WO2010115233A1

WO2010115233A1 - Process and system for producing potable water

Info

Publication number: WO2010115233A1
Application number: PCT/AU2010/000380
Authority: WO
Inventors: Gheorghe Emil Duta; Linda Tran; Adnan Mohammad-Ali; Jonathan Emmanuel Marcel Arulappen
Original assignee: Dewatering Filtration Technology Services Pty Ltd
Priority date: 2009-04-07
Filing date: 2010-04-01
Publication date: 2010-10-14
Also published as: AR076212A1; AU2010234213A1

Abstract

A process for producing potable water comprises the steps of : (a) speciating raw water to be purified to determine unit operations for purification of said water to potable standard; and (c) conducting unit operations for purification of the water to potable standard wherein a plurality of said determined purification unit operations are conducted in a treatment vessel within a treatment stage and wherein said treatment vessel is configured for conducting those unit operations requiring treatment of water with at least one gas of predetermined composition.

Description

PROCESS AND SYSTEM FOR PRODUCING POTABLE WATER

The present invention relates to a process and system for producing potable water, involving purification of water for human consumption or water which is to be put to domestic use. There is a shortage of clean drinking water, particularly in communities in developing countries though remote communities located in many places around the world may encounter such shortages of drinking water. Such communities typically do not have access to large scale continuous water treatment plants, such plants typically being directed to treatment of fundamentally clean water held in reservoirs which are also managed with a view to maintaining a high quality of feed water for treatment and distribution to households. Such reservoirs may not be available as water resources for such communities. It has been estimated that drinking water shortages affect more than 1 .1 billion people around the world. It has also been estimated that over 135 million people could die as a result of water related illnesses by 2020.

In addition, even if reservoirs of water are available as a resource, there is the problem of a lack of infrastructure to treat water to provide potable or drinking water. Currently used technologies for water purification are land and energy intensive. For example, the Winneke Water Treatment Plant, which treats between 10 and 30% of the water requirement of Melbourne, Australia, has capacity to treat well in excess of 100 GL p.a water. The plant has multiple units, for example including 14 sand filters alone for the filtration step (see, for example, A Walewijk and R Carty, Sugarloaf Pipeline: Preparing the Winneke Treatment Plant for an Increase in Capacity, 71 ^st Annual Water Industry Engineers and Operators Conference, Bendigo, Australia, 2-4 September 2008). Such a plant could not be operated in many parts of the developing world for reasons of both capital and operating cost.

Where employed, membrane based purification technologies to treat large quantities of water are high in both capital and operational costs due to the pressures and operating conditions required. That is, membrane treatment processes employed for water purification on the large scale require high amounts of power to operate the membrane treatment plant. This power is used to operate pumps which generate the pressures necessary to achieve membrane purification. In many places, power shortages and interruptions of supply are regular occurrences and therefore membrane plant may be unreliable. Other issues arising with conventional water treatment methodology are discussed in the Applicant's PCT Publication No. WO 2006/069418 published 6 July 2006 and the contents of which are hereby incorporated herein by reference.

As to the water resources which are available to remote communities and many communities in developing countries, these are likely to be impermanent and relatively small in volume as well as being subject to high levels of pollution or contamination. Such pollution may be generated by human and animal activity, water being often contaminated with excreta, particularly if an available water body is used as a latrine or receives contaminated water from latrines as a result of poor sanitation. Such contamination includes pathogen contamination including contamination by cholera, typhoid, and E. coli bacteria as well as other microorganisms including protozoa such as Giardia and Cryptosporidium spp. Industrial and agricultural activity may also give rise to pollution in the form of high nutrient and metal content. Water may also have a high chemical and biological oxygen demand, having high levels of iron and sulphur containing compounds, these compounds adversely affecting potability of water in some circumstances, particularly where there are interactions between these and other compounds. While the need for disinfection of water - whether by boiling, chlorination or treatment with mixed oxidants - is recognised, particularly in smaller portable or Point of Use (POU) systems, such systems, often tailored to individual use, assume homogeneity of composition of water which may not in fact be achievable. Appropriate treatment methodology for water purification also includes steps other than disinfection. Disinfection, while important, is a necessary but usually insufficient condition for producing a desired quality of potable water. Boiling of water is also not an appropriate treatment scheme for providing bulk quantities of water for distribution to individuals within a community for both reasons of cost and safety. In addition, while boiling may inactivate pathogens, it may reduce heavy metals and arsenic to more toxic form. This is because the dissolved oxygen in boiling water is practically zero. Insoluble hydroxides in their reduced form become soluble and As (III) is much more toxic than As (V). It is apparent that many processes and systems for producing low volumes of potable water do not have the flexibility to conduct the unit operations necessary to purify what may be a significantly contaminated water source, which has no practical substitute. There is therefore a need for a process and system for producing potable water which offers treatment flexible with the nature and volume of the raw water to be purified while offering the option of low capacity water purification and potable water production.

