R. Goel, T. Tokutomi and H. Yasui Kurita Water Industries, Research and Development Center, 7-1, Wakamiya, Morinosato, Atsugi, Kanagawa, Japan 243-0124 (E-mail:
[email protected]) Abstract Source minimization of excess sludge production by economical means can be considered an attractive option to deal with the problem of sludge disposal under strict disposal standards. In this paper long-term operational results for a process that combines the oxidative ozone pretreatment with anaerobic sludge digestion are described. The ozone pretreatment solubilized around 19% and 37% of the solids at 0.015 and 0.05 gO3/gTS ozone dose. The solubilization ratios during ozonation did not show any significant difference for the sludge concentrations ranging from 1.8–2.6%. The TVS concentrations after ozone treatment were observed to be about 3% lower than the feed sludge concentrations suggesting only partial mineralization during ozonation. The ozone pretreatment resulted in improved solid reduction efficiencies during anaerobic digestion leading to higher methane recovery. The TVS removal efficiencies during anaerobic digestion were observed to increase by a maximum of 35–90% depending on the applied ozone dose during ozone pretreatment. The improvement in TVS degradation efficiency at different applied ozone doses correlated well with the extent of solubilization during ozonation. Long-term data also suggested that biomass acclimation to ozonated sludge was necessary before higher degradation efficiencies could be achieved. Keywords Activated sludge; anaerobic digestion; hydrolysis; methane; ozone; sludge treatment
Introduction
Sludge treatment and disposal from biological wastewater treatment plants is troublesome and require a significant financial outlay for both capital and operational purposes. Reduction in sludge production from treatment plants is desirable and can be achieved either during wastewater or sludge treatment processes. Theoretically, the specific sludge production (gVSS/gCOD) in wastewater treatment plants can be reduced by: 1) channeling more energy for biomass maintenance purpose, 2) inducing cryptic growth, 3) promotion of growth of higher organisms like protozoa and ciliates, and 4) control of operating parameters like temperature and DO. Recently, many studies have shown successful application of the above principles for sludge reduction (Yasui and Shibata, 1994; Ratsak et al., 1994, Rensink and Rulkens, 1997; Low and Chase, 1999; Mayhew and Stephenson, 1998; Strand et al., 1999; Abbassi et al., 2000). In sludge treatment, anaerobic digestion is the most fundamental process for solid reduction and many studies have been focused around this process for improving solid reduction through sludge pretreatment. During anaerobic digestion of sludge, the enzymatic hydrolysis of solids is reported to be the rate-limiting step (Eastman and Ferguson, 1981). Hence, to compensate for the slow biological enzymatic hydrolysis, a chemical/mechanical sludge pretreatment step that attempts to disrupt the cell wall structure to make the biomass easily hydrolysable has been combined with anaerobic digestion. In earlier studies, thermal and thermo-chemical methods have been used for sludge pretreatment and even though substantial improvements in VS degradation following thermal/thermo-chemical treatment have been reported, problems related to odor, toxicity (Haug et al., 1978; Stucky and McCarty, 1984) and corrosion have led to only limited application of such pretreatment methods. Recent studies have tried to explore other
Water Science and Technology Vol 47 No 12 pp 207–214 © IWA Publishing 2003
Anaerobic digestion of excess activated sludge with ozone pretreatment
