GB2552854B - Wastewater treatment plant control systems - Google Patents

Description

Wastewater Treatment Plant Control Systems

FIELD OF THE INVENTION

This invention relates to methods for controlling the operation of a wastewater treatment plant, in particular to optimise the dosing of settlement-enhancing chemicals in such a plant. The invention also relates to wastewater treatment plants controlled in this way, and to suitable controllers and their software.

BACKGROUND TO THE INVENTION

We have previously described techniques for measuring the biochemical oxygen demand (BOD) of effluent (WO2014/029976), and techniques for the control of aeration in a wastewater treatment plant (WO2014/027183).

Wastewater treatment plants include municipal water recycling centres and industrial effluent treatment plants. In general it is desirable, and often regulated, for the output of a plant to be low in phosphorous. Fluid in the plant can be dosed with chemicals to remove phosphorous, which settles out. More generally any chemical which enhances settlement will tend to remove phosphorous from the fluid. Furthermore, growth of biomass also removes phosphorous as it becomes part of the growing microorganisms. Thus settlement and micro-organism growth, the former assisted by settlement enhancing chemicals, can both be used to remove phosphorous from the plant output.

However treatment plants also produce surplus activated sludge and one method for disposing of this involves anaerobic digestion of the sludge. The anaerobic digestion produces methane which can be captured and used, for example in a CHP (combined heat and power) engine to supply some of the electricity power requirements of the plant.

In principle, if sufficient sludge is produced, and hence sufficient feedstock for the anaerobic digestion, the plant may be carbon neutral and may even provide excess power back to the grid. Thus it can be advantageous to increase sludge production. This can be done by adding more settlement enhancing chemicals, although there is an economic cost in using settlement enhancing chemicals.

This is to be balanced against the potentially negative effect of removing too much “foodstock” as sludge since the foodstock is required in the (aerobic) sludge lanes. Similarly it is important not to over-strip fluid in the plant of phosphorous because this is needed for microbial growth. For industrial effluent treatment (and potentially for sewage treatment) the incoming liquor may be short of phosphorous and may need to be supplemented with additional phosphorous. Where activated sludge is provided to an anaerobic digester, again this should include some phosphorous to facilitate growth. In broad terms, known techniques for dosing with settlement chemicals are relatively crude, for example based upon visual appearance of the effluent, possibly supplemented with a measurement of phosphorous (phosphate) level.

There are various factors to take into account, including cost, and there is therefore a need for improved techniques for controlling the dosing of settlement enhancing chemicals in a wastewater treatment plant.

SUMMARY OF THE INVENTION

According to the invention there is therefore provided a method of controlling the operation of a wastewater treatment plant, the method comprising: chemically dosing a first flow in said plant with one or more settlement enhancing chemicals; making a measurement of a second flow, wherein said measurement is dependent on a biochemical oxygen demand of said second flow, wherein said second flow is downstream of said first flow, and wherein said second flow comprises a mixture of a flow from an input to said plant and an RAS (returned activated sludge) flow of said plant; and using said measurement to control a level of said dosing of said one or more settlement enhancing chemicals.

The inventors have recognised that the trade-offs described in the introduction can be mitigated by aligning the dosing of settlement-enhancing chemicals with the biochemical oxygen demand (BOD) of the plant. To put this into effect one could measure the BOD of the influent to the plant and then dose accordingly, for example to maintain a target ratio of BOD to phosphate. However it turns out that such an approach does not work well because it is incorrect to consider the activated sludge region of the plant merely as a combination of influent and micro-organisms - instead the activated sludge liquor becomes attuned to the feed stock, the feed stock affecting the different populations of micro-organisms which grow, and the different microorganisms in turn affecting the activated sludge liquor so that there is a complicated interaction between the feed stock and micro-organisms. Thus the inventors have further recognised that there is a need to take into account the activity of the biomass into which the influent is provided.

In broad terms, therefore, in embodiments of the invention the method measures a biochemical oxygen demand in an activated sludge region of the plant after, that is downstream of, a location where the settlement enhancing chemicals are added. A measurement in this location is then used to control the chemical dosing upstream of the measurement location, in embodiments dosing to the influent but additionally or alternatively dosing to an RAS (returned activated sludge) feed into an activated sludge lane or tank of the plant.

