Abstract
An economical method for recovering phosphate or phosphate and nitrogen from liquid streams. A liquid containing phosphate is introduced into a culture of autotrophic microorganisms in the presence of natural or artificial light, thereby producing a liquid effluent with elevated pH and reduced alkalinity. The alkalinity is reduced through the consumption of bicarbonate/carbonate by the autotrophic microorganisms. The effluent is then chemically treated with low-cost chemicals to provide Ca.sup.++ or Mg.sup.++ ions necessary to form a phosphate precipitate such as calcium phosphate or magnesium-ammonium-phosphate (MAP). The autotrophic microorganisms can be cultivated in ponds, lagoons, or photobioreactors. The pH of the culture is adjustable within a preferred range of 7.5 to 10.5 by adjusting the photobioreactor operation. The process includes an economical flotation separator for solid, liquid, gas separation and a means of concentrating ammonia nitrogen that may also be removed during the process of phosphate reclamation.
Claims
1. A process for separating solids, liquid and gases in a liquid influent containing solids and gases, including ammonia and carbon dioxide, comprising the steps of: (a) feeding the liquid influent into a separator, said separator having top and bottom portions joined by an intermediate portion; (b) maintaining the ammonia as ammonium by maintaining the pH of the liquid influent within the separator at less than about 8.3 and at a selected operating temperature; (c) feeding a sparging gas into the bottom and/or intermediate portion of the separator, thereby causing solids within the separator to float in the top portion of the separator as floated solids together with sparging gas, carbon dioxide and influent liquid; (d) vacuum suctioning through a vacuum outlet of the separator the floated solids and gases from the top portion of the separator into a receiver; and (e) removing from the separator a liquid effluent, said liquid effluent having an elevated pH and lowered carbon dioxide concentration compared to the liquid influent.
2. The process of claim 1, further comprising adding one or more coagulants to the liquid influent prior to step (a).
3. The process of claim 1, further comprising, in step (c) pressurizing the sparging gas with a blower or compressor.
4. The process of claim 1, further comprising removing solids from the separator through a bottom outlet thereof.
5. The process of claim 1, further comprising pumping the solids collected in a bottom portion of the separator therefrom as separated solids.
6. The process of claim 1, wherein the liquid effluent removed from the separator in step (e) of claim 1 is used to remove carbon dioxide, and any toxic gases that may be present, from a nutrient-laden, aqueous liquid containing phosphate, bicarbonate, and ammonia, said liquid having nonzero bicarbonate alkalinity.
7. The process of claim 1, further comprising alternately raising and lowering the liquid level in the separator above and below the vacuum outlet, and thereby remove flotation gas or float solids from the separator, respectively.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIG. 1 presents a schematic of the process steps for removal and or reclamation of phosphate from liquid streams. It is a four-step process with the first step being an optional step as represented by dashed lines.
(2) FIG. 2 presents a block diagram of the process with the optional step of precipitation with or without the use of chemicals within the phototrophic growth reactor.
(3) FIG. 3 presents a schematic of the process that includes nitrogen removal.
(4) FIG. 4 presents a schematic of an integrated process to remove both nitrogen and phosphate as calcium phosphate.
(5) FIG. 5 presents a schematic of an integrated process to remove both nitrogen and phosphate as MAP.
(6) FIG. 6 presents the prior art pretreatment gas solid separation unit for removing both suspended solids and CO.sub.2 gas.
(7) FIG. 7 A presents a plan view of a rotating photobioreactor.
(8) FIG. 7 B presents an elevational view of the rotating photobioreactor of FIG. 7A.
(9) FIG. 8 presents a process for concentrating the stripped ammonia for reuse or other productive purpose.
(10) FIG. 9 presents an improved gas solids separator for continuously removing suspended solids.
