TAKEISHI Masashi1 OGATA Yoshikazu1 TSURUNOSONO Ryuuichi1 KIYONO Shinjirou1
The development of the MLSS analyzer was an important breakthrough in process analyzers for the sewage field. The latest model, the SS400 MLSS analyzer, provides highly reliable measurement over concentrations ranging from 0-500 mg/ℓ to 0-20000 mg/ℓ. The measurement of low concentrations was made possible by adopting a measurement method that compares transmitted light with scattered light. The effect of disturbance light was eliminated from the measurement by devising an optical system that uses pulse-driven light emission. During this development, we also reviewed the maintainability of the liquid analyzer and introduced measures such as a self-cleaning floating holder and simplified calibration to facilitate maintenance management. This paper reports on the characteristics and technical background of the SS400 MLSS analyzer.
- Environmental & Analytical Product Business Division
INTRODUCTION
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| Figure 1 Flow of the Standard Activated-sludge Process |
Recently, the focus of the sewage treatment industry has started to shift away from "quantity" and towards "quality." Until as recently as a few years ago, the greatest priority was the construction of sewage treatment facilities in various parts of Japan as a means of dispersing sewage systems. These days, public attention has been drawn to the treatment methods as well as the facility construction, for reasons of environmental preservation and improved water quality. This has resulted from an addition to the list of items prescribed by environmental and emission standards and tighter government regulation. In order to comply with the regulation, the majority of plants have adopted more advanced treatment technology.
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| Figure 2 External View of the SS400 MLSS Analyzer |
Almost all of the Japanese sewage treatment methods are based on the activated-sludge process that uses active sludge (i.e., an aggregate of microbes and primitive creatures that feed on organic substances or other sources of nutrition). Among the various types of process, the standard activated-sludge process has been in wide use among the large- and medium-scale facilities within the largest cities in Japan for a long time. This process feeds sewage and active sludge into the aeration tank, as shown in Figure 1. The tank is then agitated under a supply of air so that organic matter included in the sewage is decomposed and removed.
In recent years, many medium- to small-scale facilities in rural areas have adopted an advanced process in which the decomposition cycles of both aerobic and anaerobic microbes are used to remove nitrogen and phosphorous, as well as the organic matter in the sewage. Treatment plants using the standard activated-sludge process also support this advanced process by separating their aeration tank into aerobic and anaerobic tanks.
The critical question then is "what is it that is needed in this age of quality?" The answer is highly reliable water quality control and hence a process liquid analyzer that allows us to know the precise level of water quality.
From the viewpoint of enhanced reliability, maintainability and controllability, this paper describes the improved functionality of the recently developed MLSS analyzer, which is an important process analyzer in the field of sewage treatment. It also discusses the improved functionality of such accessories as the sensor holder that accompanies the analyzer.
FEATURES OF THE SS400 MLSS ANALYZER
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| SS = Concentration of suspended solids m = Scatter factor C = Concentration of coloring components k = Absorption factor f = Shape factor due to aperture |
| Figure 3 Measuring Principle of the SS400 MLSS Analyzer |
As discussed earlier, active sludge is absolutely essential for sewage treatment. The concentration of the sludge is defined as the MLSS in the Japanese Industrial Standard JIS B 9944, "Testing Methods of Activated Sludge Process Equipment," and is one of the most important managerial indexes for consistent sewage treatment. MLSS, which stands for mixed liquor suspended solids, is the concentration of suspended matter in an aeration tank of an active sludge treatment system. The MLSS analyzer is an instrument used to continuously measure the concentration by taking advantage of the scatter and attenuation of light.
The measuring principles of modern MLSS analyzers include the transmitted light method, scattered light method, scattered light comparison method and transmitted/scattered light comparison method. Each method has both advantages and disadvantages in terms of their operating principle.
The recently developed SS400 MLSS analyzer uses the scattered/transmitted light comparison method, rather than the conventional scattered light comparison method. This enables it to analyze objects of low concentrations that were difficult to measure using conventional methods. Furthermore, we were successful in eliminating various factors that served as disturbances in MLSS measurement. This has improved the reliability of the analyzer. To ascertain whether the maintainability and controllability of instruments for analyzing water quality improved, we reviewed the maintainability of existing analyzers, in regard to the cleaning and calibration methods. Figure 2 is an external view of the MLSS analyzer.
The MLSS analyzer features:
- a wide measuring range from low concentrations in the 0-500 mg/ℓ range to high concentrations in the 0-20000 mg/ℓ range;
- a reduced effect from external light;
- a reduced effect from light reflected at the boundary surface;
- an extended service life of the light source; and
- a simplified calibration procedure.
