WO2015185910A1

WO2015185910A1 - Siphon decanter for a sequencing batch reactor

Info

Publication number: WO2015185910A1
Application number: PCT/GB2015/051595
Authority: WO
Inventors: David De Hoxar
Original assignee: Plantwork Systems Consulting Llp
Priority date: 2014-06-05
Filing date: 2015-06-01
Publication date: 2015-12-10
Also published as: GB2526848A; GB201409960D0; GB2526848B

Abstract

A siphon decanter (10) for a sequencing batch reactor (302), the siphon decanter comprising an inlet (12), an outlet (14) and a connector pipe (18), wherein the inlet, outlet and connector pipe form a substantially inverted "U" shape siphon. The siphon decanter (10) further comprises an outlet chamber (20) arranged in fluid communication with the outlet (14), a drainage aperture (22) and an overflow weir (26). The siphon decanter (10) has no motors of moving parts and requires little or no maintenance. The siphon decanter (10) enables the decant and fill stages to be overlapped when performing an activated sludge process in a sequencing batch reactor (202) thus reducing overall process time and increasing capacity.

Description

SIPHON DECANTER FOR A SEQUENCING BATCH REACTOR

The present invention relates to a siphon decanter for a sequencing batch reactor (SBR).

Embodiments of the present invention relate particularly, but not exclusively, to the use of a siphon decanter for decanting clean water from the top of a sequencing batch reactor (SBR) tank employing an activated sludge process.

Legislation requires that sewage and wastewater is treated. Wastewater treatment includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants. An objective of wastewater treatment is to produce an environmentally safe fluid waste stream and a solid waste suitable for disposal or reuse, for example, as fertiliser. In general, wastewater will undergo several treatment stages. A preliminary treatment stage may involve screening, grit removal, flow-split and storm water storage. Primary treatment normally involves a sedimentation stage that is performed in a large tank which is used to settle sludge. Grease and oils that rise to the surface of the water can also be removed.

A secondary treatment stage is directed at degrading biological components present in the wastewater. An activated sludge process is the standard choice for treating sewage and other wastewaters that have organic pollutants. The activated sludge process is generally considered the "normal" biological treatment against which other processes may be compared. The process can be used for oxidising carbonaceous biological matter, oxidising nitrogenous matter, removing phosphates and/or driving off entrained gases such as CO2, NH₄, N2 etc. Since the 1970's, the activated sludge process has been implemented in the majority of new municipal sewage works serving over 2000 people.

The activated sludge process is most commonly performed as a continuous flow process using a series of tanks or a tank subdivided by baffles for different treatment zones. A minimum of two tanks or zones are required; a reactor tank and a settlement tank. More complex water treatment processes with more tanks or zones are often required to produce high quality treated water and a stable treatment process.

Continuous flow activated sludge is preferred where the quantity of water to be treated is particularly large and the water treatment requirements are not especially onerous. It is also preferred when the required quality of water treatment and the treatment process is unlikely to change. Continuous flow activated sludge processes are simple to operate and work well where the inflow of water is constant or varies only slowly. Alternatively, the activated sludge process may be performed in batches using one tank, for example, using a sequencing batch reactor (SBR).

Sequencing batch reactors (SBRs) are well known industrial process tanks used for the treatment of wastewater and provide an attractive alternative to continuous flow. They have recently gained popularity for wastewater treatment using the activated sludge principal. SBRs are particularly suited to small/medium sized water treatment works where the inflow of water varies significantly or if there are stringent water treatment requirements. SBRs are highly flexible and can readily accommodate changes to the water treatment process.

The activated sludge process when performed in a SBR tank comprises four main stages: a) fill; b) aerate; c) settle; d) decant.

Unlike the continuous flow process, in a sequencing batch reactor (SBR) the treatment stages are separated by time rather than by tank walls; for part of the time the SBR tank acts a reactor tank and for another part of the time as a settlement tank. The more complex the process requirements, the more favourable SBR technology becomes vs continuous flow processes since continuous flow processes require a separate tank or zone for each treatment stage.

A method of removing (decanting) clean treated water from the top of the SBR tank at the end of the settlement stage is required. The method must not allow dirty water out of the tank during the other stages of the water treatment process or contaminate the clean water.

Several methods for decanting clean water from the top of an SBR tank are currently used. Typical decanters in the prior art have an arrangement of two actuators (or one pump and an actuator). Pumps and actuated valves require both cabling and a control section in the electrical panel as well as a section of code in the software all of which significantly increase overheads and running costs. The decanters often have hinged joints. They are usually made of stainless steel and require occasional inspection and maintenance inside the tank. Maintenance can be costly and may require the reactor to be taken out of service.

In one method, a decanter on a pivoted arm with a float may be operated using an actuated valve at the water outlet (normally outside the tank). In another method, a decanter on a pivoted arm using a variable buoyancy float may be operated using an air compressor and air-line to change the buoyancy of the float. In another method, a fixed submerged outlet launder may be operated using an actuated valve on the outlet (normally outside the tank). Alternatively, a pump may be suspended in the tank. All of the above removal systems may store dirty water from the stages of the activated sludge process performed within the SBR tank. Dirty water can easily contaminate the pipes of the systems or collect within the pump and may require a "first squirt" arrangement to return this dirty water to upstream of the reactor before clean water can be discharged. In addition, all of the removal systems described above have moving parts and require pumps or actuators as well as software control and monitoring.

A significant part of the perceived extra complexity of SBRs is the control of flow batches and, especially for larger works, the complexity of decanting mechanisms as exemplified above.

The cost of these decanting systems as well as their maintenance and upkeep is also a significant proportion of the cost of a SBR.

In view of the above-mentioned problems, there is a need to provide a simple and cheap way of removing clean water from a SBR tank that requires little or no maintenance.

According to an aspect of the present invention as seen from a first aspect, there is provided a siphon decanter for a sequencing batch reactor (SBR), the siphon decanter comprising: an inlet, an outlet and a connector pipe, wherein the inlet, outlet and connector pipe form a substantially inverted U-shaped siphon; an outlet chamber arranged in fluid communication with the outlet; the siphon decanter further comprising a drainage aperture and an overflow weir.

The substantially inverted "U" shape formed by the inlet, outlet and connector pipe is arranged to create siphon. From hereon, we will refer to the inverted "U" shape as "the siphon".

The siphon decanter of the present invention provides a method of decanting clean treated water from the top of a sequencing batch reactor (SBR). The siphon decanter has no moving parts, works automatically as a result of its hydraulic design and requires no actuators, valves, motors, pumps or software for its operation. In contrast to existing decanters, the siphon decanter of the present invention requires no maintenance inside the sequencing batch reactor.