DETAILED DESCRIPTION OF THE INVENTION
(11) The essence of this invention is to reduce the use of costly chemicals commonly used to raise the pH of a liquid stream for the removal of phosphate and the creation of phosphate products that are removed through any of a variety of separation techniques commonly used in chemical engineering applications. Typically, the chemicals used to precipitate soluble orthophosphate include NaOH, MgO, MgOH and CaOH. Those chemicals raise the pH and supply the calcium or magnesium ions necessary to form magnesium-ammonium-phosphate (MAP), apatite, monocalcium (Ca(H.sub.2PO.sub.4).sub.2) dicalcium (CaHPO.sub.4), or tricalcium (Ca.sub.3(PO.sub.4).sub.2.) phosphate. Air stripping is also used to remove carbon dioxide and thereby increase the pH of the solution to 8.4±. This invention uses autotrophic or phototrophic microorganisms that consume bicarbonate and thereby increase the pH of the liquid stream while decreasing the bicarbonate alkalinity, the two requirements for rapid and complete removal of phosphate from solution. The economic value of the process is predicated on developing a substantial and rapidly growing biomass to consume the carbon dioxide and bicarbonate as quickly as possible. The growth of autotrophic microorganisms requires suitable environmental conditions, such as pH, temperature, and absence of toxic constituents.
(12) As shown in FIG. 1, the first step (A) of the process preferably is to remove constituents that may adversely impact the process by uneconomically inhibiting the growth of autotrophic or phototrophic organisms. Environmental conditions of concern here include: a) turbidity or the ability to transfer light to the autotrophic microorganisms; b) ammonia nitrogen that both buffers the solution and is toxic, as NH.sub.3, to phototrophic organisms; and c) temperature that may not be suitable for the microorganisms being grown. Consequently, the optional unit processes that can be carried out in the first step are dewatering to remove suspended solids, extraction of carbon dioxide and other toxic gases that may be present, and/or increasing or decreasing the temperature to meet the needs of the phototrophic organisms. FIG. 6 presents a prior art device that accomplishes several of those goals. FIG. 9 presents an improved device for solids flotation and gas stripping. The optional step A may also include dilution to reduce the concentration of ammonia or alkalinity. Ammonia nitrogen removal can be accomplished through a wide variety of processes that include nitrification/denitrification, nitrification, nitration, Anammox®,.sup.11 ion exchange, ammonia stripping, etc. .sup.11 Anammox® is a registered mark of World Water Works, Inc. of Oklahoma City, Okla., and denotes an ammonium removal technology wherein nitrite and ammonium are converted directly into dinitrogen gas by bacteria in the phylum Planctomycetes.
(13) After completion of step A of FIG. 1 (optional preconditioning), the liquid containing phosphate (PO.sub.4.sup.3−), and bicarbonate HCO.sup.3−, reduced quantities of ammonia NH.sup.4+, Mg.sup.++, or Ca.sup.++, is introduced to a reactor—for example, the rotating phototrophic bioreactor depicted in FIGS. 7A and B. The reactor (A) in FIG. 2 can be a lake, pond, lagoon, photobioreactor, a fixed film photobioractor, or advantageously a rotating photobioreactor as depicted herein in FIGS. 7A, 7B and as described in U.S. Pat. No. 8,637,304 and U.S. Pat. No. 8,895,279. Light can be provided from a variety of sources, including natural light. The photobioreactor normally incorporates a light/dark cycle wherein bicarbonate is consumed by the photo autotrophs during the day with nitrification occurring during dark periods, further reducing alkalinity. Referring to FIGS. 7A, 7B, a rotating photobioreactor (9) is seen to comprise a vessel, a shaft (8) mounted for rotation within the vessel about a shaft axis, a plurality of axially spaced-apart, growth plates (5) attached to the shaft, each of the plates having surfaces to which a fixed film of the microorganisms is attached, means (e.g., an electric motor drive (7)) for rotating the shaft and plates about the axis, illumination means (e.g., artificial lights (6) or sunlight through the cover (10)) for shining light upon the microorganisms, and means (25) for harvesting the microorganisms from the growth plates, such as doctor blades and/or a vacuum suction device. In addition to light, inputs to the rotating photobioreactor are a nutrient-laden, influent (1), in which the rotating disks (5) are partially submerged, and carbon dioxide gas (3) that serves as a carbon source for the microorganisms. Outputs from the rotating photobioreactor are an effluent (2) with elevated pH caused by the microorganisms' consumption of bicarbonate/carbonate in the influent, oxygen gas (4) produced by the microorganisms, and the microorganisms' biomass, which is removed from the photobioreactor (9) by the harvesting means (25). For a more detailed description of a rotating photobioreactor, see U.S. Pat. No. 8,895,279, the entire contents of which are hereby incorporated by reference. If sufficient calcium or magnesium is present in the influent to form a phosphate precipitate no additional metal ions need to be added. Supplemental calcium ions or magnesium ions may be added directly to the reactor (A) in FIG. 2 if insufficient metal ions are not available to sufficiently precipitate calcium phosphate or MAP. The phototrophic organisms will consume the bicarbonate and increase the pH during the day. Calcium and/or magnesium and ammonia may be added as necessary to the photobioreactor's effluent in a downstream reactor (B) to form calcium phosphate, KAP, or MAP. The calcium phosphate may be flocculated within a reactor (B) by slow stirring in a solids contact reactor or other means or crystallized to form MAP within a tubular or fluidized bed reactor (B). A variety of suitable reactors have been previously described by Ferguson.sup.12. The aggregated or crystallized phosphate precipitate is then reclaimed in a second downstream separator (C). .sup.12 Jenkins, Eastman, and Ferguson, Calcium Phosphate Precipitation at Slightly Alkaline pH Values, Journal WPCF Vol. 45, No. 4, April 1973, page 627.
