1999 - 1900no info

Ardern (first from the left in the back row) and Lockett (first from the right in the first row) described all of the essential components of the activated sludge wastewater treatment process, as it is used today, and as it rapidly became the most widely used biological wastewater treatment process. They made the first reported use of the term “activated sludge” to refer to biological solids that they settled out of aerated wastewater and recycled back into the treatment process.

Essentials of the Activated Sludge Process

The essentials of the activated sludge process, which were described in Ardern and Lockett’s paper (and are still in use today), are

  1. aeration of wastewater in the presence of aerobic microorganisms
  2. removal of biological solids from the wastewater by sedimentation
  3. recycling of the settled biological solids back into the aerated wastewater

This set of processes favors aerobic microorganisms that flocculate to form settleable solids that can be removed by sedimentation and sent back into the aeration process, while allowing non-settleable aerobic microorganisms to wash out of the system. Thus a concentrated culture of aerobic microorganisms can be maintained in the aeration tank, so that the organic matter in the incoming wastewater will be oxidized to carbon dioxide and water with a manageable hydraulic residence time (and thus a reasonable tank volume).

In the first half of the nineteenth century, the use of sanitary sewer systems in cities was just emerging in Europe and the U.S. as an alternative to pit privies, ditches, dumping chamber pots into the street, or cesspits. The sewer systems typically simply conveyed the wastewater for discharge into a body of water.


  • Ardern, E., Lockett, W.T. (1914a) Experiments on the Oxidation of Sewage without the Aid of Filters. J. Soc. Chem. Ind., 33, 523.
  • Ardern, E., Lockett, W.T. (1914b) Experiments on the Oxidation of Sewage without the Aid of Filters, Part II. J. Soc. Chem. Ind., 33, 1122.
  • Ardern, E., Lockett, W.T. (1915) Experiments on the Oxidation of Sewage without the Aid of Filters, Part III. J. Soc. Chem. Ind., 34, 937.