GB2537836A

GB2537836A - Method and apparatus for analysing the efficiency of air diffusers in wastewater aeration basins

Info

Publication number: GB2537836A
Application number: GB1507095.6A
Authority: GB
Inventors: Plano Salvatore; Berrisford Michael
Original assignee: Plano Eng Ltd
Current assignee: Plano Eng Ltd
Priority date: 2015-04-27
Filing date: 2015-04-27
Publication date: 2016-11-02
Also published as: GB201507095D0

Abstract

The invention relates to the field of wastewater treatment plants involving biological processes carried out in aerobic reactors with submerged air diffusers. A method of obtaining a value for the oxygen transfer efficiency of submerged air diffusers in wastewater aeration basins is provided, wherein the wastewater can be aerated with a quantity of air provided by a controllable aeration system. The method comprises the steps of: aerating the wastewater; measuring an airflow value for the aerating air; measuring the dissolved oxygen content of the wastewater; measuring the temperature of the wastewater; retrieving from a reference source a previously stored special value corresponding to the measured airflow rate; calculating the oxygen transfer efficiency of the basin using the special value, the temperature of the wastewater and the dissolved oxygen content of the wastewater, wherein the reference source is obtained from a prior basin characterisation stage. An apparatus for measuring the oxygen transfer efficiency of wastewater in a basin of a wastewater treatment plant is also claimed.

Description

Method And Apparatus For Analysing The Efficiency Of Air Diffusers in Wastewater Aeration Basins

BACKGROUND

Technical Field of the Invention and its uniqueness The invention relates to the field of wastewater treatment plants involving biological processes carried out in aerobic reactors with submerged air diffusers. The invention is directed to analysing the diffuser efficiency. An aeration system comprises air blowers or compressors, air-pipes, blower-valves, diffuser-valves and air-diffusers. The aeration system setup is at its most efficient if it provides the DO level as required for the basin for the minimum possible power (and therefore energy cost). The overall efficiency of the aeration system is affected by the design and functioning of each of these elements. However, once the basin is built the most significant element driving the overall efficiency is usually the performance of the air-diffusers. When air diffusers performance efficiency falls too far it is necessary to invest in cleaning the diffusers either using chemicals or if necessary (and more expensively) by draining the basin and cleaning the diffusers directly -and ultimately to replace the diffusers.

OTE (Oxygen Transfer Efficiency) data provides the only definitive indication of the efficiency of the air diffuser performance (other data is useful for other purposes but does not address the measurement of diffuser efficiency). The uniquely innovative pad of this invention is the determination of Oxygen Transfer Efficiency by simply combining a) dissolved oxygen and water temperature values from a dissolved oxygen sensor in the basin (NB basins all already have DO sensors in place) and b) a look-up table of values of a new dimensionless parameter obtained with a prior stress test of air diffusers (this table therefore takes account for example of airflow rate and pressure in the aeration system). The new methodology therefore enables information to be obtained on the performance of air-diffusers without the use of unwieldy hood and expensive apparatus which are required for a// existing methodologies for measuring OTE. With the new invention plant operators in the future will be able to monitor the efficiency of the submerged diffusers not only much more cheaply and easily than is currently possible -but also at any point where dissolved oxygen sensors are installed. This information is extremely valuable for the operator as not only does it enable the operator to maintain optimal aeration efficiency by optimising cleaning schedules for the diffusers (including determining what kind of cleaning is required and when) but also optimal maintenance and accurate monitoring data allows the life of diffusers to be extended whilst maintaining acceptable performance.

Description of Related Art

Background

Sewage is a water-carried waste, in solution or suspension, that is intended to be removed from a community. Also known as wastewater, it is more than 99% water and is characterized by volume or rate of flow, physical condition, chemical constituents and the bacteriological organisms that it contains.

The invention can be used in all types of biological reactors -including with all types of biological growth: a) Different types of biological reactors Wastewater biological reactors with submerged air diffusers can also be grouped into three main types according to their hydraulic characteristics and performances: i. In a complete-mixed reactor, the incoming wastewater is rapidly distributed throughout the tank so the concentration of a particular pollutant in the effluent will be the same throughout the basin. In such reactors, the oxygen demand and dissolved oxygen concentration (Co) are uniform throughout the basin.

ii. In the plug-flow reactor wastewater enters at one end and leaves at another end, with a continuous horizontal velocity in the liquid through the tank caused by gravity with little or no longitudinal dispersion of particles, although some mixing is caused by the horizontal flow of water. In the plug-flow reactor oxygen demand and dissolved oxygen concentration (Ci, C2, C3, CN) changes along the basin.

