TERI Advanced Oxidation Technology (TADOX^®) to treat industrial wastewater with integration at pre- and post-biological stage: case studies from India

Industrial wastewater treatment is still a major challenge in developing countries such as India where waste streams are not adequately treated and free from dissolved organics and recalcitrant compounds, which prevents reuse and recycling of treated water. To improve the current state of treated water reuse, the Central Pollution Control Board (CPCB) under the Ministry of Environment, Forests and Climate Change (MOEF&CC), Government of India (GoI) in 2015 came out with regulation of complying with ‘Zero Liquid Discharge’ (ZLD) for grossly polluting industries such as tanneries, slaughterhouses, chemical industries, pharmaceutical manufacturers, pulp and paper, textile and dyeing etc. (CPCB (Ministry of Environment and Forests 2015). So far, the guidelines of ZLD have been opposed by various national, regional, and local industrial associations because the way ZLD is achieved is highly resource and energy intensive, unaffordable and unsustainable. This is mainly because the current technology providers, regulators and end-users are accustomed to a conventional type of biological degradation-based treatment and the same treatment is given to all streams alike without understanding their matrix, composition and treatment requirements. Due to this generic approach, the performance of various treatment systems is greatly reduced, leading to low=quality treated water, which when reaches for subsequent tertiary treatment, involving RO and MEE etc., puts up an additional load, thus making ZLD compliance unachievable.

In this context, The Energy and Resources Institute (TERI), New Delhi has developed advanced oxidation nanotechnology (AON) - based wastewater treatment technology called TERI Advanced Oxidation Technology (TADOX^®), which has proved to be beneficial enough to be integrated in current systems to fill the gaps. Complete details of the technology and the process flow have been published and discussed in earlier reports (Bahadur & Bhargava 2019; Bahadur et al. 2020; Das et al. 2020). Some of the advantages of the advanced oxidation process (AOP)/AON approach are the minimal addition of chemicals, recovery and reuse of used nanomaterials and simultaneous destruction of recalcitrant organics and bio-pollutants including pathogens such as viruses and bacteria (Oturan & Aaron 2014).

This integrated approach may also result in enhancement of quality of RO permeate, reduce number of RO cycles and reduce overall load on MEE. Earlier studies on polishing of tannery effluent (Panizza & Cerisola 2004; Hasegawa et al. 2014; Sivagami et al. 2018; Zhao & Chen 2019), pharmaceutical effluent (Janssens et al. 2017; Talwar et al. 2018; Fanourakis et al. 2020)and slaughterhouse effluent (Bustillo-Lecompte & Mehrvar 2016; Bukhari et al. 2018; Ozturk & Yilmaz 2019; Vidal et al. 2019) have shown promising results, which means that TADOX^® implementation in a similar way can improve the scenario for these grossly and highly polluting industries.

This paper presents a total of six case studies showing versatility in integration and implementation of TADOX^® at the pre-biological stage and post-biological stage, as per the nature of the effluent under analysis. To the best of our knowledge, there is no other publication that has given such an in-depth and versatile integration approach for any AOP-based technology. Case studies of integration of TADOX^® at the pre-biological stage include three case studies of chemical effluents from 2,4 D pesticide manufacturing plants, speciality chemical anisole plant and hydrocarbon-rich stream from oil drilling waste pits with an aim to improve the performance of downstream biological treatment with improving biodegradability of the effluent (increased BOD/COD ratio) and reduction in toxicity of the effluent. Three case studies of integration at the post-biological stage includes, bio-treated pharmaceutical effluent, tannery effluent and slaughterhouse effluent. Further, this opens tremendous opportunities in implementation at the polishing stage, which is expected to improve legal compliance and enable high-end reuse of treated water, thereby improving water reuse efficiency in the plant.

As shown in Figure 3 For primary treatment or Stage 1, to a 10–100 L/hr continuous flow wastewater sample, previously optimized dose of an alkali earth metal oxide and saturated solution of polyelectrolyte was added (proprietary formulation) under fast stirring for 5 min and later slow stirring for 15 min. Thereafter, the formed flocs could settle for about one hour or until a clear supernatant is obtained. Thereafter, a supernatant was aerated for mixing of n-TiO₂ powdered nanomaterial to be added subsequently. The obtained suspension was vigorously stirred and fine bubble aeration continued for 30 min. Thereafter, the suspension was left undisturbed for 30 min to attain adsorption-desorption equilibrium in the contact tank.

