Technology for use of animal wastes in socially responsible agricultural practice. H.C. Willers, R.W. Melse, A.J.A. Aarnink and P. Hoeksma Agrotechnology and Food Innovations, P.O. Box 17, 6700 AA, Wageningen, NL Tel. (+31) 317 476591; Fax: (+31) 317 425670 Email:
[email protected] Web: www.agrotechnologyandfood.wur.nl ABSTRACT Reuse of nutrients from manures should be stimulated in a sustainable agriculture. Technology for manure processing should aim at separation of nutrients, minimal nutrient losses, low energy consumption and reduction of hygienic risks. Anaerobic digestion, mechanical separation, separate collection of faeces and urine an water evaporation were assessed for their potential to produce fertilisers from manure. Most of the phosphorus excreted by animals can be concentrated in solid fractions. Concentration of liquid fractions to liquid nitrogen fertilisers can be achieved by water evaporation but the energy consumption of the two processes for concentration of liquid fraction should be reduced. Manure treatments as anaerobic digestion and concentration of liquid fractions will reduce hygienic risks with pathogens and weed seeds when processed manure is applied to crops. The extend of this risk reduction should be further investigated.
INTRODUCTION Compliance with the EU Nitrates Directive (EEC, 1991) will face the Netherlands with a manure surplus once again. In several regions in Europe, as well as in the United States, methods for processing of manures have been subject to research. A wide variation of techniques have been looked at (Rulkens et al., 1998; Burton and Turner, 2003, Angelidaki and Ellegaard, 2003; Melse and Verdoes, 2004). For various economical and technical reasons, manure processing has not become a success in the Netherlands. A socially responsible agriculture produces food in a sustainable manner, minimising mineral losses, consumption of non-renewables and health risks. For sustainability in agriculture, it is essential to balance mineral inputs and crop requirements and to shortcut mineral cycles. The necessity of reuse of nutrients from manures should be stressed, as the production of chemical fertilisers depletes natural resources like fossil fuels (nitrogen fixation) and phosphate rock. This paper intends to present manure processing techniques that have the potential to produce replacements for chemical fertilisers. This potential is illustrated using some examples that are currently assessed in the Netherlands. APPROACH Three examples of manure processing techniques were chosen to illustrate the potential of obtaining fertilisers that could replace or, at least, supplement chemical fertilisers: [A] Co-digestion to produce methane and an odourless slurry from a mixture of manure and other wastes (Angelidaki and Ellegaard, 2003). This treatment was chosen because it is promoted in several European countries like Denmark and Germany. Recently the Dutch Government has also adjusted legislation to stimulate co-digestion.
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[B] Mechanical separation of pig slurry and evaporation of water from the liquid fraction in combination with acid scrubbing and condensation of ammonia vapour. Products made from the liquid fraction are ammonium sulphate solution and a liquid concentrate containing both nitrogen and phosphorus (NPK-concentrate). This process was chosen because it was tested on two pig farms in the Netherlands (Melse and Verdoes, 2004). [C] Separate collection of pig faeces and urine followed by concentration of urine with energy from exhaust ventilation air in combination with acidification (Aarnink et al., 2000; Willers et al., 2003). This treatment originates in the Dutch Hercules project, where several separation techniques were tested. Simplified diagrams of the three processes are given in Figure 1. Some of the important factors in the evaluation of these processes are addressed in this paper. Their liquid products are compared with chemical N-fertiliser as a reference.
animal slurry [A] co-substrate
[B]
animal slurry
thermophylic anaerobic digestion
decanter centrifuge
digestate
solid fraction N concentrate liquid fraction evaporator NPK concentrate
faeces [C] urine
evaporation with exhaust ventilation air
concentrate
Figure 1: Simplified diagrams of three options for animal manure processing
The three manure treatments were assessed for their potential to: 1. Separate nutrients: Nutrient requirements of crops vary. Separation of nitrogen and phosphorus creates possibilities for application of these nutrients to crops in other ratios than they are present in the raw manure. 2. Minimize energy use: Manure processing uses energy for pumps, heaters etc. Fossil fuels are used for transportation of manure and its products. On the other hand, energy can be produced from the organic matter in wastes. 3. Reduce hygienic risks: Manure or its products may contain pathogens and weed seeds that restrict its use as a fertiliser.
