[Research project]
Assessing the medium term impact of permeable pond covers on pond performance and odour management
Project funded by Australian Pork Limited
Background
Intensive piggeries are a major source of gaseous emissions, the most obvious of which are odours. Emissions from anaerobic ponds constitute up to 80% of total odour emissions from typical Australian piggeries (Smith et al. 1999). Reducing odour emissions from Australian piggeries necessitates reducing odour emissions from anaerobic treatment ponds specifically.
Piggery odours raise concerns from a number of perspectives, including restriction of industry expansion, impact on amenity values and concerns regarding health impacts. While the pig industry has been aware of factors such as poor public perception and the impact of odour emissions on the ability to expand, little has been done to identify practical methods to reduce odour emissions.
A recent comprehensive review of air quality issues associated with intensive livestock operations concluded: "The most effective and economically feasible strategy for reducing emissions from outside storage units...is accomplished by covering the entire surface area of the storage unit" (Iowa State University and The University of Iowa Study Group, 2002). The report drew attention to the role of impermeable and permeable covers, including straw and polymer-based materials in odour management. This indicates that cover technology is viewed as the only practical solution to odour emission from existing waste storage and treatment facilities.
The technology underpinning impermeable covers has been developed over many decades. A number of industries manufacture and deploy these covers. In contrast, permeable covers have been identified only recently for odour management. Basic information and experience regarding methods of construction and techniques for deployment is required so that these covers can be recommended as simple, low-cost alternatives to impermeable covers.
While straw-based covers have been promoted in Canada (PAMI, 1993), it was on the basis of qualitative assessment and anecdotal evidence only. Australian Pork Limited (APL) project 1473 Efficacy of supported straw covers for odour reduction in piggery effluent ponds provided the first quantified data and defensible information regarding the efficacy of straw based covers under laboratory and field conditions. Specific results from the field investigations included the following:
- Supported straw covers and a polypropylene (PP) fabric cover reduced odour emission by 85-90%;
- Straw based covers performed consistently over a 10-month period, despite obvious deterioration of the straw;
- PP fabric covers appeared to offer cost advantages over supported straw covers;
- PP fabric covers would probably be easier to manufacture, deploy and manage;
- The costs of permeable covers appeared very favourable compared to the costs of impermeable covers.
While permeable covers were very effective in reducing pond odour emission, the uncertainties regarding:
- longevity of the covers (ultimately determining the true cost of these covers)
- possible negative impacts on pond performance
- likely increased emission of odours from housing during flushing
necessitated more comprehensive investigation before this technology could be recommended unreservedly for odour management. APL has committed funds to the task of improving our knowledge base, which is being undertaken by DPI&F's intensive livestock environmental management officers in Toowoomba.
Project objectives
- Following trials over a period of 30 months, produce a comprehensive "how-to" guide to enable the construction, deployment and maintenance of an effective permeable pond cover by producers.
- Determine and report the life expectancy and costs of a permeable pond cover.
- Characterise and report the impact permeable pond covers have on anaerobic pond treatment processes over a three year period.
- Identify the basic process whereby permeable pond covers reduce odour emissions.
- Investigate the relationship between ambient air odorant concentrations and olfactometry.
- Investigate whether odour emissions from housing and effluent irrigation areas are increased following the deployment of a permeable pond cover on a treatment pond.
- Assess the impact of permeable pond covers on the intensity and offensiveness of pond odour emissions.
Expected outcomes
It will be demonstrated that installing a permeable pond cover is a cost-effective, efficient technique to reduce odour emissions from anaerobic treatment ponds.
Pig producers will be able to implement permeable pond cover technology at their facility with very high confidence regarding likely performance, purchase and operating costs, and cover life expectancy.
Information regarding the deployment and mode of operation of permeable pond covers will be transferable to other waste streams and treatment processes.
Summary of methodology
Experimental facilities
The technology is being trialled at three different piggery operations.
Piggery A is situated near Toowoomba. It is a "niche" operation with a small, highly loaded anaerobic pond and secondary, facultative pond. A prototype permeable cover was trialled at the site over a period of 12 months prior to the commencement of this project. For the current study, a supported polypropylene cover was manufactured from commercial products.
Piggery B is a grow-out operation situated near Brisbane. The single anaerobic pond was used to acquire all the field data for the preliminary assessment of permeable cover technology (Efficacy of supported straw covers for odour reduction in piggery effluent ponds). The entire pond was covered in two sections. Roughly half of the pond surface was covered with a supported polypropylene cover identical to that used at Piggery A. The other half was covered with a supported straw cover.
Piggery C provided a unique facility to trial pond cover technology. Waste from the piggery buildings is split equally between two identical anaerobic ponds. One of the ponds was completely covered with a supported polypropylene cover, while the other was left uncovered as a control.
Odour emission rate assessment
Odour samples will be collected using standard DPI&F techniques. A UNSW wind tunnel (Wang, Jiang, et al. 2001) will be used to collect odour samples to determine the rates at which odour is emitted from the pond liquor surface or cover surface at each site.
