Sewer quality modeling – a dry weather approach

Sewer quality modeling – a dry weather approach

Urban Water 2 (2000) 295±303 www.elsevier.com/locate/urbwat Sewer quality modeling ± a dry weather approach Jes Vollertsen *, Thorkild Hvitved-Jacob...

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Urban Water 2 (2000) 295±303

www.elsevier.com/locate/urbwat

Sewer quality modeling ± a dry weather approach Jes Vollertsen *, Thorkild Hvitved-Jacobsen Environmental Engineering Laboratory, Aalborg University, Sohngaardsholmsvej 57, 9000 Aalborg, Denmark Received 22 March 2000; received in revised form 27 October 2000; accepted 1 February 2001

Abstract An integrated conceptual sewer model for microbial transformations of organic matter and sulfur components in wastewater is presented. The concept and its validation are outlined. The reconstruction of an open sewer system in Germany, the Emscher River, is used as an example of how the concept in terms of a corresponding model can be used as an engineering tool. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Aerobic; Anaerobic; Emscher; Hydrogen sul®de; Model; OUR; Sewer; Wastewater

1. Introduction Increased public awareness of environmental problems has resulted in the development of European Union receiving water quality directives. Consequently, the thresholds for what are acceptable urban pollution loads and impacts have been reduced. Today, direct discharge of urban dry weather wastewater ¯ows into receiving waters is no longer acceptable. For sensitive receiving waters, removal of organic matter is no longer sucient and advanced wastewater treatment in terms of nitrogen and phosphorus removal is required. As a result, the structure of the urban wastewater system is changing. Wastewater is typically intercepted and transported to new, centralized treatment plants and combined sewer over¯ows are reduced or, where possible, eliminated. A sewer system and a corresponding treatment plant have traditionally been separately designed and operated as having two di€erent functions: a sewer system must convey the wastewater to a treatment plant and a treatment plant must reduce the pollution load into the receiving waters. As a result, the microbial transformations in the sewers that may cause considerable quality changes and treatment of the `wastewater' are ignored. These quality changes are often quite signi®cant; for example, substantial COD removal has been ob-

*

Corresponding author. Tel.: +45-96-35-85-04; fax: +45-98142555. E-mail address: [email protected] (J. Vollertsen).

served (Raunkjñr, Hvitved-Jacobsen, & Nielsen, 1995; Huisman, Gienal, Kh uni, Krebs, & Gujer, 1999). Very few attempts have been made to integrate the sewer system as part of the wastewater treatment process (Green, Shelef, & Messing, 1985). There has been a lack of reliable operational methods to quantify in-sewer microbial transformations of organic matter. This is a major reason why the e€ects of these processes on the subsequent wastewater treatment have not been taken into account. Although, some attempts have been made to couple an understanding of aerobic microbial transformations of organic matter with hydrodynamic sewer models, such as MOUSETRAP (Garsdal, Mark, Dùrge, & Jepsen, 1995). In this model the in-sewer organic matter degradation is based on the gross wastewater characteristics, COD and BOD. The approach has, however, not become widely used. For the simulation of anaerobic microbial transformations (i.e. hydrogen sul®de formation) empirical methods have been developed and frequently applied to solve engineering problems (Boon, 1995). In the middle of the 1990s it became clear that a conceptual model describing aerobic microbial transformations of wastewater had to take into account the active microbial biomass and several fractions of organic substrate. Measurement and subsequent simulation of oxygen uptake rates (OUR) in batches of unseeded wastewater was identi®ed as the key experimental technique. Hereby wastewater COD can be fractionated into components with di€erent microbial properties, i.e. organic substrate fractions and the biomass fraction can be found together with corresponding

