Studies on the treatment of the human urine using Electrochemical Advanced Oxidation Process

Abstract

Poor sanitation and lack of effective sustainable approach have become one of the major environmental issues of concern in developing countries like India. Moreover, with continuous discharge of recalcitrant organic pollutants from urinal wastewater into the surroundings has put human health and environment under the potential risk of threat. Therefore, a sustainable yet affordable solution for the safe disposal of urinal wastewater is required. Electro-oxidation (EO) under electrochemical advanced oxidation processes has drawn great attention for the treatment of a wide variety of recalcitrant toxic mixed effluents via generating in-situ strong reactive oxygen species (ROS) such as hydroxyl radical (OH•) and reactive chlorine species (RCS). In this study, the treatment performance of the EO process for synthetic urine/actual human urine (SU/AHU) and its metabolites in batch and continuous mode of operation using mixed metal oxide anode (MMO) and doped-mixed metal oxide (doped-MMO) anode was studied. Furthermore, best efforts have been made to integrate two processes i.e. photo-catalysis (PC) and EO within the same treatment unit for enhanced degradation of pollutants. For systemic studies, the effect of various selected operational parameters such as pH, current density, electrolyte (i.e. NaCl) dose and treatment time has been studied using response surface methodology (RSM). Box-Behnken design (BBD) under RSM has been employed for experimental designing, data analysis, the interaction between EO parameters, their optimization along with evaluation in terms of percentage Degradation (%Degradation) (Y1) and Energy consumption (Y2). A quadratic model was suggested by exploiting the sequential F-test and other adequacy measures. Furthermore, experimental data were then analyzed by multiple regression analysis of RSM and simultaneous optimization of various operational parameters for the responses was performed by the desirability function approach. The best appropriate optimized conditions for batch EO treatment of uric acid with MMO anodes was found to be pH= 2, n (NaCl dose) = 0.875 g/L, j (current density)= 7.142 mA/cm2, t (treatment time) = 6.95 min with Y1= 91.571% and Y2= 0.526 kWh/m3, which showed highest overall desirability, D = 0.899. While for creatinine, the most appropriate optimized conditions were obtained at pH= 2.4, n = 0.75 g/L, j= 12.005 mA/cm2, t = 85 min, with Y1= 85.41% and Y2= 16.826 kWh/m3 and overall desirability, D = 0.899. The optimal operating conditions for maximum removal of urea were found to be j=18.14 mA/cm2, n=1.45 g/L, pH= 4 and t= 135 min, with Y1= 94.78% and Y2= 20.54 kWh/m3 which showed combined desirability, D =0.857. The v photo-activity of MMO anode in terms of quantification of generated OH• at MMO was studied under both UV and solar radiations using Fluorescence spectroscopy. The results showed that maximum OH• production was found under UV radiation as compared to solar radiation. The strength of the present study lies in the demonstration of a significant reduction in treatment time of urine metabolites by incorporating dual effect i.e. photo-electrocatalysis (PEC) under UV radiations. The dual-process with synergistic results prompts its field-scale applications for the wastewater treatment with a significant decrease in treatment time from 7 min to 5 min (uric acid), 85 to 60 min (creatinine) and 135 to 95 min (urea). The quality of treated urine metabolites samples was validated using different analytical techniques. The transformation products of uric acid, creatinine, and urea were identified through LC-MS. Similar kind of studies was performed for the treatment of urine metabolites with doped MMO. The results depicts the maximum degradation of all urine metabolites at best appropriate optimized conditions were Y1= 95.35% (uric acid), Y1 = 90.002% (creatinine) and Y1 = 91.15% (urea) respectively with minimum Y2= 2.479 kWh/m3, Y2= 25.83 kWh/m3 and Y2 = 51.53 kWh/m3 respectively. The dual-process with synergistic results has shown a significant reduction in treatment time from 43 min to 30 min (uric acid), 140 to 120 min (creatinine) and 195 to 150 min (urea). The quality of treated urine metabolites and the transformation products of uric acid, creatinine, and urea were identified through LC-MS. Based on these identified intermediates a tentative degradation pathway for urine metabolites has been proposed in this study. The parametric study of EO (batch) treatment of SU was performed using BBD under RSM using four input operational parameters were selected such as pH (Y1), current density (Y2), treatment time (Y3) and N/Cl ratio (Y4) and %COD removed (Z1), and specific energy consumption (SEC) (Z2) as responses. The most appropriate optimized conditions for batch EO treatment of SU with MMO anodes was found to be Y1= 3.42, Y2= 30.33 mA/cm2, Y3= 8.79 h, Y4 = 0.42 with Z1= 85.25% COD removed and Z2= 11.75 kWh/kg of COD removed, which showed highest overall desirability, D = 0.985. Whereas the most relevant optimized conditions for batch EO treatment of SU with doped-MMO was found to be at Y1= 4.20, Y2= 53.91 mA/cm2, Y3= 