SENSING METHODS AND SYSTEMS

Abstract

We describe a method of determining a food-to-biomass ratio in an aqueous fluid the method comprising: providing an aqueous fluid comprising viable biomass and food for said biomass, and wherein there is insufficient available food to sustain all said viable biomass; using a sensor (for example a respirometer or a sensor for sensing an amount of ammonia, ammonium, nitrates or nitrites) to determine an amount of food in said aqueous fluid available to said biomass; determining a measure of viable biomass in said aqueous fluid by measuring polarisability of viable biomass cells in an AC electric field; and determining a food-to-biomass ratio from said amount of food and said measure of viable biomass.

Claims

1. A method of determining a food-to-biomass ratio in an aqueous fluid the method comprising: providing an aqueous fluid comprising viable biomass and food for said biomass, and wherein there is insufficient available food to sustain all said viable biomass; using a sensor to determine an amount of food in said aqueous fluid available to said biomass; determining a measure of viable biomass in said aqueous fluid by measuring polarisability of viable biomass cells in an AC electric field; and determining a food-to-biomass ratio from said amount of food and said measure of viable biomass.

2. A method as claimed in claim 1, wherein said sensor is a respirometer for determining a level of respiration of biomass within said aqueous fluid, the level of respiration representing an amount of food in said aqueous fluid available to said biomass.

3. A method as claimed in claim 2 wherein said food-to-biomass ratio is determined from a ratio of first and second values dependent on said level of respiration and on said measure of viable biomass respectively.

4. A method as claimed in claim 1, wherein said sensor is an ammonia sensor, an ammonium sensor, nitrite sensor or a nitrate sensor for determining a level of ammonia, ammonium, nitrites or nitrates in said aqueous fluid that is indicative of an amount of food in said aqueous fluid available to said biomass.

5. A method as claimed in claim 4 wherein said food-to-biomass ratio is determined from a ratio of first and second values dependent on said level of ammonia, ammonium, nitrites or nitrates and on said measure of viable biomass respectively.

6. A method as claimed in claim 1 wherein said determining of said measure of viable biomass comprises measuring a real or complex electrical permittivity of said aqueous fluid to measure said polarisability.

7. A method as claimed in claim 1 wherein said determining of said measure of viable biomass comprises measuring said polarisability at a plurality of different AC frequencies.

8. A method as claimed in claim 1 wherein said aqueous fluid comprises sewage sludge.

9. A method as claimed in claim 1, wherein said aqueous fluid comprises sewage sludge of a waste water treatment plant, the method further comprising: determining a food-to-biomass ratio of said aqueous fluid of said waste water treatment plant by monitoring said aqueous fluid in-situ in said plant; and controlling a degree of aeration of said plant responsive to said determined food-to-biomass ratio.

10. A method as claimed in claim 1 further comprising differentiating between different cell types within said biomass such that said measure of viable biomass comprises a selective measure of a selected said cell type, and wherein said determined food-to-biomass ratio comprises selective food-to-biomass ratio for a selected subset of said viable biomass in said aqueous fluid.

11. A method as claimed in claim 10 wherein said aqueous fluid comprises sewage sludge of a waste water treatment plant, the method further comprising: determining a food-to-biomass ratio of said aqueous fluid of said waste water treatment plant by monitoring said aqueous fluid in-situ in said plant; and controlling a degree of aeration of said plant responsive to said determined food-to-biomass ratio; wherein a first selected said cell type comprises a carbonaceous cell type and said selective food-to-biomass ratio comprises a first selective food-to-biomass ratio more or preferentially responsive to viable carbonaceous cell biomass than to viable nitrogenous cell biomass.

