Process for removing selenium from water

Abstract

A biological selenium removal system and process is disclosed and includes a series of MBBRs for reducing selenium species to elemental selenium. In addition, the system and process includes an automatic control system that senses redox potential in the MBBRs and based on a redox potential setpoint range, controls the dosing of external carbon to each MBBR in order to control and maintain the actual redox potential in each of the MBBRs within a redox potential setpoint range.

Description

[0001] 5952-625

[0002] SYSTEM AND PROCESS FOR REMOVING SELENIUM FROM WATER

[0003] RELATED APPLICATION

[0004] This application claims priority to U.S. Provisional Patent Application Serial No.

[0005] 63 / 720,856 filed on November 15, 2024, which is incorporated herein by reference in its entirety.

[0006] FIELD OF THE INVENTION

[0007] The present invention relates to a biological system and process for removing selenium from water, and more particularly to an automatic control system for controlling and maintaining the redox potential in the biological system within a redox potential setpoint range.

[0008] BACKGROUND OF THE INVENTION

[0009] Selenium has become a pollutant of concern around the world because of its potential effects on human health and the environment. In the United States, recently issues national pollution discharge elimination systems (NPDES) permits have forced industrial facilities to meet strict new discharge requirements for selenium (total selenium < 10 pg / L). Several state environmental quality boards have ruled that industries must achieve this selenium limitation in their surface water discharges. Globally, it is anticipated that the demand for processes that remove selenium to parts per billion (PPB) levels in industrial effluents will be significant in the coming years.

[0010] There are many sources of selenium. Selenium is found in wastewater from coal mines, oil and gas extraction, petroleum refining, coal fire power generation, various mining industries, and other industrial activities. Selenium is even present in some irrigation water and in storm water runoff from agricultural operations located in areas with seleniferous soils. Granted, selenium is even a nutrient for biological systems. However, the safety margin between being a nutrient and being highly toxic is very narrow.

[0011] It is known to use standalone moving bed bioreactors (MBBRs) to remove selenium from water. However, biological selenium removal processes carried out in MBBRs are often unable to meet the stringent selenium discharge limits normally in the PPB level required in many cases. There are numerous reasons for this, including, for example, the complex chemistry of selenium removal, limited biological reduction in some cases, slow biological reduction rates, etc. However, one particular problem in selenium removal stands out. Effective selenium reduction often requires the presence of specific electron donors, such as an organic carbon source, that certain selenium-reducing bacteria use for energy. Maintaining the right level of carbon in a selenium removal process is challenging, and an imbalance can lead to incomplete reduction, limiting selenium removal efficiency. Optimum redox potential (sometimes referred to 5952-625

[0012] as oxidation-reduction-potential or ORP) values in an MBBR configured to remove selenium are ambiguous and are not well understood. As a result, optimum redox potential values for an MBBR for selenium removal rates have not been well defined in previous studies. Importantly, most previous selenium removal studies are based on a single point carbon addition strategy with 100% of the external carbon for all of the reactors added in the influent or to a single reactor. This can result in serious operational issues in the biological selenium reduction process that employs multiple MBBRs. For example, it is not uncommon for a process that relies on dosing external carbon into a single reactor to overdose and cause the redox potential to overshoot anything close to an optimum value. The problem also exists in cases where the external carbon addition to a single reactor is inadequate and results in an underdose and a redox potential value in the MBBR that undershoots. In the end, these problems and shortcomings often result in a poor and unstable selenium removal performance in an MBBR system.

[0013] SUMMARY OF THE INVENTION

[0014] The present invention presents a biological selenium removal system and process that aims to address the shortcomings and disadvantages of conventional biological removal processes. Specifically, the present invention entails an MBBR system and process for removing selenium that includes multiple MBBRs with each having a dedicated carbon source pump for directing carbon into the MBBRs. Moreover, the present invention includes an automatic control system that senses redox potential in each MBBR and based on an optimum redox potential setpoint range, controls the dosing of the external carbon to each MBBR individually. This enables the redox potential in each MBBR to be closely monitored and controlled. Based on real time redox potential values in the MBBRs, the flow of the external carbon source to each MBBR can be closely controlled so as to maintain the actual redox potential in each MBBR within a redox potential setpoint range.

