into the river. In 1891, sedimentation tanks were added to the works, and the sludge was carried by steam boats and dumped further out into the estuary, at sea. During the 1880s, chemical engineer William Webster developed a system for the electrolytic purification of sewage (patent application filed on 22 December 1887; US patent awarded on 19 February 1889), trialled in 1888 at the Southern Outfall works which had been built by his father's firm over 20 years earlier. By 1897, additional pumping capacity was needed, and four extra pumps operated by triple-expansion steam engines were installed in an extension, designed to fit in with Bazalgette's main engine house, to the north of the older building. Later, in 1899, a further increase in London's population necessitated an increase in the efficiency of the original Watt engines, and considerable alteration to their design was carried out by Goodfellow & Co of Hyde, Manchester, for London County Council. They were converted from simple to compound engines with the original single cylinders were augmented by high and intermediate pressure cylinders. The additional steam required was provided by replacing the earlier Cornish boilers by more efficient Lancashire boilers with double flues and in 1901, the improved engines were fully working. In 1913, the triple expansion steam engines were replaced by diesel engines, which are still to be seen in the triple expansion engine house, and by 1956, the Watt-Goodfellow engines had been decommissioned, (Prince Consort having been temporarily put back in steam in 1953 to assist with draining the flooding of the eastern Royal Arsenal and Abbey Wood) and were left, with the rest of the ironwork, to rust and to vandals. Restoration The pumping station became a Grade I listed building in 1970 and will remain on the Heritage at Risk Register until the restoration is completed. The Crossness Engines Trust, a registered charity, was formed in 1987 to oversee the restoration project which was due to be completed in 2013. When the pumping station was decommissioned in the 1950s, it was not considered economic to dismantle the engines, as the cost of doing so far exceeded any scrap value. The more valuable metal items (made from brass), such as the engine oilers, much pipework, and even the handrails from the stairs, were removed. The remaining building and engines were left to suffer considerable vandalism and decay. As Prince Consort was the last