- Email: [email protected]
Estimation of the self-purification capacity of biofilm formed in domestic sewer pipes
Biochemical Engineering Journal 31 (2006) 96–101
Short communication
Estimation of the self-purification capacity of biofilm formed in domestic sewer pipes Yasunori Tanji ∗ , Rie Sakai, Kazuhiko Miyanaga, Hajime Unno Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8501, Japan Received 18 August 2005; received in revised form 3 March 2006; accepted 27 May 2006
Abstract To estimate the self-purification capacity of sewer pipe, six different types of concrete blocks were installed in a domestic sewer pipe for nine months. The concrete blocks used were plain, grain, porous, and wet concrete (no-hole, or perforated with holes of d = 1 or 12 mm). After a 79-day exposure to sewage, a heterogeneous biofilm formed on the surface of each block. The self-purification capacities of the blocks were estimated by measuring the decrease in substrate concentration in artificial sewage. The analyzed substrates were dissolved oxygen (DO), total organic carbon (TOC), NH4 + , and NO3 − . Wet concrete with holes (d = 12 mm) showed the highest substrate consumption rates: DO = 460, TOC = 480, NH4 –N = 87, and nitrogen as NO3 –N = 170, in mg substrate/(m2 h). These results indicate that sewers have a considerable potential for removing organic material and nutrients, and modification of sewer surfaces may increase these activities. © 2006 Elsevier B.V. All rights reserved. Keywords: Self-purification; Wastewater treatment; Sewage pipes; Immobilization; Nitrification; Denitrification
1. Introduction Sewage quality varies during transport from the source to the wastewater treatment plant. Changes in the biological oxygen demand (BOD) or the chemical oxygen demand (COD), according to the flow of sewage through sewer pipes, have been reported [1–5]. Several researchers have analyzed the in situ self-purification capacity of sewer pipes. For instance, Chen examined the utilization of oxygen in a sanitary gravity sewer and found that 14% of dissolved organic carbon (DOC) in the sewage was removed after flowing 1.5 km in a sewer pipe for 18 min, and he estimated that the liquid sewage phase was responsible for 40% of the DOC removal, while the sewage sediment contributed to 60% of the removal [6–8]. However, engineers do not consider the impacts of the self-purification capacity of sewer pipe on the wastewater treatment process. This is partially due to the complexity of the chemical, physical, and biological processes occurring in the sewer pipes. Furthermore, since the water quality of sewage changes by the minute,
∗
Corresponding author. Tel.: +81 45 924 5762; fax: +81 45 924 5818. E-mail address: [email protected] (Y. Tanji).
1369-703X/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.bej.2006.05.021
in situ evaluation of pipes’ capacity for self-purification is difficult. The concentration of microorganisms in a sewer system is low relative to the substrate concentration. In other words, the food/microorganisms (F/M) ratio in a sewer system is relatively high and, therefore, the substrate does not limit the growth of the microorganisms. To improve the self-purification capacity, a low F/M ratio is desirable [9], and there is a possible way to reduce the F/M ratio. That is to use sewers with rougher wall surfaces to facilitate biofilm attachment. Rougher interior wall surfaces provide more surface area for biofilm to adhere. Previously, in order to endow sewer pipes with the capacity for self-purification, a porous ceramic support was installed in a model sewer pipe and the removal of total organic carbon (TOC), NH4 − , and NO3 − from model sewage was analyzed [10]. The results indicated that the combination of aeration and biofilm development enhanced the simultaneous removal of organic carbon and nitrogen. However, the structure of the biofilm formed in the model sewer was thought to differ from that of real sewer pipe. In this study, in order to assess and improve the self-purification capacity of real sewer pipes, concrete blocks with six different types of surfaces were installed in a working sewer system and exposed to sewage for 79 days. The blocks were then used to assay
Y. Tanji et al. / Biochemical Engineering Journal 31 (2006) 96–101
the self-purification capacities of the various surfaces for nine months.
97
thetic wastewater (Fig. 1D-b). TOC was analyzed using a TOC analyzer (Shimadzu TOC-5000A, Shimadzu Co. Ltd., Tokyo, Japan).
