Environmental and economic assessment of proposed on-site wastewater management system in multi-storey residential building

Indian metro-cities have major downsides of inadequate infrastructural facilities and land unavailability for constructing a wastewater treatment plant (WWTP). Ever-increasing wastewater quantities give rise to major environmental and health problems. As against this, the treatment facilities provided are inadequate and nearly 50% of them have outdated infrastructure (CPCB 2016). A centralized approach for wastewater treatment is a costly affair, comes to 10 million Rs. per million litres daily (MLD) for infrastructure development and additional operation and maintenance (O&M) costs, which becomes challenging to the government day by day (CPCB 2016). Ever since the local government made treatment of sewage for housing society compulsory (built-up area ≥20,000 sq. meters), 30% of such existing sewage treatment plants were observed as non-functional, leading to continuous dumping of untreated wastewater to the river (MPCB 2016). This has not only adverse impacts on the environment but also downstream users are severely affected.

Many small communities of India are facing difficulties in constructing and maintaining conventional residential WWTPs because of land availability issues, limited financial resources, and plenty of problems associated with complex O&M. Thus, to tackle these problems and revolutionize waste management, systems for treating and recycling wastewater need to be on-site, cost-effective, and self-sustainable, delivering chemical-free treated effluent that would be entirely harmless to nature and residents. One such system is DOSIWAM (Decentralized On-Site Integrated WAste Management) (Mapuskar 1988). The fundamental concept of the DOSIWAM system is the profitable treatment, reuse and recycling of wastewater at the source of the generation itself.

The current study aims to propose the concept of the ‘environmental floor’ for the installation of the DOSIWAM system at intermittent levels of a multi-storeyed building. The term ‘environmental floor’ is designated for the refuge floor, where the treatment units are located, keeping the standard refuge area vacant and working on a gravity basis to fulfil the requirement for treated water used for various non-consumptive purposes like flushing, gardening, firefighting, floor washing, etc. The treatment units consist of the Malaprabha digester, the Intercepting tank, the Stabilization tank, and the Storage tank. Greywater treatment is carried out with treatment units positioned on environmental floors that receive wastewater from above located intermittent floors and pass treated water to below. Here, the system has a non-mechanised arrangement where no mechanical equipment is utilized that could cause additional weight and complications on the floors (Dewalkar & Shastri 2020).

Figure 1 shows the process flow of greywater collection from intermittent floors, treatment on the environmental floor, distribution of treated water for flushing purposes, and collection of blackwater at the underground Malaprabha digester. The refuge floors distribution is carried out as per the guidelines given by the Urban Development Department, Government of Maharashtra, India. If the height of a building is more than 39 meters, the first refuge floor should be after 39 meters and so on successively, at every 15-meters interval. The height and carpet area for current multi-storeys is 110 meters and 960 m² respectively. It is noteworthy that the area occupied by treatment units positioned at environmental floors (refuge floors) is comparatively minimum (7.8% of total carpet area) as its treatment capacity is governed by the volume of grey wastewater generated from intermittent floors, which ensure economy in cost.

Extensive life cycle assessment (LCA) studies have been carried out worldwide addressing environmental impact assessment of mechanised and exclusively activated sludge treatment process (ASP) (Machado et al. 2007; Lopsik 2013; De Feo et al. 2016; Garfi et al. 2017; Laitinen et al. 2017), but LCA-driven studies for wastewater treatment in the Indian context, are very few (Kalbar et al. 2012a, 2012b, 2013, 2014; Kamble et al. 2017; Raghuvanshi et al. 2017; Singh & Kazmi 2017; Singh et al. 2018; Kamble et al. 2019) and to date, no detailed environmental and economical studies have been attempted for the DOSIWAM system. Ahmed et al. (2010) envisioned that ‘research of LCA in wastewater extends mostly in developed countries with almost no contribution from developing countries’. Analysing LCA in the Indian context is a bit-challenging task, as availability of a national database is an issue (Kalbar et al. 2012a; Kamble et al. 2019). The software employed for LCA conduction is either generic or designed for one specific area. SimaPro and GaBi are the most widely used generic LCA software applied to analyse the wastewater treatment (WWT) system (Kulczycka et al. 2015). LCA and LCCA receive valuable importance for their decision-making ability towards environmental and economical sustainability (David & Messiha 1999).

