Human wastes have always caused problems and nuisance because of microbial and chemical activities. Spreading of diseases, odors and unaesthetic conditions have been key concerns leading to the installation of sewer networks. By conveying waterborne waste in channels or pipes, odor and health problems have been reduced significantly. However, confining the human waste to underground structures does not prevent the microbial and chemical processes to take place, it only constricts these to locations where they cause less harm.

The processes taking place in sewers during conveyance of wastewater are of physical, chemical and biological nature. Physical processes are associated with the build up and erosion of sewer sediments. Chemical and physico-chemical processes occur in case of e.g. gas transfer across the water/air boundary in terms of reaeration and emission of hydrogen sulfide and odorous substances. Moreover, chemical oxidation and precipitation of sulfide may take place. Biological processes result in transformations of wastewater compounds associated with e.g. production of odorous substances and changes in the biodegradability of the wastewater having considerable impacts on a downstream located wastewater treatment plant.

Physical and pysico-chemical processes	Transformation processes

Transformations of wastewater in sewer networks are related to processes in the wastewater phase, the slime layer (biofilm) and the sewer sediments. These biological transformations change the quality of the wastewater important for corrosion and breakdown of the sewer, odor problems, wastewater treatment and combined sewer overflows. The transformations going on in sewers are strongly interlinked, and the importance of a process cannot be predicted without knowing the impacts of all the other processes. As an example, to a question whether hydrogen sulfide will occur in a certain gravity sewer, it must first be determined if the conditions are anaerobic (absence of both oxygen and nitrate). To do so, the complete mass balance for oxygen must be established, i.e. reaeration and oxygen consumptions in the bulk water and biofilm must be known. In order to predict reaeration, the sewer geometry, temperature and flow conditions must be known, and in order to know the oxygen consumptions in bulk water and biofilms, the quality (composition and biodegradability of the COD) of the wastewater must be defined.

Such complex relations have been obstacles for prediction of the negative impact from the in-sewer processes and therefore for selection of corresponding sound scientifically based solutions. However, recent improved knowledge on the in-sewer processes opens now for model simulation related to:

	Corrosion of sewer pipes and structures
	Odor problems
	Treatment plant and sewer network interactions
	Receiving water impacts

The two first issues, corrosion and odor, are related to anaerobic conditions (no oxygen and no nitrate is present). Such conditions allow anaerobic biological processes like sulfate reduction and fermentation to proceed, which are causing corrosion and odor. Often these problems are observed under the following conditions:

After transport in pressure mains: When wastewater is transported in pressure mains, anaerobic conditions rapidly will develop as no reaeration takes place. Even at low temperatures, significant H2S and odor will be produced at typical transport times and COD concentrations. Due to volatile substances (like H2S) being stripped off the water phase, corrosion and odor problems occur downstream of the pressure main outlet, especially where high turbulence levels exist.

In gravity sewers: Odor and corrosion are seen in some gravity sewer systems. Conditions that increase the risk of such problems are: High temperatures, low flow velocities, large water depths, large biofilm to bulk water ratio, high COD concentrations, sediment deposits, and stagnant wastewater in e.g. septic tanks and detention basins. Furthermore, if such wastewater is subject to high turbulence, stripping of hydrogen sulfide and other odorous compounds take place, potentially resulting in corrosion and odor problems.

A severely corroded manhole located downstream of a pressure main.

Problems in treatment plants can be caused by H₂S and odorous substances produced in the sewer system. This is especially the case for the inlet structures of treatment plants. Furthermore, H₂S can cause extensive growth of filamentous bacteria, causing activated sludge bulking. However, problems associated with low quality (reduced biodegradability) of the wastewater COD are more common. Treatment plants need easily biodegradable COD for nitrogen removal and for biological phosphorous removal. Easily biodegradable COD is rapidly consumed during transport under aerobic conditions, but produced under anaerobic conditions. Consequently, some sewer systems (those which are aerobic and where long transport times exist) will produce poor quality of wastewater COD but little odor nuisance and corrosion; while other sewer systems (those which are anaerobic) will produce good quality of wastewater COD but often result in significant odor and corrosion problems. It is a challenge to design and operate sewer networks that works between these extremes.

