INTRODUCTION

The monitoring of fall-out dust utilising the bucket collection is internationally recognised and documented as an accepted method of determining fall-out dust from various sources.  

The standard procedure accepted internationally is the ASTM 1982 “Standard Method for Collection and Analysis for Dust Fall” (Settleable Particulates).  As a general criterion, generated dust from a source like a factory, plant, or mine rises into the air due to thermal action, wind velocity or by other means.  Depending on the particulate size and wind velocity, the dust begins to fall out as soon as the immediate thermal or other effects that lifted the dust are dissipated.

NATURAL PARTICULATE CLASSIFICATION & VARIABLES AFFECTING FUGITIVE DUST

 While many factors affect the precipitation rate, the main factors are related to wind velocity, air humidity, particulate size and dynamic shape, and prevailing ground cover.

Once the dust is airborne the factors are simplified to wind velocity and turbulence.  Dust particulate of a size greater than 80µm is unlikely to be carried any great distance unless a severe windstorm is being experienced, while dust of a 30µm diameter will only be transported about 300m by a wind velocity of 5,0m/sec in about 60 seconds. 

BUCKET FALL-OUT MONITORING

The combination of factors we have seen determine the range of particulate sizes constituting the bulk mass of settleable dust range.

From the previous example the lower limit of settleability will thus be about 5µm and particulate collected in a bucket system normally range from this size upwards.

In all models the bucket lip should be located 2,2 metres above the ground to prevent thermal interference with the precipitation phenomenon. 

The buckets are partly filled with distilled water, to which a biocide is added to prevent the growth of algae.

While detritus from birds can be a problem we have noted that the DustWatch unit largely prevents this as the rotating action of the cover discourages birds from alighting on the unit.  Birds also prefer the highest point that happens to be the tail far from the bucket sample.

The unit is available as a twin bucket model or with four buckets.  The latter version permits sampling of four different wind directions.  A degree of latitude in wind direction is permitted with the vector principle applying to slightly lateral wind directions

While most imported and locally produced units use a two bucket system, which includes a direction change counter, the units require a high degree of maintenance and are prone to battery failure that then means a total loss of the cycle samples.  Many of the systems monitor a 180° segment, arguing that the result is representative of wind fall-out from a given direction.

The DustWatch models are totally mechanical and are operated by the wind, the direction of which opens or closes the relevant sample bucket to atmosphere.  The systems offer no count facility or timer, which does not contribute to any meaningful collection of data unless wind direction is carefully monitored. 

ASSESSMENT OF RESULTS

In order to assess the results, the collected dust is filtered through a sub-micronic pre-weighed filter using a vacuum filter bench.  Once the wet filtrate has been desiccated by evaporation of any retained moisture, the filter is reweighed to ascertain the collected mass.  (Insoluble particulate)

Soluble particulate is assessed by evaporating the catch media and weighing the resulting solids.

To give an idea of the relevant mass classifications refer to the table below:

CLASSIFICATION – AMERICAN STANDARD TEST METHOD

ASTM D1739

Dust = Milligrams/day/square metre

S.A. Classification Department of Environmental Affairs & Tourism	ASTM equivalent	GERMAN DIN AIR QUALITY MONTHLY LIMIT
SLIGHT	<250	650 – Non-industrial limit
MODERATE	251 – 500
HEAVY	501 – 1200	1300 – Industrial limit
VERY HEAVY	>1200

Units are normally monitored weekly and particulate collected fortnightly or monthly if continuous monitoring is undertaken or shorter periods if localized assessment needs to be considered.

To assist in making the masses (weight) mean something we note the mass of some everyday items:

1. – Paracetamol tablet 608.83 mg

2. – After handling the Paracetamol tablet 608.63 mg

3. – Pinch of salt 140.31 mg

4. – a single drop of homeopathic medicine 75.32 mg (as the drop evaporated, the mass dropped by about 1.5 mg per second).

ASSESSMENT SERVICES AVAILABLE

Gerry Kuhn E & HE CC can undertake regular servicing of units in the field on short or longer term assessments on the following basis:

1. SALE OF DUSTWATCH SAMPLING UNITS

2. HIRE ARRANGEMENTS CAN BE MADE WHEN MONITORING ON A SHORTER TERM BASIS IS REQUIRED

3. PLACING, SERVICING, COLLECTION OF BUCKETS & ASSESSMENT OF FALL-OUT PARTICULATE AT LOCATIONS CLOSE TO OUR LABORATORY

4. COURIER SERVICE ARRANGEMENTS WORK WELL WITH CLIENTS COLLECTING THEIR OWN SAMPLES AND COURIERING THEM TO OUR LABORATORIES

5. PREPARATION OF FALL-OUT DUST REPORTS

6. SINGLE BUCKETS CAN BE DEPLOYED TO TRIANGULATE RESULTS WITH THE DUSTWATCH UNITS

CONCLUSION

As the outlined method of fall-out dust monitoring on an ongoing basis is internationally accepted, the results of an assessment of your facility will offer an accurate credible result.

Results can be related to continuous sampling using other methods like the international PM10 and PM15 standards.  Similarly the short-term gravimetric sampling can be indirectly related to bucket sampling results