Filter beds at a sewage plant. The beds consist of fine gravel, and are used to filter minute particles of matter from partially-treated sewage. This serves two functions. First, the matter removed by the filter beds is broken down by bacteria that inhabit the gravel, making the matter biologically inactive. Secondly, the resulting water is aerated by the slow trickling flow out of the boom pipes, thus increasing the oxygen content. After final testing, the water will be clean enough, and contain sufficient oxygen, to be released into nearby rivers. The photograph was taken at a sewage plant near Aughton, South Yorkshire, UK.