Stefan Ahlman

Uncertainty-based calibration and prediction with a stormwater surface accumulation-washoff model based on coverage of sampled Zn, Cu, Pb and Cd field data

A dynamic conceptual and lumped accumulation wash-off model (SEWSYS) is uncertainty-calibrated with Zn, Cu, Pb and Cd field data from an intensive, detailed monitoring campaign. We use the generalized linear uncertainty estimation (GLUE) technique in combination with the Metropolis algorithm, which allows identifying a range of behavioral model parameter sets. The small catchment size and nearness of the rain gauge justified excluding the hydrological model parameters from the uncertainty assessment. Uniform, closed prior distributions were heuristically specified for the dry and wet removal parameters, which allowed using an open not specified uniform prior for the dry deposition parameter. We used an exponential likelihood function based on the sum of squared errors between observed and simulated event masses and adjusted a scaling factor to cover 95% of the observations within the empirical 95% model prediction bounds. A positive correlation between the dry deposition and the dry (wind) removal rates was revealed as well as a negative correlation between the wet removal (wash-off) rate and the ratio between the dry deposition and wind removal rates, which determines the maximum pool of accumulated metal available on the conceptual catchment surface. Forward Monte Carlo analysis based on the posterior parameter sets covered 95% of the observed event mean concentrations, and 95% prediction quantiles for site mean concentrations were estimated to 470 μg/l ± 20% for Zn, 295 μg/l ± 40% for Cu, 20 μg/l ± 80% for Pb and 0.6 μg/l ± 35% for Cd. This uncertainty-based calibration procedure adequately describes the prediction uncertainty conditioned on the used model and data, but seasonal and site-to-site variation is not considered, i.e. predicting metal concentrations in stormwater runoff from gauged as well as ungauged catchments with the SEWSYS model is generally more uncertain than the indicated numbers.

Modelling substance flows in urban sewer systems using MATLAB/Simulink

A model for simulation of substance flows in urban sewer systems has been developed in the computer programme package MATLAB/Simulink. The model is called SEWSYS and it handles fluxes of substances within an urban catchment. SEWSYS keeps track of each substance; where it originates and where it finally ends up. Some of the substances included are water, phosphorus, nitrogen, heavy metals and PAH. SEWSYS consists of a stormwater module, sanitary wastewater module and a wastewater treatment plant with nitrogen reduction. Verifications have been carried out both for each component separately and for the model as a whole. The results show that the modelling approach is valid, but further enhancements should be made.

Strategies for management of polluted storm water from an urban highway in Göteborg, Sweden

Stormwater from urban highways contains high concentrations of heavy metals, polycyclic aromatic hydrocarbons (PAHs) and other substances that are harmful to the environment. The choice of abatement strategies is not trivial. If the stormwater is brought to the municipal wastewater treatment plant (WWTP) in combined sewers, it will cause overflows at combined sewer overflow (CSO) points, accumulation of contaminated sludge that cannot be used as fertilisers on farmland, and sometimes disturbances of the operation of the WWTP. If the stormwater is discharged to a receiving water it will carry all the pollutants directly to the water body, most often a small urban creek not suitable for such discharges. If the stormwater is allowed to infiltrate, the groundwater and in some cases the vegetation will be effected. In an action plan for the improvement of the stormwater handling all these issues have to be taken into account. Abatement of pollutants at the source is one very important strategy that should have first priority. Thus, to improve the receiving water in a sustainable way the sources of the pollution have to be identified, quantified and reduced.

Substance flow analysis of the stormwater system in Vasastaden, Göteborg

A substance flow analysis of the stormwater system in Vasastaden in the city of Goteborg has been carried out using a substance flow model called SEWSYS. The model handles fluxes of substances within an urban catchment and keeps track of each substance; where it originates and where it finally ends up. The present sewer system has been studied and the status of sustainability evaluated. Different abatement measures to achieve a system with more sustainability have been modelled and evaluated. The measures include source control and end-of-pipe solutions. From this study it may be concluded that all the studied measures individually lead towards a very high reduction of the discharges of pollution to the water body, but the source control measures are recommended as a first choice since they by definition are sustainable.