Power plants often produce wastewaters that contain salts, such as those from wet gas scrubbing, coal pile run-off, and leachate from gypsum stacks. Evaporation of those liquid wastes in a modern zero liquid discharge system produces clean water that is recycled into the plant plus a solid product suitable for landfill disposal. Here are the options to consider.
In most power plants, the largest wastewater producer is the cooling water system. Historically, natural evaporation of the cooling tower blowdown from holding ponds has been very successful, particularly in the western U.S. This approach to waste liquid disposal is a simple but effective example of a zero liquid discharge (ZLD) system. The downside is that the water is permanently lost from the system through natural evaporation, and the remaining residue must be periodically cleaned from the pond.
Because cooling tower blowdown is relatively dilute, generally less than 10,000 mg/L total dissolved solids (TDS), reverse osmosis (RO) membranes are often used to pre-concentrate the cooling tower blowdown prior to concentrating the liquid in an evaporator; the remainder is reduced to solids in a crystallizer. The salts present in cooling tower blowdown, for example, are usually composed of sodium sulfate and sodium chloride with small quantities of calcium, magnesium, sulfate, and bicarbonate. All of these salts can be readily crystallized by evaporation.
However, wastewater from wet flue gas desulfurization (wet FGD) systems and integrated gasification combined cycle (IGCC) plants contains highly soluble salts, such as calcium and ammonium chlorides, and certain heavy metal salts, which are not so easy to crystallize by evaporation. Conventional ZLD evaporation-crystallization processes for wet FGD and IGCC waste streams require clarification and extensive pretreatment. Usually, the wastewater must be treated with lime, soda ash, and other chemicals to replace the calcium, magnesium, ammonium, and heavy metal ions with sodium ions so that a crystalline solid can be produced. The pretreatment equipment and chemicals increase the ZLD system footprint as well as the capital cost and system maintenance requirements.
Burning or gasifying coal or petcoke produces a gas that can contain sulfur dioxide, hydrochloric acid, hydrofluoric acid, NOx, fly ash, and many other chemical species. In coal-fired power stations, wet FGD systems are used to remove those pollutants from the flue gas. Similarly, a gas-scrubbing step is used in most coal and petcoke gasification processes. Most use wet scrubbing, in which an alkaline agent dissolved in water reacts with and removes those noxious constituents from the flue gas. Wet FGD typically requires a continuous blowdown to limit the accumulation of corrosive salts and suspended solids absorbed from the gas stream.
The composition of wet FGD wastewaters varies widely, although they are primarily chloride solutions. There may also be a large concentration of nitrate or formate, depending on the conditions of combustion. The dominant anion in the wastewater depends on the sorbent used as the reagent in the wet FGD; typically, it is calcium carbonate (limestone), sodium hydroxide (caustic soda), ammonium hydroxide, calcium hydroxide (slaked lime), or magnesium hydroxide.
Therefore, wet FGD and IGCC wastewaters are typically solutions of highly soluble salts such as calcium chloride or sodium formate, usually in the range of 5,000 to 40,000 mg/L TDS. Discharge of these wastewaters is usually regulated due to the presence of relatively small amounts of toxic contaminants, such as heavy metals, selenium, boron, and organics. Often, some type of treatment is required to reduce or eliminate these toxins from the wastewater before discharge to the environment.
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