AU711069B2 - Treatment of water - Google Patents

Description

U"'

-1- P/00/0011 Regulation 3.2

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

S

S,

S

.5.5

ORIGINAL

Name of Applicant: Actual Inventors: Address for service in Australia: Invention Title: WATER RESEARCH COMMISSION Peter Dale ROSE, Genevieve Anne BOSHOFF, Oliver O'Connor HART and Johan Pieter BARNARD CARTER SMITH BEADLE 2 Railway Parade Camberwell Victoria 3124 Australia TREATMENT OF WATER The following statement is a full description of this invention, including the best method of performing it known to us THIS INVENTION relates to the treatment of water. More particularly, the invention relates to a process and installation for the treatment of water, suitable for, but not limited to, the simultaneous treatment of effluent waste water containing sulphate anions and heavy metal cations, and effluent waste water containing organic wastes and exhibiting a chemical oxygen demand (COD).

According to one aspect of the invention in the treatment of water containing dissolved sulphate anions and dissolved heavy metal cations, to reduce the dissolved sulphate anion content thereof and to reduce the dissolved heavy metal cation content thereof, by a process including the process steps of: subjecting the water to dissolved heavy metal cation content reduction by dosing it with sulphide anions to cause precipitation of heavy metal sulphide therefrom; o. ~feeding at least part of the water which has been subjected to the dissolved S 15 heavy metal cation content reduction into an elongated trench in a fashion which "causes the water fed into the trench to flow lengthwise along the trench; 0:00 subjecting the water flowing along the trench to a biological dissolved sulphate anion content reduction in the trench to convert the sulphate anions 0.

dissolved in the water to sulphide anions dissolved in the water; 0.

20 withdrawing the water which has been subjected in the trench to the biological dissolved sulphate anion content reduction from the trench and subjecting at least part thereof to a biological oxygenation; and separating biomass from at least part of the water which has been subjected to the biological oxygenation to provide dewatered biomass and product water, there is provided the improvement whereby the process steps are carried out sequentially, the heavy metal cation content reduction taking place before the biological oxygenation and the biomass separation taking place after the biological oxygenation and acting to provide dewatered biomass capable of use as a byproduct, the precipitated heavy metal sulphides being separated from the water before the biomass separation takes place.

BGC:JMD:#28391 10 August 1999 3 More particularly, the biological oxygenation may be an algal oxygenation, the water withdrawn from the trench and subjected to the biological oxygenation being, before it is subjected to the biological oxygenation, subjected to a dissolved sulphide anion content reduction by dosing it with heavy metal cations to cause precipitation of heavy metal sulphide therefrom; and part of the water which has been subjected to the biological oxygenation may be recycled into the trench to cover and enclose the water being subjected in the trench to the biological dissolved sulphate anion content reduction, thereby to seal off the water being subjected to the biological dissolved sulphate anion content reduction from the atmosphere, the water which has been subjected to the biological oxygenation and which has been recycled to the trench flowing lengthwise along the trench while being exposed to the atmosphere and undergoing polishing.

~Subjecting the water flowing along the trench to the biological dissolved sulphate anion content reduction may comprise feeding metabolizable carbon into the water in the trench and allowing the metabolizable carbon to be metabolized by organisms which act to cause the dissolved sulphate anion content reduction. While the metabolizable carbon source may comprise biomass from the biological oxygenation, it conveniently comprises an organic carbon source which exhibits a 20 high chemical oxygen demand (COD) and is an effluent or waste product comprising organic material dissolved and/or suspended in waste water. In a particular, feeding the metabolizable carbon into the water in the trench may comprise feeding metabolizable carbon-containing water into the trench selected from: tannery waste water; BGC:JMD:#28391 10 August 1999 brewery waste water; water forming part of sewage sludge; starch manufacture waste water; paper pulp waste water; and water containing algal biomass and which has been subjected to the high rate algal oxygenation.

These waste waters provide metabolizable organic carbon and the necessary organisms for the biological sulphate reduction in the trench, although, naturally, the trench can be seeded with suitable microorganisms for the biological sulphate 10 reduction. Using these waste waters, which typically are at least somewhat saline, has the advantage that they are purified and eventually leave the process as product water when separated from biomass from the algal oxygenation. When such waste waters are not available, however, biomass from the desalination, otherwise forming a by-product, can, as indicated above, be used instead.