The selection of treatment systems is a major concern in developing countries facing many problems due to insufficient data with respect to cost utilisation and performance evaluation, resulting in an inappropriate selection of treatment systems (Poch et al. 2004; Kalbar et al. 2012a). Such a situation is observed in India also, where wastewater treatment technologies are selected on the basis of past experience and existing cost and resources constraints (Singh & Kazmi 2017). Nowadays, residents are dealing with some major challenges with conventional residential WWT systems, such as:

• 1.

  Energy driven treatment with erratic supplies and unavailability of alternative sources to fulfil the requirements.

• 2.

  Power supply remains a major hindrance, interruption in treatment mechanism upsurges the risk of system failure.

• 3.

  Economical aspects of skilled manpower deployment for scientific handling and an annual burden to pay consent for O&M to municipal corporations create an extra burden on residents.

• 4.

  Improper allocation of the system in the premises leads to unbearable noise and vibration issues to ground floor residents.

Thus, people are in search of a simplified process-based, low-cost viable option. Hereby, the current study brings to light the importance and suitability of a virtually designed non-mechanised DOSIWAM system for a multi-storeyed building using an LCA and LCCA approach.

DOSIWAM is a sustainable, eco-friendly, hygienically safe sanitation process based on natural biodegradation, which converts waste into wealth. In this system, every grain of solid and every drop of liquid is treated hygienically by bio digestive process and natural aeration and end products are return to the soil through horticulture or agriculture in an ecologically sustainable manner.

The stabilization system works on the basis of settlement and degradation of the solids from the wastewater. A multi-sectional stabilization tank contains five main and sub-main compartments maintaining a length to breadth ratio of 1:3. Aerobic condition is provided to each compartment with sufficient freeboard and a one-day detention period to obtain proper contact time between air and wastewater. The intercepting tank where solids settle at the bottom diverts the water to the first compartment with controlled velocity within one-day detention time. It was observed that a small amount of solids get settled at the bottom of the first compartment and remain in the second compartment. Settled sludge is then digested gradually. A sludge valve can be used to remove the excess sludge, which may settle in the first two compartments and thus act as the settling zone. In the third, fourth, and subsequent compartments, some amount of dissolved solids is removed. Thus, horizontal and vertical zigzag flow patterns and gradual oxidation of pollutants using effective microorganisms in treatment units results in simpler and non-polluting effluent that can very well be used for non-consumptive purposes. The physiochemical characteristics of treated greywater are under acceptable limits of pollution parameters prescribed by the Central Pollution Control Board (CPCB) and free from obnoxious odour.

The Malaprabha digester of the DOSIWAM system is an environmentally and economically sustainable solution for blackwater management. It consists of three main components: the digestion chamber (30 days HRT) followed by displacement (15 days HRT) and the outer chamber (15 days HRT). The high retention time of 45 days leads to pathogen-free faecal sludge. Waste entered inside the digester separates in three layers with heavier solids at the bottom, lighter solids at the top, and minimal solids in the middle layer, which are then promoted to move towards the second and third chambers during the process of anaerobic digestion. A scum breaker provided help to reduce the formation of the scum layer on the top surface, which could hamper the production of the biogas from the waste. The settled sludge can be removed from the first chamber, and later on can be used as fertilizer. Evolved scum can be removed from the provided gaskets. The design of the system is such that there is little to no human intervention required to work or maintain the system. The decomposition of organic matter to methane can be effectively carried out and fully trapped in the Malaprabha digester. Integrated and complementary treatment of human night soil serves dual purposes of managing human waste and recovering energy that can replace and reduce dependency on LPG (liquefied petroleum gas). Complete oxidation of biomethane during the combustion of biogas (81.5 CO₂/MJ) is comparatively lower than LPG burning (139 CO₂/MJ) and is considered as neutral (Smith et al. 2000).

