In many locations domestic waste collection is performed by municipal employees. In others, by private companies. This article provides an overview of processes and hazards that are based on observations and experiences in the Province of Quebec, Canada. Editor.

Overview

Besides the few workers employed by municipalities in the Province of Quebec, Canada, that have their own waste collection boards, thousands of waste collectors and drivers are employed in hundreds of companies in the private sector.

Many private enterprises rely, either wholly or partially, upon jobbers who rent or own trucks and are responsible for the collectors who work for them. Competition in the sector is high, as municipal contracts are awarded to the lowest bidder, and there is a regular annual turnover of enterprises. The high competition also results in low and stable domestic waste-collection rates, and waste collection accounts for the lowest proportion of municipal taxes. However, as the existing landfills fill up, landfill costs rise, obliging municipalities to consider integrated waste-management systems. All municipal workers are unionized. Unionization of private-sector workers began in the 1980s, and 20 to 30% of them are now unionized.

Work Processes

Waste collection is a dangerous trade. If we recognize that garbage trucks are similar to hydraulic presses, it follows that waste collection is like working on a mobile industrial press under conditions much more demanding than those encountered in most factories. In waste collection, the machine travels through traffic in all seasons and workers must feed it by running behind it and tossing irregular objects of variable volume and weight, containing invisible and hazardous objects, into it. On average, collectors handle 2.4 tonnes of waste per hour. The efficiency of waste collection operations is entirely dependent on determinants of work rate and rhythm. The need to avoid rush-hour traffic and bridge line-ups creates time pressures at collection points and during transport. Speed is again of importance during unloading at landfills and incinerators.

Several aspects of waste collection influence workload and hazards. First, remuneration is on a flat-rate basis, that is, the territory specified by contract must be completely cleared of domestic waste on collection day. Since the volume of waste depends on residents’ activities and varies from day to day and from season to season, the workload varies enormously. Secondly, workers are in direct contact with the objects and waste collected. This is quite different from the situation in the commercial and industrial waste-collection sectors, where waste-filled containers are collected by either front-loading trucks equipped with automated fork-lifts or by roll-off trucks. This means that workers in those sectors do not handle the waste containers and are not in direct contact with the waste. Working conditions for these collectors therefore more closely resemble those of domestic waste drivers, rather than domestic waste collectors.

Residential collection (also known as domestic collection) is, on the other hand, primarily manual, and workers continue to handle a wide variety of objects and containers of variable size, nature and weight. A few suburban and rural municipalities have implemented semi-automated collection, involving the use of mobile domestic waste bins and side-loading collectors (figure 1). However, most domestic waste continues to be collected manually, especially in cities. The principal characteristic of this job is thus significant physical exertion.

Figure 1. Automatic, side-loading refuse collector.

Pak Mor Manufacturing Company

Hazards

A study involving field observations and measurements, interviews with management and workers, statistical analysis of 755 occupational accidents and analysis of video sequences revealed a number of potential hazards (Bourdouxhe, Cloutier and Guertin 1992).

Workload

On average, waste collectors handle 16,000 kg spread out over 500 collection points every day, equivalent to a collection density of 550 kg/km. Collection takes almost 6 hours, equivalent to 2.4 tonnes/hour, and involves walking 11 km during a total work day of 9 hours. Collection speed averages 4.6 km/h, over a territory of almost 30 km of sidewalks, streets and lanes. Rest periods are limited to a few minutes precariously balanced on the rear platform, or, in the case of driver-collectors of side-loading trucks, at the wheel. This demanding workload is exacerbated by such factors as the frequency of truck dismounts and mounts, the distance covered, travel modes, the static effort required to maintain one’s balance on the rear platform (a minimum of 13 kg of force), the frequency of handling operations per unit time, the variety of postures required (bending movements), the frequency of tosses and twisting movements of the trunk and the high collection rate per unit time in some sectors. The fact that the Association française de normalisation (AFNOR) adapted weight standard for manual handling was exceeded in 23% of observed trips is eloquent testimony of the impact of these factors. When workers’ capacities (established to be 3.0 tonnes/hour for rear-loading trucks, and 1.9 tonnes/hour for side-loading ones) are taken into account, the frequency with which the AFNOR standard is exceeded rises to 37%.

Diversity and nature of objects handled

Manipulation of objects and containers of variable weight and volume interrupts the smooth flow of operations and breaks work rhythms. Objects in this category, often hidden by residents, include heavy, large or bulky objects, sharp or pointed objects and hazardous materials. The most frequently encountered hazards are listed in table 1.

Table 1. Hazardous objects found in domestic waste collections.

Glass, window panes, fluorescent tubing

Battery acid, cans of solvent or paint, aerosol containers, gas cylinders, motor oil

Construction waste, dust, plaster, sawdust, hearth cinders

Pieces of wood with nails in them

Syringes, medical waste

Garden waste, grass, rocks, earth

Furniture, electrical appliances, other large domestic trash

Pre-compacted waste (in apartment buildings)

Excessive numbers of small containers from small businesses and restaurants

Large amounts of vegetable and animal waste in rural sectors

Extra-large bags

Prohibited containers (e.g., no handles, excessive weight, 55-gallon oil drums, thin-necked drums, garbage cans without covers)

Small, apparently light bags that are in fact heavy

Excessive numbers of small bags

Paper bags and boxes that rip

All waste that is hidden because of its excessive weight or toxicity, or that surprises unprepared workers

Commercial containers that must be emptied with an improvised system, which is often inappropriate and dangerous

Workers are greatly helped by having residents sort waste into colour-coded bags and mobile domestic bins which facilitate the collection and allow better control of work rhythm and effort.