2. Materials and methods 2.1. Concrete blocks for biofilm formation in a working sewer Six types of concrete blocks were immersed in sewage flow (Fig. 1A and B): plain Portland concrete, grain concrete, porous concrete, and three types of wet concrete. The wet concrete was either non-perforated, or perforated with d = 1 mm × depth = 5 mm (No. 5) or d = 12 mm × depth = 10 mm (No. 6) holes. The composition (wt.%) of the Portland and grain concretes was 52% 3CaOSiO2 , 24% 2CaOSiO2 , 9% 3CaOAl2 O3 , 9% 4CaOAl2 O3 Fe2 O3 , and 6% other. The grain concrete was an agglomerate of concrete granules (3–5 mm), with a porosity of 20–25%. The composition (wt.%) of the porous concrete was 78% SiO2 , 16%Al2 O3 , 1.3% Fe2 O, 0.86% CaO, and 2.6% SO3 , and the void was 88%. Wet concrete was reinforced with plant fibers, and the size of each block was 50 mm × 50 mm × 20 mm. The faces of the concrete blocks, except the front, were coated with epoxy resin. Surface area of each block was 25 cm2 (5 cm × 5 cm). Six blocks were fixed in the poly vinyl chloride (PVC) holder (Fig. 1C). Two sets of six blocks were immersed in the flow of sewage. The sanity gravity sewer examined in this study was located in the Suzukakedai campus of the Tokyo Institute of Technology, which receives material from about 3000 people. The sewage in this area is primarily domestic wastewater, and the experimental wastewater and rainwater flow separately to the domestic wastewater. Experiments were conducted between June 2004 and February 2005. During the initial 79-day study, two sets of blocks were immersed in the flow without lab experiments. Next, a set of blocks was used in laboratory experiments to analyze the selfpurification capacities of the blocks. After each experiment, the set of blocks was returned to the sewage flow for continued sewage exposure. Each lab experiment took two days. 2.2. Determination of dissolved oxygen and organic carbon uptake rates The decrease in dissolved oxygen (DO) concentrations was used to measure the activity of biofilm formed on the concrete blocks. The epoxy resin-coated faces of the blocks were washed with brushes to remove biofilm attached to the resin. After washing, each block was placed in a closed vessel (Fig. 1D-a) that was then filled with air and saturated with 300 ml synthetic wastewater of the following composition (per liter): 26.7 mg carbon as glucose, 106.7 mg carbon as polypepton, 5 mg KH2 PO4 , and 5 mg NaHCO3 . The composition of the synthetic wastewater was determined based on the analysis of TOC concentrations in the campus sewage. The DO concentration in the medium was continuously monitored using a DO sensor (OBS Digital DO controller, Tokyo, Japan). The biological activity of the biofilm was also analyzed by measuring TOC uptake rates using open vessels filled with air and saturated with 200 ml syn-
2.3. Nitrification and denitrification rates by biofilm formed on the concrete surfaces The nitrification and denitrification activities of the biofilm formed on the concrete blocks were analyzed. For the nitrification analysis, a block was placed in an open vessel (Fig. 1Db), filled with air saturated 200 ml of synthetic wastewater of the following composition (per liter): 60 mg carbon as glucose, 40 mg nitrogen as (NH4 )2 SO4 , 12.6 mg KH2 PO4 , 19.4 mg MgSO4 ·7H2 O, 84 mg NaHCO3 , 1.2 mg CaCl2 ·2H2 O, and 0.1 mg FeCl3 ·6H2 O. For denitrification analysis, a block was placed in a closed vessel (Fig. 1D-a), filled with pure nitrogen gas saturated 300 ml synthetic wastewater of the following composition (per liter): 60 mg carbon as glucose, 40 mg nitrogen as KNO3 , 12.6 mg KH2 PO4 , 19.4 mg MgSO4 ·7H2 O, 84 mg NaHCO3 , 1.2 mg CaCl2 ·2H2 O, and 0.1 mg FeCl3 ·6H2 O. The synthetic wastewater used for the nitrification and denitrification tests was sampled periodically, diluted with distilled water, and filtered through a mixed cellulose-ester membrane filter of 0.2 m mean pore size. Various nitrogenous ions were then measured by ion-chromatography (SCL-10A; Shimazu, Kyoto, Japan). 3. Results and discussion 3.1. Biofilm formation on concrete blocks Since the pH of Portland cement is between 11 and 12, the formation of biofilm on the surface of the concrete was expected to be slow. After the 79-day exposure to sewage flow, however, the front surfaces of six different concrete blocks (including Portland cement) were completely covered by biofilm. After that, lab experiments and sewage exposure continued for an additional 120 days. The voids of grain concrete and perforated wet concrete were filled by biofilm. Five holes of perforated wet concrete (d = 12 mm) were filled with living organisms. Microscopic observation revealed that the biofilm was composed of two layers: a heterogeneous non-dense layer, which was very rough and primarily composed of filamentous microorganisms, and a dense layer composed of abiotic components, compressed filamentous microorganisms, protozoa such as Vorticella and Litonotus, metazoa such as Diplogaster, and many different kinds of bacteria. Tubifex was occasionally found in the biofilm. After the 79-day sewage exposure, the exterior appearances of all the block surfaces were nearly identical. The enlargement and detachment of the biofilm from the blocks was thought to equilibrate after long-term sewage exposure. 3.2. Biofilm activity The aerobic activity of the biofilm formed on the concrete blocks was analyzed by measuring DO and TOC uptake rates. Weekly, a set of concrete blocks was transferred to the lab and
98
Y. Tanji et al. / Biochemical Engineering Journal 31 (2006) 96–101
Fig. 1. (A) Photographs of concrete blocks and (B) classification of the concrete. “No.” corresponds to the number in (A). (C) Photograph of a set of concrete blocks in the PVC holder. (D) Reactor vessels for analyzing the self-purification capacity of the concrete blocks: (a) closed vessel for measuring DO and NO3 − consumption and (b) open vessel for measuring TOC and NH4 + consumption.