In the Stage II of the treatment, the suspension was re-aerated and well mixed for 10 mins before being pumped to a photocatalytic reactor (PCR) (patented design having optimised geometry and suitable UV light radiation source) and treated under recirculation mode for 120–180 min (as per requirement). Thereafter, the treated water was passed through an in-house designed and developed assembly utilising suitable filtration to separate used nanomaterial. Finally, treated water was obtained and spent nano-catalyst was recovered (Bahadur & Bhargava 2019; Bahadur et al. 2020; Das et al. 2020; Bahadur 2021).

pH, electrical conductivity, and total dissolved solids (TDS) were analysed by Pocket Pro Plus multi tester by HACH, USA. UV–vis spectra were recorded on UV–vis spectrophotometer model DR6000 by HACH, USA. Analysis of other wastewater quality parameters were carried out at National Accreditation Board for Testing and Calibration Laboratories, India (NABL) accredited laboratory as per ISO/ IEC 17025:2015. Micropollutants and pathogens were analysed as per standard methods given in APHA 23 edition 2017 at an accredited laboratory (Rice et al. 2017).

The estimation of EE_Volume is significant to calculate the overall cost of treatment of wastewater (Bahadur et al. 2020).

For estimation of cost of treatment, three of the most important parameters are considered, (i) chemical consumption, (ii) power consumption and (iii) cost of replacement of electromechanical units.

Firstly, to compute the cost of chemicals used in the treatment, the stagewise consumption in g/m³ of coagulants, flocculants and Nanomaterials used in treatment have been estimated. Thereafter, the unit rates in INR/kg of each of the coagulants, flocculants and commercial nanomaterials have been used to compute stage wise cost of treatment for one cubic meter wastewater.

Secondly, the computation of the power consumption of the treatment has been done by considering the drawn current, rated voltage, and the operational hours for each of the electro-mechanical units such as agitator, air blower, pumps, UV lamps and nanomaterial recovery unit. The overall energy consumption is then computed in kWh per cubic meter of effluent treated and cost of treatment is then calculated in Indian rupees (INR) per cubic meter considering the INR 8 per kWh rates prevalent at the time of treatment in Haryana, India. Conversion from US dollars (USD) to INR has been done as per 1 USD equivalent to 75 INR.

Thirdly, the cost towards maintenance of the plant and machinery and cost towards the replacement of UV lamps and other electromechanical items have been estimated as per typical life of 12 months of continuous operation. Thus, computation of the total cost includes all expenses as elaborated above for the end-to-end treatment.

As evident from Figure 4 and Table 1, there is substantial difference in the aesthetics and water quality after TADOX^® treatment. Overall 5 h end-to-end treatment led to treated water having 62% reduction in BOD and 91% reduction in COD. The BOD/COD ratio, indicative of biodegradability, improved by almost four times from 0.008 to 0.035 as the result of TADOX^® treatment. Moreover, toxicity of samples is also expected to be reduced greatly because of 60% removal in total phenolic content from the treated water. Although there is a scope of further improvement in biological degradability of the stream with a better BOD to COD ratio (around 0.3) thus higher retention time of treatment could greatly benefit the results and can be planned in future. Such a toxicity reduction is essential to maintain the biological activity of the system in subsequent stage (Vollertsen & Hvitved-Jacobsen 2002; Vilar et al. 2012; Holkar et al. 2016; Zhang et al. 2020). From the picture it can be clearly seen that the sample became dark coloured, which is attributed to the oxidative degradation of the phenolic compounds and have resulted in the significant COD reduction. Odour of phenols from the sample was also reduced significantly. When treated with TADOX^®, treated samples converted into dark brown colour which is indicative of oxidation of phenols and four times improved biodegradability with COD reduction of 90% and BOD reduction of 50–60% (without biological treatment). Thus, this study indicates that when TADOX^® is integrated at a pre-biological stage it will help in downstream biological treatment and is also expected to reduce the load on subsequent tertiary treatment, thus elaborating the benefits.

E_EO for this stream has been estimated to be 15.5 kWh/order-COD·m³, which is lower than literature reported data of application of photocatalytic process in similar streams, thus this makes TADOX^® a promising approach for commercialisation. Energy requirements per unit treatment volume for this stream has been estimated to be 16.2 kWh/m³. The overall treatment cost for this stream is calculated to be 3.3 USD/m³ at laboratory scale in batch mode operation, which is expected to be reduced to 1.66 USD/m³ at a commercial scale in continuous mode operation of TADOX^® plant. The derived cost of treatment is 58% lower than the existing cost incurred by the industry for treatment of this difficult wastewater.

