Optimization of halogen dose for biofilm control

Bruce Alan Hunt, Purdue University

Abstract

Microorganisms are ubiquitous and have the ability to colonize almost any surface. This colonization often leads to microbial film accumulation (biofouling) which is undesirable in most industrial applications. Biofouling can decrease both pumping and heat transfer efficiency and increase corrosion rates. The direct result of biofouling leads to increased capital and operating costs. The economic impacts have spurred research into developing an effective biofouling control and/or prevention strategy. The primary objective of this research was to define an optimum halogen dose for biofilm prevention or control of established biofilm. Biofilm prevention and control were examined for two different halogen sources (bromine and chlorine) in separate, identical recirculating systems (termed: A and B). In each system, experiments were performed at two pH values, 7.6 and 8.6, and with two nitrogen sources, ammonia and nitrate. Linear velocity was maintained at approximately 2 feet per second (fps) throughout the study. The two systems were identical except that system A was treated with mixture of sodium bromide, in 10% stochiometric excess, and calcium hypochlorite (bromine system), while system B was treated with calcium hypochlorite (chlorine system). Heat transfer resistance (HTR) and differential pressure were monitored as indirect measurements of the degree of biofouling. Bromine was found to be more effective than chlorine in controlling and destroying the biofilm. At both pHs 7.6 and 8.6 with ammonia as the nitrogen source, chlorine required roughly twice the residual (as Cl$\sb2$) as bromine to achieve equivalent biofilm control. Bromine was more effective than chlorine at controlling and destroying the biofilm at both pH 7.6 and pH 8.6 with nitrate as the N-source. Experiments performed under both pH conditions with nitrate as the nitrogen source indicated that chlorine required roughly 1.5 times the dose as bromine for effective control of biofouling. Corrosion and Ceriodaphnia dubia acute toxicity were monitored throughout this study in order to develop a holistic approach to the optimization process. Free forms of chlorine and bromine were found to be more toxic than their respective combined forms. Halogen decay, planktonic microorganism and fixed film were also studied in order to develop a fundamental understanding of the mechanisms which led to effective control.

Degree

Ph.D.

Advisors

III, Purdue University.

Subject Area

Civil engineering|Industrial engineering|Microbiology

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