- Email: [email protected]
Biofilm in the sediment phase of a sanitary gravity sewer
Water Research 37 (2003) 2784–2788
Research note
Biofilm in the sediment phase of a sanitary gravity sewer Guang-Hao Chen*, Derek H.W. Leung, Ju-Chang Hung Department of Civil Engineering, Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China Received 25 September 2002; accepted 3 February 2003
Abstract Microbial activity of the sediment phase in a 1.5-km-long concrete sewer section with a cement pipe in a 540-mm diameter was investigated in this paper. SEM examinations and elementary composition analyses of the sediment samples have identified the presence of a biofilm layer at the sediment surface. Bacterial counting results with a DNAstaining technique have revealed that the amount of bacteria in this layer was 2.1 1011 cell g1 dry wt, which is close to that of activated sludge. ATP content in the sewer biofilm was found relatively high, demonstrating that the sewer biofilm is active. Throughout the entire 1.5-km sewer section, the biofilm activity was maintained at almost the same level. Lab-scale sediment oxygen uptake flux (SOUF) tests showed that the shear flow velocity above the sediment phase linearly increases the SOUF, which of the potential value was determined to be 32 g O2 m2 day1 at an estimated shear flow velocity of 0.055 m s1 at 25 C in the sewer line, provided that the mean flow velocity was 1.5 m s1, and the mean water depth was 220 mm. Such a high SOUF value further endorsed the existence of the active sewer biofilm. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Sanitary gravity sewer; Sediment; Biofilm; Microbial activity; Sediment oxygen uptake flux; DNA-staining cell enumeration
1. Introduction It has been unveiled that partial oxidation of organic pollutants can be achieved during sewage conveyance in sewers in the prevalence of aerobic environment [1–5]. This may explore a low-cost interim solution to the heavily water polluted developing nations such as China where the vast majority of sewage is discharged to the environment without appropriate treatment since many small cities cannot afford the treatment cost. Our recent studies using a 1.5-km section of a gravity sewer with a bottom slope of 0.0075 have shown the potentiality of using a gravity sewer to effectively abate the pollutant concentration during the sewage conveyance when DO level in the water phase is not lower than 1 mg L1 [6,1,7]. It was also observed that the sediment phase played a more important role in both organic oxidation *Corresponding author. Tel.: +852-2358-8752; fax: +8522358-1534. E-mail address: [email protected] (G.-H. Chen).
and dissolved organic carbon (DOC) utilization than the bulk liquid phase in such a sewer [1]. This may be attributed to the presence of active sewer biofilm in the sediment phase. In order to identify the sewer biofilm and quantitatively assess its microbial activity, this paper is aimed at: (1) identifying the presence of the sewer biofilm, (2) enumerating bacteria cell number in the biofilm and (3) determining ATP content as well as sediment oxygen uptake flux (SOUF).
2. Materials and methods 2.1. Outline of the sewer The sanity gravity sewer examined in this study locates in the campus of the Hong Kong University of Science & Technology. It has a bottom slope of 0.0075 and a length of 1.5 km. This sewer delivers the campus sewage to a nearby sewage treatment works. The slope parallels to the hydraulic gradient of the sewer line.