With this object in view, the present invention provides a process for producing potable water comprising:

(a) speciating raw water to be purified to determine unit operations for purification of said water to potable standard; and

(b) conducting unit operations for purification of the water to potable standard wherein a plurality of said determined purification unit operations are conducted in a treatment vessel within a treatment stage and wherein said treatment vessel is configured for conducting those unit operations requiring treatment of water with at least one gas of predetermined composition.

Unit operations to be conducted in the treatment vessel may be physical, e.g degassing, chemical, e.g pH adjustment, or physico-chemical, e.g settling or sedimentation, in nature. Unit operations to be performed in the treatment vessel may include any two or more distinct unit operations from a group including degassing (for example of CO₂, radon and/or volatile organic compounds (VOC)), aeration, sparging including sparging with a gas other than air and oxygen, clarification, coagulation, flocculation, precipitation (of metals and/or chemical compounds whether organic or inorganic), settling, sedimentation, pH adjustment, chlorination, nitrification, denitrification, oxidation (for example of H₂S and disinfection (or pathogen removal using bactericides such as chlorine, chloramines, chlorine dioxide, hydrogen peroxide and hypochlorites such as sodium hypochlorite). Other unit operations, such as water softening, may be included within this group. A water softening process scheme may involve raising pH to alkaline levels, settling of precipitates, discharge of sludge followed by reduction of pH to near neutral levels. However, neutral pH requirement for purified water is not mandatory. Heating, or boiling, of water is neither a necessary nor desirable step in the water purification scheme. All operations are conducted at or close to ambient temperature conditions.

A unit operation selected from the group consisting of coagulation, flocculation, nutrient removal (N and P being key nutrients), precipitation, settling and sedimentation (settling operations) is typically required within the treatment vessel, advantageously a single tank treatment vessel operated in-line in the primary treatment stage. This treatment vessel may be the first and only tank treatment vessel, operated in-line, within a water purification system operated in accordance with the above described process. Filtration, other than filtration at a very coarse or grizzly scale to remove tree branches, leaves and other bulk matter, is not typically conducted within a treatment vessel, particularly an advantageously single treatment vessel, used in the treatment stage. Further, the unit operation of flotation would not typically be used in accordance with the process.

Gas(es) to be used in degassing, aeration, sparging, nitrification, denitrification or oxidation may be selected through their very nature to have predetermined gas composition. For example, a unit operation requiring use of oxygen would have pure or substantially pure oxygen as the predetermined gas composition. Gas composition may also be controlled or tailored, more finely, to the requirements of the unit operation. Two classes of unit operation may be considered: 1 ) those unit operations using an oxygen containing gas and 2) those unit operations using gases excluding oxygen particularly for stripping operations.

Where air is used, air may be supplied from a blower to a diffuser, for example a fine bubble diffuser. However, to achieve further process control and efficiency, gas of tailored or controlled composition may be obtained by treating air through membrane separation or other techniques if necessary, to generate required gases of predetermined composition. In such case, gases used in a unit operation to be conducted in the treatment vessel may include oxygen, nitrogen, oxygen enriched air, nitrogen enriched air, carbon dioxide and mixtures of these gases. In such case, the treatment vessel is configured for "dual" gas operation and it may also be seen that gases may be selected from a plurality of available gases providing much flexibility in conducting purification unit operations with varying requirements for gas composition.

Nitrogen, nitrogen enriched gas (that is with nitrogen content in major proportion) or other gas inert at the low temperatures contemplated for the practice of the process, may be more effective in stripping VOC and H₂S from water to be purified. An oxygen depleted or oxygen free gas will form bubbles within the water volume present within the treatment vessel, these bubbles having larger stripping capacity than those having higher levels of oxygen. This is because oxygen may be consumed by oxidation reaction(s) within the treatment vessel. Therefore, where proportion of oxygen present in the stripping gas is lowered and stripping bubbles have lower oxygen content, less bubble shrinkage will occur increasing degassing or stripping efficiency and reducing stripping time. Residual or lower levels of VOC and H₂S, left after a degassing or sparging unit operation with an inert or low oxygen gas, may then be subjected to oxidation, whether by air, another oxygen containing gas or another oxidising gas. Such oxidation may involve a multi-stage process in which air, oxygen enriched air or oxygen is used as oxidant in a first step with a further oxidation step using an oxidant other than air and oxygen. For example, hydrogen peroxide may be used in such further oxidation step. Where dual gas generation is employed, nitrification - denitrification processes may also be implemented. For example, in a first step, contaminants may be oxidised by air with an enriched oxygen level. In a second step, anoxic conditions for denitrification may be created by stripping with an inert or reducing gas free of oxygen. Nitrogen is conveniently to be selected for the denitrification application. Oxygen and nitrogen may be generated in a membrane separation unit. The oxygen and nitrogen gases may be mixed to requisite proportions in a dual gas treatment process.

Carbon dioxide may also be usefully applied in the process. It may be used for temporarily increasing acidity and re-carburation in lime softening and other processes.

In the above cases, control over gas composition present in an atmosphere above a water level in the treatment vessel will result from the steps taken to tailor composition of the at least one predetermined gas. Other steps, such as control over pressure of the gas atmosphere may be taken as well. Control over pressure results in control over dissolved gas content in water to be purified through the operation of Henry's Law.

