|Environmental Handbook Volume II: Agriculture, Mining/Energy, Trade/Industry (GTZ/BMZ, 1995, 736 pages)|
|Trade and industry|
|51. Mechanical engineering, workshops, shipyards|
A product undergoes numerous production stages in the course of the metalworking process. The environmental impacts of these stages affect the workplace and hence the people working there. They also affect the air, water and soil.
Due to their proximity to the point of origin, it is the workforce who are most seriously exposed to the production hazards. In highly industrialised countries this is the subject of comprehensive worker protection rules. The workplace hazards are listed below, taking as examples the most important and environmentally relevant machining processes. This is followed by a description of the wider environmental effects including the problems of waste disposal.
2.1 Potential hazards of selected operations
2.1.1 Metal cutting
Machining processes such as drilling, milling, turning, cutting, honing, grinding etc. make use of oils and oil preparations for lubricating and cooling tools and workpieces, to prevent overheating and possible melting of the workpiece and tool. Oils are dosed by spraying or pouring systems at rates of up to 100 litre/min. in order to dissipate heat. The spraying of moving and sometimes very hot tools and workpieces produces vapours containing droplets known as aerosols.
Metalworking techniques require appropriate coolants which must combine several different properties (non-foaming, corrosion-inhibiting, non-decomposing etc.).
Such a wide range of properties can only be achieved through the addition of varying quantities of chemical additives. These are added to the coolants in the form of non-water- miscible cutting oils or water-miscible concentrates.
More than 300 individual substances are used as coolant components. The following table divides these into substance groups by areas of application.
|Substance group||Reason for use||Examples|
|Mineral oil||Lubrication effect||Hydrocarbons with different boiling ranges; fatty oil; esters|
|Polar additives||Enhanced lubrication properties||Natural fats and oils of synthetic esters|
|EP additives||To prevent micro-welds between metal surfaces at high pressures and temperatures||Sulphurized fats and oils, compounds containing phosphorous, compounds containing chlorine|
|Anti-corrosion additives||To prevent rusting of metal surfaces||Alkano-amines, sulphonates, organic boron compounds, sodium nitrite|
|Anti-misting additives||To prevent breakdown of the oil and thus generate less mist||High molecular substances|
|Anti-ageing additives||To prevent reactions within the coolant||Organic sulphides, zinc dithiophosphates, aromatic amines|
|Solid lubricants||To improve lubrication||Graphites, molybdenum sulphides, ammonium molybdenum|
|Emulsifiers||To combine oils with water||Surfactants, petroleum sulphonates, alkali soaps, amine soaps|
|Foam-inhibitors||To prevent foaming||Silicon polymers, tributyl phosphate|
|Biocides||To prevent formation of bacteria/fungus||Formaldehyde, phenol, formaldehyde derivatives, cathon MW|
A significant increase in certain occupational diseases has occurred parallel with the introduction of the coolants which are now commonplace. According to scientific findings, diseases of the skin and respiratory tracts and cancer may occur.
Where coolant use is unavoidable, mist extraction as close as possible to the point of origin or encapsulation is necessary. Consistent use must be made of personal protection measures such as the wearing of protective clothing and the use of special skin protection substances. Factories should produce skin protection plans.
Bacteria which can have severe effects on health can occur due to the organic nature of coolants. Bacteria formation is promoted by warm/hot ambient temperatures. Anti-bacterial additives are introduced to counter this. Timely replacement of coolants avoids the need for high doses of anti-bacterial additives, which also represent a health hazard. However, this increases the total quantity of waste to be disposed of. Proper storage of "spent" coolants and subsequent separation of emulsified oils and greases, and also of metal compounds and other components, is imperative.
Safety data sheets informing of the danger of coolants and instructions for use should be displayed in the national language(s). It is important that staff are aware of the long-term dangers of coolants; a particular difficulty here being the often creamy, pleasant smelling and seemingly harmless nature of coolants.
No generally applicable limit values exist for coolants in the breathing air. The only guide is the relevant MAK values9) for the individual substances. The management should find out which are the most environment-friendly coolants and ensure that these are procured.
9) The term MAK (maximum allowable concentration) in Germany refers to the maximum possible concentration of a substance in the air of the workplace, in the form of gas, vapour or suspended matter.
2.1.2 Cleaning and degreasing of workpieces
For subsequent surface treatment, adhesive or thermal bonding etc., workpieces have to be freed of substances such as oils, fats, resin, wax, cellulose, rubber or plastics. Solvents are widely used for this purpose. Workpieces can be degreased and cleaned by various methods, for instance by cold, hot and/or vapour degreasing or combined processes.
