AU654132C - Improved method for solute transfer between solid and liquid phases - Google Patents

Description

IMPROVED METHOD FOR SOLUTE TRANSFER BETWEEN

SOLID AND LIQUID PHASES

This invention relates in the first aspect to the treatment of contaminated effluents by sorption and, in particular, the treatment of domestic waste and in a second aspect to the desorption or leaching of selected components from a solid material and in particular, the extraction of metals from solid materials containing them.

In many residential areas where conventional central sewerage collection and treatment systems are not available, it is customary to use septic tanks wherein the domestic effluent is initially broken down biologically to a liquid and that liquid is released into the surrounding natural ground through leach drains. However, in many areas the use of conventional leach drains would result in contaminants in the effluent from the septic tank entering the natural subterranean water and from there flowing into local rivers and wetlands causing, as a result, environmental damage. It is now common for local authorities to prohibit the use of conventional leach drains in areas where the water table is less than a minimum specified distance below the natural ground level, although these considerations are generally based on engineering constraints rather than on a sound technical approach to the control of contaminant migration and environmental impact. Normally the authorities require a distance of at least one and a half to two metres between the base of the leach drain and the normal upper level of the water table. Also there are areas where the existing nutrient level in the ground, groundwater or surface water is such that effluent discharge thereinto should or must be treated prior to discharge to reduce the nutrients therein so any increase in the existing nutrient level is minimised and any potential for increased environmental impact is minimised. It will thus be appreciated that there are many desirable residential areas where conventional reticulated sewerage systems are not available and where the soil quality, water table and/or nutrient conditions are such that domestic septic tank systems with conventional leach drains may not be usable in order to protect the quality of the local environment. It is therefore the object of the present invention to provide a method and apparatus for the treatment of contaminated waste liquid which is environmentally acceptable, effective in operation and can operate in areas of sensitive prevailing environmental conditions, such as where the normal water table is of a height which does not permit the use of conventional septic system leach drains where the natural soils have no appreciable treatment abilities, or where the underground water is used for potable purposes, or in catchments where there is excessive nutrient enrichment of surface and groundwater.

There is thus provided a method of treating waste liquid containing known contaminants comprising delivering the liquid into a reservoir containing a pervious treatment medium, the reservoir being lined with a material at least impervious to the liquid to prevent direct leakage from the reservoir to the ground, maintaining intimate contact between the liquid and the treatment medium for a period sufficient to interact to reduce selected contaminants in the liquid to predetermined levels.

Optionally, the treated liquid is allowed to pass through a sand layer covering the reservoir to evaporate or pass into the adjacent ground after reduction of said selected contaminants. The required residence time of the waste liquid being treated in the reservoir is dependent on the nature and relative quantities of the contaminants to be removed by the treatment medium, and upon the predetermined levels of contaminants allowable in the released liquid. Factors relevant to the residence time are, the size of the reservoir, the input rate of effluent, the ratio of evaporation compared rainfall, the location of the delivery point of the waste liquid into the reservoir, the length of the flow path through the treatment medium to the sand layer and the hydraulic head differential between the leach drain and the outlet. Depending upon these factors and the excess of evaporation, there may be no overflow from the reservoir at all. This does not prejudice the efficiency of effluent control. Conveniently there is also provided an apparatus for treating waste liquid when used in carrying out the above described method comprising a reservoir lined with a material at least impervious to the waste liquid to be treated, a mass of a treatment medium in the reservoir, waste liquid distribution means located in the reservoir and arranged to disperse said liquid into the treatment medium, a sand layer covering the medium in the reservoir, the arrangement being such that the waste liquid delivered into the reservoir must travel through the treatment medium, at least a predetermined distance selected to provide a resident period for the liquid in the treatment medium to reduce selected contaminants in the liquid to the predetermined level.

Preferably, the waste liquid is delivered into the reservoir in the vicinity of the bottom thereof, or at least below the surface of the treatment medium to provide a substantial residence time. Optionally, baffles may be provided extending from above the upper level of the treatment medium to near the bottom of the reservoir to increase the length of the flow path and hence residence time of the liquid.

