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SAFETY AND HEALTH IN WWTP 7.1 PHYSICAL HAZARDS 7.1.1 Slips and falls 7.1.2 Other accidents 7.1.3 Drowning 7.1.4 Fires 7.1.5 Oxygen deficiency or enrichment 7.2 CHEMICAL HAZARDS11 7.2.1 Aluminum sulfate Al2(SO4)3 7.2.2 Ferric chloride Fe(Cl)3 7.2.3 Anhydrous ammonia 7.2.4 Caustic soda – NaOH 7.2.5 Calcium hydroxide Ca(OH)2 7.2.6 Chlorine - Cl2 7.2.7 Polymers 7.2.8 Solvents 7.2.9 Phosphate Na3PO4 7.2.10 Fuels 7.2.11 Toxic or suffocating gases or vapors 7.3 BIOLOGICAL HAZARDS 7.3.1 Viruses 7.3.2 Bacteria 7.4 HAZARDS ASSOSIATED WITH EXPLOSIVE GAS MIXTURES 7.5 HAZARDS ASSOSIATED WITH LOUD NOISES 7.6 HAZARDS ASSOSIATED WITH ELECTROMECHANICAL EQUIPMENT & ELECTRIC ENERGY 7.6.1 Safety rules 7.6.2 Emergency procedures 7.6.3 Basic Lockout / tagout procedures 7.7 HAZARDS ASSOSIATED WITH LABOR IN LIMITED SPACE40 7.7.1 Manholes 7.7.2 Pumping stations 7.8 SPECIFIC HAZARDS IN A WWTP 7.8.1 Excavations 7.8.2 Bar screens, Grit and sand removal units 7.8.3 Aeration tanks 7.8.4 Settling tanks and Clarifiers 7.9 QUESTIONS & ANSWERS


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7. SAFETY AND HEALTH IN WWTP This chapter refers to the main objects relevant to the health and safety of workers in WWTP.

Particularly important factors affecting workers in a wastewater treatment plant are: •

recognition of the physical, chemical and biological risks, risks related to the noise and electro-mechanical equipment, and risks arising when working in indoor / enclosed spaces

knowledge of how to address these risks.

The health and safety programs in a sewage treatment plant, usually designed according to guides who rely on the applicable laws or regulations. These programs are designed to protect workers from












While managers and supervisors of the unit, play an important role in the development and implementation of safety plans, every employee regardless of their position-bear personal responsibility for their safety. Every one in collaboration with the management of the facility shares the responsibility for their personal safety and that of his colleagues. Each should be primarily interested in their own safety and to undertake major initiatives to ensure safety in the workplace.

7.1 PHYSICAL HAZARDS The most common physical hazards associated with the operation of a wastewater treatment plant is slips and falls, drowning, injuries such as sprains and dislocations, injuries from moving machinery, electric shocks from damage wiring and the risks resulting from the use of compressed air .

Main physical hazards in WWTP – Precautionary measures

7.1.1 Slips and falls •

Proper lighting of the installations of the unit

Use suitable footwear such as work boots with nonslip soles

Open and unobstructed access to the tanks and buildings

Removing debris and tools after the work is completed

Immediate cleaning up spills


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attention to the often wettable areas which are slippery due to algae growth - drying is recommended in all crossing corridors

Frequent washing of access areas around the bioreactors due to possible presence of fatty foam from the aeration tank, which make these areas slippery

Frequent removal of frost from access corridors, stairs and areas around the bioreactors and secondary sedimentation tanks - Distribution of sand or salt to remove ice and improve traffic conditions.

Attention for possible leaks in pipelines of liquids. There should be immediate repair and clean the spill.

Principal Safety equipment: antislipery surfaces, guardrails, handrails stairs, safety chains at entry points and lighting for night operations and areas of low visibility.

The necessary safety equipment should be readily accessible

8-1 Attention! Use of safety boots is mandatory

8-1Typical safety ladder with cage enclosure


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7.1.2 Other accidents •

During maintenance work which is necessary to move parts of the machinery, the use of lifting and use of protective helmets specially fortified with iron is strongly recommended.

Maintain safe distance from the equipment with moving parts and avoid loose clothing that can get caught in moving parts of equipment.

The main axes of rotation of the pump can also be dangerous. For this reason, the protective parts have to be replaced before the machine parts are assembled and put into operation again. Accustomed security checks can identify gaps or loose protective gear for which action must be taken to replace or repair them.

The mechanisms drive the scraper settling tanks can create potentially dangerous situations where safety components such as switches and pins fuses don’t operate correctly. If these devices do not automatically stop operation of the equipment when developing high speed excessive force may be developed , and so cause injury.

The work in systems containing pressurized air also may entail risks. Additional measures and precautions to be taken during the durability test pressure of a pipeline or at work on the side of the blower from which air is supplied to the tank or to any other part of the distribution system of the air. If the pressure in the distribution system of air reaches high levels, there is the risk of explosion and ejection of parts of the system with excessive force. To minimize the risks associated with the use of compressed air, always in the air system is isolated and depressurized before starting work on it.

The work in systems containing pressurized air also may entail risks. Additional measures and precautions should be taken during the durability test pressure of a pipeline or at work on the side of the blower from which air is supplied to the tank or to any other part of the distribution system of the air. If the pressure in the distribution system of air reaches high levels, there is the risk of explosion and ejection of parts of the system with excessive force. To minimize the risks associated with the use of compressed air, the air system must always be isolated and depressurized before starting work on it.


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NIREAS VOLUME 7 6 8-2 Attention! Use of protective helmet is mandatory

7.1.3 Drowning •

The rails must be compatible with the requirements of all relevant codes and safety chains must be fastened at all access points.

It is necessary to have lifejackets and ropes and poles rescue

The bioreactors constitute a special risk because the increasing amount of air in the liquid mixture decreases the buoyancy of the human body thereby increasing the risk of drowning

Whenever possible, the reactor should cease to operate and empty, especially when maintenance tasks are performed

When the work is carried out over the reactor, it is recommended to stop the operation of the air supply system.

When the work is done on or around open tanks for example when inspecting equipment or pipes, when samples are taken, when the depth of the sludge is measured or equipment is accessed protective measures should be taken

The required safety equipment must be readily accessible


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NIREAS VOLUME 7 8-2 Life jacket work vest


7.1.4 Fires Burns can be very serious and cause painful injuries. Structural damage from fires can be very costly. Every facility should develop a fire prevention plan, with input from the local fire marshall, fire chief, and insurance company. The plan may be very simple or very complex, depending on the specific facility needs. Some items that may be included in any plan are:

1. Regulate the use, storage, and disposal of all combustible materials/substances. 2. Provide periodic cleanup of weeds or other vegetation in and around the plant. 3. Develop written response procedures for reacting to a fire situation, to include evacuation. 4. Provide required service on all fire detection and response equipment (inspection, service, hydrostatic testing). 5. Routinely inspect fire doors to ensure proper operation and unobstructed access. 6. Immediately repair, remove, or replace any defective wiring. 7. Restrict the use of any equipment that may provide a source of ignition in areas where combustible gases may exist. 8. Maintain clear access to fire prevention equipment at all times. 9. Develop a written hot work procedure and permit to provide written authorization to perform operations (for example, welding, cutting, burning, and heating) that involve a source of ignition. Regardless of the size of the facility, each operator should know the location of fire protection equipment in their work area and must be trained in the proper use of fire extinguishing equipment


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and methods of extinguishing fires. Training must be provided upon initial employment and at least annually thereafter.

A portable extinguisher must also be visually inspected monthly and must receive an annual maintenance check. Maintenance checks must be documented and the records must be retained for one year. Hydrostatic testing of extinguishers is also required every 5 to 12 years, depending on the type of extinguisher. Remember, always have extinguishers serviced promptly after use so they will be ready if you need them again.

8-1Types of fire and fire extinguishers

(John M. Stubbart, 2006 ) 8-3 No Smoking


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8-4 Naked flames forbidden


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7.1.5 Oxygen deficiency or enrichment Low oxygen levels may exist in any poorly ventilated, low-lying structure where gases such as hydrogen sulfide, gasoline vapor, carbon dioxide, or chlorine may be produced or may accumulate. Oxygen in a concentration above 23.5 percent (oxygen enrichment) also can be dangerous because it speeds up combustion. Oxygen deficiency is most likely to occur when structures or channels are installed below grade (ground level). Several gases (including hydrogen sulfide and chlorine) have a tendency to collect in low places because they are heavier than air. The specific gravity of a gas indicates its weight as compared to an equal volume of air. Since air has a specific gravity of exactly 1.0, any gas with a specific gravity greater than 1.0 may sink to low-lying areas and displace the air from that area or structure. (On the other hand, methane may rise out of a manhole because it has a specific gravity of less than 1.0, which means that it is lighter than air.) You should never rely solely on the specific gravity of a gas to tell you where it is. Air movement or temperature differences within a confined space may affect the location of atmospheric hazards. The only effective way of ensuring safe atmospheric conditions prior to entering a confined space is to test the atmosphere with an appropriate monitor(s) at various levels and locations throughout the space.

