Abstract
The anaerobically digested dairy wastewater (ADDW) contains a significant quantity of organic and inorganic pollutants, nutrients, total suspended solids, and microorganisms. The anaerobic digestion process converts organic nitrogen into inorganic nitrogen, which increases the concentration of ammonia nitrogen (NH3-N) in the influent. Discharge of the ADDW without appropriate treatment can cause serious environmental issues like eutrophication and oxygen level depletion problems in surface water ecosystems, which cause harmful effects on human health as well. Therefore, there needs to be a sustainable wastewater treatment approach that will resist further environmental consequences after discharge. Among various treatment approaches, the electrochemical treatment process is considered a clean, promising, and adaptable solution for treating ADDW. In this study, electrochemical treatment approaches have been applied to remove and recover NH3-N from ADDW. The electro-oxidation (EO) treatment technique was applied to remove chemical oxygen demand (COD), ammonia nitrogen (NH3-N), total phosphorus (TP), orthophosphate (OP), total nitrogen (TN), and total Kjeldahl nitrogen (TKN) from ADDW. The major operational parameters, viz., current density, electrolyte concentration, treatment time, and mixing speed, were optimized using response surface methodology (RSM). Optimal conditions were determined to be a current density of 90 mA cm−2, an electrolyte concentration of 0.08%, a treatment time of 180 minutes, and a mixing speed of 400 rpm. Under optimal conditions, the removal efficiencies for COD, NH3-N, TP, OP, TN, and TKN were 78.36%, 63.93%, 87.41%, 92.39%, 67.01%, and 81.42%, respectively. The reaction rate adhered to the first-order kinetic model for pollutants removal, exhibiting correlation coefficients (R²) approaching 1. The findings indicate the potential of the EO process for treating high pollutant-laden ADDW and suggest the need for further studies to validate these outcomes on a pilot scale.
The combination of chemical coagulation and photo electro-Fenton treatment is regarded as an effective technology for the removal of NH3-N from ADDW. This study employed Taguchi design to reduce the operating factors from five to three. The three operating factors were subsequently optimized for maximum NH3-N removal efficiency through the application of response surface methodology (RSM) in conjunction with the Box–Behnken design (BBD). Under optimal conditions of 0.51 mM Fe2+, current density of 49.44 mA/cm2, and treatment time of 118.60 minutes, a removal efficiency of 92.13% NH3-N from ADDW was attained, with a 90% selectivity of N2 during validation experiments. The degradation rate of NH3-N was determined to be 0.0229 min⁻¹ using a pseudo-first order kinetic model. The findings indicate the effectiveness of the integrated chemical coagulation and photo electro-Fenton process in substantially decreasing NH3-N concentration in ADDW.
A transmembrane electro-chemisorption (TMECS) ammonia recovery system was employed to extract NH3-N from the ADDW. Three operational factors, namely current density (0-150 A/m²), catholyte concentration (0.25-1.00 M), and the anolyte to catholyte ratio (1:1-3:1), were utilized to assess their impact on NH3-N recovery from ADDW. The system exhibited broad applicability for current densities between 0 and 150 A/m², attaining NH3-N removal and recovery efficiencies of 30.34%-99.49% and 9.84%-76.45%, respectively, throughout an 8-hour treatment period. The generation of authigenic acids and bases via electrolysis in anolytes and catholytes, respectively, is crucial for converting ammonium (NH4+) into free ammonia (NH3). Fourteen distinct machine learning (ML) models were utilized to assess the predictive efficacy of NH3-N recovery efficiency. The feedforward artificial neural network (ANN) model surpassed other machine learning models, achieving the highest coefficient of determination (R²=0.9816) and the lowest root mean squared error (RMSE=2.6062). The impact of input factors was also assessed using the feature significance plot, partial dependency plot, Shapley additive explanation (SHAP) dependence plots, and summary plot. The evaluation of feature importance revealed that the anolyte to catholyte ratio is the most significant parameter influencing NH3-N recovery. This study may facilitate the prediction of electrochemical NH3-N recovery from ADDW, thereby improving the optimization, scalability, and commercialization of TMECS technology to achieve the nitrogen cycle.
The TMECS system was operated at different load ratios to evaluate ammonia recovery efficiency. The load ratio (Ln) represents a key operational parameter in optimizing the TMECS -driven recovery of total ammonia nitrogen (TAN) from ADDW. It is defined as the proportion of the applied current density relative to the TAN loading rate. The findings indicate that higher load ratios (Ln > 2) significantly enhance TAN recovery efficiency and increase TAN flux across both the cation exchange membrane (CEM) and the gas-permeable membrane (GPM). This improvement can be attributed to the elevated generation of hydroxide ions (OH⁻) in the catholyte and protons (H⁺) in the anolyte, which facilitate the conversion of ionic ammonium into volatile ammonia. Moreover, regression models were developed to describe the relationship between acid/base production rates and the load ratio. These models provide a predictive framework for optimizing TMECS operation, thereby improving system performance and cost-effectiveness, and supporting the transition toward larger-scale applications based on integrated experimental and modeling insights.
This research advances the development of sustainable approaches for NH3-N removal and recovery from ADDW. The implementation of an electrochemical-based TMECS system offers an effective and environmentally sustainable approach for NH3-N recovery, thereby supporting the transition toward a circular economy and more resource-efficient wastewater treatment.