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model parameters (Bjerre, Hvitved-Jacobsen, Teichgr aber, & Heesen, 1995). The concept used to fractionate wastewater COD has its origin in the activated sludge model describing heterotrophic transformations (Dold, Ekama, & Marais, 1980). However, some modi®cations of the growth processes and the substrate compounds were necessary as substrate and biomass conditions di€er radically between activated sludge treatment systems and sewer systems. Without such modi®cations, simulations gave unsatisfactory results and a poor agreement between measured and simulated biomasses was obtained (Bjerre et al., 1995; Vollertsen & Hvitved-Jacobsen, 1998). In order to simulate microbial transformations in sewers, other important processes must be taken into account, namely bio®lm processes, sediment interactions and the reaeration of the bulk water. Together these processes de®ne the dissolved oxygen (DO) balance in a sewer. In a ®rst approach, sediment interactions have been omitted and only sewers without signi®cant sediment deposits were addressed. An investigation of a 5 km long intercepting gravity sewer without side connections and with a rather uniform slope shows that under dry weather conditions a concept including bulk water transformations, bio®lm processes and reaeration can be validated with a sound result. To simulate anaerobic conditions as well, it is necessary to extend the concept. This has been done by introducing fermentation and integrating the concept with an empirical description of hydrogen sul®de formation (Hvitved-Jacobsen, Vollertsen, & Tanaka, 1998b). Under ®eld conditions Tanaka and HvitvedJacobsen (1998) and Tanaka, Hvitved-Jacobsen, and Horie (2000) validated this concept with good results and it has been applied to solve engineering problems (Stemplewski et al., 1999; Hvitved-Jacobsen & Vollertsen, 1998c). Further concept improvements and extensions for the simulation of dry weather transformations should be incorporated. Anoxic transformations and alternating aerobic±anoxic±anaerobic conditions especially need further investigation. Furthermore, Ashley, Hvitved-Jacobsen, Vollertsen, McIlhatton, and Arthur (1999) described how to include wet weather conditions and couple sediment related in-sewer processes with wet weather impacts on receiving waters.

ideal if identical concepts could be used for the simulation of both systems. However, the substrate and biomass conditions di€er radically between the two systems. In sewer systems, large concentrations of easily degradable organic matter are present together with comparatively low concentrations of biomass, while the opposite is the case in activated sludge treatment plants. Activated sludge models include two processes that in¯uence the heterotrophic biomass concentration. These are growth and decay. The growth process is described as a ®rst-order process in the biomass, modi®ed with saturation type equations to account for substrate limitations, for example. This approach yields good results for the simulation of in-sewer wastewater transformations when substrates do not limit the growth process corresponding to an exponential increase in OUR. Also, the transition to organic substrate limited conditions is described well with a saturation type equation (Vollertsen, 1999). However, it has been problematic to validate the decay process when simulating microbial transformations in batches of unseeded wastewater (Bjerre et al., 1995). Vollertsen (1998) observed that suspended sewer solids might consume substantial quantities of oxygen without an increase in biomass, corresponding to an unrealistic high decay rate if these ®ndings are to be accounted for by the growthdecay concept. Instead Hvitved-Jacobsen, Vollertsen, and Nielsen (1998a) and Vollertsen (1999) suggested introducing a non-growth-related substrate uptake of the heterotrophic biomass ± a maintenance energy requirement ± as a substitution for the decay process. The maintenance energy requirement process is described as a ®rst-order process in the biomass. This concept (Fig. 1) was validated with respect to the biomass concentration. It was found that simulations of suspended sewer solid OURs still yielded correct biomass concentrations after 100±200 h (Vollertsen, 1998). In the activated sludge models, the use of several fractions of hydrolyzable substrate is known to improve the description of the hydrolysis process (Sollfrank, 1988). However, typically, hydrolysis is described satisfactorily using only one fraction (Henze et al., 1995). When characterizing wastewater organic matter by simulation of measured OUR in batch reactors, 2±3

2. Characterization of wastewater COD It is necessary to characterize the wastewater in agreement with the concepts applied for the simulations when simulating transformation processes in sewers and microbial processes in activated sludge. As an assessment of the interaction between the dry weather runo€ and the wastewater treatment plant is one of the main purposes of simulating in-sewer processes, it would be

Fig. 1. Microbial transformations of wastewater for simulation of insewer transformations.

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Table 1 The model concept for characterization of wastewater based on a simulation of OUR measurements in a batch of unseeded wastewater SS Aerobic growth in bulk water Maintenance energy requirement Aerobic hydrolysis, fast Aerobic hydrolysis, slow A

1=YHw )1 1 1

XS1

)1

XS2

XBw 1 ()1)A

SO …1 1

Process rate YHw †=YHw

)1

Eq. Eq. Eq. Eq.

