Patent Application
20240343625
The present invention relates to systems and processes for removing selenium, and more particularly to biological and physio-chemical processes for removing selenium from wastewater.
Selenium has become a pollutant of concern around the world because of its potential effects on human health and the environment. In the USA, recently issued National Pollution Discharge Elimination System (NPDES) permits have forced industrial facilities to meet strict new discharge requirements for selenium (total selenium <10 μg/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 ppb levels in industrial effluents will be significant in the coming years.
There are many sources of selenium. Selenium is found in wastewater from coal mines, oil and gas extraction, petroleum refining, coal fired 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.
As water quality standards become stricter, conventional treatment processes are constrained in reducing selenium to sub-ppb levels. Current state-of-the-art technologies do not offer economically viable processes for reducing selenium to these levels. One example of a selenium removal process is the “ABMet” process offered by General Electric. See U.S. Pat. Nos. 6,183,644 and 8,163,181. This process and other commercial selenium removal processes have serious drawbacks. For example, many of these processes require the removal of nitrates and/or nitrites prior to the removal of selenium. Also, it is typical for the kinetics of commercially available biological processes to be slow. This results in high capital costs to build effective treatment plants. Also, in many cases, biological selenium removal processes rely on the removal of elemental selenium. Processes that require the removal of elemental selenium are challenging to perform efficiently. Finally, virtually all biological selenium removal processes are prone to produce significant concentrations of organic selenium, compounds that are far more toxic than selenates and selenites. The direct consequence of this is that the treated water may actually be more toxic than the water prior to treatment.
Therefore, there is a need for an efficient and cost effective selenium removal process which minimizes the production of elemental selenium, selenium −2 and organo-selenium species.
The present invention relates to a biological selenium removal process for removing selenium and particularly selenium +6 species (selenates) from the water. Water is directed to a first biological reactor containing biomass and operated under anaerobic or anoxic conditions. Selenium +6 species are biologically reduced by the biomass to selenium +4 species (selenites) or absorbed on the biomass. Thereafter, the water containing the selenium +4 species is directed to a precipitation reactor. A coagulant, such as a ferric or aluminum salt, is mixed with the water. Solids having adsorption sites precipitate from the water. Selenium +4 species are adsorbed onto the adsorption sites of the solids. Thereafter, the solids having selenium +4 species adsorbed thereto, in addition to the sloughed biomass containing adsorbed selenium, are separated from the water. The water is further treated in a second biological reactor under aerobic conditions where the water is subjected to reoxygenation resulting in oxidizing the organo-selenium and any residual selenium +4 species back to selenium +6 species, which are generally considered to be less toxic than selenium +4 species and much less toxic than organo-selenium species.
In one embodiment, a biodegradable material, such as a carbon source, is added to the water in the first biological reactor to promote the biological reduction of selenium +6 species to selenium +4 species. Also, in some cases the water includes nitrates (N—NO3) or nitrites (N—NO2). NOx is used herein to refer to nitrates, nitrites or nitrates and nitrites. To minimize the reduction of selenium to elemental selenium and selenium −2 and to minimize the production of organic selenium, the dosage of the biodegradable material is controlled. Control is based on the ratio of chemical oxygen demand (COD) to NOx fed into the first biological reactor. It was found that production of elemental selenium and organic selenium can be minimized or reduced by dosing the biological reactor such that the ratio of COD to NOx is maintained in the range of 6-15.
In one specific embodiment, a process is described herein for removing selenium from water. This process entails directing the water containing selenium into a first biological reactor containing biomass. The water in this reactor is maintained under anoxic or anaerobic conditions. A carbon source is mixed with the water in the first biological reactor which gives rise to biologically reducing selenium +6 species to selenium +4 species while at least some of the selenium may be incorporated into the biomass. Thereafter, the process entails directing the water containing the selenium +4 species and the excess biomass from the first biological reactor to a downstream precipitation reactor. Here a coagulant is mixed with the water, causing solids having surface complexation binding sites to precipitate from the water. In the precipitation reactor, the selenium +4 species are adsorbed onto the complexation binding sites of the solids. At this point, the water containing the solids having the adsorbed selenium +4 species, as well as the biomass, is directed to a solids-liquid separator that separates the water from the solids having adsorbed selenium +4 species and the biomass. After this separation process, the water, substantially free of solids, is directed from the solids-liquid separator to a downstream second biological reox reactor operated under aerobic conditions. In the second biological reactor, the process entails oxidizing the water in the presence of air and removing most of any residual carbon source, and oxidizing most of the remaining selenium species in the water, including organo-selenium, to selenium +6.
