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Effect of glutaraldehyde biocide on laboratory-scale rotating biological contactors and biocide efficacy Lakkana Laopaiboon* Niphaphat Phukoetphim Pattana Laopaiboon *Corresponding author Financial support: Research Center for Environmental and Hazardous Substance Management, Khon Kaen University, Thailand and National Research Center for Environmental and Hazardous Waste Management, Chulalongkorn University, Thailand. Keywords: acclimation, aldehyde biocide, biofilm, RBC, wastewater treatment.
Glutaraldehyde (1,5-pentanedial) is a non-oxidizing biocide which is widely used as an antimicrobial agent in a variety of applications such as in cooling water systems, paper-pulp industry, oilfield operation, leather tanning industry, poultry industry, cosmetic field, microbiological field, food industry and medical area. The widespread application of biocides in the industries to control microbial growth may cause environmental and ecological problems (Juergensen et al. 2000; Nishihara et al. 2000). Therefore, knowledge of the effects of biocides on wastewater treatment units and their efficacy against bacterial cells is important. Rotating biological contactors (RBCs) are one of the standard methods of biological wastewater treatment in which degradation and bio-oxidation are predominantly achieved by a biofilm which is attached to the rotating discs (Banerjee, 1997; Casey, 1997). It is widely accepted that sessile bacteria in a biofilm are more resistant to biocides than their planktonic counterparts (Stewart et al. 1998, Laopaiboon et al. 2002). Therefore laboratory-scale RBC units may be used to predict the effect of the biocide on the operation of full scale RBC, the ability of full scale RBCs to degrade biocides and the ability of biocides to control and eliminate biofilms in continuous systems. In this study the effects of 50% glutaraldehyde, a commercial biocide widely used in paper and pulp industry, on biofilms in RBCs and on biofilm establishment in RBCs after acclimation had been investigated. The effects of the biocide on the reduction in COD and glutaraldehyde concentration of the synthetic wastewater applied to the RBCs as well as on the activity and survival of the biofilms and planktonic cells in the system were determined. The development of resistant / acclimatized microflora to the biocide and the effects that such microorganisms would have on biocide degradation as a sole carbon source and RBC treatment efficiency were observed. Differences in susceptibility of planktonic and biofilm bacteria to glutaraldehyde were also compared. When the synthetic wastewater (Laopaiboon et al. 2001) containing 0, 40, 80 and 180 ppm glutaraldehyde was introduced to the RBCs at a dilution rate of 1.60 h-1, the results showed that the biofilms became acclimated to glutaraldehyde and eventually could degrade it. Acclimation to the biocide took longer at the higher biocide concentrations. The degree of biocide degradation and COD removal depended on acclimation period, the presence of other organic matters and the amount of mineral salts available (Figure 1, Figure 2, Figure 3 and Figure 4). Glutaraldehyde at up to 80 ppm had no effect on treatment efficiency and populations of biofilms and planktonic phase of the system whereas glutaraldehyde at 180 ppm caused a progressive decline in all measured values. However, no glutaraldehyde concentration used in the study was sufficiently high to kill microorganisms in the RBC system. Applying high biocide concentration (180 ppm glutaraldehyde) to a wastewater treatment plant might inhibit simple carbon source utilization by microorganisms in the system (Table 1). The results also clearly showed that the presence of biofilm provided additional resistance to glutaraldehyde to bacteria because the biocide had to penetrate through biofilm to reach bacteria. In this study, there was at least a seven-fold difference in susceptibility of planktonic and biofilm cells to glutaraldehyde. Higher organisms in the biofilms, protozoa and rotifers, were found to be more sensitive to glutaraldehyde than bacterial cells in the biofilms. The resistant bacterium to glutaraldehyde was tentatively identified as predominantly of the species Burkholderia cepacia. The increased resistance of bacteria to glutaraldehyde due to acclimation should be considered in biocide applications.
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