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Overview

If you own livestock, even on the small-scale such as for family subsistence or a hobby-farm, the death of livestock is inevitable. In cases where you are raising animals for meat or hide, their death is even predictable. The most severe cases for concern dead animal disposal when the animals have to be destroyed for reasons of infectious-disease control. Improper disposal of animal remains can easily lead to a pest-management problem, or worse, disease outbreak.

Disposal

In order to limit pests, prevent the spread of disease, and minimize the smell - what is the most sustainable way to dispose of animal or livestock remains? To be sustainable, a proposed solution should address:

  • pest potential
  • cost
  • availability
  • environmental impact
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Methods

There are several methods of livestock disposal to choose from, each has its own benefits and drawbacks. While there may be no correct answer for everyone, there are better approaches - especially as we factor in sustainability. Inspiration for this Q&A layout is drawn primarily from Amundson (2013). While this book concerns husbandry (see ) of goats, I have tried to fill out her proposals to make them more applicable to a general audience (to include pet-disposal).

No part of this answer shall be construed as providing legal advice.

Incineration

This solution is often preferred where there has been large-scale contamination or infection by disease (see "Creutzfeldt–Jakob disease" in Rendering below). This method often results in an undesirable smell, and can be noxious to neighbors. Has low biosecurity risks, but true incineration must occur at high temperatures.

Within sustainable-frameworks, incineration could be combined with some type of energy-recovery from the waste-heat. Wikipedia offers some suggestions, but they seem more suited to industrial incinerators:

Incineration with energy recovery is one of several waste-to-energy (WtE) technologies such as gasification, pyrolysis and anaerobic digestion. While incineration and gasification technologies are similar in principle, the energy product from incineration is high-temperature heat whereas combustible gas is often the main energy product from gasification. Incineration and gasification may also be implemented without energy and materials recovery. {Wikipedia article, "Incinerate"}

In many jurisdictions, open-burning is prohibited.

Composting

A terrific treatment of meat-scrap composting has already been presented by @Jay-Bazuzi on this site for the question: "Why shouldn't meat be placed in my compost pile?" I will attempt to expand upon this answer to cover a larger scale (entire animal at-a-time).

Sawdust, straw, or other carbon should be increased to at least one foot (0.3 meter) underneath and surrounding the animal corpse. Straw is convenient because it might already be on-hand for animal bedding or plant insulation (to help a plant "winter").

Composting methods are often prescribed by law within the US, to include: primary and secondary composting (decomposition) bins, structural impermeability, setback distances from established water-sources, etc.

Livestock that perish from suspect neurological concerns are prohibited (at least in Illinois) from being composted and must be incinerated.

Burial

Another least-desirable method of animal disposal, second in my opinion to rendering, is burial. Burial is the unscientific dumping of an animal corpse beneath the ground. This method has a high potential for ground-water or well-water contamination. If buried too shallow, scavengers or pests will come to visit and potentially spread disease.

Rendering plants

Rendering is a process that converts waste animal tissue into stable, value-added materials. Rendering can refer to any processing of animal products into more useful materials, or more narrowly to the rendering of whole animal fatty tissue into purified fats like lard or tallow. Rendering can be carried out on an industrial, farm, or kitchen scale.

The majority of tissue processed comes from slaughterhouses, but also includes restaurant grease and butcher shop trimmings, expired meat from grocery stores, and the carcasses of euthanized and dead animals from animal shelters, zoos and veterinarians. This material can include the fatty tissue, bones, and offal, as well as entire carcasses of animals condemned at slaughterhouses, and those that have died on farms, in transit, etc. The most common animal sources are beef, pork, sheep, and poultry.

While at first-glance, rendering might be seen as the embodiment of the reduce-reuse-recycle motto, biosecurity concerns have made rendering one of the least-sustainable disposal methods (in my opinion).

One of the most widely-covered, biosecurity events in recent times was the "Mad-Cow" disease outbreak. Properly called Bovine spongiform encephalopathy, this disease was spread to humans through contaminated beef which could not be rendered safe through cooking. When passed to humans, it is referred to as "variant Creutzfeldt–Jakob disease". The wiki article extract is below:

Creutzfeldt–Jakob disease or CJD is a degenerative neurological disorder that is incurable and invariably fatal. CJD is at times called a human form of mad cow disease (bovine spongiform encephalopathy or BSE). However, given that BSE is believed to be the cause of variant Creutzfeldt–Jakob (vCJD) disease in humans, the two are often confused. {excerpt from Wikipedia} [emphasis mine]

References

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    Anaerobic digestion would be another route. Provides biogasand fertilizer, but requires (with lifestock remains) a reliable pasteurization/heat treatment. probably out of reach for small scale husbandry. If AD is still relevant for your question, I can maybe elaborate as an additional answer. – mart Jan 26 '15 at 10:32
  • @mart Yes please, it certainly deserves a more thorough look - and I know little about it. – Clayton Jan 26 '15 at 10:55
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Note: This still needs info on when carcasses should only be incinertaed, and when which heat tgreatment is neccessary.

Animal leftovers can be used as a substrate for anaerobic digestion, to generate biogas. They are high in fat in protein and thus very energy rich. However, There are severyl considerations that will, in all likelyhood, place AD out of the range of a small operation.

Process considerations
I use the European biowaste directive as a guideline. Animal leftovers need to be heat treated to reduce pathogens. Before heat treatment, the leftovers need to be crushed to bits smaller than 12mm. There are several ways to do this:

  • Dedicated equipment: a heated tank where the leftovers are either heated to 70° and held at that temp. for one hour, or to 120°C and held for 20 minutes.

  • thermophilic AD: If the digester temperature is higher than 50°C and a minimum retention time can be guaranteed, this can be sufficient

  • Composting: The digestate of the AD stage can somtimes be composted for heat treatment. Typically, one will need to add carbon rich substrate and the digestate is often too moist for composting

The latter two approaches will need to be tested on a plant by plant base to prove that actual hygienization does occur.

The main advantage of AD is the actual energy generation: One ton of chicken heads will get you about 250m³ biogas at 58% methane. This is about 1200-1300 kWh of energy. This is easily more than the heat and power needed for the process itself.
Also, animal leftovers have a high fertilizer value and can replace fossile fuel based fertilizers. After the heat treatment the leftovers are safe for land application.
The high nutrient content is also a downside, as too high nutrients (especially nitrogen) will poison the bacteria. So you will need additional substrates (manure, plant based stuff) and possibly water to balance them. Another problem is that handling animal leftovers can be dangerous due to pathogens, so the operator needs equipment to mitigate these risks. Waste plants are typically expensive.
I find it hard to give an actual number for a size, but I don't think a dedicated plant for less than a few hundred tons a year makes sense, economically. It will be also hard to build and operate such a plant on small scale within the legislative framework usually found in industrialized countries, as there are several health & safety as well as public health concerns.

One way to go would be to have a rendering plant and use the product not as fodder but as an energy rich substrate in AD. You would solve the preparation, heat treatment etc centrally. This is more economical and safer. In germany, using leftovers as fodders has been outlawd in many instances. At least one renderer started selling his animal derived hog fodder to biogas plants (still under the name "porky power"). This is a sensible approach, but happened due to unique market conditions.

While AD is a good way to get some value from animal leftovers, it is probably out of reach of most small scale operations. However, I think several small scale farmers that use biogas for manure disposal could band together to share heat treatment and other rendering equipment. Ultimately, this can be one stepping stone in closing local nutrient cycles and supplanting fossile energy.

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