Nineteenth-century cities piped sewage away in combined sewers but generally discharged it raw into rivers and seas. Biological treatment changed this: trickling filters were developed in the 1890s and the activated-sludge process, demonstrated by Ardern and Lockett in Manchester in 1914, used aerated microbial cultures to clean wastewater before discharge. Treatment works became a standard, land-hungry component of urban infrastructure, usually sited downstream on the city edge. Effluent standards drove their spread through the twentieth century.
Sewage treatment broke the link between urban growth and downstream pollution, making large concentrated cities environmentally sustainable on shared rivers.