Climatic conditions and the nature of objects transported

Wet paper bags and poor-quality plastic bags that rip and scatter their contents over the sidewalk, frozen garbage cans and domestic bins stuck in snow banks can cause mishaps and dangerous recovery manoeuvres.

Work schedule

The need to rush, traffic problems, parked cars and crowded streets all can contribute to dangerous situations.

In an attempt to reduce their workload and maintain a high but constant work rhythm in the face of these constraints, workers often attempt to save time or effort by adopting work strategies that may be hazardous. The most commonly observed strategies included kicking bags or cardboard boxes towards the truck, zigzagging across the road to collect from both sides of the street, grabbing bags while the truck is in motion, carrying bags under the arm or against the body, using the thigh to help load bags and garbage cans, hand-picking of waste scattered on the ground and manual compaction (pushing garbage overflowing the hopper with the hands when the compacting system is incapable of processing the load rapidly enough). For example, in suburban collection with a rear-loading truck, almost 1,500 situations were observed per hour that could result in accidents or increase workload. These included:

• 53 mounts and dismounts from the truck’s rear platform

• 38 short runs

• 482 bending movements

• 203 tosses

• 159 twisting movements

• 277 potentially hazardous actions (including 255 work strategies aimed at reducing workload by saving time or effort)

• 285 instances of increased workload, including 11 mishap- recovery activities

• 274 dangerous or heavy objects or containers.

Collection with side-loading trucks (see figure 1) or small mobile domestic bins reduces the manipulation of heavy or dangerous objects and the frequency of situations that could result in accidents or an increase in workload.

Use of public thoroughfares

The street is the collectors’ workplace. This exposes them to such hazards as vehicular traffic, blocked access to residents’ waste receptacles, accumulation of water, snow, ice and neighbourhood dogs.

Vehicles

Rear-loading trucks (figure 2) often have excessively high or shallow steps and rear platforms that are difficult to mount and render descents perilously similar to jumps. Hand-rails that are too high or too close to the truck body only worsen the situation. These conditions increase the frequency of falls and of collisions with structures adjacent to the rear platform. In addition, the upper edge of the hopper is very high, and shorter workers must expend additional energy lifting objects into it from the ground. In some cases, workers use their legs or thighs for support or additional power when loading the hopper.

Figure 2. Back-loading enclosed compactor truck.

National Safety Council (US)  The packer-blade comes down within centimetres of the edge of the platform. The blade has the capacity to cut protruding objects.

The characteristics of side-loading trucks and the operations related to their loading result in specific repetitive movements likely to cause muscle and joint problems in the shoulder and upper back. Driver-collectors of side-loading trucks have an additional constraint, as they must cope with both the physical strain of collection and the mental strain of driving.

Personal protective equipment

While the theoretical value of PPE is beyond question, it may nevertheless prove inadequate in practice. In concrete terms, the equipment may be inappropriate for the conditions under which collection is carried out. Boots, in particular, are incompatible with the narrow utilizable height of rear platforms and the high work rhythm necessitated by the manner in which collection is organized. Strong, puncture-resistant yet flexible gloves are valuable in protecting against hand injuries.

Work organization

Some aspects of work organization increase workload and, by extension, hazards. In common with most flat-rate situations, the main advantage to workers of this system is the ability to manage their work time and save time by adopting a rapid work rhythm as they see fit. This explains why attempts, based on safety considerations, to slow down the pace of work have been unsuccessful. Some work schedules exceed workers’ capacities.

The role of the myriad variations of residents’ behaviour in the creation of additional hazards merits a study in itself. Prohibited or dangerous wastes skilfully hidden in regular waste, non-standard containers, excessively large or heavy objects, disagreements over collection times and non-conformity with bylaws all increase the number of hazards—and the potential for conflicts between residents and collectors. Collectors are often reduced to the role of “garbage police”, educators and buffers between municipalities, enterprises and residents.

Collection of materials for recycling is not without its own problems despite a low waste density and collection rates far below those of traditional collection (with the exception of the collection of leaves for composting). The hourly frequency of situations that could result in accidents is often high. The fact that this is a new type of work for which few workers have been trained should be borne in mind.

In several cases, workers are obliged to perform such dangerous activities as mounting the truck’s compaction box to get into the compartments and move piles of paper and cardboard with their feet. Several work strategies aimed at speeding up work rhythm have also been observed, e.g., hand re-sorting of the material to be recycled and removing objects from the recycling box and carrying them to the truck, rather than carrying the box to the truck. The frequency of mishaps and disruptions of normal work activity in this type of collection is particularly high. These mishaps result from workers doing ad hoc activities that are themselves dangerous.

Occupational Accidents and Prevention

Domestic waste collection is a dangerous trade. Statistics support this impression. The average annual accident rate in this industry, for all types of enterprise, truck and trade, is almost 80 accidents for every 2,000 hours of collection. This is equivalent to 8 workers of every 10 suffering an injury at least once a year. Four accidents occur for every 1,000 10-tonne truckloads. On average, each accident results in 10 lost workdays and accident compensation of $820 (Canadian). Indices of injury frequency and severity vary among enterprises, with higher rates observed in municipal enterprises (74 accidents/100 workers versus 57/100 workers in private enterprises) (Bourdouxhe, Cloutier and Guertin 1992). The most common accidents are listed in table 2.

