Passing the NPDES Litmus Test
Power plant operators must control pH limits and contaminants in their wastewater streams to meet permit guidelines
When people think of water issues at electric utilities, the first thought that usually comes to mind is production of high-purity water for steam generation. Even though a power plant may not have myriad fluid processes like a refinery or petrochemical facility, water discharge from a steam-generating facility is usually considerable. Chemistry in discharge streams must be carefully controlled to prevent pollution of receiving bodies of water or surrounding land.
Consider the water streams that discharge from a power plant. Water discharge sources may include ash ponds, coal pile runoff ponds, roof and floor drains, boiler blow-off tanks, and chemical unloading facilities. Human-made water discharges from all power plants in the United States fall under National Pollutant Discharge Elimination System (NPDES) guidelines, under the Clean Water Act. While each plant has its own permit guidelines, some items are common among most facilities. For example, the typical pH range allowed by NPDES guidelines is 6.0 to 9.0 (Distilled water, which is regarded as neutral, has a pH of 7; values between 7 and 14 indicate alkalinity). Another universally regulated parameter is total suspended solids (TSS). This value may be limited to 30 miligrams per liter (mg/l) or less. Yet another pollutant that must be monitored and controlled is oil and grease (O&G).
Solving pH Problems
With regard to pH issues, a problem that sometimes plagues power plants is 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 potentially toxic chemicals to downstream aquatic life. A practical and often 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 refillable tank with cooling system, piping to the injection point, and a pressure regulating valve on the discharge line to control flow.
The efficiency of the system can be enhanced by placing a distribution grid on the piping discharge. This enhances CO2/water mixing. Several systems are able to perform admirably in preventing high-pH excursions from ash ponds.
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 the pH down, and if the rinse water is allowed to exit through normal plant drains, the result will be NPDES violations. Experience suggests that plant personnel tend to forget about this issue during outages because these events keep most plant personnel very busy and, at times, rather frazzled. Solutions to water-wash waste disposal can sometimes be problematic because large quantities of wastewater are typically generated during the process. One possibility 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 such disposal is allowed in the plant's environmental permit.
Reining in Fine Particulates
An issue that plagues 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. A technique that works well to reduce carryover is injection of a settling agent into the stream or streams that enter the ponds. Polymers are the choice for this application. In general, high-solids effluents require high-molecular-weight polymers of 12,000,000 to 16,000,000 or more. These polymers are typically based on the polyacrylamide structure.
This compound can be further modified to form either a cationic or anionic polyacrylamide for the desired application.
With low-suspended-solids wastewaters, low-molecular-weight, high-charge-density polymers, such as the poly-diallymethyl ammonium chlorides (DADMACS) may be employed.
The polymer molecules act as bridges between solids particles, and as the polymers collect solids, the overall particle weight increases such that settling occurs. The chemical is often so effective that only a slight residual of perhaps one or two parts per million is sufficient for solids agglomeration. An aspect of importance is that solids 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 a heavy rainfall. Environmental authorities are never happy when this occurs.
Policing O&G Concentrations
Another potentially troublesome contaminant in discharges is oil or heavier organics such as grease and thick lubricants. A common source is floor drains in the plant where leakage of oil from heavy equipment may enter. A well-designed oil/water separator is often sufficient to maintain O&G concentrations well below the plant's NPDES limits. However, 100-percent reliability must never be taken for granted. Cases have been documented 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 6 to 8 inches of rain in a few hours, but in areas near the Gulf Coast the total could be much higher.
This article originally appeared in the 05/01/2006 issue of Environmental Protection.