[0015] Moreover, the selenium removal system and process of the present invention includes a redox potential controller programmed to communicate with and control multiple dedicated pumps that supply external carbon to the individual MBBRs. The redox potential controller is programmed to maintain the redox potential in each MBBR within a redox potential setpoint range. Each MBBR is provided with a redox potential measuring probe that continuously or intermittently measures in real time the redox potential in the respective MBBRs. A signal representing the real time value of the redox potential in each MBBR is directed from each of the probes to the redox potential controller that is operatively connected to each of the dedicated carbon source pumps that serve the respective MBBRs. The redox potential controller computes how much, if any, carbon is needed at any one time for any one of the MBBRs in order to bring the actual redox potential in the MBBR within the selected redox 5952-625

[0016] potential setpoint range. After computing this, the redox potential controller is configured to drive or control the respective carbon source pump in accordance with the dictates of the redox potential controller. The redox potential controller may be in the form of a proportional-integral-derivative (PID) controller and effectively maintains the redox potential in all of the MBBRs within a selected redox potential setpoint range.

[0017] Other objects and advantages of the present invention will become apparent and obvious from a study of the following description and the accompanying drawings which are merely illustrative of the invention.

[0018] BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a schematic illustration of the biological selenium removal process of the present invention.

[0020] Figure 2 is a schematic illustration of an automatic control system for controlling the redox potential in the biological selenium removal system.

[0021] Figure 3 is a graphic illustration showing exemplary data for selenium concentrations versus removal rate for the selenium removal system and process of the present invention.

[0022] Figure 4 is a graphic illustration showing the redox potential or ORP profiles for the selenium removal system that includes three MBBRs in series as shown in Figure 1.

[0023] DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0024] The present invention fundamentally relates to a process for biologically reducing various selenium species, such as selenate (selenium+6) and selenite (selenium+4) to elemental selenium (Se). The system and process for reducing selenium is indicated generally by the numeral 10 and shown in Figure 1. The system includes multiple MBBRs which, in the case of the example shown in Figure 1 , includes three MBBRs in series which are referred to as R1 , R2 and R3. Each MBBR includes a tank. The tank is provided with a carbon source inlet 12 through which a source of external carbon is supplied to the MBBR. Further, each MBBR includes a mixer 14 and a redox potential probe 16. Each MBBR includes biomass and in a preferred embodiment, the biomass is of the fixed film type supported on biocarriers. It should be noted, however, that suspended biomass in combination with fixed film biomass can be employed in the biological reactors shown in Figure 1. Structural details of an MBBR are not dealt with here because MBBRs are well known and appreciated by those skilled in the art and the particular structure of the MBBR is not in and of itself material to the present invention.

[0025] The MBBRs of the present invention are operated under anoxic or anaerobic conditions. Biomass in the biological reactors R1 , R2, and R3 can serve various functions. In the case of the present invention, the biomass serves the primary function of biologically reducing selenium 5952-625

[0026] species to elemental selenium. However, if nitrates or nitrites are present in the wastewater, the biomass can denitrify the wastewater by reducing them to nitrogen gas.

[0027] To support biomass growth in MBBRs R1, R2, and R3, a carbon source, such as acetate, ethanol, methanol, glucose, glycol, is directed into the MBBRs to promote the biological reduction of selenium. Biomass that reduces selenium need energy to drive the biochemical reactions involved in converting selenium from its soluble forms (selenate or selenite) to an insoluble form, elemental selenium. Carbon acts as an energy source for the biomass, enabling the biomass to perform these reduction reactions efficiently. As noted above, in the case of the present invention, selenium removal relies on a reduction process, where selenium compounds are converted to a lower oxidation state. Carbon-based compounds (such as organic molecules) serve as electron donors in this process, providing the electrons needed for the reduction of selenium.

[0028] Downstream from the MBBRs is a solids-liquid separator indicated by the numeral 20 and which is configured to remove the elemental selenium from the water. Elemental selenium is in a solid form and can be removed by numerous types of solids-liquid separation systems. For example, a ballasted flocculation system, such as that marketed by Veolia Water Technologies under the name “Actiflo”, can be employed to remove the elemental selenium. See U.S. Patent Pub. 2010 / 0096335 which shows and describes a ballasted flocculation system. The disclosure of this patent publication is expressly incorporated herein by reference. Another option for a solids-liquid separation system that can be used downstream of the MBBRs is a rotary disc filter of the type marketed by Veolia Water Technologies under the name “Hydrotech”. See U.S. Patent 8,961,785 which discloses a rotary disc filter of the type that could be used in this application. The disclosure of this patent is expressly incorporated herein by reference. While the elemental selenium is in a solid form, in some cases it may be beneficial to add a coagulant and / or flocculant to the water which will facilitate the separation of the elemental selenium from the water. The combined use of coagulants and / or flocculants enhances the efficiency of separating the elemental selenium.

