Practical Determination And Location Of Windblown Dust Sources And How To Establish A Good Monitoring Program – Publication in Chemical Technology December 2003 Edition
Practical Determination and Location of Windblown Dust Sources and How To Establish a Good Monitoring Program
New dust-monitoring technology developed
A cost-effective alternative to conventional dust fall-out monitoring technology has been designed and developed by local company DustWatch and Hygiene Engineering (DustWatch) and is in operation at numerous mines and industrial sites in South Africa and Botswana.
Fall-out dust is dust that actually settles.
The patented DustWatch fall-out dust monitoring units, which are used where material is conveyed or moved in such a way that fine dust particles are liberated, collect dust samples using the bucket-collection method (an internationally recognised method of determining fall-out dust from various sources).
Fall-out dust monitoring is a legal requirement by the chief pollution control officer, under the Atmospheric Pollution Prevention Act.
Under this act, all manufacturing concerns are responsible for the dust at the perimeter of the area.
If dust levels measured are above the specific limits, then engineering solutions must be implemented to decrease the fall-out dust levels. DustWatch was developed from concepts rather than designed from scratch as a single stand-alone product.
Development involved much wind-testing and site prototype development.
“Many existing dust fall-out monitoring technologies employ electronics to allow operation of the systems, which we found are difficult to maintain and most have their shortcomings,” notes DustWatch principal Gerry Kuhn.
“We saw the need to develop a simple unit, which requires little maintenance to do the required sampling, that can be used anywhere, is cheap in capital cost and can be operated with little training.
“We wanted a unit that could eliminate most, if not all, of the drawbacks of other systems,” he outlines.
The provisional patent was registered after almost 18 months of exhaustive trials and, in September, the company’s PCT international application was lodged.
While locally-produced twin- direction electronic models cost about R20 000 to R30 000, with additional costs for solar panel and battery-pack installations, the DustWatch unit costs R4 000, says Kuhn.
While the present production mode has been standardised on two or four wind directions, prototypes have been constructed to suit particular prevailing wind patterns, offering three or more directions.
Most imported and locally- produced units use only a two-bucket system.
Kuhn made use of aerodynamics to eliminate grit from the samples and, as a result of this thinking, the unit collection bucket is 2,2 m above the ground.
The units are totally mechanical and are operated by the wind, the direction of which opens or closes the relevant sample bucket to the atmosphere.
There are currently over 60 of these units in operation, predominantly in the Western Cape, but also in the Northern Cape, Gauteng and Botswana.
These are in place on diamond-, dolomite-, gold- and iron-ore-mines, cement factories, smelter operations and lime works, as well as at a quarry operation, a foundry and for town import sampling.
The units have been used in ascertaining and quantifying dust import or export, in quantifying dust precipitation models and quantifying dust movement.
DustWatch has also had success in monitoring saprophyte-type airborne bacteria migration from composting plants using the DustWatch units and has had results up to four kilometres from the contamination source.
“We hope to expand this programme using unmodified Dustwatch units, but still need to undertake further research,” observes Kuhn.
While the product works well, there was a need to develop the sampling techniques, the dust collection and the development of a software application programme to optimise the value of information collection – much of this has been done in parallel with the development of the unit.
Kuhn regards the combination of units together with efficient service and periodic auditing to be one of the most cost-effective methods of dust-monitoring, meeting the requirements of the chief pollution control officer.
A typical dust report issued by the company would include the actual values precipitated on an ongoing basis.
From this, a trend line can be produced with the results viewed against previous months or years, an indication of organic substances and pollen-count can be given, an analysis can be undertaken of the captured particulate and the position of polluting sources can be located and the efficiency of controls evaluated.
‘We believe that we have provided a solution to dust- monitoring in its simplest form,” explains Kuhn.
AIRBORNE POLLUTANTS – CURRENT & FUTURE ISSUES IN ENVIRONMENTAL MONITORING
SYNOPSIS AND INTRODUCTION
With the ever increasing sophistication in the technology and therefore cost of environmental monitoring putting it out of reach, the need for developing more cost effective and acceptable methods has to become an international necessity if serious attempts are to be made to improve health and the environment.
This paper examines current trends and offers a cheaper alternative following development and ongoing research monitoring.
With production monitoring taking place at over 50 monitor stations Southern Africa wide for periods up to three years, the monitoring technology is presented as a working model.
THE AIMS OF MONITORING
Without a means of monitoring the pollutant levels we cannot consider whether the condition of the atmosphere or environ at any position is improving or getting worse. We cannot ascertain with any degree of certainty if any place is starting to become a health hazard before actually waiting for the health deterioration patterns to set in without a means of monitoring the area. It is perhaps an indictment of industry and governments that irreversible damage to the health of a residential population or workers is regarded as a time to consider ” doing something about the problem” or even only of an indication that “there may be a problem”.
