Interceptor sewer for abatement of pollution in surface water

Due to rapid urbanization and the inflow of migrants to the urban areas for earning their livelihood, environmental problems are increasingly severe across Asia, especially in India. It is attributed to the rising human population, urbanization, and economic activities. Unplanned population growth and poor wastewater management resulted in reduced water resources in a country like India ((Nallathiga 2008) and completely outpaced the augmentation of civic infrastructure. Physical and financial constraints in providing civic infrastructure like water, power supply sanitation, sewage treatment, housing, etc., at the same pace at which growth and in-migration occur, resulted in the discharge of wastewater in the nearby water bodies, i.e., ponds, drains, rivers, etc.

One of the major factors contributing to the pollution in water bodies and low capacity utilization of sewerage treatment plants is the absence of a sewerage network in large unplanned colonies of cities. For example, in Delhi, around 55% population lives in slum areas and is deprived of sewerage facilities. Providing sewerage infrastructures in these congested areas, which have developed in an unplanned manner over a period of time, is also highly complicated and difficult in terms of feasibility, legality, and time-consuming. Hence, wastewater generated in these colonies is discharged untreated into nearby waterbodies through drains.

The population of Yamuna basins' towns and cities was estimated to be 118.34 million in the year 2018. It resulted in huge concretization, reduction in green areas, size and the number of water bodies, which were the source of water to meet the demand of the urban population (Chandramouli & GR 2011; Kumar et al. 2020) Along the banks of rivers, there are many illegal colonies within floodplains that discharge untreated waste and fecal residues causing pollution in river water (Supplementary material, Figure S1a, b). The embankments of all smaller and major drains have also been encroached upon, wherein habitats discharge their domestic waste directly into drains. Most of the households have constructed their toilets on the banks of these drains and are directly discharging their soil wastes into the drain, thus increasing fecal coliform in drain water.

Storm water drains are meant for carrying rainwater only. But, in the absence of sewerage facilities in slums, wastewater is discharged into these drains that ultimately outfalls into waterbodies and contaminates the available fresh water, reducing the water resources. In Delhi, 23 drains carrying wastewater of planned and unplanned colonies discharge into river Yamuna. According to CPCB, the river water quality standard of river Yamuna in the Delhi stretch has become class C. This classification is done according to the best use designated to the surface water (Sharma & Singh 2009). River water must have acceptable quality for the survival of aquatic life, flora, and fauna. As water for processing and drinking is available in abundance, there are substantial industrial activities in those cities situated along the banks of rivers, i.e., manufacture of sugar, distilleries, textile, leader, pulp and paper, food processing, chemicals, pharmaceuticals, oil refineries, thermal power, etc. resulting into the massive discharge of industrial waste (Sharma et al. 2020). The statewise list of industrial cities, their percentage contribution of pollution, and the total industrial discharge generated (Table 2). Despite government regulations for setting up effluent treatment plants at the outlets of such industries, the industrial effluent is bypassed and discharged into nearby drains untreated, which ultimately contaminates river water and further ground water, when it percolates into the ground.

A large amount of solid waste is thrown by the residents into and on the banks of drains, rivers, and ponds. In Delhi, solid waste generation was estimated to be 17,000–25,000 TPD for an anticipated pollution of 22.4 million by 2021. Despite efforts to decompose the waste by composting, incineration, etc., a minimum residue of 4,000–5,000 TPD, i.e., 20%, will require a large area of landfills (MoEF GOI 2001). Hence, there will be a lot of pressure on the municipal bodies to manage even the residue of municipal waste in the future. Without an adequate solid waste management infrastructure in Delhi and less awareness programs, the residents throw their domestic waste in nearby water bodies, i.e., drains and rivers, ultimately increasing the pollution in water bodies.