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Of course there are many more aspects to manure processing, e.g. logistics, scale of operation, costs, required skills, labour and ability to deal with environmental issues like heavy metals and gaseous emissions. Regarding the quality of fertilisers, other components than nitrogen and phosphorus, e.g. potassium and organic matter are important. These aspects are only discussed briefly in the paper but should be addressed as well when manure processing is considered. RESULTS AND DISCUSSION The potential of manure processing techniques [A], [B] and [C] with regard to nutrient separation, energy consumption and hygiene are summarised in Table 1. Their liquid products are compared to chemical fertiliser under the following assumptions: • Nutrients in chemical fertilisers are available in the required form and they are not contaminated with useless or harmful components. • The production of chemical N-fertiliser costs 45 MJ/kgN (Maurer et al., 2003). • Chemical fertilisers contain no pathogens or weed seeds. The results are discussed in this paragraph. Table 1: Nutrient separation (% N and P to liquid product), energy consumption and qualitative indications of hygienic risk for liquid products from manure processing systems [A], [B] and [C] as compared to chemical N-fertiliser. Energy for Type of N-fertiliser Nutrient separation Volume Hygiene 3) production Nitrogen Phosphorus reduction2) (MJ/kgN) % in product % in product Chemical 100 0 45 + + Raw Manure 100 100 0 Digested pig manure [A] 100 100 -60 1) + Liquid fraction [B] 73 21 6 N-concentrate [B] 50 0 + + 39 NPK-concentrate [B] 15 24 +/+ Pig urine fraction [C] 50 8 low Pig urine concentrate [C] 50 8 39 +/+ 1)
Negative energy consumption by biogas production Significant volume reduction= +, no volume reduction = 3) Low hygienic risk = +, hygienic risk = 2)
[A] Co-digestion Nutrient separation Co-digestion of manure and other substrates does not separate nutrients. The cosubstrate may add to the nutrients or dilute the nutrients, e.g. bone meal from slaughterhouses, will increase the phosphate content of the digested slurry. During the digestion, organically bound nitrogen is converted to ammoniacal nitrogen. This enhances the direct availability of nitrogen to crops but also increases the risk of ammonia volatilisation during manure storage and application. Energy Co-digestion results in a net production of energy (Table 1). The methane in the biogas can be used for heating or for production of heat and power in a combined heat and power unit. Where anaerobic digestion is used in combination with other techniques, the energy produced may be used, e.g. for water evaporation.
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Hygiene Anaerobic digestion reduces pathogens in the substrate. Especially thermophylic digestion (50-55°C) creates conditions that are unfavourable for survival of pathogens (Angelidaki and Ellegaard, 2003). Heat produced from the methane can be used in an additional preor post treatment to pasteurise or sterilise substrate or digested slurry. [B] Mechanical separation and evaporation of water from the liquid fraction Nutrient separation Mechanical separation of pig slurry with a decanter centrifuge results in a solid fraction (15% by mass) and a liquid fraction (85% by mass). Some results of this kind of separation are shown in Table 2. These results were taken from a field study. Most of the phosporus (79% by mass) and part of the nitrogen (27% by mass) in the slurry ends up in the solid fraction. The evaporation process produces a nitrogen concentrate (ammonium sulphate) and a concentrated liquid fraction containing the other components (NPK-concentrate). Together these liquids contain 88% of the nitrogen and 28% of the phosphorus in the original slurry. Energy Electrical energy is needed to operate a decanter centrifuge. This was measured to be approximately 25 MJ per m3 of pig slurry (Melse and Verdoes, 2004). The evaporation requires 170 MJ per m3 of raw slurry. Hygiene No results are reported of the effects of mechanical separation on survival of pathogens and weed seed viability in manure fractions. In the evaporator, the liquid fraction is heated to 100°C. The retention time was not reported, but if it was more than 1 hour, the evaporator complies with the EU legislation that requires 1 hour at 70°C for pathogen reduction. Table 2: Composition of pig slurry and fractions after separation with a decanter centrifuge. Results were taken from results of Hercules experiments that were not yet published. Numbers are mean values of the separation of three different charges. nm=not measured. Parameter Unit Raw pig slurry Solid fraction Liquid fraction Mass % 100 15 85 Total Nitrogen g/kg 6.3 9.4 5.4 Total Ammoniacal Nitrogen g/kg 4.1 4.8 3.8 Total Phosphorus g/kg 1.2 5.4 0.3 Total Potassium g/l 4.1 3.7 4.3 Total Solids g/kg 72 244 35 Volatile Solids g/kg 55 196 23 Total Suspended Solids g/kg nm nm 6.8 pH 7.8 nm 7.9
