Nitrification of the liquid phase of digestate: The transfer of the process from laboratory to pilot plant and full scale conditions

This paper aimed to verify the laboratory research results related to the liquid phase of digestate (LPD) treatment using nitrification at pilot scale conditions and to suggest an optimal method for full-scale system start-up. A reactor applying the nitrification of LPD was operated for 358 days. Significant free ammonia and free nitrous acid inhibition towards nitrite-oxidizing bacteria (NOB) increased N-NO2− concentrations up to 2300 mg/L in LPD. As a result of pH and a nitrogen loading rate control combined with the re-inoculation of the reactor, NOB activity was fully restored after 90 days, where the nitrogen oxidation rate and nitrogen oxidation efficiency reached up to 0.26 kg/(m3⋅d) and 99.9%, respectively, at N-NO2− concentrations not exceeding 45 mg/L. Thus, the feasibility of stable operation of the system even without any dilution of LPD was confirmed. The procedure of full-scale reactor start-up consisting of a separate unit serving to produce nitrifying biomass was suggested. It was shown that the volume of such a reactor could be divided into three compartments to optimize the start-up. Simple modeling proved the possibility of completing the start-up process in 34 days under specific conditions (total ammonia concentrations in LPD 2 kg/m3, daily production of LPD 20 m3/d, nitrogen loading rate reaching 0.25 kg/(m3⋅d)). The results could be used to accelerate the method transfer into practice. [Display omitted] •A pilot plant reactor was operated for the nitrification of the liquid digestate.•Free ammonia and free nitrous acid inhibition limited the nitrification start-up.•Nitrogen loading rate and pH control enabled the complete restoration of the process.•A start-up technique allowing the complete prevention of inhibition is presented.•A suitable method for a full scale reactor start-up is suggested.