"Kills 99.9% of bacteria". Sounds impressive. Consumers will have seen this claim on supermarket shelves on all kinds of household biocide chemicals such as hand soaps and detergents. In reality, what does this actually mean? Is a kill target of 99.9% an effective biocide? On the face of it, it sounds very good. However, when referring to microbiological populations, like bacteria, this requires context. What was the original number that was reduced by 99.9%? When quantifying microbiological populations, it is common to use extremely large numbers, referenced in logarithms (1.0 × 105 or 1.0 × 106 cells).
Reducing bacterial populations by 100% using batch dosed biocide chemicals is an impossible feat. It will never be possible to entirely eliminate 100% of bacterial populations, as some will always survive and potentially recover to pre-dose populations. Therefore, selection of the most efficient biocide chemical is of the utmost importance.
In the example scenario of a hand soap, the target of 99.9% is very difficult to achieve. The biocide chemical is applied to the target surface area (hand) and almost immediately diluted and washed off by a running stream of water. The chemical might get a contact time of up to 1 minute to kill 99.9% of bacteria under normal circumstances.
Although hand soap is not directly related to control of bacterial populations in oilfield process systems, the theories behind the testing of all biocide chemicals are the same. It boils down to three parameters that need to be tested; chemical, concentration and contact time, tested against microbiological populations. The questions any operator should ask before using a biocide chemical should be;
Which biocide chemical?
What concentration should be applied?
How long should the biocide chemical be added for?
By answering these questions, the operator will have learned key points as to the efficiency of the biocide chemicals under test in relation to the required conditions.
This paper will discuss a recent example of a biocide chemical evaluation carried out by a Malaysian operator, in which fourteen different biocide chemicals were tested to determine the most efficient biocide chemical. Each biocide chemical was tested under controlled laboratory conditions against both planktonic and sessile mixed microbial consortia populations, and ranked for efficacy.