Nitrogen removal from digested manure in a simple one-stage process

A process based on partial nitrification and recirculation into the anaerobic digester was studied to remove nitrogen from digested manure and thus reduce enhanced gaseous ammonia emissions due to on-farm biogas production. An anaerobic reactor representing an anaerobic manure digester was fed with a nitrite solution and digested manure liquor. Nitrite was efficiently removed from the influent and ammonium formation was observed first. Ammonium was subsequently eliminated up to a maximum of 90% of the influent concentration indicating anaerobic ammonium oxidation activity. This activity however decreased again and was lost at the end of the 4 months operation period. In an 1.5-L aerobic CSTR that was fed with digested manure liquor, ammonium was efficiently removed from the influent. Nitrite and nitrate formation was observed but mass balances indicated significant N-removal. Accumulation of suspended solids was observed at the end of the experiment suggesting presence of oxygen-free environments. In a second test in a 15-L CSTR where suspended solids sedimentation could be avoided, low N-removal rates were observed in the absence of biofilm carrier elements whereas high N-removal rates were achieved in their presence. A simple one-stage process based on immobilized biomass could therefore be installed downstream of agricultural anaerobic digesters in order to mitigate undesirable gaseous ammonia emissions

Nitrogen removal from digested manure to decrease ammoniac emissions from agriculture

Christof Holliger

Biogas production transforms organic matter into methane, carbon dioxide and ammonia (NH3). The aim of this study was reducing NH3 emissions from liquid effluents of agricultural anaerobic digesters. An alternative to classic nitrification-denitrification was tested: partial nitrification of the ammonium-rich effluent and its recycling into the digester for nitrite conversion into dinitrogen gas (N2) via denitrification or anaerobic ammonium oxidation (Anammox). Two laboratory-scale bioreactors were set-up to this effect. In the first reactor, short residence time and oxygen-limited conditions were guaranteed. The second was filled with digested manure simulating the final stage of a plug-flow anaerobic digester. The feed was artificially amended with nitrtite (NO2-), simulating the effluent of the first reactor. Ammonium (NH4+) removal efficiencies higher than 80% were reached in the aerobic reactor, producing a NO2--rich effluent. However, mass balance showed a large percentage of “nitrogen losses”. This can either be explained by denitrification or Anammox occurring in anoxic zones inside the reactor or by aerobic denitrification, an interesting but not yet fully understood process. The anaerobic reactor removed about 90% of nitrite fed and during a certain period also approximately 80% of ammonium was degraded. Using FISH analysis, Anammox bacteria were identified in the microbial community of this reactor.

2007

Robust and sustainable cultivation of microalgae: recycling of nitrogen from liquid digestate and control of biological contamination

Adrian Pulgarin

The present doctoral thesis aimed to achieve a robust cultivation of microalgae by recycling nitrogen from a liquid digestate. However, the use of non-sterile nutrient source increases the risks of contamination of undesired organisms such as bacteria, viruses, protozoa, and rotifers. Therefore, the thesis also aimed to develop a cheap and bio-compatible decontamination method, which can be applied to microalgae during their cultivation. To reach these objectives, three projects were carried out. In the first project, microalgae (Chlorella vulgaris) were successfully cultivated at high-density with liquid digestate from anaerobic digestion as a nitrogen source. The cultivation was carried out at laboratory and pilot scales in glass-column and thin-layer photobioreactors, respectively. Biomass dry weight and productivity reached values up to 18.6 grams per liter and 0.93 grams per liter per day, respectively. To obtain these results, a cultivation method was developed to overcome inherent issues associated to the use of liquid digestate such as the acidification of the water, ammonium toxicity, turbidity, and nutrient imbalance. The nitrogen mass balance was carried out during the cultivation and it shows that only 40 to 60 % of the nitrogen supplied to the cultures were assimilated in the biomass. Surprisingly, accumulation of nitrogen in the supernatant accounted only for approximatively 3 %. Therefore, a large fraction of the nitrogen was lost to the atmosphere.In the second project, a citrate-modified photo-Fenton (PF) process was used to treat microalgae cultures contaminated with the bacterium Escherichia coli. The aim was to set experimental conditions where physiological and morphological differences can be used to favor the inactivation of bacteria and to minimize the loss of microalgae. Results showed that the citrate-modified PF was more effective against bacteria than a solar light/hydrogen peroxide treatment, while being less effective against microalgae. If the treatment was applied at an early-stage contamination, bacteria were fully inactivated, while the microalgae loss of viable cells was about three orders of magnitude. In the third project, the citrate-modified PF process was used to treat microalgae cultures contaminated by the rotifer Brachionus calyciflorus, a predator of microalgae, which can lead cultures to failure due to its high consumption rate of algal cells. Then, treated cultures were cultivated up to 14 days to assess the efficacy of the treatment. Results show that the citrate-modified PF and solar light/hydrogen peroxide treatments had equivalent efficacy to inactivate rotifers. However, the PF process remained less harmful for microalgae. When the citrate-modified PF treatment was applied to an early-stage contamination (5 rotifers per mL), the decontamination was effective and no regrowth of rotifers was observed. However, the cultivation of contaminated cultures, which were not treated, failed due to an increasing rotifer population (>1000 individuals per mL after 14 days of cultivation).As a whole, the present doctoral thesis contributes to the development of more robust and sustainable cultures of microalgae. Methods were developed to cultivate microalgae at high-density with liquid digestate and to control biological contaminants. Finally, challenges and opportunities were discussed as a perspective for further works.

