Formation And Control Of Disinfection By-Products In Chlorination Of B-G Algae And Algal Organic Matter (AOM)

The incidence of algal blooms in source waters has increased significantly in both the number and magnitude over the past few years as a result of unbalanced rapid economical development but poor nutrient control in both wastewater effluent discharge and excessive agricultural fertilization in China. Fresh, vivid algal blooms by blue-green algae in 2007 hit the nation's several major water systems including Taihu Lake, Caohu Lake, Dianchi Lake. Algal blooms cause lots of challenges in drinking water treatment. One aspect is from algae cells, which can cause poor settling, clogging filters and breakthrough of small-size algae from sand filters. And the other aspect is from algal organic matter (AOM), which was not readily removed by coagulation or pre-oxidation enhanced coagulation processes. Algal blooms also cause water quality problems in water supplies including obnoxious taste and odors and release of algal toxins. In addition, algae cells can serve as precursors to form disinfection by-products (DBPs) during chlorination/chloramination. The aim of this study is to characterize algae cells and AOM, to assess the role of AOM in the formation of nitrogenous DBPs (N-DBPs) and carbonaceous DBPs (C-DBPs) upon adding chlorine or monochloramine, and to get the relationships between AOM and their DBP formation.The EEM fluorescence spectra of algae cells centered at the Ex/Em of 230/334, 280/334, and 620/642 nm/nm, which represent the aromatic proteins, the tryptophan-like proteins and the chlorophyll. The presence of NO₃^-, NO₂^-, Fe³⁺ and NOM affected the EEM spectra of algae cells greatly. FL_230/334 decreased with the negative exponential functions of the concentrations of NO₃^- and NO₂^-. Both FL_230/334 and FL_280/334 decreased with the negative exponential functions of the concentrations of Fe³⁺ increased. FL_230/334, FL_280/334 and FL_620/642 decreased with the negative linear functions of the concentrations of NOM increased. The fluorescence property of algae cells can be applied to detect the concentration of algae cells both in axenic algae solutions and in natural water.AOM was characterized by UV absorbance spectra, EEM fluorescence spectra, organic nitrogen and organic carbon analysis, molecular weight distribution, polarity distribution and free amino acids/aliphatic amines analysis. AOM was enriched in organic nitrogen (org-N). Org-N in AOM contains both high (13500 Da) and low molecular weight (MW) (70-1000 Da) organic matter, while the organic carbon (org-C) contains relatively lower MW organic matter with MW from several dozens to several hundreds Da. Specific free amino acids and aliphatic amines only occupied 2.5% and 11.3% of the total org-N in EOM and IOM, respectively. IOM posed different characteristics from EOM, such as IOM had higher concentrations of total org-N, free amino acids, but lower concentrations of aliphatic amines than EOM; the org-N in IOM contained more proportion of higher MW and more hydrophobic contents than EOM.Formation of carbonaceous disinfection by-products (C-DBPs), including trihalomethanes (THMs), haloacetic acids (HAAs), haloketones (HKs), chloral hydrate (CH), and nitrogenous disinfection by-products (N-DBPs), including haloacetonitriles (HANs) and trichloronitromethane (TCNM) from chlorination of microcystis aeruginous, a blue-green algae, under different conditions was investigated. Factors evaluated include contact time, chlorine dosages, pH, temperature, ammonia concentrations, bromide ion concentrations and algae growth stages. Increased reaction time, chlorine dosage and temperature enhanced the formation of the relatively stable C-DBPs (e.g., THM, HAA, and CH) and TCNM. Formation of dichloroacetonitrile (DCAN) followed an increasing and then decreasing pattern with prolonged reaction time and increased chlorine dosages. The pH affected DBP formation differently, with THM increasing, HKs decreasing, and other DBPs having maximum concentrations at certain pH. The addition of ammonia significantly reduced the formation of most DBPs, but TCNM formation was not affected and 1,1-dichloropropanone (1,1-DCP) formation was higher with the addition of ammonia. Increasing bromide concentrations increased the incorporation of bromine into DBPs, but did not necessarily increase the total molar concentrations of DBPs. Most DBPs increased as the growth period of algae cells increased. Chlorination of algae cells of higher organic nitrogen content generated higher concentrations of N-DBPs (e.g., HANs and TCNM) and CH, comparable DCAA concentration but much lower concentrations of other C-DBPs (e.g., THM, TCAA and HKs) than did natural organic matter (NOM).Organic chloramine concentrations in AOM chlorination/chloramination were much higher than those from SNOM. During chlorination of AOM, organic chloramine formed in the beginning 2 hours, and inorganic chloramines showed a max concentration on 1d of chlorination. The inorganic chloramines formed in AOM chlorination included monochloramine, dichloramine and trichlormine. The remaining organic chloramine concentrations were about 1.0 mg/L (as Cl₂) after 3-day chloramination of AOM with performened monochloramine. During the chlorination of AOM, organic chloramines were not detected after 1 day and more nitrogenous DBPs (N-DBPs) and haloaldehydes and less carbonaceous DBPs (C-DBPs) were formed than the chlorination of SNOM. Organic chloramines were found after 3-day chloramination of AOM and the formation of most N-DBPs and C-DBPs were much less than that of SNOM. EOM formed less DBPs (except for TCNM) than IOM and algae cells in chlorination and chloramination.The content of organic nitrogen (DON) of AOM showed linear relationships with the cumulative normalized EEM volumes at regionsâ… ,â…¡andⅣΦ_{â… +â…¡+â…£,n}, and negative linear relationship with SUVA.Φ_{â… +â…¡+â…£,n}, which represents the organic nitrogen contents, decreased rapidly in chlorination, while theΦ_{â…¢+â…¤,n} decreased much slower thanΦ_{â… +â…¡+â…£,n}. The concentrations of specific DBPs had some correlations, such as the concentrations of DCAN, CH and TCAN, DCAA, TCAA and DCAN had good positive correlations, which implicated that these DBPs came from the similar precursors. DON showed positive relationships with HAAs, CH and HANs yields, and it showed negative relationships with TOX yields in 3h, but it had no relationships with the yields of TCM, TCNM and HKs. SUVA had no relationships with DBPs, but the variation of UVA₂₅₄ and UVA₂₇₂ before and after chlorination had positive linear relationships with DBPs. Organic nitrogen in AOM is the precursors of N-DBPs and C-DBPs, and organic carbon can also be the precursor of N-DBP in the presence of monochloramine.Preoxidation by ozone and potassium permanganate enhanced the algal removal in the coagulation/sedimentation process. Preozonation destructed the algal cells and caused the release of IOM dramaticly. The released IOM was not readily removed by the following coagulation process, and played as the precusors of DBPs. The formation of various DBPs increased dramaticly in the water treated by preozonation-coagulation-sedimentation compared to that treated by coagulation-sedimentation, such as the formation of CH and TCNM increased by 18 and 43 times, respectively, and other DBPs such THMs, HAAs and HKs increased 2⁶ times. Preoxidation by potassium permanganate changed the surface of the algal cells, but did not detruct the algal cells serverely. The dissolved organic matter decreased slightly in the water treated by permanganate preoxidation-coagulation-sedimentation compared to that treated by coagulation-sedimentation. The formation of various DBPs decreased about 10³0% in the water treated by preoxidation-coagulation-sedimentation compared to that treated by coagulation-sedimentation, except that HKs and TCNM increased about 10⁷0% and 50%, respectively, and CH remained unchanged. In summary, preoxidation by permanganate was a better strategy to control the algal cells and the DBP formation in the treatment of algae-containing water.