9.5.3 Meat Rendering Plants

9.5.3.1 General1

Meat rendering plants process animal by-product materials for the production of tallow, grease,

and high-protein meat and bone meal. Plants that operate in conjunction with animal slaughterhouses

or poultry processing plants are called integrated rendering plants. Plants that collect their raw

materials from a variety of offsite sources are called independent rendering plants. Independent plants

obtain animal by-product materials, including grease, blood, feathers, offal, and entire animal

carcasses, from the following sources: butcher shops, supermarkets, restaurants, fast-food chains,

poultry processors, slaughterhouses, farms, ranches, feedlots, and animal shelters.

The two types of animal rendering processes are edible and inedible rendering. Edible

rendering plants process fatty animal tissue into edible fats and proteins. The plants are normally

operated in conjunction with meat packing plants under U. S. Department of Agriculture, Food Safety

and Inspection Services (USDA/FSIS) inspection and processing standards. Inedible rendering plants

are operated by independent renderers or are part of integrated rendering operations. These plants

produce inedible tallow and grease, which are used in livestock and poultry feed, soap, and production

of fatty-acids.

9.5.3.2 Process Description1-3

Raw Materials —

Integrated rendering plants normally process only one type of raw material, whereas

independent rendering plants often handle several raw materials that require either multiple rendering

systems or significant modifications in the operating conditions for a single system.

Edible Rendering —

A typical edible rendering process is shown in Figure 9.5.3-1. Fat trimmings, usually

consisting of 14 to 16 percent fat, 60 to 64 percent moisture, and 22 to 24 percent protein, are ground

and then belt conveyed to a melt tank. The melt tank heats the materials to about 43°C (110°F), and

the melted fatty tissue is pumped to a disintegrator, which ruptures the fat cells. The proteinaceous

solids are separated from the melted fat and water by a centrifuge. The melted fat and water are then

heated with steam to about 93°C (200°F) by a shell and tube heat exchanger. A second-stage

centrifuge then separates the edible fat from the water, which also contains any remaining protein

fines. The water is discharged as sludge, and the "polished" fat is pumped to storage. Throughout the

process, direct heat contact with the edible fat is minimal and no cooking vapors are emitted. For this

reason, no emission points are designated in Figure 9.5.3-1.

Inedible Rendering —

There are two processes for inedible rendering: the wet process and the dry process. Wet

rendering is a process that separates fat from raw material by boiling in water. The process involves

addition of water to the raw material and the use of live steam to cook the raw material and

accomplish separation of the fat. Dry rendering is a batch or continuous process that dehydrates raw

material in order to release fat. Following dehydration in batch or continuous cookers, the melted fat

and protein solids are separated. At present, only dry rendering is used in the United States. The wet

rendering process is no longer used because of the high cost of energy and of an adverse effect on the

9/95 Food And Agriculture 9.5.3-1
Figure 9.5.3-1. Edible rendering process.

9.5.3-2 EMISSION FACTORS 9/95
fat quality. Table 9.5.3-1 shows the fat, protein, and moisture contents for several raw materials

processed by inedible rendering plants.

Batch Rendering Process —

In the batch process, the raw material from the receiving bin is screw conveyed to a crusher

where it is reduced to 2.5 to 5 centimeters (cm) (1 to 2 inches [in.]) in size to improve cooking

efficiency. Cooking normally requires 1.5 to 2.5 hr, but adjustments in the cooking time and

temperature may be required to process the various materials. A typical batch cooker is a horizontal,

cylindrical vessel equipped with a steam jacket and an agitator. To begin the cooking process the

cooker is charged with raw material, and the material is heated to a final temperature ranging from

121° to 135°C (250° to 275°F). Following the cooking cycle, the contents are discharged to the

percolator drain pan. Vapor emissions from the cooker pass through a condenser where the water

vapor is condensed and noncondensibles are emitted as VOC emissions.

The percolator drain pan contains a screen that separates the liquid fat from the protein solids.

From the percolator drain pan, the protein solids, which still contain about 25 percent fat, are

conveyed to the screw press. The screw press completes the separation of fat from solids, and yields

protein solids that have a residual fat content of about 10 percent. These solids, called cracklings, are

then ground and screened to produce protein meal. The fat from both the screw press and the

percolator drain pan is pumped to the crude animal fat tank, centrifuged or filtered to remove any

remaining protein solids, and stored in the animal fat storage tank.

Continuous Rendering Process —

Since the 1960, continuous rendering systems have been installed to replace batch systems at

some plants. Figure 9.5.3-2 shows the basic inedible rendering process using the continuous process.

The system is similar to a batch system except that a single, continuous cooker is used rather than

several parallel batch cookers. A typical continuous cooker is a horizontal, steam-jacketed cylindrical

vessel equipped with a mechanism that continuously moves the material horizontally through the

cooker. Continuous cookers cook the material faster than batch cookers, and typically produce a

higher quality fat product. From the cooker, the material is discharged to the drainer, which serves the

same function as the percolator drain pan in the batch process. The remaining operations are generally

the same as the batch process operations.