In use, it is envisaged that the siphon decanter will be fitted to a sequencing batch reactor comprising a tank. The inlet will be positioned inside the sequencing batch reactor at the top of the tank and the outlet and outlet chamber will preferably be outside the tank with the connector pipe extending through a wall of the tank. Alternatively, the outlet and/or outlet chamber may be positioned inside the tank, possibly adjacent to the inlet. The outlet chamber is in fluid communication with the outlet and is arranged to receive water exiting the outlet downstream of the inlet and connector pipe. The SBR may be filled with wastewater by an inflow feed pump, or alternatively, by gravitational fill from an upstream water tank preferably arranged at a greater height than the water in the SBR. Where the gravitational fill method is used, an actuated valve or similar may be used to control flow rate in the SBR. When the siphon is primed, water will pass out of the SBR via the inlet, through the connector pipe and out the outlet where it will be received by the outlet chamber and then collected for discharge outside the SBR.

Preferably, the overflow weir is located within the outlet chamber adjacent to the outlet and separates the outlet chamber into first and second compartments. The overflow weir controls the maximum water level in the reactor. Preferably, the overflow weir is at a suitable height to allow the drainage aperture to discharge less than the minimum flow rate that fills the SBR.

Preferably, the drainage aperture is an opening located adjacent to the base of the outlet chamber. The drainage aperture allows clean water to exit the first compartment of the outlet chamber. Preferably, the drainage aperture is appropriately dimensioned according to the size of the SBR tank and siphon decanter. Preferably, the drainage aperture is sized so that at the minimum inflow rate to the reactor, water over the drainage aperture will build up to at least siphon prime level. If the drainage aperture is too big, it will be difficult to prime the siphon. If the drainage aperture is too small, the time taken for the siphon to break after SBR filling stops will be too great.

In use, water flowing out of the outlet is received by the first compartment of the outlet chamber. The second compartment receives water exiting the first compartment from the drainage aperture or flowing over the overflow weir if the water level is high enough. Preferably, the outlet chamber further comprises an outlet pipe arranged to allow water to exit from the second compartment and to be discharged and collected for use.

Preferably, the inlet has a bell shape. Preferably, the inlet has an opening with a large sectional area to control the velocity of inlet water into the connector pipe. The large sectional area also minimises the risk of the sludge blanket that forms in the settlement phase of an activated sludge process from being lifted during the decanting process. Preferably, the inlet and outlet have walls that are substantially vertical. Preferably, the angle of the walls are substantially over 55° and more preferably over 60° relative to the horizontal. It has been found that this orientation will prevent solids from settling on the walls of the inlet and will instead slide back into the SBR tank. Preferably, an inner wall of the inlet comprises a sloped portion adjacent to the connector pipe. The sloped portion ensures that splashes of dirty water occurring inside of the SBR tank during the aeration stage are returned to the tank and do not contaminate the connector pipe.

In an embodiment of the present invention, the siphon decanter further comprises a breather pipe arranged to set a siphon break level. Preferably the breather pipe comprises a flexible hollow tube. The breather pipe may be located on a top portion of the outlet, inlet or the connector pipe. The height of the bottom of the breather pipe determines the level at which air can get into the siphon from the outlet chamber. In use, it is envisaged that the breather pipe will be clamped in different positions relating to different siphon break levels. Alternatively, the siphon break level is set by the level that air can get into the connector pipe from the inlet or outlet.

Alternatively, the siphon break level may be set by raising or lowering the outlet. This will determine when the water level in the reactor reaches the bottom of the outlet and the level at which air can get in through the outlet. In an embodiment of the present invention, the outlet further comprises an air valve arranged to release air from the outlet. Where conditions are not onerous, the air valve is preferably made of a simple soft tube of rubber on a small diameter spigot. In use, the air valve is arranged to prime the siphon by letting air out of the siphon but not letting air back in. Air leaving the siphon is able to exit the outlet via the tube but the tube will collapse if there is a partial vacuum stopping air getting back in. This drives air out to prime the siphon. It is envisaged that an air valve will be used in scenarios where the inflow rate to the reactor in not high enough to prime the siphon.

In an embodiment of the present invention, the siphon decanter further comprises a drainage aperture insert arranged to be positioned over the drainage aperture to change the effective size of the drainage aperture. The drainage aperture insert is preferably fixed in place on the drainage aperture. The ability to change the size of the drainage aperture has several advantages, for example, a uniform sized outlet chamber with a uniform drainage aperture can be manufactured and installed on a wide range of SBR sizes and a drainage aperture insert used to change the effective size of the drainage aperture to the appropriate size. Drainage aperture inserts also allow the performance of the siphon decanter to be fine-tuned during plant commissioning by trying different inserts and choosing the most appropriate effective size for the drainage aperture. In addition, the drainage aperture insert can readily be changed if the treatment process in the SBR changes and requires an alternative size drainage aperture or if the siphon decanter is moved to another reactor or another site. Preferably, the siphon decanter is made of polypropylene and the components may be joined by plastic welds. Advantageously, the siphon decanter is light weight and is relatively cheap to manufacture. Alternatively, the siphon decanter may be made of another suitable material.

It has been found that for SBR tanks with a diameter up to 4m and water depth up to 6m, a single siphon decanter is sufficient. For larger SBR tanks, there is a risk that sludge will be drawn up from the sludge blanket if one inlet is used. The arrangement of inlets on SBR is particularly advantageous for SBR tanks with large diameters and improves flow characteristics through the siphon decanter. Alternatively, two or more siphon decanters with single inlets may be used on the SBR tank, preferably with their inlets arranged at equally spaced locations around the tank.

According to an aspect of the present invention as seen from a second aspect, there is provided a sequencing batch reactor (SBR) comprising a tank and a siphon decanter as hereinbefore described, wherein the siphon decanter is arranged to decant water from the top of said tank.

Preferably, the outlet and outlet chamber are arranged outside the sequencing batch reactor tank. Preferably, the connector pipe is arranged through a wall of the sequencing batch reactor tank. Alternatively, the outlet and outlet chamber may be arranged inside the tank adjacent to the inlet and clean water is discharged to the outside of the tank.

In use, it is envisaged that the sequencing batch reactor (SBR) will be filled with wastewater and will be used to perform an activated sludge process to treat the wastewater. After the settlement stage, a layer of clean water will be present at the top of the tank and a layer of sludge below. An inflow pump will then start (or valve opened if using gravitational fill) and the tank will then be filled with fresh wastewater from the bottom of the tank. When the water level in the tank reaches the invert of the siphon i.e. the bottom of the connector pipe, water will flow from the inlet through the connector pipe and will exit the outlet into the outlet chamber. When the water level in the tank reaches siphon prime level, all the air will be driven out of the siphon, priming the siphon. Water will continue to flow from the tank through the connector pipe and out through the outlet into the outlet chamber while the siphon is primed.

Water will exit the outlet chamber via the drainage aperture that is preferably located at the bottom of the outlet chamber. The overflow weir is preferably located within the outlet chamber and preferably divides the outlet chamber into first and second compartment, wherein the first compartment receives water from the outlet. Preferably, the drainage aperture is located at the bottom of the outlet weir between the first and second compartments. The drainage aperture is appropriately sized so that the rate of clean water exiting the first compartment of the outlet chamber to the second compartment is lower than the rate of water entering the SBR tank. The water level in the outlet chamber builds up until it reaches the top of the first compartment the overflow weir and will then overflow into the second compartment. The maximum water level in the SBR tank is determined by the height of the overflow weir.

