Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: APPARATUS AND METHOD FOR TREATING FGD BLOWDOWN
OR SIMILAR LIQUIDS
FIELD OF THE INVENTION
[0001] This invention relates to water treatment including biological
water treatment and treatment of feeds having high concentrations of
inorganic and organic contaminants, for example scrubber blow down water
from a flue gas desulfurization (FGD) operation in a coal fired power plant.
BACKGROUND OF THE INVENTION
[0002] The following background discussion does not imply or admit
that any process or apparatus described below is citable as prior art or part
of
the knowledge of people skilled in the art in any country.
[0003] Scrubber blow-down water from a flue gas desulfurization
operation in a coal-fired power plant contains a wide range of inorganic
contaminants removed from the flue gas. The blow down water may also
contain organic contaminants, such as di basic acid (DBA), and ammonia
added as part of or to enhance the FGD process. The FGD scrubber blow-
down water may have very high total dissolved solids where the main anions
are chlorides and the main cations are calcium, magnesium and sodium. The
rate of blow-down may be controlled to maintain a desired chloride
concentration causing the blow-down water to have a high, but generally
stable chloride concentration. The concentration of other contaminants may
vary widely as influenced, for example, by burning coal from different sources
even in a single power plant. However, the concentration of TDS, TSS, Ca
and Mg hardness, nitrate, ammonia, and sulfur for example as sulphate are all
likely to be high, and various heavy metals may be present, making the blow
down water very difficult to treat, particularly to achieve very low levels of
contaminants.
[0004] Current methods of treating blow down water rely heavily on
physical and chemical processes to remove inorganic contaminants. The
physical and chemical processes also involve costly chemicals and produce
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large amounts of sludge. Arsenic, mercury and heavy metals may also be
present in the blow down water at above regulated levels. Further, some
jurisdictions have recently regulated selenium concentrations in effluents
discharged to the environment. The permitted concentration of selenium may
be 0.5 ppm or less or 200 ppb or less while the blow down water may contain
1-20 or 2-10 ppm of selenium which is not removed in conventional treatment
plants.
[0005] In U.S. Patent No. 6,183,644, entitled Method of Selenium
Removal and issued on February 6, 2001 to D. Jack Adams and Timothy M.
Pickett, dissolved selenium is removed from contaminated water by treating
the water in a reactor containing selected endemic and other selenium
reducing organisms. Microbes may be isolated from the specific water or
imported from other selenium contaminated water. The microbes are then
screened for ability to reduce selenium under the site specific environmental
conditions. The selected microbes are optimized for selenium reduction, then
established in a high density biofilm within a reactor. The selenium
contaminated water is passed through the reactor with optimized nutrient mix
added as needed. The elemental selenium is precipitated and removed from
the water. Products using this or a similar process have been sold as the
ABMetTm process by Applied Biosciences Corp of Salt Lake City, Utah, U.S.A.
SUMMARY OF THE INVENTION
[0006] The following summary is intended to introduce the reader
to the
invention but not to define it. The invention may reside in any combination of
one or more of the apparatus elements or process steps described anywhere
in this document.
[0007] It is an object of this invention to improve on, or at
least provide
a useful alternative to, the prior art. It is another object of the invention
to
provide a wastewater treatment process or apparatus. Other objects of the
invention are to provide an apparatus or process for treating FGD blow down
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water or other wastewaters having selenium or nitrate and selenium, or a
process or apparatus for biologically removing inorganic contaminants, for
example nitrogen, selenium, arsenic, mercury or sulphur, from waste water.
[0008] In one aspect, the invention provides a process having steps
of
aerobic treatment to remove COD and nitrify a waste stream, anoxic
treatment to denitrify a waste stream, anoxic treatment to remove selenium
and anaerobic treatment to remove heavy metals or sulphur or both. Removal
of heavy metals is possible because SO4 is present and converted to sulphur
by anaerobic SO4 reducing bacteria. The process may further include one or
more of (a) membrane separation of the waste stream upstream of the anoxic
digestion to remove selenium, (b) dilution upstream of the biological
treatment
step, (c) physical/chemical pretreatment upstream of the biological processes
or dilution step to remove TSS and soften the waste stream, for example
through the addition of lime or sulfides and the removal of precipitates or
(d)
ammonia stripping upstream of the biological treatment steps or dilution step.
