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High-Purity Water Production Is Not the Only Power Plant Water Issue

Posted on: Wednesday, 12 October 2005, 03:01 CDT

By Buecker, Brad

Water treatment articles in Power Engineering often focus on production methods and chemical treatment programs that ensure high- purity water enters and circulates through steam generators. However, treatment of other water streams at a plant is important, albeit for other reasons.

Consider discharge water streams. These may come from ash ponds, coal pile runoff ponds, roof and floor drains, boiler blowoff tanks, and other sources. Manmade water discharges from all power plants in the United States fall under National Pollutant Discharge Elimination System (NPDES) guidelines. While each plant has its own permit guidelines, some items are common to most facilities. For example, the typical pH range allowed by NPDES guidelines is 6.0 to 9.0. Another universally regulated parameter is total suspended solids (TSS). This value may be limited to 30 mg/1 or less. Oil and grease also must be monitored and controlled.

Some power plants are plagued by discharges that exceed the 9.0 pH limit. These violations are often due to alkaline minerals in ash pond waters or to algae growth on ponds during warm weather. Treatment of an algae-infested pond with an algaecide may not be possible due to discharge of chemicals potentially toxic to downstream aquatic life. A practical and usually simple solution to lower the discharge pH is to inject gaseous carbon dioxide into the pond upstream but near the discharge point. Often just a small flow of CO2 is enough to lower pH below the 9.0 limit. The simplest systems require only a refutable tank with cooling system and safety vent, piping to the injection point, and a pressure regulating valve on the discharge line to control flow. Placing a distribution grid on the piping discharge can enhance system efficiency, improving CO2- water mixing.

Serious pH excursions can occur during unit outages if plant personnel or contractors water blast boiler tubes and the boiler backpass to remove slag and ash. Sulfurous compounds in the ash drive pH down, and if the rinse water is allowed to exit through normal plant drains, the result will be NPDES violations. Plant personnel tend to forget about this issue during outages, as these events keep most of them busy and at times rather frazzled. Solutions to water-wash waste disposal can sometimes be problematic, as large quantities of wastewater are typically generated during the process. One possible solution is to route the waste through a mix tank, or even an accessible manhole, where a neutralizing compound such as sodium carbonate can be added. If the plant has self- contained ponds that do not discharge to outside bodies of water, the waste can be pumped or trucked to these locations if the plant's environmental permit allows such disposal.

An issue that affects many plants is control of fine particle discharge from coal pile runoff ponds and ash ponds. Very fine particles tend to remain in suspension and carry over to discharge streams. Injecting a settling agent into the stream or streams that enter the ponds works well at reducing carryover. Polymers are the choice for this application. Gary Antony, an expert in this chemistry points out that, "In general, high solids effluents require high molecular weight polymers of 12,000,000 to 16,000,000 plus. These polymers are typically cationic polyacrylamides with high charge densities." The polymer molecules act as bridges between solid particles, and as the polymers grab solids, the overall particle weight increases causing settling to occur. The chemical is often so effective that only a slight residual of perhaps one or two parts-per-million is sufficient for solids agglomeration.

Solids can build up rather rapidly when the settling polymer is doing its job. It is critical that the settling take place in areas of the pond or even inlet ditches where dredging or removal with a backhoe is practical. Inability to remove solids from the pond can result in situations where the pond can overflow during heavy rainfall. Environmental authorities are never happy when this occurs.

Oil or heavier organics such as grease and thick lubricants are other potentially troublesome discharge contaminants. Oil leaking from heavy equipment into floor drains is a common source of these impurities. A well-designed oil/water separator is often sufficient to maintain oil and grease concentrations well below the plant's NPDES limits. However, 100 percent reliability must never be taken for granted. cases have occurred where extremely heavy rainfall overloaded the separator and allowed some oil to overflow with the water discharge. Systems need to be designed to handle maximum rainfall events. In the Midwest, this could be six to eight inches of rain in a few hours, but in areas near the Gulf Coast rainfall totals could be much higher.

By Brad Buecker, Contributing Editor

Copyright PennWell Publishing Company Sep 2005

Source: Power Engineering

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