Conveniently, the dosing with sulphide anions of the water being subjected to the dissolved heavy metal cation content reduction is by means of water containing dissolved sulphide anions, which water has been subjected to the biological dissolved sulphate anion content reduction in the trench and has been withdrawn from the trench. In accordance with the invention, heavy metal sulphide selected from the heavy metal'sulphide precipitated from the water being subjected to the dissolved heavy metal cation content reduction, from heavy metal sulphide precipitated from the dissolved sulphide anion content reduction, and from mixtures thereof, may be converted to heavy metal salts and by-product sulphur; and these heavy metal salts arising from the conversion of the heavy metal sulphides into the by-product sulphur may be dosed into the water withdrawn from the trench, before it is subjected to the biological oxygenation, thereby to reduce the dissolved sulphide anion content of the water by precipitating heavy metal sulphides therefrom.

The algal oxygenation may be a so-called high rate algal oxygenation by means of algae, conveniently in a high rate algal oxygenation pond, for example as described in the Applicant's South African Patent No 92/4896.

Feeding the water which has been subjected to the dissolved heavy metal cation content reduction into the trench mray into one end of the trench, the water in the trench flowing along the trench and being withdrawn from the opposite end of the trench prior to being subjected to the dissolved sulphide anion content 10 reduction. Feeding the water from the high rate algal oxygenation into the trench may be into said opposite end of the trench, the polished water being withdrawn from said one end of the trench before being recycled to the algal oxygenation, so that the water undergoing polishing and the water undergoing biological dissolved sulphate anion content reduction flow countercurrent to each other, in opposite directions, along the trench. More particularly, feeding the water which has been subjected to the dissolved heavy metal cation content reduction may be into one end of the trench, the water flowing along the trench to the opposite end of the trench, from which it is withdrawn prior to being subjected to the biological dissolved sulphate anion content reduction, water which has been subjected to the *20 biological oxygenation being fed into the opposite end of the trench to cover and enclose the water in the trench being subjected in the trench to the biological dissolved sulphate anion content reduction, thereby to seal off the water being subjected to the biological dissolved sulphate anion content reduction from the atmosphere, the water which has been subjected to the biological oxygenation then flowing along the trench, countercurrent to the flow of the water being subjected to the biological dissolved sulphate anion content reduction, to the one end of the 6 trench, from which it is withdrawn and then recycled to the biological oxygenation.

Conveniently these waters flowing in opposite directions are separated by an impermeable membrane therebetween over the length of the trench and over a major proportion of the width of the trench, there being some hydraulic continuity, to a limited degree, between the waters on opposite sides of the membrane, along the longitudinal edges of the membrane.

Hydrogen sulphide gas may be withdrawn from the water below the membrane and any sulphide ions escaping past the membrane from the water below the membrane into the water above the membrane will typically be oxidized 10 to sulphate ions by oxygen dissolved in the water above the membrane, which water has been oxygenated and is thus oxygen-rich compared with the water below the membrane. The water above the membrane also serves to insulate the water below the membrane from heat loss.

Heavy metal sulphides precipitated in the dissolved heavy metal cation content reduction of the water and heavy metal sulphides precipitated in the dissolved sulphide anion content reduction of the water withdrawn from the biological dissolved sulphate anion content reduction can be combined and converted together as suggested above to sulphur, as a by-product, and to heavy metal salts, useful in said dissolved 9slphide anion content reduction.

During the dissolved sulphate anion content reduction organic sediment will precipitate in the trench and the process may thus include recycling water and sediment in the trench below the membrane in an upstream direction, water being removed from the trench at a plurality of positions and reintroduced into the trench at positions upstream of the positions where it is removed, there being a series of the recycle flow lines along the length of the trench, each flow line optionally having its inlet adjacent the outlet of the flow line downstream thereof, and having its outlet adjacent the inlet of the flow line upstream thereof, to form a plurality of flow loops in the trench below the membrane, the loops dividing the length of the trench into a series of portions which give an approximation of a plurality of interconnected stirred chambers. In other words, part of the water in the trench which is being subjected to the biological dissolved sulphate anion content reduction may be removed from the trench, being recycled in an upstream direction, relative to the direction of flow along the trench of the water being subjected to the biological dissolved sulphate anion content reduction, the removal 10 of the recycled water taking place at a plurality of positions spaced along the length of the trench and the recycled water being reintroduced into the trench at a plurality of positions spaced along the trench, any sediment in the water removed Sfrom the trench simultaneously being recycled.