0043-1354/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0043-1354(03)00083-6
G.-H. Chen et al. / Water Research 37 (2003) 2784–2788
There are 14 manholes with air-tight covers, located along the sewer line at a distance interval of 100 m. Due to manpower constraint, only five of the 14 manholes were selected as the sampling and observation stations, which were at 0, 331, 552, 872, and 1278 m from the beginning of the sewer. The details of this sewer system on its configuration, the characteristics of sewage quality and quantity, and the hydraulic conditions can be found in our previous papers [1,6]. Field observations at each manhole revealed that the sediment phase substantially developed along the entire sewer line. Sediment sampling was conducted with a sewer shovel, and the samples were collected at these five locations periodically. 2.2. Microscopic observation of the sediment samples A scanned electronic microscopy (SEM) equipped with LaB6 filament and a second electron detector (Philips XL30+EDAX) was employed in this study to examine the sediment samples. Preparation of the sediment specimens for examination was made as follows: (1) vacuuming sediment samples using a freeze dryer (Edwards) under 40 C for 2 hr; (2) returning it to a room temperature in a dessicator; and (3) coating the specimens with a gold film. 2.3. Bacterial enumeration and ATP analysis To determine the total cell number of bacteria in both sewage and sediment phases, cell DNA-staining technique using 40 , 6-diamidino-2-phenyl indole (DAPI) counting was adapted from Saby et al. [8], which can stain DNA of all bacteria cells. The top layer with 0.1mm thickness was collected from the surface of the samples with 4-mm thickness, using a micro-cutter under a stereoscope (Olympus). The top layer samples were then dispersed using a sonicating probe (Sonics & Materials), placed at a depth of 1 cm, in a 2-min
2785
operation with a power of 40 W. Under such operation conditions, the respiratory activity of bacteria will not be affected [8]. Upon the completion of the dispersion, the treated samples were immediately diluted in sterile saline water and then subjected to the DAPI-staining treatment for generating fluorescent cells. Cell number was finally counted under an epifluorescence microscope (objective 100, Olympus BX40) with a UV light at a wavelength of 330–385 nm. Counting was conducted in 30 fields so as to secure an acceptable accuracy and the result can be expressed as cells g1 dry wt. ATP content in the above top layer was determined using a modified trichloroacetic acid (TCA) extraction method [6]. Prior to the determination, the samples were dispersed with the same technique as described above. The solids concentration in the samples was determined according to the Standard Methods [9]. 2.4. Determination of sediment oxygen uptake flux (SOUF) At present, it is almost impossible to conduct in- situ measurement of the SOUF in a sewer. Thus, a benchscale SOUF measurement system was employed in this study, as shown in Fig. 1. This system was developed from our previous study on the measurement of benthal oxygen uptake flux for tidal river sediment [10]. It has a rectangular chamber made of acrylic plastics with a total volume of 2.9 L. Three perforated baffles were installed to create a uniform flow in the chamber. A pump was used to circulate the water at a desired flow rate. Three sediment samples can be placed in the measurement chamber. The total water/sediment contact area was 71.3 cm2. Prior to each measurement, the overlying water was aerated for seconds in order to obtain a high initial level of DO. Air bubbles were then removed before the whole system was tightened for the measurement of DO depletion. After the preparation, continuous circulation was carried out at a specific flow rate in
Fig. 1. Measurement setup of sediment oxygen uptake flux.
2786
G.-H. Chen et al. / Water Research 37 (2003) 2784–2788
a temperature-constant room at 25 C, that is the mean temperature of the local sewage. The shear flow velocity just above the sediment surface was determined by a laser flow velocity meter (2D-LDV, Polytec GmbH). The overlying water used the sewage filtered with 1-mm membrane filter (Whatman). During the operation, the DO level was monitored by a DO meter (YSI). The measurement continued until the DO level dropped to a relatively low level (about 2 mg 11). Before and after each measurement, suspended solids (SS) and dissolved organic carbon (DOC) concentrations were analyzed according to the Standard Methods [8]. Upon the completion of each measurement, the overlying water was used for measuring the specific oxygen uptake rate (SOUR) of the water. Based on the DO level difference during the measurement, the SOUR, the measurement surface areas and the value of SOUF can then be determined.
3. Results and discussion 3.1. Biofilm identification Fig. 2 shows a typical SEM picture of the crosssection of the entire sediment samples. The sediment surface was present smoothly, which might be attributed to the presence of a thin bacterial layer at the sediment surface. The mean thickness of this thin layer was determined to be 10–20 mm through the SEM examinations of various sediment samples. Elementary composition analysis of the thin layer using the EDAX equipment indicated that the carbon, oxygen, and nitrogen compositions in this layer were 46.1%, 30.7% and 14.5%, respectively, all of which are close to that in bacterial cells (C: 47.3%, O: 27%, and N: 11.3% [11]. Bacteria counting with the DAPI-staining technique showed that the total cell number of bacteria in this thin
layer was 2.1 1011 cell g1 dry wt. Such a cell amount is close to that in activated sludge (5 1011 cell g1 MLSS) [12]. Comparatively, in the water phase the total cell number was only 3.3 108 cell g1 SS. It is thus apparent that this thin layer is a biofilm layer developed at the sediment surface. 3.2. Microbial activity of the biofilm Fig. 3 shows the variation of the ATP content in the biofilm layer at the five sampling locations of the 1.5-km sewer section. It was found that the ATP content was relatively high compared to that in activated sludge [13,14]. This means that the sewer biofilm has a high microbial activity, which is consistent with the reported results [15]. The presence of active biofilm at the sediment surface endorses the possibility of achieving an effective removal of DOC in sewers in aerobic conditions. Fig. 3 also indicates that over the entire sewer section, the sewer biofilm activity did not decline significantly. This may be due to a high flow velocity induced by this high slope, which made the biofilm updated periodically over the sewer line. Thus, it is understood that the whole sewer line is able to contribute to DOC oxidation during the course of sewage conveyance when oxygen limitation does not exist. Measurements of the SOUF are shown in Fig. 4. This flux reflects the overall microbial activity of the sewer biofilm. It is found that an increase in the shear flow velocity present just above the biofilm linearly increases the SOUF, as described by SOUF ¼ 519:8 Us þ 4:9
ð1Þ
Where Us is the mean value of the shear flow velocity (m s1). The above relationship between the shear flow velocity and the SOUF can be explained such that the SOUF may be limited by the oxygen transfer rate across
Fig. 2. A typical cross-section SEM picture of the sediment samples.
