NPDES COMPLIANCE OF COOLING TOWERS BLOWDOWN AT POWER PLANTS WITH RECLAIMED WATER AS SOURCE WATER Nathan Schmaus, P.E. *, Joseph Viciere, P.E., BCEE, CDM Smith CDM Smith, 1715 North Westshore Boulevard, Suite 875, Tampa, FL 33607 * Corresponding Author, CDM Smith, 1715 North Westshore Blvd., Suite 875, Tampa, FL 33607 ABSTRACT Over the years, municipal reclaimed water has been used as a reliable source of makeup water by many power plants for their cooling towers. However, because of the nutrients discharged with the wastewater and their increased concentrations in cooling towers due to evaporation of the water, power plants may often have to operate their cooling towers in such a way to still maintain compliance with their National Pollutant Discharge Elimination System (NPDES) permit. CDM Smith has conducted studies to evaluate the occurrence of high nutrient concentrations (ammonia-n, nitrate-n) in cooling towers using reuse wastewaters and their impact on cooling tower operation. These studies include an evaluation of the wastewater plant processes where the reclaimed water originates, making an engineering judgment as to whether corrective actions need to be taken to reduce the occurrence of high nutrient levels in the effluent wastewater even when it meets the discharge permit effluent concentrations, and proposing recommendations, as necessary, to allow power plants to maintain compliance with their discharge permit limits. The objective of this paper is to present a review of the conditions, design, and performance history of the treatment plants providing reclaimed wastewaters for cooling towers and to assess the reliability of these facilities. This review includes calculating average and maximum concentrations for ammonia-n and nitrate-n in the source reuse water and developing trend analyses for each facility to estimate expected water quality. Using the expected water quality estimates and cooling tower operational information a mass balance around the cooling tower system is developed in each case study to estimate expected effluent concentrations of ammonia- N and nitrate-n across a range of cycles of concentration. Historical data from wastewater treatment plants effluents considered as source waters for the cooling towers are analyzed to determine if improvements to the operations of these facilities would lower the concentrations of nutrients in their reclaimed waters. Similarly, operation parameters of cooling tower systems are scrutinized to determine the expected effluent quality of their blowdown based on the nominal cycle of concentration at which these systems are normally operated. Trend analysis and mass balance analysis are performed on the cooling tower systems. A series of recommendations for possible actions that could increase the reliability of source waters from treatment plants and cooling tower system operations is presented in this paper for two different case studies. However, an engineering and economic evaluation in each case would be necessary to determine the feasibility and benefits associated with these recommendations. KEYWORDS Feed Water, Makeup Water, Cycles of, Blowdown, Cooling Tower 1-1
INTRODUCTION Thermal power plants utilize various types of energy sources (nuclear, municipal wastes, natural gas or coal, to name a few) to heat feed water in high pressure boilers to generate steam. Once pressurized, the steam enters a steam turbine coupled to an alternator to convert the mechanical energy into electricity. Leaving the turbine, the exhaust steam is converted into water (to be reused as feed water) in a condenser by using reclaimed water as cooling water recycled through cooling towers. The recirculating cooling tower system reduces the temperature of the circulating water, dissipating the heat by forcing ambient air through the cascading water. The cooled water is then recirculated and mixed with the feed water. The following diagram (Figure 1-1) shows the basic diagram for a thermal power plant. Feed Water Pump Cooling Tower Boiler Steam Turbine Figure 1-1 Thermal Power Plant Diagram Makeup water, which may be supplied from potable water, surface water, groundwater, or reclaimed water, is used to offset the water lost by evaporation, drift loss, or blowdown in the cooling tower system, as shown in Figure 1-2. Recirculating water is removed from the system by blowdown in order to control the water quality in the cooling tower system. Evaporation Makeup Water Circulating Water Drift Loss Blowdown Figure 1-2 Cooling Tower Circulating Water Mass Balance METHODOLOGY The frequency and amount of water removed from the system as blowdown are determined by the following factors: 1. Initial Makeup Water Quality 2. Operating Water Quality Parameters 1-2