Thus in one embodiment the first flow chemically dosed with one or more settlement enhancing chemicals comprises a flow from the input to the plant, in particular into a primary tank or tanks of the plant. In embodiments the second flow comprises a flow in an activated sludge lane or tank of the plant. In embodiments the RAS flow comprises a third flow, and the second flow, where the measurement is made, is downstream of both the first and third flows, both the first and third flows may be dosed.

In embodiments, in particular where the influent is dosed, the BOD measurement in the second flow is compensated for a proportion of RAS in the second flow. Preferably an adjustment is also made for the rate of the first flow. More particularly a dosing value derived from the BOD measurement may be adjusted in proportion to the rate of the first flow.

In embodiments the dosing controls a level of phosphorous (phosphate) in the second flow and/or an output of the plant, and/or a rate of settlement of activated sludge from the second flow. Thus the dosing may comprise dosing with a polymer, more particularly a polyelectrolyte and/or a metal salt such a ferric chloride, ferric sulphate, ferrous sulphate, or aluminium sulphate.

The particular target to be achieved by the dosing may depend upon the type of plant and its operating conditions. Thus for example in one approach the dosing may be controlled to target a desired ratio of phosphorous to biochemical oxygen demand in the activated sludge lane/tank. The skilled person will appreciate that the particular dose of chemical (to achieve a desired target level of phosphorous/phosphate) may be determined by an initial calibration process. This may involve a lab measurement of a level of phosphorous, in combination with data from a chemical manufacturer which defines the amount of chemical to add to target a particular phosphorous level or to remove phosphorous from a fluid. The amount of chemical to add can be adjusted up or down in proportion to the flow rate of the flow to which the chemical is being added. A similar adjustment can be made when dosing with a chemical such as a polyelectrolyte as again manufacturers data is available to define a (flow-rate-dependent) dose to reduce/remove phosphorous. Similar considerations apply if dosing to control to a particular target level of phosphorous in the output of the plant. Where dosing is to control a rate of settlement or generation of activated sludge from the second flow again manufacturers data may be employed to achieve a desired target rate, once an initial calibration has been made. In general the skilled person will understand that the absolute quantity of a chemical or chemicals with which to dose will normally be determined by an initial calibration procedure for any particular plant.

In some preferred embodiments the measurement of biochemical oxygen demand is made by obtaining a sample of the fluid and providing this to a sealed chamber, such that the sample incompletely fills the chamber leaving a headspace. The sample is then incubated in the chamber and the pressure change in the headspace is monitored to determine a value for the biochemical oxygen demand, for example as described in our WO2014/029976 (incorporated by reference).

Embodiments of the above described method can be particularly advantageous in the context of a plant in which activated sludge from the plant is provided to an anaerobic digester. The anaerobic digester digests the sludge to produce bio gas (primarily methane), which in turn is used to generate electricity to partly or wholly power the plant and, preferably, to provide surplus electricity to the grid. In such a system a method as described above can be employed to enhance extraction of the activated sludge for more efficient operation of the overall system. In embodiments the plant may thus become substantially carbon neutral and may, in embodiments, provide a source of mains electricity.

There is also described a wastewater treatment plant comprising: a primary tank; an input to provide an input flow into said primary tank; one or more sludge lanes or tanks; a fluid connection between said primary tank and said one or more sludge lanes or tanks; a biochemical oxygen demand (BOD) measurement device to measure a BOD parameter of fluid at a measurement point in said one or more sludge lanes or tanks; and a controller, coupled to or incorporated into said BOD measurement device, to process said measured BOD parameter to determine a parameter specifying a quantity of settlement-enhancing chemicals to dose at a location in said plant elsewhere than in said one or more sludge lanes or tanks.

There is further described a controller for a wastewater treatment plant as described above.

