Biocides are also known as disinfectants, preservatives, sterile and, anti-microbial agents and antiseptics. The strict definition of 'biocide' is: it will kill living cells. Whether it can successfully “kill” depends upon several variables not least of which is the dose (i.e. the concentration of biocide) and the time it is in contact with microorganisms. Most biocides can also be regarded as biostatic. That is, at concentrations lower than that required to kill, the biocide inhibits cell growth, whilst it is present. Once the chemical is removed, the bacteria will continue to grow again. At doses lower than biostatic the biocide can even become a source of nutrition and therefore encourage growth.
In oilfield operations, it is usually necessary to dose the system, either periodically or continuously, with biocides that prevent microorganisms growing or kill them outright. Oilfield use of biocides tends to be insignificant (perhaps 3% of the total market) and hence products tend not to be produced specifically for oilfield applications. Data from 1987 shows that the oil industry spends over $40 million per year on biocides. Biocides represent typically around 6% of the total chemical bill.
Although there is large range of commercially available biocides for application in field operations, these chemicals are based upon only a few basic active agents. The user must be aware that the same or similar compound will be marketed by many suppliers, each giving it a different trade name or code.
How do biocides work?
The mode of action of biocides is not always fully understood. It takes a great deal of research to find out why or how a chemical is toxic. However, those compounds used in the medical and health fields have been studied extensively. Some operate by disrupting the cell wall, others by attacking the internal cell order.
In general, non-oxidizing biocides function primarily by altering the permeability of the cell walls of the microorganisms and interfering with their biological processes.
Most common biocides in oilfield
The most common non-oxidizing organic biocides in the oilfield are glutaraldehyde (glut) and tetrakis-hydroxymethyl-phosphonium (THPS), with smaller amounts of formaldehyde and acrolein being used.
Selection should include the following steps:
- Assessment of the technical data provided by biocide manufacture/supplier.
- Laboratory testing to determine the in use efficacy.
- Field trials.
- Field use.
Emphasis is placed on step (1) since the testing procedure is costly. Often, oil companies simply rely upon use of tried and tested chemicals without going through the above procedure.
Laboratory tests identify appropriate biocides. Each test incorporates a control biocide, that is one of known efficacy, in order that comparisons can be made. There is evidence that biocide concentrations and contact time which are effective against planktonic bacteria are not sufficient to kill sessile populations. More sophisticated test procedures are required to evaluate biocides against sessile populations and more emphasis is placed upon performance in field trials.
In general, the biocide suppliers are in a position to provide much of the data required to assess which biocide is the most suitable. In addition to the Materials Safety Data Sheet (MSDS) the following information should be sought from the biocide supplier:
|Biocidal activity:||This should be determined in a series of laboratory tests against the target microorganisms, incorporating biocides of known efficacy. The test conditions should simulate, as far as practically possible, those areas to be treated. The data obtained should also include time/dose response curves, which are generally not linear.|
|Compatibility with brines:||Biocidal activity can be considerably less in sea water and formation water than that in fresh water. This parameter should be evaluated in the laboratory tests.|
|Activity changes with pH:||Biocidal activity of some compounds can alter with changes in pH. Therefore, the activity spectrum with respect to pH should be known.|
|Activity changes with temperature:||Biocidal activity of some compounds can be affected by temperature. Although an increase in temperature can lead to increase in activity, there are some biocide chemicals that decay at temperatures of 30°C and above.|
|Sensitivity to light:||Some compounds are vulnerable to degradation by light (photolysis). This can be an advantage for chemicals released into the open environment, providing the toxicity of the breakdown products is less than the original compound. However, such a property can lead to discrepancies between laboratory tests, usually carried out in the light, and offshore use which is usually dark.|
|Activity with surfaces:||Some biocides, or components of biocides, have an affinity for surfaces. This is a desirable characteristics if it is a surface that requires treatment, e.g. in a continuous flow system. However, consideration should be given to compatibility with filming corrosion inhibitors if they are present in the area being tested.|
|Compatibility with other field chemicals:||A biocide might react with other chemical additives which may already be present in the water. Such reactions may produce solids, which can cause problems, or the reaction may be entirely in solution. The reaction of a biocide with another chemical may mean the efficacy of one or both chemicals is removed. An example of this is the reaction of chemical oxygen scavengers, sulphites or bisulphite, with chlorine, thus removing the chlorine residual from the point of oxygen scavenger injection. Oxygen scavengers also react with glutaraldehyde.|
|Partition coefficients:||Most biocides under consideration will be water soluble/miscible since it is the water phase that supports microbial growth. However, even these compounds may partition into a hydrocarbon phase to some extent. Hence biocides dosed into a water phase can be found in crude oil which has been in contact with the water. This can present several problems; lack of biocide containment, leaching of biocide from crude oil back into the water at sub lethal doses and interference with refining processes.
Alternatively, an oil dispersible/water miscible biocide might be required. It is essential to know the partition coefficient and the rate of partitioning so that a lethal biocide dose is quickly obtained in the target waterphase.
|Corrosivity:||Some biocidal compounds are also corrosive, e.g. chlorine. It would not be good practice to unnecessarily enhance one problem whilst reducing another.|
|Emulsion formation:||The formation of water/oil emulsions is actively discouraged. Therefore, the use of a biocide which encourages emulsion formation is not good practice.|
|Degradation of biocides:||Water treated with biocide cannot always be contained and must consequently be released into the open environment. Many biocides are described as degradable, assuming that this makes them more acceptable for release. This assumption must be backed by data on; the routes of degradation, any breakdown products, the half life and toxicity of such products.