3. NPDES Permit Limitations 4. Cost of Water Supply 5. Other Regulatory Restrictions (Water Use Restrictions). Initial Makeup Water Quality The initial makeup water quality is determined by the source water used by the facility. Reclaimed water is the focus of our analysis. It can be an attractive option since it is often an underutilized resource that is becoming more valuable due to increasing water use restrictions and resource protection. Since using a limited and expensive resource such as potable water, in industrial applications, is no longer a sustainable and cost-effective option, reclaimed water is often considered to offset the use of potable water. The quality of the reclaimed water is dependent upon the wastewater treatment facility treatment process where it originates and the regulatory maximum discharge limitations for that treatment facility. Regulation Requirements for Reclaimed Water Used in Cooling Towers In the State of Florida, Rule 62-610.668(2), Florida Administrative Code, regulates the use of reclaimed water in open cooling towers. At a minimum secondary treatment is required to achieve the effluent limitations specified paragraph 62-600.420(1)(a), F.A.C. These effluent limitations provide a baseline water quality of the reclaimed water that may be available for use as makeup water. Reclaimed Water Quality and Operational Impacts Some of the typical compounds that are often found in moderate concentrations in various reclaimed waters are Alkalinity, Silica, Iron, Calcium, Magnesium, Ammonia-N, Nitrate-N, Phosphate, Coliform, Sulfate, Total Dissolved Solids (TDS), Total Organic Compounds (TOC), ph, Biological Oxygen Demand (BOD), and Total Hardness. High concentrations of these compounds due to several cycles of recirculating the reclaimed water through the cooling tower system (cycles of concentration) can lead to a number of operational problems including scaling, corrosion, biological growth, fouling, and foaming. Operating Water Quality Parameters The cooling tower system water quality parameters are imposed by the equipment manufacturers based on the equipment materials to reduce operation and maintenance costs of the equipment due to the problems mentioned above. However, these maximum concentrations are those measured in the water circulating through the cooling tower system at the optimum cycles of concentration. Once the concentration of the parameters in the circulating water reaches the maximum allowable concentrations, the circulating water is removed from the cooling tower system as blowdown. There needs to be a balance between the cooling tower system recirculating water quality and the allowable number of cycles of concentration. By reducing the cycles of concentration to maintain adequate water quality, the power plant s water use will increase along with the volume of blowdown discharged, which may also affect the NPDES permit effluent flow limitations. By increasing the cycles of concentration to reduce the use of reclaimed water, the power plant s operation staff will have to utilize chemicals that can help in reducing corrosion and calcium carbonate precipitation potential governed by temperature, ph, sulfate, TDS, and calcium, of the recirculating water. 1-3
