Purpose
Ammonia (NH3) found in anaerobically digested dairy wastewater (ADDW) could pose a serious threat to the environment. Various methods, including ion exchange and reverse osmosis, have been employed to remove ammonia from ADDW. While these techniques can be effective, they have significant drawbacks, particularly the generation of highly concentrated wastewater as a byproduct. This concentrated effluent poses a considerable challenge for safe disposal, potentially leading to secondary environmental impacts if not managed appropriately. As a result, while these methods address ammonia removal, they often shift the burden to another critical area, necessitating the development of more sustainable and holistic wastewater treatment solutions. In recent years, an electrochemical approach has garnered significant attention as an innovative and efficient alternative for wastewater treatment. This method is gaining growing recognition for its effectiveness in degrading a broad spectrum of pollutants, including ammonia, with minimal chemical additives. Its versatility, coupled with the potential for on-site application and reduced secondary waste production, makes electrochemical treatment a compelling solution for addressing the challenges posed by traditional wastewater treatment technologies. Different active radicals (•OH, OH–) generated during electrochemical process are used to oxidize NH3 to nitrogen gas (N2) and increase the selectivity of N2 (Eq. 1-4). The selectivity of N₂ in ammonia decomposition measures how much of the nitrogen from NH₃ is converted into N₂ gas instead of forming other nitrogen-containing byproducts.
(1) 2NH3 + 6OH– → N2 + 6H2O + 6e–
(2) NH3 + •OH → •NH2 +H2O
(3) NH2 + •NH2 → N2H4
(4) N2H4 → N2 + 2H2
Not many studies have looked into how ammonia breaks down during electrochemical treatment or how to predict this process. One common problem is that ammonia undergoes oxidation beyond the desired or controlled extent, leading to the formation of undesirable products like nitrate (NO3–), nitrite (NO2–) etc. Achieving high ammonia removal efficiency and selective conversion to non-reactive N₂ gas is critical for optimizing electrochemical treatment. The purpose of this research was to investigate the viability and kinetics of electrochemical treatment for improving dairy wastewater quality through ammonia removal at different current densities.
What Did We Do?
Anaerobically digested dairy wastewater was sourced from a commercial dairy facility in southern Idaho and was stored at 39.2°F prior to the experiment. The concentrations of ammonia, nitrate, and nitrite in the collected wastewater were measured using a Hach DR 5000 spectrophotometer.
In the electrochemical reactor, a niobium-based boron-doped diamond (BDD/Nb) electrode was used as the anode, while a graphite plate served as the cathode (Fig. 1). Both electrodes had a working surface area of 3.10 in2 (20 cm²), with the interelectrode gap kept constant at 0.39 inch (1 cm).
Different levels of electric current (20, 30, 40, and 50 mA/cm2) were applied to study their effect on how ammonia was efficiently removed. The breakdown process of ammonia was analyzed using a mathematical model called pseudo-first-order kinetics. Additionally, changes in ammonia, nitrate, and nitrite levels and production of N2 gas were recorded over a 120-minute treatment period. The connection between the reaction speed and the applied current was also examined.
What Have We Learned?
Figure 2 illustrates the effect of applied current density on the removal of ammonia during the electrochemical treatment of ADDW. The removal of ammonia increased substantially with higher applied current densities (from 20 to 50 mA/cm2), with removal efficiency of 80.12% to 98.26% during a 120-minute treatment time. The applied current density is a critical operating factor that influences the electrochemical reaction by regulating the generation of active radicals on the electrode surface. This trend can be attributed to the fact that higher current densities enhance the formation of active radicals, which in turn accelerates the ammonia oxidation rate.
Figure 3 shows that ammonia removal at various current densities followed the pseudo-first order kinetic model. The relationship between the reaction rate constant (min-1) and applied current density (mA/cm2) demonstrated an exponential function with a high correlation coefficient value (R2= 0.98) (Fig. 4). This supports the accuracy of the pseudo-first order kinetic model in describing ammonia removal from ADDW. From the concentration profile, it is clear that a substantial amount of nitrogen was released from the system into the gas phase, primarily as N2 gas. This nitrogen loss from the system was estimated based on the total nitrogen mass balance. Ultimately, the selectivity of nitrogen reached to 90%. It was noted that the concentration of NH3 declined over time during the electrochemical treatment, with only a small amount of NO3− and NO2− being produced. The final concentration of NO3− and NO2− were 140 mg/L and 0.87 mg/L respectively. It has been documented that NO2⁻ can undergo reactions with NH3 to form N2 or be oxidized by oxygen gas (O2) to produce NO3⁻. This likely explains why the final concentration of NO2⁻ was lower compared to that of NO3⁻. All of these findings clearly demonstrate that the electrochemical treatment can effectively remove ammonia from ADDW and achieve high nitrogen selectivity.
Future Plans
In the future, we will work on nitrogen and phosphorus recovery simultaneously from dairy liquid manure by applying electrochemical treatment approach.
Authors
Presenting author
Ashish Kumar Das, Ph.D. Student, Environmental Science Program, College of Natural Resources, University of Idaho
Corresponding author
Dr. Lide Chen, Professor, Department of Soil and Water Systems, Twin Falls Research and Extension Center, University of Idaho, lchen@uidaho.edu
Acknowledgements
This research was funded by the USDA Sustainable Agricultural Systems Initiative through the Idaho Sustainable Agriculture Initiative for Dairy (ISAID) grant (Award No. 2020-69012-31871).
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