Ammonia emissions subsequent to anaerobic treatment
Due to the upcoming energy crises, the generation of bioenergy from different biomass sources gains more and more importance. Anaerobic treatment is one possibility for bioenergy generation.
Ammonia (NH3) predominantly develops as a result of farming and the employment of fertilisers. It contributes to atmospheric contamination, in particular in rural regions. Besides its intensive odour and its toxic effect, NH3 is problematic as it can be transformed easily into other harmful N-containing compounds. In addition, NH3 immissions can lead to over-fertilisation.
NH3 emissions can also occur during biological waste treatment. An anaerobic fermentation (digestion) is commonly carried out with the aim of the generation of CH4-rich biogas. The digestate may be treated to gain a humus rich product. During the anaerobic degradation processes proteins are converted into NH4 +/NH3 through ammonification, leading to a potential of NH3 emissions. The fate of NH3 subsequent to anaerobic treatment during post-composting of digestates was investigated. For this purpose, various series of N balances were established. These included NH3 exports, N leaching as well as the different N compounds in the substrate. The investigations were carried out on pilot plant scale. One set of experiments (Exp. 1a, b) covered a mesophilic resp. a thermophilic anaerobic fermentation stage for a model biowaste with subsequent treatment through composting. In a second set (Exp. 2a, b, c) the same waste was treated by sole composting using different aeration rates. Furthermore, a digestate from an technical scale anaerobic dry-fermentation plant was composted directly resp. after mixing with structural material (Exp. 3a, b).
It could be shown that, anaerobic fermentation combined with composting (Exp. 1a, b) led to higher NH3 emissions compared to sole composting (Exp. 2a, b, c). The NH3 releases occurred relatively quick with the beginning of posttreatment by composting. When comparing the different combined variants (Exp. 1a, b), the experiment with a thermophilic stage at the begin showed higher emissions than the experiment with a mesophilic stage. The known impact factors on NH3 emissions are NH4 +/NH3 content, Norg content, pH, temperature and water content of the substrate as well as the aeration rate. All factors were evaluated and proved not to be the reason for the different trends in the experimental sets of Exp. 1 and Exp. 2. It is assumed, that during aerobic treatment a high degree of Nimmobilization occurs. The intensive growth rate of microorganisms during this process may induce medium and long term N-immobilization into hardly degradable microbial compounds (e.g. Chitin, RNA, DNA). In contrast, low Nimmobilization occurred during the anaerobic phases. The lower growth rate of microorganisms in anaerobic processes induces only a limited N-immobilization, but nevertheless, microorganisms did depolymerise organic N-compounds into amino acids and NH4 +/NH3. These compounds deliver a large pool of N available for NH3 releases in aerobic phases. In the case of amino acids NH3 releases can occur after amino acid transformation via ammonification. A procedure to prove the assumption of the different N immobilization degree in anaerobic resp. aerobic processes was suggested.
During the Exp. 3a and b no NH3-emissions could be detected during post-composting. The reason was, that the NH4 +/NH3 contained in the digestate presumably partly was lost during a storage step at the facility. Furthermore, the substrate degradation was very advanced. For that reason during aerobic post-treatment no significant degradation and with it ammonification occurred. In consequences no further NH4 +/NH3 was set free from organic N compounds and therefore no source for NH3 releases was available. Based on the results, strategies for avoidance of NH3-emissions and for utilization of released NH3 as a source for fertilizers become suggested.
