Reduction of excess sludge production by ozonation

Transcription

1 Appendix D Scientific article Reduction of excess sludge production by ozonation Klas Svensson Department of Water and Environmental Engineering, Lunds Institute of Technology, Lund University, Sweden Abstract The literature on reduction of excess sludge production by ozonation is summarized. Two sequencing batch reactor experiments where sludge is ozonated for this purpose are performed. The purpose of these is to evaluate the prospects of using ozonation to reduce excess sludge production in two different kinds of sludge. Urban sewage sludge is used in the first, and paper mill sludge in the second. Both experiments are allowed to run for 12 days, and the chemical changes in the sludge are measured. It is concluded that urban sewage sludge has more potential for this kind of treatment than paper mill sludge. Keywords: Ozonation. Excess sludge. Introduction Activated sludge processes, which are used to treat waste water, generate excess sludge. This has to be removed from the process. Removal and disposal of this sludge is costly and cumbersome, which is why methods of reducing the excess sludge production are being researched. One such method is sludge ozonation. In recent years, some researchers have reported that less or no excess sludge is produced when activated sludge is exposed to small concentrations of ozone. The body of research on this subject is relatively small, and has been summarized in table 1. Table 1: Reduction of excess sludge production in the literature study articles. Article Excess sludge production Yasui et al. (1995) Decreased to zero Sakai et al (1997) Decreased by 66% Kamiya et al. (1998) Decreased by 7% Kaindl et al. (1999) Not measured Scheminski et al.(2) Not measured Déleris et al. (22) Decreased by 7% Ahn et al. (22) Not measured Salhi et al. (23) Decreased to zero Boehler et al. (25) Decreased by 46% Jiang et al. (25) Decreased by 54% Dytczak et al. (25) Decreased. Aerobic: 6.3% Anoxic/Aerobic: 14.7% According to the literature ozone causes bacterial cell walls to become more permeable, which makes the cell swell and burst due to osmotic pressure. Filamentous bacteria seems to be especially susceptible to this kind of disruption. When cells are degraded in this way, chemical changes can be seen in the sludge. Cell content is released into the sludge liquor. Solubilization of organic material increases. The biodegradability of solubilized organics also increases, making them more likely to be mineralized. The sedimentation properties of the sludge improve. Excess sludge production is reduced. Bearing this in mind, two experiments were made in order to evaluate the feasibility of reducing excess sludge production in two different sludges. 47

2 Materials and Methods Sludge from two different waste water treatment plants were used. The first was taken from the Källby plant, which treats sewage from the city of Lund. The second was taken from the pulp mill Södra Cell Mörrum. They were used in one experiment each. 2 cylindrical sequencing batch reactors were set up, each containing 3 L of sludge mixed with waste water in equal proportions. One reactor was used as control. In this reactor, the process was allowed to run its course without ozonation. The other reactor was ozonated (for 4 minutes) twice a day for 12 days at a dose of.15 kg O3/ kg Dry Substance. The reactors were run in a 12-hour cycle consisting of an inflow phase, an ozonation + reaction phase, a sedimentation phase and an outflow phase. 2 cycles were run per day. During each cycle,.5 L waste water was exchanged in both reactors to keep the process going. Analysis led variables were Total Chemical Oxygen Demand (TCOD), Soluble Chemical Oxygen Demand (SCOD), Suspended Solids (SS), Volatile Suspended Solids (VSS), ammonium nitrogen (NH4-N) and the Sludge Volume Index (SVI). 25 ml of sludge solution was taken out of each of the reactors every day. Measurements were made on these samples. TCOD and SCOD was measured with a Dr. Lange test kit (LCK114). TCOD samples were diluted ten times to get it within the test kit range (15-1 mg COD/l), and then put in a cuvette, heated for 2 hours at 148 C, and then measured in a Dr. Lange spectrophotometer. SCOD samples were filtrated before use, and not diluted, but were otherwise measured in the same way as TCOD. NH4-N was also measured with a Dr. Lange test kit (LCK33, range 2-47 mg/l). Whatman glass microfibre filters were used to measure SS and VSS. Samples were vacuum filtered through these. The filters were dried at 15 C, and then weighed to determine SS. To determine VSS, they were dried at 55 C, to allow volatile substances to evaporate. The sludge level after 3 minutes of sedimentation was measured, and used in the SVI calculations. 48

3 Results and discussion SCOD (g/l) Figure 1: SCOD. Källby experiment. and ozonated reactor values. SCOD (g/l) Figure 2: SCOD, Södra Cell Mörrum experiment. and ozonated reactor values. As can be seen in Figures 1 and 2, ozonation definitely increased solubilization of organic material in the sewage sludge, but not in pulp mill sludge. Why the pulp mill sludge wasn t affected isn t clear. The reason could be that pulp mill sludge has a higher content of fibrous material, which may not increase solubilization when oxidized. Too much ozone may be lost in degrading wood fibres that too few bacterial cells are degraded to have an impact on the TCOD and SCOD level. 49

