Determination of the enzyme activity of activated sludge by methylene blue reduction

Many attempts have been made to find simple and reliable methods for determining the activity of activated sludge. The most simple method is a measure of mixed liquor suspended solids (MLSS) or mixed liquor volatile suspended solids (MLVSS). Neither of these measurements, however, measures the cell activity but only the cell weight, and they do not dis tinguish between living and dead cells. More specific methods for measuring the active cell mass have been used by several workers, methods ranging from stan dard plate counts to the measuring of the cell constituents present only in living cells. A review of these works is presented in a paper by Weddle and Jenkins.1 None of these methods is simple or suitable for serial analysis. In the same paper, Weddle and Jenkins stated that a significant measure of the activity of activated sludge is the rate of substrate removal. In aerobic systems, such as in the activated sludge process, the substrate is removed by biological oxydation, with oxygen as the final electron acceptor. The rate of substrate re moval may, therefore, be equivalent to the oxygen uptake rate (OUR). The OUR is an extensively used measure for the activity of activated sludge in research laboratories. The OUR can be measured by the manometric technique of Warburg, but because this technique requires special equipment and skilled technicians, it has been used little in controlling wastewater treatment plants. The development of the oxygen electrodes has made measuring OUR much easier and time-saving, and it is surprising that the use of oxygen electrodes in measuring OUR in activated sludge plants has not totally replaced other methods. If a method for measuring the activity of activated sludge is to serve as a practical controlling measurement, it must express the total oxygen uptake capacity for well-conditioned, unpoi soned sludge, as well as for sludge which has been influenced by components toxic to the microorganisms in the sludge. Furthermore, a method which is to be widely used in treatment plants must be simple to carry out and must not need expensive equipment. This work is an attempt to develop such a method.