Water Quality Management - River Dynamics and Integrated River Management - page 583

Membrane Diffusers inject atmospheric oxygen supplied by blowers located on shore that travels trough
a manifold system to the instream diffusers located near the bottom of the river. There is no known use of
membranes for waterway aeration, however, this technology is being considered for possible application
in the Chicago Waterway System because they are generally very flexible and resist fouling because of the
following characteristics (CTE, 2007):
z The membranes are normally closed until sufficient air pressure opens the units to begin operation.
z When the air is interrupted, the membranes close preventing liquid/solids entry.
z Membrane diffusers have only an exterior surface phenomena as the liquid and air interface is at
the exterior surface of the membrane compared to the interior of a rigid material (such as ceramic
diffusers).
z Operation of a membrane unit involves major flexing during on/off operation with major flexing
even during normal airflows. This flexing tends to minimize the accumulation of surface inorganic
materials.
z The surface of some membrane materials is quite smooth and slick. These smooth, slick surfaces
minimize or eliminate calcium carbonate and other contaminant build up.
z Membrane diffusers also require relatively low electrical energy. Typically membrane diffusers have
an OTE of 10%-30%.
Jet Ejectors mix air and water together using a Venturi and provide a jet of water containing air
bubbles. The horizontal travel of the plume maintains a gas/liquid transfer interface for a much longer
period of time than for conventional diffused aeration systems. The horizontally mixed plume is enriched
with fine bubbles which will rise slowly to the surface providing for excellent oxygen absorption. Jet
aerators have not been applied to waterway aeration, but this method of air diffusion has been proven to be
reliable and effective in wastewater treatment tanks. Particularly because jet aerators are much less likely
to clog compared to fine bubble diffusers this technology is being considered for possible application in
the Chicago Waterway System (CTE, 2007). Typically jet aerators have an OTE of 10%-25%.
U-Tubes: The downward water velocity is designed to exceed the buoyant velocity of the air or oxygen
bubbles that are released into the water column. Consequently, the bubbles are transferred downward and
around the end of the baffle at the bottom, thus, the name U-Tube (Fig. 9.15). This process temporarily
pressurizes the bubbles via the large increase in hydrostatic head within the U-Tube. This increases the
saturation concentration which, in turn, increases the DO deficit thereby creating a greater driving force
for the adsorption of oxygen into the water column. At sea level, a 34-ft head of water creates approximately
two atmospheres of pressure inside a gas bubble (one due to air pressure and one due to water pressure).
Typically U-tubes can produce OTE's as high as 90%.
Fig. 9.15 Schematic diagram of a compressed air U-Tube contactor (figure provided by AECOM-CTE, Chicago, 2008)