Removal of organic micropollutants at PPB levels in laboratory activated sludge reactors under various operating conditions: biodegradation

Abstract

The biodegradation behaviour of 5 model chemical contaminants, 2,4-dichlorophenoxy acetic acid (2,4-D); 2,4,6-trichlorophenol (TCP); pentachlorophenol (PCP); 4-nitrophenol (4-NP) and lindane, spiked into synthetic peptone sewage to μg/l levels (5–1000 μg/l) have been studied in laboratory activated sludge reactors under a range of operating conditions (sludge ages of 1–32 days and sludge loadings of 0.09–0.9 mg BOD 5/mg MLSS/d). The reactors were continuously stirred tanks operated under a computer controlled fill and draw cycle of 4 h. Experiments were started with sludge collected from a municipal treatment plant. In most experiments, a gradual adaptation took place resulting in increases in biodegradation rates by an order of magnitude or more compared to initial biodegradation rates and rates obtained in comparative batch experiments, respectively. Times required for adaptation varied among chemicals and experiments and ranged from 2–5 days for 4-NP to 1–2 months for 2,4-D and lindane. No concentration thresholds for adaptation were observed, and the biodegradation generally followed first-order kinetics. Removals by biodegradation in successfully adapted systems were generally within a range of about 40 to about 95% except for 4-NP, which was degraded to concentration levels below the analytical detection limit. For estimation of the part of the total removal which was due to biodegradation the amounts of PCP, TCP and lindane wasted with the sludge were calculated from sorption isotherms. Volatilization was negligible. The operational parameters influenced the biodegradation differently for the different chemicals. PCP was degraded best at high sludge ages and was not degraded significantly at sludge ages below approx. 8 days. This suggests catabolic degradation by slow growing specific degraders. TCP was consistently degraded at high sludge ages (low sludge loadings) and at random at lower sludge ages, but rate constants were highest at intermediate sludge loadings. A similar pattern was seen for 2,4-D. Lindane was degraded poorly at high sludge ages and to a much greater extent at intermediate and at high sludge loadings. These findings suggest co-oxidation as the dominant degradation mechanism for lindane and as a significant mechanism for TCP and 2,4-D.