In treating waste liquid released from domestic septic tanks, the principal contaminants to be removed are bacteria, phosphorus and nitrogen, and therefore the mud is of a composition that has high bacteria phosphorus and nitrogen absorption capacities. A typical medium consists. for example, of about 70% sand, about 30% red mud (alumina refining residue) and a small amount of gypsum, say 1% to 3%. The term "Red Mud" is well known in the aluminium processing industry and is the residue from the extraction of aluminium oxide (Al₂0₃ ) from Laterite or Bauxite ores by the Bayer Process. The presence of iron oxides, aluminium oxides and aluminosilicates, the very small particle size, and other specific chemical characteristics of this residue results in the high contaminant removal capacity. Other suitable media include, amongst others, the following or combinations of the following: sand, natural clays, loam soils, activated carbon, alum, specific ion exchange resins, iron filings, heavy mineral processing wastes. Conveniently the reservoir may be fully below ground or partly above the level of the surrounding ground and the sand surrounds that part of the wall of the reservoir above ground level. In this way the treated liquid may migrate through the sand into the ground without creating surface water on the ground adjacent the reservoir, The high efficiency which can be achieved in contaminant removal is due to two inherent fundamental features of the flow structure:

(1) There can be no overflow until all of the treatment medium has become fully saturated with effluent and therefore solid/liquid contact is extremely high.

(2) There is a long contact time between solid and liquid.

The degree of intimacy of solid/liquid contact and the length of contact time are fundamental features in determining the rate and efficiency of solute phase transfer either in terms of solute deposits on solid phase or solute leaching from the solid to the liquid phase. Furthermore the apparatus offers the opportunity to control contact time and flow rate by simply modifying the height of the perimeter wall or the head at the liquid delivery point. In the preceding discussion reference has been specifically made to the removal of contaminants from a liquid, however, it is to be understood that the method and apparatus may also be applied to the leaching of selected components from solid material, such as separating metals from ore.

Accordingly there is also provided a method of treating solid material containing at least one selected component by a leaching liquid, comprising delivering a liquid into a reservoir containing said solid material, the reservoir being lined with a material at least impervious to the liquid to prevent leakage from the reservoir to the ground, maintaining the solid material in the reservoir flooded with the liquid for a period sufficient to allow the liquid and solid material to interact to enable transfer of said selected component from the solid material to the leaching liquid, and allowing the leaching liquid containing the selected component to overflow from the reservoir and be collected. Preferably the leaching liquid is introduced adjacent one end of the reservoir below the surface of the solid material and passes out of the reservoir at or adjacent the opposite end of the reservoir above the level of the solid material. Conveniently there is also provided an apparatus for treating solid material when used in carrying out the above described method, comprising a reservoir lined with a material at least impervious to the leaching liquid, a mass of a solid material in the reservoir leaching liquid distribution means located in the reservoir and arranged to deliver leaching liquid into the reservoir below the surface of the solid material, and an outflow area in the wall of the reservoir arranged so the leaching liquid must travel at least a predetermined distance to the outflow area through the solid material within the reservoir, said predetermined distance being selected to provide sufficient residence time for the leaching liquid in the solid material to remove a portion of the selected component from the solid material.

The reservoir may be formed on the surface of the ground or by digging an appropriate sized hole in the ground and lining that hole with a liquid impervious sheet material, such as a plastic sheet, which in view of the size of the reservoir, will normally be required to be of a fabricated bonded or welded construction to eliminate the risk of leakage of liquid into the surrounding ground at joins in the sheet. The plastic sheet is conveniently of a thickness of the order of up to 2 mm or more to have the required strength to resist tearing or puncturing when exposed to the conditions that will be normally encountered in the construction and operation of the effluent reservoir. The invention will be more readily understood from the following description of one practical arrangement thereof as illustrated in the accompanying drawings. In the drawings: Figure 1 is a plan view of the treatment reservoir; Figure 2 is a longitudinal sectional view along the line 2-2 in Figure 1; and

Figure 3 is a transverse cross-sectional view along the line 3-3 in Figure 1.

Referring now to the drawings, the reservoir 10 as illustrated is of a rectangular shape which is convenient for the construction of the impervious liner, however, the reservoir may be of any desired shape. Also as illustrated, the reservoir is divided into two independent sections by the central wall 12 as will be referred to in more detail hereinafter. Depending upon the conditions of use, and requirements of application this feature may be included, however, some authorities responsible for the control of effluent disposal require two leach systems for each on-site effluent disposal system. Also as illustrated, the reservoir is constructed above the level of the surrounding natural ground and again, this is merely one form of reservoir which may equally be constructed partly or fully below ground level.

