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3. Design and equipment recommendations for small- to medium-sized abattoirs


The ideal abattoir operation uses the line-slaughter system. “Line slaughter” entails hoisting up the carcass at an early stage, preferably beginning with the bleeding. All subsequent slaughtering and dressing procedures are carried out with the carcass suspended on and moving along an overhead rail (or line). Line slaughter is suitable for bovines, small ruminants and pigs.

This method is in stark contrast to the booth-slaughter system common in the past. In the booth system, abattoirs or slaughter spaces are divided into many spots (“booths”), which allow for the simultaneous slaughtering of a certain number of animals (“batch”). A team of men attends each slaughtering spot. The animals in a batch are each taken to a defined floor area, slaughtered and dressed in that spot. The simultaneous slaughtering often creates congested conditions and hygienic problems. The equipment is typically limited, in some places only a knife and axe are used while others have hoists for lifting the carcasses. Skinning cradles for flaying are used with cattle, and they help keep the carcasses from floor contact.

Batch slaughter also remains a frequently used system in APHCA-member countries, especially for large ruminants. The old central abattoirs still in use were designed for this system and have not converted their 50-year-old technology. But even some relatively new medium-sized municipal or private abattoirs were designed for batch slaughter because it is cheap to construct compared with a line-slaughter facility. In some places, slaughter personnel who have long used the batch practice have been adamant in maintaining their traditional way and have refused to use the modern slaughter lines. Part of the problem has been that, in some cases, the newer line-slaughter system was not properly designed and thus difficult to operate. Such inadequate installations were abandoned by the workers who returned to the booth- or batch-slaughter systems.

In very small slaughter operations where the line system is not warranted, the principle of booth slaughter is acceptable and widely applied. In small-scale conditions, the slaughtering and dressing of animals on the same spot can be done hygienically if the necessary equipment, such as hoists and skinning cradles, are used together with an overhead railing to facilitate the dispatching of the carcasses. This refers in particular to large and small ruminants. For pigs, due to the need for scalding, a slightly different system has to be applied (see page 31).

Slaughter systems for small ruminants

Small ruminants, at least those breeds that are short-haired (most tropical breeds), can be hygienically slaughtered even in field or small-scale conditions. In the simplest case, the bleeding is done on the ground on a cover of banana leaves, on a table (Fig. 71) or something similar. Then a rack with standard hooks or gambrels (hanger-like hooks) is used to keep the carcass in a vertical position off the ground for skinning and eviscerating (Fig. 16 and Annex A5). The branch of a tree may be sufficient for slaughtering on a farm premise (Fig.17).

Small ruminants’ skin, such as sheep skin, is usually firmly attached to the carcass and is usually not easy to manually pull off. The use of a knife is prohibited because of the risk of skin damage or damaging the superficial fascia (a fine membrane that improves carcass appearance and reduces surface shrinkage). Thus, it often requires the butcher to punch his fist forcefully between the skin and the carcass surface to detach the skin (“fisting”). Fisting is hygienically critical: The butchers must take care to frequently wash their hands and arms and not touch the dirty outside of the animal’s skin while removing the skin this way. A proposed solution to fisting is the use of compressed air; a compressed air pipe is introduced between the skin and carcass surface and the air pressure gradually detaches the skin. The air must pass through a filter in order to reduce the micro-organisms present in the air, which otherwise can constitute a source of contamination.

Fig. 16: Gambrel with dual function for small ruminant dressing: A shackle is inserted into one hind leg and then into the central loop of a gambrel for the skinning of the free (unsuspended) hind leg (1). Then the skinned legs are hooked into the gambrel (called “legging”) (2, 3). All the skin is removed (4), followed by evisceration (5). This type of gambrel can be used for moving a carcass on a rail (as shown here) or left stationary (see
Annex A5).

In many population centres of Asia’s developing countries, in particular the Muslim countries where sheep and goat meat are in high demand, large-scale batch slaughter facilities can be found operating with a daily throughput of thousands of small ruminants. Unlike the batch slaughter of cattle, small ruminant batch operations fortunately can be carried out more hygienically because, after bleeding, the carcasses can be suspended vertically on hanging racks with hooks or suspension devices (Fig. 18). Batch slaughter of small ruminants in large-capacity municipal abattoirs probably will not be easy to replace because this system allows butchers to control and identify their own animals, which might be difficult to do in the line-slaughter system.

Fig.17: Farm slaughtering of small ruminants

Fig. 18: Batch-slaughtering system for small ruminants: This facility has a huge slaughtering floor with a multitude of hanging devices for vertical skinning and eviscerating.

Wherever possible in medium- or large-sized operations, line slaughter (Fig. 19) of small ruminants is the best option. In addition, a separate bleeding line should be available (Figs. 19 and 20), at the end of which the legging is carried out. This involves removing the skin around the hock, exposing the tendon dorsal of the hock (Achilles tendon) and cutting off the foot. The carcasses are transferred from the bleeding rail to the dressing rail by inserting spreading hooks or gambrels at the Achilles tendon. In this position, evisceration is easy and hygienic. In contrast to batch slaughter, meat inspection in line slaughter can be done without difficulty.

Conventional vertical line slaughter of sheep breeds with a large amount of wool can cause hygienic problems. A great deal of meat-surface contamination occurs during the manual skinning of the hind legs of such sheep because it is difficult to avoid contact between the hair and the meat surface. Also, butchers have difficulty keeping their hands and arms clean during the procedure.

For those woolly animals, a double-railing system that allows the suspending of the carcass by all four legs has proven very useful (Fig. 21). The legs and belly region are skinned in this position and then the forelegs are released. The remaining still-attached skin parts can be hygienically removed and the carcass can be eviscerated in this vertical position.

Fig. 19: Line slaughtering of small ruminants with bleeding rail (left) and dressing rail (right): Legging takes place at the transfer station. Bleeding hooks roll back by gravity on the sloped rail. Transport on rail moves either manually for a maximum of 30/h with manual skinning (a) or by an overhead conveyer for a maximum of 60/h, although a mechanical skinning device (b) is needed.

Fig. 20: Bleeding on rail: An animal is suspended on one leg by shackle or by plastic rope, as shown here.

Fig. 21: Manual skinning of sheep with much wool: Suspension is from a double rail to avoid contamination during skinning.

Fig. 22: Hanging of dressed carcasses for dispatching or cooling

Slaughter systems for bovines

One of the principles of hygienic slaughtering is the lifting of carcasses off the floor at the earliest possible stage. Bovines, mainly cattle and buffaloes and in some areas also yaks, have to be lifted up mechanically. But many slaughter operators do not have the means for this or for proper carcass suspension. This results in a multitude of hygienic problems in bovine slaughtering.

Farm or field slaughtering of bovines is typically not very common, but it does take place, especially in an emergency situation due to drought or infectious disease. In the case of yaks, those that are kept in high altitudes are usually not near established slaughter facilities.