In a batch reactor wastewater is transferred into a tank which is then sealed for a period whilst the biomass functions. In batch-processing reactor (characterized by not steady-state conditions) the oxygen demand and dissolved oxygen concentration are changing with the time.

b) Different types of biological growth processes The removal of major constituents found in wastewater can be accomplished in a number of aerobic treatment processes which can be classified as either suspended growth or attached (fixed film) growth treatment processes. In suspended growth processes, the microorganisms responsible for treatment are maintained in liquid suspension by appropriate mixing methods. The most common suspended growth processes are the Activated Sludge Processes, Aerated Lagoons and Aerobic Digestion. In attached growth processes, the microorganisms responsible for the conversion of organic material or nutrients are attached to inert packing material. The most common reactors for supporting aerobic attached growth processes are the Packed-Bed Reactors and Granular Sequencing Batch Reactors.

This invention is concerned with both suspended and attached growth processes whenever air is provided by submerged air-diffusers (which is most often the case). The incoming settled wastewater may contain recycled sludge (biomass) recovered from the bottom layer of a secondary settler.

Aeration systems as a whole An aeration system can be broken into three parts: airflow generation, airflow distribution, and aeration control. Airflow generation consists of air blowers. Airflow distribution consists of air piping and air diffusers. Aeration control consists of blower control, blower control valves, diffuser control valves and dissolved oxygen control.

All four parts need to work well together for efficient plant operation.

An efficient and effective aeration system must also be capable of reliably and automatically varying the air supply to meet the variable diurnal loading, changing wastewater input constituents, seasonal ambient air conditions, and water temperature fluctuations in order to manage the energy consumption of the process with optimal efficiency.

DO control and its weakness The most common aeration control system involves choosing a Dissolved Oxygen (DO) set-point value and manually or automatically adjusting blower airflows to maintain DO at that set-point value, at the point in the basin where it is measured using a DO sensor. Effluent nitrogen levels are sometimes also monitored to refine blower airflows as needed to increase or decrease dissolved oxygen concentration. DO control as described above is indeed an indirect measure of adequate airflow to the biological process requirements. However, a DO sensor doesn't provide information on the respective performance of air-diffusers.

Conventional Off-gas testing and some of its weaknesses Conventional off-gas testing, was introduced by Redmon et al in 1983 to monitor the performance of air diffusers by measuring Oxygen Transfer Efficiency (OTE) at field conditions. The technique involves a floating hood to capture the gas emerging from the surface of the aerated basin, an oxygen detector able to analyse the fraction of oxygen content in air and a crew composed at least of two expert engineers.

These off-gas techniques provide a snapshot of OTE data only at the time of the visit from a costly specialist. Optimisation of the efficiency is attempted at that moment by adjusting both blowers and valves. However, this has inter alia the following weaknesses unless these measurements are carried out by the specialist frequently and regularly, decay in the performance of a diffuser may not be identified.

Jenkins attempt to overcome weaknesses of conventional off gas testing US2009230055 (Jenkins et al) attempts to overcome the lack of ongoing efficiency data by describing a method of using off-gas measurements to calculate OTE using a permanent fixed hood and a gas detector in order to optimise the aeration efficiency of a VV1/UTP. Although this does provide the ongoing data necessary for more effective monitoring of diffuser performance, it has its own considerable weaknesses: a) The hood itself is very expensive, and it is necessary to purchase a hood with annex 02 analyser for every basin to be monitored b) The hood is a cumbersome piece of apparatus which is permanently in the basin which creates substantial risks, both to any operator who must maintain the hood or for any breakage of the hood falling into the basin.

Bhaskar attempts to overcome weaknesses of conventional off-gas US 8221631 (Bhaskar et al) also includes a fixed hood in his description of a VWVTP system where DO is used to control the aeration of a basin. This is a slightly different approach to Jenkins. However, the requirement for a fixed hood means it suffers from the same 2 weaknesses of Jenkins as described above.

The invention overcomes the weaknesses of all the above approaches by calculating OTE on an on-going basis without the need for a fixed hood. The invention uses off-gas technique but does not require introducing any new fixed instrumentations into the basin. It requires only a first stress test at various airflow rates using off-gas technique with a floating hood in order to characterise the basin by calculating and using a new adimensional parameter for the basin. This provides a more complete picture of the aeration efficiency than conventional off-gas techniques, and thereafter OTE is calculated on an on-going basis simply from DO and water temperature data taken together with a look-up table set up using the data and parameter established in the stress test.