In a first application, in which one or more, commonly two cells are to be used, the reservoir 10 has a continuous peripheral wall. Each working reservoir has provided therein a leach drain 15, located centrally within the reservoir, each connected to a flow diverter 17 which enables the incoming liquid to be selectively diverted into either or both of the leach drains 15 and 16. Optionally, the leach drain may be located on a base of sand (not shown) or other suitable pervious material in order to gain more effluent head relative to the perimeter wall 11, and to immerse the infiltration area.

In a second application, the reservoir 10 has a continuous peripheral wall 11 and a central longitudinal dividing wall 12, which together define two working reservoirs 13 and 14. Each working reservoir has provided therein respective leach drain units 15 and 16, each connected to a flow diverter unit 17 which enables the incoming liquid to be selectively directed into either or both of the leach drains 15 and 16.

The perimeter wall 11 of the reservoir is conveniently of concrete which may be preformed and assembled on site, or constructed in situ by use of appropriate form moulds, and the construction of the wall preferably provides an inverted V-shape to the finished wall for stability. Alternatively the perimeter wall may be formed by mounding soil or any other suitable material to form a shape over which the impervious membrane may be placed. It will be noted in Figure 3 that the central wall

12 is slightly higher than the perimeter wall 11, and this is to ensure that when the working reservoirs are being operated in sequence, there is no flow of liquid from one working reservoir to the other. All flows are outwardly over the perimeter wall of the reservoir and preferably over the selected portion 27 located opposite to the leach drains 15 and 16 and which is lower than the level of the remainder of the perimeter wall. In this way, a preferred location of discharge of liquid from the reservoir is provided. The respective leach drains 15 and 16 are of identical construction extending substantially the full length of the respective working reservoirs and are located off centre with respect of each of the working reservoirs towards the dividing wall 12. The leach drains 15 and 16 may be of any suitable construction and shape, for example, circular, to provide the required distribution of the incoming contaminated liquid substantially evenly over the full length of the working reservoir. A circular leach drain shape, also commonly referred to as a soak well positioned vertically may also be used, as also can a circular shaped reservoir.

In a third application, each reservoir has a containing wall equivalent to the central wall 12. The leach drain is positioned close to the central wall 12 and flow is generally in the direction away from the containing wall.

A layer of coarse particle material, such as coarse sand, is optionally provided to surround or enclose the leach drains 15 and 16 to provide a jacket to minimise accumulation of slime that would restrict the outflow from the drains.

In the embodiment illustrated, a single drain is shown in each working reservoir, however, a series of parallel drains can be provided distributed across the reservoir, or a series of secondary leach drains, branching from the main leach drains 15 and 16, may be provided as shown in broken outline in Figure 1. All such drains are optionally enclosed in coarse sand or like coarse material as previously referred to. The increased leach drain distribution is considered desirable in order to achieve desired throughflow rate when the pressure head generating the flow into the drains and through the treatment medium is relatively small, when the permeability of the treatment medium is inherently low, or when the clogging of the treatment medium surrounding the leach drains occurs following extended usage reducing the hydraulic permeability of the treatment medium and hence the flow rate is low. In this regard, it may also be desirable to provide a reduced height area in the perimeter wall, particularly in situations where the pressure head is low, to thereby obtain the desired flow rate through the reservoir. When such a reduced height area is provided, it is preferably located at a maximum distance from the entry of the liquid into the reservoir, so the liquid has a long flow path and hence an increased residence time in the reservoir. The reduced height of a section of the perimeter wall_^ is indicated in broken outline at 21 in the working reservoir 23 in Figure 3 of the drawing.

For other applications, it may be desirable to increase the diameter (height) of the leach drain, and the adjacent bed of treatment medium in order to achieve desired flow rate, if a low permeability treatment medium is to be used.

Where there is an excess of rainfall over evaporation, a membrane should be provided to prevent the infiltration of rainwater. The liner 20 is prefabricated from polythene sheet material or other suitable impervious material to the required shape to cover the entire internal surface of each of the working reservoirs 13 and 14 and to extend to the top of the perimetal walls. The liners for the respective working reservoirs 13 and 14 may be made individually or of a one piece construction.

