25 May 2005
1. Table 2-2 provides only general guidelines. The final choice of treatment
system must be based upon complete raw water analysis, feedwater
requirements, and overall economics, including both external and internal
treatment and blowdown. External treatment may be necessary to reach
recommended levels of total dissolved solids (TDS) without exceeding
other parameter limits for causticity, silica, or SS.
2. Separate deaeration is required for all boilers with pressure over 0.103
megapascal (15 pounds per square inch gauge), except where lime-soda
softeners are designed to provide adequate deaeration as well as
3. Degasification is required after hydrogen zeolite treatment.
4. The filtration process may require clarification and aeration.
Aeration. Well water can contain high levels of dissolved iron (1 to 5
ppm). Although this quantity of dissolved iron may seem small, it can produce excessive
precipitates when the iron comes in contact with air. If these precipitates are deposited
in system lines, they will restrict flow and heat transfer. Soluble iron can be removed by
filtration after contact with air (aeration), a process that causes the soluble iron to be
converted by oxidation to insoluble iron, which then precipitates. Aerators are designed
to mix air and makeup water in equipment that contains slats or trays to provide
thorough mixing (i.e., aeration of the water). They are usually of the coke tray or wood
slat design. Coke tray aerators consist of a series of coke-filled trays through which the
water percolates. A forced-draft fan supplies air for aeration during the percolation
process, with the water free falling from one tray to the next. Wood slat aerators are
similar to small atmospheric cooling towers with staggered slats to break the free fall of
the water and thereby increase the surface contact with air. Wood slat aerators can also
be equipped with a forced-draft fan to increase efficiency. In addition to oxidizing iron,
aeration can also strip or remove dissolved gases such as carbon dioxide, hydrogen
sulfide, and methane. Aerators also contribute to a reduction in dissolved manganese
by causing it to be oxidized to an insoluble salt.
Filters and Filtration. A variety of filters can remove particles in a wide
range of sizes, from course to very fine. These solids or particles may include soluble
iron that has been precipitated, residual calcium carbonate particles, sand, dirt, debris,
and some microbiological organisms.