(a) (b) (c), n ˆ 1 (c), n ˆ 2

If SS is not present in sucient concentration XBw is used to supply the remaining COD (endogenous respiration).

lH SS =…KSw ‡ SS †XBw ;

…a†

qm XBw ;

…b†

khn …XSn =XBw †=…KXn ‡ XSn =XBw †XBw :

…c†

fractions of hydrolyzable substrate characterized by their di€erent hydrolysis rates can be observed. Typically, 2 fractions are needed to yield an acceptable simulation of an OUR measurement (Hvitved-Jacobsen et al., 1998a). In both the activated sludge concept and the wastewater characterization concept the same type of surface-limited reaction is assumed for the hydrolysis processes. Summarizing, the concept for characterization of wastewater consists of the four processes: growth of the heterotrophic biomass, maintenance energy requirement of the same biomass, hydrolysis of the fast hydrolyzable substrate and hydrolysis of the slowly hydrolyzable substrate (Fig. 1). In Table 1, a mathematical formulation of the concept applying the `matrix' notation common for activated sludge models is shown. In Fig. 2 this model is applied for simulation of OUR measured in wastewater. The result of such a simulation is a fractionation of the wastewater COD into 4 fractions of organic matter: heterotrophic biomass …XBw †, readily biodegradable substrate (SS ), fast hydrolyzable substrate …XS1 † and slowly hydrolyzable substrate …XS2 †. While OUR measurements may look quite dissimilar, generally the model can simulate di€erent OUR measurements rather well. In the ®rst measurement, shown in Fig. 2, SS is present and in the second SS is absent. Observing that the OUR increases exponentially at the start of the measurement allows this conclusion. Four hours into the ®rst measurement, the SS is consumed, which can be seen from the rapid decrease in OUR. From the OUR at time zero, the biomass concentration can be calculated when SS is present and the shape of the OUR `tail' allows the determination of XS1 . XS2 can be determined by a mass balance on the total COD (Vollertsen, 1998).

3. In-sewer aerobic transformations Having characterized the wastewater in terms of COD fractions, the next step is to identify which in-

Fig. 2. Two OUR measurements on unseeded wastewater in a batch reactor. Simulations are performed with the model shown in Table 1. Observe that the two examples di€er in both the content of readily biodegradable substrate and in the level of dilution.

sewer wastewater transformations will occur. To do so, the processes in the bulk water must be combined with the processes in the bio®lms, the sediments, and the air± water gas exchange (reaeration). Steady-state bio®lm properties are assumed and sediment beds are either absent or assumed to behave like a bio®lm.

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The bulk water transformation processes under aerobic conditions are identical to the processes taking place in wastewater during an OUR measurement in a batch reactor. However, oxygen limitations will occur at low DO concentrations. Hence, these processes can be described as shown in Fig. 1 and Table 1 with additional saturation type equations accounting for the oxygen limitations. Bio®lm processes are allowed for in a somewhat different way. As is the case for the bulk water processes, conceptual models for microbial transformations in bio®lms may be formulated. However, for bio®lms it is necessary to take into account di€usion into the bio®lm and subsequent microbial transformations of di€erent electron donors and acceptors inside the bio®lm (Characlis & Marshall, 1989). This results in a rather large number of model parameters that cannot readily be determined for engineering applications. Therefore, a `semi-conceptual' approach has been chosen. The bio®lm is treated as a homogeneous, partly penetrated ®lm with a smooth surface and without a di€usive boundary layer. Transformations inside the ®lm are assumed to follow zero-order kinetics. In this case, consumption of SS and DO can be described as a half-order reaction in the bulk water concentration of the consumed component (Jansen & Harremoes, 1984). Hydrolysis in the bio®lm is described as in the bulk water but modi®ed with an eciency factor multiplied with a hypothetical bio®lm biomass concentration as little is known about hydrolysis in sewer bio®lms (Hvitved-Jacobsen et al., 1998a). Furthermore, the bio®lm is described as being in steady state with respect to the biomass concentrations in the ®lm; i.e. all the biomass produced are immediately released into the bulk water. The description of gas exchange between the sewer atmosphere and the bulk water is limited to reaeration. Reaeration is described as an empirical equation based on the two-®lm model developed by Lewis and Whitman (1924). The empirical formula used is a modi®cation of the work by Parkhurst and Pomeroy (1972). Jensen (1995) improved their equation based on their data and new data obtained using radioactive tracers in gravity sewers. The temperature plays an important role for all insewer transformation processes. Temperature is taken into account using the Arrhenius expression. 4. In-sewer integrated aerobic±anaerobic transformations When neither oxygen nor nitrate are present, the wastewater becomes septic (anaerobic). The main problem associated with anaerobic conditions is the formation of hydrogen sul®de …H2 S†, which has negative e€ects on the sewer itself, its surroundings and the subsequent wastewater treatment. H2 S is very toxic and

