AU2011213910A1 - Systems and Methods for the Treatment of Water - Google Patents

Abstract

The present invention provides a system for the treatment of stormwater comprising: (i) an underground water storage means, and (ii) a water treatment means in fluid connection with the water storage means, wherein the water storage means is adapted to be positioned above the water treatment means, with the underground water storage means being adapted to provide water to the water treatment means at a high hydraulic head. Fig 5/5 Ir riI

Description

SYSTEMS AND METHODS FOR THE TREATMENT OF WATER FIELD OF THE INVENTION The present invention relates to the treatment of storm water. In particular, the present invention relates to systems and methods for the removal of contaminants from 5 stormwater using BACKGROUND TO THE INVENTION When rain falls over undeveloped land, about 50% of the stormwater infiltrates the ground, about 40% is carried back to the atmosphere by the combined effects of evaporation and transpiration, leaving only around 10% as runoff. Urbanisation has resulted in an increase in .0 land area covered by impervious surfaces such as roads, buildings, compacted soil, and footpaths; these surfaces limiting the amount of rain that can infiltrate into the ground. Accordingly, developed land provides for a greater amount of stormwater runoff, sometimes to a level as high as 55%. This increased storm water runoff can result in severe and frequent flooding. .5 Rainwater passing over impervious surfaces typically becomes contaminated by contact with material such as rubbish, animal faeces, nutrients, pesticides, oil, and other toxic chemicals. The contaminated runoff may then flow into surrounding land which is pervious to water leading to contamination of the land and/or groundwater. The runoff may also run directly into adjacent waterways such as rivers and creeks. 20 Regulations for stormwater treatment have been implemented for new developments and redevelopments including residential, commercial and even industrial projects. Water Sensitive Urban Design (WSUD) is one way that the quality of stormwater runoff may be improved to minimise the effects on receiving water ecosystems. WSUD has been implemented by many municipal councils in Australia, and is becoming increasingly adopted 25 in Europe and America. WSUD guidelines require removal of total suspended solid (TSS) total Phosphorus (TP), total Nitrogen (TN) by at least 80%, 45%, 45% respectively. Systems and methods for the control and treatment of stormwater runoff are known in the art. For example, buffer strips may be used as a source control measure, particularly for 1 management of road runoff. They are effective in the removal of coarse to medium size !0 sediments and can be used as an effective pre-treatment measure for bioretention systems. They also can assist in reduction of peak flows for smaller events and may promote infiltration dependent upon the underlying soil conditions. Vegetated Swales are open channel systems which use vegetation to aid the removal of 15 sediment and suspended solids. The removal efficiency is dependent on the density and height of the vegetation in the channel. As for buffer strips, the vegetation can assist in reducing peak flows for a range of events and may also be beneficial in volumetric reduction through infiltration, dependent upon the underlying soil conditions. o Wetlands are an effective stormwater treatment measure for the removal of fine suspended solids and associated contaminants, as well as soluble contaminants. They can also provide significant storage for a range of storm events. These systems use a combination of physical, chemical and biological processes to remove stormwater pollutants. 5 Bioretention Systems (also known as biofiltration systems or rain-gardens) promote the removal of particulate and soluble contaminants by passing stormwater water through a filter medium, either for infiltration into surrounding soils, or for collection by an underdrain. Well designed bioretention systems can provide both flow management and water quality benefits. A range of factors affect the treatment performance of the 50 bioretention systems, including the type and composition of filter media (e.g. loamy sand), the presence and type of vegetation used, and the presence of design enhancements such as the use of a saturated zone to enhance denitrification, and the health of vegetation. Infiltration systems reduce the volume of stormwater conveyed downstream by means of 55 infiltration into surrounding soil, and hence decrease the frequency of runoff and the mass of contaminants carried. In general, the use of vegetated infiltration systems is advocated wherever possible, these systems behaving as biorententions. In unvegetated systems (and also in unvegetated bioretention systems that allow infiltration), coarse particulates are deposited on the floor of the basin. Dissolved material and very fine particulates infiltrate 60 into the soil, hence the potential for contamination of groundwater is a problem. Inflows in 2 excess of the storage and infiltration capacity of the basin will overflow and continue downstream. Media filtration systems are often used for filtering stormwater. These systems typically use i5 a simple sand media, or a more specialised engineered media, and may be specifically tailored to provide water quality suitable for stormwater harvesting. Ponds are stormwater treatment measures such as open water bodies (without significant shallow vegetated areas in the predominant flow paths) and ornamental ponds. The 'O treatment of stormwater is predominantly associated with temporary detention to reduce peak flows and facilitate settling of suspended solids. Other treatment processes promoted in pond systems include phytoplankton assimilation of soluble nutrients and ultra-violet disinfection. '5 Sedimentation