During filling, as the level in the SBR tank increases above siphon prime level and water flows freely into the outlet chamber. Initially, flow out of the outlet chamber is controlled by the drainage aperture. Once the water level builds up to the height of the overflow weir, flow continues through the drainage aperture and now over the overflow weir as well. The siphon is now primed.

When filling stops, the water level in the SBR tank starts to drop. As the water level falls below siphon prime level, the siphon continues to operate under partial vacuum until siphon break level, when the siphon substantially empties of water. The outlet chamber empties of water through the drainage aperture. Siphon break level is sufficiently below siphon prime level to leave an air gap between the water level in the SBR tank and the invert of the connector pipe

Preferably, at siphon break level, the inlet is still marginally below water level to avoid scum exiting through the decanter. Preferably, the siphon break level is sufficiently below the invert of the connector pipe so that expansion during aeration does not send dirty water out of the decanter. The siphon decanter uses the siphon principle in a hydraulic design that allows clean water out (primed by inflow of wastewater into the SBR tank) and which continues to operate until the siphon is broken. The siphon is preferably set to be broken at a level low enough so that, during the fully mixed aeration stage, expansion of the water volume and splashing in the SBR will not send dirty water out through the siphon decanter. Advantageously, the siphon decanter of the present invention operates by relying on a partial vacuum to keep the connector pipe full of water. The partial vacuum is initiated by priming the siphon and means that clean water is continually drawn from the top of the SBR tank into the connector pipe which remains full of water while the siphon is primed. Advantageously, the siphon is primed as a result of changes in the level of water in the SBR that occur as a consequence of the normal operation of the activated sludge process, negating the need for a motor, pump or to manually prime the siphon. According to an aspect of the present invention as seen from a third aspect, there is provided a method of decanting water from the top of a sequencing batch reactor (SBR), the method comprising: providing a siphon decanter of the present invention and a sequencing batch reactor comprising a tank, wherein the inlet is arranged inside the tank; filling the sequencing batch reactor tank so that the water level is above a siphon prime level; decanting clean water that exits from the outlet chamber for collection.

Preferably, the decanting step overlaps with part or all of the filling step. Preferably the filling step is performed as part of the activated sludge process for the treatment of wastewater in the sequencing batch reactor. Preferably, the clean water is generated from the activated sludge process.

Preferably, the filling step comprises filling the sequencing batch reactor tank with wastewater ready to be treated in a further cycle of the activated sludge process. Since the siphon decanter of the present invention is primed and operates as a result of changes in the level of water in the SBR that occur through the activated sludge process, there is little or no monitoring or maintenance required. Once the sequence of filling the SBR tank has been optimised, water is simply collected from the outlet chamber as and when the process completes each cycle. To use the siphon decanter for the activated sludge process for water treatment in a sequencing batch reactor (SBR), there is an overlap in the decant and fill stages of the SBR cycle as inflow of wastewater that fills the SBR tank is used to prime the siphon. So instead of clearly separating the decant and fill stages, the filling stage is used to push clean water up the tank (piston effect). Overlapping the decant and fill stages advantageously reduces overall process time and increases capacity. Because the tank is always "full", there is more scope for a gravity discharge than for a reactor with a varying level. The inflow pump arranged at the bottom of the SBR tank for filling is preferably designed so that the fill flow is spread across the tank. This prevents vertical mixing in the tank and reduces the chance of the sludge rising into the layer of clean water. By reducing the cost of the decanter on SBR tanks, it is envisaged that it will become advantageous to use SBR tanks rather than continuous flow for providing the activated sludge process for water treatment.

Embodiments of the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 illustrates a simplified version of the activated sludge process for water treatment in a continuous flow process that forms part of the prior art;

Figures 2a-d illustrate of the stages of the activated sludge process for water treatment in a sequencing batch reactor (SBR) that forms part of the prior art; Figure 3 illustrates a siphon decanter for a sequencing batch reactor (SBR) according to an embodiment of the present invention;

Figures4 a-j illustrate the siphon decanter shown in Figure 3 in use on a sequencing batch reactor (SBR) performing an activated sludge process;

Figures5 a-c illustrate the main stages of the activated sludge process when performed in a sequencing batch reactor (SBR) with the siphon decanter, as shown in Figure 3;

Figures 6a-f illustrate features of alternative embodiments of the siphon decanter in Figure 3;

Figures 7a-c illustrate a sequencing batch reactor with a siphon decanter according to an alternative embodiment of the present invention wherein the inlet of the siphon decanter comprise multiple inlets; Figures 8 a-f illustrate the stages of a complex water treatment process when performed in a sequencing batch reactor (SBR) with the siphon decanter, as shown in Figure 3.

Referring to Figure 1 there is shown the stages of an activated sludge process 100 in simplified form. Dirty wastewater (sewage) 102 is first supplied to a reactor tank 104 containing biological mass (biomass), otherwise known as 'sludge' or 'activated sludge' 106. The sludge 106 is a biological soup of bacteria and higher micro-organisms such as saprotrophic bacteria, protozoan flora, petritrichs, rotifiers and/or filter feeders which act to reduce organic content in the wastewater. This biomass eats the pollutants in the wastewater to thrive and multiply producing more sludge 106 and cleaning the water. All of the micro-organisms in the sludge 106 exist naturally in the environment. In the reactor tank 104, wastewater 102 and sludge 106 is mixed by mechanical means and the combination of wastewater 102 and biological mass 106 in the reactor tank 104 is commonly known as mixed liquor 108. The mixed liquor 108 is also aerated in the reactor tank 104, for example, by using air diffusers to bubble air into the mixed liquor 108. The addition of oxygen to the mixed liquor 108 encourages the multiplication of aerobic bacteria in the biological mass, facilitating the conversion of nitrogen from its reduced form (ammonia) to oxidized nitrite and nitrate forms, a process known as nitrification. In some treatment processes, aluminium sulphate or ferric sulphate may also be added during the aeration stage, both of which readily react with solubilised phosphorus to form non-soluble precipitates, thus facilitating the removal of phosphorus from the water.

The mixed liquor 108 is then passed to a further settlement tank (FST) 110 where the mixed liquor 108 is allowed to separate into clean water 112 and sludge 106. The clean water 1 12 leaves the system from the top of the FST 110 and sludge 106 is drawn off the bottom of the FST 110. The majority of sludge 106 is returned to the reactor tank 104 to maintain the desired concentration of sludge 106 ('returned activated sludge') and a small proportion of sludge 106 is disposed of ('surplus activated sludge'). The continuous return of sludge 106 builds up a culture of organisms that are best suited to the treatment conditions i.e. those that thrive best on the mixture of pollutants in the wastewater 102. The activated sludge treatment process 100 can be tuned by changing the conditions in the reactor to produce different treated water qualities.

