Evaluating Alternative Cooling Tower Makeup Water Options
A best practices approach to comparing four alternative approaches.
As water resources become increasingly stressed by drought and intensifying demand from competing uses, industries that use significant quantities of water will face greater pressure to adopt water-efficiency strategies that can decrease fresh-water withdrawals. In industrial operations, cooling towers present a viable opportunity for water conservation. Traditionally, the vast majority of industrial plants that operate open evaporative, recirculating cooling systems rely on municipally supplied water of potable quality, on-site well water or locally available surface water (adjacent rivers/lakes) as the principal makeup water source. One approach that facilities can adopt to reduce their draw from these supplies — thus reducing stress on drinking water reserves — is to use alternative makeup water sources.
This analysis focuses on the evaluation and best practices application of four major alternative source options: sodium zeolite softened water, reverse osmosis (RO) permeate, high phosphate gray water and municipally treated wastewater. The concept of mitigating fresh-water usage by utilizing a nontraditional makeup water supply is readily welcomed at many U.S. facilities. However, difficulties emerge when performing a cost/benefit analysis of a selected option and, subsequently, the implementation of a project in terms of defining the practical method of application to an existing open evaporative cooling system.
Herein, an approach is presented which analyzes the key elements that best define whether an alternative is suitable for an application. It includes how that alternative can fit within the scope of the water-conservation goals determined by a facility or external jurisdiction.
In assessing each makeup water source option, a case study approach analyzing operational and performance data, based upon an actual site application, was employed. The goal was to present a clear, nonbiased comparison, enabling others to adopt a similar process and, ultimately, to make an informed choice among makeup water source options when faced with that highly consequential task.
It should be emphasized that no single nontraditional makeup water option will always present itself as the best solution. The benefits, limitations and total cost of implementation associated with each alternative must be objectively evaluated on a case-by-case basis with respect to site-specific conditions and the unique cooling system under consideration. Trade-offs will certainly factor into the overall analysis and, in many cases, the indirect costs of each option cannot be fully anticipated.
Alternative Water Source Drivers
Beyond the water-conservation benefits, inherent drivers are present for encouraging cooling tower system owners/operators to consider nontraditional makeup water sources. With municipally supplied potable water, prices have risen substantially in recent years, especially in water-stressed regions, making this option increasingly cost-prohibitive in those particular regions. Well water, depending on the geographical location, can present performance challenges related to mineral scaling or iron-related fouling.
Additionally, locally sourced surface water contains a high degree of variability and poses key risks related to higher silt and turbidity potential. Surface water sources also impose higher microbiological/organic loading demand on recirculating cooling water, requiring additional biological monitoring and treatment.
Case 1: Sodium Zeolite Softened Makeup Water
The main purpose of analyzing this option is to demonstrate how softening locally supplied city water — in lieu of using that same city water in its raw form — can drastically decrease the amount of makeup water supply required and blowdown to the municipal sewer (figure 1).
With this example, the cost of softened water at the facility is $3.54/1,000 gal. The base city water cost is $2.17/1,000 gal, and the cost of sewerage is $5.68/1,000 gal. The difference of $1.37/1,000 gal is reflective of the annualized softener operating costs (salt, extra regeneration water supply and sewerage).
Hypothetically, if this cooling tower system were to run on raw city water without softening, the cycles of concentration would be restricted to 2.0. By utilizing softening, the facility can achieve 6.0 cycles of concentration — the optimum number of cycles based on a mass balance curve analysis. Based on brief cost analysis of this comparison (table 1), the net yearly savings for using 100 percent sodium zeolite softened city water at this facility is $50,796.61.
At 6.0 cycles of concentration, highly alkaline conditions with essentially no calcium or magnesium hardness are present at this facility. But even with elevated pH (theoretically at 9.6), the potential still exists for low carbon steel corrosion to occur. Another factor is the elevated temperature of the heat exchanger skin/surface, which contributes to an even higher potential for corrosion.
Because of the corrosion-related risks, the benefits are counterbalanced against the requirement to maintain robust concentrations of corrosion inhibitors and dispersants. With softening, it also is imperative that tight monitoring and control of pH and conductivity are maintained.