basins are open water bodies aimed predominantly at the removal of coarse and medium particles. The sediment basins are often used in conjunction with wetlands design for pre-treating water prior to entering the wetland. The treatment of stormwater in sedimentation basins is achieved almost entirely by temporary detention to facilitate settling of suspended solids. Detention Basins are open or closed storages aimed primarily !0 at reducing downstream peak flows although they also offer some removal of coarse and medium particles. An infiltration basin is a type of best management practice that is used to manage stormwater runoff, prevent flooding and downstream erosion, and improve water quality in 85 an adjacent waterway. It is essentially a shallow artificial pond that is designed to infiltrate stormwater though permeable soils into the groundwater aquifer. Infiltration basins do not discharge to a surface water body under most storm conditions, but are designed with overflow structures (pipes, weirs, etc.) that operate during flood conditions. A common problem of prior art systems for is their requirements for significant areas of 90 land in a development to be set aside for treating and/or storing runoff. Typically 1% to 2% of the total area of a housing development must be dedicated to this purpose. For example, 3 a development of 10 ha requires 1000 m2 to 2000 m 2 to be dedicated to receiving runoff. This requirement reduces the options available for the developer to utilise precious land. Many prior art systems also have difficulty in adequately dealing with high in-flows of )5 stormwater runoff. Furthermore, some prior art systems are not capable of treating stormwater runoff to a desired quality, while others require considerable maintenance or a significant input of power. It is an aspect of the present invention to provide systems and methods to overcome or ameliorate one or more problems of the prior art. It is a further aspect of the present )0 invention to provide an alternative to the systems and methods provided by the prior art. The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it )5 existed before the priority date of each claim of this application BRIEF DESCRIPTION OF THE FIGURES Fig. 1 is a diagrammatic representation of a preferred system of the present invention in plan view showing entry point for stormwater and exit point for treated water. Fig. 2 is a diagrammatic representation of the system shown in Fig.1, in a side cutaway view. 10 Fig. 3 is a diagrammatic representation of a preferred system of the present invention in plan view showing the path of water through the system. Fig. 4 shows a diagrammatic representation of the system shown in Fig. 3, through line B-B (upper panel) and A-A (lower panel). Fig. 5 shows a diagrammatic representation of a system with no overlap between the 15 footprints of the treatment means and the storage means. 4 SUMMARY OF THE INVENTION In a first aspect the present invention provides a system for the treatment of stormwater .0 comprising: (i) an underground water storage means, and (ii) a water treatment means in fluid connection with the water storage means, wherein the water storage means is adapted to be positioned above the water treatment means, with the underground water storage means being adapted to provide water to the water treatment means at a high hydraulic head. In one embodiment the storage means is a tank having a depth of at least .5 about 0.3 m or 0.4 m and/or a capacity of at least about 5 kL. In one embodiment, the water treatment means is a filtration or infiltration bed comprising a medium or media capable of removing at least a portion of a target contaminant. The filtration or infiltration bed has an area of about 1 m2 to 10,000 M 2 , and a height of about 0.5 m to about 5 m. In one embodiment, the hydraulic head of the system is at least about 0 0.3 m or 0.4 m. In one embodiment the system comprises water energy dissipation means, which may be a pipe having a diameter larger than that of an in-flow pipe. A further embodiment of the system comprises water dispersal means such as a perforated pipe. Yet a further embodiment of the system comprises a gross pollutant trap. 5 The system may be adapted to be inherently water-impermeable or water-permeable, and may further comprise means for collecting the treated water. In a second aspect the present invention provides a method for installing a system for the treatment of stormwater runoff, the method comprising the steps of providing a system as described herein, placing the system into a cavity, the cavity being of sufficient depth such 40 that the system is able to be buried. In a third aspect the present invention provides a method for treating stormwater runoff, the method including the steps of: providing a system as described herein and allowing a stormwater runoff to enter the system. A fourth aspect of the present invention provides a treated water produced by a method 45 described herein. 5 DETAILED DESCRIPTION OF THE INVENTION After considering this description it will be apparent to one skilled in the art how the invention is implemented in various alternative embodiments and alternative applications. However, although various embodiments of the present invention will be described herein, 0 it is understood that these embodiments are presented by way of example only, and not limitation. As such, this detailed description of various alternative embodiments should not be construed to limit the scope or breadth of the present invention. Furthermore, statements of advantages or other aspects apply to specific exemplary embodiments, and not necessarily to all embodiments covered by the claims. 