Power plants need a consistent, reliable source of water for cooling tower makeup that is also cost effective and sustainable. In many cases, cooling tower makeup water must be treated to soften or remove solids and, in some cases, deal with organics. With properly treated water, the potential for scaling in the cooling tower is reduced significantly. In turn, this reduces cooling circuit cleaning requirements, extends the life of the cooling equipment, and reduces the cooling tower blowdown flow to the environment.
For treating cooling tower blowdown, systems that remove silica, hardness and other suspended solids are used. Additional technologies can be implemented to further treat the water before it is returned to the cooling towers, thus significantly reducing wastewater and sludge volume.
Choosing the Right TechnologiesAmong other things, the choice of technologies and the specific treatment train depend on:
- The raw feed water quality.
- The desired makeup water quality.
- The flow requirements.
- The target cycles of concentration for the cooling tower.
- The target blowdown flow from the cooling tower.
- The discharge limits.
- The sludge treatment/disposal method.
Treatment technologies for cooling tower makeup can include clarification and softening systems. However, traditional clarifiers rely on long detention times and rise rates of 0.5 to 1.5 gallons per minute (gal/min) per square foot to deliver the required effluent quality. These systems typically include a solids contact clarifier or a mixer/clarifier/flocculation tank and a sludge thickener for waste handling. Because these steps can use separate basins, the system has a large footprint - approximately 1 gal/min/ft2 or less in some applications. These systems also take longer to produce usable water from startup due to longer basin retention times, which can be a disadvantage to peaker plants.
High-Rate Clarification TechnologyTechnologies have been introduced that allow higher rate systems with reduced detention times and waste volume. These include rise rates up to 6 to 8 gal/min/ft2 and even higher in some stages of the process treatment train, while maintaining excellent effluent levels. One such unit is a high-rate clarifier incorporating a thickening process. While traditional clarification systems use separate basins for the different process steps, the high-rate clarifier consists of a solids contact reaction chamber, clarifier with tube settler and gravity thickening in a single process train or unit. This allows a greatly reduced footprint and as much as 10 times higher sludge concentration. For flow rates up to 1 million gal/day, the high-rate unit is contained in a single, round, steel basin; for larger flows, a concrete process train is used. The footprint reduction is important for power plants with limited space for expanding their existing water treatment systems.
Power plants that need rapid increases in water during peak demand require a system that can quickly provide sustained quality levels for water use. Due to its process, the high-rate clarifier can provide this, and it is capable of continuous sustained operation. It uses a simple controller that does not require a PLC unless chemical feed systems or sludge dewatering systems are added. Limited operator monitoring is needed once an appropriate sludge level is developed in the reactor. It aids suspended solids and heavy metals removal and can provide softening. The effluent water produced may not require additional filtering before the water can be used or discharged.
When raw water enters the process train, it is treated with a coagulant and a polymer to facilitate flocculation formation. Softening will require lime or caustic addition. The clarifier’s reaction chamber is designed with a flow rate approximately 10 times that of the influent flow. The combination of high shear mixing in the chamber and flocculation dewatering allows for high sludge settling rates and sludge densification. While a conventional clarifier produces a sludge density of 1 to 2 percent, the high-rate clarifier produces a sludge density of approximately 10 percent without having to use a thickener basin. In a properly designed and operated high-rate clarifier system, effluent water consistently contains less than five true color units, and turbidity levels of less than 2 NTU. The system can be used ahead of a filter, but the operator should pay attention to the amount of polymer carry-over that may occur. Excessive levels of coagulant carry-over can result in reduced filter run times, increasing the wastewater amount.
Packaged Treatment SystemsWhen lime softening is not required but the plant’s source water quality varies significantly, packaged water treatment systems that incorporate a multi-barrier design are well suited. For instance, one packaged system uses a two-stage clarification process followed by a mixed media filter to successfully treat raw water with high turbidity levels.
In this system, coagulant and polymer are added in the first stage to aid flocculation. The dosed water is forced up through settling tubes where large flocculation particles settle to the bottom of the tank. Periodically, a sludge blowdown is performed. In the second stage, the water is pumped through a buoyant media. Because buoyant media is used, this type of clarification cannot be used for lime softening due to the scaling effects. However, it is effective for solids removal, particularly in cold water environments, because the solids do not need to settle to the bottom of a basin.
The first stage reduces the turbidity level in the water by approximately 95 percent, while the secondary clarifier traps and retains smaller flocculation particles and reduces the effluent from this stage by an additional 95 percent. In general, systems recirculate around 10 percent of the sludge collected in the first clarification stage. By recirculating a percentage of the separated sludge into the influent stream, large and/or rapid changes in raw water turbidity result in small changes seen by the treatment system. This process results in efficient use of chemicals and improved flocculation and retention in the clarification stage. For example, the water coming from a river may be 100 NTU, but by adding sludge from the first stage, the incoming turbidity is elevated to around 300 NTU. Now, if the level in the water rises to 150, the amount of sludge is reduced to maintain the turbidity around 300. Also, because the sludge that is recirculated has already been doused with chemicals, any unused amount will be reused in the second pass and also enhance flocculation.
After the water passes through the clarification stages, it is filtered through a mixed media gravity filter. The media is tailored to meet the effluent requirements, based on raw water conditions. The filter must be periodically backwashed to waste, typically once every 24 to 48 hours, to remove contaminants trapped by the filter. When further polishing of the water is not required, the filter can be removed from the system and the unit will still deliver quality water.
The packaged system provides a net water production above 95 percent while removing up to 70 percent TOC and generating low total waste volume. For flow rates below 3 million gal/day, complete fabricated steel packages are available. For larger flows, a concrete process train is typically used.
In many cases, this high-effluent quality water can be combined with raw source water and still meet applicable limits.
In conclusion, initial capital equipment costs may be higher for high-rate water treatment systems; however, these systems provide reduced installation costs because major assemblies can be prefabricated before being transported to the site. They may provide automated methods for reducing phosphorus, true color, suspended solids, BOD, and COD from surface or ground water.