A study by Tissa Fernando, Flo-Dry Engineering Ltd, Auckland, New Zealand. Paper given at the 3rd International Symposium of the "Australian Renderers Association Inc., Sydney, 1995.

BACKGROUND

ODOUR CONTROL SYSTEMS

CONDENSERS

SCRUBBERS

AFTERBURNERS

BIO-FILTERS
DISPERSION STACKS
COST COMPARISON
CONCLUSION

Rendering is perceived by the public to be an unpleasant, "smelly", industry and over the last decade regulatory authorities around the world have specified methods for control or abatement of odour from rendering plants.
The raw material for rendering is not treated as a valuable by-product and hence unlike meat or edible by-products rendering material is not refrigerated nor is not normally preserved. Thus, the raw material putrefies giving rise to a number of odourants.
Odourants are chemical compounds, and their threshold levels are given in the table below (Danielson, 1967).
ODOUR THRESHOLD CONCENTRATIONS OF SELECTED COMPOUNDS
This table emphasises the extremely low levels of odourants that are smelt by the human nose!
Amines, mercaptans and the sulphides are generally expected to be present in rendering plant fugitive gases.
It should be well understood that if an odour abatement system is to be satisfactory, then the very the odourants must be reduced to levels well below those given in the above table. Also it is imperative that the whole array of chemical compound must be removed.


Universally accepted odour control systems for the rendering industry are:

CONDENSERS
 SCRUBBERS
 AFTERBURNERS(INCINERATORS)
 BIO-FILTERS
 DISPERSION STACKS


In rendering plants the strongest odours emanate from cooking and drying processes in which steam (boiled off the raw material) is present. This steam must be firstly treated in a condenser to remove the condensables and to reduce the temperature of the non-condensables to around 35 degrees celsius to 40 degrees celsius. Condensation is a primary odour control system and removes up to 90% of the odours. Secondary odour control treatment reduces the odour to below 99% of the input to the condenser.

As shown below, cooling water is used to condense the condensable steam that is contaminated with water soluble odorous chemical compounds. Hot water can be recovered in the condensation process saving heat energy.
SCRUBBER TYPES


Below is shown two types of scrubbers. The venturi type scrubber is for low intensity odours and is not used extensively in the rendering industry. The packed bed type scrubber is used in the with various chemicals as the scrubbing liquid.
ODOUR CONTROL SYSTEM FOR EXHAUSTS WITH STEAM
In order for the scrubbers to be effective the correct combinations of chemicals must be used as shown in the table below (Snow, 1975).
PACKED TOWER EXPERIMENTS WITH VARIOUS SOLUTIONS
PERCENTAGE OF ODOURANT REMOVED
ODORANT
SOLUTION
1
2
3
4
5
6
7
A
30
10
>90
30
>90
0
10-30
B
80-90
>90
>90
>90
0
C
0
>90
0
10-25
10
0
0
D
>90
E
>90
F
80-90
80
75
40-80
75
80
0-60
G
0
20
0
25
KEYS:
Odorants
Solutions
A
 Valeraldehyde (aldehyde)
1
 Water
B
 Trimethylamine (amine)
2
 1% Sodium Hypochlorite
C
 Dipropyl Sulphide (sulphide)
3
 3% Potassium Permangonate
D
 Butryric Acid (fatty acid)
4
 5% Sodium Bisulphate
E
 Butanedione (ketone)
5
 5% Sodium Bisulphate
F
 Amyl Alcohol (alcohol)
6
 5% Hydrochloric Acid
G
 Heptadiene (unsaturated alkane)
7
 5% Sodium Hydoxide
For rendering plant applications, an acid pre-wash (by using dilute sulphuric acid, pH 1.6) is used in the first stage scrubber. This prevents the generation of odorous chlorinated compounds from ammonia and amines. Then, a second stage scrubbing is required with strong alkaline (pH 12-13) sodium hypochlorite with considerable excess of available chlorine.
Alternatively, in the first stage scrubbing acidic sodium hypochlorite is used (pH 5.0) together with sodium hydrogen sulphite to remove aldehydes followed by sodium hydroxide (second stage).
Second stage scrubbing is required to give 99% plus odour reductions.
It is important to note that in chemical scrubbing , for satisfactory reductions of odour levels, second stage scrubbing is required with chemicals used in each stage. Thus chemical scrubbing requires an annual cost of chemicals which could be a significant cost and also the application of the chemicals must be controlled. Generally, the control of chemical dosage is not simple.


Afterburning is a widely used method. This method of odour destruction works well provided the afterburner is well designed and the afterburning parameters are adhered to.

