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Wastewater quality changes during transport in sewers — An integrated aerobic and anaerobic model concept for carbon and sulfur microbial transformations
~
Pergamon
Wal. Sci Tech. Vol. 39. No.2. pp. 233-249. 1999.
PH: S0273-1223(99)OOO37-2
e 1999IAWQ Publishedby Elsevier ScienceLId Printed in Great Britain.All rights reserved 0273-1223/99 $19'00 + 0'00
ERRATA Water Science and Technology 38(10),249-256
AEROBIC MICROBIAL TRANSFORMATIONS OF PIPE AND SILT TRAP SEDIMENTS FROM COMBINED SEWERS J. Vollertsen, T. Hvitved-Jacobsen, I. McGregor and R. Ashley
Water Science and Technology 38(10),257-264
WASTEWATER QUALITY CHANGES DURING TRANSPORT IN SEWERS - AN INTEGRATED AEROBIC AND ANAEROBIC MODEL CONCEPT FOR CARBON AND SULFUR MICROBIAL TRANSFORMATIONS T. Hvitved-Jacobsen, J. Vollertsen and N. Tanaka
It is regretted that the text of the above articles was transposed. The publishers would like to apologise for this error and the corrected versionsof the articlesfollow.
233
WASTEWATER QUALITY CHANGES DURING TRANSPORT IN SEWERS - AN INTEGRATED AEROBIC AND ANAEROBIC MODEL CONCEPT FOR CARBON AND SULFUR MICROBIAL TRANSFORMATIONS T. Hvitved-Jacobsen, J. Vollertsen and N. Tanaka Environmental Engineering Laboratory, AalborgUniversity, Sohngaardsholmsvej 57, DK-9000Aalborg, Denmark
ABSTRAcr A conceptual model for wastewater quality changes during transport in sewers is presented. The model concept includes reaeration and main aerobic and anaerobic microbial processes in water phase and sewer biofilm. Emphasis is on microbial transformations of heterotrophic biomass and soluble and particulate fractions of organic substrate; the inclusion of sulfate respiration in the model concept is outlined. The model concept has been tested in gravity sewers as well as in pressure mains. Oxygen utilization rate measurements of wastewater samples from sewers are used for model calibration. The model is exemplified as a tool for evaluation of wastewater quality changes in an intercepting gravity sewer. The model concept can be used in the design process of sewers taking into account quality aspects. It is recommended to consider sewer processes when addressing functioning of the sewer, wastewater treatment and combined sewer overflow effects. @ 1999 IAWQ Published by Elsevier Science Ltd. All rights reserved
KEYWORDS Wastewater; microbial transformation; aerobic process; anaerobic process; sulfide; sewer; conceptual model; simulation. INTRODUCTION A sewer system interacts with wastewater treatment as well as receiving waters. Wastewater is transported through the sewers to treatment and overflow of untreated wastewater from combined sewers into receiving waters take place during storm events. Microbial processes in wastewater of sewer systems proceed during transportation, (Nielsen et al., 1992). These processes take place in the water phase, in the biofilms and in temporarily settled sewer sediments, Figure I. Soluble as well as particulate wastewater compounds undergo transformation processes. The dissolved oxygen (DO) concentration in the wastewater determines whether processes take place under aerobic, anaerobic or varying conditions. Understanding of complicated microbial processes, interactions and conditions is a comer stone for taking into account several types of sewer problems, e.g. odour and health aspects, hydrogen sulfide corrosion and malfunctioning of wastewater treatment due to input of low quality wastewater. A comprehensive understanding of a sewer system as being a reactor for microbial processes is therefore needed for sewer design and operation taking into account functioning of the sewer, wastewater treatment and combined sewer overflow effects. The necessity for such a comprehensive understanding is well accepted for wastewater treatment, however, not yet fully realized for wastewater transport in sewers. The urban wastewater systems and management hereof causes several desired as well as undesired effects which have there origin in sewer microbial processes. Important examples are: 242
243
Wastewater quality changes duringtransport in sewers
• Functioning of the sewer: considerations concerning the sewer itself and the surroundings within the catchment area; examples are related to odour and corrosion problems due to sulfide formation. Another important example is sediment accumulation. • Wastewater treatment: COD fractions are removed and transformed depending on the design and management of the entire wastewater system. One example relates to production or removal of readily biodegradable substrates when addressing nitrogen and biological phosphorus removal in wastewater treatment plants. Another example relates to in-sewer wastewater treatment, (Green et al., 1985). • Wet weather impacts on receiving waters: impacts from storm water runoff and combined sewer overflows, e.g. caused by discharge of resuspended sewer sediments.
WAS1EWATER FROM HOUSEHOLDS AND INDUSTRY
WAS1EWATER TREATMENT
TREATED WASTEWATER SLUDGE
SEWER SYSTEM
Figure 1. Wastewater transport within the urban wastewater system. Contrary to what is known about microbial processes in sewers, the hydraulics and to some extent also solids aspects of wastewater transport are better understood. Model simulation of wastewater and solids transport is therefore also common practise for design and analysis of sewer system performance during both dry and wet weather flow. Knowledge on physical, chemical and biological processes in sewers affecting wastewater quality suffer from a lack of both theoretical and empirical basic understanding. Except for sulfate reduction under anaerobic conditions resulting in sulfide formation and for sewer solids transport, processes in sewers have not been considered - and almost not discussed - among scientists and practitioners within the area of wastewater treatment and combined sewer overflow management. In terms of quality aspects, sewers are typically just considered systems for wastewater supply at the boundaries where they are connected with wastewater treatment plants and overflow structures discharging into receiving waters.
