|Environmental Handbook Volume II: Agriculture, Mining/Energy, Trade/Industry (GTZ, 1995, 736 p.)|
|Trade and industry|
|54. Mills handling cereal crops|
Given the processing techniques employed today, it may generally be assumed that large volumes of air are required to produce milled and hulled products (flour, wholemeal products, flakes, grains etc.), in addition to power for cleaning, hulling, grinding (milling) and the transport of intermediate and end products.
This air is used mainly for vertical and horizontal transfer inside the milling or hulling system and for dust extraction from the processing units and the entire mill complex. Furthermore, under certain climatic conditions cool air is required to ventilate power plant and processing machinery as well as the entire building complex.
Industrial wastewater is produced only in the cereal washing department in the mill industry, and even then only where granular or wholemeal products are to be produced. The modern mill industry makes particular use of a dry cleaning process which separates out impurities by means of screens and weighing sorters. If the plant also produces bulgur and parboiled rice, process water with a low starch content is produced.
The wastewater from waste-recycling power generating plants, particularly that from rice husk gasification for the production of lean gas for gas-engine powered plants, has a phenol content of over 0.03 mg/l. When husks are burnt to produce steam, a residual quantity of 18% ash in relation to the quantity input must be disposed of. The same applies to gas plants.
It can therefore generally be stated that the environmental impacts of the mill operation lie in the following areas:
- dust emission,
- noise nuisance,
- hazard of dust explosions and fires,
- odour nuisance to a limited degree,
- hazard of toxic gas,
- recycling of residual substances and waste disposal,
- process water.
2.1 Cereal storage and handling
2.1.1 Port and transshipment silos, mill silos
Storage installations of this kind are used for the storage and transshipment of cereal for import and export. They are found in all major ports where imported cereals (wheat, maize, rice, millet etc.) as well as raw products and semi-finished products for the food and animal feed industry are put into store for intermediate storage, and from which the domestic industry is supplied with raw materials or goods for export are shipped (maize, rice, millet, tapioca etc.).
The following table shows the dust content of the service air from the various mill sections and admissible emission values in Germany.
Table 1 - Pollutants produced and admissible emission values in Germany
Type of mill industry
Dust content of service air
| || ||
Permissible emission values
Silo installations Drying plants Mills handling cereal crops Hulling mills Seed cleaning
12 to 15 15 to 18 approx.96 6 to 8 8 to 10
g/m3 g/m3 g/m3 g/m3 g/m3
50 mg/m3 50 mg/m3 50 mg/m3 50 mg/m3 50 mg/m3
In storage installations with preliminary cleaning plants and in mills, dust emissions are collected in aspiration pipe systems during cleaning, and separated with the help of cyclones and filters. To achieve the best possible removal of dust from machines and buildings, all equipment handling materials and machinery should be enclosed and fitted with appropriate aspiration connections. The extraction of dust with so-called mass separators or filter separators is described and explained in the guidelines nos. 3676 and 3677 of the Association of German Engineers VDI. The safety measures in these guidelines should be observed.
With the high degree of mechanisation in modern mills, the only workplaces where dust is a problem are the loading and packing operations; here too, extraction devices must be used wherever possible.
All the dust from aspiration systems and cleaning in transshipment silo installations is collected and bagged.
The cleaning waste, which may contain live pests, is to be destroyed immediately.
In mill-cleaning plants, dust waste and granular cleaning waste is treated and added to mill afterproducts (bran) (feed ingredient).
Noise is another environmental problem. The increasing use of high-speed technical equipment and the intensive use of machines in the smallest possible space give rise to an increasing noise nuisance which is becoming a hazard to man.
Precautions must be taken to protect employees and local residents. Structural measures, such as the lining of ceilings and walls with soundproofing materials, must be taken, and vibration isolation materials must be used for machine foundations.
The TA-L [Technical Instructions on Noise Abatement] in Germany lays down, for the various industrial and residential areas and mixed use areas, safety guidelines for immissions which must be observed in the planning and erection of industrial plants.
Personnel must be issued with hearing protection where they are constantly exposed to noise levels of over 70 dB.
Information and training must therefore be provided for personnel, and compliance with safety measures monitored.
People, buildings and the machine stock can be at risk from dust explosions and fires. Following any such explosion, there is a chemical conversion of a dust/air mixture, which accelerates as heat is generated, causing a sudden pressure effect from existing or newly formed gases. Three components constitute the basis for a dust explosion: dust, air (oxygen) and ignition energy; the latter can be in the form of heat or electricity (electrostatic charging).
