anaerobic digestion: a review comparison with two types of aeration systems for manure treatment and...
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Agricultural Wastes 10 (1984) 117-133
Anaerobic Digestion: A Review Comparison with Two Types of Aeration Systems for Manure Treatment and
Energy Production on the Small Farm
A. Well inger
Biogas Project, Swiss Federal Research Station for Farm Management and Agricultural Engineering,
8355 T/~nikon, Switzerland
ABSTRACT
The performance of an anaerobic digestion plant is compared with aeration systems with direct and heat I;ump energy recovery on the basis of energy production and utilization. The success of the plants in production of fertilizer and pollution control is also discussed in terms of chemical changes of the manure, crop growth, weed control and odour reduction.
I N T R O D U C T I O N
With the increasing size of animal production units, odours have become a serious problem in many places. Manure treatment was therefore first applied to decrease or eliminate odour emissions. Lately, renewed interest has been shown in the improvement of manure for fertilizer because of the increasing cost of chemicals. Finally, after the OPEC oil crisis in 1973, manure treatment with respect to energy production has been actively promoted. Hence 'manure treatment' is a very heterogenic term and before the different possible procedures can be rated the purpose of the treatment has clearly to be stated.
Essentially manure can be considered as feedstock for four different products (Fig. l): (a) waste which has to be disposed of, (b) feedstuff, (c) fertilizer and (d) energy source. The present review will be restricted to the terms 'energy source' and 'fertilizer' and the respective treatments.
117 Agricultural Wastes 0141-4607/84/$03.00 © Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Great Britain
118 ,4. Wellinger
I FEEDSTUFF I
l
1 ENERGY I SOURCE
Fig. 1. The four possible uses of manure.
MANURE AS AN ENERGY SOURCE
The systems considered
Basically, there are two ways of obtaining energy from manure, either by an oxidative or by a reductive treatment. During aeration heat is produced by aerobic bacteria. When sufficient oxygen is introduced high growth yields are achieved during the breakdown of organic material. Because the efficiency of the energy utilization is usually lower than 40 ~o in biological systems, considerable amounts of heat are released which can be recovered with the aid of heat exchangers. Depending on the reaction temperature this heat can be either utilized directly for heating purposes or indirectly with the aid of a heat pump.
During anaerobic fermentation the complex organic matter is degraded to the relatively clean and easily purified gaseous products carbon dioxide and methane, with a fairly small growth yield of bacteria (McCarty, 1964). Hence very little heat is released. Thus a large amount of organic matter is destroyed, comparable to aerobic degradation, but about 90 ~o or more of the substrate energy is retained in the form of methane.
A medium sized Swiss farm of 30 cattle units (CU) was taken as the basis for a comparison of the three energy yielding systems: (1) aeration with direct heat recovery, (2) aeration with heat recovery by a heat pump and (3) biogas production. One CU corresponds to a dairy cow of 600 kg, or to approximately 6-7 fattening pigs with an average weight of 60 kg,
Review comparison of anaerobic digestion with two types of aeration systems 119
with respect to volumetric waste as well as gas production. A recent survey has shown (Wellinger et al., 1981) that if water use is restricted in the housing a cow deposits about 68 litre of manure per day with a Total Solid (TS) content of 8.7 ~o and a Volatile Solid (VS) content of 77 ~ of the TS. The corresponding figures for an average fattening pig are 12 litre of manure with 4. l ~o TS and 70 ~o VS. The comparison assumes that the heating requirement (including warm water production) of the farmhouse would be equivalent to 3.5 t light oil per year which could possibly be replaced by the respective renewable energy source. The monthly distribution of the energy need is indicated in Fig. 2.
Other parameters used for an economical consideration were chosen according to Kaufmann (1982) as compiled in Table I.
(MJ/day)
Fig. 2. Heating requirement of an average Swiss farm (3.5 t ofoi l per year) versus net gas production from 30 CUs (RT = 25 days, insulation U-value = 0.45 W/m 2 °C).