We see the deterioration of urban, industrial and residential areas to a point where the persons working and living in the areas have to start dying of pollutant induced cancers, chronic lung diseases and other consumptive causes before actions are taken.
We have fortunately seen a great improvement to direct working conditions with the present and draft legislation covering the health of workers. Occupational hygiene is becoming an integral aspect of every mine, ship, industry and to some extent even within the agricultural industry. The gap between conditions within the working place and the residential area now has to be concentrated on to a degree never considered before and even if this aspect has not been totally legislated or is not being adequately policed, we now have to consider aspects of pollution very carefully if we are to be accepted as suppliers to the developed nations or even as recipients in a trade pact.
|In the near future mines will have to prove that they
are monitoring their emission levels and are doing
this to acceptable standards and that they are reducing
emissions in a structured systematic way.
|The science of determining the effect or impact of
pollutants on the environment as well as the health
of people and animals will similarly have to improve.
|In the same way, industry whether local or aimed at
some export market will have to unify standards and
monitor their emissions satisfactorily.
|Farming methods will also have to be modified to limit
the creation of dust, the liberation of pesticides where
these are used and to limit veld-burning operations.
|Finally, monitoring must be as simple as possible, the
equipment cost should not be too expensive and the
cost of running a programme as low as possible. In the
South African or indeed the African context or other
developing areas it is vital that as many persons as
possible can be trained to service and run a monitoring
programme. A requirement for programmes to be run by
scientists only is just not feasible.
As this paper outlines concepts associated with airborne pollutants and concentrated on particulate forms of airborne pollutants, we will not consider gasses although some forms of fumes and vapour are considered in some detail, as these can result in chemical reactions within the atmosphere, which manifest as particulates.
With the above in mind it is important to consider what has to be achieved before deciding on the method to be used to carry out the monitoring. Integral to this thinking, the following questions should be asked when embarking on a sampling programme:
|How does the pollutant cause a problem?|
|What is the problem associated with this pollutant?|
|If the pollutant settle out will it be easier and more
representative to measure the fall-out settlement or
should the airborne load be determined?
|If the airborne load is determined, can we accurately
determine where and under what conditions it will
settle out or precipitate or even whom it will affect?
|Can a single means of monitoring be used or is there
a necessity to employ a multiplicity of methods to
determine the pollutant load or concentration?
From the above questions it should then be possible to determine if capture of the particulate must be achieved within the airflow or to wait for the particulate to settle out before capturing. To illustrate this important point further let us ask a further question:
v Will the captured particulate fall-out within the area where monitoring is required and thus cause a problem or will it cause a problem while in suspension?
Generally the answer to this question will tend to dictate how the particulate must be captured, enabling the best method to be selected.
AIRBORNE PARTICULATE CAPTURE
As the amount of particulate to be captured is very small and the particulate size is also likely to be limited, it is unlikely that any form of cyclonic separation will either be size selective or will be efficient enough to be of any practical use.
Purpose designed micro-cyclones could however be considered to establish particulate size distribution or to guarantee that size selection takes place.
An example of the use of a micro-cyclone is within the personnel gravimetric sampling train used to establish the extent of respirable dust ingestion on a person sampled for a period of time. In this example, the cyclone is used as a method of removing any particulate of a size in excess of that constituting respirable dust (+ about 8µm).
TOTAL SPECTRUM DUST CAPTURE – GRAVIMETRIC SAMPLING
As with the above arrangement given as an example, it is possible to capture the total spectrum of airborne dust with the standard gravimetric dust sampling train providing that precautions are taken to negate the possibility of wind blown excessively sized particulate entering the sample – grits or sand. With this type of arrangement it is also important that the capture filter is permitted to catch the sample as evenly and uniformly as possible to prevent increases in filtering velocities or decreases in the sampling flow rate.
The illustrations – Figure A – indicate the requirement of the sampling trains for both respirable as well as the total spectrum dust capture rigs using standard gravimetric dust sampling equipment available on a mine with added precautionary requirements.
PM10 PARTICULATE MONITORING
The PM10 or sub 10µm particulate capture sampling has been in general use for some time, constituting a means of sampling potentially respirable particulates on a longer term basis. The stipulation of monitoring sub 10µm particulate superseded the original primary and secondary National Air Quality Standards (NAAQS) for particulate matter (PM) which were promulgated in 1971 (EPA). The original primary standards for PM, measured as total suspended particulates or TSP, were 260-microgram µg/m3. The 24-hour average was not to be exceeded more than once per year with an annual geometric mean of 75µm/m3. The NAAQS was revised in 1987 with the following criteria changed:
|TSP was replaced as an indicator that included
particles with an aerodynamic diameter of less
than 10µm – PM10.
|The 24 hour primary TSP standard was replaced
with a 24 hour PM10 standard of 150µm/m3.
|The annual primary TSP standard was replaced
with an annual average PM10 standard of 50µm/m3.
|The secondary TSP standard was replaced with
24 hour and annual PM10 standards identical in
all respects to the primary standards.
