In-shed pasteurisation of spent litter is a means of heat treating the litter to significantly reduce pathogen load before a fresh batch of chicks is raised on that litter. The following is a standard operating procedure for pasteurising spent litter, based on the available literature, Australian experience and project research, extracted from Walkden-Brown, S., Islam, A., Laurenson, Y., Hunt, P. & Dunlop, M. (2015). Methods to quantify and inactivate viruses in poultry litter. Final Report of Project 2.2.3, p. 189. Poultry CRC, Armidale. A litter temperature prediction model & pasteurisation decision support tool developed as part of the Poultry CRC project, “LitterHeatMap“, is available directly from Dr Laurenson (Animal Science, University of New England) at firstname.lastname@example.org.
Most of the heat and ammonia in heaped litter are generated by a mixture of aerobic mesophilic and thermophilic bacteria and fungi which require nutrients, water, oxygen and a suitable pH. The closer to optimum these conditions are, the quicker and higher the temperatures achieved. Some of the background and guiding principles for the process are summarised below:
The pasteurising heats of 50˚C and above obtained in heaped poultry litter above are due largely to aerobic microbial activity requiring oxygen. Aerobic oxidation of carbon substrates releases large amounts of heat energy, water and CO2 while anaerobic fermentation yields much less energy and CO2 and large amounts of energy rich methane (CH4) (Haug 1993). Aerobic oxidation of glucose produces 677 kcal/mol of energy while anaerobic oxidation produces only 96 kcal/mol of energy. Putrefactive odours are a common by-product bacteria of anoxic and anaerobic composting with sulfur and nitrogen acting as electron acceptors rather oxygen.
The following description of the microbial kinetics during composting is adapted from Beffa et al. (1996). A large variety of mesophilic, thermotolerant and thermophilic aerobic microorganisms (including bacteria, actinomycetes, yeasts, molds and various other fungi) have been reported in composting and other self-heating organic materials at temperatures between 20-60°C. At an early phase of the composting process (temperatures between 20-40°C) mesophilic/thermotolerant fungi, principally yeasts and molds, and acid producing bacteria are the dominant active degraders of fresh organic waste. Mesophilic bacteria (which prefer temperatures of 20-40˚C) are predominant in the early stages of the process, soon giving way to thermophilic (high temperature) bacteria, which inhabit all parts of the stack where the temperature is satisfactory. Thermophilic fungi usually appear after 5 to 10 days followed by actinomycetes. Mesophilic microorganisms are killed or inactivated during the initial thermogenic stage (temperatures between 40-60°C), where the number and species diversity of thermophilic/thermotolerant bacteria, actinomycetes and fungi increase. The optimal temperature for thermophilic fungi is 40-55°C, with a maximum at 60-62°C. Fungi are killed or are present as spores at temperatures above 60°C. Thermophilic actinomycetes are generally more tolerant than fungi to high temperatures but at temperatures above 60°C their number and the species diversity also decreases, and their importance in the degradation process becomes negligible. Thermophilic bacteria are very active at 50-60°C, and at temperatures above 60°C the degradation process is performed essentially by these microorganisms.
Heat inactivation of pathogens relies on complex time-temperature relationships (Haug 1993). Higher temperatures require shorter periods to cause inactivation. Moist heat and higher levels of hydration are more effective than dry heat, or low moisture microbial forms. In general, parasites and their eggs are inactivated more readily than bacteria which in turn are inactivated more readily than viruses although there is wide variation and overlap between these. Heat in the temperature range able to be achieved during thermophilic litter composting is a potentially effective means of inactivating many pathogens including viruses, bacteria, fungi, protozoa and metazoan parasites. Exceptions include prions, bacterial spores (genera Clostridia and Bacillus) and the eggs of some helminth parasites and certain protozoal cysts. In general, vegetative bacteria are destroyed after 5-10 min at 60-70˚C and pasteurization at 70˚C for 30 minutes destroys most pathogens (including viruses) found in sewage sludge (Haug 1993).
Temperature acts primarily by denaturing proteins and irreversible protein cross-linking and coagulation after denaturation is solvent dependent, requiring higher temperatures as material becomes more desiccated. This likely explains both the greater efficacy of moist heat over dry heat for sterilization and the extended survival of very resistant life forms such as spores, lyophilized virus etc. Viruses which are non cellular, are inactivated by a) the collapse mechanism leading to breakdown of hydrogen bonds and collapse of the secondary structure of DNA or protein capsid (DNA viruses) or b) the chain break mechanism resulting in a break or change the nucleic acid chain at a single point following some chemical reaction (mostly RNA viruses) (Woese 1960).
