WEF Residuals and Biosolids Conference 2017

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1 The sludge ozonation for different types of mixed liquor under high and low ph conditions by a plug-flow reactor Xiaoyu Zheng and Eric R. Hall Department of Civil Engineering, The University of British Columbia, 6250 Applied Science Lane, Vancouver, BC Canada V6T 1Z4,craeszheng@gmail.com, ehall@civil.ubc.ca ABSTRACT Sludge disintegration by ozone treatment with a high degree of solid solubilization at a low dosage is desirable. However, sludge ozonation is a complicated process and many factors affect its efficiency. In this study, a plug-flow ozone reactor was used to test the performance of sludge ozonation by different TSS concentrations, three types of sludge, the carbonate or bicarbonate addition, under high and low ph conditions. It was found that higher degree of sludge disintegration was achieved with lower TSS concentration. There was an optimum TSS concentration at which the maximum soluble COD concentration could be achieved. The types of sludge had a significant impact on the efficiency of sludge ozonation. Both the size and the structure of particles in sludge had significant impact on the efficiency of sludge ozonation. As one of the main scavengers, the carbonate and the bicarbonate inhibited sludge disintegration while causing high cellular inactivation due to ph increase. The highest degree of sludge solubilization was achieved under high ph condition, while more than 90% cellular inactivation was observed under low ph condition during sludge ozonation. KEY WORDS Sludge, Ozonation, Disintegration INTRODUCTION As a strong chemical oxidant, ozone can be used to disintegrate excess sludge and high degree of solid solubilization can be achieved 1. The main drawback for sludge ozonation is its high energy consumption and associated high operating cost. The ozone dosage, generally expressed by g O 3 /g TSS treated, is one of the most important parameters to evaluate the efficiency of sludge ozonation. A low dosage with a high degree of disintegration is desirable. However, the dosage reported in literature varied greatly. For example, it was reported that an ozone dosage lower than 0.015g O 3 /g VSS was not enough to cause cell rupture 2 ; no alternation in bacteria DNA was detected at the dosage of 0.02 g O 3 /g TSS and destruction of bacteria occurred mainly at ozone dosage above 0.08 g O 3 /g TSS 3. By measuring Oxygen Uptake Rate (OUR), at an ozone dosage of around 0.02 g O 3 /g TSS, 80% of microbial respiration activity was lost 4. Another study 5 reported that around 70% of sludge bacteria activity was lost at ozone dosage of g O 3 /g TSS. The discrepancy of the ozone dosages for sludge disintegration may be due to many other factors which may be overlooked. Sludge is a three-phase complex matrix of flocs, living cells and organo-mineral materials, and the interactions between ozone and activated sludge are 872