According to another aspect of the invention there is provided an installation 15 for the treatment of water in accrdance with the process described above, the installation comprising a dissolved heavy metal cation precipitation stage having a precipitate outlet and a water outlet separate from the precipitate outlet, the water e*O S outlet leading to an elongated trench forming a biological dissolved sulphate anion content reduction stage, the trench having an outlet in turn leading to a biological 20 oxygenation stage, and the biological oxygenation stage in turn having an outlet leading to a biomass separation stage.

The trench may have 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 biological dissolved sulphate anion content reduction chamber, below the membrane, which chamber forms the biological dissolved sulphate anion content reduction stage, and an upper water polishing chamber, above the membrane, the chambers being in hydraulic communication along the length of the trench on opposite side edges of the membrane.

The membrane may be supported on a porous grid or mesh below the membrane. The grid or mesh may slope upwardly from opposite sides of the trench to a central ridge extending lengthwise along the trench, Gas off-takes may be provided from below the membrane at spaced positions along the trench, conveniently from the ridge of the grid.

Gutters may extend along the walls of the trench, the membrane extending across the width of the trench from one gutter to the other, the hydraulic 10 communication between the upper and lower chambers taking place via the gutters and past opposite side edges of the membrane which are spaced from the trench walls and which overhang the gutters. A series of pumps may be associated with the trench, spaced along its length, each pump being arranged to recycle water and sediment from the lower chamber in an upstream direction relative to flow along the lower chamber, each pump being associated with a flow line along which it pumps water and sediment, the flow line having an inlet from the lower chamber and an outlet into the lower chamber which is upstream of said inlet. The inlet and the outlet of each flow line may be movable longitudinally along the lower chamber and the outlet of each flow line may be adjacent the inlet of a similar flow line upstream thereof, the inlet of each flow line correspondingly being adjacent the outlet of a similar flow line downstream thereof.

Typically the trench will have a depth of 2 6 m, preferably 3 5 m, below the membrane, a depth of 0.5 2 ri, preferably 1 2 m above the membrane, and a width of 10 30 m, preferably 15 25 m, having a length of up to one or more 9 km, eg 1 4 km, typically 2 3 km. The series of pumps associated with the trench may comprise 2 5 pumps, conveniently 2 4 pumps.

The installation will further comprise the dissolved heavy metal cation precipitation stage for the precipitation of dissolved heavy metal cations from water being treated as the sulphides thereof, this precipitation stage having a raw water inlet and a sediment outlet leading to a precipitate conversion stage for converting heavy metal sulphide precipitate into sulphur and heavy metal salts, said precipitation stage having a water outlet feeding into the lower chamber of the trench, preferably at one end thereof.

10 The installation may further comprise a dissolved sulphide anion precipitation stage for the precipitation of dissolved sulphide anions as heavy metal sulphides from water withdrawn from the trerh, preferably from the opposite end of the trench to that into which the dissolved heavy metal cation precipitation stage feeds, the dissolved sulphide anion precipitation stage having a sediment outlet 15 conveniently leading to said precipitate conversion stage and a water outlet conveniently leading to a an algal oxygenation stage such as an algal high rate oxygenation pond.

The precipitate conversion stage may have a sulphur by-product outlet and a heavy metal salts outlet feeding into the dissolved sulphide anion precipitation stage and a flow line may be provided for recirculation of water withdrawn from said opposite end of the trench, upstream of the dissolved sulphide anion precipitation stage, into the water entering the dissolved heavy metal cation precipitation stage, to provide sulphide anions for dissolved heavy metal precipitation.

As indicated above, the installation may include a biological oxygenation stage such as a high rate algal oxygenation pond, having an inlet fed from the dissolved sulphide anion precipitation stage and an outlet optionally leading to a biomass separation stage where biomass is separated from purified water product.

The oxygenation stage may further have a water outlet leading into the upper chamber of the trench, conveniently at said opposite end of the trench, a return flow line leading from the upper chamber at the one end of the trench, back to the oxygenation stage. An organic carbon feed line may lead to the lower chamber of the trench, conveniently at the one end of the lower chamber, and a flow line optionally leads from the biomass separation stage to the organic carbon feed line, for allowing biomass to be used. as organic carbon feed.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a schematic flow diagram of an installation according to the 15 invention, for carrying out the method of the invention; Figure 2 is a schematic cross-section of a trench forming part of the installation of Figure 1, along line I1-11 in Figure 3; and Figure 3 is a schematic longitudinal section of the trench whose crosssection is shown in Figure 2, along line IIl-Ill in Figure 2.