For slaughtering an animal in a farm or field situation, the so-called “tripod” (Fig. 23) has been developed. It allows for a carcass to be lifted up by means of a manual hoist, also called a “chain block”, after the animal has been stunned, bled and flayed on the ground. The tripod is composed of extendable aluminium posts, which are lightweight for carrying. However, special precautions must be taken when using this system to quarter the carcass: There may be a temptation to drop the quarters to the ground because there are no supporting rails. Also, if an entire herd of bovines has to be slaughtered, the tripod will create a bottleneck in terms of the time needed. This slowness may encourage the temptation to slaughter the animals simultaneously on the ground in order to speed up the process, but it will be at the expense of meat hygiene.

There were attempts in the past to construct mobile slaughter facilities, including the creation of a mobile container for slaughtering chickens. But the methods for slaughtering large animals either failed or were not very practical because the concept involved slaughter lines and equipment on trucks or in containers. If there is a need for a mobile slaughter facility in a field situation, the recommended approach calls for designing slaughter-line components, such as a rail system and hoists, in a way that they can be assembled and disassembled wherever livestock are to be butchered.

In some developing countries where programmes for upgrading slaughterhouses have been initiated, the introduction of the line-slaughter system has proven technically possible at a reasonable cost, even in small municipal communities, and acceptable to slaughter workers. However in some countries there was opposition to the method as pointed out in Table 1, for example in Indonesia and Pakistan. The opposition is among those workers and their employers, who are meat dealers or butcher shop owners, who are not prepared to change their traditional practices – much to the detriment of food safety and consumer protection.

The differences between booth-slaughter and line-slaughter of bovines are schematically drawn in Fig. 24. In booth slaughtering (see diagram “A” in Fig. 24), all operations take place in one spot. Bleeding and the initial flaying is carried out on the floor; if a hoist is available, the rest of the operation (completion of flaying, eviscerating, carcass splitting and quartering) can be carried out with the carcass in a vertical position. For cutting the hind and forequarters, an overhead rail of approximately 2.2 m height is useful.

Fig. 24: Features of the bovine booth-slaughtering and line-slaughtering systems

The booth (and batch) system can be modified to include some elements from the line-slaughter system (Fig. 24-B2 and Fig. 27) and thus provide another acceptable alternative to the simple booth system (Fig 24-A). For instance, bleeding can be done in a separate location (Fig. 24-b). As shown in system B2 (Fig. 24), a circular rail (3 to 3.5 m high) with several mobile hoists can be used in one of the following ways:

Alternative 1: After bleeding the animal at the blood-catching area and initial flaying and hock cutting on the ground, the carcass can be hoisted up and moved along the rail to a free space where the rest of the flaying, eviscerating and splitting can be done with it in a vertical position. The circular rail allows for returning the mobile hoists to the stunning and blood-catching area to be used again for the next carcass. The disadvantage of this method is that flaying has to be started on the floor.

Alternative 2: The animal, after stunning, is lifted by means of a special hoist for bleeding in a vertical position. After the bleeding, the hoist then moves to a mobile skinning cradle and the carcass is placed on the cradle, where the initial flaying operation starts and the feet are cut. The cradle moves to any of the remaining hoists and the carcass is hoisted up and into a vertical position then moved along the line to where the flaying, eviscerating and splitting are done.

Fig. 25 depicts the sequence of the slaughtering and dressing operations in the booth system. Operations a1 to a5 are carried out in the same booth. The manual lifting shown is done with a wall-attached hoist, which in this case is preferable to a simple manual chain block. Instead of manual hoists, electric hoists can be used. If a skinning cradle is used (Figs. 26 and 28), the hoist of the booth system is used two times: first for lifting up the carcass and to lower it onto the skinning cradle. Second, the hoist is needed to lift up the partly flayed carcass (Fig. 32) to a vertical position to complete the flaying as well as for eviscerating and splitting.

Fig. 25: Production flow (schematic) in booth slaughtering, with possibility of hoisting up the carcass: All operations depicted from a1 to a5 take place in the same spot, or “booth”.

Fig. 26: A simple booth- slaughtering facility: Cables linked to the hoist support the carcass spreader on both ends. The skinning cradle here (foreground) is made of concrete, and there is a cattle restrainer for stunning (left). Booth slaughtering can be acceptable in small-scale conditions.

Fig. 27: Modified booth-slaughtering system: The circular rail holds several manual hoists.

Fig. 28: Skinning cradles for small ruminants (the smaller one) and bovines.

For a simple bovine booth-slaughter facility, efficient manual or electric hoists are indispensable for lifting carcasses off the ground – the most critical feature in bovine slaughtering. But slaughter workers often refuse to use the hoists because they are too difficult to operate. If the hoists do not keep a carcass in a relatively stable, suspended position, the various slaughtering steps (flaying, eviscerating and splitting) cannot be done accurately. In such cases, the butchers will lower the carcass in a way that makes it manageable but that places the neck or shoulder parts in contact with the floor (Fig. 8) and with all the negative consequences for meat hygiene. Or they may even give up trying to work with an impractical hoist and resort to floor slaughtering.

Therefore, technically appropriate equipment must be provided. Spreaders attached only from the middle section of the device to the overhead structures (Fig. 29) do not achieve sufficient stability. A wall-mounted hoist, which has two suspension chains or cables, each one attached to both ends of the spreader, is the most suitable piece of equipment (Fig. 26 and Fig. 30).

Fig. 29: Inadequate suspension of carcass: A carcass partly flayed on the floor is being hoisted up. Due to the spreader suspension from the middle of the devise, the carcass is difficult to stabilize in a vertical position and thus allow for accurate work.

Fig. 30: Good carcass suspension: A spreader with connections on both ends, assisted by an electric wall hoist, maintains a stable position.

Although the middle-suspended spreader typically is not easy to work with, good carcass stability can be achieved if this type of device is used with an overhead rail. Using a specific landing device on the rail provides stability for accurately removing the hide as well as eviscerating and splitting the carcass (Fig. 31).

Line-slaughter systems for bovines can be subdivided into 1) simplified line slaughter, or semi-line slaughter (Fig. 24-B1) and 2) continuous–line, or full-line, slaughter (Fig. 24-C). The two differ in the position of the carcass: In the continuous-line slaughter, the carcass, once hoisted up, remains in a vertical position on the line throughout the entire process (Fig. 34), while semi-line slaughter uses vertical and horizontal positions (Fig. 33); the horizontal operations take place with the carcass on a skinning cradle. For this purpose, the carcass has to be lowered from its vertical position to the horizontal position on the cradle, where the initial flaying operation starts and feet are cut. Having done this, the carcass is gradually lifted up until it returns to the vertical position on the rail for complete flaying, eviscerating and splitting (Figs. 32 and 33).

Fig. 32: Sequence of flaying operations on skinning cradle in which the carcass is gradually lifted for complete flaying

Fig. 33: Semi-line slaughtering: The carcass is temporarily taken from the line and placed on a skinning cradle. Using the cradle avoids contact with the ground.