Odour samples will be assessed using the DPI&F research olfactometer according to Australian Standard 4323.3 (Standards Australia & Standards New Zealand 2001).
Volatile organic compound (VOC) assessment
The identity and relative concentrations of individual odorant chemicals will be assessed to determine the efficiency and mode of operation of the permeable covers. A US EPA dynamic emission chamber ("flux chamber") is used to this end. Gas samples derived from flux chambers are considerably more concentrated than those collected from wind tunnels. The higher concentrations in flux chamber samples will facilitate analysis of discrete odorants using instrumental methods of analysis such as gas chromatography and/or mass spectrometry.
Laboratory-scale assessments
Laboratory-scale assessments will be undertaken to generate information regarding critical elements of the project, including aspects such as the efficiency of odour reduction by the various covers, the mode of odour reduction (biofilter versus physical barrier, etc), potential for production or reduction of greenhouse gases and investigations regarding the microbiology of odour reduction.
Physico-chemical assessment of impact on pond performance
Pond liquor samples will be collected at approximately five-week intervals and analysed for a range of standard water quality variables. These data will be used to identify the impact a permeable pond cover may have on pond treatment performance.
Assessment regarding accumulation of odorants in pond liquor
The concentration of odorants in pond liquor will be assessed by directly analysing the liquor derived from covered and uncovered liquor using instrumental methods of analysis such as gas chromatography and/or mass spectrometry.
Likely impacts on odour emission from housing following flushing will be assessed using custom-built apparatus.
Project progress
General
All pond covers are being monitored according to a regular sampling programme. Monitoring activities include physical assessment of cover condition, collection and analysis of pond liquor samples and collection, and assessment of gas emissions from open liquor and cover surfaces.
The measurement of odour emission rates is proceeding smoothly, utilising standardised sampling procedures and equipment (Hudson, Galvin, et al. 2004). Assessment procedures have been expanded to incorporate odour intensity assessments utilising a technique that is being developed in-house.
Collection of samples for Volatile Organic Compound (VOC) analysis has centred on use of the US EPA dynamic emission chamber (flux chamber). Collaborating researchers within DPI&F have developed analytical procedures incorporating a concentration technique to improve the detection of a range of volatile chemicals. The concentration technique involves trapping the volatile compounds on a multi-component sorbent trap, from which trapped material is flushed into the inlet of a gas chromatograph (GC). Materials eluted from the GC are detected by a mass spectrometer (MS).
Cover performance and maintenance requirements under local weather conditions
Cover performance (physical appearance and intactness) has been documented since the covers were initially deployed. Maintenance requirements have been limited to physical mowing or chemical spraying of grasses surrounding the pond and cover. Grass will grow out onto a pond surface if unchecked. The same behaviour was observed on the covers. Grass encroachment on the covers is more severe because the covers offer a support for the grass. It was noted that producers do not regularly manage the margins of ponds - serious encroachment by vegetation may become an issue, leading to submersion of the cover around the pond margins.
Rainfall does not pose any significant threat to the integrity of the cover. After heavy rainfall, rainwater percolates through the cover material, leaving only scattered pools across the cover surface. The matrix of flotation devices bound these pools, preventing extensive submersion of the cover.
Rainfall does not appear to impact adversely on the cover performance. While sample collection has not targeted the periods immediately prior to or following rainfall, no deterioration in odour reduction has been observed from the data collected or from anecdotal reports.
Wind associated with thunderstorms does not appear to be an issue either. No evidence of wind damage or cover disturbance has been observed. Covers are actually very heavy once they have made contact with the liquor surface, therefore, it is considered highly unlikely that wind will be able to disturb the cover provided the cover is adequately secured around the pond margin.
Permeable pond cover life expectancy and costs
There is reason for concern regarding the ultra-violet (UV) stability of spun-fibre polypropylene geofabric. Although vendors assured us that UV stability would not be an issue, certain batches of product have degraded very quickly. Product failure has been restricted to areas where the cover fabric is not in physical contact with the pond liquor. This occurs around the pond margin and on the upper surface of the floats. In these areas, the fabric has degraded in patches to the extent that it has disappeared completely. In areas where the fabric is in contact with the liquor, an extensive biofilm develops. This protects the fabric from solar radiation and no physical damage has been observed.
We are exploring two strategies to eliminate UV damage to the cover. Polyethylene shade cloth is manufactured and used under conditions of full sunlight exposure in a variety of permanent applications. Commercial shadecloth is manufactured and sold according to two basic criteria:
- transparency to sunlight (graded in percentages), and
- guaranteed life expectancy (typically five or fifteen years for "handyman" and commercial applications respectively).
We have installed a supplemental cover over the PP cover at Piggery A using a commercial quality shade cloth that reduces sunlight penetration by at least 95%. Similar material is scheduled for use at the Piggery C and Piggery B ponds. Parts of the original covers will be left exposed to sunlight to assess the efficacy of the additional cover in reducing structural damage.
The second strategy will involve an assessment of the shade cloth as a pond cover material in its own right. This will be done at Piggery B, where a strip of the pond liquor about 3 m wide separates the existing polypropylene and straw covers. This strip will be covered using a shade cloth cover supported in the same fashion as existing polypropylene covers.