WASTEWATER QUALITY CHANGES DURING TRANSPORT IN SEWERS During transportation of wastewater in sewers, microbial transformation of organic matter takes place. Such quality changes take place under aerobic as well as anaerobic conditions associated with biomass in the water phase, the biofilms and the sediments. In terms of removal of readily biodegradable organic matter, transformations are especially related to aerobic conditions. These microbial changes of wastewater substrate as well as of biomass under sewer conditions have been investigated and quantified by Bjerre et al. (1995), Bjerre et al. (1998a), Bjerre et al. (l998b), Vollertsen and Hvitved-Jacobsen (1998), Tanaka and HvitvedJacobsen (1998), Hvitved-Jacobsen et al. (1998), Tanaka et al. (1998) and Tanaka and Hvitved-Jacobsen (subm.). The concept of microbial transformations in a sewer is based on the description used in the activated sludge model, i.e. biomass growth and substrate utilization, (Henze et al., 1987). In this respect an important relationship between wastewater microbial transformations taking place in a sewer and in a successive wastewater treatment plant is established. This fact is a fundamental requirement when considering a sewer
244
T. HVITVED-IACOBSEN et al.
system and a wastewater treatment plant as an entity in terms of treatment. However, microbial processes in a sewer and in an activated sludge system differs in the detailed 'description. This is especially the case concerning the fact that different fractions of particulate organic matter in a sewer are available as a source for hydrolysis products and that biomass decay is replaced by biomass maintenance energy requirement, (Bjerre et al., 1998b; Hvitved-Jacobsen et al, 1998; Vollertsen and Hvitved-Jacobsen, 1998). Such conceptual differences affect the defmition of stoichiometric and kinetic parameters as well as their numerical values. This fact is, however, no obstacle for integration of sewer and wastewater treatment processes. Traditionally, COD removal or transformation in a sewer is considered in terms of changes in the total COD concentration or alternatively BOD or dissolved COD, although uncertainties of measurements from real situations often makes this practically unrealistic. Furthermore, such COD values are of limited interest when dealing with wastewater microbial processes in sewers. Assessment of quality changes need a total overview in terms of transformations of biomass and substrate components including associated processes. This overview can only be obtained from model simulations based on a detailed description of the microbial system including realistic and intensive laboratory and field investigations.
Transformation under aerobic conditions Under aerobic conditions in a (gravity) sewer, wastewater microbial transformations are primarily caused by growth and maintenance of heterotrophic biomass associated with carbon and oxygen uptake as well as hydrolysis of particulate organic substrate. The driving and governing process for these transformations is the continuous supply with oxygen by reaeration, (Jensen and Hvitved-Jacobsen, 1991; Jensen, 1995). These transformations are illustrated in Figure 2.
Inpurof
components ~
Bulk water maint9118f1C8
Fast
erJetgy
frydtrJIyzabIe
19QulremeI1t
COD
OuIputof components
1---" •
Figure 2. The simplified aerobic wastewater system under gravity sewer conditions (Hvitved-Jacobsen et al., 1998). Readily biodegradable substrate (Ss) includes two fractions: Fermentable substrate (SF)and fermentation products (SA), i.e. volatile fatty acids (VFA's).
Transformation under anaerobIc conditions Wastewater transformations under anaerobic conditions in sewers have been investigated by Tanaka and Hvitved-Jacobsen (1998), Tanaka et al. (1998) and Tanaka and Hvitved-Jacobsen (subm.), These transformations have been integrated with the aerobic processes as shown in Figure 2, Figure 3. Contrary to aerobic heterotrophic transformations, the rate of transformations are relatively low under anaerobic conditions. Volatile fatty acids (VFA's) are produced under anaerobic conditions from fermentable readily biodegradable organic matter. (Hvitved-Jacobsen et al., 1995). It is, therefore, needed to classify readily biodegradable substrate (Ss) into two fractions: Fermentable substrate (SF) and fermentation products (SA>. i.e. VFA's, Figure 3. The VFA's may be substrate for methane producing bacteria. The sulfur cycle in terms of the sulfate respiration processes can be integrated with the anaerobic carbon cycle, Figure 3, Tanaka and Hvitved-Jacobsen (subm.). Fractionation of readily biodegradable substrate (Ss) into SF and SA fits well to the anticipation that mainly SF is used by sulfate reducing biomass in sewer biofilms. By integrating the sulfide formation in a process description, a conceptual approach is obtained
Wastewaterquality changes during transport in sewers
245
instead of the traditional empirical descriptions, (Thistlethwayte, 1972, Boon and Lister, 1975, Pomeroy and Parkhurst, 1977, Hvitved-Jacobsen et al., 1988).
/ttputot
components. ~ _ _ _ _ _ _ _ _ _ _ _ _
! Methane
~~ ../
: producing : bicmISS
: :
I
,
!._---- ------ ~
Figure 3. The simplified anaerobic system for wastewater transformations in a sewer considered as an extension of the aerobic system shown in Figure 2. A model concept for microbial wastewater transformations Integration of the aerobic and the anaerobic system for wastewater transformations in sewers makes it possible to describe changing aerobic/anaerobic conditions, (Tanaka and Hvitved-Jacobsen, 1998). Such changing conditions may take place in pressure mains when injecting air or pure oxygen and in gravity sewers with varying slope. A combined aerobic/anaerobic process model concept is shown in Table I, based on the systems outlined in Figure 2 and 3. The process model is the basis for integration with a transportdispersion model for wastewater transportation in sewers. The model outlined in Table 1 is based on investigations and considerations published in Bjerre et al. (I 998b), Vollertsen and Hvitved-Jacobsen (1998), Tanaka and Hvitved-Jacobsen (1998), Hvitved-Jacobsen et al, (1998), Tanaka et al. (1998) and Tanaka and Hvitved-Jacobsen (subm.). Table 1. Integrated processes for transformation of wastewater organic matter in a sewer where aerobic as well as anaerobic processes may proceed. Symbols are defined in Table 2, 3 and 4. -Sg Xew (I-YH..)lYH.. Aerobic growth in bulk water -IIY H.. Aerobic growth in biofilm -IIY HI (I-YHI)lYHI Maintenanceenergy requirement I -I -1 Aerobic hydrolysis.fast I ·1 Aerobic hydrolysis,slow I -I Anaerobichydrolysis.fast I -I Anaerobic hydrolysis,slow I -) Ferment.lltion in bulk water and biofilm Hydrogensulfide production Reaeration -I a: ~H (Sp+SA)/(Ks+(SP+SA)) S
SQ:>'
process rate Eg. • Eg.b Eg.c Eg.d,n=I
Eq.d,n=2 Eq. e, n=I
Eq.e,n=2 Eg. f
Eq.g Eg.h
246
T. HVITVED-JACOBSEN et al.
The model includes aerobic growth and maintenance energy requirement in the bulk water, aerobic growth in the biofilm, aerobic and anaerobic hydrolysis, fermentation, sulfide production and reaeration. Interaction with the sediments is disregarded, Figure I.
