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Review of the use of aerobic thermophilic bioprocesses for the treatment of swine waste
Livestock Science 102 (2006) 187 – 196 www.elsevier.com/locate/livsci
Review article
Review of the use of aerobic thermophilic bioprocesses for the treatment of swine wasteB Pierre Juteau * INRS-Institut Armand-Frappier, Universite´ du Que´bec, 531 boulevard des Prairies, Laval, Que´bec, Canada H7V 1B7
Abstract For 40 years, aerobic thermophilic (AT) processes have been presented as a possible way of treating liquid manure, especially swine wastes. If they are well designed, bioreactors can be self-heating and a temperature up to 75 8C (but preferably in the 55–65 8C range) can be reached. Claimed benefits are the efficient destruction of pathogens, the simplicity of the process, its robustness, a higher reaction rate (and consequently smaller bioreactors), the conservation of nitrogen and the possibility of heat recovery. However, there are very few examples of implementation of AT technologies for this particular application. The aim of this review is to present the knowledge that has been acquired over the last 40 years through experimentations of AT processes with livestock wastes. D 2006 Elsevier B.V. All rights reserved. Keywords: Aerobic thermophilic treatment; ATAD; Animal waste; Environment; Nitrogen fate; Phosphorus fate
1. Introduction The autothermal thermophilic aerobic digestion (ATAD) is one of the well-recognized technologies for the treatment of sludge produced by municipal wastewater treatment plants. In this type of bioreactor, the temperature rises over 50 8C due to the conservation of a part of the heat produced by the aerobic metabolism of the microorganisms that consume the abundant organic material present in the sludge. The B This paper is part of the special issue entitled bBiosecurity of Livestock EffluentsQ, Guest Edited by Dr. Jose´ Martinez and Dr. Franc¸ois Madec. * Tel.: +1 450 687 5010; fax: +1 450 686 5501. E-mail address: [email protected].
1871-1413/$ - see front matter D 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.livsci.2006.03.016
main benefit of ATAD is its efficiency to kill pathogenic organisms. However, compared to other processes, the number of full scale installations is low: in 2000, it was estimated that 40 wastewater treatment plants were using ATAD in Europe, 25 in the United States and 10 in Canada (Metcalf & Eddy inc. et al., 2003). The first process of this type was developed in Germany in the 1960s (Fuchs, 1973). In the 1990s, ATAD has been recognized in the United States as a process capable of fulfilling class A pathogen requirements in order to produce a biosolid that can be used as soil amendment with few restrictions (USEPA, 1990, 1992). Since its first steps of development, aerobic thermophilic digestion has been proposed as a process that could be used for the treatment of livestock wastes
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that are in liquid form. This applies mainly to pig manure but also, in certain cases, to cattle manure. In addition to the pathogen-killing effect, claimed benefits were the simplicity of the process, its robustness, a higher reaction rate (and consequently smaller bioreactors), the conservation of nitrogen and the possibility of heat recovery. Nevertheless, full-scale aerobic thermophilic plants for liquid manure are scarce and most of them have been set for experimental purposes. Examples can be found in Germany (Po¨pel and Ohnmacht, 1972; Martens et al., 1998), United Kingdom (Burton and Turner, 2003), Scandinavia (Skjelhaugen, 1999), Czechoslovakia (Hojovec, 1990), United States (Terwilleger and Crauer, 1975) and Canada (Pilon, 1984). Unfortunately, the available information about these systems is limited and not always easily accessible since most of the data have been presented in conferences or in non-English reports (many in German, some in Norwegian and at least one in French). More peer-reviewed papers presenting laboratory studies have been published, but their numbers are still relatively low compared to other technologies. Although it has not often been used, the interest for the aerobic thermophilic treatment of livestock wastes still exists today. It is especially true in regard to the increasing concern about the possibility of pathogen dissemination by the use of non-hygienised manure. Consequently, the aim of this review is to present a summary of the knowledge that has been gathered over the 40 years of experimentation with this technology. We focus specifically on livestock waste treatment with some references to other use of aerobic thermophilic processes in order to place particular problems in a more general perspective. A reader interested in the application of aerobic thermophilic processes to other types of wastewater should consult the excellent review of LaPara and Alleman (1999). A detailed analysis of ATAD processes for municipal sludge has also been made by the US Environmental Protection Agency (USEPA, 1990).
2. Nomenclature Different names have been used to designate the aerobic thermophilic treatment of organic wastes that content enough water in order to be pumped (from
high-strength wastewater to sludge). This includes autothermal thermophilic aerobic digestion (ATAD) or treatment (ATAT), thermophilic aerobic digestion (TAD), aerobic thermophilic stabilization (ATS), aerobic thermophilic sequencing batch reactor (ATRBS), aerobic thermophilic treatment (A-T treatment), aerobic thermophilic processing, Fuchs process, thermophilic aeration, hot fermentation, LICOMk system, liquid composting and wet composting. It is important to note that the two latter terms are sometime used for naming systems that are not thermophilic. In this review, for simplicity and in order to include all the possible variations of the technology, we will use the term aerobic thermophilic (AT) treatment. It is also important to note that the current review does not cover solid-phase composting processes. Obviously, bthermophilicQ means bhotQ. However, behind that, the definition is not so clear. The term comes from a classification of microorganisms as psychro-, meso- and thermophiles, which are low, medium and high-temperature loving organisms, but the limits between each group vary somewhat in different textbooks. For this review, we consider a process as thermophilic if it reaches at least 50 8C.
3. Heat production One of the first concerns in the development of AT processes was to determine if complete autoheating can be achieved, and in which conditions. An early American study based on mathematical simulations supported this possibility (Kambhu and Andrews, 1969). The experimental confirmation had already been done in Germany in the 1960s but results were presented in English only in the early 1970s (Po¨pel and Ohnmacht, 1972). Since then, self-heating up to the thermophilic range has been shown for different organic wastes, including liquid manure, even in cold climates (Terwilleger and Crauer, 1975; Pilon, 1984; Hojovec, 1990; Martens et al., 1998; Skjelhaugen, 1999; Oechsner and Doll, 2000; Burton and Turner, 2003; Juteau et al., 2004; Heinonen-Tanski et al., 2005). A minimal quantity of organic material is needed to sustain self-heating, but an absolute value is difficult to establish. Excluding any heat loss, for a 40 8C increase (heating from 20 to 60 8C), a theoretical minimum of 24 g COD/l can be calculated using
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a specific heat of 4.2 kJ/kg 8C, a heat production of 20,000 kJ/kg of volatile solids destroyed (Metcalf & Eddy inc. et al., 2003), an oxygen consumption of 1.42 kg O2/kg organic matter oxidised (Henze et al., 2002), a COD/BOD ratio of 2 and a specific weight of 1 kg/l of slurry. However, as we will discuss below, heat losses are important and depend on reactor design. Nevertheless, the experience has clearly shown that the way in which the livestock production facilities are operated in industrial countries generates wastes that are concentrated enough for self-heating AT processes. Self-heating is dependent on energy conservation and one of the most important loss is related to the aeration (Kambhu and Andrews, 1969; LaPara and Alleman, 1999). This is mainly due to water evaporation and, to a lesser extent, to heat extraction by the air. Consequently, it is important to choose a highly efficient aerator in order to minimize the quantity of air that has to be injected in the bioreactor. A minimum oxygen transfer efficiency (OTE) around 10% to 15% has been suggested for sludge digestion (Jewell and Kabrick, 1980). These are values that are achievable by some aerators used for wastewater treatment. Initially, some authors pointed out that thermophilic conditions could result in lower oxygen transfer rates due to the decrease of oxygen solubility at high temperature (Graczyk and Kolaczkowski, 1980). Further studies showed that this phenomenon is completely compensated by a concomitant increase of the K La, which is the overall oxygen transfer coefficient (Aiba et al., 1984; Boogerd et al., 1990; Vogelaar et al., 2000). Actually, the combination of a thermophilic temperature and a high organic load (sludge, manure) seems to increase the oxygen transfer rate since OTE between 70% and 100% have been reported by different authors (USEPA, 1990; Skjelhaugen, 1999). If an aerator is not efficient enough, oxygen transfer can be improved by recycling the air (Norcross and Yanlong, 1996; Blackburn, 2001, 2002). The maximum temperature that can be reached by self-heating AT processes is around 75 8C. This is due to a large reduction of the microbial activity at this temperature (Fig. 2). Optimal operating temperature is also well below that, as we will discuss below. Consequently, in some cases, there is a need to limit the temperature. This can be done by controlling the
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volume of fresh air injected in the bioreactor (Breider and Drnevich, 1981; Norcross and Yanlong, 1996) or by adjusting the amount of shear generated through a jet aeration device (Pressley, 1999). Another possibility is to extract the extra heat by a heat exchanger or a heat pump and use this calorific energy for other purposes, like the production of hot water (Evans et al., 1982; Baines et al., 1986; Svoboda and Fallowfield, 1989; Svoboda and Jones, 1999). This has been recently proposed for a farm scale implementation of an AT process for pig manure (Blackburn, 2001, 2002). This would permit the recovery of 3 to 5 kW of energy per 100 hogs as hot water at 55 8C (Blackburn, 2000).