Cold cleaning frequently involves the use at room temperature in open baths of solvent mixtures whose precise composition is not known to the user. Mixed with air, the vapours of these solvents or solvent mixtures can be explosive. Most solvents represent a health hazard for man.
Solvents are classified as organic compounds such as hydrocarbons, halogenated hydrocarbons, ethers (diethyl-ether, tetrohydrofuran, dioxan), ketones (acetones, methylethylketone) and organic alkalis (sodium hydroxide solution, ammonia) and acids (hydrochloric acid, nitric acid, sulphuric acid).
The most important halogenated hydrocarbons are chlorinated hydrocarbons (CHCs), such as tri-, tetra-, perchloroethylene, dichloromethane, tetrachloroethane etc.10) On account of their grease-dissolving properties and high volatility, CHCs are used in almost every branch of metal working as cleaning agents in cold cleaners and in hot degreasing. The high volatility ensures quick drying after cleaning, but also means it is necessary to monitor solvent concentrations in the workplace. Through skin contact and inhalation, CHCs can damage mucous membranes, central nervous system, liver, kidneys and lungs.
10) The best known are CFCs (chlorofluorocarbons) used in other application e.g. as refrigerants. CFCs are partially responsible for the destruction of the vital ozone layer in the atmosphere. CFCs and carbon tetrachlorides and certain other chlorinated hydrocarbons are banned in Germany under the CFC halogen prohibition directive of 6 May 1991 and the chloro-aliphatic compounds directive.
In addition, most solvents are inflammable and represent a particular pollution hazard for water.
Alternative processes use alkaline aqueous solutions (with surfactants and other washing components in varying concentrations) or water (high-pressure cleaning).
Apart from the need for worker protection, it should be remembered that practically all solvents seriously pollute the environment. Particular problems in this regard include damage due to solvent evaporation, soil and groundwater pollution and the difficulties of disposing of used solvents and solvent sludges.
Foremost among modern methods of alleviating the problems of disposal are primary measures to prevent wastewater occurring in the first place, rather than subsequently treating highly contaminated bath and flushing water before it enters the drainage system. Membrane filtration and ion exchange processes can be used to regenerate process baths and extend their useful life. Similarly, flushing water can be used several times over with continuous dirt and oil separation (recycling via ion exchangers, emulsion cracking and cascade flushing techniques). Resulting wastewater quantities and pollutant loads are reduced. One might also attempt to process and reuse solvents in a closed solvent circuit. This technique is rarely successful in the case of reprocessing surfactants, so the improvement of their biodegradability is an important factor. Management should optimize selection of solvents based on technical and environmental factors11).
11) Only wastewater experts can definitively optimise the choice of solvents. Information is available in: Dagmar Minkwitz, "Ersatzstoffe für Halogenkohlenwasserstoffe bei der Entfettung und Reinigung in industriellen Prozessen" (serial publication of the Bundesanstalt für Arbeitsschutz (German Federal Institute for Occupational Safety and Health) GA 38) Dortmund, Bremerhaven 1991 (Wirtschaftsverlag NW) ISBN-3-89429-086-2. See also "Zeitschrift Oberflächentechnik, Bezugsquellennachweis für die Oberflächentechnik mit Trendübersichten und Tabellen, Munich, 4th edition 1991) (Seibt Verlag), ISBN 3-922948-70-7.
The following precautions should be taken where degreasing is carried out with organic solvents:
- do not use substances which are unknown;
- use enclosed equipment where possible;
- ensure effective ventilation and aeration of work rooms;
- ensure good extraction at the workplace;
- avoid skin contact;
- use protective equipment;
- as solvents are heavier than air, they force the breathing air out of trenches, cellars, containers and depressions in the ground; suffocation can be avoided by means of floor openings and ventilation;
- use only non-combustible washing vessels with self-closing covers for cleaning small parts with inflammable solvents;
- keep only quantities of flammable solvents at the workplace as are required for the work and store in suitable containers with effectively sealed covers;
- avoid electrostatic charges;
- in operating manuals indicate the solvents used, limitations on use and safety precautions, instruct personnel;
- secure and lock installations when not in use;
- avoid hand-spraying of degreasing agents with spray guns;
- avoid blow-drying with compressed air of surfaces which have been treated with chlorinated solvents;
- with open degreasing equipment, note the quantities of solvent displaced on immersion of the workpiece and dimension the system accordingly;
- workpieces should leave the system free of solvents.
