Unraveling oxidation behaviors for intracellular and extracellular from different oxidants (HOCl vs. H2O2) catalyzed by ferrous iron in waste activated sludge dewatering

Cell lysis in sludge pretreatment by advanced oxidation process (AOP) has a great effect on sludge dewaterability. Cell lysis caused by reactive radicals (e.g. hydroxyl radical) was dependent on the reaction site of AOP. However, little is known about the accurate radical generation site of AOP in sludge pretreatment. In this study, two kinds of oxidation behaviors from different oxidants (HOCl vs. H2O2) catalyzed by ferrous iron were comparatively investigated. Higher amount of living cells (84.3%) and hydroxyl radicals (9.86 x 10(-5) M), and more fragmentized sludge flocs (particle sizes of D50 was 50.1 vs. 573 mu m of RS) were detected in sludge conditioned by Fe2+/H2O2 which implied that Fenton reaction mainly happened at surface and outside of sludge flocs (such as EPS layer and liquid phase). Thus, it could be regarded as "extracellular oxidation". Fewer living cells (undetectable), fewer amount of hydroxyl radicals (undetectable in sludge), and more integrated sludge flocs (particle size of D50 was 56.1 vs. 57.3 mu m of RS) were determined in sludge conditioned by Fe2+/Ca(ClO)(2). Hence, it could be regarded as "Intracellular oxidation". In addition, sludge pretreatment based on Fe2+/Ca(ClO)(2) could achieve simultaneous deep-dewatering performance and total coliforms inactivation. Based on response surface methodology, the optimal dosages of Fe2+ and Ca(ClO)(2) were proposed as 106.1 and 234.5 mg/g volatile solids respectively, without any acidification of sludge. Under these optimal dosages, the water content of dewatered sludge cake was 51.9 +/- 0.1 wt% and the pH of the final filtrate was 5.8 +/- 0.2. Total coliforms of sludge could be inactivated in 10 s after Fe2+/Ca(ClO)(2) addition. (C) 2018 Elsevier Ltd. All rights reserved.