Dust emissions from a beef cattle feedlot on the Darling Downs

Summary

Dust in beef cattle feedlots may impact on the health and wellbeing of workers and animals, and community amenity. Unfortunately, there is currently no data available on dust characteristics and emissions from Australian feedlots. The introduction of the NPI handbook for beef cattle feedlots in 1999 and subsequent revision in 2001 (National Pollutant Inventory 2001), resulted in some feedlots reporting emissions of particulate matter less than 10 ?m (PM10). Under NPI methodology, emission rates for feedlots are estimated based on American data, however, it has recently been proven that such American data overestimates the actual emissions from American feedlots. Additionally, there is no data on typical dust fall levels for Australian beef cattle feedlots.

A feedlot was selected for collection of Australian data. It was representative of larger Australian feedlots in terms of design and management. The feedlot was located on the Darling Downs in Southern Queensland. The capacity of the feedlot is approximately 15,000 standard cattle units (SCU), making it the eleventh largest feedlot currently operating in Australia. Dust fall was monitored monthly at seventeen sites over a twelve-month period. Additionally, a week of intensive monitoring was undertaken using a pair of co-located total suspended particulate and PM10 high volume samplers. Sampling was also undertaken in real time during this period using a DustTrak real time PM10 analyser. Dust fall gauges capture the larger dust particles that are deposited relatively close to their source. These have been shown to be a major cause of complaints (Vallack and Shillito 1998).

The data indicated:

Dust fall was greatest near roads in and around the feedlot (average 27.7 g/m2/month of insoluble solids) followed by;

The background sites (average 12.5 g/m2/month of insoluble solids);

Sites within the feedlot (average 9.6 g/m2/month of insoluble solids); and

Generally, the intermediate sites located on the edges of the feedlot pens area had the lowest dust fall (average 3.2 g/m2/month of insoluble solids).

This showed that even thought the dust deposition rate in the feedlot pens was high; it was unlikely that the larger particles would be transported over a large distance by local winds. For the dust fall results, low variability was observed between months. The feedlot and intermediate sites indicated that dust generation from both pens and internal roadways in the feedlot was relatively constant. In contrast, the roadway and background sites showed a large amount of variation between months, indicating that the emission rate of dust near these sites was not constant over time, possibly influenced by factors such as vehicle movements. The PM10-TSP ratio is used in the literature to define the fraction of total airborne dust that has a particle diameter of less than 10 ?m. The published values indicate that 20 to 40% of the total dust in American feedlots is less than 10 ?m in diameter.

This project has shown that for an Australian feedlot, approximately 60% of the total suspended particulate matter is less than 10 ?m. During the high volume sampling rain fell and the results recorded during this period indicated that the rain did not influence the PM10-TSP ratio. Additionally, it was shown that for a single rainfall event where approximately 10mm of rain fell, the PM10 concentrations were suppressed for up to 10 hours. Overall, the results provide an insight into dust emissions in an Australian context. The results show that the current NPI emission rate data may not accurately reflect the true emissions of PM10 from Australian feedlots. This is based on back calculated emission rates for the feedlot in question and the fact that Australian feedlot dust has a higher proportion of PM10 than that published for American feedlots. Because of this, it is recommended that further research be undertaken, namely: particle size distribution of dust from Australian feedlots, and determination of seasonal PM10 emission factors for Australian feedlots.