NPDES Permit Discharge Limitations If the thermal power plant discharges the cooling tower system blowdown to a regulated body of water, canal, river or stream, the concentrations of pollutants and flow of the effluent water must be maintained below the maximums specified in the NPDES Permit. Table 1-2 provides an example of exceedances of a pollution discharge permit related to Ammonia-Nitrogen for a virtual power plant facility. Month/Year Table 1-2 Example of Effluent Violations Ammonia Nitrogen DAV Outfall No. 001 Ammonia Nitrogen DMAX Ammonia Nitrogen DMAX Loading Limit = 1.0 mg/l Limit = 3.0 mg/l Limit = 23 lb/d April 2011 1.200 c c December 2011 1.740 4.97 23.1 January 2012 1.464 4.95 23.8 DAV = Daily Average; DMAX = Daily Maximum mg/l = milligrams per liter; lb/d = pounds per day; c = compliant Even though a water quality parameter may be below the maximum concentration for good operational practice, it may lead to a violation of the permit daily average or maximum daily concentrations. Given the above water quality requirements, it is paramount that the influent reclaimed water quality be reliable to prevent operational problems and permit violations. The treatment facility s historical effluent concentrations can be analyzed to provide greater insight into the reliability of the reclaimed water quality. RESULTS Overview of the Reliability Assessment of Reclaimed Water Quality The historical results for that virtual plant are reviewed to determine the frequency of the treatment facility operational upsets. For example, Figure 2-1 displays the occurrence of a large spike in the Ammonia-N effluent concentration. This corresponds to permit violations in December 2011 and January 2012 presented in Table 1-2. Comparing this to the treatment facility s records, it was determined that this was due to a process change at the facility. Note, that before the process change from January 2011 to November 2011, the Ammonia-N concentrations often exceeded the daily average discharge concentrations of the NPDES permit and that led to a violation in April 2011. After the treatment facility s process change in January 2011, the effluent Ammonia-N concentrations stabilized and were consistently below the daily average discharge limitation. 1-4
Figure 2-1 Example of Reclaimed Water Quality from the Treatment Facility For any given water quality parameter, the standard deviation can be determined to provide insight into the reliability of reclaimed water quality and how that may affect the operation of the recirculating cooling tower system. Applying this to the example, from February 2012 through September 2012, the mean daily average concentration of ammonia-n was 0.15 mg/l with a standard deviation of 0.05 mg/l and the maximum daily average concentration was 0.24 mg/l. During this same period, the mean daily maximum concentration was 0.28 mg/l with a standard deviation of 0.11 mg/l and the maximum daily average concentration was 0.46 mg/l. Since the sample size is relatively small, the mean concentrations are reported with their respective standard deviations. The standard deviation is the degree to which the concentrations (daily average and daily maximum) for each month differ from their respective sample mean. Table 2-1 Ammonia-N s Average Standard Deviation Typical Range Daily Average 0.15 mg/l 0.05 mg/l 0.10 mg/l to 0.20 mg/l Daily Maximum 0.28 mg/l 0.11 mg/l 0.17 mg/l to 0.39 mg/l The historical concentrations of ammonia-n from February 2012 to September 2012 are used to develop the probability curves that can provide a visual indication of the reliability the reclaimed water quality that can then be used to optimize the system cycles of concentration thereby limiting water use and discharge. 1-5
1.00 Ammonia Nitrogen s (Feb. 2012 to Sep. 2012) 0.90 0.80 Cumulative Probability 0.70 0.60 0.50 0.40 0.30 0.20 Daily Average Cumulative Probabilities Daily Average Ammonia-N Mean Daily Average Daily Maximum Cumulative Probabilities Daily Maximum Ammonia-N Mean Daily Maximum 0.10-0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 Figure 2-2 Cumulative Probabilities of Treatment Facility s Effluent Ammonia-N s (February 2012 to September 2012) Cooling Tower System Model In order to determine the recommended cooling tower operating parameters a mathematical model is developed using the mass balance equation (Equation 2-1) and the cycle of concentration equation (Equation 2-2). The model is executed by inputting the average cooling tower water circulation flow rate, the average rate of evaporation, rate of drift loss, and the influent nitrate-n concentration (specified by the manufacturer). The model is executed in Microsoft Excel using the solver analysis tool at