Some of the biological treatment steps may be performed in a fixed film
reactor, for example a granular activated carbon bed. One or more of the
biological treatment steps may also be performed in a suspended growth
reactor such as a membrane bioreactor. Each biological treatment step may
be performed in a distinct reactor optimized to perform a step or two or more
of the biological treatment steps may be performed in a multiple purpose
reactor.
[0009] In another aspect, the invention provides an apparatus having
one or more reactors configured to provide aerobic treatment of COD,
nitrification, denitrification, selenium and heavy metals removal and sulphur
removal by biological treatment. The reactors may include a membrane
bioreactor or a fixed film reactor. The fixed film reactor may comprise a
granular activated carbon bed. The apparatus may further have one or more
of an inlet for diluting the feed water to the biological processes, a system
for
adding lime or sulfides to the wastewater upstream of the biological reactors,
a precipitate remover, or an ammonia stripper.
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[0010] The invention is particularly suited for treating FGD blow
down
water to produce an effluent with low concentrations of selenium, for example
1 ppm or less or 10 ppb or less, and low concentrations of total nitrogen, for
example 1 mg/L or less or 10 ppm or less, in the effluent. However, the
invention may also have applications in treating blow down water when
selenium concentration in the effluent is not a concern. The invention may
also be useful for treating other wastewaters having selenium, for example
mining, contaminated ground or surface water streams, or petroleum refinery
waste streams, particularly where the waste stream also has significant
concentrations of one or more of COD, nitrate, ammonia, TDS, TSS,
hardness, CaSO4, or sulphate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Examples of embodiments of the invention will be described
below with reference to the Figures described below.
[0012] Figure 1 is a schematic flow diagram of an apparatus and
process for treating water.
[0013] Figure 2 is a more detailed process flow diagram of an
exemplary system for treating FGD blow down water.
[0014] Figure 3 is a schematic diagram showing an alternate
embodiment for part of the system and process of Figure 1 or 2.
DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0015] Table 1 shows the contaminants, and their concentrations,
assumed for FGD scrubber blow-down water in the design of the following
exemplary embodiments. FGD blow-down water may exist with other
contaminants or other concentrations of contaminants. The composition of
FGD blow-down can also vary widely over time for a specified coal-fired
power plant as influenced, for example, by changes in the source of coal.
However, FGD blow-down water is generally characterized by very high total
dissolved solids (TDS) where the main anion is chloride and the main cations
are calcium, magnesium and sodium. The blow-down also contains significant
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concentrations of fine suspended solids, including CaSO4 fines. The blow-
down also contains a wide range of inorganic contaminants, including
ammonia, which is added for selective catalytic reduction in the scrubbing
process, as well as some organics, particularly DBA (dibasic acid) added to
enhance scrubber efficiency. In the exemplary embodiments, the effluent is
intended to have a total nitrogen (TN) content of 10 ppm or less and selenium
concentrations of 0.4 ppm or less.
