Conventional and zero liquid discharge treatment plants for textile wastewater through the lens of carbon footprint analysis

The concentration of greenhouse gases (GHGs) in the earth's atmosphere has increased at an alarming rate in recent years due to various anthropogenic activities (Althor et al. 2016). The concentration of GHGs above the carrying capacity of the earth's atmosphere directly leads to global temperature rise, which in turn causes sea-level rise, flooding, and extreme weather phenomena (Pandey et al. 2011).

In the year 2010, global greenhouse emissions were reported as 48,629 metric tCO₂eq, of which nearly 3% of the global emissions were contributed by the waste management sector (Singh & Maurya 2016). In India, the contribution from the waste management sector was 3.34%, which is found to be above the global average (Singh et al. 2016). It underlines the immediate need for a well-developed methodology to quantify GHG emissions from this sector to implement emission mitigation practices further.

The wastewater treatment plants have a positive contribution towards environmental impact mitigation through the wastewater treatment, but their energy consumption must also be considered to understand the overall environmental impact. Recent developments in the concept of zero liquid discharge (ZLD) facilities are important for the resource recovery approach. High energy consumption by the reject management system in ZLD facilities demands a well-developed methodology to quantify the environmental impact of the wastewater treatment plants.

The wastewater treatment aspects become a major concern in the case of industrial effluent discharge to the natural receiving bodies. The textile industry has a major contribution to the economic development of India as it is the second-highest exporter of textiles after China. Textile industries simultaneously lead to massive production of textile manufacturing effluents containing various dyes and auxiliary chemicals (Pattnaik & Dangayach 2019).

Coimbatore city is also known as the Manchester of India as the textile manufacturing has thrived in southern parts of the country (Manikandan et al. 2015). Enough availability of raw materials and cheap labor further enhance the socio-economic growth associated with textile manufacturing activities. With the ever-increasing textile manufacturing practices in southern India, the adverse environmental impact becomes a major concern for regulatory authorities.

The practice of setting up the common effluent treatment plant (CETP) for textile manufacturing industry clusters has been implemented by the regulatory bodies in India. In the textile dyeing process, salts like NaCl and Na₂SO₄ are added to enhance dye fixation on the cellulose fiber (Holkar et al. 2016). The combined effluent from textile manufacturing processes is highly colored, having high organic load and extremely high total dissolved solids. As the CETP generally provides treatment only up to the secondary level, the total dissolved solid content was not completely removed from the discharged treated textile effluent. The higher salt content is a major concern for the natural receiving water bodies. Therefore, regulatory bodies made it mandatory for all the textile industries with effluent discharge of more than 25 m³/day to set up a ZLD facility.

ZLD refers to the installation of facilities and systems which will enable an industrial effluent for absolute recycling of permeate and converting solute (dissolved organic and inorganic compounds/salts) into residue in the solid form by adopting a method of concentration and thermal evaporation (CPCB 2015). Conventionally, the secondary treated textile effluent from the effluent treatment plant (ETP) was being discharged at various places, which would ultimately find its way into the receiving natural water bodies. The ETP treatment scheme was not designed to remove the dissolved solid content and heavy metals in the textile effluent. Salt content beyond the specified levels in the water can have an adverse impact on the receiving agricultural lands. The textile effluent contains toxic heavy metals like chromium, and its release to natural water bodies can lead to possible bioaccumulation and biomagnification concerns (Paździor et al. 2017).

