Patent Application
20120261338
The invention relates to wastewater treatment for precipitating dissolved materials from wastewater. For example, the invention may be applied in struvite precipitation reactor systems. Embodiments relate to methods and apparatus for inhibiting struvite formation and scaling problems upstream of a precipitation reactor while allowing and/or enhancing the recovery of struvite or other phosphorus-containing compounds in the precipitation reactor.
Reactors in general and fluidized bed reactors in particular have been used to remove and recover nutrients (i.e. ammonia and phosphorus) from wastewater that contains significant concentrations of phosphorus, often in the form of phosphate. Such wastewater may come from a wide range of sources. These include sources such as leaching from landfill sites, runoff from agricultural land, effluent from industrial processes, municipal wastewater, animal wastes, and the like. Such wastewater, if released into the environment without treatment, can result in excess effluent phosphorus levels.
Various phosphorus removal and recovery technologies exist. Some of the technologies provide fluidized bed reactors for removing phosphorus from aqueous solutions by producing struvite (MgNH4PO4 6H2O) or struvite analog or a phosphate compound in the form of pellets. Struvite can be formed by the reaction:
Mg2++NH4++PO43−+6H2OMgNH4PO4.6H2O
Examples of reactors used to remove and recover phosphorus from wastewater solutions have been described in various references. They include:
Regy et al., Phosphate recovery by struvite precipitation in a stirred reactor, LAGEP (March to December 2001) includes a survey of various attempts to remove phosphorus and nitrogen from wastewater by struvite precipitation.
Trentelman, U.S. Pat. No. 4,389,317 and Piekema et al., Phosphate Recovery by the Crystallization Process: Experience and Developments, paper presented at the 2nd International Conference on Phosphate Recovery for Recycling from Sewage and Animal Wastes, Noordwijkerhout, the Netherlands, Mar. 12-13, 2001, disclose a reactor and method for precipitating phosphate in the form of calcium phosphate, magnesium phosphate, magnesium ammonium phosphate or potassium magnesium phosphate.
Ueno et al., Three years experience on operating and selling recovered struvite from full scale plant (2001), Environmental Technology, v. 22, p. 1373, discloses the use of sidestream crystallization reactors to remove phosphate in the form of magnesium ammonium phosphate (also known as struvite).
Tsunekawa et al., Patent Abstracts of Japan No. 11-267665 discloses a reactor for removing phosphorus from water.
Koch et al., fluidized bed wastewater treatment, U.S. Pat. No. 7,622,047.
One problem with wastewater treatment systems and reactors is that struvite or scale having other compositions may form undesirably in effluent piping systems or otherwise upstream of the precipitating reactor. It is known to use certain inhibitors like polyphosphates, phosphonates, polymers, or other compounds or mixtures to help to limit or stop struvite formation in pipes but these inhibitors also inhibit the desired struvite formation downstream in the reactor. A cost effective solution is needed to address this problem.
This invention has a number of aspects. One aspect provides wastewater treatment systems and components thereof. Another aspect provides methods for wastewater treatment. Another aspect provides methods for recovering struvite, struvite analogs or other phosphorus-containing solids from wastewater.
One aspect provides a wastewater treatment system for producing struvite or another phosphorus-containing solid from a wastewater solution. The system comprises, in combination, at least two of a digester, a liquid/solid separation device, a settling tank and a reaction tank, and a piping system. The system comprises an injector arranged to inject H3PO4 into the wastewater in any one or more of: the digester, the liquid/solid separation device, the settling tank and the piping system. The H3PO4 may be injected upstream of the reaction tank.
In some embodiments the system comprises an automatic controller to regulate addition of H3PO4 such that scaling is inhibited. In some embodiments the system further comprises a probe for measuring the pH of the wastewater. The probe may be configured to send signals to a control system for controlling H3PO4 injection responsive to signals received from the probe. The system may be configured, for example, to maintain the wastewater pH between 7.0 and 8.5.