Model application The model concept shown in Table I is applied on a big intercepting gravity sewer line characterised with the following system parameters : length: 20 km; diameter: 1.20 m; slope 0.03 and 0.3%. The temperature and flow conditions chosen are: temperature 18°C and flow 0.3 m3/s - the average load from about 125,000 person equivalents. The wastewater chosen in this example is diluted wastewater transported aerobically for a few hours. COD components, model parameters and reaeration and flow characteristics used in a simulation with the model is shown in Table 2, 3 and 4. These parameters and characteristics are selected according to findings based on laboratory and field investigations . Table 2. COD-components and dissolved oxygen for wastewater at an upstream location of the intercepting sewer line used for model simulation . Wastewater includes soluble (S) and particulate (X) components. Component
characteristic value.wastewater 20-100
example; diluted 40
unit
gCOD/mJ heterotrophic activebiomassin the waterphase heterotrophic active biomass in the biofilm gCOD/mz hydrolysible substrate. fast biodegradable So-l00 40 gCOD/mJ hydrolysible substrate. slowlybiodegradable300 -4S0 210 gCOD/mJ s, readily biodegradable substrate o-SO 30gCOD/m3 So dissolvedoxygen 0-4 2 g07lmJ COD total COD about600 320 gCOD/mJ - Include veryslowlybiodegradable and inert organicmatter. - - 5s = SF + SA; Sf = 20 gCOD/mJ and SA = 10 gCOD/mJ XBw XBr XSI Xsz
Table 3. Model parameters used in the sewer process model outlined in Table 1. Symboland definition J.lH maximum specificgrowthrate for heterotrophic biomass YHw suspended biomass yieldconstantfor heterotrophies Ks saturation constantfor readily biodegradable substrate Ko saturation constantfor DO lXw temperature coefficient in the waterphase maintenance energyrequirement rateconstant q.. kin 1/2order rate constant YHr biofilmyieldconstant for heterotrophic biomass Ksr saturation constantfor readilybiodegradable substrate E efficiency constantfor the biofilmbiomass a, temperature coefficient in the biofilm khl hydrolysis rate constant.fraction I (fast) khZ hydrolysis rate constant.fraction 2 (slow) KX1 saturation constantfor hydrolysis. fraction 1 Kxz saturationconstantfor hydrolysis. fraction 2 Tlr. anaerobichydrolysis reduction factor qr. maximumrate for fermentation Kre saturation constantfor fermentation w: waterphase;f: biofilm
value unit d·1 7 O.sS gCOD/gCOD 1.0 gCOD/mJ O.S g07lmJ 1.07 1.0 d·1 gOz°-'m,(Ulfl 2.5 0.55 gCOD/gCOD 1.0 gCOD/mJ 1.03 4.0 1.0 0.5 0.2 0.14 3 20
d" d'i gCOD/gCOD gCOD/gCOD
d'i gCOD/mJ
247
Wastewater quality changes during transport in sewers
Table 4. Reaeration, flow and system characteristics used in the sewer process model outlined in Table 1. Symbol and definition oxygen transfer coefficient T temperature Sos dissolved oxygen saturation concentration at T ·C F Froude number = u/(g dills u mean flow velocity g gravity acceleration s slope dm hydraulic mean depth ex. temperature coefficient for reaeration A cross-sectional area or pipe V wastewater volume
value
unit
Ku
m1s
9.81
m1s2
m1m m
1.024
•
Results of model simulations are shown in Figure 4. Changes in the DO concentration and in the concentrations of readily biodegradable substrate defined as total readily biodegradable substrate (Ss), fermentable substrate (SF) and fermentation products (SA), where Ss SF + SA, are shown versus distance from the upstream point in the sewer. The DO concentration - determined by the degree of reaeration and depending on the slope of the sewer line - has considerable influence on the removal of the readily biodegradable substrate. At the lowest DO concentration (slope s =0.03%), Ss is maintained whereas even a DO concentration of about 0.3 mg/l results in a very low concentration of Ss at the end of the sewer line. A DO concentration above 0.2-0.5 mg/l is expected to prevent sulfide in wastewater of sewer systems. As seen from this example, the sewer line with slope 0.03 is expected to give sulfide problems; the sewer line with slope 0.3% is within the "gray" area.
=
In Table 5 it is further illustrated which COD-components - depending on the DO concentration - are changed during transport in the intercepting gravity sewer as a result of the transformations taking place. The table illustrates the fact that it is not the transportation time which determines the magnitude of COD transformations and removal but the supply with oxygen by reaeration. It is basically not a change in the total COD concentration which is important but the fact that the biodegradability of the COD components is reduced. This may cause serious problems depending on requirements for nutrient removal at a wastewater treatment plant. Under other circumstances - e.g. where only mechanical treatment is required - processes may be preferred where soluble, readily biodegradable COD components are transformed into particulate and less biodegradable fractions. Table 5. COD components before and after transportation in the gravity sewer used in this example. Transportation time is 5 hours at a slope of 0.3% and 11 hours at 0.03%.
COD (glm3) biomass: X Bw substrate: Ss (Sp+S.J X SI X S2 Total COD:
okin 40 30 (20+10) 40 210 320
20km slope: 0.03% slope: 0.3% 49 62 27 (5+22) 3 (3+0) 30 28 206 205 312 298
248
T. HVITVED-IACOBSEN et al. Slope, 0.03%
g/m 3 DO 2.0
gCOD/m 3 Readily biodegradable substrate
30
1.5
20
1.0 0.5 Station
0 0
10
5
15
Station
0 0
201
5
10
15
201
Slope, 0.3%
g/m 3
gCOD/m 3 Readily biodegradable substrate
DO
30
2.0
1.5
20
1.0
10 "-
0.5
tion
o
.~..~...~...~......