The technical background of these features is detailed in the following sections.
MEASURING PRINCIPLE OF THE SS400 MLSS ANALYZER
Figure 3 illustrates the measuring principle of the SS400 MLSS analyzer.
Light beams (at incident light intensity I0), when emitted into the liquid under test, scatter and are absorbed because of the suspended substances (at concentration SS) and coloring components (at concentration C) in the liquid. From the scattering theory of Rayleigh and Mie and the transmission theory of Lambert-Beer, scattered light J1 and transmitted light J2, which has passed through the liquid and thereby become attenuated, are determined by:
J1 = f1 [I0 exp(-kCl1) exp(-mSSl1) mSSl] exp(-kCl3) exp(-mSSl3)
and
J2 = f2 I0 exp[-kC (l1 + l2)] exp[-mSS(l1 + l2)]
Now, we find the ratio of the scattered light to transmitted light (J1 /J2) to obtain:
J1 /J2 = (f1 /f2) (mSSl) exp[kC(l2 - l3)] exp[mSS(l2 - l3)]
From these, we can determine the positioning of the photodetector unit by assuming (l2 = l3) and making the following formula hold true.
SS = [f2 /(f1 ml)] (J1 /J2)
This means that the concentration SS is always proportional to the ratio J1 /J2, irrespective of the incident light intensity and the concentration of coloring components, if the liquid under test (scattering factor m) is uniform.
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| Figure 4 Output Characteristics of Transmitted and Scattered Light |
Figure 4 shows the output characteristics of scattered and transmitted light that were obtained in response to the concentration SS of kaolin, a standard turbidimetric substance. Figure 5 is a graph showing the output characteristics of the ratio of transmitted light to scattered light.
The output level of transmitted light falls logarithmically as concentration SS increases. In contrast, the output level of scattered light rises over low concentrations as concentration SS increases. When concentration SS becomes high, the output level of scattered light reaches a point of saturation and starts to fall.
Theoretically, the ratio of scattered light to transmitted light (J1 /J2) is assumed to have a linear relationship with concentration SS. The actual result however is that the relationship is represented as a curve having no inflection points, as shown in Figure 5. This is due to the secondary and multiple scattering of light that take place as a result of an increase in concentration SS. This curve can be reproduced as long as the suspended solids being measured share the same characteristics. For this reason, the SS400 MLSS analyzer is designed to base its calculation of the concentration of suspended solids on this curve relationship when processing the signals of transmitted and scattered light coming from the detector.
As discussed above, the SS400 MLSS analyzer can measure low concentrations in the 0-500 mg/ℓ range and high concentrations in the 0-20000 mg/ℓ range, which both proved difficult to measure using the existing MLSS analyzer.
REDUCED EFFECT OF DISTURBANCE FACTORS
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| Figure 5 Characteristics of Ratio of Transmitted Light to Scattered Light |
Normally, aeration tanks based on the standard activated- sludge process are constantly exposed to aeration and agitation. The concentration of suspended solids, therefore, does not decrease much and is mostly unaffected by external light (sunlight and indoor light) during MLSS measurement.
In modern treatment plants that use such advanced treatment methods as the activated-sludge batch process, tank aeration and agitation and the sedimentation of active sludge are all carried out in the same tank. Consequently, depending on where the MLSS analyzer is positioned, the results of measurement may be affected by external light when, during sedimentation, there is a decrease in the concentration of suspended solids or when light is reflected at the boundary surface between the layers of supernatant liquid and active sludge.
- Reduced Effect of External Light
The SS400 MLSS analyzer is equipped with a visible light cutoff filter (for wavelengths shorter than 800 nm) in its photodetector unit and a near-infrared LED (880-nm emission wavelength) in its light-source unit, both of which eliminate the effects of visible disturbing light. In addition, the analyzer uses a pulse-driven light emission method to eliminate signals resulting from disturbing light, by finding the difference between the photoelectric signal V1 for the active-light-source level E1 and the photoelectric signal V0 for the inactive-light-source level E0, as shown in Figure 6. - Reduced Effect of Light Reflected at Boundary Surface
In earlier MLSS analyzers, the way their optical system was arranged was somewhat susceptible to the effects of the light reflected at boundary surfaces that occur between the layers of supernatant liquid and active sludge during sedimentation. In the case of the SS400 MLSS analyzer, its optical system is arranged as shown in Figure 7, so that any light reflected at the boundary surface does not enter the photodetector unit. This construction reduces the effects of light reflected off the boundary surface.