In the ZLD effluent treatment facility, reverse osmosis is provided as the tertiary treatment. The permeate stream can be reused as the process water by textile industries. The reject stream is further fed to a multiple effect evaporator to enhance water recovery. The reject from a multiple effect evaporator is sent to the Agitated Thin Film Dryer (ATFD). The water recovered from the ATFD unit is reused as the boiler feed, and colored powder is considered as hazardous solid waste. The wastewater from textile manufacturing activities can effectively be reused by the implementation of the concept of ZLD (Vishnu et al. 2008; Lu et al. 2010; Tong & Elimelech 2016; Chavan 2017). The uniform water quality parameters of the reused water are advantageous to the industries, as they can get a constant source of high-quality process water. As the conventional treatment plants are not designed to remove heavy metals and complex xenobiotic organics, implementation of ZLD treatment facilities would eliminate the discharge of these emerging pollutants through industrial effluents (Yaqub & Lee 2019). Figure 1 presents a general schematic flow diagram of the ZLD wastewater treatment plant for the textile industry. After the secondary treatment, effluent is passed through multiple stages of the reverse osmosis process to remove the dissolved solids efficiently. There are two primary sources of hazardous solid waste in the ZLD facility. Firstly, the sludge generated from primary and secondary settling tanks, and second the residual salt obtained from a multiple effect evaporator (De Benedetto & Klemes 2009). An effective hazardous solid waste management system is required at such facilities. It is important to quantify the direct emissions from the secondary treatment stage as they can be effectively mitigated by process modifications (de Haas et al. 2011). By avoiding the formation of anaerobic zones in the aeration tank of biological treatment, direct CH₄ and N₂O emissions can be effectively controlled (Ramírez-Melgarejo et al. 2019). A ZLD wastewater treatment facility effectively reduces the pollution load on land and natural water bodies. In the case of the ZLD wastewater treatment facility, effluent discharge is totally avoided, ensuring water resource recovery from the industrial effluent (Yan et al. 2016). Considering the added advantage of the uniform quality of the reused water over available groundwater with variable quality, textile manufacturers are supporting the establishment of ZLD treatment facilities. Hence, critical analysis of energy required, environmental impact, and pollution potential are to be evaluated based on scientific analysis to comparatively evaluate these aspects.

The quantification of GHG emissions due to certain human activity is important as only after a proper quantification is proper monitoring possible. This quantification process, in case of a product manufacturing step, product disposal option, or for a treatment plant, cannot be a subjective analysis as it will hinder the comparativeness of different studies. Therefore, the application of the concept of carbon footprint is required as it provides a platform to compare the GHG emissions from various considered sources reported globally. The use of the term carbon footprint was first started as a subset of ‘ecological footprint’ proposed by Wackernagel & Rees (1996). Ecological footprint refers to the biologically productive land and sea area required to sustain a given human population expressed as global hectares. In similar lines, the carbon footprint can be interpreted as the land area required to assimilate the entire CO₂ produced by humankind during its lifetime.

As per the Intergovernmental Panel on Climate Change (IPCC), the carbon footprint can be defined as a measure of the total amount of GHG emissions of a defined person, organization, or region associated with certain activities, production processes, and life cycle of a product as a whole. The specific time horizon considered for carbon footprint calculation is generally 100 years (IPCC 2019).

There are various globally accepted methodologies for the calculation of carbon footprint. Some major publications in the above context include PAS 2050, ISO standards, and IPCC guidelines. The publicly available specifications-2050 (PAS 2050) of the British Standard Institution (BSI) is used for the assessment of the life cycle GHG emissions associated with the goods and services (BSI 2011). ISO 14064 (parts 1 and 2) is an international standard for the determination of boundaries, the quantification of GHG emissions, and removal. The standards for designing GHG mitigation projects are also provided (ISO 2006a, 2006b). The ISO 14067 document specifies the guidelines for quantification and reporting of the carbon footprint of a product. The GHG emission quantification methodology specified by the IPCC 2006 guidelines for National Greenhouse gas inventories was found to be most robust and applicable for wider scenarios, considering data availability constraints. The methodology follows the ‘top-down’ or input–output analysis approach, which is most suitable for the complex systems like wastewater treatment plants.

