The influence of novel disinfection strategies in the removal of humic substances
Sustainable process-product development & green chemistry
Sustainable & Clean Technologies - Ib: Extraction & Remediation (T1-4b)
Keywords: drinking water, trihalomethanes, mixed oxidants, humic substances, full-scale water treatment
During the disinfection of drinking water the reaction between disinfectants and natural organic matter (NOM) leads to the formation of chlorinated disinfection by-products (DBP), such as trihalomethanes (THM). The nascent chlorine, ozone and chlorine dioxide combination is known to present greater oxidative power (synergy) than conventional chlorine. However, very little is known about the effect of this mixed-oxidants system on NOM, and therefore, its potential to form DBPs. The present paper studies the effect of such a pioneering oxidation strategy on the content, nature and reactivity to form THMs of natural organic matter (NOM).
The study was carried out in a full scale water treatment plant (WTP) which uses a mixed-oxidant system. WTP includes preoxidation, coagulation-flocculation, intermediate oxidation, sand filtration and disinfection. The four THMs species were analyzed by direct injection GC-ECD. Effectiveness of treatment was analyzed by measuring the concentration of humic substances (HS) at different WTP positions. HS were measured by using two different resin adsorption procedures; first using non-ionic XAD resins and second using ionic DEAE resin. THM formation capability, defined as the THM formation potential (THMFP), of each fraction was measured by four different standardized chlorination tests: THMFP3h, THMFP15h, THMFPF, SDSTHMFP.
NOM content of raw water was high enough to exceed maximum contaminant level of 100 microgramTHM/L established by the European Directive 98/83/EC. Measured HS concentration in raw water strongly varied depending on the used adsorption procedure (from 55 to 80 % of the THMFPF). Moreover, the fractionation procedure using XAD resin did not reach 100% mass balance for many of the performed analysis. This behavior was specially marked for filtered water. In general, it was noteworthy the contribution of non-humic material to THMFPF.
WTP removed up to 70% of UV absorbance, reflecting a high reduction in aromatic structures. Also, dissolved organic carbon progressively decreased throughout treatment. However, THMFPF reduction was minimal being 113 microgramTHM/L for raw water and 105 microgramTHM/L for finished water. It was observed that non humic (hydrophilic) fraction significantly increased along WTP. Data obtained for three hours chlorination tests (THMFP3h), representative of “quick formers”, followed a distinct trend and increased in finished water up to 53%. This behaviour was attributed to the ozone present in mixed oxidants system which is effective in reducing hydrophobic substances while hydrophilic and neutral fractions are unaltered. Substances such as aminoacids, polysaccharides, alcohol and short chain ketones have proved to be more refractory to conventional treatment processes. Moreover, the obtained results from THMFP3h tests indicate that they could have even higher reactivity during first stages of chlorination.
In general, coagulation-flocculation was the most effective step for the removal of THMFPF. Sand filtration was not effective, even more, sand-filtering increased THM formation capability of water. In conclusion ozone induced alterations that undergo NOM together with the preferential removal of humic fraction of NOM, convert hydrophilic fraction in the main source of THMs during the disinfection step.
See the full pdf manuscript of the abstract.
Presented Wednesday 19, 16:00 to 16:20, in session Sustainable & Clean Technologies - Ib: Extraction-Remediation (T1-4b).