Current continuous systems may employ evaporators operated under vacuum to remove

moisture from liquid fat obtained using a preheater and a press. In this system, liquid fat is obtained

by precooking and pressing raw material and then dewatered using a heated evaporator under vacuum.

The heat source for the evaporator is hot vapors from the cooker/dryer. The dewatered fat is then

recombined with the solids from the press prior to entry into the cooker/dryer.

Blood Processing And Drying —

Whole blood from animal slaughterhouses, containing 16 to 18 percent total protein solids, is

processed and dried to recover protein as blood meal. At the present time, less than 10 percent of the

independent rendering plants in the U. S. process whole animal blood. The blood meal is a valuable

ingredient in animal feed because it has a high lysine content. Continuous cookers have replaced

batch cookers that were originally used in the industry because of the improved energy efficiency and

product quality provided by continuous cookers. In the continuous process, whole blood is introduced

into a steam-injected, inclined tubular vessel in which the blood solids coagulate. The coagulated

blood solids and liquid (serum water) are then separated in a centrifuge, and the blood solids dried in

either a continuous gas-fired, direct-contact ring dryer or a steam tube, rotary dryer.

9/95 Food And Agriculture 9.5.3-3
Table 9.5.3-1. COMPOSITION OF RAW MATERIALS FOR

INEDIBLE RENDERINGa

Source

Tallow/Grease,

wt %

Protein Solids,

wt %

Moisture,

wt %

Packing house offalb and bone

Steers 30-35 15-20 45-55

Cows 10-20 20-30 50-70

Calves 10-15 15-20 65-75

Sheep 25-30 20-25 45-55

Hogs 25-30 10-15 55-65

Poultry offal 10 25 65

Poultry feathers None 33 67

Dead stock (whole animals)

Cattle 12 25 63

Calves 10 22 68

Sheep 22 25 53

Hogs 30 28 42

Butcher shop fat and bone 31 32 37

Blood None 16-18 82-84

Restaurant grease 65 10 25

a Reference 1.

b Waste parts; especially the entrails and similar parts from a butchered animal.

9.5.3-4 EMISSION FACTORS 9/95
Figure 9.5.3-2. Continuous rendering process.

9/95 Food And Agriculture 9.5.3-5
Poultry Feathers And Hog Hair Processing —

The raw material is introduced into a batch cooker, and is processed for 30 to 45 minutes at

temperatures ranging from 138° to 149°C (280° to 300°F) and pressures ranging from (40 to 50 psig).

This process converts keratin, the principal component of feathers and hog hair, into amino acids. The

moist meal product, containing the amino acids, is passed either through a hot air, ring-type dryer or

over steam-heated tubes to remove the moisture from the meal. If the hot air dryer is used, the dried

product is separated from the exhaust by cyclone collectors. In the steam-heated tube system, fresh air

is passed countercurrent to the flow of the meal to remove the moisture. The dried meal is transferred

to storage. The exhaust gases are passed through controls prior to discharge to the atmosphere.

Grease Processing —

Grease from restaurants is recycled as another raw feed material processed by rendering plants.

The grease is bulk loaded into vehicles, transported to the rendering plant, and discharged directly to

the grease processing system. During processing, the melted grease is first screened to remove coarse

solids, and then heated to about 93°C (200°F) in vertical processing tanks. The material is then stored

in the processing tank for 36 to 48 hr to allow for gravity separation of the grease, water, and fine

solids. Separation normally results in four phases: (1) solids, (2) water, (3) emulsion layer, and

(4) grease product. The solids settle to the bottom and are separated from the water layer above. The

emulsion is then processed through a centrifuge to remove solids and another centrifuge to remove

water and any remaining fines; the grease product is skimmed off the top.

9.5.3.3 Emissions And Controls1-5

Emissions —

Volatile organic compounds (VOCs) are the primary air pollutants emitted from rendering

operations. The major constituents that have been qualitatively identified as potential emissions

include organic sulfides, disulfides, C-4 to C-7 aldehydes, trimethylamine, C-4 amines, quinoline,

dimethyl pyrazine, other pyrazines, and C-3 to C-6 organic acids. In addition, lesser amounts of C-4

to C-7 alcohols, ketones, aliphatic hydrocarbons, and aromatic compounds are potentially emitted. No

quantitative emission data were presented. Historically, the VOCs are considered an odor nuisance in

residential areas in close proximity to rendering plants, and emission controls are directed toward odor

elimination. The odor detection threshold for many of these compounds is low; some as low as 1 part

per billion (ppb). Of the specific constituents listed, only quinoline is classified as a hazardous air

pollutant (HAP). In addition to emissions from rendering operations, VOCs may be emitted from the

boilers used to generate steam for the operation.

Emissions from the edible rendering process are not considered to be significant because no

cooking vapors are emitted and direct heat contact with the edible fat is minimal. Therefore, these

emissions are not discussed further.

