Abstract
A growing trend has been observed in dairy manure nutrient management practices where a significant part of nitrogen (N) often ends up in wastewater, leading to large-scale influxes of reactive N into the water bodies and ultimately posing serious health and environmental risks. Dairy manure adds a substantial amount of nitrogen to wastewater due to its high levels of associated nutrients. Specifically, undigested dairy manure is characterized by high concentrations of ammonia (NH3), which leaches and runs off to ground and surface waters. This compound is acknowledged for its adverse environmental effects and the potential harm it may cause to aquatic ecosystems. Consequently, dairy manure management is a key aspect of environmental regulations, specifically targeting challenges such as nutrient runoff and odor management. In this context, recovering and reusing ammonia nitrogen (NH₃-N) from undigested screened dairy manure (USDM) is highly enviable. Studies reported that compared to the conventional NH₃-N removal and recovery technologies from waste streams, the vacuum thermal stripping-acid absorption process (VTS-AA) can be more efficient and effective. This process significantly removes NH3-N from waste streams by reinforcing liquid–gas phase NH3 mass transfer at a temperature lower than the normal boiling point within a shorter time with the assistance of induced vacuum and eventually favors an enhanced NH3 stripping. Furthermore, a marketable fertilizer product is produced in form of ammonium sulfate through the absorption of stripped ammonia into a sulfuric acid solution.
To date, no research has specifically applied this technology for removing and recovering ammonia nitrogen (NH3-N) from undigested dairy manure. Hence, this study was focused on the VTS-AA process for NH3-N removal and recovery from undigested screened dairy manure (USDM), followed by modeling and optimization. Four operational parameters viz. temperature, pH, vacuum pressure, and treatment time were optimized employing a central composite design (CCD) based response surface methodology (RSM) to determine the optimal conditions for NH3-N removal to obtain maximum output (NH3-N removal efficiency) and study the interactive effects of operational variables on process performance. Using the response surface methodology (RSM) based central composite design (CCD) approach, the critical operational parameters of the vacuum thermal stripping process: temperature (50–70°C), pH (9–11), vacuum pressure (35–55 Kpa), and treatment time (60–90 min) were optimized. With the specified parameters set at temperature 69.9°C, pH 10.5, vacuum pressure 53.5 Kpa, and treatment time 64.2 min, the NH₃-N removal efficiency attained was 98.58 ±1.05%, aligning closely with the model prediction.
Furthermore, the recovered ammonium sulfate ((NH4)2SO4) closely matched their commercial counterparts, confirming the effectiveness of the VTS-AA process in recovering NH₃-N from USDM. The distinct advantage of this technology lies in the concurrent energy demand reduction achieved by introducing a vacuum system. These findings contribute valuable insights into the practical implementation of the VTS-AA process for treating USDM, particularly in large-scale operational contexts. Moreover, it also provides useful implications regarding NH3-N removal and paves a path for ensuring environmental and agricultural sustainability.