Y. Tanji et al. / Biochemical Engineering Journal 31 (2006) 96–101
99
Fig. 2. (A-1 and A-2) Decrease in DO concentrations in the closed vessel. (B-1 and B-2) Decrease in TOC concentrations in the open vessel. Open circle, plain concrete; open triangle, grain concrete; open square, porous concrete; closed circle, wet concrete (plain); closed triangle, wet concrete (d = 1 mm); closed square, wet concrete (d = 12 mm); n = 4.
used for experiments. Decreases in DO and TOC in the vessels filled with air saturated artificial sewage are shown in Fig. 2. To normalize the dates, values relative to the initial DO and TOC concentrations are used in the figure. DO experiments were conducted on 10 randomly selected days, and the average initial DO concentration was 8.49 g/m3 . DO decreases were linear under these experimental conditions. We initially predicted that biofilm formation on the plain concrete might be scarce compared to that of other conditions. However, biofilm formed on the surface of the plain concrete and showed DO uptake activ-
ity. The slope of each line in the figures indicates the oxygen consumption rate of the particular block. Porous (void, 88%) and wet concrete (d = 12 mm) showed relatively high rates of oxygen consumption. After exposure to sewage, the voids of the porous concrete were filled with microorganisms, enhancing this activity. Perforation of the big holes on the block surfaces substantially increased the surface area, increasing the DO uptake rate. However, the differences in DO uptake rates between the six different blocks were not great, indicating that the surface material has little effect on DO uptake rates.
Fig. 3. (A-1 and A-2) Decrease in NH4 + concentrations in the open vessel. (B-1 and B-2) Decrease in T NO3 − concentrations in the open vessel. Open circle, plain concrete; open triangle, grain concrete; open square, porous concrete; closed circle, wet concrete (plain); closed triangle, wet concrete (d = 1 mm); closed square, wet concrete (d = 12 mm).
100
Y. Tanji et al. / Biochemical Engineering Journal 31 (2006) 96–101
TOC uptake rates were examined and are shown in Fig. 2B. Experiments were conducted on four randomly selected days, and the average initial TOC concentration was 81.4 g/m3 . The trend in decreasing TOC was similar to that of DO, and the porous and wet concrete (d = 12 mm) had relatively high rates of TOC consumption compared to the other types of concrete. The aerobic oxidation of TOC consumed DO in the artificial sewage. 3.3. Biofilm conversion of nitrogen Since the NH4 –N concentration in the sewage was relatively high, we analyzed nitrogen conversion by the biofilm (Fig. 3). Nitrogen elimination from sewage proceeds via two steps: nitrification and denitrification [11,12]. Most of the nitrogen in sewage exists as NH4 + . Therefore, nitrification must occur if nitrogen is to be eliminated from sewage. Nitrification experiments were conducted two times, and denitrification three times, on randomly selected days. NH4 + decreases in the open vessel saturated with air were proportional to incubation time. The composition and void of the concrete blocks did not influence the decrease in NH4 + (Fig. 3A-1). After 8 h incubation, the NH4 –N conversion was 15–22%. Wet concrete with large holes indicated a conversion rate of 22%, and the others showed approximately 15%. Since the NH4 –N conversion is an aerobic reaction, the surface area exposed to the DO saturated bulk liquid contributed to the reaction. The denitrification reaction was rapid relative to the nitrification reaction. Although the initial concentration of nitro-
gen in this test was the same as in the nitrification test, the NO3 − conversion rate after 6 h incubation was estimated as greater than 80% for all conditions (Fig. 3B). During the NO3 − consumption experiment, no NO2 − was detected. Differences between the six conditions were minimal. Of note, the nitrifying bacteria grow much slower than the heterogeneous bacteria that dissolve organic carbon. At the same time, nitrification needs an oxygen supply, whereas denitrification proceeds under anoxic conditions and requires organic carbon. Nitrifying bacteria located within the upper area of the biofilm utilize DO and NH4 + in the sewage. On the other hand, denitrificans were located primarily within the innermost area of the biofilm [13–15]. Biofilm formed on the concrete blocks and showed both nitrification and denitrification activities. However, the rate-limiting step for the removal of sewage nitrogen appears to be nitrification. The use of concrete with a rough surface, such as wet concrete with large holes, may enhance the nitrification reaction and eventual nitrogen removal. 3.4. The self-purification capacity of sewer pipe Based on the results of this study, we estimated the selfpurification capacities of sewer pipe. The substrate uptake rates (Figs. 2 and 3) of the concrete blocks showed a zero-order kinetic dependency on substrate concentration during the initial stage of the reaction. The biological activities of the biofilm might be proportional to the film’s surface area. The conversion rates of DO, TOC, NH4 + , and NO3 − were recalculated as a rate
Fig. 4. Substrate consumption rates by concrete block. Substrates are: (A) DO; (B) TOC; (C) NH4 + ; (D) NO3 − .
