WO2018157065A1

WO2018157065A1 - Anoxic membrane filtration of iron precipitated metals and oxyanions

Info

Publication number: WO2018157065A1
Application number: PCT/US2018/019739
Authority: WO
Inventors: Michael Warren SOWELL
Original assignee: Aecom
Priority date: 2017-02-25
Filing date: 2018-02-26
Publication date: 2018-08-30

Abstract

A liquid waste stream is treated to remove metallic oxyanions and nitrate simultaneously by adding an aliquot of an aqueous solution of an iron salt and an aqueous solution of a source of organic carbon to the effluent. The resulting mixture is then incubated under anoxic conditions by controlling the rate of addition of organic carbon. Nitrate in the mixture is readily reduced to nitrogen gas and metallic oxyanions in the waste stream are reduced to insoluble compounds which are separated from the waste stream by membrane filtration.

Description

Anoxic M®mbmm Filtration of Iron Precipitated Metals and Oxyanions

Background of the Invention

Area of the Art

[0003] The present invention is in the area of water pollution control and is directed to a process for removing metals and oxyanions from a waste stream.

Summary of the Invention

[0004] A method for simultaneously treating a liquid waste stream to remove metallic oxyanions and nitrate by adding an aliquot of an aqueous solution of an iron salt and an aqueous solution of a source of organic carbon to the effluent. The resulting mixture is then incubated under anoxic conditions by keeping the oxygen-reduction (redox) potential of the mixture between about -150mV and -200mV. This is achieved by controlling the rate of addition of organic carbon. Under these conditions nitrate in the mixture is readily reduced to nitrogen gas which escapes to the atmosphere. At the same time any metallic oxyanions in the waste stream are reduced to insoluble compounds which are separated from the waste stream by membrane filtration. This results in an aqueous stream that is depleted of nitrate, metallic oxyanions and insoluble material and can be safely released to the environment or otherwise recycled.

Description of the Figures

[0005] FIGURE 1 shows a diagram of the inventive system.

[0008] FIGURE 2 is shows a diagram of an alternative embodiment in which the membrane system is sparged with air.

Detailed Description of the invention

[0007] The following description is provided to enable any person skilled in the art to make and use my invention and sets forth the best modes I have contemplated in carrying out my invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the general principles of the present invention have been defined herein specifically to provide an improved system for removing aqueous pollutants.

[0008] My work in the oil refining Waste Water Treatment sector over the past 6 years and tightening National Pollutant Discharge Elimination System (NPDES) limitations for metals and oxyanions (selenate, arsenite, etc.) has caused me to think about how to integrate the what is typically achieved in multiple steps into a single treatment step, thereby simplifying the process and facilitating retrofitting existing systems. [0009] Simultaneous removal of nitrate-nitrogen, phosphorus, mercury, selenium, arsenate, and other particulate metals in a single process is the goal of the present invention. Without the present invention treatment of waste-water from most existing oil refineries would require at least two additional treatment steps. My invention is of particular application to sites that are facing regulatory pressures that are driving the chosen treatment system toward membrane filtration and or water reuse because my system produces a Reverse Osmosis (RO) ready treated wastewater stream. A key part of this invention is operating the membrane filtration unit in an anoxic condition that has the Oxidation Reduction Potential (ORP) in range of -150 to -200 millivolts (mV), a milieu where denitrification and selenium reduction occur. However, for less restrictive discharge selenium limits, it is possible to operate the membrane filtration unit in the more conventional aerobic mode to lower the costs associated with nitrogen gas sparging of the membrane surface. That is, for anaerobic conditions the liquid and membrane surfaces are bubbles and purged ("sparged") with nitrogen whereas for aerobic conditions the liquid can be purged with air.

[0010] The invention may have applicability outside of the oil refinery sector (such as anoxic membrane denitrification of municipal effluent; treatment of effluents from fracking or mining and agricultural waste water). However, I will focus on the oil- refining sector as an example, realizing that one of ordinary skill in the art can readily apply my invention to other situations.

[0011] Each site's treatment requirements are unique because the exact mix of pollutants varies considerably. In general, to remove mercury (Hg) to low levels a refinery would have to add a chemical treatment followed by a multimedia filtration step. If nitrate is a significant pollutant and nitrogen removal to < 3 ppm is required, then the refinery would have to add a deep bed denitrifying sand filter. If oxyanion removals are required, the refinery might install a Anoxic Fluidized Bed Reactor (FBR) that could remove nitrate nitrogen and convert selenium oxyanions to elemental selenium, but this would necessitate a final membrane solids separation process to remove the biological solids and elemental selenium.

[0012] The present invention allows the nitrate-nitrogen, mercury, selenium, arsenic, phosphorus, and other particulate metals to be removed with a single process - thus eliminating the use of secondary clarifiers, multimedia filters, FBR reactors and their subsequent solids separation processes. Therefore, it requires less equipment to operate and maintain. [0013] The invention particularly applies to oil refineries (but has applicability in the municipal denitrification market as well) that are considering membrane systems for mercury and or other colloidal metals (e.g., arsenic) control, and that are also facing existing or anticipating future selenium discharge limits. Many existing plants do not have the ability insert a Modified Ludzack-Ettinger (MLE) process because the MLE requirement for high recirculation rates are beyond the existing equipment's hydraulic piping design constraints even if the waste stream contains sufficient biologically available carbon to make such recirculation useful. Therefore, my solution is to perform second stage denitrification by inserting a mechanically mixed denitrification tank ahead of the membrane filtration units and then to operate the membrane filtration tanks in an anoxic mode with nitrogen gas (N2) sparging. Alternately, if selenium limits allow, the membrane filtration unit may be operated in an aerobic mode (Fig. 2).

