Odour Control


Excerpt from Integral Hydroponics (the Ozzie Bible) by G.Low…




Controlling Odour in Indoor Grow Rooms


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.




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 neutralise 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.



Activated Carbon Filtration




The basic principle of activated carbon filtration is that when volatile organic compounds enter carbon filters they are intercepted and held/absorbed. This means that air is filtered of volatile organic compounds as it passes through activated 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 hook 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.
  • On this note, 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 or mixed flow 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 amount of moisture in the air.


There are two commonly used methods where activated carbon air filtration is concerned.


  1. Carbon scrubbing: A filter and fan are located in the growing environment and the air is constantly recycled/ventilated through the carbon filter. Through this method volatile organic compounds are removed/intercepted and the growroom air is kept odour free via constant recycling/cleansing of the air in the growroom environment. Carbon scrubbing is ideal in cooler climatic zones.
  2. Exhausting through carbon: As the air is exhausted from the growroom it passes through a carbon filter. Through this means the volatile organic compounds (odour) present in the growroom environment are intercepted before they reach the outside atmosphere.  This method is ideal in warmer climatic zones.


With this article is a flowchart that will help you work out your carbon filter and centrifugal fan requirements for exhausting through carbon.  Please note: The flow chart was designed for use with centrifugal fans. If using mixed flow fans allow for approx 45% airflow loss. If using an axial fan allow for approx 80% airflow loss. In addition, the flowchart was developed for use in warm/hot climatic zones. Carbon scrubbing will be more ideal in cooler climatic zones.  Download Carbon Filter and Fan Combination Flow Chart


Carbon Filter and Fan Theory


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. All absorption figures are based per gram of carbon. More weight equals more absorptive capacity. However, carbon quality varies and impurities, the percentage of carbon to impurities (the quality of the activated carbon), the activation process, and carbon type will determine final outcomes pertaining to longevity and performance.


Airflow through filters is fairly easy to achieve because air has very high “diffusivity” meaning there is little resistance to flow even through very tiny holes.


The holes in carbon are referred to as macropores  (large), mesopores (medium) and micropores (small). Absorption mainly takes place in the smaller (micro) pores. It is here that the attraction forces (hooks) are most concentrated. The larger pores, although they have some absorption capacity, mainly act to conduct air or fluid to the micropores,


High humidity reduces air filter performance because carbon particles become coated with water, and water reduces the diffusivity into the pellets. However if performance drops off because a filter becomes wet, it is relatively simple to improve performance by driving warm, dry air through the filter to evaporate the water, dry out the pores within the carbon and re-expose the hooks.


When contemplating what is the ideal fan and filter combination for your environment there are several factors that need to be considered.


These are, the size of the growing environment, the amount of HID light that is present in the environment (wattage = heat), the outside ambient air temperatures, and the resistance to air flow through the environment (for instance a grow room with an exhaust fan of 200l/s and no inlet fan will only reflect a true flow of approx 70% – or 140l/s – due to air pressure displacement anomalies, while a room with a 200l/s outlet fan and an inlet fan of 200l/s will reflect airflow at 200l/s due to even displacement and replacement of air).


In hot climates such as Australia it is important that airflow is considered carefully. Temperatures will vary considerably between winter and summer and this should be given due consideration. It is relatively simple to reduce airflow when the outside air temp is cool (use a thermostat to turn fans on and off based on growing environment temp or use a fan speed controller to reduce airflow) but it is impossible to displace and replace more air than your fan/s are capable of.


As a rule of thumb, always invest in more airflow than you think you will need. Short of creating cyclonic conditions in the growing environment you cannot have too much airflow in times of heat.


Litres per second (l/s) is the standard used in Australia to measure the airflow capacity of fans.


Converting this into useable information is easy.


An ideal airflow scenario in the hot Australian climate is complete air replacement every two to three minutes (two being better than three).


This means that if your environment were a total of 36 cubic metres (36 m3) you would ideally be moving 36 m3 of air every two to three minutes.


Working out the m3 of your environment is easy; it comes down to how long, how wide, and how high? Let’s say (hypothetically) that our growroom is 4mtrs long, 3 mtrs wide, and 3mtrs high. To work out the Cubic metres simply times these numbers. Ie. 4 x 3 x 3 = 36m3 (36 cubic metres).


Now, let’s say that our inlet and outlet fans were both rated at 200 l/s, reflecting 200l/s of air replacement. This means that 200 l/s equals one cubic metre every five seconds (1000l/200l/s = 5 s). Five goes into 60 (seconds) 12 times, which means we are moving 12 cubic metres of air every minute. Our hypothetical growroom is 36 cubic metres, which means we have total air replacement every three minutes (3 x 12 = 36m3).


In smaller environments with high light levels (high heat output in a confined environment) you will ideally want to increase the airflow levels to ensure complete air replacement every one minute.  This is due to heat rising significantly faster in smaller environments.


Flow Impedance/Static Pressure


Flow impedance will be governed by many factors. The standard that tends to be set with carbon filters, where centrifugal fans are concerned, is 30% air loss (70% efficiency). After running tests to establish the validity of this figure we have concluded that the 30% air loss claim is not entirely correct. The surface area of the filter and the physical makeup of the type of impedance (filter size, surface area, carbon type etc) and the centrifugal fan design (blade pitch, wattage etc) will determine outcomes where actual flow values are concerned.


Below are some examples of flow efficiencies through carbon filters. You will see that as the (same brand) filter gets smaller the airflow efficiency is reduced.


