MANAGEMENT OF BLOWDOWN FROM CLOSED LOOP COOLING SYSTEMS USING IMPAIRED WATERS by Yinghua Feng B.S. in Environmental Science, Renmin University of China, 2008 Submitted to the Graduate Faculty of Swanson School of Engineering in partial fulfillment of the requirements for the degree of Master of Science University of Pittsburgh 2010
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MANAGEMENT OF BLOWDOWN FROM CLOSED LOOP COOLING SYSTEMS USING IMPAIRED WATERS
by
Yinghua Feng
B.S. in Environmental Science, Renmin University of China, 2008
Submitted to the Graduate Faculty of
Swanson School of Engineering in partial fulfillment
of the requirements for the degree of
Master of Science
University of Pittsburgh
2010
ii
UNIVERSITY OF PITTSBURGH
SWANSON SCHOOL OF ENGINEERING
This thesis was presented
by
Yinghua Feng
It was defended on
November 11, 2009
and approved by
Radisav D. Vidic, Professor, Department of Civil and Environmental Engineering
Willie F. Harper, Associate Professor, Department of Civil and Environmental Engineering
Jason D. Monnell, Research Assistant Professor, Department of Civil and Environmental
Engineering
Thesis Advisor: Radisav D. Vidic, Professor, Department of Civil and Environmental
Table 1. Treatment technologies and types of discharge of blowdown form power plants using reclaimed waters. ................................................................................................................. 4
Table 2. Chemical composition of the blowdown waters from pilot-scale cooling towers ............ 9
Table 4. Chemical composition of the MWW blowdown water representing 4 cycles of concentration synthesized for membrane filtration tests. .................................................. 36
Table 5. Chemical composition of the AMD blowdown water representing 4 cycles of concentration synthesized ................................................................................................. 36
Table 6. Characteristics of the membranes used in the study. ...................................................... 37
Table 8. Permeate quality for actual MWW blowdown filtered with NF membranes (Units: mg/L). ................................................................................................................................ 45
Table 9. Permeate quality for synthetic AMD blowdown with different NF membranes (Units: mg/L). ................................................................................................................................ 56
Table 10. Permeate quality for actual AMD blowdown (Units: mg/L). ....................................... 58
ix
Table 11. Permeate quality for actual AMD blowdown filtered through two NF membranes in series .................................................................................................................................. 58
Table 12. Comparison of treatment efficiency of BW30 and NF90 with synthetic MWW and AMD blowdown. .............................................................................................................. 59
Table 13. Comparison of treatment efficiency of BW30 and NF90 with actual MWW and AMD blowdown. ......................................................................................................................... 59
Table 14. OPUS system details for MWW blowdown. ................................................................ 66
Table 15. OPUS system details for AMD blowdown. .................................................................. 67
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LIST OF FIGURES
Figure 1. Schematic of a recirculating cooling water system in a coal-fired power plant. ............. 2
Figure 2. Typical wastewater treatment processes in WWTPs. .................................................... 10
Figure 3. Panda Brandywine power plant systems. ...................................................................... 11
Figure 5. Typical reverse osmosis system containing several modules. (Holiday, 1982) ............ 15
Figure 6. Schematic illustration of a typical electrodialysis system. (Holiday, 1982) ................. 17
Figure 7. Evaporation ponds. (Source: BC Technologies Ltd.) .................................................... 21
Figure 8. Schematic illustration of a typical vapor compression evaporation process. (Zeien, 1980) ................................................................................................................................. 23
Figure 9. San Juan Generating Station. (Source: PNM Company. “San Juan Generating Station.”) ........................................................................................................................... 27
Figure 12. Schematic diagram of the membrane filtration system used for blowdown treatability studies. .............................................................................................................................. 34
Figure 13. a) Sulfate and b) phosphate concentration variation in SMWW permeate ................. 41