Water subjected to the plurality of purification unit operations in said treatment vessel within said treatment stage may be directed to a further or secondary treatment stage. In the latter case, the first treatment stage is referred to as a primary treatment stage. It is advantageous that no treatment stage include more than a single treatment vessel, the aim being to provide a low cost water purification system for producing potable water. Water purified in accordance with the above process must be potable according to an accepted minimum standard such as the WHO Guidelines for Drinking-Water Quality, the contents of which are hereby incorporated where permitted by reference. The potable water may be directed for use as drinking water especially for use as a resource of drinking water by a community of people each having drinking water consumption needs of about 2 or more litres per day. Such a resource is sub-municipal in scale.

The treatment vessel, in the form of a tank or profiler tank configured to facilitate settling, is advantageously employed in-line for conducting the plurality of water purification unit operations. By "in-line" is meant that the vessel is actively used in the process. Having another vessel on stand-by or operated in reserve, or to purify a different batch of raw water is not precluded. Indeed, this is advantageous because the treatment vessel, which is ideally of low capacity on a kilolitre scale - that is intended to be no more than a few thousand litres capacity and even below 1000 litres capacity - has a small footprint meaning that consumption of land for water purification purposes is minimal, a clear difference being demonstrated from large metropolitan water purification plants treating 100 GL pa or even greater volumes of water.

One or more of the treatment vessels, though only one or a single tank vessel should be used in-line in each of the primary and secondary (if employed) treatment stages, to reduce capital cost and footprint in terms of land usage, may be operated in recirculation mode. In recirculation or "closed loop" mode, water to be purified is recycled through the vessel(s) as many times as required to effectively complete a determined purification unit operation. Recycling may be conducted to enhance mixing of water with reagents or to increase effective residence time of water to be purified within a unit operation being conducted within the advantageously one tank vessel.

Each of the treatment vessel(s) may be scheduled for conducting, within each, the plurality of purification unit operations as determined to be required from the speciation data for the water feedstock or the raw water to be purified. That is, each treatment vessel may be scheduled to conduct sequential or progressive unit operations over a determined period of time. That period of time, which may range from less than an hour to a few or several hours dependent on the quality or speciation of raw water (higher quality raw water taking less time for treatment), may be dictated by the nature of the unit operation and the time taken to perform it acceptably taking into account the raw water speciation, particularly if time variant in speciation. Other relevant parameters for the unit operation may also be taken into account. However, performance of the plurality of unit operations within a given treatment vessel may occur simultaneously or overlap in time. In such cases, the unit operations may be phased in time with a successor unit operation taking over from a predecessor unit operation as time progresses. Where a given treatment vessel is operated in recirculation or closed loop mode, and reagent(s) are required to be introduced - as required - to perform each unit operation, these reagents may be introduced in the recirculation or recycle line. Such a recycle line may be provided with feed points for one or a suite of reagents, feed points or dosing units being opened only for such reagent(s) as are required to perform the scheduled unit operations. Advantageously, only two dosing units may be provided, one to provide flocculant or coagulant dosing, the other to provide disinfectant dosing. Such reagent(s) may also be introduced at the treatment vessel (s).

Aeration, degassing or sparging, coupled with a settling operation, as above defined and as conducted in a single treatment vessel in a single treatment stage, could be sufficient in some cases to produce potable water; that is water at drinking water quality. However, depending on the level of contamination and contaminant type, risk of supplying contaminated water is high. If required, further unit operations, which may include unit operations as described above, may be conducted in the preferred secondary treatment stage which, like the primary treatment stage, is advantageously conducted within a single treatment vessel (particularly a tank or profiler tank vessel) to reduce land and capital cost requirements. However, the secondary treatment stage is preferably directed to a distinct unit operation selected from the group consisting of filtration, chlorination, disinfection (or pathogen removal), pH adjustment and oxidation. Filtration, to remove suspended solids and pathogens, is advantageously conducted in the second treatment stage of the process. A plurality of such unit operations could also be conducted in the or each treatment vessel included within the secondary treatment stage which typically improves the degree of contaminant removal and the degree of disinfection. The secondary treatment stage may be described as a polishing stage.

For a broad range of contaminants, a primary treatment stage could achieve better than 90% contaminant removal. Nonetheless, there are situations where the secondary stage may target a specific contaminant instead of playing a polishing role. For example, an adsorption unit operation could be conducted to remove fluoride using activated alumina or adsorption of a range of other contaminants using granular activated carbon. Any disinfection step, particularly a step involving chlorination is to be conducted following a precipitable contaminant precipitation step. This is particularly important to prevent formation of harmful disinfection (chlorination) by products in the presence of organic contaminants and to minimize usage of disinfectant. However, when there is no risk of formation of such potentially harmful by products, disinfectant addition may be performed before precipitation and discharge of precipitated material.

Again, the treatment vessel within the secondary treatment stage may be operated in recirculation or closed loop mode. Treatment vessels in the primary and secondary treatment stages may be operated simultaneously, filtration occurring while circulating water back through a primary treatment vessel where aeration or other unit operations may be conducted, particularly for mineralisation of residual organics. Water may be recirculated through a filtration unit for as long as necessary to achieve required filtration efficiency. In closed loop mode, the secondary treatment unit operations are also advantageously conducted in batch mode.