In semi-line slaughter, the feet are cut off while the carcass is on the cradle. To hoist the carcass out of the cradle, the spreader is inserted into the Achilles tendons of the hind legs (Figs. 29 and Fig. 40).

In the continuous-line system (Fig. 34), the animal is hoisted up for bleeding and the carcass remains in this position during the entire flaying and dressing steps. For bleeding, the stunned animal is hoisted up by means of a bleeding hook, also called a shackle (Fig. 39 and Fig. 41). An alternative to this procedure is a method used for ritual slaughtering in which the bleeding is done with the carcass in a horizontal position on the ground and then hoisting it up.

In the continuous-line system, changing from the bleeding hooks to the dressing hooks (Fig. 41) has to be done while the carcass is vertical. To accommodate this process, two rails with different heights are required: the bleeding rail (4.5 m) and the dressing rail (3.5 m). The change of hooks takes place at the transfer station (Fig. 34) in an operation called “legging” (see also Figs. 35 and 36).

Fig. 34: Continuous line slaughtering: The carcass remains on the line from the point of hoisting up for bleeding. Instead of the depicted mechanical hide pulling, simple manual flaying can also be used. Different heights for the bleeding rail (left) and the dressing rail (right) are required.

Figs. 35 and 36 illustrate the steps in the legging process. In Fig. 35, the carcass is suspended on the bleeding hook, which is positioned on the higher (4.5 m) bleeding rail (right). The hide from the other (unhooked) hind leg is then removed. The foot is cut off (as shown in Fig. 35) by using pneumatic tongs, or a “hock cutter”, but it can also be done with a knife). The dressing hook is then inserted into the Achilles tendon and connected to the lower dressing rail (3.5 m).

Fig. 36 shows the different positions of the carcass during legging. The carcass (far right) is suspended by means of a bleeding hook on the higher bleeding rail while the other leg remains free; hide removal begins here. The carcass in the middle is still suspended with one leg by the bleeding hook while the skin removal is completed on the other hind leg. After the foot is removed, the free leg is then hooked to the lower dressing rail. The carcass (far left) is suspended at the Achilles tendons of both hind legs on the dressing rail; legging has been completed on this carcass and it is ready for flaying and eviscerating.

Fig. 35: Carcass during legging

Fig. 36: Positions during legging

Comparison of two types of slaughter lines for bovines, with different capacities

As illustrated in Figs. 37 and 38, the following compares the technical details and differences in equipment and operation of two well-equipped bovine-slaughter lines for low throughput (line I, with 5–10 head per hour) and for medium throughput (line II, with 10–15 head per hour). Both types meet the requirements for line slaughter in small- to medium-sized abattoirs.

Space needed for the lines: The increase of the capacity, starting from a low level (I=5–10 head/h) to approximately twice this capacity (II=10–15 head/h) requires more space for line II (30 m versus 20 m) and more mechanization of the line equipment.

Stunning: The required size of the stunning box and equipment (preferably captive bolt pistol) is the same for both types.

Bleeding: The bleeding area should be larger for line II because it may be necessary to keep two to three bled carcasses there that are waiting for the next operation. Line I has a mobile bleeding hoist, while in line II the carcasses are suspended on shackles from the bleeding rail for bleeding.

Legging and transfer: In both types of lines, the legging is done at the transfer station.The two systems differ as follows: In line I the carcass is moved to the transfer station by a mobile bleeding hoist. Line II has a bleeding rail on which shackles (bleeding hooks, Fig. 41) hold the carcass and move it to the transfer station.

Flaying or hide pulling: In line I, flaying is done manually. Platforms at different heights allow workers to reach all areas of the suspended carcass. There are similar platforms in line II, but they accommodate only the loosening of the hide; removing the hide takes place at the mechanical hide-pulling station. For technical variatons of the mechanical hide-pulling equipment, see Figs. 44 and 45).

Eviscerating: This is carried out in line I by one operator who uses a platform that is adjustable in height. Because line II accommodates greater capacity, two operators are needed, one for the green offal and another for the red one. Both operators stand on a fixed platform, but at different heights.

Carcass splitting: Typically, electrical splitting saws are used in both lines; but given the small throughput of line I, simple tools (axes) will work sufficiently.

Inspection: For veterinary carcass inspection as well as internal quality control, a station with platforms is useful for line II. In line I, both types of carcass inspection can be done from the floor. For the meat inspection of the internal organs and the heads, both types are not large enough to justify a mobile viscera table and head transport facilities that are synchronized with the movement of the carcasses, as shown in Fig. 79. In both types, viscera carts (Fig. 76 and Annex 11) and pluck and head-fixation facilities (Figs. 77–78) are sufficient. However, carcasses and corresponding organs need to be easily identifable.

Carcass washing: In line II, a stainless steel washing tunnel with spray nozzles is useful. For line I, the carcass washing can be done by using a simple water hose; but the area for the washing should be separated to keep the washing water from spreading across the floor.

Fig. 37: Bovine slaughtering line (I) for low throughput (5–10 head/h) Minimum length: 20 m

Fig. 38: Bovine slaughtering line (II) for medium throughput (10–15 head/h) Minimum length: 30 m

Figs. 39–45 illustrate some additional features for slaughter of bovines. See also Annex A14 and 911.

Fig. 39: Hoisting up with bleeding hook

Fig. 40: Beef carcass on dressing hooks on flat rail: The carcass is hung by a simple spreading bar.

Fig. 41: Bleeding hook, or shackle, (top) and dressing hook

Fig. 42: Manual flaying: Care is needed to ensure that no knife incisions occur and that no meat remains on the inside of the hide.

Fig. 43: Damage of tanned hide: Unskillful flaying results in many knife incisions that render the hide worthless. This happens particularly during traditional floor flaying. A mechanical hide puller is the best guarantee for good hide quality.

Fig. 44: Simple mechanical hide pulling by means of an electric hoist with an endless chain: The hide is stripped off from top to bottom. This device is well suited for both lines I and II.

Fig. 45: Advanced mechanical hide pulling using a chain elevator: The hide is stripped off from top to bottom by fixing the carcass at the forelegs. This method is better suited for larger operations.


Hygienic bovine slaughtering involves mechanized equipment. The cost of setting up cattle-slaughter facilities depends on the quantity and type of technical equipment used.

Hoists are indispensable; good-quality manual hoists cost US$500–1 000 while electrical hoists are US$2 000. Other electrical equipment that are considerably useful (but can be supplemented with manual tools in the small- to medium-sized facilities) are the electrical splitting saw (imported cost: US$10 000), a breastbone saw (US$5 000) and a hock cutter (US$4 000). As well, captive bolt pistols (imported) cost US$300–400 and the ammunition runs about US$0.10 per cartridge.

The other items, such as stunning boxes, overhead rails, platforms, working tables, chutes, viscera tables and carts, hanging racks and, in most cases, the bleeding and dressing hooks, can be fabricated locally at much less expense than buying on the international market.

