A modelling approach to assess wastewater reuse potential for Delhi city

Water demand is increasing globally at a faster pace as compared with the availability of water supply. The majority of cities throughout the world including India are facing water scarcity. More than 54% of India is under water stress and the per capita water availability is decreasing rapidly (NITI Aayog 2018). Uneven spatio-temporal variation of available water resources coupled with low rainfall and frequent extreme events because of climate change, population growth, industrialization, urbanization, etc. is further leading to the depletion of water resources. Moreover, unsustainable disposal practices are further causing pollution of surface and groundwater. In India, 351 of 530 surface water bodies and 839 of 5,723 groundwater blocks are severely contaminated due to the dumping of industrial, domestic and municipal waste (Central Pollution Control Board 2015). To keep up with the growing water demand, there is a need to adopt alternative, innovative water management strategies for the augmentation of existing water resources.

With an increasing freshwater availability crisis, wastewater is viewed as a sustainable and valuable resource in many parts of the world. In the past decade, the reuse of treated wastewater has gained considerable attention worldwide, especially in water-scarce regions. Treated wastewater can be regarded as an economical, reliable and assured source of water supply along with reducing the pollution load on water bodies.

Planned and unplanned reuse of wastewater for various contact and non-contact activities is practised in many parts of the world. Worldwide, about 5% of the total wastewater generated annually is successfully reused annually mainly for agricultural and industrial applications, with notable contributions from countries like Israel, Singapore, Namibia, Australia, Germany, Japan, China etc. (Sato et al. 2013).

Studies show that the largest potential for reuse exists in agricultural irrigation. In many European countries, viz. France, Greece, Cyprus, Spain, Italy and Portugal etc. almost three-quarters of the treated wastewater is used for irrigation (Angelakis & Gikas 2014). Israel is a pioneer in agricultural reuse with 75% of treated wastewater being used in irrigation (Kamizoulis et al. 2003). The industrial applications of treated water including the use in boiler feed, cooling towers, and for washing as process water, etc. are quite popular in many parts of the world such as Japan, USA, Mexico, South Africa etc. Reclaimed water is widely used for enhancement of urban streams in Japan and golf course irrigation and environmental restoration in Canada. Hong Kong expanded its water supply portfolio by using seawater for toilet flushing and reusing wastewater (Duong & Saphores 2015). Successful cases of reuse of wastewater in environmental recreation have been reported in Greece, Italy and many parts of Australia (Angelakis & Durham 2008).

Due to technological and scientific advancements in treatment techniques, the most significant emerging trend in the field of water reuse, especially in large metropolitan cities with very high water demand, is potable reuse (Angelakis et al. 2018). Successful potable reuse cases have been observed in many parts of the world. For example, groundwater injection (IPR) in Orange County, California, USA, and several ongoing IPR projects in Brisbane, Melbourne, Australia etc. (Drewes & Horstmeyer 2016; Angelakis et al. 2018). Since 1968, DPR of highly treated reclaimed water has been executed in the city of Windhoek, Namibia (Lautze et al. 2014). The effective operation of the NEWater reclamation plant in Singapore, practising IPR, since 2003 is a notable initiative in promotion of potable reuse of wastewater (Duong & Saphores 2015).

Despite the use of treated wastewater being a primeval practice, it has not been appreciated and implemented fully due to the lack of adequate treatment and attention to possible health and environmental risks (Lam et al. 2017). Level of wastewater treatment is directly related to the intended water quality for a particular use and plays a significant role in ensuring its effective reuse. While reusing treated wastewater, it is very important to maintain the required quality standards to avoid health and environmental risks. The treatment technologies may vary from region to region depending on the quality of effluent and available technologies. Regulatory frameworks and water-quality guidelines related to safe reuse of treated wastewater for various applications have been developed by many countries and organizations, such as the USEPA Guidelines, 2012, WHO Guidelines, 2006, etc. Studies conducted in several countries have revealed that health-risk elimination, public acceptance and institutional cooperation are essential for successful implementation of water reuse projects (Wester et al. 2015; Massoud et al. 2018).

