Model systems were developed to evaluate microbial growth on surfaces and to determine the efficacy ofbiocides. Dynamic systems were used to simulate key operating parameters in industrial cooling towers. Biofouling was measured on wood, stainless steel, and heat transfer surfaces. Biocide testing in these model systems showed that ethylchloro¬methylisothiazolone was effective in controlling microbial growth in biofilms. The efficacy of the isothiazolone biocide was related to the treatment level and the nutrient content of the recirculating water. Biofilm control was monitored using viable counts of microorganisms as well as measuring specific biomass components (protein and carbohydrate) on surfaces. An on¬line fouling monitor (heat exchanger) confirmed that low biofilm growth correlated with low heat transfer resistance.
Industrial water treatment systems provide an excellent environment in regards to pH, temperature, and nutrients to support the growth of a wide range of microorganisms. A considerable amount of literature is available to document the occurrence and impact of microbial growth in industrial processes. A key concern for water treatment is biodeterioration and biofouling on industrial surfaces. This can result in microbiologically influenced corrosion, blockage of screens and valves, reduced system efficiency (heat transfer and cooling), and potential health risks. 1-3
A variety of industrial biocides are used in cooling water, papermill, and oilfield systems to control the growth of bacteria, algae, and fungi in the bulk water and on surfaces. Performance data on these biocides are generated from a combination of laboratory and field use. The efficacy of industrial biocides is most easily evaluated using measurements of microbial growth and activity with planktonic or suspended cultures. Model systems are less frequently utilized but provide additional control over key industrial process variables, environmental conditions, and dosing options. These systems also allow to monitor biofouling on industrial surface materials under specific and highly controlled operating conditions.
This paper describes a series ofbiofouling studies to evaluate microbial growth and control on surfaces using model cooling towers. The systems simulate key cooling water parameters and incorporated an on-line fouling monitor to measure heat transfer resistance. The effect of methylchloro/methylisothiazolone (MCMI) biocide on biofilm development was determined under both low and high nutrient loading. Microbial growth and biocide efficacy were measured using standard viable cell counts and biochemical analyses (protein and carbohydrate).
MATERIALS AND METHODS
The biocide evaluated in the biofouling experiments contained 1.15% 5-chloro-2-methyl-4-isothiazolin-3-one and 0.35% 2-methy-4-lisothiazolin-3-oneY) All biocide concentrations used in this paper are reported on an active ingredient (ai) basis.
Model Cooling Tower Design
The model cooling system was designed and operated to maintain a constant half-life by adjusting the makeup water flow rate to provide the desired blowdown (effluent) flow rate. A schematic of the model towers used in the low nutrient study is shown in Figure I. The system was an open design with controllers to monitor and regulate temperature, pH, and air and recirculating water flow rates. Tap water was used as the makeup water source and the system was operated at 3 cycles of concentration (Table I). Each towerreceived a daily supplement of nitrogen (27 ppm NaN°3) and phosphorus (37 ppm K2HP04) to support microbial growth. The system consisted of an acrylic plastic tower in a polycarbonate basin. The basin contained an ov