| The outbreaks of ocean green tide became more and more frequent in recent years, which posed a serious threat to coastal fisheries and tourism. Therefore, it is of great significance to explore an effective way for comprehensive utilization of Enteromorpha, as well as for the restoration and improvement of the ecological environment and development of a recycling economy. In this study, Enteromorpha polysaccharide (Ep) was applied in coagulation process as coagulant aid by the concept of treating waste by waste. The preparation method of Ep was discussed, and then the coagualtion efficiency, floc characteristics and coagulation mechanism of Ep was studied systematically for different water samples.In addition, Ep was also used in coagulation-ultrafiltration process in this paper to investigate its effect on the ultrafiltration effciency and membrane fouling, which could provide a theoretical basis for the actual application of Ep. Finally, the coagualtion efficiency, behavior, mechanism and floc characteristics of Ep were compared with that of traditional caogualnt aid. The main conclusions of this paper were summarized as follows,(1) In this paper, Ep was prepared by the method of water bath, which was low cost and easy operation. The optimal conditions which optimized by single factor experiments were shown as follows:solid-liquid ratio 1:75, bath temperature 90℃, and the extraction period 4 h. The extract of Enteromorpha was centrifugalized for 20 min and then the supernatant was concentrated by rotary evaporator. Finally the alcohol precipitation was conducted to obtain a white powder-Ep. The comparative study of the composite coagualnt and dual-coagualnt showed that Al coagulant should be added firstly at the start of rapid mixing, and then Ep dosed after 30 s. In this way, the coagulant aid effect of Ep could be maximized.(2) The results presented in this work showed that coagulation efficiency and water quality could be improved due to Ep addition as a new coagulant aid. For humc-kaolin water sample, the coagulation efficiency was not excellent when Ep was used alone as coagulant. However, the UV254 and DOC removal efficiency could be improved apparently when Ep was used with aluminium sulfate (AS). The optimum dosage of AS and Ep was 10 mg/L and 0.1 mg/L, and the best pH condition was 6.0. In addition, coagulation effect of AS-Ep was better than that of AS for the whole pH ranges. For disperse yellow RGFL and active blue K-GL water sample, the optimal aluminum coagulant was AS and aluminium chloride (AC). When Ep was used together with AS and AC, coagulation efficiencies could be enhanced obviously in a large ranges of pH. The best coagulation effect could be achieved at pH=7 in AS-Ep coagulation system, while when AC-Ep was used, the optimal pH was 6.For humic acid-silver nanoparticles (AgNPs) water samples, the addition of Ep could also play a good coagualnt aid effect. The optimal dose of aluminum coagulant-poly aluminium chloride (PAC) and Ep was 2.0 mg/L and 0.2 mg/L, respectively. Under this condition, the humic, turbidity and AgNPs were all removed effectively. The various long carbon chains of Ep could absorb more suspension colloid particles and promote bridging mechanism effectively, which usually generated larger flocs with preferable settling velocity, and AgNPs could be removed by complexing action or attaching on the floc surface. For Yellow River water sample, AS was selected as the optimum coagulant from different traditional coagulants due to its high efficiency and perfect cooperation with Ep. The turbidity, UV254 and DOC removal could be improved by around 30% when 0.3 mg/L Ep was dosed. However, further increase in the dosage of Ep resulted in lower coagulation efficiency.(3) For different kinds of simulate water, the effect of Ep as a new coagulant aid on coagulation kinetics was studied. Results indicated that floc characteristics could be improved due to Ep addition. For humic-kaolin water sample, floc generated by AS-Ep had bigger size and faster growth rate than that of AS, and meanwhile the former was stronger than the latter. For disperse yellow RGFL and active blue K-GL water sample, floc diameter and growth rate could be improved apparently when Ep was added, and meanwhile the flocs strength and recoverability were also enhanced in some degree. The best growing and recovery properties could be achieved in neutral and weak acid conditions. For humic acid-AgNPs water samples, results of flocs properties showed that floc diameter and growth rate could be improved apparently when Ep was added, while the flocs strength and recoverability were slightly decreased in some degree. The floc structure was changed when Ep was added. Floc characteristic showed a parabolic variations as pH increased, and achieved optimum coanditons at pH 6. For Yellow River water samle, the size and strength of floes could be obviously improved for the given optimum Ep dosage, and meanwhile the effect of mixing speed on flocs growth characteristics was unconspicuous in AS-Ep coagulation system. For both AS and AS-Ep, floc strength and recoverability decreased with the increase of shear forces and breakage period. However flocs produced by AS-Ep could bear stronger forces than that of AS.(4) When Ep was used in coagulation-ultrafiltration (C-UF) process,0.3 mg/L Ep dosage was selected as the optimal dosage for HA removal by C-UF system. Under this dosage condition, tuirbidity and DOC removal could be enhanced, and meanwhile the subsequent membrane flux could also be improved. The reduction of membrane fouling was due to the larger floc sizes when Ep was added. Flocs generated by PAC-Ep always had a lower Df value, therefore the loosely structured aggregates produced less resistance, which is beneficial for membrane permeability. The cake layer on membrane surface formed by PAC-Ep possess higher porosity than that formed by higher density flocs when PAC was used alone, which contributed to less filtration resistance. This is a great advantage for real water sample treatment, since the decrease of residual organic matter could reduce the risk of disinfection byproducts. Therefore the safety of drinking water could be protected better.(5) Ep was used with PAC as a new kind of coagulant aid. The coagulation effect and the floc characteristics of Ep was compared with that of traditional coagulant aid in HA-Kaolin and actual water treatment. The results indicated that Ep could play a positive aid role in coagulation process compared with that of traditional coagulant aid. The UV254 and DOC removal when Ep was used was similar to that of traditional coagulant aid. When PAC was used in conjunction with Ep, the generated flocs had bigger sizes and faster growth rates than those generated by PAC-PAM and PAC-sodium alginate (SA). Compared with PAM and SA, flocs generated by Ep possess similar strength and settle ability, but more compact structure could be found when Ep was used. For actual water treatment, UV254 removal achieved maximum when Ep was used as coagulant, and meanwhile floc size and recovery ability could also be enhanced. However, the strength of flocs generated in PAC-Ep coagulation system was lower than that of PAC-PAM and PAC-SA.(6) The preparation method of Ep was simple, and the raw material-Enteromorpha, could be obtained for free. Considering all the costs of transportation, energy, equipment and reagents, the cost of Ep was lower than that of PAM and SA. In addition, the removal of turbidity, DOC, UV254, color and AgNPs were all improved due to Ep addition. Therefore, instesd of traditional coagulant aid, Ep could be used as a new coagulant aid to reduce the cost of water treatment process. In addition, for dringking water treatment, the safty of coagulant is very important. Ep was extracted from natural plants, which has a great advantage of a safe, non-toxic, high biological security and degradation. Therefore this is an obvious superiority of Ep besides cost advantage. |
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