Since the entire quantity of raw water of river Yamuna is divided into Eastern and Western Yamuna Canals on the upstream side of the Tajewala Barrage, there is hardly any release of fresh water for ecological flow and dilution of pollutants in river Yamuna, on the downstream side of the Tajewala Barrage. On the intervention of the Hon'ble Supreme Court, and later NGT in 2017, the Haryana Government was directed to release 10 Cumec freshwater from the Tajewala Barrage as the ecological flow for the river. However, this quantity of water was so low that it percolated into the ground after traveling some distance, and no adequate freshwater was left to maintain the ecological flow up to the Delhi stretch. The surface and groundwater quality in the river basin has deteriorated alarmingly (Kumar et al. 2020). Whatever freshwater is seen in the Delhi stretch from Palla to Wazirabad Barrage is the quantity of fresh water released by Haryana for the drinking needs of Delhi, as per the memorandum of understanding signed among the basin states of river Yamuna, which is lifted at the Wazirabad Barrage for treatment at Chandrawal, Wazirabad, and Okhla water treatment plants. Hence, there is no freshwater left to be released downstream of the Wazirabad Barrage for further dilution of polluted river water. In the absence of adequate freshwater on the downstream of the Wazirabad Barrage, the quality of river water cannot be improved to the bathing standard quality (Supplementary material, Table S2), despite making all efforts to reduce pollution in the river Yamuna. This is the condition of most of the river basins in India.

On the recommendation of the High-Power Technical Committee in the year 2006, constituted by the Hon'ble Supreme Court comprising of the senior representatives of the Central Water Commission, Central Pollution Control Board, Central Ground Water Board, CPHEEO under the Ministry of Urban Development, GOI, and CEO, Delhi Jal Board being the Convener, the Hon'ble Supreme Court considered the concept of ‘Laying of interceptor sewers along the three major drains, i.e., Najafgarh, Supplementary, and Shahdara, for the abatement of pollution in river Yamuna’. The objective of this concept was to intercept sewage flowing in small subsidiary drains before it enters the major drain and the intercepted sewage to be conveyed to the nearest STPs for proper treatment to ensure that only treated sewage is discharged into these drains and ultimately into rivers. The overall concept was to lay deep and large sized diameter sewer pipes along the major drains (Najafgarh, Supplementary, and Shahdara) to trap wastewater of sub drains at sources; augmentation of the existing capacity of 10-MLD STP at the mouth of the Delhi Gate drain (constructed as a pilot project under the Yamuna Action Plan (YAP)-I) up to 77 MLD; interception of remaining drains into Bela Road, Ring Road, and other nearby trunk sewers which were rehabilitated under the YAP-II, collected wastewater to be pumped to the Okhla STP. If required, additional STPs will be constructed after utilization of the full capacity of the existing plants. Interceptor sewer system concepts were conceived to reduce the environmental impacts on receiving waters by diverting the combined sewer flow from the original sewer outfalls to Wastewater Treatment Works (WwTWs). Interceptor sewer systems comprised large pipes, and pumping stations to pump wastewater into STPs. This concept was earlier tried in the Liverpool area as part of the Mersey Estuary Pollution Alleviation Scheme (MEPAS), which significantly improved the quality of River Mersey, resulting in the comeback of flora and fauna (Thomas et al. 2000, 2004).

The key features of the interceptor sewer concept include trapping untreated sewage from around 190 sub drains of three major drains (Najafgarh, Supplementary, and Shahadra) into a 59 km long interceptor sewer to be laid along these three major drains through the micro tunneling method for the ease of construction, less inconvenience to the nearby people, and no damage to adjacent properties.

The interceptor chambers will be constructed in such a manner that during dry weather flow, the entire wastewater is trapped in the interceptor sewer. But during rain, excess wastewater beyond the capacity of the interceptor sewer is overflown into the major drains directly. It also involves the construction of trash racks to trap floating materials and screens at the intercepting chambers.

The optimization of the design of interceptor sewer systems was adopted as per the design mentioned in (Karovic & Mays 2014). Being a combined sewer system, which carries wastewater and some rainwater from drains, the overflow chamber and its modeling is followed from (Xue et al. 2013). Details on the effectiveness of the best management practices are being followed as given in the study by Kaini et al. (2012). The best management practices are measures that contribute to improve or safeguard the condition of the receiving sub drains (Rathnayake & Tanyimboh 2015). In addition, simplified hydraulic models as in other studies (Meirlaen et al. 2002; Gernaey et al. 2006) are used, which are accurate and are computationally very demanding (Tavakol-Davani et al. 2016).