5 Unless the contrary intention is expressed, the features presented as preferred or alternative forms of the invention can be present in any of the inventions disclosed as alone or in any combination with each other. Throughout the description and the claims of this specification the word "comprise" and variations of the word, such as "comprising" and "comprises" is not intended to exclude 0 other additives, components, integers or steps. The present invention is predicated at least in part on the finding that a stormwater runoff treatment system having a relatively deep water storage tank in combination with a water treatment means (such as a filter bed) provides advantages over, or an alternative to, the prior art systems. Accordingly, in a first aspect the present invention provides a system for 65 the treatment of stormwater comprising: (i) an underground water storage means, and (ii) a water treatment means in fluid connection with the water storage means, wherein the water storage means is adapted to be positioned above the water treatment means, with the underground water storage means being adapted to provide water to the water treatment means at a high hydraulic head. Thus, in operation, water runs under gravity 70 from the surrounding land or from a stormwater drain into the water storage tank. Water collects in the tank, thereby providing a high hydraulic head to a filtration bed positioned below the tank. The high hydraulic head provides for efficient filtration of storm water, with the treated water being collected or allowed to enter the surrounding soil. 6 Advantageously, the present systems can be installed completely underground thereby '5 freeing up significant amounts of land space in a development. Considering the specific circumstance referred to in the Background section, a saving of 1000 m2 to 2000 m 2 per ha can be saved by use of the present systems. Even at the underground level, the system footprint may be relatively small given the higher efficiencies of filtration provided by the high hydraulic head. 0 The present systems provide the further advantage of water storage, in that the storage means can store significant in-flow volumes and allow gravity to feed water to the filtration means over time. Prior art systems can often only cope with a rate of in-flow that can be handled by an associated water treatment means, such as a sand filtration bed. Once the in-flow exceeds the capacity of the treatment means, excess (untreated) water must be 5 diverted back to the stormwater system or forced into a natural waterway. The present systems are a significant departure from prior art systems that require significant landtake due to (i) the presence of above-ground components and (ii) a relatively large area required for water treatment. The use of a relatively deep underground storage tank provides for the low landtake and high hydraulic head required by the present 0 invention. The hydraulic head in turn provides for more efficient filtration and thereby decreasing the area required for the filtration means. Prior art stormwater treatment systems are typically fed by a shallow pool of water, the limited depth providing a relatively low hydraulic head to any associated water treatment means. 95 The underground storage means is typically a tank constructed so as to suitable for underground installation. Accordingly, the tank will possess the required strength so as to withstand the pressures exerted by burial. The tank may also be fabricated from a material that is refractory to degradation as a result of corrosion, water logging and the like. The tank may be constructed from a variety of materials, including corrugated metal pipe, 00 aluminium, steel, plastic, fibreglass, pre-cast or poured-in-place concrete, or any other suitable material. Exemplary underground storage means are the polypropylene chambers provided by Stormtech USA which are designed for large scale applications. These 7 chambers may be installed up to 2.4 meters underground, and can provide a 1.5 m storage depth above the treatment means )5 Preferably the storage means is adapted to provide for high volume storage of runoff in a small footprint area. Typically, this will be provided by a tank that is relatively deep. While relative depth provides advantage in the areas of volume and footprint, it provides the further advantage of providing water a relatively high hydraulic head. In one embodiment of the invention, the depth of the tank is at least a depth selected from .0 the group consisting of about 0.1 m, 0.2 m, 0.3 m, 0.4 m, 0.5 m, 0.6 m, 0.7 m, 0.8 m, 0.9 m, 1.0 m, 1.5 m, 2.0 m, 2.5 m, 3.0 m, 3.5 m, 4.0 m, 4.5 m and 5.0 m. In certain embodiments of the invention the depth is at least about 0.3 m or 0.4 m. The skilled person understands that the tank capacity will be dictated at least in part on the size of the catchment area, rainfall, impervious land percentage, and whether reuse schemes are incorporated. In one .5 embodiment of the system, the tank capacity is between about 5 kL (for a small development area) to 10 ML (as required in some stormwater harvesting projects). The water treatment means may be any means capable of removing a contaminant from stormwater runoff. The skilled person is familiar with many types of water treatment means, and is capable of selecting an appropriate kind based on the application at hand. .0 The water treatment means is typically a filtration or infiltration bed comprising a medium or media capable of removing at least a portion of a target contaminant. The filtration means may decrease the level of total suspended solids (TSS), nitrogenous compounds, organophosphates, metals, oil, grease, petrol, bacteria, parasites, viruses, and the like. The treatment means may be a filtration means in certain embodiments of the system. 