Toxicity of xenobiotics during sulfate, iron, and nitrate reduction in primary sewage sludge suspensions

The effect and persistence of six organic xenobiotics was tested under sulfate-, iron-, and nitrate-reducing conditions in primary sewage sludge suspensions. The xenobiotics tested were acenaphthene, phenanthrene, di(2-ethylhexyl)phthalate (DEHP), 4-nonylphenol (4-NP), linear alkylbenzene sulfonate (LAS), and 1,2,4-trichlorobenzene (1,2,4-TCB) added to initial analytical concentrations of 54–117 mg L−1. The suspensions were incubated at 30 °C for 15 weeks and rates of sulfate, iron, and nitrate reduction were estimated from the time course of hydrogen sulfide accumulation, Fe(II) accumulation, and nitrate depletion, respectively. Chemical analysis showed that the xenobiotics were persistent under the different electron acceptor regimes for the duration of the experiment. This was partly attributed to low bioavailability and microbial toxicity of the xenobiotics. Rates of anaerobic respiration in control suspensions (without added xenobiotics) showed a weekly reduction potential of 0.84 mM SO42-, 0.92 mM Fe(III), and 9.25 mM NO3-. All three processes were completely inhibited by 1,2,4-TCB (54 mg L−1) whereas there was no significant (P < 0.05) toxicity of phenanthrene (109 mg L−1) and DEHP (105 mg L−1). Sulfate reduction was inhibited completely by LAS (105 mg L−1), 76% by acenaphthene (54 mg L−1) and 57% by 4-NP (117 mg L−1), and likewise iron reduction was inhibited 62% by LAS and 55% by 4-NP (the latter though at P < 0.10). Nitrate reduction was not significantly inhibited by acenaphthene and 4-NP and furthermore was resistant to LAS toxicity (105 mg L−1). Nitrate reduction also had the highest potential for mineralization of organic matter and thus was the most robust of the tested anaerobic processes in the sewage sludge suspensions.