Challenges and Opportunities for the Future
There are some opportunities for achieving benefits from oily
wastewater such as:
the reuse of oily wastewater in steam boilers,
recycling in injection wells for the enhancement of crude oil
opportunities for the sale of the oil concentrate from oily water
treatment to oil recycling companies,
And also, opportunities for the recovery of precious metals from
oily wastewater, especially from petrochemical industries.
While, turning these opportunities
into reality has remained a complex undertaking (Jamaly
et al., 2015).
A great amount of oily wastewater is
discharged into the environment by industries. As a result of the diverse
quantities of the fluctuating compositions of different components in oily
wastewater under real operating conditions, there is no one-size-fits-all
approach for the removal and/or recovery of these components (Jamaly et al., 2015).
Even though, the presence of heavy
metals (i.e. Cr, Cd, Hg, Ag, etc.) in oily wastewater is one of the major
problems militating against the recycling or reuse of oily wastewater because
of their highly hazardous and toxic nature, limited studies concerning the
recovery or removal of heavy metals from oily wastewater has been done.
Furthermore, the presence of heavy metals along with particularly hazardous
chemicals (PHCs) in oily wastewater from petroleum refineries and petrochemical
plants could even result in more deleterious effects when discharged into the environment
(Malakahmad et al., 2011; Rocha et al., 2012).
Heavy Metals Removals and Recovery
Depend on what a petrochemical or
other industrial factories manufacture, the type and concentration of heavy
metals (i.e. Cr, Cd, Hg, Ag, Cu, Ni, Pb, Zn, Al, Fe, Ba, Mn, Sn) and other
contaminants such as PHCs (particular hazardous compounds/chemicals) and PAHs
(polycyclic aromatic hydrocarbons) vary from one factory to another. To be
specific, various technologies have been applied for removing heavy metals from
oily wastewaters and the like.
Usually physicochemical treatments
such as precipitation, filtration, ion exchange, electron-deposition, and
reverse osmosis have been used as conventional methods of heavy metals removal
from aqueous solutions (Ong et al., 2005).
There are some common problems associated with these methods e.g. they are more
costly compared to biological treatment methods and can themselves produce
other waste problems; which has limited their industrial applications (dos Santos et al., 2014).
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.
Biological Treatment of Metals
The application of biological
processes, for treating water and wastewater, is gradually attracting interests
because of reasons including reduced chemicals additives, low operating costs,
eco-friendly and cost-effective alternative of conventional techniques and,
higher efficiency at lower levels of contamination (Ahluwalia
and Goyal, 2007; Gonçalves et al., 2007; Srivastava and Majumder, 2008; Appels
et al., 2010; Malakahmad et al., 2011; Kieu et al., 2011).
Sequencing Batch Reactors (SBRs) are
an attractive alternative to conventional biological treatment methods, due to
their simplicity and flexibility in operation. In addition, in several cases
SBR was used for removing heavy metals, for instance Malakahmad
et al. (2011) removed Hg2+ and Cd2+, Sirianuntapiboon and Hongsrisuwan (2007) studied
Zn2+ and Cu2+ removal, also
Sirianuntapiboon and Ungkaprasatcha (2007) testified removal of Pb2+
and Ni2+ through a hybrid Granular Activated Carbon and Sequencing
Bach Reactor. An SBR is a periodically fill-and-draw reactor, so that five
discrete periods in each cycle are filling, reaction, settling, drawing, and
idle time (Ong et al., 2005). Reactions
start during filling stage and continue during reaction step. After reaction,
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. The time period between the end of the draw and the
beginning of the new fill is termed idle.
The high performance of Sequencing
Batch Reactor (SBR) which was based on aerobic process (like activated sludge)
was shown for treating synthetic petrochemical factory wastewater containing Hg2+
and Cd2+ (Malakahmad et al., 2011).
To detect this, COD, TSS, MLSS (mixed liquor suspended solid), MLVSS (mixed
liquor volatile suspended solid), and SVI (sludge volume index) were considered
for assessment of reactor performance. At maximum concentrations of the heavy
metals, the applied lab-scale SBR removed about 76–90% and 96–98% of Hg2+
and Cd2+, respectively. Whereas, increase of heavy metals
concentration (Hg2+ and Cd2+) led to decrease of COD
removal, as well as decrease of MLSS and MLVSS, the percentages of Hg2+
and Cd2 removal were increased not only because of microbial
adaptation, but also due to biosorption of the sludge. In other word,
percentage reduction of COD removal reveals
the poor performance of microorganisms in the SBR which it was confirmed
through MLSS and MLVSS values during addition of Hg2+ and Cd2+ (Malakahmad et al., 2011). Besides, increasing
rate of SVI indicates decrease in settleability of the sludge (Malakahmad et al., 2011). In several work, the
population of microorganism in SBR system have been detected with which the
significant role of those microorganisms (including Rhodospirilium-like
bacteria (Srivastava and Majumder, 2008; Malakahmad et al.,
2011), Gomphonema-like algae (Ivorra et al., 2002; Malakahmad et al., 2011),
and Sulfate reducing-like bacteria (McGregor et
al., 1999; Malakahmad et al., 2011)) in removing heavy metals from oily
wastewater were proven.
