Thermal hydrolysis (TH) increases the anaerobic biodegradability of waste activated sludge (WAS), but also refractory organic and nutrient return load to a wastewater treatment plant (WWTP). This could lead to an increase in effluent chemical oxygen demand (COD) of the WWTP. The aim of this study was to investigate the trade-off between increase in biogas production through TH and anaerobic digestion and increase in refractory COD in dewatered sludge liquors at different temperatures of TH in lab-scale. WAS was thermally hydrolyzed in temperature range of 130e170 C for 30 min to determine its biomethane potential (BMP). After BMP test, sludge was dewatered and sludge liquor was aerated in Zahn-Wellens test to determine its non-biodegradable soluble COD known as refractory soluble COD (sCODref). With increasing temperature in the range of 130e170 C, BMP of WAS increased by 17e27%, while sCODref increased by 3.9e8.4%. Dewaterability was also enhanced through relative increase in cake solids by 12 e30%. A conversion factor was defined through mass balance to relate sCODref to volatile solids of raw WAS. Based on the conversion factor, expected increase in effluent CODs of six WWTPs in Berlin were predicted to be in the range of 2e15 mg/L after implementation of TH at different temperatures. It was concluded that with a slight decrease in temperature, formation of sCODref could be significantly reduced, while still benefiting from a substantial increase in biogas production and dewaterability improvement.

The objective of this work was to determine the effects of thermal-pressure hydrolysis (TPH) on dewatered secondary sludge (5-7 % DR) from the wastewater treatment plant Waßmannsdorf with regard to solubilisation properties, biogas production and the formation of refractory substances. In laboratory tests, the impact of the treatment temperature on the sludge due to the TPH was investigated by varying the treatment temperatures within the range of 130-170 °C with a constant hydrolysing time of 30 minutes. Furthermore, the effect of TPH (TTH: 140-170 °C; tTH: 30 min) on digested mixed sludge was studied to quantify the total biogas production of the “Degradation-Lysis-Degradation”-process (DLD). With increasing treatment temperatures (130-170 °C), the COD solubilisation of the hydrolysates was increased linearly up to 45 % which caused higher a biogas production and improved organic matter reduction rates during the anaerobic batch tests. An average methane yield of 212 L·(kg VSS)-1 was produced by the untreated secondary sludge. TPH caused an enhancement of the methane production of additional 17-27 % with the highest surplus observed at treatment temperatures of 170 °C. The organic matter degradation of 46.6 % in the untreated secondary sludge was 2.6 to 36.5 % higher in the hydrolysed sludges and increased with higher temperatures. TPH treatment of the secondary sludge caused formation of refractory COD, that has been measured in the digested sludge filtrate after 28 days of the aerobic degradation test. The organic matter of the untreated secondary sludge was found to be transformed to refractory COD up to 3 %. For the hydrolysed sludges (130-170 °C), the transformation of the organic compounds to refractory COD amounted, temperature-dependent, to 3.9-8.4 %. Raising the TPH treatment temperature from 160 to 170 °C, showed a sharp increase in refractory COD. In order to achieve high biogas yields with moderate loads of refractory compounds in the sludge water, a TPH-temperature of 150-160 °C is recommended. Applying the TPH to the DLD-configuration, hydrolysed sludges showed 20-30 % greater methane yields as well as 16-27 % higher biodegradation rates compared to the untreated digested sludge. At a treatment temperature of 170 °C of the digested sludge, 372 L·(kg VSS)-1 methane were produced with a organic matter reduction of 67.6 %. Comparing the test results of TDH at 170 °C and the Thermo-alkaline Hydrolysis (TaH) of secondary sludge, dosing 0,08 g NaOH·(g DR)-1 at a treatment temperature of 70 °C, the highest achievable methane yields were in the same range of approx. 270 L·(kg VSS)-1. TaH caused a 50 % lower refractory compound formation than TDH. However, the enhanced dewaterability of TDH treated sludge, compared to TaH treatment, provides cost-saving potential.