When oxygen deficiency or enrichment is discovered, the area should be ventilated with fans or blowers and checked again for oxygen deficiency/enrichment before anyone enters the area to work. Ventilation may continue to be provided by fans or blowers. You should follow confined space procedures before entering and during occupancy of any suspect area. ALWAYS get air into the confined space BEFORE you enter to work and maintain the ventilation until you have left the space.

Equipment is available to measure oxygen concentration as well as toxic and combustible atmospheric conditions. You must use this equipment whenever you encounter a potential confined space situation. Ask your local safety regulatory agency or wastewater association about sources of this type of equipment in your area.

Never enter an enclosed, poorly ventilated area, whether a manhole, sump, or other structure, without first following confined space entry procedures.


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7.2 CHEMICAL HAZARDS Working in a WWTP may be accompanied by risks arising from the use of chemicals. For this reason safety requirements should be adopted associated with the use of chemicals which are likely to be used for a variety of purposes, such as adjusting the pH, the nutrient supply, control of filamentous organisms or disinfection of treated wastewater.

The chemicals used for the treatment of wastewater may be hazardous to humans and the environment. May be combustible, explosive or corrosive. Many are toxic and pose a risk to humans if swallowed, inhaled or absorbed through the skin.

Strong acids, bases, and chlorine are examples of toxic and harmful chemicals that operators may encounter working in and around treatment plants and laboratories. Be very careful when handling and using these chemicals. Chemical hazards may be present in many forms—vapors, dusts, mists, liquids, gases, and particles. The seriousness of a chemical hazard depends on exposure time and the concentration of the chemical, as well as which chemical you are exposed to. To avoid injury, be sure all hazardous chemicals are clearly labeled; obtain and read the health and safety data about the chemical BEFORE using the chemical; and learn and practice safe handling procedures and precautions.

Main chemical hazards in WWTP – Precautionary measures

In general •

Existence of adequate labeling and instructions for safe management of materials, which contain vital information about the risks posed by chemicals and the necessary actions to deal with emergencies.

Appropriate training of staff about technical and management practices of the chemicals used in each WWTP

No eating, drinking and smoking during or after working with hazardous or toxic materials is allowed. Smoking can have devastating effects when flammable or explosive materials are administered . Also there are chances of contamination of the human body through the transfer of germs from hands.


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After leaving the workplace, the removal of all personal protective equipment and washing hands and face is strongly recommended.

8-5 Commercial warning label of chemicals

(California State University, 2008)

Chemical substances


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The most common chemical used in a WWTP are:

7.2.1 Aluminum sulfate Al2(SO4)3 Aluminum sulfate is used to control the amount of filamentous organisms. •

The liquid is corrosive and inhaling the fumes should be avoided

The solid phase is a powder and is highly corrosive

Eye protection is a basic condition for use, as it may cause damage to vision. In case of accident it is necessary to direct clean with plenty of water for some time.

The contact between the aluminum sulfate and calcium oxide (CaO) should be avoided. Reaction between the two chemicals is direct (exothermic reaction) releasing hydrogen and creating explosive conditions in the atmosphere, such as to cause fire

You need to use breathing mask or respirator that covers the entire face, goggles and face shields resistant to acid, rubber gloves, rubber aprons and overalls and rubber boots

8-6 Attention! Use of eye protection is mandatory

7.2.2 Ferric chloride Fe(Cl)3


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The ferric chloride is used to control the amount of filamentous organisms, the addition of nutrients in the effluent and as a flocculant •

It is available in liquid and granular form

In the presence of light and at high temperatures, is dissolved in hydrochloric acid, which is highly corrosive

The contact of substance with skin or eyes causes severe burns.

You need to use breathing mask or respirator that covers the entire face, goggles and face shields resistant to acid, rubber gloves, rubber aprons and overalls and rubber boots.

In warehouses of storage, there is the need for installation of washing systems for eye and body at a distance of 8m from these areas

First washes should be done slowly and then very quickly to minimize burns Neutralization with water containing bicarbonate ions (HCO3) is also recommended.

All the tanks, pipes and valves which come into contact with the ferric chloride should be either rubber or acid-resistant plastic.

The storage facilities must be well ventilated.

The floors, walls and equipment coming into contact with the ferric chloride need to be covered with anticorrosion paint or rubber mat.

8-7 Attention! Use of safety gloves is mandatory

7.2.3 Anhydrous ammonia


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The anhydrous ammonia is used to add nutrients to waste . •

A colorless gas with a sharp odor

Reacts very easily with chlorine (risk of explosion), and acids and is a moderate risk of causing fire.

When working with ammonia

the use of masks or breathing apparatus and protective

gloves and eye components is strongly recommended. •

In warehouses of storage, there is the need for installation of washing systems for eye and body at a distance of 8m from these areas

7.2.4 Caustic soda – NaOH

Caustic soda is used for adjusting the pH •

Available in dry or liquid form.

Mixing of any form with water leads to a chemical reaction which generates heat and hydrogen in gas phase, creating potentially explosive conditions in the atmosphere.

Mixing of caustic soda in a closed container can cause bursting.

Not compatible with acids and some metals, such as tin, zinc and aluminum causing violent reactions.

The neutralization of leakages is attenuated with the aid of acids and large quantities of water.

Caustic soda is highly corrosive to eyes and skin and is highly recommended to use protective clothing and goggles. Respiratory masks should be used to avoid inhalation of dust, fumes or droplets.

In warehouses of storage, there is the need for installation of washing systems for eye and body at a distance of 8m from these areas


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NIREAS VOLUME 7 16 8-8 Do not extinguish with water

7.2.5 Calcium hydroxide Ca(OH)2 The calcium hydroxide is used for adjusting the pH, for flocculation, coagulation, precipitation and removal of carbonate ions. •

Used as a powder, which can be particularly annoying to the skin and lungs.

In areas of management ventilation and dust extraction systems should be installed and operated.

Can create burns when mixed with any form of moisture including sweat. In this case, the contact area must be washed, with soap and water and then with vinegar, and the burn must be covered with suitable bandage.

It is recommended to use the necessary protective equipment for eyes, also respiratory masks and appropriate clothing.


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7.2.6 Chlorine - Cl2 The chlorine used to control the amount of filamentous organisms, and for disinfection of water. Although many treatment plants have switched to ultraviolet or other disinfection methods, the majority of plants use some form of chlorine for disinfection—chlorine gas, sodium hypochlorite, calcium hypochlorite, or chlorine dioxide.

The use of chlorine has saved millions of lives by

preventing the spread of waterborne diseases. But it’s critical to understand the dangers when working with chlorine at your plant. •

Available in gas, liquid or powder form (usually calcium hypochlorite - Ca (ClO) 2.

Is incompatible with many materials and with the reactions that may cause fires or explosions.

It is toxic even in low concentrations (lethal concentrations of chlorine gas in the range of 30mg / L air)

All forms of chlorine are corrosive when mixed with water and is dangerous if inhaled or come in contact with eyes or skin.

The use of respiratory masks or equipment, protective clothing and special reinforced butyl gloves are essential.

Special precautions, including venting and leak detection systems are needed in storage and management of chlorine. They must be kept separate from other parts of the plant. Also need to be secure and protected from adverse weather conditions and with good access, while there must be emergency equipment, such as special breathing equipment, eye wash station and shower. It is recommended to avoid the presence of moisture, grease and oil in the storage of chlorine.

Because it is heavier than air, always store chlorine on the lowest floor; it will collect at the lower level. For the same reason, never stoop down when a chlorine smell is noticed.

Immediate medical assistance for workers poisoned by chlorine is needed. In case of ingestion or inhalation should be directly caused vomiting.


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8-2 Effects of chlorine gas exposure

(John M. Stubbart, 2006 )


This section will focus on gas chlorine.