It will be appreciated that the more onerous the requirements, the more complex the treatment plant and process will be and may require more tanks or compartments with different operating conditions. The process may require the addition of chemicals, addition of trace nutrients, pH correction or additional chemical energy for example, for the chemical precipitation of specific pollutants. Further operating parameters include tanks that are aerated or un-aerated, and mixed or quiescent.

For each additional stage in the activated sludge wastewater treatment process 100, an additional tank will be required. Additional stages (not shown) would commonly be performed after the main aeration stage of the treatment cycle and before it reaches the final settlement tank 110. Figures 2a-d show the main stages of an activated sludge process 200 when performed in a sequencing batch reactor (SBR) tank 202. Each stage of the process is performed in the same tank, the SBR tank 202. The process has four main stages: a) Fill; b) Aerate; c) Settle; and d) Decant. Arrows on the illustrations point to water levels in the SBR tank 202 and direction of water flow.

Figure 2a illustrates the fill stage. The SBR tank 202 containing biological mass (sludge) is filled with wastewater 102 which enters from an inflow pipe 203 with use of an inflow pump at the bottom of the SBR tank 202 to create the mixed liquor 204. Mixing may be provided by mechanical means (not shown). During the fill stage, the water level in the SBR tank 202 rises from "low" to "full". The fill is typically by about 40% of the total volume of the SBR tank 202.

Figure 2b illustrates the aeration stage. The mixed liquor 204 in the SBR tank 202 is aerated by the use of fixed or floating mechanical pumps or by transferring air through bubble diffusers. During aeration, the volume of the mixed liquor 204 in the SBR tank 202 typically increases by about 0.5%. Splashes at the surface mixed liquor 204 are also common. Figure 2c illustrates the settlement stage. During settlement, a blanket of sludge 206 forms at the bottom of the SBR tank 202 and clean water 208 remains at the top of the SBR tank 202. The depth of the clean water 208 increases with time throughout the settlement stage until the blanket of sludge 206 has completely settled and separated from the clean water 208. The sludge is typically allowed to settle until the clean water 208 on top makes up about 60% by volume of the contents of the SBR tank 202.

Figure 2d illustrates the decant stage. Clean water 208 is decanted from the top of the SBR tank 202 using a decanter (not shown) and collected from an outlet pipe 210. The water level in the SBR tank 202 falls from "full" to "low" as clean water 208 is decanted.

It is common for the decanter to contain solids and contamination from the aeration stage (Figure 2b), so a 'first squirt' may be required which returns the first decant to the SBR tank 202 or inflow pipe 203 before clean water 208 is collected from the outlet pipe 210. The decanting stage commonly involves slowly lowering a scoop or 'bowl' into the SBR tank 202 to remove water from the tank and to the outlet pipe 210. Decanting systems commonly comprise two motorised drives and moving parts. As the bacteria multiply and die, the sludge 206 within the tank increases over time and surplus activated sludge (SAS) is removed via a sludge outlet 214 for further treatment. The quantity of sludge 206 within the SBR tank 202 is closely monitored. After the decant stage, the activated sludge process 200 starts again with a fresh fill stage (Figure 2a). The sludge 206 that has multiplied in the SBR tank 202 can be reused for a subsequent cycle of the activated sludge process 200.

It will be appreciated that for a simple wastewater treatment plant, a single SBR tank can be used for the whole process 200 as described above. For larger treatment plants, the rate of wastewater inflow often cannot be matched to amount of wastewater in treated in each batch. It is therefore common to provide either a balance tank or to provide a number of SBRs in parallel, or both. More complex wastewater treatment processes can also be achieved in a SBR by a more complex series of operations. In contrast to the continuous flow process, in the SBR tank process these conditions can be provided in the same reactor tank (SBR tank) simply by changing the sequence of operations.

In the SBR, different treatment conditions are separated by time. In the continuous flow process, treatment conditions are separated into tanks or compartments. This means that, in the SBR treatment processes can be changed by altering the computer control program whereas for the continuous process, it is necessary to change the size, number of shapes or tanks.

Referring to Figure 3, there is shown a siphon decanter 10 fitted to a sequencing batch reactor (SBR) 302 according to an embodiment of the present invention. The siphon decanter 10 has no moving parts or motors. The siphon decanter 10 comprises a large inlet 12 that is positioned inside the SBR tank 302 and an outlet 14 that is positioned outside the reactor, downstream of the inlet 12. The inlet 12 has a relatively large cross sectional area to keep the inlet velocity low to avoid drawing up sludge that has settled to the bottom of the SBR tank 302. A connector pipe 18 extends through a wall of the SBR tank 302 and connects the inlet 12 to the outlet 14 to form a substantially inverted hollow "U" shaped siphon (from hereon, we will refer to the inverted "U" shape as "the siphon"). The shape of the inlet 12, connector pipe 18 and outlet 14 allow the siphon decanter 10 to operate as a siphon under partial vacuum.

The siphon decanter 10 further comprises an outlet chamber 20 outside the SBR tank 302 that is in fluid communication with the outlet 14. The connector pipe 18 extends through a wall of the SBR tank 302 and the outlet 14 sits inside the outlet chamber 20.

Three significant water levels WL1 , WL2 and WL3 are also illustrated in Figure 3. Water level 1 (WL1) indicates the maximum water level in the SBR tank 302 and outlet chamber 20 of the siphon decanter 10. When water inflow into the SBR tank 302 is at its maximum, the WL1 level will be maintained by the siphon decanter 10.

Water level 2 (WL2) indicates the siphon prime level. This is the water level required to drive all the air out of the siphon of the siphon decanter 10.

Water level 3 (WL3) indicates the siphon break level. This is the water level at which air can get back into the inverted "U" shaped siphon of the siphon decanter 10. At the siphon break level, flow of water through the siphon is interrupted.

It will be appreciated that the water levels WL1 , WL2 and WL3 may differ depending on the embodiment and configuration of the siphon decanter 10 that is employed on the SBR tank 302.

The outlet chamber 20 comprises a drainage aperture 22 at the base of the outlet chamber 20 and an outlet pipe 24. The outlet chamber 20 also comprises an overflow weir 26 situated inside the outlet chamber 20 that separates the outlet chamber 20 into two compartments 28,30. The first compartment 28 is arranged to receive water from the outlet 14 and the second

compartment 30 is arranged to receive water from the first compartment 28 via the drainage aperture 22 or by water flowing over the overflow weir 26. The outlet pipe 24 is arranged to allow water to exit from the outlet chamber 20.

The height and position of the overflow weir 26 controls the maximum water level (WL1) in the SBR tank 302 and outlet chamber 20. The drainage aperture 22 is suitably sized such that at the minimum inflow rate to the SBR tank 302, water will build up in the outlet chamber 20 to a level sufficient to prime the siphon (WL2).