Aeration with direct heat recovery
For direct heat recovery the manure is continuously introduced into a well insulated (10 cm of polystyrene), partly covered reaction vessel which is equipped with metal water/slurry heat exchangers and an aerator. The feed of fresh manure displaces an equal amount of fermented manure into a storage pit. Retention time (RT) should not be longer than 5 days in
120 A. Wellinger
TABLE 1 Data used for Consideration of the Economics of the Three Energy-yielding Manure
Treatments
Prices: Heating oil 70 SFr per 100kg* Electricity 0.15 SFr per kWh
Rates: General inflation rate 7 Energy inflation rate 7 ~o Interest rate 6 ~o
Machines Plastic Concrete Pay back period (yr) 8-10 15-20 20~25 Maintenance rate (~ ) 5-10 2 3 1-2
* 100 SFr corresponds approximately to £32.40 or US$45.60.
order to achieve the required temperature of about 50 °C (G6bel, 1981). A temperature of about 45 °C is needed to run a low temperature heating system. For the manure of 30 CUs a fermenter of 20 m 3 is needed which will be filled to approximately two-thirds. One-third of void volume is necessary as a buffer for the probable foam formation. The aeration of pig manure from 200 fattening pigs (equivalent to 30 CUs) provides an energy maximum in the form of hot water of approximately 900 MJ per day (G6bel, 1981) which is slightly lower than the average energy need in January (Fig. 2). Thus for the coldest winter days an additional heating source is required. For the calculations of the energy costs (Table 2) the price of electrical heating in connection with a hot water store (2 m 3) was therefore included.
In summertime, out of the heating season, hot water is produced by electricity only, which is far cheaper than using the aerator. Only one of the described systems has been built and monitored so far in Switzerland by Alfa-Laval and it is described by G6bel (1981).
Aeration with heat recovery by a heat pump
For heat recovery with the aid of a heat pump an qrdinary, covered sunk- in-ground manure pit of 250 m 3 is furnished with a 2 kW aerator and a steel-pipe heat exchanger which is connected to a heat pump of 8.6 kW. This primary circuit is filled with glycol to avoid possible freezing in winter. The secondary circuit after the heat pump leads to a hot water
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122 A. Wellinger
store which is in turn connected to the central heating system (floor heating).
Experience has shown that at manure temperatures of 20-30°C electricity to heat ratios of 1:2.9 can be achieved with the heat pump which gets close to the maximum yields of 1:3 to 1:4. The aerator is temperature controlled and runs on average at intervals of about 15 min per hour.
Since 1 m 3 of manure has a heat potential of about 100W, enough energy can be recovered from 200 pigs to cover the heating requirement for the assumed farm dwelling, even in winter.
Only one such aeration/heat pump system is currently operating in Switzerland, and was built by LBA, Brugg two years ago. However, the heat produced is not utilized in the farmhouse but in the farrowing sheds and in parts of the fattening pig house.
Biogas production
For comparison with the aeration procedures a new and well insulated biogas plant (U-value: 0.45 W/m 2 °C) has been chosen which is properly integrated into the farm system. The digester is assumed to run at mesophilic temperatures (28-35°C). The process energy should not require more than 4 0 ~ of the gas produced as a yearly average. It is assumed to be a continuous-flow system with an average retention time of 25 days. The gas yield should be 0.27m 3 per kgVS added for cattle manure or 0.61 m 3 for pig manure (1013 mbar, 20°C) (Wellinger et al., 1981).
A comparison of the monthly net gas production with the heating requirement of an average farm shows that there is a surplus of gas in summer and a deficiency during the five winter months. An additional heating source is therefore required. In consequence a dual fuel burner (gas/oil) and a small plastic oil store were included in the calculation of the energy costs.
COMPARISON OF THE THREE SYSTEMS
The comparison of the three above described energy-yielding manure treatments with a conventional oil heating system reveals that the renewable energies still are by far more expensive than the traditional
Review comparison of anaerobic digestion with two types of aeration systems 123
heating (Table 2). With increasing independence from external energy (oil, electricity) the cost of the system and hence the yearly running cost increases as well, as already stated earlier (Kaufmann, 1982). If the energy is just produced to cover the heating requirements of the farm dwelling then biogas is about twice as expensive as oil.
