Abstract
To meet the requirement for ammonia nitrogen removal in dairy wastewater and anaerobic digestate could be a bottleneck to dairy farmers. Manure has high concentration of ammonia. To improve the situation, a new process for ammonia removal and recovery was developed in this study. Firstly, thermal stripping was coupled with acid absorption to recover ammonia from filtrate of digested and undigested dairy manure. Later, a vacuum system was coupled to thermal stripping setup to recover ammonia from liquid digested manure. And lastly, a pilot scale setup was developed to test ammonia recovery from digestate of liquid manure, foodwaste, and municipal sludge, and landfill leachate.
When thermal stripping-acid absorption was used to test the effects of temperature, pH, and dissolved solids, it was found that ammonia recovery increased with temperature. At boiling point temperature (102 oC), ammonia was completely stripped in approximately 4 h with a half-life of 1.6 h for the digested manure and in 3 h with a half-life of 1.4 h for the undigested manure. Ammonia mass transfer coefficient increased exponentially with temperature. The average ammonia mass transfer coefficient upon boiling was 31.4 mm/h in filtrate of digested manure and 30.3 mm/h in filtrate of undigested manure.
When vacuum was coupled to thermal stripping - acid absorption, about 96 – 100% of the ammonia in the digestate of dairy manure was removed in 4 h. Both ammonia mass transfer coefficient KL and saturation concentration CS were similar across the combinations of boiling point temperature 65 – 100 °C and vacuum pressure 25.1 – 101.3 kPa. The optimum boiling point was at temperature 65 °C and vacuum pressure 25.1 kPa. At 65 °C and 25.1 kPa, more that 95% of ammonia could be stripped out of digestate in 1.5 h with an ammonia mass transfer coefficient as high as 37.3 mm/h. Stripped ammonia was absorbed in an acid solution (H2SO4). By controlling sulfuric acid concentration in the absorption solution, (NH4)2SO4 content of the harvested solid product was up to 94.3 – 106.8%.