(14) FIG. 3 presents a typical process for nitrogen removal followed by phosphate recovery. An influent waste stream such as anaerobic digestate containing ammonia, phosphate and metal ions enters a nitrogen processing reactor (A) that can be an ammonia stripping reactor or nitrification, ammonia oxidation, or Anammox® reactor that converts the ammonia or a portion thereof to N.sub.2 gas or nitrate (NO.sub.3.sup.−). The intent of the selected nitrogen removal process is to reduce the adverse impacts of ammonia nitrogen on the autotrophic organisms in the photobioreactor as well as reducing bicarbonate alkalinity. The process may include ammonia stripping, partial ammonia stripping, ammonia oxidation or nitrification. If nitrification takes place, nitrate will enter the photobioreactor with the phosphate and metal ions. In the photobioreactor (B) the pH will be increased and bicarbonate alkalinity decreased by phototrophic organisms during the day. Under dark conditions nitrification may further reduce the alkalinity. After photobioreactor treatment the effluent enters a separate reactor (C) where calcium chloride or magnesium chloride and perhaps ammonia, if a residual does not exist, are added to produce calcium phosphate or MAP. The configuration of the precipitation reactor can be any of the reactors or crystallizers described by Ferguson and herein.
(15) FIG. 4 presents a schematic of a representative process for the removal and recovery of both ammonia and calcium phosphate. A majority of the ammonia is gas stripped for recovery in a flash stripping unit (A) for the removal of a majority of the nitrogen as desired by the operator through control of the process recycle rate. A portion of the effluent from the flash stripping unit (A) passes through an ammonia-stripping, photobioreactor (B) and thence through a calcium phosphate precipitation reactor (C). The liquid effluent of the precipitation reactor (C) is received by the stripping unit (A). The recycled effluent_from the phosphate precipitation reactor (C) has high pH and low bicarbonate. The flash stripping unit (A) operates at a higher pH, an average between the pH of the reactor (B) and the influent stream. For example, if CO.sub.2 has been removed from the influent stream, the pH will be approximately 8.3. The photobioreactor will have a pH of approximately 9.2, which will be reduced slightly when passing through the phosphorus removal reactor (C). If the recycle rate is 100%, the average pH in the flash stripping unit (A) will be 8.75, a pH sufficient to remove approximately 75% of the ammonia at digestate temperatures. The temperature of the stripping reactor may be increased to improve stripping. Reactor (A) may also be operated under a vacuum to increase ammonia removal efficiency. The stripping reactor may be any of a number of commonly-used stripping reactors as well as a rotating photobioreactor described in U.S. Pat. Nos. 8,637,304 and 8,895,279 as well as a sweep gas distillation process.sup.13. .sup.13 Ammonia removal by sweep gas membrane distillation, Xie et al., Jan. 18, 2009, Water Research, 43 (2009) 1693-1699.