In India, ASP is the most commonly used secondary sewage treatment system (Kamble et al. 2019). The treatment consists of an aeration tank, which acts as the biological reactor, with aerobic microorganisms used to produce a biological floc, and a final clarifier system, which separates solid and treated wastewater. Sludge acts as an adsorbent for organic and suspended solids and microorganisms carry out the oxidation of organic matter. Figure 2 shows a process-based flow diagram of both the studied systems.

LCA is an analytical tool that measures any product's or activity's environmental impacts from its raw material acquirement phase to its demolition phase (Tillman & Baumann 2004). The ISO 14040 series (ISO 2006a, 2006b) LCA standard elaborates all phases of life cycle assessment with the ‘cradle-to-grave’ and ‘cradle-to-gate’ approach. The methodology associated with LCA has four major components.

The goal of the current study is to perform a comparative analysis of two different systems: DOSIWAM (non-mechanised) and ASP (mechanised) designed for the multi-storeyed building (G + 30) with 890 equivalent habitats, where the DOSIWAM system's greywater and blackwater treatment units positioned on the environmental floors and underground respectively and the ASP system provided at the premises of building under study. This comparative study assesses the critical source of environmental impacts associated with mechanised versus non-mechanised wastewater treatment and follows a ‘cradle to gate’ approach to carry out LCA.

In the present study, to measure the comparative performance of both the systems, the functional unit was chosen as 1 m³ of treated wastewater, commonly used by many researchers (Godin et al. 2012; Kamble et al. 2019) over plant lifespan of 50 years.

The system boundary considered as per study objectives follows the ‘cradle to gate’ approach, where LCA evaluates impact from extraction, production, and processing of materials for infrastructure, operational equipment, and chemicals along with the energy required to carry out the operational process. In most of the studies, the end of life phase was excluded due to scarce information associated with dismantling and recycling of used materials. Furthermore, the demolition phase of the WWTP has a lower environmental impact than the construction and O&M phase (Lundin et al. 2000; Machado et al. 2007; Lopsik 2013). Moreover, the construction phase was also not considered in some studies, with an assumption that the impact of this phase is considerably lower than the O&M phase particularly for treatment plants with a long span of life. However, this considered assumption may not stand for some treatment technologies such as natural treatments like construction wetlands, an infiltration process where the construction phase has a significant impact and even to a greater extent than the O&M phase (Kobayashi et al. 2020). Thus, it is equally essential to consider the construction phase for environmental impact assessment (De Feo et al. 2016). In the current study, the emphasis was put on LCA of the construction and O&M phases and demolition phase, and sludge treatment was excluded; however, construction materials required for the sludge thickening bed of the ASP system were considered. In the case of the DOSIWAM system, the remaining floors above the topmost environmental floor were served by pumping treated water with a very low capacity pump, thus electricity consumption is taken into consideration. Further, during the phase of construction, transportation of construction materials, which were manufactured locally, contributes minor impact, and is thus excluded.

