Field monitoring and numerical modeling as tools in effect-evaluation of waste heat and chlorination byproducts via cooling water discharges


Power plants and other industrial facilities apply cooling water to remove waste heat from their condensers and heat exchangers. These industries are obliged to predict the thermal and environmental effects on the water bodies on which the discharge takes place in order minimize negative impact on aquatic organisms. The general approach of EC Water Framework Directive acquires the evaluation of these effects by industrial releases of priority (hazardous) substances and waste heat. For all water bodies environmental quality standards have been determined. Also, corresponding guidelines have been formulated by the national Dutch authorities to regulate waste heat released by chemical - and power plants. In order to evaluate the effect of discharge of waste heat and chemicals applied in cooling water for fouling control and system optimization, environmental risk assessments are obliged for acquiring the permit. To assess the impact of environmental releases, it is necessary to estimate the extent to which the pollutant disperses in the aquatic environment.

Chlorine is worldwide still the most applied method to prevent fouling in a cooling water system. However, chlorination of (sea) water results in the formation of compounds such as chloroform, bromoform and trihalomethanes. Although previous studies in the past showed that there was no proven negative effect of CBP's in the outlet area, it is beneficial to get more insight in the fate and effects of these compounds in the outfall area before allowing these discharges seen in the light of the consents.

This paper gives backgrounds and examples of environmental impacts due to the discharge of heat and chlorination byproducts. To gain insight, 3D modeling is applied, specifically the modeling system THREETOX, a thermo-hydrodynamic model, which allows to simulate the transport and mixing of cooling water, in both the freshwater and the marine environment. This code initially was developed within the framework of the EU-decision support system RODOS (Real-time Online DecisiOn support System) for supporting the emergency response to nuclear accidents, as one of a large set of aquatic dispersion models for runoff, rivers, lakes, coastal regions (marine environment), groundwater. Later on, the model, was adapted to the specific problems of heat dispersion in surface waters and was applied in many different studies. It has been enhanced with specific sub-modules for sluices and locks, tidal plains, the effect of bottom-vegetation, drying-and-wetting, and the effect of shipping on the vertical mixing in canals. The model originally developed for radionuclides, water quality, and heavy metals, has been used to model chemical discharges, including chemical first and second order decay and chemical reactions. The objective of the modeling approach is to incorporate spatial and temporal scales into exposure and risk assessment.

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