration, salinity (NaCl)  (Xu
and Zhu, 2004). Moreover, Santos et al. (2006) applied full-scale
electrocoagulation reactor for organic components removal from oily wastewater,
consequently about 57% COD removal was achieved after 70 hr reaction.

Fouad (2014) did a study to separate cottonseed oil from oil–water emulsions
using an electrocoagulation method, where the power consumption was calculated.
He reported that the power consumption increased from 0 to 0.9 kW/kg-oil
removed when the current density increased from 0.0009 to 0.02 A/cm2.
Moreover, he showed that the oil removal percent was higher with lower sodium
chloride concentration. Removal was 90% with fresh water containing 85 mg/L
NaCl, whereas, with seawater containing 3.5% NaCl, the oil removal was 80%.
However, higher concentrations of NaCl were preferred as the power consumption
was lower for seawater with 3.5% NaCl, 0.017 kW/kg-oil removed compared with
fresh water containing 85 mg/L NaCl, 0.022 kW/kg-oil removed (Fouad, 2014).

Among electrochemical oxidation
processes, electrocoagulation has been found to be more effective than many
other treatment technologies for heavy metal removal from oily wastewater,
because it requires no addition of chemical compound, low capital cost, and
enhances the settling of the oily sludge produced (dos Santos et al., 2014). However, electrocoagulation requires high operating cost because
of the electrical energy requirement. Apart from this, there is a release of
high quantities of metals into the oily sludge produced, thereby making the
sludge more hazardous and creating another environmental concern.

(IV) 
Electroflotation

The removal
of finely dispersed oil from oil-water emulsions of different Egyptian oil
crudes by either batch wise or continuous processes was analyzed to identify
the effect of various operating and design parameters under experimental
condition in which electroflotation cells had been equipped with a set of
electrodes mounted in the cell bottoms (Ibrahim et al., 2001). The recommended conditions for
operating batch runs were current density from 5 to 20 mA/cm2,
temperature from 30 to 40°C, and pH =6. Achieved data from continuous runs
demonstrated that, at almost complete separation of oil, the minimum power
consumption was 0.08 kWh/m3 of a 200 mg/L emulsion flowed at 300
mL/min (Ibrahim et al., 2001).