Analysis from monitoring in the United States as well as locally indicates that fugitive dust constitutes about 90% of the PM10 emissions but as most monitoring is undertaken by industry there is some loading of the fugitive dust fraction due to the higher incidence of dust creation from process requirements and materials or ore handling. Only a very small percentage of this particulate manifests as very fine particulate, which led to the requirement for PM2.5 monitoring for fume and smoke particulate capture.
The Environmental Protection Agency (EPA) revised the primary health based PM standards by adding a new annual PM 2.5 standard set at 15µg/m3 and a new 24 hour PM 2.5 standard set at 65µg/m3 (July 1997). The PM10 standards were not changed.
PM 2.5 PARTICULATE MONITORING
As already implied above, there was a necessity to scrutinise specific health issues of respirable dusts and fume and the PM 2.5 system was devised.
Of concern in the establishment of both PM 10 and PM 2.5 programmes, especially the latter, there is a degree of doubt as to the accuracy of the available equipment, equating the means of air ingestion physiologically with that of the monitor unit.
Much research went into building pumps and monitoring rigs for PM 10 including aspiration rates, while the 2.5 µm filter was achieved using an 0.45µm membrane filter in addition to the 8.0 µm filter capturing the <10µm >8 particulate. The primary induced air was selected to contain only 10µm particulate with approved design impinger or cyclones.
Monitoring using the PM 10 and PM 2.5 methods utilises 24 hour sampling on each third consecutive day in order to achieve a form of statistical acceptance. The labour intensive nature of undertaking this operation together with the fact that continuous monitoring is not undertaken as well as the high cost factor leads to an unwieldy expensive system.
At least one South African manufactured unit is available to compete against imported models.
FALL-OUT PARTICULATE MONITORING
The monitoring of fall-out dust establishes the degree to which airborne particulate is precipitated out and then has an opportunity of exposure to human beings, animals or plant life. This monitoring further establishes a means of studying the movement of most sizes of dust including particulate of a size exceeding 10µm, which constitutes nuisance fugitive dust. This larger dust particulate poses the greatest local area influence.
Fall-out monitoring also has the advantage of offering a continuous means of monitoring, negating the need to estimate how representative the results are.
For a greatly decreased cost, multiple position monitoring can take place, forming a good network of monitor station in preference to only one or two.
Full direction monitoring indicates from which direction the emission is imported and use of multiple units can establish patterns of import and export dust, which is extremely useful in establishing dust sources. Continuous monitoring offers a far greater chance of detecting very low pollutant concentrations.
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. At least two production units employing the ASTM 1739 method are available locally with both offering merits and demerits.
The system offers an opportunity of sampling airborne precipitant particulate as well as soluble airborne particulate prevalent at the coast and offers with minimal effort, the means of indicating both values from each sample.
The system also offers the potential of biological monitoring.
The system is acceptable in terms of ISO 14001 standards.
CONTINUOUS PARTICULATE LASER COUNT MONITORING
With the continuous developments occurring with laser scan technology, several portable instruments offer instantaneous readings of particulate concentration. Once a means of recording the values has been added, the resulting data is of use in establishing area dust concentrations. While such instruments can record much of the information that will enable a good monitoring programme to be run, they only offer a grab sample at best before being transported to the next sampling position where further samples need to be taken. As the instrumentation is expensive, simultaneous multiple sampling is not achieved, leading to doubt about the representivity of the results.
Monitoring positions and the equipment needs to be secure to guard against loss of expensive equipment, rending the method even more costly.
Most units are humidity sensitive, which could possibly limit the use under certain conditions.
Chemical and physical quantitative analysis of dust is not possible with this method.
FALL-OUT MONITORING – PARTICULATE DUST
DustWatch® MONITORING SYSTEM
The DustWatch monitor units form an inexpensive means of monitoring fall-out dust with a minimal maintenance requirement, low sample loss rate, no supervision requirements and all-wind velocity particulate classification to prevent grits and sand capture at high wind velocities. The directionality of sampling encompasses a narrow angle offering increased direction accuracy. As the sample represents a continuous sample taken over an extended time period, the collected material can be subjected to qualitative and quantitative chemical and physical analysis in addition to microscopic organics recognition scanning.
The unit is not affected by rainfall and samples are not lost under abnormal weather conditions.
The units offer fall-out monitoring of either two or four incoming prevailing wind directions simultaneously, offering many monitoring options:
FIGURE 1 Import ambient dust from upwind of a monitoring site together with a corresponding export dust towards the same area.
FIGURE 1 Four bucket units can thus indicate the imported dust from four different incoming sources.
