Due to the intensification of the livestock sector to meet the growing global meat consumption, higher emissions of pollutants are released into the air. One of the most important is ammonia. Its release causes acidification and eutrophication, both of which lead to biodiversity loss. In 2010, 94% of the anthropogenic ammonia emissions in Europe were attributed to agriculture (UNECE/LRTAP, 2012), putting this sector for a large challenge. As a response, the BREF guideline and AEA guideline required large pig and poultry housing facilities to be ammonia emission-low. Besides ammonia, the exhaust air from these buildings also contains various odorous compounds and dust particles, leading to possible odour nuisance in nearby residential areas and public health risks. Animal building emissions also involve nitrous oxide and methane, which are strong greenhouse gases contributing to climate change. While air scrubbers were in the first place introduced to reduce ammonia emissions, they were soon also regarded as a valuable technique to reduce other pollutants such as odour and dust. While optimizing air scrubbers to meet new requirements concerning odour and particulate matter besides ammonia, it is therefore recommended to anticipate the growing attention towards greenhouse gases by investigating their possible removal and/or production in these systems.
Chapter 1 provides a state-of-the-art on the application of air scrubbers and biofilters as a mitigation technique at animal housing systems. The properties of the incoming exhaust air of pig and poultry housing facilities were summarized to get an overview of the disturbances to be dealt with. When confronting the legally required minimal removal efficiencies (especially for ammonia) with the performances of full-scale air scrubbers, it was clear that not all installed air scrubbers reach these requirements. The process design and control options for air scrubbers were subsequently assessed in detail to gain more knowledge on the underlying reasons and in view of possible optimization strategies.
Following the literature review, the objectives of this doctoral research work were formulated (Chapter 2). This work focuses on the design and operation of chemical, biological and multi-stage (combi) air scrubbers for the removal of ammonia from pig housing facilities. Besides ammonia, attention was paid to emissions of the strong greenhouse gases nitrous oxide and methane and to emissions of hydrogen sulphide as a model odour component. The optimization of air scrubbers was investigated by combining modelling and simulation with experimental measurements at full-scale installations, as they work synergistically. Even though the performance of air scrubbers can be highly varying over time (Chapter 1), it is in literature mostly evaluated based on point measurements. Therefore, in this work, both diurnal fluctuations and spatial variations were investigated during intensive measurement campaigns at a chemical air scrubber, a biological air scrubber and two-stage biological air scrubber installed at pig housing facilities in Flanders (Chapter 3). Ammonia, nitrous oxide and methane at the inlet and outlet of the air scrubbers were monitored on-line for one week using a photoacoustic gas monitor. Only one of the three air scrubbers performed well for ammonia removal. Nitrous oxide was produced inside the biological and two-stage scrubber and methane could not be removed in the different air scrubbers because of its low water solubility. Modelling and simulation are useful tools to investigate the influence of design parameters and control handles on process performance in a time-efficient and cost-effective way. A mechanistic model for a countercurrent chemical air scrubber was set up in Chapter 4. Mass balances for water, ammonia, hydrogen sulphide, nitrous oxide and methane were implemented. Besides, heat balances were introduced to estimate water consumption and asses the effect of temperature. Air scrubbers are exposed to highly changing inlet conditions in terms of ventilation rate, air temperature and pollutant load. The model allowed to study the separate influence of each of these variables, which are interrelated in practice. The ventilation rate was found to have the largest influence on the ammonia removal efficiency in chemical air scrubbers. A mechanistic model of a biological air scrubber was set up and applied (Chapter 5) to investigate in detail the role of the biological conversions in the removal efficiency. It was found that the effect of nitrification on the driving force for ammonia mass transfer mainly lies in the associated pH decrease,rather than in the ammonia conversion as such. Without pH control, only about 50% of the absorbed ammonia is oxidized to nitrite and nitrate, balancing the pH increase from ammonia absorption by the protons produced during nitrification. Complete conversion of ammonia was possible when the pH was controlled at around 7, maintaining the driving force for mass transfer and consequently the ammonia removal efficiency. The results of a long-term online monitoring campaign performed at two biological air scrubbers for one year, are presented in Chapter 6. In addition to continuous monitoring of the gas phase concentrations of ammonia and nitrous oxide, the most important washing water characteristics were analysed each week as well: pH, electrical conductivity (EC) as a measure for the total nitrogen content, as well as total ammoniacal nitrogen (TAN), nitrite and nitrate. Doing so, the air scrubber’ performance could be linked with the washing water characteristics, thus increasing process knowledge for both the start-up period and during full operation. Additionally, the influence of inoculation with activated sludge was investigated as possible cheap and easy method to increase the performance of biological air scrubbers. The inoculated air scrubbers, immediately showed nitrifying activity, thereby reducing the pH and thus increasing the driving force. Furthermore, less nitrite accumulation was observed. During the start-up of these two biological air scrubbers, the microbial community structure was investigated as well by applying metabarcoding (Chapter 7). Analysis of different samples of the inoculum, biofilm and washing water over five months showed that both biological air scrubber evolved to a rather comparable community structure, compared to the activated sludge. Small differences, mostly in abundancy, were observed between both air scrubbers and between the different stages. Nitrite oxidizing bacteria were found only abundant in the inoculated air scrubber, corresponding with the lower nitrite accumulation found in this system as seen in Chapter 6.
Chapter 8 offers some final considerations, conclusions and perspectives on the design and operation of chemical, biological and multi-stage air scrubbers, including suggestions for future research.