Impact of Biofiltration on Biostability of Drinking Water Distribution System
Date Issued
2010
Date
2010
Author(s)
Wu, Chia-Chen
Abstract
Biofiltration received much attention in recent years because it could remove organic matters and reduce formation potential of disinfection byprducts (DBPs) efficiently. By lowering prechlorination dosage in conventional drinking water treatment plant, the rapid sand filters could be converted to biofilters without renovation and supply better water quality. A pilot plant with three sets of rapid sand filters was established to study the impacts on the water quality resulted from biofiltration. The three filters received various prechlorination dosages and were packed with granular activated carbon (GAC), anthracite, and ceramic beads, respectively. Each filter effluent disinfected for maintaining the residual chlorine of 1 mg/L-Cl2 and entered to a simulated distribution system.
The results showed that the prechlorination dosage directly affected the removal of non-purgeable dissolved organic carbon (NPDOC) and assimilable organic carbon (AOC) in distribution system. The removal of NPDOC rose from 40% to 60% as the chlorine residual of the filter influent was under 1 mg/L. The averages of AOC decreased from about 200 to 50 ug acetate-C/L as the chlorine residual of the filter influent was below 0.1 mg/L. By Quantitative Polymerase Chain Reaction analysis (Q-PCR), no Escherichia coli and Enterococcus sp. were found as 0.31 to 0.78 mg/L of residual chlorine were maintained in the distribution system.
The detected trichloromethanes (THMs) species was chloroform, which was ranged from 8.5 to 24.1 μg/L in finished water. The dichloroacetonitrile (DCAN) was the only detectable haloacetonitriles chosen as the indicator of nitrogenous DBPs (N-DBPs) in this study. With 2 mg/L prechlorination dosage applied, the filtration process formed lower concentration of DCAN due to the low chlorine residual remained in the influent of filters. The DCAN formation potential ranged from 1.56 to 9.84 μg/L in the distribution system, which resulted the regeneration of DCAN from 0.91 to 2.49 μg/L with the chlorine residual.
In conclusion, this study provides evidences that biofiltration not only reduced possibility for microbial regrowth but also released no pathogens. However, the biofiltration system in this study could not remove the precursors of N-DBPs. Therefore, DCAN could be regenerate by DCAN formation potential and chlorine residual remained in the distribution system.
Subjects
biofiltration
distribution system
prechlorination
biostability
formation potential
Type
thesis
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