The Delhi Jal Board conducted the pre-feasibility report before the preparation of a detailed project report (DPR) (EIL 2008) including demarcation of the alignment of the interceptor sewer (IS) along the portion of the major drains to intercept the sub drains and to convey trapped sewage into the nearest STP; A topographical survey along the proposed alignment of the interceptor sewer and up to 1 km of each sub drain; Soil investigation to find the water table, soil profile, and bearing capacities up to 50 m depth of the average ground level; proposed L-section of the interceptor sewer alongside drains; Locations of intercepting chambers and manholes; Measurement of dry weather flow in all the sub drains proposed to be trapped into the interceptor sewer; Collection of wastewater samples from the sub drains and major drains to find out the water quality, and subsequently, decide the best sewage treatment technologies; Encroachment observation, if any, along and across the alignment of the entire interceptor sewer; and Title and ownership of the land coming across the proposed alignment of the interceptor sewer along the major drains.

The methodology adopted in the construction of this entire project was the state-of-art. Modern technology, i.e., micro tunneling has been adopted in laying the sewer line and in the construction of lift stations and interceptor sewer chambers (Supplementary material, Figure S3). The entire length of the interceptor sewer has been laid through the micro tunneling method, causing minimum nuisance to the public and nearby structures. In fact, the micro tunneling method is costlier, but it is a better option in terms of social and environmental conditions, which cannot be worked out in terms of money, as it does not obstruct the movement of traffic and prevent environmental pollution. It also saves the cost of cutting of roads and their restoration. All the lift stations were constructed from top to bottom, which eliminated wider excavation and collapse of trenches. The big interception chamber has been provided with mechanical screening and scrapers to remove the floating material to avoid the chocking of sewer lines. Since most of the construction work of the interception chambers was carried out within the bed of drains, balloon types of cofferdams were used for the diversion of the flow of drains.

To ascertain the timely construction of this project, there was a three-tier monitoring mechanism, i.e., Project Monitoring Committee headed by a member (technical) to monitor the monthly progress as per the bar chart and coordination among various departments, the Project Monitoring Unit headed by the Chief Executive Officer, Delhi Jal Board to ascertain the smooth flow of funds as required, to complete the project in time, and the Apex Committee headed by the Chief Secretary, Delhi, once in 3 months, to monitor the progress and resolve the issues pending with other departments of Delhi.

The entire work was divided into six packages with the following scope of work as summarized in Table 3. Thus, the work on all the segments could be started simultaneously and completed as early as possible in a time-bound manner. The packages were divided in such a manner that every package was linked with a particular sewage treatment plant (Supplementary material, Figure S4).

Apart from the above three major drains, the Delhi Jal Board took the initiative to trap the following drains into the nearby sewerage systems or set up STP at the mouth of drains: Magazine Road Drain, Sweepers Colony Drain, and Khyber Pass drains. These three drains have been trapped at the Arunanagar sewage pumping station, and sewage is pumped to the Oklha STP through the Nigam Bodh SPS and the Ring Road Trunk sewer. Metcalf House Drain: The connection of the drain to the Bela Road Sewer was not technically feasible. Hence, a small pump house near the drain has been constructed and is being pumped to the Bela Road Trunk sewer. Qudesia Bagh Drain: Not much discharge in the drain, as Masonic Club has shifted its connection to the Bela Road sewer. Mori Gate Drain: The Delhi Jal Board has proposed an STP of 45 MLD capacity at the outfall point and asked the Delhi Development Authority to allot the land. Tonga Stand Drain: Partially trapped into the Nigam Bodh pumping station. Civil Mill Drain: Very low discharge, and the drain has been connected to the Ring Road sewer. Delhi Gate Drain: An additional STP of 68 MLD was constructed and commissioned in 2015, in addition to the existing 10-MLD capacity STP to treat the entire discharge of the drain. The Sen Nursing Home Nalah + Nalab 12A: 10 MLD is being treated at STP at its mouth and remaining trapped in the Ring Road sewer. Drain No. 14: The drain has been trapped in the Ring Road sewer. Barapula Drain: Barapula Drain is also one of the big drains which starts from Meharoli Badarpur road and, after traversing the entire south Delhi and receiving the discharge of hundreds of sewered and unsewered colonies, meets with Yamuna near Sarai Kalekhan. The Delhi Jal Board ultimately has proposed setting up 180 MLD STP at the mouth of this drain to treat wastewater of the Barapula Drain. Maharani Bagh Drain: Partially trapped in the existing sewer. Kalkaji Drain: Trapped in the existing sewer. Tehkhand Nalah: Not feasible for trapping. Tughlakabad Drain: Trapped.

