ASHRAE Journal - October 2013 - 28

Technical feaTure
fea

Total Dissolved Solids in
Reclaimed Water
By Daniel H. nall, P.e., Faia, MeMBer asHrae; roBert seDlak, P.e.

More than half of human water demands can be met with recycled nonpotable water, which opens up the possibilities for many water consumption
reduction strategies. Recycled sources include HVAC condensate, rooftop
storm water, gray (wash) water, and local sewage treatment plant effluent.
Some of these are relatively clean, requiring only antimicrobial treatment,
while others require filtration or removal of organic contaminants.
Levels of total dissolved solids (TDS) are often overlooked in these non-potable sources, resulting in challenges for end-uses such as cooling tower makeup and
irrigation. Excessive levels of TDS can result in corrosion, fouling and scale in cooling towers and in toilet
fixtures, and mortality to irrigated plants.
While some non-potable water sources, such as harvested storm water run-off and HVAC condensate,
have very low TDS, other sources, such as treated sewage effluent (TSE) and recovered wash water can have
quite high TDS. In areas with “hard” potable water
supplies, TDS in the incoming city water may approach
the acceptable upper limits for cooling tower makeup,
before chlorides, nitrates, bicarbonates and sulfate salts
are added by human use. Successful use of recycled
water sources for non-potable water demands requires
management of TDS from those widely varying sources.
Cooling tower water consumption is affected by the TDS
level in the makeup water. Conventional limits for TDS in
basin water range from 1,000 to 1,500 ppm, consistent with
a conductivity of 2400 μS/cm. Cooling tower blowdown
wastes water from the basin, removing dissolved solids to
maintain the desired maximum TDS concentration.
Blowdown rates may sometimes be fixed to control
maximum levels of certain impurities present in the

makeup water rather than maintaining a TDS upper
limit. The ratio of cooling tower makeup flow to blowdown flow is called the cooling tower cycles of concentration (COC). The maximum allowable COC is usually
the ratio of the maximum allowable basin TDS to the
makeup water TDS.
With conductivity or other concentration measurement sensors, blowdown rate can be controlled dynamically to maintain the desired TDS in the cooling tower
basin. For conventional cooling tower water treatment
protocols, higher levels of makeup water TDS result in
higher rates of blowdown. Two strategies to reduce cooling tower water consumption are:
• Provide makeup water from low TDS water sources,
maximizing the cycles of concentration for the makeup
water.
• Use treatment protocols that enable basin TDS levels
to exceed conventional levels.

TDS in Potable Water Sources
TDS in potable water is generally less than 500 mg/L, a
recognized threshold above which excessive hardness,
unappetizing taste, scaling and corrosion may occur. The
range of TDS for potable water supplies can be from as
low as 30 mg/L in streams running through igneous rock

About the Authors Daniel H. Nall, P.E., FAIA, is a senior vice president at Thornton Tomasetti Group, New York, and Robert Sedlak, P.E., is a senior vice president at WSP USA, New York.
28

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ASHRAE Journal - October 2013

