Abstract
The pre-treatment of waste activated sludge (WAS) has become more common since it often results in improved bioconversion to methane, in both rate and extent. However, thorough insights on the possible effects and mechanisms of mild pre-treatment techniques, such as temperatures <100 °C combined with the addition of H2O2, are still limited. This study reports the effects of the addition of 5–30 mgH2O2/g TS and its interaction with thermal pre-treatment at 70 °C on methane production, using WAS as the substrate. It was found that the addition of H2O2 increased the methane production rate, coinciding with a decrease in apparent viscosity of WAS, which probably improved mass transfer under non-ideal mixing conditions. While H2O2 solubilized proteins and carbohydrates and mineralized a small fraction of the humic substances in WAS, these biochemical transformations did not suffice to explain the observed extent and rate of methane production. A decreased particle size, the presence of Fenton’s reagent, and the presence of cationic polymers in the WAS were discarded as the reasons for the observed decrease in apparent viscosity. It was concluded that the pre-treatment conditions applied in the present study might be a strategy to enhance mixing conditions in full-scale anaerobic digesters.
Highlights
Waste activated sludge (WAS) is the main by-product of conven tional activated sludge (CAS) wastewater treatment plants (WWTP)
During pre-treatment, the breakdown of H2O2 by catalase (Wang et al, 2009) will result in misused reagent; catalase must be inactivated via thermal denaturation to ensure H2O2 is available for other reactions
None theless, the observed decline in pH (
Summary
Waste activated sludge (WAS) is the main by-product of conven tional activated sludge (CAS) wastewater treatment plants (WWTP). Thermal pre-treatment techniques have been popularized and are commonly categorized into high- and lowtemperature using 100 ◦C as division In this regard, low-temperature thermal pre-treatment has a lower energy requirement and avoids the use of pressured vessels, steam and sophisticated control systems (Pilli et al, 2014). The observed effects of microwave-H2O2 pre-treatment have been attributed to oxidizing by-products or side-reactions, such as Fenton’s reagent and superoxide radicals (Ambrose et al, 2020; Xiao et al, 2012; Yu et al, 2016) Such claims have not been thoroughly researched, probably because of the difficulty of measuring oxidizing species such as hydroxyl radicals. It is unclear how each pre-treatment individually contributes to the reported improvements in solubiliza tion, methane production and to the decreased WAS viscosity
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