12. A method as claimed in claim 10 wherein said aqueous fluid comprises sewage sludge of a waste water treatment plant, the method further comprising: determining a food-to-biomass ratio of said aqueous fluid of said waste water treatment plant by monitoring said aqueous fluid in-situ in said plant; and controlling a degree of aeration of said plant responsive to said determined food-to-biomass ratio; wherein a second selected cell type comprises a nitrogenous cell type, and said selective food-to-biomass ratio comprises a second selective food-to-biomass ratio more responsive to viable nitrogenous cell biomass than to viable carbonaceous cell biomass.

13. A method as claimed in claim 10 wherein a first selected said cell type comprises a carbonaceous cell type and said selective food-to-biomass ratio comprises a first selective food-to-biomass ratio more responsive to viable carbonaceous cell biomass than to viable nitrogenous cell biomass; the method further comprising: determining first and second selective food to biomass ratios of said aqueous fluid of said waste water treatment plant by monitoring said aqueous fluid in-situ in said plant; and controlling said waste water treatment plant responsive to both said first and second selective food-to-biomass ratios.

14. A method as claimed in claim 1, further comprising determining a rate of growth of said viable biomass by determining a plurality of measures of viable biomass in said aqueous fluid over a first period of time, and calculating a rate of change of said measure of viable biomass over said first period, said rate of change of said measures of viable biomass being indicative of a rate of growth of said viable biomass.

15-17. (canceled)

18. A method as claimed in claim 1, further comprising determining a rate of conversion of food to viable biomass by: determining a plurality of measures of an amount of food in said aqueous fluid available to said biomass over a second period indicative of a rate of change of amount of food over said second period of time; and determining a plurality of measures of viable biomass in said aqueous fluid over said second period of time indicative of a rate of change of viable biomass over said second period; and calculating a rate of conversion of food to viable biomass by comparing the rate of change of food and rate of change of biomass over said second period of time.

19-21. (canceled)

22. A system for determining a food-to-biomass ratio in an aqueous fluid, the system comprising: a sensor to determine an amount of food in said aqueous fluid available to said biomass; cell polarisability measuring apparatus to determine a measure of viable biomass in said aqueous fluid by measuring polarisability of viable biomass cells in an AC electric field; and a system to determine a food-to-biomass ratio from said amount of food and said measure of viable biomass.

23. A system as claimed in claim 22, wherein said sensor is a respirometer for determining a level of respiration of biomass within said aqueous fluid, the level of respiration representing an amount of food in said aqueous fluid available to said biomass.

24. A system as claimed in claim 22, wherein said sensor is an ammonia sensor, an ammonium sensor, nitrite sensor or a nitrate sensor for determining a level of ammonia, ammonium, nitrites or nitrates in said aqueous fluid that is indicative of an amount of food in said aqueous fluid available to said biomass.

25-27. (canceled)

28. A system for differentiating between, or providing a selective response to, different types or classes of viable biomass in activated sludge (AS), the system comprising: a probe for electrically probing said activated sludge (AS); and cell polarisability measuring apparatus, couplable to said probe, to determine a measure of viable biomass in said aqueous fluid by measuring polarisability of viable biomass cells in an AC electric field; wherein said apparatus is configured to differentiate between said different types or classes of viable biomass organisms by operating at one or more selected frequencies or frequency ranges of said AC electric field.

29. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] 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:

[0041] FIGS. 1a and 1b show, respectively, a high level schematic diagram of a waste water treatment plant, and schematic block diagram of a control system for closed-loop control of a waste water treatment plant according to an embodiment of the invention;

[0042] FIGS. 2a and 2b show a culture vessel which may be adapted for use in embodiments of the invention, showing the vessel under, respectively, normal atmospheric pressure and reduced pressure;

[0043] FIG. 3 shows the variation of pressure with time when incubating influent over a period of hours;

[0044] FIG. 4 shows a model of a sewage treatment plant usable for calculating a food:biomass ratio;

[0045] FIGS. 5a and 5b, show, respectively, growth of biomass in four zones of an activated sludge lane, and an example of an organism growth pattern;

