ABSTRACT

Biocide dose response studies are commonly conducted on water solutions containing bacteria to determine the effect of chemical treatments before application. Biocide product labels provide broad guidelines for dosing. However, site water chemistry and bacteria biology make the minimum effective dose differ for each location difficult to determine. A large volume culture of sulfate reducing bacteria (SRB) was prepared and allowed to grow until 4 log bacteria were present. The sample as then split into four identical 500 ml samples. The four samples were dosed at 0, 5, 50 and 400 ppm of tetrakis(hydroxymethyl)-phosphonium sulfate (THPS) based biocide. The effect on bacteria levels were tested using an enzyme-based bacteria metabolism test after 0.2, 1, 8, 24 and 96 hr. This study showed that a single application of 50 ppm of biocide could be as effective as a 400 ppm on high numbers of planktonic SRB.

INTRODUCTION

Tetrakis(hydroxymethyl)-phosphonium Sulfate (THPS) is a very common active ingredient in oil and gas biocides.1 While product labels provide broad guidelines application dosing the lowest effective dose of THPS is difficult to determine. Site water chemistry and bacteria biology variability will affect the dose need to achieve the desired level of bacteria population control. For these reasons biocide dose response studies are commonly conducted on solutions containing bacteria to determine the effect of treatments before application.

The bacteria used in dose response testing can come from standard cultures or collected from site.2 Given that there can be hundreds of species of bacteria present in a field site, stock culture testing can only yield results of limited value.3

Biocides are typically dosed in batch or continuous application and the evaluation of a biocide dosed can be challenging. While genomic testing can provide extremely accurate results the cost and delay associated with this type of testing make it undesirable. Adenosine triphosphate (ATP) based testing also has limitations. ATP testing must be combined with adenosine monophosphate (AMP) testing to determine both live and inactive (dormant/inactive or stressed bacteria populations).4 This complication of ATP based testing adds to the cost and complication of its use. The use of culture-based testing to evaluate biocide dose response is inherently flawed in that there is no way to evaluate the effect contact time has on the bacteria. This study employed an enzyme-based assay that can determine the total living bacteria content of a sample in 30 minutes.

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