2.2 Mechanical engineering and operation of workshops and shipyards

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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)
- drying

Emissions into the air can be divided into:

- coarse and fine dust
- aerosols
- 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 separator)
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).

2.2.2 Wastewater

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.

2.2.4 Soil

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).

2.2.5 Noise

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.

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