rather low bulk water concentrations result in lethal equilibrium concentrations of H2 S gas in the sewer atmosphere. The gas is also strongly smelling, giving rise to odor problems. Microorganisms will oxidize H2 S into sulfuric acid when oxygen is present. This can result in severe corrosion of concrete and metals in the sewer system (Boon, 1995). However, bene®cial e€ects of septic wastewater are also known. Fermentation processes take place under anaerobic conditions producing volatile fatty acids (VFA) which are required for biological phosphorus removal. Also, hydrolysis continues under anaerobic conditions, although, at a lower rate. Therefore, a net increase in the concentration of readily biodegradable substrate is seen under anaerobic sewer conditions, which is bene®cial for biological nitrogen removal processes in wastewater treatment plants (Henze et al., 1995). The integrated aerobic±anaerobic concept for insewer organic matter transformations is an extension of the aerobic concept, by taking into account hydrolysis under anaerobic conditions, fermentation and hydrogen sul®de formation (Fig. 3). Hydrolysis under anaerobic conditions is assumed to be identical to hydrolysis under aerobic conditions except for an eciency factor. This approach is similar to activated sludge models and has been validated for batches of unseeded wastewater (Stemplewski et al., 1999; Tanaka et al., 2000). The fermentation process is included as a ®rst-order process in the biomass modi®ed with a saturation type expression, taking into account substrate limitations due to fermentable products …SF †. Again, this expression is equivalent to what typically is used in activated sludge models (Henze et al., 1995). The description of the hydrogen sul®de formation is an empirical description presented by Hvitved-Jacobsen et al. (1998b). This description is a further development of an equation for prediction of hydrogen sul®de formation in pressure mains suggested by Nielsen, Raunkjñr, and Hvitved-Jacobsen (1998). The important parameters in the description of the H2 S formation are the concentration of sulfate, the concentration of degradable organic matter and the temperature. Typically, wastewater sulfate contents are high and not limiting. Hence, only degradable organic matter and temperature are taken into account. In the model the organic matter that can be used by the biomass for sulfate respiration is assumed to be XS1 ; SF and SA (Hvitved-Jacobsen et al., 1998b; Tanaka et al., 2000). It should be noted that the H2 S formation does not take into account oxidation of H2 S or release into the atmosphere in gravity sewers. Therefore, a model simulation gives the total H2 S produced, which for gravity sewers is not equivalent to measured bulk water concentrations. In the model, all anaerobic processes contain a switch turning the processes on if DO is absent. The switch is equivalent to what is typically used in activated sludge

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Fig. 3. An integrated concept for the transformation of organic matter and sulfur components under aerobic and anaerobic conditions.

Table 2 Integrated processes for transformation of wastewater organic matter in a sewer where aerobic as well as anaerobic processes may proceed SA

SF Aerobic growth in bulk water Aerobic growth in bio®lm Maintenance energy requirement Aerobic hydrolysis, fast Aerobic hydrolysis, slow Anaerobic hydrolysis, fast Anaerobic hydrolysis, slow Fermentation Hydrogen sul®de production Reaeration *

1=YHw 1=YHf )1 1 1 1 1 )1

XS1

)1 )1 1

XS2

XBw

SH2S

1 1 ()1)

SO …1 …1 1

process rate YHw †=YHw YHf †=YHf

)1 )1 1

)1

Eq. (a) Eq. (b) Eq. (c) Eq. (d), n ˆ 1 Eq. (d), n ˆ 2 Eq. (e), n ˆ 1 Eq. (e), n ˆ 2 Eq. (f) Eq. (g) Eq. (h)

If SS is not present in sucient concentration XBw is used to supply the remaining COD (endogenous respiration).