In Figure 1 of the drawings, reference numeral 10 generally designates an installation in accordance with the invention, for carrying out the method of the invention. A major feature of the in'stallatibn 10 is a biological dissolved sulphate anion reduction stage formed by a trench generally designated 12 and described in more detail hereunder with reference to Figures 2 and 3. The trench 12 has a membrane 14 extending along its length and across its width, as also described in more detail hereunder with reference to Figures 2 and 3.

11 In Figure 1 of the drawings, a raw water feed line is shown at 16, feeding into a dissolved heavy metal cation precipitation stage designated 18 and in the form of a flocculator. A flow line 20 is shown feeding from the flocculator 18 into one end of the trench 12, below the membrane 14. A further flow line 22 is shown extending from the flocculator 18 to a heavy metal sulphide conversion stage 24.

The conversion stage 24 has a sulphur product outlet flow line 26 and a dissolved heavy metal salts outlet flow line 28. A further heavy metal sulphide feed line 30 is shown feeding into the conversion stage 24, 10 The trench 12 has an outlet flow line 32 feeding into a dissolved sulphide ik* anion precipitation stage 34 in the form of a flocculator similar to the flocculator 18. The flow line 28 from the conversion stage 24 feeds into the flow line 32 between the trench 12 and flocculator 34, and the flow line 30 flows from the flocculator 34 to the conversion stage 24. A sulphide feed flow line 36 is shown feeding from the flow line 32, between the trench 12 and the position where the flow line 28 joins the flow line 32, into the flow line 16, upstream of the flocculator 18.

A flow line 38, for feeding a source of organic carbon to the trench 12 below the membrane 14, is shown feeding into the flow line 20 between the flocculator 18 and the trench 12.

A flow line 40 leads from the flocculator 34 to an algal high rate oxygenation pond 42 which is shown having an outlet flow line 44 leading to a separation stage 46 for separating algal biomass from product water, the stage 46 having a product water outlet flow line 48 and an algal biomass outlet flow line 50. The algal biomass outlet flow line 50 is shown feeding into the flow line 38.

The pond 42 has a further outlet flow line 52 for feeding water into the trench 12 above the membrane 14 at the end of the trench 12 opposite the one end of the trench which receives water from the flocculator 18, and there is a flow line 54 from said one end of the trench which receives water from the flocculator 18, for feeding water from above the membrane 14 in the trench 12 back to the pond 42.

In Figures 2 and 3 the trench is generally designated 12, the membrane 10 again being designated 14. The trench 12 is in the form of a ditch having a floor 56 and side walls 58, with end walls 60 being provided at opposite ends of the trench. At one end of the trench, designated 62, the flow line 20 is shown entering the trench 12 below the membrane 14 and the flow line 54 is shown leaving the trench 12, above the membrane 14. At the opposite end of the trench, designated 64, flow line 32 is shown leaving the trench 12 below the membrane 14, and the flow line 52 is shown entering the trench 12, above the membrane 14, The membrane 14 divides the trench into a lower chamber generally designated 66 and an upper chamber generally designated 68.

In Figures 2 and 3 the membrane 14 is shown resting on and supported by a metal mesh or grid 70, the grid sloping upwardly from gutters 72 which extend along opposite side walls 58 of the treifih 12, to a central ridge or apex at 74. The grid 70 is supported at its apex 74 by a plurality of longitudinally spaced pillars 76.

It is also supported at its outer longitudinal edges by a plurality of longitudinally spaced supports 78, shown in Figure 2 as bolts connected to the side walls 58 above the gutters 72. The gutters 72 have upper edges which are spaced below 13 the grid 70 and below the membrane 14, and the outer edges of the grid 70 and membrane 14, at 80, are spaced horizontally from the walls 58, so that hydraulic continuity is provided at opposite ends of the membrane 14, as indicated by arrows 82. Gas outlet pipes 84 are provided from the lower chamber 66 through the membrane 14 at the ridge 74.

