Dust Monitoring Legislation

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.