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 such invention.
The present invention is a system and process for removing selenium from water or wastewater. As used herein, “water” encompasses wastewater. That is, the terms “water” and “wastewater” are interchangeable. Fundamentally, the process relies heavily on biologically reducing selenate (selenium +6) to selenite (selenium +4) and minimizing the further reduction of selenium to elemental selenium or selenium −2. To remove the selenite from the water, solids are formed having surface complexations that serve as adsorption sites for selenite. Hence, selenite is absorbed onto the solids and the solids are subjected to a solids-liquid separation process where the solids having the adsorbed selenite are separated from the water. At this point, the water may still include residual selenium including organo-selenium, as well as residual biodegradable material that might have been used to facilitate the initial biological reactions that reduced selenate to selenite. To address these contaminants, the water is subjected to a second biological process that is operated under aerobic conditions to remove residual biodegradable material, as well as to oxidize residual selenium, including organo-selenium, back to selenate. Effluent from the second biological process is substantially free of selenium except for the possibility of a very small amount of selenate.
Biomass in biological reactor 4 serves two principal functions. First, the biomass reduces selenate to selenite. Secondly, if NOx are present in the wastewater, the biomass denitrifies the wastewater by reducing NOx to nitrogen gas. To support biomass growth in the biological reactor 4, a phosphorus source (stream 2) might be added to the reactor. Also, a biodegradable material (stream 3), such as a carbon source, is added to the biological reactor to promote the biological reduction reactions. The carbon source is generally a liquid sugar, such as glucose or glycerol. One example of an effective carbon source is glucose monohydrate.
Water containing the selenite is pumped from the biological reactor 4 to a mixed precipitation reactor 7. In the precipitation reactor 7, the water is mixed with a coagulant, a ferric or aluminum salt (stream 6), which results in the precipitation of solids having surface complexation binding sites. Selenite in the water is adsorbed onto the binding sites of the solids. It is preferable to exercise pH control over the water in the precipitation reactor 7. Generally, the pH should be maintained at neutral or slightly acidic. As shown in
Water containing the adsorbed selenite, biomass from the biological reactor 4, and solids from the contaminated wastewater stream 1 is pumped to a solids-liquid separator 9. If required, a polymer (stream 8) can be mixed with the water in the solids-liquid separator 9 to facilitate the separation of solids from the water. Various types of solids-liquid separators can be employed. One such solids-liquid separator is a sand ballasted flocculation process marketed by Veolia Water Technologies under the name ACTIFLO. Other solids-liquid separation systems, such as ultrafiltration units, multimedia filtration units, filter presses, centrifugal separation units such as hydrocyclones or centrifuge, gravity separators such as settlers and decanters as well as disc and drum filters, dissolved air flotation (DAF) and dissolved gas flotation (DGF) units can be employed in the process depicted in
Effluent from the solids-liquid separator 9 is substantially free of solids but may contain organic selenium, residual selenite and residual carbon source, if a carbon source is added to the biological reactor 4. This effluent is directed into a biological reox reactor 15 which includes biomass in the form of fixed film biomass and/or suspended biomass. Air is supplied via line 14 to the biological reox reactor 15 so as to maintain aerobic conditions in the reactor. This results in the removal of any residual carbon source, as well as the oxidation of organic selenium and residual selenite back to selenate. It is desirable to maintain the pH of the water in the biological reox reactor 15 in the range of 6 to 8. Accordingly, one or more pH adjustment reagents can be directed into the biological reox reactor 15 via line 13. Depending on the coagulant added in reactor 7, a very small dosage of phosphorus might also need be added to reactor 15.
To efficiently remove selenium according to the processes described above, it is desirable to minimize the formation of elemental selenium, selenium −2, as well as organic selenium. During testing, it was discovered that the formation of these forms of selenium could be reduced or minimized by controlling the dosage of the reducing agent (for example, the carbon source).
It was found that the initial dosage of the reducing agent could be estimated based on the ratio of COD (expressed as mass of COD per unit of time) to NOx (expressed as mass of NOx as N per unit of time) fed to the biological reactor 4 and maintaining the ratio of COD to NOx at 6-15, and preferably 8-12.