Receiving water impacts relate to storm events and consequently to the resuspension of sediments in combined sewer networks. Sewer sediments that are deposited during dry weather become partly degraded to more easily degradable COD. When resuspended during a storm event, this easily degradable COD will form part of the first flush during the event. At the discharge points, a rapid depletion of receiving waters dissolved oxygen may occur.

The prediction of the different integrated in-sewer processes calls for a simulation tool – a computer model. The WATS model (Wastewater Aerobic/anaerobic Transformations in Sewers) is a dynamic model taking all the important biological and chemical processes in sewers into account. The model yields information on e.g. wastewater quality, hydrogen sulfide and corrosion rates for any combination of time, space and network configuration.

It is important to notice that the in-sewer processes proceed under quite different conditions compared with the processes under “artificially controlled” conditions in activated sludge of a treatment plant. It has therefore been important to study the in-sewer transformations of wastewater under “sewer conditions” in details and to transfer this understanding to a simulation tool that reflects the specific characteristics that exist in a sewer.

The SPN-Group has studied in-sewer processes during the past 20 years. These studies are in terms of the conceptual understanding that is established today our basis for prediction and simulation of the in-sewer processes and their ultimate effects. Along with these studies, the conceptually formulated WATS sewer process model was developed. These in-sewer process studies have resulted in more than 150 publications.

Selected publications of in-sewer processes:

General information:

Hvitved-Jacobsen, T. (2002), Sewer Processes – microbial and chemical process engineering of sewer networks. CRC Press, pp. 237. ISBN 1-56676-926-4.

 

Hvitved-Jacobsen, T., J. Vollertsen and J.S. Matos (2002), The sewer as a bioreactor – a dry weather approach, Water Science & Technology, 45(3), 11-24.

 

Hvitved-Jacobsen, T. (2004), Sewer Processes – microbial and chemical process engineering of sewer networks. Gihodo Shuppan Co., Ltd., Japan, pp. 227. ISBN 4-7655-3194-5. (In Japanese language).

Concept and characteristics of the WATS model:

Hvitved-Jacobsen, T., J. Vollertsen and P.H. Nielsen (1998), A process and model concept for microbial wastewater transformations in gravity sewers, Water Science & Technology, vol. 37, no. 1, 233-241.

 

Hvitved-Jacobsen, T., J. Vollertsen and N. Tanaka (1998), Wastewater quality changes during transport in sewers - an integrated aerobic and anaerobic model concept for carbon and sulfur microbial transformations, Water Science & Technology, vol. 38, no. 10, 257-264 (read text pp. 249-256). Errata: Water Science & Technology, vol. 39, no. 2, 242-249.

 

Vollertsen, J. and T. Hvitved-Jacobsen (1999), Stoichiometric and kinetic model parameters for microbial transformations of suspended solids in combined sewer systems, Water Research, 33(14), 3127-3141.

 

Vollertsen, J. and T. Hvitved-Jacobsen (2002), Biodegradability of wastewater – a method for COD-fractionation, Water Science & Technology, 45(3), 25-34.

WATS model descriptions:

Vollertsen, J., T. Hvitved-Jacobsen and A.H. Nielsen (2004), Stochastic modeling of COD transformations in gravity sewers, accepted for Water Environment Research, ISSN 1061-4303.

 

Vollertsen, J., A.H. Nielsen, W. Yang and T. Hvitved-Jacobsen (2004), Effects of in-sewer processes – a stochastic model approach, proceedings from the 4th IWA (International Water Association) International Conference on Sewer Processes and Networks, Funchal, Madeira, Portugal, November 22-24, 2004, pp 8.

 

Nielsen, A.H., C. Yongsiri, T. Hvitved-Jacobsen and J. Vollertsen (2004), Simulation of sulfide buildup in wastewater and atmosphere of sewer networks, proceedings from the 4th IWA (International Water Association) International Conference on Sewer Processes and Networks, Funchal, Madeira, Portugal, November 22-24, 2004, pp 8.