Deep shafts are enhanced biological wastewater treatment systems. They are based on the process of activated sludge systems but particularly adapted where land is in short supply. Oxygen is injected into a return sewage stream, which is injected into the base of a deep columnar tank buried in the ground. 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 microorganisms contained in the activated sludge. The rising oxygen and injected return sludge provide the physical mechanism for mixing. Mixed sludge and wastewater is decanted at the surface, the sludge is again enriched in oxygen while the supernatant continuously flows out of the system. The efficiency of deep shaft treatment can be high but they require high skilled labour for both, construction and operation and maintenance and large amount of energy.