The need for water fordomestic, industrial and agricultural purposes increases accordingly withpopulation growth and economic development.
The liquid wastes originating fromproduction and consumption activities by humans primarily cause environmentalproblems and quality deterioration when they are discharged into receivingwater bodies such as rivers, lakes, seas and groundwater. Specifically, water qualityproblems are associated with municipal and industrial wastewater discharges. Industrialwastewaters are the discharge from industrial plants and manufacturingprocesses. Industrial discharges may consist of very strong organic wastewaterswith a high oxygen demand or contain undesirable chemicals that can damagesewers and other structures. Thewastewater 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-treatmentof a waste is to protect man’s health, the sewer system, the receivingenvironment and animals living in it (Türkman and Uslu, 1991). Presently,techniques for industrial wastewater treatment mainly include membranefiltration, adsorption, coagulation, biological treatment and biofiltering.Even with use of these technologies it is still difficult to achieve effluentqualities that satisfy discharge standards (Suty, de Traversay and Cost, 2004).
Since various physical and chemical pretreatmenttechniques produces only limited improvement to the biodegradability of suchwastewater and biological systems are not able to handle high strengthwastewater, Advanced Oxidation Process (AOP) is sort after as it is one of themost promising treatment processes, specifically ozonation. Ozone has a high oxidation potential and has beenwidely used for disinfection and the removal of organics for water andwastewater treatment (Khadhraoui et al., 2009; Staehelin and Hoigne, 1982; Cameland Bermond, 1998).The present ozonationtreatment process is limited by low ozone dissolution and a slow mass transferrate, leading to low utilization efficiency of gaseous ozone and thus highoperation costs (Chu et al., 2008). Microbubble wastewater treatment has become increasingly attractivefor its small bubble size (less than 50?m), huge interfacial area, long stagnation time, lower bubble risingspeed, and high interior pressure (Agarwal, Ng and Liu, 2011).In thisstudy, catalytic microbubble ozonation was used to treat a petrochemicalwastewater, 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 hasmore oxidation potential than ozone and readily reacts with organic compoundssuch as inactivated aromatics (Khuntia,Majumder, & Ghosh, 2016).
The objective of this research is toinvestigate the efficiency of catalytic microbubble ozonation by optimizingvarious parameters, namely effects by coagulant addition, catalyst addition andpH changes. Theefficiency was investigated in terms of COD removal.The experimental apparatus is shownin 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 oxygenconcentrator (NewLife Intensity 10, LifelineCorporation Pte. Ltd., Singapore) and fed to an ozone generator (SGL-50G,ProMedUSA Pte.
Ltd., Singapore) with anoutput 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 themicrobubbles. Microbubbles were constantly generated in the wastewater for 4hours; samples were taken every hour and analyzed for COD content. Due to thedetrimental properties of ozone to humans upon exposure to copious quantities,the experiment was conducted in a fume hood to remove the excess ozone tomitigate the effects of ozone on the human health.The COD removal efficiency was measured via thepercentage removal of COD. The make-up of the reagents in the digestion vessel are1.
5 mL digestion solution (K2Cr2O7), 3.5 mLAgSO4-H2SO4 solution and 2.5 mL of 500 times dilutedextracted sample. The vessels were refluxed for 2 hours in a digital reactorblock (HachUSA, n.d.) at 150OCand left to cool to room temperature after. The absorption was measured using awavelength of 600nm in the DR6000 spectrophotometer (HachUSA, n.
d.) for each sample. The spectrophotometer wascalibrated to zero using a blank sample. The percentage of COD removal can bedetermined from Equation (1):% of COD Removal = ———- (1)Where CODt and CODoare the COD values at time = t and 0 respectively.
The COD analysis methodadheres to the standards and procedures outlined by ASTM International (2012).To determine the amount of residual dissolved ozonepresent in the reactor, the spectrophotometric volumetric method was used. Themake-up of the reagents in the reaction vessel are 7 mL of ultrapure water, 1mL of Indigo solution and 2 mL of extracted sample. The samples were analyzedin the spectrophotometer with a wavelength of 600nm. The amount of residualozone in the sample can be calculated using Equation (2):O3mg/L = ———- (2)Where: = difference in absorbance between sample andblank, b = path length of cell, 1 cm, V = volume of sample (normally 90 mL) andf = 0.42.
The factor f is based on a sensitivity factor of20,000/cm for the change in absorbance (600 nm) per mole of added ozone perlitre.Alternatively, residualdissolved ozone can be calculated using Equation (3) based on theoretical output of an ozone generatorusing oxygen feed gas (A2Z Ozone, n.d.):O3mg/L = Where: LPM = flowrate ofozone in litres per minute, O3 % = concentration of ozone generatedby weight by the ozone generator.
A process flow wasestablished to treat the petrochemical wastewater to produce the highestremoval efficiency from ozonation. Firstly, coagulation was done to lower thehigh COD content in the wastewater (approximately 30 – 50% as seen from theinitial values in Graph 1) to improvethe efficiency of the ozonation process and the amount of coagulant added wasoptimized. During ozonation, a catalyst was added and the amount of catalystadded was optimized followed by a variation of the working pH of the wastewaterduring ozonation.
Similarly, working pH of the wastewater was optimized. Thefollowing sections discussed in detail the processes mentioned.FeCl3was used as the coagulant and was added in varying amounts ranging from 23.3g/Lto 83.3g/L. The solution was allowed to coagulate for 15 mins while beingstirred gently with a magnetic stirrer bar before undergoing vacuum filtration througha 40-micron glass fiber filter to obtain the filtrate. The filtrate was bubbledwith ozone next.
From graph 1, 83.3g/L has the best removal efficiency of 90.1% however it is not practical touse such a large quantity of coagulant as a great amount of foaming wasobserved 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 coagulantoptimization, ozonation was carried out with GAC added into the wastewater.
Varyingamounts of GAC, ranging from 5g/L to 20g/L, were used and kept in suspension bya magnetic stirrer bar. Fromgraph 2, 10g/L showed the highest COD removal of 55.2% in the wastewater.
Hence, 10g/L of catalyst was chosen as theoptimized amount.The pH of the wastewater was adjustedusing hydrochloric acid solution (1M HCl) to reduce the pH or sodium hydroxidesolution (1M NaOH) to increase the pH. pH was measured using a laboratory pHmeter (Metrohm,n.d.). From graph 3, wastewater treated at pH 11 was the most promising with aremoval 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 pH7 since it the pH after coagulation is relatively close to pH 7 and does notrequire the addition of NaOH.The optimal conditions to obtain the highest efficiency of COD removalfor 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 thedischarge to public sewers in Singapore. Hence further treatment is requiredfor SS wastewater. Electrolysis was suggested as the next treatment step.
Theexperimental setup and procedures are being formulated and in progress.Meanwhile, another type of wastewater will be treated using the samemethod to verify the removal efficiency of catalytic microbubble ozonation.This other wastewater, H1 wastewater, is rich in ammonia content and has athird of the COD content as SS wastewater. Conditions for H1 wastewater aresimilar in nature to SS wastewater as it cannot be treated by biologicalprocess directly and it is of high strength.