2011-01-24_REPORT - C1980005 - Laserfiche WebLink

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w Table 1 - Proportions of HZS and HS " in dissolved sulfide pH Proportion o un-ionized HZS ' fac or Proportion of HS " 5.0 0.99 0.01 6.0 0.91 0.09 6.2 0.86 0.14 6.4 0.80 0.20 6.6 0.72 0.28 6.8 0.61 0.39 7.0 0.50 0.50 7.2 0.39 0.61 7.4 0.28 0.72 7.6 0.20 0.80 7.8 0.14 0.86 8.0 0.09 0.91 8.2 0.059 0.941 8.4 0.039 0.961 8.6 0.025 0.975 8.8 0.016 0.986 9.0 0.010 0.99 Toxicity of HZS -1.1 .y95 -7.3 , 33 5 --i.s .-A `I -7.7 . Il $ , .o7S -8.3 .oil 9 `6.5 .033, %.7 ®a I 8.5 ,a 13 Note: The proportion of un-ionised H2S is referred to as the "i factor" when used in the predictive equations in Part 3 Everyone is familiar with the odour of H2S in its natural occurrences. Because of this familiarity there has been a lack of appreciation of its toxic character, and many deaths have resulted from carelessness in dealing with it. The threshold odour concentration of H2S is very low - between 1 and 10 ng/I. It is potentially very dangerous because its smell is quickly lost as the concentration increases. In oil refineries, tanneries, viscose plants, and many other chemical industries, men have occasionally been exposed to H2S in concentrations that have resulted in death, and there have been many deaths in sewers on this account. Even the H2S from swamps and from natural hot springs can be deadly. Several lives have been lost as a result of bathing in hot sulfurous spring waters in closed rooms. There is evidence that a concentration of 0.03 % (300 parts per million) of H2S in the air has caused death. It should be noted that this is the concentration that could arise from water containing 1 mg/I of un- ionized H2S. Fortunately these hazards are now more widely recognised, and the frequency of fatal accidents has been greatly reduced. Other forms of sulfide in waste waters Up to this point the discussion has been about hydrogen sulfide and its ionized form, HS -. Sulfur combines with metals, too, producing compounds which are generally insoluble, such as zinc sulfide, ZnS, two copper sulfides, CuS and Cu2S, several iron sulfides, etc. In all such combinations, as well as in H2S and HS -, sulfur is in an electronegative state. In this state it is simply called sulfide. In waste waters of normal pH values (6.5 to 8), sulfide may be present partly in solution as a mixture of H2S and HS -, and partly as insoluble metallic sulfides carried along as part of the suspended solids. In analyses of waste waters, a distinction is made between dissolved sulfide and insoluble sulfide. The sum of these forms is called total sulfide. The concentrations are normally expressed in terms of the sulfur content. The amount of insoluble metallic sulfide does not ordinarily exceed 0.2 to 0.3 mg/1 if the sewage is of residential origin, but the amount may be larger in sewers containing trade wastes.