Pig slurry options too dear
Their cost analysis has shown that alternative technologies are not currently cost effective in Ireland, but they may have future potential.
For example, anaerobic digestion would be cost effective on large units (2,000 sows plus), or if centralised treatment plants were developed, and if the renewable energy feed in tariff for energy sold to the grid was increased.
Pig farmers are challenged by the immense restrictions on the use of pig manure on intensive grassland and cereal farms, imposed by the nitrates directive. Few pig farmers own sufficient suitable land for utilisation of the manure generated, and they supply manure instead to customer farms, where it replaces chemical fertiliser.
However, many of these farms are no longer suitable, because the organic N loading from grazing livestock is already at or approaching the 170 kg/ha limit.
After 2017, it is estimated that a further 45 to 50% of land-spread area will be required, when the full limit on use of pig manure, depending on soil test P results, will be applied.
Despite the significant financial value of pig manure as a fertiliser, customer farmers may in some instances be reluctant to use pig manure, because they fear loss of single farm payment entitlements if they inadvertently make mistakes.
Tillage farmers, in particular, were initially concerned about the variable availability of N in pig manure; the short window for application of manure to tillage crops; the effect of manure application on permitted fertiliser N application in subsequent years; and a fear that N will become available too late in the season, possibly contributing to lodging. However, these concerns have been largely dispelled in recent years.
In addition, concentrated pockets of pig production occur around the country (for example, Cavan or Mitchelstown), where the quantity of pig manure available exceeds availability of suitable land for spreading. Pig manure has to be exported from these areas, at considerable cost.
The alternatives studied and costed by Teagasc researchers are anaerobic digestion of pig manure and grass silage, with solid-liquid separation of the digested material, composting of the solid fraction, and treatment of the liquid fraction by means of integrated constructed wetlands (ICW) and woodchip filters.
They looked at the costs for a 500-sow integrated pig farm producing 16 cubic metres of liquid pig manure per sow per year (8000 m3 in total per year), at 4.5% dry matter. Calculations for anaerobic digestion indicated that a 500-sow integrated unit digesting grass silage and pig manure at a 1 to 20 ratio (fresh basis) would lose €60,019 per year. However, it is important to remember that, in the case of an anaerobic digestion plant, econom- ies of scale apply. For example, a 2,000 sow unit with an investment cost of €958,579 would have a payback time of 32.6 years. Furthermore, if the price paid for the electricity was to increase from €0.15/kWh to €0.22/kWh, the payback time would be 138 years for the 500-sow unit, but about 12 years for the 2,000 sow unit.
Calculations for the solid and liquid separation of the anaerobically digested pig manure (and grass silage) by decanter centrifuge, without grant aid, indicated a total cost of €10.87 per m3 of manure. In the case of compost, in order to break even, a 500-sow integrated pig farm would need to sell compost at €0.71/litre.
For the treatment of the liquid fraction of separated pig manure from a 500-sow integrated unit, an ICW would require a land area around the pig unit of between five and ten hectares. Woodchip filters, although a lower cost technology, have the limitation that their effluent cannot be directly discharged into a water body, because of high levels of P and N. One solution to this that could increase the efficiency of both technologies, while reducing costs, would be to pre-treat the liquid fraction in the woodchip filters. This would reduce the volume of extraneous water required to dilute the liquid before discharge to the ICW, and consequently reduce the land area required for the ICW.
The background to the Teagasc research project includes reducing greenhouse gas emissions from stored pig manure, by capturing methane during anaerobic digestion. In addition, production of renewable energy from pig manure is carbon neutral and offsets carbon dioxide that would otherwise come from fossil fuels — thus helping to meet Ireland’s targets to reduce carbon dioxide emissions.
Anaerobic digestion can also help reduce pathogen levels in pig manure. However, it does not reduce the P and N content of manure. Moreover, as the manure will most likely be co-digested with other biomass, the N and P content of the digested material will likely be even higher than that of the raw manure.
