Proceedings of the 10th International Conference on Environmental Science and Technology Kos island, Greece, 5 – 7 September 2007 ADVANCED TREATMENT BY OZONATION AND SONOLYSIS FOR DOMESTIC WASTEWATER REUSE H. SELCUK1, V. NADDEO2, L. RIZZO2,* and V. BELGIORNO2 1 Department of Environmental Engineering, Pamukkale University, 20017, Kinikli, Denizli, Turkey, 2 Depart. of Civil Engineering, University of Salerno, Via Ponte don Melillo, 84084 Fisciano (SA), Italy; *corresponding author, e-mail: l.rizzo@unisa.it EXTENDED ABSTRACT In many countries, especially in arid and semi-arid areas, because of the leak of natural water resources the wastewater reuse is an important integrative/alternative resource for not drinking uses (such as urban, agricultural and industrial). Disinfection is one of the most important step for the reuse of treated domestic wastewater to eliminate infectious diseases and chlorination is for sure the most used disinfection process. Unfortunately, the formation of hazardous chlorination by-products restrict the usage of chlorine for the disinfection of wastewater. In Italy a very stringent limit for chlorination by products such as trihalomethanes (30 μg/L) was set for wastewater reuse. Accordingly, the use of alternative oxidation/disinfection systems should be evaluated as possible alternative to chlorine. Ozone is a powerful disinfectant for the inactivation of a wide range of microrganisms as well as oxidizing a wide range of organic and inorganic pollutants. Recently ultrasounds were found to be effective as pre-treatment for wastewater disinfection by UV irradiation as well as the combined affect of ozone and ultrasounds provided good efficiency for disinfection of well water. In this context, we investigated the advanced wastewater treatment by ozonation and ultrasounds in terms of bacteria inactivation (in particular Escherichia coli) as well as colour removal (as UV absorbance at 436 nm, UV436). Domestic wastewater samples taken from the effluent of wastewater treatment plants, were used in ozonation and sonolysis experiments. The disinfection efficiency by ozone increased from 68.6% to 97.1% as contact time increased from 3 to 15 min respectively. According to the investigated conditions (max ozone dose: 28 mg/(L min); max contact time: 30 min), the ozonation process alone was not enough to decrease the E. coli colonies under the limit set for wastewater reuse in Italy (10 CFU/100 mL). Sonolysis improved ozonation process; after 15 min contact time E. coli colonies decreased under 10 CFU/100 mL. Moreover, ozonation process was effective for colour removal; the UV436 decreased from 13 to 41% as contact time increased from 3 to 15 min. On the opposite, because the sonolysis process alone increased colour, not significant differences in terms of UV436 were observed comparing ozonation and combined sonolysis/ozonation process. Keywords: Escherichia coli, colour removal, ozonation, sonolysis, wastewater reuse. B-771 1. INTRODUCTION In many countries, especially in arid and semi-arid areas, because of the leak of natural water resources the wastewater reuse is an important integrative/alternative resource for not drinking uses (such as urban, agricultural and industrial). Moreover, in some countries in order to meet the very stringent limits set for wastewater disposal, the upgrading costs of wastewater treatment plants (WTPs) are so high that wastewater is too valuable to be wasted, so the wastewater reuse seems to be an attractive alternative option. On the other hand, wastewater reuse regulation might be stringent as well. Disinfection method is one of the most important part for the reuse of treated domestic wastewater to eliminate infectious diseases. Recently hazardous chlorination by-products restricted the use of chlorine for the disinfection of wastewater (De Feo et al., 2006). In Italy a very stringent limit for chlorination by products such as trihalomethanes (30 μg/L) was set for wastewater reuse. Accordingly, the use of alternative oxidation/disinfection systems should be evaluated as possible alternative to chlorine. Ozone is a powerful disinfectant for the inactivation of a wide range of microrganisms as well as oxidizing a wide range of organic and inorganic pollutants. In order to improve oxidation/disinfection efficiency and decrease contact time as well as applied doses (which can increase both the treatment costs and formation of undesirable by products), ozone was also used together with UV radiation, H2O2, and more recently ultrasounds (Jyoti and Pandit, 2004). Ultrasounds produce strong cavitation in aqueous solution resulting in formation of shock wave (Parlitz, et al. 1995) and reactive free radicals (Hua, et al. 2000) because of the violent collapse of the cavitation bubbles. These effects should contribute to the physical disruption of microbial structures (Radel, et al. 2000) and inactivation (Mason, et al. 2003, Naddeo et al. 2005) as well as the decomposition of toxic chemicals (Joyce, et al. 2003). The physical-chemical water characteristics significantly contribute to ultrasound disinfection efficiency (Futura, et. al. 2004), and in any case, the implosion of the cavitation bubbles can drastically increase the water turbidity (Naddeo et al. 2007). Recently ultrasounds were found to be effective as pre-treatment for wastewater disinfection by UV irradiation (Neis and Blume, 2002) as well as the combined affect of ozone and ultrasounds provided good efficiency for disinfection of well water (Jyoti and Pandit, 2004). In this context, we investigated the advanced wastewater treatment by ozonation and ultrasounds in terms of bacteria (Escherichia coli) inactivation as well as colour removal. 