varying blowdown flow rates to determine the corresponding cycles of concentration. X f Cycles _ of _ = X 0 CW + X mw MW X 0 BW 0 EW X 0 DL X f = X mw CW EW DL BW + MW Where, X f = Nitrate-N concentration at Outfall 001, mg/l X o = Initial nitrate-n concentration in the system, mg/l X mw = Nitrate-N concentration in the makeup water, mg/l CW = Cooling tower water circulation flow rate, mgd MW = Makeup water flow rate, mgd EW = Rate of evaporation, mgd DL = Rate of drift loss, mgd Cooling Tower Operation Applying this to our example, the operation of the recirculating cooling tower system with respect to the cycles of concentration is evaluated to determine if current operation will enable the power plant to meet its permit limits or if operational adjustments are needed. The average cycles of concentration presented in Table 2-2 are based on the ammonia-n concentrations and 1-6
the nitrate-n concentrations that were detected in the samples taken from the Treatment Facility s reclaimed water, the power plant s makeup water storage pond, and the power plant s cooling tower outfall, during the period from February 2012 through September 2012. Also factored into the cycles of concentration calculations are the portion of makeup water provided to the cooling tower system from the blowdown out of the heat recovery steam generator, the evaporative coolers, and the wastewater from the boiler water treatment system (reverse osmosis generation reject stream and backwash from the ion exchange). The power plant typically adds a total of 4 gallons of 19 percent active aqueous ammonia-n solution to the steam cycle. Ammonia-N was detected at a concentration of 1.71 mg/l and nitrate-n was not detected. The typical flow rate of this combined stream was recorded to be 0.12 mgd. Table 2-2 Cycles of (February 2012 through September 2012) Cycles of Parameter Mean Standard Deviation Typical Range Ammonia-N 2.6 1.3 1.3 to 3.9 Nitrate-N 4.3 0.9 3.4 to 5.2 Based on the recent operational history of the cooling tower system from February 2012 through September 2012, the cycles of concentration based on ammonia-n would typically range between 1.3 and 3.9, compared to a range between 3.4 and 5.2 based on nitrate-n (Figure 2-3). The lower cycle of concentration determined for the cooling tower system based on ammonia-n is due to the fact that some of the ammonia-n input to the system reacts with the chlorine fed to the cooling tower system as sodium hypochlorite. Thus, the ammonia-n concentration of the recirculated water after a number of cycles of concentration is lower than what it would have been if that reaction did not take place. Consequently, the determination of the number of cycles of concentration in the cooling tower system is best determined based on nitrate-n. 1.00 Cycles of Cumulative Probability Cumulative Probability 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 68.27% 68.27% Ammonia-N Mean (Ammonia-N) Nitrate-N Mean (Nitrate-N) 0.00-1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 Cycles of 1-7
Figure 2-3 Cycles of Cumulative Probability Water Quality Data Analysis and Mass Balance Using the expected influent concentrations and the current cycles of concentration a mass balance around the cooling tower system was developed to determine the expected cooling tower effluent ammonia-n and nitrate-n concentrations. The expected ammonia-n concentrations in the cooling tower effluent at the outfall are estimated in Table 2-3 given the current cycles of concentration and the expected influent water quality. Table 2-3 Expected of Ammonia-N at the Outfall Expected Daily Average Expected Daily Maximum Cooling Tower Influent Ammonia-N Current Cycles of Outfall Ammonia- N Permit Discharge Limitations Mean 0.15 2.6 +/- 1.3 0.39 +/- 0.20 1.0 Mean Plus 1 STDEV 0.20 2.6 +/- 1.3 0.52 +/- 0.26 1.0 Maximum 0.24 2.6 +/- 1.3 0.62 +/- 0.31 1.0 Mean 0.28 2.6 +/- 1.3 0.73 +/- 0.36 3.0 Mean Plus 1 STDEV 0.39 2.6 +/- 1.3 1.01 +/- 0.51 3.0 Maximum 0.46 2.6 +/- 1.3 1.20 +/- 0.60 3.0 Given that ammonia-n concentrations from the Treatment Facility remain within the expected range, based on the data obtained since February 2012 and the assessment that the new, upgraded facility will be able to maintain this level