Page 250 - Industrial Wastewater Treatment, Recycling and Reuse

224 Industrial Wastewater Treatment, Recycling, and Reuse

Figure 5.9 Bayer Tower Biology unit with separate secondary clarifier.

pressure of the water column combine to produce optimum oxygen transfer
and utilization. Up to 80% of the oxygen in the air can be dissolved in the
water in the Tower Biology process. This means that, in comparison with
surface aeration in open basins, only one-fifth to one-seventh as much air is
needed to introduce the same quantity of oxygen into the process.
This reduces the amount of energy required and the amount of off-gas
produced. The closed tank design significantly reduces stripping effects and
the formation of aerosols that result in unpleasant odors in open basins.
By building tall tanks instead of flat basins, the amount of ground space
required for a Tower Biology unit is some 30–50% less than is needed for a
conventional treatment unit. It is also much easier to check for leaks, an
important consideration for the protection of the groundwater.
The sludge/water mixture that leaves the activation tank is de-aerated in
a special cyclone before it is transferred to the secondary clarifiers. This also
results in further flocculation of the sludge, which helps improve sedimen-
tation during secondary clarification.
Secondary clarification usually takes place in funnel-shaped tanks with
vertical through-flow known as Dortmund cones. These secondary clarifiers
are made from steel and rest on a concrete column under which there is room
to construct soundproofed storage facilities for the necessary machinery.