FIGURE 1 Two units located on opposite sides of a site will indicate the imported precipitation arriving at the site as well as the corresponding export from the site in both directions.
FIGURE 1 By extrapolating the import result from one unit with the export result from the second unit, an indication of the exact generated dust can be made. The undertaking can then establish exactly how much of the exported dust they are responsible for.
FIGURE 1 Two or more units can be positioned in line at regular intervals to ascertain the exact extent of dust precipitation from a dump or other dust producing feature or operation, enabling a detailed precipitation model to be prepared.
SAMPLE CAPTURE AND ASSESSMENT
In order to capture and retain the precipitant dust, the capture buckets are partly filled with a capture medium to which an algaecide has been added.
Once the sample bucket has been retrieved the sample is filtered to remove any large +0.50mm organic particles or insects, which do not constitute dust.
The sample is then filtered through a wet strength nitro-cellulose filter of pore size 1.0µm, which is weighed both before filtration and again after desiccation of the filtrate and filter. The mass of captured filtrate is thus determined.
Should the soluble chlorides in coastal samples be required, a known volume of the filtrate water is desiccated and the resulting salts weighed. By calculation the mass of solubles can be determined.
As the cross-sectional area of each collection bucket is known, the precipitation rate per m2 can be determined by calculation and the result indicated in any units to the time weighting preferred.
As most standards require the results to be reflected on a Milligram/day/square metre basis it is preferable to report results in this format.
While it is possible to ash each sample to determine the exact carbon or organic constituent, this is a lengthy process and should rather be dispensed with in favour of microscopic analysis that will permit the analyst to determine the following:
The type of recognisable organic particulate and pollens present, their size as compared to 5µm graticle spacing and an estimate of the amount of organic matter. A range of magnifications from 80 to 150 is ideal and an old Konimeter microscope and stage provides an ideal combination of specifications. An estimate of the percentage of respirable particulate can also be made. A cursory scan for fibrous material can be undertaken.
If all of the samples taken over an extended period (say 1 year) are combined to make up to four composite samples for each monitoring unit or station, these can be analysed for a variety of elements as a “finger print” operation. There is usually enough sample, once all of the filters are ashed, to make up a pellet for further analysis. Such quantitative analysis could include:
|Scanning electron microscopy – SEM|
|X-ray diffraction – XRD|
|X-ray fluorescents – XRF|
The latter option permits 36 and 39 element packages to be run on an XRF spectrometer with major elements analysed and reported on as oxides using an energy dispersive X-ray analyser.
A further ICP ME 46A or M analysis can be used in a qualitative capacity but the method has limitation as sample masses in excess of 20g are needed.
Finally for metal recognition with very small samples, the AAGEOBM method utilising flame AAS analysis can be considered.
All of this detailed analysis can be undertaken by several laboratories that specialise in assay, soil sampling and environmental analysis at various prices.
PRESENTATION & EXTRAPOLATION OF THE RESULTS
As each sampling period will produce results, these can be added to the previous data and the ongoing hyperbolic trend indicated. Once a year’s results have been plotted it is possible to overlay the monthly results as these become available in order to compare the monthly results with of the corresponding month a year earlier. Similarly annual trend lines can be compared.
The hyperbolic trend curve will thus indicate the degree of improvement or deterioration on an ongoing basis.
A mobile unit can be used to monitor on an ad-hoc basis any area not adequately covered by fixed monitors. The units can also be used for area investigations.
Trend graphs can be plotted from the programme (Microsoft Excel based) that we have developed to chart results from the forty odd units under our direct supervision.
In the same way that units on both sides of an installation or dust source will indicate the degree of export of dust, so units along the same geographical bearing can be assessed to establish how dust is lifted from one source to be deposited further along the line and then a further dust source replenishes the load to be deposited elsewhere. In investigations of this type “finger printing” of certain characteristics is required to establish the true extent of travel of the particulate and how dilution occurs over distance.
While we do not advocate that all mines or industrial concerns need to go to the limits outlined above, it is worth considering that the units installed around an undertaking may provide extremely valuable information that could be of use in research work.
As described above the consideration of networking the directional particulate movement can provide extremely valuable information for research of the following nature:
The encroachment of desertification in dry arid areas.
Cross contamination in multi product open stockpiles.
The health effects of crop spraying and the re-exposure of persons during other operations involved with the sprayed crops – reaping, ploughing or burning the stubble.
The incidence of allergens and ongoing pollen counts, which have proved valuable in warning small community susceptible persons of pending high pollen counts is extremely valuable involving only an additional scrutiny of the samples.
Cross contamination of industrial concerns that may be exporting dust towards each other.
Finally, much research is being carried out on intercontinental sub-micronic dust migration and African red dust is resulting in the death and destruction of sea corals off the Florida Coast (USA).