4 14 12 NH4-N (mg/l) Figure 3: NH4-N. Källby experiment. and ozonated reactor values. Figure 3 shows that NH4-N levels increased in the sewage sludge. Furthermore, it increased more in the ozonated than in the control reactor. So it is pretty clear that ozonation caused more NH4-N to be dissolved. In fact, nitrogen levels were so low in the latter case that no reliable measurements could be made at all, since the nitrogen level was out of the measuring kit s range. As in the previous case, this probably means that cells were degraded to a higher extent in the sewage sludge. No mineralization whatsoever could be detected in either of the cases. The ozone dose was probably too low to cause this kind of effect. According to the SS-measurements, no excess sludge was produced in either of the experiments. SS-levels showed sinking trends, probably as a result of changed environmental conditions for the sludge. Comparing the differences between the ozonated and the control reactors, ozonation caused SS-reduction in sewage-, but not in pulp mill sludge. Previous results show that increased solubilization of organic material, which indicates cell degradation, was not detectable in the ozonated pulp mill sludge. In other words, cells are probably not degraded here. This can be the reason why SS-levels aren t affected by ozonation SVI (ml/g) Figure 4 : SVI. Källby experiment. and ozonated reactor values. 5

5 SVI (ml/g) Figure 5: SVI, Södra Cell Mörrum. and ozonated reactor values. Ozonation significantly improved the sedimentation speed of both sludges. All ozonated values are below the control values in Figure 4 and 5. Maybe because ozonation is harmful to filamentous bacteria, which are known to make a sludge hard to sediment in too large concentrations. Conclusions Judging from the results, there is more to be gained by ozonating Källby sludge than by ozonating Mörrum sludge at the used ozone dose. Ozonation increased solubilization, decreased SS-content and improved sedimentation properties of Källby sludge. According to the literature study, these results indicate that the mechanics of excess sludge reduction have been at work. The same conclusion cannot be drawn about the Mörrum sludge, since the only beneficial effect caused by ozonation was the improvement of the sedimentation properties. This does not mean that excess sludge production in Mörrum sludge can t be reduced by ozonation, but at the dose used in the experiment, the prospects do not look promising. Acknowledgements The author would like to thank his supervisor Jes la Cour Jensen and everyone else at the Department of Water and Environmental engineering at the Lund Institute of Technology. Their help has been greatly appreciated. References Articles used in the literature study: Effect of ozone on survival and permeability of Escherichia Coli. D.B. McNair Scott and E.C Lesher, Journal of bacteriology, vol 85, 1963 Reducing sludge production and improving denitrification through ozonation. M.Dytczak, K.L.Londry, H. Siegrist, and J.A.Olezkiewicz, 25, Conference compendium. IWA specialized conference for nutrient management in wastewater treatment, processes and recycle streams,

6 More insights into the understanding of reduction of excess sludge production by ozone. Mostapha Salhi, Stéphane Déléris, Hubert Debellfontaine, Philippe Ginestet and Etienne Paul. Biosolids 23 An innovative approach to reduce excess sludge production in the activated sludge process. H.Yasui and M.Shibata. Water science and technology. Vol.3.No.9. pp Enhancement of capacity and efficiency of a biological waste water treatment plant. Nikolaus Kaindl, Ulf Tillman and Christian H. Möbius. Water science and technology. Vol.4, No 1112, pp , 1999 Oxidative treatment of digested sewage sludge with ozone. A. Scheminski, R.Krull and D.C. Hempel. Vol. 42. No.9.pp New combined system of biological process and intermittent ozonation for advanced wastewater treatment. T. Kamiya, J. Hirotsuji. Water science and technology. Vol. 38. No. 89. pp , 1998 Ozonation of wastewater sludge for reduction and recycling. K.-H. Ahn, K.Y. Park, S.K. Maeng, K.-G. Song and S. Choi. Water science and technology. Vol.46. No.1. pp Reduction of sludge by ozone treatment and production of carbon source for denitrification. K-H. Ahn, I.-T. Yeom, K.-Y. Park, S.-K. Maeng, Y.Lee, K.-G. Song and J.-H. Hwang. Water Science and technology. Vol.46. No pp Improvement of nitrogen removal and sludge reduction by MBR process combined with sludge ozonation. Y.-F. Jiang, L.Wang, B.-Z. Wang, S. Liu, and S.-B. He, 25 An activated sludge process without excess sludge production. Y. Sakai, T. Fukase, H.Yasui and M. Shibata. Water science and technology. Vol.36. No.11. pp , 1997 Partial ozonation of activated sludge to reduce excess sludge, improve denitrification and control scumming and bulking. M.Boehler, H. Siegrist. Water Science and Technology. Vol.49, pp Potential of activated sludge disintegration. M. Boehler, H. Siegrist. 25, Conference compendium. IWA specialized conference for nutrient management in wastewater treatment, processes and recycle streams, 25. Minimization of sludge production in biological processes: an alternative solution for the problem of sludge disposal. S. Deleris, V. Geaugey, P. Camacho, H. Debellefontaine and E. Paul. Water science and technology. Vol. 46. No.1. pp