The scope one emission includes the GHG emission due to on-site fossil fuel use within the system boundary. The indirect emissions accounted under scope two and scope three are caused due to activities of the facility, but not directly emitted at the site. The major emissions widely considered under scope two are the emissions due to the generation of purchased electricity (IPCC 2019). The emissions due to associated transportation of employees, chemicals, and waste material disposal are accounted for under the scope three category. The carbon footprint analysis was used by various researchers to quantify the GHG emissions from domestic wastewater treatment plants (Pagilla et al. 2009; Mo & Zhang 2012; Parraviciniak et al. 2016). There exists a clear lack of quick and reliable tools for carbon footprint calculation, considering the data limitations in the case of developing countries. The understanding of the concept of carbon footprint and its appropriate field-scale application is important for the reporting of anthropogenic greenhouse emissions from the ETPs.

The objective of the present study was to study the environmental impact associated with the treatment of industrial textile effluent. The two possible scenarios considered in the present study included a conventional treatment system and a ZLD treatment system. Carbon footprint was used as the parameter to comparatively analyze the environmental impact of considered scenarios. Carbon footprint calculation methodology was developed for the application to the wastewater treatment sector in developing countries. Actual field-scale data were collected through extensive industrial visits for the carbon footprint analysis. The expected research outcome included significant inputs for the decision-making authorities towards the establishment of either the conventional or ZLD effluent treatment system.

The following section on methods and techniques includes the details of the proposed carbon footprint calculation methodology. Various steps of carbon footprint analysis applied to conventional and ZLD treatment systems are presented in this section. It is followed by the results and discussion on the finding of comparative carbon footprint analysis. Lastly, the conclusion section presents the collective findings of present research along with the key inputs to textile manufacturers on the selection of appropriate textile effluent treatment system.

In the present research work, a comparative analysis of carbon footprint for a ZLD textile effluent treatment facility and a conventional treatment facility discharging the secondary treated effluent to the receiving natural water bodies was performed. The overall treatment capacity of the considered plant in Tirupur district, Tamil Nadu, India, was 4.4 million liters per day, of which 2.2 million liters per day was operational capacity at the time of data collection. Out of the treatment capacity of 2.2 million liters per day, 78% of the effluent was recycled back by the ZLD facility.

In the identification of significant GHG sources, system boundary plays an important role. Only the direct and indirect GHG emission sources considered in the system boundary will be considered for the calculation of carbon footprint. The definition of the boundary condition should include both the special and temporal scales. The system boundaries for the present study are shown in Figure 2.

The data for the present comparative study were collected in the year 2016. On the temporal scale, we are considering an annual operational period for both types of treatment systems. Considering the lack of data on embodied emissions for different types of construction materials being used, the analysis was limited to the operation phase of the considered wastewater treatment plants.

The treatment capacity of the wastewater treatment plant is a key parameter to determine possible GHG emissions from the treatment process. To avoid the misleading results due to different treatment plant capacity values, the treatment plant activities up to secondary treatment were considered the same in the present study for both the scenarios. GHG emissions associated with the tertiary treatment and reject management were considered only for the ZLD facility. Transportation of settled sludge and residual solids to the Transport, Storage, and Disposal Facility (TSDF) was included in the carbon footprint calculations. In the case of the conventional treatment plant, methane emissions associated with the discharge of secondary treated effluent were considered. Based on plant data of percentage effluent recycled, an equivalent amount of groundwater extraction and associated GHG emissions were considered for the conventional treatment plant scenario.

The methodology of the calculation of carbon footprint for the wastewater treatment plants can be divided into three steps. The first step includes the identification of key sources of direct and indirect GHG emissions. The second step can be termed as the quantification step in which one attempts to quantify the emissions from the identified source. The final step involves the use of appropriate emission factors (EFs) to report the data in terms of carbon dioxide equivalents calculated over a time horizon of 100 years.

