Development of water reuse: a global review with the focus on India

BCM

Billion cubic metres

MCM

Million cubic metres

MLD

Million litres per day

Mt

Million Tonnes

ha

Hectares

kL

Kilo-litres

mg/L

Milligrams per litre

mL

Millilitres

L/d

Litres per day

NTU

Nephelometric Turbidity Unit

MPN

Most Probable Number

GDP

Gross domestic product

STP

Sewage treatment plant

PPP

Public private partnership

BOD

Biochemical Oxygen Demand

ASP

Activated sludge process

UASB

Upflow anaerobic sludge blanket

SBR

Sequential batch reactor

BIOFOR

Biologically active fitration systems

FAB

Fluidized aerobic bio-reactor

SAFF

Submerged Aerobic fixed film reactor

MBBR

Moving bed biofilm reactor

TPP

Thermal power plant

bgl

Below ground level

WSP

Waste stabilization ponds

GHG

Greenhouse gas

IPR

Indirect potable reuse

DPR

Direct potable reuse

CPHEEO

Central Public Health and Environmental Engineering Organization

CWC

Central Water Commission

CPCB

Central Pollution Control Board

NMCG

National Mission for Clean Ganga

IOCL

Indian Oil Corporation Ltd

ISO

International Organization for Standardization

WHO

World Health Organization

FAO

Food and Agriculture Organization of the United Nations

USEPA

United States Environmental Protection Agency

UNICEF

United Nations International Children's Emergency Fund

Growing water demand is leading to water scarcity, creating a threat to economic growth, human rights, and environmental integrity in developed as well as developing countries, especially India. Despite having 4% of the world's freshwater resources and a widespread river network, the availability of water resources is quite complex throughout the country (Central Water Commission 2016). There is a noticeable difference in the volume of water resources available and the volume that can be put to use (Table 1) due to limitations of physiography, topography, inter-state administrative issues, and the existing technologies to utilize the water resources sparingly (NITI Aayog 2018).

Out of the total annual rainfall, a limited amount could be used effectively because of variations over time and space, causing water shortage (Central Water Commission 2016). The impact of climate changes on water resources also affects water availability. In the year 2018, a deficiency of 9% has been recorded in the cumulative annual rainfall (Indian Meteorological Department 2018). The surface water pollution has further aggravated the problem of water shortage (NITI Aayog 2018).

Based on the current demand and availability trends, India is expected to encounter a huge water deficit in the future, mainly in the irrigation sector (Table 1). As per the estimates, the anticipated water demand would be 1498 billion cubic metres (BCM) by the year 2030, contrary to the expected water availability of 744 BCM, creating a huge gap between the demand and availability (NITI Aayog 2018). This anticipated water crisis could become severe unless scientifically planned and environment-friendly water use strategies and management plans are implemented.

Reuse of treated wastewater is one of the most promising water management strategies, which serves the two-fold purpose of augmenting water resources as well as reducing the environmental impacts of disposal of untreated wastewater (Pereira et al. 2002; Massoud et al. 2018). During the old times (3200–1100 BC), ancient civilizations used wastewater generated from urban settlements in agriculture (Pedrero et al. 2010; Jaramillo & Restrepo 2017). Around the world, depending on the resources available, strategic and unintentional reuse of treated water for numerous applications such as agriculture, industrial, environmental and recreational, domestic including indirect potable reuse, has been identified (Ilemobade et al. 2009; Adewumi et al. 2010; Angelakis & Gikas 2014; Wilcox et al. 2016; Chen et al. 2017; Sgroi et al. 2018; Voulvoulis 2018; Hagenvoort et al. 2019; Jodar-Abellan et al. 2019; Lee & Jepson 2020; Rebelo et al. 2020; Takeuchi & Tanaka 2020; Van Rossum 2020). Several researchers have explored the viability of direct potable reuse with varying degrees of success (Grady et al. 2014; Burgess et al. 2016; Cotruvo 2016; Drewes & Horstmeyer 2016). Figure 1 represents a comprehensive study of global experiences of water reuse (Sato et al. 2013; Aleisa & Zubari 2017; BCC Research Report 2017; Sanz 2019).

