Defensa de la Tesi de Cecilia Polizzi
13.06.2021  - 

El proper dia 17 de juny, presentarà la seva Tesi Doctoral Cecilia Polizzi, desenvolupada entre el grup GENOCOV del Departament d'Enginyeria Química, Biològica i Ambiental i la Universitat de Florencia.

La defensa de la Tesi de Cecilia Polizzi, amb el títol “Towards an integrated removal of nitrogen and sulphur in biological treatments of tannery-like wastewaters” es durà a terme en el marc del 39è Plenary meeting of the International PhD school in civil & environmental engineering i es podrà seguir de forma virtual a través del enllaços següents:

Els director de la Tesi han estat Giulio Munz (Universitat de Florencia) i David Gabriel (Universitat Autònoma de Barcelona ) i el Tribunal que avaluarà la tesi estarà format per:

  • David Gabriel, President, UAB
  • Giulio Munz, Secretary, Uni Florence
  • Elena Ficara, vocal,  Polytechnic University of Milan
  • Piet Lens, vocal, National University of Ireland
  • Xavier Gamisans, vocal, Universitat Politecnica de Catalunya
  • Giovanni Esposito, vocal, University of Naples Federico II


The emerging challenges in wastewater biological treatments are enclosed in the global debate on climate change, energy saving and circular economy. Energy autarchy, reduction in GHGs emissions and compact configurations are among the novel targets to be met by Wastewater Resource Recovery Facilities (WRRFs). In response of such an urgency, different innovative solutions have been proposed in the last decades, ranging from the adoption of novel technologies, such as granular sludge instead of floc-based activated sludge, to rethinking of the conventional integration of Carbon (C), Nitrogen (N), Phosphorous (P) and Sulphur (S) cycles in the wastewater treatment line.

The present work focuses on the integration of novel processes for the biological removal of Nitrogen (N) and Sulphur (S), specifically the anammox process and the sulphide-based autotrophic denitrification. The novel integration of PAD with the anammox process (PAD/A) is of particular interest for the treatments of wastewaters highly rich in nitrogen and sulphur compounds, such as those derived from petroleum refineries, tanneries and fish processing. Within this framework, the present thesis aimed at gaining insight on the application of the novel PAD/A process, in high-strength wastewaters. The anammox and the PAD process have been studied in parallel lines of work, addressing different issues related to the two innovative treatments.

Industrial wastewater originated from vegetable tannery industry was selected as target stream for PAD/A implementation and the potential inhibitory drawbacks on the anammox granular biomass were studied. A fast start-up of an anammox gas-lift reactor was achieved with volumetric loads up to 0,48 gN-NO2-/l/d, maximum specific nitrogen load of 0,28 gN-NO2-/gVSS/d and stable nitrite removal higher than 95%. Mineral

precipitation on the granules’ surface was observed after 130 days of operation. An extensive study was conducted in order to characterise precipitate composition as well as its impact on reactor performance, biomass activity and microbial population. The results offer useful knowledge for the long-term stable operation of anammox granular systems, especially when treating wastewaters prone to precipitate formation.

It is concluded that there is no technical limitation for the application of anammox process on vegetable tannery wastewaters.

The PAD process was studied in a CSTR reactor, operated as an ideal chemostat and fed by nitrate and sulphide. The work aimed at evaluating the effect of influent S/N ratios and SRT conditions on the nitrite accumulation performance. Specifically, moderate to strict S-limiting conditions were studied and SRT was progressively decreased, from 40 to 23 to 13 hours. Successful nitrite accumulation was observed after one week of stable and strict S-limiting condition and maintained throughout the experimental work. Nitrite accumulation efficiency of 70-100% and nitrite conversion efficiency of 70% were observed at all the experimental phases, calculated over the converted and influent nitrate, respectively. Results indicate that S/N ratios as low as 0,6-0,8 gS/gN were a sufficient condition to ensure high nitrite accumulation efficiencies, whereas the SRT showed no significant impact on the process performance, at the studied values. Nevertheless, short SRT and the feeding conditions were crucial to obtain a highly selected sulphide-oxidizing biomass. A clear microbial population shift was observed: Sulfurimonas genus showed 90% relative abundance in the seeding sludge and, after 80 days of operation, was almost completely out-competed by Thiobacillus, exhibiting a relative abundance as high as 83%. It is speculated that microbial bioenergetics is a possible underlying reason of the clear population shift at the peculiar conditions applied, i.e. low SRT and stable electron donor deficiency. A discussion on the metabolic differences among the two genus is provided in support of such an assumption. A thermodynamic-based study was conducted in order to study the catabolic change observed in the experimental work, i.e. from full denitrification to partial denitrification (denitratation). Results indicate that even though denitritation is a highly favourable reaction, the sole denitratation could be a valuable energetic response in case of electron donor limitation, as it was the case of the experimental work.

Moreover, catabolic and full-metabolic stoichiometry have been solved according to theoretical models and a critical comparison of experimental and theoretically-derived biomass yields is provided. Aggregated information on process catabolic stoichiometry are presented as support tool for process design and optimization of the complex S-based denitrification process, over all the intermediate reactions and possible combinations of e-donor and e-acceptor. It is believed that the complexity of the S-driven denitrification due to the many possible intermediate reactions actually infers high flexibility to the combined PAD/A; indeed, a wide range of S and N loads can be successfully exploited according to the case-specific requirements.

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