Introduction Water is an environmental and natural resource without which life cannot exist on Earth. It is one of the four components of ecosystems that provide a wide range of ecosystem services contributing to human well-being. Water is finite and must therefore be recycled. The hydrological cycle is the simplest of all biogeochemical cycles involving the chemical elements of nature. It consists of the alternation between water vapour formed through the evaporation of water by solar radiation and the condensation of water vapour into rain droplets that fall to the ground as rainfall. The sustainability of water resources is maintained by ecological processes that take place in natural ecosystems. However, humans have been changing natural ecosystems, resulting in their loss and degradation. This has led to the degradation of natural water sources, making water use by humans increasingly unsustainable. For example, the unregulated extraction of water by human communities from rivers and groundwater makes water resources unsustainable. This article outlines the factors that contribute to the unsustainability of water resources and the strategies that can contribute to their sustainability and, in turn, to water security. Two nature-based solutions that contribute to water-resource sustainability are also explained. Water Resources and Factors Affecting Their Sustainability About 70% of the Earth’s surface is covered by water. Of this, only approximately 3% is freshwater, and the bulk of it is not available for human use because it is locked up in glaciers and deep aquifers. Only about 0.5% of freshwater is available for human use. About 2 billion people around the world do not have access to safe water. Many countries have been facing acute water scarcity, which may become a disaster due to prolonged droughts and high surface temperatures resulting from anticipated climate change. This is particularly true for urban centres across the world. Water scarcity results from the unsustainable use of water resources, reductions in stream flow, poor water quality due to pollution, and the loss of water sources such as wetlands, springs, small streams and small rivers. In fact, land degradation alone is undermining the well-being of 3.2 billion people—approximately 40% of the global population. It has also been reported that 87% of the world’s wetlands have been lost due to land degradation. Land degradation is threatening food security and access to clean water and air for people across the world (Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services [IPBES], 2018). The excessive withdrawal of water from rivers, coupled with the loss and degradation of river ecosystems—particularly floodplain ecosystems—the degradation of catchments and watersheds, groundwater depletion, changes in the dynamic exchange between groundwater and river water, and alterations in the local hydrological cycle also contribute to the unsustainability of water resources. The dynamic exchange between groundwater and river water includes the flow of groundwater into rivers during the lean season, known as lateral or base flow, and the movement of water from rivers into groundwater during the monsoon season or flooding. Strategies for the Sustainability of Water Resources The sustainability of water resources can be achieved through the following strategies: Restoration of degraded catchments and watersheds through the redevelopment of vegetation, including grasslands. Revival of surface-drainage channels by restoring connectivity and undertaking desilting. Restoration of wetlands, particularly floodplain wetlands. Development of small dams of the run-of-the-river type. Ensuring the dynamic hydrological exchange of water between groundwater and rivers through the maintenance of connectivity within river ecosystems. Protection of the local hydrological cycle through vegetated catchments, watersheds and river basins. Regulated and sustainable withdrawal of water from rivers and extraction of groundwater, while ensuring access for all communities irrespective of their economic status. Conservation of water resources. Some Government Initiatives It is pertinent to mention the strategies followed by the Government of India to ensure water security. The Hon’ble Union Minister of Jal Shakti stated in The Hindu (18 June 2026, p. 7) that water security is central to a Viksit Bharat. He mentioned several schemes and initiatives launched by the Government of India, ranging from the Jal Jeevan Mission—the world’s largest rural drinking-water supply programme—to the Jal Sanchay Jan Bhagidari initiative, which involves water conservation and groundwater recharge through 1.55 crore rainwater-harvesting and groundwater-recharging structures. He also referred to the Ken–Betwa River Linking Project; programmes of the National Mission for Clean Ganga (NMCG), which demonstrate that environmental restoration and development can advance together; and initiatives involving the treatment of sewage through sewage-treatment plants. According to the statement cited, 4,260 million litres per day (MLD) of sewage is treated by sewage-treatment plants, and the Ganga has attained bathing-quality water. He summarised that water challenges can be addressed only through an integrated approach that takes into account drinking water, sanitation, river conservation, irrigation efficiency, groundwater recharge, wastewater reuse and climate resilience. These elements constitute a connected ecosystem, and such an integrated approach is essential for water security. Critical Strategies In addition to the strategies mentioned above, two major strategies are critical for water-resource sustainability: Recycling and reuse of wastewater. Rejuvenation of rivers. Both strategies are explained in the following sections. 