Chlorine Gas

Chlorine gas is still a commonly used disinfectant. It’s highly effective and comparatively low-cost. Depending on the amounts stored and used, a Risk Management Program (1000 kg or more on site) and/or Process Safety Management Program (600 kg or more on site) may need to be in place. The reportable quantity for a spill is 5 kg .

Here are some key items to know about chlorine gas safety.

Physical Properties •

Color – greenish yellow

Odor – pungent, similar to laundry bleach. Detectable by smell at concentrations of 0.2 to 0.4 ppm.

Specific gravity – Approximately 2.5. This means gas chlorine is 2.5 times heavier than air, and will sink to the lowest level in a building or area.

Boiling point is –1oC. Liquid chlorine that escapes from a cylinder or ton container will immediately convert to gas.

One volume of liquid chlorine converts to 460 volumes of gas.


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Liquid chlorine that vaporizes on skin can reduce the temperature enough to cause frostbite.

Chlorine is not explosive or flammable, but can react violently with many substances.

Chlorine reacts with water to form hypochlorous and hydrochloric acids, with hypochlorous acid being the main disinfectant.

How Chlorine Exposure Affects Humans

Chlorine is a severe irritant. As noted above, when mixed with water (including moisture in mucous membranes, eyes and skin), it forms an acid. The primary route of exposure is through the eyes and respiratory system. Exposure to chlorine has effects ranging from irritation to death, depending on the concentration and time of exposure.

The EPA ceiling level (the maximum limit of any worker exposure) is 1.0 ppm.

A level of 10 ppm is considered Immediately Dangerous to Life and Health under the National Institute for Public Safety and Health in the USA (NIOSH).

At low levels for a short time, chlorine can cause eye irritation, coughing, sneezing and throat irritation. At higher levels, labored breathing and vomiting may occur. Death can result from suffocation.

Dangerous Chlorine Reactions

Chlorine has the potential for violent or explosive reactions with certain substances. It is very important to separate chlorine from the following: •

Ammonia and ammonia compounds

Hydrocarbons – oils, greases, solvents, even in small amounts.


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NIREAS VOLUME 7 20 Reaction of Chlorine Gas with Sodium Video


Always secure cylinders and ton containers to protect them from falling, rolling or being dropped.

Both cylinders and ton containers have fusible metal plugs that will melt when the temperature gets between 60 and 75 oC to relieve pressure. These pressure relief valves keep the containers from rupturing during a fire. Chlorine may be stored indoors or outdoors, though shading from sunlight is recommended for outdoor storage. Storage areas should be away from Air Condition intakes, as chlorine gas could be distributed throughout a building in case of a leak.

Separate the chlorine storage area from incompatible materials, especially ammonia, sulfur dioxide, and hydrocarbons like fuels and oils.

The chlorine storage area should have a well-maintained chlorine gas detector installed, complete with alarm and call-out capability if a leak occurs when the plant is unmanned.

Unloading Chlorine Cylinders and Ton Containers

All employees receiving chlorine cylinders and containers must be properly trained.

Always use proper equipment to unload cylinders and ton containers. Chain cylinders to a hand truck, or move with a forklift if already secured in a storage rack.


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Make sure the protective valve housing is on securely.

Never lift a chlorine cylinder by its protective valve housing!

The hoist and cables must be in good operating condition. Have a professional inspect the hoist each year and repair or replace it when necessary.

Never stand under a hoisted container. Stand to either side.

Once the containers or cylinders are unloaded, secure them properly at the site. Always store cylinders in an upright position. Store ton containers with the two valves lined up vertically. Chlorine Leak Detection

Check for chlorine leaks by using a plastic squeeze bottle with a solution of ammonium hydroxide in the bottom. Squeeze to allow only the ammonia vapor (NEVER the liquid) to detect the presence of chlorine. If a leak exists, it will form a white cloud.

In addition, chlorine leak detection instrumentation is recommended, and required in some states. Leak detectors are typically connected to an alarm system with call-out capability for unmanned plants. Leak detectors must be properly maintained, calibrated and tested.

Chlorine Leak Response

NEVER respond to a chlorine leak unless you have been properly trained and have the necessary safety equipment—including a self-contained breathing apparatus and protective suit.

Otherwise, call whatever agency is listed in your plant’s Emergency Response Plan.

To speed response and recovery, each treatment plant should have the appropriate Chlorine Institute Emergency Kit at the site:

Kit A: for 45 or 70 kg cylinders Kit B: for ton containers Kit C: for tank cars and tank trucks


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Cylinder containment vessels: for 45 or 70 kg cylinders

Whether a chlorine leak is handled in-house or not, your Emergency Response Plan should detail the exact procedure. Most importantly, practice, practice and practice the procedure. Finding the Emergency Response Plan and reading it is not a good option in the middle of a chlorine gas leak.

Connecting and Disconnecting Chlorine Cylinders and Containers

Changing cylinders or containers is one of the most likely opportunities for exposure to chlorine. It is extremely important to make sure all operators are thoroughly trained before attempting the task. 45 and 70 kg Cylinders:

Make sure the cylinder is upright and properly secured. The yoke and adapter connects the cylinder valve outlet to the feed system. A gasket must be used on the valve face and MUST be replaced with every new connection. Failure to replace the gasket will often lead to a chlorine leak. Do NOT reuse the old gasket.

Ton Containers:

Ton containers should be secured in a horizontal position in a cradle, with the two valves aligned vertically. The top valve will feed gas, the bottom will feed liquid. A yoke and adapter connects the cylinder valve to the outlet feed system. As with the cylinders, a gasket must be used on the valve face and MUST be replaced with every new connection.

If the vacuum regulator connects directly to the cylinder or container, follow the manufacturer’s instructions for connection. Otherwise, follow the directions in Section 6 of the Chlorine Institute’s Water and Wastewater Operators Chlorine Handbook.

Make sure the appropriate personal protective equipment is available and worn during the procedure. •

Self contained breathing apparatus with full face mask


Hard hat


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Safety shoes

Long sleeved shirts


Clothing and gloves should be free of oil or grease.

7.2.7 Polymers

The polymers used to improve the capacity of the sludge settling •

used in the form of powder, granules or gel.

Can cause irritation to eyes, nose and skin. Chemical burns can be created if the polymers come into contact with the eyes or skin.

Use of goggles, gloves and suitable long-sleeved clothing is essential in order to avoid contact, also necessary is the use of respiratory masks or filters to prevent inhalation of dust or fumes.

Leaks can create extremely slippery conditions and should be cleaned immediately.

7.2.8 Solvents The solvents are used for equipment maintenance and cleanliness of workplaces •

Enough solvents are flammable, corrosive or harmful to the human body.

The use must always be in accordance with the posted instructions on their packaging and after use and consumption they must be removed by appropriate techniques.

It is recommended to use both, ventilation systems and respiratory masks or devices appropriately protected and protective gloves.

Near the areas of use and storage of solvents must be placed fire extinguishers

storage of solvents is highly recommended to be in special fireproof chambers or rooms where there are warning signs that ban smoking. In larger storage areas fire protection systems must be installed. The storage of solvents beside acids, caustic or chlorine compounds should be avoided.


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7.2.9 Phosphate Na3PO4 The phosphate is used to add nutrients to wastewater •

Moderately alkaline compound that can cause burns to the eyes and irritation of the mucous membranes.

The use of respiratory masks and goggles are necessary

In warehouses of storage, there is the need for installation of washing systems for eye and body at a distance of 8m from these areas

Exposure to sodium phosphate can cause chronic dermatitis

8-9 Types of air-purifying respirators

(EPA, 1985)


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Fuels may be dumped or drained into storm sewers connected to sanitary sewers or they may enter the collection system as a result of a leaking underground fuel line or tank. Fuel oil and gasoline usually float on the surface of wastewater and are not diluted by mixing. Therefore, most floating fuels tend to collect in wet wells and can create both explosive and toxic conditions or atmospheres.

To reduce the chances of an explosion in a wet well and downstream enclosed structures, a combustible-gas detector should be installed in wet wells to sound an alarm and transmit a signal to the main control panel before explosive conditions are reached. Explosion-proof wiring, equipment, and fixtures will help to prevent fires and explosions in hazardous areas such as wet wells. Adequate ventilation also is essential. Oil skimmers should be installed in wet wells to remove floating fuel oil and gasoline. This equipment may be rarely used, but can remove explosive fuels without exposing operators to hazardous conditions.