SUMMARY OF THE INVENTION

An embodiment of the invention therefore provides a method of obtaining a value for the oxygen transfer efficiency of submerged air diffusers in the wastewater of a wastewater aeration basins, wherein the wastewater can be aerated with a quantity of air provided by a controllable aeration system, the method comprising the steps of; aerating the wastewater, measuring an airflow value for the aerating air, measuring the dissolved oxygen content of the wastewater, measuring the temperature of the wastewater, retrieving from a reference source a previously stored special value corresponding to the measured airflow rate, calculating the oxygen transfer efficiency of the basin using the special value, the temperature of the wastewater and the dissolved oxygen content of the wastewater, wherein the reference source is obtained from a prior basin characterisation stage.

The method may include a basin characterisation stage for obtaining the reference source comprising; providing air to the wastewater at a selected airflow value, measuring and recording the selected airflow value, allowing the dissolved oxygen level to stabilise, measuring the stabilised dissolved oxygen level at the selected airflow value, calculating the oxygen transfer efficiency using the conventional off-gas technique, measuring the temperature of the wastewater at the selected airflow value, calculating the special value using the oxygen transfer efficiency, the dissolved oxygen level and the temperature of the wastewater, recording the special value corresponding to the selected airflow value, providing air to the wastewater at a plurality of further airflow values, and for each of the further airflow values repeating the measurements and calculations to obtain a special value for each further airflow value, compiling a table of special values corresponding to each of the selected airflow values.

The off-gas technique may includes measuring the proportion of oxygen and carbon dioxide in the atmosphere, measuring the proportion of oxygen and carbon dioxide in the gas emitted from the surface of the aeration basin at the selected airflow rate using a gas collecting hood, calculating the oxygen transfer efficiency of the wastewater using the oxygen and carbon dioxide proportions in the atmosphere and in the air emitted from the surface of the wastewater.

The stored values for the airflow values and corresponding special values may be interpolated to provide continuous data.

The oxygen transfer efficiency may be calculated as the product of the special value corresponding to the airflow rate and an oxygen factor, wherein the oxygen factor represents the ability of the wastewater to absorb oxygen and is calculated for a particular airflow value by taking the difference between the oxygen saturation at field conditions and the dissolved oxygen content of the wastewater and dividing this difference by the oxygen content in the air.

The special value may be calculated by taking the oxygen transfer efficiency calculated using the off-gas method and dividing this by the oxygen factor at the selected airflow value The airflow value may be the airflow rate or the airflux.

The temperature of the wastewater may be used to calculate the oxygen saturation at field conditions. The temperature of the airflow at an air inlet may be measured and used to calculate the oxygen content in the air.

The wastewater may be treated in a plug flow type basin having a plurality of diffuser zones, where the method steps of the embodiment are performed in each of the diffuser zones to calculate the oxygen transfer efficiency in each of the diffuser zones.

In a further embodiment of the invention, apparatus is provided for measuring the oxygen transfer efficiency of wastewater in a basin of a wastewater treatment plant, wherein the wastewater can be aerated with a quantity of air provided by a controllable aeration system, comprising; a dissolved oxygen detector interface for receiving data from a dissolved oxygen detector, a water temperature detector interface for receiving data from a water temperature detector, an airflow meter interface for receiving data from an airflow meter, a memory unit, a processor unit and an output interface, wherein the memory unit is arranged to store a reference source comprising reference data for a specific wastewater treatment basin, the reference data comprising special values and airflow rates, where each special value corresponds to a particular airflow rate, wherein the processor is arranged to receive an airflow value from an airflow meter when connected to the airflow meter interface, a dissolved oxygen value from a dissolved oxygen detector when connected to the dissolved oxygen detector interface and a water temperature value when a water temperature detector is connected to the water temperature detector interface, the processor further arranged to retrieve from the memory unit the special value corresponding to the input airflow when the memory unit is loaded with the reference source, the processor further arranged to calculate the oxygen transfer efficiency using the retrieved special value, airflow value, the dissolved oxygen value the water temperature value, and the processor further arranged to output the value of the oxygen transfer efficiency to the output interface.

An interface may be provided for receiving a value for air temperature from a temperature detector located at an air inlet and wherein the processor is arranged to use the value of air temperature in the calculation of oxygen transfer efficiency.

