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© IWA Publishing 2015 Water Science & Technology
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Study of the sludge reduction in an oxic–settling– anaerobic activated sludge process based on UNITANK L. P. Sun, J. F. Chen, W. Z. Guo, X. P. Fu, J. X. Tan and T. J. Wang
ABSTRACT An oxic–settling–anaerobic process (OSA) can effectively reduce sludge production, but most of the research studies on the OSA process have been either under laboratory test conditions or based on synthetic wastewater, which cannot fully reflect the performance and sludge reduction efficiency in existing OSA process. Thus, aiming at examining the sludge reduction efficiency and the stability of the OSA process, UNITANK and UNITANK–OSA processes were performed in a 120 m3/d pilot-scale system using actual sewage. The results indicate that UNITANK–OSA achieved a 48% reduction of the sludge compared to the reduction due to UNITANK, not considering the accumulation of the effluentsuspended solids. The effluent quality was not found to change significantly, except that the total phosphorus concentration increased slightly. The extracellular polymeric substances metal floc theory may, to some extent, explain this reduction in this study. The OSA process could be used to reform the classic wastewater treatment process to get lower sludge mass. Key words
| pilot-scale study, sludge reduction, sludge source reduction, wastewater treatment
L. P. Sun (corresponding author) J. F. Chen J. X. Tan T. J. Wang School of Environmental Science and Engineering, Sun Yat-sen University, No.135 Xingang Road West, Guangzhou 510275, China E-mail: [email protected] L. P. Sun Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Guangzhou 510275, China W. Z. Guo X. P. Fu Foshan Water Group, Foshan 528000, China
INTRODUCTION Conventional activated sludge processes are widely used in sewage treatment because of three main advantages: (1) high organic pollutant removal efficiency; (2) stable operation; and (3) low-operation cost. However, a large amount of excess sludge is generated due to the degradation of organic pollutants (Ye & Li ). The treatment and ultimate disposal of excess sludge are expensive, which usually accounts for 20–60% of the total operational cost in a conventional activated sludge treatment plant (Low & Chase ; Liu ). Meanwhile, the ultimate disposal of the sludge through the use of landfills and/or incineration is facing difficulties, such as exceeding the landfill capacity and the secondary pollution caused by either incineration of the sludge or the landfills (Chen et al. ). Therefore, new approaches that could reduce the excess sludge have received wide attention and are under extensive study (Low et al. ; Hendrickx et al. ). Those approaches have achieved different levels of sludge reduction but have not yet been widely used in practice due to the problems of expensive operational cost, negative effects on sludge characteristics, or the effluent quality. Westgarth first modified a conventional activated doi: 10.2166/wst.2014.474
sludge process in the laboratory by inserting a sludge holding tank in the sludge return circuit to form an oxic–settling–anaerobic process (OSA process), which effectively reduces the sludge production. Subsequently, research studies on the OSA process have been conducted and have achieved 20–80% reduction of the sludge (Chudoba et al. ; Ghigliazza et al. ; Copp & Dold ; Chen et al. ). Three main scenarios have been addressed to explain the reduction of sludge in the OSA process: (1) energy uncoupling (Chen et al. ; Liu ); (2) sludge decay in the sludge holding tank (Martinage & Paul ; Chen et al. ; Zhu & Chen ); and (3) extracellular polymeric substances (EPS) metal floc theory (Novak et al. ; Park et al. ). However, most of the research studies on the OSA process are either under laboratory test conditions or based on synthetic wastewater, which cannot fully reflect the performance and sludge reduction efficiency in the existing OSA process. Thus, aiming at examining the sludge reduction efficiency and the stability of the OSA process, we conducted a pilot-scale experiment using actual sewage to promote a full-scale practical application.
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METHODS Experimental apparatus Based on a UNITANK setup (Liao et al. ), we constructed a 120-m³/d pilot-scale UNITANK–OSA setup, as illustrated in Figure 1. The sizes of B, C, and D basins were same: they were 2.3, 2.3, and 4.2 m in length, width, and height, respectively, and the effective volume of one basin was about 20 m³. The size of the OSA anaerobic basin was 2.5, 2.0, and 1.7 m in length, width, and height; the effective volume of the OSA anaerobic basin was about 7.1 m³. The hydraulic residence time (HRT) of the UNITANK process was 12 hours, and the HRT of the OSA anaerobic basin was 5 days. Sludge from the B basin and the D basin of the UNITANK was pumped to the OSA anaerobic reactor, the electric reflux valve fitted on B and D basins would be opened automatically when they were in the final stage of the settling process. After the anaerobic reaction, the sludge was pumped from the OSA anaerobic reactor to the C basin of the UNITANK. An oxidation–reduction potential meter, a flow meter, and a pH online monitor were fitted on the OSA anaerobic reactor, and all were automatically controlled by a sequence controller.