25 Stormwater filters include a diverse range of treatment methods which utilize an artificial media, such as sand, peat, grass, soil or compost to filter out pollutants entrained in urban stormwater. These filters are typically designed solely for pollutant removal (quantity bypassed). The filtration means may be based on the use of sand as a medium. Sand filters remove 30 constituents from stormwater runoff primarily through a physical process of filtering out particulates from the water. The type of media used and its grain size distribution determine how small of a particle is strained out. Coarser sands have larger pore spaces that have high 8 flow-through rates but pass larger suspended particles. A very fine sand, or other fine media filter, has small pore spaces with slow flow-through rates and filter out smaller total TSS 5 particles. Some composite sand media, such as peat-sand mix, may also provide ionic adhesion or exchange for some dissolved constituents which further enhances effluent quality. The filtration means may also lower TN and TP, as required by WSUD. The system may incorporate the use of physically or biologically engineered filtration media. The prior art provides a range of tailored filtration media engineered to suit the specific soil O and water properties of a particular application. Some contain bio-engineered naturally occurring micro-organisms that biologically degrade and remediate toxic chemicals that are the result of daily urban and industrial activities. Toxic chemicals are transformed in the process into natural, non-toxic elements. The toxic chemicals treated include PCBS, PAHS, organophosphates, coal tars, pesticides and herbicides. These toxins are carcinogenic to 5 humans and their accumulation in soils and waterways are a major health and environmental concern. Some heavy metals such as arsenic, chromium and selenium can also be biologically degraded using such media. As briefly referred to above, the underground water storage means is disposed above the treatment means, with the means being fluidly connected. By this arrangement, water that 0 enters the storage means passes under the force of gravity into the treatment means. While the storage means may be disposed directly above the treatment means, it is nevertheless contemplated that the two means may be remote from each other. The precise disposition of the storage and treatment means is not critical, so long as water in the storage means is capable of flowing under gravity to the treatment means. A workable 55 system with separated storage and treatment means is shown in Fig 5. In certain embodiments of the invention, the treatment means is a filtration bed having an area of from about 1 m2 to 10,000 m2 , and a height of about 0.5 m to about 5 m. The filtration bed may be any convenient or advantageous shape such as a rectangle, circle, oval etc. 60 The fluid connection may be effected by the use of any conduit such as a pipe. Typically, the fluid connection has a maximal flow rate that is higher than the filtration rate of the media, thereby avoiding the treated flow being "throttled" by the fluid connection. Where the 9 water storage means and water treatment means abut, the fluid connection may not be embodied any hardware, with the arrangement simply being that water in the storage 15 means flows out of an aperture or passage in the floor of storage means and directly into the treatment means. It may even be the case that the storage means has no floor, being completely open thereby allowing water to enter the treatment means without even the need to pass through an aperture. The water storage means is adapted to provide water to the water treatment means at a '0 high hydraulic head. In one embodiment this is achieved by (i) the storage means having a minimum depth, and/or (ii) positioning the storage means significantly higher than the water treatment means. In certain embodiments of the invention, the high hydraulic head is at least 0.1 m, 0.2 m, 0.3 m, 0.4 m, 0.5 m, 0.6 m, 0.7 m, 0.8 m, 0.9 m, 1.0 m, 1.5 m, 2.0 m, 2.5 m, 3.0 m, 3.5 m, 4.0 m, '5 4.5 m and 5.0 m. In certain embodiments of the invention the hydraulic head is at least about 0.3 m or 0.4 m. For embodiments using the Stormtech chamber, the hydraulic head may be at least about 1.5 m. Maximum hydraulic head is measured under conditions where the storage means is full. A stormwater treatment system is preferably sized to obtain a desired hydraulic head using MUSIC software simulation, whereby the hydraulic head is 0 measured and used for treatment rate calculation at each time step. An example is of the simulation is shown in the Table below. Time step of Inflow volume Hydraulic Treatment Outflow Storage simulation (based on head (based rate (based volume (accumulated) = (example catchment on storage on hydraulic ( based on Inflow volume only) characteristics, depth) head, outflow volume treatment rainfall, infiltration rate) (dictate the region) media) hydraulic head) 10 minute 20 m3 1.0m 100 1/min 1m3 19m3 20 minute 0.5 m3 0.9m 90 I/minute 0.9m3 18.6m3 30 minute 3 m3 1.3m 130 1/min 1.3m3 20.3m3 10 40 minute 15 m3 2.5m (max) 250 1/min 2.5m3(max) 32.8m3(max (max) storage) 15000 2 m3 0.9m 90 1/min 0.9m3 18.6m3 minute 15010 0.1 m3 0.8m 80 I/minute 0.8m3 17.9 m3 minute In one embodiment the system comprises water energy dissipation means. It will be understood that water entering the system may be fast flowing stormwater having 5 considerable energy. It is desirable to keep water in a low energy state for at least the reasons (i) to allow larger particles to settle or (ii) to minimise agitation of any filtration medium. The skilled person is familiar with many methods for lowering the energy of water including the use of baffles, grates and the like. In one embodiment of the system, the water energy dissipation means is a pipe having a diameter larger than that of an in-flow 0 pipe. In one embodiment of the system, the water energy dissipation means are permanently full of water to diminish hydraulic gradient of the inflow, thereby facilitating settling of finer particles. For example, where the stormwater