Two parameters are required: a residence time and a minimum temperature.
RATE OF POLLUTION OXIDATION FOR AN AFTERBURNER
The graph above shows the effect of the residence time and temperature combinations.
The minimum requirements are a residence time of 0.5 second and a temperature of 750 degrees celsius.
In calculating the residence time it is essential to allow a safety factor of 50% by increasing the volume of the afterburner and ensuring that the minimum temperature is achieved by the use of a temperature controller and a temperature recorder.
As the afterburner requires a high temperature the cost of fuel would be high unless the air is pre-heated by the use of the final exhaust gases and hot water is produced for the use in the abattoir.
The diagram below shows the Flo-Dry after burner system which uses preheat and the cooling of the final exhaust gases to produce hot water. This system has been operating successfully at PVL Proteins Ltd, Auckland for the last 3 years. In order to obtain approval from the relevant licensing authority it was required to prove the residence time by the use of isotope tracers and the exhaust gases were tested for hydrogen sulphide, mercaptans and amines.
FLO-DRY AFTERBURNER
The afterburning temperature can be reduced by the use of a catalyst as shown below.
AFTERBURNER TIME TEMPERATURE SPECIFICATIONS
NON CATALYTIC
CATALYTIC
Temperature
750 degrees celsius
450 degrees celsius
Residence time
0.5 second
0.5 second
The type of catalysts used are based on platinum. The cost of the catalyst can be offset by the low temperature of operation. At the lower temperatures the materials of construction would be less costly. The catalyst has to be replaced after some time.


A widely used method for reduction of odour is the bio-filter. The odorous air is passed through a bed of organic material such as peat moss, compost, bark, woodchips, rice hulls or a combination of these. Generally, the substrate is filled with stone (road metal or scoria) and the organic material is placed on top of the stones.

In some instances the filling material is soil (Prokop et al, 1985).
The diagram below gives the arrangement of a typical bio-filter.
TYPICAL BIOFILTER
The filter material adsorbs the odorous compounds that are then broken down to non-odorous compounds by microbial action.
For the efficient operation of bio-filters the following is necessary:

Humidity
Oxygen
A high count of microbes
An even distribution of the gases through the bed.
An open porous structure of the bed in which the pressure drop does not increase.
A good drainage system under the bed to ensure that the bed is not waterlogged.
Control and recording of the temperature of the gases entering the bed.
Depending on the strength of the odourants in the gas the rate of gas through the bio-filters varies between 10 to 120 m3 /h/m2 of filter area.
The bio-filter is an environmental friendly system with the running costs being mainly the electrical energy for the fan. Thus this system has been used extensively in rendering plants around the world.
Generally, the filter life is in excess of 5 years provided the filter is constructed to the correct design. As the bio-filter appears to be a fairly simple system, in many instances amateurish designs of the bio-filter have been found to fail mainly by compaction of the bio-filter. Tests carried out on the Flo-dry bio-filter systems both in Australia and New Zealand have indicated 99% plus odour reductions.


This method depends on diluting the odours with air by the use of a stack to such an extent that the malodours are not detected at ground level or in buildings occupied by humans.

It is important to note that the total quantity of the odorous gases or pollutants are released to the atmosphere and thus there is an analogy to highly polluted liquid wastes directed to the sea.
However, in certain locations provided the plant, topographical and meteorological effects are favourable to this method of odour discharge, the method could be effective and economical .
The effects of various factors on dispersion stacks is given below.
FAVOURABLE AND UNFAVOURABLE ASPECTS OF FACTORS
IMPORTANT IN AIR POLLUTION FROM STACKS

SOURCE: Perrys Chemical Engineers Handbook


ANNUAL RUNNING COST ESTIMATES
Based on 100,000m3/h
Odour Eliminater Type
Cost $/Year
Scrubber
250,000
Afterburner(no heat recovery)
1,160,000
Afterburner (with heat recovery)
230,000
Biofilter
120,000
As shown in the above table, the bio-filter is the cheapest to run. In terms of capital costs, bio-filters and scrubbers are similar. After burners are viable only if fuel is extremely cheap or if the afterburner is equipped with heat recovery systems. Generally the cost of an afterburner with heat recovery systems can be 4 to 5 times that of a bio-filter for the same input volume of odorous gases. Thus, bio-filters appears to be the best choice provided land is available and land is cheap.


There are a number of proven odour control technologies for the rendering industry. Thus, rendering can be carried out without being a public nuisance. It is important to design the appropriate system and operate the system with due care and diligence. Also, the renderer must always strive to render "fresh" or "stabilised" raw material and not process decaying and putrefying raw material.

Danielson, J.A. (Editor) (1967): Air Pollution Engineering Manual, U.S.Public Health Service, Publication 999-AP-40.
Snow, R.H. (1975) Investigation of Odor Control in the Rendering Industry, In Cheremisinoff,
P.N. & Young , R.A.(Eds): Industrial Odor Technology Assessment, Ann Arbor Science, pp, 147-174.
Rolke, R.W. et al (1972) Afterburner Systems Study, U.S.Environmental Protection Agency, Publication EPA-R2-72-062.
Prokop,W.H.,Bohn, H.L., (1985) Soil Bed System for Control of Rendering Plant Odours, JAPCA, 35, 1332-1338.
Perry, Robert H., Green, D.W (1988) Perry's Chemical Engineers' Handbook, 6th ed, PP, 26-22
   
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