One reason for these differences is the fact that most of the renewable energies are so called 'band' energies, i.e. they are produced at equal rates throughout the year. Hence at peak demand there is still a need for an additional energy source. The main explanation, however, for the differences in price is the number of units produced per unit time. Whereas thousands of oil burners are manufactured per year, only a few alternative energy systems are constructed. Hence there is a high chance that with increasing numbers the latter will become relatively cheaper as well.
Although the energy produced from the manure of 30 CUs is sufficient to cover the major part of the heating requirement of the farm dwelling, from an economical point of view this farm size is rather small. Because the installation costs which are more or less independent of the amount of manure treated account for close to half of the total investment costs, the running costs for smaller units become rather excessive when expressed per CU. However, most of the livestock in Switzerland is kept in units which are equal to or smaller than 30CUs (Kaufmann, 1980).
The comparison among the alternative systems seems to indicate that both forms of aeration produce heat at a considerably lower cost than the anaerobic digestion. However, it has to be pointed out that the figures for the biogas installation are based on 18 offers to the canton of Freiburg in Switzerland (Kaufmann, 1980) and on a follow-up of already built digesters (Oehler, 1980), whereas the aeration systems are based essentially on only one installation of each. Thus the absolute figures for the cost of the biogas production are much more significant than those for the aeration.
The yearly monetary input to the two aeration systems is about equal. The use of a heat pump makes the whole system more susceptible to technical failures. On the other hand, however, the high process temperature of the direct heat utilization is a definite disadvantage of the system. At 50°C all the ammonia volatilizes, with the result that the quality of the manure as a fertilizer is drastically lowered (see next section).
Even though energy production through biogas is the most expensive
124 A. Wellinger
of the possibilities discussed, from the point of view of political economics it still seems to be the most reasonable: it is the source of energy with the lowest external energy demand. In fact, if the overall energy balance is considered it is the only true energy producer. The heat pump recovers only two-thirds of the energy produced by heat from the manure treatment. But these two-thirds only replace the energy lost during the production and the transport of the one-third of electricity which has an efficiency factor of 0.33. Though roughly calculated the overall energy production is close to zero. Methane as a product has a much higher exergy than hot water. 'Exergy' describes the quality of energy with respect to its potential to carry out mechanical work. Methane production has also the highest energy yields with energy factors up to 40 (Kaufmann, 1980) which comes close to hydro-electricity production in a river power station which is known as the most efficient energy producer. An energy factor is defined as the amount of energy produced during the lifetime of the unit divided by the amount of energy utilized for the construction and the operation of the unit. Small heating pumps (4 kW), for example, have energy factors of not more than 8-12 (Reutimann, Soci6t6 Suisse pour l'Energie Solaire; personal communication).
MANURE AS A FERTILIZER
Independent of energy production, farmers have tried for some years to improve manure quality by addition of chemicals or by an aerobic or anaerobic treatment.
Since the two latter procedures are essentially the same as for energy production, the fertilizer quality of the treated manure also plays an important r61e in the calculation of the process costs. However, at present there are not enough data available to allow an economical assessment of fertilizer quality after a treatment. In the following section we have therefore tried to compile the few published results in order to compare the influence of the two different treatments on the chemical parameters of the fertilizer.
The aims of a manure treatment with respect to fertilization are a reduction of odour emissions during storage and application to the land, a decrease in viscosity, a stabilization and preservation of the nitrogenous compounds and, finally, improvements in the plant compatibility, the efficiency of nutrient utilization and growth yield.
Review comparison of anaerobic digestion with two types of aeration systems 125
FERTILIZER VALUE
Chemical parameters
The purpose of a manure treatment with respect to fertilization is entirely different from a treatment of a waste material for pollution control and volume reduction where as much as possible of the organic matter has to be degraded. The object of a manure improvement is to obtain a low carbon to nitrogen ratio and a reduction in the easily degradable organic substances in order to keep the manure relatively stable over a longer period of storage time. The removal of these compounds also helps to avoid nitrogen barriers for the plants because it prevents fast growth rates of the soil bacteria which would consume the nitrogen.
Hence the contents of ammonia (NH4-N), total nitrogen (Ntot) and organic substances (VS), as well as the composition of the latter, are the most important factors which determine the fertilizer value of the manure, besides the levels of potassium and phosphorus which remain more or less stable during any treatment.