In some embodiments the system comprises a plurality of injectors arranged for injecting H3PO4 at more than one location in the system upstream of the reaction tank.
Some embodiments further include one or more injectors arranged to inject CO2 into the wastewater upstream from the reaction tank (e.g. in any one or more of the digester the solid/liquid separation device, the settling tank and the piping system).
Some embodiments further include a metering mechanism for metering a Mg-containing material into the wastewater. A controller may be configured to control the metering mechanism for adding the Mg-containing material at a rate determined at least in part by an amount of H3PO4 injected upstream of the reaction tank.
The above features may be combined with one another and with other features as described herein in any suitable combinations .
Another aspect of the invention provides a method for treating wastewater to produce struvite or another phosphorus-containing solid. The method comprises introducing wastewater into a wastewater treatment system; and injecting H3PO4 and/or CO2 into the wastewater at one or more points in the wastewater treatment system upstream of a precipitation reactor in an amount to prevent or limit to formation of struvite upstream of the reactor.
Some embodiments of the method further comprise controlling the injection of the H3PO4 into the wastewater, in response to one or more signals received from one or more probes, to maintain a predetermined level of H3PO4 in the wastewater. The predetermined level may be selected to be a level sufficient to substantially inhibit the formation of struvite in the treatment system upstream of the precipitation reactor. Some embodiments comprise maintaining the pH of the wastewater between 7.0 and 8.5.
In some embodiments the wastewater treatment includes a digesting step and the method comprises: after introducing the wastewater into the wastewater treatment system, digesting the wastewater in a digester; from the digester, transferring the wastewater to a solid/liquid separation device; from the solid/liquid separation device removing solids and from the solid/liquid separation device transferring the wastewater to a clarifying tank;
from the clarifying tank transferring the wastewater to a reaction tank for the formation of struvite; and removing effluent from the reaction tank. In such embodiments the method may inject H3PO4 into the wastewater during or between one or more of the foregoing steps.
In some embodiments both CO2 and H3PO4 are injected into the wastewater. In such embodiments injecting CO2 into the wastewater may be performed upstream from the precipitation reactor. For example CO2 may be injected into one or more of a digester, a solid/liquid separation device a piping system and a settling tank of the treatment system.
Some embodiments comprise controlling relative amounts of CO2 and H3PO4 injected into the wastewater based at least in part on a production of struvite or other phosphorus-containing solids by the precipitation reactor.
Some embodiments comprise adding a Mg-containing material to the wastewater at a rate determined at least in part by an amount of H3PO4 injected upstream of the reaction tank.
Further aspects of the invention and features of example embodiments are illustrated in the appended drawings and described in the description.
The accompanying drawings illustrate non-limiting embodiments of the invention.
Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well-known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense.
Some embodiments of the invention in the following description relate to reactor apparatus or methods wherein phosphorus in wastewater is precipitated in the form of struvite or struvite analogs or a phosphate compound. This choice of example coincides with an aspect of the invention believed to have significant commercial utility. The scope of the invention, however, is not limited to these examples.
An embodiment finds particular application in wastewater treatment systems comprising a fluidized bed reactor of the type described in Koch et al., U.S. Pat. No. 7,622,047, entitled “Fluidized Bed Wastewater Treatment”, which is hereby incorporated by reference. Such systems may produce pellets of struvite, struvite analogs or other phosphorus-containing solids from wastewater.
For convenience, the term “wastewater” is used in the following description and claims to describe aqueous solutions such as industrial and municipal wastewater, leachate, runoff, animal wastes, effluent or the like. The term “wastewater” is not limited to effluent from municipal sewage, animal waste, or any other specific source. Some embodiments provide methods for treating municipal sewage and/or animal waste. Some embodiments provide methods and apparatus for treating other kinds of wastewater. Indeed, the term “wastewater” should also be considered to include any solution having certain properties and constituents of wastewater (i.e. any wastewater-like solution) which could optionally be manufactured from raw materials strictly for use in the production of struvite.