Optionally, air or other gas can be injected to increase the efficiency of the effluent treatment process.

The working reservoirs are each filled with an appropriate material as indicated at 23, commonly referred to as treatment medium, which will support and/or permit the necessary physical, chemical and/or micro-biological activity to remove the selected contaminants from the contaminated liquid delivered into the reservoirs. The treatment medium within the reservoirs and the reservoir outer walls are optionally covered completely with a layer 22 of porous sand material with a bank 25 of sand surrounding the complete perimetal wall 11 of the reservoir forming a continuation of the layer 22 of sand overlying the reservoirs. The form of the sand bank and overlying layer are clearly seen in Figures 2 and 3. This construction of the enclosing sand layer and banks provides facility for the treated liquid to be evaporated from the exposed surface of the sand layer and bank and also for the treated liquid to pass downwardly through the sand bank to enter the surrounding natural ground. These processes may occur acceptably without a sand cover layer in certain applications.

As the leach drains 15 and 16 deliver the contaminated liquid into the lower regions of the respective working reservoirs 13 and 14 below the surface of the mud and adjacent the dividing wall 12, the flow path for the liquid through the treatment medium will be generally in a direction from the inner lower portion of the reservoir to the outer upper end where the liquid can pass over the perimeter wall into the overlying sand layer. In order to control short circuiting of the contaminated liquid baffles such as indicated at 26 in Figure 3 may be provided so that the contaminated liquid may not be discharged over the perimetal wall of the reservoir without first passing beneath the lower end of the baffle.

Baffles may also be arranged to cause the liquid to follow a non-linear path as viewed from above, which may be of serpentine form, through the reseroir.

The provision of two working reservoirs 13 and 14 with respective leach drains 15 and 16 enables the working reservoirs to be used alternately so that while one is in use, the other may be dried out and if necessary, the treatment medium therein may be removed and replaced with fresh treatment medium. Also under periods of unusually high input of contaminated liquid, the flow diverter unit 17 may be set to distribute the incoming liquid to the leach drains in both working reservoirs.

It will be appreciated that the contaminated liquid treatment system as above described would enable the use of conventional septic effluent treatment tanks in residential areas where the water table is too high to permit the use of conventional leach drains buried directly in the ground in the conventional manner or where the geochemistry of the underlying sediments causes them to be poor contaminant filters and so that the nature and, quantities of contaminants that may ultimately reach the natural underground water is within acceptable limits.

It is to be also appreciated that the contaminated liquid treatment system disclosed herein not only enables conventional septic effluent treatment tanks in residential areas where the water table is too high, but also in areas where this height is not necessarily a problem, but water quality is a problem, in that it must be maintained at potable quality to support existing groundwater abstraction schemes or public water supply and therefore the use of conventional leach drains is a constraint in that conventional leach drains may contaminate the groundwater or surface water beyond potable limits. In addition, groundwater may not be the constraint that is overcome, but surface water also, in terms of its nutrients status on an overall catchment basis may be, and is in many instances, a constraint to the utilisation of conventional septic leach drains as they do not remove nutrients. The system disclosed herein facilitates use of on-site waste disposal systems where the catchment condition is one of excessive nutrient enrichment and further development based on traditional technology is therefore constrained. In general the system disclosed provides an alternative to retic -¹ited sewage systems connected to wastewater treatment plants where supply of reticulated sewage is constrained by engineering difficulties such as high expenditure and work programs.

In the preceding description, reference has been specifically made to the removal of contaminants from a liquid. However, the method and apparatus is equally applicable to a desorption or leaching of a selected component from a solid material. In such an operating mode, many of the features described above are of equal relevance. However, the treatment medium above referred to is replaced in such an embodiment with a solid material containing a selected component, for example, raw . or milled ore containing a small concentration of gold. The leach drains 15 and 16 provide the required distribution of a dilute cyanide leaching liquid substantially over the full length of the reservoir, to ensure intimate contact between the leaching solution and the ore so that gold is liberated into the solution, now referred to as pregnant liquid.