G.-H. Chen et al. / Water Research 37 (2003) 2784–2788
2787
which is slightly greater than the actual SOUF (27 g O2 m2 day1) that was estimated from the sewer oxygen budget analysis [6]. The estimated SOUF value is for the entire sewer section, while the SOUF determined from the bench scale data and Eqs. (1) and (2) reflects a potential SOUF value in the sewer. Thus, the determining approach for a SOUF value is reasonable. Such a SOUF value is close to that of a wastewater biofilm [18], which also endorses a high microbial activity of the sewer biofilm in the sanitary gravity sewer. Fig. 3. Variation of the ATP content in the biofilm along the sewer line.
Fig. 4. Effect of the shear flow velocity on the SOUF.
the diffusion or shear layer present at the biofilm surface: when the diffusion layer thickness is reduced by the shear flow velocity, the mass transfer resistance then decreases accordingly. As a result, a high shear flow velocity is in favor of a thinner diffusion layer, which in turn promotes the oxygen transfer rate from the bulk phase into the biofilm, thereby increasing the SOUF. Such a finding also implies that the internal organic sources could be the major cause for oxygen utilization in the sewer biofilm. The SOUF measured in the above cases does not indicate its actual level since the shear flow velocity in the sewer pipe is unknown. In order to estimate this velocity so as to determine the corresponding SOUF in the sewer, the following formula derived from pipe hydraulics [16] was employed for calculating a mean shear flow velocity (Us) at the pipe bottom: 0:5 Ds Um ð2Þ Us ¼ 1:63 Dm where Dm and Ds are the mean values of the water depth and the shear layer thickness at the pipe bottom (mm); and Um is the mean flow velocity (m s1) of the sewage. The Dm and Um were found to be 220 mm and 1.5 m s1 [1]. It is assumed that the Ds be 0.1 mm [17]. Therefore, the Us in the existing sewer section is found to be 0.052 m s1. With this shear flow velocity, the SOUF is thus determined to be 32 g O2 m2 day1,
4. Conclusions The biofilm in a sanitary gravity sewer was investigated in this study. It has been confirmed that a biofilm layer is present at the sediment surface, in which the total cell number of bacteria was found to be 2.1 1011 cell g1 dry wt. Throughout the entire 1.5-km sewer section, a high microbial activity was maintained. The SOUF measurements at different shear flow velocities indicated a linear relationship between this velocity and the SOUF. The potential SOUF in the sewer pipe was determined to be 32 g O2 m2 day1 at 25 C with respect to the estimated shear flow velocity at 0.055 m s1, when the mean flow velocity was 1.5 m s1 and the water depth was 220 mm in the sewer pipe. Such a high SOUF value supported the existence of the active biofilm in the sewer.