In Figures 2 and 3 the trench is shown roughly to scale, having a width between the side walls 58 of about 20 m, a depth below the outer edges of the membrane of about 4 m and a length of about 1 72 km.

A layer of sediment 86 is shown on the floor 56 of the trench 12 (Figure 3), 10 having a maximum depth at the end 62 of the trench and a minimum depth at the end 64 of the trench.

The trench 12 is provided with a plurality of pumps, namely three pumps 88, and 92, associated with respective flow lines 94, 96 and 98. The pumps 88 92 are arranged to withdraw sediment and water from the trench 12, and to pump 15 it in a direction away from the end 64 and towards the end 62 of the trench, ie in an upstream direction relative to water flow in the trench 12 below the membrane 14 from the end 62 to the end 64 of the trench.

The pump 88 is closest to the end 64 of the trench and withdraws water from adjacent said end 64, and reintroduces it into the trench 12 at a position adjacent and downstream, relative to water flow along the trench 12 under the membrane, from that at which the central pump 90 withdraws water from the trench 12. The pump 92 closest to the end 62 of the trench 12 similarly withdraws water from the trench 12 adjacent arid upstream, relative to water flow along the trench 12 under the membrane 14, from the position at which the central pump reintroduces water into the trench 12, and said pump 92 reintroduces water into the trench 12 adjacent said end 62 of the trench 12. Thus, the outlet of the flow line 94 of the pump 88 is downstream, relative to water flow along the trench 12 under the membrane 14, from the inlet to the flow line 96 of the pump 90, the outlet of the flow line 96 being downstream, relative to water flow along the trench 12 under the membrane 14, from the inlet to the flow line 98 of the pump 92, The flow lines 94, 96 and 98 are all roughly of the same length, ie about 0.5 km each.

The process of the invention will now be described with reference to Figures 1 3 of the drawings, particularly with reference to Figure 1, 10 In accordance with the process of: the present invention raw water feed, such as mine effluent waste water containing heavy metal cations such as ferrous cations and sulphate anions is fed along flow line 16 Into flocculator 18, the raw water feed receiving water containing dissolved sulphide anions from flow line 36 upstream of the flocculator 18. In the flocculator 18 heavy metal sulphide 15 precipitation takes place, such as ferrous sulphide precipitation. Heavy metal sulphide is withdrawn from flocculator 18 along flow line 22 to heavy metal sulphide conversion stage 24 where' it is combined with further heavy metal sulphide entering stage 24 along flow line 30, being converted in said stage 24 to sulphur and heavy metal salts such as ferrous salts. The sulphur is withdrawn from stage 24 along sulphur product line 26, and heavy metal salts dissolved in water issue from stage 24 along flow line 28.

Water whose dissolved heavy metal cation content has been reduced in the flocculator 18 passes along flow line 20 into the trench 12 at the upstream end 62 of the trench 12 below the membrane 14. This water receives an organic carbon feed from flow line 38 which feeds into flow line 20. The water fed to the trench 12 along flow line 20 flows along the length of the trench from the end 62 of the trench to the opposite end 64 thereof, under the membrane 14, where it undergoes a biological dissolved sulphate anion content reduction, with the production of sulphide anions in the water. Gases produced by microorganisms under the membrane 14 are removed via the pipes 84 for treatment or disposal thereof.

Water issues from below the membrane 14 in the trench 12 along flow line 32 to flocculator 34. Dissolved sulphide-containing water is withdrawn from flow line 32 into flow line 36 and thence into flocculator 18.

Flocculator 34 is fed with water containing dissolved sulphide ions along 10 flow line 32, and is fed with dissolved heavy metal salts along low line 28.

Sulphide precipitation takes place in flocculator 34. Heavy metal sulphide precipitated in flocculator 34 passes along flow line 30 into conversion stage 24.

Water whose dissolved sulphide anion content has been reduced by precipitation of heavy metal sulphides in the flocculator 34 passes along flow line 40 into pond 42 where it undergoes a so-called algal high rate oxygenation.

Product water, containing algal biomass, issues from pond 42 along flow line 44, into separation stage 46 where algal biomass is separated from product water.

Product water issues from separation stage 46 along product flow line 48 and algal S biomass in turn issues from separation stage 46 along flow line 50 which feeds into flow line 38 and thence into the trench 12 below the membrane 14. If desired, and if a separate source of organic carbon is available, the algal biomass in flow line 50 may be withdrawn as a by-product, the other organic carbon source, such as tannery effluent wa'ste water, being used to supply organic carbon to the trench 12 below the membrane 14.