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[C] Separate collection of pig faeces and urine and evaporation using exhaust ventilation air Nutrient separation Separate collection of pig faeces and urine was achieved with a convex belt running underneath slatted floors in the pig house. This belt system performed well in a pilot and results were reported (Willers et al., 2003). However, the belt system under the floor was considered a technical risk in a full scale pig house. Parallel with the first belt experiments, some experiments were made with a grooved floor system and a scraper. Results of the nutrient separation of the grooved floor and scraper system are shown in Table 3. Approximately 50% of the nitrogen and 7% of the phosphorus excreted by the pig are collected in the urine fraction. This urine fraction could be concentrated 4-6 times by creating a large contact area between the urine and the exhaust ventilation air of a pig house using a scrubber system (Willers et al., 2003). Nitric acid was used for acidification of the urine to prevent loss of ammonia in the scrubber. The nitric acid provided extra nitrogen. Energy The electrical energy required for the grooved floor system is low but was not measured at the time of the experiments. The exhaust ventilation air of the pig house provides the energy (pig metabolic heat) for water evaporation. The process unit that creates the contact surface between liquid and air requires a pump to circulate the liquid. The energy consumption of this pump in a Hercules experimental pig house was 33 MJ for every kg of nitrogen produced. The process requires nitric acid that is produced industrially. One kg of nitrate-nitrogen is required for every kg of ammonia-nitrogen in the liquid. The indirect energy consumption of the nitric acid production is assumed to be the same as for chemical nitrogen fertiliser production (45 MJ/kgN). Thus, the total energy consumption is 39 MJ for every kg of nitrogen produced. Hygiene There is no knowledge on the hygienic aspects of this system. Conditions in the concentrate that is produced from the urine fraction are very unfavourable for pathogens, as the pH is approximately 4 and salt concentrations are very high. Table 3: Composition of fractions after separate collection of pig faeces and urine using a grooved floor and a manure scraper (Aarnink et al., 2000). Parameter Unit Faeces Urine Mass % 37 63 Total Nitrogen g/kg 13.1 7.8 Total Ammoniacal Nitrogen g/kg 3.2 6.7 Total Phosphorus g/kg 4.4 0.2 Total Potassium g/l 6.5 8.2 Total Solids g/kg 307 36 Volatile Solids g/kg 241 16 pH 6.4 9.3
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General Discussion Nutrient separation Two manure separation techniques were compared as part of systems [B] and [C]: slurry separation with a decanter centrifuge and separate collection of faeces and urine. The first produces a liquid fraction that is 85% of the slurry by weight. The urine fraction that is collected with the grooved floor system and scraper is 63% of the total mass excreted by pigs. Both separation techniques are effective in separating phospohorus to a solid fraction although separate collection (92%) is superior to centrifugation of slurry (79%). On the other hand, separation with a decanter centrifuge produces more liquid fraction that contains more nitrogen (73% of the nitrogen in the slurry) than the urine fraction that contains 50% of the excreted nitrogen. Energy The energy consumption by separation of pig waste into a solid and a liquid fraction is relatively low (see Table 1). Besides nutrient separation, volume reduction is an important way to reduce energy consumption in the Netherlands, because manures and slurries are transported over long distances from animal farms to crop areas. It is remarkable that both systems that concentrate a liquid fraction, [B] and [C], consume approximately the same amount of energy as the production of chemical N-fertiliser. The quality of these concentrates will always be poorer than that of chemical fertiliser due to a lower concentration, other (unwanted) compounds, smell and colour. If volume reduction is required, e.g. to reduce transport costs, energy consumption by concentration techniques should be reduced. One option is to incorporate anaerobic digestion in the process. The energy from biogas can than be used efficiently for evaporation of water. The energy produced from typical Dutch pig manure by digestion is approximately 450 MJ/m3, based on a volatile solids concentration as in Table 2. Water evaporation requires 2850 MJ/m3 in a process without heat losses. So a maximum of 15% of the