ABSTRACT
As a general rule, as more water is withdrawn from a surface water body, more aquatic organisms are likely to be impinged on the intake structure and entrained through the cooling-water system. Following this assumption, in most cases a cooling system that reduces the volume of water withdrawn will concomitantly reduce the number of organisms injured or killed by impingement and entrainment. Because this relationship is not linear (e.g., 10 times less flow does not necessarily result in 10 times less impingement and entrainment), the actual effects must be evaluated on a site-specific basis. Moreover, not all impinged and entrained organisms are killed or otherwise removed from the ecosystem. Nevertheless, the adverse impact associated with impingement and entrainment will usually be reduced when less cooling water is withdrawn. The highest impact in this category is associated with once-through cooling systems. Power plant once-through cooling systems typically withdraw in the range of tens of millions to billions of gallons per day. The volume of makeup water required for wet cooling towers is many times less than that used in once-through systems. The percentage reduction varies but generally falls in the range of 1% to more than 10% of the once-through flow volume. Dry towers use even less cooling water than wet cooling towers. Cooling systems that rely on moving large volumes of air by fans – mechanical-draft wet towers and dry towers – may create their own form of impingement and entrainment. Insects and birds can be drawn into the intake plumes of large fans. Larger organisms can be trapped on the exterior of the fans or their intake coverings like insects caught on automobile radiators. Smaller organisms can be pulled through the moving fan and injured or killed. The authors are not aware of any published literature quantifying this impact, but the parallels to aquatic impingement and entrainment are obvious. Potentially, large batteries of fans may inflict harm to local populations of endangered insects or birds or important pollinator species. To place this in perspective, a dry tower installed to cool a power plant-sized unit might have banks of fans that cover several acres. For example, a dry tower system at a 40-MW geothermal power plant near Reno, Nevada, employs 240 fans covering a large surface area. Photographs of that facility are available at https://home.nvbell.net/sbgeo/steamboat.html. Impingement and entrainment, when they result in death or harm to an organism, create an adverse impact to that organism. However, they do not necessarily create an adverse impact on the population or ecosystem at large. The principle of compensation – enhanced reproductive output by populations that have experienced loss of young members of the population – is welldocumented in the literature. Compensation may serve to dampen the population-wide effects of impingement and entrainment. A thorough review of
compensation in fish populations was submitted to the EPA by the Utility Water Act Group as part of its comments on the Section 316(b) new-facility proposed rule[8].