Most spray paints and brush paints contain considerable quantities of hydrocarbon and chlorinated hydrocarbon solvents (spray paints as much as 90%, normally 50 - 70%) which evaporate on spraying and drying. Paints also contain finely dispersed pigments. Some of these are highly toxic. Depending on the application, paints may have to satisfy a wide range of quality requirements. Available paint systems are accordingly diverse.
There are three possible ways of avoiding solvent emissions from painting installations; these can be used separately or in combination:
- use of low-solvent paints
"High solids" paints, water soluble paints and dispersion paints have been developed for this purpose. A further alternative is solvent-free powder paint, for which new applications are constantly being found.
- use of high-performance application methods
Solvent emissions depend not only on the paint formulation but also on the application method. An important evaluation criterion is the application efficiency factor, which is defined as the ratio of paint remaining on the product to the total quantity of paint used. Lower efficiency means higher paint consumption and thus higher solvent emission. Application efficiency is primarily determined by the process and by the form of the parts to be painted.
The following application efficiency values can be taken as guidelines for the painting of large surface areas by various methods:
- compressed air spraying 65%
- "airless" spraying 80%
- powder painting 98% (with recycling of spraying loss)
- electrostatic spraying 95%
- dipping, flooding 90%
- rolling, pouring approx. 100%
- brush, roller application 98%
The choice of application method depends on certain quality requirements, e.g. coating thickness, surface roughness etc. and is hence closely related to the purpose of the painted object.
The various levels of waste gas resulting from the different methods can be greatly reduced by enclosing the application zone and additionally by air circulation. This will reduce the outlay on waste gas cleaning.
- collection and cleaning of waste gas (with solvent recycling).
To obtain different surface properties (surface refining), workpieces are electroplated with chromium, zinc, tin, copper, cadmium, lead or brass. For this the selected metallic coating is deposited from an electrolyte solution in an electro-chemical process. To enable the metal coatings to be applied, workpieces must be cleaned and degreased.
Where cold cleaning and degreasing are carried out, the hazards from cold cleaners must be taken into account (see 2.1.2). The boil-off technique is also used for rough cleaning. Strong alkalines such as sodium or potassium hydroxide solutions are used for this purpose. These alkalines can damage eyes, skin and respiratory tracts if splashed or given off as mist or dust. An electrolytic process is often used for subsequent fine cleaning. Electrolytes are often alkaline solutions (5% sodium hydroxide solution) or cyanidic salts. Apart from the dangers posed by the boil-off technique, extraction ventilation is necessary in view of the large quantities of hydrogen produced, so as to avoid reaching the explosibility limit of the air-hydrogen mixture. Safety in the workplace is increased by installing gas warning devices.
Pickling degreasers and pickles are used to remove oxidation layers and casting or rolling scale from metal surfaces. These are acids (sodium hydroxide solution for aluminium) such as sulphuric, hydrochloric, phosphoric, hydrofluoric or nitric acid which attack and dissolve the workpiece surface. The main health hazards are skin diseases; dangerous vapours and gases can be inhaled in the case of inadequate extraction. Especially dangerous are nitrous gases which can occur when using nitric acids, also fluorine compounds from hydrofluoric acid and hydrogen chloride from hydrochloric acid.
Cyanides are used for cleaning in salt baths (fluorides), pickling (removal of thin surface films), with chemical and electrolytic polishing or burnishing, and also with surface coating and thermo-chemical hardening processes. These can cause hydrocyanic poisoning as well as skin diseases when solutions containing cyanide come into contact with acids. Therefore baths containing acids and cyanide must be covered and separated by partitions. Containers and equipment are to be clearly marked to prevent carry-over of substances which can mutually react. In all cases one should check to determine whether cyanide can be replaced by substances less hazardous to health.
The actual electroplating of the workpiece can be done in countless different process variants and stages. Materials posing just about every conceivable danger can be used in electroplating. The dangerous properties result both from the main components of the bath fluid and from different additives such as emulsifiers, foam inhibitors and wetting agents.
Strong aerosols can occur during bath filling and further preparation. Dangerous substances may enter the breathing air due to the production of gas (hydrogen) during the electrolytic process.
The main hazards with coatings are skin complaints and in particular allergies due to nickel and chromates. If consumed, both nickel and chromates can be carcinogenic. Nickel in fluid particle form is subject to a TRK value12) 4 of 0.05 mg/m³ breathing air.
12) TRK value: German technical directive on concentration of carcinogenic substances
Welding is the joining of materials using heat and/or force, with or without the use of welding fillers (anti-oxidation substances).