water in pig slurry can be evaporated using the biogas potential of the slurry. However the energy production can be increased significantly by using co-substrates with a high organic energy content (Angelidaki and Ellegaard, 2003). Another option is to search for methods that can create a contact area between a liquid and a gas phase using less energy than the pump that is used in system [C]. If that is possible, the use of exhaust ventilation air of animal housing to evaporate water will become attractive. Hygiene All treatments discussed in this paper are likely to have positive effects on hygiene. However, the hygienic aspects are not well documented, except for anaerobic digestion. Given the present discussions in the Netherlands on diseases and risk management, it seems essential that all manure treatment and application strategies are assessed for their abilities to reduce hygienic risks. Other aspects of manure processing Many aspects of manure processing were not addressed in this paper. They should be taken into consideration when a treatment system is designed. As compared to chemical fertilisers, manure products contain organic matter. As some soils are depleted in organic matter, it can be substantiated that the organic matter in manure products have a fertilising value. The investment costs of mechanical treatments vary significantly. Logistics and
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scale of operation are important factors in cost optimisation. Operational costs are affected by labour. Manure can be processed on farms utilising the labour at hand. But operation may require skills that are not available on farms. Environmental issues like heavy metals and gaseous emissions have to be dealt with. When manure or manure fractions are concentrated, heavy metal concentrations may exceed legal application limits. Emissions of ammonia, greenhouse gases, dust and odours from manure treatment may be considerable and extra technical measures are often required to reduce these emissions. CONCLUSIONS Reuse of minerals from animal waste shortcuts mineral cycles and thus contributes to a more sustainable agriculture. The results in this paper show that nitrogen and phosphorus in pig manure can be separated. The separation enables balancing of manure product nutrients and crop requirements. The separation of phosphorus is more efficient than that of nitrogen. Separated solid fractions contain most of the phosphorus excreted by animals. Concentration of liquid fractions to liquid “N-fertilisers” can be achieved by water evaporation. The energy consumed by the production of these concentrates is in the same range as the energy consumed by the production of chemical nitrogen fertiliser. Ways must be sought to reduce the energy consumption of the concentration process. One option is the incorporation of anaerobic (co)digestion in the manure treatment. Manure treatments as anaerobic digestion and concentration of liquid fractions will reduce hygienic risks with pathogens and weed seeds when processed manure is applied to crops. The extend of pathogen elimination should be further investigated. REFERENCES [1] EEC (1991). Council Directive of 12 December 1991 concerning the protection of waters against pollution caused by nitrates from agricultural sources (91/676/EEC) [2] Rulkens, W.H., A. Klapwijk and H.C. Willers (1998) Recovery of valuable nitrogen compounds from agricultural liquid wastes: potential possibilities, bottlenecks and future technological challenges, Environmental Pollution, 102(S1): 727-735 [3] Burton and Turner (2003) Manure management. Treatment strategies for sustainable agriculture, 2nd edition, Silsoe Research Institute, UK, ISBN 0 9531282 6 1 [4] Angelidaki, I. And L. Ellegaard (2003) Codigestion of Manure and Organic Wastes in Centralized Biogas Plants. Status and Future Trends, Applied Biochemistry and Biotechnology, 109(1-3): 95-105 [5] Melse, R.W. and N. Verdoes (2004) Evalution of four farm-scale treatment systems for liquid pig manure: products, emissions, and economics, submitted for publication [6] Aarnink, A.J.A., W. Kroodsma, D. Swierstra, H.W.. Houwers and N.W.M. Ogink (2000) Separation of faeces and urine in a piggery by a convex belt or grooved floor system, IMAG report V 2000-83 [7] Maurer, M, P. Schwegler and T.A. Larsen (2003) Nutrients in urine: energetic aspects of removal and recovery, Water Science and Technology, 48 (1): 37-46 [8] Willers, H. C., R.W. Melse and N.W.M. Ogink. (2003). Concentration of Urine from Fatteners combined with Ammonia Removal by Scrubbing Exhaust Air of a Pig House,. Ninth International ASAE Symporium on Animal, Agricultural and Food Processing Wastes (ISAAFPW), 12-15 October 2003, Triangle Park, Raleigh, North Carolina, USA. 584-589.
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