Conventional processes used to treat water and wastewater mainly removes the suspended solids, pathogens and biodegradable organic matter. The majority of persistent organic pollutants are not generally removed by these processes.
Persistent organic pollutants (POPs) constitute a class of anthropogenic substances (manmade) and can be found as trace quantity elsewhere in environment. They are toxic and bio-accumulate in humans, plants, animals, and have significant adverse impacts on human health and the environment, even at very low concentrations. They may cause cancer and disorders in the reproductive and immune systems as well as affecting the human developmental process. POPs do not readily break down in the environment with half-lives in soils in the order of years, although they may be transformed both physically and chemically over long periods of time. They exist in agricultural runoff, drainage to the sewerage system and industrial discharge.
In this study, three organic pollutants were selected for investigation humic acid as natural organic matter (NOM), metsulfuron methyl herbicide as POP, and biological treated sewage effluent (BTSE).
In the first part of the study, removal of humic substance representing NOM was investigated with various types of photocatalytic reactors. The percentage of dissolved organic carbon (DOC) removal with a batch reactor with titanium dioxide (TiO2) as the photocatalyst ranged from 20 to 60 %. When powdered activated carbon (PAC) was added together with TiO2 in the photo reactor, an improvement of more than twice DOC removal was noticed compared with the same amount of TiO2 used alone. From these results, the use of PAC - TiO2 demonstrated superior removal of humic substance within a shorter contact time and higher removal efficiencies compared with using TiO2 alone. Solid phase micro extraction coupled with Gas Chromatography and Flame ionisation detector (SPME/GC FID) equipped with DB-5 column was used to investigate the intermediate photo products during the photo-catalytic reaction. The manner in which intermediate photoproducts evolve and transform was demonstrated by the GC FID peak. The photo reaction can be summarised in the following way. The photo resistant by-products was adsorbed on the PAC-TiO2 surface as shown in GC peak results. From DOC measurements, it is estimated that less than 25 % of the initial material remained. It is noted that during the PAC-TiO2 batch process humic substance was removed immediately without forming a large amount of intermediate macromolecules of humic substance.
In the photocatalysis continuous reactor, the humic substance removal efficiency was studied at different detention time (different flowrates). Better results were achieved at longer detention times as there was more contact time. When the PAC was added, the results also indicate that the photo-catalytic adsorption hybrid system removed a significant amount of humic substance (80% DOC removal) within a shorter contact time compared with using TiO2 alone.
In a recirculated continuous plug flow reactors the factors for controlling removal rates in heterogeneous catalysis are mass transfer and surface reaction controls. These factors were improved when a high recirculation flow rate of 250 mL/min was used where flow is turbulent. When a small amount of PAC was added in addition to TiO2, DOC removal improved to 80% in a shorter operation time of less than 10 minutes. The results with various types of reactors indicate that recirculated continuous reactor gave the highest efficiency for removal of NOM (humic substance) in a shorter detention time.
In the second part of the study, the removal efficiency of metsulfuron methyl representing persistent organic pollutants (POPs) was studied. Batch reactor experiments conducted with different doses of TiO2 and a small amount of PAC of 0.05 g/L revealed that the TOC removal efficiency can be significantly increased up to 80%. Further, the concentration profile and the rate constant showed superior photocatalysis performance in the presence of PAC. The PAC added during the photo-oxidation absorbed the intermediate compounds and thereby promoted the photocatalytic oxidation. The photooxidation with a detention time of 0.5 to 2 hours resulted in intermediate products of smaller molecular weight substances. In this study, a detailed analysis with SPME/GC (solid phase micro extraction/gas chromatography) was made to study on the photo oxidation intermediates. Following 10 min of residence time in the batch reactor the MM partitioned to smaller molecular weight compounds (or substrate) which occurred at different peak times during the GC (12.10, 14.25, 17.40, 19.63 and 20.18 minutes). After 5 hours of residence time in batch reactor, same substrate was found to be degraded. The photo oxidation was faster when activated carbon was used together with TiO2. The substrate that occurred at the peak times of 19.96 and 18.32 minutes during the GC had nearly disappeared, while the peak at time 14.27 minutes was lower. Some anionic by - photoproducts was investigated by using ion-chromatography. Nitrate and nitrite ions were formed as by-photoproducts. The formation of NO3- and NO2- anions occurred was faster when PAC was added to the photo-oxidation. Similarly, SO42- ions form during the photo-oxidation of MM. Where PAC is present in the reactor, the concentration of SO42- ions peaked earlier at approximately 50 min and thereafter reduces its (SO42-) concentration. The reduction in concentration of SO42- after 50 min may be due to a portion being adsorbed on the PAC-TiO2 surface and a portion being transformed to SO2. In this study, the increase in efficiency of MM degradation is similarly attributed to the adsorption of photo-products on the more surface available with TiO2 coupled with the PAC and active sites available to react with the pollutants. This reduces the competitive adsorption on active sites of PAC-TiO2 increasing efficiency of degradation of MM. However, complicated photo-oxidation and by-products occur during these processes, and it is difficult to determine the actual mechanism of photo-catalytic reaction on the PAC-TiO2 surface and the role of active sites because sophisticated instruments are required to do this. Experiments with recirculated continuous reactor were also conducted by using TiO2 and TiO2-PAC. The coupling of PAC with continuous heterogeneous TiO2 photocatalysis leads to a faster degradation of MM than the heterogeneous TiO2 photocatalysis alone. The incorporation of a small amount of PAC of 0.05 g/L with 1.5 g/L of TiO2 led to 78% removal even with a short residence time of 5.25 minutes.
The granular activated carbon (GAC) filter was found to be very effective as a pretreatment for the removal of herbicide (MM). Fixed bed column experiments packed with GAC were conducted with different GAC bed heights (5, 10 and 15 cm) and different effluent velocities. The GAC photocatalytic hybrid system showed up to 90% removal with GAC bed depth of 10 and 15 cm. The 10 and 15 cm deep GAC columns showed a steady state of effluent concentration. The retention time of GAC followed by photoreaction was less than 10 minutes.
Recirculated photocatalytic batch reactor experiments conducted with the biological treated sewage effluent showed effective DOC removal. After start up, with the recirculated flow of 60 mL/min the effluent DOC was reduced by 60% in a period of 180 min, and became relatively stable. There were no large differences between results obtained with various recirculation flow rates. About 70 to 75 % DOC removal was achieved using flow rates of 100 mL/min and 250 mL/min. However, with a recirculation flow of 250 mL/min, DOC removal decreased to 65% down from the 73% DOC removal obtained with that of 100 mL/min rate. This can be explained in terms of the characteristics of the plug flow reactor. The flow rates used in this study were large enough to keep the catalyst in suspension, and to promote good mass transfer between the reactants. When a small amount of PAC (0.05g/L) was added, a complete removal of DOC was observed after 250 and 300 min operation times. The addition of 0.05 g/L of PAC adsorbent to the recirculated continuous reactor facilitated better organic removal than titania photocatalyst alone. DOC removal was further increased to 75% within 30 min of operation.
The membrane photocatalysis hybrid system was used to separate catalyst from the effluent. The membrane flux was very low and fouling was high when TiO2 was tried to be filtered through MF filter. To facilitated TiO2 separation, (i) pH adjustment and (ii) flocculation of TiO2 slurry were used. Although the use of pH adjustment achieved effective improvement to membrane operation, it was 15% less effective than applying a pre-treatment of flocculation of TiO2 slurry. Photocatalysis and flocculation pretreatment processes before MF/UF also resulted in high (over 90%) DOC removal, surpassing those achieved with mixed TiO2 and PAC photocatalyst.