The Rhodospirilium cells, the
sulfate reducing bacteria, and the diatom Gomphonema algae are originally or
mostly anaerobic, and they are able to be adapted to aerobic condition.
While achieved outcomes revealed
that in the SBR removal efficiency of cadmium was higher than mercury, when the
concentrations of mercury and cadmium were increased by magnitude 5 as a
significant shock for the SBR system, for mercury removal efficiency of SBR
reduced sharply to 40%, and for cadmium reduction was decreased slightly to
76%, but due to adaptation of microorganisms with new condition, they could
respectively reach more than 80% and 95% for Hg2+ and Cd2+
after passing several days (Malakahmad et al.,
However, 95% mercury removal and 99%
cadmium removal have been gained by means of the SBR throughout the process
even with considerable shortage of microorganisms. It verifies that in addition
to activities of microorganisms, some portion of heavy metals concentration was
removed from wastewater as a result of biosorption which plays a significant
role in biological treatment of metals compounds confirmed formerly by other
researches (Al-Qodah, 2006; Anayurt et al., 2009;
Sar? and Tuzen, 2009; Tuzen et al., 2009).
Some important notes on Malakahmad et
Testifying a synthetic petroleum wastewater (COD 110 mg/L; urea 33
mg/L; Hg2+ 0.1-9 mg/L; Cd2+ 0.1-15 mg/L etc.). In fact,
analyzing real wastewater would reveal more logical and authentic results to be
considered in industrial sites.
Only two types of heavy metal were studied – mercury and cadmium – which the reason has
been due to the intended petroleum refinery wastewater:
As it has been ascertained that SBR act as both biological process
and biosorption of sludge (Malakahmad et al., 2011),
hence it could be implied that heavy metals bear the potential ability to be
adsorbed, so devising of a novel biopolymer flocculants system as pretreatment
or the like might be helpful in enhancing the performance and performance of
the reactor, even though there sre many researches about the performance of
bioadsoption processes in removing heavy metals (Yan
and Viraraghavan, 2001; Alluri et al., 2007).
Despite high performance of SBR in removal of Hg2+ and
Cd2+ presented by Malakahmad et al.
(2011), none of contaminant removal reached discharge criteria: based on
US EPA (1995) the amount of mercury and
cadmium concentrations of effluent discharge are 0.013 mg Hg2+/L and
0.73 mg Cd2+/L, respectively. To do so, presenting a cutting-edge
technology is required by which not only discharge standards should be met, but
also shocks originated from variation of contaminant concentration should be
Impact of operational factors (such as COD concentration, each
heavy metal concentration, temperature, aeration speed, etc.), as well as the
interaction among factors have been ignored. Finally, the amount of optimal
condition could be evaluated by means of DX 7 (Design Expert Software – version
7), by which the correlation between factors could be achieved too.
do so, the impact of each contaminant concentration on each other, and their
interaction with each other as well as COD removal should be scrutinized
precisely. As it has been found out that by increase of heavy metal to oily
wastewater the percentage of COD removal decrease, and the toxicity and
inhibitory impact of heavy metal on COD removal during biological treatment (Malakahmad et al., 2011),
it would be a good opportunity to discover a better arrangement for removing
heavy metal through a selective pretreatment methods (like biopolymer
flocculants, AOP (electro-fenton reactor)) or removing COD by an anaerobic
reactor such as EGSB, UASB, and so forth, if it is possible to divide the
as a significant shock for the SBR system, when the concentrations of mercury
and cadmium were increased by magnitude 5, for mercury removal efficiency of
SBR reduced sharply to 40%, and for cadmium reduction was decreased slightly to
76%, but due to adaptation of microorganisms with new condition, they could
respectively reach more than 80% and 95% for Hg2+ and Cd2+ after passing
several days (Malakahmad et al., 2011). As Malakahmad et al. (2011) has
illustrated whenever there was a shock during increase of heavy metal
concentration, the performance of reactor for Hg2+ and Cd2+
decrease significantly till the microorganism would be adapted to the situation
after several days. For industrial purposes, that circumstance should be solved
to avoid unwilling discharge of pollutants into environment.