During the technical procedure of inventory analysis, LCA models for both treatment systems were constructed as per governed specifications in defined goal and scope. Inventory analysis enumerates raw materials, energy consumption, and environmental emissions to air, soil, and water throughout the life span of a treatment plant (Ahmed et al. 2010). In the case of the ASP treatment system, all foreground input data (i.e. infrastructural inputs, O&M resources, effluent characterization) required for life cycle inventory (LCI) were collected from documents of the existing plant situated at the premises of building under study. Infrastructural input data for a virtually designed DOSIWAM system were based on the detailed hydraulic design executed in the frame of a study and effluent characterization that was performed by collecting samples from a similar DOSIWAM system currently in operation and located in Pune, India. The O&M phase primarily required electricity, which was taken from the Indian energy mix database of Ecoinvent 3. Chemicals (sodium hypochlorite), input materials, building equipment for soil excavation, and transportation were taken from the global database of Ecoinvent 3. Guidelines given in the Intergovernmental Panel on Climate Change were used to estimate air emissions (Bartram et al. 2019). Anaerobic degradation of COD and nitrogen removal are the sources of biogenic CH₄ and N₂O emissions respectively. Besides, net CH₄ emission was obtained as a result of subtracting recovered methane from gross emission (Bartram et al. 2019). The mechanised treatment system's operational report suggests the replacement of electromechanical equipment; thus, substitution was considered every 12 years throughout the 50 year life span of the system. Input and output inventories per functional unit for both systems are presented in Table 1.

LCA modeling and impact assessment of the studied systems were performed by SimaPro software with Ecoinvent 3 database (Pre-sustainability 2018). The method adopted was Impact 2002+. Impact assessment and damage assessment (midpoint and endpoint categories) were interpreted in the current study and all the critical sources of the impact associated with the input-output scenario are discussed below. The results are relevant only if the assumptions and limitations of the current study are taken into consideration. As discussed earlier, the study is limited to construction and operation phases. Sludge treatment, disposal, and heavy metal emissions were excluded due to a lack of data availability. Coal-based power generation (IN-Western grid medium voltage) was the only Indian database used in modeling and a global database was used for other background data due to a lack of appropriate Indian life cycle inventories. As per IPCC guidelines, CO₂ emission was excluded due to its biogenic nature, which results in no contribution to climate change (Coats et al. 2011).

The most significant impact category is global warming (GW). High energy consumption and chemical dosing during the O&M phase contribute the highest environmental burden for the ASP system (1.017 kg CO₂ eq.), whereas the DOSIWAM system has a significantly lower contribution (−0.126 kg CO₂ eq.). Capturing biogas mitigates greenhouse gases (GHG) by preventing CH₄ emission, which is 25 times more detrimental than CO₂. The Malaprabha digester as a source of biogas energy reduces fossil CO₂ emission by substituting fossil fuel energy, which results in the reduction of the impact category by −0.451 kg CO₂ eq. Indian energy production is mainly coal-based thermal power (IN-Western grid medium voltage) accounting for 85.88% and chemical dosing 7.02% emissions during the operation phase of the ASP system. On the other hand, DOSIWAM tanks with RCC construction (cement 70.25% and reinforcing steel 28.54%) are responsible for the high share of GW impact in its construction phase.

Aquatic eutrophication is closely associated with enrichment of recipients mainly due to phosphorous, nitrogen, and COD as organic matter, contributing to biomass formation. Aquatic eutrophication of the DOSIWAM system (0.0783 kg PO₄ P-lim.) dominates over the ASP system (0.0124 kg PO₄ P-lim.) due to effluent discharge of the human night-soil based Malaprabha digester with a high concentration of toxic ions (sodium, potassium, chloride, sulfur, ammonia, magnesium). In addition, the weaker efficiency of the Stabilization tank for phosphate removal (>42%) has a predominant effect on the water environment.

Much like the impact of GW, the aquatic and terrestrial eco-toxicity impact is mainly due to heavy metal emissions from Indian coal-based electricity generation where landfill is dumped with coal mining spoils (Kalbar et al. 2013). It is noteworthy that ASP has substantial contribution towards electricity consumption, which leads to 80.2% impact and 12.7% impact is due to the upstream manufacturing process of sodium hypochlorite thus results in comparatively higher impact (95.762 kg TEG water and 22.67 kg TEG soil) than DOSIWAM (23.24 kg TEG water and 6.22 kg TEG soil) where heavy metal emissions during cement and steel production are the largest contributor.

