Denitrification of the electron donors toluene-C (15-100 mg/L), m-xylene-C (15-70 mg/L), benzene-C (5-25 mg/L), and acetate-C as experimental reference (50-140 mg/L) was carried out in batch culture. An initial concentration of 1.1 +/- 0.15 g of volatile suspended solids/L of denitrifying sludge without previous exposure to aromatic compounds was used as inoculum. The results showed toluene and nitrate consumption efficiency (ET and EN, respectively) of 100%. Toluene was completely mineralized (oxidized) to CO2. In all cases, the N2 (YN2) and HCO3-yields (YHCO3) were 0.97 +/- 0.01 and 0.8 +/- 0.05, respectively. The consumption efficiency (EX) of m-xylene (53 +/- 5.7%) was partial. The YN2 and YHCO3 were 0.96 +/- 0.01 and 0.86 +/- 0.02, respectively. Benzene was not consumed under denitrifying conditions. The specific consumption rates of toluene (qT) and m-xylene (qX) were lower than that of acetate (qA). The differences in specific consumption rates were probably owing to the negative effect of benzene, toluene, and isomers of xylene on the cell membrane.
The adaptation of the technique for measuring the contact angle (CA) in order to follow the changes of sludge settleability of a denitrifying sludge surface is described. Denitrifying sludge was continuously fed into an upflow anaerobic sludge blanket (UASB) reactor with acetate and nitrate. The loss of settleability in the sludge was induced by decreasing the hydraulic residence time (HRT) from 1 d to 0.125 d. CA was measured with air and nitrogen bubbles. A comparison between sludge volume index (SVI) and the CA and their correlation with sludge settleability was carried out. Results showed that CA method had a high correlation with sludge settleability. Moreover the CA method showed to be simpler and less time consuming than the SVI method.
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