Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
1999-07-30
2001-03-06
Barry, Chester T. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S631000, C210S757000, C210S803000, C210S804000
Reexamination Certificate
active
06197196
ABSTRACT:
THIS INVENTION relates to the treatment of water. More particularly, the invention relates to the treatment of waste water. Still more particularly, the invention relates to a process for treating sulphate- and metal-containing waste water, and to a process for treating sulphate-containing waste water.
According to a first aspect of the invention, there is provided a process for treating sulphate- and metal-containing waste water, which process comprises
adding a sulphide compound to the waste water, with the sulphide compound reacting with at least one metal in the waste water to form a corresponding metal sulphide and with the metal sulphide precipitating from the waste water;
separating the precipitated metal sulphide from the waste water, to obtain sulphate-containing waste water; and
subjecting the sulphate-containing waste water to biological sulphate reduction in which sulphates in the waste water are converted to sulphides.
The sulphate- and metal-containing waste water may be mine effluent or waste water containing dissolved heavy metal cations, such as ferrous cations, and dissolved sulphate anions. Instead, however, the waste water can be any other dissolved sulphate- and metal-containing waste water. These waste waters may also be acidic.
The sulphide compound may be in liquid or gaseous form. For example, it may be hydrogen sulphide.
The separation of the precipitated metal sulphides from the waste water may be effected in a separation stage, which may comprise a settler.
The biological sulphate reduction may be effected in a reaction stage. A metabolizable carbon source may be added to the waste water in and/or before the reaction stage to form a waste water mixture having a solids component and a liquid component, with the carbon source being metabolized by organisms involved in the biological sulphate reduction. The metabolizable carbon source may comprise an organic carbon source which exhibits a high chemical oxygen demand (‘COD’). In particular, the organic carbon source may be an effluent or waste product comprising organic material dissolved, suspended and/or carried in waste water, such as sewage, settled sewage, settled sewage solids, tannery waste water, brewery waste water, starch manufacture waste water, and paper pulp waste water. Such waste waters provide metabolizable organic carbon and the necessary organisms for biological sulphate reduction in the reaction stage.
The reaction stage may, in particular, comprise an accelerated hydrolysis reactor in which, as the waste water containing the carbon source passes through the reactor, e.g., from one end thereof to the other, hydrolysis of the solids component thereof into non-digestible COD material or RefCOD material; slowly digestible COD material or SBCOD material, and readily biodegradable COD material or RBCOD material, takes place. The RefCOD and SBCOD material may have a larger particle size than the RBCOD material, which typically has a particle size of the order of about 0.1 &mgr;m or less. The RefCOD and SBCOD material settles to the bottom of the reactor as the waste water passes through the reactor, while the RBCOD material is withdrawn from the reactor with the waste water. The waste water from the reactor may be subjected to polishing, in a polishing stage, e.g., to remove nitrogen and phosphorus therefrom.
The accelerated hydrolysis reactor may comprise, at or in its bottom or base, a plurality of valleys in which the settled material collects. The settled material may be recycled, preferably with shearing thereof, to the reactor. The shearing may be by means of a pump, e.g., a high shear pump. In this fashion, in addition to the hydrolysis of the solids component, fractionation of the RefCOD and SBCOD material, into RBCOD material, occurs.
Typically, the accelerated hydrolysis reactor may comprise three of the valleys. The settled material from each of the valleys may be recycled to the inlet end of the reactor. Instead, however, the settled material of the second and third valleys can be recycled to the reactor downstream of its inlet end, e.g., to above the second and third valleys respectively.
At least one sulphide compound will normally be formed in the accelerated hydrolysis reactor during the hydrolysis of the solids component. The process may thus include removing this sulphide compound from the accelerated hydrolysis reactor. At least one of the sulphide compounds which is formed may be gaseous hydrogen sulphide which will thus collect in a head space of the reactor. The removal of the hydrogen sulphide from the reactor may then include purging the head space with an inert gas, such as nitrogen, and withdrawing a combined hydrogen sulphide/inert gas gas stream from the reactor head space. The hydrogen sulphide may then optionally be recovered from this gas stream. The recovered hydrogen sulphide can then typically be used as the sulphide compound required for precipitation of the metal from the raw waste water. Instead, if desired, the gas stream can be subjected to sulphide oxidation, thereby to obtain sulphur as a product.
While the waste water from the accelerated hydrolysis reactor, and which contains the RBCOD material, can be directly subjected to the polishing, it may instead pass through a further reactor in which it is subjected to further biological sulphate reduction, i.e., further biological conversion of sulphates to sulphides. Further settling of solid material can also take place in this reactor. This reactor may also be subjected to hydrogen sulphide removal, e.g., by means of a nitrogen or other inert gas purge, as hereinbefore described for the accelerated hydrolysis reaction. The further reactor may comprise a baffle reactor, a UASB (upflow-anaroebic-sludge-blanket) reactor, an expanded bed granular reactor, a stirred reactor, or the like.
The waste water, before being subjected to the polishing, may also pass through an elongated trench reactor wherein any residual settleable material can settle out, and wherein further biological sulphate reduction can be effected, if necessary. It will be appreciated that the waste water from the accelerated hydrolysis reactor can, instead of passing to the further reactor as hereinbefore described, pass directly to the trench reactor.
The elongated trench reactor typically has a depth of 2-6 m, a width of 10-30 m, and a length of up to one or more kilometers, e.g., 1-4 km. The waste water enters one end of the trench reactor, passes along the trench reactor, and is withdrawn at its other end. A series of pumps may be associated with the trench, with the pumps being spaced apart along its length. Each pump may be arranged to recycle waste water and sediment from the bottom of the trench reactor in an upstream direction relative to the flow of waste water along the reactor. This will keep the bacterial population in the reactor in continuous movement. Thus, each pump is associated with a flow line along which it pumps waste water and sediment, the flow line having an inlet from the trench reactor and an outlet which is upstream of the inlet. All the outlets may be located in proximity to the waste water inlet end of the trench reactor. The trench reactor may thus be as described in ZA 98/3970 or AU 65949/98 which both claim priority from ZA 97/4165, and which are hence incorporated herein by reference, or a modification thereof. For example, when the trench reactor is in accordance with that described in ZA 98/3970/AU 65949/98, i.e., having a membrane extending along its length and across its width at a level below the tops of its side walls, the membrane dividing the trench into a lower sulphate reduction chamber below the membrane and in which the flow line inlets are located, and an upper waste water polishing chamber above the membrane, with the chambers being in hydraulic communication along the length of the trench at opposite side edges of the membrane, the polishing stage may thus be provided by the upper chamber above the membrane.
However, in a modification of the trench reactor of ZA 98/3970/AU 65949/98, the pro
Hart Oliver O'Connor
Rose Peter Dale
Barry Chester T.
James Ray & Associates
Water Research Commission
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