This article originally appeared in WQP October Commercial Water 2020 issues as "Farewell to Phosphorus"
According to an United Nations (UN) study, 20% of all water used is expended in industrial processes, and in industrialized nations, this number can be as high as 80%. A large portion of that water is used for cooling operations where once-through systems and cooling towers utilize it to cool critical processes, such as power plant condensers, oil refinery and chemical plant systems and HVAC/comfort cooling systems. With the demand for water accelerating globally and the stress it places on availability of freshwater resources, many technologies have been developed to reduce water use and improve the recycling of these precious water resources. While significant progress has been made in reducing water use, there have been limited advancements in technologies that reduce the environmental impact of the concentrated discharge water streams that result from that reduced water use. These discharge streams can upset the delicate balance of nutrients in the environments downstream of industrial processes, negatively impacting the local ecosystem, aquatic life and everything that depends on it. A cost-effective solution is required to address these challenges, and for many years, the technology eluded water treaters. Now, options exist that are cost-effective, tested, proven and available to be implemented immediately.
The Use of Phosphate in Cooling Water
Since the regulation and elimination of hexavalent chromium-based cooling water treatment, the industry has relied on the use of phosphorus-based technology for mild steel corrosion inhibition and deposition control in cooling water systems. These organic and inorganic phosphate-based molecules protect the operating assets of production facilities, whether they are heat exchangers, surface condensers, transfer piping, etc., from failure. The addition of this chemical treatment also enables those facilities to reduce water usage by 75 to 90%.
However, phosphate-based chemistry also has significant challenges. Without the addition of advanced polymer dispersants, these materials can cause the failure of production assets through deposition, flow restriction and under-deposit corrosion. Significant improvements have been made in these polymers, including the development of dedicated terpolymer dispersants, like stress tolerant polymer (STP) technology. This technology, in combination with polymers like alkaline enhanced chemistry (AEC), reduces phosphate contribution to systems and discharge (in this case, by up to 60%). Still, even the smallest amount of remaining phosphate in concentrated cooling water discharge streams can become an issue.
Phosphate can also have a significant impact on downstream ecosystems. As a limiting nutrient in biological growth, any excess phosphate in the water can result in additional growth of biological organisms that would not have grown if the phosphate was not present. In other words, the intentional use of phosphate additives can lead to unwanted algae and biological growth.
Algae blooms can be harmful to other organisms and negatively impact the surrounding environment. It has been found that one pound of phosphorus can fuel the growth of up to 500 pounds of wet algae. That algae growth can directly impact the ability of a facility to clean up its water prior to discharge into the environment, resulting in discharge violations, fines and even the potential loss of production. With such a significant impact on the environment, regulating authorities are currently, or in the process of, restricting the phosphorus discharge limits from facilities, eliminating the ability to use phosphate technology.
Non-Phosphorus Chemical Treatment Approaches
There are a few different approaches with varying benefits that can be applied to meet phosphorus discharge restrictions. As a result of implementing these new technologies, many companies can improve safety, increase production rates and further safely concentrate and reuse water in cooling systems.
Zinc is well-known and has been used for more than 30 years to improve corrosion inhibition in stressed systems. Some companies began exploring the use of zinc and carboxylic acid-based polymers in the 1990s as one of the first solutions for non-phosphorus treatment. Unfortunately, zinc can cause deposition issues that can lead to system failures and is a U.S. EPA priority pollutant deemed harmful to the environment.
Many providers now offer stannous chloride (tin) with a reactive starch, polyaspartic or saccharic type acid for mild steel corrosion control. Tin is typically fed at two-to-three times the levels required for zinc and also comes with a significantly higher unit cost. Tin can provide an improvement in mild steel corrosion control versus an untreated system but only in waters that do not have an oxidizer like hypochlorite (chlorine) or bromine present. Oxidizers are commonly fed to cooling water systems and are a recognized standard practice to aid in controlling biological growth as well as the dangerous Legionella bacteria.
Tin materials are quickly oxidized and precipitate in the bulk water, rendering it useless for corrosion control. Tin also has a KSP similar to that of calcium phosphate salts, meaning it can cause deposition and failures, just like a phosphate-based material. The final nail in the coffin for tin materials is that they can cause direct, galvanic corrosion to occur. Attempts have been made to use carboxylic acids to sequester the tin and keep it in a soluble form. This tends to work well in a neat, formulated chemical product, but upon introducing the product to the cooling water system, the carboxylic acid releases the tin, where it is affected by oxidizers and system metals and is ultimately rendered useless.
New Technology for Non-Phosphorus Cooling Water Treatment
Over the last 15 years, new research methods have resulted in a novel understanding of chemistry in cooling water systems and advancements in chemical design. This new understanding of how to engineer and control passivation films in aqueous systems is changing fundamental assumptions of the last 40 years and is opening new avenues for technology development. By leveraging a series of techniques to understand every layer of a corrosion-inhibiting, passivation film less than 150 nm thick on a metal surface, the team developed new chemistries to engineer a robust, non-phosphorus, protective film in an aqueous solution that does not inhibit heat transfer. SUEZ’s E.C.O.Film*, or Engineered Carboxylate Oxide (E.C.O.), technology, is the result of this new understanding.
E.C.O.Film uses polymeric technology based solely on carbon, hydrogen and oxygen (CHO). In a cooling water system, the CHO technology primes the metal surface and aids in promoting a passivated metal oxide layer before helping to cap and protect the metal oxide with a dynamic matrix layer. The film formation takes minutes to hours, not days or weeks, and is self-limiting in thickness. This means that the protective film will never grow to a point where deposition could cause production problems. In some applications, a patented surface film formation catalyst (SFFC) is used to enhance corrosion protection. This catalyst selectively targets only the metal surface and helps to develop a stronger passivated metal-oxide layer in corrosive water conditions. SFFC is typically fed at 92% lower levels than zinc and 96% lower levels than tin. Unlike tin technology, E.C.O.Film technology is 100% oxidizer stable, so regardless of how much hypochlorite or oxidizer is fed to control biological growth, E.C.O.Film will continue to work and provide protection against harmful corrosion and deposition.
A chemical production plant in North America had been using a traditional phosphate-based treatment for corrosion control in its cooling water systems. Due to production requirements and the dynamics of the system, the plant struggled to maintain to meet its key performance indicator (KPI) targets. The plant either experienced calcium phosphate deposition in the heat exchangers, or unacceptable corrosion rates. Both of which would negatively impact the plant and asset life in the long term. Historical corrosion rates had been only 1-2 milli-inch per year (mpy) but with unacceptable pitting.
The customer switched to the E.C.O.Film technology and immediately saw a difference in performance. Cleaning frequency was reduced, desired flow rates were maintained, and mild steel corrosion rates were drastically improved. The first 30-day corrosion coupon analysis after implementing E.C.O.Film exhibited less than 0.1 mpy (a 10 to 20 times reduction in corrosion rate) and no pitting (See images on left). E.C.O.Film solved the deposition issues related to phosphate technology, mitigated the corrosion issues and eliminated the need to use phosphate.
Non-phosphorus technology allows production facilities to minimize phosphorus contributions to outfalls, which in turn, prevents harmful algae growth and detrimental eutrophication of freshwater resources. It can help facilities meet or exceed new phosphorus discharge regulations and can prevent costly deposition associated with phosphate-based materials. Overall, facilities can be a better partner in their community, improve their environmental and brand image and meet long-term sustainability goals.