of operation, the daily average effluent concentrations of ammonia-n at the outfall are expected to remain below the permit discharge limitation of 1.0 mg/l. Likewise, the daily maximum effluent concentrations of ammonia-n at the outfall are expected to remain below the permit discharge limitation of 3.0 mg/l. The expected nitrate-n concentrations in the cooling tower effluent at the outfall are estimated in Table 2-4 given the current cycles of concentration and the expected influent water quality. Expected Daily Average Expected Daily Maximum Table 2-4 Expected of Nitrate-N at the Outfall Cooling Tower Influent Nitrate-N Current Cycles of Outfall Nitrate-N Permit Discharge Limitations Mean 33.06 4.3 +/- 0.9 142 +/- 30 137 Mean Plus 1 STDEV 35.66 4.3 +/- 0.9 153 +/- 32 137 Maximum 35.84 4.3 +/- 0.9 154 +/- 32 137 Mean 35.90 4.3 +/- 0.9 154 +/- 32 289 Mean Plus 1 STDEV 39.44 4.3 +/- 0.9 170 +/- 35 289 Maximum 41.30 4.3 +/- 0.9 178 +/- 37 289 1-8
Given that nitrate-n concentrations from the Treatment Facility remain within the expected range, based on the data obtained since February 2012 and the assessment that the new, upgraded facility will be able to maintain this level of operation, the daily average effluent concentrations of nitrate-n at the outfall are expected to exceed the permit discharge limitation of 137 mg/l. However, the daily maximum effluent concentrations of nitrate-n at the outfall are expected to remain below the permit discharge limitation of 289 mg/l. DISCUSSION Cycles of to Meet Permit Discharge Limitations Given that approximately 84 percent of future daily average concentrations of nitrate-n are expected to be below 35.66 mg/l in the makeup water pond, based on the mean plus one standard deviation of the data set, in order for the nitrate-n daily average concentration at the outfall to remain below the permit discharge limitation of 137 mg/l, the cooling tower cycles of concentration are required by the analysis to remain below 3.8 (Table 2-5). Table 2-5 Maximum Cycles of to Meet Discharge Limitations Mean Plus 1 STDEV Nitrate- N Conc. Permit Discharge Limitations Maximum Cycles of to Meet Discharge Limitation Daily Average 35.66 137 3.8 Daily Maximum 39.44 289 7.3 Maximum Nitrate-N at Cooling Tower Influent Looking at it another way, in order to maintain the nitrate-n concentrations in the cooling effluent at the outfall below the permit discharge limitations of 137 mg/l for the daily average and 289 mg/l for the daily maximum, the maximum daily average and daily maximum nitrate-n concentrations in the cooling tower influent would need to remain below 31.9 +/- 6.7 mg/l and 67.2 +/- 14.1 mg/l, respectively, given that the current cooling tower cycles of concentration remain at 4.3 +/- 0.9 (Table 2-6). Table 2-6 Recommended Maximum Nitrate-N Influent Permit Discharge Limitations Current Cycles of Recommended Maximum Nitrate- N Influent Daily Average 137 4.3 +/- 0.9 31.9 +/- 6.7 Daily Maximum 289 4.3 +/- 0.9 67.2 +/- 14.1 Analysis of Cooling Tower Operating Range As shown in Figure 2-4 and Figure 2-5, in order to not exceed the daily average permit discharge limitation 137 mg/l for the nitrate-n concentration the cooling tower operating range is to remain below 3.8 cycles of concentration corresponding to the blowdown flow rate ranging from 0.90 mgd to 1.84 mgd. However, it should be noted that per the discharge permit that the outfall maximum daily average flow rate is to be less than 0.92 mgd and the maximum daily maximum flow rate is to be less than 1.84 mgd. The blowdown flow rate of 0.92 mgd corresponds to the cooling towers operating at 3.77 cycles of concentration. Therefore, the ideal 1-9
operating point is at 3.8 cycles of concentration that corresponds to the blowdown flow rate of 0.90 mgd. The blowdown flow rate may be increased periodically to as high as 1.84 mgd (daily maximum flow rate permit limitation), if there are high influent concentrations of nitrate-n ranging from 36 mg/l to 121 mg/l (Table 2-7 and Figure 2-5). Mean Plus 1 STDEV Nitrate-N in Cooling Tower Influent Table 2-7 Cooling Tower System Model Output Blowdown Flow Rate (mgd) Predicted Nitrate-N at Outfall Cycles of Maximum Permitted Influent Daily Avg. Nitrate-N Maximum Permitted Influent Daily Max. Nitrate-N 35.66 1.84 85 2.4 57 121 35.66 1.70 89 2.5 55 115 35.66 1.27 107 3.0 46 96 35.66 1.02 125 3.5 39 83 35.66 0.90 137 3.8 36 75 35.66 0.85 143 4.0 34 72 35.66 0.73 161 4.5 30 64 35.66 0.64 178 5.0 27 58 35.66 0.61 185 5.2 26 56 35.66 0.57 196 5.5 25 53 Figure 2-4 Blowdown Flow Rate and Nitrate-N versus Cycles of 1-10
