Air quality maintenance through dust fall monitoring
The Hillside and Bayside smelters are located at Richards Bay, South Africa and are fully owned and operated by BHP Billiton which is among the world’s largest producers of major commodities, including aluminium, copper, energy coal, iron ore, manganese, metallurgical coal and nickel to name a few.
Land use around the smelters is predominantly characterised by industrial development. The Hillside Smelter is surrounded by gravel roads and tarred roads which are not extensively used. The Bayside Smelter is also surrounded by gravel roads and tarred roads where the latter is used extensively. Average dust fallout concentrations are monitored on a monthly basis. Monthly table and graph reports are submitted to ASA, which display the dust fallout levels relative to the South African National Standards (SANS) four-band scale for dust deposition.
Dust fall monitoring legislative framework
Dust fall monitoring is an aspect of air quality management, which entails the measurement of dust deposition in the ambient air particularly as a result of mining or industrial activity which may adversely influence the surrounding residential environment and inhabitants.
Currently, South Africa does not have prescribed legislation regarding the collection and measurement of dust fall. The American Standard for Testing and Materials (ASTM D1739, 2005) is the current framework being utilised for this purpose (ASTM D1739, 1998). The Department of Environmental Affairs (DEA) has proposed the Draft National Dust Control Regulations (Government Gazette, 2011) for the control of dust in all areas, including; residential and light commercial areas in May 2011.
The draft regulations prescribe conditions for which all areas need to be in compliance, namely;
No person may conduct any activity in such a way as to give rise to dust in such quantities and concentrations that:-
- The dust, or dust fall, has a detrimental effect on the environment, including health, social conditions, economic conditions, ecological conditions or cultural heritage, or has contributed to the degradation of ambient air quality beyond the premised where it originates; or
- The dust remains visible in the ambient air beyond where it originates; or
- The dust fall at the boundary or beyond the boundary of the premises where it originates exceeds:-
- 600 mg/m2/day averaged over 30 days in residential and light commercial areas measured using reference method ASTM D1739; or
- 1200 mg/m2/day averaged over 30 days in areas other than residential and light commercial areas measured using reference method ASTM D1739.
Furthermore, the SANS 1929 (2005) guidelines for dust fall incorporate the ASTM D1739 specifications to guide South African dust air quality, specifically in relation to dust fall (Table 1 and Table 2). In terms of these guidelines, no industry may operate within the fourth band (alert band) as specified in Table 1. Industry may operate within the third band (action band) for a limited period of time provided they have received written authorisation from the relevant authorities. This authorisation may, however, only be granted by the authorities if it is deemed essential in terms of practical operational reasons and provided that an appropriate dust suppression technology is applied for the duration of the required operation.
Dust fall that exceeds the specified guidelines may be discounted by the authorities for enforcement and control purposes if they are shown to be the result of an extreme weather or geological event. Such an extreme event may be characterised by excessive dust fall over an entire metropolitan area and not be localised to a particular operation. Natural seasonal variations will not be considered as extreme events and will not be discounted.
Table 1: Four-band scale evaluation criteria for dust deposition (SANS 1929)
| Band Number | Band Description Label | Dust Fall Rate (D) (mg/m2/day), 30-day average) |
Comment |
| 1 | RESIDENTIAL | D < 600 | Permissible for residential and light commercial |
| 2 | INDUSTRIAL | 600 < D < 1200 | Permissible for heavy commercial and industrial |
| 3 | ACTION | 1200 < D < 2400 | Requires investigation and remediation if two sequential months lie in this band, or more than three occur in a year. |
| 4 | ALERT | 2400 < D | Immediate action and remediation required following the first exceedance. Incident report to be submitted to the relevant authority. |
Table 2: Target, action and alert threshold for ambient dustfall (SANS 1929)
| Level | Dust Fall Rate (D) (mg/m2/day), 30-day average) |
Averaging Period |
Permitted Frequency of Exceedance |
| TARGET | 300 | Annual | - |
| ACTION RESIDENTIAL | 600 | 30 days | Three within any year, no two sequential months. |
| ACTION INDUSTRIAL | 1200 | 30 days | Three within any year, not sequential months. |
| ALERT THRESHOLD | 2400 | 30 days | None. First exceedance requires remediation and compulsory report to the authorities. |
Execution of the project
Gondwana used the American Society for Testing and Materials standard method for collection and analysis of windblown dust deposition (ASTMD1739). This method uses a simple devise consisting of a 5 litre capacity cylindrical bucket filled three quarters with distilled water. An inorganic biocide, such as a copper sulphate solution is added to the distilled water to prevent algae growth.
The bucket is placed on a stand comprising of a ring supported by four stabilizing bars above the base plate to prevent contamination of the sample by perching birds. A stainless steel insect screen is placed over the bucket opening to prevent further contamination by insects and large pieces of course organic detritus. A security clamp locks down the stainless steel insect screen and bucket onto the base plate to prevent theft. The base plate is connected to a 2 meters long steel pole, which is either attached to a pre-existing fence post or to a base plate stand, which is buried to a depth of approximately 0.5 metres.
The buckets are left exposed in the field for 30 days, in various locations around the smelters, to collect the windblown dust deposition, which is trapped in the distilled water. On return to the laboratory, the buckets are rinsed to remove external contaminants. The bucket sample is filtered through a mesh course filter with a pore size of approximately 1 mm to remove insects and course detritus, which may have passed through the insect screen. The sample is then filtered through a pre-weighed paper filter to remove the dust fallout (i.e. the insoluble fraction). To ensure that all dust collected in the bucket is removed; the sides and base of the bucket are rinsed with distilled water. This secondary sample is also passed through the paper filter. This process is repeated until no dust remains in the bucket. The filter, containing the dust fall, is then dried in an oven and once dry, gravimetric analysis is conducted to determine the insoluble fraction based on specific equations.
Recent similar projects
Gondwana has an extensive track record in air quality management projects. Amongst their portfolio of projects in dust fallout monitoring, is the Lonmin Platinum mines in Marikana (Rustenburg) and Limpopo as well as an ongoing project for ESA Assmang. Another project is the New Largo monitoring site for proposed mining activity.