Page 112 - Membranes for Industrial Wastewater Recovery and Re-Use

Industrial waters 9 1

Figurr 3.1 3 In-linr addition of sulphuric arid to a rerirculatingcooling system in St. Petersburg. Florida

1978; Wiatr, 2002). Once a biofilm forms, it provides a protective habitat for
microorganisms (Fig. 3.14). Biocides can be used to control biofilms as part of the
internal chemical treatment process, the type and required dosage depending on
the organic and nutrient content of the make-up water. The most commonly
used biocide is chlorine, though other chemical approaches are also effective.
Ozone is a powerful biocide effective for control of bacteria, viruses, and protozoa,
but can exacerbate problems of scale adhesion since by-products from the
oxidation of biofilms can serve as binding agents for scale on heat exchanger
surfaces.
When reclaimed water is used for cooling, the assurance of adequate
disinfection is a primary concern to protect the health of workers and individuals
exposed to aerosols from the cooling towers. The disinfection requirements for
the use of reclaimed water in cooling towers are site specific and based on the
potential for exposure to aerosols from cooling operations and prevention ofbiofilm
growth. Limited data are available on relative quantities of microorganisms in
recirculating cooling systems. Pathogen survival depends on the source water
quality, pretreatment mechanisms, and the type and dosages of biocides used in
the facility (Levine et al., 2002). While there are no universal standards, the most
frequently monitored bacteria include total and faecal coliforms and Legionella
pneumophilia. Typically disease outbreaks are associated with levels over 1000
cfu (colony forming units) per ml in cooling towers. A comparison of the levels of
Legionella pneumophilia in recirculated cooling water is shown in Fig. 3.15. This
facility uses a pro-active approach by conducting quarterly monitoring. Typical
values range from non-detectable to 300 cfu ml-l. Monitoring can provide
insight into the effectiveness of disinfection practices.
Water velocities below 0.3 m and temperatures over 50°C tend to promote
biofilm formation and the associated fouling reactions. Control of fouling is