For reuse in each category, secondary treatment followed by disinfection is a must that may vary regionally according to the quality of effluent and available technologies. The most preferred treatment technology used in urban cities in India is the activated sludge process followed by an anaerobic process, mainly upflow anaerobic sludge blanket (UASB) or trickling filters depending on land availability. Constructed wetlands and waste stabilization ponds are generally used as standalone treatment units for low-end uses like irrigation or as a polishing unit or tertiary treatment to remove nutrients. Nowadays, ASP modifications like SBR, extended aeration, and so on are also being used. Membrane bio-reactors (MBRs) integrated with filtration (ultra, micro, or nano) is the most preferred technique for obtaining high-quality effluent, but has high capital costs. Chemical treatment processes such as reverse osmosis, electro-dialysis, chemical precipitation, advanced oxidation process, and so on are used for producing water of drinkable quality. Some of the other techniques include BIOFOR, FAB reactor, facultative aerated lagoons, SAFF, MBBR, etc. Soil aquifer treatment is also used as an advanced treatment technology for producing high-quality effluent. Treatment technologies are generally selected based on land, skill and power availability, costs, and desired quality of effluent.

In many countries, governing water quality parameters and regulatory guidelines/policies are available for different applications. This may be due to the difference in topography, basic climatic and local variations, technical infrastructure, economic and social considerations, and so on.

Owing to the requirements of large amounts of water, the largest reuse potential exists in agriculture. In almost every country of the world, planned or unplanned reuse of wastewater is being done. In many countries in Europe, almost 75% of the treated wastewater is being used for growing crops, for instance, Greece, Cyprus, Belgium, Spain, Italy, Portugal, and so on (Angelakis & Gikas 2014). In Cyprus and Spain, the reuse of approximately 90% of the generated wastewater is being done. Agricultural irrigation is quite popular in France as well. In Brazil, aquaculture and crop irrigation have been done using treated wastewater since 1992. China has the first rank in terms of reuse by volume, especially in agriculture (Duong & Saphores 2015). In Australia, treated wastewater is used to irrigate vineyards, grow vegetables and crops along with landscape irrigation (Lautze et al. 2014). Israel has revolutionized water reuse. Approximately 90% of its wastewater is being reused with 75% usage in irrigation (Kamizoulis et al. 2003). Agricultural use is the most preferred option for most of the countries throughout the world including developing countries like Jordan, Tunisia, India, Thailand, Mexico, etc. But, it is essential to adhere to the minimum quality norms to eliminate public health and other environmental risks. A comparison of available international guidelines and standards might aid in developing guidelines for other areas as well (summarized in Table 2). Apart from the discussed guidelines, there are regional guidelines available for agricultural reuse in countries like Tunisia, Mexico, Chile, Cyprus, France, Italy, Greece, Portugal, Spain, Jordan, and so on.

Industrial reuse of wastewater has gained popularity over the last few years throughout the world. This may be due to many factors such as depletion in groundwater, high costs of extracting water, etc. Many countries have shifted the focus on reusing treated wastewater in water-intensive industries, mainly, textile, pulp and paper, leather, tanneries, thermal power plants, etc. (Lautze et al. 2014). Typical industrial applications include reusing treated water as feed water in boilers, cooling towers, and as process water for washing purposes, according to the industry. In Durban, South Africa, successful working of wastewater recycling and reuse plant is being executed, where the treated water is utilized in a paper industry, which has reduced the costs and resulted in augmentation of water supply (Indian Institutes of Technology 2011). Industrial reuse is also quite popular in Japan. Reclaimed water is being treated and reused in Singapore as process and cooling water (Gulamussen et al. 2019). In the USA and Mexico, there are successful cases of reuse of treated water as boiler feed in thermal power plants. Because of high water consumption by industries, the use of treated wastewater for industrial applications is one of the most sustainable options, which can further be strengthened by strict water quality guidelines. Unfortunately, quite a few countries have specific guidelines for industrial reuse (Table 2). To support these guidelines, ISO standards i.e. ISO/TC 282/SC 4 for industrial water reuse have also been developed (International Standards Organization 2020).

Environmental use of reclaimed water includes the construction of wetlands, enhancement of wetland habitat and its restoration, stream augmentation, and so on. Recreational uses include landscape impoundments, golf course irrigation, recreational impoundments, development such as lakes, fountains, and maintenance of parks, gardens (USEPA 2012). In California, USA, 10% of the environmental and recreational demand is fulfilled by treated wastewater. Other examples include the city of West Palm Beach, Florida wetlands reclamation project, Pomona Water Reclamation Plant with Integrated Aquaculture-Wetland Ecosystem (AWE), Los Angeles County, and the Eastern Municipal Water District constructed wetlands in Riverside County, California (Cleaves 2005). Treated wastewater is extensively used in Japan for the enrichment of urban streams and in Canada, for golf course watering and environmental restoration (Duong & Saphores 2015). Successful cases of water reuse in environmental and recreational activities have been observed in countries like Greece, Italy, and several parts of Australia (Angelakis & Durham 2008). To maintain the effective functioning of such reuse systems, it is important to develop and follow specific guidelines related to water quality parameters, as discussed in Table 2.