In an APHCA-member country, building a slaughter facility (building only) requires an estimated US$300 per square meter. Thus, a physical structure housing a small- to medium-sized bovine slaughter line occupying 20x10 m would cost approximately US$60 000. The most expensive part of a modern slaughter facility is the refrigeration units. Building and installing a normal-sized carcass “chiller” (refrigeration unit) would cost around US$140 000. Carcass chillers are not used in traditional meat marketing practices. However, with the gradual shifting of consumer preference for chilled meat sold in quality shops and supermarkets, there will be an increasing need for carcass chillers.

Approximate costs of equipment for booth slaughter of bovines (in US$):

Manual hoist

1 000

Electric hoist

2 000

Skinning cradle



1 000

Railing, hooks

1 000 (up to 3 000)

There are several options for equipping a bovine booth slaughter facility, from staying small and requiring only one hoist, no cradle and a short rail to the much larger system that involves a bleeding hoist, one or several cradles, one or several skinning hoists and an extensive rail system. The total expense generally will vary from US$3 000 to 15 000.

Approximate costs for a low-throughput (10–15 head/h) bovine slaughter line (in US$):

Stunning box

3 000

Captive bolt pistol


Rodding equipment

1 000

Hock cutter

4 000

Electrical hoists (3)

6 000

Platforms (galvanized, stationary)

6 000

Overhead rails (galvanized)

10 000

Breastbone saw

5 000

Splitting saw

10 000

Head-washing cabinet

5 000

Carcass-washing cabinet

6 000

Chutes, viscera carts,

racks, tables

10 000

Hooks, small tools

5 000


71 400

The above figure reflects the minimum cost for a low-capacity, partly mechanized installation that allows for the efficient line slaughter of bovines. For a slaughter line complete in all aspects, especially to comply with hygienic requirements, some additional equipment will be necessary: meat inspection tools and installations, hand-washing basins and thermostat-regulated hot water receptacles for sanitizing knives, saws and other tools. Expenses for such a good-quality slaughter line can run as high as US$100 000.

Slaughter systems for pigs

Farm slaughter of pigs is very common and easier to do than with bovines because they weigh less (around 110 kg for a 4-month-old pig). Of course, sows and boars are much heavier and hence more difficult to handle without proper slaughtering facilities.

In contrast to other livestock, pig skin is commonly eaten. This requires the removal of the hair during slaughtering, which is done by scalding the pigs in hot water (60°–62°C) or by steaming the skin in more advanced operations. Both procedures serve to loosen the hair, which is then removed by a scraping process. In small- to medium-sized operations, a knife is used for scraping, but in larger operations machines are used.

Once the hair is removed from a pig, there should be no further contact between the floor and the skin (which will become food for human consumption). There are ways to maintain this principle in a farm process: Scalding can be done by either pouring hot water onto the carcass or submerging it in a container filled with hot water. As shown in Fig. 46, the problem of loading and unloading the carcasses can be solved by using a wire mesh carrier. The carcass is then placed on a ladder-type device, in a horizontal position, for dehairing and then turned vertically up. In this position, eviscerating and carcass cutting can be hygienically carried out (Fig. 47).

Fig. 46: Using a stainless wire mesh device for loading and unloading of pig into a scalding vat

Fig. 47: After scalding and dehairing, place the carcass in horizontal position on an ordinary ladder (1) and then turn the ladder vertically (2) for eviscerating and cutting (3).

Commercial pig-slaughter operations in APHCA-member countries usually have a minimum daily throughput of 50 animals in a small facility and up to 400 or more in a larger place. Small-scale pig slaughter for commercial purposes is rare. There is evidence of this level of operation in Nepal, and the small-scale slaughter method just described (Figs 46 and 47) would be applicable. However, in those operations, there appears to be inadequate separation of live pigs from the carcass dressing and fresh meat selling (see the Nepal country report in Annex B).

In medium-sized and larger pig-slaughter operations, which are common in most Asian countries, hygienic slaughtering can easily be achieved by equipping the facility with a modern set-up (Fig. 54): railing systems with elevating devices, electrically heated scalding vats, lifting devices for moving carcasses into and out of the vat, dehairing machines with mechanical loading and unloading devices and electrical splitting saws.

However, in most of the pig-slaughter facilities in APHCA-member countries, modern systems are not in place and are not likely to be introduced any time soon. As long as the pork goes unchilled to the markets and consumers, such a system may not be necessary. What is critical is the elimination of the unhygienic batch slaughter of pigs (shown on pp. 14-15). This can be done by installing a semi-line system, which enables hygienic slaughtering and carcass dressing without electro-mechanical equipment. Such systems have been developed to good standards in the Philippines, for example.

In many pig-slaughter facilities in APHCA-member countries, there are no proper lifting devices and carcasses are dragged along the floor from the stunning and bleeding stages to the scalding and onto the scraping tables. For example, in the facility illustrated in Fig. 48, pigs are stunned behind the fencing at the left-hand side. Carcasses are then dragged across the floor and manually lifted into the scalding vats, which stand 1 m high. From there, carcasses are dragged onto the scraping tables where they are also eviscerated horizontally. They are only hung on rails for splitting.

A great deal of improvement can be achieved if the pig slaughtering and dressing are carried out on floor surfaces constructed at different levels (a “terraced” or multitiered system). In this approach, the killing of the pigs takes place at the highest level and carcasses are gradually lowered during the various procedures. This system utilizes gravity and facilitates moving the carcasses without complicated electro-mechanical elevating equipment.

In a small-scale facility, two different tiers, or levels, of the floor may be sufficient. However, evisceration must be done with the carcass in a horizontal position after completing the manual dehairing (Fig. 49), although horizontal evisceration is not the best hygienic solution. Also, if carcass splitting is carried out (which is not the case everywhere), it may be necessary to do so in a horizontal position on the table.

Fig. 49: Small-scale pig slaughtering on a two-tiered floor

Much better working and hygienic conditions can be achieved with three tiers, or levels, of floor surface (Fig. 50). The bleeding of the pigs after stunning and the collection of the blood have to be done carefully for hygienic reasons. Blood is usually collected in a receptacle that is held close to the bleeding wound. Care should also be taken that the scalding water is renewed or supplemented with fresh water from time to time and maintained at the correct temperature (60°–62°C).

In the two- and three-tiered systems, the overhead railing starts at the scraping and gambrelling area. From here, all procedures are organized in a straightforward and hygienic manner. Above the scraping table, the rail can be kept low for easier gambrel lifting of the carcass (with the hind legs of the pig hooked). The eviscerating and splitting procedures take place on another lower level of floor, which enables the operators to work more easily because the rail height at their level is approximately 2.6 m high. In this method, there is no risk of the carcass touching the floor.