0 0
10
5
15
201
0
5
--Ss
10
Station
15
201
Figure4. Example of model simulation, cf. text. DISCUSSION The integrated aerobic and anaerobic model conceptfor carbonand sulfur microbial transformations which is presented is a result of several years of research related to sewer processes, cr. reference list. The model concept can be extended and applied on sewer sediment transport in suspension, (Vollertsen and HvitvedJacobsen, 1998; Vollertsen et al., 1998). This fact is important whendealing with combined seweroverflow eventsand relatedDO depletion in receiving waters. Determination of COD components and stoichiometric and kinetic parameters included in the model is based on the combined use of experimental techniques and model calibration. Essential results originate from oxygen utilization rate (OUR) measurements, (Bjerre et al., 1995; Bjerre et al., 1998a; Bjerre et al., 1998b; Vollertsen and Hvitved-Jacobsen, 1998; Tanaka and Hvitved-Jacobsen, 1998). The different stages of the OUR curve are determining main biomass and substrate characteristics. OUR measurements are carried out on identical wastewater samples originating from both upstream and downstream stations in sewers. Such OUR measurements include the effectof biomass activities from bulk wateras well as biofilm. Furtherimportant information originate fromVFAandsulfideanalysis of wastewater samples. Under aerobic conditions in sewers, the consumption rate for readily biodegradable substrate (Ss) used for biomass growth and respiration may typically vary between 10 and 30 mgCOD/(l h). Supply of Ss by hydrolysis will occur, however, with a significantly lower rate resulting in a net Ss removal. Therefore, wastewater transportation under aerobic conditions in relatively long gravity sewers will typically result in depletion of readily biodegradable substrate and to some extent also fast hydrolyzable substrate affecting a successive treatment process in termsof denitrification and biological phosphorus removal. Underanaerobic conditions, however, e.g. by transport in pressure mains, readily biodegradable COD is generally preserved. Although the anaerobic heterotrophic processes mayremove a minorpart,anaerobic hydrolysis will typically resultin a small (1-2 mgCOD/(l hj) net production of readily biodegradable substrate, (Tanakaand HvitvedJacobsen, subm.),
Wastewater quality changes during transport in sewers
249
CONCLUSIONS Recent development in the understanding of wastewater transformation processes in sewers including heterotrophic biomass and substrate components under aerobic as well as under anaerobic conditions has made it possible to quantify organic matter transformations and establish a corresponding conceptual model. Procedures exist for model parameter determination. Hydrogen sulfide formation is integrated in the model concept. Aerobic conditions in relatively long intercepting gravity sewers may cause considerable reduction of biodegradable organic COD-fractions whereas anaerobic conditions only result in minor changes . The model concept developed can be used for evaluation of wastewater quality changes during transportation in a sewer, e.g, related to successive wastewater treatment concerning availability of fast biodegradable substrate needed for denitrification and biological phosphorus removal - or the opposite: related to production of particulate organic fractions with low biodegradability which can be removed at mechanical wastewater treatment. Based on the model concept it is possible to include wastewater quality changes as a useful parameter in the design process of sewers.
REFERENCES Bjerre, H.L.• T. Hvitved-Jacobsen, B. Teichgrliber and D. te Heesen (1995). Experimental procedures characterizing transformations of wastewater organic mailer in the Emscher river. Germany. War. Sci. Tech., 31(7), 201-212. Bjerre, H.L., T. Hvitved-Jacobsen. S. Schlegel and B. Teichgrliber (1998a). Biological activity of biofilm and sediment in the Emscher river, Germany. War. Sci. Tech. , 37(1), 9-16. Bjerre, H.L.• T. Hvitved-Jacobsen, B. Teichgraber and S. Schlegel (l998b). Modelling of aerobic wastewater transformations under sewer conditions in the Emscher river. Germany. Accepted for publication 10 War. Environ. Res.• pp. IS. Boon, A.G. and A.R. Lister (1975). Formation of sulphide in rising main sewers and its prevention by injection of oxygen. Prog. War. Tech., 7(2), pp. 289-300. Green, M.• G. Shelef and A. Messing (1985). Using the sewerage system main conduits for biological treatment, War. Res .• 19(8). 1023-1028. Henze. M.• C.P.L. Grady Ir.• W. Gujer, G.v.R. Marais and T. Matsuo (1987). Activated sludge model no. I. Scientific and technical report no. I. International Association on Water Pollution Research and Control. Hvitved-Jacobsen, T.• B. Iiitte and N.Aa. Jensen (1988). Hydrogen sulfide control in municipal sewers. In H.H. Hahn and R. Klute (eds.), Pretreatment in Chemical Water and Wastewater Treatment, proceedings of the 3rd International Gothenburg Symposium, Gothenburg. Sweden. June 1·3, 1988. Springer-Verlag, 239-247. Hvitved-Jacobsen, T.• K. Raunkjer and P.H. Nielsen (1995). Volatile fally acids and sulfide in pressure mains. War. Sci. Tech.• 31(7), 169-179. Hvitved-Jacobsen, T.. J. Vollertsen and P.H. Nielsen (1998). A process and model concept for microbial wastewater transformations in gravity sewers. War. Sci. Tech., 37(1), 233-241. Jensen, N.Aa. and T. Hvitved-Jacobsen (1991). Method for measurement ofreaeration in gravity sewers using radiotracers. Res. J. WPCF, 63(5), 758-767. Jensen (1995) . Empirical modelling of air-to-water oxygen transfer in gravity sewers. War. Environ. Res., 67(6), 979-991. Nielsen. P.H.• K. Raunkjer, N.H. Norsker, N.Aa. Iensen and T. Hvitved-Iacobsen (1992). Transformation of wastewater in sewer systems - a review. War. Sci. Tech., 25(6). 17-31. Pomeroy, R.D. and 1.0. Parkhurst (1977). The forecasting of sulfide buildup rates in sewers. Prog. War. Techn.• 9(3). 621-628. Tanaka. N., T. Hvitved-Jacobsen, T. Ochi and N. Sato (1998). Aerobic/anaerobic microbial wastewater transformations and reaeration in an air-injected pressure sewer. Accepted for the 71st Annual Water Environment Federation conference & Exposition. WEFrEC '98. Orlando, Florida, USA. October 3-7. pp. 12. Tanaka, N. and T. Hvitved-Iacobsen (1998). Transformation of wastewater organic mailer in sewers under changing aerobic/anaerobic conditions . War. Sci. Tech., 37(1), 105-113. Tanaka. N. and T. Hvitved-Iacobsen (subm.), Anaerobic transformations of wastewater organic matter under sewer conditions. submitted for the 8th International conference on Urban Storm Drainage, Sydney. Australia, August 30 - September 3. 1999,pp.9. Thistlethwayte. D.K.B. (ed.) (1972) . The control of sulfides in sewerage systems. Butterworth. Sidney. Australia. Vollertsen, I . and T. Hvitved-Jacobsen (1998). Aerobic microbial transformations of resuspended sediments in combined sewers a conceptual model. War. Sci. Tech.• 37( 1).69-76. Vollertsen, J.. Hvitved-Jacobsen, T .• McGregor, I. and Ashley. R. (1998). Aerobic microbial transformations of pipe and silt trap sediments from combined sewers. Ibid.
