By Colin Frayne, for Association of Water Technologies and Aquassaurance inc.
May 1, 2011
Increasing cycles of concentration can allow processors to reuse waters and minimize water use. The typical practical contaminant maximums in recirculating cooling water are shown.
- Alkalinity: total alkalinity (M alkalinity). A practical limit is often 500 mg/L as CaCO3, although up to 800 mg/L CaCO3might be achievable, depending upon water chemistry.
- Ammonia (NH3).Up to 20 to 40 mg/L can be tolerated if the copper content is low, water temperatures are not too high, good microbiological control is maintained, and waterside surfaces are kept clean.
- Chemical Oxygen Demand (COD). Say, less than 100 mg/L
- Calcium. A practical limit for total hardness is often 600 mg/L as CaCO3, although with top quality inhibitors and tight control, up to 1200 mg/L as CaCO3 might be achievable.
- Chlorides. The lower the better, as chloride is a depassivating ion and reduces the corrosion resistance of many constructional metals. Perhaps 500-600 mg/L chloride in carbon steel systems, but only 200 mg/L maximum in systems containing 304 stainless steel. Also, the system metal surfaces must be kept scrupulously clean!
- Iron. Iron salts (and to a lesser extent manganese salts) are often to be found in recovered waters and can be ignored unless the level rises to perhaps 0.3 mg/L or more.
- Oil, Solvents and Hydrocarbons. Even small traces of oil can reduce chemical inhibitor performance and impede heat transfer and therefore must be eliminated.
- pH. Typically, from pH 7.0 to 90.0
- Phosphate. Phosphate in recovered waters can often be used as the basis of a chemical inhibitor program. Say 2-3 mg/L total PO4.
- Silica. The limit of solubility in recirculating cooling water is typically around 150 to 175 mg/L and should not be exceeded.
- Sulfate. Sulfates are causative agents (along with oxygen, hydrogen, etc.) of various types of concentration cell corrosion, Usually, say, 1800 mg/L is the maximum limit but this varies with several factors. Up to 2,300 mg/L has been tolerated in suitably conditioned systems.
- Suspended solids (SS). Maximum tolerated levels of SS in recirculating water is perhaps 50 to 60 mg/L.