Fig. 50: Medium-scale pig slaughtering on a three-tiered floor

The great advantage of the terraced (multi-tiered) floor is the lack of need for electro-mechanical lifting or moving devices. An overhead railing system that starts at the gambrelling operation (at the scraping table) is sufficient and can be manually operated.

To further facilitate these manual operations, clever designers have created some simple devices that supplement the technically good and hygienic traditional slaughtering of pigs, such as in Indonesia and illustrated in Figs. 51 and 52. Moving the pig carcass from the scalding vat up to the scraping table requires physical strength. As shown, rollers installed on the outlet side of the scalding vat facilitate such work (Fig. 51). If mechanical lifting of pig carcasses is required at any stage in the slaughter line, the device shown in Fig. 52 can be useful.

Fig. 51: Scalding vat (stainless steel) with metal rollers: The rollers facilitate the transfer of pig carcasses from vat to scraping table.

Fig. 52: Manual lifting device for pigs: This device is useful for transferring carcasses from the scraping table to an overhead rail.

Properly done traditional pig slaughter on a three-tiered floor results in hygienically impeccable pork. The system is essentially semi-line slaughter because, starting from the gambrelling operation, all processes take place on the rail. Investment costs are low because only a few pieces of technical equipment are needed (beyond the physical building): electrical stunning tongs, a scalding vat (preferably electrically heated with thermostat for maintaining a constant temperature), a scraping table and overhead rails with gambrels.

Fig. 53: Core equipment for a “terraced”, or multitiered, slaughtering line: This entails the scalding vat (bottom), scraping table (middle) and the overhead rails.

Approximate costs of equipment for a terraced pig-slaughter line (in US$) (one line with 20 head/h):

Electrical tongs (imported)

2 000

Scalding vat (stainless steel, locally made)

3 000

Scraping and gambrelling table (stainless steel, locally made)

1 500

Railing system (galvanized, locally made)

3 000

Hooks/gambrels (stainless/galvanized, locally made)

1 500


11 000

In contrast to the traditional lines, modern pig-slaughter lines (Fig. 54) consist of the following technical devices for smooth production and adequate hygiene:

Electric breastbone and splitting saws are commonly used to replace the knife and axe. More sophisticated lines also use vacuum bleeding with a hollow knife, carcass singeing and polishing machines, and automated conveyer systems.

Fig. 54 and Annex A6-8 illustrate the basic equipment and production steps of a modern pig-slaughter line.

Fig. 54: Modern pig-slaughtering line with basic equipment (from 12/h): The line length is 20 m.

Bottlenecks, or production jams, which determine the throughput per hour, tend to occur at the scalding vat stage. Minimum scalding time for one pig should be four minutes. If the scalding is done one by one, 12 pigs can be put through per hour – on the assumption of five minutes for one pig (scalding plus transfer time). To increase the frequency, machines that enable simultaneous scalding of several pigs have been developed (Fig. 55). Nowadays, these are widely used in medium- to larger-sized pig-slaughter facilities.

Also, larger operations have replaced the relatively slow manual or electrical hoists used in the bleeding and at gambrelling operations (as shown in Fig. 52 and Fig. 54) with continuous electrical elevators (Fig. 56). These are very practical because bleeding hooks and gambrels can be easily hooked on to lift up the carcasses.

Fig. 55: Scalding machine for simultaneous scalding of several pigs: The slowly rotating drum moves pigs through the scalding bath in exactly the time necessary for efficient scalding. Left: side view; right: front view.

Fig. 56: Continuous electrical elevator

The gambrelling from the scraping table enables the suspending of the carcasses in a spread position (Fig. 57), which in a medium-sized operation allows for the final hair removal (by knife), singeing (manually by torch), eviscerating and splitting.

However, the spread position also can be achieved with ordinary dressing hooks in two ways: either by inserting a metal bar (“spreader”) between the hooks (Fig. 58) or by installing a double rail, with one hook attached to each rail (Fig. 59).

Fig. 57: Gambrels


Fig. 59: Double-rail system: The carcass is hoisted up on two converging rails; the horizontal rails are parallel (approximately 1 m apart).

Fig. 58: Spreader: Hooks on each hind leg are separated by the spreader.

Facilities in APHCA-member countries generally need to import (from outside the region) some of the core equipment necessary for a modern pig-slaughter line, such as the electrical stunning tongs, scalding machines with rotating drums and dehairing machines. However, manufacturers recently began producing electrical stunning equipment in the Philippines (Fig. 60) and electrical scraping and dehairing machines in Vietnam (Fig. 61) for use in commercial pig slaughtering. A little more time is needed to determine if this regional production is of equal quality with the equipment brought in from outside the region.

Fig. 60: Electrical stunning equipment manufactured in the Philippines

Fig. 61: Dehairing machine manufactured in Vietnam

Approximate costs of equipment for a low-capacity modern pig-slaughter line (in US$) (similar to Fig. 54 and Annex A7, Fig. 99)

Electric stunning equipment (imported)

2 000

Electric hoist (for bleeding and loading vat)

1 500

Scalding vat 1.5x2.5 m (electrical heating  and discharger) 

3 000 

Dehairing machine                          

30 000

Scraping/gambrelling table (stainless steel)

1 500

Gambrelling hoist                            

1 000

Railing system (galvanized)               

6 000

Hooks, gambrels (galvanized/stainless steel)

3 000

Platforms (galvanized)                     

2 000

Electric splitting saw (imported)

5 000

Breastbone saw                               

2 000

Miscellaneous equipment                  

5 000


62 000

The dehairing equipment used will considerably affect the total expense. If imported from a developed country, a basic dehairing machine will cost around US$30 000 (or up to US$60 000 for a simple but higher capacity one). The costs can be considerably higher if a more expensive imported dehairing machine is integrated into the line. However, by using locally fabricated items elsewhere in the line, if available, and using manual tools (axe and knife) for the splitting, the overall investment can be considerably lower. And if the regional production (Vietnam) of a dehairing machine proves successful, the expense for a basic machine can be cut by half.

Certainly highly mechanized slaughter lines greatly facilitate proper pig slaughtering; but they are also prone to technical defects that might not be easy to repair under the conditions of a developing country (lack of spare parts and technical expertise). Also, investment costs for the modern slaughter lines are much higher than for the traditional system. The trend in Asian countries, especially in private-sector enterprises operating ambitious high-quality meat production, is a shift to the mechanized slaughter lines. However, the many private and public slaughterhouses that rent out their facilities to meat dealers with their own butchering teams will certainly continue running traditional lines, which are technically less complicated, less subject to breakdowns and more durable in harsh day-to-day use. For these facilities, a shift to the terraced-floor method (at three levels, as shown in Fig. 50) that requires only a minimum of mechanical equipment is recommended.

Multispecies slaughter systems

Slaughter systems for multiple species do not seem to be in high demand in APHCA-member countries. In theory, multispecies refers to a combination of pig and small ruminants or bovine and small ruminants. Because small ruminant slaughtering is of importance mostly in Muslim countries (where it is also an option for meat export), the combination with pig slaughtering can be ruled out.

