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A transition from conventional centralized to hybrid decentralized systems has been increasingly
advised recently due to their capability to enhance the resilience and sustainability of
urban water supply systems. Reusing treated wastewater for non-potable purposes is a promising
opportunity toward the aforementioned resolutions. In this study, we present two optimization
models for integrating reusing systems into existing sewerage systems to bridge the supply–demand
gap in an existing water supply system. In Model-1, the supply–demand gap is bridged by introducing
on-site graywater treatment and reuse, and in Model-2, the gap is bridged by decentralized
wastewater treatment and reuse. The applicability of the proposed models is evaluated using two
test cases: one a proof-of-concept hypothetical network and the other a near realistic network based
on the sewerage network in Chennai, India. The results show that the proposed models outperform
the existing approaches by achieving more than a 20% reduction in the cost of procuring water and
more than a 36% reduction in the demand for freshwater through the implementation of local on-site
graywater reuse for both test cases. These numbers are about 12% and 34% respectively for the
implementation of decentralized wastewater treatment and reuse.
Structural resilience describes urban drainage systems’ (UDSs) ability to minimize the
frequency and magnitude of failure due to common structural issues such as pipe clogging and
cracking or pump failure. Structural resilience is often neglected in the design of UDSs. The current
literature supports structural decentralization as a way to introduce structural resilience into UDSs.
Although there are promising methods in the literature for generating and optimizing decentralized
separate stormwater collection systems, incorporating hydraulic simulations in unsteady flow, these
approaches sometimes require high computational effort, especially for flat areas. This may hamper
their integration into ordinary commercially designed UDS software due to their predominantly
scientific purposes. As a response, this paper introduces simplified cost and structural resilience
indices that can be used as heuristic parameters for optimizing the UDS layout. These indices only
use graph connectivity information, which is computationally much less expensive than hydraulic
simulation. The use of simplified objective functions significantly simplifies the feasible search space
and reduces blind searches by optimization. To demonstrate the application and advantages of the
proposed model, a real case study in the southwest city of Ahvaz, Iran was explored. The proposed
framework was proven to be promising for reducing the computational effort and for delivering
realistic cost-wise and resilient UDSs.