Webcast Q&A: How Lakeland Electric Maintains Production Efficiency, Lowers Cost, and Conserves Fresh Water With Reuse of Reclaimed Water for Cooling: Questions and Answers
Following the Dec. 11 Plant Engineering Webcast, "How Lakeland Electric Maintains Production Efficiency, Lowers Cost, and Conserves Fresh Water With Reuse of Reclaimed Water for Cooling,"there were a number of viewer questions that could not be answered during the Webcast itself.
Ken Riddle, Supervisor of Chemical Processes for Lakeland Electric and Kevin Milici, Global Marketing Leader, GE Water & Process Technologies, provided answers to the viewer questions. To review the full Webcast, go to www.plantengineering.com/Webcasts or click on this link:
Q: What was the cost of installation?
A: In the case of reusing municipal wastewater, the cost of installation depends on many factors such as the location of the municipal plant, location of available pipelines, and if any additional treatment equipment will be required to reuse the water. In some instances, the municipal water source may be close to the plant and of high quality so minimal tie-ins and no additional treatment equipment are required. In other instances, a pipeline may need to be run and additional treatment may be required.
Q: What is the total cost of ownership?
A: Cost of ownership will depend on the cost and quality of the municipal water versus the cost and quality of the current water source as well as any capital costs that will be required. A first pass analysis would factor in influent water cost + chemical treatment cost + discharge water costs + capital costs. The local municipal authority would be able to provide an estimate on charges for the influent water. A GE Water representative can help organize a complete analysis.
Q: What are the problems to be on the lookout for?
A: Some typical problems that need to be considered include modification of the cooling water microbiological program, estimation of new cycles of concentration, saturation index modeling to determine the plan for controlling deposition, varying levels of phosphate and ammonia in the makeup water, potential for higher corrosion rates, need for improved chemical feed and results monitoring and discharge permit issues. A company with direct experience in this area can provide answers to each of these as well as assisting in identifying other issues.
Q: What are the different types of feedwater sources?
A: As we covered in the Webcast, the potential sources for reuse/reclaim water can come from outside of the plant, and/or from inside the plant. An example from outside the plant would be secondary and tertiary treated municipal waste water. From within the plan, common sources are reject water from reverse osmosis systems, boiler blowdown and process wastewater streams.
Q: What can be done with the blow down water that has 9+ turns and conductivity of 1400 ppm?
A: Water at ‘9 turns’ and 1400 micromhos would not necessarily be precluded from reuse applications. However, a careful analysis of the composition (concentration of specific impurities) would be warranted. After reviewing the analysis, a list of options can be developed.
Q: How do you control micro load in reuse applications?
A: This is typically done with a combination of oxidizing and non-oxidizing biocides, or in some cases, just a based oxider. The point of application is the cooling tower basin or in some instances, the cooling water makeup system. A biosurfactant is also used on occasion. Common oxidizing biocides that have been used in reuse applications include bleach, bromine, and chlorine dioxide. The overall plan should factor in plant requirements and capabilities, feed frequency, monitoring and control, as well as biocide selection. A GE Water representative can review the options with you. Control can be maintained based on targeted specifications for oxidant level, often in combination with biological activity monitoring methods.
Q: How is the polymer measured?
A: The actual functional chemistry of the polymer is measured directly with the GE’s TrueSense for Cooling automation and control technology. In addition, the polymer can be measured with an off-line test method as well.
Q: What method do you use for measuring biocide? ORP, DPD analyzer or something else?
A: The method that’s used to measure the non-oxidizing biocide residual will depend on which non-oxidizer is used and on the water chemistry. In this application, sodium hypochorite (bleach) is the sole biocide. The TrueSense for Cooling automation measures free chlorine directly. In addition, the onboard Oxidation-Reduction Probe (ORP) measures that as well.-. We have seen cases where ORP is used in combination with DPD. In other cases, free and total chlorine residual is monitored with DPD in both the makeup water and cooling tower. And if chlorine dioxide is the primary non-oxidizing biocide, then an on-line ClO2 monitor and/or manual ClO2 residual tests may be run.
Q: Have you had success in using reverse osmosis reject as cooling water make up? We have a water softener in front of our reverse osmosis machine.
A: Yes, using RO reject as a partial or complete source of cooling water makeup is a very common practice. However it should be evaluated by an expert such as a GE Water representative to uncover any potential problems. For example, the cycled up concentration of chloride should be determined if there’s stainless steel in the system.