[0046] FIG. 6 shows a sewage treatment plant control system according to an embodiment of the invention;

[0047] FIG. 7 shows electrical permittivity measurements as a function of frequency in four different zones along an activated sludge (AS) lane of a waste water treatment plant; and

[0048] FIGS. 8a and 8b illustrate the use of food-to-biomass sensing systems in a waste water treatment plant.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0049] Activated Sludge Monitoring

[0050] FIG. 1a shows, at a high level, a schematic diagram of the operation of a waste water treatment plant 10. Thus the plant accepts influent 12, fluid from which the solids have been substantially removed, containing a high level of ‘food’ for bacteria, protozoans rotifers, fungi and the like (biomass') and having a high biochemical oxygen demand (BOD). The output from the plant has two components, a clear component 14 which may be provided to a water course and a biological component 16 comprising living biological material referred to as returned activated sludge (RAS), typically at around 60% concentration. The RAS is provided back to the input side of the plant to help maintain the eco system. Often the RAS lane 16 joins the influent lane 12 to form a combined, activated sludge (AS) lane 18.

[0051] FIG. 1b shows a block diagram of a closed loop based water treatment control system 200 to implement real time closed loop control of a sewage treatment plant based on a combination of a respirometer and cell polarisability measuring apparatus. The respirometer may be configured to perform a pressure and/or composition measurement of the gases in the headspace of a closed vessel/sealed chamber, as described later, or may operate in some other manner, for example using an oxygen electrode to measure respiration.

[0052] In an example implementation a culture vessel as described with reference to FIG. 2, below, is used as a respirometer 100 to measure the quantity of food in the activated sludge lane 18 (where there is an excess of biomass). The respirometer 100 responds to overall changes in gas pressure/composition within the respirometer. The quantity of biomass is also measured in the activated sludge lane 18, by cell polarisability measuring apparatus 250, although it may alternatively be measured in the RAS lane 16. In some embodiments the respirometer and cell polarisability measuring apparatus may be combined into a single unit—for example a floating respirometer may be combined with a probe bearing one or more electrodes for making a capacitance measurement for the apparatus. In other embodiments they may be separate modules. For example the respirometer may be a device as shown in FIG. 2 and the cell polarisability measuring apparatus may comprise a separate probe into the activate sludge.

[0053] The outputs from respirometer 100 and apparatus 250 are each provided to a data processor 210, for example a general purpose computer under software control. Thus measures of both food and biomass, at one or more locations within the plant, are provided to the data processor 210. The data processor may output one or more parameters indicating the food-to-biomass ratio and/or BOD (biochemical oxygen demand) at one or more locations in the system, for example on a screen for an operator to use in controlling the plant or to an aeration control system 220 to automatically control the aeration such that it is sufficient, but not significantly in excess of that required given the amount of food/biomass the plant is coping with. This in turn enables the plant to operate efficiently but also to react to shock loads and variations in food/biomass levels over time periods of one or more days, weeks, months or years.

[0054] In embodiments, as described later, optionally the food-to-biomass ratio and/or BOD (biochemical oxygen demand) data is provided for different classes of organism, potentially at different monitoring locations, for example carboniferous and nitrogenous organisms.

[0055] We have previously described a system for monitoring the metabolism/growth of microorganisms, the system comprising a sealed chamber with a flexible diaphragm to provide sensitive pressure measurements of gas pressure in the headspace above a culture liquid. For details reference may be made, for example, to our U.S. Pat. No. 8,389,274.

[0056] It is helpful to outline details of such a device since a similar pressure measuring system may be adapted for inclusion in embodiments of the invention described later. Thus FIGS. 2a and 2b show, schematically, an embodiment of a similar device 100 to that in U.S. Pat. No. 8,389,274 under, respectively, normal atmospheric pressure and negative pressure (in operation either negative pressure or positive pressure may be produced). Thus a culture 102 of biological material undergoes metabolism and growth during which it exchanges gases with the aqueous liquid (water) carrying cells depending upon various factors gas may be used and/or produced, for example the cells may produce carbon dioxide during respiration. 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 by hardware, software or a combination of the two. As illustrated the device includes a sealable inlet/outlet port 114; it also includes an agitator 112, and may incorporate temperature control (not shown). 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.