Table 2. Most common accidents in domestic waste collection, Quebec, Canada.

Collectors typically suffer hand and thigh lacerations, drivers typically suffer sprained ankles resulting from falls during cabin dismounts and driver-collectors of side-loading trucks typically suffer shoulder and upper back pain resulting from tossing movements. The nature of the accidents also depends on the type of truck, although this can also be seen as a reflection of the specific trades associated with rear- and side-loading trucks. These differences are related to equipment design, the type of movements required and the nature and density of waste collected in the sectors in which these two types of truck are used.

Prevention

The following are ten categories in which improvements could make domestic waste collection safer:

1. management of health and safety (for instance, the development of accident-prevention programmes based on workers’ knowledge of occupational hazards which are better adapted to actual tasks)
2. training and hiring
3. work organization, organization of collection and workload
4. vehicles
5. training and work conditions of auxiliary, occasional and temporary workers
6. collection contracts
7. public management
8. collaboration between employer associations (municipal and private), workers and municipal or regional decision-making bodies
9. stability of the workforce
10. research on personal protective equipment, ergonomic design of trucks, subcontracting jobbers and safety.

Conclusion

Domestic waste collection is an important but hazardous activity. Protection of workers is made more difficult where this service is contracted out to private sector enterprises which, as in the province of Quebec, may subcontract the work to many smaller jobbers. A large number of ergonomic and accident hazards, compounded by work quotas, adverse weather and local street and traffic problems must be confronted and controlled if workers’ health and safety are to be maintained.

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Adapted from 3rd edition, Encyclopaedia of Occupational Health and Safety.

Waste water is treated in order to remove pollutants and to comply with the limits set by law. For this purpose an attempt is made to render the pollutants in the water insoluble in the form of solids (e.g., sludge), liquids (e.g., oil) or gases (e.g., nitrogen) by applying appropriate treatments. Well known techniques are then used to separate the treated waste water to be returned to the natural waterways from the pollutants rendered insoluble. The gases are dispersed into the atmosphere, while the liquid and solid residues (sludge, oil, grease) are usually digested before being submitted to further treatment. There may be single or multi-stage treatments according to the characteristics of the waste water and to the degree of purification required. Waste water treatment may be subdivided into physical (primary), biological (secondary) and tertiary processes.

Physical Processes

The various physical treatment processes are designed to remove insoluble pollutants.

Screening

The sewage is made to pass through screens which retain coarse solids that may block or damage the treatment works equipment (e.g., valves and pumps). The screenings are processed according to local situations.

Sand removal

The sand contained in the waste water has to be removed as it tends to settle in the pipework on account of its high density and cause abrasion to the equipment (e.g., centrifugal separators and turbines). Sand is generally removed by passing the waste water through a channel of constant cross-section at a velocity of 15 to 30 cm/s. The sand collects on the channel bottom and may be used, after washing to remove putrescible matter, as an inert material, such as for road building.

Oil removal

Oils and non-emulsifiable fats have to be removed because they would adhere to the equipment of the treatment works (e.g., basins and clarifiers) and interfere with the subsequent biological treatment. Oil and fat particles are made to collect on the surface by passing the waste water at an appropriate velocity through tanks of rectangular cross-section; they are skimmed off mechanically and may be used as a fuel. Multi-plate separators of compact design and high efficiency are frequently used for oil removal: the sewage is made to pass from above through stacks of flat inclined plates; the oil adheres to the bottom surfaces of the plates and moves to the top where it is collected. With both these processes, the de-oiled water is discharged at the bottom.

Sedimentation, flotation and coagulation

These processes enable the solids to be removed from the waste water, heavy ones (greater than 0.4 μm in diameter) by sedimentation and light ones (less than 0.4 μm) by flotation. This treatment, too, relies on the differences in density of the solids and of the flowing waste water which is passed through sedimentation tanks and flotation tanks made of concrete or steel. The particles to be separated collect in the bottom or at the surface, settling or rising at velocities which are proportional to the square of the particle radius and to the difference between the particle density and the apparent waste water density. Colloidal particles (e.g., proteins, latexes and oily emulsions) with sizes from 0.4 to 0.001 μm are not separated, as these colloids become hydrated and usually negatively charged by adsorption of ions. Consequently the particles repel each other so that they cannot coagulate and separate. However, if these particles are “destabilized”, they coagulate to form flocks greater than 4 μm, which can be separated as sludge in conventional sedimentation or flotation tanks. Destabilization is obtained by coagulation, that is, by adding 30 to 60 mg/l of an inorganic coagulant (aluminium sulphate, iron (II) sulphate or iron (III) chloride). The coagulant hydrolyses under given pH (acidity) conditions and forms positive polyvalent metal ions, which neutralize the negative charge of the colloid. Flocculation (the agglomeration of coagulated particles in flocks) is facilitated by adding 1 to 3 mg/l of organic polyelectrolytes (flocculation agents), resulting in flocks of 0.3 to 1 μm diameter which are easier to separate. Sedimentation tanks of the horizontal-flow type may be used; they have rectangular cross-section and flat or sloping bottoms. The waste water enters along one of the head sides, and the clarified water leaves over the edge at the opposite side. Also vertical flow sedimentation tanks can be used which are cylindrical in shape and have a bottom like an inverted right circular cone; the waste water enters in the middle, and the clarified water leaves the tank over the top indented edge to be collected into an external circumferential channel. With the two types of tank, the sludge settles on the bottom and is conveyed (if necessary by means of a raking gear) into a collector. The solids concentration in the sludge is 2 to 10%, whereas that of the clarified water is 20 to 80 mg/l.