Just by way of example, a typical wastewater treatment system 10 (
Examples of solid/liquid separation devices that may be used are centrifuges, clarifiers, thickeners, gravity belt thickeners, belt presses and the like. From solid/liquid separation device 16 effluent may pass to a further tank 18, which may be termed a clarifying/settling tank or equalization/storage tank, from which the effluent may be transferred to precipitation reactor tank 22 through inlet 24. In most cases, between these various elements the wastewater is pumped by means of one or more pumps 20 and passes through various valves, pipe fittings, and instruments.
Struvite or other phosphorus-containing compounds may be precipitated in reactor tank 22 in a variety of ways including through the process described in Koch et al., U.S. Pat. No. 7,622,047. Fully treated effluent is removed from reactor tank 22 at outlet 26.
In systems for treating wastewater containing dissolved materials that tend to precipitate at higher pH levels, scale formation in effluent piping can be a problem. An example is a system for recovery of phosphate in the form of struvite from liquid effluents of anaerobic processes (e.g., anaerobic digester liquors, dewatering liquors at municipal wastewater treatment plants, etc.). The solubility of struvite is a function of pH and decreases when pH increases. As pH increases, struvite precipitates from the wastewater. In such systems, struvite formation may be encouraged as a result of high pH increases and it is thus desirable to reduce pH upstream of the reactor tank.
One way in which pH increases is when carbon dioxide is released from the wastewater. Carbon dioxide tends to be released when wastewater cascades down drains or flows in partially-full drain pipes in the effluent piping system. Carbon dioxide is typically present at elevated levels in entering wastewater due to the high fraction of carbon dioxide in the sealed atmosphere in anaerobic treatment tanks that may precede the phosphorus recovery process in a wastewater treatment plant. Once the wastewater is exposed to ambient air, and especially when mixed turbulently with air, or when the fluid pressure is reduced (e.g. in pump suction piping or near piping flow restrictions etc.) the carbon dioxide tends to offgas, causing pH increase in the wastewater. The carbon dioxide offgassing and the resultant pH increase can therefore lead to increased struvite scale formation in the effluent piping system upstream from a reactor.
This scale formation is not necessarily a wide-spread phenomenon, as turbulent fluid flow in pipes can cause small localized variations in pH sufficient to trigger struvite precipitation and/or scale formation, for example, at the location of a valve or other feature (an elbow, for example) that causes the local turbulence or local pressure drop. Struvite scale then can build up at such a location.
In struvite/phosphate recovery systems pH can be controlled to promote the formation of struvite in a reactor and to reduce effluent phosphate levels. One preferred range of pH is between 7.0 and 8.5. The carbon dioxide that can be present at elevated levels in the wastewater results in low pH conditions that are unfavorable to the formation of struvite in the reaction tank. In order to counter this problem, one can add alkaline (basic) substances such as sodium hydroxide (NaOH), magnesium hydroxide (Mg(OH)2), ammonium hydroxide (NH4OH), anhydrous ammonia (NH3) or the like to the system in or upstream from the reaction tank to increase the pH of the wastewater and to promote struvite formation in the reaction tank. However, purchasing such materials and supplying and maintaining equipment to introduce such materials into the process adds to the cost of operating a wastewater treatment system.
One way to inhibit premature struvite formation is to add CO2 to the system, decreasing pH, as described in a co-pending application filed by the applicants entitled “METHODS AND APPARATUS FOR STRUVITE RECOVERY USING UPSTREAM CO2 INJECTION”, which is incorporated herein by reference. One other way to decrease pH, it has been determined, is to add phosphate in the form of phosphoric acid, H3PO4, either on its own or in combination with CO2 as discussed below.
