Ammonia stripping process via solar assisted heating for swine manure biogas slurry based on CaO regulation

Biogas slurry treatment has transformed from 'pollutant removal' to 'nutrient recovery' in the context of 'sustainable development' and 'circular economy'. The ammonia stripping process can be expected to efficiently remove the ammonia nitrogen for the nitrogen transformation and the nutrient recovery from the biogas slurry. The cost of alkali addition (especially sodium hydroxide (NaOH) and heating) can account for more than 70% of the total cost in the stripping process, leading to higher operation costs. Therefore, this study aims to reduce the energy consumption in the ammonia stripping of swine manure biogas slurry. The calcium oxide (CaO) was used instead of NaOH to adjust the pH value of biogas slurry, due to its lower price. Solar heating was used to treat the biogas slurry, instead of the traditional heating coupled ammonia stripping. The results showed that the CaO adjusted the pH value of biogas slurry, indicating a certain flocculation effect. A better performance was then obtained in the form of calcium hydroxide. Once the added concentration was 5 g/L, the pH value of the biogas slurry exceeded 12.0, where the removal rates of turbidity, COD, and TP reached 88.0%, 50.1%, and 93.8%, respectively. The influencing factors were ranked in the descending order of pH, temperature, and gas-liquid ratio on the efficiency of ammonia stripping. In the swine biogas slurry with the low buffering capacity, the pH of the swine biogas slurry should be controlled above 10.8 for the stripping, in order to ensure≥70% ammonia nitrogen removal rate. Meanwhile, there was an increase in the temperature for the higher efficiency of ammonia stripping at the higher pH of the biogas slurry. Only when the pH and temperature of the biogas slurry were adjusted to the optimal ranges, the increasing gas-liquid ratio significantly promoted the ammonia stripping efficiency. The lowest running cost was achieved in the ammonia stripping process under the condition of a high pH value of 12.0 without heating (25 ℃) was about 2.46 Yuan/m³. Compared with the temperature-driven ammonia removal rate, the cost was reduced by 52.1%-68.7% among various stripping parameters (ammonia nitrogen removal rate>80%). However, the heating further improved the ammonia nitrogen removal rate. The coupling of solar energy and ammonia stripping significantly increased the ammonia nitrogen removal rate. The biogas slurry was adjusted by CaO in the form of 10% calcium hydroxide with an additive concentration of 5 g/L. After that, the biogas slurry and air temperature increased to 40.7-48.2 ℃ and 43.6-61.5 ℃ using solar collectors, respectively. The ammonia nitrogen removal rate of≥97.5% was obtained by the self-developed solar-assisted heating ammonia stripping device in summer. Even in winter, the average air and biogas slurry temperatures after heating reached 37.6 and 31.4 ℃, respectively. The removal rate of ammonia nitrogen reached more than 86%. This finding can provide a promising coupled solar application for the removal of low-cost ammonia nitrogen.