AU2022282798A1 - Treatment of a hydro-excavated mud - Google Patents

Abstract

A transportable system is described for handling a hydro excavated mud created during a wet excavation. The system comprises a two-decked screen receiving hydro excavated mud, the two-decked screen comprising an apertured upper deck which receives the hydro excavated mud and an apertured lower deck arranged to receive hydro excavated mud from the apertured upper deck, wherein undersized material that passes through the two-decked screen is collected as screened hydro excavated mud. A dewatering cyclone receiving screened hydro excavated mud and removing a first portion of dirty water therefrom. A dewatering screen receiving screened hydro excavated mud and removing a second portion of dirty water therefrom. A filter press receiving one or both of the first portion of dirty water and second portion of dirty water, the filter press producing a pressed filter cake and substantially clean water. The substantially cleaned water from the process is collected for reuse.

Description

Treatment of a hydro-excavated mud

This document claims priority from AU2021901530 filed on 24 May 2021 the contents of which are hereby incorporated by reference in their entirety.

Technical field

The present invention relates to the treatment of hydro-excavated mud. In an embodiment, the treatment involves dewatering of the hydro-excavated mud to reclaim water which can be reused in e.g. a hydro-excavation. In an embodiment, the solids from the mud such as sand, rock and soil are reclaimed from the water phase and are suitable for reuse.

Background

Excavation is a process of removing soil, rock and other material to create a hole, trench or tunnel. It is sometimes necessary to excavate material from a site to prepare it for e.g. the foundations of a building or to install some other structure such as a road or railway. Excavation is a learned skill, and the process must be undertaken by a skilled practitioner. The work must comply with legislation which, in Australia, includes the Health and Safety at Work Act 2015 (HSWA) and all relevant regulations, including the Health and Safety in Employment Regulations 1995 (the HSE Regulations) and the Health and Safety at Work (General Risk and Workplace Management) Regulations 2016 (the GRWM Regulations).

The material that is removed during an excavation is generally considered as waste unless it is intended to be reused on the site and/or it can be reused elsewhere as backfill without further treatment. If the excavated material must be processed before it is reusable, a duty of care applies in relation to how the waste material is handled. In some instances, the excavated waste material must be disposed of according to waste management guidelines. Some soil and rock-based waste material is sent to landfill.

The process of wet excavating sometimes referred to as hydro-excavation or Non- Destructive Digging (NDD) produces quantities of rock, soil and liquid. The wet excavation process typically makes use of high-pressure water to excavate a site. The waste material generated comprises up to 50 or 60% water. Due to its non destructive nature, hydro-excavation has become recognised as the most effective way of drilling around underground utilities such as cables and mains. The wet slurry of waste material created during hydro-excavation process is typically vacuumed into a tank mounted on the hydrovac tanker that undertook the excavation process.

Due to its solid-to-liquid state, the hydro excavated mud collected in the truck’s tank can be difficult to dispose of. The solids can contain both contaminants and useful materials, such as sand, fine aggregates, organic matter and an elevated level of fine silts and clays. The disposal of the wet muddy waste is not guaranteed at every landfill site in Australia; and where it is, it incurs a large cost due to its composition and weight.

There exists a need for an improved way of dealing with hydro excavated mud that may be cheaper and/or more reliable than existing processes. In preferred processes, the waste recovered from the hydro excavated mud can be turnable into a product that is reusable on site if required.