The individual processes most commonly used are gas welding, arc welding and inert gas shielded welding.
Polluting factors in welding workshops are:
- chemicals in the generated gases, vapours and dusts
- high temperatures (approx. 3,200°C - 10,000°C)
- radiation (ultraviolet radiation): eye damage, severe inflammation of unprotected skin;
infrared radiation: can penetrate the vitreous body of the eye, reaching the retina and causing cataracts)
- noise (up to 110 dB(A))
Diverse hazards occur depending on the fuels, inert gases, filler materials, workpiece coatings etc. in use. The following table summarizes the pollutants occurring with the different welding methods. The carcinogenic and mutagenic elements chrome and nickel are especially relevant. Certain hazardous elements are detectable in welding fumes in concentrations of over 1% and can lead to health damage. Clinical and epidemiological investigations indicate a frequent occurrence amongst welders of chronic bronchitis and increased impairment of the respiratory tracts.
Pollutants occurring in various welding processes include:
|Pollutant||Causes||Welding process||MAK * |
|Lead||PbO||Welding of lead or lead-coated workpieces||all||0.1|
|Chromium||Cr2/ 3||Welding with alloyed electrodes, Cr Ni steel||all|
|Carbon monoxide||CO||Welding with basic coated electrodes, gas flame||all||30|
|Carbon dioxide||CO2||Gas welding with coated electrodes, inert gas||all||5000|
|Copper||CuO||Welding of copper, copper-coated workpieces||all||0.1|
|Manganese||Mn O||Welding of workpieces containing Mn, all electrodes||all||5|
|Nickel||NiO||Welding of Cr Ni steel, alloyed electrodes||all|
|Nitrogen||NO2||Welding in confined spaces, trenches, tanks||all||9|
|Zinc||ZnO||Welding of zinc, galvanized workpieces, zinc paint||all||5|
|Aluminium||Al2O3||Welding of aluminium, almost all types of electrodes||Arc welding||-|
|Iron||Fe2O3||Welding of steels, all electrodes||Plasma arc welding||8|
|Fluorides||F||Welding with basic and alloyed electrodes||Arc welding||2.5|
|Calcium||CaO||Welding with coated electrodes||Arc welding||5|
|Sodium||Na2 OH||Welding with coated electrodes||Arc welding||2|
|Oxygen (ozone)||O3||Strong UV radiation||Plasma arc welding||0.2|
|Titanium||TiO2||Welding with coated electrodes||Arc welding||8|
|Vanadium||V2O3||Welding workpieces containing vanadium||Arc welding||0.5|
* MAK: maximum allowable concentration
The welding of metals with anti-corrosion coatings may also have adverse toxicological consequences. Pollutants may be released depending on the type of coating.
Alkyl resins: acrolein, butyric acid
Phenolic resins: phenols, formaldehyde
Polyurethane: isocyanates, hydrogen cyanide
Epoxy resins: phenols, formaldehyde, hydrogen cyanide.
Although the inert gases carbon dioxide, argon and helium are not toxic, in poorly ventilated rooms they can displace the breathing air and under extreme conditions cause suffocation. Ozone may be produced during arc welding. Even low concentrations (0.1 parts per million (ppm)) of ozone can cause irritation to the eyes and upper respiratory tracts and in the event of exposure to 5-10 ppm over several minutes, pulmonary oedema.
At high temperatures nitrogen oxides are formed and emitted from the nitrogen and oxygen in the air on the periphery of the welding flame. Nitrogen oxides are highly toxic and, after a relatively long asymptomatic period, can lead to radical lung changes, pulmonary oedema and death. If the workpiece has been degreased with solvents containing chlorine and not properly dried, phosgene may be produced during welding. Phosgene is highly toxic and can also cause pulmonary oedema after a long asymptomatic period.
Since the welding of plastics is not yet widespread in many countries, it is not dealt with here. It must be pointed out however that the hazards for man and the environment are also considerable with the welding of plastics. Protective measures and special disposal procedures are necessary to guard against the release of solvents and other waste gases containing pollutants.
Soldering is the thermal joining of two materials using a material (solder) whose melting point is below that of the workpiece.
If the solder melts above 450°C the process is termed "hard soldering or brazing" and at lower temperatures "soft soldering". Apart from additional hazards due to the base material binders, the hazards involved in soldering are mainly associated with the flux and the solder.