The three major GHGs widely considered for the carbon footprint analysis of wastewater treatment plants include carbon dioxide, methane, and nitrous oxide (Wiedmann & Minx 2008). The individual emission rate of other GHGs is considered much lower when compared with that of these three GHGs together (UK POST 2006). The capacity to trap the energy in the atmosphere is different for different GHGs. It is quantified in terms of ‘global warming potential’ which is based on the radioactive effect of 1 kg of the gas over 100 years, compared with the effect of 1 kg of CO₂, and it is expressed as CO₂ equivalents. As per the IPCC fifth assessment report published in 2014, the global warming potential values for methane and nitrous oxide are 28 and 265, respectively.

The EF associated with electricity consumption to be used for Indian conditions is 573.88 kg CO₂-eq per MW of electricity consumed (Singh & Maurya 2016). This factor depends on the electricity generation profile used in the country and associated emissions. Direct emissions on the wastewater treatment plant site include the wood used as a boiler feed material and the use of diesel for backup power generators on-site. The EF used for wood in the proposed methodology is 1.644 tCO₂eq per ton of wood used. In the case of the diesel use on-site as a generator feed, the EF of 0.00255 tCO₂eq per liter of diesel can be used (Sridhar 2010). GHG emissions due to vehicular transport depend on the type of vehicle. The EFs for different types of vehicles used in India are given in Table 1.

In a conventional wastewater treatment plant, it is general practice to discharge the secondary treated effluent to the natural receiving water body. The GHG emissions associated with the treated effluent discharge can be quantified, as discussed below.

The emissions associated with groundwater extraction include two major parts as the emissions associated with the electricity consumption for pumping and emissions from the transportation of groundwater from the pumping site to the textile manufacturing facility. The groundwater requirement was estimated based on the actual quantity of water required and from where the groundwater has been extracted and transported. The worked-out figures were in line with published data by Shah et al. (2004); Subburaj (2008); and Grönwall & Jonsson (2017). These data were also corroborated with industry experts. During our interaction with the textile manufacturers, they have clearly mentioned that they prefer to use the reclaimed water for the textile processing than the groundwater as the quality of the groundwater varies considerably according to the pumping site. The water is transported in lorries from the pumping site to textile facilities, and currently, around 172 such lorries are operating in Tirupur district, with 15 km as average distance traveled per trip. The proposed carbon footprint calculation methodology can be summarized as shown in Figure 3.

The following section presents the results obtained from the comparative analysis of carbon footprint for a ZLD effluent treatment facility and a conventional effluent treatment facility. From the field visits and data provided by plant operators, the major carbon emission sources were identified. Direct and indirect GHG emissions were calculated as per the IPCC guidelines.

Table 2 presents the data on the quantification of GHG emission sources used in both the scenarios compared in the present analysis. Sludge transportation requirements for a ZLD treatment facility were found to be more than that for the conventional treatment plant, as the transport of both dewatered settled sludge and residual salts was considered.

The GHG emissions for the quantified sources are calculated by multiplying with respective EFs, as discussed in the Methods and Technique section. Table 3 represents a model calculation of the carbon footprint for a conventional treatment plant as per the proposed methodology.

The results of the calculated carbon footprint for a ZLD treatment facility and a conventional treatment facility are presented in Table 4. The total carbon footprint was observed to be 10,598.31 tCO₂eq/year for a ZLD treatment facility, the same the conventional treatment facility was found to be 7,761.22 tCO₂eq/year. The total transportation presented in row three of Table 4 included the transportation associated with sludge disposal and groundwater transport in the case of a conventional treatment facility.