Domestic use of reclaimed water includes toilet flushing, cleaning, and other non-contact activities. Toilet flushing accounts for approximately 30–40% of domestic water usage. The use of treated wastewater for domestic non-contact applications may help in a significant reduction of freshwater usage. In addition to households, toilet and urinal flushing using treated wastewater may be implemented in institutional, commercial, and industrial buildings (USEPA 2012). Though there is a social barrier concerning the use of treated wastewater for in-house applications, high-quality treatment, and public education and awareness is leading to the rising use of treated wastewater for toilet flushing and non-contact activities. The water availability portfolio of Hong Kong has been expanded by using saline water for flushing toilets and implementing non-potable wastewater reuse (Duong & Saphores 2015). In Greece, toilet flushing using a dual reticulation system is quite popular. One of the main applications for water reuse in Canada and Australia include toilet flushing and urban uses (Ryan & Foster 2016). Some of the guidelines developed by different nations indicating water quality criteria for domestic use have been discussed in Table 2.

Nowadays, potable reuse is significantly emerging, especially in cosmopolitan cities with very high water demand (Angelakis et al. 2018). Potable reuse may be indirect potable reuse (IPR); that is, storing treated water in an environmental buffer and direct potable reuse (DPR), in which treated water is directly fed into the water distribution network (USEPA 2012). The major barrier related to potable reuse is the risk associated with public health. With the advancement of treatment technologies, high-quality water is being obtained which is safe for human consumption. Successful cases have been witnessed in various regions throughout the world.

For example, groundwater injection (IPR) in Orange County, California, USA, Big Springs, Texas, USA, several ongoing IPR projects in Brisbane and Melbourne in Australia, and so on. (Drewes & Horstmeyer 2016; Angelakis et al. 2018). Since 1968, DPR of highly-treated wastewater is being accomplished in the city of Windhoek, Namibia (Lautze et al. 2014). In Singapore, treated wastewater (branded as NEWater) has been supplied by the public utilities since 2003. It is a notable initiative in promoting indirect potable reuse (Duong & Saphores 2015). Reclaimed wastewater project for groundwater recharge in Flanders, Northern Belgium, has been satisfying the drinking water demand of its residents since 2003 (Kamizoulis et al. 2003). There is a dearth of guidelines and regulations related to potable reuse of wastewater which acts as an obstruction in the implementation of such projects. To maintain treated water quality for groundwater recharge and augmentation of water supplies, USEPA provides reuse guidelines, last updated in 2012 (Table 2). Recently, WHO has published a guidance document on potable reuse in 2017 based on drinking-water quality (GDWQ) (World Health Organization 2017).

Even though the use of treated wastewater is primitive, it has been neglected due to the lack of management and review of social concerns and health and environmental risks (Lam et al. 2017). Studies conducted in numerous countries show that apart from health risk elimination, social willingness and acceptance and institutional support are also important for the effective operation of water reuse projects (Friedler et al. 2006; Hurlimann & Dolnicar 2010; Ross et al. 2014; Wester et al. 2015; Garcia-Cuerva et al. 2016; Massoud et al. 2018)

Unfortunately, no study showing a detailed analysis of the water reuse situation and challenges in India has been performed which leads to failure of such projects at the local level and causes huge economic losses as well. The objectives of this paper include an overview of the existing wastewater reuse practices throughout the world with a focus on India. Also, the factors driving the cause, together with the challenges and opportunities associated with the expansion of reuse have been elaborated as an effort to provide a comprehensive background that may be used for future regulatory policy formulation.

Due to differences in baseline climatic conditions, existing water resources, and level of economic and social development, there is a regional variation in factors affecting the reuse of treated wastewater, except rising population which has taken its toll on the whole world. In North America, limited water availability has given rise to water reuse (Diemer 2007) whereas, high agricultural demand is responsible for water reuse in South America (Jiménez 2008). Escalating urbanization, salinization, and climate change are the major driving factors in Europe. Increasing industrial, urban, and agriculture needs, shifting focus to greener technologies have led to water reuse in Australia (Po et al. 2003). In the Middle East, Africa, and Asia, depleting water resources, deteriorating water quality, climate change, and food security are giving rise to the reuse of treated water (Bahri 2008; Hamoda 1996; Visvanathan 2018). Water stress is of prime concern in most Indian cities. Issues like water shortage due to the increased per capita usage, industrial development, other environmental and financial challenges, etc. emphasize water reuse. The prime factors contributing to the reuse of treated wastewater in India have been analyzed in this section.