Regarding bacterial inactivation in litter using composting, the thermophilic temperatures achieved are well above the thermal death points of mesophilic pathogens, such as E. coli O157:H7 and Salmonella spp. (Chen and Jiang 2014). In several studies enteric bacteria such as Salmonella spp., E. coli, Campylobacter spp., vegetative Clostridium perfringens and Listeria monocytogenes were undetectable in composted litter or reduced to undetectable levels by poultry litter composting (Brodie et al. 1994; Kwak et al. 2005; Macklin et al. 2008; Silva et al. 2009).
The die-off of pathogens during composting may not be uniform and persistence of pathogens in poultry compost has also been reported in many studies with the surface of fresh compost being identified as the critical location for pathogens to extend survival (Chen and Jiang 2014). In open air full composting environments regrowth of bacterial pathogens due to the recontamination is a risk but this is not a major risk for short term litter pasteurization in sheds.
Regarding virus inactivation in poultry litter Newcastle disease virus and avian influenza virus in chicken faeces, feed and litter in porous nylon bags were inactivated by day 3 in composting litter which reached temperatures of 50˚C to 65˚C by day 7 (Guan et al. 2009). Infectious laryngotracheitis virus was reduced to undetectable levels by normal litter composting for 5 days or heating at 38˚C for 48 hours (Giambrone et al. 2008). Walkden-Brown et al. (2010) reported that Fowl Adenovirus 8 was largely inactivated in litter after 6–7 days of litter pasteurisation by heaping while chicken anaemia virus and infectious bursal disease virus were largely inactivated after 6-10 days. Marek’s disease virus retained significant infectivity at days 9–10. There was little evidence of any litter transmission of infectious bronchitis virus or vaccinal Newcastle disease virus at all. Coccidial oocysts appeared to be inactivated by first sampling after 3 days of pasteurisation.
The selection of heating 55˚C for 3 days in this SOP is based on this information and broad guidelines provided in USA and Australian regulations relating to the inactivation of pathogens in sewage sludges and composts as summarised below.
In its Part 503 Biosolids Rule the EPA differentiates between Class A and Class B treated sludges (EPA 2012). If pathogens (Salmonella sp. bacteria, enteric viruses, and viable helminth ova) are below detectable levels, the biosolids meet the Class A designation. Biosolids are designated Class B if pathogens are detectable but have been reduced to levels that do not pose a threat to public health and the environment as long as actions are taken to prevent exposure to the biosolids after their use or disposal. To meet Class A and B conditions using composting the following requirements must be met (EPA 2012):
Clause 3.2.la of the AS4454 Australian Standard on Compost and Produce Standards in Australia (Standards Australia 2012) specifies the following process criteria for pasteurisation on the basis of all material being subjected to sufficiently high temperature for a sufficient duration to cause thermal death:
A carbon to nitrogen ratio (C:N) between 15 and 30 is recommended. Above 30 microbial growth is impaired. Below 15, high temperatures are achieved but nitrogen is in excess and given off as ammonia. Broiler litter typically has a C:N of 10-15 and this ratio reduces with increased litter reuse. Adding a high C low N source will reduce ammonia emissions.
Optimum moisture content in most compostable material is generally between 40 and 60%. Excessive moisture limits porosity and oxygen availability and insufficient inhibits microbial growth. Used poultry litter typically has a dry matter content of 20-35%. Poultry CRC research in Australia has shown variable temperature responses to moisture addition to litter, with significant temperature responses only seen in very dry litter (<20% moisture content). In the USA on farm responses to moisture addition to litter of 25-26% initial moisture content have also been variable with increasing moisture level often not producing the expected increased temperatures and creating subsequent issues related to ammonia production and litter and caking (Lavergne et al. 2006). Excessive ammonia production following addition of water to litter to bring moisture content up to 28-34% has also been observed in Australia. It is possible that excessive ammonia production during poultry litter composting inhibits thermophilic bacteria at lower moisture contents than optimal for other materials with a higher C:N ratio. A guide to estimating moisture content of litter provided in Table 1.
Table 1. Descriptions provided to litter of different moisture contents (McGahan et al. 2014).
|Description||Moisture content (%)|
|Dry to friable||15-20|
|Friable to moist||20-30|
|Sticky, beginning to cake||30-40|
|Wet and sticky, heavy caking||40-50|
|Very wet and sticky||>50|
Pasteurisation in heaped litter is largely an aerobic process as outlined above. Oxygen can become limiting if particle size is small and porosity poor, or if moisture content is too high. Compression in large heaps will reduce porosity and oxygen availability deep in heap or windrow. Turning increases oxygen availability but cools the heap significantly producing a saw-tooth like temperature profile.
