Treatment of sulphate- and metal-containing water

Details Treatment of sulphate- and metal-containing water Treatment of sulphate- and metal-containing water

1999-10-14

2001-05-08

Simmons, David A. (Department: 1724)

Liquid purification or separation

Processes

Treatment by living organism

C210S603000, C210S610000, C210S631000, C210S912000, C435S262500, C423S140000, C423S153000, C423SDIG001, C423SDIG001

Reexamination Certificate

active

06228263

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.
According to the invention, there is provided a process for treating sulphate- and metal-containing waste water, which process includes
subjecting, in a reaction stage, the sulphate- and metal-containing waste water to biological sulphate reduction in which sulphates in the waste water are converted to sulphides, with metals present in the waste water precipitating out and treated waste water being obtained;
withdrawing the precipitated metals from the reaction stage;
withdrawing treated waste water from the reaction stage; and
subjecting the treated waste water to polishing and/or to nutrient removal.
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 are also normally acidic.
The process may include adding metabolizable carbon to the sulphate- and metal-containing waste water in and/or before the reaction stage, for metabolization by the 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’), and may be water having organic material dissolved, suspended and/or carried therein. In one embodiment of the invention, the water may be an effluent or waste product. The effluent or waste product may be sewage, such as primary sewage sludge and/or secondary sewage sludge; settled sewage; settled sewage solids; tannery waste water; brewery waste water; starch manufacture waste water; winery waste water; and/or paper pulp waste water. In another embodiment of the invention, the water may have the organic source suspended therein, such as fine lignocellulosic material suspended in water. These waters all 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, with the sulphate- and metal-containing waste water entering the reactor at or near an inlet end thereof and the treated waste water being withdrawn at or near an outlet end thereof. As the sulphate- and metal-containing waste water, containing the metabolizable carbon source admixed therewith, flows along the reactor from its inlet end near the inlet end of the reactor, with hydrolysis of the solids component thereof into non-digestible or refractory COD material, hereinafter also referred to as ‘RefCOD material’; slowly biodegradable COD material, hereinafter also referred to as ‘SBCOD material’, and readily biodegradable COD material, hereinafter also referred to as ‘RBCOD material ’, also taking place in the reactor, mainly downstream of the metal precipitation. At least some of the RefCOD and SBCOD material has a larger particle size than the RBCOD material, which has a particle size which is typically of the order of bout 0.1 &mgr;m or smaller. Typically, the RefCOD and SBCOD materials have particle sizes in the range 60 to 1000 microns. At least some of the RefCOD and SBCOD material thus settles to the bottom of the reactor as the waste water passes along the reactor, while at least some of the RBCOD material is withdrawn from the reactor with the treated waste water. Typically, substantially all of the RefCOD and SBCOD material settles, while substantially all of the RBCOD material is withdrawn with the treated waste water.
The accelerated hydrolysis reactor may thus comprise a precipitation section at or near its inlet end and a subsequent hydrolysis section. The reactor may include, at or in a bottom or base thereof and in the precipitation section, at least one primary valley or trough in which the precipitated metals collect, and, at or in a bottom or base thereof and in the hydrolysis section, at least one secondary valley or trough. The primary and secondary troughs or valleys thus extend transversely to the direction of water flow through the reactor. A plurality, eg three, of the primary troughs or valleys, located adjacent one another, may be provided. Likewise, a plurality of the secondary troughs or valleys, located adjacent one another, may be provided.
Thus, the precipitated metals which collect in the primary valleys or troughs will be withdrawn, eg by means of pumps, in the form of a slurry.
The settled material in the secondary troughs or valleys can be recycled to the reactor, preferably with shearing thereof, eg by means of a high shear pump. In this fashion, in addition to the hydrolysis, fractionation of the RefCOD and SBCOD material, into RBCOD material, occurs.
Typically, the accelerated hydrolysis reactor may comprise three of the secondary valleys. The settled material from each of the secondary valleys may be recycled to the inlet end of the reactor. Instead, however, the settled material of the second and third secondary valleys can be recycled to the reactor downstream of its inlet end, eg to above the second and third secondary valleys respectively.
The process may include removing sulphides from the accelerated hydrolysis reactor. At least some of sulphides which form during the biological sulphate reduction are in the form of gaseous hydrogen sulphide, which collects in a head space of the reactor. The removal of the hydrogen sulphide may then include purging this head space with an inert gas, such as nitrogen, and withdrawing a combined hydrogen sulphide/inert gas stream from the reactor head space. Hydrogen sulphide can then be recovered from this gaseous stream. The recovered hydrogen sulphide can then typically be used as the sulphide required for precipitation of metals 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 pass directly to a polishing stage in which the polishing and/or nutrient removal is effected, it may instead pass through a further reactor where it is subjected to further biological sulphate reduction, ie further biological conversion of sulphates to sulphides. Further setting of solid material can also take place in this reactor. This reactor may also be subjected to hydrogen sulphide removal, eg 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 reactor, an expanded bed granular reactor, a stirred reactor, or the like.
The waste water from the further reactor can then, if desired, pass through an elongated trench reactor, before passing to the polishing stage. 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. In the trench reactor, any residual settleable material can settle out, and biological sulphate reduction can be effected therein, if necessary.
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, eg 1-4 m. 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 lone along which it pumps waste water and sediment, the

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