Power cables with aluminium, use of copper for the electricity supply grid in the case of the ASP system (1.57E-02 MJ surplus), are foreground contributors to mineral extraction. This result is in line with a previous study (Lopsik 2013; Singh et al. 2019). While usage of iron, nickel, chromium, and manganese for the production of steel and various minerals used for cement production required for construction of DOSIWAM tanks (1.36E-02 MJ surplus) are responsible contributors to this impact indicator.

Carcinogens (ASP: 7.33E-03 kg C₂H₃Cl eq. and DOSIWAM: 4.78E-03 kg C₂H₃Cl eq.) and non-carcinogens (ASP: 2.27E-02 kg C₂H₃Cl eq. and DOSIWAM: 5.88E-03 kg C₂H₃Cl eq.) impact of human toxicity potential is mostly due to the aromatic hydrocarbons and dioxin compounds emitted mainly from the production of PVC pipes, reinforcing steel, chemicals for disinfection and electricity generation.

Ozone layer depletion (OLD) impact recorded for the ASP system (5.31E-08 kg CFC-11 eq.) is almost five times higher than the DOSIWAM system (1.11E-08 kg CFC-11 eq.), this is due to the fact that the production of chemicals for disinfection is the principal contributor for OLD, accounting for 77.09% of the impact whereas DOSIWAM has a chemical-free treatment process.

Respiratory organics impact is associated with sodium hypochlorite chemical, which has a major role in the treatment process for ASP, releasing non-methane volatile organic compounds that are responsible for 1.49 E-04 kg C₂H₄ eq. impact, whereas particulate pollution mainly during steel production for the DOSIWAM tanks results in 8.34E-05 kg C₂H₄ eq. impact. The combustion process of coal for electricity production, which releases harmful emissions like particulates, nitrogen oxides, sulfur dioxides, arsenic, barium, selenium, zinc, chromium, benzene, etc. primarily causes respiratory inorganic impact for ASP (2.06 E-03 kg PM2.5 eq.) and in the case of DOSIWAM (3.26E-04 kg PM2.5 eq.) emissions, mainly particulates, nitrogen oxides, sulfur dioxides during clinker production, have a most adverse effect on respiratory inorganics.

Coal-based power generation is the largest source of sulfur dioxide leading to the formation of aquatic and terrestrial acidification. When emissions of acidifying pollutants are exposed to water they form sulfuric acid. Aquatic and terrestrial acidification for the ASP system was found to be 4.52E-03 kg SO₂ eq. and 1.66E-02 kg SO₂ eq. respectively. Whereas clinker incineration is mainly responsible for sulfur dioxide emissions in the case of the DOSIWAM system, which has 1.37E-03 kg SO₂ eq. and 7.89E-03 kg SO₂ eq. impacts respectively.

Land occupation is mainly affected due to the usage of land for the construction of the treatment system (Lopsik 2013). The land occupation impact category is affected more in case of the ASP system (9.17E-03 m²org.arable), which being an intense aeration process-based technology occupies a significant area for tanks (Risch et al. 2015). Whereas DOSIWAM's Stabilization tanks installed at environmental floors and the Malaprabha digester, positioned underground, showed an effective reduction of 79.4% in the land occupation category (1.89E-03 m²org.arable).

The ASP system was found to be the alternative for depleting non-renewable energy (13 MJ Primary), almost 6 times higher than DOSIWAM (2.02 MJ Primary), which is due to depletion of non-renewable resources; that is, hard coal utilization for electricity generation accounts for 82.4% impact in this category. DOSIWAM's non-mechanised treatment process and potential resource recovery lead to a beneficial effect on the total environmental impact index.

In addition, results from analysis reveal that impacts of categories like ozone layer depletion, respiratory organics and inorganics have a comparatively lower impact than other impact indicators. Figure 3(i) shows comparative midpoint characterisation between studied systems with impact 2002+ method and Figure 3(ii) shows normalization performed as per factors offered by global normalization LCIA methods, where the human health endpoint category showed the most significant impact for both the studied systems.

