Water and steam sit at the heart of nearly every chemical and power plant, yet cooling towers and makeup water systems often operate in out-of-the-way corners until something fails. That was the message from Brad Buecker, senior technical consultant with SAMCO Technologies, during October's eChem Expo Field Notes Lunch & Learn.

Buecker focused on microbiological fouling, which he said causes the fastest and most severe problems in cooling water systems. Microbes enter through makeup water and, in systems with cooling towers, from the air, which the tower scrubs very effectively. Once free-floating (planktonic) organisms settle onto surfaces, they form sessile colonies protected by a polysaccharide slime that traps silt and debris. These biofilms can impede heat transfer more than almost any scale. Within the deposits, sulfate-reducing bacteria can generate acids and hydrogen sulfide, driving microbiologically influenced corrosion (MIC) that perforates metal with little overall metal loss.

He cited a power plant that left cooling water stagnant in a 316L stainless steel condenser during a month-long outage. On restart, the tubes were riddled with pinhole leaks, and the entire condenser had to be retubed. In another case, biomass and trapped silt added so much weight to cooling tower fill that it collapsed.

Sessile colonies also serve as breeding grounds for Legionella. Buecker noted that outbreaks can originate from any system producing fine droplets, not only cooling towers, and that low-flow areas and dead legs raise risk.

Treatment Options and Tradeoffs

Oxidizing biocides remain the primary defense, but chlorine has limitations. Its active form, hypochlorous acid, dissociates into the much weaker hypochlorite ion as pH rises; at pH 8, only about 20% remains active. Reacting chlorine with sodium bromide to form hypobromous acid preserves activity at alkaline pH. Halogens can also be consumed by slime, can form regulated halogenated organics, and are often limited by permits to a roughly two-hour daily feed window intended to protect aquatic life at the discharge.

Alternatives include monochloramine and chlorine dioxide, which can penetrate biofilms, and on-site bleach and peroxide generation. Supplemental non-oxidizers such as isothiazolinones, DBNPA and quaternary amines, along with biodispersants, can target organisms that halogens miss. Buecker urged plants to work with experienced water treatment suppliers to identify the organisms present and to involve environmental regulators before testing any new chemical.

He also warned that reclaimed municipal effluent, increasingly used as makeup water, can carry high levels of ammonia, phosphate, hardness and chloride. On one project, upgrading treatment with a moving bed bioreactor proved cheaper than enlarging the cooling tower. He also cautioned against reflexively specifying 304L or 316L stainless steel without accounting for chloride concentration in cycled-up water.

Detect Problems Early

Buecker recommended routine monitoring of approach to wet bulb temperature, recirculating pump flow and current, and condenser performance to catch fouling before it becomes catastrophic.

Buecker will continue his water conversation at the next Lunch & Learn on Nov. 19. The topic: corrosion. Register here.