Y. Tanji et al. / Biochemical Engineering Journal 31 (2006) 96–101
per unit of surface area and time, and are compared in Fig. 4. Divide volumetric sewage flow-rate by self-purification capacity per unit area gives possible biological sewage change during transportation. Voided materials, such as grain and porous concrete, demonstrated improved conversion rates of DO, TOC, NH4 + , and NO3 − . Perforations in the wet concrete were also effective at enhancing this capacity. The perforations with big holes (d = 12 mm), especially, were more effective than perforations with small (d = 1 mm) holes. Wet concrete with large holes demonstrated substrate consumption rates of DO = 460, TOC = 480, NH4 –N = 87, and nitrogen as NO3 –N = 170 (in mg substrate/(m2 h)). Perforations of five holes (d = 12 mm, depth = 10 mm) on the surface of the 50 mm × 50 mm increase the actual surface area by 75%, contributing to the increase in biological activity. No differences between self-purification capacities were observed between plain concrete and nonperforated wet concrete, indicating that the composition of the concrete did not affect the self-purification capacity. Acknowledgments This study was supported in part by a grant (16760629) from the Japanese Ministry of Education, Culture, Sports, Science, and Technology. We thank Aidan Synnott for critical reading of the manuscript. References [1] H.G. Leu, C.F. Ouyang, J.L. Su, Effects of flow velocity changes on nitrogen transport and conversion in an open channel flow, Water Res. 30 (1996) 2065–2071.
101
[2] K. Raunkjaer, P.K. Nielsen, T. Hvitved-Jacobsen, Acetate removal in sewer biofilms under aerobic conditions, Water Res. 31 (1997) 2727–2736. [3] M.A. Warith, K. Kennedy, R. Reitsma, Use of sanitary sewer as wastewater pre-treatment systems, Waste Manage. 18 (1998) 235–247. [4] N. Tanaka, T. Hvitved-Jacobsen, Transformation of wastewaterorganic matter in sewer under changing aerobic–anaerobic conditions, Water Sci. Technol. 37 (1998) 105–113. [5] N. Tanaka, K. Takenaka, Control of hydrogen sulfide and degradation of organic matter by air injection into a wastewater force main, Water Sci. Technol. 31 (1995) 273–282. [6] G.H. Chen, D.H.W. Leung, Utilization of oxygen in a sanitary gravity sewer, Water Res. 34 (2000) 3813–3821. [7] G.H. Chen, D.H.W. Leung, J.C. Hung, Removal of dissolved organic carbon in sanitary gravity sewer, J. Environ. Eng. 127 (2001) 1–7. [8] G.H. Chen, D.H.W. Leung, J.C. Hung, Biofilm in the sediment phase of a sanitary gravity sewer, Water Res. 37 (2003) 2784–2788. [9] M.A. Warith, K. Kennedy, R. Reitsma, Use of sanitary sewer as wastewater pre-treatment systems, Waste Manage. 18 (1998) 235–247. [10] I.W. Marjaka, K. Miyanaga, K. Hori, Y. Tanji, H. Unno, Augmentation of self-purification capacity of sewer pipe by immobilizing microbes on the pipe surface, Biochem. Eng. J. 15 (2003) 69–75. [11] B. Walter, C. Haase, N. R¨abiger, Combined nitrification/denitrification in a membrane reactor, Water Res. 39 (2005) 2781–2788. [12] X.H. Xing, B.H. Jun, M. Yanagida, Y. Tanji, H. Unno, Effect of C/N values on microbial simultaneous removal of carbonaceous and nitrogenous substances in wastewater by single continuous-flow fluidized-bed bioreactor containing porous carrier particles, Biochem. Eng. J. 5 (2000) 29–37. [13] A. Wachtmeister, T. Kuba, M.C.M. Van Loosdrecht, J.J. Heijnen, A sludge characterization assay for aerobic and denitrifying phosphorus removing sludge, Water Res. 31 (1997) 471–478. [14] W. Raucha, H. Vanhoorenb, P.A. Vanrolleghem, A simplified mixedculture biofilm model, Water Res. 33 (1999) 2148–2162. [15] P. William, M. Barber, C.D. Stuckey, Nitrogen removal in a modified anaerobic baffled reactor (ABR): 1, denitrification, Water Res. 34 (2000) 2413–2422.