4. Fate of pathogens AT processes are clearly superior to mesophilic and psychrophilic treatments in order to eliminate pathogens. Reviews on this topic are available (USEPA, 1990; Bicudo and Goyal, 2003; Mohaibes and Heinonen-Tanski, 2004). The pasteurization effect of AT processes is obviously due to the temperature but also to the pH, which normally increases up to 9.0–9.3 during treatment. Pathogens that have been reported to be quickly destroyed in this type of treatment include bacteria (coliforms, Salmonella, Campylobacter, Clostridium perfringens), viruses (Aujesky’s Disease-, Swine Vascular Disease-, Foot and Mouth Disease-, Enteric Cytopathogenic Bovine Orphan-, Equine Rhino-, Pseudoragibies-, reo-, adeno- and entero-virus), parasites (Ascaris and Taenia eggs) and protozoa (Cryptosporidium). Fecal streptococci and Bovine Parvovirus were reduced in a full-scale AT process operated at 55 8C but they were not completely eliminated (Martens et al., 1998). For an efficient pasteurization effect, the targeted pathogens must be subjected to an elevated temperature during a minimum of time. The hydraulic regime has an impact on this parameter. In a continuously fed stir tank, not all the liquid stays inside the vessel for the same period of time. This concept is known as bResidence time distributionQ (RTD) and it means that some pathogens will escape the vessel sooner than the mean hydraulic residence time (HRT), which is equal to the theoretical HRT (volume of reactor/volumetric flow rate) for an ideal
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completely mix reactor. Continuous feeding also opens the door to different phenomena (short-circuiting, formation of dead zones, etc.) that will affect the RTD and even reduce the performance of the treatment. Consequently, many authors have stated that batch processes give a better control on the pasteurization than continuous feeding (Burton, 1992; Martens et al., 1998). This has been illustrated by Oechsner and Ruprich (1989) who reported a reduction of faecal streptococci from 105/g to 100/g in batch mode whereas, in continuous mode, counts in the effluent were from 104 to 105/g. The use of a plugflow design instead of a completely mix tank could be another way of insuring a minimum of time for pasteurization but this reactor type does not seem to have been tried for AT bioprocesses.
5. Fate of carbon An argument that is often presented to support the use of AT processes is a faster specific transformation rate of organic material at high temperature, which should mean smaller bioreactors (Kambhu and Andrews, 1969; Yi et al., 2003). However, experiments have not always supported this hypothesis. For example, Couillard and Zhu (1993) reported a higher maximum specific utilization rate of substrate (slaughterhouse effluent) at 58 8C whereas LaPara et al. (2000) found no significant difference between 25 and 65 8C with a synthetic wastewater. With pig manure, Hissett et al. (1982) made respirometric measurements using bacterial consortia adapted to temperatures between 5 and 50 8C. They reported the results as an Arrhenius plot that showed a typical straight line with a positive slope between 5 and 40 8C; however, after that, the slope become negative and an optimum appears around 45 8C (Fig. 1). Another important aspect is the reduction level of organic material (expressed as biological oxygen demand (BOD) or chemical oxygen demand (COD)) that can be achieved. For this parameter, many authors have reported that, in the mesophilic–thermophilic range, the performance falls as the temperature rise. This means that the residual BOD or COD is higher for an AT process than for a treatment at lower temperature. This is true for livestock wastes (Graczyk and Kolaczkowski, 1980; Evans et al., 1983,
Fig. 1. Arrhenius plot showing the effect of temperature on the respiration rate. Measurements were made with a manometric (D, o) or continuous flow (E, .) respirometer, and with the whole piggery slurry (o, .) or its supernatant fraction (D, E). Reprinted from Hissett et al. (1982), with permission from Elsevier.
1986; Juteau et al., 2004) as well as for other substrates (Tripathi and Grant Allen, 1999; Suvilampi et al., 2005). This is attributed to a possible decrease of the microbial diversity (Evans et al., 1983). This suggests that the best of the two worlds can be obtained by splitting the treatment in two steps, the first one being thermophilic and the second mesophilic. This approach does not seem to have been investigated for livestock waste. However, its efficiency has been shown with other types of high strength wastewater (Suvilampi et al., 2003, 2005; Quesnel and Nakhla, 2005a,b), even though it is not often used. It is important to note that this type of design is exactly the opposite of many AT processes for municipal sludge digestion (btwo-stage ATADQ) since they involved a mesophilic reactor followed by a thermophilic one (USEPA, 1990). On the other hand, the combination of AT pre-treatment with anaerobic mesophilic digestion of municipal sludge is common (USEPA, 1990; Metcalf & Eddy inc. et al., 2003). When tested with pig waste, a dual system of this type showed a better reduction of COD, solids and coliforms than with a single anaerobic bioreactor (Pagilla et al., 2000). Also, more biogas was produced with less H2S, and the sludge showed a better dewaterability.
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In an AT batch process, the oxygen uptake rate (OUR, mg/l min) shows important variations (Fig. 2). It is very intense at the beginning when the most easily biodegradable material is consumed. Thereafter, it slows down. This is a disadvantage from an economical perspective since the aeration equipment, if designed to fulfill the oxygen demand during the intense OUR period, will be oversized for the rest of the time. For that reason, authors have concluded that a continuous AT process is more economically sound
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than an AT batch process. However, as stated above, this is conflicting with the objective of pasteurization for which a batch process is better suited. Aerobic processes are capable of greatly reducing the offensive odour that is associated with livestock wastes. For this aspect, on a short-term basis, lowtemperature treatments and AT processes perform equally (Evans and Thacker, 1985; Evans et al., 1986). However, as mentioned above, the residual BOD is higher in an AT process. Consequently, there is a potential for odour regeneration due to fermentation during long-term storage. Yet this is not what has been reported by Skjelhaugen (1999) who found no odour regeneration over a 10-month storage period of a cattle slurry that had been treated in an AT bioreactor (43–50 8C). The authors did not mention the residual BOD but they stated that the process was designed to only moderately reduce the organic matter. Finally, a parameter that is interesting but that have few, if any, practical significance is the colour. In AT processes, pig manure passes from grey-green to a dark-brown (Graczyk and Kolaczkowski, 1980; Pilon, 1984; Juteau et al., 2004). A higher temperature gives a more intense colour (Juteau et al., 2004). This appears similar to the humus-making process that takes place during solid-phase composting.