lH …SF ‡ SA †=…KSw ‡ …SF ‡ SA ††SO =…KO ‡ SO †XBw aw…T k1=2 SO0:5 YHf =……1

20†

…a† …T 20†

YHf †A=V …SF ‡ SA †=…KSf ‡ …SF ‡ SA †††af

qm SO =…KO ‡ SO †XBw a…T w

20†

…b†

:

…c†

khn …XSn =XBw †=…KXn ‡ XSn =XBw †SO =…KO ‡ SO †…XBw ‡ eXBf A=V †aw…T

20†

gfe khn …XSn =XBw †=…KXn ‡ XSn =XBw †KO =…SO ‡ KO †…XBw ‡ eXBf A=V †aw…T qfe SF =…Kfe ‡ SF †KO =…SO ‡ KO †…XBw ‡ eXBf A=V †aw…T …T 20†

kH2S 10 3 …SF ‡ SA ‡ XS1 †0:5 aS KL a…SOS

…d† 20†

…e†

20†

…f†

KO =…SO ‡ KO †24A=V

…g†

SO † where KL a ˆ 0:86…1 ‡ 0:20F2 †…su†3=8 dm 1 a…T r

20†

24

models. The mathematical formulation of the concept (Fig. 3) is shown in Table 2. 5. How to apply the concept Investigating in-sewer wastewater transformations using the concept makes it possible to design the

…h†

wastewater quality to suit a certain purpose. If mechanical treatment or treatment for nitrogen and phosphorus removal is planned, di€erent wastewater quality aspects are desired. For example, having the largest possible fraction of the COD in particulate form enhances mechanical treatment; while readily biodegradable compounds are needed for nitrogen and phosphorus removal. Depending on the conditions

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imposed by the sewerage system, quality aspects related to microbial processes can be enhanced or suppressed and potential hydrogen sul®de problems can be assessed. However, to obtain these bene®ts, knowledge of the actual wastewater quality must be obtained and the concept calibrated to the actual problem. It is necessary to characterize the wastewater in agreement with the concept used for simulation of the in-sewer transformations; i.e. by the method described for fractionation of wastewater COD. Such a study has been conducted in the Emscher catchment, Germany, (Fig. 4) for the Emschergenossenschaft who operate a large part of the sewerage system in the Ruhr district (Hvitved-Jacobsen & Vollertsen, 1998c). Previously, intense coal mining activity resulted in frequent collapses of the surface making it dicult to maintain closed sewers. Instead, the River Emscher and its tributaries were developed as open sewers where 60± 80% of the dry weather ¯ow was wastewater (Stemplewski et al., 1999). Today, the mining activities have moved to other areas and ground conditions have stabilized. Now an open sewer system is no longer acceptable and it has been decided to return the Emscher and its tributaries into as natural a state as possible by intercepting the wastewater in closed sewers.

Fig. 4. The Emscher catchment.

The wastewater produced in the catchment is now treated at plants located in Dinslaken at the Emscher mouth, in Bottrop and in Dortmund. In order to distribute the wastewater to the 3 treatment plants an approximately 50 km long intercepting sewer with a capacity of several cubic metre per second has to be established. Di€erent wastewater conveyance options have been evaluated based on three criteria: no permanent sediment beds; no serious hydrogen sul®de problems; and, a good quality of the wastewater for biological nitrogen and phosphorus removal. The latter two criteria were assessed using the concept in Fig. 3 and Table 2. Model components and parameters were determined from measurements on the Emscher and its tributaries. An average wastewater COD composition was used to model the organic matter composition of the tributaries based on 30 OUR measurements on wastewater in main tributaries to the Emscher (Table 3). To cover the diversity of wastewaters found in the catchment it was necessary to use 3 fractions of hydrolyzable substrate. Model parameters (Tables 4 and 5) were partly obtained from the same 30 OUR measurements and partly obtained from previous, related studies (Jensen, 1995; Bjerre et al., 1995; Bjerre, Hvitved-Jacobsen, Schlegel, & Teichgraber, 1997; Bjerre, Hvitved-Jacobsen, Teichgraber, & Schlegel, 1998; Tanaka & Hvitved-Jacobsen, 1998; Tanaka, Hvitved-Jacobsen, Ochi, & Sato, 1998b; Tanaka et al., 2000). Two sewer scenarios were tested and compared: a gravity sewer and a pressure main. It was concluded that both scenarios would result in anaerobic sewer conditions and, hence, in the formation of H2 S. In the pressure main, H2 S concentrations will be somewhat higher than in the gravity sewer. However, in both scenarios the concentrations are expected to give no or only minor problems (Fig. 5). The quality of the organic matter with respect to nitrogen and phosphorus removal will be improved by