As per the findings by Gupta & Singh (2012), the GHG emission from a domestic wastewater treatment plant in New Delhi, India, was found to be nearly 3,028 tCO₂eq/year. The reported GHG emissions were less when compared with the present study findings because the emissions associated with the discharge of treated wastewater were not considered within the system boundaries by Gupta & Singh (2012). In the case of the domestic wastewater treatment plant at Patna, India, the GHG emissions were quantified as nearly 6,584 tCO₂eq/year by Singh & Maurya (2016). The on-site sludge digestion unit was considered by Singh & Maurya (2016), leading to higher on-site methane emissions. In both the mentioned studies, the system boundary was limited to the wastewater treatment plant premises boundary. From Table 4, it can be observed that for the fuel use on-site and electricity consumption, the emissions from the ZLD treatment facility are higher than those from the conventional treatment facility based on the positive value of percentage difference of carbon emissions with respect to the conventional treatment facility, calculated as the (Emissions from the ZLD treatment facility − Emissions from the conventional treatment facility)/Emissions from the conventional treatment facility. Percentage individual emission contribution from considered GHG emission sources is presented in Figure 4(a) and 4(b) for the ZLD treatment facility and conventional treatment facility, respectively. In the case of a ZLD treatment facility, fuel use on-site and electricity consumption can be identified as the GHG emission hotspots. In the case of a conventional treatment facility, methane emissions due to treated effluent discharge contribute the second-highest percentage of GHG emissions underlining the importance of water recycling by establishing a ZLD treatment facility. The complex organic compounds present in the textile effluent are considered as a potential threat to human health and aquatic organisms (Makene et al. 2019). Therefore, it is important to develop strategies that would minimize the carbon footprint of the designed ZLD wastewater treatment plants for industrial effluents, which in turn would minimize the overall environmental impact.

The availability of freshwater resources for any industrial production is particularly important. Considering the scarcity of natural freshwater resources, wastewater reuse potential has become an important factor for sustainable production (Cetinkaya & Bilgili 2019). The establishment of ZLD ETPs results in a reliable continuous process water source for the textile manufacturing industries. Also, the uniformity in water quality parameters of reused water further proves advantageous.

Based on the results of the present work, it was found that it is important to minimize the energy requirements for ZLD wastewater treatment plants. The energy consumption by reject management units can be significantly reduced by source segregation of the effluent streams. Only the effluent streams containing a high concentration of dissolved solids can be treated separately and supplied to the tertiary treatment units. The source segregation leads to a reduced volume of reject stream to be handled by reject management systems. This can lead to a reduction in the total operation time of reject management units, resulting in reduced energy consumption. The use of renewable energy sources should be encouraged along with the establishment of ZLD wastewater treatment plants, which can ensure a reduced carbon footprint by the reduction of scope two carbon emissions. The carbon footprint of a given ZLD treatment facility can be used effectively as a performance indicator to further limit the energy consumption on site.

The textile manufacturing facilities in southern India are thriving with the rising demand, and associated environmental impact is a major concern for regulators. The concept of ZLD treatment plants has been well established for such polluting industries but needs further critical analysis in terms of associated climate change impact due to higher energy consumption. Any well-quantified efforts towards the reduction of the carbon footprint of ZLD treatment facilities can further reduce the overall environmental impact of textile manufacturing activities. The conclusions of the present research work can be summarized as follows:

• 1.

  Carbon footprint analysis can be used as an effective tool to quantify carbon emissions from the ETPs, which supports further understanding of the pollution hotspots at the ETPs.

• 2.

  The total carbon footprint was estimated as 10,598 tCO₂eq/year for a ZLD treatment facility, whereas for the conventional treatment facility, it was found to be 7,761 tCO₂eq/year.

• 3.

  Second- and third-highest GHG emission contribution in terms of percentage was found to be associated with the groundwater extraction and treated effluent discharge in case of a conventional treatment plant, indicating the importance of effluent recycling.

• 4.

  The performance of various ZLD treatment plants can be compared using carbon footprint as a performance indicator to compare the energy efficiency of various units.

• 5.

  It is concluded that the carbon footprint analysis is an important tool for comparing alternate strategies in wastewater treatment as well as to evaluate the tradeoff between the environmental protection and energy requirement.

The carbon footprint of the ZLD treatment facility was found to be nearly 1.36 times the carbon footprint of a conventional treatment plant. However, efforts should be made to develop technologies that would minimize the carbon footprint, in line with the low-carbon technologies which are being attempted by many researchers across the globe.

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