One aspect of the present invention provides methods and systems which add phosphate to the system, preferably by way of injection of H3PO4 in any of the elements of the treatment system upstream of the reactor, including into the piping system 14. The addition of H3PO4 decreases pH and inhibits struvite formation. It has been determined that struvite precipitation in a water treatment system can be largely delayed until the effluent reaches the reactor if enough H3PO4 is added throughout the system.
It will be appreciated that H3PO4 could be injected into the water treatment process at any point in the process upstream of the reactor 22, for example at stage “A” as shown in
Scale formation can also be detected by measuring pressure in the piping system, and the dose of reagent (e.g. one or more of CO2/H3PO4 in each appropriate application) can be adjusted in response to measured pressure signals. For example, fouling in a pump would result in lower discharge pressure for the same pump speed, or fouling in a piping system would result in a higher pump discharge pressure upstream in the piping system for the same flow.
It will be appreciated that one can easily measure the pH of the effluent at one or more points in the system to control the rate of flow of any injected H3PO4. One such suitable point is at or near the inlet 24 of reactor 22, as shown in
The following experimental data show how pH decreases in a centrate following phosphate addition through addition of H3PO4:
At one or more stages of the process, H3PO4 and/or CO2 is injected into the system, for example at one or more of steps 120, 122, 124 and 126. A control device 130 may continuously control the flow of H3PO4 and/or CO2 to accomplish a desired level of H3PO4 and/or CO2 in response to signals received from one or more probes 132.
Among the advantages of injecting H3PO4 to reduce pH in a wastewater treatment system to produce struvite are that:
only relatively small quantities of H3PO4 are required, and H3PO4 is inexpensive
adding H3PO4 of course adds phosphate which is a required compound in the production of struvite (more struvite can be produced in a reactor which is not already at capacity.)
adding H3PO4 in a reactor which is not already at capacity results in capture of more ammonia, which is almost always in excess in a wastewater treatment system, so the resulting effluent is cleaner.
one can control Mg injection into the end reactor based in part on an amount of H3PO4 added upstream. In some embodiments a controller is configured to control a metering mechanism for adding a Mg-containing material at a rate determined at least in part by an amount of H3PO4 added upstream. In fact, one can add enough Mg to precipitate all reactor influent phosphate (the centrate phosphate plus any added phosphate)—this keeps ammonia removal constant.
To deal with localized variances in pH, the goal is to add enough H3PO4 to lower pH enough so that even with microfluctuations the pH in the pipe is lower than the pH at the inlet of the pipe, preventing struvite and scale formation.
At one or more stages of the process, H3PO4 is injected into the system, for example at one or more of steps 120, 122, 124 and 126.
Again, the H3PO4 can be added in conjunction with CO2 injection. A control device 130 may continuously control the flow of CO2 to accomplish a desired level of carbon dioxide in response to signals received from one or more probes 132. In some embodiments the relative amounts of CO2 and H3PO4 added are controlled based at least in part on a production of the reactor. This control may be provided automatically and/or by human adjustment. If the reactor is at capacity, one can increase the relative amount of CO2 injected to decrease pH. If reactor is not at capacity, one can use more H3PO4 to reduce pH while simultaneously providing more phosphate to use the unused capacity of the reactor and increase the yield of struvite.
One problem with wastewater treatment systems used to produce struvite is that there can be a large percentage of loss of struvite in the form of ‘fines’—small struvite crystals that form but are so small they get carried off with effluent from the reactor. It is desirable to reduce upstream scale formation without creating a situation where too many fines form. At the reactor pH may change in a graduated manner and it is thought that this is beneficial for reducing formation of fines.
As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof.
This application claims the benefit under 35 U.S.C. §119 of U.S. Provisional Patent Application No. 61/443,183 filed on Feb. 15, 2011 entitled METHODS AND APPARATUS FOR STRUVITE RECOVERY USING UPSTREAM PHOSPHATE INJECTION, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61443183 | Feb 2011 | US |
