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This paper describes a method of evaluating the biological reaction of the activated sludge process based on the respiration rate. We have developed a respiration rate measuring device (rr analyzer) for process control, and verified its effectiveness through continuous measurements at a municipal wastewater treatment plant. INTRODUCTIONThe activity has been regarded as key to the evaluation of the substrate removal capability cf an activated sludge process, and in particular at an aeration tank. Some proposed measuring methods have been made, such as ATP activity, respiration rate, etc. Though some devices have been devised to automatically measure rr in activated sludge, few automatic measuring devices have been available for long-term monitoring at a sewage treatment plant or process controls. Against this technical background, the authors have developed a respiration rate measuring device (rr analyzer), employing a DO meter, for long-term use in such a process. This device has made continuous measurements at a municipal wastewater treatment plant (processing capacity: 30,000 m 3 day" 1 ). As a result, we have disclosed that rr activity can be an adequate index for evaluating the effect of DO control, and that it is useful for the supervision of a proper DO setting value. This fact has been made clear through measurements and analyses by a mathematical model, to be outlined below. Fig. 1 shows the construction of the rr analyzer sensor. The sensor block consists of DO electrodes with a temperature compensator, a measuring tank equipped with an agitator, and two pinch valves (Vj and Vj)• The principle of rr measurement is based on a method deriving from the Standard Methods for the Examination of Sewage (1). The procedure for rr measurement and operation is as follows: RESPIRATION RATE MEASURING SYSTEM1) Feed air from the bottom part of the DO meter and sample an activated sludge mixed liquor into the measuring tank in the air-lift mode. 2) Shut pinch valve V 2* 3) Maintain aeration until a required DO level (about 5 mgZ~*) is attained. Inlet Outlet Fig. 2 Schematic representation of DO distribution in the aeration tank , (1) Inlet of air (2) DO electrode @ Agitator (3) Pinch valve (V[) (5) Pinch valve (V 2 ) (6) Measurement tank (7) Inlet of sampling (8) Outlet of sampling Fig. 1 Sensing element of the rr analyzer 4) At the required DO level, stop aeration and remove bubbles from the measuring tank. Close pinch valve Vj. 5) Stirring activated sludge in the measuring tank with the agitator, check the decreasing mode of DO concentration within the measuring tank, using the DO meter. 6) Calculate the amount of decrease in DO concentration per unit of time. The resultant figure is the rr value.This measuring procedure of 1) **» 6) is usually repeated. When intending to repeat measurements on the same sample, however, a repetitive measurement sequence for 3) * 6) has been adopted. DO SETTING VALUES AND DO DISTRIBUTION CHARACTERISTICS WITHIN THE AERATION TANKIn the case of uniform aeration in a plug flow, the DO dist...

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