EPFL2021

Wastewater treatment processes modelling for anaerobic digestion effluents

Miriel Steudler

The general topic of this research project is the modelling of wastewater treatment processes for anaerobic digestion effluents, which are characterized by high nitrogen concentrations. In order to avoid eutrophication of lakes and rivers caused by nutrient enrichment as it happened in Switzerland a few decades ago, the implementation of efficient nitrogen removal processes for those effluents remains nowadays of importance. Moreover nitrogen removal will even become a pre-requisite for efficient micropollutants techniques to be implemented. The first objective of this master project is the modelling of the biological nitrogen removal process Anammox in the software gPROMS. This process converts ammonium and nitrate (close to 1:1 influent concentration ratio) into dinitrogen and nitrate by Anammox bacteria. In order to obtain an influent with the proper concentration ratio, this process can be coupled to a Sharon reactor operated for partial nitrification (two stages operation). In this study, the Anammox biological developed model has been integrated into a continuous stirred tank reactor (CSTR) combined with a particles separator. The two modeled reactors, Sharon and Anammox, have then been run in series and steady-state results have been obtained. Through a comparison with literature results (lab experience and pilot-plant), the steady-state simulation results in terms of nitrogen removal efficiency and effluent concentrations have been considered as reliable. However, a calibration of the model with real data still needs to be investigated. The second objective of the project is the study of key influencing parameters (pH and temperature) on the operation of the two stages Sharon-Anammox process. A sensitivity analysis showed that temperature and pH are influencing the nitrite to ammonium ratio Sharon effluents and consequently affecting the efficiency of the Anammox reactor; the growth and selection of bacteria in the Sharon reactor are determined by those two parameters (chemostat principle). The optimal range of operation for the Sharon reactor in order to maximize the efficiency of the Anammox reactor are found to be between 306K and 308K and between 7.8 and 8.0 for the pH according to the steady-state simulations; those results are coherent with literature findings and operation of full-scale Sharon reactors. The final objective of the project is the comparison of the Sharon-Anammox process with an activated sludge system in terms of energy consumption and nitrogen removal efficiency. By minimizing in both cases the mmonium effluent concentration and playing with the aeration needs, savings of 75% in benefit of the Sharon-Anammox system have been observed; the order of magnitude obtained by optimization is coherent with the stoichiometry implemented in the model. The biological Anammox model itself will be implemented in other types of reactors models (one stage technologies involving biofilms) and the models predictions will be compared with operational data (at pilot or full-scale). Modelling wastewater treatment processes is a help to better understand the processes themselves, their interactions, their optimal operation conditions both by taking into consideration the energy consumption and the water quality through optimization techniques. The challenge of modelling will however still remain the modelling of bacteria interaction and inhibition, as well as a deep understanding and quantification of various mass transfer processes involved. This will need a close collaboration with expert scientists in order to better understand these phenomena. Modelling is a good support for experimental studies, as it can help to plan efficient experimental campaign by minimizing the required experimentations run.