Performance of ammonia recovery from biogas slurry by direct contact membrane distillation crystallization

Biogas slurry is known as the effluent from livestock and poultry farms. There is a substantial amount of nutrients and water beneficial for agricultural purposes. The direct application of biogas slurry into farmland is often confined to environmental concerns. It is often lacking the necessary cropland to absorb the waste products in large-scale farms, which generate significant volumes of biogas slurry. Moreover, the biogas slurry can be treated for irrigation. But the high costs associated with the treatment are hindered by the high nitrogen content. Prior to the treatment as wastewater, the recovery of ammonia from the biogas slurry can be expected to represent a viable solution. Among the various technologies available for ammonia recovery, thermal stripping can serve as a cost-effective and low-carbon emission suitable for biogas slurry treatment. Nevertheless, conventional thermal stripping can typically yield ammonia in liquid form via gas stripping or membrane distillation, resulting in low fertilizer products in market value. In this study, an innovative approach was proposed to recover ammonia nitrogen from the biogas slurry in a crystalline form using membrane distillation-crystallization. The temperature of the receiving solution increased to equalize the vapor pressures on both sides of the membrane. A favorable ammonia mass transfer rate was achieved to reduce the rate of water vapor transfer. As a result, the receiving solution was supersaturated to allow for the recovery of ammonium salt crystals by cooling to room temperature. The results revealed that the temperature of the nearly saturated phosphoric acid diammonium solution needed to be raised to 47 °C when the temperature of the biogas slurry on the feed side was 40 °C. Conversely, the temperature of the nearly saturated ammonium sulfate solution had to be elevated to 65 °C, when using sulfuric acid as the absorbent. Moreover, the increasing temperature of the acid solution enhanced the ammonia flux, which rose from 10.7 g/(m²·h) at 40 °C to 14.9 g/(m²·h) at 70 °C. Higher ammonia nitrogen concentrations on the feed side significantly boosted the ammonia mass transfer flux. With phosphoric acid diammonium as the absorbent, the ammonia nitrogen recovery in the crystals reached 77.60% after 6 h of testing. However, the recovery ratio increased to 92.20% using sulfuric acid as the absorbent. The higher ammonia recovery ratio was primarily attributed to the lower pH value of sulfuric acid. Furthermore, the ammonia nitrogen recovery in the crystals even reached as high as 116.70%, when the treatment time was extended. The membrane distillation-crystallization can be expected for the ammonia nitrogen recovery from biogas slurry. A valuable reference can also provide for the efficient recovery of ammonia nitrogen from biogas slurry, although it remained a preliminary concept. Additional research should be conducted to examine the correlation between water and ammonia mass transfer, in order to reduce heat conduction. Particularly, it is also required for a better understanding of the crystallization mechanism of nitrogen fertilizer, due mainly to the heterogeneous nucleation properties inherent in the membrane process.