is fed into the system by a pipe of diameter about 225 mm , it is then directed to a pipe having a diameter of 525 mm (and permanently filled with water) , causing the energy to be lowered considerably. The 95 system may have 1, 2, 3, 4 or more of these larger diameter pipes in certain embodiments. The system may further comprise water dispersal means. The energy dissipation means may feed water into a water dispersal means before it eventually enters the storage means. The function of the water dispersal means is to minimise the preferential delivery of water to certain region(s) of the storage means. An even distribution of water is preferred 00 because it provides more even dispersal over the treatment means in certain embodiments of the system. Typically, the water dispersal means is a perforated pipe. In one embodiment of the system, and where multiple water energy dissipation means are used, 11 the water dispersal means is in fluid connection with each of the water energy dissipation means. 15 In one embodiment of the system, a gross pollutant trap (GPT) is included as a primary screen to remove larger items such as take away containers, leaves, bottles and plastic bags. Smaller pollutants, such as dirt, chemicals, heavy metals and bacteria are typically not collected directly by the GPT; however, some small particles are caught up with the larger items in the trap any small particles that have bypassed the GPT will eventually deposit in .0 the energy dissipation pipes as described above, and are thus prevented from reaching the storage means. The GPT may be adapted to store a dry or wet load; collected items are either stored above (dry) or below (wet) standing water levels. Traps that store trapped items in a dry state are generally preferable because they are cheaper to operate as the collected material can be delivered to local landfill facilities without issue. In systems using .5 the polypropylene chambers for storage such as the Stormtech chamber, GTP is incorporated in the form of isolator rows. Some factors to be considered in selecting a GPT for use with the present invention include size of particles to be caught in that location, physical space available for the trap, frequency of storms or other major water influxes, average flow rates over a year, maintenance .0 requirements, the ease and safety of access for maintenance work, frequency of maintenance that is practical in the location, estimated loading in the area, safety and aesthetics of the trap being exposed or enclosed, installation and operating costs. The system may be adapted to be inherently water-impermeable. For example, all 25 components of the system may be water-impermeable, or the components may assemble into a unitary water-impermeable structure. Alternatively, the system may further comprise a dedicated water-impermeable. Embodiments of the present systems that are water impermeable are able to efficiently collect the treated water generated by the system. In some circumstances, it may be desirable to store (or otherwise control) the treated water. 30 Alternatively, the system may be water-permeable in which case treated water is simply left to infiltrate the surrounding soil. 12 The system may further comprise means for collecting the treated water. The means typically comprises an area under the treatment means from which water exiting under the force of gravity collects. The collection area may be a void, or a matrix having spaces 5 allowing water to flow such as crushed rock. In one embodiment of the system, the means for collecting the treated water is a network of agricultural pipes laid in a coarse sand/gravel layer under the treatment means. The collected water may collect toward a low point in the collection area, and may be conducted away from the system using standard pipes. In one embodiment of the system O the collected water is conducted to a pump pit containing a pump. The pump may be configured to conduct the treated water to the surface, a stormwater drain, a storage tank or any other containment means. In a further aspect, the present method provides a method for installing a system for the treatment of stormwater runoff, the method comprising the steps of providing a system as 5 described herein, placing the system into a cavity, the cavity being of sufficient depth such that the system is able to be buried. As mentioned elsewhere herein, the components of the system are adapted for burial by way of the materials and design embodying the system. The system may be buried in any convenient location, but is advantageously located under an area or structure that would otherwise preclude use for runoff treatment 0 or storage. Advantageously, the system may be located under a building structure, a pedestrian plaza, a parking lot, a road, a playground, or designated parkland. Thus, the present invention allows for an area of land to have a dual purpose, this not being achievable with systems of the prior art that are not capable of burial. The skilled person understands that components of the system may need to be constructed 55 from materials or designed to withstand significant stresses occasioned by an overlying structure, or vehicles travelling over the system. Furthermore, it may be necessary to first stabilize the floor of the cavity, (using concrete or "road base" for example), to prevent movement of any system component once installed. An engineer of ordinary skill is capable of achieving a suitable installation given the requirements of any particular site. 60 As mentioned supra one embodiment of the system may be adapted to retain treated water. Where the system is not water-impermeable, it may be installed within a water 13 impermeable lining or other barrier. The skilled person is familiar with a range of materials useful for lining, including pond Liner PVC, EPDM, and butyl rubber. In a further aspect