6. Fate of nitrogen
Fig. 2. Evolution of the temperature and the microbial activity in self-heating sequencing batch bioreactors treating pig wastes. When no temperature control is used (A), it quickly increases over 70 8C. This is due to the intense microbial activity, as it can be seen by the evolution of the oxygen uptake rate (OUR). However, when the temperature reaches 74–75 8C, the OUR drops, which causes a temperature decrease. After a while, the microbial activity resumes and the temperature increases again up to the inhibitory level. When the temperature is limited to 70 8C (B), 60 8C (C) or 50 8C (D), there are no such oscillations. Symbols: n, OUR; thin line, temperature; bold line, dissolved oxygen (DO). Reprinted from Juteau et al. (2004), with permission from Elsevier.
One characteristic revealed in the early investigations of AT processes is the absence of nitrification (NH4+YNO3 ) over 40 8C (Terwilleger and Crauer, 1975; USEPA, 1990; Blackburn, 2000). This is an advantage if the objective is to conserve the nitrogen since no loss can happen due to nitrification– denitrification (NH4+YNO3 YN2) activities. Consequently, the effluent of an AT bioreactor contains two main forms of nitrogen, ammonia and organic nitrogen. In addition, an inorganic precipitate (struvite, MgNH4PO4d 6H2O) should be present. The conclusion about the absence of nitrification is based on the fact that neither nitrate nor nitrite is normally detected in thermophilic conditions. This means that ammonification (organic NYNH4+) would be the only biological transformation of nitrogen in an AT process. This view has been recently challenged by Korean researchers who reported on two
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occasions the presence of nitrous oxide (N2O) in the gas phase of a laboratory AT bioreactor treating swine waste (Yi et al., 2003; Lee et al., 2004). Increase of N2 has also been noted, but nitrite and nitrate were absent from the effluent. Nitrous oxide is a well-known intermediate of the denitrification process and N2 is its final product, but denitrification occurs normally in anoxic conditions whereas the results come from a bioreactor in which the dissolved oxygen was 2–3 mg/l. Some autotrophic nitrifiers are also capable of aerobic denitrification during which they can produce N2O from nitrite under oxygenlimiting conditions, but it is generally considered as a marginal process (Colliver and Stephenson, 2000). Furthermore, the production of N2O by autotrophic nitrifiers during nitrification (in contrast with nitrifiers making N2O from aerobic denitrification) has been proposed based on experiments with pig slurry (Burton et al., 1993; Pahl et al., 2001). Another hypothesis suggested by the authors is a direct aerobic deammonification (NH4+YN2) that could be carried out by aerobic heterotrophic denitrifiers (Patureau et al., 2000). However, they investigated the microbial community of their reactor using a molecular approach (16S rRNA gene library) and found neither autotrophic nitrifiers nor aerobic heterotrophic denitrifiers (Lee et al., 2004). Instead, based on the presence of Bacillus and Alcaligenes species and since nitrification by a thermophilic Bacillus-like strain has already been reported (Me´vel and Prieur, 2000), they proposed that ammonia could be nitrified by heterotrophic nitrifiers and then quickly denitrified. More work has to be done on this aspect in order to complete our understanding of the nitrogen fate in AT conditions. What is clear is the fact that an important part of the nitrogen is in the form of ammonia. In a laboratory study on the AT treatment of pig manure, Beaudet et al. (1990) showed that the ammonia can be completely volatilised due to the high temperature and the alkaline pH. Generally, AT processes are not designed to catch the ammonia from gaseous effluent (USEPA, 1990). This could be acceptable for municipal sludge but not for liquid manure in which the ammonia concentration is very high. Without such a control, ammonia volatilisation was found to cause very bad smells around a full-scale AT reactor treating swine waste (Pilon, 1984). This also represents a source of
atmospheric pollution. These problems can be avoided by recuperating ammonia vapours with a scrubber that operates with an acid solution. In this way, nitrogen is conserved but a part of it takes the form of a concentrated ammonium salt solution. This has been experimented at the full-scale AT treatment plant of the Federal Agricultural Research Centre in Germany (FAL process). Total nitrogen reduction by stripping was up to 30%, which was below the expectations (Burton and Turner, 2003). On the other hand, a two-stage air scrubber allowed an efficient recuperation of the volatilised ammonia, producing an ammonium sulphate solution (nitrogen content up to 65 g/kg). In its patent application, Blackburn (2002) has also planned to use such an air scrubber. Also, ammonia emission can be minimized without the use of a scrubber. Alfa-Laval has developed a small AT bioreactor, well suited for on-farm operation, in which nitrogen loss by stripping is highly reduced (Skjelhaugen, 1999). Low volatilisation (b1%) is due to a condenser that catches water vapour and ammonia from the exhaust air. A low air flow rate (0.5 m3/m3 h), being made possible by the use of a highly efficient aerator (OTE N 90%), also contributes to the minimization of ammonia stripping. A small peat filter placed on top of the reactor adsorbs the small amount of ammonia that passes through the condenser. Also, some nitrification takes place in the peat bed. Oechsner and Doll (2000) reported similar low values for ammonia stripping in a two-stage AT process that used an air flow rate of 0.4 m3/m3 h. Clearly, the design of the reactor has a strong impact on ammonia volatilisation, especially the choice of the aerator. In this way, an objective of ammonia stripping, which can be achieved by intense aeration, could become conflicting with the objective of heat conservation, which calls for low aeration with highly efficient aerators.
7. Fate of phosphorus Swine waste is very rich in phosphorus and, in some regions, this element can represent a greater problem than nitrogen. Plants need much less phosphorus than nitrogen, and soils do not lose phosphorus in any gaseous form like the way a part of the nitrogen is removed by denitrification and
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ammonia volatilisation. Consequently, fulfilling the nitrogen needs of crop with swine wastes could generate an overfertilisation in term of phosphorus. If the soil is already saturated with phosphorus, runoff can drive this nutrient into rivers in which it will cause eutrophication. In the province of Que´bec (Canada), new rules on the use of manure as fertilizer are based on phosphorus and producers need to know the removal efficiency of any treatment technology for this element. Phosphorus appears in both soluble and particulate forms in manure. Consequently phosphorus can be reduced in the bulk volume by solid separation. To further decrease the phosphorus content of the liquid phase, a process will have to favour in some way the precipitation of soluble phosphate before solid separation. This can be done by chemical coagulation using additive like ferrous salts, alum, lime, etc. Biological processes have also an impact on phosphorus. A part of the soluble phosphate is incorporated into the cellular material of microorganisms. The phosphorus content of bacteria is however low, around 1.5% to 2.0% on a dry weight basis (Metcalf & Eddy inc. et al., 2003). Better removal efficiency can be obtained by encouraging growth of Phosphorus accumulating organisms (PAO), which contain up to 30% of phosphorus. This is accomplished by enhanced biological phosphorus removal (EBPR) systems in which the biomass is exposed to repetitive anaerobic and aerobic conditions. EBPR processes have been tested with piggery wastewater at room temperature (Bortone et al., 1992; Ra et al., 2000). However, there is no report on EBPR bioreactors operated in thermophilic conditions. Actually, tests conducted between 20 and 35.5 8C showed that PAOs do not seem to be capable of adapting to high temperatures (Panswad et al., 2003). A simple way of precipitating phosphorus is through aeration. Even a low-level aeration of swine waste causes the formation of insoluble hydroxyapatite and struvite (Suzuki et al., 2002; Ndegwa et al., 2003). This crystallisation seems to be due to the increase of pH induced by aeration. This rise of pH is explained by the stripping of CO2 and, to a lesser extent, to the production of ammonia during the degradation of organic matter (Stevens and Cornforth, 1974). Consequently, any aerobic biological treatment
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of swine waste in which the pH can become alkaline will cause phosphorus precipitation. The phenomenon is temperature dependent in both psychrophilic (Ndegwa et al., 2003) and thermophilic (Juteau et al., 2004) range. This means that AT processes would be more efficient for phosphorus precipitation. In a laboratory test, centrifugation at 4000g decreased the total phosphorus of an AT bioreactor effluent by 85% compared to 77% for the untreated swine waste (Juteau et al., 2004). Effluent centrifugation was also used at the FAL treatment plant, which gave an overall phosphorus removal of 79% (Burton and Turner, 2003).
