tool development for cost effective control...

TOOL DEVELOPMENT FOR COST EFFECTIVE CONTROL

STRATEGIES IN URBAN WATER SYSTEMS

INDIVIDUAL RESEARCH PROJECT ER3

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THE CHALLENGE

Urban water systems (UWS) are sources of various pollutions for receiving waters including effluent of wastewater treatment plants, diffuse pollution, effluent of separate sewers and combined sewer overflows (CSO). CSOs can have an important impact on the receiving waters and are to be considered with high priority to increase the quality of rivers and receiving waters as requested by the Water Framework Directive. In Flanders, most of the sewer systems are combined sewers, equipped with multiple CSO structures to release excess water to the rivers. Aquafin is responsible on behalf of Flemish Region of 281 wastewater treatment plants (WWTP), 5 335 km of sewers, 1 394 pumping stations and detention basins and over 3000 CSO structures. The impact of these CSO structures on the receiving waters varies considerably and thus requires decision making on a case to case basis for proper management. In order to take the best decision, it is crucial to evaluate the load released to the environment during rain events and the pollution impact on the receiving waters.

Fellow in charge: Bertrand ValletSupervisors: Stefan Kroll, Marjoleine WeemaesAQUAFIN

THE PROJECT

In order to help managers to choose the best and most cost-effective solution, the objective of this project was to design a CSO generator able to model the water quality released from CSOs to the receiving waters and to give guidance on the development of a decision tool based on simulation results from this CSO generator in order to prioritise different structures according to their potential for the reduction of the impact on the receiving water, taking into account the uncertainty on the model parameters.

SANITAS SUSTAINABLE AND INTEGRATED URBAN WATER SYSTEM MANAGEMENTWWW.SANITAS-ITN.EU

METHODOLOGY

The project was divided in two parts: the CSO generator development and the decision tool guidance. The CSO generator is an extension of the BSM influent generator already existing for simulating the influent of waste water treatment plants. It is based on the accumulation of particulate pollutants on the catchment and resuspension of these pollutants during wet weather when the flow rate increases. It has been extended by introducing different particle classes according to different resuspension flows owing to the more complex hydraulic situations encountered at CSO locations. The model has been calibrated using the database from the Flemish Environmental Agency (VMM) which is monitoring several CSO locations with both flow and turbidity measurements.

In order to understand the processes and to calibrate the model, a monitoring campaign was performed on 4 different locations with the turbidity probes. Measurements were done both in the sewer (pump pit or throttle pipe) and at the overflow wall to verify the hypothesis of complete mixing in the sewer system. In one of the monitored locations, an evaluation of the data quality from the VMM database was made by monitoring in parallel the turbidity with the same type of probe, previously calibrated and with a high level of maintenance. Complementary parameters were also measured to characterise better the quality of the water with UV-vis spectrum analysis.

Regarding the model development, different levels of complexity were tested, introducing multiple particle classes and multiple catchments in series.

This project intended also to develop a decision tool based on simulation results in order to test the effect of different solutions for CSO management on the load spilled. The idea of the tool is to associate different existing CSO structures with characteristic parameter value distributions by running Markov Chain Monte Carlo (MCMC) simulations on the CSO generator model. The parameter value distribu-tions allow to compute uncertainty around the modelling result and to give confidence in the result of the solution tested for decision makers.

MAJOR RESULTS

MEASUREMENT CAMPAIGN

• High maintenance request to obtain valuable data.

• Choice of the location crucial in the calibration process for both good conditions for the probe safety and good CSO event frequency to perform on-site calibration.

• Confirmation of the hypothesis of completely mixed reactor, which is of high importance for the development of any water quality related model.

• Lack of maintenance on the VMM probe in Meerhout lead to an underestimation of the turbidity and less dynamics (conclusion requiring further investigation on other locations to be confirmed).

VMM DATABASE

• 73 locations out of 168 gave acceptable results (45%) but only 33 CSO locations (20%), wide spread over Flanders have been identified as suitable for modelling and load computing highlighting the difficulties to obtain relevant data for multiple locations.

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The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme FP7/2007-2013, under REA agreement 289193. This publication reflects only the authors’ views and the European Union is not liable for any use that may be made of the information contained therein.

Coordinator: Joaquim Comas, LEQUIA - Universitat de Girona, [email protected]


BENEFITS

This project provides:

• An analysis of the complexity related to the implementation of monitoring devices to measure water quality parameters in CSO.

• A model able to reproduce water quality of CSO for single events and time series.

• A methodology for decision support on sewer management taking into account the uncertainty around the prediction of a CSO model.

DECISION TOOL METHODOLOGY

The idea of the tool is to associate different existing CSO structures to characteristic parameter value distributions by running Markov Chain Monte Carlo (MCMC) simulations on a simplified CSO generator model. The distributions are determined according to a database of 33 CSO structures monitored by the Flemish Environmental Agency (VMM). Once the bank of parameter distributions is built, the effect of one structure can be tested by modifying a model using the parameter distribution of the structure to test. The MCMC simulations will thus provide the uncertainty around the model predictions and by comparing different solutions with their implementation cost, it will be possible to advise decision makers. During the project, the tools were developed to run the MCMC simulations. Applicability to small catchments was demonstrated but the computational demand for the simulations of more complex catchments (usual situation in Flanders) is prevents its use at this stage. Further investigation may improve the technical solution.

APPLICABILITY AND POLICY IMPLICATIONS

Policy implementation requires monitoring. The Water Framework Directive objective of reaching good ecological status for rivers in Europe demands consideration the impact of CSOs. The measurement campaign allowed to show that the implementation of water quality probes in sewers, especially to monitor CSO events that present very hard conditions for probes, requires a very high level of maintenance associated to cost and manpower. The implementation turbidity probes done by VMM and analysed during this study showed that only 20% of the implemented probes give valuable results from which loads can be computed. The temptation to implement wide and complex monitoring systems or to design regulation based on intensive measurements should be balanced by the cost and the constraints of their implementation.

Modelling is offering a good complement to evaluate the effect of CSOs on the environment but it should always be used associated the uncertainty of the prediction in order to take care of the unknown conditions that could influence the modelling results. During this study, it was attempted to develop a tool able to take into account the complexity and the variability of the configuration present in sewers over Flanders. The methodology is very relevant though its implementation remains difficult and requires further development. The policies should consider modelling solution when asking for monitoring their implementation. On a long term it is less costly than the measurements. However, detailed guidance is essential in order to reach the proper level of confidence in the modelling results.

• 2 713 CSO events analysed and showing wide range of data for TSS event mean concentration, total TSS load, maximum TSS load, total volume spilled, maximum flow rate and CSO duration without correlation to the characteristics of the catchment.

MODELLING RESULTS

• 4 configurations tested: 3 different particle classes + modelling of the upstream catchments.

• Level of complexity dependent on the location.

• Calibration was feasible but parameter values are unrealistic compared to the observed values, suggesting that the Hill function used for the resuspension is not appropriate to describe the behaviour of pollutant in the sewer. Adding a constant mass released over the whole event above a threshold flow rate may help to improve the modelling results.

TOOL DEVELOPMENT FOR COST EFFECTIVE CONTROL STRATEGIES IN URBAN WATER SYSTEMS

INDIVIDUAL RESEARCH PROJECT ER3


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