For decades, the most common cooling water treatment programs for large industrial cooling tower-based systems have relied on a combination of inorganic and organic phosphate (PO₄) chemistry to maintain the quality of cooling water to ensure system efficiencies. However, important factors are driving an evolution away from phosphate-based chemistry towards non-phosphate/non-zinc treatment methods to improve cooling system reliability and efficiency at many plants.
This article examines challenges with phosphorous-based programs, key factors to controlling cooling water chemistry and the advantages of phosphorous- and zinc-free cooling water treatment technology.
Interest in Phosphorus and Zinc-free Water Treatment Methods Grows
Inorganic and organic phosphate programs, such as those that relied on phosphonates, phosphinates, and polyphosphates, etc., emerged as the technology of choice when chromate and zinc-based corrosion inhibitors were phased out due to environmental concerns. However, this transition did not come without difficulties, including the issue of precisely controlling the chemistry to avoid scale formation in heat exchangers. In addition, phosphate-based treatments promote algae growth on cooling tower wetted locations exposed to sunlight and in holding ponds, resulting in increased biocide feed.
Two additional factors has also driven an evolution away from phosphate-based chemistry toward non-phosphate treatment methods. One is the increasingly problematic issue of phosphorus discharge and its effects on the generation of toxic algae blooms in receiving bodies of water. The second is growing evidence that well-formulated non-phosphate programs are more effective, from both a technical and economic standpoint, than phosphate/phosphonate chemistry for scale prevention and corrosion protection.
Influence of Phosphate in the Natural Environment
Phosphorus, along with nitrogen and carbon, is a macronutrient that is essential for all life forms. In fact, it is often the limiting nutrient for growth in aquatic systems because it is present in very low concentrations relative to that required by plants and microorganisms. Yet it in high concentrations it can be problematic.
Algae derive their carbon requirements from inorganic bicarbonate and carbonate, utilizing energy from sunlight to convert the inorganic carbon into organic carbon for cellular tissue growth. Some species of algae are also capable of “fixing” atmospheric nitrogen gas, using the nitrogenase enzyme to convert N2 into ammonia and other compounds required for the biosynthesis of nucleic acids and proteins. Common among the photosynthetic nitrogen fixing species are cyanobacteria, commonly referred to as “blue-green algae.” Cyanobacteria are known for their extensive and highly visible green blooms. An example is a cyanobacteria bloom in the shallow western basin of Lake Erie that occurred in 2011. The unpleasant and unsightly algae growth resulted in fouled beaches, sharply reduced tourism, and a decline in fish populations. Apart from their noxious sensory impact, cyanobacteria also produce microcystins and other cyanotoxins that are toxic to fish, birds, and mammals.
Subscription users only!
Subscribers are able to view the whole article. Please register/subscribe (it's free and easy) to read all articles.