2. MATERIALS AND METHODS 2.1. Wastewater samples Domestic wastewater samples were taken from three WTPs located in province of Salerno (southern Italy). The sewage loading of WTP1, WTP2 and WTP3 comes from 8000, 1000 and 500 people respectively. Due to surrounding tourist area, the loading of WTP1 and WTP2 doubles in summer season. The wastewater characteristics of the WTP effluents are shown in table 1. The wastewater are characterized by high fluctuation in several parameters, such as BOD5 (WTP1), COD (WTP1, WTP3), Cl- (WTP1), NH4 and E. coli. The wastewater samples used for ozonation and sonolysis tests were taken downstream of activated sludge process, just upstream of final disinfection step by chlorine. B-772 Table 1: wastewater quality characteristics (average ± standard deviation) of the investigated WTPs Parameter Units WTP1 WTP2 WTP3 pH 7.34 ± 0.22 7.5 ± 0.1 7.7 ± 0.2 TSS mg/L 11.8 ± 6.3 9.9 ± 6.2 13.6 ± 10.2 BOD5 mg/L 35.4 ± 42 12.7 ± 6.1 20.3 ± 11.4 COD mg/L 113.6 ± 186 40.7 ± 23.1 90.2 ± 77.8 Free Cl2 mg/L 0.12 ± 0.04 0.1 ± 0.05 0.1 ± 0.06 SO4mg/L 23.5 ± 11.4 39.6 ± 7.9 21.0 ± 7.4 Clmg/L 168 ± 279 99.9 ± 6.9 62,3 ± 11.3 Total P mg/L 0.82 ± 0.32 1.7 ± 0.7 1.1 ± 0.5 NH4 mg/L 0.84 ± 1.09 3.8 ± 8.5 3.0 ± 4.3 NO2mg/L 0.11 ± 0.15 0.1 ± 0.4 0.1 ± 0.2 NO3mg/L 12.2 ± 5.5 13.8 ± 5.1 10.0 ± 7.5 CFU/100 mL 7682± 10238 500 ± 727 4860 ± 9945 E. coli 2.2. Analysis Membrane filter method (acetate cellulose type filter with 0.45 µm) was used to count bacteria according to Standard Methods (AWWA-APHA-WEF, 1998). Bacterial count was carried out using TBX media (Oxoid) for detection of Escherichia coli (E. coli). The plates were incubated at 44°C for 24 h. The results were expressed in colony forming units per 100 mL (CFU/100 mL). Colour removal was evaluated by means of UV absorbance measurements at 436 nm (UV436); the effect of the processes was investigated in terms of behaviour of UV absorbance spectra too. Absorbance measurements were performed using a λ12 UV-Vis spectrophotometer from Perkin Elmer. Turbidity was detected by HACH turbidimeter (model 2100 N). The applied ozone was estimated according to Standard Methods (AWWA-APHA-WEF, 1998). 2.3. Ozonation tests Ozonation experiments were carried using an ozone generator (Microlab model by Aeraque I.T., Italy) characterized by a maximum mass flow rate of 1.5 g O3/h, maximum gas flow rate of 200 NL/h, and a work pressure of 0.5 bar. The tests were carried out adjusting ozone dose at 28 mg L-1 min-1. Samples were taken at different contact time to be analysed for E. coli and absorbance. 2.4. Ultrasound tests The ultrasound device used was a Sonics VCX-750, equipped with a horn tip of 1.3 cm diameter which is operated at 20 kHz. Electrical power in the range of 6-50 W was applied, and power density ranged from 25 to 200 W/L. The tests were carried out with (TC) and without (NTC) temperature control. The temperature was monitored during the tests. Samples were taken at different sonication time to be analysed for E. coli and absorbance. 2.5. Ozonation-ultrasound combined tests The ozonation-ultrasound combined tests were carried out putting ozone diffuser and ultrasound probe in the same reactor just in controlled temperature conditions. B-773 3. RESULTS AND DISCUSSION 3.1. Escherichia coli inactivation The disinfection efficiency by ozone increased from 68.6% to 97.1% as contact time increased from 3 to 15 min respectively (Figure 1). 100 E.coli inactivation (%) 90 80 70 60 50 40 30 20 10 0 3 5 7.5 10 15 ozonation time (min) Figure 1: E.coli inactivation by ozone. According to the investigated conditions (max ozone dose: 28 mg/(L min); max contact time: 30 min) and wastewater characteristics, the ozonation process alone was not enough to decrease the E. coli colonies under the limit set for wastewater reuse in Italy (10 CFU/100 mL). The use of ultrasound improved ozonation process after 15 min contact time (Figure 2); in wastewater sample characterized by 700 CFU/100 mL initial E. coli density the combined process ozonation/sonolysis allowed to decrease the E. coli colonies under 10 CFU/100 mL. 100 E. coli inactivation (%) 98 96 94 92 90 88 86 84 82 80 O3 US (TC) US (NTC) US+O3 (TC) Figure 2: E.coli inactivation by ozone, ultrasound (with (TC) and without temperature control (NTC)), ultrasound and ozone simultaneously. However, the experiments showed that the increased temperature due to ultrasound application may