Operation of the UNITANK–OSA system First, the UNITANK setup was operated independently for 60 days. After the 60-day operation, the UNITANK process became stable. The UNITANK was then modified as a UNITANK–OSA system by inserting an OSA anaerobic reactor, which was run another 60 days to compare the effluent quality and the sludge production with those of the UNITANK in the first 60 days. The operation cycle for the UNITANK and the sludge interchange cycle for the UNITANK–OSA were 8 and 4 hours, respectively. The phases of the UNITANK–OSA operation are as follows: (1) the sludge was pumped from the bottom of basin B
Figure 1
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Schematic diagram of the UNITANK–OSA setup.
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or D into the OSA anaerobic reactor in sequence for 15 minutes every 4 hours; and (2) the sludge remained in the OSA anaerobic reactor for 5 days before it was pumped out to the C basin. The sludge interchange rate (which was the rate of interchange sludge quality from the OSA anaerobic reactor into the UNITANK and the total sludge quality in the UNITANK tank), the sludge retention time, and the sludge interchange times of the OSA anaerobic reactor were 10%, 5 days, and six times per day, respectively. The sludge withdrawal rate was precisely determined each time to maintain the mixed liquor suspended solids (MLSS) level of the C basin at 2000–3000 mg/L. Materials and methods of analysis The sewage used in this pilot-scale experiment was from the grit chamber of the Zhenan sewage treatment plant in Foshan city. The collection system of the plant is a combined system that includes a certain amount of industrial wastewater as well. As a result, the influent quality during the operation fluctuated significantly. The activated sludge taken from the aeration tank of the Zhenan sewage treatment plant was seeded to the UNITANK setup. Measurements of the parameters of chemical oxygen demand (COD), suspended solids, total nitrogen (TN), NH4-N, total phosphorus (TP), metallic ions, soluble protein, MLSS, and mixed liquor volatile suspended solids were measured according to the standard methods (State Environment Protection Administration ).
RESULTS AND DISCUSSION Effluent quality COD removal efficiency The UNITANK process maintained a high and stable removal rate for COD over the entire operation period, as
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shown in Figure 2. This result indicates that the insertion of the OSA reactor did not have negative effects on the organic removal efficiency of the UNITANK process.
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period, and the average concentration of NH4-N was 1.1 mg/L. TP removal efficiency
Ammonium stabilization efficiency The microbial nitrogen removal of the conventional activated sludge process is based on aerobic nitrification and anaerobic/anoxic denitrification (Akunna et al. ; Khin & Annachhatre ). In the UNITANK process, the sewage was first treated in the anaerobic D/B basin and was then fed to the aerobic C basin, which facilitated the nitrogen removal to some extent. Figure 3 shows the ammonia removal efficiency over the entire operation period. The insertion of the OSA reactor was found to not have negative effects on the ammonia nitrogen removal efficiency of the UNITANK process, as the NH4-N effluent concentration remained below 6.0 mg/L over the entire operation
Figure 2
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Change in the COD removal rate over the entire operation period.
Figure 3
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Change in the NH4-N removal rate over the entire operation period.
Figure 4 shows the TP removal efficiency over the entire operation period. Before the insertion of the OSA reactor, the TP effluent concentration was between 0.33 and 2.27 mg/L (average of 1.25 mg/L), with a removal rate of 46%. After the insertion of the OSA reactor, the TP effluent concentration was between 0.97 and 2.77 mg/L (average of 1.89 mg/L), with a removal rate of 31%. The results indicate that the TP removal efficiency deteriorated after the insertion of the OSA reactor. The following scenario may explain the deteriorated TP effluent rate. Biological phosphorus removal is achieved by enhanced storage in the biomass as poly-P bacteria. A high phosphorus removal efficiency can be achieved by withdrawing the sludge with high phosphorus content (Machnicka et al. ). With the sludge reduction achieved by the UNITANK–OSA process, the sludge withdrawal rate was substantially reduced, leading to phosphorus accumulation in the process. As a result, the TP effluent rate was correspondingly increased. To reduce the TP effluent concentration of the UNITANK–OSA process, phosphorus removal agents (liquid aluminum sulfate, 20 mg/L by aluminum trioxide) were added into the sludge in the OSA anaerobic reactor so that the TP concentration of the interchange sludge was reduced, as described in Table 1. A significant release of phosphorus in the OSA anaerobic reactor was observed, as the TP concentration measured from the supernatant of the sludge withdrawn from the aerobic basin of the
Figure 4
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Change in the TP removal rate over the entire operation period.
