Environmental Engineering Reference
In-Depth Information
Process water reuse
Water reuse can be problematic in food processing. In the case of fruits and vegetables,
countercurrent washing can be implemented in some cases. In the meat and poultry industries,
water reuse is more complicated because of the dangers of pathogenic bacteria contamination.
Poultry-processing plants use large volumes of water with opportunities for reuse. For instance,
immediately after evisceration, birds are cooled quickly to below 4.4
C to minimize the develop-
ment of bacteria that pose a risk to human health. Chilling is performed in big horizontal tanks
equipped with some type of conveying system filled with water at temperatures below 4.4
°
C.
In the United States in accordance with US Department of Agriculture (USDA) regulations,
1.9 liters of water per bird need to be constantly renewed with fresh cold water. Some instal-
lations have the chilling operation divided in two sections, prechilling and chilling, so in a
countercurrent design, cleaner water from the chiller could be used to feed the prechiller.
However, the USDA requires that if reused water is used upstream in the process (or any
process in a poultry facility), then that water needs to be incorporated in the HACCP plan to
avoid the risk of upstream contamination (Russell, 2005).
Frequently, canning plants use once-through water cooling systems to cool down the
product after sterilization and send the water after only one pass directly to the drain.
Significant amounts of water, money, and carbon emissions can be saved by installing a system
that recirculates cooling water, which is indirectly cooled by a cooling tower and supplemen-
tarily cooled with a chiller if needed.
To avoid the accumulation of impurities in boilers, water is periodically drained in volumes
of less than 10 percent of the feed water, but it can be more than that in cases of poor make-up
of water quality. The process is called “boiler blowdown,” and the water is frequently dis-
charged down to the drain and then discarded with wastewater. Instead of discarding it, this
water can be treated with reverse osmosis and returned to the boiler. Blowdown water reuse
not only saves water but also energy and chemicals.
°
Water recycling
There are many internal water sources that can be captured, retreated, and used in processes
that require potable water, or in other applications that do not need potable water. Some poten-
tial sources include:
Filter backwash water.
Cooling tower blowdown water.
Boiler blowdown water.
Reverse osmosis reject streams.
Lost condensate recovery and reuse (General Electric [GE], 2007).
Coca-cola in Australia recycles filter backwash water by sending it back to the water
treatment plant, so after treatment is used in the manufacturing process. The backwash water
generated by this plant was up to 200,000 liters a day that before retrofitting with the recycling
system was sent to the sewer (Judd and Jefferson, 2003).
Cooling towers are flexible in terms of the source of makeup water. With the adequate
reverse osmosis treatment, reject streams, boiler blowdown, and refrigeration equipment
condensates can be used as make up water for cooling towers. Scrubbers are even more
flexible because they can receive blowdown water from cooling towers in addition to all the
other sources mention before (GE, 2007).
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