[Research project]

Efficacy of supported straw covers for odour reduction in piggery effluent ponds

Project jointly funded by Australian Pork Limited and the Department of Primary Industries and Fisheries.

Supported straw and permeable polypropylene covers reduce odour emissions from anaerobic ponds by up to 90%, according to this project completed by DPI&F's intensive livestock envionmental management officers.

The two-year project showed that supported straw covers were effective in reducing odour emissions from anaerobic ponds, and while the ultimate life expectancy of the covers are not known at present, a single application of straw appears to last at least 12 months.

The research also showed that polypropylene fabric (commonly sold as geomembranes and as a weed retardant) was equally as effective as the supported straw covers in reducing odour emissions.

While a range of cover types has been trialed in North America, this is the first trial of its kind under Australian conditions. This was also the first study to provide a truly quantitative assessment of the efficacy of these covers over a prolonged period.

Project background

Overseas and local odour emission research identified that waste storage and treatment ponds were responsible for the bulk of odour emitted from piggery operations. According to Australian research, approximately 80% of total odour emissions are from pond systems. Pond systems are particularly important odour sources for small to medium sized piggeries (less than 1200 sows).

While several options have been proposed for odour reduction generally, few have specific application for ponds. Of these, covers of various sorts appeared most promising. Canadian researchers have successfully used straw as an unsupported cover on pig effluent storages. Chopped straw was blown onto the surface of ponds to provide odour control lasting weeks to months. It was necessary to reapply straw to the pond surface as the straw sank into the effluent. While no qualified data was produced to verify the efficacy of straw covers, consistent anecdotal evidence indicated that straw covers might have a role in reducing odour emission from effluent ponds. The straw however would need to be supported under Australian conditions to provide a permanent odour solution.

Before straw covers could be promoted for odour management in Australia, a number of questions had to be answered:

• How well do straw covers actually work in reducing odour emission?
• How long would a straw cover last, and would it reduce odour over its entire life?
• How would a straw cover be supported?
• Would a supported straw cover be cost effective relative to an impermeable cover?

Research program

The research was undertaken in two phases:

• Laboratory-scale trials to determine in-principle effectiveness and to help identify cover materials and methods for supporting them, and
• Field-scale trials of potential cover types on working ponds, to determine their effectiveness.

The odour emissions were measured using DPI&F's Dynamic Olfactometer. Olfactometry allows the odour-reducing effectiveness of different cover materials to be compared reliably.

Laboratory-scale trials

A series of laboratory scale anaerobic digesters were established and operated to evaluate the efficacy of various cover types and methods of support. 

Digesters containing effluent are used to mimic effluent ponds for the testing of covers and supports.

Laboratory-scale digester with supported sugarcane cover.

Cover materials trialed during the laboratory phase included:

• Barley straw
• Lucerne hay
• Sugarcane mulch
• Polystyrene beads.

Pilot-scale field trials

Pilot-scale field trials were conducted to establish potential design criteria for the pond covers for use in the pond-scale trial.

Pond-scale field trials

The most promising cover materials and methods of support were field tested to provide information regarding durability, life expectancy and costs of various filter and support combinations.

Preliminary trials had indicated that a range of materials could be used to conveniently construct a cover support. Monofilament polyethylene hail or bird netting was light and cheap and provided an effective support based for straw or other cover materials. Closed cell polyethylene backer rod, widely used in the construction industry for caulking joints, proved to be a convenient buoyancy material. It was available in a range of diameters and in lenghts of up to 200 m. Spacing and support configurations were determined to support a 100 mm straw bed, and four covers with different support configurations wre prepared for field trials.

6000 mm x 6000 mm trial covers were constructed and deployed on the effluent pond at DPI&F's Wacol Pig Research Centre. A similarly sized cover manufactured from spun fibre needle punched polypropylene fabric (PP fabric) was also assessed. Initial trials of this material in the United States were not favourable, but more recent information indicated that it was effective in reducing odour emissions.