It has been estimated that the population is likely to reach 1.66 Billion by the year 2050, making India the most populated country in the world. Population increase has a direct effect on food demand (Table 3). Moreover, with projected national gross domestic product (GDP) growth up to 6.0% per annum by the year 2050 respectively, the per capita income will increase by 5.5% per annum, which in turn will upsurge the demand for food (Central Water Commission 2017). For an agriculture precedent country like India, the farmers have a political priority to use water for safeguarding food production, which will lead to increased water demand.

Apart from uneven distribution, the existing surface and groundwater sources are also facing an availability crisis. The surface water level is declining and the rivers are drying up and facing excessive water pollution (NITI Aayog 2018). The level of water storage has reduced in some of the major rivers as compared to the average of the last decade (Figure 2). The River Ganga has observed a dip of 0.24% in the water level, which is unfavorable to sustain the rising water demand (Central Water Commission 2021b). Most of the rivers and urban lakes are severely contaminated because of the dumping of industrial and municipal waste (Dutta & Bhaskar 2018). Declining trends are being observed in the groundwater, which is responsible for 45% of irrigation water and 80% of the drinking water (Figure 3). In the last 10 years, over-exploitation and unregulated use, especially in industries, have led to a more than 50% dip in groundwater level which has created water shortage, predominantly in the agriculture-intensive belts (Central Ground Water Board 2017). The situation has become particularly serious in the Northwestern region, where the levels of groundwater have fallen from 8 m to 16 m below the ground level. For example, Punjab and Haryana states have only 20.32 BCM and 9.79 BCM of annual groundwater availability respectively and they extract 34.88 BCM and 13.05 BCM respectively annually (Mohan 2016). Moreover, critical contamination has been detected in 839 of 5,723 groundwater blocks across the country, rendering the water unfit for use (Dutta & Bhaskar 2018).

The research indicates that the available water resources are in poor condition both in terms of quantity and quality. Thus, there is an impending need to deeply understand the availability of water resources and their usage and shift the focus to interventions such as reuse of treated wastewater that would make water usage more efficient and sustainable to avoid declining agricultural outputs and a severe shortage of water.

In the last 15 years, due to the variations in climate, the hydrologic processes and local water resources have been seriously impacted, affecting the rainfall pattern in the country (Shah 2016). The number of rainy days in a year is also reducing presumably due to climate change (United Nations 2018). In the last decade, more than half of the glaciers in the Himalayan mountain ranges have withdrawn due to global warming, affecting water availability (NITI Aayog 2018). Another major issue is the uneven distribution of rainfall in the spatio-temporal region, ranging from as low as 150 mm/year in the north-western region to as high as 2,500 mm/year in the north-eastern region, which affects the surface and groundwater recharge (NITI Aayog 2018). India, being an agrarian economy, is particularly vulnerable to these changes, which further hampers the planning and management of water sources.

Urban India is growing at a fast pace. It is expected that by the year 2050, the population of urban residents will have an increase of approximately 404 million (Chauhan et al. 2016). Rapid urban growth is interrelated with increasing energy and water demands. Growing water consumption patterns, mainly in the chemical, pharmaceutical, power plants, food, and textile industries, has created a challenge for existing water resources. It is visible in industrial metropolitans such as Chennai, Bengaluru, and New Delhi, where a shortage in water availability has given rise to the cost of freshwater production and industrial water charges (UNICEF, FAO, and South Asia Consortium for Interdisciplinary Water Resources Studies 2013). Due to these reasons, allocating water for industries is at least a priority in India, thus impacting industrial development. Treated wastewater, being a constant source of water supply, can aid in fulfilling the water requirement.

The discussed factors strongly support the need to investigate alternative sources to support and augment the existing water supplies. To address the situation of water shortage and promote water reuse in India, it is necessary to analyze the challenges involved in the designing and implementation of these systems.

Even though water reuse is an attractive and sustainable option, there are many challenges associated with converting this concept into an application. Globally, these barriers are often related to lack of high treatment technologies, public health, and risks, institutional and economic deficiencies, and so on (Duong & Saphores 2015). Depending on the local conditions and community perspectives, these challenges have been handled differently in various countries. In North America, the technical and regulatory barriers have been managed with the support of stringent wastewater disposal and water reuse regulations both at the state and national levels. The issues related to community satisfaction have been handled by implementing strong public awareness programs and encouraging users by providing financial incentives (Freedman et al. 2014; Lautze et al. 2014; Al-Ali & Filion 2015). In Australia, community resistance had emerged as the major barrier in water reuse implementation, which has been eliminated with the help of effective marketing schemes, robust governmental initiatives, public campaigns and price incentives to users. The biggest advantage being that the water and wastewater is handled by the same utility in the country, thus eliminating any coordination gap (Freedman et al. 2014; Adapa et al. 2016).

