Study On Ultrafiltration-Nanofiltration Combined Membrane Process In High Quality Drinking Water Treatment

In recent years,with the continuous growth of Chinese economic strength,people's living standards have also been greatly improved.The large-scale and high-intensity investment of the state in the water supply infrastructure has solved the problem of insufficient water supply.The main contradiction in the water supply industry has shifted from the contradiction of water supply shortage to the contradiction of how to further improve water quality.How to guarantee water quantity and water quality at the same time have become the consensus of water industry researchers and local water authorities.In the current process,The usual method in response to problems such as microorganisms and organic pollutants is to add coagulant and add to the oxidation and disinfection process.Although the factory water meets the Standards for drinking water quality,but the disinfection by-products which produced by oxidative disinfection and the ability of the current process to remove trace organics are urgently needed to produce in the production of high quality drinking water.As a purely physical process,the UF-NF combined membrane process greatly reduces the dosage of the disinfectant and solves the problem of chlorination by-product to some extent,It is a greater guarantee of chemical safety in water quality.In addition,the UF-NF combined membrane process has significantly improved the removal of pathogenic microorganisms and trace organic matter in water compared with the current process.Therefore,based on the demand of high-quality drinking water,this paper carried out a pilot-scale study on the treatment of the East of Lake Taihu by UF-NF combined membrane process,and carried out the experiment of the odorant substance?2-methyli-soborneol?2-MIB?and Geosmin?GSM??in the raw water.The main findings of this test are as follows:?1?The annual average turbidity of the East of Lake Taihu is 55.73 NTU,the annual average permanganate index(COD_Mn)is 4.24 mg/L,the annual average ultraviolet 254(UV₂₅₄)is 0.1015 cm^-1,and the annual average dissolve organic matter?DOC?is 2.69 mg/L.It can be seen from the above data that the East of Lake Taihu has a high content of organic matter and it is a typical Class III water.The molecular weight distribution of organic matter in water is mainly concentrated in the region of 1k-3kDa,followed by two regions of less than 1kDa and3k-10kDa;organic matter in the East of Lake Taihu is mainly composed of strong-sparing components and extremely polar components.The strong-dissolving components are composed of humic organic matter.The polar hydrophilic components are composed of aromatic protein;the analysis of the three-dimensional fluorescence of raw water shows that there are both substances caused by algae and biodegradation in water,as well as protein organic matter caused by production and living.?2?The raw water is filtered through a fine grid of 5 mm and directly enters the UF-NF combined membrane process,and the production water of this process is 5.57 m³/d.Through the uninterrupted pilot test for 13 months,it is proved that the UF-NF combined membrane process can operate stably.The UF system performs a hydraulic backwash every 30 minutes and rinse time is 30 seconds;a chemically enhanced backwash is performed every 24 hours.This cleaning frequency reduces the contamination rate of the UF membrane and creates good water inlet conditions for the NF system.The NF system will regularly add fungicide during operation?60ml/3-4 days in summer;60ml/7 days in winter?.The addition of fungicide can alleviate the contamination rate of the NF membrane and provides a guarantee for the stable operation of the double membrane process.?3?After the raw water passes through the UF membrane,the effluent turbidity average is0.12 NTU,the effluent COD_Mn average is 2.39 mg/L,the effluent UV₂₅₄ average is 0.0509 cm^-1,and the effluent DOC average is 2.47 mg/L.UF has a high removal rate of raw water turbidity.The final effluent turbidity of the UF-NF combined membrane process was below 0.05 NTU,the average effluent COD_Mn was 0.58 mg/L,the effluent UV₂₅₄ average was 0.0057 cm^-1,and the effluent DOC average was 0.38 mg/L.The UF-NF combined membrane process can remove more than 80%of COD_Mn and DOC,and more than 90%for UV₂₅₄ removal.This shows that the combined process has a good removal effect on the organic matter.?4?The analysis of the characteristics of the organic matter produced by the UF-NF combined membrane process shows that the UF mainly removes the macromolecular protein substances and weak hydrophobic components of the medium molecules,the removal rates are72.4%and 26.8%respectively.At last,the process removes most of the macromolecules and medium molecular substances in the water,only some small molecule humic acids and microbial metabolites are remained.The removal rates for each molecular weight range are96.7%,84.2%and 71.4%respectively.At the same time,the results of three-dimensional fluorescence and hydrophobicity separation also showed that UF had better removal effect on the peak of the fluorescent protein B peak and the C peak which represents the hydrophobic fluorescent substance,the removal rates were 24.4%and 25.3%respectively.The final combination process removes more than 90%of the hydrophilic protein-based fluorescent material,the removal of the hydrophobic fulvic acid fluorescent and humic acid fluorescent substances is also more than 70%.The final process water production has been in line with the Standard of pure drinking water in bottles?barrels??GB17324-2003?,which guarantees the biosafety and chemical safety of the produced water to the greatest extent.?5?The annual average dissolved solids?TDS?of raw water is 205.24 mg/L.After passing through the UF membrane,TDS in water is 198.11 mg/L,the removal rate was 4.5%.TDS of the final water produced by the UF-NF combined membrane process was 129.29 mg/L,and the removal rate of TDS in water was about 35%.This indicates that the combined process removes the total amount of dissolved solids in water mainly by NF membrane.After ion detection of raw water,UF produced water and NF produced water,it was found that the NF membrane has a good removal effect on divalent anions and cations in water.The contents of Ca²⁺,Mg²⁺and SO₄^2-in water were 38.1mg/L,8.6mg/L and 50.8mg/L respectively,and decreased to 26.6mg/L,5.9mg/L and 5.7mg/L after NF membrane.The removal rates were 30.1%,31.4%and 88.7%respectively.The high removal rate of SO₄^2-and retention of Ca²⁺and Mg²⁺can reduce the risk of scale formation on the NF membrane and meet the dual requirements of water quality and mouthfeel.?6?In order to study the effect of the combined process on the removal of olfactory substances in water,the concentration of GSM and 2-MIB in raw water was adjusted to 55.3ng/L,76.3 ng/L,98.2 ng/L,150.2 ng/L and 191.7ng/L.After the UF-NF combined membrane process,the effluent concentrations were 8.96 ng/L,18.97 ng/L,21.72 ng/L,35.37ng/L and 42.54 ng/L.The final conclusions indicate that the removal rate of the two olfactory substances in the combined process is between 70%and 80%.The combined process removal of odorant substances mainly depend on NF membranes.The average removal rates of nanostrain for GSM and 2-MIB are 70.6%and 77.2%.The concentration of 2-MIB and GSM after the UF of raw water decreased by 1.6%and 5.47%,respectively,indicating that UF has a weak ability to remove odor substances in water.