In India, unplanned and unregulated use of wastewater for irrigation has been practised for a long time but the interest in proper planning and management strategies for reuse of treated wastewater has only evolved recently. Reuse of treated wastewater is quite conspicuous in industry, but in municipal and other sectors it is still in its early stages. Many wastewater reuse projects have been implemented in several cities, namely Chennai, Pune, Delhi, Mumbai, Hyderabad, Jaipur, Nagpur, Bengaluru, Surat, etc., mainly for industrial and other non-potable applications (CPHEEO Manual 2013). However, the integration of the concept at the planning stage of water supply scheme development is still missing due to a lack of comprehensive policy at national and state levels.

Realizing the high stress on existing water resources and their distribution, treated wastewater has been acknowledged by the Government of India as a valuable resource. Several schemes and policies have been launched at administrative levels to support and encourage the reuse of treated wastewater. Yet the idea of the reuse of treated wastewater does not garner mainstream appeal, for numerous reasons. There are several planning and management issues related to the development and implementation of wastewater reuse projects, including lack of required infrastructure, poorly maintained sewerage networks, site conditions, financial cost–benefit analysis, social acceptance, public health, and stakeholder coordination (Central Pollution Control Board 2015; Ravishankar et al. 2018). Due to the intermixing of sewage and industrial effluents, maintaining optimum quality of treated wastewater as per the application criteria also becomes challenging. Therefore, effective implementation of water reuse projects requires an integrated approach considering technical feasibility, economic viability and public acceptance along with the development of a regulatory policy framework which will result in an improved water availability scenario in the country.

There has been substantial difficulty in estimating the potential quantum of wastewater reuse due to limitations of both knowledge and data (Chu et al. 2004). The appropriate scale of water reclamation is a critical design variable (Lee et al. 2018). Quantitative estimation of the amount of water required by various sectors, such as industrial, agricultural, commercial, municipal, etc., becomes difficult due to the non-availability of volumetric data, especially in developing countries (NITI Aayog 2018). The aim of the paper is to develop a modelling approach for designing a framework to find the amount of water available for reuse under various physical constraints based on water demand by different sectors and the total amount of wastewater generated with the help of a linear programming model. The study aims to establish the wastewater reuse potential from the functional point of view considering various governing parameters. It is an attempt to provide an in-depth analysis of wastewater reuse planning taking into consideration the constraints that affect reuse schemes to help water managers in estimating water needs for various applications.

The study area selected for this analysis is one of the megacities and the capital of India, i.e. Delhi. Situated on the western banks of the river Yamuna, which divides the city into East Delhi, known as Trans Yamuna Area, and New Delhi, the city extends from 28°23′17″N to 28°53′00″N and 76°50′24″E to 77°20′37″E. Inhabited by a population of 20 million, spread over 1,483 km², the city mostly consists of residential zones with significant commercial and industrial spaces. Delhi has the highest population density amongst all States/Union Territories in the country. The city is characterized by a semi-arid climate with harsh summers and severe winters and low to moderate rainfall. Water supply and wastewater management are handled by an independent government body, Delhi Jal Board (DJB). DJB is responsible for ensuring adequate water supply and the establishment of sewerage facilities for management and safe disposal of wastewater.

Delhi is currently experiencing acute pressure on the existing water resources. There is an ever-increasing demand for water due to the rapidly growing population and urbanization. The average water consumption in Delhi is estimated to be as high as 240 litres per capita per day (L/cap.d) in some areas whereas it is much below 100 L/cap.d in other areas (rainwaterharvesting.org 2003). In Delhi, in addition to water from nearby canal networks (eastern Yamuna Canal, upper Ganga Canal, western Yamuna Canal, etc. (Figure 1(a)), the extraction of groundwater is done for water supply. It has been estimated that the city will run out of groundwater by the year 2020 (NITI Aayog 2018). Moreover, the pollution load in the Yamuna River has increased because of the disposal of sewage and industrial effluents. The background information required for model development has been collected for the study area (Table 1).

DJB currently supplies 3.4 Mm³/d of water for drinking and other purposes against an expected demand of 4.5 Mm³/d (as per the Central Public Health & Environmental Engineering Organisation (CPHEEO) manual) (CPHEEO 2012). The sewerage network is spread throughout the city (Figure 1(b)), treating approximately 68% of the total wastewater generated. Realizing the importance of wastewater as a valuable resource, DJB has taken several initiatives for using treated effluent in various applications, mainly for irrigation and horticulture (Economic Survey of Delhi 2019). At present, approximately 0.34 Mm³/d of treated wastewater is being reused in Delhi. Considering the current and anticipated water demands, there is a need to explore the possibility of reusing treated effluent on a larger scale.