Filtration may be conducted with various media, whether sand, stone, mixed media and catalytic media. Preference is advantageously given to catalytic materials with catalytic surfaces based on elements which, in case of accidental dissolution in water, pose less health risk. Oxides of iron, manganese, aluminium and silicon in various combinations are a suitable choice. Such materials may be combined with simple elements resistant to oxidation and dissolution in water, for example platinum group elements including platinum. Such media may be supported for example on oxides of silicon, aluminium or simple carbon such as activated carbon. Adsorbent media could be used as well. Where catalytic media are employed both for filtration and catalytic oxidation, catalytic oxidation may follow a process scheme as described in PCT Publication No. WO 2006/069418 as previously referred to. Post filtration and any further disinfection step, a membrane treatment step, such as reverse osmosis (RO), may be included. An RO step would allow removal of highly soluble salts such as chlorides. Other unit operations, for example adsorption (of fluoride or other contaminants as above mentioned) could be included. Water to be purified in accordance with the above scheme may be sourced from groundwater, such as available from bores and wells, or surface water. Where ground water is used, pathogen content may be variable but such water often contains gases such as hydrogen sulphide and/or radon as well as volatile organic compounds. Surface water may be contaminated with various chemical and biological species and differences in the water purification process scheme may derive from the differences in speciation. Generally, however, water to be treated by the process would not have a high biological oxygen demand ("BOD ') or level of nutrients, for instance of an order of magnitude comparable with household sewerage. In accordance with another embodiment of the present invention, there is provided a system for producing potable water by a plurality of purification unit operations comprising: a treatment stage for conducting unit operations in the purification of water to potable standard wherein said treatment stage includes a single treatment vessel in which a plurality of purification unit operations are conducted and wherein said treatment vessel is configured for conducting those unit operations requiring treatment of water with at least one gas of predetermined composition. Product water is then preferably directed to a secondary treatment stage.

As gases used in the unit operations may include air, oxygen, nitrogen, carbon dioxide and mixtures of these, as above described, the system may comprise a membrane separation unit for producing these gases. The gases may have components in controlled proportion as required by a unit operation.

The system may include a pressure regulator for controlling pressure of gases in an atmosphere above a water level in said treatment vessel to control dissolved gas content of water to be purified. The pressure regulator may comprise a pressure relief valve.

Advantageously, the primary treatment stage consists of a single "in line" treatment vessel in the form of a tank. By "in-line" is meant that the vessel is actively used in the purification process to be conducted within the system. Further treatment vessels may be available in stand by or reserve mode. Such further treatment vessels may also be dedicated to treatment of different batches of raw water where the system is operated, as preferred, batchwise. A batchwise mode of operation allows better treatment of water at small capacity.

Unit operations to be used within the system have been described above. Typically, a unit operation selected from the group consisting of coagulation, flocculation, precipitation, settling and sedimentation will be conducted within the said at least one treatment vessel. Therefore, this treatment vessel is advantageously provided in the form of a tank or profiler tank which has a base of tapering cross-section or conical section as is preferred for settling type unit operations. Although a cylindrical tank with such tapering base is preferred as the profiler tank, the treatment tank of the Applicant's co-pending Australian Provisional Patent Application No. 2008902802 filed 3 June 2009, and the contents of which are hereby incorporated herein by reference, could be used instead. In any case, provision is to be made for recovery and disposal (further treatment) of sludge from the base of the tank.

As described above, the secondary treatment stage also advantageously includes a single in-line treatment vessel. As this secondary treatment vessel will often be used for filtration type unit operations, it may advantageously include a filter. In such case, the treatment vessel may include a bed of filtration media and may be provided as a column or other suitable vessel shape for conducting the filtration unit operation. The bed may include a single filtration medium or a mixed filtration medium. The water purification system may include a contact or intermediate storage stage, made up of one or more tanks, in which treated water may be stored prior to distribution for use by a community served by the water treatment system. In the contacting stage, water may be disinfected by inactivation of pathogens by a bactericide, this unit operation requiring a certain amount of time of contact between bactericide and pathogen. For example, where the bactericide is chlorine, minimum contact time may be - for example - 30 minutes. Contacting is also appropriate where highly toxic contaminants, posing serious risk, have been present in the water to be purified. Treated water batches may be held in buffer storage tank(s) to be checked for contaminant level before water distribution.

The process and system for producing potable water of the present invention may be more fully understood from the following description of preferred and non-limiting embodiments thereof, the description referring to the following figures in which: Fig. 1 is a process diagram of a system for producing potable water operated in accordance with one embodiment of the present invention;

Fig. 2 is a schematic illustration of a contact tank system which may be included within the water purification system of Fig. 1 ;

Fig. 3 is a schematic illustration of a dual gas feed assembly that may be used within the water purification system of Fig. 1 ;

Fig. 4 is a process diagram of a system for producing potable water including a single profiler tank within the primary treatment stage and according with a further embodiment of the present invention; and Fig. 5 is a process diagram of a system for producing potable water according with a still further embodiment of the present invention.