(16) FIG. 5 presents a schematic of a process to remove and recover most of the ammonia and phosphate as MAP. The influent is first delivered to an ammonia flash stripping unit (A) that may be operated at higher temperatures and gas stripping rates. The flash stripping unit (A) also receives a recycle flow from the photobioreactor (B) that has a substantially higher pH and reduced alkalinity. The blended contents of the flash stripping unit (A) will have a lower alkalinity, higher temperature, pH, and perhaps stripping gas flow rate, the conditions necessary for rapid removal of ammonia gas. It is expected that at least 60% of the ammonia will be removed by the flash stripping unit. The effluent from the flash stripping unit (A) is recycled to a photobioreactor (B) that will further increase the pH and by gas stripping remove the remaining ammonia gas. The effluent from the photobioreactor (B) is recycled to the flash stripping unit (A). A portion of the effluent from the gas stripping unit (A) is delivered to a phosphate reclamation reactor (C) where magnesium ions are added to precipitate flocculated or crystallized MAP or KAP. The residual ammonia from the recycle stream (A-B) should be sufficient to create MAP in the presence of magnesium ions. If a proper recycle ratio is chosen, substantially all of the ammonia and phosphate will be removed as MAP or KAP.
(17) As depicted in FIG. 4, the ammonia in the stripping gas may be recovered in an ammonia recovery unit of a kind described previously by Burke, U.S. Pat. Nos. 8,637,304 and 8,895,279. The ammonia gas water vapor may simply be condensed or condensed in the presence of CO.sub.2 or other agents as described previously by Burke, U.S. patent application Ser. No. 13/506,249.sup.14. The ammonia recovery (D) will produce various ammonia products such as ammonium hydroxide, concentrated ammonium hydroxide or ammonium bicarbonate or ammonia-acid-precipitated compounds such as ammonium sulfate. A portion of the recovered ammonia may be used to meet the ammonia concentration requirements for MAP precipitation in reactor C. .sup.14 “Ammonium Bicarbonate Fiber Explosion Process,” U.S. patent application Ser. No. 13/506,249, filed on Nov. 7, 2012.
(18) Alternatively, the stripped gas (FIGS. 4, 5) may be used to produce a concentrated ammonia product. FIG. 8 illustrates such a process. As previously discussed, reclaiming phosphorus utilizing a biological process will more often than not involve the removal and or reclamation of ammonia nitrogen. The value of the recovered ammonia nitrogen is directly proportional to the concentration of the product. FIG. 8 presents the process of condensation of ammonia, water, and carrier stripping gas for the production of a highly concentrated ammonia gas or ammonium hydroxide product that can be used to remove carbon dioxide from biogas, pretreat biomass, produce selective catalytic reduction (SCR) fluid, or produce ammonium carbonate/bicarbonate for the ammonium bicarbonate fiber explosion process energy storage, or hydrogen production. After stripping, the carrier gas will contain ammonia gas, water vapor, and perhaps nitrogen and oxygen. The process of concentrating the stripped ammonia consists of optionally removing water through a membrane separation system (A) followed by condensing the remaining water vapor and ammonia gases in a condenser (B1) to produce an ammonia-rich condensate. Typically, chilled condensate is used to wash ammonia and water vapor from the stripping gas in the condenser. But, that process will rarely produce concentrations exceeding 2% ammonia. The process can be improved by dissolving carbon dioxide in the chilled wash fluid, thereby producing a weak carbonic acid that will more effectively concentrate the ammonia gas. The ammonium carbonate/bicarbonate solution that forms in vessel (B2) can then be decomposed in a heated pressure vessel (C) that will, upon release of the pressure, preferentially release concentrated ammonia gas in accordance with Graham's Law. The lower molecular weight of ammonia (MW=17) means that ammonia will escape solution at a significantly higher rate than carbon dioxide (MW=44). The ammonia gas can then be collected separately from the carbon dioxide to produce ammonium hydroxide. Some or all of the carrier gas is recycled from the heated pressure vessel (C) through the condenser (B1).
(19) In FIG. 8, the stripping gas containing NH.sub.3, H.sub.2O and a carrier gas are optionally passed through a water-permeable membrane (A) that removes water vapor under vacuum. The stripping gas then passes through a chiller that reduces the gas temperature. The chiller is followed by a condenser (B1) where the gas is cooled and washed by a low-pH carbonic acid wash fluid. The ammonium bicarbonate wash fluid (B2) is then pumped through a heat exchanger under pressure to a thermal pressure decomposition separator (C) where the dissolved ammonium bicarbonate/carbonate is decomposed to CO.sub.2 and NH.sub.3. Upon release of pressure, the ammonia gas is recovered and compressed while the wash fluid is cooled and recirculated to the condenser (B1). The chilling and heating are preferably performed by a heat pump.