The outlet 14 comprises an adjustable breather pipe 32 at the top of the outlet 14. The siphon break level WL3 may be set by the breather pipe 32. In an alternative embodiment where adjustment is not needed, the outlet 14 may be set at a higher or lower level so that air can get in through the outlet 14 when the level in the reactor reaches the bottom of the outlet 14, such that the siphon break level WL3 is higher or lower respectively.

The outlet 14 further comprises an air valve 34 adjacent to the breather pipe 32 at the top of the outlet 14. The air valve 34 is arranged to let air out of the inverted "U" (the siphon), but not let air back in. The siphon can be primed by driving air out of the siphon through the air valve 34. In some embodiments where the rate of water flow into the SBR tank is high enough, the rate of water flow may be sufficient to drive air out of the inverted "U" (the siphon) to prime the siphon and an air valve 34 may not be required. The inlet 12 further comprises a sloped portion 36 between the inlet 12 and the connector pipe 18. The sloped portion 36 means that, if there is splashing in the SBR tank 302 during the aeration stage, any splashes of dirty water or sludge will fall back onto the sloped portion 36 and back into the SBR tank 302 rather than contaminating the connector pipe 18. It is envisaged that in alternative embodiments, the breather pipe 32, the air valve 34 and the sloped portion 36 may not be required. The necessity of these features depends on the overall hydraulic design of the siphon decanter 10 and the fill/decant process stages.

In this particular embodiment, the connector pipe 18 extends through the wall of the SBR tank 302 and the outlet 14 and outlet chamber 20 are positioned outside the SBR tank 302. In alternative embodiments, some or all of these parts may be positioned inside or above the SBR tank 302. In alternative embodiments, the breather pipe 32 may be positioned on the inlet 12 of the siphon decanter 10.

In use, after the settlement stage of an activated sludge process, when the top of the reactor 302 is full of clean water, an inflow pump (not shown) at the bottom of the SBR tank 302 starts filling the tank with new wastewater. Alternatively, the SBR tank 302 is filled by gravitational flow of water from a water tank upstream of the SBR 302 and inflow of water is controlled with an actuated valve or similar.

When the water level in the SBR tank 302 reaches the invert of the siphon (i.e. the connector pipe 18), clean water flows through the connector pipe 18 and begins to fill the outlet chamber 20. Water subsequently starts to drain out through the drainage aperture 22. The drainage aperture 22 is appropriately sized so that the water level has to build up in the first compartment 28 of the outlet chamber 20 until it reaches the top of the overflow weir 26. The water level in the SBR tank 302 and outlet chamber 20 continues to rise while the SBR tank 302 is being filled and the overflow weir 26 ensures a constant maximum water level (WL1). The hydraulic arrangement of the outlet chamber 20 facilitates siphon priming without the need to prime the siphon manually or by an alternative means. The siphon is primed when the water level in the SBR tank 302 reaches WL2 and all air is driven out of the siphon. As the outlet chamber 20 fills to weir level (the height of the overflow weir 26), air is pushed out of the siphon, priming the siphon. Water will continue to flow from the SBR tank 302 through the inlet 12, connector pipe 18 and out through the outlet 14 into the outlet chamber 20 while the siphon is primed. Water will continue to flow over the overflow weir 26 until the inflow of wastewater stops. When SBR filling stops, the water level in the SBR tank 302 drains down to siphon break level (WL3) and the siphon is broken leaving an air gap between the invert of the connector pipe 18 and the water in the SBR tank 302.

The siphon break level is either set by the breather pipe 32 or by the position of the outlet 14. Either can determine the level at which air can get into the connector pipe 18 from the outlet chamber 20.

At siphon break level (WL3), the bottom of the inlet 12 will be around 100 mm below water to avoid scum passing into the siphon decanter 10. The maximum expansion during the aeration stage in a 5 m deep SBR tank is typically 30 to 35mm. It is envisaged that the siphon break level (WL3) will be at around 75 mm below the invert of the siphon connector pipe 18 so that expansion during aeration would not cause water to pass into the siphon decanter 10.

Figures 4a-j show the functional stages of the siphon decanter 410 when being used to remove clean water from a sequencing batch reactor (SBR) tank 402 as part of an activated sludge process. The siphon decanter 410 differs from siphon decanter 10 in that it does not comprise breather pipe 32 or an air valve 34. The three significant water levels WL1 , WL2 and WL3 (as previously defined) are indicated in the illustrations and arrows point to the water level in the SBR tank 402 and siphon decanter 410 at various stages in the process.

Before the activated sludge process begins, the SBR tank 402 containing biological mass (sludge) is filled with wastewater. During aeration (Figure 4a), the water level in the SBR tank 402 rises above the siphon break level WL3 as the volume of the entrained air bubbles increases the tank volume, typically by about 0.5%. In a 6 m deep reactor, this would increase the water level by about 30 mm. Surface splashes of water are common during aeration. It will be appreciated that there must be enough distance (freeboard) between the maximum water level during aeration and the invert of the siphon i.e. the connector pipe 418 so that water cannot splash up and pass through the siphon decanter 410. In this particular embodiment, the invert of the siphon of the siphon decanter 410 is 75 to 100 mm above the maximum water level during aeration. The sloped portion 436 returns any slashes inside the inlet 412 to the reactor tank 402.

After aeration, there is a settlement phase (Figure 4b) that typically lasts about 1 hour. The water level in the SBR tank 402 drops back to the siphon break level WL3. During the settlement phase, the sludge settles to form a blanket of sludge at the bottom of the SBR tank 402 and the depth of clean water above the sludge blanket gradually increases. After settlement, the fill phase starts (Figure 4c-f). The SBR tank 402 is filled with new wastewater from the base of the tank 402 and the water level increases. The rate of filling must not be enough to break up the sludge blanket or force incoming wastewater through the sludge blanket. This is avoided by spreading the water filling across the cross section of the bottom of the tank 402.

When the water level reaches the invert of the connector pipe 418 in the wall of the SBR tank 402, clean water starts to flow through the siphon decanter 410 (Figure 4c). Water flows through the connector pipe 418 and into the outlet chamber 420 outside the reactor 402 and then starts to pass through the drainage aperture 422 and away out the outlet pipe 424 (Figure 4d). The drainage aperture 422 is sized so that it passes water at a lower rate relative to the rate of filling the SBR tank 402.

In use, the drainage aperture 422 may need to be occasionally cleared due to a build-up of slime and debris. This can be achieved by simply inserting a brush or cleaner into the drainage aperture 422 from outside of the tank 402. A blockage or partial blockage of the drainage aperture 422 can be determined by monitoring the drainage rate or time compared to recorded levels. Due to the lack of moving parts on the decanter 410, there is no requirement to perform maintenance inside the SBR tank 402, thus reducing cost and time.