The flotation tanks are usually cylindrical in shape and have fine bubble air diffusers installed in their bottoms, the sewage entering the tanks in the centre. The particles adhere to the bubbles, float to the surface and are skimmed off, while the clarified water is discharged below. In the case of the more efficient “dissolved-air floating tanks”, the waste water is saturated with air under a pressure of 2 to 5 bars and then allowed to expand in the centre of the floating tank, where the minute bubbles resulting from the decompression make the particles float to the surface.

Compared to sedimentation, flotation yields a thicker sludge at a higher particle separation velocity, and the equipment required is therefore smaller. On the other hand, the operating cost and the concentration of solids in the clarified water are higher.

Several tanks arranged in series are required for coagulating and flocculating a colloidal system. An inorganic coagulant and, if necessary, an acid or an alkali to correct the pH value are added to the waste water in the first tank, which is equipped with an agitator. The suspension is then passed into a second tank equipped with a high-speed agitator; here, the polyelectrolyte is added and dissolved within a few minutes. The flock growth takes place in a third tank with a slow-running agitator and is carried out for 10 to 15 minutes.

Biological Processes

Biological treatment processes remove organic biodegradable pollutants by use of micro-organisms. These organisms digest the pollutant by an aerobic or anaerobic process (with or without supply of atmospheric oxygen) and convert it into water, gases (carbon dioxide and methane) and a solid insoluble microbic mass which can be separated from the treated water. Especially in the case of industrial effluents proper conditions for the development of micro-organisms must be assured: presence of nitrogen and phosphorus compounds, traces of microelements, absence of toxic substances (heavy metals, etc.), optimum temperature and pH value. Biological treatment includes aerobic and anaerobic processes.

Aerobic processes

The aerobic processes are more or less complex according to the space available, the degree of purification required and the composition of the waste water.

Stabilization ponds

These are generally rectangular and 3 to 4 m deep. The sewage enters at one end, is left for 10 to 60 days and leaves the pond partly at the opposite end, partly by evaporation and partly by infiltration into the ground. The purification efficiency ranges from 10 to 90% according to the type of effluent and to the residual 5-day biological oxygen demand (BOD₅) content (<40 mg/l). Oxygen is supplied from the atmosphere by diffusion through the surface of the water and from photosynthetic algae. The solids in suspension in the waste water and those produced by microbial activity settle on the bottom, where they are stabilized by aerobic and/or anaerobic processes according to the depth of the ponds which affects the diffusion both of oxygen and sunlight. The oxygen diffusion is frequently accelerated by surface aerators, which enable the volume of the ponds to be reduced.

This type of treatment is very economical if space is available, but requires clay-like soil to prevent the pollution of underground water by toxic effluents.

Activated sludge

This is used for an accelerated treatment carried out in concrete or steel tanks of 3 to 5 m depth where the waste water comes into contact with a suspension of micro-organisms (2 to 10 g/l) which is oxygenated by means of surface aerators or by blowing in air. After 3 to 24 hours, the mixture of treated water and micro-organisms is passed into a sedimentation tank where the sludge made up by micro-organisms is separated from the water. The micro-organisms are partly returned to the aerated tank and partly evacuated.

There are various types of activated-sludge processes (e.g., contact-stabilization systems and use of pure oxygen) which yield purification efficiencies of greater than 95% even for industrial effluents but they require accurate controls and high energy consumption for oxygen supply.

Percolating filters

With this technique the micro-organisms are not kept in suspension in the waste water, but adhere to the surface of a filling material over which the sewage is sprayed. Air circulates through the material and supplies the required oxygen without any energy consumption. According to the type of waste water and to increase efficiency, part of the treated water is recirculated to the top of the filter bed.

Where land is available, low-cost filling materials of appropriate size (e.g., crushed stone, clinker and limestone) are used, and on account of the weight of the bed the percolating filter is generally constructed as a 1 m high concrete tank usually sunk in the ground. If there is not enough land, more costly lightweight packing materials such as high-rate plastic honeycomb media, with up to 250 square metres of surface area/cubic metre of media, are stacked in percolating towers up to 10 m high.

The waste water is distributed over the filter bed by a mobile or fixed sparging mechanism and collected in the floor to be eventually recirculated to the top and to be passed into a sedimentation tank where the sludge formed can settle. Openings at the bottom of the percolating filter allow for air circulation through the filter bed. Pollutants removal efficiencies of 30 to 90% are achieved. In many cases several filters are arranged in series. This technique, which requires little energy and is easy to operate, has found widespread use and is recommended for cases where land is available, for instance, in developing countries.

Biodiscs

A set of flat plastic discs mounted parallel on a horizontal rotating shaft are partially immersed in the waste water contained in a tank. Due to the rotation the biological felt that covers the discs is brought into contact with the effluents and atmospheric oxygen. The biological sludge coming off the biodiscs remains in suspension in the waste water, and the system acts as activated sludge and sedimentation tank at the same time. Biodiscs are suitable for small to medium-sized industrial factories and communities, take up little space, are easy to operate, require little energy and yield efficiencies of up to 90%.