The reference source may be obtained using the method of the embodiment of the invention described above.

The output value of oxygen transfer efficiency may be displayed on a display or the output value of oxygen transfer efficiency used to control and/ or monitor operating parameters of the wastewater treatment plant. The operating parameters include airflow rate or sludge recycling rate. If the wastewater treatment plant is of the orbal biological type the operating parameters may be the airflow rate and the horizontal flow velocity. If the wastewater treatment plant is of the discontinuous biological reactor type the operating parameters may be the airflow rate and the feed rate as determined by the feed pump.

The apparatus may be remote from the basin.

The apparatus may include a dissolved oxygen detector, a water temperature detector and/or an airflow meter.

In a further embodiment a computer readable medium is provided containing a reference source of data obtained using the method of the above embodiment.

An embodiment of the invention therefore provides a method of managing the overall energy efficiency of delivering oxygen to a wastewater aeration plant, said aeration plant having at least one basin containing wastewater, wherein the wastewater is aerated with a quantity of air provided by a controllable aeration system, the aeration system comprising at least one air blower providing air to air diffusers submerged in the wastewater and wherein the amount of air is measured by airflow meter. The invention is directed to monitor and improve the efficiency of submerged air-diffusers by performing a stress test. The stress test is used to determine the dimensionless parameter referred to as a special value or the "Plano Number" essential for the on-going monitoring of diffuser efficiency and maintenance of air-diffusers.

Stress Testing (or load test) Controllable operational parameters such as blower speed, diffusers-valve opening, may be varied and the steps of measuring the proportion of oxygen and carbon dioxide in the gas emitted from the surface of the wastewater, measuring the amount of oxygen dissolved in the wastewater, measuring the temperature of the wastewater, optionally measuring the temperature of inlet airflow, measuring airflow, calculating the Oxygen Transfer Efficiency using the oxygen and carbon dioxide proportions, calculating Oxygen factor using the dissolved oxygen, oxygen saturation and oxygen content in the inlet airflow values, calculating a characteristic parameter by dividing the Oxygen Transfer Efficiency by the Oxygen factor.

Airflow rate may be increased stepwise and the DO measurement allowed to stabilise before changing the airflow rate again. The indicator of airflow rate may be the actual airflow rate as measured. For each airflow rate, airflux (airflow rate divided by total actual surface of air-diffusers) is calculated. The Plano Number plotted against the Airflux determines the variation of the efficiency of air-diffusers at changing airflow rate.

On-going monitoring and controlling of the aeration basin performance In a further embodiment, apparatus for monitoring and controlling the performance of a wastewater aeration basin on an on-going basis is provided, the apparatus may include a dissolved oxygen sensor, a temperature sensor for sensing the temperature of the wastewater, optionally a temperature sensor for sensing the temperature of the air in the inlet air pipe, optionally a pressure sensor for sensing the pressure in the atmosphere, a data processor, a memory arranged to store at least one basin-specific parameter, wherein the data processor is arranged to receive as input dissolved oxygen values from the dissolved oxygen sensor, wastewater temperature values from the temperature sensor (normally included in the DO sensor), optionally atmospheric pressure values from a pressure sensor, optionally air temperature values of inlet airflow from a temperature sensor (normally included in the airflow meter), airflow rate values from an airflow meter and to calculate a value for Oxygen Transfer Efficiency from concurrent dissolved oxygen concentration, wastewater temperature and oxygen content in the incoming air values and a one basin-specific parameter may be obtained from a look-up table derived from the stress testing. The data processor may be arranged to output the calculated Oxygen Transfer Efficiency. The apparatus may include a display for displaying the Oxygen Transfer Efficiency. The output of this graph is used to determine the cleaning time of submerged air diffusers. The data processor may be located remotely from the wastewater aeration basin.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a graph showing the relation between the Plano Number and the Airflux.

Figure 2 is a schematic view of an aeration tank showing the apparatus used during the stress test with airflow rate measurements.

Figure 3a is a graph of airflow rate against time, which shows an example of airflow rate profile applied to a basin over time.

Figure 3b is a graph of dissolved oxygen against time which shows how dissolved oxygen varies with time for the airflow rate profile shown in Figure 3a.

Figure 3c is a graph of Oxygen Transfer Efficiency against time which shows how Oxygen Transfer Efficiency varies with time for the airflow rate profile shown in Figure 3a.

