![]() ![]() Lack of oxygen is not the only hazard in a confined space. ![]() Even if rescued and resuscitated, permanent damage can occur (Wilkenfeld et al. Without rescue they can die within minutes. If the oxygen depletion is great enough individuals can become unconscious after one breath. When individuals enter an atmosphere with oxygen levels below 16.5% they may immediately become too disoriented to get themselves out and eventually succumb to unconsciousness. Emotional instability and impaired judgement may occur at oxygen levels somewhere between these two points. Below about 11% the person experiences nausea, vomiting, inability to move and unconsciousness. When the percentage of oxygen in breathing air falls below about 16.5% a person’s breathing becomes more rapid and more shallow, the heart rate increases and the person begins to lose coordination. The earth’s atmosphere consists of 21% oxygen at sea level. Because low levels of oxygen in confined spaces cannot be detected by unaided human observation it is extremely important to use an instrument that can determine the level of oxygen before entering any confined space. Decreased oxygen levels can be the result of a variety of conditions including the replacement of oxygen with another gas, such as methane or hydrogen sulphide, the consumption of oxygen by the decay of organic material contained in the waste water or the scavenging of oxygen molecules in the rusting process of some structure within the confined space. Hazards occur when the confined space is associated with a deficiency of oxygen, the presence of a toxic chemical or an engulfing material, such as water. In general, however, a confined space is an area with limited means of entry and egress that was not designed for continuous human habitation and that does not have adequate ventilation. A universal definition of a confined space is elusive. Empty tank in a sewage treatment plant.Ĭonfined-space entry is one of the most common and one of the most serious hazards faced by sewage treatment workers. Wet surfaces, often encountered in sewage treatment plants, contribute to slipping and falling hazards.įigure 2. Mixing equipment, sludge rakes, pumps and mechanical devices used for a variety of operations in sewage treatment plants can maim, and even kill, if they are inadvertently activated when a worker is servicing them. Most sewage treatment plants, however, have confined spaces which include underground or below grade vaults with limited access, manholes (figure 1) and the sedimentation tanks when they have been emptied of liquid content during, for example, repairs (figure 2). Physical hazards vary with the design of the plant. The result of an encounter with a physical hazards can often be immediate, irreversible and serious, even fatal. Physical hazards include confined spaces, inadvertent energizing of machines or machine parts and trips and falls. The key to preventing and/or minimizing the adverse effects associated with working in sewage treatment plants is to anticipate, recognize, evaluate and control the hazards. (See also the article “Sewage treatment” in this chapter.) The specific hazard associated with each process varies depending on the design of the treatment plant and the chemicals used in the different processes, but the types of hazard can be classified as physical, microbial and chemical. There are a variety of different processes that are used to remove contaminants from waste water including sedimentation, coagulation, flocculation, aeration, disinfection, filtration and sludge treatment. The goal of waste water treatment plants is to remove as much of the solid, liquid and gaseous contaminants as possible within technically feasible and financially achievable constraints. ![]() Although some waste treatment plants are exclusively dedicated to handling one or the other type of material, most plants handle both animal and industrial waste. Industrial solid waste production rates range from 0.12 tons per employee per year at professional and scientific institutions to 162.0 tons per employee per year at sawmills and planing mills (Salvato 1992). Humans excrete about 250 grams of solid waste per capita per day, including 2000 million coliform and 450 million streptococci bacteria per person per day (Mara 1974). Two basic types of waste enter a treatment plant, human/animal waste and industrial waste. Although reduction of the amount of waste is important, the proper treatment of waste is essential. ![]() Without treatment of waste the current concentration of people and industry in many parts of the world would very quickly make portions of the environment incompatible with life. ![]()
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