SWQM - A simple river water quality model for assessment of urban ...

12 nd International Conference on Urban Drainage, Porto Alegre/Brazil, 10-15 September 2011
EXAMPLE APPLICATION
As an example of application of SWQM, a hypothetical river described in the literature
(Schütze et al., 2002) has been modelled. It consists of a 40 km stretch of a river with
identical profile in all river sections. It has a base flow (from upstream) of 1500 l/s. The
wastewater treatment plant effluent has been determined by simulation of adaptations of the
example sewer system (A128 example) and wastewater treatment plant of the Simba
simulation system (see ifak, 2009); its average flow amounts to 318 l/s. In a similar way, CSO
discharges have been obtained by simulation. Figure 3a illustrates the simulation set up.
Baseflow is assumed to enter the river at Section E0, whilst CSO discharges are into Section
E6 and wastewater treatment effluent discharges into Section E11. Each section is assumed to
have a length of 1 km. Hydrodynamic simulation was carried out using an extended version of
SWMM, which has been integrated into SIMBA, thus allowing arbitrary biochemical
transformation processes (such as the SWQM) to be modelled. As wastewater treatment
modelling took place using the Activated Sludge Model No. 1 (Henze et al., 2000), modelling
results (using the pollutant fractions of ASM1) had to be converted into the variable set of
SWQM. Appropriate converter functions have been implemented. In a similar way, sewer
system modelling results had been converted from sewer system variables (COD, TKN) into
the SWQM variables in order to describe the discharges into the river in an appropriate way.
1000
0 1 2 3 4 5
Time [d]
6 7 8 9 10
6 SWQM - A simple river water quality model
[l/s]
7000
6000
5000
4000
3000
2000
Selected river sections
link E07 flow
link E13 flow
link E20 flow
link E27 flow
link E34 flow
link E40 flow
Figure 3. River hydraulics a) Model setup b) flow results in various river sections
Figure 3b illustrates the flows in the river system, with the CSO discharges into Section E6
being clearly visible. Figure 4a displays DO concentrations at selected river sections vs. time.
The small influence of the diurnal variation of discharges as well as of photosynthetic activity
can be seen. DO sag curves are clearly visible, also showing that, for the given river and
discharge characteristics, the minimum DO concentration lies far downstream of the
discharge location. Finally, Figure 5 illustrates the evaluation of river water quality, based on
simulation of a real one year rainfall series. Here, the evaluation according to the Urban
Pollution Management Manual (FWR, 1998), counting exceedances of certain threshold
values of critical DO and NH3-N concentrations over exposure times of 1 hour, 6 hours and 1
day, indicates that the river has problems with low oxygen concentrations, showing itself as
frequent short-duration occurrence of critically low DO concentrations. A similar analysis
with regard to NH3-N concentrations yields that no problems with regard to ammonia are
present (not shown here). The evaluation routine has been set up in a buffered way, hence
enabling it to process arbitrarily long time series (ifak, 2009).