8. Foam formation A technical parameter that should be mentioned is the problem of foaming. Production of foam occurs in all treatments in which liquid manure is aerated. In AT treatment of municipal sludge, foam is considered beneficial to the process since it provides a thermal isolation (USEPA, 1990). Bioreactors for this application are sized to accommodate a 0.5- to 1.0-m depth foam layer and mechanical foam cutters seem to be efficient to control it. However critical problems with foam have often been reported for AT processes treating liquid manure (Pilon, 1984; Skjelhaugen, 1999; Heinonen-Tanski et al., 2005). It is generally presented as a detail but people who have worked with this technology know that the intense foaming represents a serious obstacle to its implementation. Even if there is no design criterion for this aspect, it is important not to underestimate the need for foam abatement. Different devices for foam control in AT bioreactors are available or have been proposed like mechanical cutters (USEPA, 1990), liquid sprayers (Stover, 1998) and collecting pipes connected to a Venturi injector (Pressley and Williamson, 2001, 2003). Nevertheless, there is a need for scientific investigations (and publications) on that topic.
9. Conclusion AT processes are valuable options for the treatment of liquid manure but they have been clearly under-
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used. Among their specific advantages, we can highlight the following: ! The quality of the organic fertilizer that is produced. The whole effluent or its separated liquid and solid constituents have characteristics similar to those of compost, especially the near complete absence of pathogens. This is a big advantage since farmers, environmentalists and the public in general consider that compost is a noble product for soil amendment, which is not necessarily the case for other treatment residues that are suspiciously referred as biosolids. ! The conservation of nitrogen. It is generally believed that there is no nitrification in AT processes. If it is true (this has been recently challenged), it means that almost all the nitrogen can be conserved. It can be kept into the slurry by minimizing the volatilisation of ammonia or it can be partially separated by maximizing this volatilisation process and by using a scrubber to produce a concentrated ammonium salt solution. This latter can then be sold as a mineral fertilizer. ! The robustness of the process. The thermophilic biomass takes place rapidly in an AT process and there is no particular difficulty for its maintenance. This is in contrast with coupled nitrification/ denitrification processes, in which nitrifiers are slow growers and are sensitive to inhibition, or with methanogenic fermentation, which relies on fragile syntrophic relations between different types of bacteria. However, if the concept of AT treatment of liquid manure is clearly validated, experiences with this technology appear relatively scarce and there is room for process improvement. Also, even though a respectable number of pilot and full-scale experiments have been conducted, the available information about them is often limited to conference proceedings and hard-to-find reports. Consequently, there is a need to better communicate these results in scientific journals.
Acknowledgements This review has been made as part of a research project supported by a grant (STPGP 269701) from
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P. Juteau / Livestock Science 102 (2006) 187–196 Fifth International Symposium on Agricultural Wastes. Chicago, pp. 454–460. Evans, M.R., Svoboda, I.F., Baines, S., 1982. Heat from aerobic treatment of piggery slurry. J. Agric. Eng. Res. 27, 45 – 50. Evans, M.R., Deans, E.A., Hissett, R., Smith, M.P.W., Svoboda, I.F., Thacker, F.E., 1983. The effect of temperature and residence time on aerobic treatment of piggery slurry— degradation of carbonaceous compounds. Agric. Wastes 5, 25 – 36. Evans, M.R., Deans, E.A., Smith, M.P.W., Svoboda, I.F., Thacker, F.E., 1986. Aeration and control of slurry odours by heterotrophs. Agric. Wastes 15, 187 – 204. Fuchs, H., 1973. Biological decomposition of organic material. United States patent 3,745,113. Graczyk, M., Kolaczkowski, S.T., 1980. Aerobic thermophilic stabilization of hog manure. Proc. Livestock Waste: A Renewable Resource: 4th International Symposium on Livestock Wastes. Amarillo, TX, pp. 342–345. Heinonen-Tanski, H., Kiuru, T., Ruuskanen, J., Korhonen, K., Koivunen, J., Ruokoja¨rvi, A., 2005. Thermophilic aeration of cattle slurry with whey and/or jam wastes. Bioresour. Technol. 96, 247 – 252. Henze, M., Harremoe¨s, P., la Cour Jansen, J., Arvin, E., 2002. Wastewater Treatment: Biological and Chemical Processes. Springer, Berlin. Hissett, R., Deans, E.A., Evans, M.R., 1982. Oxygen consumption during batch aeration of piggery slurry at temperatures between 5 and 50 8C. Agric. Wastes 4, 477 – 487. Hojovec, J., 1990. Possibilities of hygienizing animal manure under thermophilic aerobic stabilization. In: Hall, J.E. (Ed.), Proc. Recent Developments in Animal Waste Utilization: Consultation of the European Cooperative Research Network on Animal Waste Utilization. Bologna, Italy, pp. 12–17. Jewell, W.J., Kabrick, R.M., 1980. Autoheated aerobic thermophilic digestion with aeration. J. WPCF 52, 512 – 523. Juteau, P., Tremblay, D., Ould-Moulaye, C.B., Bisaillon, J.G., Beaudet, R., 2004. Swine waste treatment by selfheating aerobic thermophilic bioreactors. Water Res. 38, 539 – 546. Kambhu, K., Andrews, J.F., 1969. Aerobic thermophilic process for the biological treatment of wastes—simulation studies. J. WPCF 41, R127 – R141. LaPara, T.M., Alleman, J.E., 1999. Thermophilic aerobic biological wastewater treatment. Water Res. 33, 895 – 908. LaPara, T.M., Konopka, A., Nakatsu, C.H., Alleman, J.E., 2000. Effects of elevated temperature on bacterial community structure and function in bioreactors treating a synthetic wastewater. J. Ind. Microbiol. Biotechnol. 24, 140 – 145. Lee, J.W., Lee, H.W., Kim, S.W., Lee, S.Y., Park, Y.K., Han, J.H., Choi, S.I., Yi, Y.S., Yun, Z., 2004. Nitrogen removal characteristics analyzed with gas and microbial community in thermophilic aerobic digestion for piggery waste treatment. Water Sci. Technol. 49, 349 – 357. Martens, W., Fink, A., Philip, W., Weber, W., Winter, D., Bo¨hm, R., 1998. Inactivation of viral and bacterial pathogens in large scale slurry treatment plants. In: Martinez, J., Maudet, M.-N. (Eds.), Proc. RAMIRAN 98: 8th International conference on manage-
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ment strategies for organic waste use in agriculture. Rennes, France, pp. 529–539. Metcalf & Eddy Inc., Tchobanoglous, G., Burton, F.L., Stensel, H.D., 2003. Wastewater Engineering: Treatment and Reuse. McGraw-Hill, Boston. Me´vel, G., Prieur, D., 2000. Heterotrophic nitrification by a thermophilic Bacillus species as influenced by different culture conditions. Can. J. Microbiol. 46, 465 – 473. Mohaibes, M., Heinonen-Tanski, H., 2004. Aerobic thermophilic treatment of farm slurry and food wastes. Bioresour. Technol. 95, 245 – 254. Ndegwa, P.M., Zhu, J., Luo, A.C., 2003. Influence of temperature and time on phosphorus removal in swine manure during batch aeration. J. Environ. Sci. Health, Part B, Pestic. Food Contam. Agric. Wastes 38, 73 – 87. Norcross, K.L., Yanlong, L., 1996. Thermophilic aerobic waste treatment process. United States patent 5,587,081. Oechsner, H., Doll, L., 2000. Inactivation of pathogens by using the aerobic-thermophilic stabilization process. Proc. Animal, agricultural and food processing wastes: Eighth International Symposium on Animal, Agricultural and Food Processing Waste. Des Moines, IA, pp. 522–528. Oechsner, H., Ruprich, W., 1989. Aero-thermophile behandlung von Flu¨ssigmist (Thermophilic aerobic treatment of slurry). Landtechnik 44, 328 – 330. Pagilla, K.R., Kim, H., Cheunbarn, T., 2000. Aerobic thermophilic and anaerobic mesophilic treatment of swine waste. Water Res. 34, 2747 – 2753. Pahl, O., Burton, C.H., Dunn, W., Biddlestone, A.J., 2001. The source and abatement of nitrous oxide emissions produced from the aerobic treatment of pig slurry to remove surplus nitrogen. Environ. Technol. 22, 941 – 950. Panswad, T., Doungchai, A., Anotai, J., 2003. Temperature effect on microbial community of enhanced biological phosphorus removal system. Water Res. 37, 409 – 415. Patureau, D., Zumstein, E., Delgenes, J.P., Moletta, R., 2000. Aerobic denitrifiers isolated from diverse natural and managed ecosystems. Microb. Ecol. 39, 145 – 152. Pilon, A., 1984. Proce´de´ Fuchs (Fuchs process). Ministry of environment, Que´bec, QC. Report RD–85–16, Envirodoc 850680. Po¨pel, F., Ohnmacht, C., 1972. Thermophilic bacterial oxidation of highly concentrated substrates. Water Res. 6, 807 – 815. Pressley, R.L., 1999. Process for treating biosolids from wastewater treatment. United State patent 5,948,261. Pressley, R.L., Williamson, J.D., 2001. Process for controlling foam in a treatment reactor. United States patent 6,168,717. Pressley, R.L., Williamson, J.D., 2003. Process for controlling foam in a treatment reactor. United States patent 6,514,411. Quesnel, D., Nakhla, G., 2005a. Utilization of an activated sludge for the improvement of an existing thermophilic wastewater treatment system. J. Environ. Eng.-ASCE 131, 570 – 578. Quesnel, D., Nakhla, G., 2005b. Characterization and treatability of aerobic bacterial thermophilically treated wastewater by a conventional activated sludge and granular activated carbon. Water Res. 39, 677 – 687.