Summary of invention

In a first aspect there is provided a transportable plant for handling a hydro excavated mud created during a wet excavation, the transportable plant adapted to perform a process comprising: a first dewatering step comprising discharging the hydro excavated mud onto at least a two-decked screen, the two-decked screen comprising an apertured upper deck which receives the hydro excavated mud and an apertured lower deck arranged to receive hydro excavated mud from the apertured upper deck, wherein hydro excavated mud passes through the upper deck and oversized material is passed along the upper deck to a stockpile, and undersized material is discharged onto the lower deck; a second dewatering step comprising receiving hydro excavated mud from the upper deck onto the lower deck wherein oversized material is passed along the lower deck to the stockpile, and undersized material is discharged through the lower deck as screened hydro excavated mud; a third dewatering step comprising treating the screened hydro excavated mud in a cyclone to remove dirty water, thereby forming a cyclone treated hydro excavated mud which can be added to the stockpile, a fourth dewatering step comprising passing the cyclone treated hydro excavated mud to a secondary dewatering screen to remove further dirty water and to provide a further dewatered product; and a fifth dewatering step comprising filtering the dirty water using a filter press to filter out fine solids, the filtering resulting in a pressed filter cake which can be added to the stockpile, and substantially clean water.

The first dewatering step comprises treating hydro excavated mud to separate water from large solids including roots, rocks and other aggregate. The large solids dewatered are those that are too large to pass through upper deck. Typically, the large solids are those larger than about 15 to 20 mm, but in some instances even smaller pieces can be removed. The dewatered product from the first dewatering step is hydro excavated mud that is passed from the upper deck onto the lower deck of a two-decked screen.

The second dewatering step comprises treating hydro excavated mud received from the upper deck to separate water from medium sized solids including roots, rocks and other aggregate. The medium solids dewatered are those that are too large to pass through lower deck. Typically, the medium solids are those larger than about 5 mm but smaller than about 20 mm (or whatever upper limit was selected for the upper deck). The dewatered product from the second dewatering step is screened hydro excavated mud.

The third dewatering step comprises treating screened hydro excavated mud in a cyclone to separate water from small sized solids. The small solids dewatered are those that are removed from the cyclone. Typically, the small solids are those larger than about 0.1 or 0.075 mm but smaller than about 5 mm. Particles smaller than e.g. about 0.075 mm remain in the dirty water liquid discharged from the cyclone. The dewatered product from the third dewatering step is dirty water (liquid) and a cyclone treated hydro excavated mud (solid).

The fourth dewatering step comprises passing the solid material slurry discharged from the cyclone (the cyclone treated material) onto a dewatering screen. The dewatering screen further dewaters the solid material. The dewatered product from the fourth dewatering step is dirty water (liquid) a further dewatered hydro excavated mud (solid).

The fifth dewatering step comprises treating the dirty water from the third and fourth dewatering steps in a filter press to separate water from fine solids. The fine solids are those less than about 0.075 mm. The dewatered product from the fifth dewatering step is substantially clean water (liquid) and a pressed filter cake of fine solid material.

In an embodiment, the process is continuous, and each dewatering step is undertaken in sequence. The process can make use of multiple conveyor belts and multiple stockpiles. The process can make use of one common conveyor belt and a single stockpile. An advantage of common conveyors and a single stockpile is that less equipment is required and all waste solid is on one place for further processing. The present process can be useful for hydro-excavated mud slurry that comprises materials of all sizes from small particles of sand to large sized rocks. The present process, in embodiments, is particularly useful for the treatment of clay slurry that results from the hydro-excavation of a clay based soil. Clay based slurries can be the most difficult to dewater where the mud is very thick. The product in the stockpile (or stockpiles) can be referred to as a waste material. The stockpile(s) can be a waste stockpile. Alternatively, the product in the stockpile(s) can be referred to as a recycled or conditioned material since it may no longer comprise “waste” and may have further uses.

Following the first and second dewatering steps, the large and medium sized rock and aggregate is removed so that the cyclone can operate without risk (or with decreased risk) of internal damage from the solid items. Once treated by the cyclone in a third dewatering step, all that remains is dirty water with suspended fine solids. This dirty water with fine solids is suitable for reuse around site, such as in earlier stages of the process which might require additional liquid where it is not of concern that there is some fine solid suspension. By making use of the dirty water, the present process can be a closed loop in which water put into the system from hydro-excavation can be recovered and then re-used.

