2. Inherent Cooling Water Environmental problems
Most of the evaporative types of cooling water systems consist of cooling towers of different
types and heat exchangers. The common environmental problem is hydrocarbon leakage into the
cooling water from the aging heat exchangers, which subsequently causes high Biological
Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) levels at the blowdown. Other
problems are possible leaching of lumber treatment chemicals, particulate from the Total
Dissolved Solids (TDS) as carryover in the drift loss; uncontrolled pH, high discharge
temperature, TDS and residual chlorine in the cooling tower blowdown.
Hydrocarbon leakage could be due to the failure of tube sheets, and the rupture of exchanger
tubes caused by corrosion or poor materials of construction. Depending on the size of the leaks,
the leakage is normally unnoticed by operations and maintenance personnel and the plant is kept
in production until the next planned turnaround. Should there be leakage in the cooling water
system, the light hydrocarbons usually find their way out the cooling tower exhausts by means of
air stripping which may cause air pollution problems in terms of VOC or HAPs emissions.
Heavier hydrocarbons can find their way out via the cooling tower blowdown and if discharged
into the public water streams can cause water pollution problems in term of high BOD, COD and
priority pollutants levels at the outfall.
In most of the refinery plants that usually handle heavier hydrocarbon chemicals and involve
multiple plant units, the cooling tower blowdown is usually collected and treated with other
wastewater streams before discharged to the outfall. Volatile hydrocarbons considered as fugitive
VOC emissions sometimes are unavoidable due to an unexpected heat exchanger leakage and
may find their way out via the cooling tower exhausts. Chemical plants handling lighter
chemicals are usually considered cleaner plants and usually discharge their cooling tower
blowdown directly into the outfall without treatment.
Particulates resulting from the TDS in the drift loss containing salts and minerals, once being
dried in the ambient air will become very fine particulates, which are usually smaller than 10
micron in size. They could have some impact on sensitive areas in terms of PM
10
and PM
2.5
regulations concerning their impact on the local vegetation, soil contamination, and hazards to
personnel, automobiles, and plant equipment. Visible steam plumes from the cooling tower
exhaust stacks if not controlled may also impact on the visibility of the nearby air and ground
transportation vehicles.
Other cooling tower problems are the free oil and grease (O&G) leaked from the exchangers
handling the hydrocarbons and need to be skimmed off in the cooling tower basin. Sludge build
up from the suspended solids containing in the makeup raw water, ambient air scrubbing, algae,
and piping corrosion products could be contaminated by leaking hydrocarbons and free O&G
and accumulated in the bottom of the cooling tower basin and pump sump. Such contaminated
oily sludge may become hazardous and must be collected and disposed of in a sound
environmental manner. Changes in environmental rules, such as Resource Conservation
Recovery Act (RCRA) and the 1990 Clean Air Act Amendment, NESHAPS and HON rules
have forced the plant management and the Design Engineers to change the old way of using