In Europe, the reorientation of water governance towards integrated management, establishment of guidelines for water reuse, and development of cost recovery reuse business models have led to an increase in wastewater reuse programs in most of the countries. In Windhoek, Namibia, the DPR project is working successfully due to effective planning and management strategies at the governmental level. Organizing water management campaigns with media visibility and risk perception and continuous monitoring has helped in removing health-related public concerns. In other parts of Africa, except South Africa, the challenges include low collection and treatment facilities, lack of public education and awareness, overlooking of the operation and maintenance costs of treatment and conveyance, and so on. In the Middle East region, except for Israel, Kuwait and Jordan, the challenges are similar.

Israel is a global leader in water reuse owing to highly efficient treatment technologies and the formulation of strict reuse legislations followed by obtaining public trust. Jordan follows restricted water source choice for the users by adopting blending of treated water at the conventional water sources. China has observed enhanced reclaimed water use in recent years mainly due to strong governmental intervention in terms of issuance of a technical guide for reusing municipal wastewater, mandates to implement reuse policies, penalizing the defaulters, strengthening of public awareness, and building cooperation among the stakeholders (Lyu et al. 2016).

India is still in the starting phase of intended wastewater reclamation and reuse. Thus, it is imperative to ascertain the barriers in planning and implementation of water reuse projects, which can serve as an input for the feasibility and management studies to make water reuse a success in the country and achieve sustainability.

Various technical issues act as barriers to water reuse. Based on the consideration of several case studies, the key factors limiting the applicability of water reuse are discussed in this section.

There is a backlog in terms of laying affordable and scalable wastewater collection and treatment networks (Table 4). In most of the cities, communities have developed without sewerage infrastructure, and sewers are already built, swarming and haphazard areas create a difficult and onerous task. The amount of wastewater generated is approximately 62,000 million litres per day (MLD) which is 60–70% more than the treated volume (Central Pollution Control Board 2015). Moreover, the research shows that the existing sewerage infrastructure in most of the cities is characterized by obsolete and faulty sewer networks, insufficient treatment capacity, sub-optimal capacity utilization, non-conforming of quality standards, and so on, resulting in a limited quantity of wastewater available for reuse. Though there has been an increase in the treatment capacity from 11787 MLD in 2009 to 26,066.31 MLD in 2018 (Sewage Treatment Market in India 2018; Segment analysis outlook and opportunities 2018), a large gap exists between the generated and treated wastewater volume that needs to be bridged.

Governmental efforts to reduce water pollution remain ineffective due to the mixing of large quantities of wastewater with surface and groundwater. In most cities, the wastewater generated is a mixture of both domestic and industrial wastewater, which causes difficulty in providing risk mitigation and reuse recommendations. The lack of effective and low-cost treatment technologies further obstructs the reuse process. There is a scarcity of reliable treatment trains for the removal of undesirable contaminants along with restricted accessibility to trained personnel familiar with the working of water reuse systems.

Mekala et al. (2008) studied the reuse of wastewater for irrigation in Hyderabad city, near Musi River. Evidence shows that the production of rice has reduced by 40–50% and the groundwater in the area was not fit for consumption due to the presence of high salt levels. Further, high groundwater tables and waterlogging due to the blocking of pores in soil have also been observed. In New Delhi, Jamwal & Mittal (2010) investigated the effluent quality of 16 sewage treatment plants (STPs) to explore reuse and found that though the reuse potential exists for horticulture and industrial uses, there is a need for tertiary treatment of secondary effluent, which is currently unavailable. Starkl et al. (2015) studied the feasibility of reuse systems for irrigation in Hyderabad. It was found that the unlawful practice of using untreated sewage for irrigation and lack of quality monitoring is causing health risks. Governmental support in terms of treatment train setup and other incentives would be instrumental in the successful implementation of reuse projects. In Varanasi city, the Ministry of Agriculture has released notification against the use of treated wastewater even for agricultural purposes as the intermixing of municipal wastewater with effluent from household textile industries has contaminated the water with heavy metals (Notification for not using treated wastewater for agriculture Uttar Pradesh Jal Nigam 2015). The studies indicate that at-source water quality and the designated treatment process play a major role in planning for water reuse. Also, there is a lack of quality and performance monitoring plans at reclamation facilities.

