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Odour Control

Odour control can be an important issue for indoor gardeners. That is, undesirable odours can occur in the grow room and are often vented into the outside environment and can travel on air for significant distances.

Odour

Odours are volatile organic compounds that are “tasted” by the nose. Everything we can smell is volatile, meaning it evaporates (converts to gas) and mixes with the air we breathe in. Disguise these odours with another odour, or break these compounds down into something else, or intercept these compounds prior to them exiting into the outside environment and odours cease to be a problem.

How Do We Deal With Odour?

There are three commonly used methods of dealing with odour.

• 1) Ozone
  
• 2) Carbon Filtration
  
• 3) Using a fragrance to disguise/mask another smell
  

Let’s look at ozone first.

Ozone (O3)

In nature ozone occurs when UV light comes into contact with O2 (oxygen). The result of this is O3 (ozone). The earth’s ozone layer is a result of the interaction between 02 and UV light. The conversion of 02 to 03 via this interaction is referred to as a photochemical reaction.

In addition to this, ozone is also formed when O2 passes between electrical arcs. 03 occurs when 02 passes through a corona (an electrical arc) causing the 02 bond to split, freeing two 01 molecules which then collide and bond with 02 molecules. 02 + 01 = 03. For this reason, abundant amounts of ozone are present straight after (and during) an electrical storm.

Ozone is also created by motor vehicles when nitric oxide that is formed in the engine passes into the atmosphere and oxidizes quickly, forming nitrogen dioxide. Nitrogen dioxide reacts with other atmospheric components, forming 0, which combines with 02 – forming 03 (ozone).

Ozone is a gas that is classified as an oxidant or a substance that converts organic material into their base compounds. Chlorine, fluorine, and hydrogen peroxide are other examples of oxidants. Ozone is the second most powerful oxidant after fluorine.

Ozone is effective as a means of odour control due to its ability to break these organic substances down into base compounds. Ozone is a very fast acting oxidant and very quickly converts odours (or a percentage of the odour) into inert substances.

Ozone also has the ability to destroy/inactivate microorganisms that are present in the air (as well as in water that is treated with ozone). For this reason ozone is commonly used in hospitals for it’s germicidal properties. Where plants are concerned, ozone has the (potential) ability to eliminate/inactivate fungal spores, viruses and pathogens that are present in the grow room air.

Ozone is an unstable substance and quickly reverts back to O2 (within, approximately 20 – 90 minutes). Because of this ozone cannot be stored as gas and must be produced on sight. There are two methods that are commonly used to produce ozone.

1)UV lamp unit. UV light in combination with 02 results in 03.

2)Corona discharge; an electrical arc is formed between two points creating ozone. Corona discharge units will generate more ozone than UV lamp units. However, corona discharge units will also create nitric acid and nitric oxides. Therefore, corona discharge units should never be placed in the grow room environment, or in any other place where you or your plants will be for extended periods.

Ok, so that’s some basic theory on ozone… but does it work?

Well firstly, ozone’s ability to neutralize odours is determined by a few things.

• 1) The type of smell (it’s organic composition)
  
• 2) The degree of the problem (and)
  
• 3) The amount of ozone that is present in the treatment area (to combat the organic compounds/odour)
  
• 4) The ability of the ozone to mingle/come into contact with the organic compounds (odour)
  

Now that we’ve established this… How effective is ozone as an odour control agent? To be blunt, ozone doesn’t come with any guarantees. The problem is, ozone will only eliminate a percentage of the odour that you, ideally, wish to eliminate completely. This percentage will be governed by the variables that we have just covered. This may, or may not, prove to be satisfactory.

In addition, ozone is also an air pollutant and is potentially harmful to plants and humans (namely, your plants and you!). That’s not to say that ozone should be avoided at all costs. It really means that, too often, ozone is promoted as an odour combatant without other important information.

So what should we know?

First and foremost is that ozone can harm plants (and you). Ozone is an oxidant and a corrosive. This makes its’ use, potentially, hazardous. Secondly, to reiterate on an earlier point, ozone may or may not produce the desired results (ozone may not neutralize enough of the smell) and, last but not least, there is a more effective method (and less problematic method) of odour control in the form of carbon filtration.

Using Ozone Safely

Ok, so we’ve identified that ozone can be detrimental to the health of you and your plants, but how much ozone can we safely use, and how do we avoid these possibly harmful effects?

Well firstly, short exposure times to ozone, in the levels that are likely to be present in the grow room probably don’t pose too much of a risk to you. The same can’t be said so easily about the risks posed to any plants that are present in the room, as their tolerance to ozone is lower than yours and their exposure time will be far greater than yours (unless you are living in the grow room 24 hours a day, like your plants).