2 complicated and poorly understood 6. Except for the ozone dosage, the efficiency of sludge ozonation may depend on many factors including (1) sludge characteristics, (2) ozone mass transfer efficiency in the reactor and the configuration of the ozone reactor, (3) flow rate and concentration of ozone in the gas-phase, (4) flow rate and solid concentration in the treated sludge, (5) contact time between ozone and sludge, (6) scavengers in sludge, and (7) ph. In this study, three different types of sludge were tested to investigate the sludge disintegration efficiency. Three different solid concentrations of mixed liquors were also tested with the same amount of ozone applied. The effects of the carbonate and bicarbonate (the scavengers) addition and ph adjustment on the performance of sludge ozonation were also tested. Hydroxyl radicals (OH ) generation during ozonation was assumed to be one of the critic factors for sludge solubilization and cellular inactivation 3, 6. The carbonate and bicarbonate ions are the most common scavengers in sludge, which react with OH, stop producing superoxide radicals (O 2 /HO 2 ), and terminate the chain reaction. At high ph condition, the main reaction in sludge ozonation is the hydroxyl radical reaction; and at low ph condition, the ozone reaction is dominated by direct ozone oxidizing reaction. In this study, batch tests were conducted to investigate sludge disintegration with addition of the carbonate/bicarbonate, base and acid. MATERIALS AND METHODS A plug-flow ozone reactor was built, which included a pump (Moyno 331), a Venturi injector (Mazzei Model 283) and a coil mixer. The coil mixer was made of 1 foot PFA tube with ½ inch internal diameter, and the coil diameter was 4 inch. The flow rate for sludge was about 3.0 L/min, and the ozone injection rate was about 80 mg/min. The retention time of sludge in the reactor was less than 15 seconds. Three types of sludge were tested: (1) sludge from the aerobic tank in UBC MBR (membrane bioreactor) pilot plant at 40 days SRT, (2) the mixed liquor from aerobic phase of the control SBR (20 L lab-scale) at SRT = 50d, and (3) the mixed liquor from aerobic phase of the SBR (20 L lab-scale) combined with a sludge ozone treatment unit at nominal SRT = 50 d. Sludge from UBC MBR pilot plant had high solids concentration (TSS=11580 mg/l). It was diluted by 75% and 50% with its effluent to composite two different solid concentrations for sludge ozonation test. The sludge samples from aerobic phase of the control SBR were prepared and completely mixed as: (1) 2 L mixed liquor ml distilled water, (2) 2 L mixed liquor ml 0.4 N Na 2 CO 3, (3) 2 L mixed liquor ml 0.2 N NaOH ml distilled water, (4)2 L mixed liquor +100mL 0.8N HCl + 400mL distilled water. The ph values for sludge samples were 7.4, 10.4, 11.1, and 2.0, respectively. The sludge samples from aerobic phase of the ozonated SBR were prepared and completely mixed as: (1) 2L mixed liquor + 500mL distilled water, (2) 2 L mixed liquor ml 0.4 N Na 2 CO 3, (3) 2 L mixed liquor + 100mL 0.2 N NaOH ml distilled water, (4) 2 L mixed 873

3 liquor +100 ml 0.8 N HCl ml distilled water. The ph values for sludge samples were 7.2, 10.2, 11.0, and 2.0, respectively. Each sludge sample was then treated by the plug-flow reactor with and without ozone injection. Table 1 lists types, composition and characteristics of sludge for ozone treatment. Table 1 Types, composition and characteristics of sludge for the batch test Types of Sludge Mixed liquor from aerobic tank in UBC MBR Pilot Plant Mixed liquor from aerobic phase of the control SBR (SRT=50 d) Mixed liquor from aerobic phase of the SBR combined with sludge ozonation (SRT=50 d) Sludge prepared for treatment by the plug-flow reactor 2 L no dilution 2 L 75% diluted by effluent 2 L 50% diluted by effluent 2 L mixed liquor ml distilled water 2 L mixed liquor ml 0.4 N Na 2 CO 3 2 L mixed liquor ml 0.2 N NaOH ml distilled water 2 L mixed liquor +100 ml 0.8 N HCl ml distilled water 2 L mixed ml distilled water 2 L mixed liquor ml 0.4 N NaHCO 3 2 L mixed liquor ml 0.2 N NaOH ml distilled water 2 L mixed liquor +100 ml 0.8 N HCl ml distilled water Solids concentration TSS(VSS) (mg/l) (8280) 8740 (6300) 5700 (4300) 4920 (4100) 4613 (3680) VSS to TSS ratio (%) Particulate Size by Volume D(0.1)=10.6 um, D(0.5)=23.0 um, D(0.9)=116.6 um D(0.1)=33.7um D(0.5)=128.1um D(0.9)=339.8um D(0.1)=31.9um D(0.5)=87.0um D(0.9)=181.3um Each time, about 2 L sludge was treated in a once-through way by the plug-flow reactor. Sludge was sampled before and immediately after treatment. Triple samples were taken from each sludge for mixed liquor suspended solids (MLSS) and mixed liquor volatile suspended solids (MLVSS) measurement. The soluble constituents were sampled after filtration through 0.45 μm filters of the supernatant from sludge centrifuge treatment. The sludge samples were analysed on the same day of the batch tests. The soluble constituent samples were stored at 4 C until the analyses were performed at the Environmental Engineering Laboratory at UBC. MLSS, MLVSS, the chemical oxygen demand (COD) were measured according to the Standard Methods 7. Particle size distribution measurement was measured by a Malvern Instrument (Westborough, MA, USA) Mastersizer 2000 analyzer, with a Hydro S (Malvern) automated 874

