Involvement of humic substances in regrowth

There appear to be interactions in the distribution system that complicate the ability to use AOC/BDOC as an independent assessment of regrowth potential. Two such complications are the limitation of the assays themselves and the potential interaction between the organic carbon concentration with the presence of disinfectants and pipe materials. To address these interactions, a series of experiments spanning several years have been conducted in model distribution systems at the Center for Biofilm Engineering (CBE) using soil-derived humics. When compared to easily utilized organics, humic substances supported the same order of magnitude of biofilm organisms. As carbon concentration was increased from 500 to 1000 to 2000 ppb, there was no increase in growth rate of the organisms, suggesting zero-order kinetics. If the system was chlorinated, there was less biomass, but growth rates were higher. In the presence of corrosion products, humic-fed systems supported more organisms than a control system fed biologically treated water. When free chlorine was maintained at a residual of about 0.2 mg/l, biofilm numbers on the surfaces were reduced. Phosphate alone did not result in fewer bacteria, while a combination of chorine and phosphate had the best results (lowest biofilm numbers). Adjustment to PH 9 was not effective. Recently completed work compared increasing levels of humic substances in the presence of free chlorine and monochloramine on biofilm growth on a number of surfaces (PVC, epoxy, cement, ductile iron). As the concentration of humic substances was increased from 0, 0.5 to 2 mg/l, there was an increase in biofilm numbers on all surfaces. This effect was the most pronounced on iron surfaces. These results illustrate that carbon compounds not measured by the BDOC or AOC tests may profoundly influence biofilm numbers. In addition, iron surfaces are at much higher risk for elevated biofilm counts in the presence of humic substances, even if disinfection is practiced. However, corrosion control may mitigate this interaction. (C) 2003 Elsevier B.V. All rights reserved.