Figure 3d is a graph of Airflux against time which shows how Airflux varies with time for the airflow rate profile shown in Figure3a.

Figure 3e is a graph of Oxygen factor against time which shows how Oxygen factor varies with time for the airflow rate profile shown in Figure 3a.

Figure 3f is a graph of Plano Number (calculated from the values of OTE and Oxygen factor shown in Figures 3c and 3e) against time for the airflow rate profile shown in Figure 3a.

Figure 4 is a graph of Oxygen Transfer Efficiency against time. This Figure shows the comparison between the OTE measured with the normal off-gas technique and the OTE computed by Equation 16 Figure 5 is a schematic view of an aeration tank showing the apparatus used during the on-going monitoring of air diffusers maintenance, aeration system and biological activity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Introduction

Background

Most biological wastewater treatment plants employ aerobic processes to ensure effluent quality standards fixed by the law. Aeration is the highest energy footprint for municipal and industrial wastewater treatment plants, ranging from 50 to 75% of total energy consumption.

In the last two decades, with the increasing energy costs, more efficient fine pore diffusers have replaced coarse pore diffusers and surface aerators. Although fine pore diffusers can improve the energy efficiency of aeration basins in the short term, they are subject to fouling and scaling, resulting over the time in increased head loss and reduced Oxygen Transfer Efficiency, both contributing ultimately to increased plant energy costs and costs of regular cleaning and downtime. The determination of optimum diffuser cleaning frequency is an object of an embodiment of the present invention.

When evaluating a given aeration system (blower + air pipes + air diffusers + diffuser valves + blower valve), a number of factors intrinsic to the aeration devices will affect the performance of diffusers including the process layout, the mode of operation of process, the control methodology used and the maintenance of the equipment. For submerged air diffusers these factors include: diffuser type, diffuser placement, diffuser density, gas flow rate per diffuser, basin geometry, wastewater composition, process type and flow regime, process loading, DO control, deterioration of diffusers, mechanical integrity of diffusers.

Definition of main existing Aeration Parameters 1. Oxygen Transfer Rate (OTR) Oxygen Transfer Rate (OTR) is related to the amount of air supplied in a wastewater aeration basin in a given time and the Oxygen Transfer Efficiency (OTE), by the equation: OTR=(pw.),"2.)* Qin).* OTE (1) At standard conditions, i.e. clean water at 20 deg C, by: SOTR =(p" ",)* Qa"* SOM (2)

At field conditions for used air diffusers by:

(aF)SOTR=(p 02)* Q * (aF)SOTE (3) where: oxygen content in the air, (0.299 at Normal Conditions) [kg m-3].

alpha factor, (0.3 -0.8) adimensional fouling factor of air-diffusers, (0.7 -0.9) adimensional airflow rate. Airflow rate values are normally provided in normal meter cubic per hour, [Nm3 * OTE = Oxygen Transfer Efficiency, [%] OTR is effected by, a) the bubble's size created by the air diffusers (which depend on the gas velocity, type of diffusers, maintenance of diffusers, hydrodynamic conditions of the basin etc.), b) the parameters of the water (temperature, impurity, viscosity, depth etc.) and c) the actual dissolved oxygen concentration.

2. Oxygen transfer efficiency (OTE) OTE is the percentage of total oxygen input into the basin which is dissolved into the water and can be calculated as an average across the whole basin or calculated for a specified sub-area of the basin. OTE makes visible how much of the oxygen input is wasted in terms of oxygen content in the incoming air not transferred into the water,%. OTE is a useful measurement parameter because it can be measured directly at field conditions using the off-gas technique. It can be measured using a floating hood placed on the surface of the wastewater to collect gas and an oxygen detector able to measure the percent of oxygen in the leaving air from the hood. OTE measured in a specific location of the aeration basin is computed by the following well known equation: MR -MR. (4) OTE - ' MR, where: * MRi = molar ratio of inlet oxygen to the inert gas fractions. It is a-dimensional * MR, = molar ratio of outlet oxygen to the inert gas fractions. It is a-dimensional MR, 1- Yco," -YET 20,1 Y, is the fraction of oxygen of inlet airflow, %, Yco2i is the fraction of CO2 at the inlet, YH201 is the fraction of water at the inlet. MR, -

I Ye YCO, Y11,0" Y, is the fraction of oxygen of out airflow, %, Yccce is the fraction of CO2 at the outlet, YH20e is the fraction of water at the outlet.

