Im Projekt E-VENT wurden innovative Verfahren der kommunalen Abwasserreinigung und Klärschlammbehandlung untersucht, um damit den Energieverbrauch von Klärwerken und die damit verbundenen Emissionen von Treibhausgasen (THG) zu senken. Nach einem Screening verschiedener Technologien wurden dazu Labor- und Pilotversuche zur thermischen Hydrolyse von Klärschlamm und zur Abwasserbehandlung mit granuliertem Belebtschlamm im Nereda®Verfahren durchgeführt. Aufbauend auf den Versuchsergebnissen wurden für ein Berliner Klärwerk verschiedene Varianten für einen zukünftigen Neubau modelliert und bewertet, um das Potential der innovativen Verfahren zur Senkung von THG-Emissionen unter den spezifischen Randbedingungen in Berlin abzuschätzen. Abschließend wurden auch die Investitions- und Betriebskosten der neuen Verfahren mit einer konventionellen Referenzvariante verglichen. Die Versuche zur thermischen Hydrolyse zeigen, dass der Faulgasertrag aus dem Klärschlamm damit deutlich erhöht werden kann (bis zu +26%). Gleichzeitig erhöht sich aber auch die Rückbelastung des Faulschlammzentrats mit Phosphor, Stickstoff und refraktären organischen Stoffen. Neben dem erhöhten Aufwand zur Behandlung des Zentrats kann vor allem der refraktäre organische Anteil die Ablaufqualität der Klärwerke deutlich verschlechtern, besonders bei Thermo-Druck-Hydrolyse. Bei thermo-alkalischer Hydrolyse konnte im Pilotversuch ein Mehrgasertrag von +19% im Jahresmittel sowie eine moderate Belastung des entstehenden Zentrats gezeigt werden, das die Ablaufwerte des Klärwerks nicht signifikant verschlechtert. Im Pilotversuch zum Nereda®-Verfahren wurde ein stabiler Betrieb mit granuliertem Belebtschlamm erreicht, der eine gute biologische Reinigungsleistung für Phosphor und Stickstoff zeigte. Die hohen Anforderungen an die Ablaufwerte konnten jedoch nicht zuverlässig erreicht werden. Wie auch im konventionellen Belebtschlammverfahren ist dabei die Verfügbarkeit von Kohlenstoff (CSB/N-Verhältnis) ein möglicher limitierender Faktor für die biologischen Prozesse und die erreichbare Ablaufqualität. Darüber hinaus wurde ein erhöhter Anteil von Feststoffen im Ablauf des Nereda®-Verfahrens festgestellt, der zur Erreichung der vorgegebenen Zielwerte eine Nachreinigung über Filtration erforderlich macht. Vor einer großtechnischen Umsetzung sind daher weitere Untersuchungen in größerem Maßstab notwendig, um die zuverlässige Einhaltung der geforderten Überwachungswerte zu prüfen. Die Messung von Lachgas ergab relativ hohe Emissionsfaktoren dieses starken THG für die Nereda®-Pilotanlage. Die Bewertung der Verfahren für einen zukünftigen Neubau des Klärwerks Stahnsdorf zeigen, dass die innovativen Verfahren die Energiebilanz gegenüber einer konventionellen Referenz weiter verbessern können. Dabei werden die möglichen Vorteile einer thermo-alkalischen Hydrolyse im Faulgasertrag durch den Mehraufwand auf dem Klärwerk und auch durch geringere Energierückgewinnung in der Klärschlammentsorgung im Modell ausgeglichen. Beim Nereda®Verfahren sinkt der Verbrauch an Strom und Fällmitteln und verbessert so die Energiebilanz und senkt die damit verbundenen Emission von Treibhausgasen. Dabei ist zu beachten, dass wichtige Eingangsdaten weiter validiert werden sollten, um zu einer abschließenden Bewertung dieser Verfahren zu kommen. Die Schätzung der Investitions- und Betriebskosten ergab, dass die innovativen Verfahren Kostenvorteile bieten können. Insgesamt zeigte das Projekt, dass die hier untersuchten innovativen Verfahren ein Potential zur Senkung der THG-Emissionen der Abwasserreinigung bieten. Für den betrachteten Neubau des Klärwerk Stahnsdorf konnten dieser THG-Fußabdruck um bis zu 72% gesenkt werden, was einer Einsparung von 3700 Tonnen CO2Äquivalenten entspricht. Bei einer zukünftigen Einführung solcher innovativen Verfahren ist jedoch immer die zuverlässige Einhaltung der vorgegebenen Ablaufwerte als Primärziel der Abwasserreinigung zu garantieren und dafür in großtechnischem Maßstab zu überprüfen.