Pergamon
Wal. Sci Tech. Vol. 39. No.2. pp. 233-249. 1999.
PH: S0273-1223(99)OOO37-2
e 1999IAWQ Publishedby Elsevier ScienceLId Printed in Great Britain.All rights reserved 0273-1223/99 $19'00 + 0'00
ERRATA Water Science and Technology 38(10),249-256
AEROBIC MICROBIAL TRANSFORMATIONS OF PIPE AND SILT TRAP SEDIMENTS FROM COMBINED SEWERS J. Vollertsen, T. Hvitved-Jacobsen, I. McGregor and R. Ashley
Water Science and Technology 38(10),257-264
WASTEWATER QUALITY CHANGES DURING TRANSPORT IN SEWERS - AN INTEGRATED AEROBIC AND ANAEROBIC MODEL CONCEPT FOR CARBON AND SULFUR MICROBIAL TRANSFORMATIONS T. Hvitved-Jacobsen, J. Vollertsen and N. Tanaka
It is regretted that the text of the above articles was transposed. The publishers would like to apologise for this error and the corrected versionsof the articlesfollow.
233
WASTEWATER QUALITY CHANGES DURING TRANSPORT IN SEWERS - AN INTEGRATED AEROBIC AND ANAEROBIC MODEL CONCEPT FOR CARBON AND SULFUR MICROBIAL TRANSFORMATIONS T. Hvitved-Jacobsen, J. Vollertsen and N. Tanaka Environmental Engineering Laboratory, AalborgUniversity, Sohngaardsholmsvej 57, DK-9000Aalborg, Denmark
ABSTRAcr A conceptual model for wastewater quality changes during transport in sewers is presented. The model concept includes reaeration and main aerobic and anaerobic microbial processes in water phase and sewer biofilm. Emphasis is on microbial transformations of heterotrophic biomass and soluble and particulate fractions of organic substrate; the inclusion of sulfate respiration in the model concept is outlined. The model concept has been tested in gravity sewers as well as in pressure mains. Oxygen utilization rate measurements of wastewater samples from sewers are used for model calibration. The model is exemplified as a tool for evaluation of wastewater quality changes in an intercepting gravity sewer. The model concept can be used in the design process of sewers taking into account quality aspects. It is recommended to consider sewer processes when addressing functioning of the sewer, wastewater treatment and combined sewer overflow effects. @ 1999 IAWQ Published by Elsevier Science Ltd. All rights reserved
KEYWORDS Wastewater; microbial transformation; aerobic process; anaerobic process; sulfide; sewer; conceptual model; simulation. INTRODUCTION A sewer system interacts with wastewater treatment as well as receiving waters. Wastewater is transported through the sewers to treatment and overflow of untreated wastewater from combined sewers into receiving waters take place during storm events. Microbial processes in wastewater of sewer systems proceed during transportation, (Nielsen et al., 1992). These processes take place in the water phase, in the biofilms and in temporarily settled sewer sediments, Figure I. Soluble as well as particulate wastewater compounds undergo transformation processes. The dissolved oxygen (DO) concentration in the wastewater determines whether processes take place under aerobic, anaerobic or varying conditions. Understanding of complicated microbial processes, interactions and conditions is a comer stone for taking into account several types of sewer problems, e.g. odour and health aspects, hydrogen sulfide corrosion and malfunctioning of wastewater treatment due to input of low quality wastewater. A comprehensive understanding of a sewer system as being a reactor for microbial processes is therefore needed for sewer design and operation taking into account functioning of the sewer, wastewater treatment and combined sewer overflow effects. The necessity for such a comprehensive understanding is well accepted for wastewater treatment, however, not yet fully realized for wastewater transport in sewers. The urban wastewater systems and management hereof causes several desired as well as undesired effects which have there origin in sewer microbial processes. Important examples are: 242
243
Wastewater quality changes duringtransport in sewers
• Functioning of the sewer: considerations concerning the sewer itself and the surroundings within the catchment area; examples are related to odour and corrosion problems due to sulfide formation. Another important example is sediment accumulation. • Wastewater treatment: COD fractions are removed and transformed depending on the design and management of the entire wastewater system. One example relates to production or removal of readily biodegradable substrates when addressing nitrogen and biological phosphorus removal in wastewater treatment plants. Another example relates to in-sewer wastewater treatment, (Green et al., 1985). • Wet weather impacts on receiving waters: impacts from storm water runoff and combined sewer overflows, e.g. caused by discharge of resuspended sewer sediments.
WAS1EWATER FROM HOUSEHOLDS AND INDUSTRY
WAS1EWATER TREATMENT
TREATED WASTEWATER SLUDGE
SEWER SYSTEM
Figure 1. Wastewater transport within the urban wastewater system. Contrary to what is known about microbial processes in sewers, the hydraulics and to some extent also solids aspects of wastewater transport are better understood. Model simulation of wastewater and solids transport is therefore also common practise for design and analysis of sewer system performance during both dry and wet weather flow. Knowledge on physical, chemical and biological processes in sewers affecting wastewater quality suffer from a lack of both theoretical and empirical basic understanding. Except for sulfate reduction under anaerobic conditions resulting in sulfide formation and for sewer solids transport, processes in sewers have not been considered - and almost not discussed - among scientists and practitioners within the area of wastewater treatment and combined sewer overflow management. In terms of quality aspects, sewers are typically just considered systems for wastewater supply at the boundaries where they are connected with wastewater treatment plants and overflow structures discharging into receiving waters.
