Sep 13th 2019, Updated May 28th 2021
Cooling towers are used by industrial plants to eject waste heat to atmosphere. Evaporation of water removes heat and results in concentration of ions and metals in the cooling water that remains. A portion of the cooling water is blown down after a number of cycles, before the ions and metals reach their concentration scaling limits. This cooling tower blowdown (CTB) water requires management. Meanwhile, the water lost as blowdown or from evaporation or drift must be replaced with makeup water, typically from a freshwater source.
There are a number of ways to minimize blowdown generation and freshwater consumption by safely increasing the number of cooling tower cycles before the need for blowdown. The number of cycles before blowdown can be optimized using sensors and controls, or by targeting specific contaminants-of-concern such as chlorides or scaling ions.
Before deciding how to manage CTB, key project information must be gathered first:
Considering these details and any other important plant information will allow a critical assessment of options and their economics.
Once the CTB has been generated, treatment solutions vary from relatively minor, targeted interventions, to comprehensive full treatment trains that combine membrane and thermal systems to achieve minimal liquid discharge (MLD) or zero liquid discharge (ZLD).
Understanding the cost of alternate (non-treatment) CTB management options is important for properly comparing total treatment costs and what treatment options are possible within a given budget. To keep these total costs low, it is usually beneficial to achieve the highest RO recovery possible, thus minimizing more expensive volume reduction further downstream in evaporators or offsite transport for disposal.
It should be noted that seemingly small changes in recovery percentages have a large, non-intuitive impact on brine volume. For example, boosting recovery from 95% to 97.5% may seem like gaining “only 2.5%”. However, this means halving the volume of the remaining brine and therefore 50% fewer trucks needed for disposal (or 50% smaller evaporation pond capacity or a 50% smaller downstream evaporator). Savings from boosting recovery can be enormous, as demonstrated in the CTB treatment case study below.
A North American power plant was sending their CTB to evaporation ponds. However, the ponds were reaching capacity and offsite disposal was an expensive option. The power plant sought a treatment option to reduce offsite disposal volumes in order to lower costs. The treated water needed to meet utility water reuse requirements such that TDS < 500 mg/L. The water chemistry is summarized in Table 1.
With a TDS of 1,750 mg/L, the raw CTB is suitable for treatment by reverse osmosis (RO). 80% recovery is achievable with a conventional, primary RO system, reducing CTB volume by 5x. After the primary RO, the volume can be further reduced and recovery increased using other technologies, including ultra-high pressure reverse osmosis. A flexible pilot system with multiple process configurations to analyze performance and cost at incrementally increasing recoveries was tested on the CTB.
Figure 1 shows a simplified process flow diagram of the pilot system. The incremental process steps and their costs are explored below for volume reductions (and recoveries) of 10x (90%), 20x (95%), 40x (97.5%), and 70x (99%). Brine concentration and composition changes with each incremental step in recovery improvement are shown in Table 1. Table 2 shows the recovery at each stage, the volume reduction, the technology to achieve it, and the cost guidance.
| State Point | Volume Reduction | Recovery | Technology | Cost Guide: OpEx + CapEx |
|---|---|---|---|---|
|
A |
5X |
80% |
RO-I with anti-scalants, concentration polarization management, and flushes |
$1–2 /m3 |
|
B |
10X |
90% |
Above + high pH, UF & RO-II |
$2–3 /m3 |
|
C |
20X |
95% |
Above + some soda ash |
$3–3.5 /m3 |
|
D |
40X |
97.5% |
Above + more soda ash |
$3.5–5.8 /m3 |
|
E |
70X |
99% |
Above + UHP-RO |
$4–8 /m3 |
This pilot study and others like it provide cost and performance data for a broad range of process configurations and recoveries. Varying the quantity of soda ash (sodium bicarbonate) alone provided a lot of flexibility in achieving the desired recovery. It is generally advisable to stay below 80% calcium sulfate saturation in the brine to prevent irreversible scaling. Adding soda ash can increase recovery but is typically a costly approach and should therefore be controlled carefully. Saltworks has technologies available to help with keeping chemical consumption costs down.
Besides flexibility with process steps, the proper integration of pilot plant elements is also important for this kind of study. Although each step can be viewed as a unit operation, properly assessing the performance of a treatment chain requires considering an integrated plant with unified process controls. This is recommended for the following critical reasons:
Readers are offered the following reminders and guidance when considering CTB and brine treatment economics:
Project owners do not need to become experts in advanced brine treatment or economic optimization. Contact Saltworks for help and guidance on completing these calculations.
Saltworks Technologies is a leader in the development and delivery of solutions for industrial wastewater treatment and lithium refining. By working with customers to understand their unique challenges and focusing on continuous innovation, Saltworks’ solutions provide best-in-class performance and reliability. From its headquarters in Richmond, BC, Canada, Saltworks’ team designs, builds, and operates full-scale plants, and offers comprehensive onsite and offsite testing services with its fleet of mobile pilots.
Saltworks can solve your cooling tower blowdown challenges. We can add novel sensors and controls to your cooling tower to minimize water consumption and blowdown production. We can treat blowdown for re-use at your cost-optimized freshwater recovery or achieve zero liquid discharge.
Real-time measurement provides prompt feedback for controls. A real-time sensor can help to optimize your process, reduce risk, and minimize operational and maintenance demand. Saltworks has developed a new real-time sensor—that is simple and robust—to operate in the high-TDS range.
Reverse osmosis-based brine concentrators are reaching new performance levels not seen before. Saltworks is pleased to report that a substantial pilot plant is currently demonstrating 99% freshwater recovery on cooling tower blowdown (CTB).
Saltworks delivers reliable solutions for data center cooling that maximize uptime and energy efficiency while helping meet water sustainability goals. Our modular, low footprint systems can optimize cooling cycles, reduce scaling, and minimize wastewater disposal volumes.