Thermal alkaline pretreatment (TAP) of waste activate sludge (WAS) was carried out in pilot-scale over a year to investigate its seasonal effects on anaerobic digestion and its impact on dewaterability, sludge liquor quality and formation of soluble refractory COD (sCODref). Temperature of TAP was set at 65–70 °C and pH was increased by initial dosing of sodium hydroxide [NaOH] (50% w/w, 1–2.5 mL/L sludge) as alkali agent following 2–2.5 h reaction time. Pilot digesters were fed with primary sludge (PS) and hydrolyzed WAS (HWAS) and compared to a reference digester fed with PS and untreated WAS. Biogas yield increase due to TAP of WAS showed a sinusoidal trend throughout the year with maximum in summer (+42%), minimum in winter (+3%) and average of +20%, indicating a strong seasonal effect on TAP efficiency. Ammonium [NH4+-N], orthophosphate [PO43--P] and sulphate [SO42-] in sludge liquor increased by 34.6%, 17.0% and 21.6% with TAP, respectively. Centrifugation tests showed no significant difference in dewaterability of both digestates with respect to total solids of sludge cake. Normalized capillary suction time of digestate increased due to TAP, indicating a lower capability for water release. Furthermore, detected sCODref after batch aerobic biodegradation tests showed an increase of 30.3% with TAP. Hence, implementation of TAP of WAS in full-scale will potentially lead to an increase of 0.8–1.1 mg/L of sCODref in effluent of six wastewater treatment plants (WWTP) in Berlin. In conclusion, TAP of WAS leads to increase in biogas production with a slighter negative impact on effluent COD quality than high-temperature thermal hydrolysis.

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.

Wastewater treatment plants (WWTPs) are evolving towards a more sustainable manner, by which not only the effluent quality and operational costs but also the greenhouse gases (GHG) potential is incorporated into the assessment inventory. GHG emissions from the WWTPs include CH4, CO2 and N2O, of which the N2O is of special interest due to 265-fold CO2-equivalent. Thus, even a low amount of N2O is undesired. Aerobic granular sludge (AGS) is a promising biological nutrient removal technology due to considerable structural and microbiological distinctions compared with conventional activated sludge (CAS) flocs, leading to huge improvements of carbon footprint saving. Nevertheless, the N2O formation from the AGS reactor is likely higher than that from the CAS, in terms of sequence batch reactor (SBR) configuration and inherent complex mechanism. In addition, there wasn’t any long-term monitoring campaign on the AGS reactor focusing on N2O emissions so far.This study focusses on a N2O emission online monitoring campaign, which was carried out in a Nereda® AGS reactor treating domestic wastewater from the Berlin region, lasted more than 6 months, including two different phases, namely feeding with pre-treated and raw wastewater after aerated sand trap and 2mm sieve box. The off-gas was sucked from the top of the SBR reactor and measured with online gas analyzer. Then the emission factor (EF) was calculated based on the correlated influent nitrogen load, which was converted from the influent NH4-N concentration by fixed ratio of 0.8. During the first phase, the EF was equal to 2.97%, while during the second phase, the EF was equal to 4.52%. Generally, the EF calculated in terms of both phases was 3.71%. Compared with other long-term campaigns based on CAS and SBR processes, higher GHG potentials could be induced, which also challenges the predominance of the AGS reactors from the perspective of minimizing GHG when only considering the energy consumption into scope. In-depth analysis indicated that the hydroxylamine oxidation pathway was the most likely over the monitoring course and EF calculated during main aeration incorporate negligible fraction of N2O produced from the pre-denitrification phase. Correlation test combining two specific time frames showed the moderate positive correlation between temperature and EF, which was in contrast to what has been assumed before but coincided with the inference from the micro-level analysis of our study. The weak negative correlation ship of COD/N ratio and EF was reported for each individual phase. Due to the insignificant impact from pre-denitrification and exclusion of the post-denitrification phase, it could not be considered as reliable. In terms of narrow range of DO and no accumulation of nitrite, the weak negative correlation ship of DO and EF could not infer to any further conclusion. In addition, it should be noted that some uncertainties may distract the reliability of our results. High resolution online measurement should be applied for the determination of off-gas flow, COD and TNb concentration, instead of correlation method or infrequent laboratory analysis. The detection of dissolved N2O along the course are needed to provide more insights about the N2O formation during the process and to distinguish the contribution between aerated phase and non-aeration phase. At last, more frequent monitoring of the significant precursor nitrite and hydroxylamine is demanded to figure out the dominant pathway for AGS reactors.

