text: Where land is in short supply sewage may be treated by injection of oxygen into a pressured return sludge stream which is injected into the base of a deep columnar tank buried in the ground. Such shafts may be up to  deep and are filled with sewage liquor. As the sewage rises the oxygen forced into solution by the pressure at the base of the shaft breaks out as molecular oxygen providing a highly efficient source of oxygen for the activated sludge biota. The rising oxygen and injected return sludge provide the physical mechanism for mixing of the sewage and sludge. Mixed sludge and sewage is decanted at the surface and separated into supernatant and sludge components. The efficiency of deep shaft treatment can be high.
  Surface aerators are commonly quoted as having an aeration efficiency of 0.5–1.5 kg O/kWh (1.1–3.3 lb O/kWh), diffused aeration as 1.5–2.5 kg O/kWh (3.3–5.5 lb O/kWh). Deep Shaft claims 5–8 kg O/kWh (11–18 lb O/kWh).
  However, the costs of construction are high. Deep Shaft has seen the greatest uptake in Japan, because of the land area issues. Deep Shaft was developed by ICI, as a spin-off from their Pruteen process. In the UK it is found at three sites: Tilbury, Anglian water, treating a wastewater with a high industrial contribution; Southport, United Utilities, because of land space issues; and Billingham, ICI, again treating industrial effluent, and built (after the Tilbury shafts) by ICI to help the agent sell more.
  DeepShaft is a patented, licensed, process. The licensee has changed several times and currently (2015) Noram Engineering sells it.