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pretreatment options like mechanical disintegration using pressurized jets (Choi et al., 1997), thickening centrifuge (Dohanyos et al., 1997) and ball milling (Kopp et al., 1997). Other pretreatment options like ultrasound treatment (Tiehm et al., 1997); and ozone oxidation (Weemaes et al., 2000) have also been reported. Recent reports (Yasui et al., 1996) on full-scale applications of ozone treatment to completely eliminate excess sludge production from full-scale activated sludge treatment plants signifies the role ozone can play in sludge hydrolysis and enhancement of biodegradability. Considering this, ozonation was also considered as an attractive pretreatment for solid hydrolysis before anaerobic digestion. In this work, the main objectives were to study: 1) the aspects related to solubilization and mineralization of activated sludge with solid content of 2–3% at low ozone doses (0.015–0.05 gO3/gTS), 2) the effect of ozone dose and solid retention times on solid degradation efficiencies during anaerobic digestion, and 3) the behavior of VFA, ammonia and inorganic solids, through long-term continuous experiments. Since earlier study (Weemaes et al., 2000) using ozone pretreatment has presented degradability data based on batch experiments only, long-term operational data accounting for factors such as biomass acclimatization was another consideration in this study. Experimental methods
The scheme used for carrying out the experiments is shown in Figure 1. The sequencing batch reactor (SBR) was used for cultivating the activated sludge for subsequent ozonation and digestion. The harvested sludge was ozonated in a completely mixed reactor and used for feeding to anaerobic digesters. Four 2 l jar fermenters were operated at 35°C. Two of the digesters served as control reactors and the other two were used for digesting ozonated substrate. Detailed description of each step is given in the following sections. Activated sludge sequencing batch reactor (SBR)
A 160 l activated sludge SBR was operated for cultivating the activated sludge required for feeding to the anaerobic digesters. The SBR was operated at a sludge age of 10 days. The 500 ml of concentrated substrate (BOD5 300,000 mg/l) containing yeast extract, fish extract, fructose syrup and phosphoric acid was batch fed to the reactor daily. The SBR reactor was operated in a cycle of 1 day consisting of feeding (30 minutes), aeration (16–18 h), settling (4–6 h) and excess sludge withdrawal (5 minutes). The MLSS in the reactor was around 5,000–6,000 mg/l. Ozonation of activated sludge
The sludge ozonation were performed at pH of 2 in this study. The pH of the fraction to be ozonated was adjusted to 2 by using 4N HCl acid and was put in the ozone contactor. A precalculated ozone dose (gO3/gTS) of either 0.015 or 0.05 was applied during ozonation. After ozonation the pH was readjusted to 7–7.2 using 4N NaOH. The raw activated sludge
Synthetic wastewater
Off gas to ozone meter
Ozonated activated sludge
Ozone generator Ozone contactor SBR
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Biogas to gas collection Digested sludge for disposal Ozonated sludge fed AD Raw sludge fed AD (control)
Figure 1 Schematic of the experimental setup, the sludge transfer was performed in batch mode manually
and ozonated sludge were stored in refrigerator. The amount of stored sludge was enough for 3–5 days feeding to the jar fermenters. Batch-fed continuous anaerobic sludge digesters
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Two-litre jar fermenters equipped with temperature and mixing control were used for the experiments. The temperature of the digesters was controlled at 35°C. The startup of the anaerobic reactors was done using a mixture of ground UASB sludge, thermophilic anaerobic digested sludge and mesophilic anaerobic digested sludge from a full-scale treatment plant. The AS and ozonated sludge were batch-fed once every day to the digesters after removing an equivalent amount from the digesters. Analytical methods
