Citation: Ramaiah, M.; Avtar, R.; Kumar, P. Treated Wastewater Use for Maintenance of Urban Green Spaces for Enhancing Regulatory Ecosystem Services and Securing Groundwater. Hydrology 2022, 9, 180. https://doi.org/10.3390/ hydrology9100180 Academic Editor: Miao Jing Received: 14 September 2022 Accepted: 12 October 2022 Published: 17 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). hydrology Article Treated Wastewater Use for Maintenance of Urban Green Spaces for Enhancing Regulatory Ecosystem Services and Securing Groundwater Manish Ramaiah 1 , Ram Avtar 1,2 and Pankaj Kumar 3, * 1 Graduate School of Environmental Science, Hokkaido University, Sapporo 060-0810, Japan 2 Faculty of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan 3 Department of Adaptation and Water, Institute for Global Environmental Strategies (IGES), 2108-11 Kamiyamaguchi, Hayama 240-0115, Japan * Correspondence: kumar@iges.or.jp Abstract: Rising land surface temperature (LST), urban heat island (UHI) effects, and stress on surface-, processed-, potable-, and ground-water resources are some undesirable ecological changes due to rapid urbanization. Treating and reusing city-generated wastewater for maintaining urban green spaces (UGS) helps in reducing/preventing groundwater extraction, ensuring sufficient supply of potable water, and bringing down LST. However, the benefits of reusing treated wastewater in UGS for enhancing regulatory ecosystem services (RES) and ushering in a circular economy are yet to be realized. In view of these, the transportation costs of treated wastewater for irrigating the UGS of Panaji city—proposed to be developed as a smart city—were assessed. Field surveys were conducted at seven gardens/parks to collect the primary data on vegetation type (ground cover, hedge plants, and trees) and their daily water requirement. As the main focus of this study, a cost–benefit analysis of (a) drawing the groundwater using borewells versus use of treated wastewater from the city’s STP, and (b) two modes of treated wastewater transport: water tankers vs. pipeline was performed. Our analyses suggest that the copiously available 14 MLD treated wastewater from the STP, which meets all the safety standards, is far in excess of the current requirement of 6.24 MLD for watering the vegetation in all 17 parks/gardens in the city. Pipeline is an efficient (less energy, labor, and time) and economical (~47% more than water that is tanker-based) transportation mode. By utilizing the otherwise unused treated wastewater, which is processed at a cost of over USD half a million annually, the RES offered by the use of treated wastewater are (a) partially curtailing a combined loss of ~16 MLD due to the extraction of groundwater plus evapotranspiration (@8.86 mm d −1 ) from Panaji city’s 1.86 km 2 UGS, and (b) reduction in LST ~3–4 ◦ C in all of Panaji city. In addition, with the proficient and sustainable management of UGS and the meeting of many UNSDGs, the enhanced vegetation growth plus elevated carbon sequestration rates in the UGS are possible through the reuse of treated wastewater. Keywords: urban green spaces; treated wastewater; sustainable urban vegetation; cost–benefit analysis; circular economy; UNSDGs 1. Introduction In many developing countries experiencing rapid urbanization, over 80 or even 90% of sewage and other wastewaters are discharged directly into open streams or coastal water, thus, severely polluting and harming the environment, leading to water-borne diseases, along with hindering tourism and economic development [1–3]. The United States Envi- ronmental Protection Agency [4] identifies that different patterns of the utilization of water resources in urban settlements play a significant role in the preservation, development, and maintenance of urban green spaces (UGS). Although some studies [5,6] have focused on how the water resource management practices in urban settlements affect the UGS Hydrology 2022, 9, 180. https://doi.org/10.3390/hydrology9100180 https://www.mdpi.com/journal/hydrology