Compost microorganisms operate best under neutral to acidic conditions, with pH’s in the range of 5.5 to 8 although pH up to 9 supports adequate microbial composting. Composting chicken litter tends to be slightly alkaline (pH > 8) and to acidify slightly during the composting process. Ammonia production increases rapidly as pH increases above 8. The composting process is somewhat self-buffering and deliberate modification of pH is rarely justified.
There are a number of practices that can be implemented to potentially influence litter pasteurisation.
Litter cake is typically removed to reduce moisture content and condition litter. Removing cake is also likely to increase the C:N ratio. Cake removal may be done before or after heaping and pasteurisation. The effects of inclusion or removal of cake on the thermal properties of heaped litter have not been investigated. However recent Poultry CRC research (Experiment 2.3 in this report) has shown that pasteurisation for 7 days had no effect on reducing the size of cake pieces. It is probably more practical and beneficial to decake prior to pasteurisation.
Long composting cycles typically involve turning, both to aerate the core and to mix in the cooler drier outer layers to produce a more uniform product. The benefits of turning for shorter litter pasteurisation periods is less clear. Poultry CRC research in Australia has shown that over a 9-day period, turning large heaps (height 2.5-2.8 m) at day 3 resulted in a sustained increase in mean temperature following turning whereas there was no benefit in turning smaller windrows (height 0.8-1.2m) (Walkden-Brown et al. 2010). However, more recent work over a shorter 7-day pasteurisation period and a range of heap sizes presented in this report has shown that turning of litter on day 3 led to a significant reduction in temperature on the day after turning, with a rebound increase resulting in higher temperatures on days 6 and 7 with the two effects cancelling themselves out. The imperative to mix to ensure more uniform exposure to pasteurising temperatures is also reduced with short pasteurisation times as shown in the same CRC studies. Over a 7-day pasteurisation period in heaps of a wide range of sizes the time spent at 55˚C or higher was significantly greater at a depth of 5 cm from the surface than at 100 cm deep in the heap. This indicates that the thin, cool “rind” on pasteurised litter heaps is thin.
In the three on-farm studies covered in recent CRC research presented in this report significant overall benefits of covering on temperatures were only seen under conditions when litter was very dry (16% moisture), heaps were very small, and ambient conditions were very cold (mean temperature in the shed of 12.6˚C). Under higher moisture conditions in the same experiment there was no beneficial effect of covering. Under a wider range of conditions on two other commercial farms, no major benefit of covering was observed. This contrasts with a single report from the USA in which covering increased temperatures in heaps of higher moisture content (37-40%) but only following moisture addition. Ambient temperatures were as cold as those seen in the Australian study in which a response to covering was observed.
Passively aerated systems are designed to eliminate the need for physical turning during normal composting. Aeration is achieved through perforated plastic pipes embedded at 12- to 18- inch intervals in the base of each windrow. Air is drawn into the pipes from outside the pile and is forced through the pile from the chimney effect created by the hot gases escaping from the windrow. The effects of such systems on temperature profiles in pasteurizing litter have not been evaluated.
Forced aeration involves air being forced through the pile from the base through a system of pipes. This can speed up the whole compost process but is capital-intensive. One small CRC study (Expt 2.2.2 in this report) investigated the effects of forced ventilation on temperatures in heaped litter and found that while aeration led to increases in mean temperatures over 7 days of 3.1 to 9˚C at 0, 5 and 50 cm depths, but not 10 cm.
As noted in section 8.2.3, addition of moisture to heaped litter has had very variable results in both Australia and the USA with negative effects on final litter moisture and ammonia production from the pasteurised litter. The one unequivocal improvement observed in our studies involved very dry litter (16% moisture) in which increasing moisture content to 28% led to a large increase in average temperatures of 8.9˚C. In covered heaps in the same experiment no response to additional moisture was observed demonstrating that the two effects are not additive. On a commercial farm with large heap sizes and 2nd use litter with mean initial moisture content of 18%, there was no clear beneficial increasing moisture content to 28% or 34%. Significant adverse effects due to excessive ammonia were observed in chicks reared on pasteurised litter including that from the high moisture treatments. On another farm with a range heap sizes and initial moisture contents ranging from 13-26˚C the association between initial moisture content and temperatures during pasteurisation was negative rather than positive.
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