In order to treat the dirty water so as to make it suitable for reuse in the hydrovac truck for the wet excavation process, the dirty water is subject to further or fifth dewatering step using a filter press. The filter press can remove fine solids. The fine solids can wear the high-pressure water pump used in the wet excavation process and cause it to run less efficiently or to cease to work over time. By making use of the substantially clean water, the present process can be a closed loop in which water put into the system from hydro-excavation can be recovered and then re-used in a further excavation process.

Accordingly, the substantially clean water that can be produced by the stages of sequential dewatering in the present process is suitable for reuse in a wet excavation process utilising one or more high-pressure pumps.

The plant is designed to be transportable from site to site with minimal setup and without the use of expensive traffic escorts. All items can be mounted on a single frame with container lock downs or tie down brackets and lifting lugs for cranage to and from the truck. The transportable unit can be a one lift totally self-contained unit with electrical plug for genset or mains power. The items in the system may not require dismantling or containerising for transport. Furthermore, the system may be mobilised within about an hour when on site and demobilised within about an hour when leaving the site. The system may take longer than an hour to mobile and or demobilise such as 2, 3, 4 or 5 hours.

The difficulty in making the unit transportable is to design the many required items to work efficiently while being of a size to fit within an envelope. The size needs to be small enough to meet road regulations as to not require expensive escorts or height requiring telecommunications and power authority involvement. In an embodiment, the plant is at most about 13 to 14 meters (preferably 13.7 meters) in the longest length dimension. In an embodiment, the plant is about 3 to 4 meters (preferably 3.6m in height) and about 3 to 4 (preferably 3.45 meters) in width. Finally, the plant should be designed such that it can be easily serviced and maintained so that the unit is working to its most efficient processing. In an embodiment, the cyclone and dewatering screen are provided together in a module desanding plant.

In an embodiment, the hydro excavated mud is delivered into the process directly from the hydrovac truck. Alternatively, the hydrovac truck can deliver its load into a vessel and then the hydro excavated mud can later be passed into the process. Whilst it is possible for the hydro excavated mud to pass into the process by any means, it is preferable that the mud is discharged from directly the hydrovac truck since there are fewer process steps involving moving the mud slurry that is difficult to move due to its solid/liquid nature.

In an embodiment, in order to discharge the hydro excavated mud, the truck with the product of wet excavations reverses up a ramp and discharges the contents onto the two-decked screen system. The hydro excavated mud passes first onto the surface of the upper deck. The out flow of hydro excavated mud from the truck can be controlled by a truck mounted valve. When the flow from the truck stops (mostly a liquid portion), the rear of the tank can be opened up so that the remaining contents from the truck (mostly a solid portion) can be washed out onto the surface of the upper deck. This sequence of events may assist in minimising or eliminating any contamination due to spillage during transferring of the hydrovac mud slurry from the truck to the vessel housing the upper decking process.

The two decked screen, or double decked screen can comprise an upper deck arranged above a lower deck. Each screen has apertures or openings to allow material to pass through the screens. The apertures of the upper deck are larger than the apertures of the lower deck. Each of the screens can be vibrating which assists in causing the solids in the hydro excavated mud to pass through the apertures if they are undersized. The upper deck can be a woven screen or a perforated plate. The upper deck can comprise openings in the form of apertures over most of its surface. The material that is received onto the surface of the screen can either pass through openings in the screen or be conveyed along the screen for subsequent removal as solid material. The apertured upper deck can have openings that allow passage of material that is less than about 30, 25, 20, 15 or 10 mm in one measured dimension. In an embodiment, the apertures in the screen are 25 mm. Objects that have a dimension that is larger than the openings (e.g. greater than 25 mm) will be unable to pass through the screen and will therefore be captured by the screen for removal. Accordingly, in an embodiment, the first dewatering process removes solids that are larger than about 25 mm in one dimension. In an embodiment, the apertures in the screen are 16 mm. Objects that have a dimension that is larger than the openings (e.g. greater than 16 mm) will be unable to pass through the screen and will therefore be captured by the screen for removal. Accordingly, in an embodiment, the first dewatering process removes solids that are larger than about 15 mm in one dimension.