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Table 1
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TP concentration of the interchange sludge after dosing phosphorus removal agents (mg/L)
Dosing
Supernatant of precipitation
Supernatant
Dosing in the
in the
Measurement
sludge of D
of sludge in
supernatant
OSA
date
tank
OSA
of OSA sludge
sludge
18 October
1.55
59.6
1.05
1.16
22 October
1.47
60.2
0.91
1.22
Mean
1.51
59.9
0.98
1.19
UNITANK process was approximately 1.5 mg/L whereas in the OSA anaerobic reactor it approximately 60 mg/L. After precipitation of the anaerobic sludge, we added aluminum sulfate into the supernatant and into the sludge separately, and the TP concentration was decreased to 1 mg/L and 1.2 mg/L, respectively. The results indicate that the OSA process can achieve a desirable TP removal efficiency by adding aluminum sulfate into the supernatant or into the interchange sludge directly. Note that with the high phosphorus content, the sludge precipitation can be utilized as a type of resource. Sludge reduction efficiency Sludge yield is a key parameter to evaluate the sludge reduction efficiency. The sludge yield is determined by Yobs ¼
(MLSS2 MLSS1 ) × Va þ W n P (COD1 COD2 ) × Q
(1)
1
where MLSS2 and MLSS1 are the mean MLSS concentrations of the final phase and beginning phase of the UNITANK process in the experiment, respectively, Va the active volume (60 m3) of the UNITANK process, W is the withdrawn sludge quality of the system over the entire operation period, COD1 and COD2 are the mean influent and effluent concentration every day, respectively, Q is the trean P ted water volume every day, and (COD1 COD2 ) × Q is 1 the total treated organic mass over the entire operation period. Not considering the accumulation of the effluent suspended solids, the sludge yields of the UNITANK and UNITANK–OSA were 0.288 kg-MLSS/kg-COD and 0.151 kg-MLSS/kg-COD, respectively. Compared to the UNITANK process, the UNITANK–OSA process achieved a 48% sludge reduction, which sharply decreased the amount of withdrawn sludge.
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Taking the effluent suspended solids into account as a means of sludge withdrawal, the actual sludge yields of the UNITANK and UNITANK–OSA were 0.336 kg-MLSS/kgCOD and 0.211 kg-MLSS/kg-COD, respectively. Regarding the accumulation of the effluent suspended solids, the UNITANK–OSA process achieved a 38% sludge reduction compared to the UNITANK process. In contrast to the 30–80% sludge reduction efficiency achieved by previous relevant studies (Liu & Tay ; Wei et al. ), the 48 or 38% sludge reduction efficiency achieved in this study was desirable. In addition, this reasonable sludge reduction efficiency was achieved under a low organic loading rate, whose COD influent rate was approximately 100 mg/L. This result implies a great economic benefit if applied to a full-scale sewage treatment plant. Sludge reduction mechanism Prior research indicated that EPS accounts for 80% of the total mass of the sludge (Frolund et al. ), with protein and polysaccharide accounting for 70–80% of the EPS mass (Dignac et al. ). Novak found that metals in EPS can act as bridging agents to chelate protein and polysaccharides (Novak et al. ). To analyze the connection between EPS release and soluble metal, we conducted a study on the soluble protein and metal of the sludge in the system; the results are presented in Table 2. Due to the extremely low concentration of soluble polysaccharide, which was difficult to detect, the soluble polysaccharide concentration is not presented in Table 2. The results indicated that EPS had a certain degree of release under anaerobic conditions, as the concentration of soluble protein in anaerobic sludge was almost twice that in aerobic sludge. The release of Mg and K exhibited the same trend as that of the EPS release under anaerobic conditions. Given that protein coagulated with Mg2þ, namely the lectin-like material as one of the existing forms of protein in sludge (Novak et al. ), the result indicated that soluble protein was released from EPS. EPS released a large quantity of degradable protein and 2þ Mg under anaerobic conditions when the interchange sludge was fed into the OSA anaerobic reactor, while the
Table 2
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Concentrations of soluble protein and metals in the sludge (mg/L)
Protein
Mg
Ca
Fe
Al
K
Na
Aerobic sludge
39.77
38
45
2.89
0.72
9.96
20.16
Anaerobic sludge
77.22
136
25
0.43
0.11
25
19.19
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concentration of Kþ was increased because a large number of microbes were dissolved (Bakker ). Because the anaerobic sludge returned to the UNITANK aerobic unit, the released protein was thoroughly degraded. Meanwhile, we postulated that Mg2þ was re-coagulated with protein to form lectin-like materials and that Kþ was absorbed into the microbial cytoplasm. Because of the constant interchange of sludge, the lectin-like materials were in a dissociation–degradation–complexation cycle, so that the sludge was reduced in the study. However, Novak found that EPS with iron were more likely to be degraded under anaerobic conditions, while degradation of EPS with magnesium was more likely under aerobic conditions (Park et al. ). In this study, the EPS release under anaerobic conditions was with magnesium. There are two reasons for the difference: (1) Novak’s study was based on a long-term aerobic digestion reactor that could last for a 50-day uninterrupted aerobic digestion process, aimed at the release of calcium and magnesium under aerobic conditions, unlike our study in which the release is not likely to occur because the implementation of aerobic conditions was part of an intermittent short-term process; and (2) the previous study was based on the use of a synthetic wastewater, while in this study the sewage was from a sewage treatment plant with a relatively high concentration of magnesium. In addition, there are research studies presenting the same conclusion as ours (Sun et al. ). The favorable condition of complexation and dissociation for EPS and metals remains to be further investigated.
