Removal of Pollutants by Sulfide Precipitation at Three Plants

Pollutant Raw Wastewater Treated Effluent Percent Removal

Plant 1

Chromium, hexavalent (mg/L) 26,000

Chromium (mg/L) 32,000

Plant 2

Chromium, hexavalent (mg/L) 22

Chromium (mg/L) 2400

Nickel (mg/L) 680

Plant 3

Chromium, hexavalent (mg/L) 11,000

Chromium (mg/L) 18,000

Source: U.S. EPA, References 1, 2, and 5. BDL, below detection limit. a Approximate value.

BDL BDL BDL BDL

Most of the pollutants may be effectively removed by precipitation of metal hydroxides or carbonates using a reaction with lime, sodium hydroxide, or sodium carbonate. For some, improved removals are provided by the use of sodium sulfide or ferrous sulfide to precipitate the pollutants as sulfide compounds with very low solubilities. After soluble metals are precipitated as insoluble flocs, one of the water-solid separators (such as dissolved air flotation, sedimentation, centrifuga-tion, membrane filtration, and so on) can be used for flocs removal.9-11 The effectiveness of pollutant removal by several different precipitation methods is summarized in Tables 5.15-5.17. Table 5.18 presents the removability of pollutants by two types of skimming systems:

1. The API system

2. The TEB system

API stands for the American Petroleum Institute and TEB stands for Thermal Emulsion Breaking. Figures 5.1 and 5.2 show two typical types of oil-water separators, gravimetric and parallel plate.7 A dissolved air flotation (DAF) clarifier is commonly used for polishing the effluent from an oil-water separator.8-1013

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