Albany Filter Beds (water purification), Hong Kong
University of Bristol - Historical Photographs of China reference number: FH01-308. The Albany Filter Beds were built in 1888 and demolished in 1954. The structure included a vaulted underground water storage chamber. The ‘slow sand’ method of filtering water was developed by British civil engineer James Simpson (1799-1869) in 1829. Slow sand filtration includes a slimy ‘schmutzdecke’ of algae, plankton and bacteria etc, above a sand bed, which purifies the water by natural processes. See Gwulo.com/node/4972 (https://gwulo.com/node/4972).
Black and white photograph