(20) In another embodiment of the present invention, during preconditioning (e.g., FIG. 1, suspended solids removal), solids are separated from anaerobically digested liquid waste using a solids, liquid, gas separator similar to that disclosed in Burke's U.S. Pat. No. 6,893,572. Bias, et al., in U.S. Pat. No. 8,404,121 duplicated and used the same vessel. FIG. 3 of Burke's U.S. Pat. No. 6,893,572 is presented herein in FIG. 6 as prior art. Claim 26 of Burke's U.S. Pat. No. 6,893,572 stated in relevant part: “The method . . . further comprising passing digested liquid of step (a) through a liquid/solids separator prior to step (b) and passing a purge gas through the liquid solids separator, thereby producing a carbon dioxide enriched gaseous effluent from said liquid/solids separator.” As shown in FIG. 6, digestate or a liquid containing suspended solids, ammonia, and CO.sub.2 enters a separator. Gas from a blower or compressor is also introduced into the separator, below the digestate influent, to supplement the gas necessary for flotation. At the same time, a vacuum pump removes a CO.sub.2-enriched gas and a gas deficient in ammonia. The effluent liquid having a higher pH and a lower concentration of CO.sub.2 continuously exits the separator while the separated solids are intermittently removed. Suspended solids are removed while the pH of the effluent increases to approximately 8.3±. The higher-pH, carbon dioxide-deficient liquid produced by the solids, liquid, gas separation process depicted in FIG. 6 can be used as a preconditioned influent to any of the processes depicted in FIGS. 1 through 5.
(21) FIG. 9 presents an improved version of the process/apparatus described previously by Burke and shown in FIG. 6. It provides for the continuous removal of suspended solids without the difficulties of separating solids using an internal circular weir as described by Bias, et al., in U.S. Pat. No. 8,404,121. It has been repeatedly shown that thickened solids do not flow over a weir plate without mixing dilute water flowing under the float blanket with the thickened solids. The thickened float solids float on the surface of the water, a portion of which floats below the water surface. Consequently the mass of float solids will not float over a weir without accompanying water, resulting in a reduced concentration of concentrate from the separator, which is undesirable. That is why all flotation separators use a scraper and ramp to lift the thickened solids from the flotation separator water surface. The process shown in FIG. 9, however, utilizes Burke's process/apparatus described in U.S. Pat. No. 6,893,572, but supplemented with vacuum flotation that removes dissolved gases that promote flotation of solids within the separator. The process_depicted in FIG. 9 also supplements the limited dissolved gas in the liquid influent with sparged gas delivered through fine air diffusers or spargers, with or without a blower or compressor to power the spargers. The float solids are also removed from the top of the flotation separator through a vacuum pipe assembly and thence into a receiver in communication with the inlet side of a vacuum pump. The vacuum pump sucks the solids from the top surface of the liquid influent within the separator into and through the vacuum receiver, which separates the solids from the liquid. Solids can be removed from the surface of the influent liquid in the separator by simply raising and lowering the top surface of the liquid influent within the separator to submerge the solids below the vacuum pipe or pipes for removal. Coagulants can be added to the liquid influent prior to its entering the separator to encourage coagulation of the solids therein. Optionally, the solids can also be removed through a bottom outlet of the separator for cleaning or intermittent solids removal as previously described by Burke. Solids that collect in a bottom portion of the receiver preferably are pumped out by a pump as separated solids. The higher-pH, carbon dioxide-deficient liquid produced by the improved solids, liquid, gas separation process depicted in FIG. 9 can be used as a preconditioned influent to any of the processes depicted in FIGS. 1 through 5.
(22) The foregoing description has been directed to particular embodiments of the method of the present invention in order to comply with the requirements of the United States patent statutes. It will be apparent to those skilled in this art, however, that many modifications and changes in the method and the apparatus for practicing the method will be possible without departing from the spirit and scope of the invention. It is intended that the following claims be interpreted to embrace all such modifications and changes as will be apparent to one having ordinary skill in this technology.
