CASE NOTES

The feasibility of composting animal carcasses in NSW in an emergency animal disease response

Kevin Wilkinson¹ and Byron Stein²

¹ Director, Frontier Ag & Environment
² Policy & Project Officer, Animal Biosecurity, Department of Regional NSW

Posted Flock and Herd July 2023

Introduction

For the first time in Australia, the applicability of carcass-grinding and composting was evaluated for mass disposal in an emergency animal disease (EAD) outbreak. NSW DPI has successfully completed a composting field trial on a large commercial piggery in Southern NSW, providing important preliminary data on process performance and insight into how grinding and composting could be implemented effectively in an EAD outbreak.

The feasibility of mass disposal options for an EAD outbreak in piggeries is disease and site-specific. On-farm disposal is likely to play a prominent role in cases where restrictions are placed on the movement of infected materials including carcasses (e.g., for African swine fever (ASF) or foot-and-mouth disease (FMD) outbreaks).

Composting is an approved mass disposal method under AUSVETPLAN (Animal Health Australia, 2021), but its practical application in an Australian EAD is hampered by a number of issues, including (Price et al., 2020):

Uncertainty about its effectiveness at eliminating diseases especially when the process is managed sub-optimally
Lack of nationally consistent guidance material on the subject
Lack of experienced subject matter experts in the country
Disagreement between environment and biosecurity agencies about its suitability relative to other disposal methods
Concerns around complex logistics, especially for sourcing sufficient co-composting (carbon) materials
Time required to complete this method of disposal and associated delays to resume business operations, for example, versus options such as burial or landfilling.

Yet, mass disposal of large animals (e.g., pigs, sheep and cattle) is likely to be more challenging than has often been assumed in the past. Aside from issues to do with movement controls placed on carcasses in an EAD, gaining access to sufficient landfill space is not guaranteed, and there are concerns around the sustainability of on-farm trench burial as a mass disposal method.

For this reason, both industry and government agencies are committed to investigating alternative carcass disposal options, particularly now that both ASF and FMD are pressing concerns. The trial reported in this paper makes a significant contribution to the growing body of work currently underway on this subject.

Methods

Three separate composting windrows were set up in early March 2022. The 'control' treatment (Treatment 1) involved the traditional layered method of carcass composting in which pig carcasses were placed on a bed of straw followed by a layer of spent bedding and a second layer of carcass material (Figure 1). The whole pile was then capped with straw. The second windrow (Treatment 2) was set-up just like the first, but the contents of the whole windrow were fed through an industrial horizontal grinder after 14 days of composting (Figure 2). After grinding, this windrow was reformed to continue composting. The third treatment (Treatment 3) involved the pre-grinding of carcasses before windrow formation. The carcasses were ground up and then combined with spent bedding using a front-end loader. This mix was then placed on a bed of straw with more straw placed on top. Windrow temperatures were monitored with wireless probes and each windrow was subjected to six turns with a front-end loader.

Figure 1: Second layer of carcass applied

Figure 2: An industrial grinder was evaluated in the trial

At the time of this composting trial, the piggery was affected by an outbreak of Japanese encephalitis (JE). This resulted in a mix of mortalities comprising mainly of sows, some boars, neonatals, aborted foetuses and entrails.

Immediately upon completion of windrow set-up, 4 motion-sensitive infra-red cameras (Swift Enduro, wide angle outdoor cameras) were installed at each corner of the site to monitor any activity by scavenging animals during the trial.

Samples of raw material and compost were taken at various times throughout the trial for microbiological testing including faecal coliforms, E. coli, Listeria monocytogenes, Salmonella sp., Campylobacter spp., Yersinia enterolitica, Streptococcus suis, Clostridium perfringens, coagulase positive Staphylococci and Adenovirus (PCR).

Results and discussion

Composting for 14 days resulted in a mean 99.97% reduction in faecal coliform counts, 99.98% reduction in E. coli and complete elimination of Adenovirus. In addition, Salmonella was undetected after 132 days composting. After 202 days composting, E. coli counts were below the established limit of <100 cfu/g but faecal coliform counts consistently exceeded the accepted threshold of 1,000 cfu/g. Furthermore, elimination of other pathogens (e.g., Listeria monocytogenes and Leptospira sp.) and Helminths was proven to be less reliable even though standard time-temperature requirements for pathogen reduction had been met. Given the weight of global experience supporting the efficacy of mass mortality composting in eliminating pathogens (Costa et al., 2019; Guan et al., 2010; Pepin et al., 2021; Wilkinson, 2007), the cause of pathogens persisting in this trial is probably due to a lack of process efficiency. Meeting this KPI quickly is highly desirable from a biosecurity perspective, and the project has identified how the composting processes used in this trial can be better optimised.

