Thesis submitted in fulfillment of the requirements for the degree of Doctor at the University of Girona (PhD programme: Experimental Sciences and Sustainability). The CSO volume, peak and duration were used to evaluate the goodness of the calibrations.
INTRODUCTION
- Historical background
- Legal background on the protection of water quality
- Motivation and objectives
- Thesis structure
A complex consisting of a sewage system (common or separate), a treatment plant (WTP) and receiving water is usually called a municipal wastewater system (MWW) in the literature (Figure 1-1). What are the possible uses of occurrence and duration data obtained by low-cost methods.
OBJECTIVES
The aim of this thesis is to develop a method based on inexpensive sensors to determine the occurrence and duration of combined sewer overflows and to effectively use data obtained from this method for the assessment and maintenance of CSSs and in the calibration of hydrodynamic sewer models. Therefore, the specific objectives, in accordance with the justifications presented in the introduction, are threefold:.
MATERIALS AND METHODS
Locations
An overview of the La Garriga municipal wastewater system is given in Figure 3-6 below. Installation and instrumentation of the measuring station in the CSO structure of the western catchment of Graz.
Data collection
In the La Garriga case study, Hach-Lange's 'Sigma 950' was used for the verification of the temperature method (see chapter 4.1). Precipitation data were obtained using a weather station, model David Vantage Vue (Figure 3-11), located on the roof of the town hall of La Garriga, right in the center of the village.
General description of the methodology
The data on the occurrence and duration of CSO events from the 14 CSO structures of the La Garriga CSS were collected over a period of 11 months (from July 2011 to May 2012). To characterize the performance of the sewer system, the following parameters were calculated for each social structure in the network over the entire data collection period: (i) total number of overflows, (ii) total duration of overflows (sum of the duration of all overflows in the period ), (iii) the average flooding duration, (iv) the average chronological order in which a CSO structure starts flooding compared to all other structures in the network, and (v) the flooding probability. The methodology in this section focused on calibrating a modeled CSO structure to properly describe the flow at its discharge.
The CSO structure in the Graz urban catchment was used as a case study to demonstrate the usefulness of the new calibration approach. A calibrated SWMM model of the Graz catchment (Gamerith et al., 2011) was used as a reference model. In the spillway approach, the CSO structure model was calibrated using the flow measurements at the spillway, whereas in the duration approach, the CSO structure model was calibrated using CSO duration data.
RESULTS AND DISCUSSION
Field validation of a new low-cost method for determining occurrence and duration of
- Description of the method
- The case study
- Verification of the method on a single CSO structure
- Evaluation of the method on multiple CSO structures
- Advantages, limitations and potential applications of the proposed single
- Automatic detection of CSOs
- Alternative approaches for CSO monitoring
For the transverse structures, the sensor was placed at the bottom of the center of the discharge pipe. Vertical dashed lines indicate the start and end points of the CSO event obtained from the flowmeter measurements. The analysis of the temperature signals for the 13 structures and the 57 episodes was performed manually.
Percentage distribution of temperature shift magnitudes for each CSO structure for the entire study period. A mathematical technique was used to illustrate the temperature time series of the CSO1 structure. When the CSO occurs, the two temperature profiles converge (because the temperature in the inlet channel and in the CSO structure would be the same), indicating the total duration of the CSO event.
Using data from monitoring combined sewer overflows to assess, improve, and
- Data collection
- Methodology for CSS evaluation
- Results and discussion
Total number of overflows and total overflow duration for each CSO structure for the 53 rain episodes in the evaluated period. Evaluated parameters of the CSO structures (dots represent the mean value and lines represent the 95% Ci). The x-axis is each structure of CSS (note that structures 9 and 13 do not have values because they were not used in this study).
Most of the structures in the 11-month case study exceeded the recommended number of overflows per year. For the case study of La Garriga CSS, a decision tree was constructed for each CSO structure. A plot of the predicted and observed responses of each CSO structure for 53 rain episodes that occurred in La Garriga.
Using the duration of combined sewer overflow events for the calibration of sewer
- Case study and data
- Sensitivity Analysis
- Model calibration and validation
- Results and discussion
- Final thoughts
The suitability of using durations from CSOs for calibrating a hydrodynamic model was demonstrated in the urban catchment of the city of Graz (Austria).