Air is driven out of the siphon by the flow of water through the siphon to fully prime the siphon (Figure 4e). Filling continues for up to 2 hours (Figure 4f). As flow into the reactor 402 continues, the water level in the outlet chamber 420 increases to above siphon prime level WL1 and, as it rises further, water flows over the overflow weir 426. Only if and when the water level in the outlet chamber 420 exceeds the height of the overflow weir 426 (i.e. siphon prime level WL1) will water exit the outlet pipe 424 at the same or similar rate of filling the SBR tank 402.

When filling ends (Figure 4g), the water level in the SBR tank 402 starts to drop. The outlet chamber 420 drains to below the weir level but the inverted "U" shaped siphon remains full and water flowing. The rate of clean water flowing out of the SBR tank 402 through the siphon decanter 410 is now controlled by the drainage aperture 422 (Figure 4h).

When the water level drops below bottom of the outlet (or, in embodiments comprising a breather pipe, the bottom of the breather pipe) the siphon is broken (Figure 4i). Air gets into the inverted "U" shaped siphon and water flow through the siphon stops. There is now an air gap between the water level in the SBR tank 402 and the invert of the siphon i.e. the bottom of the connector pipe 418 thus preventing flow leaving the reactor 402. The step shown in Figure 4j is the same as the step shown in Figure 4a demonstrating how the activated sludge process in a SBR tank 402 using the siphon decanter 410 is continuous and can be cycled over and over again with no down-time. The process shown by Figures 4a-j is activated by filling the SBR tank 402 with water. The process is further regulated by the rate of water inflow into the SBR tank 402, the length of the fill stage (Figure 4c-f) and the hydraulic design of the siphon decanter 410.

No additional input or programming is required to activate the decanting process which occurs between siphon priming (Figure 4e) and siphon breaking (Figure 4i).

The siphon decanter 410 does not move and has no moving parts or motors. Little or no maintenance is required to operate the siphon decanter 410. The siphon decanter 10 will continue to remove clean water from the top of the SBR tank 402 with every cycle of the activated sludge process (Figure 4 a-j) performed in the SBR tank 402.

Figures 5a-c show the operating stages of a sequencing batch reactor (SBR) tank 502 comprising a siphon decanter 10 performing an activated sludge process water treatment. The process is similar to the process shown in Figures 4a-j but has been simplified into three stages: 1. Fill/decant (Figure 5a); 2. Aerate (Figure 5b); 3. Settle (Figure 5c). The three significant water levels WL1 , WL2 and WL3 (as previously defined) are indicated in the illustrations and arrows point to the water level in the SBR tank 502 and siphon decanter 10 at various stages in the process. In Figure 5a, the fill and decant stages (equivalent to the stages shown in Figures 4 c-i) have been simplified into one stage. Fill/decant starts after the settlement stage when clean water 508 sits at the top of the SBR tank 502 and the sludge blanket 506 is settled at the bottom of the SBR tank 502. Inflow of wastewater 102 from the inflow pipe 503 into the SBR tank 502 fills the tank and primes the siphon on the siphon decanter 10. When filling stops, the water level in the SBR tank 502 slowly falls to siphon break level WL3.

In Figure 5b, aeration of the mixed liquor 504 (comprising mixed newly filled wastewater 102 and sludge 506) in the SBR tank 502 commences. During aeration, the volume in the SBR tank 502 increases by 0.5 % and there are surface splashes so the siphon break level WL3 must be sufficiently below the invert of the siphon i.e. the bottom of the connector pipe 18/418. Figure 5c illustrates the settlement phase. The water level in the SBR tank 502 falls back to siphon break level WL3 as a new sludge blanket 506 forms with clean water 508 on top. The fill/decant phase (Figure 5a) recommences and the siphon decanter 10 removes the new layer of clean water 508 from the top of the SBR tank.

The siphon decanter 10 differs from all the existing decanting systems because it has no moving parts. It uses water flow and an increase in water level in the SBR tank 502 to prime the siphon. As the filling of the SBR tank 502 primes the siphon decanter 10 and clean water is removed from the SBR tank 502, the fill and decant stages of the cycle are overlapped i.e. the SBR 502 is always full.

It will be appreciated that overlapping two process steps saves time which increases the capacity of the SBR 502. The batch volume of the SBR 502 can also be increased which increases capacity. However, the process must be controlled and care must be taken not to decant sludge and solids out through the siphon decanter 10 if the fill and decant stages occur at the same time.

Advantageously, the sludge blanket continues to consolidate during filling and will trap solids entering the reactor in the inflow of wastewater. Care in design at the bottom of the reactor, for example, to spread the inflow of wastewater across the SBR tank 502 base area during filling, will help reduce the risk of flushing out solids and help trap inflow solids in the blanket.

Referring to Figures 6a and 6b, there are shown alternative arrangements of the WL2 (siphon prime) and WL3 (siphon break) fill levels on a sequencing batch reactor (SBR) 602 with a siphon decanter 610 designed for alternative filling techniques. Figure 6a illustrates the setup of a SBR 602 with a siphon decanter 610a where the first fill of the SBR tank 602 is very fast. The fast fill means that the first part of the fill from WL3 to WL2 must have no outflow through the siphon decanter 610a. Then there is a period when the flow out through the decanter builds up until the siphon is fully primed. This means that there is little risk in using a first fast fill. Note also that fast inflow until the siphon is fully primed will help to prime the siphon. If a significant part of the fill to the SBR tank 602 can be put in quickly at the start of the fill, the fill can be slowed towards the end to avoid lifting of the sludge blanket until the end of the fill.

In Figure 6b illustrates the setup of a SBR 602 with a siphon decanter 610b where the gap between the siphon prime WL2 and siphon break level WL3 has been increased. One way that this can be achieved is by increasing the length of the inlet 612b that extends into the SBR tank 602. Increasing the gap between the siphon prime WL2 and siphon break level WL3 means that there is scope to have a further fill stage later in the treatment process. This may be necessary in certain situations, for example, to achieve a very low level of total nitrogen in the treated water, the SBR tank 602 can be filled during an additional non-aerated period in the treatment process. The extra chemical energy in the incoming wastewater fill will help de-nitrification i.e. to reduce nitrates in the water to nitrogen which is consequently released from the SBR as a gas. After the siphon has been primed, the "fill" stage may be stopped early and the siphon allowed to break. Filling may then be resumed with mixing, but not aeration. This sequence provides anaerobic conditions in the SBR tank 602 which is necessary for treatment stages such as phosphorus release (which is part of the process for phosphorous removal).

If the inlet 612b and outlet 614b are extended to give scope for further fill stages in the cycle, it will be more difficult to prime the siphon using flow only and an air valve may be required. It will be appreciated that the inlet 612b and outlet 614b cannot be extended indefinitely as the inlet will get too close to the sludge blanket.

