ESEC is top power plant in the world, plant Using reclaimed water as a makeup supply for cooling tower systems, but it comes with its own challenges because of its unique chemistry. This paper introduces the success and challenge with reclaimed large cooling tower operation
One option is treated municipal wastewater, or reclaimed water, which can help relieve strains on diminishing freshwater supplies. In recent years, the use of reclaimed water as a makeup water source in power plants has become more prevalent, especially in drought-prone areas
However, the use of reclaimed water for cooling tower applications represents a dramatic departure from freshwater. Not unexpectedly, reclaimed water presents a number of operational risks and challenges related to the propensity for elevated and variable concentrations of phosphate, ammonia, organics, chlorine, select metals, and other potentially problematic constituents. Refer to table 1
Table 1 cooling operation risk
Municipal effluent treatment is driven by city bylaw, which set discharge limits on a range of contaminants and pollutants. Wastewater treated to at least a secondary treatment level includes standards that are placed on fecal coliform, five-day biochemical oxygen demand, residual chlorine, total suspended solids, and pH parameters. However, what’s important to note is that secondary treatment does not set limits or requirements on hardness, phosphate, or total dissolved solids—parameters that are very important for cooling water treatment.
one mechanical draft-cooling towers—each of which comprises 14 cells, a high-efficient film fill, and a recirculation rate of approximately 212,000 gallons per minute—are utilized at the plant to meet cooling needs. A local municipal wastewater treatment facility supplies the plant with reclaimed water treated to a secondary level, which accounts for the entirety of the plant’s makeup water supply.
Table 1: City water quality
| City water EFFLUENT | ||||
| DATE | Flow to ESEC | TSS | CBOD5 | Fecal |
| Energy Centre | Coliform | |||
| 1000m3/day | mg/L | mg/L | CFU/100 mL | |
| CDACS | ASOP-304 | ASOP-316 | ASOP-702 | |
| Average | 16.2 | 8 | 3 | |
| Minimum | 13.9 | 4 | <2 | <1 |
| Maximum | 18.8 | 11 | 5 | <2 |
| Geomean | 1 | |||
| Permit Limit | ≤20 | ≤15 | ≤200 |
1 Suspend solids control
Due to quality of water from city, the average TSS is 10 ppm, So The cloth media filter installation recommendation is based upon the following conditions (as shown on the design sheet): 10 mg/l average daily influent TSS, 20 mg/l peak influent TSS, and an acceptable upstream process such as an activated sludge plant with a minimum SRT of 5 days.
filtration occurs as the amount of particulates on and within the pile cloth media increases, the static pressure required to pass water through the pile cloth media increases. This results in an increased water level within the filter basin and increased differential pressure on the pile cloth media. Upon reaching a specific basin level (83%) or elapsed time period, the backwash mode will be automatically initiated to clean the pile cloth media, the basin level down to 71% when back flash completed. A quiescent environment during filtration, combined with the outside-in filtration flow path, allows heavier particulates to settle to the bottom of the filter basin. Upon reaching a specific number of backwash cycles performed, or an elapsed time, the solids waste mode will be automatically initiated
Figure 1 installed disk filtration
2, Cooling foam control
ESEC facility averages 16-17 million liters of make-up water per day. To promote water conservation efforts, raw water to the plant is sourced from reclaimed water via Calgary’s Wastewater Treatment Plant. It includes high degree of suspended Solids, Total Dissolved Solids (TDS), high organic material. Increased cooling tower bacteriological activity, Cooling water allowing high movement and constant air-water interface, which those factors to cause the white foam produced at the cooling process, it blown into the air and around the facility, making it visible to the surrounding equipment, flying foam creating potential health and environmental hazards
At circulation water intake structure, the foam grows up quickly and occupy the circulation water intake basin and spilling over in this area, block walk way, cause the false basin level alarm and potential trip the circulation water pump and trip the entire plant, due to foam cause the plant reliability issue so each shift operator needs manual quench the foam down with industrial defoamer several time, each time need at least 1 hours.
Fig 2 foam on circulating water pump suction basin
Solutions.
Cooling tower foaming:
- 3 x 50% cloth disc filters were proactively installed on the reclaimed water make up line to the basin. This very likely helped a lot, but it was not enough.
- With assistance from our chemical supplier, multiple anti foam products were tested to determine which was the most effective. A temporary injection system to the basin was simultaneously set up to use
- Twinned our cooling water free chlorine analyzer and installed a cartridge style sample pre filter to increase analyzer reliability
- Permanent skid installed with injection into the condenser cooling water outlet pipe
Fig 3 new installed cooling tower anti-foam pump skid
Results.