There is further described software for controlling a wastewater treatment plant as described above, in particular in the form of processor control code for a dedicated or general purpose computer system. The code is provided on a non-transitory physical data carrier such as a disc or programmed memory. The code (and/or data) may comprise source, object or executable code in a conventional programming language. As the skilled person will be aware, such code and/or data may be distributed between a plurality of coupled components in communication with one another.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which:

Figure 1 shows a simplified block diagram of a wastewater treatment plant configured to implement a dosing control technique according to an embodiment of the invention;

Figure 2 shows a flow diagram of a dosing control procedure according to an embodiment of the invention; and

Figures 3a to 3c show a preferred embodiment of a biochemical oxygen demand (BOD) measuring device for use with the plant/procedure of Figures 1 and 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to Figure 1, this shows a simplified block diagram of a wastewater treatment plant 10 incorporating a chemical dosing control system as described later. The plant has an input 12 to receive crude sewage or, in related applications, industrial effluent. The influent is provided to a set of screens 14 which act as a course mechanical filter and an outflow 16 from the screens provides an input to one or more primary settlement tanks 18. These primary tanks are typically designed for a retention time of order hours to allow some initial settlement and sludge removal prior to biological treatment. An output 20 from these tanks provides an input to one or more activated sludge lanes 22a,b or tanks. These lanes/tanks also provide a returned activated sludge (RAS) flow 24 which is combined with the output 20 from the primary tanks so that the activated sludge lanes receive a combined RAS and primary effluent (PE) input. The combination of effluent and activated sludge in the lanes/tanks is generally referred to as Mixed Liquor Suspended Solids (MLSS). Typically the proportion of RAS is around 50% (in the range 20%-70%). A variety of biological processes takes place in the MLSS. Typically there is an initial low oxygen region in which anaerobic processes take place and a later aerated region in which aerobic processes take place. Anaerobic processes include denitrification which can result in the release of nitrogen; aerobic processes include nitrification (which generates nitrate) and the conversion of organic carbon to carbon dioxide. Growing micro-organisms use organic compounds, oxygen and other nutrients to generate new biomass, releasing carbon dioxide and water. Typically phosphorous is present in a variety of different forms such as polyphosphates, orthophosphate, and a range of organic phosphorous compounds. Typically during treatment phosphorous compounds are converted to orthophosphate which is in turn used to form various constituents of the growing micro-organisms, thus removing phosphorous from the wastewater. Other phosphorous can be removed by chemicophysical precipitation using settlement enhancing chemicals such as polyelectrolytes and/or metal salts such as ferric chloride sulphate, ferrous sulphate and aluminium sulphate.

Ferric sulphate is relatively fast acting and may be used on the influent; it forms a hydroxide and hence a chemical cage around the phosphorous. Ferrous sulphate acts more slowly (because it is first oxidised to ferric sulphate). This is therefore advantageous to use in the activated sludge processing region because as microorganisms die they release phosphate. Aluminium sulphate removes phosphate indirectly by a physicochemical process. It can be advantageous to employ polyelectrolytes, optionally in combination with metal salts; these remove phosphorous by promoting settlement. There is a range of suitable, commercially available polyelectrolytes intended for sewage treatment. Since the metal salts themselves promote settlement these chemicals can generally also be employed to increase the quantity of activated sludge extracted from a lane/tank. However it is important not to extract too much activated sludge as this is important for proper biological processing.

The output from the activated sludge lanes/tanks is typically provided to a secondary clarifier (not shown in Figure 1), which may be funnel shaped so that a substantially clear treated effluent output 26 can be extracted from the top of the clarifier whilst activated sludge can be extracted from the bottom. Some of this activated sludge is provided to the RAS flow 24; the remaining activated sludge may provide a SAS (surplus activated sludge) output 28. The treated effluent 26 may be provided to a watercourse optionally after further tertiary treatment. The SAS output 28 may be provided to an anaerobic digester 30 producing biogas, typically mostly methane, which may then be used to power an electrical generator 32 to provide an electrical power output to the plant and/or grid mains. The generator may burn the biogas in a boiler coupled to a turbine, for example in a combined heat and power (CHP) system or the biogas may be used in fuel cells.

Thus in a municipal wastewater treatment plant or “Water Recycling Centre”, sludge settlement can occur in both the primary tanks and the activated sludge process. Chemical dosing can be used to promote separation of the sludge from the water, to achieve increased sludge production to act as a feedstock, for example for the anaerobic digester of a CHP plant and/or reduced phosphate levels in the final effluent output. An industrial effluent treatment plant may be similar to the plant shown in Figure 1 but may employ a dissolved air flotation (DAF) water treatment process to separate sludge from the effluent output.