[0014] The process adds organic carbon (methanol or other suitable carbon source such as ethanol, glycerol or similar well-known sources) and iron salt such as ferrous sulfate or ferric chloride to the anoxic tank. The biological activity, primarily of bacteria, drives the ORP negative (controlled between -150 and -200 mV); in the absence of oxygen and nitrate, nitrogen as nitrite is biologically converted to N2 gas, and the iron salt reacts and co-precipitates iron selenite, iron arsenate, and iron phosphate. Carbon beyond the stoichiometric denitrification amount is feed to the anoxic tank to take the ORP low enough so both denitrification and selenate reduction to elemental selenium or selenite (which contributes more selenite to the iron selenite precipitation reaction) occurs. If the ORP goes low enough for sulfate reduction to hydrogen sulfide to occur, the excess iron can scavenge that as iron sulfide (FeS). The solids are filtered by the membranes to produce a high quality permeate that is re-aerated in the permeate tank and the concentrated Recycle Activated Sludge (RAS) and Waste Activated Sludge (WAS) are sent to their respective process discharge points.

[0015] Fig. 1 shows a diagram of the system. At the right side of the drawing a Denitrification Tank receives the effluent to be processed. This effluent contains nitrate as well as selenium that must be removed prior to effluent discharge. As explained above, the effluent may also contain mercury and other oxyanions to be removed. The tank is not aerated so that bacterial metabolism soon renders the solution anoxic. The Denitrification Tank is mechanically agitated and denitrifying bacteria use any organic carbon already present as well as added carbon (e.g., methanol from the methanol tank) for growth (i.e., synthesis of new bacteria). These bacteria use nitrate as an electron acceptor for their anaerobic respiration. This reduces the nitrate to nitrite and eventually to N2 gas. This denitrifying mixture flows by gravity or is pumped to the Membrane Filtration tank, which is constantly sparged with N2 gas (from a nitrogen gas source) to ensure anoxic conditions are maintained in the membrane filtration tank. A Knock Out (KO) filter removes water from the N2 gas recycling system. If the selenium limits are high enough, the user could elect to sparge the membranes with air to avoid the installation of a nitrogen gas system. FIG. 2 show the process adopted to use air sparging of the membrane system so that it operates in an aerobic mode..

[0016] Under these highly anoxic conditions the ORP becomes strongly negative reducing the selenate present to selenite or elemental selenium. The ferric chloride (FeC ) that has been added earlier precipitates the selenite as well as other reduced metal anions that may be present. The ferric chloride also precipitates any sulfide produced by reduction of sulfates in the effluent. The effluent passes through the membrane while the filter retains the precipitated selenium, other metals and other solid materials. The exiting effluent is sampled by an ORP sensor, which thereby measures the redox potential in the Membrane Filtration tank. The ORP sensor is connected to an Analytic Instrument Controller (AIC). If the ORP is insufficiently low, the controller signals the organic carbon metering pump to increases its pumping rate. Similarly, if the ORP becomes excessively negative, less organic carbon is added. An Analytic Instrument Transmitter (AIT) with an attached nitrate-detecting electrode measures and transmits the nitrate level to ensure and document that the effluent is sufficiently depleted in nitrate. The flow rate of effluent through the membrane filtration unit is controlled to ensure proper nitrate depletion. The effluent then passes into a final aeration tank for reaeration prior to discharge.

[0017] The retained solid materials are scoured from the membrane surfaces as sludge, which can be separated into recycled activated sludge (RAS) which is sent back through the treatment system and waste activated sludge (WAS) which is disposed of. The disposal route will depend on the level of toxic metals present in the sludge.

[0018] The following claims are thus to be understood to include what is specifically illustrated and described above, what is conceptually equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention. Those skilled in the art will appreciate that various adaptations and modifications of the just described preferred embodiment can be configured without departing from the scope of the invention. The illustrated embodiment has been set forth only for the purposes of example and that should not be taken as limiting the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.

Claims

What is claimed is:

1. A method for simultaneously treating an effluent to remove metallic oxyanions and nitrate comprising the steps of:

adding an aliquot of an aqueous solution of an iron salt and an aqueous solution of a source of organic carbon to the effluent to form a mixture;

incubating the mixture under anoxic conditions;

keeping the oxygen reduction potential of the mixture between about -150mV and -200mV by controlling a volume of the organic carbon added whereby nitrate in the mixture is reduced to nitrogen gas and the metallic oxyanions are reduced to insoluble compounds; and

separating insoluble material from the mixture so that the mixture is depleted of nitrate, metallic oxyanions and insoluble material.

2. The method of claim 1 , wherein the step of separating is accomplished by using a membrane filtration system.

3. The method of claim 1 , wherein the iron salt is ferric chloride.

4. The method of claim 1 , wherein the source of organic carbon is methanol or a comparable easily biodegraded organic compound.

5. The method of claim 1 , wherein the step of incubating under anoxic conditions is accomplished by sparging the mixture with nitrogen gas.

6. The method of claim 1 further comprising a step of aerating the mixture after the step of separating.

7. The method of claim 1 , wherein the oxyanions are selenite