WASTEWATER QUALITY CHANGES DURING TRANSPORT IN SEWERS During transportation of wastewater in sewers, microbial transformation of organic matter takes place. Such quality changes take place under aerobic as well as anaerobic conditions associated with biomass in the water phase, the biofilms and the sediments. In terms of removal of readily biodegradable organic matter, transformations are especially related to aerobic conditions. These microbial changes of wastewater substrate as well as of biomass under sewer conditions have been investigated and quantified by Bjerre et al. (1995), Bjerre et al. (1998a), Bjerre et al. (l998b), Vollertsen and Hvitved-Jacobsen (1998), Tanaka and HvitvedJacobsen (1998), Hvitved-Jacobsen et al. (1998), Tanaka et al. (1998) and Tanaka and Hvitved-Jacobsen (subm.). The concept of microbial transformations in a sewer is based on the description used in the activated sludge model, i.e. biomass growth and substrate utilization, (Henze et al., 1987). In this respect an important relationship between wastewater microbial transformations taking place in a sewer and in a successive wastewater treatment plant is established. This fact is a fundamental requirement when considering a sewer
244
T. HVITVED-IACOBSEN et al.
system and a wastewater treatment plant as an entity in terms of treatment. However, microbial processes in a sewer and in an activated sludge system differs in the detailed 'description. This is especially the case concerning the fact that different fractions of particulate organic matter in a sewer are available as a source for hydrolysis products and that biomass decay is replaced by biomass maintenance energy requirement, (Bjerre et al., 1998b; Hvitved-Jacobsen et al, 1998; Vollertsen and Hvitved-Jacobsen, 1998). Such conceptual differences affect the defmition of stoichiometric and kinetic parameters as well as their numerical values. This fact is, however, no obstacle for integration of sewer and wastewater treatment processes. Traditionally, COD removal or transformation in a sewer is considered in terms of changes in the total COD concentration or alternatively BOD or dissolved COD, although uncertainties of measurements from real situations often makes this practically unrealistic. Furthermore, such COD values are of limited interest when dealing with wastewater microbial processes in sewers. Assessment of quality changes need a total overview in terms of transformations of biomass and substrate components including associated processes. This overview can only be obtained from model simulations based on a detailed description of the microbial system including realistic and intensive laboratory and field investigations.
Transformation under aerobic conditions Under aerobic conditions in a (gravity) sewer, wastewater microbial transformations are primarily caused by growth and maintenance of heterotrophic biomass associated with carbon and oxygen uptake as well as hydrolysis of particulate organic substrate. The driving and governing process for these transformations is the continuous supply with oxygen by reaeration, (Jensen and Hvitved-Jacobsen, 1991; Jensen, 1995). These transformations are illustrated in Figure 2.
Inpurof
components ~
Bulk water maint9118f1C8
Fast
erJetgy
frydtrJIyzabIe
19QulremeI1t
COD
OuIputof components
1---" •
Figure 2. The simplified aerobic wastewater system under gravity sewer conditions (Hvitved-Jacobsen et al., 1998). Readily biodegradable substrate (Ss) includes two fractions: Fermentable substrate (SF)and fermentation products (SA), i.e. volatile fatty acids (VFA's).
Transformation under anaerobIc conditions Wastewater transformations under anaerobic conditions in sewers have been investigated by Tanaka and Hvitved-Jacobsen (1998), Tanaka et al. (1998) and Tanaka and Hvitved-Jacobsen (subm.), These transformations have been integrated with the aerobic processes as shown in Figure 2, Figure 3. Contrary to aerobic heterotrophic transformations, the rate of transformations are relatively low under anaerobic conditions. Volatile fatty acids (VFA's) are produced under anaerobic conditions from fermentable readily biodegradable organic matter. (Hvitved-Jacobsen et al., 1995). It is, therefore, needed to classify readily biodegradable substrate (Ss) into two fractions: Fermentable substrate (SF) and fermentation products (SA>. i.e. VFA's, Figure 3. The VFA's may be substrate for methane producing bacteria. The sulfur cycle in terms of the sulfate respiration processes can be integrated with the anaerobic carbon cycle, Figure 3, Tanaka and Hvitved-Jacobsen (subm.). Fractionation of readily biodegradable substrate (Ss) into SF and SA fits well to the anticipation that mainly SF is used by sulfate reducing biomass in sewer biofilms. By integrating the sulfide formation in a process description, a conceptual approach is obtained
Wastewaterquality changes during transport in sewers
245
instead of the traditional empirical descriptions, (Thistlethwayte, 1972, Boon and Lister, 1975, Pomeroy and Parkhurst, 1977, Hvitved-Jacobsen et al., 1988).
/ttputot
components. ~ _ _ _ _ _ _ _ _ _ _ _ _
! Methane
~~ ../
: producing : bicmISS
: :
I
,
!._---- ------ ~
Figure 3. The simplified anaerobic system for wastewater transformations in a sewer considered as an extension of the aerobic system shown in Figure 2. A model concept for microbial wastewater transformations Integration of the aerobic and the anaerobic system for wastewater transformations in sewers makes it possible to describe changing aerobic/anaerobic conditions, (Tanaka and Hvitved-Jacobsen, 1998). Such changing conditions may take place in pressure mains when injecting air or pure oxygen and in gravity sewers with varying slope. A combined aerobic/anaerobic process model concept is shown in Table I, based on the systems outlined in Figure 2 and 3. The process model is the basis for integration with a transportdispersion model for wastewater transportation in sewers. The model outlined in Table 1 is based on investigations and considerations published in Bjerre et al. (I 998b), Vollertsen and Hvitved-Jacobsen (1998), Tanaka and Hvitved-Jacobsen (1998), Hvitved-Jacobsen et al, (1998), Tanaka et al. (1998) and Tanaka and Hvitved-Jacobsen (subm.). Table 1. Integrated processes for transformation of wastewater organic matter in a sewer where aerobic as well as anaerobic processes may proceed. Symbols are defined in Table 2, 3 and 4. -Sg Xew (I-YH..)lYH.. Aerobic growth in bulk water -IIY H.. Aerobic growth in biofilm -IIY HI (I-YHI)lYHI Maintenanceenergy requirement I -I -1 Aerobic hydrolysis.fast I ·1 Aerobic hydrolysis,slow I -I Anaerobichydrolysis.fast I -I Anaerobic hydrolysis,slow I -) Ferment.lltion in bulk water and biofilm Hydrogensulfide production Reaeration -I a: ~H (Sp+SA)/(Ks+(SP+SA)) S
SQ:>'
process rate Eg. • Eg.b Eg.c Eg.d,n=I
Eq.d,n=2 Eq. e, n=I
Eq.e,n=2 Eg. f
Eq.g Eg.h
246
T. HVITVED-JACOBSEN et al.
The model includes aerobic growth and maintenance energy requirement in the bulk water, aerobic growth in the biofilm, aerobic and anaerobic hydrolysis, fermentation, sulfide production and reaeration. Interaction with the sediments is disregarded, Figure I.