CONCLUSIONS Based on a 120-m3/d pilot-scale experiment, using actual sewage, this study first performed the UNITANK process and then activated the OSA reactor to proceed with the UNITANK–OSA process. Over the entire operation period, the effluent quality did not change significantly, except that the TP effluent rate deteriorated. Compared with the UNITANK process, the UNITANK–OSA process achieved a 48% sludge reduction, not considering the accumulation of the effluent suspended solids, while the reduction was 38% if the effluent suspended solids accumulation was taken into account. Comparing the concentrations of protein and soluble metals in the aerobic unit with those in the anaerobic unit, magnesium and protein were found to be released under anaerobic conditions; subsequently, the soluble protein was degraded and the magnesium was chelated with new
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protein. It is in this cycle that the UNITANK–OSA process achieved the desirable sludge reduction efficiency.
ACKNOWLEDGEMENTS This work was supported by the Science & Research Program of Guangdong (contract no. 2012A032300005, 2012B091000029), the Science & Research Development Program of Foshan (contract no. 2012HY100531, 2012AA100091), and the Chancheng District Research Special Fund (2012B1002).
REFERENCES Akunna, J. C., Bizeau, C. & Moletta, R. Nitrate and nitrite reductions with anaerobic sludge using various carbon sources: glucose, glycerol, acetic acid, lactic acid and methanol. Water Res. 27 (8), 1303–1312. Bakker, E. P. Alkali Cation Transport Systems in Prokaryotes. CRC Press, Boca Raton, FL, USA. Chen, G. H., Mo, H. K., Saby, S., Yip, W. K. & Liu, Y. Minimization of activated sludge production by chemically stimulated energy spilling. Water Sci. Technol. 42, 189–200. Chen, G. H., Mo, H. K. & Liu, Y. Utilization of a metabolic uncoupler, 3,30 ,40 ,5-tetrachlorosalicylanilide (TCS) to reduce sludge growth in activated sludge culture. Water Res. 36 (8), 2077–2083. Chen, G. H., An, K. J., Saby, S., Brois, E. & Djafer, M. Possible cause of excess sludge reduction in an oxic-settlinganaerobic activated sludge process (OSA process). Water Res. 37 (16), 3855–3866. Chudoba, P., Morel, A. & Capdeville, B. The case of both energetic uncoupling and metabolic selection of microorganisms in the OSA activated sludge system. Environ. Technol. 13 (8), 761–770. Copp, J. B. & Dold, P. L. Comparing sludge production under aerobic and anoxic conditions. Water Sci. Technol. 38 (1), 285–294. Dignac, M. F., Urbain, V., Rybacki, D., Bruchet, A., Snidaro, D. & Scribe, P. Chemical description of extracellular polymers: implication on activated sludge floc structure. Water Sci. Technol. 38 (8–9), 45–53. Frolund, B., Palmgren, R., Keiding, K. & Nielsen, P. H. Extraction of extracellular polymers from activated sludge using a cation exchange resin. Water Res. 30 (8), 1749–1758. Ghigliazza, R., Lodi, A., Converti, A., Nicolella, C. & Rovatti, M. Influence of the ratio of the initial substrate concentration to biomass concentration on the performance of a sequencing batch reactor. Bioproc. Biosyst. Eng. 14 (3), 131–137. Hendrickx, T. L. G., Temmink, H., Elissen, H. J. H. & Buisman, C. J. N. The effect of operating conditions on aquatic worms eating waste sludge. Water Res. 43 (4), 943–950.