A pilot-size experiment in batch-fermenters of 3.2 m 3 with cattle and pig manure showed that after a reaction time of 40 days the VS were about equally degraded (20 30 Yo) by digestion and by intermittent aeration for 7min every 28min (Besson et al., 1982a) (Table 3). The results of VS degradation in continuous-flow digesters are even better. In an inquiry over one and a half years on 10 full-size on-farm digesters (Wellinger et al. , 1981) an average degradation of cattle manure of 40Yoo was determined (Table 3). The breakdown of pig manure at approximately 18 Yo was slightly lower than in the batch experiment, probably as a result of the short retention time (17 days).
Higgins et al. (1982) showed in a comparative batch experiment with sewage sludge that after complete biological degradation not only the percentages for the VS breakdown of an aerobic and an anaerobic treatment but also the degrees of degradation of the different groups of compounds were equal (Table 4). Some care is required, however, with the absolute figures, which are probably slightly too high, since lignin is-- other than indicated--anaerobically undegradable and aerobically only to a small degree (Ginnivan et al., 1981). The pool sizes of the various groups of compounds in fresh cattle manure are fairly different from the sludge values and so are the percentages of degradation as measured in a 5 m 3 continuous-flow digester (Table 4). However, it is interesting to note
126 A. Wellinger
TABLE 3 Manure Composition of Fresh Pig and Cattle Excreta and the Relative Changes During
Aerobic and Anaerobic Treatment
Type o f excreta/treatment Unit VS N,o , NHa-N
1. Batch pilot-size plant Pig g/litre 45-7 5' 12 3.72 Aeration ~o change -20 .0 -5 .3 -4 .3 Pig g/litre 46.9 5.02 3.72 Digestion ~o change -23-7 -0 .5 +9.5 Cattle g/litre 47.7 1.83 0.91 Aeration ~o change - 20.7 - 6.9 - 20.2 Cattle g/litre 43.7 1.88 1.01 Digestion ~o change - 28.8 - 1.0 + 5.8
2. Continuous rid#size plant Pig g/litre 32.5 3' 52 2-46 Digestion* ~o change - 17.9 + 1.50 -4 .90 Cattle g/litre 64.6 2-82 1.28 Digestion** ~ change -41.9 - 14-9 +7.5
* Retention time 17 days, temperature 35 °C (mean values of 2 plants). ** Retention time 40 days, temperature 32°C (mean values of 4 plants). Data after Besson et al. (1982a) and Wellinger et al. (1981).
TABLE 4 Pool Sizes of the Major Chemical Classes in Sewage Sludge and Cattle Manure and Their
Relative Reduction During Aerobic or Anaerobic Treatment
Organic Ran' Aerobic Anaerobic Fresh Anaerobic constituent sewage reduction** reduction** cattle reduction**
sludge* manure*
VS 7.50 46.8 43-4 71.0 25.0 Hemicellulose 2.5 100 100 19.3 85.0 Cellulose 32.2 68.9 66.6 14.5 22.5 Lignin 13.6 52.8 39.1 8.2 0.0 Crude protein ND ND ND 15.6 14-4 Lipids 11.0 69.9 71.4 7.5 27.6
* In ~o of total solids. ** Relative reduction during treatment in o/. ND, Not determined. After Higgins et al. (1982) and Wellinger & Sutter (unpublished results).
Review comparison of anaerobic digestion with two types of aeration systems 127
that the succession of degradability is the same in both substrates. The comparable results of a thermophilic (55°C) aerobic treatment of pig slurry over 4 days are 57 .4~ reduction for hemicellulose, 29-7 9/0 for cellulose, none for lignin and 45.7 9/0 for crude protein (Ginnivan et al., 1981).