Short communication
Estimation of the self-purification capacity of biofilm formed in domestic sewer pipes Yasunori Tanji ∗ , Rie Sakai, Kazuhiko Miyanaga, Hajime Unno Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8501, Japan Received 18 August 2005; received in revised form 3 March 2006; accepted 27 May 2006
Abstract To estimate the self-purification capacity of sewer pipe, six different types of concrete blocks were installed in a domestic sewer pipe for nine months. The concrete blocks used were plain, grain, porous, and wet concrete (no-hole, or perforated with holes of d = 1 or 12 mm). After a 79-day exposure to sewage, a heterogeneous biofilm formed on the surface of each block. The self-purification capacities of the blocks were estimated by measuring the decrease in substrate concentration in artificial sewage. The analyzed substrates were dissolved oxygen (DO), total organic carbon (TOC), NH4 + , and NO3 − . Wet concrete with holes (d = 12 mm) showed the highest substrate consumption rates: DO = 460, TOC = 480, NH4 –N = 87, and nitrogen as NO3 –N = 170, in mg substrate/(m2 h). These results indicate that sewers have a considerable potential for removing organic material and nutrients, and modification of sewer surfaces may increase these activities. © 2006 Elsevier B.V. All rights reserved. Keywords: Self-purification; Wastewater treatment; Sewage pipes; Immobilization; Nitrification; Denitrification
1. Introduction Sewage quality varies during transport from the source to the wastewater treatment plant. Changes in the biological oxygen demand (BOD) or the chemical oxygen demand (COD), according to the flow of sewage through sewer pipes, have been reported [1–5]. Several researchers have analyzed the in situ self-purification capacity of sewer pipes. For instance, Chen examined the utilization of oxygen in a sanitary gravity sewer and found that 14% of dissolved organic carbon (DOC) in the sewage was removed after flowing 1.5 km in a sewer pipe for 18 min, and he estimated that the liquid sewage phase was responsible for 40% of the DOC removal, while the sewage sediment contributed to 60% of the removal [6–8]. However, engineers do not consider the impacts of the self-purification capacity of sewer pipe on the wastewater treatment process. This is partially due to the complexity of the chemical, physical, and biological processes occurring in the sewer pipes. Furthermore, since the water quality of sewage changes by the minute,
∗
Corresponding author. Tel.: +81 45 924 5762; fax: +81 45 924 5818. E-mail address: [email protected] (Y. Tanji).
1369-703X/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.bej.2006.05.021
in situ evaluation of pipes’ capacity for self-purification is difficult. The concentration of microorganisms in a sewer system is low relative to the substrate concentration. In other words, the food/microorganisms (F/M) ratio in a sewer system is relatively high and, therefore, the substrate does not limit the growth of the microorganisms. To improve the self-purification capacity, a low F/M ratio is desirable [9], and there is a possible way to reduce the F/M ratio. That is to use sewers with rougher wall surfaces to facilitate biofilm attachment. Rougher interior wall surfaces provide more surface area for biofilm to adhere. Previously, in order to endow sewer pipes with the capacity for self-purification, a porous ceramic support was installed in a model sewer pipe and the removal of total organic carbon (TOC), NH4 − , and NO3 − from model sewage was analyzed [10]. The results indicated that the combination of aeration and biofilm development enhanced the simultaneous removal of organic carbon and nitrogen. However, the structure of the biofilm formed in the model sewer was thought to differ from that of real sewer pipe. In this study, in order to assess and improve the self-purification capacity of real sewer pipes, concrete blocks with six different types of surfaces were installed in a working sewer system and exposed to sewage for 79 days. The blocks were then used to assay
Y. Tanji et al. / Biochemical Engineering Journal 31 (2006) 96–101
the self-purification capacities of the various surfaces for nine months.
97
thetic wastewater (Fig. 1D-b). TOC was analyzed using a TOC analyzer (Shimadzu TOC-5000A, Shimadzu Co. Ltd., Tokyo, Japan).