The cooling tower foam controlled successfully, the inlet of pump basin no foam built up and top of the cooling can’t see foam, and operator has very happy feedback regards this project and not worry the foam during working in cooling tower to breath the fly foam in the air, the cooling tower basin level is more stable due to foam is not build up on the front of the basin. Significant manpower savings in foam monitoring, abatement and clean up
3 , Cycle increase
SEC condenser use stainless material A249-316, The combination of tensile stress and a specific corrosive environment can crack stainless steels. This mode of attack is termed stress corrosion cracking (SCC). The most common environmental exposure condition responsible for SCC of stainless steels is the presence of chlorides, so chloride limit initially specified by KBV (plant engineering for construction) is 500ppm. The cooling tower operated at approximately 3.2 cycles of concentration due to chloride limitation
Cooling tower use Calgary municipal reclaim water which includes high level of dissolved organic compounds, another factor to impact cycle increase is Organic compounds in circulating cooling water react with sodium hypochlorite disinfectant to form by-products, including trihalomethanes (THMs) (specifically chloroform<0.05ppm).
Further improvements to existing water conservation efforts by increasing our cooling tower cycles of concentration, while staying within condenser metallurgic tolerances as well as city bylaw requirements for plant discharge water is challenge.
Solution.
- Complete a detailed engineering study to determine limits of chlorides, sulphates, hardness, scale inhibitor capacity, as well as chloroform formation, increased chloride limits to 1000ppm no stress corrosion cracking,
- Establish a scaling model to understand the corrosion potential and mitigate with cooling water pH control
- Small, gradual increases to COC with two weeks of monitoring between every increase
- Increased frequency of onsite water quality monitoring with bench tests as well as third party lab tests
- Installed a slip stream blow down line on the cool side of auxiliary cooling water exchanger supply. This flow was incorporated into our total blowdown flow control. Flow was optimized to keep chloroform formation minimal.
- Based on results of water quality monitoring, automatic controls to adjust COC based on ambient temperature were implemented
Results.
Cycles were increased from 3.2 to 5.2 overall. Water chemistry is comfortably within all required parameters. Water consumption has decreased by about 30%. Raw water reduced by about 800 million liters annually and discharge water reduced by about 400 million liters annually. A bonus is that total water cost savings are at least $1 million per year.
4 , Microbiological control
As such, effective microbiological control is extremely important for maintaining efficient and reliable equipment operation. If left unchecked, microorganisms can rapidly accumulate and form biofilms and algae on tower fill, decking, transfer piping, and heat transfer surfaces. This can diminish heat transfer efficiency and promote under-deposit corrosion. Some microbes also pose significant health concerns. To inject the sodium hypochlorite to the cooling tower to control the residue free chlorine to 0.5 ppm, bleach is pumped to cooling tower then use the HAC CL17 online meter with DPD method to test free chlorine then control the beach feed rate. CL17 simple every 2.5 minutes
Online free chlorine test results large depends on the sample cleanliness and microorganism’s contamination, city bylaw requests continuously monitory the free chlorine level
Challenge 1: the accuracy of the online chlorine meter and offline free chlorine test
Solution: install secondary online free chlorine meter and install a cartridge filter to keep the free chlorine test chamber cleaning to provide accuracy reading to provide over feed to cooling tower
Challenge 2, large amount of bleach management
Solution: The truck transport the bleach form 400 kilometers away from Edmonton, the usage for the leach is average per days is 4400kg/days, so every 5 days has a truck to delivery to site, yearly 80 trucks delivery to site, so plant is considering to install larger bleach tank which always maximize the truck transport volume.
Challenge 3: city supply bad water to cause the plant bleach consumption and scale inhibitor increase
When city water treatment has failure in a Bioreactor which is causing the problems. NaClO feedrate is up from ~100L/Hr to ~297L/Hr. Inhibitor feed is also almost triple. City water trying to increase NaClO and ammonia feed rate to help with the bad water. While if the increase too much ammonia on the cooling tower feedwater which reacts with free chlorine to form the chloramine until reach the break points, the cooling tower bleach demanded is equal total chlorine minus the free chlorine, so test the total chlorine can understand how much the ammonia add in then notice city water to control the feed rate of ammonia in the situation of the bioreactor failure. And in the meaning time the order to bleach will more frequency than normal.
In the meaning time the two much ammonia residual in the cooling tower which neutralizes scaling inhibitor 1-Hydroxyethylidene-1,1-diphosphonic acid (HEDP) to cause the inhibitor consumption increase.
5, Corrosion monitory
The chemistry of reclaimed water can also vary significantly over time, with fluctuating levels of problematic contaminants that increase the tendency for deposition/scaling, corrosion, and microbiological growth—all of which can directly damage cooling water system components or impede the ability of traditional cooling water treatment chemicals to function effectively, cooling tower is material is fiber glass, no corrosion issue, condenser is stainless steel, cooling tower riser and circulation water pipe is steel, so there is corrosion issue. So there is no corrosion inhibitor to add in cooling tower
Challenge: to monitory the cooling tower steel material corrosion
Solution: the inside of the condenser chamber sacrifice anode and circulation water pipe and riser pipe is coal tar epoxy coating.