the present invention provides a method for treating stormwater runoff, 5 the method including the steps of: providing a system as described herein and allowing a stormwater runoff to enter the system. The present methods may provide a flow rate of from about 0.15 1/s/sq.m to about 0.4 1/s/sq.m, dependent upon storage depth and selection of filtration media. In one embodiment, the method further comprises the step of collecting a treated water produced by the system. '0 Yet a further aspect of the present invention provides a treated water produced by a method as described herein. An especially preferred embodiment designed for a typical 10 ha residential development of 60% impervious area will now be described by reference to the figures. Turning first to Fig. 1 there is generally shown a plan view of a system of the present invention comprising an '5 underground treatment and storage unit 2, an in-flow pipe 4 (of diameter 225 mm) having a GPT unit 6 disposed inline. The GPT unit is of the standard type manufactured by CDS or Rocla.The in-flow pipe accepts water from drain pipe 8 carrying storm water runoff, conducting it to the storage component of the underground treatment and storage unit 2. The system is lined (not shown), with treated water being collected into a pump pit 10 and 0 returned to the drain by raising main 12. The combined underground treatment and storage unit has the following dimensions: 20 m x 15 m x 3 m (LxWxH). The cutaway side elevation shown in Fig. 2 reveals further detail. In particular, the underground treatment and storage unit 2 comprises an upper storage tank 14 which is disposed immediately above an infiltration media bed 16. The storage tank is capable of 85 holding 540,000 litres of water, to a depth of 180 cm. Sandwiched between the tank 14 and media bed 16 is layer 20 cm thick of aggregate size 20 mm wrapped in a non-woven geotextile 18. The path of water through the system is shown by arrows with the stormwater runoff exiting the GPT unit, and being conducted to the storage tank 14. Under conditions of sufficiently high in-flow the tank 14 fills (a representative water level is shown 90 by the broken line). The water is present a relatively high hydraulic head, and so water flows at some force through the passage holes 28 into the aggregate layer 18 and into the 14 media bed 16. A network of agricultural pipes (not shown) laid in a sand/ gravel layer, separated from the filtration media by an open-weave shade cloth, to collect the treated flow. The out-flow pipe 20 conducts treated water to the pump pit 10, with the pump 22 15 conducting water away via the raising main 12. The pump may be replaced by a gravity-fed outlet pipe should the terrain be sloping. Materials for filtration media as recommended by Faculty for Advancing Water Biofiltration (FAWB) are shown in the Table below: material (size) % weight Clay & Silt (<0.05 mm) 2 Very Fine Sand (0.05-0.15 mm) 23 Fine Sand(0.15-0.25 mm) 20 Medium to Coarse Sand (0.25-1.0 mm) 45 Coarse Sand (1.0-2.0 mm) 8 Fine Gravel (2.0-3.4 mm) 2 Q0 The filtration media footprint is congruent with that of the tank: 20m long x 15m wide. It is beneficial to maximise the filtration footprint as this minimises the concrete tank size and the cost of filtration materials is substantially less than concrete tank. Figure 3 shows the preferred embodiment, at the level of the floor of the storage tank. Again, arrows within the pipes show the direction of water flow. This figure shows the in 15 flow pipe 6 conducting water to two large diameter pipes (filled with water) 24 that act to dissipate energy of the water. The pipes 24 terminate in a perforated pipe 26 that acts to evenly disperse water across the full width the tank. The tank floor has passage holes 28 of dimension 300mm x 300mm that allow water to pass from the tank into the filtration bed. The in-flow pipe 6 and the perforated pipe 26 are retained in position by fixing to the 10 concrete tank wall 30 by way of brackets 32. Fig. 4 shows a cross sectional view of the underground treatment and storage unit, the upper panel at the section B-B (looking toward the perforated pipe), the lower panel at section A-A (looking toward the in-flow pipe) of Fig. 3. This view shows more clearly the passages 28 in the floor of the tank through which water flows from the tank to the 15 underlying treatment means. This view also shows a cross-section through the larger pipes 15 (filled with water) 24 demonstrating the presence of sediment therein. The sediment is allowed to settle due to the decrease in flow rate and velocity of water through the larger pipes 24. The invention may be said broadly to consist in the parts, elements and features referred to .0 or indicated in the specification of the application, individually or collectively, in any or all combinations of two or more of said parts, elements or features. Wherein the foregoing description reference has been made to integers or components having known equivalents thereof, those integers are herein incorporated as if individually set forth. .5 It should be noted that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the spirit and scope of the invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications be included within the scope of the invention. O The above description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles described herein can be applied to other embodiments without departing from the spirit or scope of 5 the invention. Thus, it is to be understood that the description and drawings presented herein represent a presently preferred embodiment of the invention and are therefore representative of the subject matter which is broadly contemplated by the present invention. It is further understood that the scope of the present invention fully encompasses other embodiments that may become obvious to those skilled in the art. 40 16