Table 3 COD-components and dissolved oxygen for wastewaters from Emscher River sub-catchments used in the sewer process model outlined in Table 2

a b

Component

Fraction of total COD

Unit

XBw : Heterotrophic active biomass in the water phase XBf : Heterotrophic active biomass in the bio®lm XS1 : Hydrolyzable substrate, fast biodegradable XS2 : Hydrolyzable substrate, medium biodegradable XS3 : Hydrolyzable substrate, slowly biodegradableb SF : Fermentable substrate SA : Fermentation products (i.e. VFAs) SS : Readily biodegradable substrate …SF ‡ SA † SO : Dissolved oxygen COD: Total COD

COD  0.040a 10 COD  0.080a COD  0.165a COD  0.695a COD  0.010 COD  0.010 COD  0.020a 0.0

gCOD=m3 gCOD=m2 gCOD=m3 gCOD=m3 gCOD=m3 gCOD=m3 gCOD=m3 gCOD=m3 gO2 =m3 gCOD=m3

The fractions are based on 30 OUR measurements on samples from main catchments in the Emscher area. Including very slowly biodegradable and inert organic matter.

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Table 4 Model parameters used in the sewer process model outlined in Table 2a

a b

Symbol and de®nition

Value

Unit

lH : Maximum speci®c growth rate for heterotrophic biomass YHw : Suspended biomass yield constant for heterotrophic biomass KS : Saturation constant for readily biodegradable substrate KO : Saturation constant for dissolved oxygen (DO) aw : Temperature coecient in the water phase qm : Maintenance energy requirement rate constant k1=2 : 1/2 order rate constant YHf : Bio®lm yield constant for heterotrophic biomass KSf : Saturation constant for readily biodegradable substrate e: Eciency constant for the bio®lm biomass af : Temperature coecient in the bio®lm kh1 : Hydrolysis rate constant, fraction 1 (fast) kh2 : Hydrolysis rate constant, fraction 2 (medium) kh3 : Hydrolysis rate constant, fraction 3 (slow) KX1 : Saturation constant for hydrolysis, fraction 1 KX2 : Saturation constant for hydrolysis, fraction 2 KX3 : Saturation constant for hydrolysis, fraction 3 gfe : Anaerobic hydrolysis reduction factor qfe : Maximum rate for fermentation Kfe : Saturation constant for fermentation kH2S : Hydrogen sul®de production rate constant aS : Temperature coecient for hydrogen sul®de production

6. 7b 0.55 1.0 0.05 1.07 1.0 4 0.55 5 0.15 1.05 12b 5b 0.4b 1.5b 0.5b 0.1b 0.14 3 20 2 (3) 1.030

d 1 gCOD/gCOD gCOD=m3 gO2 =m3 ± d 1 0:5 gO0:5 d 1 2 m gCOD/gCOD gCOD=m3 ± ± d 1 d 1 d 1 gCOD/gCOD gCOD/gCOD gCOD/gCOD ± d 1 gCOD/gCOD gS2 =m2 ±

w: water phase; f: bio®lm. Determined from 30 OUR measurements on samples from main catchments in the Emscher area.

Table 5 Reaeration and ¯ow characteristics used in the sewer process model outlined in Table 2a Symbol and de®nition

Value

KLa : Oxygen transfer coecient T : Temperature SOS : Dissolved oxygen saturation concentration at T °C F : Froude number ˆ u=…gdm †0:5 u: Mean ¯ow velocity g: Gravity acceleration s: Slope dm : Hydraulic mean depth A=V : Ratio of bio®lm area to bulk water volume ar : Temperature coecient for reaeration a

9.81

1.024

Unit d 1 °C gO2 =m3 ± m/s m=s2 m/m m m 1 ±

Parameters subject to variation during transport are not indicated with a value.

transport in the pressure main, as well as by transport in the gravity sewer. However, the pressure main will produce nearly twice as much readily biodegradable substrate compared with the gravity sewer (Fig. 5). The Emschergenossenshaft have proposed that the e‚uent ¯ow will ultimately decrease due to water-saving e€orts in the catchment. Therefore, additional simulations were performed assuming lower ¯ow but the same transported organic matter loads. These simulations showed that for this scenario H2 S problems will also be manageable and the wastewater treatable. Furthermore, simulations for the normal temperature range in the catchment indicate that the sewerage system can be operated satisfactorily all year.