[0057] FIG. 3 shows the general shape of a pressure-time curve for a sample of liquid from a sewage treatment plant. Thus there is an initial period during which the pressure can vary and results appear unreliable. This typically lasts up to around 10 minutes. 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 (the graph of FIG. 3 is not to scale). The initial rate of pressure drop appears to be related to the concentration of food present, a faster drop being observed with more “food” present. (Thus either the pressure drop or the rate of pressure drop may be measured). Without wishing to be bound by theory it is surmised 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 may be a measurement of both BOD and COD (chemical oxygen demand)—but if so this is potentially advantageous for aeration control.

[0058] In embodiments this approach provides a “BOD5” test proxy. More particularly the area under the pressure-time curve to this point may also be used as an indication of the amount of food available, and in embodiments may provide a better proxy for a BOD5 test.

[0059] Thus, broadly speaking, a closed vessel pressure measurement can be used as a measure of oxygen utilisation by a given body of biomass with time, consistent with the food availability. Additionally or alternatively it can be useful to control based on a food to biomass ratio. If necessary a measurement of the biomass may either be made by heating a sample, for example by microwaving the sample, to determine the dry weight of biomass or by measuring the amount of biomass indirectly by culturing the biomass.

Food-to-Biomass Ratio Measurement

[0060] The inventors have correlated MLSS to Permittivity/Capacitance and have thus been able to characterise MLSS in real-time on-line. More particularly the volatile or viable MLSS is measured because this only sees the live cells and not dead cells and other organic and inorganic matter that makes up MLSS. This facilitates advising on the amount of Return Activated Sludge required to seed the process at any point in time, and doing this dynamically.

[0061] In addition to the techniques we describe herein, this is also important because in a toxic event where cells are being killed, or in poorly operating plant, the MLSS may have only a small concentration of live cells and thus conventional dry weight methods of measuring the MLSS will fail (sampling a volume from the AS lane, filtering it to concentrate the solids, then drying).

[0062] An important control parameter for the activated sludge process of a sewage treatment plant is the relationship between the load (this may be measured in kg/day) of BOD or bacterial ‘food’ entering the aeration plant, and the ‘mass’ of bacteria in the aeration tank available to treat the incoming BOD. This is referred to as the Food to (bio)Mass Ratio (F:M ratio), sometimes also referred to as the Sludge Loading Rate (SLR). The F:M ratio is perhaps the single most important parameter in controlling the activated sludge process; it may notionally be defined as the kg of BOD5 applied per kg MLSS per day.

[0063] The amount of biomass within the reactor is referred to as the Mixed Liquor Suspended Solids (MLSS) (mixed liquor combines raw/unsettled waste water and activated sludge). One approach to establish the MLSS is by filtration and drying at 105° C. to constant weight; it is then possible to calculate the F:M ratio using the model shown in FIG. 4.

[0064] Referring now to FIG. 4, Food to (bio)Mass Ratio according to this model may be calculated as follows:

FB:M=[(Bi×Qi)/SMLx Va)]×10.sup.−3×24

where [0065] Influent flow=Qi (m.sup.3/hr) [0066] Influent BOD=Bi (mg/l) [0067] Aeration tank volume=Va (m.sup.3)

[0068] MLSS=SML (g/l)

and where, in this example, FB:M refers to the loading measured by the BOD5 technique.