Anaerobic processes

Anaerobic processes are carried out by two groups of micro-organisms—hydrolytic bacteria, which decompose complex substances (polysaccharides, proteins, lipids, etc.) to acetic acid, hydrogen, carbon dioxide and water; and methanogenic bacteria, which convert these substances to a biomass (that can be removed from the treated sewage by sedimentation) and to biogas containing 65 to 70% methane, the remainder being carbon dioxide, and having a high heat value.

These two groups of micro-organisms, which are very sensitive to toxic contaminants, act simultaneously in the absence of air at an almost neutral pH value, some requiring a temperature of 20 to 38^oC (mesophilic bacteria) and other, more delicate ones, 60 to 65^oC (thermophilic bacteria). The process is carried out in stirred, closed concrete or steel digesters, where the required temperature is held by thermostats. Typical is the contact process, where the digester is followed by a sedimentation tank to separate the sludge, which is partially recirculated to the digester, from the treated water.

Anaerobic processes need neither oxygen nor power for oxygen supply and yield biogas, which can be used as a fuel (low operating costs). On the other hand, they are less efficient than aerobic processes (residual BOD₅: 100 to 1,500 mg/l), are slower and more difficult to control, but enable faecal and pathogenic micro-organisms to be destroyed. They are used for treating strong wastes, such as sedimentation sludge from sewage, sludge in excess from activated sludge or percolating-filter treatments and industrial effluents with a BOD₅ up to 30,000 mg/l (e.g., from distilleries, breweries, sugar refineries, abattoirs and paper mills).

Tertiary Processes

The more complex and more expensive tertiary processes make use of chemical reactions or specific chemicophysical or physical techniques to remove water-soluble non-biodegradable pollutants, both organic (e.g., dyes and phenols) and inorganic (e.g., copper, mercury, nickel, phosphates, fluorides, nitrates and cyanides), especially from industrial waste water, because they cannot be removed by other treatments. Tertiary treatment also enables a high degree of water purification to be obtained, and the water thus treated may be used as drinking water or for manufacturing processes (steam generation, cooling systems, process water for particular purposes). The most important tertiary processes are as follows.

Precipitation

Precipitation is carried out in reactors made of an appropriate material and equipped with agitators where chemical reagents are added at a controlled temperature and pH value to convert the pollutant to an insoluble product. The precipitate obtained in the form of sludge is separated by conventional techniques from the treated water. In waste water from the fertilizer industry, for instance, phosphates and fluorides are rendered insoluble by reaction with lime at ambient temperature and at an alkaline pH; chromium (tanning industry), nickel and copper (electroplating shops) are precipitated as hydroxides at an alkaline pH after having been reduced with m-disulphite at a pH of 3 or lower.

Chemical oxidation

The organic pollutant is oxidized with reagents in reactors similar to those used for precipitation. The reaction is generally continued until water and carbon dioxide are obtained as final products. Cyanides, for instance, are destroyed at ambient temperature by adding sodium hypochlorite and calcium hypochlorite at alkaline pH, whereas azo- and anthraquinone-dyes are decomposed by hydrogen peroxide and ferrous sulphate at pH 4.5. Coloured effluents from the chemical industry containing 5 to 10% non-biodegradable organic substance are oxidized at 200 to 300°C at high pressure in reactors made of special materials by blowing air and oxygen into the liquid (wet oxidation); catalysts are sometimes used. Pathogens left in urban sewage after treatment are oxidized by chlorination or ozonisation to render the water drinkable.

Absorption

Some pollutants (e.g., phenols in waste water from coking plants, dyes in water for industrial or drinking purposes and surfactants) are effectively removed by absorption on activated carbon powder or granules which are highly porous and have a large specific surface area (of 1000 m²/g or more). The activated carbon powder is added in metered quantities to the waste water in stirred tanks, and 30 to 60 minutes later the spent powder is removed as a sludge. Granulated activated carbon is used in towers arranged in series through which the polluted water is passed. The spent carbon is regenerated in these towers, that is, the absorbed pollutant is removed either by chemical treatment (e.g., phenols are washed out with soda) or by thermal oxidation (e.g., dyes).

Ion exchange

Certain natural substances (e.g., zeolites) or artificial compounds (e.g., Permutit and resins) exchange, in a stoichiometric and reversible manner, the ions bound to them with those contained, even strongly diluted, in the waste water. Copper, chromium, nickel, nitrates and ammonia, for instance, are removed from waste water by percolation through columns packed with resins. When the resins are spent, they are reactivated by washing with regenerating solutions. Metals are thus recovered in a concentrated solution. This treatment, though costly, is efficient and advisable in cases where a high degree of purity is required (e.g., for waste water contaminated by toxic metals).

Reverse osmosis

In special cases it is possible to extract water of high purity, suitable for drinking, from diluted waste water by passing it through semi-permeable membranes. On the waste water side of the membrane the pollutants (chlorides, sulphates, phosphates, dyes, certain metals) are left as concentrated solutions which have to be disposed of or treated for recovery. The diluted waste water is subjected to pressures up to 50 bars in special plant containing synthetic membranes made of cellulose acetate or other polymers. The operating cost of this process is low, and separation efficiencies of greater than 95% may be obtained.

