upgrading of the first activated sludge plant to use the 'simplex' system took place between 1970 and 1973. because of the proximity of the m60 motorway and the risk that deep excavations might damage it, seven of the final settling tanks were built above ground. effluent from the activated sludge plant was raised up to the level of the tanks by screw pumps, and the increased hydraulic head improved their performance. the upgrade also included five primary settlement tanks and eight sludge consolidation tanks. two new sludge boats were purchased in 1971 and 1972, and a new dock was constructed on land which had formerly been used for secondary beds.