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Ra, C.S., Lo, K.V., Shin, J.S., Oh, J.S., Hong, B.J., 2000. Biological nutrient removal with an internal organic carbon source in piggery wastewater treatment. Water Res. 34, 965 – 973. Skjelhaugen, O.J., 1999. Thermophilic aerobic reactor for processing organic liquid wastes. Water Res. 33, 1593 – 1602. Stevens, R.J., Cornforth, I.S., 1974. The effect of aeration on the gases produced by slurry during storage. J. Sci. Food Agric. 25, 1249 – 1261. Stover, E.L., 1998. Biochemically enhanced thermophilic treatment process. United States patent 6,036,862. Suvilampi, J., Lehtomaki, A., Rintala, J., 2003. Comparison of laboratory-scale thermophilic biofilm and activated sludge processes integrated with a mesophilic activated sludge process. Bioresour. Technol. 88, 207 – 214. Suvilampi, J., Lehtomaki, A., Rintala, J., 2005. Comparative study of laboratory-scale thermophilic and mesophilic activated sludge processes. Water Res. 39, 741 – 750. Suzuki, K., Tanaka, Y., Osada, T., Waki, M., 2002. Removal of phosphate, magnesium and calcium from swine wastewater through crystallization enhanced by aeration. Water Res. 36, 2991 – 2998. Svoboda, I.F., Fallowfield, H.J., 1989. An aerobic piggery slurry treatment system with integrated heat-recovery and high-rate algal ponds. Water Sci. Technol. 21, 277 – 287.
Svoboda, I.F., Jones, A., 1999. Waste management for hog farms— review. Asian-Australas. J. Anim. Sci. 12, 295 – 304. Terwilleger, A.R., Crauer, L.S., 1975. Liquid composting applied to agricultural wastes. Proc. Managing livestock wastes: Third International Symposium on Livestock Wastes. St-Joseph, MI, pp. 501–505. Tripathi, C.S., Grant Allen, D., 1999. Comparison of mesophilic and thermophilic aerobic biological treatment in sequencing batch reactors treating bleached kraft pulp mill effluent. Water Res. 33, 836 – 846. USEPA, 1990. Autothermal thermophilic aerobic digestion of municipal wastewater sludge. Environmental Protection Agency, Washington, DC. Report EPA/625/10-90/007. USEPA, 1992. Control of pathogens and vector attraction in sewage treatment. Environmental Protection Agency, Washington, D.C. Report EPA/625/R-92/013. Vogelaar, J.C.T., Klapwijk, A., VanLier, J.B., Rulkens, W.H., 2000. Temperature effects on the oxygen transfer rate between 20 and 55 8C. Water Res. 34, 1037 – 1041. Yi, Y.S., Kim, S., An, S., Choi, S.I., Choi, E., Yun, Z., 2003. Gas analysis reveals novel aerobic deammonification in thermophilic aerobic digestion. Water Sci. Technol. 47, 131 – 138.
Review article
Review of the use of aerobic thermophilic bioprocesses for the treatment of swine wasteB Pierre Juteau * INRS-Institut Armand-Frappier, Universite´ du Que´bec, 531 boulevard des Prairies, Laval, Que´bec, Canada H7V 1B7
Abstract For 40 years, aerobic thermophilic (AT) processes have been presented as a possible way of treating liquid manure, especially swine wastes. If they are well designed, bioreactors can be self-heating and a temperature up to 75 8C (but preferably in the 55–65 8C range) can be reached. Claimed benefits are the efficient destruction of pathogens, the simplicity of the process, its robustness, a higher reaction rate (and consequently smaller bioreactors), the conservation of nitrogen and the possibility of heat recovery. However, there are very few examples of implementation of AT technologies for this particular application. The aim of this review is to present the knowledge that has been acquired over the last 40 years through experimentations of AT processes with livestock wastes. D 2006 Elsevier B.V. All rights reserved. Keywords: Aerobic thermophilic treatment; ATAD; Animal waste; Environment; Nitrogen fate; Phosphorus fate
1. Introduction The autothermal thermophilic aerobic digestion (ATAD) is one of the well-recognized technologies for the treatment of sludge produced by municipal wastewater treatment plants. In this type of bioreactor, the temperature rises over 50 8C due to the conservation of a part of the heat produced by the aerobic metabolism of the microorganisms that consume the abundant organic material present in the sludge. The B This paper is part of the special issue entitled bBiosecurity of Livestock EffluentsQ, Guest Edited by Dr. Jose´ Martinez and Dr. Franc¸ois Madec. * Tel.: +1 450 687 5010; fax: +1 450 686 5501. E-mail address: [email protected].