Hvitved-Jacobsen and Vollertsen (1998c) concluded that transport of wastewater from Dortmund to Dinslaken in either the gravity sewer or the pressure main option is expected to result in insigni®cant changes in the total COD compared with the inputs to these sewers. Concentrations of readily biodegradable substrate will increase during transport. The general level of readily biodegradable substrate and fast hydrolyzable substrate in the tributaries was found to be relatively high. Therefore, wastewater transported to the treatment plants by either of the proposed alternative intercepting sewers will be well suited for advanced biological treatment in terms of both nitrogen removal and biological phosphorus removal. Minor problems relating to

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a certain set of performance criteria related to both the subsequent treatment processes and the sewer system itself. Based on wastewater quality measurements, integrated aerobic and anaerobic processes can be simulated with a model concept that takes into account both organic matter transformations and hydrogen sul®de formation. Therefore, today, the state of the art is to design and operate sewer systems which are integrated with the subsequent wastewater treatment processes.

References

Fig. 5. Concentrations of hydrogen sul®de and readily biodegradable substrate at the inlet to the Bottrop treatment plant.

hydrogen sul®de formation might be expected. These problems are, however, expected to be manageable in terms of hydrogen sul®de precipitation. Therefore, from a process point of view, the general conclusion is that wastewater transport in an intercepting gravity sewer or an intercepting pressure main from Dortmund to Dinslaken is possible. Subsequently, it has been decided to establish a gravity sewer parallel to the Emscher based on these simulations and other more traditional decision tools and criteria.

6. Conclusion Di€erent types and levels of wastewater treatment de®ne di€erent quality criteria for the wastewater to be treated. For example, if wastewater is treated mechanically, as much organic matter as possible should be of a particulate form. On the other hand, when wastewater is treated biologically to remove nitrogen and phosphorus, a high content of readily biodegradable compounds is necessary. These di€erent wastewater quality criteria should be taken into account when designing and operating sewer systems. Today, the knowledge of in-sewer microbial processes is at a level where sewer systems can be designed to meet

Ashley, R.M., Hvitved-Jacobsen, T., Vollertsen, J., McIlhatton, T., & Arthur, S. (1999). Sewer solids erosion washout and a new paradigm to control solids impacts on receiving waters. In Proceedings of the eighth international conference on urban storm drainage (Vol. 1, pp. 171±178). Sydney, Australia. Bjerre, H. L., Hvitved-Jacobsen, T., Teichgraber, B., & Heesen, D. te. (1995). Experimental procedures characterizing transformations of wastewater organic matter in the Emscher River, Germany. Water Science Technology, 31(7), 201±212. Bjerre, H. L., Hvitved-Jacobsen, T., Schlegel, S., & Teichgraber, B. (1997). Biological activity of bio®lm and sediment in the Emscher river, Germany. Water Science Technology, 37(1), 9±16. Bjerre, H. L., Hvitved-Jacobsen, T., Teichgraber, B., & Schlegel, S. (1998). Modelling of aerobic wastewater transformations under sewer conditions in the Emscher river, Germany. Water Environmental Research, 70(6), 1151±1160. Boon, A. G. (1995). Septicity in sewers: causes, consequences and containment. Water Science Technology, 31(7), 237±253. Characklis,W. G., & Marshall, K. C. (1989). Bio®lms. In W. G. Characklis, & K. C. Marshall (Eds.), Wiley series in ecological and applied microbiology, USA, 1989. Dold, P. L., Ekama, G. A., & Marais, G. R. (1980). A general model for the activated sludge process. Progress in Water Technology, 12(6), 47±77. Garsdal, H., Mark, O., Dùrge, J., & Jepsen, S. E. (1995). Mousetrap: modelling of water quality processes and the interaction of sediments and pollutants in sewers. Water Science Technology, 31(7), 33±41. Green, M., Shelef, G., & Messing, A. (1985). Using the sewerage system main conduits for biological treatment greater Tel-Aviv as a conceptual model. Water Research, 19(8), 1023±1028. Henze, M., Gujer, W., Mino, T., Matsuo, T., Wentzel, M. C., & Marais, G. R. (1995). Activated sludge model No. 2. Scienti®c and Technical Report No. 3 International Association on Water Quality. Huisman, J.L., Gienal, C., Kh uni, M., Krebs, P., & Gujer, W. (1999). Oxygen mass transfer and bio®lm respiration rate measurement in a long sewer evaluated with a redundant oxygen balance. In Proceedings of the eighth international conference on urban storm drainage (Vol. 1, pp. 306±314). Sydney, Australia. Hvitved-Jacobsen, T., Vollertsen, J., & Nielsen, P. H. (1998a). A process and model concept for microbial wastewater transformations in gravity sewers. Water Science Technology, 37(1), 233± 241. Hvitved-Jacobsen, T., Vollertsen, J., & Tanaka, N. (1998b). Wastewater quality changes during transport in sewers ± an integrated aerobic and anaerobic model concept for carbon and sulfur microbial transformations. Water Science Technology, 38(10), 257±264. Hvitved-Jacobsen, T., & Vollertsen, J. (1998c). An intercepting sewer from Dortmund to Dinslaken, Germany prediction of wastewater

