Issues of water scarcity and how it can be curbed continues to be a challenge to the world. There is continuous aggravation of water pollution due to population growth and its corresponding effects on industrialization, agriculture, and domestic activities. This has led to radical exploration of alternative and sustainable means of obtaining water from unconventional means, such as water re-use through wastewater treatment [1
]. Because wastewater production is inevitable, wastewater treatment for re-use has become the most convenient way of stemming water scarcity. One of the major industrial water pollution sources is the oil refinery industry. Reports indicate that for every 1 barrel of oil processed, about 10 barrels of oily wastewater is produced. In addition, large volumes of oily wastewater is generated during transport and refinery of oil [2
]. Apart from salts being the highest in concentration (1000–3500 mg/L) of oily wastewater, organic compounds follow with a concentration range of 15–1500 mg/L, then oil and grease ranging from 2 to 565 mg/L [3
]. Consequently, high levels of chemical oxygen demand (COD), total suspended solids (TSS), total dissolved solids (TDS), soap oil and grease (SOG), and traces of metals are found in the oily waste streams, which make their treatment problematic and cause huge pollution problems [4
Over the years, several treatment methods have been employed to treat oily wastewater. These include membrane filtration, coagulation-flocculation, hydro-cyclones, and biological treatment systems, among others [5
]. With a focus on organic component removal, biological systems have been preferred due to their low cost of operation and eco friendliness [7
]. However, biological treatment processes have shown limitations at high contaminant concentration. In addition, some recalcitrant organic pollutants (such as inert COD) show high resistance to biological degradation processes [8
With advancements in research, advanced oxidation processes (AOPs) are being explored for many wastewater treatment applications and to fill the gaps left by conventional treatment processes. AOPs are chemical processes that make use of hydroxyl radicals to effect oxidation [9
]. Figure 1
shows a brief description of contaminant degradation in an AOP (photo-catalysis). Photo-catalysis is essentially the activity that occurs when a light source interacts with a semiconductor metal oxide [9
]. During photo-catalysis, an incident light (UV) causes excitation on the surface of a photo-catalyst. This happens when electrons in the valence band absorb energy higher than their band gap energy and therefore move into the conduction band. When this happens, a hole (h+
in the valance band) and electron (e−
in the conduction band) are produced simultaneously. These two species (e−
) then either recombine later or they react with oxygen molecules to form peroxide or with water molecules to form hydroxyl species. The peroxide and hydroxyl formed then attack and degrade organic pollutants into more manageable compounds such as water or carbon dioxide [10
Photo-catalysts such as TiO2
have been used extensively in wastewater treatment due to their ability to function at ambient temperature, their accessibility, cost effectiveness, and their ability to oxidize most organic contaminants to safer and more manageable compounds such as CO2
]. In an experiment to treat olive mill wastewater, El Hajjouji et al. [13
] used a TiO2
/UV system to degrade 94% phenol over a 24 h period with 1 g/L of the catalyst. In a similar study, Chatzisymeon et al. [14
] studied the effects of operating parameters, including initial organic load of influent, contact time, and concentration of TiO2
in media on the removal of COD, total phenols, and color from black table olive processing wastewater. The findings showed that the efficiency of the TiO2
photo-catalysis increased with increasing contact time, catalyst dosage, and decreasing initial organic load.
Zeolites are mainly known for their peculiar ion-exchange, molecular sieving, and adsorption abilities [15
]. Zeolites have been applied in diverse ways in water purification. They have been applied in heavy metal removal from wastewater [17
] and for dye removal for textile effluent [19
]. Zeolites act as photo-catalysts when the zeolite framework is photo-activated or incorporated with other materials such as semiconductors [21
]. Their unique ability of promoting stabilization of photochemically generated redox species and controlling charge transfer and electron transfer processes makes them worth exploring for many photo-catalytic applications [22
In this study, the efficiencies of zeolite (photo-activated) and TiO2
were studied and compared on the basis of the removal of COD and SO42−
from oily wastewater in a photo-catalytic system. These two contaminants are higher than the permitted discharge limits of 1000 ppm (COD) and 500 ppm (SO42−
), respectively [1
]. Three operating parameters were varied catalyst dosage (0.5–1.5 g/L), reaction time (15–45 min), and mixing rate (30–90 rpm). Response surface methodology (RSM) was used to optimize and study the interactions between the operating parameters. The effluent used for the experiment was collected from a local oil refinery effluent treatment plant in South Africa.
4. Optimum Conditions
Numerical optimization was done to determine the optimum conditions of the three parameters for contaminant removal. The numerical optimization technique explores the entire design space on the basis of the developed models to detect the optimum factor conditions for the given range. Equations (2)–(5) served as the objective functions, whereas the three independent variables served as constrains. These constrains were set within the given range. The goal for the optimization was to achieve maximum contaminant removal.
represents a ramp graph showing the optimum conditions for the operating parameters and the desirability obtained from the zeolite experiment. As can be inferred from the graph, to achieve maximum COD removal of 92% and SO42−
removal of 87%, all operating parameters were at the maximum with a desirability of 91.7% contaminant removal. This translates into maximum time and energy requirements to achieve the set goal of contaminant removal for the given range of factors.
shows the ramp graph for the optimum conditions of the operating parameters for the TiO2
experiment. Unlike zeolite, a catalyst dosage of 1.5 g/L, reaction time of 15 min, and mixing rate of 30 rpm achieved a removal efficiency of 91.21% of COD and 85.5% removal efficiency of SO42−
, leading to a desirability of 91%, which translates into lesser energy and time requirements for achieving maximum contaminant removal for the given range of factors.
shows a comparative study of work done by other authors in photo-catalytic degradation of petroleum refinery effluent. It can be seen that much focus was given to only the organic (COD) aspects of the petroleum refinery effluent without consideration for the possibility of removing other inorganics such as salts simultaneously. This study however considered the possibility of removing both organics (COD) and inorganics (SO42−
) simultaneously from oil refinery effluent. The conditions of the experiment and the results obtained in each case are shown in the table.