Ammonia stripping to boost anaerobic digestion of nitrogen-rich waste(waters)

Resumo

Anaerobic digestion is an effective and widely used biotechnology to treat wastes and wastewaters but, like all technologies involving the activity of microorganisms, is sensitive to a number of toxicants, whose presence or accumulation in high concentrations should be avoided. One of the most recurrent inhibitors is ammonia, thus limiting the efficient treatment of nitrogen-rich waste(waters). The present thesis studies the application of ammonia stripping with air, both as pre-treatment and side-stream treatment, in order to improve anaerobic digestion of two typical N-rich waste(waters): pig manure (co-digested with food and crop waste) and the supernatant from the anaerobic digestion of the organic fraction of municipal solid waste. Two scenarios were taken into consideration that required the implementation of different digester-stripping column configurations. Side-stream configuration (65°C) was used when treating a proteinaceous mixture (pig manure, food and crop waste) in thermophilic conditions (55°C) due to both the need of increasing ammonium fraction before applying the stripping procedure and the possibility of taking advantage of the digestate temperature. The stripping column worked as pre-treatment at higher temperature (75°C) when ammonium concentration in the waste(water) was already high (anaerobic supernatant), which also enabled to increase organic matter biodegradability through thermal solubilisation. After having defined the stripping column working parameters and their effects on anaerobic biomass (side-stream configuration) and waste(water) characteristics (pre-treatment configuration), the efficiency of the selected technology was successfully proved with experiments in continuous reactors. Air side-stream stripping proved to be an effective tool to control ammonia concentration in thermophilic anaerobic digestion. It benefitted from digestate temperature and, when using high alkalinity substrates, such as pig manure, chemical addition to increase pH was avoided. Despite the thermophilic biomass suffered the harsh conditions of the stripping procedure (high temperature and pH, oxygen at saturation levels), the likely inactivation of part of the microbiome did not influence continuous digester. Once the stripping column working parameters were fixed, an adequate recirculation flowrate between the column itself and the digester had to be selected in order to control ammonia concentration in the anaerobic reactor and, at the same time, to leave minimum alkalinity for smooth digestion. Mesophilic anaerobic post-digestion of supernatant was effectively IV boosted by the combination of ammonia stripping and low-temperature thermal pre-treatment. Control of ammonia concentration in the post-digester assured stable high methanogenic activity while organic matter solubilisation improved the methane yield. The obtained post-digestate (both with and without pre-treatment) could be compatible with the application of the low-energy intensive and environmental-friendly partial nitritation/anammox process if a previous organic matter-removal step is foreseen. However, the stringent nitrate discharge limit could be respected only if the incoming ammonium into the anammox tank was maintained below certain levels and this was only possible when the ammonia stripping pre-treatment was applied. This same treatment scheme, when compared to the costs of classical and widely implemented nitrification/denitrification, permitted limiting global expenditures as well. To sum up, this thesis provide a thorough demonstration of ammonia stripping flexibility to control ammonia levels in anaerobic reactors treating nitrogen-rich waste(waters).