J. Vollertsen, T. Hvitved-Jacobsen / Urban Water 2 (2000) 295±303 transformations during transport. Environmental Engineering Laboratory, Aalborg University, Denmark. Jansen, J. la. C., & Harremoes, P. (1984). Removal of soluble substrates in ®xed ®lms. Water Science Technology, 17(2±8), part 1, 1±14. Jensen, N. A. (1995). Empirical modeling of air-to-water oxygen transfer in gravity sewers. Water Environment Research, 67(6), 979± 991. Lewis, W. K., & Whitman, W. G. (1924). Principles of gas absorption. Industrial Engineering Chemistry, 16(12), 1215±1220. Nielsen, P. H., Raunkjñr, K., & Hvitved-Jacobsen, T. (1998). Sul®de production and wastewater quality in pressure mains. Water Science Technology, 37(1), 97±104. Parkhurst, J. D., & Pomeroy, R. D. (1972). Oxygen Absorption in Streams. Journal of the Sanitary Engineering Division American Society of Civil Engineers, 98(SA1), 101±124. Raunkjñr, K., Hvitved-Jacobsen, T., & Nielsen, P. H. (1995). Transformation of organic matter in a gravity sewer. Water Environment Research, 67(2), 181±188. Sollfrank, U. (1988). Bedeutung organischer Fractionen in kommunalem Abwasser im Hinblick auf die mathematische Modellierung von Belebtschlammsystemen. Dissertation ETH No. 8765. Z urich 1988. Stemplewski, J., Schlegel, S., Stein, A., Geisler, W., Schmelz, K.-G., Hvitved-Jacobsen, T., & Vollertsen, J. (1999). Restructuring the

303

Emscher system. In Proceedings of the 11th EWPCA (the European Water Pollution Control Association) symposium: Sewerage systems cost and sustainable solutions, 4±6 May 1999, Munich, Germany, p. 14. Tanaka, N., & Hvitved-Jacobsen, T. (1998). Transformations of wastewater organic matter in sewers under changing aerobic/ anaerobic conditions. Water Science Technology, 37(1), 105±113. Tanaka, N., Hvitved-Jacobsen, T., Ochi, T., & Sato, N. (1998). Aerobic/anaerobic microbial wastewater transformations and reaeration in an air-injected pressure sewer. In Proceedings of the 71st annual water environment federation conference and exposition (Vol. 2, pp. 853±864), WEFTEC'98, Orlando, FL, USA, 3±7 October. Tanaka, N., Hvitved-Jacobsen, T., & Horie, T. (2000). Transformations of carbon and sulfur wastewater components under aerobicanaerobic transient conditions in sewer systems. Water Environment Research, 72(6), 651±664. Vollertsen, J., & Hvitved-Jacobsen, T. (1998). Aerobic microbial transformations of resuspended sediments in combined sewers ± a conceptual model. Water Science Technology, 37(1), 69±76. Vollertsen, J., & Hvitved-Jacobsen, T. (1999). Stoichiometric and kinetic model parameters for microbial transformations of suspended solids in combined sewer systems. Water Research, 33(14), 3127±3141.