POWER March 2017 - 35

WATER & WASTEWATER
Table 1. Chemistry of reclaimed water. Source: GE
Parameter
Unit
Conductivity
Cl
SO4
Turbidity
M-Alkalinity
Ca
Mg
o-PO4
Al
Fe
Zn
Ammonia, free
Ammonia, free and fixed
Nitrate
Chemical oxygen demand
Total organic carbon
uS/cm
ppm
ppm
NTU
ppm as CaCO3
ppm as CaCO3
ppm as CaCO3
ppm
ppm
ppm
ppm
ppm as N
ppm as N
ppm as NO3
ppm O2
ppm as C
Secondary treated Tertiary treated
600-2,700
60-400
30-120
1-12
600-1,900
100-400
20-170
1-10
70-180
80-240
30-120
2-35
0-2.6
0-2.2
0-2.4
0-15
2-21
50-170
10-220
2-52
Table 2. Asset risks of reclaimed water. Source: GE
Asset risk
Deposition & scaling
30-145
70-220
30-110
0.5-20
0.4-2.1
0.4-6.7
0.1
0-15
2-21
50-170
10-65
1-10
and risks must be considered and addressed.
Even though governmental regulations prescribe
water quality parameters, not all reclaimed
water is created equal. Depending
on the source and the pretreatment methods
used, the concentrations of problematic
contaminants-such as iron, phosphate, and
ammonia-can vary significantly. Moreover,
each water source can have different
concentrations of hardness, silica, and other
contaminants that may limit the maximum
concentration ratio (cycles of concentration)
at which cooling towers may safely operate.
The chemistry of reclaimed water can also
vary significantly over time, with fluctuating
levels of problematic contaminants that
increase the tendency for deposition/scaling,
corrosion, and microbiological growth-all of
which can directly damage cooling water system
components or impede the ability of traditional
cooling water treatment chemicals to
function effectively. The risks (and their causes)
associated with the use of reclaimed water for
tower cooling are summarized in Table 2.
Of particular concern is the possibility of
Cause
High and varying phosphate levels
High levels of calcium hardness, alkalinity, and silicon dioxide
Presence of silt, iron, aluminum, manganese, and copper
High total dissolved solids, sulfate, and chloride (pitting corrosion on
stainless steel)
Corrosion
Presence of ammonia (attacks copper, causing pitting and cracking)
Presence of copper (pitting of low-carbon steel)
Presence of manganese
Microbiological growth
High chemical oxygen demand and biochemical oxygen demand, presence
of ammonia, nutrients (phosphorus and nitrogen), and bacteria
(biofilm formation, leading to pitting and deposition)
High demand for oxidant biocide (bleach), leading to chemical degradation
and high chloride concentration
the U.S., reclaimed water can be treated to
secondary or tertiary levels, as well as to the
minimum requirements as specified by Title
22 of the California Administrative Code.
The chemistries of these varying reclaimed
waters are shown in Table 1.
Secondary treatment includes oxidation
and settlement processes to remove organics
such as oil and grease, while tertiary treatment
adds precipitation and filtration steps
for addressing scale-forming species. Reclaimed
water treated to Title 22 standards-
known as " Title 22 Water " -incorporates
final chlorination to meet specific disinfection
requirements.
Municipal effluent treatment is driven by
Clean Water Act regulations, which set discharge
limits on a range of contaminants and
pollutants. Wastewater treated to at least a
March 2017 | POWER
secondary treatment level includes standards
that are placed on fecal coliform, five-day
biochemical oxygen demand, residual chlorine,
total suspended solids, and pH parameters.
However, what's important to note is
that secondary treatment does not set limits
or requirements on hardness, phosphate, or
total dissolved solids-parameters that are
very important for cooling water treatment.
Still, with only about 6% of the total available
municipal wastewater volume presently
reused in the U.S., the opportunity to utilize
reclaimed water as a makeup water source
for cooling towers is substantial.
Challenges and Risks
In evaluating the feasibility of using reclaimed
water as a cooling tower makeup
water source, a range of unique challenges
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elevated phosphate concentrations, which
present a severe risk of calcium phosphate
deposition in the cooling system. Reclaimed
water can also carry high amounts of fine
particles and heavy metals/metals, which
increase the tendency for scale formation
and fouling. The presence of copper, even at
a very low level-as low as 0.1 ppm-can
cause significant pitting corrosion of lowcarbon
steel. Manganese may create special
concerns with respect to deposition and subsequent
severe pitting corrosion on metals
such as stainless steel. At concentrations
higher than 0.2 ppm, manganese deposition
is very likely in the absence of an effective
treatment. Reclaimed water at detectable
manganese concentrations (0.02 ppm) may
also cause manganese deposition problems.
Another significant challenge includes the
potential for microbiological activity in the
circulating water system. In using reclaimed
water, large populations and numerous types
of microorganisms are carried to the cooling
tower. With high levels of chemical oxygen
demand and total organic carbon loading, including
the presence of ammonia, nitrogen,
and phosphate, cycled cooling water is an
ideal environment for microbiological growth.
As such, effective microbiological control is
extremely important for maintaining efficient
and reliable equipment operation. If left unchecked,
microorganisms can rapidly accumulate
and form biofilms and algae on tower fill,
decking, transfer piping, and heat transfer surfaces.
This can diminish heat transfer efficiency
and promote under-deposit corrosion. Some
microbes also pose significant health concerns.
35
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POWER March 2017

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