Studies indicate that many a time, despite having the required infrastructure, the projects fail due to institutional barriers associated with planned water reuse. The major institutional challenges encountered in India are discussed in this section.

There is a lack of coordination between the authorities in charge of water supply and wastewater management. This power fragmentation causes delays in strategic implementation, interdepartmental disagreements, resource division, and other complexities that make the resulting water reuse project far more complex than required (Lautze et al. 2014). Limited synchronization affects the data availability in terms of coverage, robustness, and efficiency. A detailed record is not available for several critical sectors, mainly water use patterns, discharge rate and sources of wastewater, social acceptability, and willingness. Moreover, the existing data is usually of inferior quality, inconsistent, and unreliable due to the use of obsolete methodologies. Such issues have a direct impact on policy formulation, infrastructure management and limit research and innovation related to water reuse (NITI Aayog 2018).

There are no strict policies or guidelines regarding the reuse of treated wastewater. While policy and guiding frameworks in India acknowledge the need for reuse of treated wastewater, the design and operation and maintenance aspects for the management of such projects and tariff structure for the sale of treated water for various applications have not been taken up aggressively. Lack of institutional and legal structure further affects the foundation of water reuse systems.

Shastri & Raval (2012) suggest that recycling and reuse is the most preferred option for the Pune city which can only be achieved by mandating all the Urban Local Bodies to recycle and reuse at least 50% of the sewage produced by them. Saldías et al. (2015) studied the institutional and policy requirements for wastewater reuse in agriculture in Hyderabad and observed an increase in health and environmental risks due to informal and unplanned reuse. It was suggested that proper planning at all the authoritative levels is needed. Sharma et al. (2016) found that the lack of policies and coordination among the stakeholders has led to the inadequacy of resources to develop an integrated water management system for Shimla city. Strict guidelines leading to water reuse may reduce the freshwater demand and result in economic gain. Research done by Drangert & Sharatchandra (2017) in Bengaluru indicates that a poorly developed and implemented water management system is leading to water scarcity in the city and there is a need for planned water reuse supported by strict guidelines.

One of the key reuse barriers is public perception. Generally, public education and awareness are neglected, which leads to the foundering of water reuse projects. Moreover, the lack of community trust in authorities conjoined with incongruous treatment technologies further aggravates the citizens’ concerns to reuse treated wastewater because of potential health risks. Literature review suggests that public acceptance is one of the major obstacles in reusing treated wastewater, especially for domestic purposes (Duong & Saphores 2015). Throughout the world, water reuse has faced opposition due to psychological repulsion, which arises out of lack of public knowledge (Baghapour et al. 2017).

Devi & Samad (2008) attempted to determine the extent and the nature of the gap between the formal and informal practices of wastewater disposal, its use, and the associated impacts in Hyderabad. It was found that there is a wide gap between the two due to the failure to attend to socio-economic realities like a poor water and sewerage tariff system and no public education and awareness. This is one of the major reasons responsible for the low usage of treated wastewater in many states. Ravishankar et al. (2018) investigated the willingness of residents to reuse treated wastewater for non-potable purposes in Bengaluru and found that negligence of public education has made it difficult to implement water reuse in the city. The studies clearly indicate that there is a strong need to develop public outreach programs to build awareness and improve the state of water reuse in India. Research shows that it is important to integrate public education and consultation in water reuse planning to gain the popularity of the water reuse programs (Cleaves 2005).

Another challenge faced while planning for water reuse is to ensure the financial sustainability of the project. Investment costs, construction costs, and operation and maintenance costs of treatment and distribution (pipe, pumping, storage, etc.), and the sale price of reclaimed water play a pivotal role in the designing of water reuse systems. In India, the investment costs are quite high. Moreover, access to funding and desired expertise for the establishment of such projects is also limited. Market viability acts as another barrier to water reuse. Many reuse programs worldwide have failed due to a lack of end-users (Duong & Saphores 2015). Moreover, there is sparse data available regarding the financial feasibility analysis of such projects, which obstructs proper planning and management.

Rai (2011) investigated the factors affecting urban water demand for reuse in residential, commercial, and industrial sectors for Delhi. It has been observed that price is the major contributing factor to the need for treated wastewater. It can be inferred that to increase the demand, it is important to ensure that the price of reclaimed water is lesser compared to alternative sources without compromising the quality. Proficient planning and designing an appropriate pricing policy, considering monetized as well as non-monetized costs and benefits, is essential to stimulate the growth of water reuse. Studies show that many countries have overcome the economic constraints in water reuse by applying the full cost recovery principle; that is, allowing the project to sustain commercially along with providing social and monetary benefits (Bixio et al. 2006).

