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The use of
hexavalent chromium as a corrosion inhibitor of HVAC cooling and heating
systems was finally prohibited for cooling tower use in New York City in
January 1990, and elsewhere in the U.S. over some years prior.
For decades, chromate was the
overwhelming treatment chemical of choice - providing low corrosion rates
typically around 1 mil per year (MPY) for even open cooling tower systems, and
controlling microbiological growths through its own inherent toxicity. Building
and plant engineers relied solely upon the use of chromate based chemical
additives to provide the required corrosion protection of steel piping systems.
With even the most inferior application methods, often nothing more than an
unmeasured scoop of chromate powder dumped into the cooling tower sump at
irregular intervals, corrosion rates could often be maintained at or below 1
MPY.
Although its use in boilers and open
cooling tower systems was almost totally banned, continued use in closed
systems was permitted under stringent regulatory conditions - an option many
building owners and plant operators have taken advantage of given the proven
superior corrosion control provided by hexavalent chromate.
Removing
hexavalent chromate from a closed circulating system prior to maintenance,
drain down, or a change in chemical treatment program presents enormous
problems since it is illegal to discharge chromate treated water into sewers,
streams, rivers, or ground surfaces anywhere in the United States. State and
federal regulations are quite strict.
Up
until now, the only legal solution for those having older circulating systems
treated with chromate has been to repeatedly drain and flush the entire volume
into 55 gallon drums, and dispose of them as a hazardous waste in a sanitary
landfill or through incineration - all at extraordinary cost.
Rather than remove thousands of gallons
of chromate treated water and pay for the disposal of tens of thousands of
pounds of hazardous waste material, it is possible to selectively filter the
hexavalent chromate from any vessel or piping system.
Through a special
procedure, the hexavalent chromate is adsorbed onto various specialty resin or
ion exchange compounds - thereby reducing any toxic waste disposal problem ten
thousand fold or more. In most cases, the problem can be reduced to the weight
of just little more than the original chemical content itself.
At a typical concentration of 1,000 PPM
for a closed system treated with hexavalent chromate, the actual amount of
chemical is still only one tenth of one percent of the volume of the water - or
1/1,000 the weight of the water. A 5,000 gallon closed system, for example,
would require the disposal of approximately 42,250 lbs. of chromate treated
water on just the first draining - multiple drainings would be required to
reduce the final chromate level to acceptable limits. Again for this example,
91 drums, 55 gallons each, would need to be filled and carted away just on the
first drain down. As many as 300 drums might be ultimately
required.
However, only about 42 lbs. of hexavalent
chromate actually exists as the source of the problem, and can be slelectively
removed by perhaps a few dozen ion exchange cartridges. This reduces the
hazardardous waste problem dramatically - thereby also reducing handling and
disposal costs. In addition, relatively few pounds of hazardous waste will not
likely title a building or plant facility as a hazardous waste point
source.
Preliminary
testing of the water to identify its chemical characteristics and an estimate
of the overall contaminant problem is recommended - since other factors, some
of them hidden or unknown, can influence chromate removal
efficiency.
This simple and inexpensive
filtration system can be easily installed, setup and maintained by building
personnel. Depending upon the volume of the system, the concentration of
chromate to be removed, and the removal capacity of the particular type of
filtering system selected, the chemical content can be reduced in as little
time as a few days to a few months.
The
chromate removal system typically consists of a series of five cartridge
elements; each having the purpose of removing a specific component part of the
chemical contaminant. Cartridges are individually housed to allow changing only
those elements which have reached their maximum capacity - thereby reducing the
cost of unnecessarily replacing partially used elements. For larger systems,
refillable housing containing significantly more chromate removal resin can be
used instead of the individual cartridges.
A flowmeter is
incorporated into the piping layout in order to ensure the most effective
chromate removal by maintaining the required retention time through the
filtering media. When operated under design parameters, this chemical removal
system will reduce chromate levels from 1,000 PPM or more to 0 PPM - thereby
fulfilling all federal and/or state discharge requirements.
This chemical specific filtering system
may be installed on a once through pass to a drain in order to satisfy a
maximum allowed chemical discharge limit. Another option is to install it
across one or more circulating pumps whereby the clear filtered water is
returned to the system. After full completion of the chemical removal process,
it is necessary to only dispose of the used cartridges by following recommended
hazardous material procedures. Since the chromate is adsorbed onto resins which
are contained within "sealed" plastic filter cartridges
typically, worker exposure and site contamination is
minimized.
Given knowledge of the total
system volume, flow rate, contaminant concentration, and rate of cleaned fluid
returned to the system, it is possible to calculate an estimate of the time
required to clean any piping system down to any level of removal.
The below
schematic flow diagram illustrates the basic mechanism for setting up an
effective chromate removal system as described above. Other variations exist.
Critically important is the need to prefilter the liquid using a fine 1-5
micron sediment cartridge in order to protect the microfine pores on the ion
exchange resin from clogging - thereby limiting their removal capacity.
While this
Technical Bulletin specifically relates to the removal of hexavalent chromium,
the same basic selective filtration technology is applicable to remove other
chemical contaminants as well. In most cases, a large volume contaminant
problem can be reduced down to the original volume or weight of chemical
applied.
©
Copyright
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