2. Materials and methods 2.1. Concrete blocks for biofilm formation in a working sewer Six types of concrete blocks were immersed in sewage flow (Fig. 1A and B): plain Portland concrete, grain concrete, porous concrete, and three types of wet concrete. The wet concrete was either non-perforated, or perforated with d = 1 mm × depth = 5 mm (No. 5) or d = 12 mm × depth = 10 mm (No. 6) holes. The composition (wt.%) of the Portland and grain concretes was 52% 3CaOSiO2 , 24% 2CaOSiO2 , 9% 3CaOAl2 O3 , 9% 4CaOAl2 O3 Fe2 O3 , and 6% other. The grain concrete was an agglomerate of concrete granules (3–5 mm), with a porosity of 20–25%. The composition (wt.%) of the porous concrete was 78% SiO2 , 16%Al2 O3 , 1.3% Fe2 O, 0.86% CaO, and 2.6% SO3 , and the void was 88%. Wet concrete was reinforced with plant fibers, and the size of each block was 50 mm × 50 mm × 20 mm. The faces of the concrete blocks, except the front, were coated with epoxy resin. Surface area of each block was 25 cm2 (5 cm × 5 cm). Six blocks were fixed in the poly vinyl chloride (PVC) holder (Fig. 1C). Two sets of six blocks were immersed in the flow of sewage. The sanity gravity sewer examined in this study was located in the Suzukakedai campus of the Tokyo Institute of Technology, which receives material from about 3000 people. The sewage in this area is primarily domestic wastewater, and the experimental wastewater and rainwater flow separately to the domestic wastewater. Experiments were conducted between June 2004 and February 2005. During the initial 79-day study, two sets of blocks were immersed in the flow without lab experiments. Next, a set of blocks was used in laboratory experiments to analyze the selfpurification capacities of the blocks. After each experiment, the set of blocks was returned to the sewage flow for continued sewage exposure. Each lab experiment took two days. 2.2. Determination of dissolved oxygen and organic carbon uptake rates The decrease in dissolved oxygen (DO) concentrations was used to measure the activity of biofilm formed on the concrete blocks. The epoxy resin-coated faces of the blocks were washed with brushes to remove biofilm attached to the resin. After washing, each block was placed in a closed vessel (Fig. 1D-a) that was then filled with air and saturated with 300 ml synthetic wastewater of the following composition (per liter): 26.7 mg carbon as glucose, 106.7 mg carbon as polypepton, 5 mg KH2 PO4 , and 5 mg NaHCO3 . The composition of the synthetic wastewater was determined based on the analysis of TOC concentrations in the campus sewage. The DO concentration in the medium was continuously monitored using a DO sensor (OBS Digital DO controller, Tokyo, Japan). The biological activity of the biofilm was also analyzed by measuring TOC uptake rates using open vessels filled with air and saturated with 200 ml syn-
2.3. Nitrification and denitrification rates by biofilm formed on the concrete surfaces The nitrification and denitrification activities of the biofilm formed on the concrete blocks were analyzed. For the nitrification analysis, a block was placed in an open vessel (Fig. 1Db), filled with air saturated 200 ml of synthetic wastewater of the following composition (per liter): 60 mg carbon as glucose, 40 mg nitrogen as (NH4 )2 SO4 , 12.6 mg KH2 PO4 , 19.4 mg MgSO4 ·7H2 O, 84 mg NaHCO3 , 1.2 mg CaCl2 ·2H2 O, and 0.1 mg FeCl3 ·6H2 O. For denitrification analysis, a block was placed in a closed vessel (Fig. 1D-a), filled with pure nitrogen gas saturated 300 ml synthetic wastewater of the following composition (per liter): 60 mg carbon as glucose, 40 mg nitrogen as KNO3 , 12.6 mg KH2 PO4 , 19.4 mg MgSO4 ·7H2 O, 84 mg NaHCO3 , 1.2 mg CaCl2 ·2H2 O, and 0.1 mg FeCl3 ·6H2 O. The synthetic wastewater used for the nitrification and denitrification tests was sampled periodically, diluted with distilled water, and filtered through a mixed cellulose-ester membrane filter of 0.2 m mean pore size. Various nitrogenous ions were then measured by ion-chromatography (SCL-10A; Shimazu, Kyoto, Japan). 3. Results and discussion 3.1. Biofilm formation on concrete blocks Since the pH of Portland cement is between 11 and 12, the formation of biofilm on the surface of the concrete was expected to be slow. After the 79-day exposure to sewage flow, however, the front surfaces of six different concrete blocks (including Portland cement) were completely covered by biofilm. After that, lab experiments and sewage exposure continued for an additional 120 days. The voids of grain concrete and perforated wet concrete were filled by biofilm. Five holes of perforated wet concrete (d = 12 mm) were filled with living organisms. Microscopic observation revealed that the biofilm was composed of two layers: a heterogeneous non-dense layer, which was very rough and primarily composed of filamentous microorganisms, and a dense layer composed of abiotic components, compressed filamentous microorganisms, protozoa such as Vorticella and Litonotus, metazoa such as Diplogaster, and many different kinds of bacteria. Tubifex was occasionally found in the biofilm. After the 79-day sewage exposure, the exterior appearances of all the block surfaces were nearly identical. The enlargement and detachment of the biofilm from the blocks was thought to equilibrate after long-term sewage exposure. 3.2. Biofilm activity The aerobic activity of the biofilm formed on the concrete blocks was analyzed by measuring DO and TOC uptake rates. Weekly, a set of concrete blocks was transferred to the lab and
98
Y. Tanji et al. / Biochemical Engineering Journal 31 (2006) 96–101
Fig. 1. (A) Photographs of concrete blocks and (B) classification of the concrete. “No.” corresponds to the number in (A). (C) Photograph of a set of concrete blocks in the PVC holder. (D) Reactor vessels for analyzing the self-purification capacity of the concrete blocks: (a) closed vessel for measuring DO and NO3 − consumption and (b) open vessel for measuring TOC and NH4 + consumption.
