Chitosan-alumium Chloride Dual-coagulants Removal Technology For Toxic Cyanobacteria In Drinking Source Water Treatment Processes

The proliferation of harmful cyanobacteria is considered as a serious environmental issue since its ability to produce toxins as well as taste and odor compounds which adversely affect aquatic ecosystems and human.Microcystins(MCs)produced by M.aeruginosa are hepato-toxins which pose a health threat on humans.Other algal organic matters(AOM)could also compromise the safety of drinking water since they are precursors leading to the development of disinfection by-products(DBPs)during chlorination.Generally,most AOM including MCs are contained within cells which are classified as intracellular organic matters(IOM),and those releasing metabolites deriving from cyanobacterial cells are categorized as extracellular organic matters(EOM).The cell contained IOM can also be released into the environment once cell integrity is damaged.The removal of dissolved AOM(including MCs)is more difficult than those in the cells of M.aeruginosa.Therefore,an efficient method to remove M.aeruginosa cells without damaging the membrane meanwhile adsorb the extracellular MCs and EOM is significant to drinking water treatment.Coagulation is considered as the most important process for algal removal in conventional drinking water treatment.Even though the traditional coagulant would effectively remove the M.aeruginosa cells without causing cell lysis,it was ineffective at the removal of extracellular MCs.The organic polymer coagulant has been proposed as a substitute for conventional coagulants in water treatment.However,the high cost has limited its application as a sole coagulant in practice.Thus,in this study,we prepared dual/composite coagulants by combining chitosan(organic polymer coagulant)and aluminium chloride,and to investigate their effects on the removal of M.aeruginosa cells,extracellular MCs and EOM,dewatering of cyanobacteria-laden sludge and reduction of cyanobacteria-related DBPs.The goal of this study is to optimize the dual-coagulants removal technology for treatment of cyanobacteria related pollution in drinking water treatment processes.(1)The effects of chitosan(CTS)on cell integrity and extracellular MCs of M.aeruginosa cells during flocculation and flocs storage processes were investigated.The impacts of CTS addition,flocculation stirring and flocs storage time were comprehensively detected to prevent or reduce cell lysis and MCs release.Under CTS concentration 7.31 mg/L,fast mix speed 227 rpm,fast mix time 2 min,slow mix speed 19 rpm,slow mix time 12 min,99%of M.aeruginosa cells were integrated removed.Furthermore,an amount of extracellular MCs were adsorbed by CTS polymers in this process.With CTS flocs protect,though cells showed some damage,extracellular MCs concentration in flocculated samples was lower than background level within first 2 days.However,lots of MCs release was observed after 4 days which may result from CTS degradation and cells lysis.Therefore,CTS flocs should be treated within 4 days to prevent the adsorbed MCs releasing again.(2)The effects of two CTS and aluminum chloride(AC)dual-coagulants with different dosing orders and a composite coagulant chitosan-aluminum chloride(CTSAC,CTS and AC premixed)on removal of M.aeruginosa cells and second metabolites in cyanobacteria-laden drinking water treatment were investigated,and in comparison with AC and CTS applied alone.Response surface method was employed successfully to optimize and model the coagulation processes using different coagulants.Of note,negligible cell lysis was observed during all the dual and composite coagulation processes.The CTSAC removed cells was the most effective for stronger entrapment and bridging ability.The optimal coagulation performance was obtained when CTSAC was set as 2.6 mg/L CTS plus 7.5 mg/L AC,under which 97.8%of intact cells,53.08%of extracellular MCs and almost all EOM were simultaneously removed.Overall,the combination of CTS and AC coagulation process has proven to be a better substitute for conventional coagulation in cyanobacteria-laden drinking water treatment.