This composting trial was successfully conducted despite many challenges faced due to the nature of the mortalities (being affected by Japanese encephalitis), minimal equipment (e.g., only one loader and the weighbridge could not be used as originally planned), resources not being available 'on call' and project budget limitations. Contrary to conventional 'wisdom' (Bonhotal et al., 2022; Oshins et al., 2022), we found that composting can be conducted at low C:N ratios with the resultant end-product having similar composition to straw-based piggery spent bedding.

All treatments successfully reached thermophilic conditions (i.e., >45°C) within a few days of set-up (Figure 3, Table 1). Temperatures remained high in all treatments until about day 100, after which each windrow began to cool off. This does not necessarily indicate that the composting process had been completed, however, since the windrows were no longer actively managed after the last turn on day 84. The main objective was not to produce a manure compost, but to dispose of carcasses in a biosecure manner.

Table 1: Key performance data for each treatment

Temp probe position	Days >55°C			Max temp recorded (°C)			Mean temp for trial (°C)
Temp probe position	Treat 1	Treat 2	Treat 3	Treat 1	Treat 2	Treat 3	Treat 1	Treat 2	Treat 3
Base	63	9	54	72	62.9	70.4	52.9	46.5	50.7
Upper	87	60	89	74.5	68.7	72.1	58.4	55.3	56.8
All probes	72	38	61	74.5	68.7	72.1	57.2	51.2	53.8

Figure 3: Temperature development during composting. Example results shown for Treatment 1 - whole carcass composting

Conclusions

There are sufficient grounds showing that grinding combined with composting may have a significant role to play in a mass mortality disposal operation involving pigs. This trial has provided some useful insight into the applicability of the different processes used, as well as what equipment and other resources are needed for process optimisation and validation.

When comparing the various treatments, the following conclusions can be drawn:

Whole carcass composting (Treatment 1) is a robust and relatively simple, low technology option. It is ideally suited for application in an EAD where co-compost is readily available and where the process does not need to be completed quickly.
Grinding after a set period of composting (Treatment 2) is perhaps the most promising option for the following reasons:

It is potentially safe since the grinding process is conducted on material that has already been subjected to a period of thermophilic composting.
It is cleaner and quicker than raw carcass grinding. Cleaning of grinders after processing raw carcass can be very challenging. In contrast, processing of the partially composted material was potentially quicker and created less gross contamination of the machine and at the composting site.
It could be suitable for a wider range of EADs (e.g., FMD perhaps) since aerosolisation of virus particles would be less likely to occur after a short period of thermophilic composting has been completed.
The end-product was finer, and arguably, a higher grade of product (i.e., more uniform, with fewer bones and straw etc.) than the other products tested.
It could be used cost-effectively since the efficient operation of the grinder will not be held up by the potentially slow rate of carcass supply to the disposal site after destruction since the grinder is used on completed windrows.

Grinding raw carcasses (Treatment 3). Due to the cost of hiring a grinder, this method is probably only suitable for use in a very large outbreak in which the machine is operating constantly (or close to). This may work at a large commercial piggery but the carcasses would have to be supplied to the grinder at a rapid rate, and this will be dependent on the resources available for destruction. Alternatively, the grinder could be located in a centralised, temporary location, with carcass material from several large farms brought to it for processing and composting. However, movement controls of carcasses during an EAD may restrict the volumes that can be processed in this way.

References

Animal Health Australia (2020) AUSVETPLAN Operational manual: Disposal (Version 3.2) Australian Veterinary Emergency Plan (AUSVETPLAN), Edition 3, Canberra, ACT
Bonhotal J, Schwarz M, Rynk R, Biala J, Gilbert J & Michitsch R (2022) Chapter 8 - Composting animal mortalities, Editor(s): Robert Rynk, The Composting Handbook, Academic Press, Pages 307-340, ISBN 9780323856027 doi.org
Costa T & Akdeniz N (2019) A review of the animal disease outbreaks and biosecure animal mortality composting systems Waste Management 90:121-131 doi.org
Guan J, Chan M, Grenier C, Brooks BW, Spencer JL, Kranendonk C, Copps J and Clavijo A (2010) Degradation of foot-and-mouth disease virus during composting of infected pig carcasses Canadian Journal of Veterinary Research 74:40-44
Oshins C, Michel F, Louis P, Richard TL & Rynk R (2022) Chapter 3 - The composting process, Editor(s): Robert Rynk, The Composting Handbook, Academic Press, Pages 51-101, ISBN 9780323856027 doi.org
Pepin B, Williams T, Polson D, Gauger P & Dee S (2021) Survival of swine pathogens in compost formed from preprocessed carcasses Transboundary and Emerging Diseases 68(4):2239-2249 doi.org
Price J, McOrist S, Wilkinson KG (2020) Development of decision support tools for on-farm mass herd euthanasia, disposal and decontamination (DDD) for the Australian Pig Industry in the event of an exotic disease outbreak - Literature Review. Report for Australian Pork Limited
Wilkinson KG (2007) The biosecurity of on-farm mortality composting. Journal of Applied Microbiology 102:609-618