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Khin, T. & Annachhatre, A. P. Novel microbial nitrogen removal processes. Biotechnol. Adv. 22 (7), 519–532. Liao, Z. L., Zhu, B. R. & Ye, J. F. Progress of UNITANK process and mechanism. Technol. Water Treat. 35 (1), 20–23, 29. Liu, Y. Effect of chemical uncoupler on the observed growth yield in batch culture of activated sludge. Water Res. 34 (7), 2025–2030. Liu, Y. Chemically reduced excess sludge production in the activated sludge process. Chemosphere 50 (1), 1–7. Liu, Y. & Tay, J. H. Strategy for minimization of excess sludge production from the activated sludge process. Biotechnol. Adv. 19 (2), 97–107. Low, E. W. & Chase, H. A. Reducing production of excess biomass during wastewater treatment. Water Res. 33 (5), 1119–1132. Low, E. W., Chase, H. A., Milner, M. G. & Curtis, T. P. Uncoupling of metabolism to reduce biomass production in the activated sludge process. Water Res. 34 (12), 3204–3212. Machnicka, A., Grubel, K. & Suschka, J. Enhanced biological phosphorus removal and recovery. Water Environ. Res. 80 (7), 617–623. Martinage, V. & Paul, E. Effect of environmental parameters on autotrophic decay rate (bA). Environ. Technol. 21 (1), 31–41.
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Novak, J. T., Sadler, M. E. & Murthy, S. N. Mechanisms of floc destruction during anaerobic and aerobic digestion and the effect on conditioning and dewatering of biosolids. Water Res. 37 (13), 3136–3144. Park, C., Muller, C. D., Abu-orf, M. M. & Novak, J. T. The effect of wastewater cations on activated sludge characteristics: effects of aluminum and iron in floc. Water Environ. Res. 78 (1), 31–40. State Environment Protection Administration. Monitoring and Analysis Methods of Water and Waste Water, 4th edn. China Environmental Science Press, Beijing. Sun, L. P., Randall, C. W. & Novak, J. T. The influence of sludge interchange times on the oxic-settling-anoxic process. Water Environ. Res. 82 (6), 519–520. Wei, Y. S., Van Houten, R. T., Borger, A. R., Eikelboom, D. H. & Fan, Y. B. Minimization of excess sludge production for biological wastewater treatment. Water Res. 37 (18), 4453–4467. Ye, F. & Li, Y. Oxic-settling-anoxic (OSA) process combined with 3,30 ,40 ,5-tetrachlorosalicylanilide (TCS) to reduce excess sludge production in the activated sludge system. Biochem. Eng. J. 49 (2), 229–234. Zhu, H. & Chen, J. H. Study of hydrolysis and acidification process to minimize excess biomass production. J. Hazard. Mater. 127 (1–3), 221–227.
First received 1 August 2014; accepted in revised form 10 November 2014. Available online 22 November 2014
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© IWA Publishing 2015 Water Science & Technology
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71.1
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2015
Study of the sludge reduction in an oxic–settling– anaerobic activated sludge process based on UNITANK L. P. Sun, J. F. Chen, W. Z. Guo, X. P. Fu, J. X. Tan and T. J. Wang
ABSTRACT An oxic–settling–anaerobic process (OSA) can effectively reduce sludge production, but most of the research studies on the OSA process have been either under laboratory test conditions or based on synthetic wastewater, which cannot fully reflect the performance and sludge reduction efficiency in existing OSA process. Thus, aiming at examining the sludge reduction efficiency and the stability of the OSA process, UNITANK and UNITANK–OSA processes were performed in a 120 m3/d pilot-scale system using actual sewage. The results indicate that UNITANK–OSA achieved a 48% reduction of the sludge compared to the reduction due to UNITANK, not considering the accumulation of the effluentsuspended solids. The effluent quality was not found to change significantly, except that the total phosphorus concentration increased slightly. The extracellular polymeric substances metal floc theory may, to some extent, explain this reduction in this study. The OSA process could be used to reform the classic wastewater treatment process to get lower sludge mass. Key words
| pilot-scale study, sludge reduction, sludge source reduction, wastewater treatment
L. P. Sun (corresponding author) J. F. Chen J. X. Tan T. J. Wang School of Environmental Science and Engineering, Sun Yat-sen University, No.135 Xingang Road West, Guangzhou 510275, China E-mail: [email protected] L. P. Sun Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Guangzhou 510275, China W. Z. Guo X. P. Fu Foshan Water Group, Foshan 528000, China
INTRODUCTION Conventional activated sludge processes are widely used in sewage treatment because of three main advantages: (1) high organic pollutant removal efficiency; (2) stable operation; and (3) low-operation cost. However, a large amount of excess sludge is generated due to the degradation of organic pollutants (Ye & Li ). The treatment and ultimate disposal of excess sludge are expensive, which usually accounts for 20–60% of the total operational cost in a conventional activated sludge treatment plant (Low & Chase ; Liu ). Meanwhile, the ultimate disposal of the sludge through the use of landfills and/or incineration is facing difficulties, such as exceeding the landfill capacity and the secondary pollution caused by either incineration of the sludge or the landfills (Chen et al. ). Therefore, new approaches that could reduce the excess sludge have received wide attention and are under extensive study (Low et al. ; Hendrickx et al. ). Those approaches have achieved different levels of sludge reduction but have not yet been widely used in practice due to the problems of expensive operational cost, negative effects on sludge characteristics, or the effluent quality. Westgarth first modified a conventional activated doi: 10.2166/wst.2014.474
