The need for water for
domestic, industrial and agricultural purposes increases accordingly with
population growth and economic development. The liquid wastes originating from
production and consumption activities by humans primarily cause environmental
problems and quality deterioration when they are discharged into receiving
water bodies such as rivers, lakes, seas and groundwater. Specifically, water quality
problems are associated with municipal and industrial wastewater discharges. Industrial
wastewaters are the discharge from industrial plants and manufacturing
processes. Industrial discharges may consist of very strong organic wastewaters
with a high oxygen demand or contain undesirable chemicals that can damage
sewers and other structures. The
wastewater must be fully treated before being allowed to flow into a river and,
if necessary, pre-treated before flowing into a sewer. The reason for the pre-treatment
of a waste is to protect man’s health, the sewer system, the receiving
environment and animals living in it (Türkman and Uslu, 1991).


techniques for industrial wastewater treatment mainly include membrane
filtration, adsorption, coagulation, biological treatment and biofiltering.
Even with use of these technologies it is still difficult to achieve effluent
qualities that satisfy discharge standards (Suty, de Traversay and Cost, 2004). Since various physical and chemical pretreatment
techniques produces only limited improvement to the biodegradability of such
wastewater and biological systems are not able to handle high strength
wastewater, Advanced Oxidation Process (AOP) is sort after as it is one of the
most promising treatment processes, specifically ozonation. Ozone has a high oxidation potential and has been
widely used for disinfection and the removal of organics for water and
wastewater treatment (Khadhraoui et al., 2009; Staehelin and Hoigne, 1982; Camel
and Bermond, 1998).
The present ozonation
treatment process is limited by low ozone dissolution and a slow mass transfer
rate, leading to low utilization efficiency of gaseous ozone and thus high
operation costs (Chu et al., 2008). Microbubble wastewater treatment has become increasingly attractive
for its small bubble size (less than 50?m), huge interfacial area, long stagnation time, lower bubble rising
speed, and high interior pressure (Agarwal, Ng and Liu, 2011).In this
study, catalytic microbubble ozonation was used to treat a petrochemical
wastewater, SS wastewater, having excessive amounts of Total Dissolved Solids
(TDS) and Chemical Oxygen Demand (COD). Ozone is being used with a catalyst, Granular Activated Carbon (GAC), to generate hydroxyl radicals, which has
more oxidation potential than ozone and readily reacts with organic compounds
such as inactivated aromatics (Khuntia,
Majumder, & Ghosh, 2016). The objective of this research is to
investigate the efficiency of catalytic microbubble ozonation by optimizing
various parameters, namely effects by coagulant addition, catalyst addition and
pH changes. The
efficiency was investigated in terms of COD removal.The experimental apparatus is shown
in Figure 1. The reactor was prepared with a beaker filled with 200 ml of wastewater.
Compressed pure oxygen from an oxygen tank was supplied to an oxygen
concentrator (NewLife Intensity 10, Lifeline
Corporation Pte. Ltd., Singapore) and fed to an ozone generator (SGL-50G,
ProMedUSA Pte. Ltd., Singapore) with an
output rating of 100% and finally bubbled into the reactor at a rate of 1 L/min.
An air stone attached to the tip of a gas bubbler was used to generate the
microbubbles. Microbubbles were constantly generated in the wastewater for 4
hours; samples were taken every hour and analyzed for COD content. Due to the
detrimental properties of ozone to humans upon exposure to copious quantities,
the experiment was conducted in a fume hood to remove the excess ozone to
mitigate the effects of ozone on the human health.The COD removal efficiency was measured via the
percentage removal of COD. The make-up of the reagents in the digestion vessel are
1.5 mL digestion solution (K2Cr2O7), 3.5 mL
AgSO4-H2SO4 solution and 2.5 mL of 500 times diluted
extracted sample. The vessels were refluxed for 2 hours in a digital reactor
block (HachUSA, n.d.) at 150OC
and left to cool to room temperature after. The absorption was measured using a
wavelength of 600nm in the DR6000 spectrophotometer (HachUSA, n.d.) for each sample. The spectrophotometer was
calibrated to zero using a blank sample.  The percentage of COD removal can be
determined from Equation (1):