[0069] In embodiments of the techniques we describe the volatile or viable MLSS is linked to to a measure of biomass activity as measured by oxygen uptake rate or similar measurement, which is representative of food availability, in particular where there are more bacteria than food for them, so that the respiration measured represents the available food. In effect biomass activity is linked with biomass live mass taking into account only the active constituents that process the effluent. Thus we can accurately calculate (automatically and dynamically) the F:M ratio on-line in near real-time. This facilitates a high degree of process efficiency, and substantial reductions in costs, energy input, and carbon emissions.

[0070] FIG. 5a shows the measured growth of biomass in four zones of an activated sludge (AS) lane of a waste water treatment plant, where permittivity is a proxy for the number of viable organisms. In FIG. 5a an increase in permittivity reflects an increase in growth in four consecutive zones of a plug flow activated sludge lane. Each zone has its own growth characteristics for that particular time point (flow, food availability and the like). As the example illustrates, the inventors' measurements have shown that biomass growth can be determined in zones along the Activated Sludge lane, demonstrating that the lane can be “mapped” to show how the process is working. This may be modelled in a controlled culture vessel to mimic the activated sludge process over time to its conclusion. This is useful as it indicates the overall biological process duration for the particular food/flow characteristics at that time. In general the growth of organisms follows a particular pattern for the given plant conditions; FIG. 5b shows an example of this.

[0071] A change in the conditions such as food input (influent concentration) or flow rate will in general affect the profile of the “map”. For example: [0072] As flow rate and food concentration changes, the position of peak biomass concentration changes in relation to time [0073] As flow rate and food concentration changes, the height of peak biomass changes [0074] As flow rate and food concentration changes, the slope of permittivity changes in growth and decline phases

[0075] These parameters can be used to further optimise the treatment process in near-real time.

Treatment Plants and Control

[0076] FIG. 6 shows an embodiment of a sewage treatment plant control system 600, illustrating a system of the type shown in FIG. 1 in more detail. Thus an activated sludge vessel 602 is provided (in this example) with three food-to-biomass sensor modules 400a, b, c each coupled to a data logging system 604. In embodiments each sensor module comprises a respirometer and capacitance/permittivity measuring apparatus as previously described.

[0077] In embodiments a sensor module may also include a temperature measuring device to provide fluid temperature data back to data logger 604. An optional controller 606 interfaces with and controls the sensor modules. A flow sensor 608 measures a rate of liquid flow into and/or within activated sludge vessel 602. A data handling and visualisation system 610 is connected to the data logging system 604 to receive data from the sensor(s), to controller 606, to control when measurements are made, and to flow sensor 608. The data handling system 610 may thus receive liquid flow data and/or temperature data and/or pressure or gaseous composition measurement data from the one or more sensor modules. The data handling system 610 may present this as raw data to the operator, for example on a graphical display and/or this data may be processed, for example to convert a measurement of gaseous pressure/composition to an indication of oxygen demand and/or an indication of a need for aeration; again one or more of these may optionally be displayed graphically or output in some other manner by module 610. In general module 610 also provides an operator interface to allow control of the sensing modules to make measurements. Optionally module 610 may also receive inputs from one or more additional sensors such as an output flow rate sensor, and/or an ammonium level sensor, and the like. Module 610 may further optionally receive additional inputs from the plant, for example an input of dry biomass weight obtained from drying a sample from one or more locations in the vessel.

[0078] In embodiments the information output by module 610 may be employed by an operator of the plant for manual control of a level of aeration and/or for control of a flow rate of sludge through vessel 602 (by controlling a pump), and/or for controlling a degree of RAS feedback (by controlling a RAS pump). In a typical activated sludge vessel aeration may be provided by a series of tubes with holes at intervals along their length provided with an air supply and located at the bottom of the sludge vessel; these tubes may run perpendicular to the flow direction and it may be possible to control aeration so that at different locations along the flow different levels of aeration are provided. Thus the data from module 610 may be employed to control a degree of local aeration, for example in the region of a particular sensor.

