Alternative Uses for Pig Manure
Economic analysis showed that land-spreading of pig manure for its fertiliser value is the most economic use for pig manure currently but work at Teagasc in Ireland has demonstrated the technological feasibility and effectiveness of several alternative uses/treatments for pig manure.
1 August 2014
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The Nitrates Action Plan introduced by S.I. No. 378 (2006) and the spiralling cost of fossil fuel prompted research into non land-spread options for pig manure. Despite restrictions and difficulties relating to the land spreading of pig manure, it is likely to be the most cost-effective use of pig manure in Ireland for the foreseeable future.
Teagasc led a three-and-a-half year project to investigate alternative non-land-spread uses for pig manure. The economic feasibility of the alternatives investigated was also assessed.
Anaerobic Digestion
Anaerobic digestion was investigated in laboratory-scale digesters at National University of Ireland Galway (NUIG) and in a pilot-scale digester at Teagasc Moorepark. The laboratory work found that grass silage could be co-digested with pig manure at a volatile solids ratio of 1:1 (manure/silage) in the feedstock, and this was found to improve the specific methane yield.
When the reactors were operated under an organic loading rate of up to 3kg volatile solids per cubic metre per day and a grass silage volatile solids ratio of up to 40 per cent, the system was found to be stable. The volumetric methane production was up to 501L cubic metres per day. In subsequent pilot-scale experiments, the specific methane yield increased from 154mL methane per gramme of volatile solids added with mono-digestion of manure to 251mL methane per gramme of volatile solids added with anaerobic co-digestion of manure and grass silage (volatile solids ratio of 1:1).
Volatile solids removal rates increased from 41.4 per cent (manure alone) to 53.9 per cent (manure + silage). The results show that co-digestion of pig manure and grass silage is preferable to monodigestion of manure alone.
Composting of Manure Solids
The separated solid fraction of pig manure was composted using different bulking agents (straw, sawdust, shredded green waste and wood chips) at different ratios. Results demonstrated that addition of a carbon-rich bulking agent is required when composting the separated solids of pig manure.
Of the bulking agents investigated, sawdust produced the best quality compost. Stable compost was produced using a carbon to nitrogen ratio as low as 16. This corresponds to a separated manure solids to sawdust ratio of 4:1 (fresh weight). In addition, microbiological analyses showed that pig manure-derived compost meets microbiological criteria for marketable processed manure products, as set out in EU regulations, as E. coli and Enterococcus were below limits and it was Salmonella-free.
Use of Solid Manure as a Fuel
A small-scale pyrolysis reactor in the University of Limerick (UL) was used to study the suitability of producing energy from the separated solid fraction of pig manure before and after composting. The use of all three end-products of pyrolysis (biochar, bio-oil and gas) to generate energy was evaluated.
The pyrolysis studies showed that the proportion of biochar, bio-oil and gas produced, and the physical and chemical characteristics of these products were influenced by both sawdust addition and feedstock composting. Increasing the sawdust content in the wood/manure mixture reduced the biochar yield and increased the bio-liquid yield. The biochar showed increased heating values, but reduced nutrient concentrations with increasing sawdust addition. The heating value of the gases produced also increased, while that of the bio-liquid was decreased with sawdust addition.
Composting of the feedstock before pyrolysis increased the biochar and bio-liquid yield, but decreased the gas yield. The biochar showed reduced heating values, while the bio-liquid heating values increased with composting.
Integrated Constructed Wetlands
Sixteen meso-scaled Integrated Constructed Wetlands (ICW) systems, each comprised of four cells, were constructed at Teagasc Moorepark to assess the treatment of the separated diluted liquid fraction of pig manure.
Different application rates and flow rates were investigated and microbiological analyses were conducted to investigate the removal of pathogenic micro-organisms.
The study demonstrated the potential of this technology to treat the separated liquid fraction of pig manure. However, due to the system’s high sensitivity to ammonium, the separated liquid fraction of pig manure had to be greatly diluted before entering the ICW. This may limit the use of ICWs on pig farms due to the high land area required to construct such systems.
