career | 08.03.2023

Elevated AOX levels in sewage sludge

Determining the causes through biological investigations

An orienting investigation was to clarify whether increased AOX (Adsorbable Organically Bound Halogens) findings in the sewage sludge of the client's sewage treatment plant are due to organohalogen compounds.

The question was whether the increased AOX values recorded in the conventional sewage sludge analysis were not falsified by inorganic chlorides from microorganisms. It was shown that many microorganisms survive the preparatory freeze-drying and grinding of the sludge. After an elution time of 24 hours, no cells capable of reproducing were present in the test samples. Apparently, the cell structures are only damaged to the extent that the cells lose vitality. Macromolecules, such as the nucleic acids stained by the dye used, therefore remain in the cells. Supplementary AOX analyses of samples did not show any significant differences.

In a second investigation module, trace analysis was used to check whether indications for the cause of the increased AOX measured values could be identified. A large number of different halogenated substances were found, which were present in higher amounts in the sample with high AOX than in the comparison sample with low AOX. This diversity of halogens as well as the diversity of halogenated substances argue against the thesis that the increased AOX values are due to the input of one or more industrial chemicals. The great diversity of chlorinated, fluorinated, brominated and iodinated products rather supports the thesis that reaction conditions have developed within a temporary framework that have promoted the unspecific halogenation of water constituents. Possibly, the temporary disturbance in the sludge settling tank had favoured the halogenation by reduction of existing iron salts. Reduced iron compounds in the return sludge can create very reactive conditions in the presence of oxidants in the aeration tank. Should organohalogen compounds be formed under these conditions, they would be non-specific chemical halogenations mediated by biological processes.