Power generation facilities can maintain the quality of their cooling water by utilizing low-power ultrasound technology to prevent algal blooms.
Cooling ponds and canals are an effective way to reject waste heat from power generation facilities, but they are not without their challenges. Algal blooms can disrupt the operation of the ponds and the entire facility, but there are solutions available to control algae, which in turn can reduce pH, turbidity, and biological oxygen demand (BOD) levels.
Cooling ponds are known for their low evaporation rate, as they primarily transfer waste heat to the atmosphere through convection. This helps to minimize water consumption by the plant. Additionally, cooling ponds are considered to have a lesser environmental impact compared to direct cooling systems.
The water source for cooling ponds can vary by facility, often including river water or treated sewage effluent. Impurities such as high levels of dissolved solids are removed by discharging a portion of the water, known as blowdown. The same volume plus evaporation loss is then taken back in, referred to as makeup. Organizations like the U.S. Environmental Protection Agency (EPA) set effluent limitations to protect receiving water quality standards. These limits are often site-specific.
Parameters such as pH, BOD, nutrient levels (nitrogen and phosphorus), and turbidity are often subject to permitting limits. When these limits are met, facilities may be required to stop blowdown to avoid permit violations. However, this can exacerbate problems in the ponds and negatively impact the performance and lifespan of the condenser by causing increased scale formation.
Causes of increased pH, BOD, nutrients and turbidity
The degradation of water quality in cooling ponds that leads to an excess of pH, turbidity, and nutrients can be caused by various factors such as the source water quality and chemical use. When treated sewage effluent is used, the water already contains high levels of nutrients, and increased levels of pH and turbidity. Chemicals added to the water to prevent scaling, corrosion, and microbial growth can also negatively affect the water quality by increasing pH and adding phosphorus.
Cooling ponds are shallow, slow-flowing, and have elevated temperatures, which along with excess nutrient levels create an ideal environment for algal growth. Algae remove CO2 from the water through photosynthesis, increasing the pH. Algal blooms also increase TSS and BOD in the water. When these blooms cause discharge permit violations, the blowdown of the plant is suspended, exacerbating the issue.
Algae are a diverse group of organisms that can be classified into different kingdoms. Some, such as eukaryotic, have cells similar to plants, while others, such as prokaryotic, are classified as bacteria. These prokaryotic algae, also known as blue-green algae or cyanobacteria, can cause significant problems with water toxicity, taste, and odor. They can lead to various illnesses, such as skin irritation, paralysis, and even suspected cases of liver cancer. Every year, many lakes worldwide are forced to close for recreational use due to blue-green algae growth. Governments must allocate substantial budgets for cleaning measures. In the U.S. alone, freshwater harmful algal blooms (HABs) are estimated to cost $4 billion annually.
Algal bloom solutions for cooling water
Preventing algal blooms is crucial to avoid exceeding discharge limits and introducing potential toxins into the receiving water. Removing phosphorus-containing additives used in treatment processes may help, but it’s not always clear how much of the nutrient loading comes from these chemicals. Also, source water used for makeup can introduce nutrients and lead to algal blooms. Other methods to control algal blooms include using additives like aluminum sulfate or metals that bind phosphorus, but these are only effective if external loads of phosphorus are not introduced. Algaecides like copper sulfate can control algal growth directly, but frequent dosing is required due to the continuous introduction of makeup water. These chemicals can also harm nontarget organisms like fish and zooplankton if the water is discharged.
Ultrasound is a method commonly used in other applications such as drinking water reservoirs and recreational lakes to control algae. It uses low-power ultrasonic mechanisms that don’t cause algal cells to lyse and don’t harm multicellular organisms. It works by fixing algal cells in the water column, preventing them from absorbing sunlight at the surface and thus preventing blooming. LG Sonic has conducted over 10 years of research on ultrasonic algae control and its effect on specific algal types in different types of water bodies and variations in water quality. They have created a database of optimal ultrasonic parameters for different algal species and water quality characteristics and continuously update it to prevent resistance. The MPC-Buoy system automatically monitors, analyzes and predicts algal presence in the water body, and adjusts ultrasonic parameters accordingly.