Reducing the environmental impacts of industrial cooling water discharges

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cooling
Left: Phosphate-based cooling water treatment program. Right: Non-phosphate-based cooling water treatment program.

By James Green

According to the United Nations, 20% of all water use in the world is for industrial processes. In industrialized nations, this number can increase to as high as 80%. A significant 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, especially for power generation, significant emphasis has been placed on developing technologies to reduce water use and improve the recycling of water resources. While significant progress has been made, there have been limited advancements in technologies that reduce the environmental impact of concentrated discharge water streams that upset the delicate balance of nutrients in the environments downstream of industrial processes.

New research methods utilized over the last 10 years have resulted in a novel understanding of chemistry in cooling water systems and advancements in chemical design. This new understanding of engineering passivation films in aqueous systems allows companies to eliminate the use of excess nutrients, such as phosphate, heavy metals, such as zinc, and hazardous acid usage. In addition, this can improve operator safety, increase production rates and the reuse of water in cooling systems.

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Since the regulation and elimination of hexavalent chromium-based cooling water treatment, the industry has relied on the use of phosphate-based technology for deposition and corrosion inhibition in cooling water systems. In various forms, these phosphate-based molecules protect the operating assets of production facilities from failure, while enabling those facilities to reduce water usage by 75% – 90%.

Still, phosphate-based technology comes with its own set of 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 the industry-leading stress tolerant polymer (STP) technology.

This technology, in combination with polymers like alkaline enhanced chemistry (AEC), leads the way in reducing phosphate levels, in some cases up to 60%. Still, even the small amount of remaining phosphate in concentrated cooling water discharge streams can become an issue.

In addition to the challenges with deposition control, phosphate can have a significant impact on downstream environments and ecology. Phosphate is a limiting nutrient in biological growth, such that any excess phosphate in the water can result in additional growth of biological organisms that wouldn’t have grown if the phosphate wasn’t present.

One of the most challenging issues highlighted currently in the Great Lakes area is excessive algae growth due to the presence of excess phosphate. Algae can be harmful to other organisms and negatively impact the surrounding ecology. It has been found that one pound of phosphorus can generate up to 230 kg of wet algae. That algae growth can directly impact the ability of a production plant to clean up its water prior to discharge into the environment, resulting in fines and 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. These discharge levels are so low that the use of phosphate-based materials in cooling water systems are ineffective and cannot achieve the desired performance.

To support the changing needs of the industry and the desire to remove phosphorus from cooling water systems in an effort to improve deposition issues, reduce algae and discharge issues, and improve the environmental profile, water treatment companies have been pursuing and introducing non-phosphorus solutions. There are a few different approaches with varying benefits that can be applied to meet phosphorus discharge restrictions.

The most common approach is to utilize zinc in combination with a carboxylic acid polymer to eliminate the need to use phosphorus. Initially used over 30 years ago, zinc is a well-known and commonly used corrosion inhibitor. Unfortunately, zinc can also cause deposition issues and is a priority pollutant that is deemed harmful to the environment. Overall, this approach works well but has significant environmental drawbacks.

In the last few years, it has become common for many companies to utilize stannous chloride (tin) in combination with a polyaspartic, glucaric or saccharic type acid. Typically fed at 2 – 3 times the levels required for zinc, at significantly higher cost, tin can provide an improvement in corrosion control versus an untreated system, but only in waters that do not have an oxidizer like hypochlorite or bromine present.

These oxidizers are commonly fed continuously to cooling water systems to control biological growth. Tin materials have multiple oxidation states and in a cooling water system where oxidizers are present, the tin is quickly oxidized to an insoluble form and proceeds to precipitate in the bulk water, rendering it useless for corrosion control. In addition, tin has a solubility product constant similar to that of calcium phosphate salts, meaning it can cause deposition and failures, just like a phosphate-based material.

The final issue for tin materials is that they can cause direct, galvanic corrosion to occur in production assets like heat exchangers. Many companies have attempted to use carboxylic acids to sequester the tin and keep it soluble, which works well in a neat, formulated chemical product.

Still, upon introducing the product to the cooling water system, the carboxylic acid releases the tin, where it is affected by oxidizers and system metals.

So how can companies meet their sustainability and environmental goals with a cost-effective solution? By using new technologies based on engineered films.

Instead of trying to develop anodic or cathodic corrosion inhibitors to control a corrosion cell effectively, a team at SUEZ redefined the known mechanisms for corrosion control. By leveraging a series of techniques to understand every layer of a corrosion-inhibiting passivation film (<150 nm thick) on a metal surface, the team spent 15 years developing methods and chemistries to engineer a robust, protective film in an aqueous solution, that doesn’t inhibit heat transfer, and is thinner than previous phosphate-based technology.

SUEZ’s E.C.O. (engineered carboxylate oxide) Film technology allows customers to meet or exceed changing environmental regulations and take significant steps towards reaching their sustainability goals.

While it may contain trace amounts, this non-phosphorus technology helps to reduce toxicity to aquatic life, minimize harmful algae growth and algae blooms and increase water reuse.

It also delivers the same performance and protection standards that cooling water system operators have come to expect from traditional chemistries.

E.C.O. Film uses polymeric technology based solely on carbon, hydrogen and oxygen (CHO) containing polymers. In a cooling water system, the CHO technology primes the metal surface and aids in promoting a passivated metal oxide layer before helping to protect the metal oxide with a protective, dynamic, heterogeneous capping 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. Instead, it stays 20 – 50 nm thick. In some applications, a patented surface film formation catalyst (SFFC) is used to enhance corrosion protection.

This catalyst targets only the metal surface (it’s not found in the capping matrix) and helps to develop a stronger passivated metal-oxide layer in corrosive water conditions. Typically fed at 92% lower levels than zinc, and 96% lower levels than tin, the SFFC is an excellent choice for a non-phosphorus, environmentally conscious approach to treatment.

In some applications, it has been documented that asset life (time to failure) can be 10 – 20 times longer due to the improved corrosion protection of E.C.O. Film. Unlike tin technology, it is 100% halogen/oxidizer stable, so regardless of how much hypochlorite or oxidizer is fed to control biological growth, it will continue to work and provide protection against harmful corrosion and deposition.

This 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 and changing phosphorus discharge regulations and can prevent costly deposition associated with phosphate-based materials.

James Green is with SUEZ – Water Technologies & Solutions. Email: j.green@suez.com

Read the full article in ES&E Magazine’s June/July issue below.

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