As with the above described embodiment, the leach drains 15 and 16 deliver the leaching liquid into the lower regions of the respective reservoirs 13 and 14 below the upper level of the ore and adjacent the dividing wall 12. The flow path for the liquid through the ore is generally in a direction from the inner lower portion of the reservoir to the outer upper end. The height of the wall is preferably selected so the liquid level in the reservoir is sufficient to cover all of the ore in the reservoir throughout the duration of the processing thereof. The pregnant liquid will pass over the perimeter wall at the outer upper end into a collection sump or drain (not shown), which replaces the sand layer of the previous embodiment, and from which the pregnant gold liquid is transferred to a gold extraction stage where, for example, the gold can be adsorbed onto carbon and recovered using any technology known to those skilled in the art. Leaching liquid may be recycled to the reservoir for further leaching after cyanide make up if required.

Such recycle is likely to be essential in many leaching applications because a single pass is unlikely to enable total extraction of the selected component.

In order to control short circuiting of the leaching solution, baffles such as those indicated at 26 in Figure 3 are provided to ensure that the solution may not be discharged over the perimetal wall of the reservoir without first passing beneath the lower end of the baffle. Other forms of baffle such as those of a form which would cause serpentine or other similar tortuous flow path through the treatment media may be provided.

The provision of two working reservoirs 13 and 14 with respective leach drains 15 and 16 enables the working reservoirs to be used alternately as described above. Hence, while one reservoir is in use, the other may be drained and, if necessary, the ore may be removed and replaced with fresh ore. The configuration described above offers an alternative to conventional heap leaching systems and offers superior liquid solid contact, residence time and control of residence time, flow distribution and hence, mass transfer characteristics and the control thereof. Whilst the above discussion has focussed n the treatment of a gold bearing ore, this is in no way intended to be limiting and it is envisaged that the system would be equally applicable to the leaching of many other metals from their ores. The system would also be suitable for leaching and desorption of many other selected components from solid host materials containing them. Where desirable, incorporation of devices which can inject air, oxygen or other oxygen-containing gas into the reservoir to improve leaching through control of the redox potential can also be accommodated within this apparatus to further improve process efficiency and yield.

Claims (27)

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A method of treating waste liquid containing known contaminants comprising delivering the liquid into a reservoir containing a pervious treatment medium, the reservoir being lined with a material at least impervious to the liquid to prevent direct leakage from the reservoir to the ground, maintaining intimate contact between the liquid and the treatment medium for a period sufficient to interact to reduce selected contaminants in the liquid to predetermined levels.

2. A method as claimed in claim 1, wherein the treated liquid is allowed to pass through a sand layer covering the reservoir to evaporate or pass into the adjacent ground after reduction of said selected contaminants.

3. A method as claimed in claim 1 or 2, wherein the liquid is introduced adjacent one end of the reservoir partly or fully below the surface of the treatment medium and passes out of the reservoir at or adjacent the opposite end of the reservoir above the level of the perimeter wall,

4. A method as claimed in any one of claims 1 to 3, wherein at at least one location in the reservoir a substantial portion of the liquid flowing through the treatment medium is passed through the treatment medium at a predetermined level spaced downward from the surface of the treatment medium.

5. A method as claimed in claim 3, wherein at least one barrier extends across substantially the full extent of the reservoir at a location intermediate the said ends and partially through the depth of the treatment medium from the top surface thereof, and all liquid flowing to said opposite end of the reservoir is passed beneath said barrier. -15-

6. A method as claimed in any one of claims 1 to 5, wherein a substantial portion of the liquid flowing through the treatment medium is caused to follow a path of non-linear form as viewed from above.

7. A method as claimed in any one of claims 1 to 6, wherein the waste liquid is septic tank effluent and the treatment medium is of a composition of phosphorus and nitrogen sorption capacity.

8. A method as claimed in claim 7, wherein the treatment medium also has a bacteria sorption or filtration capacity.

9. An apparatus for treating waste liquid when used in carrying out the method claimed in claim 1, comprising a reservoir lined with a material at least impervious to the waste liquid to be treated, a mass of a treatment medium in the reservoir, waste liquid distribution means located in the reservoir and arranged to disperse said liquid into the treatment medium, a sand layer covering the treatment medium in the reservoir, the arrangement being such that the waste liquid delivered into the reservoir must travel through the treatment medium at least a predetermined distance selected to provide a resident period for the liquid in the treatment medium to reduce selected contaminants in the liquid to the predetermined level.

