CA2517322A1 - Apparatus and method for treating fgd blowdown or similar liquids - Google Patents

Abstract

A process has steps of aerobic treatment to remove COD and nitrify a waste stream and anoxic treatment to denitrify a waste stream followed by anoxic treatment to remove selenium and anaerobic treatment to remove heavy metals and sulphur may be used to treat, for example, FGD blow down water. 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, or (d) ammonia stripping upstream of the biological treatment steps or dilutions step. These processes may be provided in a variety of suspended growth or fixed film reactors, for example a membrane bioreactor or a fixed film reactor having a GAC bed.

Description

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 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 ABMetT"" process by Applied Biosciences Corp of Salt Lake City, Utah, U.S.A.
The entirety of US Patent No. 6,183,644 is incorporated herein by this reference to it.
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 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 S04 is present and converted to sulphur by anaerobic S04 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.

(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, CaS04, 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 concentrations of fine suspended solids, including CaS04 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 ~I 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 % dry 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 ---- 20-4.0 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 CO~/HC03 1500 ppm 1-3,000 ppm Fluoride 360 ppm 61-1600 ppm Iron - Total 1400 ppm 116-6400 ppm 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 (S04) 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 130F 125-130F

[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, _ 7 _ 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 S04 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 _ $ _ 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 _g_ 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, 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.

For example, N03 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

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. The entire disclosures of each of WO 2004/071973; WO
2005/016826 and CA 2,477,333 are incorporated herein by this reference to them.

[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 ZeeWeedT""
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) 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.

Claims (8)

CLAIMS:

We claim:

1. A process having steps of anoxic treatment to denitrify a waste stream followed by anoxic treatment to remove selenium and anaerobic treatment to remove heavy metals, for example arsenic, mercury, or sulphur.

2. The process of claim 1 further comprising a step of aerobic treatment to remove COD from and nitrify the waste stream upstream of the steps of claim 1.

3. The process of claim 1 or 2 further comprising one or more steps of:
a) membrane separation of the waste stream upstream of the step of anoxic digestion to remove selenium;
b) dilution upstream of the digestion steps;
c) physical/chemical pretreatment upstream of the digestion steps or the dilution step to remove TSS and soften the waste stream; or, d) ammonia stripping upstream of the digestion steps or dilution step.

4. An apparatus having one or more reactors configured to provide aerobic treatment of COD, nitrification, denitrification, selenium removal and heavy metal and sulphur removal by biological treatment.

5. The apparatus of claim 4 wherein one of the reactors is a membrane bioreactor.

6. The apparatus of claim 4 or 5 wherein one of the reactors is a fixed film reactor

7. The apparatus of any of claims 4 to 6 wherein the fixed film reactor has a granular activated carbon bed.

8. The apparatus of any of claims 4 to 7 having one or more of an inlet for diluting the feed water to the reactors, a system for adding lime or sulfides to the wastewater upstream of the reactors, a precipitate remover, or an ammonia stripper.