Dual media system for treating wastewater containing selenium

Abstract

A bioreactor has a biofilm supporting bed with at least two types of media. An upper media is relatively porous, preferably porous enough to admit particles of a lower media. In use, wastewater flows downwards through the bed. Soluble nitrogen is reduced in the upper media and converted into nitrogen gas. Nitrogen gas bubbles rise through the upper media and escape from the bed. Selenium is reduced in the lower media and converted into elemental selenium. The elemental selenium is released periodically by backwashing the bed, which may cause fluidization or other expansion of the lower media into the upper media.

Claims

1. A reactor for treating selenium-containing water, comprising: an upstream medium in an upstream media bed and a downstream medium in a downstream media bed wherein the upstream media bed is more porous than the downstream media bed, the reactor has a bed expansion area above the downstream media bed, the upstream medium is fixed in position relative to the reactor and located in some or all of the bed expansion area, the downstream medium comprises particles, and the upstream media is sufficiently porous to admit the particles of the downstream medium into the bed expansion area.

2. The reactor of claim 1 comprising organisms capable of reducing nitrogen attached to the upstream medium and organisms capable of reducing selenium attached to the downstream medium.

3. The reactor of claim 1 further comprising a pipe in or downstream of the downstream medium connected to a supply of backwashing fluid.

4. The reactor of claim 1 wherein the upstream media bed is supported by brackets or a plate attached to the reactor.

5. The reactor of claim 1 wherein the upstream medium comprises a medium selected from the group of vertically configured trickling filter media, activated carbon cloth, rope structures, or synthetic fiber structures.

6. The reactor of claim 1 wherein the upstream medium comprises MBBR media.

7. A process for treating water containing selenium comprising the steps of reducing nitrogen in an upstream medium and reducing selenium in a downstream medium wherein in an operation mode water flows downwards through the reactor, the upstream medium is located in an upstream media bed, the downstream medium is located in a downstream media bed below the upstream media bed, and the upstream media bed has a fixed position in the reactor and in a backwash mode water flows upwards through the reactor, the upstream media bed remains in the fixed position and the downstream media bed expands into at least part of the upstream media bed.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a schematic representation of a prior art biological reactor.

(2) FIG. 2 is a schematic representation of a biological reactor embodying the features of the present invention.

(3) FIG. 3 is a schematic representation of the compacted media bed expansion during backwash.

DETAILED DESCRIPTION

(4) FIG. 2 shows a treatment system 10 which includes a biological reactor 12. The system 10 may be used to remove selenium from wastewater. Biological reactor 12 contains a denitrification zone 14 and a selenium reduction zone 16. Biological reactor 12, as shown, is organized as a downwards plug flow reactor. Other types of reactors and reactor configurations may be used.

(5) During normal operation, wastewater influent 18 enters the biological reactor 12 and travels downward by gravity through the denitrification zone 14. The denitrification zone 14 supports a population of nitrogen reducing organisms. The organisms may be located in a fixed biofilm on relatively porous media bed 20. Media used in the porous media bed 20 provides adequate surface area for microbial growth and is sufficiently porous to allow liquid flow within the biological reactor 12 without significant pressure loss even as nitrogen gas is produced. The porous media bed 20 may comprise stationary media such as vertically configured trickling filter media, stationary submerged media, activated carbon cloth, rope structures or synthetic fiber structures on which biofilm can grow. Loose media, for example MBBR media or synthetic media, may also be used. The porous media bed 20 may be supported by brackets 22 or a plate attached to the biological reactor 12. Alternatively, the porous media bed may rest on the selenium reduction zone 16.

(6) As wastewater influent 18 flows through the denitrification zone 14, the nitrogen reducing organisms convert at least a portion of soluble nitrogen contained in the wastewater influent 18 to nitrogen gas. The nitrogen gas travels upwards through the porous media bed 20 where it may be collected. Alternatively, the biological reactor 12 may be open to atmosphere.

(7) Optionally, the porous media bed 20 may also contain selenium reducing microorganisms to facilitate at least partial reduction of selenium contained in wastewater influent 18.

(8) As wastewater influent 18 leaves the denitrification zone 14, it travels by gravity to the selenium reduction zone 16. The selenium reduction zone 16 supports a population of selenium reducing organisms. The organisms may be located in a fixed biofilm on compacted media bed 24. Activated carbon may be employed as the medium and provides a large surface area available for microbial growth. The activated carbon may be in the form of GAO or pelletized activated carbon. Other media might be used, for example polymeric fibers, crushed stone, pumice, sand, plastic media or gravel.

(9) While passing through the compacted media bed 24, selenium and remaining nitrates, if any, are biologically removed from wastewater influent 14 and are retained within the compacted media bed 24.

(10) Treated effluent 26 exits the biological reactor 12 through effluent port 28. In the preferred embodiment, the effluent port 28 is located below the compacted media bed 24.

(11) During normal operation, solids accumulate in the compacted media bed 24 and the pressure drop across the compacted media bed 24 will increase deteriorating filter efficiency. Backwashing may be initiated at a selected time interval or pressure drop set point.

(12) During backwashing, backwash liquid 36 enters the biological reactor 12 through backwash port 30, which may be connected to a distribution system 32, for example one or more perforated horizontal pipes. Aggregate 34 may be installed around the distribution systems 32 below the compacted media bed 24 to aid in flow distribution while also preventing breakthrough of media to the distribution systems 32. Other systems and arrangements suitable for distributing the backwashing fluid through the compacted media bed 24 may also be used.

(13) Backwash liquid 36 travels upwards through the compacted media bed 24. The solids attached to the compacted media bed 24 are removed and entrained in the backwash liquid 36. The backwash liquid 36 and dislodged solids travel upwards through the media bed 20, and are removed through troughs 38 connected to a backwash effluent line 40. In addition to solids, gasses such as carbon dioxide, nitrogen, and hydrogen sulfide, are also released from the media bed 24 as the backwash liquid 36 travels upwards. Most of the gas separates from the backwash liquid 36 and exit though a vent to atmosphere or a treatment device attached to the biological reactor 12.

(14) The turbulence created by the passage of backwash liquid 36 may expand the compacted media bed 24 beyond its volume during normal operations. Preferably, the media bed 20 is adequately porous to allow the upwards expansion of the compacted media 24 during the backwashing cycle. A graphical representation of the compacted media bed expansion during backwash is provided in FIG. 3.

(15) While the above description provides examples of one or more processes or apparatuses, it will be appreciated that other processes or apparatuses may be within the scope of the accompanying claims.