The upper deck is preferably vibrating. The vibration of the screen can assist in moving rocks and particles and other aggregate through openings in the screen. If the upper deck is not vibrating and is a stationary screen, rocks can peg (become jammed) in the mesh of the screen. This means that the rocks have to be removed either manually, or with rams that periodically pivot the mesh. It is preferable not to use additional mechanical components for e.g. pivoting which are expensive to maintain. Furthermore, trapped rocks could come lose and roll down the screen with speed creating an OH&S issue.

The upper deck can be adjustable so as to change its angle relative to horizontal. The angle can be selected to assist with particular product flow and to optimise dewatering. If the product is flowing too quickly, the screen can be moved towards a more horizontal profile to slow the passage of product along the screen surface.

If the product is not flowing quickly enough, the screen can be declined to use forces of gravity to assist in increasing the passage of product along its surface. Upon passing through the screen, the material having smaller dimensions passes to the lower deck. The lower deck can be arranged substantially underneath the upper deck, so that the material is received onto its surface. The lower deck can be a woven screen or a perforated plate. The lower deck can comprise openings in the form of apertures over most of its surface. The material that is received onto the surface of the lower deck can either pass through openings in the screen or be conveyed along the screen for subsequent removal as solids. The lower deck can be adjustable to change its angle relative to horizontal. Typically, the upper and lower decks are adjustable together so that adjustment of one results in adjustment of the other. The angle can be selected to assist with particular product flow and to optimise dewatering. The lower deck is preferably vibrating. The vibration of the screen can assist in moving rocks and particles and other aggregate through openings in the screen. The apertured lower deck can have openings that allow passage of material that is less than about 10, 8, or 6 mm in one measured dimension. In an embodiment, the apertures in the screen are 5 or 6 mm. Objects that have a dimension that is larger than the openings (e.g. larger than 5 or 6 mm) will be unable to pass through the screen and will therefore be captured by the screen for removal. Accordingly, in an embodiment, the second dewatering process removes solids that are larger than about 5 or 6 mm in one dimension.

Over either or both of the upper deck and lower deck there can be arranged a water source for wetting the hydro-excavated mud. The water source can be selected from one or more of jets, hoses, sprinklers and bars from which water can be sprayed. The applied water can assist with solid product retention on the screen if required. The applied water can also be used for cleaning stuck mud from the solid aggregates. The applied water can also add additional water to the system that may be required to keep the mud slurry moving through the process. The skilled person will appreciate when and how much water needs to be added at this stage to encourage process flow. The amount of water can be determined by trial and error.

Oversized materials that cannot pass through the upper deck will be captured by the surface of the upper deck and can be moved by the vibrational forces along the screen towards a discharge or disposal end. Oversized materials that cannot pass through the lower deck will be captured by the surface of the lower deck and can be moved by the vibrational forces along the screen towards a discharge or disposal end. At the discharge or disposal ends of either or both of the upper and lower decks, there can be a further common conveyor or respective conveyors that collects the oversized materials. The conveyors can discharge or each discharge the oversized materials onto a stockpile or stockpiles.

Upon passing through the lower deck, the undersized material is collected as screened hydro excavated mud. The material can be collected in a hopper or tank. The hopper or tank can be full of water. The water can be dirty water recycled from further down the process stream herein described. Optionally, the level of water in the tank can be replenished or discharged according to sensors. There can be more than one sensor in the tank. A first sensor can detect when the water level is too high and require some water discharge. A second sensor can detect when the water level is too low and requires the introduction of water into the tank.

The screened hydro excavated mud comprising solids smaller than e.g. 5 to 6mm are passed into a dewatering cyclone. In fitting with the transportable nature of the apparatus, the cyclone can have a main tank volume of not more than about 1.5 m3. The cyclone can have a diameter of at most about 12 or 15 inches. The maximum water handling capacity can be at most 170 m3/h. The cyclone operates at a required pressure, which is usually provided by a pump. The cyclone requires a consistent flow rate to work correctly, this is achieved by the correctly sized pump running at the correct speed for the particular slurry. This is all determined by the production rate of requirement of each unit.