Referring now to Figs. 1 , 4 and 5, there is shown - in each case - a water purification system 100 for producing potable water by a plurality of purification unit operations comprising a treatment stage for conducting unit operations in the purification of raw contaminated water from a water source such as a shallow aquifer. The treatment stage, in this case the primary treatment stage, includes a treatment vessel 75 or 1 15 in which a plurality of purification unit operations are conducted before product water is directed, as preferred to the secondary treatment stage made up of a single treatment vessel 195. The plurality of water purification unit operations to be applied is determined following a speciation of water present in the shallow aquifer by physical and chemical analyses. Those unit operations will include treatment of water with gas(es) of predetermined composition. As the quantity of water to be purified is scarce and limited in quantity, water purification system 100 is operated to purify batches of water for distribution to a remote agricultural community in a developing country.

Although Figs. 1 and 5 show two treatment vessels or profiler tanks 75 and 1 15, each of about 1500 litre capacity, available for use in the water purification system, only one profiler tank is advantageously used to purify a given batch of water (as shown in Fig. 4 which shows a simplified water purification system 100 having only one profiler tank 75 which has a low footprint and low capital cost directed to use in developing countries where cost may be a significant issue). In the cases illustrated in Figs. 1 and 5, treatment profiler tank 75 will be treated as the operating treatment vessel with profiler tank 1 15 being left on stand-by to process a future batch or operating to part purify another batch of water. It may be understood, for the purposes of illustration, that the purification unit operations conducted in profiler tank 1 15 are the same as those conducted in profiler tank 75 though modifications to the purification unit operation schedule could be made if, for example, there were variations in speciation of water coming from the shallow aquifer. Whether raw water is directed to profiler tank 75 or 1 15 depends on the state of flow valves 55 and 85. If flow valve 85 is closed and flow valve 55 is open, raw water will be directed to profiler tank 75. If flow valve 85 is open and flow valve 55 is closed, raw water will be directed to profiler tank 1 15. Whether raw water is directed to profiler tank 75 or profiler tank 1 15, it is passed through strainer 65 or 95 to sieve out larger items such as plant matter and other detritus.

As key unit operations to be conducted in profiler tanks 75 and 1 15 involve unit operations employing predetermined gases with a controlled or tailored gas composition, as well as the requirement for dissolved gas concentration in water within the profiler tanks to be within certain bounds, the profiler tanks 75 and 1 15 are configured to receive a gas supply which may be selected from a plurality of available sources. The profiler tanks 75 and 1 15 are also closed to atmosphere. Dissolved gas concentration in water is controlled, in accordance with Henry's Law, by controlling pressure in an atmosphere or head space above water level in whichever profiler tank 75 or 1 15 is in use at any particular time. Pressure may be controlled using a pressure relief valve 41 for profiler tank 75 and pressure relief valve 42 for profiler tank 1 15. Pressure regulators in the form of pressure relief valves 41 and 42 allow achievement of a pressure differential between the atmosphere above water level in each profiler tank 75 and 1 15 and the external atmospheric pressure. The pressure relief setting may be adjusted manually or automatically in accordance with any particular gas using unit operation being performed at any given time. Use of proportional pressure relief valves is advantageous for automated process control.

In the case of Fig. 1 , the water purification system 100 is directed to purification of water speciated to contain organic compounds, particularly volatile organic compounds (VOC), suspended solids and pathogens which are likely to be harboured in the suspended solids. The VOC content is sufficiently low that aeration may be conducted in profiler tank 75 to oxidise the VOC so that it may be removed from the water to requisite standard. An aeration unit operation is conducted using a fine bubble diffuser 40, of conventional type, and delivering fine bubbles of oxygen enriched air. To that end, the fine bubble diffuser 40 is connected to a membrane separation unit ("MSU") 20 which removes nitrogen from the air introduced to the MSU 20 by an air compressor 5. Profiler tank 1 15 likewise is provided with a fine bubble diffuser 45 which may also take oxygen enriched air from MSU 20. A flow restrictor or orifice 25 in the nitrogen line provides a back pressure for oxygen enriched air tending to direct this gas through fine bubble diffuser 40 or 45 as the case may be. Flow rate of oxygen enriched air to fine bubble diffusers 40 and 45 may be controlled through appropriate setting of solenoid actuated flow control valves 30 and 35.

In the process scheme of Fig. 1 , no provision is made for nitrification, which would require injection of nitrogen as well as oxygen or oxygen enriched air; that is plural gases. If dual gas injection is required, the gas feed system shown schematically in Fig. 3 may be employed. Here, air is introduced to MSU 20 from an air compressor 5, air flow to MSU 20 being controlled by solenoid valve 10. A pre-filtration of air may be conducted at air filter 15. The MSU 20 may provide both oxygen and nitrogen outputs. If nitrogen is required for nitrification, oxygen may be vented through oxygen vent valve 21 with nitrogen being delivered through either fine bubble diffuser 40 or 45, at flow rate dictated by setting of control valve 30. On the other hand, if oxygen is required, nitrogen may be vented through nitrogen vent valve 22, with oxygen being delivered through either fine bubble diffuser 40 or 45 at flow rate dictated by setting of control valve 30. Flow restrictor 25 acts as described above.