There are a number of combined bovine- and small ruminant-slaughter lines, especially in countries with large sheep and goat populations. Such slaughter lines are primarily designed for bovines. Small ruminant slaughtering on the same line uses the same railing system but with different hooks. For small ruminant slaughtering and dressing, gambrel-type hooks with a long central straight metal bar are used; these keep the small carcasses low enough to allow skinning and dressing operations by workers positioned on the same platform, which typically are used for cattle slaughtering. In facilities that slaughter both small ruminants and bovine, the line requires modification to include a second lower line (rail) only for bleeding of the small ruminants (otherwise they are too high) that enters at a transfer station. (Such a system has similarities to the line shown in Fig. 19.) At the end of the dressing line, special hanging racks (Fig. 22) move the small ruminant carcasses along the relatively high bovine railing systems.

For very small operations in countries with beef and pork consumption, a facility for both bovine and pig slaughtering similar to those developed in Europe is recommended. This type enables the slaughtering of both species in a vertical manner.

As shown in Fig. 62, bovines are partly flayed on a cradle and hoisted up by the electric hoist (b) and beef spreader (a) for eviscerating and splitting. Pigs are first scalded and then hoisted up by the lifting device (c) for eviscerating and splitting on the twin rail (d). Of course, both operations do not take place simultaneously. For beef slaughtering, both lifting devices (a) and (c) are used, as follows: After splitting the beef carcass on device (a), the two forequarters are hooked into (c) and the hind and forequarters are separated. After moving the forequarters onto the rail (d), the hindquarters are transferred from (a) to (c) before landing on the twin rail (d).

Currently utilized stunning methods and recommended improvements

Internationally recommended methods for stunning livestock are rarely used in APHCA-member countries. Especially with the stunning of bovines, for which the practised method is to inflict a blow to the head with either a hammer (Fig. 64) or the back of an axe. For bovines, in particular buffaloes, another common practice is to stab a pointed knife, or “puntilla” (Fig. 63), into the foramen atlanto-occipitale and sever the spinal cord. The foramen is easily accessible in buffaloes, whose thick skull and skin render a hammer blow inefficient. There are abattoirs producing top-quality beef using stunning pistols with a mushroom head (acceptable to some Muslim communities, as shown in Fig. 72) or a penetrating captive bolt pistol (Fig. 67). Stunning pistols of the captive bolt type should always be used on animals that are restrained in stunning boxes (Figs. 68 and 69 and Annex A 12). See Fig. 70 for a depiction of electric stunning of cattle.

Electric stunning tongs are frequently used for slaughtering pigs, although they are mostly of the home-made type, such as the one shown in Fig. 60 (which has a transformer) or the very primitive wooden tongs connected to the mains, as shown in Fig. 65. In both cases, the electrical parameter used most likely is not the most suitable for pig stunning because they may cause a great deal of pain and thus negatively influence the biochemical properties of the meat.

Fig. 63: Using the “puntilla” for stunning

Fig. 64: Using a hammer for stunning

Fig. 65: Primitive home-made wooden tongs for stunning pigs, by directly connecting them to an electrical source, with no transformer

Fig. 66: Proper electrical stunning with suitable equipment

There is an urgent need for acquiring proper stunning equipment in facilities throughout the APHCA-member countries. This can be as simple as purchasing electrical tongs for stunning pigs (Fig. 66). If locally produced tongs are not satisfactory, imported ones that allow for the regulation of the voltage (120– 300 V, according to the size of the pigs) and other parameters (amperage and frequency) by a transformer should be used. These typically cost around US$2 000. Electrical stunning tongs can be used wherever the power supply is available and they normally do not need a great deal of maintenance and supplies.

In the case of captive bolt pistols for bovines, penetrating types are relatively cheap to acquire (US$300–400). However, typically there are problems maintaining a supply of cartridges. The different pistol manufacturers (all located in developed countries) also produce the cartridges, specifically made for each type of pistol. But importing a continuous supply of cartridges can be difficult for individual abattoirs in countries with air freight or firearm restrictions. In these cases, the government livestock department should facilitate the importation.

There was an attempt in the past in Thailand to produce cartridges for captive bolt pistols in military factories. But the enterprise failed due to a small demand for the ammunition; a pooling of needs from several APHCA-member countries could make such an attempt more economically profitable and thus successful.

Fig. 67: How the captive bolt pistol operates

Fig. 68: Stunning box with revolving door for ejecting animals

Fig. 69: Wall-attached stunning box for small-scale operations: The hinged door (a) allows animals to enter; the upward-sliding door (b) allows them to be released; protective grid (c); the steel plate (d) prevents animal from lowering its head.

Fig. 70: Electrical stunning of cattle: The photo shows an animal about to enter a neck restrainer; once the animal is restrained, electrodes from a stunning device are placed on the nose and heart. This is an accepted Halal method used in a Jakarta abattoir.

Restraining livestock for Halal slaughter

Restraining small ruminants for Halal slaughtering is easy – animals are manually laid on a bleeding table and their throat is then sliced (Fig. 71). Restraining bovines is much more difficult because they are less docile. Without supporting equipment, bovines are usually forced down by attaching ropes to the fore and hind feet of one side that are then firmly pulled from the other side.

Other livestock subject to Halal slaughter are camels, and they are even more difficult to restrain. Some butchers resort to such horrible methods as severing the hock tendons to make them collapse.

Fig. 71: Halal slaughtering of sheep

Fig. 72:  Captive bolt pistol with mushroom head

Some Muslim authorities regard the electrical stunning of small ruminants and bovines (Fig. 70) as acceptable. Some also tolerate the use of non-penetrative captive bolt pistols, which have a mushroom-shaped top on the bolt (Fig. 72) and inflict a blow to an animal’s head.

Currently, a meat scientist in Pakistan is working on behalf of Australian authorities on a comprehensive review of Halal slaughter, including potential methods to reduce animal suffering. The completed report would be worth future discussion.

Specialists within the Meat and Livestock Australia (MLA) organization have developed a cattle-restraining box (Fig. 73) for use in Asian countries. The box facilitates the laying down of cattle and the cutting of the throat. However, it still enables the animal to struggle a great deal. A cattle “restrainer” (Fig. 74) that rotates 90 degrees has been used in the Muslim south of the Philippines, though it is unclear if it is working satisfactorily.

Fig. 73: Bovine restraining box: A rope is attached to the left fore and hind feet and pulled from the opposite side, under the wall of the box; the opposite wall (b) is closed during bringing down the animal.

Fig. 74: Rotating bovine-restraining box: The box with the restrained animal can be mechanically tilted 90 degrees to facilitate Halal slaughtering.

Meat inspection

Presumably, all APHCA-member countries have introduced some type of meat inspection law. A legislative framework is necessary to enforce the regulations or directives on meat inspection. The regulations need to provide technical specifics on ante-mortem and post-mortem inspection procedures and judgement of carcasses or carcass parts as well as definitions of technical terms and registration requirements for commercial abattoirs.

