Manage manure to reduce greenhouse gas emissions
Livestock urine and manure are significant sources of methane and nitrous oxide when broken down under anaerobic conditions.
Nitrous oxide is produced during the nitrification–denitrification of the nitrogen contained in livestock waste. Anaerobic conditions often occur where manure is stored in large piles or settlement ponds to deal with waste from large numbers of animals managed in a confined area (for example, dairy farms, beef feedlots, piggeries and poultry farms).
Methane has 25 times and nitrous oxide has nearly 300 times the global warming potential of carbon dioxide.
Ruminants excrete 75–95% of the nitrogen they ingest. Ruminants on lush spring pasture commonly ingest protein (containing nitrogen) in excess of their requirements but are usually energy limited, resulting in higher ruminal ammonia concentrations being excreted in the urine as urea.
Nitrous oxide emissions from ruminants can be minimised by balancing the protein-to-energy ratios in their diets. One study reports that dairy cows on a 'high sugar' variety of perennial ryegrass excreted 18% less nitrogen in total and 29% less urinary nitrogen.
Measures to reduce livestock urinary nitrogen
- breeding animals for improved nitrogen efficiency
- using forages that have a higher energy-to-protein ratio
- balancing high protein forages with high-energy supplements.
Reducing greenhouse gas emissions from livestock manure
- manure stockpile aeration and composting reduces methane emissions
- adding urease inhibitors to manure stockpiles can reduce nitrous oxide emissions; urease inhibitors are chemical additives that stop or reduce the rate that urea (found in animal urine and manure) is converted to nitrous oxide.
Using manure to capture and use methane on-farm
Livestock industries have shown increased interest in biogas (methane) capture-and-use systems, such as covered ponds and the flaring or combustion of the captured biogas to provide heat or power. These systems are common in Europe but not in Australia, and may be profitable, regardless of offset income, because of the energy production and the trading of renewable energy certificates.
Biogas generation systems can reduce greenhouse gas emissions and improve farm productivity for intensive livestock farmers (mainly pork and dairy farmers). There are 3 eligible activities under the Emission Reduction Fund eligible emissions reduction activities.
With a biogas generation system, large volumes of manure are digested under low-oxygen conditions to produce biogas that is subsequently combusted to destroy methane and produce heat or electricity. The waste sludge is normally returned to the land as fertiliser, either as slurry or pellets.
Australian Pork Ltd has released a biogas code of practice: 'On-farm biogas production and use (piggeries) for Australian producers'. The code is focused on covered effluent ponds for the pork industry and provides a basis for appropriate and uniform standards across Australia to improve the standard of installations.
The simplest system is a covered pond to capture the gas and combust it in a flare. More complex systems offset energy costs either by combusting the gas to produce heat or by combusting it in a generator for the cogeneration of heat and electricity. If the energy generated exceeds farm requirements, the excess may be exported to the grid.
Biogas generating facilities are usually added to existing manure storage ponds. The process of using biogas is as follows:
- Waste ponds are covered to prevent methane escaping into the atmosphere. The cover consists of an impermeable membrane stretched across the pond. It is sealed at the edges in anchor trenches with gas-collecting pipes located underneath the cover. A mechanism must be installed to drain rainwater so it does not pool excessively and collapse the cover.
- Biogas is collected and piped away from the pond.
- Biogas is combusted, converting methane to carbon dioxide and water. The remaining waste solids are a concentrated form of nutrients that can be used as fertiliser.