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. surface-aerated basins a typical surface-aerated basing (using motor-driven floating aerators) most biological oxidation processes for treating industrial wastewaters have in common the use of oxygen (or air) and microbial action. surface-aerated basins achieve 80 to 90% removal of bod with retention times of 1 to 10 days. the basins may range in depth from 1.5 to 5.0 metres (4.9 to 16.4 ft) and utilize motor-driven aerators floating on the surface of the wastewater. in an aerated basin system, the aerators provide two functions: they transfer air into the basins required by the biological oxidation reactions, and they provide the mixing required for dispersing the air and for contacting the reactants (that is, oxygen, wastewater and microbes). typically, the floating surface aerators are rated to deliver the amount of air equivalent to 1.8 to 2.7 kilograms o2/kwh (4.0 to 6.0 lb o2/kwh). however, they do not provide as good mixing as is normally achieved in activated sludge systems and therefore aerated basins do not achieve the same performance level as activated sludge units. biological oxidation processes are sensitive to temperature and, between 0 and 40 °c (32 and 104 °f), the rate of biological reactions increase with temperature. most surface aerated vessels operate at between 4 and 32 °c (39 and 90 °f). sequencing batch reactors (sbrs) main article: sequencing batch reactor sequencing batch reactors (sbrs) treat wastewater in batches within the same vessel. this means that the bioreactor and final clarifier are not separated in space but in a timed sequence. the installation consists of at least two identically equipped tanks with a common inlet, which can be alternated between them. while one tank is in settle/decant mode the other is aerating and filling. aeration methods diffused aeration fine bubble diffusers in aeration tank of activated sludge sewage treatment plant near adelaide, australia sewage liquor is run into deep tanks with diffuser grid aeration systems that are attached to the floor. these are like the diffused airstone used in tropical fish tanks but on a much larger scale. air is pumped through the blocks and the curtain of bubbles formed both oxygenates the liquor and also provides the necessary mixing action. where capacity is limited or the sewage is unusually strong or difficult to treat, oxygen may be used instead of air. typically, the air is generated by some type of air blower. surface aerators (cones) surface aerator for aeration tank at activated sludge sewage treatment plant (at morro alto treatment plant, vespasiano, brazil) vertically mounted tubes of up to 1-metre (3.3 ft) diameter extending from just above the base of a deep concrete tank to just below the surface of the sewage liquor. a typical shaft might be 10 metres (33 ft) high. at the surface end, the tube is formed into a cone with helical vanes attached to the inner surface. when the tube is rotated, the vanes spin liquor up and out of the cones drawing new sewage liquor from the base of the tank. in many works, each cone is located in a separate cell that can be isolated from the remaining cells if required for maintenance. some works may have two cones to a cell and some large works may have 4 cones per cell. pure oxygen aeration pure oxygen activated sludge aeration systems are sealed-tank reactor vessels with surface aerator type impellers mounted within the tanks at the oxygen carbon liquor surface interface. the amount of oxygen entrainment, or do (dissolved oxygen), can be controlled by a weir adjusted level control, and a vent gas oxygen controlled oxygen feed valve. oxygen is generated on site by cryogenic distillation of air, pressure swing adsorption, or other methods. these systems are used where wastewater plant space is at a premium and high sewage throughput is required as high energy costs are involved in purifying oxygen. recent developments a new development of the activated sludge process is the nereda process which produces a granular sludge that settles very well (the sludge volume index is reduced from 200–300 to 40 millilitres per gram (192–288 to 38 us fl oz/oz)). a new process reactor system is created to take advantage of this quick settling sludge and is integrated into the aeration tank instead of having a separate unit outside. about 30 nereda wastewater treatment plants worldwide are operational, under construction or under design, varying in size from 5,000 up to 858,000 person equivalent. issues process upsets further information: secondary treatment § design considerations sludge bulking can occur which makes activated sludge difficult to settle and frequently has an adverse impact on final effluent quality. treating sludge bulking and managing the plant to avoid a recurrence requires skilled management and may require full-time staffing of a works to allow immediate intervention. the discharge of toxic industrial pollution to treatment plants designed primarily to treat domestic sewage can create process upsets. costs and technology choice further information: sewage treatment § types of treatment processes further information: sewage treatment § process selection the activated sludge process is an example for a more high-tech, energy intensive or "mechanized" process that is relatively expensive compared to some other wastewater treatment systems. it can provide a very high level of treatment.: 239 activated sludge plants are wholly dependent on an electrical supply to power the aerators to transfer settled solids back to the aeration tank inlet, and in many cases to pump waste sludge and final effluent. in some works untreated sewage is lifted by pumps to the head-works to provide sufficient fall through the works to enable a satisfactory discharge head for the final effluent. alternative technologies such as trickling filter treatment requires much less power and can operate on gravity alone. history the davyhulme sewage works laboratory, where the activated sludge process was developed in the early 20th century the activated sludge process was discovered in 1913 in the united kingdom by two engineers, edward ardern and w.t. lockett, who were conducting research for the manchester corporation rivers department at davyhulme sewage works. in 1912, gilbert fowler, a scientist at the university of manchester, observed experiments being conducted at the lawrence experiment station at massachusetts involving the aeration of sewage in a bottle that had been coated with algae. fowler's engineering colleagues, ardern and lockett, experimented on treating sewage in a draw-and-fill reactor, which produced a highly treated effluent. they aerated the waste-water continuously for about a month and were able to achieve a complete nitrification of the sample material. believing that the sludge had been activated (in a similar manner to activated carbon) the process was named activated sludge. not until much later was it realized that what had actually occurred was a means to concentrate biological organisms, decoupling the liquid retention time (ideally, low, for a compact treatment system) from the solids retention time (ideally, fairly high, for an effluent low in bod5 and ammonia.) their results were published in their seminal 1914 paper, and the first full-scale continuous-flow system was installed at worcester two years later. in the aftermath of the first world war the new treatment method spread rapidly, especially to the us, denmark, germany and canada. by the late 1930s, the activated sludge treatment became a well-known biological wastewater treatment process in those countries where sewer systems and sewage treatment plants were common.