TABLE 1: Typical FGD Blowdown Water
Parameter Typical Value Min - Max
Chlorides 30,000 ppm 20-40,000 ppm
pH >5.0<6.0
TDS 75,000 mg/L 50,000 ¨ 150,000 mg/L
TSS 2 % dry wt 1 ¨ 5 % d ry wt
Aluminum ¨ Total 960 ppm 80-3700 ppm
Antimony 12 ppm 0.03-49.0 ppm
Ammonia ¨ N 31 ppm 0.25-64 ppm
Nitrate ¨ N 350 ppm 200-450 ppm
Total Nitrogen 200 ppm 50-400 ppm
Arsenic ¨ Total 15 ppm 0.27-100 ppm
Barium ¨Total 100 ppm 2.0-770 ppm
Beryllium 2.1 ppm 0.06-6.9 ppm
Boron ---- 20-40 ppm
Cadmium¨Total 0.8 ppm 0.12-1.5 ppm
Calcium 18,000 ppm 10,000-30,000 ppm
Chromium ¨ Total 23 ppm 0.5-210 ppm
Chromium VI ---- 3-12 ppm
Cobalt ---- 0.05-4 ppm
Copper ¨ Total 1.7 ppm 0.3-6.6 ppm
CO3/HCO3 1500 ppm 1-3,000 ppm
Fluoride 360 ppm 61-1600 ppm
Iron ¨Total 1400 ppm 116-6400 ppm
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Lead ¨Total 19 ppm 0.2-140 ppm
Lithium 2-3 ppm
Magnesium 15,000 ppm 10,000-20,000 ppm
Manganese 10 ppm 3.6-200 ppm
Mercury ¨ Total 0.38 ppm 0.5-1.4 ppm
Nickel ¨Total 10 ppm 0.5-74 ppm
Phosphate ¨Total 1.0 ppm 0-10 ppm
Potassium 6800 ppm 5000-10,000 ppm
Selenium ¨Total 17 ppm 1.5-100 ppm
Silver ¨ Total 10.0 ppm 0.002-20 ppm
Sodium 15,000 ppm 10,000-20,000 ppm
Sulfate (SO4) 60,000 ppm 40,000-80,000 ppm
Thallium 0.76 ppm 0.02-2.2 ppm
Vanadium 1.0-11.0 ppm
Total Zinc 15.0 ppm 1.7-50.0 ppm
Temperature 130 F 125-130 F
[0016] In greater detail, treating the blow-down raises several
challenges which the invention addresses in a variety of ways, as described
generally below and then by describing examples of treatment systems and
processes. In the following description, pre-treatment refers to treatment
occurring upstream of biological process steps.
[0017] High TDS concentrations make it difficult to maintain activity
for
biological treatment. This issue is addressed by diluting the waste stream
upstream of biological treatment. The high TDS also makes it difficult to
flocculate and settle biomass in an activated sludge process. This issue is
addressed by using fixed film bioreactors or membrane bioreactors.
[0018] High hardness causes scaling due to Ca or Mg oversaturation
and any pH or temperature shifts, for example during denitrification, may
cause precipitation of calcium or magnesium sulfates or carbonates. This
issue is addresses by a softening pre-treatment, for example lime softening,
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and optionally by adding acid for pH adjustment upstream of the biological
process.
[0019] The high TSS, particularly because it is essentially inorganic,
causes problems with developing and controlling a suspended biomass and
with bioreactor and membrane plugging. This issue is addressed by pre-
treating the waste stream to coagulate or flocculate and then physically
remove (for example by settling or floating) suspended solids.
[0020] High nitrate concentrations are a concern because nitrate is a
preferred electron acceptor for biological reduction over selenate. This issue
is addressed by decreasing the nitrate concentration upstream of a selenate
reducing step. The nitrate reducing step may occur in an upstream part of a
plug flow selenate reducing reactor, as part of a multi-step biological
process
or reactor upstream of a selenate reducing process, or part of a distinct
process or reactor.
[0021] Ammonia in the blow down water is a concern because
concentration in the final effluent may be regulated and because oxidation of
ammonia may increase nitrate concentration. This issue is addressed by
removing the ammonia, for example, by stripping the ammonia as NH3 in a
pre-treatment process or removing ammonia biologically by a
nitrification/denitrification process either in a series process or with
recirculating flows.
[0022] The presence of various heavy metals, for example Cu, As or
Hg, or related oxidized contaminants are a concern because they may be
regulated in the effluent but are difficult to remove in low concentrations.
This
issue may be addressed in part by precipitating these elements out in a pre-
treatment softening step. The issue may be further addressed by biologically
reducing SO4 and precipitating these contaminants as metal sulfides after
removing nitrate and selanate and selenite.
[0023] The presence of selenium, as selenate or selenite, is a concern
because of recent regulation of selenium concentrations in effluent. The
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selenium is difficult to remove because of its low concentration and its
tendency to form selenate or selenite and dissolve in water making physical or
chemical removal difficult, costly or inefficient. Selenium is addressed in
the
invention by biologically reducing it to elemental selenium and then
precipitating it for removal.
