This paper presents a novel Inter Catchment Wastewater Transfer (ICWT) method for mitigating sewer overflow. The ICWT aims at balancing the spatial mismatch of sewer flow and treatment capacity of Wastewater Treatment Plant (WWTP), through collaborative operation of sewer system facilities. Using a hydraulic model, the effectiveness of ICWT is investigated in a sewer system in Drammen, Norway. Concerning the whole system performance, we found that the S{\o}ren Lemmich pump station plays a vital role in the ICWT framework. To enhance the operation of this pump station, it is imperative to construct a multi-step ahead water level prediction model. Hence, one of the most promising artificial intelligence techniques, Long Short Term Memory (LSTM), is employed to undertake this task. Experiments demonstrated that LSTM is superior to Gated Recurrent Unit (GRU), Recurrent Neural Network (RNN), Feed-forward Neural Network (FFNN) and Support Vector Regression (SVR).
Control overflow from the sewer system and Wastewater Treatment Plant (WWTP) is a crucial and challenging task for many cities in developed countries, such as the Drammen city in Norway. The Drammen city is located in southeastern Norway, it has two wastewater treatment plants: the Muusøya WWTP and the Solumstrand WWTP. The Muusøya WWTP has a designed treatment capacity of 33,000 PE (population equivalents), a dimensioning flow (Qdim) of 780 m 3 /h, and a maximum flow (Qmax) of 1,200 m 3 /h. The Solumstrand WWTP has a designed treatment capacity of 130,000 PE, the Qdim and Qmax for the Solumstrand WWTP is 2,000 m 3 /h and 4,000 m 3 /h respectively. Combined sewer accounts for more than 80% and less than 50% respectively in the sewer system associated with the Muusøya WWTP and the Solumstrand WWTP. Moreover, the drainage area of the Muusøya WWTP has a higher population density than the rest of Drammen due to it is located in the traditional city center. The lower WWTP capacity, a higher portion of combined sewer, and denser population have resulted in the severe overflow problem in the Muusøya area. Therefore, the Drammen city launched the Regnbyge 3M project to mitigate overflow from the sewer system and the WWTP of Drammen.
There are two types of overflow mitigation measures: structural measures and nonstructural measures (Lee et al., 2017). Structural measures refer constructing new hydraulic facilities and the rehabilitation of sewer components (e.g., expansion of sewer pipes). Nonstructural measures are methods that maximize the capacity of the sewer system with minimal changes to the infrastructure through intelligent operating strategies. As the most popular structural measures, the storage tank is still servicing in many developed cities. However, due to limited space or high investments, storage tanks cannot be always constructed in densely populated urban context (Ganora et al., 2017;Ngo et al., 2016) such as the Muusøya area. The drawbacks of structural measures have motivated the research for nonstructural methods, such as exploit the sewer in-line storage capacity (Darsono and Labadie, 2007;Grum et al., 2011;Garofalo et al., 2017), intelligent sewer control (Lee et al., 2017) and explore underground space (Wu et al., 2016).
Considering the spatial mismatch of capacity of the sewer system and WWTP between the Muusøya WWTP and the Solumstrand WWTP, as well as according to the goal of the Regnbyge 3M project, we propose a novel nonstructural method: Inter Catchment Wastewater Transfer (ICWT) for the Drammen city.
The idea of ICWT is inspired by the concept of Inter-basin Water Transfer (IBWT). IBWT refers to transfer water from basins having sufficient water (donor basin) to basins facing water shortages (receiving basin) (Wang et al., 2015, Yevjevich 2001). IBWT utilizes the differences of flow regime in different basins to create a win-win situation. Afterward, sewage from the Bragernes catchment and the Strømsø catchment merges with wastewater from the Konnerud catchment and the Kobbervikdalen catchment, then discharge to the Solumstrand WWTP.
We generalize the concept of IBWT to sewer system management, the drainage area of the Muusøya WWTP can be regarded as the ‘donor basin’, and the drainage area of the Solumstrand WWTP can be regarded as the ‘receiving basin’. If flow to the Muusøya WWTP already exceeded its capacity and the Solumstrand WWTP still has leftover treatment capacity, part of wastewater can be conveyed to the Solumstrand WWTP for treatment. The ultimate objective of ICWT is to balance the distribution of sewer flow and uneven WWTP treatment capacities in different catchments.
Therefore, three specific questions are raised: First, whether ICWT could reduce the overflow? Second, what are the individual and combined effects of ICWT and structural measures such as storage tank? Third, it is obvious that under the ICWT scheme, the Søren Lemmich pump station will become the bottleneck of the whole system. The Søren Lemmich pump station will be requested to operate with high sensitivity, if the Søren Lemmich pump station cannot pump wastewater timely, the ICWT will only bring extra burden to the Bragernes catchment rather than mitigate overflow. The operation of a pump station highly depends on the water level information. Pumps will be activated when the water level reaches the start level of pumps. The operation of a pump station can be enhanced if accurate water level prediction information can be provided (Chiang et al., 2010). To timely operate a pump, enhance decision-making or give enough response time for operators, it is imperative to find a model that can provide the multi-step ahead water level information (Liu et al., 2016;Chang et al., 2014;Chen et al., 2014).
In present practice, the assessments of the effectiveness of nonstructural measures count on hydraulic models mainly (Autixier et al., 2014;Lucas and Sample, 2015;Chiang et al., 2010;Seggelke et al., 2005).
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