TS and TVS in the feed sludge and reactors were measured according to Standard Methods. Suspended solid and volatile suspended solids were measured after twice centrifuging the sludge sample at 12,000 rpm for 10 minutes. The supernatant of first centrifugation was filtered using 0.45 µm and used for soluble fraction analysis. The TOC in the soluble fraction was measured using Shimadzu TOC 5000. The VFA concentration in soluble samples was measured using liquid chromatography along with visible detector at 410 nm. Soluble ammonia concentration was measured calorimetrically using a flow injection analyzer. The ozone gas measurements were done using an ozone meter (Nippon Ozone Co.) based on a UV detector. Results and discussion Solid solubilization during ozonation
The sludge solubilization during ozonation could be a rough indicator of improvements in biodegradability that could be achieved in subsequent biological treatment processes where hydrolysis of solids is the rate-limiting step. To assess the ratio of solubilization during ozonation, the VSS in sludge were measured before and after ozonation. A plot between VSS before and after ozonation is shown in Figure 2(a). It was estimated that at 0.05 gO3/gTS ozone dose, about 37% of the solids were solubilized. Similar experiments also indicated that at ozone dose of 0.015 gO3/gTS solubilization ratios of 19% could be expected (data not shown). From Figure 2(a), it was also observed that at a particular ozone dose, the solubilization ratios did not change for the sludge concentrations from 1.8–2.6% used in this study. Solid mineralization during ozonation
TVS before and after ozonation were measured to assess the sludge mineralization during ozonation stage. A plot between raw sludge TVS and TVS in ozonated sludge (ozone dose = 0.05 gO3/gTS) is as shown in Figure 2(b). The regression line suggests that about 5% of the TVS are mineralized during ozone treatment. To further refine these estimates, the TVS data before and after ozonation was statistically tested using standard t distribution for paired sample for mean. Calculations using 29 data sets (df = 28) indicated that only 3% reduction in TVS was statistically significant (α = 0.05). Consistent to the TVS loss, TVS/TS ratio changed from 0.92 in activated sludge to 0.84 in the ozonated sludge. On the other hand, no change in VSS/SS ratio was observed. Solid reduction during anaerobic digestion
The operation of anaerobic digesters was divided into three phases (phase I, phase II and phase III). In each phase four digesters were operated under different conditions. The details of experimental conditions used for the operation of four reactors in each phase are
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VSS in ozonized sludge (g/l)
19 y = 0.6336x
17
2
R = 0.8749
15 13 11 9 7 5
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15
20
25
30
VSS in raw sludge (g/l)
(a)
After 0.05gO 3/gTS ozone dose
TVS after ozonization (g/l)
28 26
y = 0.9481x 2
R = 0.8132
24 22 20 18 16 14 15 (b)
20
25
30
Inlet TVS (g/l) After 0.05gO 3/gTS ozone dose
Figure 2 (a) Sludge solubilization during ozone treatment (b) sludge mineralization during ozone treatment
given in Table 1. According to the operation scheme, it was possible to estimate the effect of SRT and applied ozone dose on sludge digestion efficiency and compare them with the control runs. The time course of TVS during different runs is as shown in Figure 3 and a summary of the other measured parameters is presented in Table 2. The TVS reduction efficiencies for the untreated activated sludge at 28 days SRT during different runs (I–1, II–1 and III–1) ranged between 25% and 35%. The TVS reduction efficiencies for the sludge treated with 0.015 gO3/gTS (I–2 and I–4) were only 10–30% higher than the efficiencies for untreated sludge (I–1 and I–3). As the operation was continued in phase II with an increased ozone dose of 0.05 gO3/gTS, the TVS reduction efficiencies gradually improved to 59% and 50% for the SRT of 28 and 14 days (II–2 and II–4) respectively, suggesting an improvement of 85–90% over the control efficiency respectively. It Table 1 Operating conditions for anaerobic digesters Run No.