However, having said this, where exposure to ozone is concerned always ere towards the side of caution. That is, limit exposure times and, if/where possible have the ozone generator switch off 30 minutes prior to you entering the room (via timer). Better yet place the ozone generator in a space outside of the grow room where the air that is exhausted from the grow room and the ozone can mix (eg. ceiling cavity, next to exhaust outlet etc).

As mentioned earlier, ozone can be beneficial for inactivating fungal spores etc. However, by treating the exhausted air we lose this benefit.

Definitely, never use a corona discharge unit in the grow room, or in any other area where you (or your plants) will encounter corona produced ozone. These units release some nasty by-products (nitric acid) that make their use in the growing environment highly problematic. However, corona units can be safely used in exhaust ducting (or some form of a mixing box etc) where airflow extraction is occurring (ozone should only be produced when the fans are in use).

Which brings us to our next point… ozone’s potential to degrade commonly used ducting materials. Yes, this is starting to get very complex, isn’t it?

The EPA lists ozone resistant materials as:

Austenitic (300 series) stainless steels
Glass and other ceramics
Teflon and Hypalon
Concrete

On a more positive note, while ozone will degrade air conditioning ducting (the type of ducting material commonly sold through hydroponics suppliers) this process will be dependent on the levels of ozone (hence oxidant/corrosive) present. Generally speaking, air conditioning ducting doesn’t tend to degrade too quickly when a small-scale ozone unit is in use.

Excessive levels of ozone can have several negative effects on plants:

Photosynthesis can be decreased and (hence) growth impaired
Stomata may close when ozone is in the atmosphere (limiting gas exchange – I.e. plant’s ability to take in CO2 and expel oxygen)
Respiration may be increased (resulting in reduction of carbohydrates/plant energy)

Ozone damage first appears as leaf curl (ends of leaves curling). Thereafter, tissue damage can/may be identified in the form of tip burning and/or (in more extreme cases) bronzing/rusting leaves.

The last significant factor that we need to look at is how much ozone is safe?

Where you are concerned, the ACGIH lists a maximum concentration of 0.3 mg/L (by volume) for exposure of up to 15 minutes. If you are absolutely committed to generating ozone in the growing environment then it is possible to check ozone levels through testing devices that range in sophistication and price. Suppliers of scientific testing equipment should be able to help you with regards to this type of equipment.

And where plants are concerned? Sorry folks, this will be species and genetics dependent and there is not enough research around this problem (pertinent to indoor growing situations). Therefore, practices such as mixing air and ozone outside of the growing environment are highly recommended.

Better yet, buy a carbon filter.

Carbon Filtration

The basic principle of carbon filtration is that when organic compounds enter carbon filters they are intercepted and held/absorbed. This means that air is filtered of organic compounds when it passes through carbon.

The process by which organic molecules bind to carbon does not involve chemistry – it is a physical binding caused by a force called Van der Waal’s Force. Put simply, it is the molecular structure of organic compounds which stick to carbon in the same way Velcro works – the hook bits stick to the furry bits. The carbon is the hok and the organic compounds are the furry bits.

Carbon filtration, unlike ozone, does not have any (potential) harmful effects on plants or people. In addition, when the appropriate filter is used, carbon filtration can be guaranteed (unlike ozone) to eliminate high percentages of odour. This means that carbon filtration should be the first port of call as a means of eliminating odours.

Some factors that you will need to be aware of are:

Carbon filters will restrict airflow. Therefore, equipment choices pertaining to filter and fan type are important.

Airflow dynamics.

On this note, mixed flow fans and/or centrifugal fans are the ideal fan types for a situation where a carbon filter is in use. These fans force air through lines (line pressure) or impedance’s (such as carbon filters) far more effectively than axial fans. Their cost is typically higher than axial fans. They also tend to produce more noise as noise is a result of air pressure – the more air pressure, the more noise.
Carbon filtration will (probably) cost more than ozone (depending on what type of fan/s you already own). The benefits, however, make the expense well worthwhile.
There are many types of carbon used in air filtration, and (at least) several different brands (of carbon filter) available on the market. The performance of a carbon filter is determined by the quality and amount of carbon present, by the activation process used, by the flow pattern of air through the filter, and by the moisture in the air. A given amount of carbon can only absorb a certain amount of volatile organic chemicals/compounds, so clearly the more carbon present (i.e. the denser or heavier the filter) the longer it will work.

Using A Fragrance to Disguise/Mask Another Smell

The last method of odour control that we’ll look at is disguising odours with another smell. I should point out that this is the least effective method of dealing with odours and is only of value in small-scale situations.
Other than this, I have also encountered individuals who use a combination of ozone and a masking fragrance to tackle larger problems. I believe this is more through necessity than design (the fragrance was introduced after ozone, alone, failed to tackle the problem).

There is, in fact, very little that can be said about this one. Introducing a fragrance in the hope that it disguises another fragrance pretty much speaks for itself.

Speak to your hydroponics supplier about product options.

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