References [1] Chen GH, Leung DHK, Mo HK, Saby S. Structural, chemical, and biological properties of the sediment in a sanitary gravity sewer. First IWA conference-Paris 2000, France, July 2000. [2] Pomeroy RD, Parkhurst JD. The forecasting of sulfide buildup rates in sewers. Prog water Technol 1973;9:621–8. [3] Hvitved-Jacobsen T. Vollersten J, Nielsen PH. Volatile fatty acids and sulfide in pressure mains. Water Sci. Technol 1995;31:169–79. [4] Tanaka N, Takenaka K. Control of hydrogen sulfide and degradation of organic matter by air injection into a wastewater force main. Water Sci Technol 1995;31:273–82. [5] Ochi T, Kitagawa M, Tanaka S. Controlling sulfide generation in force mains by air injection. Water Sci Technol 1998;37:87–95. [6] Chen GH, Leung DHW. Oxygen utilization of a real gravity sewer under aerobic condition. Water Res 2000;34:3813–21. [7] Chen GH, Leung DHW, Huang JC. Removal of organic matter in a gravity sewer. Environ Eng, ASCE 2001;127(4):289–95. [8] Saby S, Sibille I, Mathieu L, Paquin JL, Block JC. Influence of water chlorination on the counting of bacteria with DAPI (40 , 6-diamidino-2-phenylindole). Appl Environ Microbiol 1997;63:1564–9.
2788
G.-H. Chen et al. / Water Research 37 (2003) 2784–2788
[9] APHA.Standard methods for the examination of water and waste water. 24th ed. American Public Health Association, Washington DC, 1998. [10] Chen GH, Leong IM, Liu J, Huang JC. Study of oxygen uptake by tidal river sediment. Water Res 1999;33: 2905–12. [11] Ide T. Water quality engineering 1990. Tokyo: Gihoudou Publisher, [in Japanese]. [12] Fr^lund B, Palmgren R, Keiding K, Nielsen P. Extraction of extracellular polymers from activated sludge using a cation exchange resin. Water Res 1996;30:1749–58. [13] Brezonik PL, Patterson JW. Activated sludge ATP: effects of environmental stress. J. Sanitary Eng Div-Proc Am Soc Civil 1971; 813–24.
[14] Peter ON, Alonzo WL. Microbial viability measurements and activated sludge kinetics. Water Res 1980;14: 217–25. [15] Lemmer H, Roth D, Schade M. Population density and enzyme activities of heterotrophic bacteria in sewer biofilms and activated sludge. Water Sci Technol 1994;28:1341–6. [16] Chow VT. Open-channel hydraulics. New York: McGrawHill, 1959. [17] Williamson K, McCarty PL. A model of substrate utilization by bacterial films. J WCPF 1976;48:9–24. [18] Chen GH. Fundamental study of organic and nitrogenous removal in a biofilm Ph.D.dissertation, Kyoto University, 1990, [in Japanese]
Research note
Biofilm in the sediment phase of a sanitary gravity sewer Guang-Hao Chen*, Derek H.W. Leung, Ju-Chang Hung Department of Civil Engineering, Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China Received 25 September 2002; accepted 3 February 2003
Abstract Microbial activity of the sediment phase in a 1.5-km-long concrete sewer section with a cement pipe in a 540-mm diameter was investigated in this paper. SEM examinations and elementary composition analyses of the sediment samples have identified the presence of a biofilm layer at the sediment surface. Bacterial counting results with a DNAstaining technique have revealed that the amount of bacteria in this layer was 2.1 1011 cell g1 dry wt, which is close to that of activated sludge. ATP content in the sewer biofilm was found relatively high, demonstrating that the sewer biofilm is active. Throughout the entire 1.5-km sewer section, the biofilm activity was maintained at almost the same level. Lab-scale sediment oxygen uptake flux (SOUF) tests showed that the shear flow velocity above the sediment phase linearly increases the SOUF, which of the potential value was determined to be 32 g O2 m2 day1 at an estimated shear flow velocity of 0.055 m s1 at 25 C in the sewer line, provided that the mean flow velocity was 1.5 m s1, and the mean water depth was 220 mm. Such a high SOUF value further endorsed the existence of the active sewer biofilm. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Sanitary gravity sewer; Sediment; Biofilm; Microbial activity; Sediment oxygen uptake flux; DNA-staining cell enumeration
1. Introduction It has been unveiled that partial oxidation of organic pollutants can be achieved during sewage conveyance in sewers in the prevalence of aerobic environment [1–5]. This may explore a low-cost interim solution to the heavily water polluted developing nations such as China where the vast majority of sewage is discharged to the environment without appropriate treatment since many small cities cannot afford the treatment cost. Our recent studies using a 1.5-km section of a gravity sewer with a bottom slope of 0.0075 have shown the potentiality of using a gravity sewer to effectively abate the pollutant concentration during the sewage conveyance when DO level in the water phase is not lower than 1 mg L1 [6,1,7]. It was also observed that the sediment phase played a more important role in both organic oxidation *Corresponding author. Tel.: +852-2358-8752; fax: +8522358-1534. E-mail address: [email protected] (G.-H. Chen).
and dissolved organic carbon (DOC) utilization than the bulk liquid phase in such a sewer [1]. This may be attributed to the presence of active sewer biofilm in the sediment phase. In order to identify the sewer biofilm and quantitatively assess its microbial activity, this paper is aimed at: (1) identifying the presence of the sewer biofilm, (2) enumerating bacteria cell number in the biofilm and (3) determining ATP content as well as sediment oxygen uptake flux (SOUF).