Model application The model concept shown in Table I is applied on a big intercepting gravity sewer line characterised with the following system parameters : length: 20 km; diameter: 1.20 m; slope 0.03 and 0.3%. The temperature and flow conditions chosen are: temperature 18°C and flow 0.3 m3/s - the average load from about 125,000 person equivalents. The wastewater chosen in this example is diluted wastewater transported aerobically for a few hours. COD components, model parameters and reaeration and flow characteristics used in a simulation with the model is shown in Table 2, 3 and 4. These parameters and characteristics are selected according to findings based on laboratory and field investigations . Table 2. COD-components and dissolved oxygen for wastewater at an upstream location of the intercepting sewer line used for model simulation . Wastewater includes soluble (S) and particulate (X) components. Component
characteristic value.wastewater 20-100
example; diluted 40
unit
gCOD/mJ heterotrophic activebiomassin the waterphase heterotrophic active biomass in the biofilm gCOD/mz hydrolysible substrate. fast biodegradable So-l00 40 gCOD/mJ hydrolysible substrate. slowlybiodegradable300 -4S0 210 gCOD/mJ s, readily biodegradable substrate o-SO 30gCOD/m3 So dissolvedoxygen 0-4 2 g07lmJ COD total COD about600 320 gCOD/mJ - Include veryslowlybiodegradable and inert organicmatter. - - 5s = SF + SA; Sf = 20 gCOD/mJ and SA = 10 gCOD/mJ XBw XBr XSI Xsz
Table 3. Model parameters used in the sewer process model outlined in Table 1. Symboland definition J.lH maximum specificgrowthrate for heterotrophic biomass YHw suspended biomass yieldconstantfor heterotrophies Ks saturation constantfor readily biodegradable substrate Ko saturation constantfor DO lXw temperature coefficient in the waterphase maintenance energyrequirement rateconstant q.. kin 1/2order rate constant YHr biofilmyieldconstant for heterotrophic biomass Ksr saturation constantfor readilybiodegradable substrate E efficiency constantfor the biofilmbiomass a, temperature coefficient in the biofilm khl hydrolysis rate constant.fraction I (fast) khZ hydrolysis rate constant.fraction 2 (slow) KX1 saturation constantfor hydrolysis. fraction 1 Kxz saturationconstantfor hydrolysis. fraction 2 Tlr. anaerobichydrolysis reduction factor qr. maximumrate for fermentation Kre saturation constantfor fermentation w: waterphase;f: biofilm
value unit d·1 7 O.sS gCOD/gCOD 1.0 gCOD/mJ O.S g07lmJ 1.07 1.0 d·1 gOz°-'m,(Ulfl 2.5 0.55 gCOD/gCOD 1.0 gCOD/mJ 1.03 4.0 1.0 0.5 0.2 0.14 3 20
d" d'i gCOD/gCOD gCOD/gCOD
d'i gCOD/mJ
247
Wastewater quality changes during transport in sewers
Table 4. Reaeration, flow and system characteristics used in the sewer process model outlined in Table 1. Symbol and definition oxygen transfer coefficient T temperature Sos dissolved oxygen saturation concentration at T ·C F Froude number = u/(g dills u mean flow velocity g gravity acceleration s slope dm hydraulic mean depth ex. temperature coefficient for reaeration A cross-sectional area or pipe V wastewater volume
value
unit
Ku
m1s
9.81
m1s2
m1m m
1.024
•
Results of model simulations are shown in Figure 4. Changes in the DO concentration and in the concentrations of readily biodegradable substrate defined as total readily biodegradable substrate (Ss), fermentable substrate (SF) and fermentation products (SA), where Ss SF + SA, are shown versus distance from the upstream point in the sewer. The DO concentration - determined by the degree of reaeration and depending on the slope of the sewer line - has considerable influence on the removal of the readily biodegradable substrate. At the lowest DO concentration (slope s =0.03%), Ss is maintained whereas even a DO concentration of about 0.3 mg/l results in a very low concentration of Ss at the end of the sewer line. A DO concentration above 0.2-0.5 mg/l is expected to prevent sulfide in wastewater of sewer systems. As seen from this example, the sewer line with slope 0.03 is expected to give sulfide problems; the sewer line with slope 0.3% is within the "gray" area.
=
In Table 5 it is further illustrated which COD-components - depending on the DO concentration - are changed during transport in the intercepting gravity sewer as a result of the transformations taking place. The table illustrates the fact that it is not the transportation time which determines the magnitude of COD transformations and removal but the supply with oxygen by reaeration. It is basically not a change in the total COD concentration which is important but the fact that the biodegradability of the COD components is reduced. This may cause serious problems depending on requirements for nutrient removal at a wastewater treatment plant. Under other circumstances - e.g. where only mechanical treatment is required - processes may be preferred where soluble, readily biodegradable COD components are transformed into particulate and less biodegradable fractions. Table 5. COD components before and after transportation in the gravity sewer used in this example. Transportation time is 5 hours at a slope of 0.3% and 11 hours at 0.03%.
COD (glm3) biomass: X Bw substrate: Ss (Sp+S.J X SI X S2 Total COD:
okin 40 30 (20+10) 40 210 320
20km slope: 0.03% slope: 0.3% 49 62 27 (5+22) 3 (3+0) 30 28 206 205 312 298
248
T. HVITVED-IACOBSEN et al. Slope, 0.03%
g/m 3 DO 2.0
gCOD/m 3 Readily biodegradable substrate
30
1.5
20
1.0 0.5 Station
0 0
10
5
15
Station
0 0
201
5
10
15
201
Slope, 0.3%
g/m 3
gCOD/m 3 Readily biodegradable substrate
DO
30
2.0
1.5
20
1.0
10 "-
0.5
tion
o
.~..~...~...~......