The composition of a flux depends on the base material, the solder and the intended use. More than 300 different types of flux are currently available, all of which contain aggressive chemicals. Soldering paste usually contains colophonium, talc and salmiac, while soldering fluid contains zinc chloride or tin chloride. Chlorine and chlorine compounds cause irritation of the respiratory tracts and the skin and, in high concentrations, lung damage. Fluxes also frequently contain fluorine compounds (irritation of the respiratory tracts, burns). Fluxes often contain substances responsible for allergies. These are mainly colophonium and hydrazine. Hydrazine is additionally classed as carcinogenic.
Tin-based solders containing lead are used for soft soldering and silver solder containing cadmium for hard soldering. Flux vapours carry metal particles which can be inhaled.
Environmental protection measures to combat the emission of liberated gases and component substances of solders and fluxes include the installation of extractor systems with downstream separator filters (cyclone method). This method may also be used to contain the environmental impact of the production stage discussed in the next section.
Grinding is the cutting of a workpiece with a geometrically undefined cutting process.
Grinding processes are characterised by high temperatures, workpiece removal and abrasive wear. In addition to noise, the health hazards from grinding are principally emissions from abraded dust or particles from the abrasive tool, workpiece and any coating, and - in the case of wet grinding - from coolants. There is an attendant risk of health disorders especially affecting the skin and respiratory tracts. Additives in coolants and the metal dusts produced (e.g. from chromium, cobalt, nickel or beryllium) can result in allergies. These metals may also be carcinogenic. The following table shows the potential pollutant sources when grinding metals.
Potential pollutant sources in the grinding of metals
Grinding tool Formation of superfine dust with
- abrasive material - profiling and dressing of containing zircon grinding discs
- lead chloride, antimony - tool grinding sulphide in separating - fettling, due to adhering cutting in stationary operation mould residues
- additives in grinding - manual grinding, usually belts containing fluorine carried out without extraction ventilation
Coolant - coarse grinding additives, with respect to - use of magnesite binders toxicity, carcinogenicity and mutual reactivity
Material containing Combustion and pyrolysis
- more than 80% %/wt. products which can occur nickel (e.g. depositing with the thermal decomposition of materials) rubber or synthetic resin
- less than 80% %/wt. nickel (e.g. high grade corrosion-resistant steel)
- Lead (e.g. in automatic Accumulation of heavy metals and steel) superfine particles in coolant
- Cobalt (e.g. hard metal, co- due to inadequate filtration alloys) or overuse
- Beryllium (e.g. Ni Be alloys)
Atomization of coolant and thus also of additives, reaction products, dissolved heavy metals and superfine, non-separated particles.
Protective measures include environment-oriented selection of grinding tools, coolant and - where possible - materials, extraction of the abraded materials and personal breathing and hearing protection.
2.2 Mechanical engineering and operation of workshops and shipyards
Special environmental problems which are not found elsewhere arise in mechanical engineering, in workshops and in shipyards. This is because the work is not carried out in one location alone, and because pollutants are diffused and vaporised throughout the site. Estimation of their environmental relevance is difficult due to their often low concentration and they frequently seem harmless, so it is not easy to communicate the problem to workers and managers. Therefore environmental training measures should be taken into account as early as the planning phase. Much depends on the attitude in the workplace, the choice of working equipment and materials and the observance of worker protection measures. Planning must additionally include early integration of technical environmental protection measures (filter systems, wastewater collection installations, cleaning installations etc.).
2.2.1 Waste air
Environmentally relevant waste air flows can be released into the environment through forced ventilation (e.g. fan systems) and/or random emission13) from diverse areas of the site.
13) Emissions are defined as air impurities (gases, dusts), noise, radiation (heat, radioactivity etc.), vibration and similar phenomena given off to the environment by a (fixed or mobile) system.
These include emissions from:
- production extractor systems
- workplace extractor systems
- room air extraction
- production processes
- mechanical cutting
- thermal joining and parting (welding, cutting)
- joining (e.g. bonding, soldering)
- surface treatment (cleaning, coating, hardening and tempering)
Emissions into the air can be divided into:
- coarse and fine dust
- organic and inorganic gases and vapours
Harmful components of the waste air are essentially:
- organic solvents and halogenated hydrocarbons from metal cutting (coolants), cleaning, degreasing, bonding and painting of workpieces in the form of gases, vapours and aerosols
- dusts from the mechanical processing of materials
Whether or not cleaning of the waste air is an absolute necessity depends e.g. on the solvents in use, the presence of other contaminating operations, weather conditions etc., also therefore on ambient factors. Long-term risks for man and the environment may be posed even by relatively small workshops.
