Page 1 of 8 HV-Consult ApS Hellevangen 48, DK-9210 Aalborg SØ, Denmark The WATS model Microbial and chemical processes related to human waste have always caused problems and nuisances. 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 are reduced significantly. However, confining the human waste to underground structures does not prevent microbial and chemical transformations to take place; it only constricts the processes to locations where they cause less harm. The WATS model is the first model concept developed to model these processes. It is continuously being developed and is still today the most comprehensive sewer process concept. WATS is a concept for modeling biological, chemical, and physical processes in sewer systems. It is developed concurrently with new research on sewer systems and processes. As such it forms the conceptual backbone of a numerical sewer process model – the WATS model – which can be used to simulate a wide range of processes in sewer systems. In brief, modeling with WATS typically aims at solving the following in-sewer problems, in practice often related to an analysis of these problems and potentially including corresponding process controls and management strategies: Concrete corrosion caused by hydrogen sulfide Hydrogen sulfide impacts on human health Odor nuisance caused by hydrogen sulfide and VOCs being emitted from the wastewater and following vented into the urban atmosphere Hydrogen sulfide and VOC controls Analysis of wastewater quality at inflows to wastewater treatment plants The current state of the WATS model can be found in the sewer process book (reference below) and later journal publications. The authors of the sewer process book are the people behind the concept and give in the book a detailed description of key processes included in the model. It also gives the mathematical formulation of the concept which is needed to build a numerical instant of WATS. Hvitved-Jacobsen T, Vollertsen J, Nielsen AH (2013). Sewer Processes: Microbial and Chemical Process Engineering of Sewer Networks. Second Edition, pp 408, CRC Press, ISBN 978-1-4398-8177-4 Disclaimer: It has become known to us, the developers of the WATS model concept, that the software producer DHI (Danish Hydraulic Institute) claims that they have included the WATS model in the Mike Urban software. This is strongly misleading as DHI has only included parts of the very earliest version of the concept as it was in the late 1990’ies. The concept that DHI has included in the Mike Urban software is only adequate for a rough prediction of hydrogen sulfide formation in force mains, and cannot be applied to simulate hydrogen sulfide and its management in complete sewer networks. We, the developer of the WATS model concept, strongly distance ourselves here from. We furthermore clearly state that we have had no part, what so ever, in the doings of DHI.
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A multi-phase model The processes taking place in sewers during conveyance of wastewater are of physical, chemical and
biological nature. The transformation processes occur in wastewater, biofilms, and sewer sediments. These
biological transformations change the quality of the wastewater important for sewer corrosion, odor
problems, wastewater treatment and combined sewer overflows. The transformations taking place in
sewers are strongly interlinked, and the importance of a process cannot be predicted without knowing the
importance of the other processes. As an example: To the question whether or not hydrogen sulfide will
occur in a certain sewer network, it must first be determined if 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 bulk water, biofilms, and sediments must be known. In order to predict the
reaeration, the sewer geometry, temperature and flow conditions must be known, and in order to know
the oxygen consumptions, the quality (composition of the COD) of the wastewater must be known.
The WATS model is a multi-phase model which includes bulk wastewater, submerged biofilms, sewer
sediments, sewer atmosphere, moist sewer surfaces, as well as ventilation of sewer gas into the urban
atmosphere. It does so by defining a number of coupled differential equations describing transformation
processes in these phases as well as transport between phases. It simulates the conveyance of water as
well as sewer gas in distributed sewer networks.
The WATS concept covers microbial and chemical transformation processes related to organic matter,
sulfurous compounds, oxygen, nitrate, nitrite, and iron. It also simulates the wastewater pH which is crucial
when predicting processes related to hydrogen sulfide and sewer malodors (Table 1, Figure 1).
Table 1. Major processes simulated by the WATS model
Phase Transport and transformation process
Above the gas/water interface Gas flow along the sewer line. Ventilation of sewer gas into the urban atmosphere. Oxidation of hydrogen sulfide on the moist surfaces of the sewer walls. Concrete corrosion.
At the gas/water interface Transport between sewer atmosphere and bulk water of oxygen, hydrogen sulfide, carbon dioxide, mercaptanes and other specific malodorous organic compounds.
Below the gas/water interface and in pressure mains
Water flow along the sewer line. Transformation processes in the bulk water, the sewer biofilms, and the sewer sediments of organic matter, sulfurous compounds, specific organic compounds, oxygen, nitrate, and nitrite. Precipitation of hydrogen sulfide by iron. Chemical oxidation of hydrogen sulfide by strong oxidizing agents. pH and buffer strength of wastewater
WATS focuses on dry weather problems and not wet weather problems.
Upstream node identifier Where does the water come from? (In Figure 5, the red pipe, this is Node ‘2’)
Downstream node identifier Where is the water discharged to? (In Figure 5, the red pipe, this is Node ‘3’)
(x,y,z) coordinate upstream node
This info is used for drawing the result on a plan, indicating the system conditions
(x,y,z) coordinate upstream node
This info is used for drawing the result on a plan, indicating the system conditions
Pipe type G=gravity pipe; P=force main
Slope The slope of the pipe
Pipe diameter The inner diameter of the pipe
Equivalent sand roughness The equivalent sand roughness or alternatively the Manning number
Sewer length Between manhole centres
Pipe shape Special pipe shapes are can be defined
Acid corrodibility If the pipe can be corroded by acid attack or not. E.g. a if it is a plastic pipe, this is ‘false’ if it is a concrete pipe, this is ‘true’.
Equivalent alkalinity of the pipe material
Depending on the materials a concrete pipe is made of, it can have a higher or lower alkalinity, which acts as a neutralizer for the acid formed.
Input flow to the nodes This is not the actual flow in the pipe, but the flow that enters the trunk sewer network from the ‘outside world’. I.e. in Figure 5Fejl! Henvisningskilde ikke fundet., the red pipe, it is the flow that enters node ‘2’ from the sub-catchments lumped to it, i.e. NOT from pipes ‘0’ and ‘1’.
Height of drop structure or pressure loss due to a turbulence in a node during dry weather conditions
This is not to be confused with losses in manholes during rainfall runoff. Only where there is a pressure loss (e.g. a drop, an outlet from a pressure main or a sharp change of direction)
Ventilation The gas tightness of the sewer is chosen
Wastewater quality parameters:
COD The COD content of the input flow to the nodes
BOD The BOD content of the input flow to the nodes
Sulfate The sulfate content of the input flow to the nodes
pH The average pH of the input flow to the nodes
Alkalinity The average alkalinity of the input flow to the nodes
Temperature The average temperature
Stochastic modeling It is seldom – or more correctly: it is never – possible to determine all inputs and all process parameters for
a given sewer. This is partly due to the amount of information needed being rather large, partly due to a
large natural variability of wastewater quality and process parameters, and partly due to the fact that we
often want to simulate future scenarios. The WATS model must consequently be seen as simulating an
‘average situation’, depending on the chosen system characteristics.
The WATS model is a deterministic model. It has a level of complexity similar to the hydrodynamic models
used for simulation of stormwater runoff. For the pipe hydraulics, WATS needs similar data as any
hydrodynamic model. In addition hereto, WATS must receive input data on the quality of the wastewater.
The parameters describing the transformation processes in the WATS model are varying in space and time.
In most cases, the effort to determine all model parameters is furthermore huge, and only the most