Y. Tanji et al. / Biochemical Engineering Journal 31 (2006) 96–101
99
Fig. 2. (A-1 and A-2) Decrease in DO concentrations in the closed vessel. (B-1 and B-2) Decrease in TOC concentrations in the open vessel. Open circle, plain concrete; open triangle, grain concrete; open square, porous concrete; closed circle, wet concrete (plain); closed triangle, wet concrete (d = 1 mm); closed square, wet concrete (d = 12 mm); n = 4.
used for experiments. Decreases in DO and TOC in the vessels filled with air saturated artificial sewage are shown in Fig. 2. To normalize the dates, values relative to the initial DO and TOC concentrations are used in the figure. DO experiments were conducted on 10 randomly selected days, and the average initial DO concentration was 8.49 g/m3 . DO decreases were linear under these experimental conditions. We initially predicted that biofilm formation on the plain concrete might be scarce compared to that of other conditions. However, biofilm formed on the surface of the plain concrete and showed DO uptake activ-
ity. The slope of each line in the figures indicates the oxygen consumption rate of the particular block. Porous (void, 88%) and wet concrete (d = 12 mm) showed relatively high rates of oxygen consumption. After exposure to sewage, the voids of the porous concrete were filled with microorganisms, enhancing this activity. Perforation of the big holes on the block surfaces substantially increased the surface area, increasing the DO uptake rate. However, the differences in DO uptake rates between the six different blocks were not great, indicating that the surface material has little effect on DO uptake rates.
Fig. 3. (A-1 and A-2) Decrease in NH4 + concentrations in the open vessel. (B-1 and B-2) Decrease in T NO3 − concentrations in the open vessel. Open circle, plain concrete; open triangle, grain concrete; open square, porous concrete; closed circle, wet concrete (plain); closed triangle, wet concrete (d = 1 mm); closed square, wet concrete (d = 12 mm).
100
Y. Tanji et al. / Biochemical Engineering Journal 31 (2006) 96–101
TOC uptake rates were examined and are shown in Fig. 2B. Experiments were conducted on four randomly selected days, and the average initial TOC concentration was 81.4 g/m3 . The trend in decreasing TOC was similar to that of DO, and the porous and wet concrete (d = 12 mm) had relatively high rates of TOC consumption compared to the other types of concrete. The aerobic oxidation of TOC consumed DO in the artificial sewage. 3.3. Biofilm conversion of nitrogen Since the NH4 –N concentration in the sewage was relatively high, we analyzed nitrogen conversion by the biofilm (Fig. 3). Nitrogen elimination from sewage proceeds via two steps: nitrification and denitrification [11,12]. Most of the nitrogen in sewage exists as NH4 + . Therefore, nitrification must occur if nitrogen is to be eliminated from sewage. Nitrification experiments were conducted two times, and denitrification three times, on randomly selected days. NH4 + decreases in the open vessel saturated with air were proportional to incubation time. The composition and void of the concrete blocks did not influence the decrease in NH4 + (Fig. 3A-1). After 8 h incubation, the NH4 –N conversion was 15–22%. Wet concrete with large holes indicated a conversion rate of 22%, and the others showed approximately 15%. Since the NH4 –N conversion is an aerobic reaction, the surface area exposed to the DO saturated bulk liquid contributed to the reaction. The denitrification reaction was rapid relative to the nitrification reaction. Although the initial concentration of nitro-
gen in this test was the same as in the nitrification test, the NO3 − conversion rate after 6 h incubation was estimated as greater than 80% for all conditions (Fig. 3B). During the NO3 − consumption experiment, no NO2 − was detected. Differences between the six conditions were minimal. Of note, the nitrifying bacteria grow much slower than the heterogeneous bacteria that dissolve organic carbon. At the same time, nitrification needs an oxygen supply, whereas denitrification proceeds under anoxic conditions and requires organic carbon. Nitrifying bacteria located within the upper area of the biofilm utilize DO and NH4 + in the sewage. On the other hand, denitrificans were located primarily within the innermost area of the biofilm [13–15]. Biofilm formed on the concrete blocks and showed both nitrification and denitrification activities. However, the rate-limiting step for the removal of sewage nitrogen appears to be nitrification. The use of concrete with a rough surface, such as wet concrete with large holes, may enhance the nitrification reaction and eventual nitrogen removal. 3.4. The self-purification capacity of sewer pipe Based on the results of this study, we estimated the selfpurification capacities of sewer pipe. The substrate uptake rates (Figs. 2 and 3) of the concrete blocks showed a zero-order kinetic dependency on substrate concentration during the initial stage of the reaction. The biological activities of the biofilm might be proportional to the film’s surface area. The conversion rates of DO, TOC, NH4 + , and NO3 − were recalculated as a rate
Fig. 4. Substrate consumption rates by concrete block. Substrates are: (A) DO; (B) TOC; (C) NH4 + ; (D) NO3 − .