An important factor in the evaluation of a manure treatment is loss of nitrogen. The comparative batch experiments of Besson et al. (1982a), as compiled in Table 3, show for the methane digestion a five times smaller total nitrogen loss than for the aeration. The results of continuous-flow digesters however, demonstrate considerably higher losses at least for cattle manure such as 8-10~o for pilot plants (Wellinger & Sutter, unpublished results) or up to 15 ~o for full-scale installations (Table 3). The losses with pig manure are smaller, even though the content of nitrogen is higher. But for the aerobic treatment much higher losses have often been reported such as 5-15 ~o in pilot plant experiments (Abele, 1978) or 15 ~o and more in full-size installations (Thalmann, personal communication). In consequence, it has to be anticipated that in practice the losses with aeration are even higher than 15 ~o, since an investigation in over 80 farms (Besson et al., 1982b) has shown that the dry matter content of the manure is usually below 4 ~o, a level which by experience has proved to be the lower limit for reduced nitrogen losses. The results of a thermophilic aerobic treatment of pig manure with 4.5 ~o TS in a 15 litre continuously fed fermenter indicate an even higher loss of nitrogen. After 4 days the level of ammonia-N was reduced from 1.08mg/litre to 0.46mg/litre, i.e. by 57'4~o ( G i n n i v a n e t al., 1981).
The formation of nitrate is often used as an argument for the aerobic process. However, in all the work cited the nitrate production was always lower than 25 mg/litre, a negligible amount with respect to fertilization.
Growth yields of fertilized plants
The ultimate purpose of a manure treatment with respect to fertilization is the increase of crop yield.
For the aerobically treated manure an increased growth yield was encountered in various instances, in pot experiments as well as in field trials (Bohle & Laur, 1974; Abele, 1978) as compared to stored manure. In comparison with chemical fertilizers an increased, or at least a balanced, dry matter production was often registered. Beside the quantitative improvement there was also a qualitative amelioration in that a much
128 A. Wellinger
stronger growth of the roots was observed. In addition the fields showed a higher percentage of clover (Abele, 1978).
In order to determine the effects of the differently treated manures on prairie land, a comparative experiment is being carried out at the Swiss Federal Research Station for Agricultural Chemistry (FAC). In a five year programme the crop yields of 64 statistically distributed plots are being compared. The plots are either not fertilized at all or fertilized with stored, aerated or digested manure or with chemical fertilizer.
Further positive aspects of aeration described are the better and faster availability of the manure to the plants and a higher suitability, i.e. the manure can also be sprayed on to growing crops (head fertilization) since it scorches much less (Abele, 1978). However, both observations apply also to digested manure (Wenzlaff, 1981 and personal observations).
In summary we can say that the two methods of aerobic, or anaerobic mesophilic, manure treatment lead to similar positive results with respect to fertilization. Both of them allow a reduction in, if not even an abandonment of, the use of chemical fertilizer, which of course reduces the yearly costs as well as the general energy costs, remembering that the production of 1 kg of ammonia requires about 2 litre of oil.
ODOUR REDUCTION AND MANURE STABILITY
Odours have become a serious problem in many places with increasing animal numbers and even more with the increasing number of non- agriculturally related persons in the countryside. In an increasing number of European countries the farmer is obliged by the authorities to reduce odour emissions.
A quantitative determination of odours is very difficult, if possible at all. Most often sensory techniques are utilized, such as the 'Olfactometer' of the University of Kiel (Mannebeck, 1977). Both methods of manure treatment can lead to substantial reductions of odour emissions. Abele (1978) reported reductions for cattle manure of approximately 50 ~o with intermittent aeration. With continuous aeration of pig manure in oxidation ditches, reductions of up to 98 ~o were achieved (Thaer, 1978). During anaerobic digestion of pig manure in a full-scale continuous-flow digester odour reductions of 50~o were achieved (Wenzlaff, 1981).
Another approach to the determination of odours is the measurement of odorous compounds in the manure. According to Schaefer (1977) the main substrates responsible for objectionable odours are compounds
Review comparison of anaerobic digestion with two types of aeration systems 129
such as Volatile Fatty Acids (VFA), phenol, p-cresol, 4-ethylphenol, indole and skatole. In continuously aerated pig manure p-cresol is completely degraded and the VFAs are oxidized to a level of approximately 200ppm (Spoelstra, 1980). But also during anaerobic digestion the pool sizes of the malodorous compounds are drastically reduced. The percentage of elimination increases with increasing detention time and decreasing dry matter content in the manure (Van Velsen, 1981).