GENERAL DISCUSSION
- Main contributions
- The implementation status of in-sewer temperature measurements
- Potential application
- Outlook for the future
The use of low-cost sensors allowed the simultaneous monitoring of all CSO structures of the La Garriga CSS, which would otherwise not have been possible (taking into account the investments required). As mentioned in the first paragraph of this chapter, an important application of the method is related to environmental protection law enforcement. Depending on the end use of the model, more or less calibration should be considered.
The results of the research presented in chapter 4.3 will thus be able to help meet the requirements of existing and future guidelines for the design and evaluation of waste water systems. Another interesting line of research of great value would be the estimation of the CSO flow discharge from temperature measurements. The integrated management of UWWS is a reliable approach to reduce the negative impact on the receiving waters while not increasing treatment costs in the treatment plant (Corominas et al., 2013).
CONCLUSIONS
The effectiveness of the method was affected by the specific physical properties of some structures, which affected the signal-to-noise ratio of the temperature. An algorithm for automatic detection of CSO events from temperature signal analysis was also proposed. Each of the analyzes included in the methodology was based on direct, simultaneous measurements of overflows in all CSO structures within the CSS.
Regarding the definition of objective functions for automatic calibration of the model using optimization, the average mean error was the best option. The calibration of the CSO structure model was particularly relevant for small episodes to correctly simulate CSO flow. For the small rain episodes, the volume and peak of the CSOs were significantly better represented through the calibration of the overflow approach (especially in volume, with errors below 1%, compared to 40% with the duration approach).
Continuous event-based confluence and catchment modeling for assessing the combined impact of river sewage discharge. Impact of a combined sewer inundation on the abundance, distribution and community structure of subtidal benthos. Effects of rainfall events on the occurrence and detection efficiency of viruses in river waters impacted by combined sewage discharges.
Japan-Korea Special Workshop on Impact Analysis and Control of Combined Sewer Overflow – Associated Event of the 4th IWA-ASPIRE Conference and Exhibition. Field Validation of a New Low-Cost Method for Determining the Occurrence and Duration of Combined Sewer Overflows. The impact of an intense combined sewer overflow event on the microbiological water quality of the Seine River.
Installation and verification
Installation Setup for verification of the method consisting of a flow meter, a primary and a secondary temperature sensor.
Physical limitations
As in the case of Structure 11, part of the CSO remains in the channel of the embankment during dry weather conditions.
Alternative approaches for CSO monitoring
It can be seen how for the November 10th event, the conductivity sensor correctly detects the CSO event, but it is extended to November 16th, thus masking that the event subsequently occurred on day 11. The same phenomenon occurred with the event of the day 17, which lasted until 21 masking the event that occurred on the 19. It was related to the deposition of solid substances on the sensor that occurred during the CSO event.
Figures AIII-4 and AIII-5 (showing solids around the sensor) were taken on November 16 and 21, respectively, days when the solids were removed, leading to restoration of the conductivity signal. The conductivity sensor detected only two events due to solids attached to the sensor, producing a prolongation of the conductivity signal.
Rain inter-episode time (La Garriga CSS)
Model development and CSO duration-rain volume curves
After calibrating the model, two case scenarios were evaluated: a poorly designed and a well-designed system. First, the calibrated model was run for 61 rain episodes that occurred between July/2011 and October/2012. The case representing the well-designed system was obtained by recalibrating the model, this time considering the number of overflows as the calibration target.
The calibration parameters in this case were the maximum depth of the pipe and junction, the height of the weir crest and the weir length. Such parameters meant that physical changes were made to the modeled system to achieve the considered number of CSO events. Summary (i.e. minimum, maximum and mean) of the characteristics of the 61 rainfall episodes used to obtain the relationship between rainfall volume and CSO duration.
CSO duration-rain volume curves (results)
Support for CSS maintenance (decision trees)
In the main screen of the La Garriga CSO Network Simulator (Figure AVII-2), the user enters rain characteristic data (total volume, duration, maximum intensity, and the time since the last rain episode) in boxes on the left side of the screen. After the user has introduced the rain data and run the model, the results (predictions about whether overflow has occurred or not) of the model appear in the center of the screen. Finally, the user can select a CSO structure and his decision tree will appear on the right side of the screen.
Interface of the CSO La Garriga network simulator with an example of an overflow forecast for a rain episode with user input characteristics; an overflow prediction is made for each CSO structure (with the CSO 2 structure marked and its tree shown in the figure).
Modified SWMM (SWMM_CSO)
In the simulated file (left), an overflow event can be observed from second 480 to second 900 (8 minutes), while in the calibration file (right), the overflow event lasts for 6 minutes.
Sensitivity, hypothesis tests and calibration results
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