Fuels may be detected by permanent or portable devices that measure either hydrocarbons or the lower explosive limit (LEL). These devices must be installed in treatment plants using pure oxygen treatment processes (activated sludge). They are located in the collection system upstream from the plant, in the headworks, and at the oxidation reactors (aerators). If hydrocarbons or explosive conditions reach a given SET POINT, an alarm is usually generated. If the concentrations increase beyond this point, the oxygen gas flow to the processes is automatically shut off. The system is purged with air to prevent a possible explosion. These detection devices must be properly located and maintained at frequent intervals to provide reliable service.

If gasoline reaches the wet well in the headworks of your treatment plant:

1. Try to remove the gasoline from the surface of the wet well with skimmers (if available) or with a portable pump. 2. Apply as much ventilation as possible to prevent an explosive atmosphere from developing. 3. Monitor the atmosphere for toxic and explosive conditions. 4. Keep personnel away from the area. 5. Do not allow any flames, sparks, or other sources of ignition in the area (use nonsparking tools and explosion-proof equipment and wiring)


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Toxic or suffocating gases or vapors

The most common toxic gas in wastewater treatment is hydrogen sulfide, which is produced during the anaerobic decomposition of certain materials containing sulfur compounds. Hydrogen sulfide will tend to accumulate in the lower voids of sewers, tanks, channels, and manholes because it is heavier than air, with a specific gravity of 1.19. The only reliable method of detecting hydrogen sulfide is by atmospheric monitoring since it has the unique ability to affect your sense of smell. You will lose the ability to smell hydrogen sulfide's "rotten egg" odor after only a short exposure. Your loss of ability to smell hydrogen sulfide is known as OLFACTORY FATIGUE.

Other toxic gases include phosgene ("war or mustard gas"), chlorine, and tear-producing substances (lacrimators).

Phosgene is produced in sewers when discharges of alcohol saturated with phosgene-wasted chloroformates is back-hydrolyzed (reverse chemical reaction) to phosgene. If an industry or laundry quickly dumps a few hundred to one thousand gallons of bleach, a slug of chlorine can occur in the plant's influent. Naphtha is used as both a fuel and a solvent and can create hazardous conditions.

Tear-gas type substances (lacrimators) occasionally may reach treatment plants. Certain organic insecticide wastes, when only partially chlorinated at an industrial wastewater pretreatment plant, can form tear-gas type substances.

Toxic gases may be detected by probes that measure the concentration of a particular gas, such as hydrogen sulfide, chlorine, or sulfur dioxide. Most instruments are capable of detecting the lower explosive limit (LEL), an oxygen deficiency/ enrichment, and hydrogen sulfide. Portable sensors and monitors for toxic gases usually require daily calibration. Permanent systems usually require weekly calibration. Both systems require regular maintenance.

Other toxics, such as carbon monoxide, chlorinated solvents, and industrial toxins, may enter your plant as a result of industrial discharges, accidental spills, or illegal disposal of hazardous materials. You must become familiar with the waste discharges into your system.

8-10Hazardous material shipping symbols


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Municipal waste water containing a large number of microorganisms, some of which are harmful. Workers in a WWTP are in daily risk of exposure to infectious agents. •

Common diseases associated with contaminated from waste water: dysentery, Asiatic cholera, typhoid fever, tuberculosis and infectious hepatitis.

The risk of these diseases is relatively small if all necessary safety measures and precautions are taken

Although treatment plants and plant personnel are not expected to be pristine, personal hygiene is the best protection against the risk of infections and infectious diseases such as typhoid fever, dysentery, hepatitis, and tetanus. Immunization shots for protection against tetanus, polio, and for both hepatitis A and hepatitis Β are often available free or for a minor charge from your local health department. REMEMBER, many pathogenic organisms can be found in wastewater. Some diseases that may be transmitted by wastewater are anthrax, tuberculosis, paratyphoid fever, cholera, and polio. Tapeworms and the organisms associated with food poisoning may also be present.

The possibility that Acquired Immune Deficiency Syndrome (AIDS), which is caused by a virus, can be contracted from exposure to raw wastewater has been discounted by researchers who have found that although the AIDS virus is present in the wastes from AIDS victims, the raw wastewater environment is hostile to the virus itself and has not been identified as a mode of transmission to date. Needle sticks from potentially contaminated syringes should remain a concern to operators and maintenance personnel. Fluids in or on syringes may provide a less severe environment than raw wastewater where dilution and chlorination significantly reduce infection potential.

Make it a habit to thoroughly wash your hands before eating or smoking, as well as before and after using the restroom. ALWAYS wear proper protective gloves when you may contact wastewater or sludge in any form. Bandages covering wounds should be changed frequently. Do not wear your work clothes home because diseases may be transmitted to your family. Provisions should be made in your plant for a locker room where each employee has a locker.


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Work clothes should be placed in lockers and not thrown on the floor. Your work clothes should be cleaned as often as necessary. If you are required to wear protective clothing because of the possibility of contamination with toxic materials, you should store your street clothes and your protective clothing in separate lockers. If your employer does not supply you with uniforms and laundry service, investigate the availability of disposable clothing for "dirty" jobs. If you must take your work clothes home, launder them separately from your regular family wash. All of these precautions will reduce the possibility of you or your family becoming ill because of your contact with wastewater.

7.3.1 Viruses The presence of living cells to reproduce is mandatory. A proportion of 50% of the viruses found in wastewater is removed from the pre-treatment stage. Viruses continue to exist in liquid waste entering the bioreactors (secondary treatment).


Transmission mode


Adenoviruses Conjunctivitis Gastroenteritis


Acute respiratory

Hepatitis A+


Infectious Hepatitis

Spiral Viruses


Acute Gastroenteritis

7.3.2 Bacteria Presence of living cells is required to reproduce, but can grow directly into the wastewater.


Transmission mode


Infection through open wounds



Symptom Tetanus Gangrene Gastroenteritis Gastroenteritis Typhoid fever Gastroenteritis


Swallowing Dysenteria


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Pests: Usually there is exposure to cysts and eggs through which pests breed and are able to survive in harsh conditions.

The equipment and surfaces may become contaminated as a result of wetting by wastewater or by spraying droplets or particulates. The areas around the bioreactors are particularly prone to such infections.

MORE Precautions: Proper hygiene and cleanliness. After any contact with the effluent or the sludge must be followed by washing of the hands. The tools used for various tasks should be cleaned after each use. The protective clothing and uniforms must be changed after the job is completed. Use protective equipment: To minimize exposure to various types of biological risks. It is also mandatory in case of open wounds, cuts and burns to hands to use special protective - impervious gloves. When suspended particles or other substances appear in the workplace , staff should be wearing face masks, goggles or even breathing apparatus as appropriate. Personal clothing and work wear should be kept in separate cabinets.

All injuries must be treated immediately to prevent any infections. The small cuts should be washed thoroughly and then antibiotic ointment should be superimposed on these. The wounds should be covered with a bandage. In the event of a more serious injury or direct contact with body fluids or waste sludge, medical help and care should be seeked diphtheria and tetanus serums should be kept in the first aid kit, as well as all those required by the direction of the competent prefectural hygiene, vaccines.

Note: Regarding the possibility of exposure of staff to HIV that causes Acquired Immune Deficiency Syndrome (AIDS) and Hepatitis B, there are no reports that relate the transmission of these two viruses through water or wastewater and there are no recorded incidents of detection from contamination. The latest scientific research suggests that it is extremely difficult to AIDS to be associated with the exposure of workers to wastewater.


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7.4 HAZARDS ASSOSIATED WITH EXPLOSIVE GAS MIXTURES Explosive gas mixtures may develop in many areas of a treatment plant from mixtures of air and methane, natural gas, manufactured fuel gas, hydrogen, or gasoline vapors. The upper explosive limit (UEL) and lower explosive limit (LEL) indicate the range of concentrations at which combustible or explosive gases will ignite when an ignition source is present at ambient temperature. No explosion or ignition occurs when the concentration is outside these ranges. Gas concentrations below the LEL are too lean to ignite; there is not enough flammable gas or vapor to support combustion. Gas concentrations higher than the UEL are too rich to ignite; there is too much flammable gas or vapor and not enough oxygen to support combustion.

Explosive ranges can be measured by using a combustible gas detector calibrated for the gas of concern. Do not rely on your nose to detect gases. The sense of smell is absolutely unreliable for evaluating the presence of dangerous gases. Some gases have no smell and hydrogen sulfide can paralyze the sense of smell.

