Henriksdal sewage treatment plant is the largest plant in Stockholm with a design flow of 370 000 m3/d. In one aeration tank of eleven a new fine-bubble aeration system has been in operation since August 1985. The tank is divided into 6 equal parts. The first part is an anoxic zone and the other five are aeration zones with tapered diffusers. Several instruments are installed in the block including separate air flow monitors in each of the five zones and D.O.-probes in the inlet and outlet of the zones. Equipment for flow measurement of settled sewage and return sludge is also installed. Every instrument is connected to a computer for data acquisition.

To evaluate the efficiency of the aeration system the oxygenation transfer capacity has been calculated from the oxygen massbalance equation for each zone as a function of air flow. To solve this equation the respiration has to be known and this is done by a simple respirometer for samples of the MLSS in each zone.

When the KLa-values are known as functions of the air flow the mass balance equation can be used to calculate the respiration rate in each zone. The computer has been logging data for 2 2 months, and it is possible to calculate the respiration rates in the different zones every hour during this period.

It is very important to know the respiration along the tank and how it varies to get the optimal tapering of the diffusers when it is time to change the aeration system in the other 10 tanks. The calculations show a different pattern in the respiration over the year depending on the rate of nitrification. Another use of the calculation of the oxygenation transfer efficiency is to recognize if any long-term change occurs due to clogging of the diffusers.

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