1. Recycling and Reuse of Sewage and Wastewater One way to address acute water scarcity in urban centres is to recycle and reuse sewage and wastewater. In large cities such as Delhi, which face water shortages, approximately 12,000–13,000 MLD of sewage is generated and discharged into the Yamuna, making it an open sewer. Recycling and reusing this water for flushing toilets, horticulture, agriculture and industrial cooling can make more freshwater available for drinking and domestic use. Conventional technologies such as sewage-treatment plants (STPs), reverse osmosis (RO) and ion-exchange technologies are used to treat sewage and wastewater. Of these conventional technologies, STPs are widely used. However, STPs have several limitations. For example, an STP is expensive, requires considerable energy, may not be effective under Indian conditions, is not suited to treating small volumes of sewage generated by villages or clusters of villages, and is not efficient in removing heavy metals and some emerging pollutants, such as antibiotics and endocrine disruptors. There are other alternative or complementary technologies that are collectively known as nature-based solutions or bioremediation technologies. When plants alone are used, these are called phytoremediation technologies. When microbes and other organisms—including plants, animals and biomolecules—are used, they are called bioremediation technologies. Some bioremediation technologies used for the treatment of sewage include nanobubbling, microbial-bead technology, enzyme-dosing technology, algae-based technology and earthworm-based technology. All these bioremediation technologies have one or more limitations and may not be suitable for the treatment of sewage in every situation. When bioremediation technologies are used independently and in place of conventional technologies, they are called alternative technologies. In other words, they are used as primary-treatment technologies. When bioremediation technologies are used for secondary or tertiary treatment of sewage and wastewater, they are considered complementary to conventional technologies. Of the different bioremediation technologies used for the treatment of sewage and wastewater, the most widely used across the world is the constructed wetland (CW) system. 1.1 Constructed Wetland Systems Constructed wetland systems have several advantages over STPs. They are inexpensive, do not require energy, are effective under Indian conditions, are well suited to treating low volumes of sewage generated by a village or cluster of villages, and can be effective in removing heavy metals and emerging pollutants. A constructed wetland is a typical wetland that has been engineered for wastewater treatment. It consists of a substratum or sediment, sewage water, porous media such as sand and gravel, and microbial communities. These components interact and generate physical, chemical and biological processes that remove pollutants. The removal efficiency varies from 75% to 99% for different pollutants. There are different types of constructed wetlands, classified according to the type of plants used or the water-flow regime. The most widely used classification is based on the water-flow regime. If sewage flows freely across the surface of a constructed wetland, it is called a free-water-surface constructed wetland system. If sewage flows through the subsurface—beneath sand or gravel—it is called a subsurface-flow constructed wetland system. Subsurface-flow systems may be further classified as horizontal-flow or vertical-flow systems, depending on the direction of water movement. There are different designs of constructed wetland systems. Constructed wetland systems also have several limitations and are not widely used in wastewater engineering. They require additional land, and the porous media may become choked or clogged with sediment from sewage. Sludge may also form a floating mat in the constructed wetland, making the system dysfunctional. Integrated Constructed Wetland System Developed by CEMDE The Centre for Environmental Management of Degraded Ecosystems (CEMDE), University of Delhi, has addressed the above limitations by developing an integrated constructed wetland system that can be installed directly in the sewer or drain carrying sewage water. This is known as in-situ bioremediation. The integrated constructed wetland system developed by CEMDE consists of: One or two stabilisation units or zones, with two or three stone masonry walls. The walls are 5–15 cm