Detailed regulations on ante- and post-mortem inspection are not commonly used by meat inspectors or are even not available in some countries. This explains why, in many cases, meat inspectors lack a systematic approach when conducting inspections in the slaughter line. Some have been observed subjectively choosing organs or lymph nodes for visual scrutiny, palpating or incising checks while ignoring other parts, such as a bovine head, that should always be included in routine post-mortem inspection. The on-the-line meat inspections that were witnessed during the research period for this report were only carried out in a few abattoirs that were producing either for export or for top-quality meat shops. Inspection by an official government meat inspector was observed in only one pig abattoir visited, which is producing for the local wet markets. In the majority of the abattoirs and slaughter slabs visited during the research period, no inspection was ever carried out for wet-market meat. There were veterinarians or meat inspectors present in some places, although they were seen tending to office work and not inspecting the meat.

Also noticed during the research were a number of exhausted or obviously sick livestock brought to be slaughtered. The resulting meat was included in routine deliveries to markets or shops. These practices, which are obviously very common, underscore the need for more professional approaches in meat inspection including ante-mortem inspection in the interest of food safety and consumer protection.

The situation is made more difficult because the meat inspection proclamations or laws, where they or supplementing regulations exist, are issued by the central government. They refer to government-controlled abattoirs, such as export abattoirs. State- or municipality-run abattoirs do not seem to acknowledge that legislation as binding law. In other cases, the Ministry of Agriculture is responsible for meat hygiene in general, but the Ministry of the Interior or Ministry of Public Health has responsibility for meat inspection. Such diffusion of roles can cause a great deal of confusion, which only enables officials to pass on responsibilities to others. Such a situation used to be the case in Thailand; the Government has since delegated and made clear all meat hygiene and inspection responsibilities to the Ministry of Agriculture.

The urgent need for professional approaches and consolidated responsibility applies also to abattoirs producing for the domestic supply. These facilities should be held accountable to the same hygienic requirements as export operations, including ante- and post-mortem inspection of meat, and which should be coordinated by only one ministry.

Responding to this need begins first with the necessary updating of existing laws and/or developing meat-inspection regulations. The new Code of Hygienic Practice for Meat (CAC/RCP 58-2005, which was revised in 2005) of the Joint FAO/WHO Codex Alimentarius Commission can be an appropriate guideline. The Codex Alimentarius code consists of internationally accepted standards and recommendations for food hygiene and food technology/composition.

The second step is the training of meat inspectors. In 2001-2002, FAO conducted meat inspection training for the Asian region, and this type of initiative needs to be resumed. The APHCA would be an appropriate forum to initiate a training programme. However, in the interest of food safety and consumer protection, individual governments should also be more proactive in organizing national meat inspection training typically practised in many other areas of the world (Fig. 75).

Fig. 75: Practical training in meat inspection: Veterinary authorities in an African country organized national training courses in 2007.

Meat inspection and sanitary control in many abattoirs and slaughter slabs in APHCA-member countries currently are unsatisfactory. What is really needed in many countries is a major overhaul of improper operations; inspectors should take commonly accepted rules of meat hygiene and condemn any abattoir that does not meet those standards, on the basis of violating hygienic principles and creating hazards to food safety. If a local meat inspector is in too weak of a position to do this, government veterinary delegations should take on this task. Targeting the unsafe practices would help clean up many rough-shod productions and force entrepreneurs to upgrade their premises.

If there are unacceptable hygienic conditions at abattoirs or slaughter slabs or insufficient or no meat inspection being carried out, the inspectors should be targeted for some type of accountability. But there also needs to be improvements to the technical facilities for conducting proper meat inspection, which currently are very poor in most abattoirs. For instance, cattle heads are not inspected partly because of inspector negligence but also because they are not properly prepared – they have not been skinned and there are no racks to hold them. There is also a lack of viscera tables and viscera racks to be used for inspection purposes. Carcass inspection is frequently impeded by the cramped hanging of carcasses. Often there is not enough space or light for meat-inspection procedures. The worst situation is with the booth and batch slaughter, for which inspectors literally have to rush from one place to another to track carcass parts before they disappear into store rooms or transport vehicles. These types of operations have none of the equipment necessary for proper inspection; identifying organs to the corresponding carcasses is impossible in many places.

Although there is considerable technical reorganization needed to improve meat-inspection standards, this report offers only a few simple technical suggestions (Figs. 76–79).

Fig. 76: Viscera cart for green bovine offal: In small operations, green offal can be inspected in the cart after the evisceration process.

Fig. 77: Simple but efficient suspension on chain hooks of cattle heads and plucks for meat inspection

Fig. 78: Meat inspection in a small abattoir: Cattle heads and plucks are suspended on a rack for proper inspection.

Fig. 79: Synchronized meat inspection for larger abattoirs: Simultaneously with the beef carcasses, carcass parts (head, plucks and feet) move on an overhead conveyer to pass through the meat-inspection station.

Sanitation and effluent treatment in abattoirs


The term “sanitation” refers to the cleaning and disinfecting of abattoirs as well as the controlling of insects and rodents through the use of chemical substances.

Necessary conditions for efficient cleaning and sanitation:

During the research for this report, these conditions were rarely observed and then only in a few abattoirs producing for either the export or top-quality domestic market. Most abattoirs in APHCA-member countries are not “cleaning-friendly” (Fig. 80); they lack smooth, washable and impermeable surfaces. Instead, livestock slaughtering takes place on cracked concrete, paved slabs or even bare ground. Where cleaning and disinfecting are impossible, there will be a very high level of permanent contamination of the facility.

Fig. 80: Slaughterhouse floor and sidewalls impossible to properly clean and disinfect

Fig. 81: “Cleaning-friendly” floor and walls with smooth and impermeable surfaces

To improve the cleaning and sanitation situation in abattoirs, physical structures need to be upgraded to make them “cleaning-friendly” (Fig. 81). “Cleaning” refers to the removal of dirt and organic substances, such as fat and protein particles, from surfaces, walls, floors, tools and equipment.

For proper cleaning, the following techniques are required:

“Disinfection” is the complete removal of micro-organisms from surfaces, floors and tools. Disinfection is achieved either by using hot water (or steam, preferably) or chemical disinfectants (Fig. 82). Chemical disinfectants are the preferred compound for use in the meat industries.

Common chemical disinfectants that are recommended for use:

During the research for this report, workers in a traditional pig slaughterhouse were seen using chlorine compounds for disinfecting; most probably it was the commercially available household bleach (sodium hypochlorite). This substance is suitable and efficient for disinfecting such traditional facilities – as long as the right concentration is used – because they barely use mechanical metallic equipment or machines. Chlorine compounds, otherwise, react very aggressively (corrosion) on the metal. However, the chlorine compounds do not affect floor and wall tiles or stainless steel equipment, such as scraping tables. Highly mechanized abattoirs should use quaternary-ammonium compounds or oxygen-releasing substances when disinfecting.