[0024] Figure 1 shows a treatment system 10 having a pretreatment
area 12 upstream of a biological treatment area 14 . Feed 16, which may be
FGD blow-down water or another feed, flows into pretreatment area 12. In the
pretreatment area 12, a large portion of the TSS in the feed is removed and
Ca and Mg are removed to soften the feed 16. The pretreatment area 12 uses
physical/chemical methods to treat the feed 16. For example, lime or sulfides
or both may be added to the feed 16 to precipitate calcium, magnesium and
metals. The precipitates may be removed by clarifiers, for example single
stage or double stage clarifiers. Settling can be enhanced by the addition of
coagulants or polymers. Alternately, the precipitates can be removed by
dissolved air floatation (DAF) involving the addition of similar treatment
chemicals. The DAF process also strips some ammonia particularly if feed 16
temperature is kept above 100 degrees F or above 130 degrees F and the
DAF process maintained at a pH of 8.5 or more or 9.5 or more. This may
reduce or remove the need for ammonia removal, by nitrification, at the
biological treatment area 14 described below in jurisdictions where emitting
ammonia in a blow off gas is permitted. Optionally, the ammonia in the gas
blow-off from a DAF reactor may be recycled to a selective catalytic reduction
part of the FGD process. DAF reactors may be purchased from Infilco-
Degremont of Virginia, USA under the AquaDAF trade mark or from Clear
Water Technology of California, USA under the GEM trade mark.
[0025] Pre-treatment effluent leaves the pretreatment area 12 through
a pretreatment effluent line 20. Dilution water 18 is added to the
pretreatment
effluent. The dilution water 18 reduces the TDS of the pretreatment effluent
to
make it acceptable for biological treatment downstream. Sufficient dilution
water 18 may be added to make the TDS like that of seawater, for example
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with a TDS of 35 g/L or less. Any low TDS water can be used for dilution
water 18, for example cooling circuit blow down water from a power plant. The
dilution water 18 also cools the FGD blow-down water, for example from 50
C or more to about 40 C or less, to further facilitate biological treatment.
[0026] The diluted
pretreatment effluent then flows to the biological
treatment area 14. The biological treatment area 14 has four zones: an
aerobic zone 22; a first anoxic zone 24; a second anoxic zone 26; and, an
anaerobic zone 28. These zones 22, 24, 26, 28 are shown connected in
series in Figure 1 although one or more of them may alternately be connected
with recirculation loops. Further alternately, some of the zones 22, 24, 26,
28
may not be required in some embodiments. The zones 22, 24, 26, 28 may
also occur in separate reactors or one or more zones 22, 24, 26, 28 may be
combined into a single reactor. One or more of nutrient streams 30, 32, 34
may be used to add nutrients to any zone 24, 26, 28 downstream of a nutrient
stream 30, 32, 34, either directly or through intervening zones 24, 26. For
example, nutrients may be added in stream 30 or stream 32 to support the
growth of bacteria in zones 26 or zone 28 or both.
[0027] The aerobic
zone 22 is used to nitrify ammonia, to the extent
that ammonia has not been stripped in the pretreatment area 12, and to
oxidize organic carbon. An optional supplemental aerobic zone may also be
added downstream of the anaerobic zone 28 to remove residual nutrients
added before or in zones 24, 26, 28 and to oxidize residual compounds from
anaerobic zone 28. If there is no TN discharge limit for the effluent, or if
ammonia is stripped in the pretreatment area 12 such that TN in the effluent
will be acceptable, the aerobic zone 22 may be omitted, or replaced by an
aerobic zone downstream of the anaerobic zone 28.
[0028] In the first
anoxic zone 24, nitrate acts as a preferred electron
acceptor and is removed by denitrification. The nitrate may be removed to a
concentration which facilitates the biological reduction of selenium in the
second anoxic zone 26, considering that nitrate in high concentration will be
used as an electron acceptor over low concentrations of selenate or selenite.
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For example, NO3 may be reduced to 10 mg/L as N or less or 1 mg/L as N or
less or 10 ppm as N or less in the stream leaving the first anoxic zone 24.