Q: What biocides are you using and are they fed continuously or shot fed?
A: In this application, sodium hypochorite (bleach) is the sole biocide and it is fed automatically as needed to maintain a targeted ORP level.
Q: Do you have foaming issues and if so how do you handle them?
A: While not specific to this application, we have seen cases where foaming is an issue with reclaim water. This is usually handled with a defoaming or antifoam agent. The key to solving the problem often depends on selecting the right additive, injecting it in the right location, and coming up with a control system to optimize performance.
Q: How much of your blowdown are your recycling/reclaiming back into the towers? How are you maximizing this?
A: The Webcast addressed the use of municipal wastewater as a source of cooling tower makeup water. So there is no recycle of blowdown back to the cooling tower. In some cases, boiler blowdown is recycled to cooling towers or blowdown from one cooling tower is cascaded to another cooling tower.
GE Water does have experience recycling cooling tower blowdown back into the cooling tower. This technology needs to be evaluated on a case by case basis.
Q: Do you reuse cooling system blow down water at Lakeland?
A: We do not reuse cooling system blowdown. It is discharged to our combined discharge, goes to our wetland project, and eventually goes to another power station to our south where it is reprocessed by RO and reused again.
Question: What is the cost of water reuse versus that of fresh water? How much energy is consumed to produce 1 liter of reused water?
A: I haven’t done a balance on the power. I’m sure more energy is used in total in the reuse water since it has to be pumped multiple times, but it is reused multiple times as well. So it’s a better use of the resource. If you balance the re-pumping with the reuse, it is probably more energy efficient overall. It’s something I will have to explore. Cost of reuse water paid back to the city is $0.18/1,000 gallons. So it is more expensive per 1,000 gallons than simple pumping cost for well water. If you factor life-cycle of the pump, well, maintenance, piping, new wells, and all the costs, it is cheaper in the long-run. The previous use for domestic purposes covers those costs for you.
Q: What’s the reliability, load dispatch and stability in the water supply?
A: Since we keep about two day’s supply in storage tanks, and we have the ability to use ponds and well water if needed, we don’t have major interruptions. It is something for each site to study and mitigate prior to making a switch to reuse.
Q: Are there major fluctuations in the water quality to be treated? How often is the water analyzed? What constituents are analyzed?
A: Most parameters are relatively consistent. In this case, dilution is a solution for most problems. It takes a large event to make much of an upset. We analyze the water extensively each day and less extensively several times each shift 24/7. The main parameters are included in the presentation.
Q: Can you provide details about how the system Lakeland used to reclaim used water works?
A: In our case, we use oxidation ditches, secondary, and tertiary treatment. It is a typical municipal wastewater treatment plant with an industrial pre-treatment requirement for local industries to meet the EPA standards for industrial discharges. The only special treatment is the additional tertiary treatment step to assure suspended solids don’t enter the reuse. It is treated with chlorine in the tertiary step, but this is more and more common for municipal sewage these days.
Q: I do not know enough about Lakeland to ask any specific technical questions, but the blow down treatment is my primary interest. Can you provide information on that?
A: We don’t treat the tower or boiler blowdowns. The boiler blowdown goes to our process ponds for reuse in the plants. The tower blowdown goes to our combined sump for discharge through our treatment system then to the wetlands. It could be treated by RO and then reused again, but is not currently cost effective. Instead it is combined with other water discharges and sent to the wetland. From there it goes to another plant for additional reuse.
Q: What chemicals were used to control growth holding ponds? Is maintenance required on heat exchangers?
A: No special maintenance is needed on the heat exchangers. They actually stay much cleaner than on the well water. We do treat with bleach to control cooling tower microbiological growth as we would with well water make-up. We use tanks not ponds to store the reuse water. It is treated with chlorine for biological control before it reaches us. The dark tanks and chlorine helps control growth in the water that would be an issue in an open pond.
Q: What production efficiency was managed as a result? Can you share results?
A: The main production efficiency is the cleaner heat exchanger systems/cooling towers. Power production has not been adversely impacted.
Q: Water quality for reuse, what is the cost per quality/volume?
A: $0.18/1,000 gallons. In addition, this provides the wastewater plant (from whom the water is purchased) with an additional revenue stream.