Pig carcass composting

A pig carcass composter made from hay bales

Piggeries typically have annual mortality rates of 2.5%, 3% and 9% among weaners, growers and finishers, and breeding sows respectively. The need to dispose of these carcasses, in addition to afterbirth and stillborn pigs, presents a significant problem. In the past, the two most common methods used for carcass disposal were burning and burying. In recent years, carcass composting has become more common.

Composting is an acceptable method of recycling animal carcasses. It may be used as an option where burial is not feasible. (In some locations burial may be a problem at rock sites, where the groundwater is shallow, or where it is difficult to keep the pits dry.) The cost associated with setting up and maintaining an animal carcass composting system is relatively low.

About composting

Composting is the biological decomposition and stabilisation of the biodegradable component in organic matter under controlled conditions. It is an aerobic process (requires oxygen) that can turn pig carcasses into a rich humus that can be used as a fertiliser or soil amendment. The process itself is not difficult and works successfully, with minimal management if you follow the guidelines.

Successful composting requires carbon, nitrogen, water and oxygen to feed the organisms that process the carcasses. A carbon-nitrogen ratio of between 20 : 1 and 35 : 1 is optimal. Above that range, organisms don't proliferate, resulting in lower compost temperatures and a slowing of the process. Below 15 : 1 (not enough carbon) nitrogen will be lost as ammonia. This reduces the value of the by-product and can create an odour problem. Correct temperature is critical in the process, as carcasses will decompose about twice as fast at a temperature of 68°C, compared to a temperature of 57°C.

Location

The composter should be located away from environmentally sensitive water bodies such as water courses, gullies and drainage ditches. A buffer between the composting operation and nearby water resources will help protect those water resources from possible contamination. The buffer will depend on Queensland Government and local government recommendations.

A location at or near the crest of a hill will eliminate or minimise surface water entering the composter from higher ground. If the composter is located on the lower part of a slope it should be protected by a diversion bank or drain to keep surface water run-off out.

Siting of the composter should take into account the location of the farmer's residence and other nearby residences. While offensive odours are not usually generated in the composting process, the handling of dead swine and compost on a daily basis may not be aesthetically pleasing. When locating a composter, consideration should be given to transporting dead pigs, supplying ingredients, and removing finished compost.

Ingredients

Composting dead pigs requires the addition of a carbon source to ensure that a proper carbon-nitrogen ratio is present for the composting process. Experience suggests that sawdust is the most ideal carbon source due to its small particle size, ease of handling, absorbency qualities and high carbon content. Research with straw and a straw-manure mixture as a carbon source indicates that composting can be accomplished with both ingredients. Longer composting times with straw may be necessary and leaching of liquids from the composting mass may be more of a problem than with sawdust. Composters using straw may need to be roofed to keep rain from leaching through the pile. Sawdust seems to shed and/or absorb liquids sufficiently so that leaching and drainage from the pile is minimal.

It has been found that 0.25 m³ to 0.4 m³ of sawdust per sow place will be required annually to support the composting process in a farrow-to-finish operation. A producer with a 100-sow herd would, therefore, need between 25 m³ and 40 m³ of sawdust per year to operate the composter. Sawdust is used in pig composters in the approximate ratio of 6 m³ of sawdust per 1000 kg of dead pigs. A producer anticipating 10,000 kg of death loss per year could, therefore, expect to use approximately 60 m³ of sawdust per year.

Design

The composting process is more efficient if the right ingredients are placed in composting bins in the correct proportions, allowed to compost for a period of time, then moved to a second bin for a secondary composting phase. Field experience suggests that composting bins can be constructed using large round or rectangular bales of cheap, poor-quality hay. Bales are placed end-to-end to form walls for three-sided enclosures (bins). A minimum of two bins is required (primary and secondary composting phases). Experience suggests that 2 m² to 2.5 m² of bin area are required per 1000 kg of carcass composted annually. A pig enterprise anticipating 10,000 kg of death loss annually would need a composter with 20 m² to 25 m² in each of the primary and secondary composting bins.

