Combined sewer systems transport wastewater from urban systems to the wastewater treatment plants, and during rain episodes they also transport runoff waters through the same system. One major problem associated with combined systems is that, during intense rain episodes, the maximum capacity of the system is exceeded, resulting in the discharge of a part of the mixed waste and rain water directly into the receiving water bodies through overflow structures. These discharges are commonly known as combined sewer overflows (CSO), and due to the release of untreated wastewater they represent an important threat for both the aquatic ecosystems quality and the human health.
The European Urban Wastewater Treatment Directive from 1991 already showed the need to control these transient discharges. With the Water Framework Directive from year 2000, and its primary objective of achieving the good ecological status of all surface waters, it forced to revise the current regulations on the control of CSOs. An example at national level is the Royal Decree RD 1290/2012, approved on 2012, which introduced a new framework to control the pollution from CSOs. Thus, in this new legal context under ongoing development about the control of CSOs, the monitoring of these discharges plays an important role.
The high cost of conventional technologies for monitoring the occurrence and duration of CSOs (e.g. flow and level sensors), together with the large number of CSO structures normally present in combined systems, highlight the need of new low-cost methods if the whole system is required to be monitored simultaneously. The research reported in the thesis “Towards better management of combined sewer systems – a methodology based on low-cost monitoring” by Albert Montserrat Royuela, focuses on the application of a low-cost technology (temperature sensors) to determine the occurrence and duration of CSOs, and the processing of such data to improve the management of combined sewer systems.
The proposed (and validated) method is based on the temperature difference between the sewer gas phase and the overflowed mix of waste and rain water, so that the temperature change produced during a rain episode is related to the occurrence of a CSO. Using simple and cheap modern temperature sensors equipped with small data loggers, allowed the simultaneous characterization of the CSO events occurred in all the CSO structures from the combined system of La Garriga for a period of almost one year, successfully determining the occurrence and duration of the whole CSOs in 80% of cases.
The data obtained from La Garriga were used to develop a methodology aimed at evaluating the performance of combined sewer systems. The methodology allowed: (1) evaluate the system capacity with data from CSO duration and rain volume; (2) characterize with statistics the performance of the whole CSO structures on a simultaneous basis; (3) evaluate the compliance of the system with national regulations and guidelines; and (4) generate decision tree models to support water managers on decision making about system maintenance. The results obtained with this methodology, which can be easily applied to any system, can be of great support to managers and engineers dealing with the improvement and maintenance of combined sewer systems.
Finally, another significant contribution from this thesis is related to the simulation of the behavior of combined systems in wet weather using numerical models, which are widely used for several purposes such as the assessment of the performance of combined systems, or the evaluation of the impact from CSOs on the receiving waters. These models are commonly calibrated (and validated) in order to adjust the simulated data to the measured ones. Because of the high cost of conventional measuring devices, a common practice is to calibrate the flow discharge at a representative point of the main sewer pipe, and then assume the validity of the behavior of CSO structures. However, the truth is that the difference between the simulated and the measured behavior of CSOs can be significant if these structures are not calibrated as well. Thus, information on the duration of CSOs obtained by temperature sensors was used in the automatic calibration of a hydrodynamic sewer model, and the results were compared against a calibration using flow data. The methodology was applied to an urban catchment drained by a combined sewer system from the city of Graz (Austria). The achieved results demonstrated that, using CSO duration information (which can be obtained with low-cost sensors), it is possible to calibrate a sewer model with a similar level of certainty to the one that would be obtained if data obtained with more advanced and expensive flow sensors were used.
On the whole, the results derived from the thesis “Towards better management of combined sewer systems-a methodology based on low-cost monitoring“ directed by Dr Lluís Corominas (ICRA) and Dr. Manel Poch (LEQUIA, UdG) represent a contribution of great value for the characterization of the activity from CSOs and the management and optimization of combined sewer systems.