[0029] One of the aims of the present invention is to determine an optimum redox potential setpoint or redox potential setpoint range for the MBBRs, and provide a control system that is configured to automatically control the flow of external carbon into the MBBRs such that the redox potential setpoint or redox potential setpoint range is maintained in all of the MBBRs. Figure 2 illustrates schematically a control system for controlling the redox potential in the multiple MBBRs depicted in Figure 1. The system includes a redox potential controller 30, a controller interface 36, three carbon metering pumps 32A, 32B, and 32C, as well as the redox potential probes 16 contained in the three MBBRs depicted in Figure 1. While the specific design of the metering pumps can vary, in one example, the metering pumps can be a positive displacement pump that accurately pumps external carbon into a respective MBBR in response 5952-625

[0030] to instructions from the controller 30. Controller 30 can adjust the pump speed in speed increments. This enables the pump to deliver external carbon to the respective MBBRs at an adjustable flow rate which can be finely tuned for consistent control of external carbon dosing.

[0031] Various types of redox potential controllers can be employed. Figure 2. In one example, controller 30 is a programmed logic controller or a proportional-integral-derivative (PID) controller. Each of the redox potential probes 16 is operatively connected to an input of controller 30. Redox potential probes 16 make real time redox potential measurements in each of the MBBRs and direct a redox potential signal 38 which is representative of the measured redox potential to the controller 30. Redox potential probes 16 are well known and used in the wastewater treatment industry to measure redox potential in wastewater treatment environments. Redox potential measurements are generally an aggregate indicator that provides a snapshot of the electron transfer environment in the MBBR and as appreciated in the context of the present invention, the measured redox potential guides the control of the metering pumps 32A, 32B, and 32C in order to promote desired biochemical reactions for the reduction of selenium. Because in this example there are three MBBRs, it follows that there are three carbon source metering pumps 32A, 32B, and 32C, with each pump configured to supply external carbon to one of the MBBRs. In order to drive and control the metering pumps, there is provided a series of signal lines 34A, 34B, and 34C that are operatively interconnected between the controller 30 and the respective metering pumps 32A, 32B, and 32C.

[0032] Turning to Figure 2 and control system 10 shown therein the control system is designed to maintain the redox potential in each MBBR R1 , R2, and R3 within an optimum range. Based on tests conducted by the inventors, it was determined that for biological removal of selenium in an MBBR system that the optimum redox potential range is between -300 mV and -400 mV with the preferred range being between -325 mV and -375 mV. Control system 10 includes a controller interface 36 which in this example enables a user to input information and data and which can comprise a supervisory control and data acquisition (SCADA) component. Control system 10 enables user inputs through this interface. This enables the operator to input various information and data and, in particular, a redox potential range (i.e. setpoint range) for each MBBR that the redox potential controller 30 recognizes and controls during the course of biologically removing selenium. While in most cases it is anticipated that the redox potential setpoint range will be the same for each MBBR, it is possible to vary the setpoints ranges among the MBBRs through the controller interface 36.

[0033] In the course of reducing the selenium species, the redox potential controller 30 is programmed to collect measured redox potential values in each of the MBBRs periodically, for example, every five minutes. Once a redox potential measurement has been made by the probe 16, the probe will direct a redox potential signal 38 to an appropriate input of the redox potential controller 30. Redox potential controller 30 is programmed to analyze the redox 5952-625

