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Hydrogen sulfide (H2S) related to wastewater in sewer systems is known for causing significant problems of corrosion and odor nuisance. Sewer systems severely affected by H2S typically rely on online H2S gas sensors for monitoring and control. However, these H2S gas sensors only provide information about the H2S emission potential at the point being monitored, which is sometimes inadequate to design control measures. In this study, a comparison of three market-ready online sensors capable of liquid-phase H2S detection in sewer systems was assessed and compared. Two of the three sensors are based on UV/Vis spectrophotometry, while the other adapted the design and principles of a Clark-type electrochemical microsensor. The H2S measurements of the sensors were statistically compared to a standard laboratory method at first. Following that, the performance of the online sensors was evaluated under realistic sewer conditions using the Berlin Water Company (BWB) research sewer pilot plant. Test applications representing scenarios of typical H2S concentrations found in sulfide-affected sewers and during control measures were simulated. The UV/Vis spectrometers showed that the performance of the sensors was highly dependent on the calibration type and measurements used for deriving the calibration function. The electrochemical sensor showed high sensitivity by responding to alternating anaerobic/anoxic conditions simulated during nitrate dosing. All sensors were prone to measurement disturbances due to high amounts of sanitary solids in wastewater at the study site and required continuous maintenance for reliable measurements. Finally, a summary of the key attributes and limitations of the sensors compared for liquid phase H2S detection is outlined. [ABSTRACT FROM AUTHOR]

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 (sCOD ref). 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 [ NH 4 + − N ] , orthophosphate [ PO 4 3 − − P ] and sulphate [ SO 4 2 − ] 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 sCOD ref 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 sCOD ref 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. Image 1 • Biogas yield of mixed sludge follows a sinusoidal trend throughout a year. • Thermal alkaline pretreatment (TAP) effect on biogas differs in winter and summer. • Dewaterability of digestate did not improve after TAP implementation. • Effluent COD increase in WWTPs after TAP was predicted through modelling. • TAP shows potential of biogas increase with small impact on effluent COD of WWTP. [ABSTRACT FROM AUTHOR]

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 130–170 °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 (sCOD ref). With increasing temperature in the range of 130–170 °C, BMP of WAS increased by 17–27%, while sCOD ref increased by 3.9–8.4%. Dewaterability was also enhanced through relative increase in cake solids by 12–30%. A conversion factor was defined through mass balance to relate sCOD ref 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 2–15 mg/L after implementation of TH at different temperatures. It was concluded that with a slight decrease in temperature, formation of sCOD ref could be significantly reduced, while still benefiting from a substantial increase in biogas production and dewaterability improvement. Image 1 • Biomethane potential of WAS increased with temperature of thermal hydrolysis (TH). • Refractory soluble COD increased with temperature of TH. • Slight decrease in TH temperature significantly reduced refractory sCOD. • Impact of TH on effluent COD increase of a WWTP was predicted. • Optimum TH temperature can be found for a WWTP to comply with COD discharge limit. [ABSTRACT FROM AUTHOR]