For inedible rendering operations, the primary sources of VOC emissions are the cookers and

the screw press. Other sources of VOC emissions include blood and feather processing operations,

dryers, centrifuges, tallow processing tanks, and percolator pans that are not enclosed. Raw material

may also be a source of VOC emissions, but if the material is processed in a timely manner, these

emissions are minimal.

In addition to VOC emissions, particulate matter (PM) is emitted from grinding and screening

of the solids (cracklings) from the screw press and other rendering operations such as dryers

processing blood and feathers. No emission data quantifying VOC, HAP, or PM emissions from the

9.5.3-6 EMISSION FACTORS 9/95
rendering process are available for use in developing emission factors. Only test data for a blood

dryer operation were identified.

Controls —

Emissions control at rendering plants is based primarily on the elimination of odor. These

controls are divided into two categories: (1) those controlling high intensity odor emissions from the

rendering process, and (2) those controlling plant ventilating air emissions. The control technologies

that are typically used for high intensity odors from rendering plant process emissions are waste heat

boilers (incinerators) and multistage wet scrubbers.

Boiler incinerators are a common control technology because boilers can be used not only as

control devices but also to generate steam for cooking and drying operations. In waste heat boilers,

the waste stream can be introduced into the boiler as primary or secondary combustion air. Primary

combustion air is mixed with fuel before ignition to allow for complete combustion, and secondary

combustion air is mixed with the burner flame to complete combustion. Gaseous waste streams that

contain noncondensibles are typically "cleaned" in a combination scrubber and entrainment separator

before use as combustion air.

Multistage wet scrubbers are equally as effective as incineration for high intensity odor control

and are used to about the same extent as incinerators. Sodium hypochlorite is considered to be the

most effective scrubbing agent for odor removal, although other oxidants can be used. Recently,

chlorine dioxide has been used as an effective scrubbing agent. Venturi scrubbers are often used to

remove PM from waste streams before treatment by the multistage wet scrubbers. Plants that are

located near residential or commercial areas may treat process and fugitive emissions by ducting the

plant ventilation air through a single-stage wet scrubbing system to minimize odorous emissions.

In addition to the conventional scrubber control technology, activated carbon adsorption and

catalytic oxidation potentially could be used to control odor; however, no rendering plants currently

use these technologies. Recently, some plants have installed biofilters to control emissions.

No data are currently available for VOC or particulate emissions from rendering plants. The

only available data are for emissions from blood dryers, which is an auxiliary process in meat

rendering operations. Less than 10 percent of the independent rendering plants in the U. S. process

whole blood. Table 9.5.3-2 provides controlled emission factors in English units for particulate matter

(filterable and condensible), hydrogen sulfide, and ammonia from natural gas, direct-fired blood dryers.

The filterable PM was found to be 100 percent PM-10. Emission factors are calculated on the basis of

the weight of dried blood meal product. In addition to natural gas, direct-fired dryers, steam-coil,

indirect blood dryers (SCC 3-02-038-12) are also used in meat rendering plants. No emission data

were found for this type of dryer. The emission control system in Reference 4 consisted of a cyclone

separator for collection of the blood meal product followed by a venturi wet scrubber and three packed

bed scrubbers in series. The scrubbing medium for the three packed bed scrubbers was a sodium

hypochlorite solution. The emission control system in Reference 5 was a mechanical centrifugal

separator.

9/95 Food And Agriculture 9.5.3-7
Table 9.5.3-2. EMISSION FACTORS FOR CONTROLLED BLOOD DRYERS

EMISSION FACTOR RATING: E

Pollutant Emissions, lb/tona

Filterable PM-10b (SCC 3-02-038-11) 0.76

Condensible PMb (SCC 3-02-038-11) 0.46

Hydrogen sulfidec (SCC 3-02-038-11) 0.08

Ammoniac (SCC 3-02-038-11) 0.60

a Emission factors based on weight of dried blood meal product. Emissions are for natural gas, directfired dryers. b References 4-5. c Reference 4.

References For Section 9.5.3

1. W.H. Prokop, Section on rendering plants, in Chapter 13, "Food And Agriculture Industry",

Air Pollution Engineering Manual, Van Nostrand Reinhold Press, 1992.

2. H.J. Rafson, Odor Emission Control For The Food Industry, Food And Technology,

June 1977.

3. Emission Factor Documentation for AP-42 Section 9.5.3, Meat Rendering Plants,

EPA Contract No. 68-D2-0159, Midwest Research Institute , Kansas City, MO,

September 1995.

4. Blood Dryer Operation Stack Emissions Testing, Environmental Technology and Engineering

Corporation, Elm Grove, WI, September 1989.

5. Blood Dryer Particulate Emission Compliance Test, Interpoll Report No. 7-2325, Interpoll

Laboratories, Inc., Circle Pines, MN, January 1987.

9.5.3-8 EMISSION FACTORS 9/95