sludge process in the laboratory by inserting a sludge holding tank in the sludge return circuit to form an oxic–settling–anaerobic process (OSA process), which effectively reduces the sludge production. Subsequently, research studies on the OSA process have been conducted and have achieved 20–80% reduction of the sludge (Chudoba et al. ; Ghigliazza et al. ; Copp & Dold ; Chen et al. ). Three main scenarios have been addressed to explain the reduction of sludge in the OSA process: (1) energy uncoupling (Chen et al. ; Liu ); (2) sludge decay in the sludge holding tank (Martinage & Paul ; Chen et al. ; Zhu & Chen ); and (3) extracellular polymeric substances (EPS) metal floc theory (Novak et al. ; Park et al. ). However, most of the research studies on the OSA process are either under laboratory test conditions or based on synthetic wastewater, which cannot fully reflect the performance and sludge reduction efficiency in the existing OSA process. Thus, aiming at examining the sludge reduction efficiency and the stability of the OSA process, we conducted a pilot-scale experiment using actual sewage to promote a full-scale practical application.
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Study of the sludge reduction based on UNITANK
METHODS Experimental apparatus Based on a UNITANK setup (Liao et al. ), we constructed a 120-m³/d pilot-scale UNITANK–OSA setup, as illustrated in Figure 1. The sizes of B, C, and D basins were same: they were 2.3, 2.3, and 4.2 m in length, width, and height, respectively, and the effective volume of one basin was about 20 m³. The size of the OSA anaerobic basin was 2.5, 2.0, and 1.7 m in length, width, and height; the effective volume of the OSA anaerobic basin was about 7.1 m³. The hydraulic residence time (HRT) of the UNITANK process was 12 hours, and the HRT of the OSA anaerobic basin was 5 days. Sludge from the B basin and the D basin of the UNITANK was pumped to the OSA anaerobic reactor, the electric reflux valve fitted on B and D basins would be opened automatically when they were in the final stage of the settling process. After the anaerobic reaction, the sludge was pumped from the OSA anaerobic reactor to the C basin of the UNITANK. An oxidation–reduction potential meter, a flow meter, and a pH online monitor were fitted on the OSA anaerobic reactor, and all were automatically controlled by a sequence controller.
Operation of the UNITANK–OSA system First, the UNITANK setup was operated independently for 60 days. After the 60-day operation, the UNITANK process became stable. The UNITANK was then modified as a UNITANK–OSA system by inserting an OSA anaerobic reactor, which was run another 60 days to compare the effluent quality and the sludge production with those of the UNITANK in the first 60 days. The operation cycle for the UNITANK and the sludge interchange cycle for the UNITANK–OSA were 8 and 4 hours, respectively. The phases of the UNITANK–OSA operation are as follows: (1) the sludge was pumped from the bottom of basin B
Figure 1
|
Schematic diagram of the UNITANK–OSA setup.
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or D into the OSA anaerobic reactor in sequence for 15 minutes every 4 hours; and (2) the sludge remained in the OSA anaerobic reactor for 5 days before it was pumped out to the C basin. The sludge interchange rate (which was the rate of interchange sludge quality from the OSA anaerobic reactor into the UNITANK and the total sludge quality in the UNITANK tank), the sludge retention time, and the sludge interchange times of the OSA anaerobic reactor were 10%, 5 days, and six times per day, respectively. The sludge withdrawal rate was precisely determined each time to maintain the mixed liquor suspended solids (MLSS) level of the C basin at 2000–3000 mg/L. Materials and methods of analysis The sewage used in this pilot-scale experiment was from the grit chamber of the Zhenan sewage treatment plant in Foshan city. The collection system of the plant is a combined system that includes a certain amount of industrial wastewater as well. As a result, the influent quality during the operation fluctuated significantly. The activated sludge taken from the aeration tank of the Zhenan sewage treatment plant was seeded to the UNITANK setup. Measurements of the parameters of chemical oxygen demand (COD), suspended solids, total nitrogen (TN), NH4-N, total phosphorus (TP), metallic ions, soluble protein, MLSS, and mixed liquor volatile suspended solids were measured according to the standard methods (State Environment Protection Administration ).
