Surface-retained organic matter of Microcystis aeruginosa inhibiting coagulation with polyaluminum chloride in drinking water treatment
Introduction
Semi-closed water areas such as reservoirs are the main sources of drinking water in urbanized areas. The eutrophication of the semi-closed water areas has been one of the critical issues in drinking water production, because it brings about the excess growth of cyanobacteria, thereby causing various disorders in drinking water treatment processes (Widrig et al., 1996, Hitzfeld et al., 2000, Ma and Liu, 2002, Westerhoff et al., 2005, Huang et al., 2009). In particular, coagulation efficiency at the destabilization step is fairly susceptible to the presence of algogenic organic matter (AOM) in source water (Bernhardt et al., 1991, Cheng and Chi, 2003, Pivokonsky et al., 2006). The reduction of coagulation efficiency caused by AOM could be temporarily overcome by increasing coagulant dose, which creates subsidiary problems, including an increased cost for the coagulant and sludge treatment. It is important to elucidate the inhibitory mechanism caused by AOM, which could contribute to the establishment of effective countermeasures to coagulation inhibition.
One of the proposed inhibitory mechanisms is that AOM can form complexes with cations originated from coagulant, which seriously deteriorates the coagulation capability (Bernhardt et al., 1987, Bernhardt et al., 1991). Our previous studies showed that proteins from Microcystis aeruginosa consume polyaluminum chloride (PACl) in the coagulation process due to the formation of chelate complexes with the coagulant (Takaara et al., 2005, Takaara et al., 2007). Since cyanobacterial proteins are mainly located inside the cell, it was speculated that these inhibitory proteins released by cell lysis or prechlorination significantly affect coagulation efficiency. However, the presence of these cyanobacterial inhibitory proteins does not fully explain the involvement of AOM in coagulation inhibition, because the reduction of coagulation efficiency without accompanying cell destruction also has been reported (Ma et al., 2007). The presence of inhibitory organic matter located on the outside of cyanobacterial cells would be one of the plausible explanations for the nonproteineous substance-induced coagulation inhibition (Hoyer et al., 1985, Bernhardt et al., 1987). The exterior-located organic matter can be categorized into surface-retained organic matter (SOM) and extracellular organic matter (EOM), and EOM has been extensively studied as an inhibitory substance (Liu and Bernhardt, 1991). On the other hand, the involvement of SOM in coagulation inhibition has not been investigated well because it has been technically difficult to analyze the contribution of SOM to coagulation inhibition separately from EOM and intracellular organic matter (IOM). The evaluation of cyanobacterial SOM as an inhibitory substance could be very informative to address the problems of the reduction of coagulation efficiency in water treatment processes.
In this study, we analyzed the inhibitory potential of SOM produced by M. aeruginosa against the coagulation with PACl. Firstly, in order to confirm the presence of inhibitory substances on the cell surface of a laboratory strain of M. aeruginosa, the removal rates of M. aeruginosa cells and kaolin under various dosage of PACl was evaluated. Then, hydrophilic organic substances, including SOM, were extracted from M. aeruginosa cells by a phenol-water extraction, and fractionated by ethanol precipitation, ultrafiltration and ion exchange chromatography. RNase A treatment was also employed to eliminate RNA co-extracted with SOM in the hydrophilic organic substances from M. aeruginosa cells. The involvement of the extracted SOM in the coagulation inhibition was evaluated by the coagulation test using a kaolin suspension in the presence of the extracted SOM.
Section snippets
Materials and methods
The preparation procedure of SOM samples for the evaluation of inhibitory potential on the coagulation with PACl is summarized in Fig. 1. Details of the sample preparation and the coagulation test are described below.
Coagulation efficiency of M. aeruginosa cells with PACl
The coagulation test for M. aeruginosa cells using PACl was performed to determine the presence of inhibitory substances on the surface of M. aeruginosa NIES-91. EOM was washed away, and cells of M. aeruginosa suspended in autoclaved tap water at the turbidity corresponding to the kaolin suspension of 20 mg/L were used. The kaolin suspension at 20 mg/L was employed as a control for the coagulation test of M. aeruginosa cells. Fig. 2 shows the reduction rate of OD660 in the coagulation test of
Conclusions
The involvement of cyanobacterial hydrophilic compound (CHC) from M. aeruginosa in the coagulation inhibition was investigated. Negatively-charged CHC with a molecular weight higher than 10 kDa would have a significant role in coagulation inhibition. Lipopolysaccharide (LPS) and/or RNA from M. aeruginosa were suspected to be strong inhibitors for the coagulation. CHC treated by RNase A also exhibited strong inhibition in the coagulation using PACl, which leads to the speculation that LPS could
Acknowledgement
This work has been funded in part by Grant-in-Aid for JSPS Fellows from Japan Society for the Promotion of Science (JSPS) and Grant-in-Aid for Young Scientists (B) in the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. We thank Mr. Takeshi Suto for his technical support during this work.
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