While shade cloth may appear to be a superior product for cover manufacture, it has different physical properties to polypropylene geofabric. Shade cloth has a very open weave. This may prevent the biofiltration mechanism that contributes to the efficacy of the existing cover. Odour samples will be collected off this cover area over the next 12 months to assess odour emission control potential.
Trends in liquor chemistry
A range of physico-chemical variables were analysed in liquor samples collected during each odour sampling event. Samples collected have been from the pond liquor, as well as from the liquid that has ponded on the cover surfaces. The data are summarised as a series of graphs in Appendix 1.
At present, it is difficult to discern clear trends in concentrations of variables. Seasonal trends in concentrations are likely for most variables in response to temperature and mixing events. Data is not available to define seasonal changes in pond liquor composition. The data derived from this project will provide this information, which will also indicate trends resulting from placement of the covers.
The data for the covered and control pond at Piggery C indicates that pH values in the covered pond are 0.2 - 0.7 pH units lower than those in the uncovered pond. There is no indication of a downward trend in pH. This confirms the conclusion drawn from the previous project, which postulated that pH values would decrease to a new equilibrium value. The pH is still almost neutral, indicating that treatment performance is unlikely to be impaired.
Liquor ammonia-N concentrations in the covered pond have increased to what appears to be a new equilibrium concentration relative to the uncovered control pond.
Liquor phosphorus concentrations appear to have increased in all ponds. They have increased in the Piggery C covered pond relative to the control pond. Once more a new equilibrium is indicated, rather than an ongoing increase.
The strength of these conclusions will improve as more data becomes available over the remainder of the project.
Identification of basic processes whereby permeable covers reduce odour emissions
Investigations to fulfil this specific objective commenced in July 2004. Research will focus on the nature of odorants emitted from the liquor and cover surfaces, as well as the microbial population that becomes established on the cover surface. These investigations will involve collaboration with researchers from the Advanced Wastewater Management Centre (www.awmc.uq.edu.au), a faculty centre within the Faculty of Engineering, Physical Sciences and Architecture of the University of Queensland, St Lucia.
Investigation of relationship between ambient air odorant concentrations and olfactometry
Work toward this objective has proceeded in two directions. The literature has been reviewed to identify which techniques are most suitable for collecting samples from emitting surfaces. This has indicated that samples collected with dynamic devices such as wind tunnels are most suitable when the results are to be used for dispersion modelling purposes.
If the samples are to be used to compare relative emission rates between surfaces, or describe the specific odorants and their relative contributions, devices such as flux chambers are probably suitable. They provide samples that are hundreds to thousands of times more concentrated for selected odorants.
Investigation whether emissions from housing and effluent irrigation areas are increased following the deployment of a permeable pond cover on a pond
Limited assessments have been undertaken to date owing to delays in the delivery and installation of pond covers. Additional work is scheduled for 2004/2005.
Assessment of the impact of permeable covers on odour intensity and offensiveness
Limited progress has taken place in this area to date. The only recognised technique for assessing offensiveness is the German VDI method (VDI, 1992). Significant methodological problems are associated with this procedure and we are currently assessing alternate strategies to produce the information required.
Pond at piggery A prior to covering |
Original pond cover at Piggery A |
Replacement supported polypropylene cover at Piggery A |
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UV damage to unshaded polypropylene fabric at Piggery A |
Shadecloth cover installed on top of supported polypropylene cover at Piggery A |
Figure 6. Comparison of odour emission rate by emitting surface, Piggery A
Figure 7. Trend in concentrations of ammonia nitrogen in covered liquor, Piggery A
Figure 8. Trend in concentrations of organic nitrogen in covered liquor, Piggery A
Expected completion date: 01/03/2006
References
Hudson N, Galvin G and Lowe S, 2004 The effect of loading rate and spatial variability on pond odour emission, Final report for Australian Pork Limited, Project 1628, Department of Primary Industries & Fisheries, Queensland.
Standards Australia and Standards New Zealand, 2001, Stationary source emissions. Part 3: Determination of odour concentration by dynamic olfactometry. AS/NZS 4323.3:2001, Standards Australia: Strathfield, New South Wales.
Wang X, Jiang J and Kaye R, 2001, Improvement of wind tunnel sampling system for odour and VOC's. 147-154 p. 1st IWA International Conference on Odour and VOCs: Measurement, Regulation and Control Techniques, University of New South Wales, Sydney.
Project Team
Neale Hudson
Senior Environmental Scientist
Department of Primary Industries and Fisheries
Phone: +61 7 4688 1519
E-mail: neale.hudson@dpi.qld.gov.au
Mark Dunlop
Environmental Engineer
Department of Primary Industries and Fisheries
David Duperouzel
Experimentalist
Department of Primary Industries and Fisheries
Gary Collman
Extension Officer (Pig - Environmental)
Department of Primary Industries and Fisheries
Further information
DPI&F's intensive livestock environmental research projects
Last updated 19 October 2004