RESULTS AND DISCUSSION Effluent quality COD removal efficiency The UNITANK process maintained a high and stable removal rate for COD over the entire operation period, as
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shown in Figure 2. This result indicates that the insertion of the OSA reactor did not have negative effects on the organic removal efficiency of the UNITANK process.
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period, and the average concentration of NH4-N was 1.1 mg/L. TP removal efficiency
Ammonium stabilization efficiency The microbial nitrogen removal of the conventional activated sludge process is based on aerobic nitrification and anaerobic/anoxic denitrification (Akunna et al. ; Khin & Annachhatre ). In the UNITANK process, the sewage was first treated in the anaerobic D/B basin and was then fed to the aerobic C basin, which facilitated the nitrogen removal to some extent. Figure 3 shows the ammonia removal efficiency over the entire operation period. The insertion of the OSA reactor was found to not have negative effects on the ammonia nitrogen removal efficiency of the UNITANK process, as the NH4-N effluent concentration remained below 6.0 mg/L over the entire operation
Figure 2
|
Change in the COD removal rate over the entire operation period.
Figure 3
|
Change in the NH4-N removal rate over the entire operation period.
Figure 4 shows the TP removal efficiency over the entire operation period. Before the insertion of the OSA reactor, the TP effluent concentration was between 0.33 and 2.27 mg/L (average of 1.25 mg/L), with a removal rate of 46%. After the insertion of the OSA reactor, the TP effluent concentration was between 0.97 and 2.77 mg/L (average of 1.89 mg/L), with a removal rate of 31%. The results indicate that the TP removal efficiency deteriorated after the insertion of the OSA reactor. The following scenario may explain the deteriorated TP effluent rate. Biological phosphorus removal is achieved by enhanced storage in the biomass as poly-P bacteria. A high phosphorus removal efficiency can be achieved by withdrawing the sludge with high phosphorus content (Machnicka et al. ). With the sludge reduction achieved by the UNITANK–OSA process, the sludge withdrawal rate was substantially reduced, leading to phosphorus accumulation in the process. As a result, the TP effluent rate was correspondingly increased. To reduce the TP effluent concentration of the UNITANK–OSA process, phosphorus removal agents (liquid aluminum sulfate, 20 mg/L by aluminum trioxide) were added into the sludge in the OSA anaerobic reactor so that the TP concentration of the interchange sludge was reduced, as described in Table 1. A significant release of phosphorus in the OSA anaerobic reactor was observed, as the TP concentration measured from the supernatant of the sludge withdrawn from the aerobic basin of the
Figure 4
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Change in the TP removal rate over the entire operation period.
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Table 1
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TP concentration of the interchange sludge after dosing phosphorus removal agents (mg/L)
Dosing
Supernatant of precipitation
Supernatant
Dosing in the
in the
Measurement
sludge of D
of sludge in
supernatant
OSA
date
tank
OSA
of OSA sludge
sludge
18 October
1.55
59.6
1.05
1.16
22 October
1.47
60.2
0.91
1.22
Mean
1.51
59.9
0.98
1.19
UNITANK process was approximately 1.5 mg/L whereas in the OSA anaerobic reactor it approximately 60 mg/L. After precipitation of the anaerobic sludge, we added aluminum sulfate into the supernatant and into the sludge separately, and the TP concentration was decreased to 1 mg/L and 1.2 mg/L, respectively. The results indicate that the OSA process can achieve a desirable TP removal efficiency by adding aluminum sulfate into the supernatant or into the interchange sludge directly. Note that with the high phosphorus content, the sludge precipitation can be utilized as a type of resource. Sludge reduction efficiency Sludge yield is a key parameter to evaluate the sludge reduction efficiency. The sludge yield is determined by Yobs ¼
(MLSS2 MLSS1 ) × Va þ W n P (COD1 COD2 ) × Q
(1)
1
where MLSS2 and MLSS1 are the mean MLSS concentrations of the final phase and beginning phase of the UNITANK process in the experiment, respectively, Va the active volume (60 m3) of the UNITANK process, W is the withdrawn sludge quality of the system over the entire operation period, COD1 and COD2 are the mean influent and effluent concentration every day, respectively, Q is the trean P ted water volume every day, and (COD1 COD2 ) × Q is 1 the total treated organic mass over the entire operation period. Not considering the accumulation of the effluent suspended solids, the sludge yields of the UNITANK and UNITANK–OSA were 0.288 kg-MLSS/kg-COD and 0.151 kg-MLSS/kg-COD, respectively. Compared to the UNITANK process, the UNITANK–OSA process achieved a 48% sludge reduction, which sharply decreased the amount of withdrawn sludge.