(3)The fate of cyanobacteria and AOM during the storage of cyanobacteria-laden CTSAC coagulated sludge was investigated.The present results demonstrated that the MCs can be adsorbed onto the CTSAC coagulants for up to 6 days,along with a reduction in the biodegradation rate of MCs.The CTSAC flocs first showed evidence of providing a protective shield to M.aeruginosa cells,and then the degradation of the floes would favor growth of the cells.This is the first study to report the occurrence of growth of M.aeruginosa cells in a CTS-based composite treated system.This work also showed that the EOM could be adsorbed by the CTSAC coagulants and there was no significant release of EOM in 4 days,thus a negligible level of DBP precursors was expected.Interestingly,the concentration of the residual Al in the coagulated system decreased sharply to 100.9 ?g/L after 4 days.The PCR-DGGE analysis based on the 16S rDNA gene indicated that the relative concentration of pathogens,such as Bacillus cereus,Pseudomonas sp.and Burkholderia sp.,all being MC degraders,was reduced during the first 4 days but increased afterwards.Taken together,the CTSAC coagulation was initially(0-4 days)able to reduce the level of MCs and AOM through adsorption,but the reduction could not last due to the degradation of CTSAC after 6 days.In summary,it would be better to dispose of the sludge within 4 days for a clearer supernatant with less secondary metabolites.This study not only improves the understanding of the cyanobacterial variation,metabolites and biological risks during storage of cyanobacteria-laden CTSAC-coagulated sludge,but also is likely to facilitate improvements at drinking water treatment plants(DWTPs)in terms of cyanobacteria-laden sludge management.(4)The dewatering performance of cyanobacteria-laden sludge from the coagulation process using CTSAC composite was systematically studied in this work.The impacts of mechanical actions and chemical effects on the filtration efficiency and filtrate quality during the cyanobacteria-laden sludge volume reduction process were also determined.The following conclusions can be drawn.The vacuum filtration had rejection effects on extracellular MCs and EOM(polysaccharide,protein and humic acid)without causing cyanobacteria cell lysis during cyanobacteria-laden sludge dewatering process.The sludge from the enhanced CTSAC composite coagulation process was of high dewatering ability,sequentially followed by CTS and AC sludge.High vacuum can improve the filtration rate but also decrease the rejection effect of MCs and EOM because high pressure causes deflocculation of adsorbed MCs and EOM.Overall,for improving filtration efficiency and saving energy cost,it is better to choose low vacuum degree pressure in the vacuum filtration process of cyanobacteria-laden sludge.The floc size played a more significant effect on sludge dewaterability than their degree of compaction degree.The protein-like substances in Soluble-EPS was negatively correlated with the dewaterability of cyanobacteria-laden sludge while no clear correlation was observed between Bound-EPS and dewaterability.The reasons for the large particle size,compact structure,low EOM and high dewaterability of the sludge from the enhanced CTSAC composite coagulation process were the strong improvement in the charge neutralization and bridge ability of inorganic AC by combining organic CTS coagulant.(5)This study showed coagulation process using the novel CTSAC composite coagulant substantially enhanced the removal of both C-DBP and N-DBP precursors in comparison to coagulation using AC or CTS coagulant alone.Mean reductions in the formation of C-DBPs and N-DBPs obtained by CTSAC coagulation were respectively 63.54%and 71%.CTS coagulation contributed to moderate reduction in both C-DBP and N-DBP formation,relative to AC alone.Mean reductions in formation of the six C-DBPs,obtained by DBP precursor removal,were 38.02%and 50.75%for AC and CTS,respectively,while the equivalent mean reductions for five N-DBPs were 33.26%and 42.27%.The superior performance of CTSAC in removing C-and N-DBPs is largely ascribed to the interaction of CTS and AC,which improved the adsorption and charge neutralization ability of CTSAC over that of AC or CTS added separately.CTSAC thus displayed remarkably enhanced removal efficiencies in the low-MW and high-MW fractions of AOM,while AC coagulation alone was not effective in removing low-MW AOM.For the emerging AOM-related NDMA especially,AC coagulation alone did not significantly reduce its formation,whereas the CTSAC composite coagulant reduced NDMA by more than 72%,due to effective removal of aromatic secondary and aliphatic amines.This highlights the synergistic effect of AC and CTS,showing the CTSAC composite coagulation process is a promising drinking water treatment technology for control of both C-DBPs and N-DBPs.