Study On Characteristics Of Secondary Effluent Organic Matters In Urban Wastewater And Ultrafiltration Separation Process

It is great meaningful for solving the lack of water resources and contradictionbetween water supply and demand to reuse the secondary effluent of urban wastewater.As ultrafitration membrane has been the core of water treatment technology, theincrease of running costs and water’s costs caused by membrane fouling limits thedevelopment of ultrafitration technology. Studies have shown that the effluent dissolvedorganic matters were primary substances to influence membrane fouling. In this paper,the secondary effluent of a wastewater treatment plant in Xi’an was used to as an objectto intergrate molecular weight distribution and hydrophobic/hydrophilic classification ofdissolved organic matters with three-dimensional fluorescence spectrascopycharacteristics. Then make these characteristics instruct the study on DOM fouling ofultrafiltration membrane and discuss measures to mitigate the membrane fouling toprovide a reference for the popularization and application of ultrafiltration technology inurban wastewater reuse. The results were shown as following:(1) Hydrophobic fractions were more than hydrophilic fractions in secondaryeffluent. Hydrophobic fractions occupied about64.3%, hydrophilic fractions occupiedapproximately35.7%.(2) Ultrafiltration and high-performance size–exclusion chromatographydetermined molecular weight distribution of secondary effluent dissolved organic matterof urban wastewater. Result indicated that secondary effluent mainly contained smallmolecular weight organic matters. With high-performance size–exclusionchromatography, the result showed that molecular weight of organic matters almost allfocused on below5kDa, with ultrafiltration, the organic matters of <3kDa accounted for44.9%in TOC. The results of the two methods are basically the same, it reflected that the secondary effluent mainly contained small molecular weight organic matters.(3) The molecular weight distribution of hydrophobic/hydrophilic componentorganics was similar to the raw water, the proportion of small molecular weightsubstances were larger and the proportion of the rest interval organics were smaller, andthe molecular weight of the strong hydrophobic organic proportion are larger than thetwo organisms. The results indicated that the dissolved organic matter in the water wasmainly composed by small hydrophobic organic molecules.(4) There were humic acid, fulvic acid and protein substances in secondary effluentraw water and three hydrophobic/hydrophilic components, and the strength of humicacid was the highest. strongly hydrophobic component contained the most complexorganics and the fluorescence peak is the most obvious, followed by the hydrophiliccomponent, while the weak hydrophobic component content is lowest.(5) The influence degree of dissolved organic matters of secondary effluent onmembrane fouling was strongly hydrophobic>hydrophilic fractions>weaklyhydrophobic. The impact of ultrafiltration membrane permeable flux for differentorganic components mainly due to the resistance of each component in the ultrafiltrationprocess was different.(6) The higher molecular weight cut of ultrafiltration membrane, the fastermembrane flux decline. The reasons were that the smaller molecular weight cut of themembranes form a cake layer in initial time, and the cake layer prevent pollutants fromentering the interior of the membrane pores. To the larger molecular weight cut of themembranes, pollutants relatively easy access to the inside of the membrane pore,thereby causing the membrane pores rapid more pollution; the higher the molecularweight cut, the more organic pollutants were into the membrane per unit time, somembrane surface contamination and internal contamination of the membrane poreincreased.(7) In the ultrafiltration process, some proper pretreatments on raw water wereconducted, the molecular weight distribution and some relevant characteristics ofdissolved organic matter in water were changed; changing the antifouling properties ofthe film; at the same time, using the low pressure and constant flux filtrating modes thatcontrolled by the system developed based on membrane structure change of hydration ultrafiltration membrane hydrophilic filtration process control system and combiningcorresponding backflushing with fast membrane surface washing, then we couldachieve the membrane fouling controlling purposes, make the decline of membrane fluxslow, at last to improve the permeability of membrane and extend membrane life.