[0079] Additionally or alternatively the plant may include a control module 612 as part of a system for automatic control of aeration/local aeration and/or of sludge flow rate and/or of RAS feedback. Optionally this control may be implemented by means of an SCADA (supervisory control and data acquisition) interface module 614. Further optionally a network connection/interface 616 may be provided for remote monitoring and/or control of the system. The skilled person will appreciate that the modules 604, 606, 610, 614 and 616 may be implemented as software modules within a computer system; the air/sludge pump control module 612 may be implemented by software with an interface to a suitable electronic controller.

[0080] In an automatic arrangement broadly speaking the system may increase a level of aeration when the oxygen demand is high as indicated by a larger measured pressure drop and vice versa. The operating region of the plant may be controlled to be different at different points along the length of flow through vessel 602—for example a region of relatively reduced oxygenation may be provided at the front end of the vessel (where the influent enters) and, for example, a quantity of nitrifying organisms may be controlled so that there is a region of increased nitrification towards an end of the flow region. The skilled person will appreciate that although vessel 602 is illustratively shown as a single vessel; in practice it may comprise multiple linked tanks.

Selective Sensing

[0081] The approaches we have previously described enable the identification of changes in the microbial population over time, or throughout the process, along the lane and/or elsewhere, and may be enhanced by using a range of scanning frequencies. In single species cultures, there is a characteristic frequency profile that is optimal for measuring that organism. When using a range of scanning frequencies there are changes in the profile of a scan when the mixed population changes. This is useful because there is more than one microbiological process going on at the same time in the Activated Sludge lane. Thus an ability to visualise these different processes can be highly advantageous.

[0082] Referring now to FIG. 7, this illustrates a measurement of the electrical permittivity four different zones along an AS lane of a waste water treatment plant. The graph illustrates the central, sloping portion of an S-curve—the permittivity levels out to the left and right of the ends of the illustrated parts.

[0083] Each zone has its own permittivity profile, and this changes from one end of the process to the other (over zones 1-4). This is significant, indicating the differences between, for example, early Carbonaceous breakdown and secondary Nitrogenous processing by different microbial species. The ability to visualise the growth of these organisms in their respective process positions is useful in being able to model and/or run a plant efficiently

[0084] More particularly, the central sloping parts of the curves of FIG. 7 are relatively gently sloping; with a substantially homogeneous culture of organisms the transition between relatively higher and lower permittivity is much sharper. The gentle slope is attributed to the mixture of types of organism present. Although not easy to distinguish in FIG. 7, the sloping parts of the curves also exhibit various features which depart from a smooth curve; these are believed to be representative of the different types of organism which were present in the samples.

Rate of Viable Biomass Growth

[0085] An additional measure of the health and growth of the biomass under given conditions within a plant is the rate of growth of the viable biomass (for example division and increased cell membrane area) for the viable biomass in relation to the prevailing conditions, e.g. available food and gasses (such as oxygen).

[0086] Advantageously, this may provide an additional set of data that provides assurance that aeration/RAS control loops are only affecting the biomass in a positive way.

[0087] The method, in particular, comprises determining a rate of growth of the viable biomass by determining a plurality of measures of viable biomass in the aqueous fluid over a first period of time, and then calculating a rate of change of the measure of viable biomass over that first period of time. These measures provide the ability to calculate, in real time, a rate of change of the viable biomass, which gives a measure of the rate of growth of the viable biomass.

[0088] The measures of the viable biomass are carried out using the permittivity techniques described above, which provide a measure of the amount of viable biomass in the aqueous fluid.

[0089] The rate of growth of the viable biomass may then be used either to control the degree of aeration and/or a volume of food in a water treatment plant, for example.

[0090] As described above, it may also be possible to measure the rate of growth for specific groups of organisms by use of certain permittivity frequencies (as outlined above) therefore determining which groups are reacting to deal with a given food source.

