Groundwater recharge is becoming common in areas where the withdrawal of groundwater exceeds its natural recharge. Wastewater treatment plant (WWTP) effluent can be used for this purpose, but persistent organic compounds can only be partly removed during soil passage. This point was confirmed in degradation tests using soil columns in which the DOC of a membrane bioreactor (MBR) permeate could only be reduced by 15% and adsorbable organic iodine (AOI) by 2%. However, ozonation was found to improve biodegradation and at the maximum formation of biodegradable DOC at 2.5 mg O3/mg DOC0, the DOC values of MBR permeate (11-13 mg/L) could be reduced in aerobic degradation batch tests to the DOC of Berlin drinking water (3-5 mg/L). A combination of ozonation at 1.9 g O3/g DOC0 with soil passage could adjust DOC, UVA254, colour (436 nm) and the molecular DOC size distribution to drinking water ranges, but AOI was only reduced from 143 mg/L to 92 mg/L and remained high compared to tap water (2.2 mg/L). The extremely persistent X-ray contrast compound iopromide, which represents part of AOI, was spiked into MBR permeate at a low concentration. Iopromide was reduced by 88% during ozonation, but AOI only decreased by 23% indicating that a transformation, but not a mineralization, of iodinated organic compounds occurs.

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