| The drinking water safety problem of excessive fluoride and arsenic (As) in groundwater which is the main water resource in rural China, has been threatening the human health, which is also one of the most important issues in the Eleventh Five-Year Plan of China.The feasibility study on the commercial nanofiltration (NF) membrane application to fluoride and arsenic removal from the simulated groundwater in this paper was carried out. The influence fators on the performances of the single NF membrane-modue during fluoride and arsenic removal, such as the raw water characteristics and the operation conditions, were investigated. In addition, the NF membrane modules arrangement & optimization, and its effects on the fluoride & arsenic removal performances under once pump driving, were discussed. Besides, the NF membrane fouling & fouling components, and its effect on NF performance were analyzed. Then, the method of concentrated water disposal was studied. Finally, the benefits for the rural water environment, for the rural economy, and for the rural society of NF technology application to fluoride & arsenic removal from groundwater were analyzed.The experiment results showed that, (1) it was feasible with the single NF membrane module system application to fluoride and arsenate (As(V)) removal. With the operation pressure of NF membrane being (0.60～0.65) MPa, the conversion rates of NF membrane being 30%, and the permeate flux of NF membrane being (20～24) L/(m2-h), the TDS rejection rates reached 68.8%-70.7%, the fluoride rejection rates were approximately 70%, and the As(V) rejection rates were more than 90%. When the content of fluoride or As(V) in raw water was lower than 4 mg/L or 243.2μg/L respectively, the content of fluoride or As(V) in permeate could meat the Standard for Drinking Water Quality (GB 5749-2006) limit value in rural China.(2) Arsenite (As(Ⅲ)) rejection rate by NF was much lower than that of As(V), so it was unfeasible for the As(Ⅲ) removal and the pretreatment methods were considered. The oxidation methods such as the aeration method with the cubage ratio 5:1 between gas to water for 2 hours, and NaCIO preoxidation method with mol ratio (4-10):1 between NaCIO to As(Ⅲ) for 30 minutes, gave the prominent results of converting As(Ⅲ) into As(V), which increased the total arsenic rejection.(3) The main mechanism for fluoride & As(V) removal was the electric charge exclusion. Affected by Donnan exclusion, the NF presented the ion selectivity prominently, and some beneficial ions to human health were kept in permeate. (4) The operaton conditions had great effects on NF. With the NF transmembrane pressure (TMP) increase, the rejection rates of fluoride and arsenic were improved, and the NF flux also increase. While with the increase of NF membrane conversion rate, the fluoride rejection rate varied insignificantly, and the arsenic rejection rate even dropped prominently.(5) The feed water conditions presented different influences on NF. The increase of raw water temperature weakened the fluoride rejection, but the permeate flux increased linearly correspondingly, educing the model:JT=J25·1.026(T-25) (JT was the function of water temperature; J25 was the NF permeate flux at 25℃).With the elevation of pH values in raw water, the fluoride & arsenic rejection rates increased greatly. The suggestions were that fluoride removal should be under alkalescence condition while arsenic removal should be under neutral condition.There existed the competition effect between the co-existing anion and NF for fluoride removal. Ions selective coefficient (S) gave the effect relationships, presenting that the competition sequence as:Na+> Cl-> F-> Ca2+> SO42-, where the ions of Cl-, Na+ and SO42-demonstrated the negative effect and Ca2+ showed the positive effect. It was calculated that 1.0 mol/L co-ions of Cl- or SO42- in feed water caused additional (3.15～11.76) mmol/L or 7.22 mmol/L more F- ions to penetrate the NF membrane, and 1.0 mol/L counter-ions of Ca2+ in feed water caused an additional 8.54 mmol/L of F- ions to be rejected by NF membrane. Co-ions of Cl- had poor effect on arsenic removal performance, but co-ions of SO42- and Ca2+ had greatly influence on arsenic removal. Humic acid in raw water weakened the fluoride removal while its effect on As(V) rejection was poor.(6) Pretreatment processes were chosen according to the different quality of the feed waters. Ideal results were achieved by the cation resion application to the high hardness contained in feed water, and the hardness removal efficiency was more than 87%. While the adsorption efficiency with active carbon was less than 31% for removal of humic acid in feed water.(7) Comparison to the performance of the single NF membrane-module, the one-stage parallel connection (1:1) NF system improved 0.8 times more permeate flux, but the rejection rates for TDS, fluoride and As(V) decreased to some extent. The two-stage in series (1:1) NF system improved 1.5 times more permeate flux and the coversion rate reached 56.6%, but the rejection rates for TDS decreased. The three-stage in series (1:1:1) NF system improved 2.3 times more permeate flux and the coversion rate reached 66.4%, but the rejection rates for TDS, fluoride and As(V) were decreased to some extent, and the fluxes among the three stages were imbalanceable. The two-stage parallel connection (2:1) NF system was the best one in fluoride & arsenic removal, with its membrane convertion rate being 64.9%, its total permeate flux being improved 2.3 times more, and its rejection rates for TDS/fluoride and As(V) not being decreased.(8) The membrane fouling can be delayed by the physical cleaning method with 36 L/h-40 L/h of cleaning flow velocity,0.3 MPa of cleaning pressure, and 10～15 minutes operation time every day. However, the membrane fouling took place after 6 months. Environmental Scanning Electron Microscope. (ESEM), Energy Spectrum Analysis and X-radiation Diffraction (XRD) showed that the main composition of membrane fouling was the crystals of the calcium carbonate and calcium sulphate with inerratic figures. It deposited on the membrane face and blocked the filtration channel. The cleaning method of'citric acid (2%W)+Na2EDTA (2%W) and ammonia (pH 3～4)' operation for 30 minutes could remove the contaminant and recover the NF membrane performance (more than 98%).(9) The calcium oxide (CaO) and calcium hydroxyl (Ca(OH)2) could be used in concentrated water disposal. The mol ratio of Calcium and F- in fluoride concentrated water disposal was 25:1 while the ratio of the calcium and arsenic in arsenic concentrated water disposal was 120:1.(10) Nanofiltration technology for excessive fluoride and arsenic removal can improve the rural water environment and avoid the second pollution, so it has nice environmental benefit. The economic evaluation of the two-stage parallel connection (2:1) NF system for fluoride & arsenic removal showed that, with the NF system operation for 8 hours daily the cost of pure water production was only 26 yuan/m3 that was much cheaper than that of being purchased from the market, so NF has better economical benefit. Besides, application of the nanofiltration technology can bring the nice social benefit for that NF can improve the inhabitants'live quality, can advance the region investment circumstance, and quicken the New Rural Building of China.Nanofiltration technology can effectively remove off the excessive fluoride and arsenic in groundwater, which presents that the NF has a better prospect application to groundwater advanced treatment.
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