Silo installations are particularly at risk from dust explosions. Mechanical sparks, pockets of glowing materials, mechanical heating, hot surfaces, welding work, electrostatic discharge sparks and the like are possible ignition sources. They must be eliminated as a safety measure, and the formation of explosive dust concentrations must be prevented, for example by enclosing machines. Structural precautions can also be taken, namely the creation of compression-proof rooms and pressure release and explosion suppressing systems. The following organisational precautions are also effective in terms of fire and explosion safety:
- welding and cutting works only to be carried out during
- regular cleaning with dust-explosion-proof equipment;
- training of employees in the handling of fire-fighting equipment and
- information to employees about the causes of dust fires and explosions.
Finally, in the planning phase, provision must be made for taking all the measures required to limit the risk of explosion (cf. in Germany, VDI guideline, 2263 on dust fires and dust explosions - Staubbre und Staubexplosionen).
Gases are most commonly used to protect stocks (pest control) in the silo installations and warehouses, but under certain circumstances sprays and vapours are an option.
The types of pest control agent for cereals currently used and approved in Germany include gaseous insecticides:
- hydrogen phosphide,
- methyl bromide,
- hydrogen cyanide.
In addition to gases, fumigants and sprays can be used for the disinfestation of silos and stores - without any need to include stocks in the treatment.
The following are approved in Germany:
- piperonyl butoxide,
- and combinations of these.
The incorrect use of agents for pest control for stock protection purposes can lead to hazardous substances seeping into adjacent production or residential buildings (e.g. hydrogen phosphide). Therefore, particular attention must be paid to the technique of pest control (e.g. silo fumigation using a circulation system).
Specific bans or restrictions on the use of these agents are recorded in the plant pesticide register of the country concerned or may be requested from the registration office for these substances. The manufacturer's instructions must be strictly observed and made available in the local language.
After treatment, waiting periods must be observed to ensure that plant products do not contain higher residue levels than are permissible where they are to be brought into circulation or eaten (cf. environmental brief, Analysis, Diagnosis, Testing and Volume III, Compendium of Environmental Standards (CES)).
Authorised contractors must be employed for the application of agents to protect commodities stored in silos and warehouses; their personnel must be appropriately trained and able to use the special equipment and safety installations.
2.1.2 Cooperative stores and warehouses
Simple storage installations (including raw material stores) are warehouses for bagged commodities or for horizontal storage. Bagged commodities or loose grains are cleaned, stored, ventilated and may also be treated as a pest control measure. Most maize, rice and sorghum harvests are still stored in this way in many countries, with possible storage losses of 15% or more.
Figure 1 - Diagram of a port and transshipment silo installation
Standard warehouses should also have installations for cleaning, ventilation and fumigation.
The risk of dust explosions can be largely avoided in warehouses by a light and open design, although this does not protect against normal types of fires which can occur. Otherwise, the environmental implications are as described in 2.1.1. Pest control in warehouses may take the form of sprays although fumigation is also commonplace.
The safety measures for silo installations described in 2.1.1 are also applicable to warehouses, with the exception of measures against the risk of explosion.
Special precautions must be taken where gases are used for pest control purposes. As bagged commodities cannot be fumigated in airtight areas, gastight fumigation tarpaulins, sealed underneath with sand, are required if this type of pest control is to be used.
2.1.3 Seed cleaning installations
Seed dressing is not considered part of a mill's activities, but in many countries it is one of the services a cooperative storage facility may offer its members.
In using these facilities seed is produced with a higher grade purity thanks to air, screen and specific weight classification. The lower content of other types of grain and the improved growth conditions resulting from chemical treatment improve quality and thus yields per hectare.
The service air from seed cleaning plants contains primary dust. It and the cleaning waste produced (rejected grain, weed seeds etc.) can be used for animal feed production.
Treatment involves the wet or dry application of fungicides and insecticides which - as pesticides - protect the seed and are classified as seed treatment agents. All such treatments approved by the Biologische Bundesanstalt fd- und Forstwirtschaft (Federal Biological Research Centre for Agriculture and Forestry) in Germany are listed in the pesticide register Pflanzenschutzmittel-Verzeichnis (1990).
These plant pesticides are used in seed improvement operations either alone or in combination depending on the purpose of the treatment.
Common pesticides (active ingredients) are:
- fenfuram etc.
Environmental measures in seed cleaning operations are confined, in terms of aspiration and service air, to keeping the production rooms and the outgoing waste air clean. The filter installations listed in paragraph 2.1.1 and the emission values shown in table 1 are applicable here.
When protecting seeds, appropriate precautions must be taken to protect personnel and, subsequently, users.
The approval regulations of the individual countries must be obeyed as must the manufacturer's recommendations for use (see too Volume III, CES).