1871-1413/$ - see front matter D 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.livsci.2006.03.016
main benefit of ATAD is its efficiency to kill pathogenic organisms. However, compared to other processes, the number of full scale installations is low: in 2000, it was estimated that 40 wastewater treatment plants were using ATAD in Europe, 25 in the United States and 10 in Canada (Metcalf & Eddy inc. et al., 2003). The first process of this type was developed in Germany in the 1960s (Fuchs, 1973). In the 1990s, ATAD has been recognized in the United States as a process capable of fulfilling class A pathogen requirements in order to produce a biosolid that can be used as soil amendment with few restrictions (USEPA, 1990, 1992). Since its first steps of development, aerobic thermophilic digestion has been proposed as a process that could be used for the treatment of livestock wastes
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that are in liquid form. This applies mainly to pig manure but also, in certain cases, to cattle manure. In addition to the pathogen-killing effect, claimed benefits were the simplicity of the process, its robustness, a higher reaction rate (and consequently smaller bioreactors), the conservation of nitrogen and the possibility of heat recovery. Nevertheless, full-scale aerobic thermophilic plants for liquid manure are scarce and most of them have been set for experimental purposes. Examples can be found in Germany (Po¨pel and Ohnmacht, 1972; Martens et al., 1998), United Kingdom (Burton and Turner, 2003), Scandinavia (Skjelhaugen, 1999), Czechoslovakia (Hojovec, 1990), United States (Terwilleger and Crauer, 1975) and Canada (Pilon, 1984). Unfortunately, the available information about these systems is limited and not always easily accessible since most of the data have been presented in conferences or in non-English reports (many in German, some in Norwegian and at least one in French). More peer-reviewed papers presenting laboratory studies have been published, but their numbers are still relatively low compared to other technologies. Although it has not often been used, the interest for the aerobic thermophilic treatment of livestock wastes still exists today. It is especially true in regard to the increasing concern about the possibility of pathogen dissemination by the use of non-hygienised manure. Consequently, the aim of this review is to present a summary of the knowledge that has been gathered over the 40 years of experimentation with this technology. We focus specifically on livestock waste treatment with some references to other use of aerobic thermophilic processes in order to place particular problems in a more general perspective. A reader interested in the application of aerobic thermophilic processes to other types of wastewater should consult the excellent review of LaPara and Alleman (1999). A detailed analysis of ATAD processes for municipal sludge has also been made by the US Environmental Protection Agency (USEPA, 1990).
2. Nomenclature Different names have been used to designate the aerobic thermophilic treatment of organic wastes that content enough water in order to be pumped (from
high-strength wastewater to sludge). This includes autothermal thermophilic aerobic digestion (ATAD) or treatment (ATAT), thermophilic aerobic digestion (TAD), aerobic thermophilic stabilization (ATS), aerobic thermophilic sequencing batch reactor (ATRBS), aerobic thermophilic treatment (A-T treatment), aerobic thermophilic processing, Fuchs process, thermophilic aeration, hot fermentation, LICOMk system, liquid composting and wet composting. It is important to note that the two latter terms are sometime used for naming systems that are not thermophilic. In this review, for simplicity and in order to include all the possible variations of the technology, we will use the term aerobic thermophilic (AT) treatment. It is also important to note that the current review does not cover solid-phase composting processes. Obviously, bthermophilicQ means bhotQ. However, behind that, the definition is not so clear. The term comes from a classification of microorganisms as psychro-, meso- and thermophiles, which are low, medium and high-temperature loving organisms, but the limits between each group vary somewhat in different textbooks. For this review, we consider a process as thermophilic if it reaches at least 50 8C.
3. Heat production One of the first concerns in the development of AT processes was to determine if complete autoheating can be achieved, and in which conditions. An early American study based on mathematical simulations supported this possibility (Kambhu and Andrews, 1969). The experimental confirmation had already been done in Germany in the 1960s but results were presented in English only in the early 1970s (Po¨pel and Ohnmacht, 1972). Since then, self-heating up to the thermophilic range has been shown for different organic wastes, including liquid manure, even in cold climates (Terwilleger and Crauer, 1975; Pilon, 1984; Hojovec, 1990; Martens et al., 1998; Skjelhaugen, 1999; Oechsner and Doll, 2000; Burton and Turner, 2003; Juteau et al., 2004; Heinonen-Tanski et al., 2005). A minimal quantity of organic material is needed to sustain self-heating, but an absolute value is difficult to establish. Excluding any heat loss, for a 40 8C increase (heating from 20 to 60 8C), a theoretical minimum of 24 g COD/l can be calculated using
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a specific heat of 4.2 kJ/kg 8C, a heat production of 20,000 kJ/kg of volatile solids destroyed (Metcalf & Eddy inc. et al., 2003), an oxygen consumption of 1.42 kg O2/kg organic matter oxidised (Henze et al., 2002), a COD/BOD ratio of 2 and a specific weight of 1 kg/l of slurry. However, as we will discuss below, heat losses are important and depend on reactor design. Nevertheless, the experience has clearly shown that the way in which the livestock production facilities are operated in industrial countries generates wastes that are concentrated enough for self-heating AT processes. Self-heating is dependent on energy conservation and one of the most important loss is related to the aeration (Kambhu and Andrews, 1969; LaPara and Alleman, 1999). This is mainly due to water evaporation and, to a lesser extent, to heat extraction by the air. Consequently, it is important to choose a highly efficient aerator in order to minimize the quantity of air that has to be injected in the bioreactor. A minimum oxygen transfer efficiency (OTE) around 10% to 15% has been suggested for sludge digestion (Jewell and Kabrick, 1980). These are values that are achievable by some aerators used for wastewater treatment. Initially, some authors pointed out that thermophilic conditions could result in lower oxygen transfer rates due to the decrease of oxygen solubility at high temperature (Graczyk and Kolaczkowski, 1980). Further studies showed that this phenomenon is completely compensated by a concomitant increase of the K La, which is the overall oxygen transfer coefficient (Aiba et al., 1984; Boogerd et al., 1990; Vogelaar et al., 2000). Actually, the combination of a thermophilic temperature and a high organic load (sludge, manure) seems to increase the oxygen transfer rate since OTE between 70% and 100% have been reported by different authors (USEPA, 1990; Skjelhaugen, 1999). If an aerator is not efficient enough, oxygen transfer can be improved by recycling the air (Norcross and Yanlong, 1996; Blackburn, 2001, 2002). The maximum temperature that can be reached by self-heating AT processes is around 75 8C. This is due to a large reduction of the microbial activity at this temperature (Fig. 2). Optimal operating temperature is also well below that, as we will discuss below. Consequently, in some cases, there is a need to limit the temperature. This can be done by controlling the
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volume of fresh air injected in the bioreactor (Breider and Drnevich, 1981; Norcross and Yanlong, 1996) or by adjusting the amount of shear generated through a jet aeration device (Pressley, 1999). Another possibility is to extract the extra heat by a heat exchanger or a heat pump and use this calorific energy for other purposes, like the production of hot water (Evans et al., 1982; Baines et al., 1986; Svoboda and Fallowfield, 1989; Svoboda and Jones, 1999). This has been recently proposed for a farm scale implementation of an AT process for pig manure (Blackburn, 2001, 2002). This would permit the recovery of 3 to 5 kW of energy per 100 hogs as hot water at 55 8C (Blackburn, 2000).
4. Fate of pathogens AT processes are clearly superior to mesophilic and psychrophilic treatments in order to eliminate pathogens. Reviews on this topic are available (USEPA, 1990; Bicudo and Goyal, 2003; Mohaibes and Heinonen-Tanski, 2004). The pasteurization effect of AT processes is obviously due to the temperature but also to the pH, which normally increases up to 9.0–9.3 during treatment. Pathogens that have been reported to be quickly destroyed in this type of treatment include bacteria (coliforms, Salmonella, Campylobacter, Clostridium perfringens), viruses (Aujesky’s Disease-, Swine Vascular Disease-, Foot and Mouth Disease-, Enteric Cytopathogenic Bovine Orphan-, Equine Rhino-, Pseudoragibies-, reo-, adeno- and entero-virus), parasites (Ascaris and Taenia eggs) and protozoa (Cryptosporidium). Fecal streptococci and Bovine Parvovirus were reduced in a full-scale AT process operated at 55 8C but they were not completely eliminated (Martens et al., 1998). For an efficient pasteurization effect, the targeted pathogens must be subjected to an elevated temperature during a minimum of time. The hydraulic regime has an impact on this parameter. In a continuously fed stir tank, not all the liquid stays inside the vessel for the same period of time. This concept is known as bResidence time distributionQ (RTD) and it means that some pathogens will escape the vessel sooner than the mean hydraulic residence time (HRT), which is equal to the theoretical HRT (volume of reactor/volumetric flow rate) for an ideal
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completely mix reactor. Continuous feeding also opens the door to different phenomena (short-circuiting, formation of dead zones, etc.) that will affect the RTD and even reduce the performance of the treatment. Consequently, many authors have stated that batch processes give a better control on the pasteurization than continuous feeding (Burton, 1992; Martens et al., 1998). This has been illustrated by Oechsner and Ruprich (1989) who reported a reduction of faecal streptococci from 105/g to 100/g in batch mode whereas, in continuous mode, counts in the effluent were from 104 to 105/g. The use of a plugflow design instead of a completely mix tank could be another way of insuring a minimum of time for pasteurization but this reactor type does not seem to have been tried for AT bioprocesses.