[0091] Furthermore, this method may be used in conjunction with the above methods of determining a food-to-biomass ratio in the aqueous fluid. Alternatively, the method of determining the rate of growth of the viable biomass may be operated in isolation, that is, without the need to determine the food-to-biomass ratio.

Rate of Conversion of Food to Biomass

[0092] There are known calculations for the average conversion of food into new biomass for wastewater plants. However, these known techniques do not offer real-time calculations, which may be used to fine tune the system. By looking at the growth of organisms and when this peaks a determination can be made of when all the food is converted. The rate of conversion may for example be indicative of the type, or changing type of food source, or how ideal the conditions are for bugs to convert it, how long the conversion of the entire food source will take (big energy implications), or how adaptable the microflora is in dealing with changing food source.

[0093] By using the techniques we describe, the rate of conversion of food to biomass is provided over a time period, which provides a measure of this conversion factor under any given plant condition and can therefore be used to fine tune the system.

[0094] The method, in particular, comprises: determining a plurality of measures of an amount of food in the aqueous fluid available to the biomass over a period of time, which is indicative of a rate of change of amount of food over the period of time, and then determining a plurality of measures of viable biomass in the aqueous fluid over the period of time, which are indicative of a rate of change of viable biomass over the period of time. From these determined measurements, a calculation of a rate of conversion of food to viable biomass can be made by comparing the rate of change of food and rate of change of biomass over the period of time.

[0095] The measures of the viable biomass are carried out using the permittivity techniques described above, which provide a measure of the amount of viable biomass in the aqueous fluid.

[0096] The rate of conversion of food to biomass may then be used either to control the degree of aeration and/or a volume of food in a water treatment plant, for example.

[0097] As described above, it may also be possible to measure the rate of conversion for specific groups of organisms by use of certain permittivity frequencies (as outlined above) therefore determining which groups are reacting to deal with a given food source.

[0098] Furthermore, this method may be used in conjunction with the above methods of determining a food-to-biomass ratio in the aqueous fluid (and also with the method of determining the rate of growth of biomass). Alternatively, the method of determining the rate of conversion of food to viable biomass may be operated in isolation, that is, without the need to determine the food-to-biomass ratio or the rate of growth of viable biomass.

Alternative Sensors

[0099] The above examples describe systems and methods based on the amount of food present in the aqueous fluid being detected by a respirometer, which provides a level of respiration of the viable mass, which is indicative of the amount of viable biomass. In alternative embodiments, the sensor may instead be a sensor of the type for detecting one or more of ammonia, ammonium, nitrites or nitrates. In such embodiments, the ammonia, ammonium, nitrite or nitrate sensor replaces the respirometer, and a measure of the amount of one or more of ammonia, ammonium, nitrite or nitrate is provided, which are indicative of the amount of food present in the aqueous fluid.

[0100] Example sensors include optical sensors and ion sensors.

Example Installations

[0101] FIG. 8a illustrates a tethered sensing system 700 of the type we have previously described. In some installations there may be two sensors, one at the start and one at the end of the treatment process (and more sensors may be used). Monitoring at multiple points in a waste water treatment process enables different levels of aeration to be employed at the different locations, thus giving rise to energy savings. Thus in embodiments a waste water treatment plant may segregate treatment sections along the flow path providing separate oxygen requirement sensing and aeration control for each section. This has the potential to result in substantial energy savings. Thus FIG. 8b illustrates the use a pair of sensing systems 700a, b, each monitoring a region of immobilised biomass (using curtains) with its own respective aeration 704a, b.

[0102] A food-to-biomass sensing system of the type we have described may potentially be employed to monitor toxicity of waste water either in a sewage treatment works or in other industrial plant, or potentially in the outfall from an industrial plant. Although the food-to-biomass sensing system we have described is particularly useful in monitoring a sewage treatment plant it may, more generally, be employed to monitor other water-based processes, for example water in a hospital, water in an air-conditioning system or the like.

[0103] 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 spirit and scope of the claims appended hereto.