Removal And Recovery Of Nitrogen From Dairy Farm Biogas Slurry By Ammonia Stripping

Along with the continued growth in demand for livestock and poultry products,large-scale farming industry has been developing. At the same time, the amount of livestock and poultry waste also increased dramatically. Biogas engineering could obtain biogas by anaerobic fermentation of waste, but also produced a large quantity of biogas slurry. Restricted by the lack of land, the biogas slurry used for land given could not be widely implemented. In view of the fact that biogas slurry consists of high concentration of nitrogen and low C/N ratio, direct emissions of biogas slurry would bring serious pollution problems, and biochemical purification treatment could not get good effect. So, it is really necessary to find the effective method to remove pollutants from the biogas slurry, especially ammonia nitrogen.Aeration stripping and stripping column were studied with dairy farm biogas slurry in this paper. In order to obtain the optimal conditions of removal and recovery of nitrogen by ammonia stripping and provide the basis and reference for purification treatment of biogas slurry, the influences of temperature, air liquid ratio（aeration quantity） and initial p H on ammonia removal efficiency were investigated respectively,the recovery of nitrogen with sulfuric acid was tested. Furthermore, pilot ammonia stripping reactor was built to do the pilot experiment.The main contents and results were as following:（1） Ammonia removal of aeration stripping and stripping column were influenced by temperature, air liquid ratio（aeration quantity） and initial p H, but the influence is different. The ammonia removal rates could both be more than 85% under the reaction conditions recommended for aeration stripping and stripping column.As for aeration stripping, because the p H of biogas slurry could increase with aeration, the effect of initial p H 8⁹.5 on ammonia removal was insignificant. The orthogonal experiments showed that the order of influence factors was as follow:temperature>aeration quantity>initial p H, the reaction conditions recommended were:temperature of 40℃, aeration quantity of 3600L（air）/L（biogas slurry）, p H of 9.5, the removal rate of ammonia was 85.2%. As for stripping column, because the reaction in the stripping column was rapid and the duration of air-liquid contact was short, biogas slurry had little variation in p H, so the effect of p H on ammonia removal was significant.The orthogonal experiments showed that the order of influence factors was as follow:temperature>initial p H>air liquid ratio, the reaction conditions recommended were:temperature of 40℃, air liquid ratio of 50, p H of 10.5, the removal of ammonia was89%.（2）Adding Ca（OH）₂ and ammonia stripping could both remove a certain amount of TP and COD of biogas slurry. As for Ca（OH）₂, With the dose of less than 14.6g/L, the removal efficiencies were poor with the best removal rates of 26.5%, 17.5%respectively; with the dose of more than 14.6g/L, the removal rates could be the best value of 91.3%, 76.8% respectively.As for ammonia stripping, aeration stripping could remove a little of COD, the COD removal rate of original biogas slurry was 3.7% with air quantity of 4000L/L’; because the duration of air-liquid contact was short, some acidoid such as CO2 could not be removed by stripping, so that the stripping column had barely effect on COD removal.（3）Ammonia removal rates could be 85% in the pilot ammonia stripping experiment when the influent flow was 500L/d. The gas mixture from the stripping column was absorbed by sulfuric acid（concentration of 0.4mol/L, seed loading of0.035）, and the ammonia recovery rate could be more than 70%. The way with absorption by alkali liquor firstly and then by sulfuric acid could obtain purer ammonium sulfate crystals.（4） The treatment cost of ammonia stripping in the pilot experiment was 6.74￥/m3.In view of the fact that（NH4）2SO4 could be recovered 1.4kg/m3 from biogas slurry, and obtain 1.12 元/m3 from biogas slurry,so the treatment cost could be lower.