Figure 2 - Diagram of a seed cleaning installation
2.1.4 Drying installations
Grain drying is a thermal process in which water is removed from the pre-cleaned damp commodities (cereal, maize, rough rice (paddy), sorghum etc.) by evaporation. Obviously, an adequate supply of heat is essential. The drying of the moist harvested produce is usual in warehousing facilities and the agricultural trade (cooperatives), and mill and silo facilities often have drying installations too. Thus, wherever large quantities of damp grain (moisture content of over 15%) are supplied, rapid drying is called for. Drying installations are used where natural sun drying is not practicable in view of the weather conditions (rainy season). Only dry commodities can be safely stored for prolonged periods without any deterioration in quality.
The service air of drying installations and the preliminary cleaning machines contains coarse to fine particles of dust which need to be separated by means of the dust separators described in paragraph 2.1.1. Drying installations are only used at harvest time, and should preferably be sited close to the (sparsely populated) growing areas. Noise is another problem. Very often mill and silo facilities also have drying installations.
The safety measures listed in 2.1.1 to protect against dust and noise must be taken here too.
Figure 3 - Diagram of a drying installation
2.2 Flour mills (wheat mills)
The purpose of a flour mill is to obtain large quantities of flour which also meet flour product requirements in terms of quality. The by-products and afterproducts (bran, middling and cleaning waste) are recycled in agriculture or in the animal feed industry in the form of feed components. Mills also make wholemeal products.
In some cases, very antiquated washing machines are still used today for cereal cleaning in the wheat mill industry, requiring monitoring of wastewater quantities (up to 1000 l/t), thus safe distances from residential areas must be maintained. In the modern cereal mill, water is only used for conditioning (wetting) the cereal and it is fully absorbed by the grain. Today, the entire cleaning process is carried out by means of air, screening and weight classification. Scouring machines have largely replaced the washing systems, thus no industrial wastewater is now produced.
In conventional mills handling cereal crops, some 5 - 10 cubic metres of air are required per milled tonne. This quantity drops to just 15% where machines working on the circulating air principle are used for cleaning. All service air discharged into the atmosphere must be filtered.
There is also a fire risk due to dust explosions in mills, and mills handling cereal crops as a whole generate noise emissions which have environmental implications for humans.
All safety measures described for cereal storage are appropriate for mills handling cereal crops in all respects. If silo installations are structurally linked to the mill, not only must automatic fire valves be fitted in the interconnecting materials handling equipment, but also the connecting walls between the installations must constitute fire barriers21). A settling tank for organic substances (husks, pieces of stem, fines etc.) must be provided.
21) For this reason, safe distances from populated areas must be maintained.
Figure 4 - Diagram of a wheat mill
2.3 Hulling mills
Hulling mills handle the following cereal types: oats, barley, rice, sorghum and millet, as well as pulses. While hulling mill technology is very different from that used in mills handling cereal crops, the environmental pollution and the resultant safety measures are largely similar in both types of mill.
2.3.1 Rice mills
The process from rough rice (paddy) through to ready-to-use white rice passes from cleaning with air, screening and weighing, dehulling and the polishing process (removal of the aleurone layer) through to sizing. Some countries have their own production from low to medium capacity rice mill facilities (China, Taiwan, Malaysia, Thailand, India and some South American countries).
The environmental pollution from rice mills in these countries is considerable if there are no complete aspiration systems or such systems are not designed to appropriate technical standards. Often, cyclones alone are used for dust separation although they only achieve a separation level of 90 - 95%, with dust emissions standing at 70 to 150 mg/m3 air. Dust filters must be used.
The major disposal problem for rice mills is presented by the rice husks from the production process (20%). One possible means of economically recycling rice husks is that of pyrolysis to produce energy in steam power plants or lean gas for gas-engine plants (see environmental brief Plant Production).
Hot industrial water (approx. 65°C) and saturated steam are used to make parboiled rice (rice precooked in the husk).
Apart from the rice husks, all other by-products are either used locally as animal feed or exported (rice polish/emery flour).
A residue of about 18% ash is produced by rice hull pyrolysis. Ash can be disposed off locally as a soil structure improver and more recently rice hull ash has been used in steel works as an insulant.
Where parboiled rice is produced, organic substances are found in the wastewater, but in such small quantities that their recovery is not economical. Approx. 1 cubic metre of drinking quality water is required per tonne of rough rice (paddy), approximately 30% of which is absorbed by the grain.
Otherwise, the environmental impacts are as described in paragraphs 2.1.1 and 2.2.
The environmental protection measures to be observed for rice mills are listed here in order of priority:
- Dust emission values as applicable in mills handling
cereal crops must also be observed in rice mills, i.e. modern aspiration systems
with separators and filter installations must be used.