Also included in this example is a comparison of actual airflow where the same fan is used in testing another brand of filter (impedance comparison).


Can Fan 250 (100 watt motor)
Stated manufacturer rating 230.5lps
Tested Rating at fan = 255.0lps
Test conducted for carbon impedance efficiency and not absolute flow. Test procedure does not reflect manufacture rating as methodology differs.

Can Fan 250 with Can 50 (carbon = 16kg) = 159lps (efficiency = 62.35%, loss = 37.65%)
Can Fan 250 with Can 75 (carbon = 24kg) = 180lps (efficiency = 70.58%, loss = 29.42%)
Can Fan 250 with Can 100 (carbon = 32kg) = 197.9lps (efficiency = 77.60%. loss= 22.4%)


Can Fan in ‘Other’ 1mtr x 250 Filter (stated carbon weight = 28.1kg)

Can Fan 250 with other filter (Carbon 28.1kg) = 162.5l/s (efficiency = 63.72%loss = 36.28%)
Loss in equivalent Can Filter = 22.4%. Airflow through equivalent Can Filter is 35.4l/s higher.

Can Fan in ‘Other’ 500mm x 250 Filter (stated carbon weight = 11.5kg)

Can Fan 250 with other filter (Carbon 11.5kg) = 121l/s (efficiency = 47.45%,loss = 52.55%)
Loss in equivalent Can Filter is 37.65%. Airflow through equivalent Can Filter is 38l/s higher.


You will note significantly higher percentages of air loss through the filters of the other manufacturer. This is due to different activated carbon types (particle vs palletized) and variants in filter surface areas.


Filters that have granulated or extruded carbon require less pressure than filters made of powdered or unprocessed carbons. The reason for this is extruded carbon has much better air channeling capacity, whereas unprocessed carbons have fine particles which tend to block up the air spaces between the bigger particles, making it harder for the air to move, therefore more pressure is needed. It is just the same as when you look at water flow through regular sized particles (such as marbles) versus irregular particles (like soil, composed of gravel, sand and clay). The more fine particles (like clay) the slower the flow because the fine particles block the pore space between the large particles.


Bigger Filter = Better Airflow and Longer Life Span


The flow rate examples we have used demonstrate that larger filters equate to less resistance against airflow.


Larger filters require less pressure than smaller filters because there is a greater surface area for the same volume of air to move through.


Compare the 1mtr Can Filter against the 500mm Can Filter. This equates to 100% more carbon and 15.25% (38.9l/s) more airflow against its smaller equivalent, while the retail cost is approximately 30% more.  Bigger filters, therefore, not only allow for more efficient airflow but also represent far better value for money in the long-term.


Centrifugal Fans for the Indoor Growroom (the basics)




The theory of operation of a centrifugal fan is similar to that of a centrifugal pump. The pressure developed arises from two sources. These are centrifugal force due to the rotation of an enclosed volume of air and to the velocity imparted to the air by the blades and partly converted to pressure by the volute or scroll-shaped fan casing.


The centrifugal force developed by the rotor produces a compression of the air which in fan engineering is called static pressure. The amount of static pressure depends on ratio of the velocity of the air leaving the tips of the blades to the velocity of air entering the fan at the heel of the blades. Therefore, the longer the blades, the greater the static pressure developed by the fan.


Operating efficiencies of fans range from 40 – 70%. Operating pressure is the sum of the static pressure and the velocity head of the air leaving the fan. In SI units the pressure is usually expressed in centimetres of water. The power output of the fan is expressed as follows:

kW = 2.72 x 10-5Q p

Where kW is the fan power output, Q is the fan volume in m3 per hour and p is the fan operating pressure in cm of water.

Efficiency is determined by the ratio of air power output to shaft power input (i.e. the power the electric motor gives.)



In technical terms, the power that is required to operate a fan is a function of the speed cubed or to the third power. In practical terms, if the speed of a fan is doubled, the power required to operate the fan will be eight times greater.


What this means is that a given wattage of fan motor has set limits placed upon it by physical laws; no matter how good the design of the fan the total airflow/air pressure is ultimately determined by the level of power that drives any given fan.
Wattage is therefore an important factor in fan choice for the purpose of moving air through carbon filtration.


Blade Distance from Fan Wall


As the space between the blade and the fan wall increases the air pressure decreases. Because the surface area is calculated with a formula that involves squaring the diameter ever extra millimeter decreases the fans effectiveness dramatically.


Some fans on the market have utilized space between the blade and the fan wall to achieve lower noise levels.  These fans can still have good airflow but in reducing blade size they have compromised air pressure.


Blade Pitch


By changing the geometry (pitch) of the fans blades you are able to increase airflow. However, while airflow is increased, air pressure will be decreased and any gains in airflow will be offset greatly by a lack of air pressure.


The best fans are designed with a blade geometry that maximizes airflow potential with air pressure potential (relative to power).


Overall Quality


Quality of fans is determined by motor quality, housing quality, blade quality, and wattage relative to airflow and air pressure output. Other factors such as how close the centrifugal blades are to the inlet of the fan, blade stabilization, motor noise, bearing quality, thermal overload switching (safety), are critical in fan quality. There are many factors that quality fan manufacturers take into account when designing and manufacturing fans.


So, what’s the best quality centrifugal fan? My tip is, you typically get what you pay for in life. European (German and Swiss) motors and design tend to assure reliability and performance. Put simply!


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. Two odours instead of one.

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