Advanced Waterworks Mathematics, 2019a

UNIT 12
12.1 BLENDING AND DILUTING
Dilution is not the solution to pollution, but dilution can be used to reduce the level of a
contaminant in drinking water supplies. Blending water sources of different water quality is
common practice. However, when a water utility wants to blend sources of supply to lower a
certain contaminant to acceptable levels, they must receive approval from the governing
Health Department. A Blending Plan must be created that specifies what volumes of water from
each source will be used and what the expected resulting water quality will be. In addition, a
sampling strategy must be included in the plan. The Health Department may not allow blending
for all contaminants. For example, the local health agency may not approve a blending plan for
a contaminant that poses an acute health effect or is deemed to be too high of a risk to public
health.
An acceptable blending plan may be for reducing manganese in a source that has exceeded the
California Secondary Maximum Contaminant Level (MCL) of 0.05 mg/L. Manganese causes
black water problems for customers at levels over the secondary MCL. Additionally, an
approved blending plan may involve a Primary MCL for nitrate. Nitrates above the MCL of 45
mg/L as NO3 can cause methemoglobinimia in infants under 6 months old. These are just two
examples of blending plans.
How are blended water quality results calculated? The blending of water supplies is nothing
more than comparing ratios. For example, if 100 gallons of one source was mixed with 100
gallons of another source, the resulting water quality would be the average between the two
sources. However, when you mix varying flows with varying water quality, the calculations
become a little more complex. Using the diagram below will assist you in solving blending
problems.
Figure 11.1 47
47
Image by Marilyn Hightower is licensed under CC BY 4.0
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