As clearly shown in Figure 5 and data reported in Table 2, there is substantial change in the aesthetics of the sample and there is significant reduction in BOD (52%), COD (90%) and total phenolic content (49%). Residual parameters of the TADOX^® treated sample shows that this water is suitable for downstream biological treatment with BOD/COD ratio increased from 0.093 to 0.45, i.e. five times higher biodegradability within 5 hours of end-to-end treatment (Holkar et al. 2016). E_EO for the treatment of this stream has been estimated to be 24.4 kWh/order COD·m³, which is also lower than the previous estimations for energy consumption for photocatalytic treatment of industrial wastewater in the range of 50–200 kWh/m³ (Miklos et al. 2018). The overall cost of treatment of this stream at lab scale implementation has been estimated at 5.35 USD/m³, which is expected to be 3.46 USD/m³ at commercial scale continuous mode plant operation. The derived cost of treatment is 43% lower than the existing cost incurred by the industry for treatment of this difficult wastewater. Thus, such a pre-treatment has resulted in removal of toxicity and improved biodegradability, which has led to an increase in overall efficiency of treatment together making the treatment cost effective and energy efficient.

It is clear from above Figure 6 that TADOX^® treatment led to improvement in colour, transparency, and turbidity of the sample. UV-visible spectra also show substantial changes, which indicate the reduction of colour, COD and dissolved organic content. Detailed water quality analysis of the pre- and post-treated samples is shown in the Table 3.

Table 3 shows significant reduction in heavy metal content such as Zn (75%), Pb (99.9%) and Ni (55%) with high 52% reduction in COD with 100% increase in BOD concentration. This reduction of COD and increase in residual BOD concentration is because the treatment led to degradation of long chain hydrocarbons, which are highly non-biodegradable, into smaller and biodegradable compounds, which resulted in higher BOD values. This means that biodegradability (BOD/COD) increased more than four times from 0.09 to 0.37, so TADOX^® treated water can be efficiently degraded through downstream conventional biological treatment plants (Holkar et al. 2016). End-to-end TADOX^® treatment of this effluent stream took 3 hours, which is much lower than conventional technologies and the E_EO was estimated to be 19 kWh/order COD·m³ with overall cost of treatment at continuous scale to be 0.9 USD/m³, which is about 80% lower than the current cost incurred by the industry for pre-treating this stream. Thus, such an intervention could result in substantial improvement in the current treatment regime.

As shown in Table 4, TADOX^® treatment led to 96% removal of BOD, 93% in COD, 90% in total nitrogen and 98% in phosphorus content. TADOX^® treatment result in treated water quality suitable for reuse in applications cooling tower makeup, boiler makeup etc. Overall cost of the treatment is expected to be 1.3 USD/m³ at batch scale and may reduce to 0.86 USD/m³ at continuous scale plants and the E_EO for the treatment has been evaluated to be 6.9 kWh/Order COD·m³ with EE_volume of 8.1 KWh/m³. Existing unit incurs about 1.4 USD/m³, which translates to about 38% direct OPEX reduction through TADOX intervention in this treatment trail. Thus, when compared with application at pre-biological stage, the TADOX^® integration at post-biological appears to be more energy efficient and cost competitive.

As shown in Figure 8, the biologically treated tannery effluent had residual colour, odour and dissolved organics, whereas after TADOX^® treatment, the treated water is free from any colour and organics have been removed, which is also clearly evident from the UV-Vis spectra. Detailed water quality analysis has been carried out and tabulated in Table 5.

From the data in Table 5, it could be seen that there is reduction in COD values and increase in BOD value indicating that the residual dissolved organics are biologically degradable, and the complex organics have been converted into simpler contaminants which are naturally degradable. Treated water quality obtained to have COD less than 100 mg/L and BOD less than 30 mg/L clearly meets the Inland Surface discharge norms as per GSR 422 (E) Environment Protection Rules, 1986 (Amended 2018) by MoEF&CC, Govt. of India (Central Pollution Control Board (Ministry of Environment and Forests 2018). Moreover, this quality meets the criteria for water reuse. The overall cost of treatment is expected to be 1.3 USD/m³ with the overall energy requirement of 6 kWh/m³ for the treatment, which is an affordable way to obtain such high-quality treated water. Existing unit incurs a cost of 2.1 USD/m3 for polishing of this water prior to its discharge into the industrial cluster drain, hence TADOX^® implementation has potential to reduce cost of polishing by 38%.