In the interests of worker protection, room air pollutants occurring in the production process must not exceed certain MAK values14). Where necessary work should be carried out in enclosed equipment. Efficient aeration and ventilation must be guaranteed, or pollutants must be extracted at the point of origin. Extracted flows of (pollutant) substances are to be cleaned by suitable processes before being expelled to the environment.
14) in Germany
Possible processes are:
· Dust separation:
Dust is a mixture of particles of different grain size, particle size depending very much on the process. Various processes are used for dust separation. These are classified as follows:
A: inertia separators (cyclone, "multiclone", mechanical
B: wet type separators (scrubbers, wet separators)
C: electrical separators (dry and wet electrostatic filters)
D: filtering separators (fabric filters, cloth filters, bag filters, vibratory sheet filters and tubular filters).
· Aerosol separation:
Waste gases containing droplets are also termed aerosols and hence distinguished from dust-laden waste gases. Droplets can be separated using the same physical principles as for dust. The greater adhesion of the separated droplets compared with dust however rules out use of the principal dust separators such as electrostatic filters and filtering separators. Only wet separators, i.e. scrubbers and wet type electrostatic filters are suitable without modification for separating aerosols.
· Separation of vaporous or gaseous substances:
The principal methods for reducing emissions of gaseous inorganic and organic substances are absorption, adsorption and thermal processes. With absorption the gaseous air pollutant is absorbed by a washing fluid. Absorption is either physical or chemical, depending on whether the absorption is based exclusively on the solubility of the gas, or whether additional chemical reactions occur in the liquid phase. Absorption processes, and also thermal and catalytic processes, are used particularly for reducing levels of organic substances.
Water-soluble organic substances, e.g. methanol, ethanol, isopropanol and acetone can be effectively separated from waste gases through absorption by means of scrubbers. The contaminated washing fluid can normally be regenerated by fractionation15).
15) Fractionation is the separation of fluid mixtures by repeated distillation.
Separation of large amounts of solvents is done by the condensation process. Recently, biological processes such as biofilters or biowashers have also become popular for cleaning waste gases with highly odorous components and/or solvents.
Adsorption is the attachment or accumulation of foreign molecules on the surface of a solid (adsorbent). Regeneration of laden adsorbents is normally done by desorption of the adsorbed substances in the gas or liquid phase (so-called desorption phase), i.e. by reversal of the adsorption process. As the desorption phase (usually a gas) contains the substance removed from the waste gas in an enriched concentration, recycling or reprocessing is possible. Solvent recycling is an especially important area of application for the adsorption process. Adsorbents are mostly activated carbons.
The residual substances yielded by the separation of the solid and gaseous waste gas pollutants (filter dusts, scrubbing water residues etc.) are normally hazardous materials and must be disposed of as special waste (giving rise to waste problems). The price of solving emission problems is often soil and water contamination, and land may become so contaminated it will eventually have to be rehabilitated (see also the environmental brief Disposal of Hazardous Waste).
In mechanical engineering, the recycling of process materials from wastewater is frequently only possible with disproportionate technical effort or not at all, because of the low concentrations involved. Concentrated liquid and spent process and production materials can and must be collected and disposed of as (hazardous) waste.
Wastewater is returned to natural bodies of water (lakes, streams, rivers, sea) after preliminary and final cleaning. There, any inorganic pollutants will lead to poisoning and deposits. Organic impurities may also be toxic and/or non-degradable. Degradable, non-toxic waste substances damage the environment by initiating excessive growth (eutrophication) of bacteria and minute life forms (algae, fungi) due to the nutrient supply. In combination with cell metabolism, this results in high oxygen consumption and finally to phenomena such as the "overturning" of the water (anoxic waters).
Heavy metals mostly enter the wastewater as metal salts produced by chemical reaction of the metals with the acids. The acidic pH value which prevails in heat treating and pickling shops promotes the solubility of the heavy metals in wastewater and hinders their removal.
Heat treating and pickling shops rinse workpieces in fresh water prior to further processes. After use, pickling fluids also contain heavy metals. In electroplating operations, rinsing water contains cyanide and is polluted with the heavy metals which are used (depending on the type of surface finishing).
Halogenated hydrocarbons are insoluble in water. They mainly enter the wastewater via rinsing water after degreasing in surface treatment plants and when cleaning engines and other objects in motor vehicle and general workshops by the use of cold cleaners and pickling agents. Further emission sources are coolant carry-overs and losses, workpiece rinsing and workshop floor cleaning.
