Manure storage

Options to reduce manure emissions exist through the capture of methane produced in anaerobic ponds via bio-digestors, capture and destruction of methane via flaring, aeration of the pond which reduces methane production by creating more aerobic conditions. Where methane is captured as biogas, this can provide additional benefit by displacing fossil fuel emissions. Approximately 4% of the total emissions from the agriculture sector in New Zealand are estimated to come from manure management. Considerable effort is being put into better quantifying these emissions as it is recognised that at present data are lacking on the range of systems in use and on emissions from specific systems under different conditions.

Manure management options are well established in principle but the cost-effectiveness of many of the technologies is challenging under the New Zealand farm system.

• Biofiltration from dairy farm effluent ponds: A biofilter is a porous layer of organic material that supports a population of microbes that feed on methane, called methanotrophs. Methane is forced through this material and is converted by the microbes to carbon dioxide and water. While research has shown this approach is possible in New Zealand, several technical challenges need to be overcome to make this a cost-effective technology.
• Covered anaerobic pond: The anaerobic pond has an impermeable cover. Methane and odorous gases are collected under this cover. This biogas can be used as an energy source or simply flared (burned) off. The economics of this approach are considered challenging for New Zealand farm systems given the relatively small herd sizes.
• Anaerobic Digester: Wastes are anaerobically digested in a controlled system using digester tanks, with a controlled capture of methane produced in the process. The biogas can then be used as an energy source. The economics of bio-digesters are extremely challenging, and a key restraint is the minimum size of herds required to make this approach cost effective in a country like New Zealand.
• Surface-aerated ponds: An anaerobic pond is mechanically surface aerated that reduces the ability of the methanogens to produce methane. The amount of methane mitigation that could be achieved by this approach has not been quantified.
• Ecopond: A new process, developed in New Zealand, whereby ferric sulphate is filtered through the effluent, removing the methane. This has shown to be very effective, but issues remain around its cost-effectiveness. For more, see this presentation to the 2023 NZAGRC conference.