To maintain an effective microbial control program, it is necessary to measure the total number of microorganisms present in the system in addition to identifying and measuring the specific types. A program's effectiveness can be determined and corrective action taken as required. Although most plants maintain stringent control over the concentrations of corrosion inhibitors, they often neglect to make standard plate counts for the control of bacteria and fungi, relying instead on visual techniques. These have obvious drawbacks. When slime or biofouling is evident during inspection of a industrial tower or other piece of water-handling equipment, the condition has already become critical. A semi-quantitative technique is required to monitor microbial growths before they become apparent to the naked eye. Such techniques are available and can be conveniently used in both the laboratory and in the field. As is true in any sampling procedure, the most important consideration is that the sample be representative of the water system to be evaluated. After the sample has been collected, total bacterial and fungal plate counts can be made. In this procedure, nutrients are added to a sterile culture plate and one milliliter — or a suitable dilution of the water to be tested — is added to the plate. If present following an incubation period, the organisms will grow into a countable culture.

For bacteria, Tryptone Glucose Extract Agar may be used as a nutrient. Incubation usually takes 48 hours at 20°C or 25 hours at 37°C. Fungi are generally cultured for five days at 20°C in Sabouraud Dextrose Agar. Differential plate counts to identify specific species can be accomplished by extensive laboratory work, but for standard field monitoring this is not necessary. The development of newer techniques has simplified field testing considerably.

Sulfate-reducing bacteria are another problematic species capable of producing localized corrosion. A test procedure is available which is capable of detecting and enumerating Desulfouibrio desulfuricans, the most common species of sulfate-reducing bacteria. The procedure involves innoculating an agar growth medium and maintaining an oxygen deficient environment in the sample tube. The amount of sulfate-reducing bacteria in the system is based on a color change in the tube.

The maximum permissible concentration of microorganisms is specific for the system under consideration, the type of microorganisms involved and operating parameters of the equipment. Realistic guidelines can be established only with a knowledge of the history of the system accompanied by laboratory testing. Experience has shown that problems are normally encountered when bacterial counts exceed 100,000 organisms per milliliter in open recirculating systems, 10,000 organisms per milliliter in closed systems or 400 organisms per milliliter in a makeup supply. Periodic field tests must be run to assure compliance with pre-set maximum count levels, and should be performed with the same diligence as any other control testing program.

Selection of a microbiocide program should be based on several factors including: the type of microorganism(s) present, extent of contamination, pH, system size/exposure time, nature and extent of process-related contaminates and environmental impact. Therefore, the proper microbiocidal program cannot be assumed but should be carefully individualized to assure its success. A method employed to assist in the selection of a microbiocide program is the laboratory culture and sensitivity assay, better known as a Relative Population Density (RPD) test.

The Relative Population Density test  is a highly successful technique for evaluating the effectiveness of a microbiocide. In this procedure, representative water samples are dosed with various microbiocides at different concentrations. The percentage

decrease in organisms, commonly called the "percent reduction," is then determined by plate counts conducted before and after innoculation. The microbiocide providing the highest percent reduction at the most economical concentration is generally designated the product of choice. The Relative Population Density test can be designed with regard to a specific need. It may employ all microbiocides effective against a particular microorganism or those microbiocides effective in a prescribed pH range.

Most microbiological control programs are designed to provide reduction of 99% or better, which is accomplished by using one or more microbiocides in a formulation. Combinations of microbiocides have demonstrated synergistic effects in some situations, similar to those encountered with corrosion inhibitors. In even the best programs, a small number of microorganisms will prove resistant to a specific microbiocide. Thus, it is generally unrealistic to expect a 100% reduction.