Table of Contents for the Digital Edition of ASHRAE Journal - October 2013

Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
Sustainable Design of Water Source VRF
Economizer for Data Center
Total Dissolved Solids in Reclaimed Water
Hot, Humid Lab Hood Facility
Engineer's Notebook
Building Sciences
The Performance Gap
ASHRAE Honor Roll
ASHRAE Research Report
Technical vs. Process Commissioning: Basis of Design
Emerging Technologies
Data Centers
HVAC Applications
Energy Modeling
Refrigeration Applications
Products
Classified Advertising
Advertisers Index
ASHRAE Journal - October 2013 - Cover1
ASHRAE Journal - October 2013 - Cover2
ASHRAE Journal - October 2013 - 1
ASHRAE Journal - October 2013 - 2
ASHRAE Journal - October 2013 - Contents
ASHRAE Journal - October 2013 - Commentary
ASHRAE Journal - October 2013 - 5
ASHRAE Journal - October 2013 - Industry News
ASHRAE Journal - October 2013 - 7
ASHRAE Journal - October 2013 - 8
ASHRAE Journal - October 2013 - 9
ASHRAE Journal - October 2013 - Letters
ASHRAE Journal - October 2013 - 11
ASHRAE Journal - October 2013 - Meetings and Shows
ASHRAE Journal - October 2013 - 13
ASHRAE Journal - October 2013 - Sustainable Design of Water Source VRF
ASHRAE Journal - October 2013 - 15
ASHRAE Journal - October 2013 - 16
ASHRAE Journal - October 2013 - 17
ASHRAE Journal - October 2013 - 18
ASHRAE Journal - October 2013 - 19
ASHRAE Journal - October 2013 - Economizer for Data Center
ASHRAE Journal - October 2013 - 21
ASHRAE Journal - October 2013 - 22
ASHRAE Journal - October 2013 - 23
ASHRAE Journal - October 2013 - 24
ASHRAE Journal - October 2013 - 25
ASHRAE Journal - October 2013 - 26
ASHRAE Journal - October 2013 - 27
ASHRAE Journal - October 2013 - Total Dissolved Solids in Reclaimed Water
ASHRAE Journal - October 2013 - 29
ASHRAE Journal - October 2013 - 30
ASHRAE Journal - October 2013 - 31
ASHRAE Journal - October 2013 - 32
ASHRAE Journal - October 2013 - 33
ASHRAE Journal - October 2013 - 34
ASHRAE Journal - October 2013 - 35
ASHRAE Journal - October 2013 - 36
ASHRAE Journal - October 2013 - 37
ASHRAE Journal - October 2013 - 38
ASHRAE Journal - October 2013 - 39
ASHRAE Journal - October 2013 - Hot, Humid Lab Hood Facility
ASHRAE Journal - October 2013 - 41
ASHRAE Journal - October 2013 - 42
ASHRAE Journal - October 2013 - 43
ASHRAE Journal - October 2013 - 44
ASHRAE Journal - October 2013 - 45
ASHRAE Journal - October 2013 - 46
ASHRAE Journal - October 2013 - 47
ASHRAE Journal - October 2013 - Engineer's Notebook
ASHRAE Journal - October 2013 - 49
ASHRAE Journal - October 2013 - 50
ASHRAE Journal - October 2013 - 51
ASHRAE Journal - October 2013 - 52
ASHRAE Journal - October 2013 - 53
ASHRAE Journal - October 2013 - Building Sciences
ASHRAE Journal - October 2013 - 55
ASHRAE Journal - October 2013 - 56
ASHRAE Journal - October 2013 - 57
ASHRAE Journal - October 2013 - 58
ASHRAE Journal - October 2013 - 59
ASHRAE Journal - October 2013 - The Performance Gap
ASHRAE Journal - October 2013 - 61
ASHRAE Journal - October 2013 - 62
ASHRAE Journal - October 2013 - 63
ASHRAE Journal - October 2013 - 64
ASHRAE Journal - October 2013 - ASHRAE Honor Roll
ASHRAE Journal - October 2013 - HR 2
ASHRAE Journal - October 2013 - HR 3
ASHRAE Journal - October 2013 - HR 4
ASHRAE Journal - October 2013 - HR 5
ASHRAE Journal - October 2013 - HR 6
ASHRAE Journal - October 2013 - HR 7
ASHRAE Journal - October 2013 - HR 8
ASHRAE Journal - October 2013 - HR 9
ASHRAE Journal - October 2013 - HR 10
ASHRAE Journal - October 2013 - HR 11
ASHRAE Journal - October 2013 - HR 12
ASHRAE Journal - October 2013 - HR 13
ASHRAE Journal - October 2013 - HR 14
ASHRAE Journal - October 2013 - HR 15
ASHRAE Journal - October 2013 - HR 16
ASHRAE Journal - October 2013 - HR 17
ASHRAE Journal - October 2013 - HR 18
ASHRAE Journal - October 2013 - HR 19
ASHRAE Journal - October 2013 - HR 20
ASHRAE Journal - October 2013 - HR 21
ASHRAE Journal - October 2013 - HR 22
ASHRAE Journal - October 2013 - HR 23
ASHRAE Journal - October 2013 - HR 24
ASHRAE Journal - October 2013 - HR 25
ASHRAE Journal - October 2013 - HR 26
ASHRAE Journal - October 2013 - HR 27
ASHRAE Journal - October 2013 - HR 28
ASHRAE Journal - October 2013 - HR 29
ASHRAE Journal - October 2013 - HR 30
ASHRAE Journal - October 2013 - HR 31
ASHRAE Journal - October 2013 - HR 32
ASHRAE Journal - October 2013 - ASHRAE Research Report
ASHRAE Journal - October 2013 - 66
ASHRAE Journal - October 2013 - 67
ASHRAE Journal - October 2013 - 68
ASHRAE Journal - October 2013 - 69
ASHRAE Journal - October 2013 - 70
ASHRAE Journal - October 2013 - 71
ASHRAE Journal - October 2013 - 72
ASHRAE Journal - October 2013 - 73
ASHRAE Journal - October 2013 - 74
ASHRAE Journal - October 2013 - 75
ASHRAE Journal - October 2013 - Technical vs. Process Commissioning: Basis of Design
ASHRAE Journal - October 2013 - 77
ASHRAE Journal - October 2013 - 78
ASHRAE Journal - October 2013 - 79
ASHRAE Journal - October 2013 - 80
ASHRAE Journal - October 2013 - 81
ASHRAE Journal - October 2013 - Emerging Technologies
ASHRAE Journal - October 2013 - 83
ASHRAE Journal - October 2013 - 84
ASHRAE Journal - October 2013 - 85
ASHRAE Journal - October 2013 - 86
ASHRAE Journal - October 2013 - 87
ASHRAE Journal - October 2013 - Data Centers
ASHRAE Journal - October 2013 - 89
ASHRAE Journal - October 2013 - 90
ASHRAE Journal - October 2013 - 91
ASHRAE Journal - October 2013 - 92
ASHRAE Journal - October 2013 - 93
ASHRAE Journal - October 2013 - HVAC Applications
ASHRAE Journal - October 2013 - 95
ASHRAE Journal - October 2013 - Energy Modeling
ASHRAE Journal - October 2013 - 97
ASHRAE Journal - October 2013 - Refrigeration Applications
ASHRAE Journal - October 2013 - 99
ASHRAE Journal - October 2013 - Products
ASHRAE Journal - October 2013 - 101
ASHRAE Journal - October 2013 - 102
ASHRAE Journal - October 2013 - Classified Advertising
ASHRAE Journal - October 2013 - Advertisers Index
ASHRAE Journal - October 2013 - Cover3
ASHRAE Journal - October 2013 - Cover4
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