4 sample dispenser unit. The ph value was measured by a portable DO meter (Hach HQ30d). The sludge ATP concentration was measured by Luminultra QG21W test kits. RESULTS Solids Concentration The efficiency of sludge ozonation was related with solids concentration. Figure 1 shows the efficiencies of sludge disintegration from MBR process by ozonation at three different solid concentrations. It seems that the higher degree of sludge solubilisation and cellular inactivation ratio were achieved with lower solids concentration. Figure 1 Sludge disintegration by ozone treatment at three different solids concentration Since the same amount of ozone was applied to each sludge sample, the ozone dosage, if defined as the mass of ozone applied per mass of treated TSS, increased from 5.4 to 10.9 mg O 3 /g TSS when TSS concentration decreased from mg/l to 5699 in Figure 1(A). However, the highest soluble COD was not achieved at the lowest TSS concentration. The highest concentration of the soluble COD after ozone treatment was measured at TSS = 8740 mg/l. It seemed that there existed an optimum TSS concentration at which the maximum soluble COD could be achieved. 875

5 The degree of TSS and VSS reduction/solubilization were generally increased when the solids concentration of the treated sludge decreased, which illustrated in Figure (B) and (C). However, the percentage of solid solubilization was not significantly increased even though the concentration of solids dropped (the ozone dosage increased). Figure (D) shows that sludge from MBR process was highly susceptible to ozone treatment and about 44.9% of living cells in sludge were inactivated even at the low dosage of 5.4 mg O 3 /g TSS treated. The cellular inactivation ratio increased when the TSS concentration decreased. Types of Sludge The types of sludge had a significant impact on sludge solubilisation and cellular inactivation in sludge ozonation. Figure 2 shows the performance of sludge ozonation for three different types of sludge at different ozone dosages. At the same ozone dosage, more cells were inactivated than the particles were solubilized. In addition, sludge from the SBR process had far less degree of sludge solubilisation and cellular inactivation ratio than sludge from the MBR process even at lower TSS concentration. Sludge from the SBR combined with sludge ozonation achieved the lowest degree of sludge solubilisation and cellular inactivation ratio even its TSS was lowest and its ozone dosage was highest. The particle size distribution of sludge may also be a factor affecting the performance of sludge ozonation. Figure 3 shows the particle size distribution of three types of sludge by volume, respectively. The median diameters of the particles in sludge from MBR, the control SBR and the ozonated SBR were 23.0 µm, µm and 87.0 µm, respectively. The particle size of MBR sludge was significantly smaller than that of sludge from SBRs. The ozone mass transfer mechanism in gas/liquid systems is generally explained by the fast kinetic regime theory proposed by Danckwerts 8. The rate limitation occurs in the liquid film, where the ozone can react fast with the various dissolved organic substances and be consumed. This causes the apparent rate of ozone mass transfer to exceed the maximum rate of physical gasliquid mass transfer. Normally, the thickness of the liquid film is between 15 to 20 μm. During sludge ozonation, with the enhancement by the fast reaction, the effective thickness of the film would be reduced to less than 10 μm 9. The thin liquid film allows only smaller particles and/or periphery of large flocs to be oxidized by ozone, while the cells inside the flocs can be viable due to aggregates physical protection 10. The theory may explain why the efficiency of ozonation of MBR sludge was higher than that of sludge from SBRs. However, it cannot explain why lower sludge disintegration ratio was achieved for sludge from the ozonated SBR than from the control SBR even though it had lower particle size than that of the control (87.0 µm versus µm). 876