The aeration unit operation commences when level in profiler tank 75 is sensed to be at a required level. Level transducer 70 provides level measurement data to a control unit, of microprocessor or other type, so that this determination may be made. In the profiler tank 1 15, level transducer 90 provides the level measurement data. Minimum aeration/oxidation process duration is between 3 and 10 minutes though a longer duration could be provided for. Profiler tanks 75 or 1 15 are operated in closed loop or recirculation mode during the aeration operation to ensure good mixing. No or very limited water flow is directed to the secondary treatment stage to be described below during this closed loop or recirculation mode of operation.

Recirculation or closed loop mode of operation is achieved, during aeration, by pumping water by mix pump 150 through mix pump valve 125 and back to the operating profiler tank 75 through profiler return valve 130 which is set to return water to profiler tank 75 when in operation. Multivalve 245 and recirculation valve 200 are set to permit such recirculation. Pressure in the line may be monitored by pressure gauge 155 to ensure the water purification system is operating correctly. If profiler tank 1 15 is in operation, then profiler return valve 130 is set to return water to profiler tank 1 15. This scheme is that used whenever recirculation, recycle or closed loop mode is selected. During such recirculation or closed loop mode operation, it is also possible to dose any desired reagents into the recirculated water through delivery of reagent(s) through any one or more of dosing units 160, 165, 170 and 175. These dosing units may be set up to inject reagent(s) into the water. Operation of dosing units may be controlled through the control unit for water purification system 100.

As well as oxidation of VOC, the aeration unit operation also oxidises metal hydroxides which may be present to higher valence. For example, aquifer water typically contains iron which may be oxidised from ferrous to ferric state during the aeration operation. As such higher valence hydroxides tend to be insoluble, it is necessary to remove the solids and so the second unit operation to be performed in profiler tank 75 or 1 15 is clarification or flocculation.

For flocculation to be conducted, the water is dosed, through one or more of dosing units 160, 165, 170 and 175 with a flocculant or coagulant. As drinking water is the desired product from the water purification system 100, the flocculant/coagulant may be iron based (for example ferric chloride and ferric sulphate are useful) or aluminium based (for example alum and polyaluminium chloride are useful). Water is recirculated through the recirculation or closed loop during the clarification or flocculation purification unit operation, and bypassing the treatment vessel 195, to ensure correct dosing of water with the desired flocculant/coagulant. The amount of flocculant/coagulant to be dosed is calculated based on the volume of the batch of water being treated.

When correctly dosed, the flocculable matter including metal hydroxides and organic compounds, forms floes which settle in profiler tank 75 or 115 (where the latter is in operation instead of profiler tank 75). Therefore, after some settling or sedimentation time, a compact sludge forms at the base of the profiler tank 75. This sludge is favoured to collect, in densest state, at the bottom of the profiler tank so, in accord with usual practice for sedimentation vessels or settlers, both profiler tanks 75 and 1 15 are provided with bases of truncated conical section. At the end of the clarification or flocculation unit operation, which may take between 10 and 30 minutes, the sludge may be removed through a drain valve 1 10 (or 101 in the case of profiler tank 1 15). The drain valve 1 10 or 101 is opened for only as long as water starts to run at an acceptable level of turbidity at which time drain valve 110 or 101 is again closed. Drain time through drain valve 1 10 (or 101 ) may be of the order of 30 seconds to 1.5 minutes.

At the end of the clarification/flocculation unit operation, water is ready to be treated in the secondary treatment stage for mineralisation of residual organics. This unit operation involves cycling or recirculation of water through sand filter(s) located in treatment vessel or sand filter 195 - the first time this treatment vessel is brought into operation - as well as profiler tank 75 or 1 15 (if the latter is in operation). To ensure that water is directed from the profiler tank 75 (or 1 15 if that profiler tank is in operation), profiler return valve 130 is closed and main pump valve 120 is opened such that main pump 135 delivers water through multivalve 245, a this valve 245 being set such that water is directed through sand filter 195, back through the multivalve 245, and recirculation valve 200 which is set so that water is returned to profiler tank 75 where aeration (again through fine bubble diffuser 40) and/or other oxidation operations may be conducted. Pressure of water delivered by main pump 135 may be indicated by pressure gauge 140 or measured by pressure transducer 145 to ensure correct operation of the system.

The duration of this closed loop residual organic mineralisation unit operation, which involves treatment vessels 75 (or 1 15) and 195, is dependent on the quality of the raw water from the aquifer. Alternatively, or even in conjunction with aeration (or air injection), a non-halogenated compound can be injected at one of dosing units 160, 165, 170, 175 such as hydrogen peroxide if organic contaminant level is heavy. This is because use of chlorine or chlorine or other halogenate disinfectants may tend to promote formation of toxic by-products, this being undesirable. On completion of this unit operation, a short backwash of sand filter 195 may be conducted using both air and water for contaminant removal efficiency.

