Accumulation and transformation of dissolved organic matter in tertiary effluent during soil aquifer treatment

A laboratory-scale soil column had been set up to investigate the accumulation and transformation of dissolved organic matter (DOM) in the tertiary effluent during soil aquifer treatment (SAT). Using XAD resins, extractable DOM in the soils was fractionated into five classes: hydrophobic acid (HPO-A), hydrophobic neutral (HPO-N), transphilic acid (TPI-A), transphilic neutral (TPI-N), and hydrophilic fraction (HPI). The results indicated that high DOC removal efficiency of the SAT system was ascribed to both adsorption and biological degradation within the soil column, in which biological degradation mainly occurred within the top 0.50 m of surface soil and the adsorptions within the entire soil column. Overall, the highest DOC concentrations of HPO-A, TPI-A and HPO-N were observed at the depth of 0.25 m of the soil column, whereas that of the TPI-N and HPI were obtained at the top of the column. The results indicated that the adsorption of HPO-A, TPI-A and HPO-N by the top soil (0-0.25 m) was saturated as a result of the significant accumulation and the relatively lower bio-degradation ability, whilst the biodegradation mechanism of the top soil was effective for the removal of TPI-N and HPI. The trihalomethane formation potential (THMFP) and UV-254 of DOM fractions for the soil samples increased significantly during the SAT treatment, especially for HPO-A, TPI-A and TPI-N. Although 10%-30% increases of specific THMFP (STHMFP) were observed for the DOM fractions as a result of the SAT treatment, the increase of STHMFP was relatively lower as compared with the corresponding increase of THMFP and UV-254. In addition, the Fourier-transform infrared (FT-IR) results showed that the structure of soil DOM fractions was changed during the SAT treatment.