Case 2: RO Permeate Makeup Water
In this case study, RO permeate was chosen as the primary makeup water source to the cooling tower (figure 2). The extremely high concentration of total alkalinity present in the local city-supplied water source would essentially limit the cooling tower cycles of concentration to 2.0 if no treatment was used. By blending city water (5 percent) with softened city water (10 percent) and RO permeate (85 percent), this facility produces makeup water that is virtually free of all ionic species, including potentially corrosive monovalent anions such as chlorides. Thus, operation of the cooling tower at 15 cycles of concentration is achieved, dramatically reducing the overall makeup water usage and blowdown to the city sanitary sewer. The cooling tower also can act to “use up” excess RO machine capacity if need be.
At $10.77/1,000 gal, the cost to produce RO permeate makeup water is significant. However, in a hypothetical sense, the total overall cost of this option was still determined to be significantly lower than utilizing municipal potable water priced at $5.18/1,000 gal (assuming 2.0 cycles of concentration). A comparison is provided in tables 2 and 3.
Based on a corrosion study of this cooling tower system, an elevated corrosion potential with low carbon steel and cast iron is present, necessitating robust levels of corrosion-inhibitor components be applied. Additionally, the facility must manage the RO reject stream as part of the overall discharge flow. It should also administer RO membrane pretreatment requirements such as softeners or use chemical anti-scalant agents, which increases the total cost of operation.
Case 3: High Phosphate Gray Water
The principal reason for using high phosphate gray water as a makeup water source is the significant cost savings associated with its use (figure 3). Most facilities that adopt this strategy have access to an on-site source of process wastewater that inherently contains a high phosphate concentration. In these cases, the need to purchase municipal water is removed, and the wastewater that would otherwise have to be managed upstream in the process also is eliminated.
Another benefit is the relatively high levels of phosphate realized, which act as an excellent corrosion inhibitor for the low carbon steel piping and components in the cooling tower system. Additionally, a facility that utilizes gray water as one of its primary makeup water sources is demonstrating measurable environmental responsibility.
However, this option also presents multiple inherent challenges. First, high phosphate gray water brings the possibility of phosphate-related fouling, ushering in the need for high concentrations of polymeric dispersant chemistry to maintain the higher cycled phosphate concentrations in a soluble state. This is especially true where the calcium hardness also cycles up to an elevated concentration in the cooling tower system. Most importantly, the system pH must be monitored and controlled closely to ensure that calcium phosphate solubility is controlled. This most frequently involves some form of external pH/alkalinity reduction agent such as sulfuric acid, hydrochloric acid or, in some cases, carbon dioxide.
The cooling tower system in the case study is equipped with an automated monitoring/control system (figures 4 and 5). The pH control range is set at 7.30 to 7.50, utilizing sulfuric acid as the pH control/alkalinity reduction agent. Phosphate solubility also is effectively maintained with proper polymeric dispersant dosage and residual monitoring, thus enhancing corrosion-inhibition properties and minimizing phosphate-related fouling potential.
Case 4: Municipally Treated Wastewater
Like high phosphate gray water, municipally treated wastewater offers benefits related to reduced water costs and environmentally responsible practices (figure 6). This strategy also permits facilities to co mply with local and state regulations that mandate the use of municipally treated wastewater, a benefit realized in this case study where the facility can meet reclaim criteria under Title 22 of the California Administrative Code.
As with other makeup water alternatives, however, several built-in challenges exist. Municipally treated wastewater may contain elevated phosphate concentrations, which, in turn, drive robust polymeric dispersant demand. This option also typically carries high water conductivity. Also, due to the potentially high chloride strength, cycles of concentration can be limited if high percentages of stainless steel alloy-containing components are present in the cooling water system components or heat exchangers.
Reliable control of pH is critical when dealing with elevated phosphate concentrations in the cycled cooling water. In the case study, which operates at 5.5 cycles of concentration, a significant quantity of sulfuric acid is fed on a continuous basis to maintain a 7.50 pH target, promoting the solubility of the cycled phosphates. Corrosion monitoring was the key feedback provider with this cooling tower. Increased dosage rates of copper corrosion inhibitor are required to reduce copper corrosion rates, which can be potentially higher at neutral or near-neutral pH control ranges.
Template for Decision
As demonstrated with each of these case studies, specific factors and challenges will come into play when evaluating nontraditional makeup water source options. Armed with an objective, data-driven approach, however, facilities can objectively weigh the risks and benefits of each potential makeup water alternative in reaching a decision.
Inevitably, a series of associated costs is required for inclusion with the analysis of each alternative with respect to the steps required to keep the cooling tower system under control from a water treatment perspective. It is how well these tangible and intangible costs are managed that ultimately determines the degree of success or failure linked with the type of makeup water selected.