5. Fate of carbon An argument that is often presented to support the use of AT processes is a faster specific transformation rate of organic material at high temperature, which should mean smaller bioreactors (Kambhu and Andrews, 1969; Yi et al., 2003). However, experiments have not always supported this hypothesis. For example, Couillard and Zhu (1993) reported a higher maximum specific utilization rate of substrate (slaughterhouse effluent) at 58 8C whereas LaPara et al. (2000) found no significant difference between 25 and 65 8C with a synthetic wastewater. With pig manure, Hissett et al. (1982) made respirometric measurements using bacterial consortia adapted to temperatures between 5 and 50 8C. They reported the results as an Arrhenius plot that showed a typical straight line with a positive slope between 5 and 40 8C; however, after that, the slope become negative and an optimum appears around 45 8C (Fig. 1). Another important aspect is the reduction level of organic material (expressed as biological oxygen demand (BOD) or chemical oxygen demand (COD)) that can be achieved. For this parameter, many authors have reported that, in the mesophilic–thermophilic range, the performance falls as the temperature rise. This means that the residual BOD or COD is higher for an AT process than for a treatment at lower temperature. This is true for livestock wastes (Graczyk and Kolaczkowski, 1980; Evans et al., 1983,
Fig. 1. Arrhenius plot showing the effect of temperature on the respiration rate. Measurements were made with a manometric (D, o) or continuous flow (E, .) respirometer, and with the whole piggery slurry (o, .) or its supernatant fraction (D, E). Reprinted from Hissett et al. (1982), with permission from Elsevier.
1986; Juteau et al., 2004) as well as for other substrates (Tripathi and Grant Allen, 1999; Suvilampi et al., 2005). This is attributed to a possible decrease of the microbial diversity (Evans et al., 1983). This suggests that the best of the two worlds can be obtained by splitting the treatment in two steps, the first one being thermophilic and the second mesophilic. This approach does not seem to have been investigated for livestock waste. However, its efficiency has been shown with other types of high strength wastewater (Suvilampi et al., 2003, 2005; Quesnel and Nakhla, 2005a,b), even though it is not often used. It is important to note that this type of design is exactly the opposite of many AT processes for municipal sludge digestion (btwo-stage ATADQ) since they involved a mesophilic reactor followed by a thermophilic one (USEPA, 1990). On the other hand, the combination of AT pre-treatment with anaerobic mesophilic digestion of municipal sludge is common (USEPA, 1990; Metcalf & Eddy inc. et al., 2003). When tested with pig waste, a dual system of this type showed a better reduction of COD, solids and coliforms than with a single anaerobic bioreactor (Pagilla et al., 2000). Also, more biogas was produced with less H2S, and the sludge showed a better dewaterability.
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In an AT batch process, the oxygen uptake rate (OUR, mg/l min) shows important variations (Fig. 2). It is very intense at the beginning when the most easily biodegradable material is consumed. Thereafter, it slows down. This is a disadvantage from an economical perspective since the aeration equipment, if designed to fulfill the oxygen demand during the intense OUR period, will be oversized for the rest of the time. For that reason, authors have concluded that a continuous AT process is more economically sound
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than an AT batch process. However, as stated above, this is conflicting with the objective of pasteurization for which a batch process is better suited. Aerobic processes are capable of greatly reducing the offensive odour that is associated with livestock wastes. For this aspect, on a short-term basis, lowtemperature treatments and AT processes perform equally (Evans and Thacker, 1985; Evans et al., 1986). However, as mentioned above, the residual BOD is higher in an AT process. Consequently, there is a potential for odour regeneration due to fermentation during long-term storage. Yet this is not what has been reported by Skjelhaugen (1999) who found no odour regeneration over a 10-month storage period of a cattle slurry that had been treated in an AT bioreactor (43–50 8C). The authors did not mention the residual BOD but they stated that the process was designed to only moderately reduce the organic matter. Finally, a parameter that is interesting but that have few, if any, practical significance is the colour. In AT processes, pig manure passes from grey-green to a dark-brown (Graczyk and Kolaczkowski, 1980; Pilon, 1984; Juteau et al., 2004). A higher temperature gives a more intense colour (Juteau et al., 2004). This appears similar to the humus-making process that takes place during solid-phase composting.
6. Fate of nitrogen
Fig. 2. Evolution of the temperature and the microbial activity in self-heating sequencing batch bioreactors treating pig wastes. When no temperature control is used (A), it quickly increases over 70 8C. This is due to the intense microbial activity, as it can be seen by the evolution of the oxygen uptake rate (OUR). However, when the temperature reaches 74–75 8C, the OUR drops, which causes a temperature decrease. After a while, the microbial activity resumes and the temperature increases again up to the inhibitory level. When the temperature is limited to 70 8C (B), 60 8C (C) or 50 8C (D), there are no such oscillations. Symbols: n, OUR; thin line, temperature; bold line, dissolved oxygen (DO). Reprinted from Juteau et al. (2004), with permission from Elsevier.
One characteristic revealed in the early investigations of AT processes is the absence of nitrification (NH4+YNO3 ) over 40 8C (Terwilleger and Crauer, 1975; USEPA, 1990; Blackburn, 2000). This is an advantage if the objective is to conserve the nitrogen since no loss can happen due to nitrification– denitrification (NH4+YNO3 YN2) activities. Consequently, the effluent of an AT bioreactor contains two main forms of nitrogen, ammonia and organic nitrogen. In addition, an inorganic precipitate (struvite, MgNH4PO4d 6H2O) should be present. The conclusion about the absence of nitrification is based on the fact that neither nitrate nor nitrite is normally detected in thermophilic conditions. This means that ammonification (organic NYNH4+) would be the only biological transformation of nitrogen in an AT process. This view has been recently challenged by Korean researchers who reported on two
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occasions the presence of nitrous oxide (N2O) in the gas phase of a laboratory AT bioreactor treating swine waste (Yi et al., 2003; Lee et al., 2004). Increase of N2 has also been noted, but nitrite and nitrate were absent from the effluent. Nitrous oxide is a well-known intermediate of the denitrification process and N2 is its final product, but denitrification occurs normally in anoxic conditions whereas the results come from a bioreactor in which the dissolved oxygen was 2–3 mg/l. Some autotrophic nitrifiers are also capable of aerobic denitrification during which they can produce N2O from nitrite under oxygenlimiting conditions, but it is generally considered as a marginal process (Colliver and Stephenson, 2000). Furthermore, the production of N2O by autotrophic nitrifiers during nitrification (in contrast with nitrifiers making N2O from aerobic denitrification) has been proposed based on experiments with pig slurry (Burton et al., 1993; Pahl et al., 2001). Another hypothesis suggested by the authors is a direct aerobic deammonification (NH4+YN2) that could be carried out by aerobic heterotrophic denitrifiers (Patureau et al., 2000). However, they investigated the microbial community of their reactor using a molecular approach (16S rRNA gene library) and found neither autotrophic nitrifiers nor aerobic heterotrophic denitrifiers (Lee et al., 2004). Instead, based on the presence of Bacillus and Alcaligenes species and since nitrification by a thermophilic Bacillus-like strain has already been reported (Me´vel and Prieur, 2000), they proposed that ammonia could be nitrified by heterotrophic nitrifiers and then quickly denitrified. More work has to be done on this aspect in order to complete our understanding of the nitrogen fate in AT conditions. What is clear is the fact that an important part of the nitrogen is in the form of ammonia. In a laboratory study on the AT treatment of pig manure, Beaudet et al. (1990) showed that the ammonia can be completely volatilised due to the high temperature and the alkaline pH. Generally, AT processes are not designed to catch the ammonia from gaseous effluent (USEPA, 1990). This could be acceptable for municipal sludge but not for liquid manure in which the ammonia concentration is very high. Without such a control, ammonia volatilisation was found to cause very bad smells around a full-scale AT reactor treating swine waste (Pilon, 1984). This also represents a source of
atmospheric pollution. These problems can be avoided by recuperating ammonia vapours with a scrubber that operates with an acid solution. In this way, nitrogen is conserved but a part of it takes the form of a concentrated ammonium salt solution. This has been experimented at the full-scale AT treatment plant of the Federal Agricultural Research Centre in Germany (FAL process). Total nitrogen reduction by stripping was up to 30%, which was below the expectations (Burton and Turner, 2003). On the other hand, a two-stage air scrubber allowed an efficient recuperation of the volatilised ammonia, producing an ammonium sulphate solution (nitrogen content up to 65 g/kg). In its patent application, Blackburn (2002) has also planned to use such an air scrubber. Also, ammonia emission can be minimized without the use of a scrubber. Alfa-Laval has developed a small AT bioreactor, well suited for on-farm operation, in which nitrogen loss by stripping is highly reduced (Skjelhaugen, 1999). Low volatilisation (b1%) is due to a condenser that catches water vapour and ammonia from the exhaust air. A low air flow rate (0.5 m3/m3 h), being made possible by the use of a highly efficient aerator (OTE N 90%), also contributes to the minimization of ammonia stripping. A small peat filter placed on top of the reactor adsorbs the small amount of ammonia that passes through the condenser. Also, some nitrification takes place in the peat bed. Oechsner and Doll (2000) reported similar low values for ammonia stripping in a two-stage AT process that used an air flow rate of 0.4 m3/m3 h. Clearly, the design of the reactor has a strong impact on ammonia volatilisation, especially the choice of the aerator. In this way, an objective of ammonia stripping, which can be achieved by intense aeration, could become conflicting with the objective of heat conservation, which calls for low aeration with highly efficient aerators.