Y. Tanji et al. / Biochemical Engineering Journal 31 (2006) 96–101
per unit of surface area and time, and are compared in Fig. 4. Divide volumetric sewage flow-rate by self-purification capacity per unit area gives possible biological sewage change during transportation. Voided materials, such as grain and porous concrete, demonstrated improved conversion rates of DO, TOC, NH4 + , and NO3 − . Perforations in the wet concrete were also effective at enhancing this capacity. The perforations with big holes (d = 12 mm), especially, were more effective than perforations with small (d = 1 mm) holes. Wet concrete with large holes demonstrated substrate consumption rates of DO = 460, TOC = 480, NH4 –N = 87, and nitrogen as NO3 –N = 170 (in mg substrate/(m2 h)). Perforations of five holes (d = 12 mm, depth = 10 mm) on the surface of the 50 mm × 50 mm increase the actual surface area by 75%, contributing to the increase in biological activity. No differences between self-purification capacities were observed between plain concrete and nonperforated wet concrete, indicating that the composition of the concrete did not affect the self-purification capacity. Acknowledgments This study was supported in part by a grant (16760629) from the Japanese Ministry of Education, Culture, Sports, Science, and Technology. We thank Aidan Synnott for critical reading of the manuscript. References [1] H.G. Leu, C.F. Ouyang, J.L. Su, Effects of flow velocity changes on nitrogen transport and conversion in an open channel flow, Water Res. 30 (1996) 2065–2071.
101
[2] K. Raunkjaer, P.K. Nielsen, T. Hvitved-Jacobsen, Acetate removal in sewer biofilms under aerobic conditions, Water Res. 31 (1997) 2727–2736. [3] M.A. Warith, K. Kennedy, R. Reitsma, Use of sanitary sewer as wastewater pre-treatment systems, Waste Manage. 18 (1998) 235–247. [4] N. Tanaka, T. Hvitved-Jacobsen, Transformation of wastewaterorganic matter in sewer under changing aerobic–anaerobic conditions, Water Sci. Technol. 37 (1998) 105–113. [5] N. Tanaka, K. Takenaka, Control of hydrogen sulfide and degradation of organic matter by air injection into a wastewater force main, Water Sci. Technol. 31 (1995) 273–282. [6] G.H. Chen, D.H.W. Leung, Utilization of oxygen in a sanitary gravity sewer, Water Res. 34 (2000) 3813–3821. [7] G.H. Chen, D.H.W. Leung, J.C. Hung, Removal of dissolved organic carbon in sanitary gravity sewer, J. Environ. Eng. 127 (2001) 1–7. [8] G.H. Chen, D.H.W. Leung, J.C. Hung, Biofilm in the sediment phase of a sanitary gravity sewer, Water Res. 37 (2003) 2784–2788. [9] M.A. Warith, K. Kennedy, R. Reitsma, Use of sanitary sewer as wastewater pre-treatment systems, Waste Manage. 18 (1998) 235–247. [10] I.W. Marjaka, K. Miyanaga, K. Hori, Y. Tanji, H. Unno, Augmentation of self-purification capacity of sewer pipe by immobilizing microbes on the pipe surface, Biochem. Eng. J. 15 (2003) 69–75. [11] B. Walter, C. Haase, N. R¨abiger, Combined nitrification/denitrification in a membrane reactor, Water Res. 39 (2005) 2781–2788. [12] X.H. Xing, B.H. Jun, M. Yanagida, Y. Tanji, H. Unno, Effect of C/N values on microbial simultaneous removal of carbonaceous and nitrogenous substances in wastewater by single continuous-flow fluidized-bed bioreactor containing porous carrier particles, Biochem. Eng. J. 5 (2000) 29–37. [13] A. Wachtmeister, T. Kuba, M.C.M. Van Loosdrecht, J.J. Heijnen, A sludge characterization assay for aerobic and denitrifying phosphorus removing sludge, Water Res. 31 (1997) 471–478. [14] W. Raucha, H. Vanhoorenb, P.A. Vanrolleghem, A simplified mixedculture biofilm model, Water Res. 33 (1999) 2148–2162. [15] P. William, M. Barber, C.D. Stuckey, Nitrogen removal in a modified anaerobic baffled reactor (ABR): 1, denitrification, Water Res. 34 (2000) 2413–2422.