2. Materials and methods 2.1. Outline of the sewer The sanity gravity sewer examined in this study locates in the campus of the Hong Kong University of Science & Technology. It has a bottom slope of 0.0075 and a length of 1.5 km. This sewer delivers the campus sewage to a nearby sewage treatment works. The slope parallels to the hydraulic gradient of the sewer line.
0043-1354/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0043-1354(03)00083-6
G.-H. Chen et al. / Water Research 37 (2003) 2784–2788
There are 14 manholes with air-tight covers, located along the sewer line at a distance interval of 100 m. Due to manpower constraint, only five of the 14 manholes were selected as the sampling and observation stations, which were at 0, 331, 552, 872, and 1278 m from the beginning of the sewer. The details of this sewer system on its configuration, the characteristics of sewage quality and quantity, and the hydraulic conditions can be found in our previous papers [1,6]. Field observations at each manhole revealed that the sediment phase substantially developed along the entire sewer line. Sediment sampling was conducted with a sewer shovel, and the samples were collected at these five locations periodically. 2.2. Microscopic observation of the sediment samples A scanned electronic microscopy (SEM) equipped with LaB6 filament and a second electron detector (Philips XL30+EDAX) was employed in this study to examine the sediment samples. Preparation of the sediment specimens for examination was made as follows: (1) vacuuming sediment samples using a freeze dryer (Edwards) under 40 C for 2 hr; (2) returning it to a room temperature in a dessicator; and (3) coating the specimens with a gold film. 2.3. Bacterial enumeration and ATP analysis To determine the total cell number of bacteria in both sewage and sediment phases, cell DNA-staining technique using 40 , 6-diamidino-2-phenyl indole (DAPI) counting was adapted from Saby et al. [8], which can stain DNA of all bacteria cells. The top layer with 0.1mm thickness was collected from the surface of the samples with 4-mm thickness, using a micro-cutter under a stereoscope (Olympus). The top layer samples were then dispersed using a sonicating probe (Sonics & Materials), placed at a depth of 1 cm, in a 2-min
2785
operation with a power of 40 W. Under such operation conditions, the respiratory activity of bacteria will not be affected [8]. Upon the completion of the dispersion, the treated samples were immediately diluted in sterile saline water and then subjected to the DAPI-staining treatment for generating fluorescent cells. Cell number was finally counted under an epifluorescence microscope (objective 100, Olympus BX40) with a UV light at a wavelength of 330–385 nm. Counting was conducted in 30 fields so as to secure an acceptable accuracy and the result can be expressed as cells g1 dry wt. ATP content in the above top layer was determined using a modified trichloroacetic acid (TCA) extraction method [6]. Prior to the determination, the samples were dispersed with the same technique as described above. The solids concentration in the samples was determined according to the Standard Methods [9]. 2.4. Determination of sediment oxygen uptake flux (SOUF) At present, it is almost impossible to conduct in- situ measurement of the SOUF in a sewer. Thus, a benchscale SOUF measurement system was employed in this study, as shown in Fig. 1. This system was developed from our previous study on the measurement of benthal oxygen uptake flux for tidal river sediment [10]. It has a rectangular chamber made of acrylic plastics with a total volume of 2.9 L. Three perforated baffles were installed to create a uniform flow in the chamber. A pump was used to circulate the water at a desired flow rate. Three sediment samples can be placed in the measurement chamber. The total water/sediment contact area was 71.3 cm2. Prior to each measurement, the overlying water was aerated for seconds in order to obtain a high initial level of DO. Air bubbles were then removed before the whole system was tightened for the measurement of DO depletion. After the preparation, continuous circulation was carried out at a specific flow rate in
Fig. 1. Measurement setup of sediment oxygen uptake flux.