The dewatering cyclone can remove solid particles in the range of from about >5 or 6mm to about 0.1 , 0.075 or 0.05 mm. Accordingly, in an embodiment, the second dewatering process removes solids that are larger than about 0.075 mm in one dimension.

The purpose of the dewatering cyclone is to remove most of the water or excess water from the screened hydro excavated mud. The water removed can be collected. The collected water can be transferred to a dirty water tank. The solid material (the cyclone treated material) from the cyclone can be discharged onto a dewatering screen. The dewatering screen can be a single deck dewatering screen. The dimensions of the dewatering screen can be about 6 x 3 feet. It should be understood that other sized dewatering screens can be used as required.

The purpose of the dewatering screen is to remove as much further water as is practical. In an embodiment, the slurry received from the cyclone is fed downward onto dewatering screen. The dewatering screen can then be steeply upwardly inclined to achieve rapid drainage of the slurry. A pool of water can form in the valley between the downwardly declined feedline and the upwardly inclined screen surface as solids build up. The angle of the upwardly inclined dewatering screen can be adjusted to hold product on the screen for the required time dependent on the product make up and water content. A very wet product may require more time on the dewatering screen, so the angle of inclination will be adjusted so that the product moves slowly up the dewatering screen ramp.

The solids can be moved along the upwardly inclined dewatering screen surface while water drains through the screen media. Typical screen apertures range from about 0.3mm to about 2mm. The apertures can be of any shape including slotted. The slope of the dewatering screen surface, along with one or more discharge weirs can create a deep bed that acts as a filter medium, allowing retention of much finer solids than the screen media aperture openings. Dewatered sand and other small particles can move over the end weir and discharge as a substantially drip-free dried product.

The “dried” and dewatered product from the dewatering screen can make its way from the screen onto the common conveyor or other conveyor and collected onto the stockpile or into a new stockpile.

The water recovered by the dewatering screen can discharged into a dewatering screen tank. Once full, the dewatering screen tank water can be pumped to the dirty water tank. There may be one or more sensors to detect when the pump(s) can be activated to move the water to the dirty water tank.

The dirty water tank holds the recovered dirty water from the cyclone and the dewatering screen. The dirty water comprises fine solids. The fine solids can be held in suspension by a paddle stirrer. The fine solids in the dirty water can be further dewatered by treatment with a filter press to filter out the fine solids. The filter press can be of the type that comprises filter plates that are pressed together by a hydraulic ram.

After a press cycle the fine solid “cake” from the filter press is dropped onto the common conveyor (or another conveyor) which discharges onto the stockpile (or onto a new stockpile). An advantage of using the filter press is that the end product has considerably less water due to the filter press process. This means that the material in the stockpile is easier to handle, reuse or dispose of.

The resulting substantially clean water that passes from the filter press can be pumped to a clean water tank. The recovered substantially clean water can be used for various site processes or further processed with an ultra-fine filter to allow reuse in the high-pressure pumps that are used in the wet excavating.

In an embodiment, the apparatus is able to treat about 8m3 of slurry in about 10 minutes. In an embodiment, the apparatus is able to treat at least about 45m3 of mud slurry in about 1 hour. In an embodiment, the apparatus is able to treat about

8m3 of slurry in about 30 minutes. In an embodiment, the apparatus is able to treat at least about 45m3 of mud slurry in about 2 or 3 hours. While these times and amounts are provided as an example, it should be understood that the production rate of the plant is dependent on the material being put through it. It has been observed that a clay-based slurry can take up to 2 hours to complete a cycle due to the nature of the slurry whereas a sand-based slurry could be as quick as 30 mins.

The time to empty one hydro truck is approximately 10-15 mins. The pump rates and flow rates of the slurry can be adjusted according to trial and error to achieve the required target treatment amounts and time. The apparatus, as a transportable unit, can be ordered to site according to the amount of material to be treated. If a faster rate of treatment is required for the amount (and type) of material that requires treatment, multiple units can be ordered. For example, to treat 90 m3 of a sandy material, two of the transportable units can be required and operated parallel to one another.