Sludge Treatment

Rendering pollutants insoluble during waste water treatment results in the production of considerable amounts of sludge (20 to 30% of the removed chemical oxygen demand (COD) which is strongly diluted (90 to 99% water)). The disposal of this sludge in a manner acceptable to the environment presupposes treatments with a cost of up to 50% of those required for waste water purification. The types of treatment depend on the destination of the sludge, depending in turn on its characteristics and on local situations. Sludge may be destined for:

• fertilization or dumping at sea if it is substantially free from toxic substances and contains nitrogen and phosphorus compounds (sludge from biological treatment), using fixed outfalls, lorries or barges

• sanitary landfill into pits dug in the ground, alternating layers of sludge and soil. Impermeabilisation of peats is required if the sludge contains toxic substances that may be washed out by atmospheric precipitations. The pits should be remote from water-bearing strata. Non-stabilized organic sludge is usually mixed with 10 to 15% lime to retard putrefaction.

• incineration in rotary or fluidized-bed furnaces if the sludge is rich in organic substances and free from volatile metals; if necessary, fuel is added, and the smoke emitted is purified.

The sludge is dewatered before its disposal to reduce both its volume and the cost of its treatment, and it is frequently stabilized to prevent its putrefaction and to render harmless any toxic substances it may contain.

Dewatering

Dewatering includes previous thickening in thickeners, similar to sedimentation tanks, where the sludge is left for 12 to 24 hours and loses part of the water which collects on the surface, while the thickened sludge is discharged below. The thickened sludge is dewatered, for example, by centrifugal separation or by filtration (under vacuum or pressure) with conventional equipment, or by exposure to the air in layers of 30 cm thick in sludge-drying beds consisting of rectangular concrete lagoons, approximately 50 cm deep, with a sloped bottom covered with a layer of sand to facilitate water drainage. Sludge containing colloidal substances should be previously destabilized by coagulation and flocculation, according to already described techniques.

Stabilization

Stabilization includes digestion and detoxification. Digestion is a long-term treatment of the sludge during which it loses 30 to 50% of its organic matter, accompanied by an increase in its mineral salt content. This sludge is no longer putrescible, any pathogens are destroyed and the filtrability is improved. Digestion may be of the aerobic type when the sludge is aerated during 8 to 15 days at ambient temperature in concrete tanks, the process being similar to activated-sludge treatment. It may be of the anaerobic type if the sludge is digested in plants similar to those used for the anaerobic waste treatment, at 35 to 40°C during 30 to 40 days, with the production of biogas. Digestion can be of the thermal type when the sludge is treated with hot air at 200 to 250°C and at a pressure of more than 100 bars during 15 to 30 minutes (wet combustion), or when it is treated, in the absence of air, at 180°C and at autogenous pressure, for 30 to 45 minutes.

Detoxification renders harmless sludge containing metals (e.g., chromium, nickel and lead), which are solidified by treatment with sodium silicate and autothermically converted into the corresponding insoluble silicates.

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Adapted from 3rd edition, Encyclopaedia of Occupational Health and Safety.

Prevention of dirt-borne disease, prevention of damage to vehicles by harmful objects and the joy of viewing a neat, attractive city are all benefits derived from clean streets. Herded animals or animal-drawn vehicles, which in earlier times caused unsanitary conditions, have generally ceased to be a problem; however, the expansion in world population with the resultant upsurge in waste generated, the increase in the number and size of factories, the growth in the number of vehicles and newspapers and the introduction of disposable containers and products have all contributed to the amount of street refuse and added to the street-cleaning problem.

Organization and Processes

Municipal authorities recognizing the threat to health posed by dirty streets have sought to minimise the danger by organizing street cleaning sections in the public works departments. In these sections, a superintendent responsible for scheduling frequency of cleaning various districts will have forepeople responsible for specific cleaning operations.

Normally, business districts will be swept daily while arterial roads and residential areas will be swept weekly. Frequency will depend upon rain or snowfall, topography and the education of the populace toward prevention of litter.

The superintendent will also decide the most effective means of achieving clean streets. These could be hand sweeping by one worker or a group, hose flushing or machine sweeping or flushing. Generally a combination of methods, depending on the availability of equipment, type of dirt encountered and other factors will be used. In areas of heavy snowfall, special snow-clearing equipment may be used on occasion.

Hand sweeping is generally done in the daytime and confined to cleaning of gutters or spot cleaning of pavements or adjacent areas. The equipment used consists of brooms, scrapers and shovels. One sweeper generally patrols a specified route and cleans about 9 km of curb per shift under favourable conditions; however, this may be reduced in congested business districts.

Dirt collected by one-person sweeping is placed in a cart which he or she pushes ahead and dumps in boxes placed at intervals along his or her route; these boxes are emptied periodically into refuse trucks. In group sweeping, dirt is swept into piles along the gutters and loaded directly into trucks. Normally a group of 8 sweepers will have 2 workers assigned as loaders. Group sweeping is particularly effective for massive clean-up jobs such as after storms, parades or other special events.

Advantages of hand sweeping are: it is easily adjusted to meet changing cleaning loads; it can be used in areas inaccessible to machines; it can be conducted in heavy traffic with minimum interference with vehicle movement; it can be done in freezing weather and it can be used on pavements where surface conditions do not permit machine cleaning. Disadvantages are: the work is dangerous in traffic; it raises dust; dirt piled in gutters may be dispersed by wind or traffic if not collected promptly; and hand sweeping may be costly in labour-expensive areas.