As previously mentioned, increased sludge production/phosphate control can involve dosing with settlement enhancing chemicals. This may involve adding a ferric compound such as iron chloride to the crude sewage upstream of the primary settlement tank (location X in Figure 1) and optionally adding a ferrous compound such as ferrous sulphate to the MLSS to lock up phosphate released during the endogenous phase of the activated sludge treatment. As previously mentioned, polyelectrolytes may also be employed to increase sludge production for increased gas output from an anaerobic digester. Optionally the RAS flow 24 may also be dosed with settlementenhancing chemicals, as shown at location Y in Figure 1. In the case of an industrial effluent treatment plant a DAF process may be employed to separate the RAS from the MLSS output of the activated sludge lanes/tanks and again polyelectrolytes and/or metal salts may be employed to enhance this process.

Embodiments of the invention use chemical dosing upstream of the activated sludge lanes or tanks, in particular dosing at the start of the treatment process (for example location X in Figure 1) and allowing a few hours settlement time, but controlling the upstream dosing so that in the mixed liquor of the activated sludge which includes, for example, phosphorous (phosphate) contributions from dying micro-organisms, the quantity of phosphorous present targets that required by the growing micro-organisms. The phosphorous needs of the growing micro-organisms may be inferred from a measurement of the biochemical oxygen demand (or a parameter dependent upon this) in the activated sludge - knowing the BOD the phosphorous requirements can be estimated and then the dosing upstream controlled to meet these. This helps to avoid having either too little or excess phosphorous, both of which would be sub-optimal. Furthermore, when the growth is optimised the sludge production can effectively be maximised without excess phosphorous in the plant output, thus facilitating feedstock extraction. BOD is typically measured in milligrams of oxygen used per litre of sample. A standard measure in the so-called BOD5 value which is measured by incubation of a sample over five days. However this technique is impractical for the closed loop control employed in embodiments of the invention, and a faster test is therefore preferable. We have previously described how measurements of pressure in a sample chamber may be employed to determine a proxy for the BOD5 test but over a much shorter time period, typically just a few hours. In preferred embodiments it is this technique (described in more detail later) which is used to measure, or at least determine a proxy for, the biochemical oxygen demand of a sample of fluid from an activated sludge region of the plant.

The particular optimum ratio of BOD5 to phosphorous level depends upon the units of measurement or, equivalently, on the calibration of the measuring apparatus. However for a total phosphorous level measured in milligrams per litre, a typical target BOD to phosphorous ratio is in the range 100:0.1 to 100:10, more typically 100:0.5 to 100:5, for example around 100:1. Again, however, a particular target value may be established by experiment for any particular plant. Controlling in this manner helps to maintain settlement at around an optimum level so that the primary effluent is not left deficient in “food” or phosphate itself.

As previously mentioned a total phosphorous level (in mg/l) in the activated sludge can be linked to a measured BOD proxy by calibration and, in a similar manner, this can be linked to a target phosphorous level at the point of dosing. The level of settlement enhancing chemical with which to dose to achieve a target level of total phosphorous is typically determined from manufacturer’s data when purchasing chemicals for this purpose.

Once an initial calibration has been made this can be adjusted proportional to a measure of the flow at the point of dosing and is adjusted according to the measured BOD5 (or BOD5 proxy) in the activated sludge, thereby adjusting the system according to the nature of the influent. In a plant of the type shown in Figure 1 an adjustment is preferably also made for the RAS flow 24 on a volume-to-volume basis - if, say the primary effluent is 50% of the RAS flow then the dose is reduced accordingly. In principle the adjustment may be non-linear because of the feedback of the RAS flow but in practice this may be disregarded. The RAS BOD can be assumed to be relatively constant and simply subtracted off on a volume-to-volume basis according to the proportion of RAS in the MLSS.

In this way if the influent flow is accurately known a precise chemical dosage can be calculated, albeit with a feedback delay of, in embodiments, four hours. This is sufficiently good for process control once the system has been initially calibrated to the quality of the influent feed.

Thus embodiments of the system optimise chemical dosing, thus saving on the cost of chemicals and potentially facilitating the use of improved but more expensive materials such as polyelectrolyte. They also help to substantially reduce phosphate release by ensuring that the chemical dose is aligned to the biochemical oxygen demand of the primary effluent food source going into the activated sludge process, where the actual BOD value is measured when the primary effluent food source is combined with the activated sludge. If the BOD is known, approximately, the level (ratio) of phosphate that needs to remain can be determined and the associated (calibrated) chemical dose can thereby be controlled.