In another aspect there is provided a transportable system for handling a hydro excavated mud created during a wet excavation, the transportable system comprising: a two-decked screen receiving hydro excavated mud, the two-decked screen comprising an apertured upper deck which receives the hydro excavated mud and an apertured lower deck arranged to receive hydro excavated mud from the apertured upper deck, wherein undersized material that passes through the two-decked screen is collected as screened hydro excavated mud; a dewatering cyclone receiving screened hydro excavated mud and removing a first portion of dirty water therefrom; a dewatering screen receiving screened hydro excavated mud and removing a second portion of dirty water therefrom; and a filter press receiving one or both of the first portion of dirty water and second portions of dirty water, the filter press producing a pressed filter cake and substantially clean water; wherein the substantially cleaned water is collected for reuse.

The description and summary relating to the first aspect applies equally to this aspect unless the context makes clear otherwise.

The first and second dewatering phases of the process results in oversized solid material that is passed along the upper deck or the lower deck to a stockpile. The third and fourth dewatering phases results in a solid dewatered hydro excavated mud that can be passed to the stockpile. The filtering fifth dewatering phase results in a pressed filter cake that can be passed to the stockpile. The stockpile can be a common stockpile. Alternatively, there can be separated stockpiles. The stockpile can comprise dried mud and aggregate. The stockpile can be further treated.

The present transportable unit preferably has a common collecting conveyor that preferably transports all the material to one stockpile. This can make it more efficient for people to load the dewatered material onto trucks. An additional incline transfer conveyor can be added to allow a larger stockpile of the material.

The system can be completely automated from the point that when the hydrovac truck backs up the ramp to the time when the solids have all been collected on the stockpile. The system can start as soon as hydrovac truck is detected approaching the apparatus. An automatic detector can be installed to detect the presence of the truck. The automatic detector can be a laser pathway that is interrupted by the truck. The system can operate for a specific amount of time following activation to ensure the system has finished its cycle before shutting down. The system may operate for a predetermined period of time. The predetermined period of time will depend on the nature of the product being treated. As an example, for a given product the system could operate for 15, 20, 25 or 30 minutes which allows time for emptying of the slurry from the truck, treatment and cleaning.

The uniqueness in the present process comes from the combination of specific pieces of equipment into a transportable closed loop dewatering system. Producing reusable water and a reusable stockpile of dry material.

Brief Description of the Figures

Embodiments of the invention will now be described with reference to the accompanying drawings which are not drawn to scale and which are exemplary only and in which:

Figure 1 is a schematic view of an embodiment of the invention.

Detailed Description of Embodiments of the Invention

The hydro excavated mud (not shown) is delivered into the process directly from the hydrovac truck 10. The truck 10 can discharge (A) the mud contents onto the two-decked screen system 12. The hydro excavated mud passes first onto the surface of the upper deck 12’.

The upper deck 12’ is a vibrating mesh screen with 25 mm apertures. The screen can be inclined in upwardly in the direction of travel by up to 10 or 15 degrees. The screen can be declined downwardly in the direction of travel by up to 10 or 15 degrees. The skilled person will control the inclination or declination according to the desired treatment speeds which will vary according to the nature of the mud slurry being treated. Objects in the hydro excavated mud that have a dimension greater than 25 mm will be unable to pass through the screen 12’ and will therefore be captured by the screen for removal along path B.

Any of the hydro excavated mud that is able to pass through the screen falls under force of gravity onto the lower deck 12”. The lower deck 12” is arranged substantially underneath the upper deck, so that the material is received onto its surface. The lower deck 12” is a vibrating mesh screen with 6 mm apertures. Oversized materials that cannot pass through the lower deck 12” will be captured by the surface of the lower deck 12” and moved by the vibrational forces along the screen towards a discharge or disposal end at which point they are passed along pathway C.