While PM10 and PM 2.5 monitoring is proving valuable in establishing the local content of air, PM10 devices ensuring the directional sampling are not producing conclusive results and are subject to local site vagaries without the back-up of extensive fall-out directional dust networks.
FALL-OUT MONITORING (SOLUBLE POLLUTANTS)
THE MONITORING CONCEPT & ASSESSMENT
The assessment of soluble compounds that are arrested during the capture of fall-out particulate can be quantified if these are thought to be an issue. At the coast the incidence of dissolved chlorides (NaCl) could play a part in loading a particulate result and should the actual captured soluble compounds need to be quantified, this can be undertaken by desiccating a known volume of the remaining catch media and weighing the residue. This can then be related to the actual volume of the catch media remaining after filtration of the sample.
When chlorides have to be assessed the algaecides Sodium Hypochlorite or Potassium Permanganate should be omitted and a shorter catch period used to prevent the build-up of algae in the sample.
Analysis of the catch media for other compounds can also be undertaken using wet chemistry techniques to ascertain the presence of any other soluble compounds, iron salts and the like.
PRESENTATION AND EXTRAPOLATION OF THE RESULTS
Soluble compounds do not usually play a major part in the quantification of particulate sampling inland and we normally do not include the result after initially commenting on the conditions.
In cases where monitor units are located close to the sea we usually undertake a quantification exercise during the peak summer months and again during the middle of winter to ascertain the mass migration of salt. Each of these values is then considered to be constant. The weighted averages are then included in the import values.
There is little value in networking the results of the solubles mass as these masses fall off rapidly with an increase in distance from the sea or saline waters’ edge. On the West Coast we interestingly note that traces of salt are found only in the seaward samples, indicating that the salt laden air precipitates quickly. Correspondingly, a unit located about 2.0 km from the sea line only has a salt content during the peak onshore wind season.
FALL-OUT MONITORING (BIOLOGICAL AGENTS)
THE MONITORING CONCEPT
Although our research in this field has indicated some early promising results we need to do a lot more work to achieve a measure of confidence in our biological agent monitoring.
Our early research followed attempts to monitor the movement of airborne bacteria from composting plants and manure dumps, as we considered that there might be some migration especially during the dry Western Cape summer months.
As we were hoping to keep any bacteria alive for as long as possible, we could not use any algaecides in the catch media; we elected to use sterile filtered water and leave the catch buckets out for shorter periods of time.
Once the buckets were filtered to remove any insoluble particulate we cultured the catch media water in an attempt to locate saprophytes or other bacteria colonies. Approximately 60% of the samples yielded colonies of some sort of bacteria.
When the catch media was dosed with diluted culture nutrient the positive results were considerably higher at over 75%. In most cases buckets facing away from the installations had no trace of colony development.
We have as yet not identified any coliform bacteria but our research programme in this regard will continue during 2002.
LABORATORY WORK CO-ORDINATION
The biological analysis work requires careful co-ordination and the micro-biologist should be thoroughly briefed before a similar programme is considered as their input in the research is critical especially with regard to the preparation of the sampling buckets and catch media, which must only be decanted into sterile buckets at the last minute to prevent inadvertent contamination.
The DustWatch monitor unit in each case is also thoroughly cleaned and disinfected for the above reason.
PROJECTS EARMARKED FOR FURTHER RESEARCH
We hope to undertake a dust dispersion exercise during 2002 concentrating on the effects of topography and vegetation on the actual settlement rates noted. This will form a valuable dispersion model for future work on rock dumps.
The sampling for biological agents needs a lot more work and we hope to achieve definite results within 2002. As part of our research into the movement of bacteria we hope to be monitoring agents emanating from a coastal seal colony as well as the existing work on monitoring the composting facilities already commented on.
We hope to become part of an intercontinental research project quantifying and capturing African dust export in an international venture.
THE UNIT DESIGN AND OPERATION
The DustWatch system was developed as an affordable means of providing practical monitoring with features not available in the market place. While they are robustly constructed and early models have now been in the harshest environments for nearly three years with minimal signs of major corrosion. Other units in operation at corrosive plants have not been as fortunate and while early primed and painted models definitely lack the protection of powder coated units. We have standardised on the later units but we do note that even the corroded earlier units still operate satisfactorily and some have been painted as a refurbishment exercise.
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.
In a twin bucket unit the angle of divergence at which the incoming dust laden wind can be collected is about 14° while on the 4-bucket units there is a possibility of vector loading of two buckets with bisecting winds. Site tests on single bucket collection supports the vector calculations and thus we have accepted the criteria as representative.
Practical tests in areas of known emission have yielded analytical results supporting the vector calculations as a second means of verification. The degree of accuracy exceeded 85% on all of the tests carried out.
The accuracy exceeds that of any other directional sampling system available as many monitor an angle of 180° divergence with the twin bucket arrangement.