7. Fate of phosphorus Swine waste is very rich in phosphorus and, in some regions, this element can represent a greater problem than nitrogen. Plants need much less phosphorus than nitrogen, and soils do not lose phosphorus in any gaseous form like the way a part of the nitrogen is removed by denitrification and
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ammonia volatilisation. Consequently, fulfilling the nitrogen needs of crop with swine wastes could generate an overfertilisation in term of phosphorus. If the soil is already saturated with phosphorus, runoff can drive this nutrient into rivers in which it will cause eutrophication. In the province of Que´bec (Canada), new rules on the use of manure as fertilizer are based on phosphorus and producers need to know the removal efficiency of any treatment technology for this element. Phosphorus appears in both soluble and particulate forms in manure. Consequently phosphorus can be reduced in the bulk volume by solid separation. To further decrease the phosphorus content of the liquid phase, a process will have to favour in some way the precipitation of soluble phosphate before solid separation. This can be done by chemical coagulation using additive like ferrous salts, alum, lime, etc. Biological processes have also an impact on phosphorus. A part of the soluble phosphate is incorporated into the cellular material of microorganisms. The phosphorus content of bacteria is however low, around 1.5% to 2.0% on a dry weight basis (Metcalf & Eddy inc. et al., 2003). Better removal efficiency can be obtained by encouraging growth of Phosphorus accumulating organisms (PAO), which contain up to 30% of phosphorus. This is accomplished by enhanced biological phosphorus removal (EBPR) systems in which the biomass is exposed to repetitive anaerobic and aerobic conditions. EBPR processes have been tested with piggery wastewater at room temperature (Bortone et al., 1992; Ra et al., 2000). However, there is no report on EBPR bioreactors operated in thermophilic conditions. Actually, tests conducted between 20 and 35.5 8C showed that PAOs do not seem to be capable of adapting to high temperatures (Panswad et al., 2003). A simple way of precipitating phosphorus is through aeration. Even a low-level aeration of swine waste causes the formation of insoluble hydroxyapatite and struvite (Suzuki et al., 2002; Ndegwa et al., 2003). This crystallisation seems to be due to the increase of pH induced by aeration. This rise of pH is explained by the stripping of CO2 and, to a lesser extent, to the production of ammonia during the degradation of organic matter (Stevens and Cornforth, 1974). Consequently, any aerobic biological treatment
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of swine waste in which the pH can become alkaline will cause phosphorus precipitation. The phenomenon is temperature dependent in both psychrophilic (Ndegwa et al., 2003) and thermophilic (Juteau et al., 2004) range. This means that AT processes would be more efficient for phosphorus precipitation. In a laboratory test, centrifugation at 4000g decreased the total phosphorus of an AT bioreactor effluent by 85% compared to 77% for the untreated swine waste (Juteau et al., 2004). Effluent centrifugation was also used at the FAL treatment plant, which gave an overall phosphorus removal of 79% (Burton and Turner, 2003).
8. Foam formation A technical parameter that should be mentioned is the problem of foaming. Production of foam occurs in all treatments in which liquid manure is aerated. In AT treatment of municipal sludge, foam is considered beneficial to the process since it provides a thermal isolation (USEPA, 1990). Bioreactors for this application are sized to accommodate a 0.5- to 1.0-m depth foam layer and mechanical foam cutters seem to be efficient to control it. However critical problems with foam have often been reported for AT processes treating liquid manure (Pilon, 1984; Skjelhaugen, 1999; Heinonen-Tanski et al., 2005). It is generally presented as a detail but people who have worked with this technology know that the intense foaming represents a serious obstacle to its implementation. Even if there is no design criterion for this aspect, it is important not to underestimate the need for foam abatement. Different devices for foam control in AT bioreactors are available or have been proposed like mechanical cutters (USEPA, 1990), liquid sprayers (Stover, 1998) and collecting pipes connected to a Venturi injector (Pressley and Williamson, 2001, 2003). Nevertheless, there is a need for scientific investigations (and publications) on that topic.
9. Conclusion AT processes are valuable options for the treatment of liquid manure but they have been clearly under-
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used. Among their specific advantages, we can highlight the following: ! The quality of the organic fertilizer that is produced. The whole effluent or its separated liquid and solid constituents have characteristics similar to those of compost, especially the near complete absence of pathogens. This is a big advantage since farmers, environmentalists and the public in general consider that compost is a noble product for soil amendment, which is not necessarily the case for other treatment residues that are suspiciously referred as biosolids. ! The conservation of nitrogen. It is generally believed that there is no nitrification in AT processes. If it is true (this has been recently challenged), it means that almost all the nitrogen can be conserved. It can be kept into the slurry by minimizing the volatilisation of ammonia or it can be partially separated by maximizing this volatilisation process and by using a scrubber to produce a concentrated ammonium salt solution. This latter can then be sold as a mineral fertilizer. ! The robustness of the process. The thermophilic biomass takes place rapidly in an AT process and there is no particular difficulty for its maintenance. This is in contrast with coupled nitrification/ denitrification processes, in which nitrifiers are slow growers and are sensitive to inhibition, or with methanogenic fermentation, which relies on fragile syntrophic relations between different types of bacteria. However, if the concept of AT treatment of liquid manure is clearly validated, experiences with this technology appear relatively scarce and there is room for process improvement. Also, even though a respectable number of pilot and full-scale experiments have been conducted, the available information about them is often limited to conference proceedings and hard-to-find reports. Consequently, there is a need to better communicate these results in scientific journals.
Acknowledgements This review has been made as part of a research project supported by a grant (STPGP 269701) from
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