At standard conditions and in process water, Oxygen Transfer Efficiency is given by: (aF)SOTE = OTE C,2o 0(20rc-T,F) cs*,T-co where: * Cs.r* = oxygen saturation at field conditions, [mg El] * Cs20= oxygen saturation at standard conditions, [mg 1-1] * co = dissolved oxygen measured in the basin, [mg 11 * 0 = temperature correction coefficient, a-dimensional * T = wastewater temperature, [°C] (5) (6) (7) 4. Mass transfer coefficient (kLal Each diffuser has its own unique field mass transfer coefficient (kLa) that depends on number of factors like submergence of aerators, material of the diffuser, diameter of bubbles, intensity of turbulence, wastewater and temperature of the water. The mass transfer coefficient, otherwise known as the oxygen transfer coefficient, kLa, is the product of the liquid film coefficient, kL and the interfacial area exposed to transfer in a given liquid volume, a. In all but the simplest systems, the individual values, kL and a, are impossible to individually measure. Incorporating them into one coefficient, kLa, provides the ability to obtain a measurable value in complex field aeration systems. The most applied technique to measure the performance of diffusers is the absorption method (ASCE, 1986). The oxygen transfer coefficient, kLa is not easy to be measured in a full-scale wastewater treatment plant.

5. Dissolved oxygen saturation at field conditions

Dissolved oxygen saturation concentration, Cs, in the water is a function of temperature, the partial pressure of the gas over the liquid and chloride concentration. The chloride concentration in fresh water is negligible, so a non-linear oxygen at saturation in clean water and at 1 atm was developed by Wang (1977): C, =14, 61996 -0, 40420 * T,, + 0, 00842 * -0, 00009 * (8) where Tw is the temperature of wastewater expressed in °C. The model has been demonstrated to be accurate to 0.01 mg 1-1. For pressure other than 1 atm and in process water the saturation concentration of oxygen Cs*, is given by the published equation: (Pw,T*9*I*11+Patm-PH20 (9) Patm-Pri 2 OSt where:

* P atmosphere pressure at field conditions, [Pa]

* atm, * P atm,st atmosphere pressure standard conditions, [Pa] * PH20, PH20,st vapour pressure at field and standard conditions, [Pa] * H = depth of aeration basin, [m] * f = 0,325 -0.5, adimensional p = 0.95 -0.99, adimensional * = weight of the wastewater, [Pa] * g = gravity acceleration, 9.807 [m S-2] The density of the wastewater and the vapour pressure are affected by the temperature as shown in the following equations: MA'.0 p,,,7 (10) where: * density of water at zero degrees, [kg m"3] * T,,T = water temperature at field conditions, [°C] * E = volumetric temperature expansion coefficient, [1.113 nt3 °C-1] 7.31.3: L. CA 57 35.72.37:Tj)+ 7235 3.5-173,43,30 PH2O = (273.15+ ilvf)'2 where: * Tw,T water temperature, [°C] Definition and uses of new aeration a-dimensional parameters: Plano Number (Pi) Plano Number (Pa Plano Number can be determined in full-scale wastewater treatment plants by a group of measureable parameters as follows: P = PatrY°2 OTE (c;T-co) We call the first part of Equation 12 Oxygen factor (f02) defined as follows: f02 (c,-Co) PairY02 PairY02 8.31441*Tair where: * Lir = air temperature, [kelvin] and it is a dimensionless parameters which represents the capacity of the mixed liquor to absorb oxygen at given oxygen content in the incoming airflow. Therefore, 20 Plano Number can be expressed as follows: pairy02

OTE =

f*H+Patm-PkPw'r "2°) Co] Patm-PH20 As shown in Figure 1, for values of Airflux between 4 and 20 Nm3 m-2 h-1 (Airflux can be expressed in kg02 m211-1 if thermal mass airflow meter are used). Plano Number is constant and beyond that, the Plano Number starts to decrease. This change is related to the phenomenon of zone movement of ascending bubbles and coalescence of bubbles. In other words the volume of air retained in the aerated column grows in proportion to the airflow rate until a certain value. Further increases in the airflow rate result in decelerated increases in the air retained in the water.

0.2095*32'-Patm (12) (13) (14) (15) Description of the detailed methodology and apparatus Stress testing (or Load Test) Stress testing for optimising the whole aeration system aims to determine the look-

up table.

A possible set-up is shown in Figure 2, where a floating hood and a portable DO sensor are deployed to take measurements at a single point.

