Removals Of Dissolved Organic Matter In Secondary Effluents By Soil-Aquifer Treatment Techniques

Groundwater recharge with municipal wastewater is an increasingly valued practice for replenishing aquifers used for domestic supply. Soil-aquifer treatment (SAT) is the import component of groundwater recharge engineers. Dissolved organic matter (DOM) is of concern during SAT. DOM was fractionated using XAD resins into five fractions: hydrophobic acid (HPO-A), hydrophobic neutral (HPO-N), transphilic acid (TPI-A), transphilic neutral (TPI-N) and hydrophilic fraction (HPI). The distribution and chemical characters of these DOM fractions in secondary effluents from the wastewater treatment plant were examined by many experimental analysis ways such as chlorination experiments, infrared spectroscopy, ultraviolet (UV) spectroscopy, nuclear magnetic resonance spectroscopy and elemental analysis. Moreover, the removal and transformation of DOM during SAT, as well as the effect of the recharge on the soil DOM, were investigated.DOM in secondary effluents was fractionated and isolated using XAD resins, and each DOM fraction was characterized. The results showed that HPO-A and HPI dominated in the secondary effluent. HPO-A was the main precursors for trihalomethanes (THMs) following chlorination. The patterns of individual THM species with increased bromide concentrations were similar for all DOM fractions, which were as follows: CHCl3 decreased exponentially; CHCl2Br and CHBr2Cl increased initially and then decreased; CHBr3 increased continuously. Chlorination resulted in decreased UV absorbance across wavelengths from 250 to 280 nm for HPO-A, HPO-N, TPI-A and TPI-N. Chlorination resulted in the decreasing content of C—H, C=C and aromatic rings, the increasing content of C—O, and the production of C=O and C—Cl in HPO-A, HPO-N, TPI-A and TPI-N. The presence of bromide in chlorination led to the occurrence of C—Br for these four fractions.The removals and changes in the chlorine reactivity of DOM during SAT were investigated by soil-column simulation experiments. The results showed that the reduction of THMFP from HPO-A, HPO-N, TPI-A, TPI-N and HPI during SAT was 27.24%, 34.43%, 26.24%, 35.85% and 36.08%, respectively. Specific THMFP (STHMFP) for each DOM fraction increased across the soil columns. THMFP was strongly correlated to UV-254 for HPO-A, HPO-N and HPI, while the relationship between THMFP and UV-254 for TPI-A and TPI-N was significantly poor.The removals of DOM by the aerobic biodegradation, SAT system and a combination of sorption and anaerobic biodegradation during SAT were examined by the BDOC reaction column tests, as well as the simulation experiments using two soil-column systems, whose influents were the secondary effluent without and with NaN3 addition, respectively. The results showed that the removals of bulk DOC by the aerobic biodegradation, the SAT system and the combination of sorption and anaerobic biodegradation during SAT were 57.3 %, 70% and 27.5%, respectively. Neutral DOM fractions (HPO-N and TPI-N) were more readily removed than acid DOM fractions (HPO-A and TPI-A) by aerobic biodegradation, while acid DOM fractions were more readily removed than neutral DOM fractions by the combination of sorption and anaerobic biodegradation during SAT. The C=O functional groups in acid DOM fractions were significantly reduced by the aerobic biodegradation, whereas those in neutral DOM fractions were significantly reduced during SAT. The C―O functional groups in acid fractions were reduced by aerobic biodegradation while the combination of sorption and anaerobic biodegradation during SAT resulted in the increased relative content of C―O functional groups in HPO-A, HPO-N, TPI-A and TPI-N. The aerobic biodegradation significantly reduced amides, while the combination of sorption and anaerobic biodegradation during SAT resulted in the production of amides.The effect of influent pre-treatment on the performance of SAT systems was investigated by a comparison between the removals of DOM by two soil-column systems, whose influents were the secondary effluents without and with activated carbon adsorption treatments, respectively. The results showed that the effluent DOM concentrations as well as the DOM removals for the SAT system would decrease when the influent DOM concentrations decrased.The effect of recharge on water extractable DOM in soils was examined, by comparing the content and functional properties of water extractable DOM in soils after recharge at the different depths with those in the soils before recharge. The results showed that the recharge resulted in the increased content of water extractable DOM in the top soils (0-12.5 cm), as well as the decreased content of water extractable DOM in the soils below 12.5 cm. The recharge resulted in the decreased content of HPO-A and TPI-A as well as the increased content of HPO-N, TPI-N and HPI. The effects of recharge on functional groups of water extractable DOM fractions in the top and bottom soils were not similar.