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Taking the effluent suspended solids into account as a means of sludge withdrawal, the actual sludge yields of the UNITANK and UNITANK–OSA were 0.336 kg-MLSS/kgCOD and 0.211 kg-MLSS/kg-COD, respectively. Regarding the accumulation of the effluent suspended solids, the UNITANK–OSA process achieved a 38% sludge reduction compared to the UNITANK process. In contrast to the 30–80% sludge reduction efficiency achieved by previous relevant studies (Liu & Tay ; Wei et al. ), the 48 or 38% sludge reduction efficiency achieved in this study was desirable. In addition, this reasonable sludge reduction efficiency was achieved under a low organic loading rate, whose COD influent rate was approximately 100 mg/L. This result implies a great economic benefit if applied to a full-scale sewage treatment plant. Sludge reduction mechanism Prior research indicated that EPS accounts for 80% of the total mass of the sludge (Frolund et al. ), with protein and polysaccharide accounting for 70–80% of the EPS mass (Dignac et al. ). Novak found that metals in EPS can act as bridging agents to chelate protein and polysaccharides (Novak et al. ). To analyze the connection between EPS release and soluble metal, we conducted a study on the soluble protein and metal of the sludge in the system; the results are presented in Table 2. Due to the extremely low concentration of soluble polysaccharide, which was difficult to detect, the soluble polysaccharide concentration is not presented in Table 2. The results indicated that EPS had a certain degree of release under anaerobic conditions, as the concentration of soluble protein in anaerobic sludge was almost twice that in aerobic sludge. The release of Mg and K exhibited the same trend as that of the EPS release under anaerobic conditions. Given that protein coagulated with Mg2þ, namely the lectin-like material as one of the existing forms of protein in sludge (Novak et al. ), the result indicated that soluble protein was released from EPS. EPS released a large quantity of degradable protein and 2þ Mg under anaerobic conditions when the interchange sludge was fed into the OSA anaerobic reactor, while the
Table 2
|
Concentrations of soluble protein and metals in the sludge (mg/L)
Protein
Mg
Ca
Fe
Al
K
Na
Aerobic sludge
39.77
38
45
2.89
0.72
9.96
20.16
Anaerobic sludge
77.22
136
25
0.43
0.11
25
19.19
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concentration of Kþ was increased because a large number of microbes were dissolved (Bakker ). Because the anaerobic sludge returned to the UNITANK aerobic unit, the released protein was thoroughly degraded. Meanwhile, we postulated that Mg2þ was re-coagulated with protein to form lectin-like materials and that Kþ was absorbed into the microbial cytoplasm. Because of the constant interchange of sludge, the lectin-like materials were in a dissociation–degradation–complexation cycle, so that the sludge was reduced in the study. However, Novak found that EPS with iron were more likely to be degraded under anaerobic conditions, while degradation of EPS with magnesium was more likely under aerobic conditions (Park et al. ). In this study, the EPS release under anaerobic conditions was with magnesium. There are two reasons for the difference: (1) Novak’s study was based on a long-term aerobic digestion reactor that could last for a 50-day uninterrupted aerobic digestion process, aimed at the release of calcium and magnesium under aerobic conditions, unlike our study in which the release is not likely to occur because the implementation of aerobic conditions was part of an intermittent short-term process; and (2) the previous study was based on the use of a synthetic wastewater, while in this study the sewage was from a sewage treatment plant with a relatively high concentration of magnesium. In addition, there are research studies presenting the same conclusion as ours (Sun et al. ). The favorable condition of complexation and dissociation for EPS and metals remains to be further investigated.
CONCLUSIONS Based on a 120-m3/d pilot-scale experiment, using actual sewage, this study first performed the UNITANK process and then activated the OSA reactor to proceed with the UNITANK–OSA process. Over the entire operation period, the effluent quality did not change significantly, except that the TP effluent rate deteriorated. Compared with the UNITANK process, the UNITANK–OSA process achieved a 48% sludge reduction, not considering the accumulation of the effluent suspended solids, while the reduction was 38% if the effluent suspended solids accumulation was taken into account. Comparing the concentrations of protein and soluble metals in the aerobic unit with those in the anaerobic unit, magnesium and protein were found to be released under anaerobic conditions; subsequently, the soluble protein was degraded and the magnesium was chelated with new
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protein. It is in this cycle that the UNITANK–OSA process achieved the desirable sludge reduction efficiency.
ACKNOWLEDGEMENTS This work was supported by the Science & Research Program of Guangdong (contract no. 2012A032300005, 2012B091000029), the Science & Research Development Program of Foshan (contract no. 2012HY100531, 2012AA100091), and the Chancheng District Research Special Fund (2012B1002).
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First received 1 August 2014; accepted in revised form 10 November 2014. Available online 22 November 2014
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