8-11 Relationship between the lower explosive limit (LEL) and the upper explosive limit (UEL) of a mixture of air and gas

(California State University, 2008)

Avoid explosions by eliminating all sources of ignition in areas potentially capable of developing explosive mixtures. Only explosion-proof electrical equipment and fixtures should be used in these areas (influent/bar screen rooms, gas compressor areas, digesters, battery charging stations). Provide adequate ventilation in all areas that have the potential to develop an explosive atmosphere.


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7.5 HAZARDS ASSOSIATED WITH LOUD NOISES Wastewater treatment facilities contain some equipment that produces high noise levels, intermittently or continuously. Operators must be aware of this and use safeguards such as hearing protectors that eliminate or reduce noise to acceptable levels. In general, if you have to shout or cannot hear someone talking to you in a normal tone of voice, the noise level is excessive. Prolonged or regular daily exposure to high noise levels can produce at least two harmful, measurable effects: hearing damage and masking of desired sounds such as speech or warning signals.

The ideal method of dealing with any high-noise environment is the elimination or reduction of all sources through feasible engineering or administrative controls. This approach is frequently not possible; therefore, employers are required to identify and monitor operators whose normal noise exposure might equal or exceed an 8-hour TIME-WEIGHTED AVERAGE1 (TWA) . According to Directive 2003/10/EC of the European Parliament and of the Council of 6 February 2003 on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (noise), allowed noise exposure is : •

exposure limit values:

upper exposure action values: LEX,8h = 85 dB(A)

lower exposure action values: LEX,8h = 80 dB(A)

LEX,8h = 87 dB(A)

To ensure that the welfare of operators is not compromised and to comply with local safety regulations,

a comprehensive hearing conservation program should be implemented. All

individuals whose normal noise exposure equals or exceeds the 8-hour TWA of 85 dBA must be included in this program. The primary elements of the program are monitoring, audiometric testing, hearing protection, training in the use of protective equipment and procedures, access to noise

1 Time-Weighted Average (TWA). The average concentration of a pollutant based on the times and levels of concentrations of the pollutant. The time-weighted average is equal to the sum of the portion of each time period (as a decimal, such as 0.25 hour) multiplied by the pollutant concentration during the time period divided by the hours in the workday (usually 8 hours). 8TWA PEL is the time-weighted average permissible exposure limit, in parts per million, for a normal 8-hour workday and a 40-hour workweek to which nearly all workers may be repeatedly exposed, day after day, without adverse effect. Q Decibel (DES-uh-bull). A unit for expressing the relative intensity of sounds on a scale from zero for the average least perceptible sound to about 130 for the average level at which sound causes pain to humans. Abbreviated dB.


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level information, and recordkeeping. The purpose of the conservation program is to prevent hearing loss that might affect an operator's ability to hear and understand normal speech.

A selection of hearing protection devices must be available to operators; however, a certain degree of confusion can arise concerning the adequacy of a particular protector. To estimate the adequacy of a hearing protector, use the noise reduction rating (NRR) shown on the hearing protector package. Subtract 7 dB from the NRR and subtract the remainder from the individual's Αweighted TWA noise environment to obtain the estimated Α-weighted TWA under the ear protector. To provide adequate protection, the value under the ear protector should be 85 dB or less, the lower the better.

Typical example: ventilation equipment, which produces high levels of noise.

Protective measures: •

Use ear protection and other protective measures for hearing

entry to buildings should be limited and presence of signs warning of the impending danger is mandatory

Note: when protective measures are used to hearing the emergency alarm may not be easily understood, visual alarms must be installed too. •

Noise from the WWTP and pumping stations (G / S and pumps) is mainly due to the operation of aerators, blowers, compressors, large pumps, generators, centrifugal equipment and other machinery operating at high speeds

Source of noise can also be the pretreatment gear

Depending on the distance, mechanical aerators can cause nuisance to adjacent residences, especially during the evening hours.

The generated noise affects a) the staff of the WWTP and b) residents in nearby villages to install, depending on the distance from the WWTP.

Noted that creating nuisance from noise generated during operation of the WWTP, depends directly on the noise levels prevailing in the area around the location of the WWTP. Therefore, in the selection of the construction site of the WWTP should be labeled noise levels prevailing in the area.


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NIREAS VOLUME 7 34 The measurement of noise levels is carried out by counters which contain noise measurement networks frequency entries A-, B-, C-and D-. The A-level is generally used in field measurements while the unit is dB (decibel). (The decibel scale is logarithmic, starts from zero and ends at 130 where corresponds to the threshold of pain caused by the intensity of the generated sound). The increase of noise levels caused by a WWTP by 3 dB-A in relation to surrounding boundaries, results in a small as negligible impact on the surroundings. Increase of 3-15 dB-A, causes moderate impact, and an increase of 15 dB-A and above, have a significant impact on the surroundings. To determine the effects of a WWTP in the surrounding area, hourly and seasonal variations should be taken into account.

8-12 Attention! Use of ear protection is mandatory


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EQUIPMENT & ELECTRIC ENERGY Electric shock frequently causes serious injury. Do not attempt to repair electrical equipment unless you know what you are doing. You must be qualified and authorized to work on electrical equipment before you attempt any troubleshooting or repairs. Ordinary 120 volt electricity may be fatal; 12 volts may, on good contact, cause injury. Any electrical system, regardless of voltage, should be considered dangerous unless you know positively that it is de-energized.

8-13 Danger! Electricity

7.6.1 Safety rules Remember these basic safety rules when working around electrical equipment:

1. Keep your mind on the potential hazard at all times. 2. Always lock out and tag out any electrical equipment being serviced. NEVER remove anyone else's lock or tag. 3. Do not use portable ladders with conductive side rails. 4. Never override any electrical safety device. 5. Inspect extension cords for abrasion, insulation failure, and evidence of possible internal damage.


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6. Use only grounded or insulated (Underwriter's Laboratory (UL) approved) electrical equipment. 7. Take care not to accidentally ground yourself when in contact with electrical equipment or wiring. 8. Do not alter or connect attachment plugs and receptacles in a manner that could prevent proper grounding. 9. Do not use flexible electrical cords connected to equipment to raise or lower the equipment. 10. Wear nonconductive head protection if there is a danger of head injury from contact with exposed energized parts. 11. Use a ground-fault circuit interrupter in damp locations. 12. Do not wear conductive articles of jewelry or clothing if they might contact exposed energized parts (unless they are covered, wrapped, or otherwise insulated).


7.6.2 Emergency procedures In the event of electric shock, the following steps should be taken:

1. Survey the scene and see if it is safe to enter. 2. If necessary, free the victim from a live power source by shutting power off at a nearby disconnect, or by using a dry stick or some other nonconducting object to move the victim. 3. Send for help, calling an ambulance or the emergency number is in your community. Check for breathing and pulse. Begin CPR (cardiopulmonary resuscitation) immediately, if needed. Remember, only trained and qualified individuals working in pairs should be allowed to service, repair, or troubleshoot electrical equipment and systems.

7.6.3 Basic Lockout / tagout procedures Whenever replacement, repair, renovation, or modification of equipment is performed, general safety procedures require that all equipment that could unexpectedly start up or release stored energy must be locked out or- tagged out to protect against accidental injury to personnel. Some of the most common forms of stored energy are electrical and hydraulic energy. The energy isolating devices (switches, valves) for the equipment should be designed to accept a lockout device. A lockout device uses a positive means such as a lock to hold a switch or valve in a safe position and prevent the equipment from becoming energized or moving. In addition, prominent warnings, such as appropriate tags must be securely fastened to the energy isolating device and the


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equipment (in accordance with an established written procedure) to indicate that both it and the equipment being controlled may not be operated until the tag and lockout device are removed by the person who installed them.

For the safety of all personnel, each plant should develop a standard operating procedure that must be followed whenever equipment must be shut down or turned off for repairs. If every operator follows the same procedures, the chances of an accidental start-up injuring someone will be greatly reduced. The following procedures are intended as guidelines and can be used as a model for developing your own written standard operating procedure for lockout/tagout. Training must be provided to ensure that the purpose and function of the lockout/tagout program are understood and that the knowledge and skill required for the safe use of energy controls are gained. Each employee using lockout/ tagout must be aware of applicable energy sources and the methods necessary for their effective isolation and control. If you hire a contractor, you must inform each other of your respective lockout/tagout procedures. You must ensure that you and your employees or co-workers comply with the restrictions and prohibitions of the contractor's program.

Periodic inspections of the lockout/tagout program are also required (at least annually) to ensure that the requirements of the program are being followed. The inspection(s) must be done by someone other than the one(s) using the procedure.