Organic solvents can enter the wastewater via absorption and spray cleaning processes. Mineral oils occur with the cleaning of workpieces and floors and degreasing, and through losses during processing. Emission sources are repair, motor vehicle, factory and maintenance workshops. In surface treatment workshops they occur in the form of the workpiece anti-corrosion and rust-protection oils used in preliminary cleaning.
Acids and alkalis enter the wastewater in pickling shops and heat treatment shops in connection with degreasing. Other wastewater burdens occur due to nitrogen (ammonium) and phosphor compounds (phosphates from pickling shops).
Wastewater can be cleaned by chemical, physical and biological processes or a combination of these. Three-stage purification of industrial wastewater is now generally considered the state of the art.
Only organic and non-toxic wastewater impurities can be removed biologically. Tests in the laboratory will determine whether or not the components contained in the wastewater inhibit biodegradability.
With biological processes a distinction is made between aerobes (with oxygen) and anaerobes (without oxygen). With high burdens (chemical oxygen demand (COD) in excess of 15,000 mg/l), anaerobic processes are used for preliminary cleaning before aerobes carry out the final cleaning, since otherwise the oxygen supply costs are excessive.
High-performance biological processes with high pollutant decomposition rates are now available to build small but nevertheless efficient systems. Newly developed processes are achieving success in the biological neutralisation of organic pollutants previously considered non-biodegradable, e.g. CHCs, by optimising the living conditions for special bacteria.
Flocculation/precipitation processes can be used to remove heavy metals from wastewater, also sedimentation processes in the case of undissolved wastewater. Chemical oxidation and precipitation processes can be used for the removal and de-toxification of cyanide.
Emulsions originating from the use of coolants can be separated by the membrane filtration process in conductive wastewater (approx. 90%) and concentrate.
Ultrafiltration is used with electrostatic immersion painting for the separation of solid paint residues. This is increasingly replacing simple sedimentation with the separation of undissolved pollutants in wastewater because it is more efficient, though more costly. Wastewater containing acids and alkalines must pass through neutralization systems. Ion exchange systems cannot selectively remove metals but are highly suitable for cleaning water carried in a circuit and recycling raw materials. For recycling pure raw materials, the different waters must be carried and used separately.
2.2.3 Waste matter
Waste matters generated by these plants can be divided into three groups:
A. Residues of the used raw materials. These include both ferrous and non-ferrous (NF) waste (scrap/chips and swarf) which may be highly contaminated with coolant, cutting oil and leaked lubricating oil.
B. Waste from process residues resulting from the processing of semifinished products and auxiliary materials. Metalliferous residues are e.g. burnt slag from torch cutting, metal sludges, used salt and acid baths from electroplating or pickling shops.
C. Non-metalliferous waste can be paint and adhesive residues, oil and oily waste, organic acids, alkalis and concentrates. Finally, waste may also be produced by wastewater and waste air cleaning processes. These include purification sludges from the works own sewage treatment plant plus dusts and sludges from the cleaning of waste air and extraction flows in the form of filter residues.
Nearly all waste in the second and third groups can be regarded as hazardous waste. They demand special monitoring and special disposal methods. Waste from the first group should mostly be recycled. Separate collection of scrap types (structural steel, alloyed steel, NF metals) in different containers is important for simple and comprehensive recycling.
In order to reduce scrap quantities with torch cutting and punching, care must be taken to achieve a systematic geometrical arrangement of contours on the sheet metal. Recycling should be considered where there are high concentrations of costly raw materials in liquid or sludge waste. To reduce waste further, fluids should where possible be cleaned with filters or baths regenerated.
The effects on the soil can be problematic in terms of both quality (e.g. toxicity or persistence) and quantity (e.g. acidification or leaching). Airborne emissions are normally small in quantity, therefore the main causes of pollution are the discharging of residual and waste materials (filter dusts, washer and scrubber residues, purification sludges) and improper handling of auxiliary materials. Of the large number of chemical substances used in metal processing, only a few substance groups have to be regarded as representing a soil hazard and thus in general also a groundwater hazard:
- anions (chlorides, sulphates, ammonium, nitrates, cyanides etc., produced e.g. in heat treatment and pickling shops)
- heavy metals (lead, cadmium, chromium, copper, nickel, zinc, tin etc.).
- solvents (halogenated and pure hydrocarbons)
- other oil-containing substances
The areas in which contamination occur are:
- all production stages using the named substances
- storage of new and used chemicals
- transport, loading and unloading on the works site (containers, tanks, pipelines, extraction system)
- cleaning and repair processes.