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% of COD Removal =    ———-  

Where CODt and CODo
are the COD values at time = t and 0 respectively. The COD analysis method
adheres to the standards and procedures outlined by ASTM International (2012).To determine the amount of residual dissolved ozone
present in the reactor, the spectrophotometric volumetric method was used. The
make-up of the reagents in the reaction vessel are 7 mL of ultrapure water, 1
mL of Indigo solution and 2 mL of extracted sample. The samples were analyzed
in the spectrophotometer with a wavelength of 600nm. The amount of residual
ozone in the sample can be calculated using Equation (2):

mg/L =      ———-   (2)

Where:  = difference in absorbance between sample and
blank, b = path length of cell, 1 cm, V = volume of sample (normally 90 mL) and
f = 0.42. The factor f is based on a sensitivity factor of
20,000/cm for the change in absorbance (600 nm) per mole of added ozone per

Alternatively, residual
dissolved ozone can be calculated using Equation (3) based on theoretical output of an ozone generator
using oxygen feed gas (A2Z Ozone, n.d.):

mg/L =  

Where: LPM = flowrate of
ozone in litres per minute, O3 % = concentration of ozone generated
by weight by the ozone generator.A process flow was
established to treat the petrochemical wastewater to produce the highest
removal efficiency from ozonation. Firstly, coagulation was done to lower the
high COD content in the wastewater (approximately 30 – 50% as seen from the
initial values in Graph 1) to improve
the efficiency of the ozonation process and the amount of coagulant added was
optimized. During ozonation, a catalyst was added and the amount of catalyst
added was optimized followed by a variation of the working pH of the wastewater
during ozonation. Similarly, working pH of the wastewater was optimized. The
following sections discussed in detail the processes mentioned.FeCl3
was used as the coagulant and was added in varying amounts ranging from 23.3g/L
to 83.3g/L. The solution was allowed to coagulate for 15 mins while being
stirred gently with a magnetic stirrer bar before undergoing vacuum filtration through
a 40-micron glass fiber filter to obtain the filtrate. The filtrate was bubbled
with ozone next. From graph 1, 83.3g/L has the best removal efficiency of 90.1% however it is not practical to
use such a large quantity of coagulant as a great amount of foaming was
observed and much of the solution was lost. Hence, a moderate quantity, 50g/L,
of coagulant with a removal efficiency of 55.2%
was chosen.Utilizing the filtrate that had undergone coagulant
optimization, ozonation was carried out with GAC added into the wastewater. Varying
amounts of GAC, ranging from 5g/L to 20g/L, were used and kept in suspension by
a magnetic stirrer bar. From
graph 2, 10g/L showed the highest COD removal of 55.2% in the wastewater. Hence, 10g/L of catalyst was chosen as the
optimized amount.The pH of the wastewater was adjusted
using hydrochloric acid solution (1M HCl) to reduce the pH or sodium hydroxide
solution (1M NaOH) to increase the pH. pH was measured using a laboratory pH
meter (Metrohm,
n.d.). From graph 3, wastewater treated at pH 11 was the most promising with a
removal efficiency of 49.4%.
However, pH 7 had a removal efficiency of 48.4%,
which was very close. Therefore, it was practical to treat the wastewater at pH
7 since it the pH after coagulation is relatively close to pH 7 and does not
require the addition of NaOH.The optimal conditions to obtain the highest efficiency of COD removal
for SS wastewater are the addition of 15g/300mL of FeCl3 coagulant,
addition of 10g/L of GAC catalyst and maintain the pH of the wastewater at 7.
However, the COD values for the wastewater are still currently too high for the
discharge to public sewers in Singapore. Hence further treatment is required
for SS wastewater. Electrolysis was suggested as the next treatment step. The
experimental setup and procedures are being formulated and in progress.Meanwhile, another type of wastewater will be treated using the same
method to verify the removal efficiency of catalytic microbubble ozonation.
This other wastewater, H1 wastewater, is rich in ammonia content and has a
third of the COD content as SS wastewater. Conditions for H1 wastewater are
similar in nature to SS wastewater as it cannot be treated by biological
process directly and it is of high strength.