Best disinfection results are achieved when intensive dry/wet cleaning precedes chemical disinfection.

Pest control of insects (Fig. 7) and rodents appears to be a very rudimentary effort in abattoirs in APHCA-member countries, if at all.

Fig. 82: Disinfection of walls using portable spray equipment: For workers’ protection, the wearing of gloves and face masks are recommended.

Effluent treatment

Slaughterhouses in APHCA-member countries typically have insufficient or, in many cases, no effluent treatment facilities. Thus, they create a great deal of pollution to water flows and the surrounding environment. While medium-sized and larger abattoirs have septic tanks, they tend to be poorly built, in a poor state of repair and/or not properly maintained – not emptied at specific time intervals.

Liquids resulting from such treatments and allowed to drain into the environment are highly polluting, as the following two examples illustrate (Figs. 83 and 84):

Fig. 83: Effluents dumped untreated beyond the walls of a rural abattoir

Fig. 84: Lagoon system combined with initial solid screening: This system is not ideal but is used for effluent treatment by an abattoir producing meat for top-quality markets.

Effluent treatment in small- to medium-sized abattoirs is a difficult issue and yet a major cause of poor hygiene. Highly efficient, combined mechanical and biological treatment plants are beyond the reach of such abattoirs because they are too costly in initial investment and day-to-day running. But simple treatment facilities such as soak-away pits or septic tanks do not produce the necessary results and are not capable of significantly reducing the pollution.

One solution out of this dilemma is the introduction of biogas digesters for abattoir effluent treatment. Biogas digesters are relatively inexpensive; construction costs for a 30 cubic meter digester is approximately US$10 000. They require little maintenance and generate energy (gas to be used for energy requirements at the abattoir) and the solid residues can be used as fertilizer.

FAO has promoted this technology and initiated the construction of some prototype biogas digesters in connection with abattoirs, the most advanced installed at the Animal Products Development Center, a national meat training centre in Manila, Philippines.

Biogas digestion is the breakdown of organic substances that make up almost 100 percent of the pollutant load of farm or abattoir effluents, primarily to methane gas but also to CO2 (to a minor extent) and traces of other gases through the activity of anaerobic bacteria. Anaerobic conditions are created through the construction of air-tight chambers usually submerged in the ground. The construction materials used are hollow blocks, cement, sand, gravel, rebars and pipes. The capacity of the digester chamber depends on the volume of effluent to be treated and the retention time; but generally for small- to medium-sized abattoirs it holds 30 cubic meters. When biogas digestion is completed, most organic matter is broken down to methane, CO2 and water. Remaining solid materials are primarily sludge.

Observations made during the research for this report indicates that the biogas digesters used for farm purposes need to be modified for abattoirs because the effluents from these facilities contain higher amounts of fat residues. Such fat residues have a tendency to generate a fatty layer or scam at the surface of the liquid in the chamber, which creates problems for the proper anaerobic digestion of the effluents. To tackle this situation, a double-chamber “digester” for abattoir waste treatment was developed, as illustrated in Fig. 85. Periodic release of the gas produced creates a mixing of the liquid contained in both chambers of the fixed-dome digester and prevents the formation of the fatty scam, as the following explains:

A = Starting point of biogas production.

B = Advanced phase of biogas production; strong gas pressure under the “dome” pushes fluid inside the digester into the second fermentation chamber.

C = Release of the biogas (through a pipe on top of the dome) causes the collapse of the gas pressure and the flowing back of the fluid into the first fermentation chamber; this produces the mixing effect desirable for abattoir effluents.

D = Renewed build up of gas pressure and repetition of the mixing effect once the gas is released.

The pollutant load of effluents is measured as “biological oxygen demand” (BOD). The BOD of abattoir effluents after treatment in a biogas digester is around 80 mg/L. Some countries have established standards for treated effluents for release into water flows. This standard is, for example in the Philippines, 50mg/L BOD. This means that after the anaerobic biogas digestion (in the “fixed dome digester” shown in Figs. 84 and 85), additional treatment is recommended to comply with stringent environmental-protection laws. This could be a combination of aerobic and anaerobic treatments, such as a series of sedimentation and fermentation chambers (an “anaerobic baffle and filter reactor”, as shown in Fig. 86). For less stringent environmental-protection requirements, the anaerobic effluent treatment in the dome digester is sufficient. The retention time in the digester depends on the volume of wastewater and the pollution load and may vary from one to four weeks.

Fig. 86: Biogas digester of the fixed dome type: Side view (top), ground view (bottom). The “fixed dome digester” units (see also
Fig. 87), where the main part of the biogas digestion takes place, are combined with additional treatment units (“anaerobic baffle and filter reactor” built according to the septic tank principle) (see also Fig. 88) for further purification, if needed, of the treated wastewater.

Fig. 87: Construction of a biogas digester of the fixed dome type: Ceiling of the dome completed (middle), second fermentation chamber still open (bottom).

Fig. 88: Construction of a biogas digester: An anaerobic baffle reactor under construction.

The treatment capability and capacity of biogas digesters is typically not sufficient for effluents from larger abattoirs. For proper treatment of such wastewaters, aerobic biological treatment plants with mechanized and pumping elements (Figs. 89 and 90) are needed. There are some high-quality abattoirs in APHCA-member countries where this type of treatment plant is operating. With increasing environmental problems, other large abattoirs will have to consider this type of treatment approach as well.

Fig. 89: Schematic view of aerobic biological effluent treatment plant

The principle of effluent treatment in such plants is illustrated in Fig. 89 and entails the following procedures:

Preliminary/primary treatment: Screening of large solid parts from the wastewater as well as inclusion of fat traps and sand traps.

Secondary treatment: This is the core function of the plant, where air is injected into the aeration tank to boost the activity of aerobic bacteria for breaking down organic wastes. The resulting sludge is composed of the bacteria and organic materials. To keep the level of bacteria at a high concentration in the aeration tank, part of the sludge is recycled; the other part goes to the sludge treatment.

Sludge treatment: This can either be drying of the sludge, such as for fertilizer, or the sludge can be subjected to anaerobic digestion that produces biogas (shown in the lower part of the scheme in Fig. 89). In this treatment system, the aerobic reactions are responsible for the effluent clearance – the anaerobic reactions are supplementary treatments only.

Investments into aerobic biological treatment plants are much higher than in biogas plants. Aerobic biological treatment plants can either be assembled at the abattoir or, for smaller operations, prefabricated units are available from specialized manufacturers. Investment costs vary according to the size of the facilities and the need for additional processing units. Investment costs can range from US$50 000 to 200 000. Larger abattoirs in the Asian region must consider aerobic effluent treatment plants in their future development programmes.

Fig. 90: Compact aerobic biological effluent treatment plant: Shown here is an aeration tank (right) and sedimentation tank (left), with a pipe system for recycling sludge.

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