Referring to Figure 6c, there is shown part of a siphon decanter 610c with an air valve 634c. The air valve 634c drives air out of the siphon but does not let air back into the siphon. Where conditions are not onerous, the air valve 634c may be made of a simple soft tube of rubber on a small diameter spigot. It will be appreciated that the air valve will be designed depending on the conditions in hand. Air leaving the siphon will blow out but the tube will collapse if there is a partial vacuum stopping air getting back in. It is envisaged that an air valve 634b will be used in scenarios where the inflow rate to the reactor in not high enough to prime the siphon. Referring to Figure 6d, there is shown part of a siphon decanter 61 Od with flexible breather pipe 632d. The breather pipe 632d is situated at the top corner of the outlet and can be moved to adjust the siphon break level. In use, it is envisaged that the breather pipe will be clamped in different positions relating to different siphon break levels.

Referring to Figure 6e, there is shown a sectional view of a drainage aperture insert 40. The drainage aperture insert 40 sits over of the drainage aperture 22 of the siphon decanter 10 to change the size of the effective drainage aperture and hence the rate of water that exits the outlet chamber 20 via the outlet pipe 24.

Referring to Figure 6f, there are shown in cross-section three alternative embodiments (A, B and C) of a siphon decanter with various inverted "U" shaped siphons. The inlet and outlets are substantially bell shaped. The shape of the siphon can be changed to help exhaust air flow from the siphon and/or to improve the structural stiffness of the siphon decanter. The lower the volume of air inside the siphon that must be exhausted to prime the siphon, the easier the siphon will be to prime.

In the first embodiment (A) shown in Figure 6f, the top of the inlet 12 and outlet 14 have been sloped. This can be easily be achieved for small siphon decanters fabricated from sheets of polypropylene or similar. A stainless steel frame may be used to hold the siphon decanter in place and to provide structural rigidity. Alternatively, it is envisaged that stiffening elements such as vertical fins may be added to the siphon decanter to eliminate the need for a stainless steel frame.

In the second embodiment (B), the transition curves of the inverted "U" shaped siphon are smooth. This can be readily achieved if the siphon decanter is fabricated from glass-fiber reinforced plastic (GRP) using a mould. It is envisaged that this shape may be used for a production run of larger siphon decanters.

In the third embodiment (C), the transition curves of the inverted "U" shaped siphon are also smooth, however, the curved shape is approximated using a number of parts cut and bent from a flat sheet and welded together.

With a curved shape, it will be appreciated that some of the surfaces immersed during aeration are not vertical. It has been found that as long as any surface submerged during aeration is sloped at least 60° to the horizontal, solids should slide off the siphon and back into the SBR tank during the settlement stage of the cycle. Referring to Figure 7a, there is shown a large diameter sequencing batch reactor (SBR) 702 with a siphon decanter 710 comprising twelve inlets 712. Section B-B of the SBR 702 is illustrated in Figure 7b and section C-C is illustrated in Figure 7c. The SBR 702 has a diameter greater than 4m and a water depth greater than 6m. The inlets 712 are located inside the SBR tank 702 and are equally spaced from each other and from the tank wall. The inlets 712 are bell-shaped and all share a common connector pipe 718, outlet 714 and outlet chamber 720. The siphon decanter 710 further comprises a ring-shaped launder pipe 750 arranged to link the inlets 712. The launder pipe 750 comprises twelve apertures 752 arranged to receive water from each of the twelve inlets 712 and convey water to the common connector pipe 718 (see Figure 7c). In use, during the decanting stage, water passes from the SBR tank 702 to the inlets 712, will enter the launder pipe 750 through aperture 752 and will travel to the common connector pipe 718 where it is conveyed to the outside of the SBR tank and collects in the outlet chamber 720. For this arrangement to work effectively, the water flow through each inlet 712 should be the same or very similar. Aperture 752 is dimensioned such that the relative head loss across the aperture is large compared with the head losses in the launder pipe 750. This provides an almost identical water flow through each inlet. It is envisaged that the design of the apertures 752 may be further be refined so that the apertures 752 for inlets further from the connector pipe 718 through the SBR tank wall have a slightly larger dimension than those for inlets nearer the connector pipe 718.

It has been found that for SBR tanks with a diameter up to 4m and water depth up to 6m, a single siphon decanter is sufficient. For larger SBR tanks, there is a risk that sludge will be drawn up from the sludge blanket if one inlet is used. The arrangement of inlets 712 on SBR 702 is particularly advantageous for SBR tanks with large diameters. Alternatively, two or more siphon decanters with single inlets may be used on the SBR tank 702, preferably with their inlets arranged at equally spaced locations around the tank (not shown).

Referring to Figures 8 a-f, there is shown the stages of a complex water treatment process 800 (such as the NUTREM® process) performed in a sequencing batch reactor (SBR) 802 comprising a siphon decanter 10. The NUTREM® process 800 is based on treating sewage to a very high standard with biological nutrient removal of both nitrogen and phosphorus i.e.

chemical additions for removing phosphorus such as aluminium sulphate or ferric sulphate are not essential. The treatment process 800 has six main stages illustrated by Figures 8a-f respectively. The three significant water levels WL1 , WL2 and WL3 (as previously defined) are indicated in the illustrations and arrows point to the water level in the SBR tank 802 and siphon decanter 10 at various stages in the process 800. A fourth water level WL4 indicates the level to which the SBR tank 802 is initially filled to. Figure 8a illustrates overlapping fill/decant stages. Fill/decant starts after the settlement stage of the last process cycle. Biomass will be in the sludge blanket 806 in the bottom of the SBR tank 802 with Clear water 808 on top. Typically the sludge blanket 806 will be in the bottom 30% of the SBR tank 802.

Inflow of wastewater 102 entering the bottom of the SBR tank 802 will increase the level in the tank 802. As the level rises, flow will start to go out through the siphon decanter 10 as soon as the level reaches the invert of the connector pipe through the SBR wall but full flow will not go out through the siphon decanter 10 until the water level in the SBR 802 has reached or passes the siphon prime level (WL2) and all air has been pushed out of the siphon. When filling stops, the siphon decanter 10 will continue to discharge water until the level in the SBR tank 802 goes down to siphon break level (WL3).

For this complex treatment process 800, the siphon break level (WL3) is well below the siphon prime level (WL2) to allow space within the reactor for a supplementary fill at a later stage in the process 800. Typically, 90% of the SBR volume may be filled during the main fill (Figure 8a) to leave enough space in the reactor for the remaining 10% later in the process (Figure 8d).

Figure 8b illustrates an anaerobic stage of the treatment process 800, wherein there is no oxygen and no nitrate present. The contents of the SBR tank 802 are mixed by a mechanical mixer 809 to form a mixed liquor 804, but are not aerated. As there is no oxygen present, phosphorus accumulating organisms (PAOs) grow using oxygen stored in their bodies in compounds of phosphorus. This releases phosphorus into the water. This initial increase in phosphorus levels in the water is an essential part of the overall biological phosphorus removal process.

Figure 8c illustrates a first aerobic stage of the treatment process 800, wherein the contents of the SBR tank 802 are aerated. With abundant food and oxygen, biomass grows rapidly in this stage. Pollutants are absorbed by the biomass and ammonia is converted to nitrates.