The interceptor sewer has been designed to take care of the wastewater of the drains within the territory of NCT Delhi. But a huge quantity of wastewater is also coming from the neighboring states, i.e., Haryana and U.P. The Badshapur Drain from Gurugram carries around 400–450 MLD of wastewater and discharges into the Najafgarh drain. Similarly, drain No. 6 coming from the industrial areas of Samalkhan, Haryana also carries around 45–50 MLD of wastewater and discharges into the supplementary drain in Delhi. Khureji Khas, Loni Road, and Sahibabad drains carry approximately 270–320 MLD of wastewater from U.P. and discharge into the Shahdara Drain. The respective states must treat the wastewater from these drains before discharging it into Delhi's drains to further improve the quality of river water.

Interceptor sewers will take care of only 75% of the total pollution load in river Yamuna, i.e., North, West, and East Delhi's drains. Delhi must prepare a separate plan for trapping wastewater discharges from the remaining drains and finally implement the DSMP 2031, which is the ultimate solution to prevent sewage flow into water bodies.

It is also observed that the interceptor sewer project has been completed to the extent of 99%. Around 1,080 MLD of wastewater has been intercepted and around 500 MLD of greywater still flow into rivers through drains due to unsewered areas in South Delhi. Besides, the residents have not connected their greywater disposal arrangement with the Delhi Jal Board sewer. Hence, greywater also flow into the drains and finally into rivers. Three STPs, i.e., Kondli (205 MLD), Rithala (182 MLD), and Okhla (563 MLD) are being rehabilitated and ungraded. By the time these STPs are commissioned, the sewage trapped in these plants cannot be treated. After commissioning of these plants, the quality of river water will further improve. Delhi must upgrade all remaining STPs to the treatment standard as prescribed by CPCB to achieve the desired river water quality standards.

Sewage reaching STPs is not purely domestic sewage. It is mixed with industrial waste coming from non-conforming industries illegally set up in residential areas. Hence, many times, the treatment process of sewage is affected, and the results do not meet the desired/designed standards. The disposal of sludge from STPs is also a major challenge. The Delhi Municipal Corporation does not allow sludge at their Sanitary Land Fills (SLFs), as they have exhausted their capacities at all SLFs. Hence, digested sludge is stacked either at STPs itself or disposed of as per the goodwill of the contractors engaged in disposing of silt/sludge, which also affects the treatment process.

The interceptor sewer project is one of the robust solutions to reduce the organic (BOD) and pathogenic (coliforms) pollution in the river Yamuna. Around 1,080 MLD of wastewater has been trapped from interceptor sewers against the targeted 1,100 MLD and is being treated at the respective STPs, which has improved the quality of river water to some extent, as anticipated. If dilution is not possible, the quality of the treated effluent will have to be of tertiary standards. Thereafter, the quality of river water will further improve by a natural self-cleaning process while traveling in the drains and rivers for miles together. After improvement of water flowing in the drains and rivers, groundwater quality will also improve, thus adding a source of groundwater in the future. However, the success of the interceptor sewer depends on the lateral coordination and cooperation of various stakeholders. All the above departments, being under different authorities and governments must come under one umbrella to monitor the progress of the projects undertaken by the respective departments and to direct the respective authorities appropriately. The regulating authorities, i.e., CPCB and DPCC, must monitor river water quality at regular intervals and pass the instructions to the concerned authorities appropriately. Thus, this concept can be replicated anywhere globally. However, the quantity of freshwater availability for dilution will ensure whether the bathing quality of river water can be achieved or not. After cleaning river water to bathing quality, the same can be utilized as raw water at water treatment plants. This study is not limited to the NCT of Delhi, but can be replicated elsewhere in India and abroad, for the abatement of pollution in surface water. The concept and methodology may little bit differ for different towns or cities, depending on their geographical and hydrological conditions.

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

The authors declare there is no conflict.