The first step is to develop an urban water balance to analyze the current water availability situation in the city. The water balance is estimated to find the movement of water in the city by assessing the inflow and outflow components (Figure 2). The analysis is done to support the argument that there is a shortage of water in the city and emphasize the need for reusing wastewater.

In order to plan for any reuse activity, it is most important to find the accurate amount of wastewater available for use from various sources. Theoretically, treated wastewater may be used for any application. But, considering the social barriers and lack of appropriate treatment, the use of treated wastewater has been restricted to non-contact activities in most countries, especially in developing countries like India, with a few exceptions where treated wastewater is used for drinking purposes, e.g. Windhoek (Namibia), Orange County (California), and Singapore (Angelakis et al. 2018). Thus, along with the availability of wastewater, water demand for various sectors also needs to be known to find the reuse potential.

Various researchers have performed studies related to the estimation of the reuse potential of treated wastewater. In most of the studies, the approach used for the estimation of water demand for various sectors is not clear (Angelakis & Diamadopoulos 1995; Tselentis & Alexopoulou 1996; Barbagallo et al. 2003; Kamizoulis Bahri et al. 2003). Jödicke et al. (2001) used mixed linear integer programming to generate optimal wastewater reuse designs considering data on location and type of reuse, current water demand, etc. Zhang (2004) used network flow optimization to determine the water demand for various sectors and minimize the overall cost. Hochstrat et al. (2005) presented a model for the estimation of water reuse potential in Europe using a mass balance approach between supply and use of reclaimed wastewater. Chu et al. (2004) and Yang & Abbaspour (2007) used a linear programming model to estimate the demand for different reuse scenarios considering wastewater charges and reuse prices for different scenarios. Adewumi et al. (2009) developed a mathematical model for implementing treated effluent reuse systems in South Africa by using the global optimization method minimizing a non-linear objective subject to a set of constraints.

In the current study, a linear optimization model has been developed to estimate the reuse potential of treated wastewater. The approach used is a modified version of the model developed by Chu et al. (2004) and Yang & Abbaspour (2007) as it provides a useful framework for potential estimation under various constraints and local conditions and the methods used for demand and availability estimation are quite clear.

The objective function is determined considering the demand and availability constraints as depicted by the algorithm in Figure 3. A linear programming problem (LPP) is formulated. The maximum amount of treated wastewater available for reuse is estimated on the basis of the demand by various sectors and the amount of wastewater available for reuse given by Equations (1)–(6).

• where A_i and A_l = crop growth area and green area respectively (ha)

• W_c and W_g = crop water and green water requirement respectively (m³/ha).

• where X_i = production or generation capacity (kW-h or tons or units depending on industry)

• W_i = Water required per unit production or generation (m³/kW-h or m³/ton or m³/unit).

• where N_h = number of households

• W_h = water required for toilet flushing per household (m³)

• W_o = water required for other activities (m³).

• where N_c = number of commercial entities

• W_c = water required for toilet flushing per entity (m³)

• W_o = water required for other activities (m³).

• where W_d = total water demand (m³).

• where r₁ = ratio of municipal wastewater with the possibility of reuse (%)

• W_M = amount of municipal wastewater generated (m³)

• r₂ = ratio of industrial wastewater with the possibility of reuse (%)

• W_I = amount of industrial wastewater generated (m³).

The objective is to maximize the demand for reuse subject to constraints related to water availability and demand for different sectors (Equations (7)–(17)). The linear programming problem is formulated and three alternatives have been considered on the basis of the available wastewater and the solutions are found with the help of the freely available R software, widely used for statistical and data analysis.

Alternative I: Collection and treatment of 100% of the total volume of wastewater generated with reuse possibility ratio as 80% and 70% for municipal and industrial wastewater respectively.

Alternative II: Collection and treatment of 68% of the total volume of wastewater generated, i.e. the present scenario, with reuse possibility ratio as 80% and 70% for municipal and industrial wastewater respectively.

