Aeration

Olievera et al (1998) said  a  schematic  view  of  a  sub-surface  aeration  tank  is  presented  in  Figure  1.  The  aeration  gas  is  introduced through  diffusers  located near the bottom of the tank,  where  it  is introduced  into  the  liquid. The  diffused  gas bubbles  flow upwards,  through  the  liquid,  towards  the  tank  surface.  Gas  bubbling  produces  effective motion  and mixing of the  liquid.  It also produces  a  turbulent  liquid  surface.  Two mass  transfer  regions  can  be  distinguished,  (i)  the  turbulent  surface,  where  oxygen  transfer  occurs  between  the  liquid surface and  the atmosphere,  and  (ii) the gas bubble  dispersion zone,  located under  the liquid surface.

The gas bubble dispersion zone accounts for the majority of the mass transfer in diffused air and sub-surface mechanical aeration systems. There  is good agitation and  mixing of the  liquid phase due  to action of  the gas bubbles and/or a mechanical mixing device. Thus,  it is assumed that the liquid in the tank is perfectly mixed,  i.e.,  the  liquid composition is uniform  at any instant in  time.  It  is also assumed  plug flow of the gas bubbles, oxygen transfer rates controlled by the liquid phase,  and negligible mass transfer of gases other than oxygen.

An alternative to plug flow within a raceway is to create a completely mixed (horizontally and vertically) tank by installing a water inlet and outlet manifold along the long axis of the tank. As seen in Figure 4, water enters uniformly along the bottom of one side of the raceway and is removed along the other side. Water must enter at a high enough velocity to create a rotational flow along the short axis of the raceway (Fig. 2). The solids will move across the bottom of the raceway and into the effluent manifold. Another method of dealing with settleable solids is to keep them in suspension with continuous agitation until they enter an external settling tank. In settling tanks (or basins), water flow is very slow so that solids settle out by gravity. Settling tanks may or may not include tube or lamella sedimen- tation material. This material is constructed with bundles of tubes or plates, set at specific angles to the horizontal (usually 60^o), that reduce both the settling distance and circulation within the settling tank. Using settling plates reduces the size requirement of a settling basin, thus saving space within a facility. However, the plates make routine cleaning of settling basins more time-consuming.