PARTICLE SIZE SELECTION
As already outlined above, the selector disk achieves the upper size limit of 100µm classification.
While we have not ashed a composite sample of results yet in order to obtain a laser scan particulate size analysis, we hope to carry out such an exercise shortly. In the meanwhile an assessment of particulate sizing can be obtained by examining each filter under the microscope eyepiece graticle, which is graded in 5 to 10µm gridlines for size recognition.
COMPLIANCE WITH STANDARDS AND CODES OF PRACTICE
As already mentioned, the entire concept meets the requirements of ASTM D1739 but this does not cater for wind direction so we have retained the fundamentals of the standard, including the recommended maxima applicable.
In order to meet the requirements of ISO 14001 stipulated monitoring the entire monitoring regimen has to be presented together with standard procedures, manuals, reporting format and traceable documentation. Assistance and detailed operator and assessment training is available and accompanies the purchase of a system.
World Bank standards have similar requirements and the package offered has met these requirements as well.
The Chief Pollution Control Officer of the Department of the Environment and Tourism has accepted monitoring results based on fall-out monitoring and has specified limits based on those applicable in the USA.
The Department of Minerals and Energy, while applying strict occupational hygiene standards outlined in the requirements of the MHSA 1996, have on many occasions sanctioned DustWatch monitoring in line with the Chief Pollution Control officers requirements, especially when exported dust has become as issue.
The PM10 and PM 2.5 monitoring concentrated on by the American EPA has been specifically used to monitor urban populations and is not designed to assess potential import/export dust situations at a local level.
In our opinion no South African manufactured PM 10 or PM 2.5 units meet the EPA standards as outlined in the Federal Register 40 CFR parts 53 and 58.
TWO, FOUR AND MULTIPLE BUCKET DustWatch® UNITS
We have already outlined both the twin and four bucket units manufactured at present as product units.
While we are considering a six bucket unit as a research unit we note that the entire unit has become exponentially larger and more expensive and we are already concerned that the over lap between buckets is likely to compromise the vector principle, further negating any advantages that further points may offer.
Under such circumstances we prefer to install two 4-bucket units with one monitoring the bisection directions effectively indicating 8 incoming wind directions.
In one instance we have manufactured a prototype 3-bucket unit with the buckets located on the prevailing wind directions to customise the unit. Results have been most successful.
While we have already covered the outside agency laboratory work that can be done we offer experience and some good tips to improve the efficiency of the monitoring programme.
WEIGHING & MASS DETERMINATIONS
The five or six decimal gram micro balances that are available on the mine for personnel gravimetric dust sampling are adequate for any filter weighing.
Masses are taken in mg where possible as results are indicated in milligram units.
With the use of wet strength cellulose filter material, the moisture absorption associated with the filters is minimal providing there is at least a 12-hour acclimatisation period.
Desiccation should ideally be undertaken under ambient conditions, as accelerated desiccation over a warmer tray will result in severe curling of the filter and cracking of the filtrate.
While an allowance of 48 hours is usually made, it is possible to gauge the point at which total desiccation has occurred. Up to this point a filter will lose mass on a continuous basis showing a steadily declining mass while on the balance pan indicating that evaporation of moisture is still occurring. Once the filter reaches parity with atmospheric conditions masses become static.
As all environmental assessment filters are 47mm Ø, the Ø47 petri slides usually used for the storage and handling of gravimetric dust sampling filters can be used.
Once all of the information on filters has been captured and the filters examined microscopically these can be stored as composites in disposable Ø 65 petri’s, which are considerably cheaper and hold up to about 50 or more filters before there is any difficulty closing the lid.
Multi-elemental scans can be conducted annually to a composite sample made up of all of each of the north, south, east and west filters of a single unit to obtain finger prints of the annual input or ambient dust.
Storage after this stage will constitute a mine or industry policy decision.
IN-HOUSE SERVICING OR CONTRACTING OUT
While most mining and larger industrial concerns have elected to run sampling and monitoring programmes themselves following the initial training and equipping of in-house laboratories, most local concerns have chosen to contract out the entire programme to our laboratory.
Concerns running their own programmes can be audited periodically.
In the West coast and Cape Peninsula areas we run programme for many concerns – changing buckets, recovering the samples, assessments and the preparation of detailed reports.
In remote areas where access is a problem, our clients change their own buckets, decanting the filtrate and water into 2 litre re-useable PVC bottles with seal caps. These samples are couriered down to our Piketberg laboratory where the assessments and monthly reports are prepared.
We anticipate appointing part time agents in areas where the throughput can justify the services. The agents will undertake bucket preparation, changing buckets, and filtrate capture before sending the filters to our laboratory by courier service for final assessment in addition to maintenance of the monitoring equipment.