It will be observed that sand filter 195 need not, in contrast with profiler tanks 75 and 1 15, be equipped with pressure relief valves for controlling pressure of gas atmospheres above water level as required in earlier conducted unit operations. Following these unit operations, water is ready for treatment for removal of any remaining pathogens. In a disinfection step, water recovered to profiler tank 75 (or 1 15) is chlorinated for disinfection. The chlorine is injected into a volume of water contained in profiler tank 75 (or 1 15), the amount of chlorine to be injected (and as measured, for example, by time of injection) is calculated with reference to the volume of water in the profiler tank. Chlorine levels may be comparable with those used in larger municipal water treatment plants.

Following chlorination, which has lower chlorine consumption than would be needed without preliminary metal hydroxide precipitation, water is again pumped through main pump 135, the multivalve 245 being set to correct position such that water is passed through sand filter 195 for sand filtration of pathogens. The sand filter 195 includes a sand bed with sand sized to filter out particles down to about 8 micron particle size. This unit operation is conducted open loop, valve 200 being set to enable discharge of treated water for final polishing to be described below rather than recirculation to profiler tank 75 (or 1 15). Main pump 135 is a variable speed pump and the speed of the pump 135 is controlled so that the contact time of the water through sand filter 195 is known. The speed of the pump is varied dependent on the quality of the raw water, the object being to ensure sufficient residence time for pathogen inactivation. Pressure transducer 190 measures pressure and provides an indication of sand filter 195 operability state. Pressure above a predetermined level may indicate need for filter backwashing. An air vent valve 185 is provided to release air from sand filter 195 if required.

Final polishing involves passing water through fine filters being 5 micron filter 230 and 1 micron filter 225, provided that sampling at port 215 indicates appropriate water quality. Provision may be made for further treatment if quality requires further improvement. An isolation valve 210 is also provided to allow servicing of the filters 225 and 230. A non-return valve 235 is also provided to prevent flow back of clean water under normal operating conditions. A further sampling point 240 allows checking of water quality to ensure it meets the minimum quality standard.

After this, water treatment may be complete and water distributed as drinking water. However, there is also an option for intermediate storage using the contact tank system shown in Fig. 2. The tank system includes two tanks 260 and 280 where treated water may be stored pending distribution. Either or both tanks 260 and 280 may be used. Inlet valves 250 and 270 distribute treated water to the tank(s) as required. Tank level may be measured using level transducers 255 and 275. It is possible to use these tanks simply for storage but further treatments, and or sampling, could be conducted in them as required. Such sampling or treatment may be conducted where the raw water has highly toxic contaminants and further quality assurance is needed before distribution.

The process scheme of Fig. 4 closely resembles that for Fig. 1 , as described above. However, the process scheme is considerably less complex since only one profiler tank 75 is used in the primary treatment stage and main pump 135 may also serve as the mix pump; that is, mix pump 150 of the process scheme of Figs. 1 and 5 may be omitted. This profiler tank 75 is used to conduct the same water purification unit operations as described above and it may do so effectively. An advantage this scheme has over the scheme of Fig. 1 is the even smaller footprint and capital cost involved. The process scheme of Fig. 4 is also operationally simpler in that a membrane separation unit is not included. Air is simply provided through blower 6 to fine bubble diffuser 40 as required for aeration. Non-return valve 7 prevents water from entering the blower 6 line. Blower 6 also plays a role in enabling backwash of sand filter 195, the use of a mixture of air and water for backwashing significantly reducing the amount of water used for backwash.

Referring now to the process scheme of Fig. 5, this process scheme allows for both nitrification and denitrification, for use of nitrogen and oxygen individually; or for mixing of nitrogen/oxygen gases if desired. To that end, it may be observed that MSU 20 has nitrogen and oxygen lines to both fine bubble diffusers 40 and 45 dependent on which of profiler tanks 75 and 1 15 are in-line at any given time. Oxygen flow to either of fine bubble diffusers 40 and 45 is controlled by flow control valves 31 and 36. Nitrogen flow to either of fine bubble diffusers 40 and 45 is controlled by flow control valves 32 and 37. Flow restrictor 25 plays the role described above. The profiler tanks 75 and 1 15 continue to play the role of clarification as described above. The mode of operation has many similarities with the process scheme described in relation to Fig. 1. However, there are some process differences. For example, in the primary treatment stage, as conducted in profiler tank 75 or 1 15, nitrite present in raw water is converted to nitrate by injecting oxygen into the profiler tank 75, 115. In addition, it is possible to multi-task profiler tanks 75 and 1 15 to achieve a high capacity water treatment system, though again operated in a batch mode.