Basic Lockout / tagout procedures

1. Notify all affected employees that a lockout or tagged system is going to be used and the reason why. The authorized employee shall know the type and magnitude of energy that the equipment uses and shall understand the hazard thereof. 2. If the equipment is operating, shut it down by the normal stopping procedure. 3. Operate the switch, valve, or other energy isolating device so that the equipment is isolated from its energy source(s). Stored energy such as that in springs; elevated machine members; rotating flywheels; hydraulic systems; and systems using air, gas, steam, or water pressure must be dissipated or restrained by methods such as repositioning, blocking, or bleeding down. 4. Lock out or tag out the energy isolating device with assigned individual lock or tag. If a tagout device is used, it must be substantial enough to prevent accidental removal. The attachment means for a tagout device must be a non-reusable type, attachable by hand, self-locking, and nonreleasable with a minimum unlocking strength of no less than 25 kg.


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5. After ensuring that no personnel are exposed, and as a check that the energy source is disconnected, operate the push button or other normal operating controls to make certain the equipment will not operate. Caution: Return operating controls to the neutral or off position after the test.

8-14 Typical lockout warning tags

(California State University, 2008)


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8-15 Typical lockout devices

(California State University, 2008)


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7.7 HAZARDS ASSOSIATED WITH LABOR IN LIMITED SPACE Limited spaces are defined as areas whose size and shape is such that the entrance and exit is limited, while they re not suitable for continuous labour of workers within them.

Because these places usually contain or may contain hazardous atmosphere, work within them may result in entrapment or suffocation of workers inside them, or otherwise there are risks to the health and safety of workers, when needed special permission to enter.

Examples of such spaces are the wet chambers of the pumping stations and the pipe wells. There are also other facilities that because of toxic, flammable or lack of oxygen air environments can be approached as "Interiors" or “limited space�.

Entrance to such spaces should be permitted only after special permit. The authorization shall be signed by the supervisor, shall mention clearly the work to be done, the risks, the special clothing and equipment required, air monitoring and recording of evidence, security personnel and emergency procedures. So for all these reasons, the entrance to such spaces shall be permitted only to qualified personnel.

Before entering such an area, the atmosphere shall be controlled. For this purpose it is recommended gas detectors and security alerts to be installed if gas prices exceed a predetermined threshold. Portable multiple gas detection units may also be used,.

Test parameters to be monitored, often concern the content of oxygen in air, flammability, and the presence of toxic gases. Most times before entering these areas ventilation is required.

The training and readiness of employees are the prerequisites for a safe entry into an interior or limited space.

8-16 No access to unauthorized persons


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7.7.1 Manholes Manholes are confined spaces and the requirements for entry into a confined space and the atmospheric hazards that may be encountered in a confined space have been discussed earlier above.

The following items are examples of other hazards that may exist in manholes, depending on the design and use of the manhole: •

Mechanical hazards, such as hot or cold surfaces, steam leaks, or rotating equipment.

Electrical hazards involving conduit, energized circuits, lights, portable power tools, or moisture/water accumulation.

Engulfment hazards, such as flooding from wastewater, sludge, or chemicals.

Physical injury, such as sprains, strains, scrapes, and cuts due to uneven footing, inadequate working room, poor balance, or awkward positioning.

Infections and diseases from bacteria, parasites, and viruses found in the wastewater stream.

Bites from insects and rodents.

Toxic exposure from substances illegally or accidentally discharged into the collection system or plant drainage system.


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NIREAS VOLUME 7 42 MORE Manhole work will usually require job site protection, either a manhole safety enclosure by itself or in conjunction with traffic controls. Job site protection when working in manholes should include barricades and traffic warning devices for the safety of vehicles, bikes, pedestrians, and workers. In addition to following confined space procedures for entry and occupancy of the manhole, the atmosphere within the manhole should be tested prior to removing the manhole cover. A spark created by removing the cover could cause an explosion if a combustible atmosphere exists within the manhole. Also, it may be helpful to evaluate the conditions within the manhole before any ventilation occurs.

Remove the manhole cover with a tool specifically designed for the purpose, not your fingers. When lifting a lid, the use of the rule "Lift with your legs, not with your back" will help eliminate back strains. Once the lid is removed, leave it flat on the ground and far enough away from the manhole to provide adequate room for a working area. This is usually at least 0.6 to 1 m.

REMEMBER: Never enter a manhole without fully complying with the requirements for work in confined spaces.

Be very cautious when using ladder rungs or steps installed in the side of the manhole. Be alert for loose or corroded steps. Always test each step individually before placing your weight on it. If possible, it is much safer to use a portable ladder as a means of entering a manhole. Be certain, however, that the bottom feet are properly placed so that the ladder will not slip or twist when your weight is placed on it. The top of the ladder should extend 1 m above the ground level to facilitate getting on and off the ladder. A mechanical lifting device is the safest possible way to be lowered into a manhole, wet well, or other below-grade work area or to be lifted to an elevated work area.

If you are working in wastewater, be sure to wear properly fitted rubber gloves and boots, or approved substitutes that will provide protection from infection. Be aware of possible needle sticks and other puncture wounds. Tools and equipment should be lowered into a manhole by means of a bucket or basket. Do not drop them into the manhole for a person to catch. Attempting to carry tools in one hand while climbing up or down a ladder is an unsafe practice.


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7.7.2 Pumping stations Pumping stations may vary from small, telemetered lift stations that are visited monthly to large pumping stations that have operators on duty 24 hours a day. Regardless of the size, type, or complexity of the pumping station, safe procedures must be followed at all times. Safety precautions are very important for work in any potential confined space, including both wet wells and dry wells.

Always be aware of the possibility of oxygen deficiency/ enrichment, explosive or flammable gases, and toxic gases that could be generated in, or be discharged into, the sewers. A properly operating ventilation system is essential, particularly in below-grade pumping stations where heavier-than-air gases may collect. Before entering a potential confined space in an unattended lift station, test the atmosphere for oxygen deficiency/enrichment, explosive or flammable conditions (lower explosive limit), and toxic gases (hydrogen sulfide).

Do not work on electrical systems or controls unless you are qualified and authorized to do so. Even if you are qualified and authorized, use caution when operating and maintaining electrical controls, circuits, and equipment. Operate only those switches and electrical controls installed for the purpose of your job. Do not open or work inside electrical cabinets or switch boxes unless you are an authorized and qualified electrician.

Be aware of moving equipment, especially reciprocating equipment and rotating shafts. Moving parts that create a contact hazard to employees must be guarded. Do not wear loose clothing, rings, or other jewelry around machinery. Long hair must be secured. Wear gloves and use appropriate tools when cleaning pump casings to protect your hands from dangerous sharp objects.

When starting equipment, everyone should stand away from rotating parts. Dust and oil or loose metal may be thrown from shafts and couplings, or sections of a long, vertical shaft could come loose and whip around, especially during start-up of equipment.

Good housekeeping is a necessity in a pumping station. Common housekeeping problems include water and oil on the floor, dirty or oily rags from cleanup operations, dirty lighting fixtures leading


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"to poor visibility, and stairways with dirt or grease carried in by maintenance or repair crews. Always be sure you have properly secured the pumping station when you leave.

Design parameters •

Wet chambers of pumping stations should be located –wherever is possible- in the exterior of buildings and have adequate ventilation.

Wet chambers of pumping stations should be able to cleaned safely and with ease, while direct access in all areas must be provided.

Underground and Overground operation points and controls (eg handled valves etc) must be accessible without risk,

In the case of screw pumps, cleaning must be done via secure jobs that are accessible without risk. Also the outer sides of the windows of the engine located at the top of the screws, should be thoroughly cleaned from the inside without difficulty.

With regard to submersible pumps, appropriate

provisions should be installed, such as

pedestals, lifting cranes, etc. •

If stairs are installed in a pumping station, they should have handrails and nonslip treads. Where space limitations prevent the installation of stairs, a spiral stairway, ship's ladder, or vertical ladder should be provided. Vertical ladders must be provided with a cage enclosure if they exceed 3 meters in unbroken length. If the ladder is not provided with a safety device or cage, an intermediate platform or landing should be provided for each 2 meters of height or fraction thereof.

Lighting should be sufficient to avoid glare and shadows. Fire extinguishers must be provided in the station, properly located and maintained. They should be of a type that may be used on electrical equipment as well as on solid material or power overload-type fires. The use of liquidtype fire extinguishers should be avoided. All-purpose, ABC chemical-type fire extinguishers are recommended.


