Removal Of The Dissolved Organic Matter During Modifed Soil Aquifer Treatment

Soil aquifer treatment (SAT) with the secondary effluent of wastewater treatment plant (WWTP) is an increasingly valued practice to renovate domestic effluents for potable and non-potable purpose in those arid and semi-arid regions. Dissolved organic matter (DOM) is a major water quality issue associated with SAT systems, which is present in the recovered water and can be converted to carcinogenic disinfection by-products. DOM was fractionated by XAD-8/XAD-4 resins into five different fractions: hydrophobic acid (HPO-A), transphilic acid (TPI-A), hydrophobic neutral (HPO-N), transphilic neutral (TPI-N) and hydrophilic fraction (HPI). Removal trend of the DOM in raw wastewater (RW) during the different units of WWTP were investigated firstly. Then the reduction mechanisms of the DOM during the SAT operation were explored via the comparison of the different advanced treatments. Finally, fly ash (FA) and coal slag were used as low-cost adsorbents for further removal of the DOM in secondary effluent.XAD fractionation results demonstrated that majority DOM of the RW was contributed by HPI (60.5% in summer and 57.4% in winter), followed by HPO-A, the remainder fractions were averaged 12.1%, 5.1% and 1.9% for HPO-N, TPI-A and TPI-N, respectively. 89.7% of the bulk dissolved organic carbon (DOC), 52.6% of UV-254, as well as 86.4% of trihalomethanes (THMs) precursors were efficiently removed by WWTP in summer, while the removal efficiency of DOC and THMFP decreased to 88.2% and 85.5% in winter. Removal of HPI related organics were effective by the facilities of A/O tanks, particularly in summer. On the other hand, PST played a key role in removing the THMs precursors in acidic and hydrophobic fractions (C=C and C=O related), especially in winter.Both HPI and HPO-A were the major components in the secondary effluent (accounting for 45.5% and 30.0% of the bulk organics), and that of the HPO-N, TPI-A and TPI-N were quite low. HPO-A was the predominant THMs precursors and aromatic components in secondary effluent, accounting for 50.0% of the bulk UV-254 and 45.4% of the THMFP, respectively. The DOC of the secondary effluent decreased 65.1% after SAT operation, in which HPI was preferentially removed when the SAT operation progressed (with a removal efficiency of 76.6%). Moreover, fractions of HPO-A and HPO-N were also efficiently removed. The adsorption mechanisms of the SAT system tended to adsorb more HPO-A, HPO-N and TPI-A and could reduce the aromaticity of those DOM fractions efficiently. On the other hand, the biodegradation mechanism would remove more HPI and TPI-N and lead an increased aromaticity. The most important removal process responsible for DOM in SAT was biodegradation instead of adsorption. Thus, the combination of the biodegradation (for TPI-N and HPI removal) and adsorption (for HPO-A, HPO-N and TPI-A removal) is essential DOM removal during the practicial recharging of the secondary effluent, which could be proven by the operation of GAC+SAT.In order to enhance the removal of DOM in secondary effluent during the SAT operation, fly ash (FA) and coal slag were used as supplementary materials to improve the adsorption mechanism of the SAT system. FA and coal slag additive in SAT system would efficiently enhance the bulk removal of the DOC, UV-254 and THMs precursors in secondary effluent, ascribing to their high surface-area. Experimental results indicated that FA and coal slag additive within the SAT columns would enhance the bulk adsorption of the hydrophobic fractions, while the removal of HPI were mainly accomplished by the biodegradation within the soli. Since the additive of FA and coal slag would negatively affect the biodegradation of the soil biomass (especially for the FA modified SAT), thus, the combination of an upper 0.25 m soil layer and a mixture of FA/coal slag and soil underneath is essential. In overall, the additive of coal slag showed a high removal of DOM in comparison with that of FA additive.Experimental results revealed that 22.5% of dissolved organic carbon (DOC), 23.7% of UV-254, 25.9% of trihalomethanes (THMs) precursors in secondary effluent were efficiently adsorbed by FA at the optimum conditions (15 g/L FA dosage, 303 K and 180 min contact time). Kinetic studies indicated that the adsorption of each DOM fractions by FA fitted the pseudo-second-order kinetic model quite well; moreover, the adsorption of hydrophilic fraction (HPI) also followed the intraparticle diffusion model. The maximum adsorption capacities (Q0) and the Gibbs free energy (ΔG0) value obtained from the adsorptions showed that FA was more efficient in adsorbing the fraction of HPI, while less effective in removing the acidic fractions. In addition, Langmuir model yielded a much better fit than Freundlich model in simulating the adsorption of the acidic DOM fractions, while contrary for the adsorption of hydrophilic fraction. The adsorption of coal slag needs a longer time to reach equilibrium (12 h), and the adsorption was affected by the particle sizes of the coal slag. At the condition of 15 g/L dosage, 303 K and 12 h contact time, the coal slag with a particle size of >250 mech would lead to a 28.6% removal of the DOM in secondary effluent. More organics would be removed at the acidic condition during the coal slag adsorption. In addition, the Freundlich model could well simulate the coal slag adsorption of all DOM fractions in comparison with that of Langmuir isotherm.For a purpose of the practical designing of the FA/coal slag modifie SAT, the adsorption saturation period of the FA/coal slag additive SAT were practically evaluated. At the additive ratio of FA/coal slag and soil was 1:1, the thickness of the mixture layer was 1 m, and the top soil layer was 0.25 m, the adsorption saturation period of the FA additive SAT was 602 d, while 1541 d for coal slag additive SAT.