10.05 h, Y4 = 0.25 with Z1= 90.55% COD removed and Z2= 20.851 kWh/kg of COD removed, which showed highest overall desirability, D = 0.941. The photo-activity of MMO and the doped-MMO anode was investigated in terms of synergistic studies by performing treatment of synthetic urine using three different vi techniques which include PC, EO, and PEC at optimized conditions. The results showed the increased rate constant values for PEC i.e. 0.2991 h-1 (MMO) and 0.3264 h-1 (doped-MMO) over the other two processes (i.e. EO and PC). Furthermore, treatment time for SU through PEC was also found to be reduced from 8.8 h to 6 h (MMO) and 10.05 h to 6.5 h (doped-MMO). The results of insitu chemical analysis, cyclic voltammetry, FT-IR, and LC-MS analysis revealed that most of the organic components got eliminated and transformed into other byproducts which are non-toxic. Besides its efficacy towards oxidation of organic components, the disinfection efficiency of EO was also studied by spiking E.coli as (pathogenic microorganism) in SU. The complete elimination of the micro-organisms was achieved within 45 min (MMO) and 75 min (doped-MMO) of the EO treatment process. To visualize the successful commercialization of the proposed method, treatment technology must provide viable solutions economically and as well as socially over traditional methods. The total operating cost for batch EO treatment of SU with MMO and doped-MMO was estimated as 0.85 $/kg of COD removed and 1.50 $/kg of COD removed at optimized conditions, respectively. However, this cost and overall economy of the studied process depend upon the scale-up version and desirability of electrodes. The results depict a sustainable solution for the on-site treatment of urinal wastewaters in terms of the economic feasibility of the EO process as well as the stability of anodes. Moreover, the overall cost could be reduced further during scale-up studies by modifying the reactor design, operating conditions, recycling the urine as flush water and coupling the decentralized molecular hydrogen production accordingly. The scale-up trials were executed under continuous recirculation mode for the treatment of SU with a working volume of 2 L in a photovoltaic driven reactor at optimized conditions along with the strong possibility of harnessing the molecular hydrogen (H2). The results showed 83.43% (MMO) and 74.20% (doped-MMO) reduction in COD within 6 h of electrolysis under continuous recirculation mode. The volumetric fraction of H2 generated during 6 h of electrolysis in the range of 70.58% -1.7190% (MMO) and 59.142% – 3.340% (doped-MMO). Other by-product gases such as N2, CO2, CH4, and CO has also been generated during electrolysis of SU by both anodes. However, the volumetric molecular fraction of these gaseous byproducts was found increasing with treatment time due to the oxidation of pollutants present in SU into molecular N2 and CO2. The total per day cost for the treatment of SU came out to be 0.054 $ (MMO) and 0.055 $ (doped-MMO). vii After the successful EO treatment of SU with both types of anodes, the study was further extended for the treatment of AHU with MMO. An almost 68.33% reduction in COD was achieved when the actual effluent was treated under in photo-voltaic driven reactor under continuous recirculation mode. The results also showed the oxidation of nitrogen-based organic compounds i.e. urea (69.09%), uric acid (95.18%) and creatinine 67.95% within 6 h of electrolytic treatment of AHU. The results clearly indicate the oxidation of organic compounds present in AHU was due to the generated RCS and ROS during the treatment process. GC-MS spectra of untreated and EO treated samples of AHU, depicts the elimination of most of the organic components after the treatment process and the identified byproducts present in the treated solution was found non-toxic through toxicity analysis. The volumetric fraction of H2 and N2 generated during 6 h of electrolysis were in the range 69.74% – 2.617% and 18.019% – 62.133% respectively. Other byproduct gases such as CO2, CH4, and CO were also generated in small fractions during electrolysis. The durability and stability of the MMO and doped-MMO anodes were checked through various characterized techniques such as EM-EDX, XRD, Raman spectroscopy, XPS and comprehensive characterization studies for both anodes. These characterized techniques confirmed the durability and stability of MMO and doped-MMO even after 400 (1242.5 h) and 500 (1853.166 h) recycles, respectively. The comprehensive study shows that both electrodes are effectively suitable for actual urine effluent treatment and can be further extended for the commercial-scale applications. To the best of our knowledge, this is one of the few reported studies dealing with EO and PEC using novel MMO and doped-MMO anodes along with the potential of harnessing the commercially useful byproduct i.e. molecular hydrogen gas during scale-up trials for the treatment of urine wastewater. In addition, the proposed study can be boon for underprivileged areas in developing countries like India where either poor sanitation or no sanitation is provided. Thus, the study needs some scale-up executions for further validating the process.