Figure 14. Repeatability of permeate flux with BW30 membrane tested on synthetic MWW blowdown .......................................................................................................................... 42
Figure 15. Average permeate flux with confidence interval using BW30 membrane .................. 43
Figure 16. Permeate flux with NF membranes tested on a) synthetic MWW blowdown ............ 47
Figure 17. Water flux of the BW30 membrane for synthetic and actual MWW blowdown waters. ........................................................................................................................................... 49
Figure 18. SEM images of the BW30 membrane after filtering (a) synthetic and (b) actual MWW blowdown for 6 h at TMP = 135 psi. ................................................................................ 50
Figure 19. Impact of TMP on water flux of BW30 operated with actual MWW blowdown. ...... 52
Figure 20. Impact of unit TMP on water flux of BW30 operated with actual MWW blowdown at different pressure. ............................................................................................................. 52
Figure 21. Impact of pH adjustment on permeate flux with BW30 operated with actual MWW blowdown .......................................................................................................................... 53
Figure 22. SEM images of the BW30 membrane after filtering the actual MWW blowdown .... 55
Figure 23. Water flux of NF membranes tested on a) synthetic AMD water and b) actual AMD blowdown .......................................................................................................................... 61
Figure 24. Water flux of actual AMD blowdown for RO and sequential NF filtration. .............. 62
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Figure 25. Processes diagram of OPUS technology developed by N.A.Water Systems, ............. 63
Figure 26. Schematic of RO process in OPUS technology. ......................................................... 65
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ACKNOWLEDGEMENTS
I would like to acknowledge and thank my advisor Dr. Radisav D. Vidic for his support and
guidance on my thesis and related research project. In addition, I will give my great thanks to Dr.
Jason D. Monnell for his unreserved help for me to achieve my research and academic goals.
This invaluable help undoubtedly makes my life and research work here much easier. Of course,
a great thank also goes to Dr. Willie F. Harper for his encouragement and assistance in the whole
defense.
This study was supported by the National Energy Technology Laboratory (NETL),
USDOE. I would like to thank all my colleagues and friends, Sean Shih, Heng Li and Mingkai
Shang for their support and help with daily work and study.
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1.0 INTRODUCTION
Cooling tower operation includes periodic discharge of concentrated recirculating water in order
to maintain desired cycles of concentration and control the buildup of solids in the system
resulting from continuous influx of solids with makeup water and evaporative losses in the
system (Figure 1). This periodic discharge of recirculating water is called blowdown and it
contains elevated levels of solids as well as chemicals that are typically added to the system to
control corrosion, scaling and biofouling. Due to fairly low water quality, blowdown is typically
subjected to some level of treatment in order to meet discharge requirements that are governed
by the final disposal options. The primary objective of blowdown management is to treat
blowdown to attain quality that is equal to or better than the makeup water so that it can be
reused as makeup water. Alternatively, blowdown may be subjected to different treatment
processes to meet discharge requirements or to minimize its volume for easier disposal.
2
Figure 1. Schematic of a recirculating cooling water system in a coal-fired power plant.
3
2.0 BLOWDOWN MANAGEMENT OPTIONS
Management options available for cooling tower blowdown typically depend on its water
quality, local discharge regulations and capabilities of treatment processes under consideration.
Typical options for power plant blowdown management include:
- Discharge to surface waters. This is the main option for once-through cooling systems
that is not feasible for recirculating cooling systems because of the blowdown quality.
- Discharge to wastewater treatment plants (WWTPs): This is probably the most cost
effective management alternative but may not be feasible for many plants since the WWTP may
not accept the blowdown without any pre-treatment due to extremely high solids and presence of
other chemicals in the blowdown that were added to control corrosion, scaling and biofouling.
- Zero liquid discharge (ZLD): This alternative involves extensive treatment of
blowdown to facilitate its reuse combined with some form of volume reduction to minimize or
eliminate the need for liquid discharge. As seen in Table 1, most power plants using wastewater
for cooling would choose this option where the concentrated solids are the only waste leaving the
plant.