Figure 8d illustrates an anoxic stage of the treatment process 800, wherein nitrates are available but no oxygen (aeration is stopped). When there is food and nitrate available, the biomass will continue to grow using oxygen from nitrates. Nitrogen gas is released to

atmosphere. Denitrification recovers most of the energy used to convert ammonia to nitrate and restores alkalinity. In some cases, most commonly for sewage from areas where the domestic water supply is soft, low alkalinity can inhibit biological activity. A supplementary fill 81 1 (allowed for by increasing the vertical distance from siphon prime to siphon break levels) may be put in at the start of this step, to provide additional food for optimum biomass growth and denitrification. Figure 8e illustrates a second aerobic stage of the treatment process 800, wherein the contents of the SBR tanks 802 are aerated for a second time. Biological treatment is completed in this second aerobic stage. By the end of the second aerobic stage, the phosphorus accumulating organisms (PAOs) have reabsorbed virtually all of the phosphorus in the water to achieve virtual complete phosphorus removal in the treatment process 800. It is also important to ensure that there is enough dissolved oxygen in the mixed liquor 804 to stop phosphorus release during settlement (Figure 8e) and the next fill/decant stage (Figure 8a). Surplus activated sludge (SAS) 806 can be removed at the end of this stage via an outlet 814. Phosphorus will leave the treatment process in the SAS. This surplus sludge 806 should be separately thickened and removed from site as co-thickening with primary sludge risks re- solubilising phosphorus and returning phosphorus to the treatment process in recycled sludge liquors.

Figure 8f illustrates a settlement stage in which biomass settles to the bottom of the SBR tank 802 forming a sludge blanket 806 with Clear water 808 above. The fill/decant stage (shown in Figure 8a) recommences and the siphon decanter 10 removes the new layer of clean water 808 from the top of the SBR tank 802. It will be appreciated that the above described embodiments are given by way of example only and that various alternatives and modifications thereto may be made without departing from the scope of the invention.

Embodiments of the siphon decanter may comprise one or both of the air valve or breather pipe arranged at any point on the inlet, outlet or connector pipe (the siphon). Two methods for inflow of wastewater to the SBR have been described: using an inflow pump and by gravitational fill. It will be appreciated however, that any other method of filling the SBR could also be used.

Although several shapes of features of the siphon decanter have been described, it will be appreciated that there are many other variations in shape that may be used to improve various aspects of the invention and parameters of the SBR process.

It is envisaged that two or more siphon decanters could be used for a large SBR. This could allow more standardisation in design. Two or more siphon decanters can be used as long as they are all primed early in the fill/decant stage of the SBR cycle and are preferably primed all at about the same time. This can be done as long as the fill flow at the start of the fill/decant stage is high enough to ensure all overflow weirs in the siphon decanters operate. Keeping the overflow weir lengths short (but long enough to avoid any overtopping of the reactor or the chambers out of the reactor tank) will facilitate this.

Claims

1. A siphon decanter (10) for a sequencing batch reactor, the siphon decanter comprising:

an inlet (12), an outlet (14) and a connector pipe (18), wherein the inlet (12), outlet (14) and connector pipe (18) form a substantially inverted U-shaped siphon;

an outlet chamber (20) arranged in fluid communication with the outlet (14); the siphon decanter further comprising a drainage aperture (22) and an overflow weir (26).

2. A siphon decanter according to claim 1 , wherein the drainage aperture (22) is located adjacent the base of the outlet chamber (20).

3. A siphon decanter according to claim 1 or claim 2 wherein the overflow weir (26) is

located within the outlet chamber (20) adjacent the outlet (14) and separates the outlet chamber into first and second compartments (28, 30).

4. A siphon decanter according to any preceding claim, wherein the inlet (12) and outlet (14) have walls that are substantially vertical.

5. A siphon decanter according to any preceding claim, wherein an inner wall of the inlet (12) further comprises a sloped portion (36) adjacent the connector pipe (18).

6. A siphon decanter according to any preceding claim, wherein the siphon decanter further comprises a breather pipe (32) arranged to set a siphon break level.

7. A siphon decanter according to claim 6, wherein the breather pipe (32) is located on the U-shaped siphon.

8. A siphon decanter according to any preceding claim, wherein the outlet further

comprises an air valve (34) arranged to release air from the U-shaped siphon.

9. A siphon decanter according to any preceding claim, the siphon decanter further

comprising a drainage aperture insert (40) arranged to be positioned over the drainage aperture (22).

10. A siphon decanter according to any preceding claim, wherein the siphon decanter comprises one or more inlets (712) in fluid communication with the connector pipe (718).

1 1. A siphon decanter according to claim 10, wherein the siphon decanter further comprises a launder pipe (750) arranged to transmit water from the one or more inlets (712) to the connector pipe (718).

12. A sequencing batch reactor (202) comprising a siphon decanter according to any one of claims 1-11 and a tank, wherein the siphon decanter is arranged to decant water from the top of said tank.

13. A sequencing batch reactor according to claim 12, wherein the outlet (14) and outlet chamber (20) are arranged outside the sequencing batch reactor tank (202).

14. A sequencing batch reactor according to claim 12 or 13, wherein the connector pipe (18) is arranged through a wall of the sequencing batch reactor tank (202).

15. A sequencing batch reactor according to any of claims 12-14, wherein when the water level in the tank is at or above the level of the connector pipe (18), water flows from the outlet (14) through the connector pipe(18) and exits the outlet into the outlet chamber (20).

16. A sequencing batch reactor according to a any of claims 12-15, wherein when the water level in the tank is above a siphon prime level (WL2), a rate of clean water exiting from the outlet chamber (20) is determined by the size of the drainage aperture (22) and the height of water in the outlet chamber (20).

17. A sequencing batch reactor according to any of claims 12-16, wherein a maximum water level (WL1) in the tank is determined by the height of the overflow weir (26).

18. A sequencing batch reactor according to any of claims 12-17, wherein when the water level in the tank falls below a siphon break level (WL3), the connector pipe (18) is substantially emptied of water.

19. A sequencing batch reactor according to any of claims 12-18 as appended to claim 6 or 7, wherein a siphon break level (WL3) is determined by the breather pipe (32).

20. A method of decanting water from the top of a sequencing batch reactor (202), the method comprising: providing a siphon decanter (10) according to any of claims 1-11 and a sequencing batch reactor (202) comprising a tank, wherein the inlet (12) is arranged inside the tank;

filling the sequencing batch reactor tank (202) so that the water level is above a siphon prime level (WL2);

decanting clean water that exits from the outlet chamber (20).

21. A method according to claim 20, wherein the step of decanting step overlaps with the step of filling the sequencing batch reactor tank (202).

22. A method according to claims 20 or 21 , wherein the method is part of an activated sludge process (200) performed in the sequencing batch reactor (202), the process further comprising an aeration step and a settlement step.

23. A siphon decanter and method of decanting water from the top of a sequencing batch reactor substantially as hereinbefore described with reference to the accompanying drawings.