Efficiency And Influencing Factors Of Algae Removal With Combined Ultrafiltration Treatment Processes

In view of high removal efficiency of the submerged ultrafiltration (SUF)process to particle material, algae cells and pathogenic micro-organisms, this processhas been considered as the core unit of the current and future water treatmentprocesses. Unfortunately, in one hand, the SUF process cannot remove the dissolvedorganic matter (DOM) efficiently; in the other hand, the natural organic matter (NOM)and algogenic organic matter (AOM) in raw water will pollute the membraneseriously. In this thesis, a novelty integrated process of powered activatedcarbon/submerged ultrafiltration (PAC/SUF) has been developed to treatment themicro-polluted algal-rich raw water. The removal efficiency and mechanism of algaecells and DOM by the PAC/SUF were investigated. Meanwhile, the property of themembrane fouling was studied. Furthermore, the PAC/SUF process was hybrided withseries of pretreatment processes according to the characteristics of raw water quality.In the first study, the PAC/SUF process was directly used to treatment algal-richraw water. The experimental results indicated that the removal rate of NH3-Nincreased rapidly from9.9%to81.5%within the starting15days, then stabilizedsubsequently; By the PCR-DGGE technology, the bacterial community in the reactorof PAC/SUF was analyzed. It was indicated that heterotrophic bacteria was thedominant species, however, the autotrophic bacteria including Cyanobacteria andNitrospira were also detected, which were responsibility for NH3-N removal. In thewhole running period, the PAC/SUF process could remove DOM efficiently. Theaverage removal rate of UV254, DOC and CODMnwere32.7%,33.8%and33.2%,respectively, which increased by21.2%,8.8%and11.0%compared with the SUFprocess. Specifically for AOM, including MC-LR,2-MIB and Geosmin, the averageremoval rate could reach43.0%,53.3%and61.0%, respectively, which alsosignificantly increased by24.5%,21.8%and25.4%compared with the SUF process.Through a mechanism analysis of DOM removal, it was thinking that the physicalinterception of membrane and the gel layer on the surface of membrane, adsorption ofPAC and biodegradation contributed into the three main sources of DOM removal.Further analysis indicated that during the whole running period the removal of thetaste and odor matter could be divided into three stages. In the first stage, the averageremoval rate reached to70%, in which adsorption of PAC and aeration contributed50%and20%, respectively; In the second stage, the removal was contributed by thecombined action of adsorption, biodegradation and aeration, but the sharp decrease ofadsorption coupled with the immature biological function, led to a lower removal(40%~60%); In the third stage, the removal rate recovered to60%. Although thePAC was almost exhausted, the higher removal efficiency was contributed by the stable biodegradation function (40%) and aeration (20%).Three-dimensional fluorescence spectroscopy (EEM) was utilized to analyze themechanism of dissolved organics removal and the characteristics of membranefouling during the running period of the PAC/SUF process. Results showed that thePAC/SUF process could obtain much better removal efficiency of protein-likesubstances, soluble microbial by-products (SMPs) and humic-like substances than theseparate SUF process, which was attributed to PAC adsorption and biodegradation.The SEM was employed to observe the surface and cross-section of membrane. TheSEM photos showed that there was a porous cake layer composed by algal cells, PACand biofilm on the membrane surface in the reactor of PAC/SUF process. By theCLSM analysis, it was confirmed that the biofilm was composed of microbial cellsand polysaccharides. This cake layer was responsibility for the external membranefouling, however, it could refuse the high concentration of DOM in the reactor intothe membrane effluent.When the PAC/SUF process was used directly to treatment the algal-rich rawwater, the concentration of MC-LR,2-MIB and Geosmin in the membrane effluentusually can't meet the Standards for Drinking Water Quality. In view of which,coagulation/sedimentation units were set as the pretreatment process of the PAC/SUFprocess. Results showed that coagulation/sedimentation units could improve organicremoval of the PAC/SUF process, especially for the removal of MC-LR,2-MIB andGeosmin. Additional, increased removal rate of protein-like substances, solublemicrobial by-products (SMPs) and humic-like substances in the raw water couldfurther help to mitigate membrane fouling. However, the concentration ofmicrocystin-LR,2-MIB and Geosmin still could not reach the standard.For further improving the running effect of the PAC/SUF process, theconventional water treatment process was utilized as the pretreatment units. Resultsshowed that the conventional treatment process contributed a further improvement onthe organic removal, with concentration of MC-LR,2-MIB and Geosmin in theeffluent meet the demand of Drinking Water Quality. As a result of sharp decrease ofprotein-like substances, soluble microbial by-products (SMPs) and fulvic after sandfiltration, membrane fouling was alleviated effectively. Enhanced coagulation withpotassium permanganate pre-oxidation was utilized to intensify the performance ofconventional pretreatment. Results showed that the pretreatment alleviated thedevelopment of TMP and enhanced the removal of dissolved organic matter. However,a poor organic removal by PAC/SUF was achieved. None of MC-LR,2-MIB andGeosmin in the effluent could reach the Drinking Water Quality. In order to reduce theload of water treatment processes, potassium permanganate loaded zeolite wasdeveloped and added into the algal-rich water as pretreatment, which was proved toremove the algae and the microcystins-LR effectively. Control measures on membrane fouling caused by algal-rich water wereinvestigated. Different chemical reagents were used to study the cleaning efficienciesof PVDF membrane after long–term ultrafiltration of algal-rich water. Results showedthat the physical methods could not remove the foulants availably, while the filterperformance of membrane soaked in0.5g/L of NaOH and0.5%of H2O2successivelyexhibited a best recover. SMPs (various protein and polysaccharide included) andprotein-like were redissolved in NaOH. Both of the quality of raw water and fluxexerted obvious influence on the membrane fouling. The growth of TMP wasproportional to either of the concentration of algal cells or the flux, but inverselyproportion to the frequency of air bubbling. However, mitigation of membrane fouling,energy depletion and operating cost should be taken into account comprehensively todetermine the optimal operating parameters.