SRT/
Average
HRT (d)
solid loading
Characteristics of feed
O3 dose (gO3/gTS)
(kg TVS/m3.d)
Phase I I–11 I–2 I–3 I–4 Phase II II–1 II–2 II–3 II–4 Phase III III–1 II–2 II–3 II–4 210
Operation comment period (days)
28 28 14 14
0.77 0.77 1.54 1.54
Lab. AS Ozonated sludge Lab. AS Ozonated sludge
0.0 0.015 0.0 0.015
35 35 35 35
28 28 14 14
0.77 0.77 1.54 1.54
Lab. AS Ozonated sludge Lab. AS Ozonated sludge
0.0 0.05 0.0 0.05
120 120 120 120
Cont. of I–1
28 28 7 7
0.77 0.77 3.08 3.08
Lab. AS Ozonated sludge Lab. AS Ozonated sludge
0.0 0.05 0.0 0.05
30 30 30 30
Cont. of II–1
1 I–1 denotes reactor 1 during phase I
Cont. of I–3
TV S conc . (g/l)
22.0 20.0 18.0 16.0
21.0 17.0 13.0 9.0
14.0
5.0
Influent
15
20 25 30 Operation period(days )
Run I-1
Run I-2
Run I-3
35
Run I-4
Influent TV S
(a) 25.0 21.0 17.0 13.0 9.0 5.0 150
160
170
180
190
200
Run III-1
Run III-2
(c)
Run III-3
Run III-4
85 135 Operation period(days) Run II-1
Run II-2
Run II-3
185
Run II-4
(b) 70 60 50 40 30 20 10 0 0
Operation period(days ) Influent TV S
35
TV S rem oval effic ienc y(% )
TVS in reactors(g/l)
25.0
24.0
10
TVS c onc. (g/l)
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can be seen that after switching to phase II, it took a long time (2 months) before the difference between control and ozonated sludge fed reactors became significant. Thus it appears that some acclimation to ozonated sludge is necessary before the full degradation potential is realized. In this perspective, considering the short operation period of phase I, it is possible that degradation efficiencies for ozonated sludge at 0.015 gO3/gTS ozone dose may have been slightly underestimated. In phase III, the SRT for 14 days operated reactors was reduced to 7 days. At reduced SRT of 7 days, the TVS reduction efficiency for un-ozonated sludge dropped significantly from 27% to 17% whereas efficiency for ozonated sludge fed reactor reduced from 50% to 46% only. This slight reduction in TVS removal efficiency at lower SRT suggests the possibility of lower SRT operations with ozone-pretreated sludge, thus making it possible to operate digesters at higher solid loading. The observed TVS removal efficiencies and its relation to SRT and ozone dose are shown in Figure 3(d). The comparison of TVS degradation efficiency for ozonated sludge and control sludge at different SRTs suggests that ozone treatment produces at least two organic fractions out of which one (ca. 30% at 0.05 gO3/gTS) has significantly higher hydrolysis rates than the original sludge and is consumed during the initial phase (£ 7days) of digestion. Judging from the degradation efficiencies at higher SRT, the degradation behavior of the remaining fraction is not much different from the solids in the original sludge. The calculated VSS removal efficiencies were generally higher than the TVS removal efficiencies (Table 2), although the relative increase in removal efficiencies over the control reactors was of the same order as described for TVS removal. The TVS and VSS efficiencies reported in Table 2 should further improve if the amount of mineralization in ozone reactor is also taken in to account. The improvement in TVS degradation efficiency at different applied ozone doses in this study correlated well with the degree of solubilization. A 19% and 37% solubilization resulted in about 30% and 80% improvements in TVS removal efficiencies. Extrapolation of these results however will require further experimental evidence.
10 20 Operation S RT(d)
30
Control 0.015 gO3/gTS oz one dose 0.05 gO3/gTS ozone dos e
(d)
Figure 3 The trend of TVS during: a) phase I, b) phase II, c) phase III, and d) TVS removal efficiencies with SRT and ozone dose
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Table 2 Summary of different process parameters Run No.
SRT/
O3 dose
Solid deg.
