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BACKGOUND: Proper treatment technologies are required to address the environmental issues associated with increasing volumes of slurries. Ammonia stripping reduces the nitrogen content of the slurries and allows for its recovery in a valuable form. Herein the influence of pig slurry characteristics on ammonia stripping efficiency and the quality of the recovered ammonia solution were assessed. RESULTS: Substrates characterized by low organic matter content, below 10 g COD L -1 , resulted in ammonia stripping efficiencies greater than 80%. Changing slurry pH to 9.5 significantly improved the process, even though high COD contents kept the efficiencies below 70%. Ammonium sulfate solutions could be concentrated up to nitrogen contents greater than 40 g N L -1 , while maintaining low organic contamination. Introducing a basic trap (pH > 12) before the acid one, allowed for the retention of more than 60% of the stripped organics with less than 3% of the stripped ammonia.CONCLUSIONS: Ammonia stripping coupled with absorption proved to be a suitable technical solution for the recovery and valorization of the nitrogen contained in pig slurries. Clear enhancements in process efficiency were observed in the case of slurries with low organic matter content. The introduction of a basic trap, together with a slight increase in the operational pH level, further increased organics abatement.

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Mathematical modelling of anaerobic digestion process has been used to give new insights regarding dynamics of the long chain fatty acids (LCFA) inhibition. Previously published experimental data, including batch tests with clay mineral bentonite additions, were used for parameter identification. New kinetics were considered to describe the bio-physics of the inhibitory process, including: i) adsorption of LCFA over granular biomass and ii) specific LCFA substrate (saturated/unsaturated) and LCFA-degrading populations. Furthermore, iii) a new variable was introduced to describe the state of damage of the acetoclastic methanogens in order to account for the loss of cell-functionality (inhibition) induced by the adsorbed LCFAs. The proposed model modifications are state compatible and easy to be integrated into the International Water Association's Anaerobic Digestion Model N°1 (ADM1) framework. Practical identifiability of model parameters was assessed with a global sensitivity analysis, while calibration and model structure validation were performed on independent data sets. A reliable simulation of the LCFA-inhibition process can be achieved, if the model includes the description of the adsorptive nature of the LCFAs and the LCFA-damage over specific biomass. The importance of microbial population structure (saturated/unsaturated LCFA-degraders) and the high sensitivity of acetoclastic population to LCFA are evidenced, providing a plausible explanation of experimental based hypothesis.

The objective of the present study is to analyse kinetic and stoichiometric parameter values of gelatine anaerobic degradation at thermophilic range, based on an experiment designed to elucidate if volatile fatty acids (VFA) are inhibitors of the hydrolysis process. Results showed that VFA are not inhibiting the hydrolysis process. The ADM1 model adequately expressed the consecutive steps of hydrolysis and acidogenesis, with estimated kinetic values corresponding to a fast acidogenesis and slower hydrolysis. The hydrolysis was found to be the rate limiting step of anaerobic degradation. Estimation of yield coefficients based on the relative initial slopes of VFA profiles obtained in a simple batch experiment produced satisfactory results. From the identification study, it was concluded that it is possible to determine univocally the related kinetic parameter values for protein degradation if the evolution of amino acids is measured in simultaneous batch experiments, with different initial protein and amino acids concentrations.