above the water level, depending on the gradient of the drain. A screen with a 14 mm mesh is fixed to the first stone masonry wall, while a screen with a 4 mm mesh is fixed to the second or third stone masonry wall. One or two filtration units or zones, having 5–15 gabions, depending on the length of the drain. Each gabion consists of an assemblage of 60–80 cm-sized cut stones embedded in plastic mesh. The height of the gabions is above the water level, with aquatic plants placed between them. One vegetated unit or zone, consisting of ridges made of loose 40–60 cm-sized cut stones and furrows containing aquatic plants. Depending on the volume of sewage flow, the number of ridges and furrows varies from 5 to 15. The integrated constructed wetland system works on the following principles: The stabilisation unit removes suspended particulate matter through water retention and activates microbes through the enrichment of dissolved oxygen. Solid waste is removed by the 14 mm mesh screen, while floating sludge is removed by the 4 mm mesh screen. In the filtration zone, enhanced turbulence is created when sewage flows through pores of different sizes formed in a zigzag arrangement within the gabions. This increases dissolved oxygen and activates the rhizospheric microbial communities of plants, in addition to the microbes found in sewage water. In the vegetated unit, further enrichment of dissolved oxygen and enhanced microbial activity lead to the effective removal of pollutants. The integrated constructed wetland system is effective and efficient for primary, secondary and tertiary treatment in drains carrying up to 10 MLD of sewage. It is expected to be less effective and efficient in drains carrying up to 20 MLD. Both conventional technologies and nature-based solutions contribute to water-resource sustainability through the recycling and reuse of sewage and wastewater. 2. Rejuvenation of Rivers through the Development of Biodiversity Parks Another nature-based solution developed by CEMDE is the Biodiversity Park approach for the rejuvenation of river systems. This approach also contributes to the sustainability of water resources. The Biodiversity Park is an integrated approach involving the ecological restoration of river ecosystems to their natural states and the restoration of their ecological integrity and connectivity. This ensures the health of the river system, which in turn supports the quality and quantity of water. River ecosystems include: Free-flowing water ecosystems. Riparian ecosystems. Floodplain ecosystems, including forests, grasslands, marshes, swamps, catchments and treatment wetlands. Embankment ecosystems. Upland ecosystems. All these ecosystems are interconnected. A Biodiversity Park in a riverscape involves the restoration of these ecosystems to their natural states. It also includes in-situ remediation of sewage and wastewater entering the river. A Biodiversity Park is a unique landscape or riverscape where lost and degraded river ecosystems are recreated or restored to their natural states. Such parks have educational, cultural and conservation values and contribute to the sustenance of river flows and water quality. The concept was first developed by CEMDE and implemented jointly by the Delhi Development Authority (DDA) and CEMDE, University of Delhi. The DDA now has a network of seven Biodiversity Parks, of which the Yamuna Biodiversity Park (YBP) is 22 years old and was the first Biodiversity Park to be developed. YBP spreads over 457 acres along the floodplains of the Yamuna, upstream of the Wazirabad Reservoir. Within YBP, river-basin communities, floodplain ecosystems and embankment ecosystems have been recreated or restored and are fully functional, with four trophic levels. YBP has become a global model not only for urban renewal but also for the rejuvenation of rivers. The National Green Tribunal (NGT) has also ordered that Biodiversity Parks should be established along the floodplains of all rivers in India, following the model of YBP. The rejuvenation of rivers through the Biodiversity Park approach also contributes to the sustainability of water resources. Conclusions The revival of springs and small streams, rainwater harvesting, restoration of surface-drainage channels, restoration of wetlands and development of small run-of-the-river dams can make water resources sustainable and secure. Regulated and sustainable extraction of water from river and groundwater systems, ensuring the continuation of the dynamic exchange of water between groundwater and rivers, and protecting the local hydrological cycle also contribute to water security. Recycling and reuse of sewage and wastewater, along with the rejuvenation of rivers, are crucial for water-resource sustainability and security. Nature-based solutions such as simplified and integrated constructed wetland systems and the Biodiversity Park approach can substantially contribute to water security. This article is written by C. R. BABU , Centre for Environmental Management of Degraded Ecosystems (CEMDE), University of Delhi [Published in NIWAS Vartika - Vol-II, Issue-2 (Jul-Sept'26): A WASH magazine by SPM NIWAS, MoJS, GoI]