Civil Engineering Reference
In-Depth Information
Waterfall aeration, which involves the cascading of water over media, forming
droplets or thin films of water to contact with air
•
Overview of Design Considerations
Key factors affecting the removal of a compound from water by aeration include:
1. Compound and physical system factors
•
Physical and chemical characteristics of the compound
•
Temperature of the water and the air
2. Aeration process factors
•
Air-to-water ratio
Contact time
•
Available area for mass transfer
•
The ability of a compound to be removed is governed by the compound's Henry's
law constant. In general, the higher the Henry's law constant, the less soluble the
compound and the more easily the compound is removed via air stripping. (Recall that
Table 9-1 lists the Henry's constants for the most common compounds removed via
air stripping.)
An aeration system should be designed for the coldest water and air temperatures
expected to occur during the course of treatment. Most groundwater supplies exhibit
a water temperature of about 55
F (13
C). However, some northern-latitude ground-
waters may be as cold as 45
F(6
C), and others may be as warm as 75
F (24
C).
General water parameters give an indication of operational constraints that may be
encountered with the aeration of a particular water, as well as the suitability of a water
for the air-stripping process. Water quality parameters that may influence the aeration
process include hardness, iron and manganese, CO
2
, and dissolved oxygen (DO) levels.
The most frequent operational constraint associated with aeration is the buildup of
scale and slimes. Iron, manganese, and calcium carbonate can oxidize and subse-
quently precipitate on packing material and can plug diffusers and air jets. Scaling
from calcium carbonate precipitation is particularly a problem with hard waters that
have moderate to high alkalinity. High iron levels may lead to biological fouling as a
result of iron bacteria growth.
Waters with very low dissolved inorganic carbon (DIC) levels (below 2-5 mg / L)
may not be suitable for aeration. Aeration will remove CO
2
, thereby reducing the DIC
levels below the recommended minimum of 2 mg / L for adequate buffering capacity.
7
Packed Tower Aeration
General Description
In packed tower aeration, influent water enters the top of a
tower, then flows downward by gravity through packing material. Air is blown upward
through the tower in a forced or induced draft (countercurrent flow). The off-gas is
either treated or vented to the atmosphere.
The most common application of packed tower aeration in water treatment is VOC
removal from groundwater. The process provides a high air:water ratio and the highest
percent removal of the air-stripping techniques (in some cases greater than 99 percent).