Hose flushing is not considered an economical operation today; however, it is effective where there is a large amount of dirt or mud adhering to the pavement surfaces, where there are large numbers of parked vehicles or in market areas. It is generally done at night by a two-person crew, one of whom handles the hose nozzle and directs the stream and the other connects hose to the hydrant. Equipment consists of hoses, hose nozzles and hydrant wrenches.

Machine sweepers consist of motorized chassis mounted with brushes, conveyors, sprinklers and storage bins. They are generally used in late evening or early morning hours in business districts and during the day in residential areas. Cleaning action is confined to the gutters and adjacent areas where most dirt accumulates.

The machine is operated by one worker and can be expected to clean approximately 36 km of curb during an 8 hour shift. Factors affecting output are: number of times and distance which must be travelled to dump dirt or pick up sprinkling water; traffic density; and amount of dirt collected.

The advantages of machine sweepers are: they clean well, rapidly and raise no dust when sprinklers are used; they pick up the dirt as they clean; they can be used at night; and they are relatively economical. The disadvantages are: they cannot clean under parked cars or in off-pavement areas; they are not effective on rough, wet or muddy streets; the sprinkler cannot be used in freezing weather and dry sweeping raises dust; and they require skilled operators and maintenance personnel.

Flushing machines are essentially water tanks mounted on a motorized chassis which is fitted with a pump and nozzle to provide pressure and direct the stream of water against the pavement surface. The machine can be expected to clean about 36 km of 7 m wide pavement during an 8 hour shift.

The advantages of flushing machines are: they can be used effectively on wet or muddy pavements; they clean rapidly, well and under parked cars without raising dust; and they can operate at night or in light traffic. The disadvantages are: they require additional cleaning to be effective where street, litter or sewer conditions are not favourable; they annoy pedestrians or vehicle operators who are splashed; they cannot be used in freezing weather; and they require skilled operators and maintenance personnel.

Hazards and Their Prevention

Street cleaning is a hazardous occupation due to the fact that it is conducted in traffic and is concerned with dirt and refuse, with the possibility of infection, cuts from broken glass, tins and so on. In crowded areas, hand sweepers may be exposed to a considerable amount of carbon monoxide and to a high level of noise.

Traffic hazards are protected against by training sweepers in ways of avoiding danger, such as arranging work against the traffic flow and providing them with highly visible clothing as well as attaching red flags or other warning devices to their carts. Machine sweepers and flushers are made visible by fitting them with flashing lights, waving flags and painting them distinctively.

Street cleaners, and in particular hand sweepers, are exposed to all the vagaries of weather and occasionally may have to work in very severe conditions. Illness, infection and handling accidents can in part be prevented by the use of PPE and in part by training. Mechanical equipment such as that used for snow cleaning should be operated only by trained workers.

There should be a conveniently accessible central point providing good washing facilities (including showers where practicable), a cloakroom with arrangements for changing and drying clothes, a messroom and a first-aid room. Periodical medical examination is desirable.

Environmental Concerns of Snow Disposal

Snow removal and disposal introduces a set of environmental concerns related to the potential deposition of debris, salts, oil, metals and particulates in local waterbodies. A particular danger exists from the concentration of particulates, such as lead, that originate in atmospheric emissions from industrialized areas and automobiles. The danger of melt-water runoff to aquatic organisms and the risk of soil and groundwater contamination has been countered by the adoption of safe handling practices that protect sensitive areas from exposure. Snow disposal guidelines have been adopted in several Canadian provinces (e.g., Quebec, Ontario, Manitoba).

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Overview

Recycling means different things to different people. To consumers, recycling may mean putting out bottles and cans for curbside collection. To a product maker—a manufacturer of raw materials or fabricator of goods—it means including recycled materials in the process. To recycling service providers, recycling can mean providing cost-efficient collection, sorting and shipping services. For scavengers, it means culling recyclable materials from garbage and waste cans and selling them to recycling depots. To public policy makers in all levels of government, it means establishing regulations governing collection and utilization as well as reducing the volume of waste to be disposed of and deriving revenue from the sale of the recycled materials. For recycling to work effectively and safely, these diverse groups must be educated to work together and share responsibility for its success.

The recycling industry had been growing steadily since its inception a century ago. Until the 1970s, it remained basically unchanged as a voluntary private sector effort conducted largely by scrap dealers. With the advent of incineration in the 1970s, it became desirable to separate out certain materials before putting waste into the furnaces. This concept was introduced to address the emission problems created by metals, batteries, plastics and other materials discarded in urban wastes which were causing many old incinerators to be shut down as environmental polluters. The increasing concern about the environment provided the primary impetus for the organized separation of plastics, aluminium, tin, paper and cardboard from the residential waste stream. Initially, the recycling industry was not economically viable as a self-sustaining business, but by the mid 1980s, the need for the materials and the increase in their prices led to the development of many new material recycling facilities (MRFs) to handle commingled recyclable materials across the US and Europe.

Work Force

The broad range of skills and expertise makes the range of employment for a MRF very wide. Whether it is a full-service MRF or a single sorting-line operation, the following groups of workers are generally employed:

• Operators of heavy equipment (front end loaders, grapples, bull-dozers, etc.) work on the tipping floor, coordinating the movement of waste from the staging area of the tipping floor to the area where the materials are sorted.