Slaughterhouse industry, is a highly polluting processing industry, which produces meat and other animal-derived products such as fats and oils etc., which are used in several chemical and pharmaceutical industries. This industry houses a large portion of the workforce in UP, India. Regulators are pushing this industry to achieve high water quality standards for its treated water and reuse wastewater as much as possible; however, due to lack of technological options there has been a delay in the implementation of ZLD in this industry. The main problem of this effluent is the recalcitrant and residual organic content in the treated effluent, which cannot be degraded biologically and through conventional processes. TADOX^® integration at the post-tertiary stage of treatment or polishing, so to say is expected to remove the residual organics, smell and colour from the sample making it reusable into processes such as washing, scrubbing and dust suppression within the plant premises. 3-hour end-to-end TADOX^® treatment was carried out for this stream and the treatment performance has been shown in Figure 8 and wastewater quality parameters for the two samples is tabulated in Table 6.

Data from Table 6 clearly indicates that the TADOX^® treated water was almost free from organics and had COD less than 10 mg/L and undetectable levels of BOD, indicating that water may be efficiently reused back in the process without any associated health risks. Overall treatment took 3 hours, and the energy requirements were 8.3 kWh/order COD·m³ and the cost of operation has been estimated at 1.3 USD/m³ for batch-scale smaller installations and 0.86 USD/m³ for large-scale continuous flow systems. Thus, TADOX^® intervention at tertiary stage is expected to be reduce OPEX by 50% in comparison to the current chemical-based treatment used by the industry.

Based on the discussed case studies, it maybe concluded that integration of TADOX^® in existing systems is expected to improve quality of treated water, at the same time reduce operating expenses (OPEX). Implementation of TADOX^® at large-scale installations such as common effluent treatment plants (CETP) can aid in handling shock loads of pollutants, due to non-selectivity of pollutants from mixed wastewater; TADOX^® at CETPs may also be used at the pre-biological treatment stage to remove toxicity, reduce COD, and improve biodegradability. When used at the end of treatment i.e., at polishing stage, it improves reusability of treated water to comply with regulatory norms. TERI Advanced Oxidation Technology (TADOX^®) might prove superior to existing technologies in the following ways:

• (i)

  clean, green, and highly efficient technology utilising intrinsic property of nanomaterials and strong oxidising power of hydroxyl radicals as compared to other oxidising species involved in other AOPs,

• (ii)

  complete degradation and mineralisation of suspended as well as dissolved organics,

• (iii)

  highly cost effective as these have limited use of externally added chemicals,

• (iv)

  generating negligible sludge, hence mitigate associated secondary pollution issues,

• (v)

  integrated approach with existing systems for compliance with ZLD norms for recycle and reuse.

E_EO analysis for various difficult wastewater streams has been done in this study and it clearly shows that TADOX resulted in lesser electrical energy consumption promising as compared to those reported earlier by (de Sá et al. 2018; Miklos et al. 2018; Bahadur et al. 2020; Ghaffarian Khorram & Fallah 2020) who worked on similar difficult wastewaters. Thus, TADOX^® proves to be highly efficient and economical as compared to other AOPs and similar technologies in this domain given that AOPs having E_EO below 50 kWh/m³ maybe commercialised at large scale (Miklos et al. 2018; Loeb et al. 2019).

Further to the guidelines of ZLD in various polluting industries published by CPCB, GoI in 2015 wherein advanced technologies, newer and cleaner approaches for stream-specific treatment has been mentioned and introduced; TADOX^® could be a useful integration approach in wastewater treatment for such highly polluting industries. Hence, TADOX^® may be an important technology to bring in the much-needed revolution in wastewater treatment industry and promote enhanced water reuse.

Authors gratefully acknowledge that TADOX^® has been developed under DST Water Mission, Water Technology Initiative (WTI) Programme of Ministry of Science and Technology, GoI [Ref No: DST/TM/WTI/2K16/78(G)]. National Mission for Clean Ganga (NMCG), Ministry of Jal Shakti, GoI is acknowledged for funding such studies on pollution abatement under the NMCG-TERI Centre of Excellence (CoE) on Water Reuse [Ref. No: Mi/31/2021-PMC-NMCG]. These case studies are part of invited talk delivered in the Singapore International Water Week (SIWW 2021) in the Theme 3.13 - Industrial wastewater treatment and reuse. Authors are thankful to all the industries for providing the samples.

All relevant data are included in the paper.

The authors declare there is no conflict.