Figure 2-5 Effluent Nitrate-N s versus Blowdown Flow Rate RECOMMENDATIONS Overview of Potential Recommendations Based on the Model Predictions and Facility Assessment Continuing with the example, based on the model predictions and the facility assessment, the recommended operating point for the cooling tower system based on the nitrate-n concentration is at 3.8 cycles of concentration, which corresponds to the blowdown flow rate of 0.90 mgd. The blowdown flow rate may be increased periodically to as high as 1.84 mgd (daily maximum flow rate permit limitation) if there are high influent concentrations of nitrate-n ranging from 36 mg/l to 121 mg/l. Additional Treatment Additional pretreatment options can also be considered, such as coagulation, flocculation, sedimentation, and clarification. Water treatment processes and dosing requirements are determined based on the facility s specific initial water quality, existing chemical dosing systems, and the required water quality. Blending An additional method to meet the initial water quality requirements is to blend the reclaimed water with an alternative source of water, such as groundwater, potable water, or surface water. The flow rates from each source can be increased or decreased as the reclaimed water quality fluctuates. 1-11
Influent Water Quality Monitoring Many power plants have historically not monitored the influent concentrations in the reclaimed water that is provided by the wastewater treatment plant (WWTP). It is recommended that power plants begin a monitoring program so that operations can be adjusted in the event of high influent concentrations. The most robust monitoring option would be to install a probe upstream of the power plant, with a transducer that can be periodically checked or even report back to the power plant s control room. A handheld probe would be less expensive and allow for instantaneous readings, which could be used for routine checking of concentrations in a makeup water pond. The least expensive options that would allow for daily monitoring would be a lab spectrophotometer or handheld colorimeter, which would require more labor, but would still give relatively instantaneous feedback. Routinely sampling the makeup water pond to monitor the nitrate-n and ammonia-n concentrations will also allow the power plant to monitor the cooling tower performance (cycles of concentration) and identify any above average concentrations that may be coming from the WWTP. Additional Water Storage Capacity The power plant s water storage capacity determines the risk of exceeding the permit discharge limitations in the event of an upset at WWTP that causes higher concentrations of pollutants in the power plants makeup water supply. For example, say that a power plant currently receives on average 3 mgd of makeup water from the WWTP. It is recommended that power plant increase the makeup water storage volume to provide two or more days of storage, which is approximately 6 million gallons. This will allow the power plant to make the necessary adjustments if there is an operational upset at the WWTP. CONCLUSIONS Each and every facility faces its own unique challenges that require a complete analysis of the existing cooling tower system, operational requirements, available water sources, water quality, and permit requirements. To determine the best modus operandi for a facility, a complete cost benefit analysis is recommended. REFERENCES 1. Chapter 62-610 Reuse of Reclaimed Water and Land Application, Florida Administrative Code, Effective 2-16-2012. 2. Frayne, Colin (1999). Cooling Water Treatment - Principles and Practice. Chemical Publishing Company Inc. 3. Cowan, Jack C. Weintritt, Donald J. (1976). Water-Formed Scale Deposits. 4. (2009). Planning for the Distribution of Reclaimed Water - Manual of Water Supply Practices, M24 (3rd Edition). American Water Works Association (AWWA) 1-12