DESIRABLE TRENDS AND MONITORING DEVELOPMENTS
In order to establish monitoring of environmental dust and pollution it is necessary to develop technologies that are simple and cheap and can be operated by unsophisticated communities rather than high tech solutions costing millions to implement with high levels of technical skill and training to operate. It is pointless developing technologies that 5% of the world can afford and can run. Let us rather develop acceptable monitoring technology that 90% of the world can afford and can run. The remaining 10% who cannot even afford this technology can be subsidised by those nations that have the means. The high tech technology can be used to establish the finer points of determinations.
We are currently designing and developing technologies for passive monitoring of PM 10 particulates and hope to achieve commercially available production units that will meet the approval of the EPA within two years. With units of this type, monitoring will – in common with the DustWatch – offer particulate capture without the use of electronics, electrical energy and power supplies, enabling the monitors to be positioned anywhere without the necessity for mains power, battery systems, solar or wind generated power installations. This, we feel, will meet the criteria mentioned in the paragraph above and permit greater monitoring where it is required.
DUST MONITORING ECONOMICS USING FALL-OUT TECHNIQUES
VALUE FOR MONEY
With the cost of equipment minimised by mass production techniques, the cost of monitor units within the DustWatch range are between 12 and 15% of the cost of other electrically driven fall-out monitoring systems produced within South Africa. Imported units cost considerably more due to the high $/Rand exchange rate.
Assessment laboratory equipment is largely available on individual mines and additional equipment is manufactured locally or imported.
Sample capture filter material is imported but is inexpensive due to high volume purchases and imports directly from the manufacturer.
Various algaecides can be used successful and most are available commercially in bulk at a minimal cost.
While distilled water is desirable for capture media preparation most installations are being operated extremely effectively using oxidation/reduction sub-micronic filtration techniques at a fraction of the cost of producing distilled water. With multiple units in the field the changing of buckets can be staggered to allow for the purchase of smaller filter units to undertake the water filtration and to optimise labour utilisation.
A Microsoft Excel based assessment programme to run the monitoring and generate reports is also available and offers additional time savings.
As already outlined it is necessary for field assistants and air quality analysts to be trained in the techniques involved with the servicing and maintenance of the monitor units as well as the assessment work and report preparation if the monitoring has to be run within the ISO 14001 standards.
As part of the training exercise, the optimisation of monitor positioning is covered at length to enable the concern to find the best location for sampling units. Our training engineer also instructs all parties in the preparation of buckets, calculation and report preparation in addition to a technical background on the need for monitoring. He will normally install and optimise the assessment programme to suit the mine requirements and train your staff.
A detailed standard procedure manual is made available to all trained personnel as well as a certificate of attendance at the training sessions.
JUSTIFICATION AND PUBLIC RELATIONS
Many of the monitoring programmes have been started in an effort to appease lobby groups or as a means of defense from threatened legal action. In many cases the results have initially proved just how bad emission levels from the various concerns were.
As action was undertaken at the various offending dust creation points, so improvements have been quantifiable. The monthly reports have been made available at open day meetings and in some instances monitors have even been welcomed on adjacent properties as lobby groups are recruited to assist the undertakings by addressing issues of veld burning and ploughing techniques to minimise dust.
The long term monitoring has in at least one instance been instrumental in locating unsuspected sources of airborne pollutant generation as well as being a deterrent against the use of unscheduled pesticides in the agricultural areas.
With the present research being undertaken in the biological field we can hypothesise that monitoring biological emissions from fish factories, process meal and other mills will be possible as the “smells” can be quantified.
Similarly composting installations can be monitored simultaneously for both particulate export as well as potential biological agent export.
The monitoring of slimes dams and dumps is already showing some successful results and long term moisture/rain influence research is showing dependable results, enabling timeous spraying to be undertaken.
In conclusion we note that monitoring can result in public relations value in addition to ensuring a social responsibility and improving conditions to workers and staff. The international sales “bottom line” will also be of inestimable value.
We believe the cost is worth it.
|Federal register Part 1V – 40 CFR Parts 53 & 58|
|Revised requirements for Designation of Reference
and Equivalent Methods for PM 2.5 and Ambient Air
Quality Surveillance for Particulate Matter – Final Rule
|EPA Revised Particulate Matter Standards – Fact sheets|
|Air Quality Criteria for Particulate Matter –
EPA 600/P – 95/001af
|DustWatch fall-out dust monitoring, sampling and
assessment procedure manual – DustWatch CC
|ASTM D1739 – American Standard Test Method|
|Strategy for Landfill Designs in Arid Regions –
Anwar Al-Yagout & Frank Townsend ASCE
|Numerous routine reports & investigative reports|
How does vegetation affect fall-out dust?
How far do dust Particles travel?
Note that the information provided here is purely to be used for estimation purposes. If accurate calculations are required, then please look at the original sources.
