Characteristics Of Cyanobacteria-containing Sludge And Water Obtained By Dewatering Cyanobacteria-containing Sludge Formed During Drinking Water Treatment

Due to nutrient over-enrichment of surface waters by urban,agricultural,and industrial development,cyanobacterial blooms have quickly become a global epidemic recently.The water quality can be greatly diminished,for example,the aesthetics of drinking water can be compromised by taste and odor compounds produced by cyanobacteria such as geosmin and 2-methylisobomeol.The presence of toxic cyanobacteria in natural waters also poses a threat to animal and human health because they can produce many types of toxic compounds,which can cause serious and even fatal human liver,digestive,neurological,and skin diseases.Furthermore,it is a known fact that disinfectants,such as chlorine,react easily with algal organic matter(AOM)to produce numerous genotoxic and carcinogenic disinfection by-products(DBPs),including carbonaceous DBPs(C-DBPs)and nitrogenous DBPs(N-DBPs).Fortunately,coagulation,the key step in conventional drinking water treatment for pollutant removal,has been validated as an effective approach to remove algal cells,without causing damage to the cells,and the toxic compounds such as microcystins(MCs)and other AOM is usually contained within the healthy algal cells.It is notable that large amounts of drinking water sludge could be produced during the production of drinking water,equivalent to 4-7%of the total drinking water produced.Because of global water scarcity in recent times,more and more drinking water sludge is dewatered,with the extracted water being recycled into the production stream,achieving zero discharge of sewage from the drinking water treatment plant.However,in the treatment of cyanobacteria-containing drinking water sludge,damage to cyanobacterial cells may occur and will result in the release of intracellular MCs and other AOM into the recycled water,thus potentially causing problems for water recycling in drinking water treatment plants.Hence,it is essential to investigate the change of algae cells and pathogens in drinking water sludge during storage.It is also essential to study the water quality of the ’dewatering water’(DW)and ensure the safety reuse of DW in drinking water treatment plant.The characteristics of the cyanobacteria-containing sludge generated in the drinking water treatment plant and the safety reuse of the water obtained by dewatering cyanobacteria-containing sludge was investigated in this study.Because the sludge is usually discharged at intervals in the drinking water treatment plant,we studied the the trends of cell viability and integrity of cyanobacteria in the sludge with different storage time.The microbial community structure especially the pathogenic bacteria in cyanobacteria-laden drinking water sludge was investigated using Illumina 16S rRNA gene MiSeq sequencing.The changes of the water quality obtained by dewatering cyanobacteria-containing sludge,including formation of C-DBPs and N-DBPs were also investigated to ensure the safety reuse of DW.Because AICI3,FeCl3,and polymeric aluminium ferric chloride(PAFC)are the conventional inorganic coagulats,the influences of different coagulant species on the algal cells,sludge and DW were studied and compared.Results are as follows:The different behaviors of Microcystis aeruginosa(M.aeruginosa)in the sludges formed by AICl3,FeCl3,and PAFC coagulants during storage were systematically investigated.Results show that the viability of M.aeruginosa decreased gradually and the viability was low after 6 d storage.The viability of M.aeruginosa in PAFC sludge was better than that of cells in either AICl3 or FeCl3 sludge after the same storage time,while the cells’ viability in the latter two systems stayed at almost the same level.In AICl3 and FeCl3 sludges high concentrations of AI and Fe were toxic to M.aeruginosa,whereas in PAFC sludge low levels of A1 showed little toxic effect on M.aeruginosfa growth and moderate amounts of Fe were beneficial to growth.The lysis of M.aeruginosa in AICl3 sludge was more serious than that in PAFC sludge,for the same storage time.Although the cell viability in FeC1b sludge was low(similar to AICl3 sludge),the M.aeruginosa cells remained basically intact after 10 d storage(similar to PAFC sludge).The maintenance of cellular integrity in FeCl3 sludge might be due to the large floc size and high density,which had a protective effect for M.aeruginosa.The microbial community structure in cyanobacteria-laden drinking water sludge generated by different types of coagulants(including AICl3,FeCl3,and PAFC)was systematically investigated using Illumina 16S rRNA gene MiSeq sequencing.Results show that Cyanobacteria,Proteobacteria,Firmicutes,Bacteroidetes,Verrucomicrobia,and Planctomycetes were the most dominant phyla in sludge,and because of the toxicity of high Al and Fe level in AICl3 and FeCl3 sludges,respectively,the PAFC sludge exhibited greater microbial richness than that in AICl3 and FeCl3 sludges.Due to the lack of light and oxygen in sludge,relative abundance of the dominant genera Microcystis,Rhodobacter,Phenylobacterium,and Hydrogenophoga clearly decreased,especially after 4 d storage,and the amounts of extracellular MC and organic matter rose.As a result,the relative abundance of MCs and organic degradation bacteria {Bacillus,Paenibacillus,Alkaliphilus,Lactococcus,Sporosarcina,Enterococcus,andBrevibacillus)increased,including pathogens such as Bacillus cereus and Enterococcus,in particular after 4 d storage.Furthermore,the increases of Bacillus cereus and Enterococcus in AICl3 sludge were higher than those in FeCl3 and PAFC sludges after 8 d storage.This is also the first study to systematically investigate the characteristics of the water obtained by dewatering cyanobacteria-containing sludge generated in the drinking water treatment plant,including formation of C-and N-DBPs.Results showed that DW had different properties when the sludge was stored at different times.The content of dissolved organic matter(DOIM)and MCs in the DW were low when the sludge was treated or disposed of within 4 days;correspondingly,the C-,N-DBP production was also low.Furthermore,the levels of DOM,MCs,and the C-,N-DBP production were the lowest after 4 d storage.However,due to the damage of algal cells to some extent,the DOM and MC levels increased significantly for storage time longer than 4 days;the production of C-,N-DBPs also clearly increased.There were also obvious differences in the characteristics of the DW from sludges generated with different coagulant species.Due to the better protective effect of FeC13 and PAFC flocs,the DOM and MC levels and the production of C-,N-DBPs in the DW with FeCl3 and PAFC coagulation were lower than those with AICI3 coagulation,even though the sludges were stored for the same amount of time.Furthermore,because of the formation of Al and Fe hydroxides,precipitated onto the surface of flocs,the soluble A1 and Fe in the DW decreased with increasing storage time,especially in the first four days.Sludge should be disposed of within 4 d due to the better water quality of DW during the first 4 d.Because the increase of extracellular MCs,organic matter,and pathogens in AICl3 sludge was higher than that in FeCl3 and PAFC sludges,FeCl3 and PAFC may be ideal coagulants in drinking water treatment plants.Overall,this study revealed the characteristics of cyanobacteria-containing sludge and water obtained by dewatering cyanobacteria-containing sludges formed with different coagulants,and thereby provides significant indicators for utilization of the water.