The term biodegradation is sometimes used but is usually only achieved with biocide levels considerably less than those in use.
|Chemical residual analysis:||It is important to be able to detect accurately the amount of biocide present in water to be discharged into the environment. If a quick field method for biocide residuals is not available, then a marker, such as fluorescent dyes, should be added to the biocide during dosing.|
|Detoxification /neutralisation:||The toxicity of some biocides can be significantly reduced by the addition of a "neutralising agent" prior to release into the environment, e.g. sodium sulphite will reduce the toxicity of glutaraldeyhyde significantly (about 100 fold) when applied in the ratio of 2:1 v/v. This approach could permit the use and subsequent release of some biocides which would otherwise be severely restricted.|
|Chemistry:||The full chemistry, including active ingredients, solvents, "inert ingredients", stabilisers and the percentages of each should be fully known. This is important from a handling or selection viewpoint. Several compounds are marketed under more than one trade name and it is essential that time is not unnecessarily wasted retesting the same compound.|
|Toxicity data:||Toxicity data on human and relevant animal species, is essential for any biocide. The human data covers the handling procedures and the risks involved should biocide find its way into potable water supplies. Offshore potable water is usually derived by distillation of sea water from around the platforms. Biocides discharged at or near an operational platform could ultimately contaminate potable water on those platforms.|
|Safety of handling:||Other factors that may influence selection, other than performance include form of the biocide. Generally, liquid biocides are used but some eg for drilling formulations are supplied as solids or powders.|
How biocides are applied?
There are two philosophies in the use of biocides. These are:
- Batch treatment: A biocidal dose is applied. Short contact time (in hours).
- Objective: to kill all organisms present.
- Frequency of treatment: weekly to monthly.
- Continuous treatment:Biostatic dose applied. Continuous contact time.
- Object: inhibit microbial growth.
Although batchtreatments necessitate a considerably higher dose, they tend to be regarded as an "insurance" treatments and carried out routinely. Continuous treatments are costly and may necessitate periodic changes of chemical type to prevent the bacteria becoming resistant and therefore continuing to grow in the presence of a biocide.
It is important to remember that the relationship between biocide concentration and contact time is not linear. If 200 ppm for six hours has been recommended, 400 ppm for three hours may not be as effective.
The typical strategy on water injection streams is for continuous chlorination and regular biocide slugs every 2 to 4 weeks. Whilst the chlorine is not biocidal it does have some biostatic effect. Any pipeline containing seawater needs to be biocidally treated. Closed drain systems must also receive regular biocide treatments and surge tanks such as those at Cruden Bay are treated about twice a year. In general, bacterial monitoring is regarded as routine throughout oilfield operations.
Hazards and environmental risks
Because biocides are intended to kill living organisms, many biocidal products pose significant risk to human health and welfare. Great care is required when handling biocides and appropriate protective clothing and equipment should be used. The use of biocides can also have significant adverse effects on the natural environment. Anti-fouling paints, especially those utilising organic tin compounds such as TBT, have been shown to have severe and long-lasting impacts on marine eco-systems and such materials are now banned in many countries for commercial and recreational vessels (though sometimes still used for naval vessels).
Disposal of used or unwanted biocides must be undertaken carefully to avoid serious and potentially long-lasting damage to the environment.
European Community Classification
The Biocidal Products Directive 98/8/EC (BPD), the classification of biocides, is broken down into 23 product types (i.e. application categories), with several comprising multiple subgroups:
MAIN GROUP 1: Disinfectants and general biocidal products
- Product-type 1: Human hygiene biocidal products
- Product-type 2: Private area and public health area disinfectants and other biocidal products
- Product-type 3: Veterinary hygiene biocidal products
- Product-type 4: Food and feed area disinfectants
- Product-type 5: Drinking water disinfectants
MAIN GROUP 2: Preservatives
- Product-type 6: In-can preservatives
- Product-type 7: Film preservatives
- Product-type 8: Wood preservatives
- Product-type 9: Fibre, leather, rubber and polymerised materials preservatives
- Product-type 10: Masonry preservatives
- Product-type 11: Preservatives for liquid-cooling and processing systems
- Product-type 12: Slimicides
- Product-type 13: Metalworking-fluid preservatives
MAIN GROUP 3: Pest control
- Product-type 14: Rodenticides
- Product-type 15: Avicides
- Product-type 16: Molluscicides
- Product-type 17: Piscicides
- Product-type 18: Insecticides, acaricides and products to control other arthropods
- Product-type 19: Repellents and attractants
MAIN GROUP 4: Other biocidal products
- Product-type 20: Preservatives for food or feedstocks
- Product-type 21: Antifouling products
- Product-type 22: Embalming and taxidermist fluids
- Product-type 23: Control of other vertebrates
The global demand on biocides for use in industrial and consumer goods was estimated at US$6.4 billion in 2008, roughly 3% up from the previous year. Affected by the global economic crisis, the market will remain quite sluggish by 2010. The industry overall is further burdened by ever stricter regulations. The market saw a wave of consolidation in 2008, as producers are looking for measures to control cost and to strengthen market position.
- Directive 98/8/EC of the European Parliament and of the Council of 16 February 1998 concerning the placing of biocidal products on the market, http://ec.europa.eu/environment/biocides/pdf/98_8_ec_web.pdf#page=48, retrieved 2012-12-20
- Market Report: World Biocide Market, Acmite Market Intelligence, http://www.acmite.com/market-reports/chemicals/world-biocide-market.html
- Wilfried Paulus: Directory of Microbicides for the Protection of Materials and Processes. Springer Netherland, Berlin 2006, ISBN 1-4020-4861-0.
- Danish EPA (2001): Inventory of Biocides used in Denmark