Occupational Hygiene and Fallout Dust Monitoring Training - Course Contents
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The three main purposes of a fallout dust monitoring programme are:

 

1.       To meet legislative requirements.

2.       To indicate long term trends.

3.       To generate or maintain awareness of dust generating activities on site.

The method and equipment used should assist in achieving these purposes.

 

The National Environmental Management: Air Quality Act, 2004 (Act No. 39 of 2004) (NEMA) contains a schedule called the National Dust Control Regulations and the latest draft version of this document specifies the use of the ASTM D1739 method.  The definition of dust is also given as particles that have a diameter smaller than 100 micrometers. 
The definition of dust also excludes the small particles of carbonaceous matter directly emitted by a combustion process.  This definition implies that some sort of quantification in the sample of the carbonaceous matter will be required.
The schedule makes for clear allocation of the residential action level to residential and light commercial areas and by inference applies the industrial action level to all other land areas which would include heavy industrial areas, farmlands, and any other land not classified as residential or light commercial as per the local town planning scheme.
The schedule also requires that the fallout dust at the boundary or beyond the boundary, of the premises from where the dust originates, not be above 600_mg/m²/day for residential and light commercial areas and not be above 1200_mg/m²/day for all other areas. 
This implies that the dust collected in the bucket has to be identified as coming from a particular source and that the limit only applies to the dust coming from that particular source.

The single bucket unit will collect dust from all dust sources in the area and if the limit has to be applied to dust from a particular source then a directional fallout dust monitoring method is required.  The four bucket DustWatch unit was designed specifically to meet this requirement.  This is discussed in more detail below, but this is why we apply the limits specified to each individual bucket of the four bucket DustWatch unit.

The SANS 1929:2005 document specifies in section 4.8.6 that the reference method for dustfall is ASTM D1739 and includes a note saying that “Any other method which can be demonstrated to give equivalent results may be used.”  This document also specifies that modelling techniques can be used to supplement measurement techniques.  If the results are below the lower assessment threshold (presumably 300_mg/m²/day) then modelling can be used without actual measurements, although initial actual measurements will probably be required.

The SANS 1929:2005 document also specifies that the dustfall sampling points should be placed within a maximum distance of 2km of the boundary of the source.  Monitoring units placed within the boundary of the source are not subject to the dust deposition criteria in 4.8.2.

The ASTM D1739 98(2010) method is the latest version of this standard.  This standard defines the dust that is collected as material smaller than 1mm.  This method does not require water to be added to the buckets before being placed in the field, although an earlier version of this standard ASTM D1739 82 did require water to be added.  The 98 version of the standard also requires the buckets to be twice as high as the diameter of the bucket. 

The ASTM D1739 98(2010) standard specifies that this method is crude and non-specific, and useful in the study of long term trends.  These comments need to be kept in mind when operating a fallout dust monitoring programme.

The United States of America does not have any air quality standards for particulate matter larger than a d50 of 10 micron (PM10).³  The progress in the technology of fallout dust monitoring continues to move forward in countries where fallout dust monitoring is included in their environmental legislation.  Australia, the UK and South Africa are among the countries that have fallout dust monitoring in their environmental legislation with application action levels.

The wind shield design described in the ASTM 98(2010) standard is a shield that surrounds the bucket and prevents wind from blowing directly over the lip of the bucket.  The shield provides a sheltered area for the bucket to stand in and also allows for dust that enters this sheltered area to land around the bucket and not be collected in the bucket.  Only dust that settles into the actual bucket is collected, the dust that settles within the shield but outside the diameter of the bucket is not collected.  This design needs to be considered when comparing it to the wind shield design of the Dust Watch unit.

The windshield design of the DustWatch unit was born out of experience by Gerry Kuhn in the mining industry and working with fan designs used to move dust-laden air from one area to another.  To prevent dust from wearing the edge of the fan blades away, the shape of the blades had to be designed to limit the dust particles from hitting the blade.  The design of the DustWatch wind shield acts in a similar way and redirects the wind blowing towards the lip of the bucket up and over the bucket.  In addition to the physical change in the direction of the air and dust, a low pressure above the bucket is created and as a result the air from below the bucket moves upward around the end of the bucket and further acts to prevent the collection of the dust during windy conditions.