TVS/TS
Specific Soluble
HRT (d)
(gO3/gTS)
Efficiency
ratio
methane ammonia TOC
Soluble
VFA COD (major component)
production
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Phase I1 I–1 I–2 I–3 I–4 Phase II2 II–1 II–2 II–3 II–4 Phase III3 III–1 III–2 III–3 III–4
(%)
(%)
(–)
(L-CH4/gTVS fed)
(mgN/l)
(mg/l)
(mgCOD/l)
28 28 14 14
0.0 0.015 0.0 0.015
35 37 24 31
38 42 28 36
0.85 0.83 0.87 0.84
0.20 0.21 0.12 0.14
745 824 647 770
68 129 493 689
– 90 (IB)4 1506(P) 1924(P)
28 28 14 14
0.0 0.05 0.0 0.05
31 59 27 50
35 65 30 60
0.87 0.73 0.89 0.74
0.12 0.25 0.11 0.20
830 1357 718 1302
1013 1876 743 2126
2136(A,P) 3791(A,P) 1833(A,P) 3691(A,P)
28 28 7 7
0.0 0.05 0.0 0.05
25 58 17 46
– 60 – 52
0.87 0.65 0.89 0.73
0.15 0.31 0.07 0.19
643 1224 422 1055
113 466 217 1590
– – – –
1 Startup, control reactors 1 and 3 operated at different SRT, ozonated sludge fed reactors 2 and 4 operated
at SRT same as 1 and 3 respectively. The applied ozone dose was 0.015 gO3/gTS 2 The ozone dose for sludge fed to reactors 2 and 4 was increased from 0.015 to 0.05 gO3/gTS 3 The SRT for reactor 3 and 4 was reduced from 14 days to 7 days. Trace metals of Fe, Ni and Co were also
added in the fed sludge. 4 IB-iso-butyric acid, P – propionic acid, A – acetic acid
As given in Table 2, significantly lower values of TVS/TS for reactor fed with ozonated sludge during phase II and phase III were observed. These lower values are consistent with the observed higher TVS reduction in these reactors during these phases. Interestingly though, the VSS/SS ratios for all reactors varied only in a narrow range of 0.92–0.95, suggesting the composition of sludge solids with respect to organic content does not change for reactors fed either with ozonated or un-ozonated sludge. Combining both the parameters of TVS/TS and VSS/SS, it can be inferred that the inorganic fraction remains as soluble and escapes with the effluent. The improvement in solid degradation efficiencies observed after ozone pretreatment were significantly higher than improvements reported for other pretreatment options like mechanical disintegration (Kopp et al., 1997), ultrasound treatment (Tiehm et al., 1997) and mechanical jet smashing (Choi et al., 1997). The level of improvement, though, seems to be better than thermal/thermo-chemical treatment at low pretreatment temperature and comparable at higher temperatures of 175°C. As the improvements in degradation efficiencies will depend on the sludge source, energy inputs during pretreatment and other operational parameters, thus the above comparisons are only indicative. Soluble and gaseous components during anaerobic digestion
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The soluble TOC and VFA (expressed in terms of COD) followed almost similar trends as shown in Table 2, suggesting that most of the soluble organic fraction was in the acidified form. Very high VFA concentrations in all the reactors were observed during phase II. The major components were found to be acetic acid and propionic acid suggesting limitation in activity of both H2 and acetate utilizing methanogenic bacteria. In phase III, addition of trace metals of Fe (5 mg/l), Ni(50 µg/l) and Co(50 µg/l) was started in the feed. After this change, the TOC levels in all the reactors except reactor 4 dropped significantly. As the SRT for this reactor was reduced from 14 days to 7 days during the same time, the level of TOC in reactor 4 reduced only slightly. The observed ammonia levels in the reactors fed
Table 3 Typical running cost for ozone pretreatment system Parameters
Control system
With ozone treatment
1,000 0.85 0.35 702 3,600 – 72,000 – 72,000
1,000 0.85 0.60 490 2,450 50 49,000 15,000 64,000
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Daily solids fed (kgDS/d) VSS/SS (–) VSS removal efficiency (%) Daily solids required to be disposed (kgDS/d) Daily sludge to be disposed @ 20% solid conc. (kg wet solids/d) Daily ozone required (kg/d) Sludge disposal cost @20,000 ¥/wet ton Cost for ozone production @20 kWh/kgO3 and @15 ¥/kWh Total treatment and disposal cost (¥)
with ozonated sludge were consistently higher than those observed in the control and corroborated well with the higher degradation efficiencies in the reactors fed with ozonated sludge. The observed specific methane production (L-CH4/gTVSfed) was slightly lower (within 10–15%) than that expected based on the TVS removal efficiencies. Partial loss in CH4 recovery may have happened due to organic mineralization and change in oxidation state of organics during ozonation stage. Cost comparison
