Power generation need not be another source of strain on diminishing freshwater resources.
Well-established water reclamation technologies are enabling producers to conserve resources and money.
Highly available, sewage is a virtually risk-free source. It has consistent quality and temperature compared to surface waters. Because secondary effluent is relatively consistent in quality, the treatment process, and the design and operation of the water treatment system, become easier. In addition, the cooling tower blowdown may be able to be returned to the municipality, eliminating one of the waste streams requiring treatment at the power plant.
On the operational side, unique treatment issues posed by utilizing reclaimed water include the need to determine cleanliness levels that must be achieved to satisfy a plant’s individual operational requirements. Reclaimed water’s chemical elements can cause problems like mineral scaling, corrosion, stress cracking, and biofouling. These problems can increase in closed-cycle cooling systems when water evaporates and leaves behind higher concentrations of constituents.
- Does the plant need to meet a specific legal or regulatory performance requirement?
What is the level of water risk in the local area?
- What are the costs?
The costs of technologies can vary widely. Investment in disc filtration, for example, even for a very large flow may be as low as $500,000 to $1 million. While ultrafiltration costs may be three to four times as expensive, other savings such as from running cleaner water through the system or lowering chemical costs may favorably impact life cycle costs. The costs of freshwater resources also are rising in some places and beginning to reflect the true costs of water.
- What are the benefits?
These may include less immediately tangible, but still important, benefits to the power company’s image with key stakeholders.
- Are there publicly owned treatment works (POTWs) nearby to keep the costs of transporting wastewater sufficiently low?
A study by the University of Pittsburgh found that 97% of power plants proposed in the U.S. could meet their cooling needs by utilizing secondary treated wastewater from POTWs located within 25 miles.
- What technical option is the best approach?
Alternatives could include clarifying systems, disc filters, biological processes, submerged microfiltration, or ultrafiltration membranes.
- Are there special issues that need to be addressed?
For example, some POTWs have low ammonia levels, while others can be quite high. Chlorine treatment is one option, but breaking down the ammonia requires high levels of chlorine, creating new risks and adding costs. Biological systems may negate some of these concerns, although requirements to maintain a minimum flow circulation even during planned power outages can be a drawback.
- Is the power company comfortable operating the water treatment system, especially if it’s biological?
If not, alternative solutions may be possible, such as having the POTW host and operate it, or outsourcing the operation to the system supplier
Based on the responses to these and other questions, the right combination of primary and secondary systems to provide the appropriate level of water purity at a reasonable cost can be determined with the help of an expert systems solution provider.
Power award Wave
A pretreatment strategy was the approach applied in the growing city of Mankato, Minnesota. The city installed a new water reclamation facility (WRF) to treat effluent from its wastewater treatment plant (WWTP), which would supply the cooling tower needs of an electrical generation plant. In addition to providing quality reuse water for the energy center, the WRF needed to meet new state phosphorus removal regulations.
The city turned to Veolia Water Technologies, a global expert in optimizing water use and wastewater treatment. Veolia provided a twostage treatment process using a combination of its Actiflo® and Hydrotech™ Discfilter processes.
The first-stage Actiflo® process is a compact, extremely high-rate clarification system that utilizes the combination of coagulation, flocculation, and sedimentation, using microsand as a seed for floc formation. The microsand provides surface area that enhances flocculation and acts as a ballast or weight. This first stage was designed to provide phosphorus removal for all of the WWTP’s current and future needs. The second stage provides additional filtration to meet the California Title 22 water reuse requirements, which focus on suspended solids and effluent turbidity reduction.
The system enabled the city to avoid supplying water from its local surface and groundwater supplies to the power facility in order to accommodate the plant’s needs. Annual savings for the city from the process changes were estimated to be about 680 million gallons of water and $1.5 million in potable water costs. In saving its natural water supply and monetary expenses, the city was able to turn waste into a resource.
The effluent water characteristics produced by the Mankato treatment facility are:
Total phosphorus <0.4 milligrams/liter (mg/L)
Total suspended solids <5 mg/L
Turbidity <0.6 nephelometric turbidity unit
Biochemical oxygen demand <2 mg/L
The water reuse project was the first of its kind in the state of Minnesota and one of the first in the nation. The Minnesota chapter of the American Public Works Association gave the Mankato facility its Project of the Year award following the treatment process upgrade, and it was also honored with a Minnesota Government Reaching Environmental Achievements Together (MnGREAT) award.
Veolia has also helped clients in custom-designing biological processes to resolve other specific treated effluent challenges, such as ammonia. In New Jersey, Veolia furnished West Deptford Energy with a BIOSTYR® biological aerated filter and Hydrotech™ Discfilter system, allowing effluent from a municipal wastewater plant to be reused in the operation of their new environmentally friendly energy station.
The use of recycled water has proven to be a win for the environment and for West Deptford Energy in saving significant chemical oxidant costs while making their power station a model of highly efficient and sustainable energy generation.
After the power plant was operating, West Deptford Energy decided to add ultrafiltration (UF) to further treat the effluent from the Discfilters. The UF treated water was a suitable source for their existing boiler feedwater system, which further reduced their reliance on city water and the cost associated with it.
As the strain on freshwater resources intensifies and energy demand grows, the power industry is increasingly turning to reclaimed water as a highly valuable resource. Reclaimed water offers a win-win solution that ensures the continued ability to respond to rising global demand by an inseparable pair.