Effects of trace element addition on volatile fatty acid conversions in anaerobic granular sludge reactors

Abstract

The effect of the addition of trace elements on the conversion of a mixture of volatile fatty acids (Acetate, Propionate, Butyrate, in a ratio 3:1:1) by anaerobic granular sludge was investigated. Two Upflow Anaerobic Sludge Bed reactors (pH 7, 30°C) were operated for 140 days at an organic loading rate ranging from 2 g COD l−1 d−1 up to 10 g COD 1−1 d−1 and a hydraulic retention time of 12 hours. One reactor (Rl) was supplied with a trace metal cocktail in the basal medium, whereas trace metals were omitted from the influent of the second reactor (R2). As a result, the trace metal concentration in the granules from R2 steadily decreased at a rate of 48 μg metal g−1 TS d−1 down to 35% of their initial value. In contrast, trace metals accumulated in granules present in Rl. At the end of the experiment, the COD removal efficiencies were 99 and 77% for, respectively, the control (R1) and deprived (R2) reactors. This difference was due to lack of propionate conversion by sludge from R2. No difference in the acetate and butyrate conversion capacity of both reactors was observed. The conversion of acetate, propionate and methanol were stimulated by the continuous addition of metals to the influent, as sludge of R1 had higher maximum specific activity values compared to sludge of R2. However, both sludges had a similar maximum specific activity with buryrate. Surprisingly, maximum specific activity tests using individual trace metals showed that the addition of a particular trace element in the activity test medium did not affect the degradation rates of a specific substrate, i.e. acetate, propionate, butyrate and methanol