2786
G.-H. Chen et al. / Water Research 37 (2003) 2784–2788
a temperature-constant room at 25 C, that is the mean temperature of the local sewage. The shear flow velocity just above the sediment surface was determined by a laser flow velocity meter (2D-LDV, Polytec GmbH). The overlying water used the sewage filtered with 1-mm membrane filter (Whatman). During the operation, the DO level was monitored by a DO meter (YSI). The measurement continued until the DO level dropped to a relatively low level (about 2 mg 11). Before and after each measurement, suspended solids (SS) and dissolved organic carbon (DOC) concentrations were analyzed according to the Standard Methods [8]. Upon the completion of each measurement, the overlying water was used for measuring the specific oxygen uptake rate (SOUR) of the water. Based on the DO level difference during the measurement, the SOUR, the measurement surface areas and the value of SOUF can then be determined.
3. Results and discussion 3.1. Biofilm identification Fig. 2 shows a typical SEM picture of the crosssection of the entire sediment samples. The sediment surface was present smoothly, which might be attributed to the presence of a thin bacterial layer at the sediment surface. The mean thickness of this thin layer was determined to be 10–20 mm through the SEM examinations of various sediment samples. Elementary composition analysis of the thin layer using the EDAX equipment indicated that the carbon, oxygen, and nitrogen compositions in this layer were 46.1%, 30.7% and 14.5%, respectively, all of which are close to that in bacterial cells (C: 47.3%, O: 27%, and N: 11.3% [11]. Bacteria counting with the DAPI-staining technique showed that the total cell number of bacteria in this thin
layer was 2.1 1011 cell g1 dry wt. Such a cell amount is close to that in activated sludge (5 1011 cell g1 MLSS) [12]. Comparatively, in the water phase the total cell number was only 3.3 108 cell g1 SS. It is thus apparent that this thin layer is a biofilm layer developed at the sediment surface. 3.2. Microbial activity of the biofilm Fig. 3 shows the variation of the ATP content in the biofilm layer at the five sampling locations of the 1.5-km sewer section. It was found that the ATP content was relatively high compared to that in activated sludge [13,14]. This means that the sewer biofilm has a high microbial activity, which is consistent with the reported results [15]. The presence of active biofilm at the sediment surface endorses the possibility of achieving an effective removal of DOC in sewers in aerobic conditions. Fig. 3 also indicates that over the entire sewer section, the sewer biofilm activity did not decline significantly. This may be due to a high flow velocity induced by this high slope, which made the biofilm updated periodically over the sewer line. Thus, it is understood that the whole sewer line is able to contribute to DOC oxidation during the course of sewage conveyance when oxygen limitation does not exist. Measurements of the SOUF are shown in Fig. 4. This flux reflects the overall microbial activity of the sewer biofilm. It is found that an increase in the shear flow velocity present just above the biofilm linearly increases the SOUF, as described by SOUF ¼ 519:8 Us þ 4:9
ð1Þ
Where Us is the mean value of the shear flow velocity (m s1). The above relationship between the shear flow velocity and the SOUF can be explained such that the SOUF may be limited by the oxygen transfer rate across
Fig. 2. A typical cross-section SEM picture of the sediment samples.
G.-H. Chen et al. / Water Research 37 (2003) 2784–2788
2787
which is slightly greater than the actual SOUF (27 g O2 m2 day1) that was estimated from the sewer oxygen budget analysis [6]. The estimated SOUF value is for the entire sewer section, while the SOUF determined from the bench scale data and Eqs. (1) and (2) reflects a potential SOUF value in the sewer. Thus, the determining approach for a SOUF value is reasonable. Such a SOUF value is close to that of a wastewater biofilm [18], which also endorses a high microbial activity of the sewer biofilm in the sanitary gravity sewer. Fig. 3. Variation of the ATP content in the biofilm along the sewer line.