It was also found that the dosage of the reducing agent could be further optimized by maintaining the residual COD concentration in reactor 4 between 20 and 200 mg COD/L, or, alternatively, by keeping the redox potential in reactor 4 between −100 and +80 mV compared to a standard hydrogen electrodes. This is especially useful if there is little or no NOx in the selenium contaminated water (stream 1).
It was found that by both maintaining the ratio of COD to NOx at 6-15, and preferably 8-12, and controlling the residual COD concentrate or redox potential in reactor 4 by varying the dosage of the reducing agent, that the formation of organo-selenium, elemental selenium and selenium −2 is minimized. Based on this, control can be carried out by continuously determining the mass per unit time of COD and NOx fed into the biological reactor 4, determining the resulting ratio of COD to NOx, and measuring the residual COD concentrate or redox potential in reactor 4 and varying the dosage of the reducing agent directed into biological reactor 4 to maintain these control parameters.
A laboratory test was run to reproduce the process as illustrated in
The selected biological treatment for this example was the moving bed biological reactor (MBBR), a fixed film and completely mixed biological treatment. The laboratory apparatus to reproduce the MBBR process at laboratory scale was a 5 L double-walled glass reactor, allowing temperature control using an industrial chiller.
The flowrate for the two biological reactors (reactor 4 and reactor 15) was maintained around 7-8 L/d to provide sufficient retention time to promote complete denitrification. A glucose monohydrate solution was dosed in reactor 4 (mechanically mixed anoxic denitrification reactor) at a rate of 6-10 g COD/g NOx and its dosage was manually adjusted according to the measured soluble residual COD in reactor 4. Water temperature was maintained at 6° C. through the final testing phase in both biological reactors, when the biological system was stable (mass balance are closing), on a 3 months old biomass. Reactor 15 was aerated using an air compressor, providing both oxygen and mixing to the system. The initial source of biomass for the two biological reactors came from seeded carriers taken in a nitrification application for municipal wastewater treatment.
The precipitation reactor step, as well as the solids separation step, were tested in batch conditions due to laboratory limitations. The selected technology for the solids separation was ballasted flocculation (reactor 9), with a precipitation reactor (reactor 7) upstream. The selected chemistry for the physico-chemical step was a ferric chloride coagulant, at a dosage of 62 mg Fe/L, used at an optimal pH of 6.5 for antimony and selenium removal. No pH adjustment was required in this particular example. No sludge recirculation was completed at laboratory scale; however, sludge recirculation should enhance the efficiency of metal removal and at the same time decrease coagulant requirements. Solids separation was aided using a dry anionic polyacrylamide polymer solution, as well as silica sand for high rate ballasted flocculation.
Water from the solid separation step was pumped in the biological reox reactor (reactor 15) at the moment of final water characterization, which was otherwise fed directly from the biological denitrification reactor (reactor 4) to provide sufficient biomass growth during laboratory testing.
The outflow was sampled in each of reactors 4 and 15, as well as at the exit of reactor 9 for performance assessment. The water samples were sent to specialized external laboratories according to their sampling and preservatives recommendations for characterization.
With the flowrate indicated above, which is believed to be close to the optimal design flowrate on a juvenile biomass, the relevant content of the water at the outlet of reactor 4 (biological denitrification reactor) and reactor 9 (solids separation), both operated in stable conditions, were:
Selenium concentration for this example was lowered from 47.9 μg/L to 3.6 μg/L using the combination of biological selenium reduction and physico-chemical removal. Most of the removal was observed through the solids separation step, as most of the selenium out of the biological step leaves as particulate (but not as elemental selenium as it was not detected by the external laboratory in charge of the selenium speciation). The balance of the dissolved selenium, mostly present as selenite, was removed through surface complexation in the precipitation reactor (reactor 7) and then removed from the stream in the solids separation step (reactor 9).
The impact of reactor 15 (biological reox) can be mostly assessed looking at the evolution of the species at the various points, since it could not be operated in continuous conditions as for the other processes. The dissolved inorganic selenium proportion out of reactor 9 was dropped to 32%, due to a good removal of the selenite portion (thus 68% of dissolved organic selenium species). After the biological reox step (reactor 15), the dissolved organic selenium fraction drops down to 8%, as 92% of the dissolved selenium is now in its inorganic form (mostly as selenates with some selenites). It is believed that the oxidation of the organic selenium to inorganic selenium significantly lowers its possible toxicity to the receiving environment.
The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered 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.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/068650 | 7/5/2022 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63227366 | Jul 2021 | US |