Malakahmad et al. (2011) have considered some variations for operational parameters and
based on those random data the operational condition have been designated. I am
wondering why the authors have not used other useful techniques and software
such as “Design of Experiments” to have more extensive views of experiment and
better outcome analyzing.
Hydraulic Retention Time (HRT 15 day) = Reactor volume (24
L)/Feeding (1.6 L/day) has been mentioned that HRT was 15 days Malakahmad et al. (2011) pp-120. But what does it
mean? I mean how the runs are done?
In this study, the impact of aeration variations in aerobic tank
has been ignored. How the aeration speed has been changed?? From X vvm to Y vvm?
A compressor with capacity of 150 L is vague!
Al-Qodah (2006) removed heavy metal ions from aqueous solutions by activated
sludge. Moreover, Anayurt et al. (2009) and, Sar? and Tuzen (2009) investigated the removal of
Pb(II) and Cd(II) from aqueous solution by green alga (Ulva lactuca) and
macrofungus (Lactarius scrobiculatus) biomasses. And also, lichen
(Xanthoparmelia conspersa) biomass was applied for the removal of Hg2+
from aqueous solution (Tuzen et al., 2009).
126.96.36.199 Hybrid Reactor for Heavy metals Treatment
and Ungkaprasatcha (2007) testified
removal of Pb2+ and Ni2+ through a hybrid Granular
Activated Carbon and Sequencing Bach Reactor.
research, a field pilot plant consisting UASB-constructed wetland systems was
investigated for long-term removal of heavy metals from municipal wastewater (De la Varga et al., 2013). They analyzed the
evolution of heavy metals removal from the water stream over time (over a
period of 4.7 year of operation) and the accumulation of heavy metals in
UASB sludge and constructed wetland sediments at two horizons of 2.7 and
4.0 year of operation. High removal efficiencies were achieved for some
metals in the following order:
Sn > Cr > Cu > Pb > Zn > Fe
(63–94%). Also, medium removal efficiencies were found for Ni (49%), Hg (42%),
and Ag (40%), and finally Mn and As showed negative percentage removals.
According to obtained outcomes, removal efficiencies of total Heavy metals were
higher in UASB (De la Varga et al., 2013).
Removal or Recovery of Heavy Metal from Anaerobic and Aerobic
Sludge by Bioleaching and Biochar
been illustrated to be a feasible method for removing heavy metals from sludge (Xiang et al., 2000; Yoshizaki and Tomida, 2000; Wong et
al., 2002). According to Wong et al. (2002),
the effect of pH requirement for isolated indigenous Thiobacillus
ferrooxidans for bioleaching heavy metals from wastewater sludge
has been studied, because based on fundamental concepts the leaching medium
needs to be pre-acidified to less than 4. They used isolated sludge-indigenous
iron-oxidizing bacteria for the bioleaching experiments to find out the
dissolution behavior of heavy metals, including Zn, Cu, Ni and Cr, from sludge
set at an initial pH (3–7) with the purpose to decline the consumption of
acid. In another paper, bioleaching of
heavy metals from from anaerobically digested sludge using isolated indigenous
iron-oxidizing bacteria has been testified by Wong
et al. (2004), in which the impact of using FeS2 as an energy
source, on the bioleaching of Zn, Cr, Cu, Pb, Ni, as well as nutrients such as
nitrogen and phosphorus. Observations revealed that addition of FeS2
accelerated the acidification of sludge and raised the oxidation–reduction
potential of sludge medium, thus resulting in an overall increase in metal
The removal of
heavy metals from anaerobically digested sewage sludge was studied by using
ferric sulfate and it was compared with the using sulfuric acid at pH 3 in
order to clarify the effect of ferric iron as an oxidation reagent on elution
of heavy metals (Ito et al., 2000). This
research showed that the addition of ferric sulfate to the sludge led to the
acidification of the sludge and the elution of heavy metals from the sludge.
Also, the pH of the sludge decreased through increase of the added iron and
decrease of the sludge concentration. Comparative results revealed that ferric
iron eluted cadmium, copper and zinc more effectively than sulfuric acid. This
effective elution of heavy metals was caused by the oxidation of the sludge
solid by ferric iron added. Therefore, it could be inferred that ferric iron
played a role to acidify the sludge and to oxidize metallic compounds in the
sludge and this new chemical method was useful for removing heavy metals from
anaerobically digested sewage sludge (Ito et al., 2000).