Bin configuration is not critical, however bins should be laid out so that the bin contents are easily accessible with a front end, or skid-steer loader. Square bins offer the greatest opportunity for reduced side effects (heat loss through walls etc.); however, long, narrow bins, which can be accessed through both ends have also been used. Primary and secondary bins should be located at close distance, or adjacent to each other (perhaps with a common wall), to facilitate moving compost from bin-to-bin.

A minimum of two bins is required. However, more bins may be required on large operations or with different management schemes. Excessively large bins should be avoided. Experience has shown that bins with 12 to 15 m² of surface area work well. A layout of two round bales deep and three bales wide provides the approximate area and has worked well. Consideration should be given for an additional bin for storing sawdust.

In higher rainfall areas (>1000 mm annual average), it may be necessary to provide a roof over the composting facility. Composting should only be carried out on a prepared pad of low permeability to prevent any leachate from contaminating groundwater resources.

Any run-off from the site should be collected and contained in the existing effluent treatment system or through the use of appropriate bunding or a collection dam.

Operation and maintenance

  1. Start a primary composting bin by placing sawdust in the bin so that there is at least 300 mm of sawdust under and around the first carcasses placed in the bin.
  2. Place carcasses in the primary bin as necessary, using sufficient sawdust so that each carcass is covered on all sides with a minimum 300 mm of sawdust. Expect to use sawdust at the rate of 6 m³ of sawdust per 1000 kg of dead pigs. Large, mature carcasses may need to be re-covered with sawdust after a day or two, as the sawdust settles around the carcass. Slashing larger carcasses to release gas build-up in the abdomen may help to prevent carcasses from exposing due to bloat. Small carcasses may require less cover, but sufficient sawdust is needed to prevent odours and cover exposed parts of carcasses. Always keep the carcasses covered with at least 300 mm of sawdust to avoid odour generation and scavenging by feral and domestic animals.
  3. Continue placing and covering carcasses as necessary until the bin is full. Experience suggests that the last carcasses placed in the primary bin should be allowed to compost a minimum of three months before moving the contents of the primary bin to a secondary bin for the second stage of composting.
  4. Allow the secondary bin to compost for at least three months. After this secondary stage of composting is completed, the compost should appear as dark, nearly black, humus-like material with very little odour. Some large resistant carcass parts (teeth, skull, etc.) may still be identifiable, but should be soft and easily crumbled after the process is finished.
  5. After secondary composting is completed the compost can be carted away and spread on the land with conventional manure spreading equipment. Spread compost at agronomic sustainable rates according to the nutrient content of the compost. Sawdust from the finished composted bins may be re-used in new bins. This will save on purchasing sawdust.
  6. Replace the bales that form composter bins as needed.
  7. Keep fresh sawdust as dry as possible because dry sawdust works much better in the composting process. Fresh sawdust in a pile will shed water reasonably well if the pile is mounded, with no pockets or depressions.
  8. Keep the area around the composter mown and free of tall weeds and brush. Watch for any leachate from the composter, and take steps to eliminate any leaching which might occur. Using more sawdust in the bottom of the bins can help eliminate leaching.
  9. In higher rainfall areas or where the bulking agent allows leakage and drainage following rainfall, composting should be conducted under a roofed structure.
  10. Ensure that all stock, dogs, feral cats and native fauna are fenced out of compost site. Electric fencing may be useful for this.

Composition

The analysis or composition of finished compost depends on the ingredients used and the ratio of dead animals to the amount of other ingredients in the composting process. The following table is a single analysis of finished pig-compost using only sawdust as a carbon source. No other ingredients were added.

Table 1. Typical composition of composted carcasses  
Nutrient Nutrient content of wet weight
% kg/tonne
Total nitrogen (TKN-N) 1.28 13.00
Ammonia (NH3-N) 0.22 2.00
Phosphorus (P) 0.27 2.84
Potassium (K) 0.28 2.90

The finished compost should be applied to land in a manner similar to that used for spreading animal manure. Compost should be spread at agronomic rates so that applied nutrients do not exceed the uptake capabilities of the crop being grown. Conventional agricultural manure spreaders are ideal for handling and spreading compost. Care should be taken not to spread compost in or near sensitive areas such as watercourses, gullies and public roads.

References

Adapted with permission from a paper by Charles Fulhage 'Composting dead swine', Proceedings of the 1993 Livestock Waste Management Conference, March 1993.

Further information