an early feature was a laboratory, where trials of various types of filter were carried out, and incoming effluent was analysed. attempts to improve the treatment process proved successful in 1914, when two chemists, ardern and lockett, discovered the activated sludge process, which was soon in use worldwide. a second deep level sewer, started in 1911, eventually reached the works in 1928, and to cope with the increased flows, half of the sewage was fed into a new activated sludge plant. three separate operating systems were installed, so that comparisons on their efficiency could be made. a second activated sludge plant was built between 1955 and 1966, and the control system on the first was upgraded between 1970 and 1973.