6 Figure 2 Sludge solubilization and cellular inactivation vs. ozone dosage for different TSS concentrations and different types of sludge The photos of three types of sludge from microscopic observation (Figure 4) may explain the discrepancy. Figure 4 illustrates the structure differences among three types of sludge. The MBR sludge was composed of many small loose flocs, fine particles and filamentous bacteria (Figure 4(A)). The flocs with large sizes were observed in sludge from both SBRs (Figure 4 (B) and (C)). The filamentous bacteria were obvious in sludge from the MBR and the control SBR. No filamentous bacteria were found and the flocs were compact in sludge from the ozonated SBR, which was similar with many articles on sludge ozonation 11. The loose structure of flocs and larger surface areas of filamentous may make it more chances for microorganisms to contact and to be exposed to ozone and/or hydroxyl radicals; and thus high degree of sludge disintegration and cellular inactivation ratio could be achieved. The microorganisms from the ozonated SBR may be adapted to long-term ozone exposure and formed solid and compact flocs structure which may resist the attack from ozone. Therefore, the lowest degree of TSS and VSS solubilization and the lowest cellular inactivation ratio were measured for sludge from the ozonated SBR. 877

7 Figure 3 The particle size distribution by volume for three types of sludge Figure 4 The microscopic photos of three types of sludge 878

8 Sludge Ozonation with Carbonates/bicarbonates addition, high ph and low ph adjustment The results of sludge disintegration for the control SBR by adding carbonates, sodium hydroxide and hydrochloric acid with and without ozone were shown in Figure 5. Figure 6 shows the similar results of sludge disintegration for the SBR combined with the ozone treatment by adding bicarbonates, sodium hydroxide and hydrochloric acid with and without ozone. Figure 5 Sludge solubilization and cellular inactivation of the sludge from the control SBR by carbonate, base, and acid addition with and without ozone treatment Figure 6 Sludge solubilization and cellular inactivation of the sludge from the ozonated SBR by adding bicarbonate, base, and acid with and without ozone treatment 879

9 The carbonates/bicarbonates addition had impact on sludge ozonation. In Figure 5, sludge reacted with high concentration sodium carbonates due to ph increase. About 6.9% TSS and 5.5% VSS were solubilized. The cellular inactivation ratio was 67.8% which was significantly higher than sludge ozone treatment only. When carbonates were added to sludge for ozonation, TSS solubilization ratio reduced from 11.4% to 9.8% and VSS solubilization ratio decreased from 11.2% to 9.9% compared with ozone treatment only. The carbonates/bicarbonates normally act as one of the hydroxyl radical scavengers and reduce ozone radical reaction, which reduced the efficiency of sludge ozonation. However, the cellular inactivation ratio increased from 22.6% to 73.4% which may be due to the increased ph (10.4) by carbonates addition. Either base or acid addition alone can be used to disrupt sludge. When they were applied to sludge alone, high degree of TSS and VSS solubilization can be achieved (Figure 5). Compared with sludge ozone treatment, high cellular inactivation ratio was observed either with base or acid treatment. From Figure 5, 61.5% and 87.1% of microorganisms were inactivated by sodium hydroxide and by hydrochloric acid, respectively; while only 22.6% by sludge ozonation. If combined with ozone treatment, the efficiency of sludge disintegration was generally improved. High or low ph treatment may inactivate higher percentage of cells than ozone treatment. In Figure 6, ozone treatment alone inactivate only 19.2% cells, while addition of bicarbonates, sodium hydroxide and hydrochloric acid separately inactivated 34.5%, 52.7%, and 97.6% of cells, respectively. It seems that sludge may be more tolerant of low dosage ozone treatment than under high ph ( 10) or low ph ( 2) condition. When sludge ozonation combined with sodium hydroxide addition, the degrees of TSS and VSS solubilization was significantly improved. The high ph favours the hydroxyl radicals generation and sludge ozonation may be dominated with ozone chain reactions, which promotes efficiency of sludge disintegration. From Figure 5 and Figure 6, sodium hydroxide addition combined ozone treatment achieved the highest degrees of TSS and VSS solubilization. It is possible that ozone treatment at high ph condition, which favours ozone indirect hydroxyl radical reaction, improves the efficiency of sludge disintegration. When sludge ozonation coupled with acid addition, more than 90% cells were inactivated. The degrees of TSS and VSS solubilization also increased but less than those under high ph condition. In Figure 5 and Figure 6, hydrochloric acid addition coupled with ozone treatment obtained the highest cellular inactivation ratio. It is possible that low ph may be more lethal to microorganisms than under high ph condition during sludge ozonation. CONCLUSIONS The efficiency of sludge ozonation depends on many factors including TSS concentration, sludge type, carbonates and bicarbonates, and ph. The lower TSS concentration, the higher degree of sludge solubilisation and cellular inactivation ratio were expected to be achieved. An optimum TSS concentration may exist at which the maximum soluble COD could be achieved. 880