[0029] In the second anoxic zone 26, selenium is removed by
biological
reduction and removal, for example by precipitation into flush flow water or
waste sludge. These steps may occur, for example, according to the process
described in US Patent No. 6,183,644 or in other fixed or suspended bed
reactors. The reactors may be seeded with selenium reducing organisms.
[0030] In the anaerobic zone 28, sulfate-reducing bacteria reduce
sulfates and produce sulfides in the form of H2S or HS-. Part of the HS- may
react with soluble metals to form insoluble metal sulfides which may
precipitate out of solution. In this way the anaerobic zone removes heavy
metals. The off gas from the anaerobic step 28 can be recycled to the aerobic
step 22 or to a downstream aerobic step to reduce the production of odors
associated with H2S.
[0031] In general, the zones 22, 24, 26, 28 may be arranged into one or
more reactors. Each zone 22, 24, 26, 28 may occupy its own reactor, for
example a CSTR optimized to reduce the primary target contaminant of each
zone 22, 24, 26, 28. Alternately, for example, zones 22 and 24 can be
combined into a combined nitrification/denitrification reactor which may have
1, 2 or more tanks. Zones 24, 26 and 28 or 26 and 28 may be combined into a
ABMet reactor having one or more tanks. Other reactors may also be used.
For suspended growth reactors, the limited concentrations of the target
contaminants may be low and the presence of other contaminants may make
biomass separation difficult and so membrane bioreactors are preferred.
Alternately, fixed film reactors may be used, for example Moving Bed
Bioreactors, for example as produced by Anox Kaldnes of Norway, fluidized
bed reactors, for example as produced by Shaw Envirogen of New Jersey,
USA, biofilters as produced by Degremont of France under the trade mark
BIOFOR, granular activated carbon reactors, for example as produced by the
Applied Biosciences Corp. of Utah, USA under the ABMet trade mark, or in
membrane supported biofilm reactors (MSBR) as described in PCT
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Publication Nos. WO 2004/071973 or WO 2005/016826. Depending on the
zone, the MSBR may operate autotrophically or heterotrophically optionally
using a process of heterotrophic denitrification as described in Canadian
Patent Application No. CA 2,477,333. Membrane separation may optionally
be used with or after any fixed film reactor although there may also be no
need for it.
[0032] Figure 2
shows a process flow diagram of a treatment plant 50
for treating FGD blow-down as described in Table 1. In the pretreatment area
12, pretreatment is by lime softening, optionally with sulfide addition, and 1
or
2 stage settling in clarifiers. Such a process may be provided by WesTech of
Utah, USA. PH is adjusted in the pretreatment effluent 20 when the dilution
water 18 is added to a pH of less than 8.5, for example between 6 and 8 to
enhance biological treatment. In the aerobic zone 22, a membrane bioreactor
having an aeration tank and a membrane tank containing ZeeWeedTM
membranes from Zenon Environmental Inc. of Ontario, Canada, is used for
nitrification. The first and second anoxic and anaerobic zones 24, 26, 28 are
provided in an ABMet reactor system by Applied Bioscience Corp. of Utah,
USA, which may consist of a 2-stage reactor configuration. This reactor is an
up-flow fixed film reactor using a GAC bed operated in plug flow so that 3
biological zones corresponding to the first anoxic, second anoxic and
anaerobic zones 24, 26, 28 can be established in sequence. A single nutrient
stream 30 is used upstream of the ABMet reactor. Precipitates are removed
from this reactor by periodically flushing the GAC bed to overflow troughs.
Sludge from the pretreatment area 12 and biological treatment area 14 is fed
to a sludge thickener and dewaterer. Thickened and dewatered sludge is sent
to waste. Sludge thickening and dewatering effluent is returned to an
equalization tank to be mixed with the FGD flow down feed water 16.
[0033] Figure 3
shows a nitrification/denitrification reactor 80 used to
provide the aerobic and first anoxic zones 22, 24 of Figure 1. Nitrification/
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denitrification reactor 80 may also be used to replace the bioreactor tank and
ZeeWeed tank of Figure 2 to provide the aerobic and first anoxic zones 22, 42
and allow the ABMet reactor to be operated with the second anoxic and
anaerobic zones 26, 28 with a minimal or no first anoxic zone 24.