The stability of the manure after the treatment is significantly different between the two procedures. Well-aerated manure starts smelling again after two (Abele, 1978) to three (Besson, 1981) weeks. The products formed during an aerobic breakdown are not necessarily the same as those of an anaerobic degradation. Thus the oxidized intermediates are relatively quickly reduced again to malodorous compounds as soon as the redox potential allows the growth of anaerobic bacteria (Guenzi and Beard, 1981). On the other hand, if manure is well digested it remains stable over a long period of time. The odour values remain unchanged even after 120 days of storage (Wenzlaff, 1981). The concentration of malodorous compounds is even reduced during that time (Van Velsen, 1981).
A reasonable indicator for the microbial activity in the manure is the concentration o fVFA (Wellinger et al., 1981). Any change in or initiation of microbial degradation will lead to a temporary increase of VFA. An increased level during steady-state conditions is a good indication of process imbalance. The most common acids produced are acetic, propionic and n-butyric.
An investigation on three farms with methane digesters showed that at least in two cases the manure was degraded only slightly more during storage even though the temperature was relatively high during the summertime (Table 5). The relative VS content (VS/TS) remained fairly constant, whereas, in accordance with Van Velsen ( ! 98 l), the acids were slightly degraded. In the third case, the dairy cattle, the anaerobic fermentation did not run under optimal conditions, which was reflected by the high levels of VFA in the digested manure. The average fermentation temperature was only 21 °C at a RT of 33 days (Wellinger et al., 1981: installation H). It is therefore not surprising that the degradation was not completed and was continued during storage (decreasing ratio of VS/TS). The increase in acetic acid noted was most certainly due to the change in temperature.
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Review comparison of anaerobic" digestion with two types o[aeration systems 131
GENERAL PARAMETERS
An important parameter in the comparison of waste treatment systems is weed control. It was shown earlier (Vogtmann et al., 1979) that the germination ability of one of the most resistant weeds, R u m e x obtusi fol ius , is substantially reduced during an aerobic mesophilic (28 °C) treatment of cattle manure.
But anaerobic digestion also has a detrimental effect on weeds, as was shown by Besson et al. (personal communication). During digestion of pig manure the seeds of R u m e x were completely destroyed within one week, whereas in cattle manure it took about three weeks to achieve the same effect. Oat grains were deactivated in an even shorter period of time.
Another factor is the handling of the manure. Both procedures reduce the viscosity and thus allow an equal distribution of the fertilizer on the field. They also reduce the risks of plugged pipes.
A further point is hygiene. An inherent danger in the use of manure as a fertilizer is the possibility of transmission of pathogens. The aerobic, as well as the anaerobic systems, efficiently reduce the number of pathogenic bacteria and viruses. This aspect, however, is dealt with in more detail in other papers, for example in Strauch et al., 1981 and B6hm, 1983 for aerobic treatments and in Lund & Nissen, 1983 and Turner et al., 1983 for anaerobic digestion.
CONCLUSION
Summing up the positive and the negative aspects of the three different manure treatment systems, methane production turns out to be the most favorable. It not only demonstrates positive effects on fertilizer quality, weed control and odour reduction, as does also intermittent aeration at mesophilic temperatures, but it is the only true energy producer. However, the investment costs are remarkably high and hence the price of produced energy equivalents is still about double compared to a conventional oil burner.
A C K N O W L E D G E M E N T
The author would like to thank J.-M. Besson and R. Kaufmann for their valuable criticism and their careful review of the paper.
132 A. Wellinger
REFERENCES
Abele, U. (1978). Ertragssteigerung durch Fliissigmistbehandlung. KTBL, Schrift 224, 124 pp.
Besson, J.-M. (1981). Behandlung von Jauche und Grille. In: Oekologische Landwirtschaft, Proc. Grrines Forum Aipbach 1980, Wagner'sche Universitfitsbuchhandlung, Innsbruck, pp. 87-120.
Besson, J.-M., Lehmann, V., Roulet, M. & Edeimann, W. (1982a). Vergleich dreier Grillebehandlungsmethoden unter kontrollierten Versuchsbedin- gungen: Lagerung, Beliiftung und Methangfirung. Mitt. Schweiz. Landw., 7/82, 161-8.
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