Flow through the cells reduced mean counts of coliform, yeasts and moulds and spore-forming bacteria across all treatments, but there were no effects on Enterococcus or E. coli counts. Microbial removal was also investigated in large-scale on-farm ICW systems.
Overall, reductions in enteric indicator bacteria counts were found across nine ICW systems treating dairy and piggery wastewater, with E. coli and Enterococcus non-detectable in the final effluent. Furthermore, Salmonella, when present in the influent material, was absent in the ICW effluent.
Woodchip Biofilters
Laboratory-scale woodchip biofilters at NUI Galway were successful in removing nutrients from the separated liquid fraction of pig manure. Therefore, six pilot-scale biofilters each comprised of a one-metre aerobic woodchip layer and a 0.5-metre saturated woodchip layer, were constructed at Teagasc Moorepark to verify results and to demonstrate effects of scale, variations in temperature and rainfall.
Reductions of up to 54 per cent solids, 80 per cent total chemical oxygen demand (CODt), 93 per cent five-day biological oxygen demand (BOD5), 86 per cent total nitrogen (TN) and 79 per cent total phosphorus (TP) were achieved in the pilot-scale woodchip biofilters.
When different chemical treatments were investigated for polishing of the pilot-scale biofilter effluent, aluminium sulphate was found to be better than lime. It removed up to 84 per cent turbidity, 76 per cent CODt and 99.6 per cent TP from the effluent.
Microbiological analyses showed that E. coli and Enterococcus, although detectable in the biofilter influent, were almost always below the limit of detection in the effluent and E. coli counts were also reduced. Furthermore, Salmonella, although detected in the influent on some occasions, was never found in the biofilter effluent.
Economics
A cost-benefit analysis of the technologies investigated was also performed.
Anaerobic digestion of pig manure and grass silage (1:1; volatile solids basis) was unviable under the current tariffs, with costs at €4.80 per cubic metre of manure.
The solid-liquid separation of the digestate would cost an additional €12.40 per cubic metre of manure.
Subsequent treatment of the separated solid fraction by composting would add €2.10 per cubic metre of manure.
The use of ICWs to treat the separated liquid fraction would add €4.50 per cubic metre of manure to the treatment costs, while the use of woodchip filters would add €2.80 per cubic metre of manure.
Therefore, these technologies are currently not cost-effective.
Transport and spreading of raw manure for its fertilizer value is the most cost-effective option. For distances of up to 14km from the customer’s farm, the tractor and vacuum tanker scenario is the most cost-effective option (€4.70 per cubic metre).
For longer distances, it becomes more cost-effective to use a truck, with the cost of transporting and spreading manure within a distance of 50km to the customer’s farm calculated at €7.70 per cubic metre of manure.
Conclusion
This project demonstrated the technological feasibility and effectiveness of several alternative uses/treatments for pig manure in Ireland.
Economic analysis showed that land-spreading of pig manure for its fertiliser value is the most economic use for pig manure currently.
Nonetheless, information on the effectiveness of and design guidelines for each technology examined are now available for adoption by stakeholders should economic conditions/ supports change in the future.
Acknowledgements
This research was funded by the Department of Agriculture, Food and the Marine’s Research Stimulus Fund Programme under the National Development Plan 2007-2013.
The Project Team was made up of: Dr Peadar Lawlor, Teagasc; Dr Brendan Lynch, Teagasc; Tereza Cota Nolan, Teagasc; Tomas Ryan, Teagasc; Dr Xinmin Zhan, NUI Galway; Dr Mark Healy, NUIG; Dr Michael Rodgers, NUIG; Dr Peter Frost, AFBI NI; Stephen Gilkinson, AFBI NI; Dr Witold Kwapinski, UL; Dr J. J. Leahy, UL; Dr Gillian Gardiner, WIT; Dr Shane Troy, Teagasc, NUIG and UL; Dr Gemma McCarthy, Teagasc and WIT; Dr Caolan Harrington, Teagasc and University of Edinburgh; Dr Sihuang Xie, Teagasc and NUI Galway; and, Dr Kathryn Carney, Teagasc and NUIG. The assistance provided by the advisory panel to the project (Dr Munoo Prasad, Dr J.J. Lenihan and Dr John Finnan) is gratefully acknowledged.
August 2014