The materials collected from pathways B and C can be collected onto a common conveyor 16 that collects the oversized materials (not shown). The common conveyor 16 moves along pathway E to discharge along F onto a stockpile 18.

Over either or both of the upper deck and lower deck there can be arranged a water source 14 for wetting the hydro-excavated mud material. The water source 14 can be a spray bar 14 from which water can be sprayed. The water applied via spray bar 14 can be dirty water supplied from dirty water tank 28 along line D. The water can be pumped by pump 15.

Upon passing through the lower deck 12”, the undersized material (not shown) moves along pathway G and is collected in a tank 20. The movement along pathway G can be effected by a pump. The upper deck tank 20 can comprise water. The level of water in the tank can be replenished or discharged according to sensors 22. In Figure 1 there are shown three sensors at various levels.

The screened hydro excavated mud collected in tank 20 (comprising solids smaller than e.g. 5 to 6mm) are passed along pathway H to dewatering cyclone 24. The cyclone 24 operates at a required pressure, which is usually provided by a pump 26. The cyclone requires a consistent flow rate to work correctly, this is achieved by the correctly sized pump 26 running at the correct speed. The dirty water collected is passed along pathway J to a dirty water tank 28. The level of liquid in the dirty water tank 28 can be replenished or discharged according to sensors 22.

In Figure 1 there are shown four sensors at various levels in tank 28.

In embodiments in which the system is desirously a closed loop, water from dirty water tank 28 can be used to fill the upper deck tank 20. This is shown along pathway K, which includes a pump 30 for movement of the dirty water from one tank to the other.

The solid materials (the cyclone treated solids) from the cyclone 24 can be discharged onto a dewatering screen 32 along pathway L. The “dried” and dewatered product from the dewatering screen 32 can make its way along pathway M onto the common conveyor 16 that conveys the material to stockpile 18.

The water recovered by the dewatering screen 32 can discharged (path N) into a dewatering screen tank 34. Once full (as detected by sensors), the dewatering screen tank water can be pumped by pump 36 along path P to the dirty water tank 28.

The dirty water tank 28 holds the recovered dirty water from the cyclone 24 and the dewatering screen 32. Fine solids in the dirty water can be held in suspension by a paddle stirrer 38. The fine solids in the dirty water can be further dewatered by treatment with a filter press 40 to filter out the fine solids. The dirty water can be pumped from tank 28 along pathway Q to the press. After a press cycle the fine solid “cake” (not shown) from the filter press 40 is dropped via pathway R onto the common conveyor 16 which discharges onto the stockpile 18. The resulting substantially clean water that passes from the filter press can be pumped to a clean water tank. The recovered substantially clean water (not shown) can be pumped by pump 42 to clean water tank 44. The clean water can be pumped from tank 44 for use in various site processes or further processed with an ultra-fine filter to allow reuse in the high-pressure pumps that are used in the wet excavating.

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Any promises made in the present description should be understood to relate to some embodiments of the invention and are not intended to be promises made about the invention as a whole. Where there are promises that are deemed to apply to all embodiments of the invention, the applicant/patentee reserves the right to later delete them from the description and does not rely on these promises for the acceptance or subsequent grant of a patent in any country.

Claims (18)

1 . A transportable plant for handling a hydro excavated mud created during a wet excavation, the transportable plant adapted to perform a process comprising: a first dewatering step comprising discharging the hydro excavated mud onto at least a two-decked screen, the two-decked screen comprising an apertured upper deck which receives the hydro excavated mud and an apertured lower deck arranged to receive hydro excavated mud from the apertured upper deck, wherein hydro excavated mud passes through the upper deck and oversized material is passed along the upper deck to a stockpile, and undersized material is discharged onto the lower deck; a second dewatering step comprising receiving hydro excavated mud from the upper deck onto the lower deck wherein oversized material is passed along the lower deck to the stockpile, and undersized material is discharged through the lower deck as screened hydro excavated mud; a third dewatering step comprising treating the screened hydro excavated mud in a cyclone to remove dirty water, thereby forming a cyclone treated hydro excavated mud which can be added to the stockpile, a fourth dewatering step comprising passing the cyclone treated hydro excavated mud to a dewatering screen to remove further dirty water and to provide a further dewatered product; and a fifth dewatering step comprising filtering the dirty water and further dirty water using a filter press to filter out fine solids, the filtering resulting in a pressed filter cake which can be added to the stockpile, and substantially clean water.