The OTE, dissolved oxygen concentration, water temperature and air temperature in the inlet airflow (optionally), are measured at one or more points in the basin and the Plano number calculated using Equation 15. These measurements and calculations are repeated for variations of increasing and reducing of airflow.

Figure 3a shows the variation of airflow vs time profile. Figure 3b shows how measured DO varies with time, when the airflow vs time profile of Figure 3a is used, while Figure 3c shows how the calculated OTE varies with time for the same airflow vs time profile. Figure 3d shows the variation of Airflux versus time profile for the same airflow vs time profile. Figure 3e show how Plano Number varies with time for the same airflow variation.

On-qoing monitoring of oxygen transfer efficiency After the Plano Number has been determined during the stress test, the OTE (ongoing) can be calculated by using the following equation: (14.61996-0.40420-Tw+0.00842.Tw2-0.0009*Tw3)-Pw*Trg 1 f 1 H+Parm Pri2 0 c OTE, Patm,st-A H 20.st 0.2095.32,1'am Pt (16) S.31441,T0tr Parameters Calculated Measurements Constant 9 - 9.807 f 0.375 Patm st 101,325 PH2Osi 2,330 H - From Basin Design Spec - Pw,T Eq.10 Water Temperature - PH20 Eq.11 Water Temperature - PI Look-up Table Airflow meter - Patm - Atmosphere Pressure -Tair - Air Temperature Ty, Water Temperature C. Dissolved Oxygen Therefore, a single unit may be provided which comprises a DO sensor and temperature sensor and a processor (Figure 5), where the processor is enabled to take the airflow measurements to determine the Plano Number from the look-up table and DO, atmosphere pressure and temperature values to compute Oxygen factor and output OTE. Alternatively, airflow rate, atmosphere pressure, DO and temperature values may be transmitted to a remote location, perhaps in the plant control room, or even a remote monitoring service provider using the Internet, where parameters including Oxygen factor and OTE can be calculated. For high accuracy, thermal mass airflow meters for the determination of Plano Number are strongly recommended.

The output values may be used to control the VVVVTP system, including air blowers, air-valves, pumps, impellers etc to maintain the most efficient operating conditions, or to monitor on-going efficiency and for example warn when the diffusers need to be cleaned.