10 The types of sludge had a significant impact on sludge solubilisation and cellular inactivation in sludge ozonation. Both particle size and the structure of flocs had significant impact on sludge disintegration. The carbonate or bicarbonate addition during ozone treatment may reduce the efficiency of sludge disintegration. The degree of solids solubilization was significantly improved under high ph condition; while 90% cellular inactivation ratio was achieved under low ph condition when sludge ozonation combined with acid addition. REFERENCE 1. Muller, J. A., Pretreatment processes for the recycling and reuse of sewage sludge. Water Science and Technology 2000, 42 (9), Albuquerque, J. S.; Domingos, J. C.; Sant'Anna, G. L.; Dezotti, M., Application of ozonation to reduce biological sludge production in an industrial wastewater treatment plant. Water Science and Technology 2008, 58 (10), Yan, S. T.; Chu, L. B.; Xing, X. H.; Yu, A. F.; Sun, X. L.; Jurcik, B., Analysis of the mechanism of sludge ozonation by a combination of biological and chemical approaches. Water Research 2009, 43 (1), Chu, L. B.; Yan, S. T.; Xing, X. H.; Yu, A. F.; Sun, X. L.; Jurcik, B., Enhanced sludge solubilization by microbubble ozonation. Chemosphere 2008, 72 (2), Saktaywin, W.; Tsuno, H.; Nagare, H.; Soyama, T.; Weerapakkaroon, J., Advanced sewage treatment process with excess sludge reduction and phosphorus recovery. Water Research 2005, 39 (5), Dziurla, M. A.; Salhi, M.; Leroy, P.; Paul, E.; Ginestet, P.; Block, J. C., Variations of respiratory activity and glutathione in activated sludges exposed to low ozone doses. Water Research 2005, 39 (12), APHA; AWWA; WEF, Standard Methods for the Examination of Water and Wastewater. 22nd ed. ed.; American Public Health Association: Washington D.C., Danckwerts, P. V., gas-liquid reaction. McGraw-Hill: New York, Paul, E.; Debellefontaine, H., Reduction of excess sludge produced by biological treatment processes: Effect of ozonation on biomass and on sludge. Ozone-Science & Engineering 2007, 29 (6), Chu, L. B.; Wang, J. L.; Wang, B.; Xing, X. H.; Yan, S. T.; Sun, X. L.; Jurcik, B., Changes in biomass activity and characteristics of activated sludge exposed to low ozone dose. Chemosphere 2009, 77 (2),

11 11. Nilsson,, F.; Marinette Hagman; Artur Tomasz Mielczarek; Per Halkjær Nielsen; Jönsson, K., Application of Ozone in Full-Scale to Reduce Filamentous Bulking Sludge at Öresundsverket WWTP. Ozone: Science & Engineering 2014, 36,