Such control promotes a good, healthy biological process and also reduces the risk of overdosing with chemicals, which might otherwise need to be corrected by adding a carbon source such as methanol, which is very expensive.

Referring again to Figure 1, the illustrated wastewater treatment plant includes a wastewater treatment plant controller 34 configured to implement a dosing control procedure as described above. The controller 34 may comprise a suitably programmed general purpose computer system including a processor and working memory and non-volatile programme memory, coupled to sensors as shown and providing a control output for controlling dosage of one or more settlement enhancing chemicals at, for example, location X and/or Y.

Thus controller 34 is coupled to one or more biochemical oxygen demand sensors 36, preferably of the sealed chamber and membrane type described later, and to one or more flow sensors 38, 40 to sense the volume flow at a dosing location, for example a volume flow rate of the primary effluent or returned activated sludge. A user terminal 42 is provided as a user interface and the controller stores calibration parameters in a non-volatile data store 44. These may define, for example, a percentage of RAS in the mixed liquor, data defining a biochemical oxygen demand of the RAS, and one or more plant calibration values linking a measured BOD to a chemical dose for a calibration flow rate, optionally via a phosphorous level or a value defining a rate of settlement of activated sludge in the lanes/tanks.

The target phosphorous (phosphate) value and/or settlement rate may be user defined, for example via terminal 42 and/or stored as a parameter in data store 44 and/or hard coded into the software, for example as a 100:1 BOD:P ratio. Controller 34 provides a dose control output 46 which may comprise data and/or a control signal for example for controlling an automatic dosing device or devices (not shown) at one or more of the chemical dosing locations.

Referring now to Figure 2, this shows an example flow diagram of processor control code running on controller 34 to implement an embodiment of the described technique.

Thus at step 50 the procedure measures the BOD at one or more points in the activated sludge lanes/tanks. Although this measurement may take a few hours, for example around four hours with the later described device this is very much faster than the conventional five days of a standard BOD5 test. Furthermore, preferably the software is configured to make measurements at time intervals which are shorter than the time taken to complete the measurement - for example BOD values, in embodiments BOD5 proxies, may be measured every hour. Samples of the activated sludge may be obtained manually but some preferred embodiments employ automated sampling, in particular using a floating respirometer, for example as described in our WO2015/019083 (incorporated by reference).

The procedure then adjusts the measured BOD for the component of BOD due to the RAS (knowing the volume percentage of RAS in the activated sludge lanes/tanks (step 52). The BOD of the RAS may be determined, for example, from regular calibration lab samples. At step 54 the adjusted BOD is converted to a level of phosphorous (or, correspondingly, to an activated sludge settlement rate, which applies to the primary effluent when this is in the activated sludge region of the plant, and then the procedure determines the difference between this and a target value (step 56). This difference can then be converted to a chemical dose (step 58), for example based upon manufacturers data, and this is adjusted as necessary based upon the flow rate(s) at the one or more dosing regions (steps 60, 62). The procedure then outputs data defining a value (or range) for the chemical dosing at the one or more dosing points, for example to the primary effluent and/or RAS (step 64). This data may be displayed, provided over a network, printed or output in any other convenient manner and/or may be employed to control an automatic dosing device. The procedure then loops back to step 50 or another BOD.

As the skilled person will appreciate steps 58 to 64 may be performed for multiple different settlement enhancing chemicals where more than one dosing chemical is employed, adding their respective influences on phosphorous (phosphate) and/or settlement rate.

Figures 3a to 3c, which are taken from an earlier published application, illustrate the operating principle of a particularly advantageous type of BOD sensor. In practice a floating version of this sensor with automatic sampling may be convenient to use. The skilled person will appreciate, however, that the techniques we describe are not limited to use of this particular type of sensor: Other sensors measuring BOD, or an equivalent value, or a value dependent upon BOD such as oxygen uptake rate, may also be employed.

Thus referring to Figures 3a and 3b, these show a device 100 comprising a sealed chamber with a flexible diaphragm under, respectively, normal atmospheric pressure and negative pressure (in operation either negative pressure or positive pressure may be produced). The diaphragm provides sensitive measurements of gas pressure in the headspace above a culture liquid, which can be used as a measure of oxygen utilisation by a given body of biomass.