10. An apparatus as claimed in claim 9, wherein at least two of said reservoirs are provided in a side by side relation each with a mass of treatment medium therein, at least one liquid distribution means in each reservoir, and means operable to direct waste liquid from a source to the conduits in either or both reservoirs. -16-

11. An apparatus as claimed in claim 9 or 10, wherein the or each reservoir has a portion of the wall of the reservoir below the level of the remainder of the wall of the reservoir providing a preferred location of discharge of liquid from the reservoir.

12. An apparatus as claimed in any one of claims 9 to 11, wherein flow directing means are provided in the treatment media to direct the liquid along a non-linear path as viewed from above to the overflow area.

13. An apparatus as claimed in any one of claims 9 to 11, wherein there is provided in the reservoir between the distribution means and said outflow area at least one barrier extending across substantially the full extent of the reservoir and partially through the depth of the treatment medium from the top surface thereof and all liquid flowing through the reservoir.

14. An apparatus as claimed in any one of claims 9 to

13, wherein means are provided in the treatment medium to direct the liquid along a non-linear path as viewed from above as the liquid flows through the reservoir.

15. An apparatus as claimed in any one of claims 9 to

14, wherein the waste liquid is septic tank effluent and the treatment medium is of a composition of phosphorus and ammonia sorption capacity.

16. An apparatus as claimed in claim 15, wherein the mud also has a bacteria sorption or filtration capacity.

17. A method of treating solid material containing at least one selected component by a leaching liquid, comprising delivering said leaching liquid into a reservoir containing said solid material, the reservoir being lined with a material at least impervious to said liquid to prevent leakage from the reservoir to the ground, maintaining the solid material in the reservoir flooded with the leaching liquid for a period sufficient to allow the leaching liquid and solid material to interact to enable transfer of said selected component from the solid material to the leaching liquid, and allowing the leaching liquid containing the selected component to overflow from the reservoir and be collected.

18. A method as claimed in claim 17, wherein the leaching liquid is introduced adjacent one end of the reservoir below the surface of the solid material and passes out of the reservoir at or adjacent the opposite end of the reservoir above the level of the solid material.

19. A method as claimed in claim 17 or 18, wherein at at least one location in the reservoir, substantially all leaching liquid flowing through the reservoir is passed through the solid material at a level spaced downward from the surface of the solid material.

20. A method as claimed in claim 17, wherein a barrier extends across the full extent of the reservoir at a location intermediate the ends of the reservoir and partially through the depth of the solid material from the top surface thereof, and substantially all liquid flowing through the reservoir is passed beneath said barrier.

21. A method as claimed in any one of claims 17 to 20, wherein the selected component is a precious metal.

22. A method as claimed in any one of claims 17 to 21, wherein an oxygen-containing gas is injected into the reservoir.

23. An apparatus for treating solid material when used in carrying out the method claimed in claim 17, comprising a reservoir lined with a material at least impervious to the leaching liquid, a mass of a solid material in the reservoir leaching liquid distribution means located in the reservoir and arranged to deliver leaching liquid into the reservoir below the surface of the solid material, and an outflow area in the wall of the reservoir arranged so the leaching liquid must travel at least a predetermined distance to the outflow area through the solid material within the reservoir, said predetermined distance being selected to provide sufficient residence time for the leaching liquid in the solid material to remove a portion of the selected component from the solid material.

24. An apparatus as claimed in claim 23, wherein at least two of said reservoirs are provided in a side by side relation, each with a mass of solid material therein, at least one leaching liquid distribution means in each reservoir, and means operable to direct leaching liquid from a source to the conduits in either or both reservoirs.

25. An apparatus as claimed in claim 23 or 24, wherein there is provided in the reservoir between the distribution means and said outflow area a barrier extending across substantially the full extent of the reservoir partially through the depth of the solid material from the top surface thereof so leaching liquid flowing toward said portion of the outflow area passes beneath said barrier.

26. An apparatus as claimed in any one of claims 23 to

25, wherein the selected component is a precious metal.

27. An apparatus as claimed in any one of claims 23 to

26, wherein an oxygen-containing gas is injected into the reservoir.