It is often required to estimate how far dust particles will travel once they are liberated into the air by moving cars, wind, or fire.
The factors that affect this are:
|height at which the dust is liberated.|
Particle size is usually the most important factor because the terminal settling velocity is highly dependent on this particle size.
The information below is taken from the text book “Environmental Engineering in South African Mines” published in 1989 in association with the Mine Ventilation Society of South Africa. Chapter 12. 1
Sizes of Dust Particles
The geometric diameters of air-borne particles may vary between 0.001 µm and 100 µm. The figure below indicates the size range for a few common particles.
From the diagram it can be seen that dust particles are seldom larger than 100 µm.
Terminal Settling Velocity – Stokes’ Law
The gravitational force acting downward on a free falling sphere is:
d = the geometric diameter of the sphere (m)
Ws = the density of the sphere (kg/m3)
Wa = the density of the air (kg/m3)
g = acceleration due to gravity (m/s2)
The drag forces acting in resistance to the fall are:
|= the Velocity of the particle (m/s)|
|= viscosity of the fluid (kg/(m*s)|
If the motion of the fluid around the particle is symmetrical, the terminal velocity of the sphere is reached if G = F. Equating these two equations yields:
This is known as Stokes’ law. It applies to spheres of size below that at which their own velocity creates turbulent flow and (NReynolds greater than 1) or in other words spheres approximately less than 250 µm.
Click here to be able to determine your own settling velocities.
A unit density quartz sphere of 1 µm would require almost 13 hours to drop from a height of 1.6 metres (theoretically). When particles are very small (less than 1 µm) the actual settling could take much longer because of the bombardment by air molecules, which cause random Brownian motion. In fact particles having terminal settling velocities of the same order as the displacement caused by the Brownian motion will remain permanently suspended, even in still air.
Air pressure and moisture content will affect the terminal settling velocity to some extent, basically because of the effect these parameters have on the density of air.
For particles with a geometric mean diameter of 0.1 µm the Brownian displacement is about 15 times that of the settling velocity. For particles of 0.01 µm it is almost 900 times. This may also be of consequence in gold mines, as it has been found that nearly 80% of the dust particles in mine air are smaller than 1 µm. These particles may thus penetrate deeply into the alveolar region of a miner’s lung. Admittedly they may deposit in the respiratory tract by impingement or aggregate due to electrostatic charges and cohesion forces to form larger aggregates which will then settle at a finite terminal velocity.
While most of our focus is on dust that is generated at ground level, larger dust (greater than 250µ
|Height of stack 100 metres|
|Size of particles, 250 µm|
|Terminal settling velocity, 2.5 m/s|
|Wind speed, 10 m/s or 36 kph or 22.37 mph|
|Assumption: acceleration time is not
included in this estimation.
As the particle size increases, so Stokes’ Law is no longer applicable and other formulae need to be used. As an example the settling velocity of a 0.2 cm or 2 mm particle is about 73 m/s. This particle may be blown by the wind, but it will not blow very far unless the winds are constantly above about 100 kph or 62 mph.
As an aside, the following table is interesting taken from Perry’s Chemical Engineers’ Handbook, sixth edition, Robert H. Perry and Don Green, pg 5-64.
|Particle Reynolds Number **||Orientation|
|0.1 – 5.5||All orientations are stable when there are three or more perpendicular axes of symmetry|
|5.5 – 200||Stable in position of maximum drag|
|200 – 500||Unpredictable. Disks and plates tend to wobble, while fuller bluff bodies tend to rotate.|
|500 – 200000||Rotation about axis of least inertia, frequently coupled with spiral translation.|
Free-Fall Orientation of Particles
** Based on diameter of a sphere having the same surface area as the particle.
Irregular particles on falling, will not take up a preferred orientation as would particles having an axis o symmetry and they may fall edgewise, for example. The shape and surface of a free falling particle will thus influence its rate of fall in a sense that the particle will always attains a velocity smaller than that of a smooth, regular sphere of equal radius.
Dust Particles and Lower Explosive Limits (LEL) – Instruments
Dust particles have a minimum or lower explosive limit to almost no upper limit. Here are examples of minimum explosive limits (oz/ft3): Polystyrene (0.02) Cornstarch (0.04), Coal (0.055), Iron (0.12).
It is important to note an explosimeter gives a reading as a percentage. The reading is based on the percentage of the LEL and not the full concentration of the vapour or gas in the mixture. For example a 50% explosimeter reading of a gasoline/air mixture really translates into 0.7% gasoline (50% of 1.4%, the LEL). It is common practice to consider a 10% explosimeter reading or 10% of the LEL, as a safe working area. When taking explosimeter readings, consider the possibility that the vapour or gas may have accumulated in recessed areas (top or bottom of tank depending on the density of the gas or vapour).