| Assimilable Organic Carbon (AOC) is the fraction of organic carbon that is most easily consumed by microorganisms, resulting in a proliferation of microbial cells. It has been regarded as one of the most important quality parameters for microbiological stability in drinking water treatment and distribution systems. Removal of AOC during the production of drinking water not only deprives heterotrophic bacteria of nutrients indispensable for their survival and multiplication in the water phase, but also limits bacterial colonization in drinking water distribution system.AOC was analysed by a modification of the LeChevallier method. The maximum growth of Pseudomonas fluorescens P17 and Spirillum sp. strain NOX which were inoculated into duplicate water samples respectively was converted to the amount of AOC. The yield factor of strain P17 at high concentration remains below the level expected on the yield factors observed at low concentrations, and the Y value of strain NOX does not decrease. Yield factors for the test strains were taken from standardization calibration curves experiments and were 7.5×106 and 1.4×107 cfu·mL-1 for strain P17 at high and low concentration and 7.1×108 cfu·mL-1 for strain NOX. The AOC concentration of raw water was fractionated into seven fractions by ultrafiltration, and the AOC of molecular weight less than 1ku is the largest proportion, reaching 43%.The AOC in Songhua River source is always at a high level and ranged from 452μg·L-1 to 2714μg·L-1, depending on the season, with the highest value in summer and the lowest in winter. Coagulation was the most effective step on the AOC removal in the conventional treatments of S plant, with the removal rate of about 30% in spring and winter, and 6080% in summer and autumn. The effect of AOC removal by filtration was not stable, and the AOC was enhanced about 20% by chlorine disinfection. AOC-NOX constituted about 10% proportion of AOC in conventional water treatment, and the ratio was enhanced by chlorine in disinfection. The AOC in the effluent of the plant depends on the concentration of influent, and the AOC in the effluent (more than 400μg·L-1), was above the limits for biostability. The AOC declined to above 300μg·L-1 along the distribution system, depending on the temperature. Most likely, biofilm processes played an important role in AOC uptake, and multiplication of bactecia in distribution systems posed a potential health threat to citizen.The variation of AOC under different coagulation conditions was investigated, including coagulant dosages, pH value, poly aluminum iron sulphate coagulant, waters from different source and Ca2+ iron concentration. It was found that the removal rate of AOC during alum coagulation in raw surface water was only about 24.5% at high coagulant dosage. Higher removal rate of AOC with the case of FeCl3 coagulation (about 65.9%) was observed under the same dosages. The removal rates of AOC during alum and iron coagulation of the synthetic water were 71.8% and 93.0% at low coagulant dosages, and the case of coagulation with FeCl3 also resulted in a better coagulation effect. It indicated that the pH of coagulation is a very influential parameter for the removal of AOC. The highest AOC removal rate after coagulation was obtained at initial pH 6.5 and 7.0, reaching 86.1%; AOC was increased to 29.1% after coagulation when the water is in alkaline condition. Poly aluminum iron sulphate coagulant had limited effect on the AOC value and enhancing Ca2+ concentration increased the removal of AOC from 54.1% to 80.4%.Three oxidants were selected in studying the variation of AOC in preoxidation process, including potassium permanganate, ozone and NaClO. AOC was increased by 61.5%, 86.9% and 81.1% respectively with the increase of oxidant dosages after coagulation. The AOC was increased by 23.6%, 33.5% and 17.7% only in preoxidation before coagulation. More AOC-NOX, which represents carboxylic acids, was formed and constituted about 40% of AOC during O3 preoxidation, while permanganate and NaClO preoxidation increased the substrate available for strain P17. The AOC/TOC ratio, which indicated assimilation ability of organic carbon, was increased from 8.2% to 14.0%, 16.7% and 10.8% in the three preoxidations respectively. The variation of TOC during permanganate and O3 preoxidation resulted from the variation of BDOC, and AOC increase was responsible for TOC increase by 5.2% during NaClO preoxidation process. The concentration of NBDOC changed little with the increase of oxidant dosages in the three preoxidation processes. Permanganate and O3 preoxidation exhibited better coagulation effect with the dosage less than 1.0mg·L-1, while better condition efficientcy was achieved at higher NaClO dosages.The variation of AOC was investigated during catalytic oxidation with titanium dioxide nano-particles loaded on several selected carriers in ozonation such as cerami, silica gel and zeolite. Catalytic ozone oxidation is more effective than ozonation alone for the degradation of large molecular weight fractions of organics into lower molecular size. The AOC increased to 674μg·L-1, 847μg·L-1 and 882μg·L-1 from 300μg·L-1 by the catalytic ozonation in the presence of cerami/TiO2, silica gel/TiO2, zeolite /TiO2 respectively; and the proportion of AOC in TOC could increase to 30.5%,33.2% and 46.0% from 4.7%, respectively. The catalytic oxidation greatly enhanced the biologradability of organics in water. With ozone dosage increased, AOC reduced 13.0% because some of the low molecular sized organic carbon was mineralized. In these processes, AOC-NOX mostly consisting of carboxyl acid increase rapidly, resulting in the increase of the proportion of AOC-NOX in AOC to 90% and the majority of AOC turned over from AOC-P17 to AOC-NOX. The AOC was less (17.9%) in modified catalytic oxidation than TiO2 catalytic oxidation, and it is assumed that advanced modification could reduce AOC efficiently to a reasonable level.
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