Abstract
The impact of oxidation on mitigation of cyanobacteria and cyanotoxins in drinking water treatment sludge was investigated at the laboratory and treatment plant scales. Two common oxidants, KMnO4 (5 and 10 mg/L) and H2O2 (10 and 20 mg/L) were applied under controlled steady-state conditions. Non-oxidized and oxidized sludge was left to stagnate in the dark for 7 to 38 days. Controlled laboratory trials show that KMnO4 and H2O2 decreased cell counts up to 62% and 77%, respectively. The maximum total MC level reduction achieved after oxidation was 41% and 98% using 20 mg/L H2O2 and 10 mg/L KMnO4, respectively. Stagnation caused cell growth up to 2.6-fold in 8 out of 22 oxidized samples. Microcystin (MC) producer orders as Chroococcales and Synechococcales were persistent while Nostocales was sensitive to combined oxidation and stagnation stresses. In parallel, two on-site shock oxidation treatments were performed in the DWTP’s sludge holding tank using 10 mg/L KMnO4. On-site shock oxidation decreased taxonomic cell counts by up to 43% within 24 h. Stagnation preceded by on-site shock oxidation could increase total cell counts by up to 55% as compared to oxidation alone. The increase of cell counts and mcyD gene copy numbers during stagnation revealed the impact of oxidation/stagnation on cyanobacterial cell growth. These findings show the limitations of sludge oxidation as a strategy to manage cyanobacteria and cyanotoxins in sludge and suggest that alternative approaches to prevent the accumulation and mitigation of cyanobacteria in sludge should be considered.
Highlights
Health concerns about the occurrence of cyanobacterial cells and their associated metabolites in water sources have been raised in the past decade [1,2,3,4,5,6].Conventional treatment processes including coagulation, flocculation, sedimentation, and filtration are common approaches to respond to the presence of cyanobacteria and cyanotoxins in drinking water treatment plants (DWTPs) [7,8,9,10,11]
Several studies have reported that toxic cyanobacterial cells (e.g., Microcystis aeruginosa, Dolichospermum circinale, Oscillatoria sp., and Cylindrospermopsis raciborskii) can survive and release cyanotoxins in the stored sludge for up to 12 days [7,14,20,21,22,23,24]
Recent studies hypothesized toxic cyanobacterial cells can even grow in the stored sludge up to 16 days leading to cyanotoxin release up to four times higher than the expected concentration [25]
Summary
Health concerns about the occurrence of cyanobacterial cells and their associated metabolites (cyanotoxins, taste, and odor agents) in water sources have been raised in the past decade [1,2,3,4,5,6].Conventional treatment processes including coagulation, flocculation, sedimentation, and filtration are common approaches to respond to the presence of cyanobacteria and cyanotoxins in drinking water treatment plants (DWTPs) [7,8,9,10,11]. Health concerns about the occurrence of cyanobacterial cells and their associated metabolites (cyanotoxins, taste, and odor agents) in water sources have been raised in the past decade [1,2,3,4,5,6]. Several studies have reported that toxic cyanobacterial cells (e.g., Microcystis aeruginosa, Dolichospermum circinale, Oscillatoria sp., and Cylindrospermopsis raciborskii) can survive and release cyanotoxins in the stored sludge for up to 12 days [7,14,20,21,22,23,24]. Recent studies hypothesized toxic cyanobacterial cells can even grow in the stored sludge up to 16 days leading to cyanotoxin release up to four times higher than the expected concentration [25]
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