(58c) A New Process for Poultry Manure Revalorization: Fresh Water and Thermal Energy Savings with Effluent Recycle, Biogas Production, Thermal Hydrolysis, Anaerobic Digestion and Snad Process

Valorization of poultry manure through the anaerobic digestion (AD) generates an effluent with high ammonia concentration that is inhibitory of the AD process. This effluent must be treated for its subsequent elimination. Biogas production favors AD, though in different countries AD was implemented on a relatively large scale only when green gas subsidies were guaranteed. This is due to the high cost of fresh water and heating of bioreactor units. In addition, due to the high viscosity of manure, a pretreatment to facilitate its metabolization to methane is required. Thermal hydrolysis can be used as pretreatment of AD to accelerate the metabolization, to increase the volume of biogas produced and to decrease the amount of solids to be disposed. In addition, the process of nitrogen abatement comprising the simultaneous partial nitrification, Anammox and denitrification is capable of removing nitrogen species. Then, the effluent can be used for dilution of the poultry manure producing cost savings in fresh water and heating. However, the coupling of these units and the evaluation of the reuse of the SNAD effluent as a solvent has not been evaluated on an experimental scale.

The aim of this work was to study the effect of recycling the final effluent resulting from the stepwise process of Thermal Hydrolysis (TH) of poultry manure combined with AD and simultaneous partial nitrification, Anammox and denitrification (SNAD).

Thus, the proposed system was a coupled process of TH+DA+SNAD. First, poultry manure was subjected to a TH treatment, which solubilizes the particulate organic matter and removes the calcareous material (calcium carbonate) from the slurry. The poultry manure was heated at 180 °C and 20 psi during 3 hours; a filtration step was added, which allows the removal of larger solids. The resulting viscous slurry was first diluted with tap water, then with the final effluent of the SNAD process and anaerobically digested in a 3.5 L granular reactor. Finally, a SNAD reactor (4.3 L) previously set and operated for several months, was stepwise fed with the effluent from the DA diluted with tap water until an inlet concentration of 400 mg total ammonia nitrogen (TAN)/L was reached. As a last step, the effluent from the SNAD process was recycled to the anaerobic reactor. The effect of the recycle was evaluated through the analysis of the coupled process efficiencies and the biomass characterization (relative abundance of populations, specific activities, sedimentation rate, sludge volumetric index and granules size).

Efficiencies up to 81% of COD removal were obtained in the anaerobic digestion process using thermal pretreatment in the poultry manure. In the SNAD process fed with the effluent from the anaerobic digester, a total ammonia nitrogen and total inorganic nitrogen removal of 100% and 72%, respectively, were obtained. Considering a stoichiometric efficiency of 89% in the removal of nitrogen by the partial nitrification-Anammox process, a good performance of the reactor was reached. When recycling the effluent from the SNAD process to the DA (recycle ratio of 0.32), the efficiency of the anaerobic digester was 65% and the methane percentage in the biogas was 73%. The efficiency decrease was explained by the decrease of the COD concentration in the influent, which is probably due to the presence of nitrate in the recirculation liquid, which in turn can be reduced to nitrogen gas by denitrification when organic matter is present. Despite the removal decrease, the methane content in the biogas of the DA increased from 62% to 73% in the recycle condition. Then, the system was stable when recycling in terms of the COD concentration of the effluent and in the biogas production. On the other hand, in the SNAD system the specific Anammox and nitrifying activities were 0.210 gN-N₂ /gSSV • d and 0.344 gN-NH₄⁺ /gSSV•d using recycle, and 0.501 gN-N₂/gSSV•d and 0.057 gN-NH₄⁺/gSSV•d without recycling. The sludge volumetric index was 26 mL/gSSV; the average equivalent diameter of the granules with and without recycle was 1.285 and 1.515 mm, respectively. The relative bacteria abundance did not show a significant difference in relation to the recycle and non recycle condition, showing mostly groups of aerobic ammonium oxidizing bacteria (aeAOB) and anaerobic ammonium oxidizing bacteria (anAOB) with values above 79%.

In conclusion, The TH-DA-SNAD coupled process with recycling allows to obtain a high removal of nitrogen, without substantially changing the relative abundance of the different bacterial communities; increases specific nitrifying activity and Anammox; decreases the average equivalent diameter of the granules and allows to obtain a good sludge volumetric index. Further challenges in order to optimize the proposed process are: to increase ammonia concentration in the influent of the SNAD system or to replace the TH with a lower operational cost unit.

Acknowledgements: This study was made possible by FONDECYT (Chile) [Grant Number 1140491] and INNOVA (Chile) [Grant Number 15VEIID-45613].