To protect against contamination in these areas, the ground must be "sealed" (i.e. provided with a protective layer to prevent penetration of the materials into the ground, or with pollutant collection devices, e.g. containment basins). Insufficient attention is often paid to the storage of hazardous materials. This can result in severe environmental pollution with long term consequences, also and in particular for third parties (e.g. due to groundwater pollution). Containers and pipelines used for transporting the materials are to be regularly checked for leaks. Care must be taken to ensure an efficient flow of work and materials, with clear rules on the depositing and disposal of waste/residues (see environmental brief Disposal of Hazardous Waste, also the reference literature).
Deafness and loss of productivity result from noise pollution above a certain level. A noise level in the workplace of around 85 dB(A) or higher for the greater part of the working shift, over a number of years, is regarded as detrimental to hearing16).
16) It is as harmful to be exposed constantly to a uniformly low noise level as to a higher one for a short time.
For comparison: Leaves blown by a light wind emit a noise level of 25 to 35 dB(A); normal conversation is between 40 and 60 dB(A). Note also that the medium and higher frequencies between 1,000 and 6,000 Hz are the most damaging.
When considering noise immissions17), a distinction must be made between the direct effect on workers at the workplace and the indirect effect due to radiation and immission in the environment. In assessing noise therefore, three aspects, each requiring different reduction measures must be considered.
17) The term immission is the effect of air impurities, radiation (e.g. thermal radiation) on man, animals, plants and property.
A. Noise origination
B. Noise propagation
· Noise transmission (propagation of sound waves in different media, e.g. transmission of machine vibrations to foundations);
· Noise radiation (stimulation of air vibrations by solid-body vibrations - loudspeaker diaphragm principle).
During operations in this sector, noise is generated by machinery, by hammering, nailing, or chipping, by internal transportation processes, impact upon depositing or lifting of semifinished products, air and gas movements, fan outlets, pneumatic components, cutting torches etc.
A fan, e.g. with 50 kW, 970 rpm and a diameter of 1,800 mm, without noise damping produces a noise level of 100 dB(A). A compressed air jet produces a noise level of 108 dB(A) with an air pressure of 5 atmospheres. Welding and cutting generate noise levels of up to 101 dB(A) and pneumatic riveters and chippers produce between 100 and 130 dB(A). Manual grinders develop up to 106 dB(A). Metal band saws develop up to 106 dB(A). Turning generates between 80 and 107 dB(A). Screw presses produce noise levels up to 103 dB(A).
Noise pollution in the neighbourhood of a factory is mainly caused by radiation through the walls of the production sheds and buildings and by outward blowing fans.
Structural measures to reduce noise should therefore be taken into account as early as the planning phase (noise-absorbing walls, choice of windows, type of building materials etc.). The expected noise conditions cannot be determined simply by adding together the known noise levels of the planned machines and processes. Due to interaction and the different damping and reflection circumstances, only on-site measurements can yield accurate data on noise conditions. The maintenance of adequate distances reduces the effect on the neighbourhood.
With regard to noise protection, a distinction is made between primary and secondary measures. Active primary measures signify the use of machines constructed according to low-noise principles. For example, sheet metal forming can be made quieter by replacing impact methods with hydraulic pressing. Priority should be given to the implementation of active primary measures.
Active secondary measures are sound insulation (prevention of propagation by obstacles) and sound absorption (absorption of sound energy and its conversion to heat). A distinction is made between structure-borne and airborne noise:
- Insulation of airborne noise is achieved by partition walls, full or partial enclosure, cladding or screening.
- Insulation of structure-borne noise can be achieved by machine feet of elastic material which prevent the transfer of vibrations.
- Absorption of airborne noise over large areas can be achieved with sound-absorbing cladding material of foam or glass fibre matting. Silencers should be fitted to reduce noise at gas and air outlets. Composite silencers combining an absorber and a resonator should be provided for dust-laden gases.
- Absorption of structure-borne noise damping is achieved by means of soundproof coverings in the form of foam rubber mats on sheet metal or in sandwich form (metal - covering - metal).
Passive noise protection signifies all equipment and measures for preventing the immission of noise and vibration to the environment and the human ear. These include personal ear protection, noise protection for control rooms, noise-insulated cabins etc.
Workers must wear ear protection in the workplace where noise levels are higher than 90 dB(A). Such workplaces must display suitable warning signs; the observance of protective measures must be monitored.
Proven methods of reducing noise immissions include the use of soundproof walls or partitions and increasing the distance between industrial buildings and residential areas. With uninhibited propagation the acoustic power level is reduced by 3 (house wall) or 6 (point source of noise) dB(A) by doubling the distance.