2. A transportable apparatus for handling a hydro excavated mud according to claim 1 , wherein the substantially clean water is supplied to high pressure pumps that are used in wet excavating.

3. A transportable apparatus for handling a hydro excavated mud according to claim 1 or 2, wherein the hydro excavated mud is discharged directly from a tank of a hydrovac tanker

4. A transportable apparatus for handling a hydro excavated mud according to any one of the preceding claims, wherein the apertures of the upper deck are in the range of from 10 to 25mm.

5. A transportable apparatus for handling a hydro excavated mud according to any one of the preceding claims, wherein the apertures of the lower deck are 5-6mm.

6. A transportable apparatus for handling a hydro excavated mud according to any one of the preceding claims, wherein the primary and the lower decks are vibrating screens and wherein the oversized materials from the primary and the lower decks are passed under the forces of vibration to a conveyor, which conveys material to the stockpile.

7. A transportable apparatus for handling a hydro excavated mud according to any one of the preceding claims, wherein the dewatering step comprises: collecting the dirty water from the third dewatering step and the dirty water from the fourth dewatering step in the same tank for further processing in the fifth dewatering step.

8. A transportable apparatus for handling a hydro excavated mud according to claim 7, wherein fine solids in the dirty water are held in suspension by a paddle stirrer.

9. A transportable apparatus for handling a hydro excavated mud according to any one of the preceding claims, wherein the angle of inclination or declination of the upper deck and or the lower deck can be changed to increase the rate of dewatering.

10. A transportable apparatus for handling a hydro excavated mud according to any one of the preceding claims, wherein the pressed filter cake is passed to a conveyor, which conveys material to the stockpile.

11. A transportable apparatus for handling a hydro excavated mud according to any one of the preceding claims, wherein dirty water is used as a feedstock for the water in the dewatering cyclone.

12. A transportable apparatus for handling a hydro excavated mud according to any one of the preceding claims, wherein dirty water is used to fill a tank (upper deck tank) in which the screened hydro excavated mud is collected.

13. A transportable apparatus for handling a hydro excavated mud according to any one of the preceding claims, wherein one or more spray bars is provided to add water to the hydro excavated mud on the upper deck, wherein dirty water is used as a feedstock for one or more of the spray bars.

14. A transportable apparatus according to any one of claims 11 to 13, wherein the apparatus operates in a closed loop with all water used in the process being obtained from the process itself.

15. A transportable apparatus for handling a hydro excavated mud according to claim 2, wherein prior to supply to the high pressure pump, the substantially clean water is further treated by an ultra fine filter.

16. A transportable apparatus according to any one of claims 1 to 15, wherein the apparatus is sized with a footprint of about 10-14m in length by about 3- 4 meters in width and about 3 to 4 m in height.

17. A transportable apparatus according to any one of claims 1 to 16 wherein the stockpile is a common stockpile.

18. A transportable system for handling a hydro excavated mud created during a wet excavation, the transportable system comprising: a two-decked screen receiving hydro excavated mud, the two-decked screen comprising an apertured upper deck which receives the hydro excavated mud and an apertured lower deck arranged to receive hydro excavated mud from the apertured upper deck, wherein undersized material that passes through the two-decked screen is collected as screened hydro excavated mud; a dewatering cyclone receiving screened hydro excavated mud and removing a first portion of dirty water therefrom; a dewatering screen receiving screened hydro excavated mud and removing a second portion of dirty water therefrom; and a filter press receiving one or both of the first portion of dirty water and second portion of dirty water, the filter press producing a pressed filter cake and substantially clean water; wherein the substantially cleaned water is collected for reuse.