Clarification proceeds as described in relation to Fig. 1 though dosing of coagulant/flocculant may be varied dependent on speciation for the raw water. Then, during recirculation of water between sand filter 195 and profiler tank

75 (or 1 15 if that profiler tank is in use), nitrogen is injected through fine bubble diffuser 40 (or 45). Such nitrogen injection creates an anoxic condition conducive to denitrification. At the end of the denitrification unit operation, water is passed on to the filtration and chlorination unit operations as discussed above in relation to the process scheme of Fig. 1 with the exception that, in Fig. 5, chlorine is injected into treated water at dosing unit 175 located after sand filtration at a rate dictated by the flowrate of water from the sand filter 195. If necessary, treated water may again be directed to intermediate storage in the contact tank system of Fig. 2. Modifications and variations to the process and system for producing potable water of the present invention may be apparent to the skilled reader of this disclosure. Such modifications and variations are deemed within the scope of the present disclosure. For example, there may be other water purification unit operations, such as those involving use of predetermined gas composition, other than those recited above. Such water purification unit operations would not depart from the scope of the present invention. In addition, the process and system of the present invention could be implemented with different process schemes than those described above without departing from the scope of the present invention.

Claims

Claims:

1. A process for producing potable water comprising:

2. A process of claim 1 wherein unit operations to be performed in said treatment vessel comprise any two or more distinct unit operations from a group including degassing, aeration, sparging, clarification, coagulation, flocculation, precipitation, settling, sedimentation, pH adjustment, chlorination, nitrification, denitrification, oxidation, disinfection and water softening.

3. A process of claim 2 wherein gas composition is controlled to enable unit operations including at least one unit operation selected from the group consisting of degassing, aeration, sparging, nitrification, denitrification and oxidation to be conducted.

4. A process of claim 3 wherein gas of controlled composition is obtained by treating air to generate a gas selected from oxygen, nitrogen, oxygen enriched air, nitrogen enriched air, carbon dioxide and mixtures of these gases for use in a unit operation.

5. A process of claim 3 or 4 wherein nitrogen or nitrogen enriched gas is used as stripping gas for stripping VOC and H₂S from water to be purified.

6. A process of claim 5 wherein stripping efficiency is increased by lowering the proportion of oxygen present in the stripping gas.

7. A process of any one of claims 2 to 6 wherein water is subjected to a degassing, stripping or sparging operation with an inert gas and then subjected to oxidation.

8. A process of claim 7 wherein oxidation involves a multi-stage process in which air, oxygen enriched air or oxygen is used as oxidant in a first step with a further oxidation step using an oxidant other than air and oxygen.

9. A process of claim 3 or 4 wherein nitrification - denitrification is implemented comprising the steps of oxidising contaminants by air with an enriched oxygen level; and creating anoxic conditions for denitrification by stripping with an inert or reducing gas free of oxygen.

10. A process of any one of the preceding claims comprising control over gas composition of gases present in an atmosphere above a water level in said treatment vessel.

1 1 . A process of claim 10 comprising controlling pressure in said atmosphere above said water level to control dissolved gas content of water to be purified.

12. A process of any one of the preceding claims wherein water subjected to said plurality of purification unit operations in said treatment vessel is directed to a further or secondary treatment stage.

13. A process of any one of the preceding claims wherein each treatment stage comprises a single treatment vessel having low capacity.

14. A process of any one of the preceding claims as dependent from claim 2 wherein said plurality of determined unit operations are conducted in a single treatment vessel in a single treatment stage and comprise aeration, degassing or sparging, coupled with a settling operation to produce potable water.

15. A process of any one of the preceding claims wherein said treatment vessel(s) are operated in recirculation mode with water to be purified being recycled through a treatment vessel as many times as required to effectively complete a determined purification unit operation.

16. A process of claim 15 wherein recirculation is conducted to enhance mixing of water with reagents or to increase effective residence time of water to be purified within a unit operation being conducted within said treatment vessel(s).

17. A process of any one of the preceding claims comprising control over gas composition of gases present in an atmosphere above a water level in said treatment vessel.

18. A process of claim 17 comprising controlling pressure in said atmosphere to control dissolved gas content of water to be purified.

19. A system for producing potable water by a plurality of purification unit operations comprising: a treatment stage for conducting unit operations in the purification of water wherein said treatment stage includes a treatment vessel in which a plurality of purification unit operations are conducted and wherein said treatment vessel is configured for conducting those unit operations requiring treating water with at least one gas of predetermined composition.

20. A system as claimed in claim 19 comprising a membrane separation unit for producing gases to be used in unit operations conducted in said treatment vessel.

21 . A system as claimed in claim 19 or 20 comprising a pressure regulator for controlling pressure of gases in an atmosphere above a water level in said treatment vessel to control dissolved gas content of water to be purified.

22. A system as claimed in claim 21 wherein said pressure regulator comprises a pressure relief valve.

23. A system as claimed in any one of claims 19 to 22 wherein said treatment vessel is a single treatment vessel.

24. A system as claimed in claim 23 wherein said treatment stage is a single treatment stage.

25. A system as claimed in any one of claims 19 to 23 further comprising a second treatment stage including a further treatment vessel, preferably a single treatment vessel.

26. A system as claimed in any one of claims 19 to 25 wherein said treatment vessel is provided in the form of a tank or profiler tank which has a base of tapering cross-section or conical section.

27. A system as claimed in claim 25 or 26 wherein said vessel in said second treatment stage includes a filter.