Study On Influencing Factors Of Micro-polluted Water In Nanofiltration Process

Nanoflltration has become one of the optimum techniques in treating micro-polluted water with following characters: high option separation property, being able to remove the disinfection by products, weedkiller and pesticide, heavy metal and part of total hardness, low operation pressure, high quality effluent, low energy demand. In order to make a full use of the advantages of nanoflltration and achieve the best effect, it's necessary to know all the factors affecting the process of flux filtration. In the research, the experiment of nanofiltration was done with the raw water containing humic acid, secondary effluent and black liquor. The following factors' influences on process of the nanofiltration were studied: concentration of raw water, molecular weight distribution of organic matter in the water, the composition of organic matter, rate of cross flow, operation pressure and so on. The results were shown as follows:(1) When the concentration of organics in the raw water are 5mg/L, 10mg/L, 15 mg/L and 25mg/L, there are 5%, 12.2%, 13.4% and 15% decreasing of membrane penetration flux and initial flux in the stable state. Meanwhile, UV₂₅₄ are 0.012cm^-1, 0.019cm^-1, 0.036cm^-1 and 0.055cm^-1. Filtration resistance is 5.21×10¹³m^-1 which is its maximum. Therefore, increasing the concentration of organics in the raw water can lead to the decline of flux, worsen the quality of effluent and increasing of filtration resistance.(2) Cross flow rate were adjusted as 0.64cm/s, 1.62cm/s, 2.76cm/s respectively. When the cross flow rate is 2.76cm/s, the filtration resistance and UV254 in the effluent is relatively low, and they are 3.75×10¹³m^-1 and 0.026cm^-1, and the flux declines slowly. When the cross flow rate declined to 0.64cm/s, the filtration resistance and UV254 in the effluent increased correspondingly, and they are 3.75×10¹³m^-1 and 0.026cm^-1. The flux declines too. Therefore, with the increasing of cross flow rate, the flux decreased slowly and at the same time the quality of effluent was better. While the filtration resistance and the speed rate of membrane fouling declined after increasing the cross flow rate.(3) Treating different raw waters with high molecular weight organics, low molecular weight organics and humic acid, the specific value of stable flux and initial flux decreased 7%, 3% and 2%, UV₂₅₄ in the effluent is 0.015cm^-1, 0.061cm^-1 and 0.045cm^-1, filtration resistance is 4.57 × 10¹³m^-1, 4.44 × 10¹³m^-1 and 4.52 × 10¹³m^-1. Then we can get a conclusion that high molecular weight organics is the key factor for membrane fouling and the declination of flux.(4) In the research of treating humic acid solution, secondary effluent and diluted black liquor with nanofiltration, the flux and filtration resistance were influenced not only by molecular weight distribution, but also nature of organic.(5) Raising the operation pressure from 0.3MPa to 0.4MPa and 0.55MPa, the specific value of stable flux and initial flux decreased 6%, 5% and 2%. Then we can conclude that raising the operation pressure makes the declining of flux slow. While the filtration resistance will increased correspondingly, we can not get a sound effluent.