More simplified and understandable results were found in individual scenarios, which determine the most influential phase of a treatment system that plays a substantial role in the results. Contribution towards impact categories in the case of the ASP system is a result of electricity usage (66.72%), chemical consumption (15.99%), overall construction materials (10.33%), and building equipment (<1%). The construction phase has the lowest share, with an average of 8.81% impact, where respiratory organics, carcinogen, and mineral extraction are influential categories. While the O&M phase influences almost all impact categories, being the highest share with an average of 91.18% of environmental impact loading.

The non-mechanised way of treating wastewater results not only in the reduction of impact but positive (beneficial) environmental impact due to the bio-digestive process of the Malaprabha digester, which helps in generating bioenergy. On the other hand, improperly maintained septic tanks are believed to be a substantial source of methane emission, accounting for around 12 Tg/yr (Bartram et al. 2019). Thus, the Malaprabha digester can conveniently replace a septic tank. The DOSIWAM system has dominating impact categories like eutrophication (98%), terrestrial acidification (77.07%), and aquatic acidification (66.5%), mainly due to effluent discharge quality with a significant concentration of phosphorous and nutrients. The construction phase stands out for almost all impact categories except eutrophication and outperforms with an average of 70.63% than the O&M phase accounting for an average of 22.22%. Figure 3(iii) shows the endpoint category normalisation for the individual scenario.

DOSIWAM is a simpler system that does not require any work to be done especially by any skilled labour, while unskilful maintenance of the ASP system could be the reason for dysfunctioning of the plant's service and other severe problems (Laitinen et al. 2017). Further, no specific chemicals are required to be added. The treated effluent is under acceptable limits of pollution parameters and can directly discharge into aquifers. Treatment is free from obnoxious odour and noise, therefore it can easily be installed on refuge floors of the multi-storeyed building. The obtained results for harmful impacts associated with electricity consumption for the ASP system and the construction phase of natural systems are in agreement with other studies (Lopsik 2013; Kobayashi et al. 2020).

LCCA is an efficient and widely used decision-making tool, which assists in selecting economically feasible WWT solutions. Residential WWTPs are capital-intensive projects with considerable on-going O&M costs and a long span of life. The cost parameters (recurring and non-recurring) have brought financial pressure to residents, thus it is necessary to conduct LCCA for an effective decision-making process. The service life of both treatment systems was observed as 50 years, which was considered for LCCA with an assumption of a required construction period of one year for both treatment systems. The salvage value was considered as 10% of civil construction work. The current study attempts to foretell the economic solution by analysing data and identifying key parameters, which are responsible for cost escalation. LCCA for both the studied systems is summarised in Table 2.

The selection of the WWT system is primarily based on various components of CAPEX. It includes initial capital costs associated with land use, material consumption, construction and installation, initial electromechanical setup, plumbing, and pipe fittings. ASP, being a land-intense treatment technology, its land-use cost significantly influences CAPEX compared to DOSIWAM, where treatment units are positioned on environmental floors; however, the cost of building floor space utilized for installation of these treatment units was considered. Civil construction and installation, and labour requirements were estimated as per the Central Government Guidelines (CPWD)-District Schedule of Rate. Plumbing and pipelines, comprising the collection and distribution setup, were governed by sources of wastewater and consecutive height of floors.

The OPEX phase of the WWT system is typically dominated by the costs associated with electricity consumption, repair and replacement of electromechanical equipment, wages of workers, and chemicals (disinfectant) along with some miscellaneous costs such as the treatment plant's infrastructure maintenance and routine inspections. The replacement schedule of electromechanical equipment was considered as 12 years. Simplification was made by assuming a replacement cost equal to the price of the initial cost. Energy and chemical consumption together attribute 90% of OPEX cost for the mechanised ASP system; also, skilled workers for operational efficiency are an essential requirement that influences OPEX. The natural gravity-governed DOSIWAM system, with no chemical and energy input, has minimal to very low effect on OPEX.