- Noise emissions are a nuisance to the population living in the surrounding area and so the details per sections 2.1.1 and 2.2 apply for this industry.
- The use of biodegradation tanks is recommended for wastewater disposal from parboiling facilities because of the higher starch concentrations.
- Measures must be taken to ensure proper hull disposal. In addition to pyrolysis and soil structure improvement, the hulls can be used in the brick industry, distilleries and possibly for furfurol production. Other uses are technically feasible and should be considered in the light of the particular site.
2.3.2 Hulling and processing of sorghum and millet
The industrial processing of sorghum and millet produces flours with good keeping properties and enables quality to be controlled in the light of the end products to be made. Better quality flours and higher yields are therefore achieved.
The upturn in the fortunes of this new branch of milling has been further enhanced by the possibility of blending this flour with wheat flour (composite flour). This has enabled a number of countries to use local raw materials by producing flours of this kind (blends of up to 20%).
The pollutants produced and safety measures to be taken are based on the data in paragraph 2.2.1.
2.3.3 Pulse hulling
The produce processed in hulling mills includes a range of pulses which are grown in temperate climates as well as the tropics. Pulses such as chickpeas, lentils and local bean varieties are brought onto the market in their hulled/split form or as flour.
The pollutants produced and safety measures to be taken are similar to the substances and measures described in paragraph 2.1.1.
2.4 Location planning
When planning the location of a food industry facility, it must be assumed that medium-sized and large operations will be accommodated. In such mass-production plants where food is processed, manufactured, transported, loaded and unloaded or stored, the following environmental factors must be taken into account, for which detailed information can be found in the relevant environmental briefs:
- An organised transport system is necessary as
facilities of this kind turn over substantial quantities of raw materials and
finished products (environmental brief and Transport and Traffic Planning).
- When planning larger facilities, a sea/land/sea transshipment facility should also be provided (environmental briefs Inland Ports and Ports and Harbours, Harbour Works and Operations).
- As facilities of this kind run day and night, they must be sited at appropriate distances from residential areas. Dust, and above all noise nuisance, must be prevented (environmental brief Planning of Locations for Trade and Industry).
- There should be a reliable energy supply on site to ensure safe operation of larger plants (environmental brief Overall Energy Planning).
- Furthermore, for safety reasons, they must be sited further from other industrial plants so that in the event of incidents (fire, dust explosion) extensive damage can be avoided.
- A water supply and organised disposal facilities are absolutely essential (environmental brief Water Framework Planning, Wastewater Disposal).
The fundamental factors in site selection (e.g. avoidance of agricultural areas or rare/valuable areas of countryside) are likewise to be found in the environmental brief on Planning of Locations for Trade and Industry.
2.5 Energy from husk waste
The energy requirement of milling plants ranges from 30 to 70 kWh per tonne of end product, and that of rice mills 30 kWh. One economic and environmental objective should be the use of the husks from rice production (approx. 20%) as a source of energy.
Waste gas emissions from the chimneys of steam power plants are becoming an environmental problem due to their ash particle content. When incinerated, a residual quantity of about 18% ash remains.
Where gas is produced from husks, industrial water is used for gas scrubbing to separate tar and dust, and also as cooling water for the gas reactor. This wastewater contains up to 1.6 mg/l phenol. The ash produced by husk pyrolysis must also be disposed of.
All residues from husk burning in the ash disposal section of steam power plants should be collected in its dry form; after cooling and intermediate storage, the ash can be reused by agriculture and industry. Fly ash in the chimney must be separated by scrubbing with dust separators before it enters the waste gas chimney.
Wastewater from gas generators may only be released once it has been neutralised and freed from solids. Venturi scrubbers and biological plant tanks must be used for tar separation.
Even at the planning stage of these power plants, ash disposal, flue gas emission and wastewater disposal must be considered in the light of parallel municipal development.
2.6 Further processing of cleaning waste and mill afterproducts
Normally waste from mills handling cereal crops is immediately milled in hammer mills and then, together with mill afterproducts, supplied to the animal feed industry. Other mill afterproducts are bran and low-grade flour, and from the hulling mill, hull bran.
This industry, often regarded as a secondary operation in mills, produces fodder concentrate for livestock farming, which contains protein, carbohydrate, fats, mineral substances and vitamins as its main blending components.
2.7 Dust disposal
Dust requiring disposal is produced only in the goods inward sections of agricultural trade establishments and cooperatives. This dust actually consists of sandy impurities which are separated when the commodities enter the preliminary cleaning plant, and can, for example, be returned to the supplier.