• Material sorters, the majority of the workforce, segregate and sort recyclable materials by product and/or colour. This may be done entirely by hand or with the assistance of equipment. The sorted materials are then baled or crated.

• Forklift operators are responsible for moving finished bales from the throat of the baler to the storage area and from there to the trucks or other means of transportation.

• Maintenance workers are becoming increasingly important as the technology advances and the machines and equipment become more complicated.

Processes and Facilities

The recycling industry has been growing very rapidly and has evolved many different processes and procedures as the technology of sorting recyclable materials has advanced. The most common types of installation include full-service MRFs, non-waste stream MRFs and simple sorting and processing systems.

Full-service MRFs

The full-service MRF receives recyclable materials mixed in the residential waste streams. Typically, the resident places the recyclables in coloured plastic bags which are then placed in the residential waste container. This allows the community to combine recyclable materials with other residential wastes, eliminating the need for separate collection vehicles and containers. A typical sequence of operations includes the following procedures:

• The waste and recyclables bags are dropped from the collection vehicle onto the tipping/collection floor.

• The mixture of waste and recyclables is moved either by a grapple or a front-end loader to an in-floor conveyer.

• The conveyer moves the material into the sorting area where a rotating trommel (a cylindrical sieve) opens the bags and allows the very small particles of dirt, sand and gravel to pass through the openings to a collection container for discarding.

• The remaining materials are semi-automatically sorted by screens or disks according to weight and bulkiness. Glass is sorted by its heavier weight, plastics by their lighter weight and paper fibre materials by their bulk.

• Employees hand sort materials, typically from an elevated position above bunkers into which the materials can be stored. Materials are sorted according to the grade of paper, the colour of glass, the physical properties of plastic and so on.

• Refuse and other wastes are collected and removed by tractor-trailer loads.

• The separated materials are moved from the bunkers by forklift or by a “walking floor” (i.e., a conveyer) to a baler or a shredding and baling operation.

• The formed bale is discharged from the baler and moved to the storage area by a forklift.

• Collected bales are shipped either by rail or a tractor-trailer. Instead of baling, some MRFs loose load the materials into a rail car or a tractor trailer.

Non-waste stream MRF

In this system, only the recyclables are delivered to the MRF; the residential wastes go elsewhere. It involves an advanced, semi-automated sorting and processing process system in which all of the steps are the same as those described above. Because of the smaller volume, fewer employees are involved.

Simple sorting/processing system

This is a labour-intensive system in which the sorting is performed manually. Typically, a conveyer belt is used to move material from one work station to another with each sorter removing one type of material as the belt passes his station. A typical sequence for such a simple, inexpensive processing system would include these processes:

• Mixed recyclables are received on a tipping floor and are moved by a front-end loader to the main sorting conveyor belt.

• Glass bottles are separated manually by colour (flint, amber, green and so forth).

• Plastic containers are sorted by grade and accumulated for baling.

• Aluminium cans are removed manually and fed to either a compactor or baler.

• The remaining materials are discharged into a residue pile or container for disposal.

Equipment and machinery

The machinery and equipment used in a MRF is determined by the type of process and the volumes of materials handled. In a typical semi-automated MRF, it would include:

• bag openers

• magnetic separators

• screens (disks, shakers or trommel)

• material classification equipment (mechanical or pneumatic)

• glass crushers

• balers and compactors

• eddy current separators (for non-ferrous metal separation)

• conveyer belts

• rolling stock.

Health and safety hazards

MRF workers face a large variety of environmental and work hazards, many of which are unpredictable since the content of the waste changes continually. Prominent among them are:

• infectious disease from biological and medical wastes

• acute and chronic toxicity from household chemicals, solvents and other chemicals being discarded. This risk is not very great (except when industrial wastes find their way into the residential stream) since household chemicals are usually not very toxic and only relatively small amounts are present.

• solvents and fuel and exhaust fumes (especially vehicle operators and maintenance workers)

• exposures to heat, cold and bad weather since many MRFs are exposed to the elements

• noise at harmful levels when heavy machines operate in confined spaces

• physical hazards such as slips and falls, puncture wounds, lacerations and abrasions, muscular strains, sprains and repetitive motion injuries. Sorters usually stand continuously, while vehicle operators must sometimes contend with poorly designed seats and operating controls.

• airborne dust and particles.

Table 1 lists the most common types of injury in the recycling industry.

Table 1. Most frequent injuries in the recycling industry.

Prevention

MRF workers have the potential to be exposed to whatever wastes are delivered to it, as well as the ever-changing environment in which they work. The management of the facility must constantly be aware of the content of the material being delivered, the training and supervision of the workers and their compliance with safety rules and regulations, the proper use of PPE and the maintenance of machinery and equipment. The following safety considerations deserve constant close attention:

• lockout/tagout precautions

• general housekeeping

• egress maintenance

• emergency preparedness and, when needed, access to first-aid and medical assistance

• hearing conservation programmes

• protection against blood-borne pathogens

• preventive maintenance of machines and equipment

• traffic patterns and danger to pedestrians from the rolling stock

• confined spaces

• fire prevention and training and equipment for fire-fighting

• household hazardous waste management

• availability and use of high-quality, properly-sized PPE.

Conclusion

Municipal recycling is a relatively new industry that is changing rapidly as it grows and its technology advances. The health and safety of its workforce depend on proper design of processes and equipment and the proper training and supervision of its workers.

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