16 Water which has been oxygenated in pond 42 is withdrawn from pond 42 along flow line 52 which feeds it into the trench 12 above the membrane 14 at the end 64 of the trench. This water flows along the trench from the end 64 to the end 62 of the trench, above the membrane 14, countercurrent to the direction of water flow below the membrane 14. This water, as it flows along the trench above the membrane 14, becomes polished. It is returned from the end 62 of the trench 12 along flow line 54 to the pond 42.

It is to be noted that the water above the membrane 14 acts to seal off the water below the membrane 14 from the atmosphere, and thermally to insulate the 10 water below the membrane, There is hydraulic continuity between the water above the membrane and the'water below the membrane 14, as indicated by the arrows 82, via the gutters 72, past the outer edges of the membrane 14 where they overhang the gutters 72. The grid 70 acts to support the membrane, and to provide the membrane with a central ridge, from which the pipes 84 withdraw gas 15 from below the membrane 14. The pillars 76 in turn support the grid 70 at its ridge 74, the outer edges of the grid being supported from the walls 58 of the trench 12 by the bolts 78,.

It is further to be noted' that, below the membrane 1, countercurrent a recirculation of water takes place by means of the pumps 88 92 and their associated respective flow lines 94 98. This recirculation provides what can be regarded as three circulation loops or cells in the trench 12 below the membrane 14, roughly equivalent to three separate stirred chambers interconnected in series.

It is an advantage of the invention, as illustrated in the drawing, that the process and installation of the invention make possible the simultaneous treatment of waters containing organic wastes, and waters containing dissolved heavy metal 17 cations and dissolved sulphate anions, to produce, as products or by-products, sulphur, water which can in principle be potable if the process is carried out carefully, and biomass such as algal biomass. Provided that land area is available, the trench, in the form of a ditch, can provide an extremely effective yet economic sulphate reduction stage and water polishing stage. Sediment 86 will slowly accumulate in the trench 12 as indicated in Figure 3, more rapidly in upstream parts of the trench relative to water flow direction under the membrane 14 than in downstream parts. This sediment can, when necessary, be removed from the trench from time to time, eg using the pumps 90 92.

10 Finally, it is to be noted that the positions of the inlets and/or outlets of the

C

flow lines 94 98 associated with the pumps 88 92 can be varied in upstream and downstream directions, if desired, arid, if desired, additional pumps similar to the pumps 88 92 may be provided for the trench 12, with their associated flow lines.

C

For the purposes of this specification, including the claims, the term "comprising" shall be taken to have the meaning "including".

Claims (13)

1. In a process for the treatment of water containing dissolved sulphate anions and dissolved heavy metal cations, to reduce the dissolved sulphate anion content thereof and to reduce the dissolved heavy metal cation content thereof, the process including the steps of: subjecting the water to dissolved heavy metal cation content reduction by dosing it with sulphide anions to cause precipitation of heavy metal sulphide therefrom; feeding at least part of the water which has been subjected to the dissolved heavy metal cation content reduction into an elongated trench in a fashion which causes the water fed into the trench to flow lengthwise along the trench; subjecting the water flowing along the trench to a biological dissolved 00*0 ulphate anion content reduction in the trench to convert the sulphate anions S 15 dissolved in the water to sulphide anions dissolved in the water; withdrawing the water which has been subjected in the trench to the biological dissolved sulphate anion content reduction from the trench and subjecting at least part thereof to a biological oxygenation; and 0* ~separating biomass from at least part of the water which has been subjected 20 to the biological oxygenation to provide dewatered biomass and product water, the improvement whereby the process steps are carried out sequentially, the heavy metal cation content reduction taking place before the biological oxygenation and *o *the biomass separation taking place after the biological oxygenation and acting to provide dewatered biomass capable of use as a by-product, the precipitated heavy metal sulphides being separated from the water before the biomass separation takes place.

2. A process as claimed in claim 1, in which the biological oxygenation is an algal oxygenation, the water withdrawn from the trench and subjected to the algal oxygenation being, before it is subjected to the algal oxygenation, subjected to a BG, CJMD:#28391 10 August 1999 19 dissolved sulphide anion content reduction by dosing it with heavy metal cations to cause precipitation of heavy metal sulphide therefrom.