[0034] potential signal 38 from the probe 16 and, based on the signal, determine what amount, if any, of external carbon is needed to maintain the redox potential in a particular MBBR within the setpoint range. Once the redox potential controller 30 makes that determination, it generates a control signal (34A, 34B, 34C) that is directed from the controller to metering pumps 32A, 32B, 32C. The control signal is effective to control the metering pumps such that the metering pumps discharge sufficient carbon into the MBBRs to maintain the redox potential within the setpoint range. At certain times during the selenium removal process, the redox potential in one or more of the MBBRs may be within the setpoint range. Typically, in those cases, the relevant metering pump would not be actuated. In other cases when the measured redox potential is outside of the established redox potential setpoint range, the redox potential controller 30 will actuate the relevant metering pump or pumps and cause sufficient carbon to be discharged into the relevant MBBR or MBBRs in order to bring the actual redox potential setpoint into the setpoint range. There are various approaches to actuating and controlling the metering pumps. In one example, the redox potential controller 30 is programmed to adjust the metering pumps in increments of percentage speed. In this way, the flow rate of carbon discharged into an MBBR and the duration of the discharge can be precisely controlled by the redox potential controller 30. It should be noted that in some cases the response of the redox potential controller 30 may take into consideration other data or information other than the measured redox potential. Historical data and other information and data can be programmed or inputted into the redox potential controller 30 to augment the measured redox potential of the MBBRs.

[0035] Figures 3 and 4 are graphs that indicate data collected with respect to tests that were conducted over approximately a six-month period for the selenium removal system and process shown in Figure 1. Figure 3 is a graphic illustration reflecting the total selenium concentration of the influent to the MBBRs and the total soluble selenium removal that took place in the three MBBRs, R1, R2, and R3, over this period. The total soluble selenium removal percentage indicates that the process as a whole is efficient and effective, and this can be said to be attributable to selecting an optimum ORP / redox potential setpoint range and providing a programmable control system that is effective in maintaining the ORP / redox potential within an optimum or selected setpoint range.

[0036] Figure 4 is a graphic illustration of the ORP / redox potential profile over this six-month period. Influent ORP / redox potential is generally constant over this period. Initially, the ORP / redox potential in the MBBRs varied considerably. However, after approximately two months the ORP / redox potential stabilized and from that point on, the ORP / redox potential remained generally stable and within the range of -300 mV to -400 mV. This again is the result of the control system that effectively controls the ORP / redox potential in each of the MBBRs within an optimum setpoint range. 5952-625

[0037] There are many advantages to the present process of removing selenium from water. First, by establishing an optimum range for redox potential and providing an automatic control system for maintaining that optimum range, significant overshoots and undershoots are avoided. Secondly, as opposed to injecting carbon at a single point, the present invention provides multiple MBBRs in series and provides for multi-point carbon injection across the MBBRs, which in turn provides redox potential control in each MBBR. Together these features provide a more efficient, reliable and consistent approach to removing selenium in water to ultra-low levels that comply with the most stringent selenium discharge limits in the industry.

[0038] The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the scope and the essential characteristics of the invention. The present embodiments disclosed herein are therefore to be construed in all respects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.

Claims

5952-625CLAIMSWhat is claimed is:

1. A method of biologically removing selenium species from water in a selenium removal system comprising multiple moving bed bioreactors (MBBRs) and multiple metering pumps for supplying external carbon to the MBBRs, the method comprising:directing the water containing the selenium species into and through the MBBRs with each MBBR including biomass supported on biomass carriers;maintaining anoxic or anaerobic conditions in the MBBRs;utilizing the metering pumps to pump carbon from a carbon source into the MBBRs and mixing the carbon with the water and biomass;selecting a redox potential setpoint range for the MBBRs;biologically reducing the selenium species to elemental selenium;after reducing the selenium species to elemental selenium, directing the water containing the elemental selenium to a solids-liquid separator and separating the elemental selenium from the water;maintaining the redox potential in the MBBRs with the redox potential setpoint range by:continuously or intermittently measuring the redox potential in the MBBRs with a redox potential probe;directing redox potential signals representative of the measured redox potential to a redox potential controller programmed to control the metering pumps and the flow of carbon from the metering pumps into the MBBRs based on the redox potential signals; anddirecting control signals from the redox potential controller to the metering pumps wherein the control signals are generated by the redox potential controller to actuate and control the operation of the metering pumps such that the metering pumps discharge carbon into the MBBRs so as to maintain the redox potential in the MBBRs within said redox potential setpoint range.

2. The method of claim 1 wherein the redox potential setpoint range is -325 mV to -375 mV.

3. The method of claim 1 wherein the redox potential controller comprises a PID controller.

4. The method of claim 1 wherein the multiple MBBRs include two or three MBBRs connected in series.

Images