Three 50-L clean and sterile plastic containers were used to collect raw EORW samples from three discharge points at Egharevbe oil mill in Ebuobanosa-Benin, Nigeria (N6˚20’1.32” E5˚36’0.53”). All chemicals/ reagents used, including commercial titanium dioxide (TiO2 ) 99.5% anatase, were of analytical grade and purchased from Sigma-Aldrich Co. Ltd (Gillingham, UK), and Qualikems Fine Chemical Ltd (Vadodara, India).
The EORW samples were preserved at 4 °C in a refrigerator before treatment. Before commencing treatment, the EORW samples were removed from the refrigerator and held at room temperature (28±2 °C) for 2 h. The raw EORW samples were prepared into different initial concentrations by diluting them with deionised water. The initial concentration range was 100 mL/L to 250 mL/L. We prepared the TiO2 -EORW solution at different concentrations ranging from 0.5 g/L to 1.5 g/L.
We used a spectrophotometer 910 model; multi-meter tester 2010 model, HACH colourimeter 402 model, pH meter 3010 model dissolved oxygen meter HI 981193, BOD/COD/total organic carbon meter (Aquadax), and total dissolved solids meter H18734 to carry out analyses of parameters.
Characterisation of raw edible oil wastewater The physicochemical analyses of the raw, control and treated EORW samples were carried out using AOAC40 methods.
Photocatalytic degradation studies
Photocatalytic studies were performed with a slurry batch reactor. This reactor had a triple jacketed flow through a twin reactors system (Model: MS-H280-Pro). Lelesil Innovative Systems manufactured in India collaborate with the Small Scale Research Group, Faculty of Engineering, University of Benin, Benin City, Nigeria. The reactor system consists of two 5-L flow-through reactors. These are the primary reactor A, inside the photocatalytic reaction chamber, and the secondary reactor B outside. Also present are a peristaltic pump, hot plate with a magnetic stirrer, central jacket for UV lamps, and timer control digital clock. Hence, variables such as flow rate, temperature, agitation speed, UV irradiation, and irradiation time can be measured.31 The photocatalytic degradation studies were started by transferring the thoroughly mixed 0.25 g/L TiO2 -EORW solution into reactor B and connecting it to reactor A. The flow meter and magnetic stirrer were set at 100 mL/min and 900 rpm, respectively. We exposed reactor A to a 250- W mercury UV lamp, which was the source of UV light, and switched on the reactor system for 30 min. At the end of the reaction time, we collected the treated EORW samples, and centrifuged them at 5000 rpm for solid-liquid separation. A 200-mL supernatant was collected and used to carry out BOD5 and COD analysis. A similar procedure was used for the control, which was EORW samples without the TiO2 catalyst. Moreover, we optimised the photocatalytic degradation process of EORW by determining the effects of the initial concentration of EORW, catalyst dose, agitation speed, and irradiation time on the photocatalytic degradation of EORW. The same procedure was followed in each case, varying one of the four variables each time: 100 mL/L or 250 mL/L, 0.25 g/L or 1.5 g/L, 300 rpm or 1500 rpm, and 20 min or 90 min, for initial concentration, catalyst dose, agitation speed, and irradiation time, respectively. The BOD5 and COD for these were subsequently analysed at the end of each experiment.
We characterised the raw, control, and treated EORW samples by carrying out analysis of BOD5 , COD, total dissolved solids, dissolved oxygen, phenol, total suspended solids, Cl- , SO4 2-, oil and grease and PO4 3- using standard methods of water analysis.41 We calculated the reduction or performance efficiency (E) of pollutant removal from EORW using Equation 1: E = 1- cf / ci X 100 where Cf is final concentration, Ci is initial concentration, and E is reduction efficiency.
Each experiment was done in triplicate and the mean and standard deviation (s.d.) of n=3 replicate results were recorded The data were analysed to determine significant differences using the Kruskal–Wallis H-test or one-way analysis of variance by ranks using Statistical Package for Social Sciences (SPSS) version 20 with a significance level of p=0.05.
Edible oil refinery wastewater (EORW) is one source of environmental pollution in Nigeria. The treatment of EORW before discharge into the environment remains a significant challenge in the edible oil refinery industries. This research was aimed at photocatalytic treatment of EORW using a batch photocatalytic reactor with titanium dioxide photocatalyst. We investigated the physicochemical parameters: chemical oxygen demand (COD), biological oxygen demand (BOD5), oil and grease, phenol, chloride (Cl-), total suspended solids, sulfate (SO42-), and phosphate (PO43-) using American Public Health Association methods. The results showed that the reduction efficiency of the treated EORW with TiO2 catalyst ranged between 65.8% (PO43-) and 87.0% (COD), and the improvement in efficiency was 54.1% (pH) and 60.8% dissolved oxygen. However, the results showed no significant difference (p<0.05) in the control treatment without catalyst. The biodegradability of EORW increased from 0.196 to 0.32. It was observed that the optimum values were an initial EORW concentration of 100 mL/L, irradiation time of 90min, catalyst dose of 1.25 g/L, and an agitation speed of 900 rpm. The kinetics of the photodegradation process was well described by the pseudo-first-order equation (R2>0.96) and pseudo-second-order equation (R2>0.98). The intra-particle diffusion model fairly represented the diffusion mechanism with an R2 value of 0.806. The treated EORW met the most acceptable water quality standards for discharged effluent according to the maximum permissible limits of the Nigerian National Environmental Standards and Regulations Enforcement Agency.