The DustWatch wind shield design causes the fallout dust to be predominantly collected under low wind and still conditions.  The wind shield also provides a particle size selection cut-off at a d90 of 100 Micron.  The size cut-off can vary if a localised dust source contaminates the sample and this is clearly noticeable if particle size analysis is done.  The four graphs below show particle size distributions collected from the four-bucket DustWatch unit.  Individual buckets were used and the dust collected over a full year combined for the analysis.  The fourth graph shows the clear contamination by a localised dust source that caused particles larger than 100micron to be collected in a significant proportion.  The other three graphs show the normal expected particle size distributions from DustWatch samples.










The ASTM D1739 98(2010) does not mention the particle sizes collected in the fallout dust monitoring other than the particles are less then 1mm.  Many people have ignored the PM₁₀ fractions of the fallout dust that can be collected and the long-term trends that can be provided and studied.

The wind shield design of the DustWatch unit therefore provides the same type of shielding as the shield design described in the ASTM 98(2010) standard, but also provides size selection and the collection of PM₁₀ particulate.  The PM₁₀ fraction of the fallout dust collected is shown in the graphs above.  The use of water in the bucket throughout the monitoring period also provides retention for small PM₁₀ and PM_2.5 particles and this is also discussed in more detail below

The DustWatch is designed to meet the requirements of ASTM D1739 but this does not cater for wind direction so we have retained the fundamentals of the standard within the design of the four bucket DustWatch unit.

With the origins of monitoring having been established with the American Standard Test method (ASTM 1739D) there have been several developments in the field and improvements to the original open bucket collection methodology.

An article by G Kornelius and M Kwata compares certain aspects of the 82 and 98 versions of the ASTM D1739 standard.  This article compared the fallout dust collection with a wind shield and without a wind shield as well as the fallout dust collection with water added and not added to the buckets at the start of the monitoring period.  The wind shield was shown to increase the collection and retention of dust with in the bucket, although additional statistical analysis was recommended.  The addition of water to the buckets at the start of the monitoring period (all samples with a wind shield) was also tested and statistically analysed using linear correlation and forcing the regression line through the origin.  The article concluded that there was some evidence to indicate that adding water at the start of the monitoring programme would increase dust retention, but the statistics used did not show significance.  Additional statistics might have revealed some significance.

Based on the article by G Kornelius and M Kwata and the fact that the previous version of the ASTM standard included the addition of water to the bucket at the start of the monitoring programme, we have chosen to always add water to the buckets at the start of the monitoring programme. 

Other factors that should be considered here are that rain falls into the bucket, so there may be water in the bucket anyway.  If one considers the potential disturbance of the dust in a dry bucket when a raindrop hits the bottom of the bucket, the value of having water in the bucket is obvious. 

When dust is collected in a bucket then it settles quite quickly depending on the particle size over an estimated time of 30 minutes, after which all the particles will have settled to the bottom of the bucket except the very fine particles.

We assume that the requirement in the ASTM 98(2010) standard to have a bucket that is twice as high as the diameter is meant to keep the dust that lands in the bucket from escaping from the bucket.  As the bucket may have no water in it, this is an important requirement, but we suspect that the height would have to be significantly more than twice the diameter to achieve total capture of all dust particulate.

The design of the selector disk emulates the operation of an aircraft wing; a feature working at wind velocities exceeding 3.0 m/s. The feature results in diversion of any particulate larger than 0.5mm that is wind driven at 3.0 m/s or more over the selector opening. This feature also minimise the capture of grits while the wind is blowing. The collected dust and particulate thus only occurs when the wind velocity falls to a point where precipitation is possible. Under extremely quiet conditions the very fine dust fractions are precipitated as well.