Claims (18)

1. A system for the treatment of stormwater comprising: (i) an underground water storage means, and (ii) a water treatment means in fluid connection with the water storage means, wherein the water storage means is adapted to be positioned above the water treatment means, with the underground water storage means being adapted to provide water to the water treatment means at a high hydraulic head.

2. A system according to claim 1 wherein the storage means is a tank having a depth of at least about 0.3 m or 0.4 m.

3. A system according to claim 1 or claim 2 wherein the capacity of the tank is at least about 5 kL.

4. A system according to any one of claims 1 to 3 wherein the water treatment means is a filtration or infiltration bed comprising a medium or media capable of removing at least a portion of a target contaminant.

5. A system according to claim 4 wherein the filtration or infiltration bed has an area of about 1 m2 to 10,000 m2 , and a height of about 0.5 m to about 5 m.

6. A system according to any one of claims 1 to 5 wherein the hydraulic head is at least about 0.3 m or 0.4 m.

7. A system according to any one of claims 1 to 6 comprising water energy dissipation means.

8. A system according to claim 7 wherein the water energy dissipation means is a pipe having a diameter larger than that of an in-flow pipe.

9. A system according to any one of claim 1 to 8 comprising water dispersal means.

10. A system according to claim 9 wherein the water dispersal means is a perforated pipe.

11. A system according to any one of claims 1 to 10 comprising a gross pollutant trap. 17

12. A system according to any one of claims 1 to 11, adapted to be inherently water impermeable.

13. A system according to any one of claims 1 to 11, adapted to be inherently water permeable.

14. A system according to any one of claims 1 to 13 comprising means for collecting the treated water.

15. A method for installing a system for the treatment of stormwater runoff, the method comprising the steps of providing a system according to any one of claims 1 to 14 , placing the system into a cavity, the cavity being of sufficient depth such that the system is able to be buried.

16. A method for treating stormwater runoff, the method including the steps of: providing a system according to any one of claims 1 to 14 and allowing a stormwater runoff to enter the system.

17. A treated water produced by a method according to claim 16.

18. A system for the treatment of stormwater runoff substantially as hereinbefore described by reference to the accompanying drawings. 18