AIRR of the DOSIWAM system with 5.30% and the ASP system with 4.45% is greater than MARR (3.5%), thus both the alternatives are economical; however, while ranking the options, DOSIWAM holds a higher rank than ASP.

Uncertainty is associated with many input parameters, which have significant impacts on the final decision; this is because some parametric assumptions need to be made in the future. The discount rate is the most vital source of uncertainty, thus is varied from 1% to 15%. Figure 4 shows the impact of various real discount rates on net capital cost. Higher discount rate favours ASP, which would have lower NPW and vice versa, this is due to high OPEX, whereas DOSIWAM was found to be less sensitive towards real discount rate. The most prominent elements of LCCA with a major impact on sensitivity were cost associated with electricity requirement (40% of O&M) and capital investments. CAPEX, OPEX, and residual cost were then increased arbitrarily by 5, 10, and 15%, and results were formulated with a sensitivity slope; that is, the ratio of the percentage change in output for the given percentage change in input, shown in Figure 5 (Rawal & Duggal 2016). The ASP system's high sensitivity towards OPEX showed relative impacts on LCCA, whereas DOSIWAM was found to be CAPEX sensitive. The purpose of system comparison will suffice with single lifespan consideration, nonetheless, by varying the life span of both alternatives, the DOSIWAM system was observed to be more affordable for a long span of life due to its negligible cost disruption towards O&M compared to the ASP system. Also, it has been noted that WWT systems with large variations in operating costs are more sensitive towards variation in the value of lifespan (National Institute of Urban Affairs 2019).

In the present day context of Indian metro-cities, major downsides of the substantial drop in groundwater level and population growth result in an earnest need for water recovery. Most of the centralised WWTP across India are on the verge of cost upgrade and untenable maintenance. Thus, source-separated decentralised treatment systems are the most convenient alternative to centralised systems as their applications to residential would substantially abridge the impact on freshwater depletion. Presently, both the studied systems have different direct impacts and performance characteristics. In accordance with assumed conditions, the current mechanised conventional WWT system, although it is superior in treatment and removal efficiency; however, the high O&M cost, land occupation, and harmful environmental emissions should not be ignored. Moreover, environmental impacts generated were observed to be about three to six times higher than the DOSIWAM system, mainly due to energy and chemical consumptions. These impacts can be well eliminated by accommodating the DOSIWAM system and even further monetary benefits come from replacing LPG for cooking, a considerable reduction in land expanse, and plumbing network. These environmental and economic considerations direct the sustainability of the studied DOSIWAM system as the best suitable alternative, which can replace conventional mechanised WWT systems.

This study encourages a new outlook in the direction of providing a sustainable on-site gravity governed DOSIWAM system, which would serve as an environmentally friendly and economically affordable solution. In view of current research, a proposal can be made to the respective local planning authority to incentivize the effort undertaken for the benefit of residents as well as the government. Such sustainable activities may incentivize in terms of extra FSI or concession in taxes. The Malaprabha digester for the generation of renewable energy could be well coordinated with the ‘Clean India Mission’. Furthermore, awareness has to be raised for the use of efficient and naturally working systems among stakeholders and beneficiaries.

In the current study, only the environmental and economic aspects were evaluated. However, if sustainability is aimed for decision making on WWT system selection, the social aspect, which is a third important pillar of sustainability, should also be taken into consideration (e.g. social acceptance of night soil-based biogas plant, treatment units positioned at intermittent floors, etc.). Further, as a future scope, instead of allocating treatment units at environmental floors, an environmental shaft may be provided in the building wherein the whole shaft is composed of stories and can treat wastewater from individual floors.

We express our sincere thanks to Sinhgad Technical Education Society and Savitribai Phule Pune University, Maharashtra, India for providing help, support, and opportunity.

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