MAINTENANCE AND CONTROL
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| Figure 6 Effects of Disturbing Light in Pulse-driven Light Emission |
1. Calibration
Unlike in other water quality analyzers, no standard substance is defined as the reference substance for MLSS analyzers. Accordingly, in order to precisely measure concentrations, the MLSS analyzer must be calibrated using an actual process liquid during its startup. The procedure for calibrating conventional MLSS analyzers involves diluting the liquid under test in order to create a calibration curve on which analyzer calibration can be based.
This procedure is both time consuming and requires a great amount of labor. The SS400 MLSS analyzer, on the other hand, uses kaolin (a standard turbidimetric substance that has a correlation with active sludge) as its reference. The analyzer therefore contains the calibration curve of kaolin in its converter, enabling calibration to be done in only one-third of the time required for the earlier procedure. What the operator has to do in actual calibration is simply set the sensor parameters for compensating sensor-to-sensor variations and perform three- point calibration (sensitivity calibration) using the liquid under test.
Periodic maintenance is essential for the successful maintenance and control of an MLSS analyzer over a long period. Once calibration based on an actual process liquid is carried out, simplified calibration using a calibration plate will suffice for all subsequent applications unless there is a change in the properties of the liquid under test. In the simplified calibration procedure, span (sensitivity) calibration can be readily done by attaching the calibration plate, a standard accessory, to the detector. This strategy dramatically reduces the time required for maintenance work.
Moreover, replacing the conventional tungsten-lamp light source with a near-infrared LED light source has eliminated the need to periodically replace the light source. The new light source also helps prevent the occurrence of algae, one of the sources of contamination, thereby reducing the frequency of instrument cleaning.
2. Sensor Holders and Cleaning Units
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| Figure 7 Comparison of Optical System Arrangements |
In this subsection, we discuss the sensor holders and cleaning units that help improve the way water quality analyzers are maintained and controlled. In conventional MLSS analyzers, the sensor was integral with the holder. Accordingly, users had to lift the 2-m high holder each time the sensor had to be maintained. In contrast, the SS400 MLSS analyzer is designed so that the sensor and holder can be separated, allowing users to choose a holder best suited for their respective treatment plants. The latest lineup of holders includes a floating ball holder and a suspension holder, in addition to the conventional immersion holder and the free- standing holder with a guide-pipe, thus increasing the range of holder options available.
Among the various types, the floating ball holder is unique as its design contains a sensor in its spherical float and permits the analyzer to measure concentrations by keeping track of changes in the liquid level. Moreover, in applications involving a constant flow of liquid such as for the optical density (or OD) process or standard activated-sludge process, very little dirt accumulates on the holder's wetted pat, thanks to its self-cleaning action. This helps reduce the frequency of cleaning. In a field test using the floating ball holder, we did not observe any unreasonable drop in display readings even during two months of uninterrupted operation without cleaning the sensor. Thus, the test confirmed this holder's effectiveness for reducing the frequency of cleaning (i.e., maintenance).
The self-cleaning action of the floating ball holder is not available, however, in treatment plants based on the activated- sludge batch process that involve temporary stops of liquid flow. In this case, the holder must be cleaned periodically. The automatic cleaning units, available either as a jet cleaning unit or a wiper cleaning unit, can be attached to a suspension holder or an immersion holder. Another field test was conducted using a suspension holder equipped with a wiper cleaning unit at a treatment plant based on the activated-sludge batch process. When the wiper cleaning unit was set off, a drop in readings began only one week after the start of operation, compared to no drop in reading even after one month of operation when the holder was cleaned once every six hours using the wiper cleaning unit. This test also confirmed the holder's effectiveness for reducing the frequency of maintenance, which is a direct result of its automatic cleaning unit.
As discussed above, consistent long-term measurement is possible by choosing the optimum holder and cleaning unit for each treatment plant. This approach ensures improved reliability and reduces the amount of work required for the maintenance and control of water quality analyzers.
CONCLUDING REMARKS
As discussed in this paper, we have made functional improvements to our existing MLSS analyzer, with the aim of enhancing the reliability of these water quality analyzers and improving the way they are maintained and controlled. As discussed, the SS400 MLSS analyzer can perform highly reliable measurement over both low and high concentrations. The analyzer also has the ability to dramatically reduce the amount of time required for periodic maintenance, such as cleaning and calibration. Consequently, we are confident that the analyzer will help increase the reliability of water quality control and produce labor savings in field applications. We do not intend to rest with these achievements, but shall continue to research ways to further improve instrument maintenance and control methods.