The ability of
two biochars converted from anaerobically digested biomass to sorb heavy metals
using a range of laboratory sorption and characterization experiments was
examined by Inyang et al. (2012). Initial
evaluation of digested dairy waste biochar and digested whole sugar beet
biochar revealed that both biochars were effective in removing a mixture of
four heavy metals (Pb2 +, Cu2+, Ni2+, and Cd2+)
from aqueous solutions. Further investigations of lead sorption by the two
biochars indicated that the removal was chiefly through a surface precipitation
mechanism, which was proved by batch sorption experiments, mathematical
modeling, and examinations of lead-laden biochars samples using SEM–EDS, XRD,
and FTIR (Inyang et al., 2012).
Madani et al. (2015) Case study:
Process: Treatment of
olive mill wastewater using physico-chemical and Fenton processes
Waste water source: effluents
of an olive oil production plant located in Lushan Industrial, Gilan city, Iran
Capacity: Oil processing
capacity of this factory is 60 tons at an average waste effluent of 430,000
L/day when it works.
Type of experiments: Two
types of experiments were performed: physico-chemical treatment and advanced
oxidation studies using the Fenton process. (Detail of this experiments are
explained in (Madani et al. 2015). Chemical
oxygen demand (COD), total phenols, color, and aromaticity were examined to
check the efficiency of the methods.
efficiency of different process in treating OMW.
Figure 1: a)
effect of Fe concentration; T0 = 298 K, pH = 3, H2O2
concentration of 0.5 M, and reaction time of 4 h at a stirring rate of 90 rpm.
b) T0 = 298 K, ferrous iron initial concentration 0.02 M, H2O2
concentration 0.5 M, and reaction time of 4 h at a stirring rate of 90 rpm. c)
T0 = 298 K, ferrous iron initial concentration of 0.02 M and a
reaction time of 4 h at a stirring rate of 90 rpm.
Results: In Acid
cracking experiment 1.5 ml per one liter of wastewater sulfuric acid was required. pH
of 2.5 was the optimum number and lower pH values reduced the experimental time
but did not shows a significant difference in the removal efficiency. However,
at a low pH, more NaOH is required to adjust the pH in other steps of the
experiment to complete treatment, which is not an economic solution. After 60
minutes 47.1, 97, 29.6, 57.4, and 63.2% removal efficiency was obtained in COD,
Turbidity, Total phenol, Aromaticity, and Color respectively.
Table 1 shows the physico-chemical analysis of OMW before
processing and after acid cracking, the Fenton test, and the coagulation test. Acid
cracking and Fenton process showed high efficiency of COD (83%), total phenols
(98.6%), color (77%), and aromaticity (67%) removal from the OMW. As a result
of this study, acid cracking and Fenton process have a significant effect in
reducing the COD and total phenols from OMW.
Figure 1a shows the effect of Fe concentration in COD, total
phenol, aromaticity, and color removal efficiency. It reveals that optimum load
of ferrous iron is 0.02 M which leads to 53, 96, 30, and 16 percent removal
efficiency for COD, total phenol, aromaticity, and color respectively. In
figure 1b the effect of H2O2 concentration in Fe load of
0.02M is illustrated. As can be seen higher efficiency was obtained in 0.05M of
H2O2. In the same type of experiment, the effect of pH
was tested and results are shown in figure 1c. The optimum pH reveals to be at
The conclusions can be drawn from this study in applying acid
cracking, chemical coagulation, and Fenton processes on Olive Mill Wastewater
(1) The acid cracking at the optimum pH 2.5 removed 97, 47, 30, 63,
and 57% of the turbidity, COD, total phenol, color, and aromaticity,
respectively. Acid cracking has dual effect; sulfuric acid is a powerful
oxidant agent that results in oxidizing the OMW component also it can lower the
pH and improves the coagulation process as well. A set of experiments shows
that only 10% of the acid cracking efficiency was due to its oxidation effect,
while the other 90% was related to the coagulation effect
(2) However increasing temperature from 298 to 308 K can increase
the efficiency of phenol removal from 96 to 98%, but does not have a
considerable effect on the efficiency process and only slightly increases the rate
of the reactions.
(3) From the set of experiments it reveals that more than 83, 98.6,
77, and 67% removal of COD, total phenol, color, and aromaticity can be
obtained by applying the combination of acid cracking, chemical coagulation,
and Fenton processes. Consequently, it can be concluded that this combination
of processes can be used as a suitable way to treat OMW.