Claims

CLAIMS1. A method of obtaining a value for the oxygen transfer efficiency of submerged air diffusers in the wastewater of a wastewater aeration basins, wherein the wastewater can be aerated with a quantity of air provided by a controllable aeration system, the method comprising the steps of; aerating the wastewater, measuring an airflow value for the aerating air, measuring the dissolved oxygen content of the wastewater, measuring the temperature of the wastewater, retrieving from a reference source a previously stored special value corresponding to the measured airflow rate, calculating the oxygen transfer efficiency of the basin using the special value, the temperature of the wastewater and the dissolved oxygen content of the wastewater, wherein the reference source is obtained from a prior basin characterisation stage.
2. A method of obtaining a value for the oxygen transfer efficiency of air diffusers in accordance with claim 1, wherein the basin characterisation stage for obtaining the reference source comprises; providing air to the wastewater at a selected airflow value, measuring and recording the selected airflow value, allowing the dissolved oxygen level to stabilise, measuring the stabilised dissolved oxygen level at the selected airflow value, calculating the oxygen transfer efficiency using the conventional off-gas technique, measuring the temperature of the wastewater at the selected airflow value, calculating the special value using the oxygen transfer efficiency, the dissolved oxygen level and the temperature of the wastewater, recording the special value corresponding to the selected airflow value, providing air to the wastewater at a plurality of further airflow values, and for each of the further airflow values repeating the measurements and calculations to obtain a special value for each further airflow value, compiling a table of special values corresponding to each of the selected airflow values.
3. A method of obtaining a value for the oxygen transfer efficiency of air diffusers in accordance with claim 2, wherein the off-gas technique includes measuring the proportion of oxygen and carbon dioxide in the atmosphere, measuring the proportion of oxygen and carbon dioxide in the gas emitted from the surface of the aeration basin at the selected airflow rate using a gas collecting hood, calculating the oxygen transfer efficiency of the wastewater using the oxygen and carbon dioxide proportions in the atmosphere and in the air emitted from the surface of the wastewater.
4. A method of obtaining a value for the oxygen transfer efficiency of air diffusers in accordance with claim 3, wherein the stored values for the airflow values and corresponding special values are interpolated to provide continuous data.
5. A method of obtaining a value for the oxygen transfer efficiency of wastewater in accordance with any preceding claim, wherein the oxygen transfer efficiency is calculated as the product of the special value corresponding to the airflow rate and an oxygen factor, wherein the oxygen factor represents the ability of the wastewater to absorb oxygen and is calculated for a particular airflow value by taking the difference between the oxygen saturation at field conditions and the dissolved oxygen content of the wastewater and dividing this difference by the oxygen content in the air.
6. A method of obtaining a value for the oxygen transfer efficiency of wastewater in accordance with any one of claims 2 to 5, wherein the special value is calculated as the oxygen transfer efficiency calculated using the off-gas method divided by the oxygen factor at the selected airflow value.
7. A method of obtaining a value for the oxygen transfer efficiency of wastewater in accordance with any preceding claim, wherein the airflow value is the airflow rate or the airflux.
8. A method of obtaining a value for the oxygen transfer efficiency of wastewater in accordance with any preceding claim, wherein the temperature of the wastewater is used to calculate the oxygen saturation at field conditions.
9. A method of obtaining a value for the oxygen transfer efficiency of wastewater in accordance with any preceding claim, wherein the temperature of the airflow at an air inlet is measured and used to calculate the oxygen content in the air.
10. A method of obtaining a value for the oxygen transfer efficiency of air diffusers in accordance with any preceding claim, wherein the wastewater is treated in a plug flow type basin having a plurality of diffuser zones, and the method steps of claims 1 to 9 are performed in each of the diffuser zones to calculate the oxygen transfer efficiency in each of the diffuser zones.
11. Apparatus for measuring the oxygen transfer efficiency of wastewater in a basin of a wastewater treatment plant, wherein the wastewater can be aerated with a quantity of air provided by a controllable aeration system, comprising; a dissolved oxygen detector interface for receiving data from a dissolved oxygen detector, a water temperature detector interface for receiving data from a water temperature detector, an airflow meter interface for receiving data from an airflow meter, a memory unit, a processor unit and an output interface, wherein the memory unit is arranged to store a reference source comprising reference data for a specific wastewater treatment basin, the reference data comprising special values and airflow rates, where each special value corresponds to a particular airflow rate, wherein the processor is arranged to receive an airflow value from an airflow meter when connected to the airflow meter interface, a dissolved oxygen value from a dissolved oxygen detector when connected to the dissolved oxygen detector interface and a water temperature value when a water temperature detector is connected to the water temperature detector interface, the processor further arranged to retrieve from the memory unit the special value corresponding to the input airflow when the memory unit is loaded with the reference source, the processor further arranged to calculate the oxygen transfer efficiency using the retrieved special value, airflow value, the dissolved oxygen value the water temperature value, and the processor further arranged to output the value of the oxygen transfer efficiency to the output interface.
12. Apparatus in accordance with claim 11, wherein an interface is provided for receiving a value for air temperature from a temperature detector located at an air inlet and wherein the processor is arranged to use the value of air temperature in the calculation of oxygen transfer efficiency.
13. Apparatus in accordance with claim 11 or 12, wherein the reference source is obtained using the method of claims of 2 to 10.
14. Apparatus in accordance with any one of claims 11 to 13, wherein the output value of oxygen transfer efficiency is displayed on a display.
15. Apparatus in accordance with any one of claims 11 to 14, wherein the output value of oxygen transfer efficiency is used to control and/ or monitor operating parameters of the wastewater treatment plant.
16. Apparatus in accordance with claim 15, wherein the operating parameters include airflow rate or sludge recycling rate.
17. Apparatus in accordance with claim 15 or 16, wherein the wastewater treatment plant is of the orbal biological type and the operating parameters include the airflow rate and the horizontal flow velocity.
18. Apparatus in accordance with claim 15 or 16, wherein the wastewater treatment plant is of the discontinuous biological reactor type and the operating parameter are the airflow rate and the feed rate as determined by the feed pump.
19. Apparatus in accordance with any one of claims 11 to 18, wherein the apparatus is remote from the basin.
20. Apparatus in accordance with any one of claims 11 to 19, wherein the apparatus includes a dissolved oxygen detector.
21. Apparatus in accordance with any one of claims 11 to 20, wherein the apparatus includes a water temperature detector.
22. Apparatus in accordance with any one of claims 11 to 21, wherein the apparatus includes an airflow meter.
23. A computer readable medium containing a reference source of data obtained using the method of claims 2 to 10.