Optimization of Two-Stage Treatment of the Liquid Phase of Digestate

Bc. Liz Mabel Vargas Cáceres

Master's thesis

Abstract:

This diploma thesis is focused in the determination of nitrogen losses from the long-term storage of the untreated liquid phase of digestate (LPD) versus the long-term storage of the nitrified LPD at lab-scale conditions, highlighting the differences between the aforementioned experimental setups. It was hypothesised that the rate of nitrogen losses during long-term storage of the nitrified LPD will be significantly lower compared with the storage of untreated LPD. Moreover, this diploma thesis seeks to verify the feasibility of the combination of previous nitrification and subsequent thermal thickening by vacuum evaporation of the LPD in order to concentrate the nutrients and other chemical compounds in the thickened LPD, while simultaneously obtaining clean water and reducing the volume of the LPD. It was hypothesised that a nitrogen-rich concentrate, which can be used as fertiliser, will be produced, and a distillate characterised by low concentrations of nitrogen and other chemical compounds, which can be used as a process liquid in the biogas plants (BGPs) for the dilution of feedstocks or for irrigation, will be recovered through the thermal thickening of the nitrified LPD by vacuum evaporation. To achieve these goals, the nitrification of the LPD in a continuously stirred tank reactor (CSTR) was performed as a first step. Subsequently, four glass beakers were filled with 750 mL of untreated LPD and other four were filled with 750 mL of nitrified LPD. In each model, different temperature storage conditions were simulated by storing two of the four beakers at room temperature, one of which was continuously stirred to verify the behaviour of the sample under moderate wind conditions. The remaining beakers were stored in a thermostatic cabinet at 10.0 ± 1.0 °C with one being constantly stirred. The concentrations of total ammonia nitrogen (TAN), N-NO2-, N-NO3- and chemical oxygen demand (COD), as well as pH and dissolved oxygen (DO), were measured a minimum of two-week intervals. It was proved that during the long-term storage of the untreated LPD approximately 7% of nitrogen in the form of TAN was lost weekly whereas during the long-term storage of the nitrified LPD the losses of total inorganic nitrogen (TAN + N-NO2- + N-NO3-) did not exceed 12% in the course of the whole experimental period.Two different series of evaporation were carried out using the nitrified LPD. Samples of 200 mL of nitrified LPD each were subjected to evaporation under reduced pressure (300 mbar) and high temperature (60°C) using a heating water bath set to 95 °C. Evaporation was ceased when the volume of the thickened LPD and distillate were equal to 50% of the nitrified LPD. Next, the pH, electrical conductivity (EC), and concentrations of CODTotal and CODSoluble, N-NO3-, N-NO2-, and TAN were measured, and mass balance of the aforementioned parameters was calculated. The results of the analyses of the thickened LPD were then compared with those obtained from the untreated LPD and nitrified LPD. The total inorganic nitrogen preserved in the thickened LPD of all the samples increased in concentration more than double after the vacuum evaporation, with N-NO3- being the dominant nitrogen form (99.9%), whereas in the distillate did not exceed 2 mg/L in all the samples. Mass balance calculations indicated that 99.9% of the total inorganic nitrogen was maintained in the thickened LPD and the percentage of total inorganic nitrogen in the distillates of all the samples did not exceed 0.06%.

Keywords

Nitrification, Thermal thickening, Denitrification, Volatilisation, Liquid Phase of Digestate

Jazyk práce: English

Datum vytvoření / odevzdání či podání práce: 13. 4. 2018

Obhajoba závěrečné práce

• Vedúci: Ing. Pavel Švehla, Ph.D.

Czech University of Life Sciences Prague

Faculty of Agrobiology, Food and Natural Resources

Master programme / odbor:
Waste Technology / Technology, Processing and Utilization of Waste