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Report Tightening NPDES Guidelines: Industrial Plants Beware Multiple methods of dealing with waste stream discharges are examined in light of the recent clampdown on industrial processes. By Brad Buecker I began my career in the power industry in 1981 at a coal-fired plant in the Midwest. At that time, U.S. EPA National Pollutant Discharge Elimination System (NPDES) guidelines focused upon a small core of primary impurities in wastewater discharge streams. These included total suspended solids (TSS), oil and grease (O&G;), pH, and free chlorine (or other oxidizing biocide). A common guideline is shown below in abbreviated form. Table 1: An abbreviated NPDES example Constituent Monthly Average (Limit Or Range) Free Available Chlorine 0.2 mg/L O&G; 10 - 15 mg/L pH (range) 6.0 - 9.0 TSS 30 mg/L My former plant, like many other plants then, utilized once-through cooling, so these limits were often easy to achieve. The majority of problems arose at coal-fired power plants from the discharge of coal-pile runoff ponds and wet-ash disposal ponds. The constituents in these streams that required the most oversight tended to be TSS and pH, but straightforward methods were available to control this chemistry. The Times They Are A'Changin' In the power industry, the development of shale gas combined with the perceived "war on coal" has led to the rapid planning and installation of simple- and especially combined-cycle units for new power generation. But due to pending 316b water regulations, which are designed to protect marine life from destruction in cooling water intakes, all of the many combined-cycle RFPs I have seen in the last two years specify a cooling tower or, in some instances, an air-cooled condenser.1 For other heavy industries, cooling towers are also quite common. The key point in this regard is that new NPDES guidelines are focusing upon many additional impurities besides those outlined in Table 1 and where the concentrating nature of cooling towers can put tower blowdown in violation 18 wateronline.com ■ of new guidelines. While it is well-known that the EPA is preparing new regulations, some states have already implemented stricter guidelines, examples of which are outlined next. A general parameter that is appearing in new NPDES regulations is total dissolved solids (TDS). This has very important consequences for cooling tower operation. Consider an example provided by new friends at a power plant in one of our southern states. Prior to this year, the plant's NPDES permit only called for monitoring of TDS. The new permit will impose an average monthly limit of just slightly over 1,000 mg/L. Due to the very nature of cooling-tower operation, the impurities in the makeup increase in concentration due to evaporation of water from the tower. Say, for instance, that the makeup water contains on average 400 mg/L TDS. If the wastewater guideline is 1,200 mg/L, the tower cycles of concentration (COC) are limited to three. The effects of COC on blowdown volume are quite evident in the figure below. The larger discharge volume at lower COC can create problems in locations where the discharge quantity is also regulated, a situation that has quite notably arisen in California. Figure 1: Effect of cycles of concentration on blowdown per conditions shown on the chart. Water Online The Magazine