BY MIKE MOWBRAY AND GARY ENGSTROM, U.S. WATER SERVICES
Today, there is a heightened focus on environmental sustainability, and for good reason. Gone are the days of opening valves and discharging unregulated waste streams into the environment. Industry has seen the impact of heightened scrutiny from the public and regulatory bodies.
There are strong trends in protecting our natural resources from excessive usage and compromising their quality. Companies, and now even municipalities, are being subject to thorough review and comment periods to justify a facility’s proposed discharge rates and water composition.
In the realm of water treatment, one area that has historically come up during the environmental review cycle is the evaluation of the additives used to treat cooling tower systems. The plot of the movie “Erin Brockovich” was centered on the use of a cooling tower chemical additive (chromate). It wasn’t until later that it was discovered that chromates were carcinogenic and as a result of this discovery the use of chromates was banned.
|Scientists test a new phosphorus-free corrosion inhibitor in U.S. Water Services’ Research & Development Lab. Photo courtesy: U.S. Water Services|
Fast forward to today and there is a new focus on cooling tower additives. This time phosphorus (P) discharges have gained significant attention as a result of the impact they can have on the eutrophication of lakes and other waterways. Phosphorus has long been recognized as the controlling factor in plant and algae growth for many lakes and streams.
A minor increase in phosphorous can fuel substantial increases in both aquatic plant and algae growth. Phosphorous can originate from municipal and industrial facilities that discharge water as well as runoff from agricultural areas.
Many parts of the country, including the Great Lakes and Chesapeake Bay watersheds, are regulating the acceptable P discharge levels well below those commonly employed in traditional cooling water treatments for scale and corrosion control.
The State of Wisconsin, for example, is regulating P discharges at levels in the 0.04 – 0.075 ppm (40-75 ppb) range. This represents a significant challenge for facilities operating under direct discharge permits.
The use of phosphorus bearing compounds in industrial cooling water treatment programs has been common place since it replaced chromates as corrosion inhibitors in the early 1970s. Typical alkaline all-organic cooling water programs where phosphonates are present as scale inhibitors can have phosphorus levels from 0.3-2.5 ppm as P while stabilized phosphate programs can have phosphorus levels as high as 6.0-7.0 ppm.
Alternative corrosion inhibitor options may not be viable since they are often based on metals such as zinc (Zn) or molybdenum (Mo) which are also being closely regulated in terms of acceptable discharge levels. Without an effective corrosion and deposit (scale) control program in place, industrial cooling systems could be compromised in a relatively short period of time. Facilities could experience significantly higher operating costs as well as potentially impact production capacities due to a loss of vacuum in the surface condensers, for example.
Specialty chemical providers have recognized the challenges facing the industry for some time and have been diligently working to find an economically viable solution that can deliver the protection required. Suppliers have investigated non-metallic corrosion inhibitors and have made them commercially available.
These organic inhibitors are often based on low molecular weight polymers, phosphonates, and amino phosphonates. Some of these organic molecules have proven to be successful carbon steel corrosion inhibitors. Although they show improved environmental acceptability, they too may have some technical concerns. Organic molecules may be more susceptible to oxidizing biocides than inorganic molecules.
Many azole-based copper corrosion inhibitors are susceptible to degradation by oxidizing biocides. Hydroxyphosphonic acid has shown susceptibility to chlorine, even at low levels. This presents a problem because oxidizing biocides are a necessary part of any successful water technology program for control of potentially harmful bacteria, such as legionella. Organic inhibitors based on the phosphorus molecule do not meet the discharge requirement for P due to the reversion of some of the organic phosphonate to orthophosphate.
Unfortunately many alternative programs have failed to properly control the corrosion rates in systems without the use of phosphorus bearing inhibitors.
Table 1 illustrates a “Green Chemistry” alternative evaluation conducted by U.S. Water Services, Inc. (U.S. Water) based out of St. Michael, MN. It contains information related to the various alternatives where the values are the subjective ratings by the authors where 4 is the best score and 1 is the worst.
Based on this information, U.S. Water developed new cooling water treatment technology and worked with a refining plant in the Midwest to help them achieve their goal of reducing phosphorus concentrations in their discharge.
The plant had two different cooling towers due to different process conditions and both cooling towers utilized induced draft counter flow cooling towers.
The first cooling tower, comprised of mild steel, copper and 304 stainless steel, ran for six months from May-October at an estimated 18,000 GPM recirculation rate while the second cooling tower ran year round at an estimated 12,600 GPM recirculation rate and was comprised of only mild steel and 304 stainless steel.
Traditional water treatment programs were being utilized in the treatment of the cooling towers. The program for the first cooling tower (Tower #1) utilized an all-organic program with an azole supplement due to the copper metallurgy contained within the system.
Power plants traditionally do not contain copper metallurgies so the concerns associated with minimizing copper corrosion are not as pertinent and will not be discussed in this article. The other cooling tower system (Tower #2) did not have an azole supplement.
The results obtained from the first chemical program (All-Organic #1) were satisfactory, but U.S. Water tried an alternative all-organic program (All-Organic #2) to try and improve the results.
The corrosion rates actually increased slightly from the 1st program to the 2nd program in Cooling Tower #2. Figure 1 (on page 42) shows the corrosion rates in the cooling tower systems for the various programs:
Due to the discharge regulations, the client needed to find a viable chemical treatment alternative. With the help of U.S. Water, a new program was identified (PhosZeroTM) and the program was implemented at the plant. In order to ensure proper system protection, the corrosion rates were carefully monitored using corrosion coupons as well as an on-line corrator.
The client wanted to maintain the same water efficiency (i.e. not reduce the cycles of concentration) which yielded a Langelier Saturation Index (LSI) of approximately 2.25.
This LSI represents a relatively high scaling potential and the heat transfer surfaces were monitored closely as well. During the course of the entire trial there was no observed accumulation of scale that would impede the heat transfer efficiencies. Table 2 lists the pertinent cooling tower values during the trial.
After the zero phosphorus program was implemented the corrosion rates improved drastically within the first 5 hours of application. The on-line corrator corrosion rates for the mild steel (Figure 2 on page 43) shows the significant improvement during the implementation phase.
The corrosion rate in mils per year (mpy) is shown to fall from greater than 2.5 mpy to less than 0.5 mpy.
Corrators are good tools to measure trends and instantaneous relative corrosion rates. Corrosion coupons are an alternative method to monitor the same parameter. Corrosion coupons are samples of pertinent metal (i.e. mild steel or copper) that are pre-weighed to a high degree of accuracy.
These metal samples are put into a system and exposed to the water in the system for an extended duration. After a period of time, the metal samples are removed and sent in for analysis where they are first cleaned of debris and then re-weighed.
The difference in the metal mass (i.e. metal loss) and the length of exposure (in days) are used to calculate the corrosion rates. In this study, corrosion coupons were employed in addition to the corrator to monitor the cumulative corrosion rates over a 45-60 day exposure period. The results are summarized in Table 3.
The results were better than anticipated and the primary objective of eliminating phosphorus from the cooling tower chemical treatment program was attained.
In addition, the zero phosphorus treatment program provided a significant reduction in the corrosion rates. The corrosion rates dropped from an average ~4 mpy to an average ~1.3 mpy. The results signify a significant shift in paradigm in the water treatment industry.
The norm used to be for facilities to have to settle for compromised results or additional water use when using alternate chemicals that didn’t contain phosphorus.
Now, industry is able to meet low P concentrations in their discharges while still protecting their critical assets.