0 0
10
5
15
201
0
5
--Ss
10
Station
15
201
Figure4. Example of model simulation, cf. text. DISCUSSION The integrated aerobic and anaerobic model conceptfor carbonand sulfur microbial transformations which is presented is a result of several years of research related to sewer processes, cr. reference list. The model concept can be extended and applied on sewer sediment transport in suspension, (Vollertsen and HvitvedJacobsen, 1998; Vollertsen et al., 1998). This fact is important whendealing with combined seweroverflow eventsand relatedDO depletion in receiving waters. Determination of COD components and stoichiometric and kinetic parameters included in the model is based on the combined use of experimental techniques and model calibration. Essential results originate from oxygen utilization rate (OUR) measurements, (Bjerre et al., 1995; Bjerre et al., 1998a; Bjerre et al., 1998b; Vollertsen and Hvitved-Jacobsen, 1998; Tanaka and Hvitved-Jacobsen, 1998). The different stages of the OUR curve are determining main biomass and substrate characteristics. OUR measurements are carried out on identical wastewater samples originating from both upstream and downstream stations in sewers. Such OUR measurements include the effectof biomass activities from bulk wateras well as biofilm. Furtherimportant information originate fromVFAandsulfideanalysis of wastewater samples. Under aerobic conditions in sewers, the consumption rate for readily biodegradable substrate (Ss) used for biomass growth and respiration may typically vary between 10 and 30 mgCOD/(l h). Supply of Ss by hydrolysis will occur, however, with a significantly lower rate resulting in a net Ss removal. Therefore, wastewater transportation under aerobic conditions in relatively long gravity sewers will typically result in depletion of readily biodegradable substrate and to some extent also fast hydrolyzable substrate affecting a successive treatment process in termsof denitrification and biological phosphorus removal. Underanaerobic conditions, however, e.g. by transport in pressure mains, readily biodegradable COD is generally preserved. Although the anaerobic heterotrophic processes mayremove a minorpart,anaerobic hydrolysis will typically resultin a small (1-2 mgCOD/(l hj) net production of readily biodegradable substrate, (Tanakaand HvitvedJacobsen, subm.),
Wastewater quality changes during transport in sewers
249
CONCLUSIONS Recent development in the understanding of wastewater transformation processes in sewers including heterotrophic biomass and substrate components under aerobic as well as under anaerobic conditions has made it possible to quantify organic matter transformations and establish a corresponding conceptual model. Procedures exist for model parameter determination. Hydrogen sulfide formation is integrated in the model concept. Aerobic conditions in relatively long intercepting gravity sewers may cause considerable reduction of biodegradable organic COD-fractions whereas anaerobic conditions only result in minor changes . The model concept developed can be used for evaluation of wastewater quality changes during transportation in a sewer, e.g, related to successive wastewater treatment concerning availability of fast biodegradable substrate needed for denitrification and biological phosphorus removal - or the opposite: related to production of particulate organic fractions with low biodegradability which can be removed at mechanical wastewater treatment. Based on the model concept it is possible to include wastewater quality changes as a useful parameter in the design process of sewers.
REFERENCES Bjerre, H.L.• T. Hvitved-Jacobsen, B. Teichgrliber and D. te Heesen (1995). Experimental procedures characterizing transformations of wastewater organic mailer in the Emscher river. Germany. War. Sci. Tech., 31(7), 201-212. Bjerre, H.L., T. Hvitved-Jacobsen. S. Schlegel and B. Teichgrliber (1998a). Biological activity of biofilm and sediment in the Emscher river, Germany. War. Sci. Tech. , 37(1), 9-16. Bjerre, H.L.• T. Hvitved-Jacobsen, B. Teichgraber and S. Schlegel (l998b). Modelling of aerobic wastewater transformations under sewer conditions in the Emscher river. Germany. Accepted for publication 10 War. Environ. Res.• pp. IS. Boon, A.G. and A.R. Lister (1975). Formation of sulphide in rising main sewers and its prevention by injection of oxygen. Prog. War. Tech., 7(2), pp. 289-300. Green, M.• G. Shelef and A. Messing (1985). Using the sewerage system main conduits for biological treatment, War. Res .• 19(8). 1023-1028. Henze. M.• C.P.L. Grady Ir.• W. Gujer, G.v.R. Marais and T. Matsuo (1987). Activated sludge model no. I. Scientific and technical report no. I. International Association on Water Pollution Research and Control. Hvitved-Jacobsen, T.• B. Iiitte and N.Aa. Jensen (1988). Hydrogen sulfide control in municipal sewers. In H.H. Hahn and R. Klute (eds.), Pretreatment in Chemical Water and Wastewater Treatment, proceedings of the 3rd International Gothenburg Symposium, Gothenburg. Sweden. June 1·3, 1988. Springer-Verlag, 239-247. Hvitved-Jacobsen, T.• K. Raunkjer and P.H. Nielsen (1995). Volatile fally acids and sulfide in pressure mains. War. Sci. Tech.• 31(7), 169-179. Hvitved-Jacobsen, T.. J. Vollertsen and P.H. Nielsen (1998). A process and model concept for microbial wastewater transformations in gravity sewers. War. Sci. Tech., 37(1), 233-241. Jensen, N.Aa. and T. Hvitved-Jacobsen (1991). Method for measurement ofreaeration in gravity sewers using radiotracers. Res. J. WPCF, 63(5), 758-767. Jensen (1995) . Empirical modelling of air-to-water oxygen transfer in gravity sewers. War. Environ. Res., 67(6), 979-991. Nielsen. P.H.• K. Raunkjer, N.H. Norsker, N.Aa. Iensen and T. Hvitved-Iacobsen (1992). Transformation of wastewater in sewer systems - a review. War. Sci. Tech., 25(6). 17-31. Pomeroy, R.D. and 1.0. Parkhurst (1977). The forecasting of sulfide buildup rates in sewers. Prog. War. Techn.• 9(3). 621-628. Tanaka. N., T. Hvitved-Jacobsen, T. Ochi and N. Sato (1998). Aerobic/anaerobic microbial wastewater transformations and reaeration in an air-injected pressure sewer. Accepted for the 71st Annual Water Environment Federation conference & Exposition. WEFrEC '98. Orlando, Florida, USA. October 3-7. pp. 12. Tanaka, N. and T. Hvitved-Iacobsen (1998). Transformation of wastewater organic mailer in sewers under changing aerobic/anaerobic conditions . War. Sci. Tech., 37(1), 105-113. Tanaka. N. and T. Hvitved-Iacobsen (subm.), Anaerobic transformations of wastewater organic matter under sewer conditions. submitted for the 8th International conference on Urban Storm Drainage, Sydney. Australia, August 30 - September 3. 1999,pp.9. Thistlethwayte. D.K.B. (ed.) (1972) . The control of sulfides in sewerage systems. Butterworth. Sidney. Australia. Vollertsen, I . and T. Hvitved-Jacobsen (1998). Aerobic microbial transformations of resuspended sediments in combined sewers a conceptual model. War. Sci. Tech.• 37( 1).69-76. Vollertsen, J.. Hvitved-Jacobsen, T .• McGregor, I. and Ashley. R. (1998). Aerobic microbial transformations of pipe and silt trap sediments from combined sewers. Ibid.