Being an acutely water-stressed region, India recognizes the need to promote recycling and reuse of treated water. Though the largest reuse potential resides in agriculture, treated wastewater finds its applications in various fields as well, such as, commercial, industrial, environmental and recreational, groundwater recharge, and augmentation of potable water supplies. In India, the concept of reuse of water has existed for a considerable time. The first case of wastewater reuse in India was reported in 1964–65, wherein cost reduction had been achieved by reusing wastewater (Arceivala & Mohanrao 1969). The use of wastewater for irrigation is functional since 1970. Treated wastewater is used for various activities such as farm forestry, horticulture, toilet flushing, industrial use (cooling towers), pisciculture and indirect uses, and so on (National Workshop on Wastewater Recycle and Reuse 2014). Yet the notion of reuse does not gather mainstream appeal for several reasons such as the lack of supportive regulation to encourage and incentivize water reuse projects. Though the level of wastewater reclamation and reuse is rather low in India, there is a large potential for the same (Arceivala & Asolekar 2015).

Nowadays, there are quite a few functioning water reuse projects in cities, such as Chennai, Nagpur, Vadodra, Kohlapur, Chandigarh, Ahmedabad, Indore, Tuticorin, Vishakhapatnam, Kolkota, Nashik, Aurangabad, and so on (Ravikumar et al. 2014). Some of the water reuse experiences have been discussed in Table 5.

It is evident from the discussed success stories that the reuse of treated wastewater provides an opportunity to meet the increasing water demand along with economic gains (in terms of cost reduction, employment generation, etc.) and environmental restoration.

Wastewater reuse in India is expected to expand greatly in the future owing to various opportunities in the water and wastewater sectors. The focus is shifting to find solutions for the effective management of water resources.

According to several studies, globally recognized water companies, namely, Veolia Water, Wabag, Xylem, Black and Veatch, Nalco, and so on have invested in several joint projects in India, which may help in reducing the treatment costs and progression of wastewater treatment technologies, thereby enhancing the quality of water for reuse (European Business and Technology center 2011). A shift from conventional centralized treatment technologies like activated sludge process, UASB, oxidation ponds, waste stabilization ponds, and so on to emerging natural decentralized technologies like phytoremediation, biosorption, and so on is being encouraged in India. According to studies, these technologies are cost-effective (lesser capital and operation and maintenance costs), have high-performance efficiency, and present an integrated view of wastewater management and reuse, offering social, economic, and environmental sustainability (Matto et al. 2014).

Several industries have taken initiatives to reuse treated wastewater, such as Saint Gobain Glass, Wipro, Shree Cement, Godrej Pvt. Ltd, and so on, which has resulted in significant freshwater savings. The Mahindra & Mahindra Auto plant in Nashik meets up to 30% of its total water consumption through recycled water (Hingorani 2011). Successful implementation of projects, like the Nagpur Tertiary Treatment Reverse Osmosis (TTRO) plant, and the Bamroli TTRO suggests that industries are shifting towards investing in this sector and these projects can be viable if designed adequately and supported by strict policies (e.g. State Policy for Wastewater Reuse for Jammu & Kashmir 2017).

Many initiatives are being taken by the government, at various governmental levels to promote the reuse of treated water in form of waste management programs, budget expansion, and encouraging public-private partnerships (PPP), and so on. Maharashtra Generation Company (MAHAGENCO) and Nagpur Municipal Corporation (NMC) have invested in a 110 MLD reuse project where secondary treated water is further treated and used as industrial cooling water leading to economic gains and significant water savings (Never 2016). More than 20 PPP models are successfully working in Tamil Nadu, Gujarat, Andhra Pradesh, West Bengal, and Maharashtra. Moreover, state government policies in Tamil Nadu, Maharashtra, Andhra Pradesh, and so on also support water reuse by mandating zero liquid discharge (ZLD) (Beca et al. 2015). Individual municipal water reuse policies have also been developed in the states of Gujarat, Jammu & Kashmir, and so on (State policy for Wastewater Reuse for Jammu & Kashmir 2017; ‘Policy for reuse of treated wastewater’ 2018.).

Policies and guidelines related to wastewater management and its recycling and reuse have been formulated and recommended by various water-related sectors of the Government of India (Figure 4) (Freedman et al. 2014; ‘National workshop on wastewater recycle and reuse’ 2014; Notification for not using treated wastewater for agriculture 2015; Ministry of Power 2016; Recycling and Reuse of Treated Wastewater in Urban India: A proposed advisory and guidance document 2016; Shah 2016; Alley 2018).