significantly effect the disinfection efficiency (Figure 2), so, in order to better understand the influence of temperature, further ultrasound experiments were B-774 carried out using different power densities, with and without temperature control. In Figure 3 the E.coli inactivation results achieved in these sonication tests are plotted; the initial E. coli density was 150000 CFU/100mL. At 15 min contact time, the influence of temperature was significant for densities equal or higher than 100 W/L; in this experimental conditions, the final temperature reached a maximum value of 50°C. At 30 min contact time the temperature effect was significant for all three investigated power densities (the maximum final temperature was 80°C). Under not controlled temperature conditions, the total E. coli inactivation at 15 min contact time was achieved using a power density of 200 W/L. Under controlled temperature conditions the total inactivation was reached at 30 min using the maximum power density (200 W/L). 100 E.coli inactivation (%) 90 80 70 60 50 40 30 25 W/L (TC) 100 W/L (TC) 200 W/L (TC) 20 10 25 W/L (NTC) 100 W/L (NTC) 200 W/L (NTC) 0 15 30 Sonication time (min) Figure 3: E.coli inactivation by ultrasound with (TC) and without temperature control (NTC), using different ultrasound densities, according to contact time. 3.2. Colour removal Since sonolysis process alone increases turbidity (Figure 4), a significant effect on absorbance measurements was detected in sonolysis process as well as in combined sonolysis-ozonation process (Figure 5). The turbidity increases as sonication time and power densities increase. Moreover, the increased kinetic activity due to higher temperatures results in higher turbidity compared to experiments carried out in controlled temperature conditions for all investigated power densities as well as sonication times. 60 Turbidity (NTU) 50 25 W/L (TC) 100 W/L (TC) 200 W/L (TC) 25 W/L (NTC) 100 W/L (NTC) 200 W/L (NTC) 40 30 20 Initial turbidity 10 0 15 30 Sonication time (min) Figure 4: effect of ultrasounds on turbidity with (TC) and without temperature control (NTC), using different ultrasound densities, according to contact time. B-775 Ozonation process was effective for colour removal (Figure 5.a); the UV436 decreased from 13 to 41% as contact time increased from 3 to 15 min. On the opposite, because of turbidity interference the absorbance spectra increased as sonication time increased (Figure 5.b). The efficiency of the combined process sonolysis/ozonation showed a less UV436 decrease compared to ozonation process alone (Figure 5.c); in particular, only 3% decrease after 15 min, and 40% was detected only after 60 min contact time. 0.2 0.18 0 min 3 min 5 min 7.5 min 10 min 15 min UV absorbance (1/cm) 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 200 250 300 a) 350 400 450 wavelength (nm) 500 550 600 0.2 0.18 0 min 5 min 15 min 60 min UV absorbance (1/cm) 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 200 250 300 b) 350 400 450 wavelength (nm) 500 550 600 0.2 UV absorbance (1/cm) 0.18 0 min 5 min 15 min 60 min 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 200 300 400 wavelength (nm) 500 600 c) Figure 5: behaviour of UV spectra for ozonated (a), sonicated (b) and ozonated/sonicated (c) wastewater. B-776 4. CONCLUSIONS In this paper, the use of ozonation and sonolysis as advanced treatment for the reuse of domestic wastewater was investigated in terms of bacteria inactivation (E. coli) as well as colour removal. The results can be summarized as follows: 1. according to the investigated conditions the ozonation process alone was not enough to decrease the E. coli colonies under the limit set for wastewater reuse in Italy (10 CFU/100 mL). 2. sonolysis improved ozonation process; after 15 min contact time E. coli colonies decreased under 10 CFU/100 mL. 3. the effect of temperature in sonolysis experiments was significant. Under not controlled temperature conditions, the total E. coli inactivation at 15 min contact time was achieved using a power density of 200 W/L. Under controlled temperature conditions the total inactivation was reached at 30 min using the maximum power density (200 W/L). 4. ozonation process was effective for colour removal; the UV436 decreased from 13 to 41% as contact time increased from 3 to 15 min. On the opposite, because the sonolysis process alone increased colour, not significant differences in terms of UV436 were observed comparing ozonation and combined sonolysis/ozonation process. Thus the combined process ozonation-sonolysis has got the potential to be a valuable alternative to conventional oxidation/disinfection processes when less expensive solutions such as chlorination cannot be applied because of very stringent limits set by regulations (e.g., trihalomethanes) as well as specific uses advise against to a given oxidant/disinfectant. However, care must be take in setting up the more suitable operating conditions in order to avoid an increase of colour because of sonolysis process. 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