title: Aerobic and Electrochemical Treatment of Process Water from Hydrothermal Carbonization of Sewage Sludge
Creators: name: Blach, Tobias
Publisher: name: UNSPECIFIED
Year: 2025

description: This dissertation demonstrates possibilities and limitations of aerobic and electrochemical processes for the treatment of process water from the hydrothermal carbonisation (HTC) of sewage sludge. During HTC, sewage sludge is heated to around 190 to 250 °C, causing various reaction mechanisms to turn sewage sludge into a brown coal-like solid. The so-called hydrochar or biochar has a higher calorific value and better dewaterability compared to the untreated sewage sludge. This enables thermal utilisation, use as an adsorbent, or for carbon sequestration. However, the reaction mechanisms also release a large number of organic substances from the solids and partially transform them into substances that are difficult to biodegrade, inhibitory or toxic. Current research focuses primarily on optimizing the HTC process and only secondarily on treating the highly contaminated process water. Nevertheless, previous studies have shown that biodegradability of process water contaminats is limited and hindered by various inhibitors. This raises the issue of the robustness of aerobic processes and the efficiency of electrochemical oxidation to remove refractory substances. The main findings are summarized in four international publications.
Paper 1 visualizes the correlation of the process water load on various reaction parameters. For this purpose, the effect of the parameters reaction temperature (190 to 250 °C), reaction time (0.5 to 4 h) and pH value (3.9 to 6.1) on calorific value, dissolved organic carbon (DOC), and ammonium was tested using design of experiments and the Box-Behnken design. The temperature turned out to be the decisive parameter of the HTC reaction. The higher the temperature, the higher was the calorific value of the hydrochars and the lower was the concentration of DOC in the process water. Higher temperature also increased the percentage of ammonium in the total nitrogen due to the mineralisation of dissolved organic nitrogen (DON).
As the reaction temperature emerged as the decisive reaction parameter from paper 1, paper 2 focuses on the effect of temperature on aerobic bio-degradability. In Zahn-Wellens tests, the DOC removal for temperatures from 190 to 249 °C was about 81%. Continuous lab-scale tests in sequencing batch reactors (SBR, V=0.3 L) also showed similar DOC removals of 72% for different temperatures (190 °C and 217 °C). Due to the strong inhibition by substances formed during HTC, nitrification was only possible by diluting the process water 1:10. Inhibition tests according to DIN EN ISO 9509 (2006) revealed a stronger inhibition for process water at 217 °C than at 190 °C. However, the overall effect of temperature on aerobic biodegradability can be considered low. In addition, an exemplary mix calculation for a municipal wastewater treatment plant showed that the refractory organics could increase the effluent concentration of chemical oxygen demand (COD) by 24 mg/L in the worst case. Paper 3 takes up the findings of paper 2 and transfers them to the operation of a pilot plant in order to determine the limits of nitrification performance and organic removal. For this purpose, the sludge loading of a membrane bioreactor (MBR, V=170 L) and an SBR (V=300 L) was successively increased. In addition, reducing the dilution of the process water from 1:20 to 1:1 led to an increase in inhibitor concentration. The maximum sludge loading for nitrification as total nitrogen (TN) per mixed liquor suspended solids (MLSS) was 20 - 25 mg TN/(g MLSS·d) in both reactors. The nitrogen sludge loading during treatment of the undiluted process water was too high for nitrification. With nitrification, COD removal was 74.8 ± 1.9% (MBR) and 71.4 ± 2.6% (SBR). The activated sludge from the SBR adapted to inhibitors in the process water to a certain degree. The nitrification rate was inhibited by 50% at 50 mg DON (i.e. 4% v/v process water share), compared to 6.6 mg DON (i.e. 0.4% v/v process water share) for activated sludge from a municipal wastewater treatment plant. Although different process waters were treated in paper 2 and 3, both the COD removal and the inhibition of nitrification were similar. Paper 2 and paper 3 have demonstrated that 20 to 30% of the COD was not biodegradable. Following this, paper 4 presents results from the electrochemical oxidation (EO) of refractory COD using a boron-doped diamond electrode and introduced three possible treatment scenarios: (I) EO alone, (II) aerobic COD removal + EO, and (III) nitrification/denitrification + EO. In all scenarios, EO proved to be very effective and even reduced the COD to below 10 mg/L in scenario III. This low concentration was associated with an energy consumption of up to 534 kWh/kg COD. As the energy consumption of the EO largely depends on the COD concentration, it was significantly lower in scenario I and II with 31.3 and 46.6 kWh/kg COD. A concluding mass balance showed that scenario II had the lowest overall energy consumption due to the reduced COD load and yet high energy efficiency. The result of this dissertation suggest that aerobic treatment with downstream electrochemical oxidation is a conceivable solution for treating HTC process water. However, due to high inhibitor concentration, nitrification can in fact only be established by diluting the process water.