Descriptions and configurations of the covers trialed are summarised in Table 1.

Table 1. Supported barley straw cover treatments used for the field trials

Cover description	Internal support	External support	Support matrix	Cover material
North	Square lattice of 300 mm backer rod @ 200 mm centres	100 mm polystyrene rod	Hail netting	Barley straw, 100-125 mm
Middle	30 mm backer rod @ 300 mm centres, parallel lines	30 mm backer rod	Hail netting	Barley straw, 100-125 mm
South	30 mm backer rod @250 mm centres, parallel lines	100 mm polystyrene rod	Hail netting	Barley straw, 100-125 mm
Tensioned	Square lattice of 30 mm backer rod @ 300 mm centres	30 mm backer rod, wire under tension	Hail netting	Barley straw, 100-125 mm
PP fabric	None	100 mm polystyrene rod	None	None

Pond-scale odour measurements

Odour samples were collected from the effluent pond surface and from above the supported pond covers to determine the reduction in odour concentration atttributable to the pond covers.

Project outcomes

Figures 1 to 4 show the change in appearance in the test covers over a 10 month period. While all covers eventually became damp, additional buoyancy appeared to delay this process.

Figure 1. Covers at time of deployment

Figure 2. South cover one month later

Figure 3. Middle cover after 3 months

Figure 4. Middle cover after 10 months

The results showed that the effectiveness of the covers was independent of physical appearance. The thickness of the straw in the covers altered rapidly as they became damp, but the covers continued to reduce odour acceptably.

Costs

The full costs of supported straw covers will not be known until the ultimate life of the cover and support is known, along with maintenance requirements.

The costs for installing and operating the covers during the project life were as follows:

Supported straw covers:  $10.52 - $12/m² depending on the support configuration

Polypropylene fabric cover:  $7.33/m²

These costs compare favourably with those for impermeable covers. For impermeable covers, the costs of the cover material alone exceeds $30/m2. There is a very significant capital requirement for anchoring/attachment of impermeable covers, and a gas disposal or flaring system is also required. The cover anchor and gas collection and treatment system could add more than $50,000 to the cost of a pond cover.

Figure 5. Summary of results obtained during the field trials 

• The results indicate that the supported straw covers reduced the odour emission rate from a typical anaerobic treatment pond by up to 90%. The polypropylene fabric was equally as effective as the supported straw covers.
• The covers performed very consistently over the period of the project - the measured odour emission rate from the covers never exceeded 5 OUm/s.
• The effectiveness of the covers was independent of physical appearance - the straw covers altered rapidly as they became damp, but continued to perform acceptably.
• The straw on a supported cover appears to have a life expectancy of at least 12 months for a single application of straw, but the ultimate life expectancy of a single application remains unknown.

Issues

A number of issues have been identified which need to be addressed before permeable covers could be recommended unreservedly. The life expectancy of the covers is inadequately known at present. The impact of the covers on waste treatment processes also needs to be assessed more fully. At present, the question "Will a permeable cover compromise waste treatment processes?" could not be answered satisfactorily. Addressing these issues will satisfy regulatory concerns regarding long term performance of the covers, impact on waste treatment and sustained protection of environmental and amenity values.

Where to from here?

Proposed future research will involve ongoing monitoring of odour emission rates and cover materials to determine ultimate life expectancy of various cover types and their effectiveness under field conditions. Concerns regarding impacts on pond treatment processes will also be addressed.

Principal project officer

Neale Hudson
Senior Environmental Scientist
Department of Primary Industries and Fisheries
Phone: +61 7 4688 1519
E-mail: neale.hudson@dpi.qld.gov.au

For a copy of the research report contact Australian Pork Limited, phone toll free 1800 789 099 or e-mail library@apl.au.com, and quote project number 1473. A fee applies for postage and handling.

Further information

DPI&F's intensive livestock environmental research projects

Last updated 3 February 2004