Gas-phase biofiltration for livestock building ammonia emission mitigation

Biofiltration;ammonia mitigation;nitrous oxide;moisutre sensor;microbial community

Yang, Liangcheng

关键词: Biofiltration;    ammonia mitigation;    nitrous oxide;    moisutre sensor;    microbial community;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/45335/Liangcheng_Yang.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Biofiltration is recognized as an effective technology to mitigate certain livestock building ammonia emissions. Biofilters are bio-reactors that can absorb ammonia and then oxidize it into nitrite and nitrate using microorganisms. Woodchips and composts are often used as packing materials thus making it an affordable method. It was originally developed in Germany to treat odors in the 1950s and later became popular in both the United States and the Europe. Most previous studies worked on maximizing biofilter ammonia removal ability; however, recent reports about generation of nitrous oxide from biofilters have spurred researchers to consider the consequences of greenhouse gas emissions. This study aims to improve the basic biofilter engineering designs (media selection, airflow resistance measurement), ammonia removal efficiency, and also to examine the effect of moisture on nitrous oxide generation. Besides that, a moisture sensor was developed to control the moisture content in biofilter media in order to achieve high ammonia removal efficiency and low nitrous oxide generation.Biofilter media selection and pressure drop management affect the affordability of biofilters. In this study, physical, chemical properties and airflow resistances of eleven commonly used biofilter media, including ten organic and one inorganic materials, were characterized. The density, porosity, particle size distribution, pH, total C, total N, and organic matter content of each material were analyzed using standard methods. The airflow resistance property was tested on a large chamber (L×W×H: 1.0m×0.6m×0.6m) with cross-section airflow rate of 0-0.15 m3.m-2.s-1. Airflow resistance driving factors, including moisture content, particle size distribution, bed thickness and compaction, were experimentally evaluated. The testing results were fitted into Hukill and Ives (1955) equation, and then two equation constants, a and b, were calculated for comparison and also used as an initial database for future biofilter designs. Based on the observations of moisture, bed thickness and compaction effects on air flow resistance, an empirical modification implementing derating factors was suggested to improve the Hukill and Ives equation.In order to evaluate the function of biofilters, a baseline test was carried out to examine ammonia removal efficiency and nitrification kinetics at extreme conditions where a high ammonia loading without pre-humidifying was introduced and a high pH value was maintained in the biofilter media. Two bench-scale biofilters were built for this study and the test was composed of an N-enriching step, an N-depleting step, and a second N-enriching step. The results showed that 90-94% ammonia removal efficiencies were observed for about ten days and then decreased in the first N-enriching step. An N-depleting step, a process that reduced the concentration of nitrogen containing compounds in the biofilter media, was applied between the two N-enriching steps. The results obtained in the second N-enriching step showed that the N-depleting step partially recovered ammonia removal efficiency, but did not last long. It was found that NH4+-N, NO2--N and NO3--N accumulated in media accounted for 50-100% of the total N captured from the inlet gas. To study the nitrification kinetics, a model that considers nitrification process as two continuous first-order reactions was applied and two nitrification transformation constants, k1 and k2, were calculated. The results show that both constants were decreased in the N enriching steps, indicating the reactions were inhibited when nitrogen compounds, especially the free ammonia (FA), were accumulated in the biofilter media. The results suggested that nitrogen compounds management is critical in achieving stable and high ammonia removal efficiency.Moisture is believed to be the most important factor in determining biofilter performance. It affects both ammonia mitigation and nitrous oxide generation. Most likely, generation of nitrous oxide is caused by the incomplete denitrification inside of biofilters where anaerobic zones are created due to high moisture contents or compaction. To examine the role of moisture content in biofilter application, a four-month test was conducted on four bench-scale biofilters. The moisture contents in the treatment biofilters were manipulated from 35% to 55%, then to 63%, with a final step of 55%; while the moisture content in control biofilters were managed at 35% and then kept constantly at 55%. It was found that ammonia removal efficiency was improved when media moisture content was increased from 35% to 55%; but further increasing moisture content to 63% did not enhance ammonia mitigation much. In contrast, little increase of nitrous oxide (0.10−0.15 ppm) was observed when moisture content was increased from 35% to 55%, but further increasing moisture content to 63% caused a peak of nitrous oxide generation. Work on microbial communities showed that the ammonia oxidizer communities were resistant to the “moisture disturbance -disturbance relief” process based on the T-RFLP test results. This observation supports the relative flat of the ammonia removal efficiency in the treatment biofilters when moisture content was changed from 55% to 63% and then back to 55%. However, the bacterial communities and nosZ gene communities displayed a functional redundancy to the moisture changes. Interestingly, the real-time qPCR results showed that the quantity of nosZ gene copy was reduced significantly at 63% moisture content. This observation can explain the increasing of nitrous oxide concentrations in this step.Based on the previous study results of moisture effects on biofilter performance, it becomes necessary to manage the moisture content in the biofilter media. To achieve this goal, a moisture sensor based on media impedance measurement was developed. The sensor is composed of a sensing unit and a circuit that returns dc voltages. In a validation test, the sensor was used to measure moisture contents in two woodchips and one compost with moisture contents of 5-65%. The results fitted theoretical predictions quite well, showing that impedance can be a reliable indicator of moisture content. Besides that, temperature and compaction effects on impedance measurement were explored. It was found that increasing temperature from 22℃ to 27℃ and further to 32℃ did not change impedance of media (which is closely related to sensor reading) while compaction for eight days did. Applying the sensors in two bench-scale ammonia mitigation biofilters for one month showed that the sensors were sensitive to moisture changes. Incorporated with a water pump control strategy, the media moisture contents were successfully managed within a desired range of 44-47% according to the sensor readings. Water balance calculation based on daily water addition and water loss rates supported the sensor measured results, while moisture measurement using oven drying method at 105℃ showed slightly higher moisture contents than the sensors measured results. At the controlled moisture condition, the two biofilters reached high ammonia removal efficiencies (82-92%) and low produced nitrous oxide concentrations (0-0.32 ppm).