Wastewater treatment (WWT) is obligatory for the protection of ecosystems and human health but also produces the greenhouse gases (GHGs) nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2 ) along the process chain. According to the IPCC (2018) anthropogenic CO2 and carbon emissions must decline by 45% worldwide from 2010 levels by 2030 to keep temperatures from rising beyond 1.5 ° degrees. Currently the sector of WWT contributes about 0.11 % to the total carbon emissions in Germany and was responsible for about 5 % of global non-CO2 GHG emissions in 2005. N2O emissions in particular play the major role here. Aerobic granular sludge (AGS) for biological WWT has gained increasing interest mainly due to higher process efficiency compared to conventional activated sludge (CAS). Studies show a reduction potential of 20 – 25 % in operation costs, 23 – 40 % in electricity use and 50 –75 % in space requirements. AGS processes are implemented as sequencing batch reactor (SBR). SBRs with a small temporal and spatial variability for biological metabolism are likely to generate process conditions promoting N2O formation. A 1% increase in direct N2O emissions could already result in a 30 % increase of the carbon footprint of a WWTP. In this thesis direct GHG emissions from AGS treating domestic wastewater are studied. It was part of the project E-VENT where an AGS Nereda® pilot-plant has been operated at Stahnsdorf WWTP, Berlin (Germany). The reactor was fully-covered and GHG emissions have been monitored online over a 3 months period. A conservative approach for off-gas flow determination has been chosen to not over-estimate GHG loads. The plant was operated with domestic wastewater extracted after the primary clarifiers. At stable operating conditions maximum removal rates of 96 % chemical oxygen demand, 90 % nitrogen and 87 % phosphorous were achieved. Determined emission factors (EF) for N2O and CH4 over the complete measurement period were 2.86 % and 0.18 % respectively. Rising process temperatures from 13 – 20 °C showed a positive correlation with EFs and higher TN loads during the day lead to higher N2O complementing literature review on N2O EFs. The CO2 EFs showed that determined values for AGS are in accordance with 2.8 % ± 1.2 % found in a comparable study by Guimarães et al. (2017). Findings conclude that N2O contributes to about 95 % to total direct carbon emissions of the Nereda® plant and is a main factor for the climate impact of AGS.

Thermal-pressure hydrolysis and thermal-alkaline hydrolysis of secondary sludge, from the wastewater treatment plant Waßmannsdorf, were compared based on the physical changes of the treated sludges. For this purpose, seven parameters were determined for investigation. These were: Viscosity, particle size distribution, microscopic images, capillary suction time (CST), TR after the laboratory centrifuge test, zeta potential and foaming potential. To measure these parameters, methods were developed and then applied respectively to sludges from both treatments. The thermal-pressure hydrolysis performed better than the thermal-alkaline hydrolysis in each parameter investigation. In particular, the dewaterability of the sludges after digestion, which represents an important cost factor in sewage plant operation, could be improved by thermal-pressure hydrolysis, but not by thermal-alkaline hydrolysis.

The focus of this study investigation was laid on the plant-specific applicability of a NaOH and thermal pretreatment of activated sludge AS with following mesophilic digestion and the influence on the biomethane potential BMP. Firstly, the hydrolysis of activated sludge from the granular sludge process of the Nereda technology, which differs from conventional activated sludge in terms of sludge formation, sludge stabilization, and sludge age, was investigated for the first time. A higher dose of NaOH (0.02 - 0.08 g NaOH per gVS, 70 °C) raised the COD and phosphate degree of digestion and the digester gas yield by 22 - 47 %. Different hydrolysis temperatures (50 - 90 °C, 0.05 g NaOH per gVS) also increased the sludge parameters. However, the BMP only enhanced by 12 % at temperatures higher than 70 °C. With increasing hydrolysis temperature, the digestion time was reduced by 2 - 5 days. Despite the process-related differences between conventional AS (from the Stahnsdorf wastewater treatment plant) and AS from the granular sludge processing, comparable results were obtained in the BMP test, with and without pretreatment. Due to a lack of time, the experiments could only be carried out once or twice. As there are currently no further experience and references on this subject, additional attempts for achieving significant results will follow. In the second part, sludges from the Waßmannsdorf sewage treatment plant were used. Laboratory tests have shown that primary sludge has no influence on the digestion process. The calculated BMP of 176.5 NmL/gCSB deviates by 3% from the value of 181.9 NmL/gCSB measured in the laboratory test. It is directly related to the ratio of the used sludges. Hydrolysis according to PONDUS (70 °C; 2 h; 2.5 mL NaOH 50 % per L AS) at laboratory revealed a comparable influence on the sludge parameters as with hydrolysis on a pilot scale. During the BMP test, the laboratory sample achieved a maximum gas yield of 143 NmL/gCSB, which is a 9 % higher BMP in comparison to the pilot sample with 132 NmL/gCSB. The laboratory results can, therefore, be transferred to the pilot scale, so that the effects of changes in operation can be reliably assessed by cost, time and effort saving laboratory tests. This thesis was written within the framework of the project “Evaluation of process options for the reduction of energy consumption and greenhouse gas emissions of Berlin sewage treatment plants" at the Berlin Centre of Competence for Water.