The collection height has been selected with several features in mind. The lifting of +500µm material in a 3.0 m/s wind velocity can only in a rare aerodynamic form achieve a height of about 2.0 m. The bucket lips are positioned at 2.2m.

The buckets can be reached for ease of handling by persons of 1.5m or taller. The elevating support cradle locks in position, protecting the buckets from theft or pilfering to a degree.

The selector disk runs on a 318 stainless steel shaft running in a nylon or Vesconite bushing for a longer trouble-free life with a minimum possibility of binding and maintenance. The disc is also dynamically balanced to minimise rotation bias.

We regularly consider and evaluate the methodology of the fallout dust monitoring programme and the equipment used to collect the samples. 

We are often asked to comment on how the directionality of the four bucket DustWatch unit works and offer the following arguments.  When the wind stops blowing from a certain direction then the atmosphere above the unit will contain dust that came from that direction (the direction the wind was blowing from).  So for the period with no wind the dust collected will still ideally be from the source in the direction that the wind was blowing from.

If the wind quickly changes direction, rotates the lid and then stops, then depending on how long it blew from the new direction, there will be some cross contamination of fallout dust from the two directions in the buckets.

There is also the complication of different wind directions at different levels in the atmosphere and other wind patterns that bring dust from one direction and deposit it from another direction.

In line with the “crude and non-specific, and useful in the study of long term trends” statement in the ASTM standard, the above complications with regard to wind patterns and the operation of the DustWatch lid to open different buckets under different wind conditions seem reasonable for the value of determining directional fallout dust emanating from a particular source.

The colour of the dust on the filters and the Microscan differences noted between filters taken during the same cycle from a four bucket dust watch unit are good indicators of the DustWatch units capability to determine directional fallout dust.

Legislative Table Highlighting Key Issues

The following ideas and concepts are relevant to this discussion.

1. The sampling period, 30 days or 14 days, or extended periods of three months.  We advise our clients to change their buckets on a 14-day cycle, especially in areas where the fallout dust levels are close to or above the action level.  This provides for a more robust monitoring programme as 26 data points are obtained over a year.  As the monitoring is continuous it is possible to estimate the 30 day averages by using a combination of the 14 day averages obtained.  The 14 day cycles also allow for easier detection of fallout dust sources because results cover a shorter period. 
   
    
2. How to use the single bucket DustWatch unit and the four bucket DustWatch unit to comply with the legislation.  The residential action level is 600_mg/m²/day and the Industrial action level is 1200_mg/m²/day.  If the results are above the applicable action level as collected in a single bucket DustWatch unit then a four-bucket DustWatch unit should be used to be able to identify the source of the dust.  With the four-bucket DustWatch unit the action levels are applied to each bucket separately as the purpose of this unit is to provide directional information with regard to the fallout dust. 
   
   The summation of the four buckets from the four bucket DustWatch unit will not compare well to the single bucket DustWatch unit results.  The single bucket DustWatch unit and the four bucket DustWatch unit should be used independently.
   
   Single bucket DustWatch units will collect fallout dust from all directions and from all sources in the area.
   
   As the legislation requires that the action levels apply to the dust originating from a particular source, the action level is applied to each of the buckets of the four-bucket DustWatch unit.
   
    
3. Terminal settling velocity impacts of the ASTM single bucket fallout dust monitoring unit and how this compares to the single bucket DustWatch unit.  Terminal settling velocities can be calculated here - http://www.dustwatch.com/fallout/dustwatch/settling_rate.htm.  To visualise how dust particles move in the air is not easy and many factors need to be considered such as temperature, humidity, turbulence, wind speed, wind direction at different heights from the ground, topography, and the type of dust being emitted in the area.  The terminal settling velocity of a 16micron particle assuming it is spherical is about 2 cm/s.
   
    
4. Discussion regarding the units, g/m²/day and mg/m²/day.  To convert the units from g/m²/day to mg/m²/day is to multiply by 1000.  The legislation requires a measurement to be averaged over a 30 day period and then compared to the action levels.  Fallout dust monitoring is one of the few monitoring programmes that yields results all year round with the exception of a few minutes during each bucket change.
   
