 The Caledonian Railway rebuilt many of the sewers, as part of a deal to allow them to tunnel under the city, and sewage treatment works were opened at Dalmarnoch in 1894, Dalmuir in 1904 and Shieldhall in 1910.The works experimented to find better ways to treat sewage, and a number of experimental filters were constructed, until a full activated sludge plant was built between 1962 and 1968 at a cost of £4 million.Treated sludge was dumped at sea, and Glasgow Corporation owned six sludge ships between 1904 and 1998, when the EU Urban Waste Water Treatment Directive ended the practice.