Typical running cost estimates for two anaerobic digestion plants with and without ozonation system are presented in Table 3. Even though, the ozone pretreatment shows its cost effectiveness, the real decisions regarding the application of such system will depend on the magnitude of capital investments, energy cost and sludge disposal costs. The proposed systems seems to be interesting for larger treatment plants and for sludges that require higher disposal costs. Conclusions
Anaerobic digestion of ozone pretreated excess sludge was studied through long-term operation of laboratory-scale reactors. Ozone pretreatment was found to be effective in partially solubilizing the sludge solids and leading to subsequent improvement in anaerobic degradability. The extent of solubilization and digestion efficiency depended on the applied ozone doses. It was estimated that about 19% and 37% of the solids are solubilized at 0.015 and 0.05 gO3/gTS ozone dose respectively. At 0.05 gO3/gTS, the anaerobic digestion efficiencies improved to about 59% as compared to 31% for the control run. At lower ozone dose of 0.015 gO3/gTS, the solid reduction efficiencies showed only slight improvements. Different process indicators like specific methane production, TVS/TS ratio and ammonia concentration in the reactor also corroborated the higher observed solid degradation rates for ozonated sludge. References Abbassi, B., Dullstein, S. and Rabiger, N. (2000). Minimization of excess sludge production by increase of oxygen concentration in activated sludge floc; experimental and theoretical approach, Wat. Res., 34(1), 139–146. Choi, H.B., Hwang, K.Y. and Shin, E.B. (1997). Effects on anaerobic digestion of sewage sludge pretreatment, Wat. Sci. Tech., 35(10), 207–211. Dohanyos, M., Zabranska, J. and Jenicek, P. (1997). Enhancement of sludge anaerobic digestion by using of a special thickeing centrifuge, Wat. Sci. Tech., 36(11), 145–153. Eastman, J.A. and Ferguson, J.F. (1981). Solubilization of particulate organic carbon during the acid phase of anaerobic digestion. JWPCF, 53, p. 352.
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Haug, R.T. Stucky, D.C., Gossett, J.M. and McCArty, P.L. (1978). Effect of thermal treatment on digestibility and dewaterability of organic sludges, JWPCF, 50, 73–85. Kopp, J., Muller, J., Dichtl, N. and Schwedes, J. (1997). Anaerobic digestion and dewatering characteristics of mechanically disintegrated excess sludge, Wat. Sci. Tech., 36(11), 129–136. Low, E.W. and Chase, H.A. (1999). The effect of maintenance energy on biomass production during wastewater treatment, Wat. Res., 33(3), 847–853. Mayhew, M. and Stephenson, T. (1998). Biomass yield reduction: is biochemical manupulation possible without affecting activated sludge process efficiency? Wat. Sci. Tech., 38(8–9), 137–144. Ratsak, C.H., Kooi, B.W. and Van Vereveld, H.W. (1994). Biomass reduction and mineralization increase due to ciliate Tetrahymena pyriformis grazing on the bacterium Pseudomonas fluorescens. Wat. Sci. Tech., 29(7), 119–128. Rensink, J.H. and Rulkens, W.H. (1997). Using metazoa to reduce sludge production, Wat. Sci. Tech., 36(11), 171–179. Strand, S.E., Harem, G.N. and Stensel, H.D. (1999). Activated sludge yield reduction using chemical uncouplers. Wat. Environ. Res., 71(4), 455–458. Stuckey, D.C. and McCarty, P.L. (1984). The effect of thermal pretreatment on the anaerobic biodegradability and toxicity of waste activated sludge, Wat. Res., 18(11), 1343–1353. Tiehm, A., Nickel, K. and Neis, U. (1997). The use of ultrasound to accelerate the anaerobic digestion of sewage sludge, Wat. Sci. Tech., 36(11), 121–128. Weemaes, M., Grootaerd, H., Simoens, F. and Verstraete (2000). Anaerobic digestion of ozonized biosolids, Wat. Res., 8, 2330–2336. Yasui, H., Nakamura, K., Sakuma, S., Iwasaki, M. and Sakai, Y. (1996). A full scale operation of a novel activated sludge process without excess sludge production. Wat. Sci. Tech., 34(3–4), 395–404. Yasui, H. and Shibata, M. (1994). An innovative approach to reduce excess sludge production in the activated sludge process. Wat. Sci. Tech., 30(9), 11–20.