Operation results: cooling tower cell #3 top almost overflowing was found due coal tar epoxy coating on the pipe deboned and liberated. It is found similar issues during the 2018 outage on cell 3 as well. UT thickness tested the pipe thickness. At the time, the coating in the upper section of riser #3 had liberated. Riser #4 was inspected for comparison and appeared okay. UT thickness results were similar for both, with no readings below nominal. liner pieces in cells 13 and 14 could potentially have come from the main CWS piping runs from the condenser, being pushed to the end up the header before making their way to the cooling tower at risers 13 and 14. It can expect to find more pieces moving forward, Liner pieces plugging ports and reducing cooling tower efficiency, two potential risk will challenge for the future facing:
- Accelerated general corrosion: Relatively low risk as similar water applications with uncoated carbon steel pipe. some baseline thickness data was accumulated could refer to for a follow up survey at any point Accelerated localized corrosion
- Localized microbial corrosion under partially failed liner sections is probably the higher corrosion risk, but much more difficult to identify and detect. If this is occurring, pinhole leaks at some point in the future. With the low operating pressure and ability to isolate each riser, there is few options for patching/repair.
5, Scaling Control
In anti-scaling inject to cooling tower using Chemtreat Corrosion inhibitor (CL5437) and control the inhibitor CL-5437 reading between 70 to 80 ppm in the cooling tower, Operator manual test per day to adjust the pump speed input, the consumption rate is 10.2kg/hours , the component is:
:
Picture of scaling skids
1-Hydroxyethylidene-1,1-diphosphonic acid (HEDP) acid with its CAS number 2809-21-4 is a cost effective scale inhibitor used in various industrial applications such as industrial water treatment and detergents. It shows good stability in presence of chlorine as well as corrosion inhibition properties in presence of zinc and other phosphates and can also be used as chelating agent in the textile industry.
Physical State and Appearance: Liquid, Light Straw, Clear Specific Gravity: 1.139 @ 20°C
pH: 3.4 @ 20°C, 100.0%Density: 9.50 LB/GA Freezing Point: 32°F
6, Cooling tower fan operation
14 fans installed at top of the cooling tower to drift the air from top, the logical control the fan speed using the condenser output temperature control fan speed, Vibration sensor installed at each fan for monitory
The challenge is the monitory the cooling tower fan gear box oil sample, the plant operation installs the cartridge filter to each of the bear box to extract the oil to the filter,
In the winter time, plant site temperature can down to -32 degree C, Anti-icing logical set up to automatically reverse spin the cooling tower to suck the hot air from adjacent fan to push back to cooling tower to melt the icing at in take of louver in the winter time
Cooling tower anti-icing automatic During a review of documentation, conflicting information was discovered. In the ICT O&M manual, it is recommended to have the adjacent fans at 50% or greater speed. In a presentation it is recommended to shut down the adjacent fans. The vendor (ICT) was contacted about the de-icing sequence and made a trip to site to review the effectiveness of our current operation. The vendor had no concerns about the operation of the de-icing sequence and saw no issues with the tower. He recommended to continue our current method of operation.
Upon reviewing the de-icing sequence and how operations was using a manual timer to remind them to restart the de-icing sequence, a small change to the logic was made. The change allows the de-icing sequence to continuously operate under the following conditions:
- Ambient temperature less than 0°C
- Manual De-Icing is NOT selected (manual de-icing of single cell)
- Auto De-Icing is selected
If any of these are not true, when the sequence finishes its cycle, it will not restart. This allows operations to focus on the plant rather than re-starting the de-icing sequence.
Gear box install cartage filter and moisture absorber, at cooling tower each VFD installed suction filter to prevent the duct into electrical device, due to VFD generator a lot of heating and in summer time the electrical room will very hot and will trip the VFD which always keep 100% speed operation, On summer time using extra air conditioner connect to cooling tower electrical room for decrease room temperature .
7 cooling tower performance
The impact of the cooling tower performance has several factor, ambient temperature and humidity is most important factor to impact the cooling tower performance, form design aspect, the approach point is important, but after installed the cooling tower scaling will impact the cooling tower performance
8 Cooling tower PH control
Large amount of the bleach injection in the cooling tower will cause the cooling tower PH increase, 16% bleach PH is 12 level, so cooling tower ph control in the range is use injection 98% sulfuric acid to adjust PH level to between 7.8 to 8.2, sulfuric acid is transported from the same plant of bleach which is average injection rate is the system is operation stable, the hours feed rate to the cooling tower is 73kg/hours, the only change is online PH probe reading is drift due to the contamination of the PH probe at the cooling tower water environmental, the solution is to install pre-treatment cartridge filter then replaced to more reliable PH meter and monthly cleaning PH problem, each shift change pre-cartridge filter then can put the pump to auto operation. Through twice of PH manual test to confirm the online reading.
Challenge, cooling tower PH probe reading is not stable, pump running on manual
Solution: Change out current pH probe and transmitter to digital probe and transmitter Current probe is unreliable and prone to failure Operations currently running the acid pumps in manual because the pH probe has proven unreliable. Looking at trying a better technology. Have option to go digital with a new transmitter, or keep analog sensor with current transmitter.
PH probe pictures
9 conclusions
Due to design engineer short of experience for the stage for design the cooling tower, ESEC plant staff through years hard working to successfully overcome all of the challenge, now cooling tower operation at high efficiency and high reliable level at different season and completely meet city bylaw requirements, reach the top of power plant criteria