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7.8 SPECIFIC HAZARDS IN A WWTP Depending on the nature of labour, there are many hazards that can occur during the operation/maintenance or upgrade og a WWTP.

7.8.1 Excavations If it becomes necessary for you to excavate a sewer line, remember to contact utility companies to locate underground telephone, gas, fuel, electric, cable, and water lines before opening the excavation. If you cannot establish the exact location of the underground utilities, you must proceed with caution and you should use detection equipment, if possible, to locate the service lines. These lines can present a very significant safety hazard to the operator(s) and to the public during excavation activities.

Become familiar with the fundamentals of excavating and the proper, safe approach for shoring, shielding, and sloping and benching before excavating. Without a proper protective system the bank (wall) of a trench or excavation can cave in and kill you. It is strongly recommended that some type of adequate cave-in protection be provided when the trench or excavation is 1,3 meters deep or deeper. Types of adequate protection include shoring, shielding, and sloping and benching. MORE SHORING is a complete framework of wood or metal that is designed to support the walls of a trench. Sheeting is the solid material placed directly against the side of the trench. Either wooden sheets or metal plates might be used. Uprights are used to support the sheeting. They are usually placed vertically along the face of the trench wall. Spacing between the uprights varies depending on the stability of the soil. Stringers (or walers) are placed horizontally along the uprights. Trench braces are attached to the stringers and run across the excavation. The trench braces must be adequate to support the weight of the wall to prevent a cave-in. Examples of different types of trench braces include solid wood or steel, screw jacks, or hydraulic jacks. The space between the shoring and the sides of the excavation should be filled in and compacted in order to prevent a cave-in from getting started. If properly done, shoring may be the operator's best choice for cave-in protection because it actually prevents a cave-in from starting and does not require additional space.


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NIREAS VOLUME 7 46 Shoring conditions Sheet piling or equivalent solid sheeting is required for trenches 1,3 m or more deep. Longitudinalstringer dimensions depend on the strut braces, the stringer spacing, and the depth of stringer below the ground surface. Greater loads are encountered as the depth increases, so more or stronger stringers and struts are required near the trench bottom.

8-17 Sheet pilings for labour in trenches

(John M. Stubbart, 2006 )


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8-18 Sheet pilings for labour in trenches

(John M. Stubbart, 2006 )

8-19 Sheet pilings for labour in trenches – Hard compact ground


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(John M. Stubbart, 2006 )

SHIELDING is accomplished by using a two-sided, braced steel box that is open on the top, bottom, and ends. This "drag shield," as it is sometimes called, is pulled through the excavation as the trench is dug out in front and filled in behind. Operators using a drag shield must always work only within the walls of the shield and are not allowed in the shield when it is being installed, removed, or moved. If the trench is left open behind or in front of the shield, the temptation could be high to wander outside of the shield's protection. Shielding does not actually prevent a cave-in, as the space between the trench wall and the drag shield is left open, allowing a cave-in to start. There have been cases where a drag shield was literally crushed by the weight of a collapsing trench wall.

8-20 Trench shields


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(California State University, 2008)


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NIREAS VOLUME 7 50 8-3 A trench shield during sewer’s network escavation

SLOPING and BENCHING are practices that simply remove the trench wall itself . The amount of soil needed to be removed will vary, depending on the stability of the soil. A good rule of thumb is to always slope or bench at least one meter back for every one meter of depth on both sides of the excavation. Exact sloping angles and benching dimensions largely depend on the type of soil that is being excavated. The type of soil dictates sloping/benching requirements. A competent person must examine the work site and determine the soil classification in order to determine requirements for a specific site. Certain soil conditions can contribute to the chances of a cave-in. These conditions include low cohesion, high moisture content, freezing conditions, or a recent excavation at the same site. Other factors to be considered are the depth of the trench, the soil weight, the weight of nearby equipment, and vibration from equipment or traffic. It is worth repeating that regardless of the presence or absence of any or all of the above factors, the trench must still have proper cave-in protection if it is 1,3 meters or more deep. Excavations less than 1,3 meters deep may also require protection if a competent person determines that there are indications of a potential cave-in. The spoil (dirt removed from the trench) must be placed at least 0,6 meter back from the trench and should be placed on one side of the trench only. A stairway, ramp, or ladder is required in the trench if it is 1,3 meters or more deep. The means of leaving the trench must be placed so that no more than 7,5 meters of travel is required to exit the trench.


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Excavations and adjacent areas must be inspected on a daily basis by a competent person for evidence of potential cave-ins, protective system failures, hazardous atmospheres, or other hazardous conditions. The inspections are only required if an employee exposure is anticipated. Walkways must also be provided where personnel must cross the excavation. If the excavation is 1,8 meters or more in depth at the crossing point, guardrails must also be provided on the walkways.

Accidents at the site of trenching and shoring activities are all too common. In addition to protecting workers from the danger of a cave-in, safety precautions must also be taken to protect them from traffic hazards if the work is performed in a street. Check with your local safety regulatory agency. They can provide you with the appropriate regulations. Do not wait until an emergency arises to obtain the information.


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7.8.2 Bar screens, Grit and sand removal units These may be either manually or mechanically cleaned. When manually cleaning screens or racks, be certain that you have a clean, firm surface to stand on. Remove all slimes, rags, greases, or other material that may create slip or trip hazards. Good housekeeping in these areas is absolutely necessary.

When raking screens, leave plenty of room for the length of your rake handle so you will not be thrown off balance if the handle strikes a wall, railing, or light fixture. Wear gloves to avoid slivers from the rake handle or scraping your knuckles on concrete. Injury may allow an infection to enter your body.

Place all material in a container that may be easily removed from the structure. Be careful lifting containers full of heavy material such as grit. Use of containers sized to prevent excessive weight is recommended. It is better to make more than one trip than to risk injury. Do not allow material to build up on the working surface.

If your rack area is provided with railings, check to see that they are properly anchored before you lean against them. If removable safety chains are provided, never use these to lean against or as a means of providing extra leverage for removing large amounts of material.

A hanging or mounting bracket should be used to hold the rake when not in use. Do not leave it lying on the deck. If mechanically raked screens or racks are installed, never work on the electrical or mechanical part of this equipment without locking out the unit. Always open, lock out, and tag the main circuit breaker before you begin repairs.

The tag should be securely fastened to the breaker handle or to the lockout device to notify others that you are working on the equipment and that it must remain de-energized.

The time and date the unit was locked out should be noted on the tag, as well as the reason it was locked out. The tag should be signed by the person who locked out the unit. No one should then close the main breaker and start the unit until the tag and lock have been removed by the person who placed them there, or until specific instructions are received from the person who tagged the breaker. Your local safety equipment supplier can obtain tags and lockout devices for you.


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Design parameters for bar screens and grit removal units •

Dispose of screenings should be done easily and safely

A sufficient number of passes and floor work (which will be secured from slipping or falling) is required Over the channels

Overpasses behind the oblique gratings must be wide enough to allow placement of the container screenings and the necessary access. Also in these pathways water from the gratings should not be concentrated.

The sharp surfaces between the grid and fixed segments, and the edges of channels must be secured (eg prophylactic sheet and railings)

The automatic mobile shredders screenings, must have on-site power switch on the engine.

Design parameters for sand removal units •

If the removal and disposal of sand is indoors, there must be adequate ventilation

Gates, rotatable blinkers, and shutters at the entrances or exits of sand removal units must be accessed and maintained with safety and ease.

When cleaning canals or wells, there should be a special concern for the safety of staff, as there is a serious risk of releasing gas when sand scraping takes place.

In ventilated sand removal units with spiral water stream, and liquid depth greater than 1,30 meters, a handle support should be installed, in the entire length in the side in which water is directed downwards, immediately above the water level.


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7.8.3 Aeration tanks Guardrails must be installed on the tank side of usual work areas or walkways. If the tank is elevated above ground, guardrails should also be installed on the ground side of the tank.

When working on Y-walls, or other unguarded areas where work is done infrequently, at least two people should do the work. Approved life preservers with permanently attached handlines should be accessible at strategic locations around the aerator. The life preservers must be provided with at least 25 meters of handline and be located not more than 30 meters apart. If you work in areas where the danger of drowning exists, you must wear an approved life jacket or buoyant work vest.