    
5. Height of the bucket compared to the diameter.  The ASTM D1739 98(2010) standard requires the buckets to be twice as high as the diameter of the bucket.  The DustWatch buckets are not twice as high as they are wide with a diameter of 17.8cm and a height of 23.6cm.  The use of water throughout the monitoring period provides the retention of the dust particles that the additional height of the ASTM design is meant to achieve.  If we were to omit the water from the bucket during the sampling period then we would use a much deeper bucket in the region of four times as high as it is wide to try and retain all of the particulate that landed in the buckets.  As rain water will inevitable land in the bucket anyway, we regard the use of water in the bucket throughout the monitoring program as essential to consistent results.  With a method as non-specific and crude as the ASTM D1739 98(2010) is it will be important to keep the variables, that we can control, as stable as possible.
   
   Another complication with an empty bucket is the temperature of the dry air in the bucket.  If a dry bucket is left in the sun in a South African summer then the air temperature in the bucket could result in thermal movement of air in the bucket and loss of very fine particles from the bucket.  The use of water in the bucket will prevent air temperature fluctuations and will create a more humid environment, which will actually encourage the collection of fine particulate (lower density), PM₁₀ and PM_2.5.
   
    
6. Our company has years of experience with regard to fallout dust monitoring.  Our clients include De Beers, Pretoria Portland Cement, Anglo American Mines, Brickfields, Opencast quarry operations, Platinum Mines, and others.  Gerry Kuhn Environmental and Hygiene Engineering have been actively involved with fallout dust monitoring since 1997 and we have experience in many industries including, cement, diamond mining, gold mining, open cast mining and others.  We have investigated dust in aircraft hangers, around harbours, and around hotels being polluted by demolition activities nearby.  Often the application of the dust monitoring has to be carefully matched to the specific requirements of the dust problem on site.
   
   The Microscanning process we use allows us to detect tampering easily as the particle sizes collected are much larger if the sample has been contaminated or tampered with.  This is an advantage when fallout dust samples are being taken in situations where tampering is possible due to tensions between clients and surrounding areas or when the units are in areas where vandalism is likely.
   
    
7. Chris Loans has written his Thesis as partial completion of a Masters Degree from WITS University in Johannesburg South Africa.  Copies of this are available on request.   chris@dustwatch.com
   
    
8. An article in Creamer’s Engineering News, “New dust-monitoring technology developed”, was published in November 2002.
   
    
9. Another article was also published in Chemical Technology December 2003 Edition.
   ”PRACTICAL DETERMINATION AND LOCATION OF WINDBLOWN DUST SOURCES AND HOW TO ESTABLISH A GOOD MONITORING PROGRAM”
   
    
10. Please visit this link to read these articles and other material.  http://www.dustwatch.com/papers_and_publications.htm
    
     
11. How far from the source can particles be collected?  Depending on the size of the particles, the wind speed and many other factors, dust can travel from a few meters to many kilometres.
    
     
12. Dust is predominantly collected in the ASTM D1739 98(2010) and in the DustWatch equipment when the wind is not blowing.
    
     
13. The density of dust affects the terminal settling velocity (via the spherical particle definition) and the momentum that the particle has in the air.
    
     

References: 

1. ASTM Standard D1739, 1998 (2010), “Standard test method for the collection measurement of dustfall (settleable particulate matter).” ASTM International, West Conshohocken, PA, 2006, DOI: 10.1520/C0033-03R06, www.astm.org.za

2. Standards South Africa, 2005:  “South African National Standard:  Ambient air quality – Limits for common pollutants”, SANS 1929:2005, Edition 1.1, Pretoria:  South African Bureau of Standards.

3. Grantz DA, Garner JHB, Johnson DW. Ecological effects of particulate matter. Environ Int 2003;29:213-39.

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  Last modified: September 29, 2011
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