3. A process as claimed in claim 1 or claim 2, in which part of the water which has been subjected to the biological oxygenation is recycled into the trench to cover and enclose the water being subjected in the trench to the biological dissolved sulphate anion content reduction, thereby to seal off the water being subjected to the biological dissolved sulphate anion content reduction from the atmosphere, the water which has been subjected to the biological oxygenation and which has been recycled to the trench flowing lengthwise along the trench while 10 being exposed to the atmosphere and undergoing polishing. a

4. A process as claimed in any one of claims 1 3 inclusive, in which subjecting the water flowing along the trench to the biological dissolved sulphate anion content reduction comprises feeding metabolizable carbon into the water in the trench and allowing the metabolizable carbon to be metabolized by organisms which act to cause the dissolved sulphate anion content reduction.

A process as claimed in claim 4, in which feeding the metabolizable carbon into the water in the trench comprises feeding metabolizable carbon-containing water into the trench selected from; a tannery waste water; brewery waste water; water forming part of sewage sludge; starch manufacture waste water; paper pulp waste water; and water containing algal biomass and which has been subjected to the high rate algal oxygenation.

6. A process as claimed in any one of the preceding claims, in which the dosing with sulphide anions of the water being subjected to the dissolved heavy metal cation content reduction is by means of water containing dissolved sulphide anions, which water has been subjected to the biological dissolved sulphate anion content reduction in the trench and has been withdrawn from the trench.

7. A process as claimed in any one of the preceding claims, in which heavy metal sulphide selected from the heavy metal sulphide precipitated from the water being subjected to the dissolved heavy metal cation content reduction, from heavy metal sulphide precipitated from the dissolved sulphide anion content reduction, a 10 and from mixtures thereof, is converted to heavy metal salts and by-product sulphur, a a

8. A process as claimed in claim 7, in which the heavy metal salts arising from the conversion of the heavy metal sulphides into the by-product sulphur are dosed into the water withdrawn from the trench, before it is subjected to the biological oxygenation, thereby to reduce the dissolved sulphide anion content of the water by precipitating heavy metal sulphides therefrom, 0*

9. A process as claimed in any one of the preceding claims, in which feeding the water which has been subjected to the dissolved heavy metal cation content 0 reduction is into one end of the trench, the water flowing along the trench to the opposite end of the trench, from which it is withdrawn prior to being subjected to the biological dissolved sulphate anion content reduction, water which has been subjected to the biological oxygenation being fed into the opposite end of the trench to cover and enclose the water in the trench being subjected in the trench to the biological dissolved sulphate anion content reduction, thereby to seal off the water being subjected to the biological dissolved sulphate anion content reduction 21 from the atmosphere, the water which has been subjected to the biological oxygenation then flowing along the trench, countercurrent to the flow of the water being subjected to the biological dissolved sulphate anion content reduction, to the one end of the trench, from which it is withdrawn and then recycled to the biological oxygenation.

A process as claimed in any one of the preceding claims in which part of the water in the trench which is being subjected to the biological dissolved sulphate anion content reduction is removed from the trench, and is recycled in an upstream direction, relative to the direction of flow along the trench of the water 10 being subjected to the biological dissolved sulphate anion content reduction, the removal of the recycled water taking place at a plurality of positions spaced along the length of the trench and the recycled water being reintroduced into the trench at a plurality of positions spaced along the trench, any sediment in the water removed from the trench simultaneously being recycled.

11. A process as claimed in claim 1, substantially as described herein with reference to the accompanying drawings.

12. An installation for the treatment of water in accordance with the process of claim 1, the installation comprising'a dissolved heavy metal cation precipitation stage having a precipitate outlet and a water outlet separate from the precipitate outlet, the water outlet leading to 'ani elongated trench forming a biological dissolved sulphate anion content reduction stage, the trench having an outlet in turn leading to a biological oxygenation stage, and the biological oxygenation stage in turn having an outlet leading to a biomass separation stage.

13. An installation as claimed in claim 12, substantially as described and as illustrated herein with reference to the accompanying drawings. DATED: 19 January 1999 CARTER SMITH BEADLE Patent Attorneys for the Applicant: WATER RESEARCH COMMISSION S S S SS S. S S SS S *5SS S S *SS. 5555 S S 5555 *5 S SS S LI~ 'I. {I 3GC:JHi:#28391 19 January 1999