Fig. 4. Effect of the shear flow velocity on the SOUF.
the diffusion or shear layer present at the biofilm surface: when the diffusion layer thickness is reduced by the shear flow velocity, the mass transfer resistance then decreases accordingly. As a result, a high shear flow velocity is in favor of a thinner diffusion layer, which in turn promotes the oxygen transfer rate from the bulk phase into the biofilm, thereby increasing the SOUF. Such a finding also implies that the internal organic sources could be the major cause for oxygen utilization in the sewer biofilm. The SOUF measured in the above cases does not indicate its actual level since the shear flow velocity in the sewer pipe is unknown. In order to estimate this velocity so as to determine the corresponding SOUF in the sewer, the following formula derived from pipe hydraulics [16] was employed for calculating a mean shear flow velocity (Us) at the pipe bottom: 0:5 Ds Um ð2Þ Us ¼ 1:63 Dm where Dm and Ds are the mean values of the water depth and the shear layer thickness at the pipe bottom (mm); and Um is the mean flow velocity (m s1) of the sewage. The Dm and Um were found to be 220 mm and 1.5 m s1 [1]. It is assumed that the Ds be 0.1 mm [17]. Therefore, the Us in the existing sewer section is found to be 0.052 m s1. With this shear flow velocity, the SOUF is thus determined to be 32 g O2 m2 day1,
4. Conclusions The biofilm in a sanitary gravity sewer was investigated in this study. It has been confirmed that a biofilm layer is present at the sediment surface, in which the total cell number of bacteria was found to be 2.1 1011 cell g1 dry wt. Throughout the entire 1.5-km sewer section, a high microbial activity was maintained. The SOUF measurements at different shear flow velocities indicated a linear relationship between this velocity and the SOUF. The potential SOUF in the sewer pipe was determined to be 32 g O2 m2 day1 at 25 C with respect to the estimated shear flow velocity at 0.055 m s1, when the mean flow velocity was 1.5 m s1 and the water depth was 220 mm in the sewer pipe. Such a high SOUF value supported the existence of the active biofilm in the sewer.
References [1] Chen GH, Leung DHK, Mo HK, Saby S. Structural, chemical, and biological properties of the sediment in a sanitary gravity sewer. First IWA conference-Paris 2000, France, July 2000. [2] Pomeroy RD, Parkhurst JD. The forecasting of sulfide buildup rates in sewers. Prog water Technol 1973;9:621–8. [3] Hvitved-Jacobsen T. Vollersten J, Nielsen PH. Volatile fatty acids and sulfide in pressure mains. Water Sci. Technol 1995;31:169–79. [4] Tanaka N, Takenaka K. Control of hydrogen sulfide and degradation of organic matter by air injection into a wastewater force main. Water Sci Technol 1995;31:273–82. [5] Ochi T, Kitagawa M, Tanaka S. Controlling sulfide generation in force mains by air injection. Water Sci Technol 1998;37:87–95. [6] Chen GH, Leung DHW. Oxygen utilization of a real gravity sewer under aerobic condition. Water Res 2000;34:3813–21. [7] Chen GH, Leung DHW, Huang JC. Removal of organic matter in a gravity sewer. Environ Eng, ASCE 2001;127(4):289–95. [8] Saby S, Sibille I, Mathieu L, Paquin JL, Block JC. Influence of water chlorination on the counting of bacteria with DAPI (40 , 6-diamidino-2-phenylindole). Appl Environ Microbiol 1997;63:1564–9.
2788
G.-H. Chen et al. / Water Research 37 (2003) 2784–2788
[9] APHA.Standard methods for the examination of water and waste water. 24th ed. American Public Health Association, Washington DC, 1998. [10] Chen GH, Leong IM, Liu J, Huang JC. Study of oxygen uptake by tidal river sediment. Water Res 1999;33: 2905–12. [11] Ide T. Water quality engineering 1990. Tokyo: Gihoudou Publisher, [in Japanese]. [12] Fr^lund B, Palmgren R, Keiding K, Nielsen P. Extraction of extracellular polymers from activated sludge using a cation exchange resin. Water Res 1996;30:1749–58. [13] Brezonik PL, Patterson JW. Activated sludge ATP: effects of environmental stress. J. Sanitary Eng Div-Proc Am Soc Civil 1971; 813–24.
[14] Peter ON, Alonzo WL. Microbial viability measurements and activated sludge kinetics. Water Res 1980;14: 217–25. [15] Lemmer H, Roth D, Schade M. Population density and enzyme activities of heterotrophic bacteria in sewer biofilms and activated sludge. Water Sci Technol 1994;28:1341–6. [16] Chow VT. Open-channel hydraulics. New York: McGrawHill, 1959. [17] Williamson K, McCarty PL. A model of substrate utilization by bacterial films. J WCPF 1976;48:9–24. [18] Chen GH. Fundamental study of organic and nitrogenous removal in a biofilm Ph.D.dissertation, Kyoto University, 1990, [in Japanese]