4
Table 1. Treatment technologies and types of discharge of blowdown form power plants using reclaimed waters.
Plant Name State Type of
discharge Treatment technologies
Magnolia CA ZLD Lime-soda softening, media filtration, RO, evaporator, evaporation pond
Emery IA WWTP Panda Brandywine MD WWTP
Jones Station TX ZLD, irrigation Evaporation pond San Juan NM ZLD Evaporator, evaporation pond, RO Linden NJ WWTP Nixon CO ZLD RO, evaporator MVPP CA WWTP, recycle RO
Palo Verde AZ ZLD Evaporation pond Walnut Creek Energy Park CA WWTP
Notes: ZLD: zero liquid discharge.
WWTP: discharge to wastewater treatment plants or sanitary sewer system.
2.1 DIRECT DISCHARGE TO SURFACE WATERS
Federal, state and local regulations govern the discharge requirements for cooling system
blowdown. If allowed, direct discharge of blowdown to surface water or ground water would be
a simple and economical choice. However, the assimilative capacity of receiving water to handle
the blowdown is limited and the quality of blowdown has to meet criteria that address the
regulatory requirements for public health and environmental protection. Considering the cooling
tower blowdown quality, discharge to surface water is only feasible for once-through cooling
systems.
5
At federal level, Clean Water Act (CWA) Section 402 established the National Pollutant
Discharge Elimination System (NPDES), which requires that all point-source discharges of
pollutant to surface waters must be authorized by an NPDES permit. General NPDES permits
can be water quality based or technology based. The water quality limitations, which are mainly
concerned with the concentrations of toxic chemicals, depend on the quality of the receiving
stream and its assimilative capacity. For each point source with NPDES permit, its water quality
is calculated, monitored, and regulated based on the surface water quality limitation. Based on
different technologies implemented in cooling tower design and operation, the concentrations of
available chlorine, chromium, and zinc are likely to be the confining factors. Otherwise, the
effluent guidelines title 40, part 423 of Code of Federal Regulations (40 CFR 423) specifying a
30-30 rule for both TSS and BOD (30 mg/L each) with a maximum concentration of 100 mg/L
for TSS and are applicable to cooling tower blowdown as a categorical waste. However, site
specific evaluation is usually needed for a given point discharge.
6
The state regulations or guidelines often include restrictions that limit the level of
bacteria in blowdown (i.e., fecal or total coliforms). Furthermore, certain chemical constituents
may also be a concern when present in excessive quantities. For example, in April 2009,
Pennsylvania DEP released a “Permitting Strategy for High Total Dissolved Solids (TDS)
Wastewater Discharges” which specifies 500 mg/L (max 750 mg/L) for TDS and 250 milligrams
per liter for chlorides and sulfates, if the discharge is located in the Monongahela River
watershed.
In summary, each cooling tower is subjected to different local regulations based on the
blowdown characteristics and intended discharge alternative and there is no unifying standard for
cooling tower blowdown discharge, especially when impaired waters are used as makeup water.
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2.2 DISCHARGE TO WASTEWATER TREATMENT PLANTS
Blowdown discharge to a local wastewater treatment plant (WWTP) may be an attractive and
economical option if accepted by the WWTP. The acceptance may be contingent on chemicals
present in the blowdown, especially for the power plants that do not include tertiary treatment,
because of the potential to compromise its own NPDES permit. Blowdown discharge to a
WWTP reduces the burden on power plants but increases the demand on local WWTPs. The size
and treatment options are the two key factors that determine whether a WWTP can accept the
cooling tower blowdown from a thermoelectric power plant.
Table 2 provides chemical composition of blowdown samples that were collected from
pilot-scale cooling towers operated with two different impaired waters: treated acid mine
drainage (AMD) and secondary municipal wastewater (MWW). The data in Table 2 indicate that
high TDS in blowdown from towers operated with AMD and those operated with MWW as well
as high sulfate concentration in the towers operated with AMD are the main characteristics of
concern.