An experiment in England found that if an operator fell into a diffused aeration tank, the operator should be able to survive because air will collect in the clothing and tend to help keep the operator afloat. (California State University, 2008) Drownings apparently occur when a person is overcome by the initial shock or there is nothing to grab hold of to keep afloat or to pull oneself out of the aerator. In aerators where diffused air is supplied only along the walls, strong currents develop, which could pull anyone but a very strong swimmer under water. ALWAYS wear an approved flotation device when working unguarded over water.

When removing or installing diffusers, be aware of the limitations of your working area. Inspect and properly position hoists and other equipment used in servicing swing diffusers. Remember, an aerator is a confined space. When it is necessary to work in an empty aerator, follow confined space procedures and comply with all relevant safety requirements. Portable ladders can be awkward but they are safe if positioned and used properly. A good practice is to use a fall arrest system when climbing up or down a ladder and to secure the top of the ladder so that it cannot slip. Be extremely careful when walking in an aerator; the floor of the aerator can be very slippery.


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Design parameters •

Perimeter protection must be ensured against a fall, especially if the liquid depth is greater than 1,30 m (in this case swimming is either impossible or difficult because of the strong currents generated by the aeration system, either due to reduced buoyancy because of the generated bubbles)

Cross-protective rods are commonly used, which are placed at a distance of about 1 meter from the aerator, in case that a person fells into the tank. Additionally, an appropriate holding device can be placed below the water level consisting e.g. of a mesh made of rods per 0,3 m.

Underground and Overground operation points and controls (eg handled valves etc) must be accessible without risk,

Special measures to ensure non-skid pathways due to foaming or wetting should be taken


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NIREAS VOLUME 7 8-4Pathway in an aeration tank



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7.8.4 Settling tanks and Clarifiers The greatest hazard involved in working on or in a clarifier is the danger of slipping. If possible, maintain a good, nonskid surface on all stairs, ladders, and catwalks. This may be done by using nonskid strips or coating. Be extremely cautious during freezing weather. A small amount of ice can be very dangerous. Be careful and do not fall in.

Your housekeeping program should include the brushing or cleaning of effluent weirs and launders (effluent troughs). Effluent weirs and launders on clarifiers should be brushed or cleaned to avoid the accumulation of slippery slimes and solids. Uneven flow over an effluent weir can encourage short-circuiting.

When it is necessary to actually climb down into the launder, always use a fall arrest system . A fall may result in a very serious injury. If you work in areas where the danger of drowning exists, you must wear a approved life jacket or buoyant work vest.

Remember that a clarifier is a confined space and confined space procedures must be followed for entry and occupancy. Be cautious when working on the bottom of a clarifier. When hosing down, always hose a clean path to walk upon. Avoid walking on the remaining sludge whenever possible.

Always turn off, lock out, and tag the clarifier breaker before working on the drive unit. Adjustments should not be made on flights or scrapers while the unit is in operation. Keep in mind that, although these are moving quite slowly, there is tremendous power behind their movement. Stay clear of any situation where your body or the tools you are using may get caught under one of the flights or scrapers.

Guards must be installed over moving parts that could be contacted accidentally. Keep these in place whenever the unit is in operation.


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NIREAS VOLUME 7 8-5 Bridge in a settling tank


Design parameters •

If settling tanks are equipped with a sludge scraper that can not be completely cleaned from the drawbridge, additional maintenance platforms over the scraper must be included

For sludge scrapers, suitable devices should be placed were possible, in order to automatically clean the walls of the tank, the coronation of overflow channels, the scroll wheel scraper etc.

Sludge removal wells must be designed with railings and be equipped with chutes. Railings must also be installed along the tank side of all normal walkways.

In the round scrapers, lubrication points of the central bearing should be accessible from the bridge


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ASSIGNMENTS SECTION QUESTIONS 1. How can you help prevent the spread of infectious diseases from your job to you and your family?

2. When testing the atmosphere before entry in any confined space, what procedure should be used?

3. How do toxic and suffocating gases or vapors enter the wastewater treatment plant?

4. Why is your sense of smell not a reliable method of detecting hydrogen sulfide?

5. What do the initials UEL and LEL stand for?

6. What is a lockout device and when is it used?


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NIREAS VOLUME 7 60 7. What types of equipment or systems are potentially hazardous for operators due to their stored energy?

8. Identify the primary elements of the hearing conservation program.

9. List at least four hazards that may be encountered when entering a manhole, excluding atmospheric hazards.

10. List three protective systems for excavations.

11. Why should slimes, rags, or greases be removed from around bar screens or racks?

12. What precautions would you take when working on electrical or mechanical parts of equipment?

13. How can the danger of slipping be reduced on slippery surfaces?


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14. Why are chlorine vents placed at floor level?

15. When combustible material in a fire is metal, such as sodium zinc, phosphorous etc , which type of extinguisher you must use? A, B, C, or D ?

16. Oxygen in a concentration above 23.5 % in a closed environment can be dangerous.True or false?

17. Methane always rises in the highest points in an enclosed space because it is lighter than air. True or false?

18. The contact between the aluminum sulfate and calcium oxide (CaO) can easily cause fire, thus it should be avoided . True or false?

19. Ferric chloride must always be carried, trasported or pumped through metal piping, tanks or valves. True or false?


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20. 21. Mixing of any form

of Caustic soda with water leads to a chemical reaction, creating

potentially explosive conditions in the atmosphere. True or false?

22. Chlorine always rises in the highest points in an enclosed space because it is lighter than air. True or false?

23. Hydrogen sulfide will tend to accumulate in the lower points in an enclosed space because it is heavier than air. True or false

24. Acquired Immune Deficiency Syndrome (AIDS), can be contracted from exposure to raw wastewater. True or false?



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1. You can help prevent the spread of infectious diseases from your job to you and your family by immunization shots, thoroughly washing your hands before eating or smoking, wearing proper protective gloves when you may contact wastewater or sludge in -any form, not wearing your work clothes home, keeping your work clothes clean, storing your street clothes and your work clothes in separate lockers, and, if your employer does not supply you with uniforms and laundry service, washing j your work clothes separately from your regular family wash. 2. When testing the atmosphere before entry in any confined space, first test for oxygen deficiency/ enrichment, then combustible gases and vapors, and then toxic gases and vapors. 3. The most common toxic gas in wastewater treatment is hydrogen sulfide, which is produced during the anaerobic decomposition of certain materials containing sulfur compounds. Other toxics, such as carbon monoxide, chlorinated solvents, and industrial toxins, may enter your plant as a result of industrial discharges, accidental spills, or illegal disposal of hazardous materials. 4. Hydrogen sulfide has the unique ability to affect your sense of smell. You will lose the ability to smell hydrogen sulfide's "rotten egg" odor after only a short exposure. Your loss of ability to smell hydrogen sulfide, and certain other odors, is known as olfactory fatigue. NEVER depend on your nose for detection, regardless of the situation. 5. UEL stands for upper explosive limit and LEL stands for lower explosive limit. UEL and LEL indicate the range of concentrations at which combustible or explosive gases will ignite when an ignition source is present at ambient temperature. 6. A lockout device uses a positive means such as a lock to hold a switch or valve in a safe position and prevent a piece of equipment from becoming energized or moving. Lockout devices are used whenever equipment must be repaired or replaced to ensure that the equipment will not start up or move unexpectedly and possibly injure workers. 7. Examples of equipment or systems that are potentially dangerous for operators due to stored energy include: springs; elevated machine members; rotating flywheels; hydraulic systems; and systems using air, gas, steam, or water pressure. 8. The primary elements of the hearing conservation program are monitoring, audiometric testing, hearing protection, training, access to information, and recordkeeping.

9. Mechanical, electrical, engulfment, and toxic exposure hazards may be encountered when entering a manhole. Physical injury, infections and diseases, and insect and rodent bites are other potential hazards.


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10. Three protective systems for excavations are shoring, shielding, and benching and sloping. 11. Slimes, rags, or greases should be removed from any area because they create slip and trip hazards. 12. When working on mechanical or electrical parts of equipment, you should lock out the main circuit breaker and fasten a tag to the handle or to the lockout device to notify others that you are working on the equipment and that it must remain de-energized. 13. Slippery surfaces, such as walkways, stairs, ladders, and catwalks, can be made less dangerous by keeping them free of grease, oil, slimes, or other materials and by applying nonslip strips or coatings. To reduce slipping risks, wear safety shoes with nonslip soles and install hand holds and railings. 14. Chlorine gas is two and a half times heavier than air and is best removed when leaks occur by blowing the gas out of the room at floor level. 15. D 16. True 17. True 18. True 19. False 20. True 21. False 22. True 23. False


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Volume 7 final