Most of these guidelines endorse a minimum of 20% of water reuse, especially in industries. The focus is on the formulation of by-laws and the development of infrastructure and financial models for water reuse. The CPHEEO guidance manual on Sewerage and Sewage Treatment Systems 2013 specifies the water quality guidelines for the intended use and case studies. Ministry of Power (2016) by the Ministry of Power has made the reuse of treated wastewater compulsory for all TPPs located within a radius of 50 km from any city.

Namami Gange has been contributing actively to promote water reuse under the NMCG program since 2014. It encourages water reuse by supporting PPP models, MoUs with ministries to reuse wastewater, achieving ZLD in industries, and drafting of reuse policies in Ganga basin states, and so on.

Reusing treated wastewater has many benefits such as the recovery of nutrients like phosphorus, potassium, etc. Studies show that the use of treated wastewater in irrigation increases crop yield by 30–40%, decreases the use of fertilizers, and increases farmers’ income by 30% leading to economic benefits of approximately INR 17,000/ha (Ravikumar et al. 2014). Irrigational reuse can further reduce the load on groundwater, thereby reducing the energy needs and GHGs (Recycling and Reuse of Treated Wastewater in Urban India: A proposed advisory and guidance document 2016). It may provide a chance to tap into carbon credits for additional profits. Water reuse can also help in the environmental restoration of the water streams by reducing the pollutant load and maintaining aquatic wildlife.

A study reports that collection and adequate treatment of 80% of urban wastewater could satisfy 75% of the industrial demand and almost 25% of drinking demand. Another study shows that the collection and treatment of 38,000 MLD of wastewater annually in urban areas could generate 14 BCM of water, which may be used to irrigate an area of about 3 Million hectares (Mha) (Recycling and Reuse of Treated Wastewater in Urban India: A proposed advisory and guidance document 2016).

The reuse of treated wastewater guarantees a secure and dependable source of water. As per a study conducted by George et al. (2009) in Hyderabad, recycling of 120 MCM/year of wastewater annually combined with reuse of 90 Million cubic meters (MCM) of runoff and water harvesting would be enough to support the expected population growth @ 2.5% by the year 2031. Another study performed by Barringer (2014) reports that 42% of the municipal water demand in Pune city might be met if treated wastewater is reused. Ghosh et al. (2019) evaluated the water supply-demand management in Delhi and concluded that, if wastewater is properly collected and treated, there is a reuse potential of 78.2%.

India is undergoing economic development, leading to high water demand. The inability of existing water resources to suffice the anticipated demand has fueled the need for alternative water augmentation strategies. The study shows that water reuse is the most suitable alternative as it is environmentally and economically beneficial with the least resource consumption. Though many opportunities encourage the use of treated wastewater, several challenges obstruct the successful working of such projects. Informal and unplanned reuse often results in increased health and environmental risks. Lack of public awareness, market viability, and administrative cooperation are other important challenges currently faced by the country. Studies designate that proper pre-feasibility studies are required to overcome the social, political, technical, and financial challenges associated with wastewater reuse.

Wastewater reclamation and reuse are still emerging in India. Even though policies and legal advisories exist centrally, but the regulation and implementation at community levels need boosting, planning, awareness, and implementation. To achieve sustainable reuse, it is required to create funding sources and technical infrastructure which may help in planned collection and adequate treatment of wastewater. The planning and operating aspects for the management of wastewater and its reuse have to be explored. The study concludes that a water reuse policy with targets, regulations, and specific guidelines must be formed at the national level at the earliest. Regulatory norms for risk management and the provision of safe water are also needed to build the integrity of treated water as a reliable alternative and increase its market viability. Promotion of ongoing water reuse efforts is essential, especially in the new areas where water supply plans are to be developed. It can be achieved through raising awareness of the value of wastewater amongst the political authorities, benefactors, municipal societies, and other stakeholders. Public-private partnership models to support water reuse projects and inclusion of financial as well as non-financial incentives for the users, especially industries, may be beneficial.

The findings from the study may be used for a pre-feasibility assessment of implementing water reuse in India. The specific challenges may be studied in detail and dealt with to expand the water availability situation in the country. The research can also be extended for the development of public awareness programs to address social concerns. The inclusion of social and institutional surveys may further aid in the formulation of policies and regulations related to water reuse. More studies can be performed on the various financial and technological water reuse models and their progress in the country.

All relevant data are included in the paper or its Supplementary Information.