Usual secondary treatment in WWTPs (Figure 2) includes biological treatment step in the
aeration tank, which is the key step in treating organic compounds in municipal wastewaters. If
the TDS in the feed water is too high, it will adversely impact microbial activity in the aeration
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tank and reduce the overall treatment efficiency of the WWTP. Therefore, every WWTP has
pre-treatment requirements for each industrial contributor and will only accept the blowdown
from a thermoelectric power plant if it will not compromise its treatment efficiency.
9
Table 2. Chemical composition of the blowdown waters from pilot-scale cooling towers
(all units are in mg/L unless specified otherwise).
Constituent AMD MWW
Ca 800 140 Mg 238 50 Na 446 700 K 29 60 Al ND 0.2 Cu ND 0.2 Fe ND 0.5 Mn 0.2 0.4 Zn ND 0.2 Cl 216 1030 SO4 2930 300 SiO2 59 25 NO3-N 1.1 16 NH3-N 0.6 2.2 PO4 0.6 15
The high TDS observed in both AMD and MWW blowdown and high sulfate
concentration in AMD blowdown are the main concern in blowdown disposal. The treatment
objectives of cooling tower blowdown is to recover portion of the blowdown that will have the
quality equal to or better than the makeup water so that it can be returned to the cooling loop.
The treatment needs for the blowdown from the pilot-scale cooling towers operated with
secondary treated municipal wastewater (“MWW blowdown”) and acid mine drainage (“AMD
blowdown”) were independently assessed in this study. The two types of blowdowns were
collected from the cooling towers and analyzed for their chemical characteristics and one
treatment alternative, membrane filtration, were analyzed through feasibility studies.
71
Treatability studies with various types of membranes including MF, UF, NF and RO,
were carried out for the MWW and AMD blowdown in a bench-scale membrane filtration unit.
A variety of operating parameters, including membrane types, trans-membrane pressure (TMP),
and feed water pH, were tested for optimal treatment performance in terms of permeate flux and
water quality.
Bench-scale experiments provide evidence that nanofiltration with BW30 membrane is
an effective treatment method to reduce concentration of dissolved species, including sulfate and
TDS in MWW blowdown: TDS and sulfate decreased from 3,060 and 326 mg/L to 379 and 31
mg/L, respectively. Preliminary studies indicate that maintaining a trans-membrane pressure of
135 psi is capable of providing acceptable permeate flux; increasing TMP to 200 psi did not
show significant increases in flux due to potentially adverse impact of increased pressure on
membrane fouling. Furthermore, lowering pH of the feed water from 7.4 to 6.0 resulted in a
higher permeate flux, while increasing it to 9.0 resulted in lower permeate flux. However, a
detail cost benefit analysis of the pH adjustment is required before such approach can be
recommended.
Treatment of actual AMD blowdown required sequential filtrations by NF90 and BW30
membranes (NF90-BW30) to decrease the TDS and sulfate from 5,810 and 3,079 mg/L to 192
and 107 mg/L, respectively.
72
5.0 FUTURE WORK
Based on the results of this study, it can be concluded that it is feasible and effective to utilize
nanofiltration (separate or sequential) to treat cooling tower blowdown. However, the long-term
performance of such a system and wastewater recovery needs to be demonstrated in order to gain
confidence in the proposed approach. Similar to the preliminary design of the OPUS system, a
number of elements will have to be used to achieve water recovery above 90%. It will be
necessary to conduct pilot-scale testing on several different blowdowns to develop a relationship
between permeate flux and water recovery in order to obtain preliminary design data for a full-
scale system.
In addition, establishing the relationship between the key operating parameters (e.g.,
pressure, cross flow velocity) and permeate flux is needed to achieve optimal flux at a minimum
cost. Also, studies on preventing or reducing membrane fouling to improve the lifetime and
performance of nanofiltration system (e.g., pretreatment needs, cleaning approach and
frequency) must be conducted before full-scale implementation of the proposed approach for
blowdown treatment.
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