Thus a culture 102 of biological material undergoes metabolism and growth during which it exchanges gases with the aqueous liquid. A gaseous headspace 104 of the sealed culture chamber 106 thus experiences changes in pressure due to exchange of gas with the culture medium, and these are monitored by a diaphragm 108 and converted to an electronic pressure signal 110 which may, for example, be digitised and processed electronically. As illustrated the device includes a sealable inlet/outlet port 114 and an agitator 112; it may also incorporate temperature control. The liquid phase (sample) to gaseous phase (measured head space) volume ratio can be used to adjust the sensitivity of the device - for example a ratio of up to 1:1 liquid : gas may be employed.

Figure 3c shows the general shape of a pressure-time curve for a sample of liquid from a sewage treatment plant: There is an initial period of up to 10 minutes during which the pressure can vary results may be unreliable then the pressure then begins to fall, flattening out in a trough region 300 after around an hour. Over a further period of several hours the pressure then gradually starts to rise once more (Figure 3 is not to scale). Without wishing to be bound by theory it is believed that the pressure drop relates to the conversion of gas into living biomass and that the trough region occurs when the oxygen has been depleted (the subsequent smaller rise relating to anaerobic respiration). In practice the pressure drop and/or the area under the pressure-time curve up to point 300, and optionally beyond into the trough region, may be used as a measurement of BOD, more particularly as a “BOD5” test proxy.

Thus broadly speaking we have described techniques in which the effect of a chemical dose on microbial growth/settlement is determined downstream of the dose in an activated sludge region towards a treatment plant, the effect on the micro-organisms being determined by a measure of biochemical oxygen demands, this value then being used to control the dosing of settlement enhancing chemicals upstream of the measurement point. This provides a feedback loop which is employed to control the dose of settlement enhancing chemicals, in particular to the primary effluence, in order to optimise a level of a nutrient, in particular phosphorous (phosphate) and/or a rate of settlement of activated sludge, in a biological treatment (activated sludge) region of the plant.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.

Claims (10)

CLAIMS:

1. A method of controlling the operation of a wastewater treatment plant, the method comprising: chemically dosing a first flow in said plant with one or more settlement enhancing chemicals; making a measurement of a second flow, wherein said measurement is dependent on a biochemical oxygen demand of said second flow, wherein said second flow is downstream of said first flow, and wherein said second flow comprises a mixture of a flow from an input to said plant and an RAS (returned activated sludge) flow of said plant; and using said measurement to control a level of said dosing of said one or more settlement enhancing chemicals.

2. A method as claimed in claim 1 wherein said first flow comprises a flow from said input to said plant.

3. A method as claimed in claim 1 or 2 wherein said RAS flow comprises a third flow, wherein said second flow is downstream of both said first and third flows, and wherein the method further comprises dosing said third flow with one or more settlement enhancing chemicals using said measurement to control a level of said dosing of said third flow.

4. A method as claimed in claim 1, 2 or 3 further comprising compensating said measurement for a proportion of RAS in said second flow.

5. A method as claimed in claim 1,2, 3 or 4 further comprising compensating said measurement for a rate of said first flow.

6. A method as claimed in any preceding claim wherein said dosing is to control one or more of: a level of phosphorous in an output of said plant; a level of phosphorous in said second flow; and a rate of settlement of, or generation of, activated sludge from said second flow.

7. A method as claimed in any preceding claim wherein making said measurement comprises: obtaining a sample of said second fluid; providing said sample to a sealed chamber such that the sample incompletely fills the chamber leaving a headspace; incubating the sample in the chamber; and monitoring how pressure in said headspace changes during said incubating to determine a value dependent on a biochemical oxygen demand of said second flow.

8. A method as claimed in any preceding claim wherein said dosing with one or more settlement enhancing chemicals, comprises dosing with one or both of a polyelectrolyte and a metal salt.

9. A method as claimed in any preceding claim wherein said first flow comprises a flow into a primary tank of said plant, and wherein said second flow comprises a flow in an activated sludge lane or tank.

10. A method as claimed in any preceding claim further comprising: providing activated sludge extracted from said plant to an anaerobic digester; using electricity generated from biogas from said anaerobic digester to provide electrical power for the plant; and enhancing extraction of said activated sludge using said settlement enhancing chemicals.