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5.22 CHAPTER FIVE

Carbonate scaling of packing becomes a problem when influent water has a relatively
high calcium carbonate hardness. Deposition occurs primarily because of pressure changes
and a rise in the water's pH as CO2 is released from the water. Carbonate scale deposits
usually are a brittle, cementlike scale.
Ferrous iron in groundwater, another source of scaling, oxidizes easily in the presence
of oxygen to form insoluble ferric compounds. The result is primarily iron hydroxide,
which accumulates on the packing as a rust- or black-colored gel.
Microbial fouling is, in most cases, primarily from the presence of iron bacteria. Iron
bacteria generally thrive in a dark environment, under aerobic conditions, and at temper-
atures between 40 ° and 70 ° F (4 ° and 21 ° C). These organisms derive energy from the
oxidation of iron from the ferrous to ferric form. Some of the more common species
of iron bacteria found in groundwater are Gallionella, Crenothrix, Leptothrix, and
Sphaerotilus. Colonies of the bacteria can grow on the packing media, forming a slimy
material that, if not controlled, can completely fill all void spaces in the packing.
One method of controlling fouling involves pretreating water with chlorine or per-
manganate and then filtering to remove oxidized solids. Another pretreatment method is
to add chelating agents to inhibit formation of oxidation products. In any event, if the in-
fluent water has a high potential for fouling, the plant must provide facilities and a reg-
ular schedule of periodic cleaning of the buildup. Cleaning consists of circulating strong
chlorine or acid solutions through the media.

PILOT TESTING

Pilot plant air stripping studies are usually conducted to determine the effectiveness of an
aeration system in removing the contaminants of concern. Testing is performed using a
laboratory or bench-scale prototype unit. Municipalities, private consultants, and the USEPA
use pilot aeration tests to determine the usefulness of aeration in removing various VOCs.
Pilot tests are sometimes performed on samples "spiked" with the contaminant to be
studied. However, it is generally best to use water directly from the contaminated source,
because subtle differences in both the physical and chemical composition of the water
could have some effect on the ability of an aeration unit to remove a contaminant.

Diffused Air Pilot Studies
The main components of a diffused air pilot plant are the tank, diffuser, and blower. The
system is usually operated in a countercurrent fashion. Influent water is piped through a
rotameter to measure flow and dispersed through a liquid distributor at the top of the tank.
Water then flows down through the tank to be discharged at the bottom. Air is supplied
by a compressor, piped through a rotameter to measure flow, and dispersed by a diffuser
located at the bottom of the tank.

Packed Column Pilot Studies
A typical packed column pilot plant consists of the column with influent piping, valves,
packing media, blower, and support structure, as shown in Figure 5.12. lnfluent is pumped
through the metering valve and meter to the top of the column, where it is distributed by
an orifice-type plate. Water trickles down through the packing and is discharged at the
bottom of the column.