Hydrophilic emulsifiers waste water treatment installations and aerobic and anaerobic bacteria

September 2010
This technical paper is to deal with a recurrent trouble of Penetrant Testing lines: bad smelling from hydrophilic emulsifier tanks, especially when in warm or hot areas and that bad smelling of recycled water coming from a waste water treatment installation. We are to let you know of a very simple, cheap, efficient way to overcome most of the troubles.

This trouble has been already dealt with in one of our papers published on our Website. (*)

One technical point, not related to the topic, but that we nevertheless think is important:

Waste water from Penetrant Testing lines shall NEVER BE MIXED with waste water from surface treatment (be they acidic, alkaline, with oxidants, etc.). These two kinds of waste water need very different processes to be “cleaned”, and mixing them makes it dramatically more complex to treat them. So each of these two “kinds” of dirty waters shall be treated separately, accordingly to the respective pollutants they contain.

What may be found in a tank of hydrophilic emulsifier? Easy answer.

Up to the ‘80s, we found … many things, for example: butts, bottles of beer and so many other kinds of items! Furthermore, workers used to wash their hands in the emulsifier tank!
Times have changed as now smoking or drinking alcohol in the work place is strictly forbidden; further everyone knows that an emulsifier has nothing to do with soap!
Except for these peculiar “additives”, the tank contains all the chemicals which make formulae of the emulsifier, but also of the penetrant(s) which are emulsified! In fact, we can summarize, in view of the topic: a lot of water (say: 94/95% for an emulsifier used at a ratio of 5% in water); 5% of emulsifier, and about 1% of penetrant. Penetrant and emulsifier may be considered as what we call the organic part of the liquid. Organic, not in the agricultural meaning; organic, because all the chemicals are based on carbon. This is a chemistry wording.

So we have:
• Water, a lot of.
• Carbon-based materials.
• An affordable temperature: it is likely that the average temperature in the vicinity of the tank will be in the 18/30°C (64/86°F) range. People working on the line are more efficient in this condition, depending also on the hygrometry (50 to 60% is generally regarded as the right figure).

This situation is IDEAL for … bacteria!! We are “immersed” in a “bath of bacteria”: our skin is in contact with several dozens of different species; we inhale a lot of them every time we inspire. We eat huge quantities of them!

So bacteria find the tank of emulsifier as the right place to live - and to reproduce, to “duplicate”:
• Water.
• Food: the organic chemicals.
• The right range of temperature.

When conditions are favorable, for many species that means a 37°C (ca 100°F) temperature, a bacterium will divide in two within twenty minutes. So, within only an hour, the mass of bacteria will have been multiplied by 8!

We easily imagine that the more bacteria, the more food they eat. But another element, not yet displayed, is very important.

Bacteria come in two “types”, just to make a parallel with … penetrant testing and penetrant classification.

• The Type 1 is aerobic bacteria, i.e. bacteria which NEED oxygen to oxidize the food they have, the organic materials, to make them easily assimilated. This digestion produces energy, water and carbon dioxide (CO2).

• The Type 2 is anaerobic bacteria, i.e. bacteria which do not need oxygen to multiply. Oxygen may even be considered as toxic for them.

Let us come back to the tank of emulsifier. A certain amount of air is dissolved in water. Hence, oxygen. Air is introduced every time parts are immersed into the tank.

Aerobic bacteria then have plenty of what they need: they multiply, very quickly. And they consume oxygen at an increasing rate. Even with the air coming in every time parts are immersed, finally a time comes when there is not enough oxygen for them to multiply. They do not die. They come to a state called dormancy, in which they often become what is called spores. When they are again in favorable conditions the spores come back to life … and they multiply.

Consider the situation in the tank of emulsifier. Aerobic bacteria have multiplied to the point where there is no longer oxygen enough to have them multiply. But there is still plenty of water and food, and agreeable temperatures. What does happen?

The Type 2 bacteria awake! These bacteria were as spores as long as there was oxygen in water. But now, the situation has changed. These bacteria, which have existed probably before the Type 1 on Earth, need an oxidizing agent to oxidize the carbon in the food. And for millenniums, for hundreds millenniums, for billions years … they have known … the Mendeleiev's periodic table of the elements!!!  They know that, in the same group 16 (old style VI-A) of chalcogens as oxygen, just under oxygen, there is an oxidizing agent, very common in our environment: sulphur!

