Civil Engineering Reference
In-Depth Information
of dissolved metals such as iron or manganese, as well as force a re-equilibration of
the corrosion scales on distribution system piping and premise plumbing. Re-
equilibration of the corrosion scales may result in the conversion of the existing metal
solids to more oxidized forms, which in turn may alter both the physical and chemical
character of the scale.
Problems related to iron oxidation are usually immediately apparent upon chlori-
nation, but generally occur at concentrations substantially higher than those associated
with manganese (greater than 0.2 mg / L). Oxidation of ferrous iron (Fe
2
) to ferric
iron (Fe
3
) is rapid, producing ferric oxide particulates that are the cause of almost all
red water problems. The removal of oxidized iron and manganese by filtration at the
water source is the commonly practiced method of removing color related to these
two constituents (see Chapter 14, ''Iron and Manganese'').
Colored water may also be caused by corrosion of iron (red water problems) or
copper (blue water problems) piping in the distribution system. Please refer to Chapter
21, ''Distribution System Corrosion: Monitoring and Control,'' for a discussion of
corrosion control.
Bacterial Regrowth in the Distribution System
Bacterial growths tend to be more of a problem in the summer for several reasons:
15
Algal blooms in the raw water may increase the amount of biodegradable organ-
ics.
•
Reaction rates are faster.
•
Increased water demand increases the nutrient and disinfectant flux in the distri-
bution system, increases the shear forces on biofilms, and creates shorter residence
times.
•
The amount of biodegradable organic matter in the finished water has a direct effect
on the potential for bacterial growth in the distribution system. The use of ozone for
disinfection forms readily biodegradable low-molecular-weight compounds when it
oxidizes the less-degradable humic and fulvic acids. Systems that have replaced chlo-
rination with ozonation have found that previously biologically stable waters now
cause significant biological growths in the distribution system. In these cases or in
cases where the raw water contains significant concentrations of biodegradable organ-
ics, it may be necessary to incorporate biological treatment in the water treatment
process. This is typically done by modifying the conventional filtration process to
achieve biodegradation of organics within the filter media. The design criteria for and
results of biological filtration are presented in Chapter 12, ''Filtration.''
Effective system flushing can reduce this problem. Maintenance of disinfectant re-
siduals is also used to limit biological growths in the distribution system. Increasing
the level of free chlorine has not always been successful. In some cases, disinfection
has been ineffective for growths on iron pipes even at levels of 5 mg / L of free chlorine
for several weeks. Free chlorine reacts with electron donors of lower redox potential,
such as corrosion products, and chlorine residuals may rapidly decrease. Monochlor-
amine is a weaker oxidant and reacts more slowly with these electron donors. As a
result, monochloramine can penetrate deeper into the biofilm before being consumed.
Monochloramine therefore is more effective in controlling biofilms than is free chlo-
