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Abstract
Thermal hydrolysis (TH) and biological hydrolysis (BH) are two main and growing anaerobic digestion pretreatment technologies. In this study, municipal wastewater sludge samples were collected from the Guelph Wastewater Treatment Plant (WWTP) in Ontario, Canada. The effects of temperature on BH treatment, including BH at 42 °C (BH42), 42 °C followed by 55 °C (BH42+55), 55 °C followed by 42 °C (BH55+42), and 55 °C (BH55) were evaluated for anaerobic digestion performance enhancement and compared with TH treatment at 165 °C. The TH, BH42, BH42+55, BH55+42, and BH55 treatments caused the reduction of volatile suspended solids (VSS) by 22.6%, 17.5%, 24.6%, 23.1%, and 25.9%, respectively. The soluble chemical oxygen demand (sCOD) content of the sludge increased by 377.5%, 323.8%, 301.3%, 286.9%, and 221.7% by the TH, BH55, BH42+55, BH55+42, and BH42 treatments, respectively. Volatile fatty acids (VFA) constituted around 40% of the sCOD in the BH-treated sludge and 6% in the TH-treated sludge. The cumulative methane yields (NmLCH4/g COD fed) of sludge treated by BH55+42 and TH were respectively 23% and 20% higher than that of the untreated sludge. For BH pretreatment, sludge treated by BH55+42 produced more methane than those treated by BH42+55, BH55, and BH42. The methane yields of the combined sludge treated by the TH and BH55+42 treatments were in the ranges of 248.9 NmLCH4/g COD to 266.1 NmLCH4/g COD fed, and 255.3 NmLCH4/g COD to 282.2 NmLCH4/g COD fed, respectively.
Highlights
The anaerobic digestion (AD) of wastewater sludge has become increasingly attractive due to its capacity to recover energy from wastewater [1,2]
With a direct comparison between Thermal hydrolysis (TH) and biological hydrolysis (BH), this study showed that the BH treatment at temperatures between 42–55 ◦ C for three days and the TH treatment at 165 ◦ C for 30 min can achieve comparable biogas production enhancements that range from 10% to 23%, which are comparable to the results reported by other research studies
The results showed that the BH temperatures and the sequential order of BH processes of different temperatures can significantly affect the hydrolysis performance and biogas production
Summary
The anaerobic digestion (AD) of wastewater sludge has become increasingly attractive due to its capacity to recover energy from wastewater [1,2]. Of WAS is typically between 60–80% of the total solids with an energy content of 19–23 kJ/gVSS [2]. Anaerobic digestion converts sludge organic content into energy-rich methane gas through rather specific reactions, including hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Hydrolysis is a critical extracellular enzymatic reaction step to break down particular solids and macromolecules into small soluble compounds, which makes them utilizable by organism cells for methane production through intracellular metabolic reactions. Since WAS mainly consists of aggregated bacterial cells and extracellular polymeric substances (EPS), the hydrolysis of WAS has been widely accepted to be a rate-limiting step for methane production from WAS.
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