And what does happen when carbon-based products are oxidized by sulphur? The carbon atoms give rise to carbon disulfide, a very volatile solvent, with a repellent odour. This solvent is used in the rubber industry.
The hydrogen atoms of the chemicals react with sulphur to become … hydrogen sulphide (also named sulfane), H2S, which is gaseous, extremely soluble in water, with a rotten-eggs odour that the human nose detects at less than 1 ppm in the air. Hydrogen sulphide is an acid, very corrosive.
These anaerobic bacteria are sulphate-reducing bacteria such as those of the Desulfovibrio type.

Do you see the problem? The tank of emulsifier contains a lot of anaerobic bacteria which use the sulphur coming from air: H2S is given off in small quantities … but enough for our noses being able to detect its presence!

That is why many tanks smell bad, especially when weather is hot … because the hydrogen sulphide is then less soluble in water than when temperatures are low … and its molecules float in air reaching the sensitive cells of the noses of the guys who work on the PT line!

So, what to do?

Many people, as soon as we talk them about bacteria, have only one thought: “KILL’EM”!
And many suppliers supply PT users with bactericides to be added to the emulsifier.
But, wait a minute: what about the halogens (chlorine and bromine mainly) content of MANY of these bactericides? Are they listed as approved materials for PT? No.

To overcome this problem, some (not all) manufacturers add a bactericide in the hydrophilic emulsifier(s) that are listed as approved. As a matter of fact, almost all the manufacturers market bactericides from the same “family” (a consequence of the chemists’ turnover from one PT materials manufacturer to an other one, a quite usual custom in this business).  These very efficient bactericides are halogen-free and obviously sulphur-free.

However, everyone knows about the ability of bacteria to develop resistance to what is assumed to kill them. Further if there are biofilms in some areas of the tank, of the pumps, etc., bactericides will not be efficient. What is a biofilm? Bacteria develop as a film of several hundreds layers of bacteria in areas where they can cling on rough surfaces: welds, for instance. The many layers protect the bottom layers of any contact with the bactericide. Well, in any case, within few months, bacteria will mutate to another strain which will resist to the product. Users will have to choose another bactericide, which several months later … etc.

The right answer to the problem, as always, is: “FIRST THINK!”

The anaerobic bacteria are the “target”. They do not develop when there is oxygen. And the aerobic bacteria do not seem to be a problem. So, we have the right answer! Help aerobic bacteria to stay “alive”, and prevent anaerobic bacteria to take precedence!

At this point of the paper, you have the answer: air!! The only thing to do is to have 24 hours a day, 7 days a week, an air influx in the tank! Small enough to not generate foam. Many holes in small pipes at the bottom of the tank.
Air influx eases the renewal of the emulsifier in contact with the penetrant layer on the parts.
Never, never stop the air influx, even at night or during week-ends. Environmental managers could tell you that this is energy wasted (compressed air consumed when there is nobody working on the line), but you must stay stubborn: never, never stop it!

If you have a tank already “polluted” by H2S, when you put the air influx in service, for several hours, the situation will be worse: the odour will intensify!! Yes, this is due to the gas going out of the tank. Better to begin the “treatment” when the line is not used for a while: for instance, the end of afternoon on a Friday. When users come back the next Monday morning, everything will be OK.

Is this means cheap, easy to install, more neutral to the environment (more environment-friendly) than any bactericide? If you answer “Yes”, try it!

And the same reasoning may be applied to waste water treatment installations when water is recycled. Quite often users complain about the bad smelling of the water sprayed in the washing/rinsing stations.
The dirty water tank is full of carbon-based materials, of water, all at favourable temperatures. Oxygen is sparse in this tank. So anaerobic bacteria multiply, even if the chemicals are low in sulphur.
Keep in mind that:
• H2S is very soluble in water.
• The human nose is very sensitive to the odour of this molecule.

When sprayed, water lets the gas go and, just by breathing the air filled with small droplets of water containing H2S, the operator says “it smells badly”. The clean water does not contain bacteria, but only H2S.
If an air influx is used in the dirty water tank, this could help. Another way is to send air in the clean water tank to “push” the H2S out of water. Do not forget that this chemical is very corrosive, and that some materials could be corroded when rinsed with clean water containing even small traces of H2S!

Reference

(*) Patrick DUBOSC and Pierre CHEMIN, Post-emulsifiable Penetrant Testing: hydrophilic emulsifier, DPCNewsletter N°001, May 2008: On our Website.