The urban water balance has been calculated for the city considering the net inflows and outflows (Table 2). The result indicates that at present, there is a water deficit in the city. According to the DJB, approximately 45% water is lost in the distribution system through leakage and for other reasons. The water losses can be assumed to be one of the major reasons contributing to the water deficit followed by high amounts of runoff caused by the increase in impervious surfaces due to urbanization. As a result of growing water demand and depleting water resources, it can be said that the deficit is bound to increase with time. The analysis promotes the need to reuse treated wastewater for Delhi city.

Several methods and techniques have been used to collect and analyze the background information used for fulfilling the objective of the study (Table 3). For calculation of agricultural demand, the commercially available tool CROPWAT 8.0 developed by the Food and Agriculture Organization (FAO) has been used. Landscape and commercial demands have been calculated using the data collected from DJB. The value of industrial demand has been collected from the literature survey. For domestic demand, a survey has been conducted to find the amount of water required for non-contact activities and the same has been compared with the values obtained from the literature (Shaban & Sharma 2007).

The quantity of potential reuse has been estimated by solving the linear programming problem considering the sector-wise water demand and the available amount of municipal and industrial wastewater.

Based on the above three cases, input variables have been fed into Equations (7)–(17) and the results have been obtained.

As shown in Table 4, the optimal quantity of wastewater reuse derived from the LP model for alternative I is 969 Mm³/yr annually. It can be observed that if 100% collection and treatment of the wastewater is achieved, there is a possibility of 100% reuse of wastewater with complete reuse in agriculture and landscape irrigation, reuse potential of 92% in domestic activities like toilet flushing, house cleaning, car washing, etc. and approximately 31% in industry. The low potential in industrial reuse might be due to the lack of appropriate collection and treatment of industrial wastewater and lack of agreements and institutional support. The reuse for commercial and fire demand is zero, which may be due to the absence of economic viability and infrastructural feasibility for the same. For alternatives II and III, the maximum quantities for reuse are found to be 827 Mm³/yr and 680 Mm³/yr with 100% reuse potential in irrigation and 31% in industry. The reuse potential for domestic purposes is 60% and 28% respectively, indicating a lesser quantity of total wastewater available for reuse because of the wide gap in wastewater generation and collection and treatment. The current amount of treated wastewater reuse in Delhi is 148 Mm³/yr. It can be observed that the results obtained from the model are quite high as compared with the current reuse scenario. It can be said that the status of the reuse of treated wastewater might be improved with the development of infrastructure and proper management and regulation of the generated wastewater.

Wastewater reuse is a sustainable option for integrated water resources management to deal with water stress. The study provides a framework for evaluating the potential quantity of treated wastewater for reuse in various sectors under demand and availability constraints. A linear programming model has been used to quantify the potential. Different alternatives have been considered and the values have been estimated under several constraints. The results of the linear programming model suggest that the agriculture and landscape irrigation sector has the maximum possibility of wastewater reuse. The current reuse of wastewater in Delhi is limited mostly to the horticulture and irrigational sectors. Reuse in industry is also practised but it has certain limitations. The results indicate that it would be beneficial to explore the domestic sector for the possibility of reuse at least for non-contact applications.

In order to effectively implement water reuse systems, technical feasibility and economic viability have to be ensured. Political and administrative willingness plays a major role in the success of water reuse projects. Moreover, there is a strong need to overcome the barrier of social acceptance through proper awareness and education, which can be supported through a sound regulatory framework with strict policies, focusing on encouraging the reuse of treated wastewater in an integrated water management system. Considering the growing water stress and the environmental issues in Delhi, there is a strong need for such a shift from a linear to a circular loop.

Lastly, the figures presented in this research may be seen as approximations due to uncertainties in demand and availability constraints, which may change from time to time. However, the study has developed a framework that may be used for evaluating the potential reuse of treated wastewater under various constraints. The results may further improve by considering the economic and other constraints along with the uncertainties encountered in the future. Such models would aid in the planned and sustainable reuse of treated wastewater and promote economically and environmentally sound practices.

The first author is thankful to the Department of Hydro and Renewable Energy, IIT Roorkee for giving technical support to carry out the research at the Indian Institute of Technology (IIT), Roorkee.